Nowhere is the continental crust evolving more rapidly than in the core of orogenic belts where the thickened crust typically undergoes partial melting and orogenic collapse, leading ultimately to crustal stabilization. While the processes of accretion and collision may take 10-100 Myr, the collapse instability, involving flow of deep crust and formation and emplacement of metamorphic core complexes and migmatite domes, is short lived (1-10 Myr). Material transfer during orogenic collapse is achieved by (1) lateral flow near the base of the evolving continental crust, where highly sheared partially molten rocks remain buried, and (2) upward flow within metamorphic core complexes (MCCs) below bounding extensional fault systems. These MCCs are commonly cored by migmatite domes that provide exceptional windows into the dynamics of the deep orogenic crust. Therefore, extensional detachment systems and migmatite domes are prime recorders of how the mechanical and thermal instability introduced by crustal thickening and melting, transitions to a gravitationally equilibrated crust. Some notable research results from these systems include the record of fluid-rock interaction across detachments and the provenance of partially molten crust as observed in migmatite domes.

MCCs are bounded by detachment shear zones that record large strain and metamorphic gradients as well as effective fluid-rock interaction enhanced by intense deformation and recrystallization processes at the grain scale. Stable isotope analyses across detachment systems have delineated the limit between a zone dominated by surface-derived (meteoric) fluids above and a zone of prevailing metamorphic fluids below. In some cases, the meteoric fluid is consistent with a surface fluid that precipitated at high elevation and was involved in convective flow from the surface down to the detachment shear zone, likely during the initial stages of orogenic collapse. The rocks within migmatite domes are varied and include refractory lithologies, typically mafic enclaves or pods, that inform the provenance of partially molten crust. In some domes, mafic pods are made of eclogite, and the age of eclogite metamorphism (pressures of 1.5-2.0 GPa) is close to the age of migmatite crystallization. In other cases, eclogite that was formed in subduction zones was incorporated as blocks or pods and transported laterally within the partially molten crust over long distances across the orogen before being exhumed in domes. The presence of eclogite within migmatite suggests that the partially molten crust is sourced at near-Moho depths and is extremely mobile, with the ability to travel laterally (order of >100 km) as well as vertically. Phanerozoic orogenic cores provide a template for understanding the extent of reworking of continental material and the flow of this material during the stabilization of continental crust over geologic time.

How to cite: Teyssier, C.: At the Core of Orogens, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7238, https://doi.org/10.5194/egusphere-egu26-7238, 2026.

In the Chundi–Malakonda–Ayyavaripalle region of the Nellore Schist Belt, part of the Dharwar Craton, six north-south (N–S) trending, elongated Banded Magnetite Quartzite (BMQ) bands have been identified. These bands are exposed within meta-rhyolite and quartzite formations and are associated with biotite–muscovite schist. The BMQs vary in width from 20 to 40 meters and extend in length from 1.5 to 4.5 kilometres. ​ High-resolution aeromagnetic surveys with a terrain clearance of 80 meters have revealed significant magnetic anomalies over the study area (source: https://geodataindia.gov.in). These anomalies range from –3,900 to +5,000 nanoteslas (nT), indicating a high concentration of magnetic minerals within the exposed BMQs, designated as Bands 1 to 6. In addition to these exposed bands, a concealed, parallel, N–S trending BMQ band has been identified through detailed analysis of aeromagnetic data. 2D and 3D Interpretation of the magnetic anomalies suggests that meta-rhyolites exist up to an average depth of 250 m from the surface and might be associated with BIF bands at depth. This depth extent highlights the substantial vertical continuity of the magnetite-rich formations in the region. The integration of geological mapping and aeromagnetic data provides a comprehensive understanding of subsurface geology, highlighting the potential for significant mineralization within the Nellore Schist Belt.

How to cite: Dharavathu, S., Kosuri, S. K., Vappangi, P. K., and Kumar, P.: Unveiling concealed Banded Magnetite Quartzites (BMQs) through high-resolution aeromagnetic surveys: New insights from the Nellore Schist belt of Eastern Dharwar Craton, India, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2063, https://doi.org/10.5194/egusphere-egu26-2063, 2026.

Within the comprehensive framework of the Sun–Earth system, plasma environments exhibit an exceptionally wide range of physical conditions. These encompass the ultra-high-temperature, high-pressure, and high-density liquid metallic outer core of Earth, which generates the geomagnetic field through the geodynamo process; the tenuous, partially ionized ionosphere; and the magnetosphere, which provides essential shielding against energetic cosmic and solar radiation while exerting substantial influence on human technological systems, most notably microwave communication infrastructure. In addition, transient ultra-high-temperature plasmas generated by solar flares and coronal mass ejections (CMEs) represent the primary drivers of disturbed space electromagnetic environments, as they propagate through interplanetary space and subsequently interact with Earth's magnetosphere.Although prior research has extensively employed the first-principles quantum Monte Carlo method coupled with the lattice Boltzmann approach (FPQM-LBM) to address various theoretical and computational aspects of plasma behavior in this context, no existing modeling framework has successfully integrated — within a single consistent methodology — the extreme conditions of the Earth's outer core plasma, the low-density ionospheric plasma, the magnetospheric plasma, and the highly energetic, transient flare/CME plasmas. As a result, a unified and comprehensive understanding of particle transport mechanisms and internal structural properties across the full spectrum of plasma regimes in the Sun–Earth system remains elusive.The present study aims to address this critical gap by developing novel theoretical frameworks and advanced computational methodologies for elucidating the particle migration mechanisms and structural characteristics of space electromagnetic plasmas throughout the panoramic Sun–Earth system. To this end, we will enhance the first-principles quantum Monte Carlo–lattice Boltzmann method (FPQM-LBM) to establish robust techniques capable of modeling particle transport under the complex electromagnetic conditions prevailing in space environments. The improved FPQM-LBM framework will be systematically applied to simulate particle dynamics across the aforementioned plasma regimes — namely, the ultra-high-temperature/pressure/density outer core plasma, the low-density ionosphere, the magnetosphere, and transient flare/CME plasmas — with particular emphasis on ionic characteristics, microstructural evolution, fine-scale particle transport processes, internal structural transformations, and the response of plasma properties to external electromagnetic perturbations. The anticipated results are expected to furnish a solid theoretical foundation and valuable predictive capabilities for advancing solar–terrestrial space physics and enhancing electromagnetic monitoring and forecasting in space weather research.

How to cite: Zhu, B.: Theoretical Foundations and Methodological Developments in the Study of Particle Transport Mechanisms and Microstructural Evolution Employing the Hybrid Quantum Monte Carlo–Boltzmann Transport Model, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3254, https://doi.org/10.5194/egusphere-egu26-3254, 2026.

Space weather can affect the operation of high voltage AC transformers in power grids by applying an offset DC current during periods of heightened geomagnetic activity. Modelling GIC requires knowledge of magnetic field variation, the response of the local subsurface geoelectric field (related to conductivity) and a representation of the connections between transformers and various resistance parameters of the power network. Presently, in Britain, the largest uncertainty in this chain applies to the resistance parameters of the network, as these values come from open-source data which are known to have many approximations.

Recent work with a transmission network operator in the UK has provided us with an improved dataset of resistance parameters of transformers, power lines and substation grounding. The grounding resistance at electrical substations has not been known before and so historically was set at 0.5 Ω in our models. The new dataset of 110 sites around central Scotland reveals substation grounding resistance varies from 0.04 Ω to 11.7 Ω with a mean of 0.54 Ω but a median of 0.2 Ω. Combined with line and transformer resistance information, we have created an improved representation of the power grid in Scotland.

Using GIC measurements from three sites (Torness, Strathaven and Neilston) for the largest geomagnetic storms in the past 25 years (October 2003, September 2017 and May 2024), we are able to validate the new model, demonstrating its improved accuracy.

The new model demonstrates that our previous assumptions of grounding resistance were too high but our estimate of line resistance was too low, thus balancing out the overall GIC magnitude on average. However, in detail, some locations show large differences in GIC compared to the original model. This highlights the importance of using accurate resistance information to correctly capture GIC.

How to cite: Beggan, C., Richardson, G., and Lawrence, E.: Improving GIC modelling and validation with high-quality information on power network parameters, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5611, https://doi.org/10.5194/egusphere-egu26-5611, 2026.

During intense solar atmospheric activity—such as major solar flares and geomagnetic storms—magnetic energy is converted into plasma kinetic and thermal energy through three-dimensional turbulent magnetic reconnection within large-scale, extended current sheets. This process releases enormous amounts of stored energy, often accompanied by the rapid ejection of high-energy particles into interplanetary space. These high-energy particles include electrons, protons, helium nuclei, and heavier ions, forming a complex multi-component, multi-abundance, and multi-isotopic population. Their energies span from ~100 keV to ~100 MeV and even into the GeV range, making them a primary driver of space weather hazards. Understanding the sources and acceleration mechanisms of these particles remains one of the most critical challenges in space weather research. Previous studies have shown that high-energy particle acceleration is highly complex, involving multiple species, a variety of mechanisms, and interactions across scales. It remains an open and challenging problem in solar and plasma physics. This paper provides a systematic review and forward-looking perspective on recent advances in high-energy particle acceleration during the fine-scale evolution of large-scale current sheets. The discussion is organized around three key pillars: theory, observations, and numerical simulations. First, we summarize the turbulence-fractal model as it applies to typical solar atmospheric events. We focus on acceleration mechanisms in turbulent magnetic reconnection within large spatiotemporal current sheets, with particular emphasis on: Turbulent (second-order) Fermi acceleration, Turbulent shock acceleration, and Turbulent wave-particle resonant acceleration. These mechanisms operate synergistically in the turbulent environment generated by reconnection, enabling efficient energy transfer to particles. Second, we review recent progress in coupling macroscopic (hydrodynamic and magnetohydrodynamic) dynamics to microscopic kinetic processes in high-energy particle acceleration. This includes multi-scale modeling of turbulence, reconnection, and particle transport. Finally, we outline promising future research directions, including improved multi-spacecraft observations, higher-resolution simulations that incorporate kinetic effects, and integrated models that bridge MHD turbulence and particle-in-cell approaches. We also highlight several urgent unresolved issues, such as the relative contributions of different mechanisms across energy regimes, the role of fractal structures in particle trapping and escape, and the origin of observed abundance enhancements in heavy ions. This review synthesizes recent theoretical, observational, and computational developments to provide a comprehensive framework for understanding high-energy particle acceleration in large-scale turbulent current sheets, with implications for solar flares, space weather forecasting, and broader astrophysical plasma processes.

How to cite: Zhu, B.: Research Progress on SEPs on the Fine Structures of the Large Temporal-spatial Current Sheets in Solar Flares/CMEs, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6969, https://doi.org/10.5194/egusphere-egu26-6969, 2026.

Lago del Desierto (49°02′S, 72°51′W) is located in a climatically sensitive sector near the Southern Patagonian Ice Field (Argentina). Three sediment cores collected from two sites in the lake were analyzed using a multi-proxy approach to reconstruct past environmental variability (Kastner et al., 2010). Numerous turbidites were identified in the sedimentological record. After excluding these event layers, a new age–depth model was developed for the first 3 sections of the core DES05-3 (289 cm), and paleosecular variations (PSV) were reconstructed for the interval between ~1000 and 3500 cal. BP.

Standard paleomagnetic measurements (alternating-field demagnetization) were performed on 125 samples from the same core. In addition, rock-magnetic measurements, including Anhysteretic Remanent Magnetization (ARM, 100 mT peak AF, 0.05 mT DC field), Isothermal Remanent Magnetization (IRM, acquisition up to 1.5 T and backfield curves), hysteresis loops and thermomagnetic analyses, were applied to extract complementary paleoenvironmental information from the sediment cores.

Rock-magnetic measurements indicate that the magnetic mineralogy is dominated by a low-coercivity component (magnetite-type), accompanied by a secondary high-coercivity fraction (hematite/goethite-type). This downcore distribution mirrors the paleoenvironmental shift described by Kastner et al. (2010): the lower part of the sequence shows an enhanced contribution of high-coercivity Fe oxides, consistent with more stable and chemically weathered catchment conditions. In contrast, the upper part shows an increasing dominance of detrital magnetite, indicating strengthened minerogenic supply and enhanced erosion, matching the onset of warmer conditions and glacier retreat during the Medieval Climate Anomaly as inferred from geochemical and lithological proxies. This agreement between magnetic and non-magnetic sediment parameters suggests coherent changes in the provenience of the sediment and in catchment dynamics over the last millennia. As expected from the catchment instability and the numerous turbidites in the upper part of the sequence, this interval could not be used for PSV reconstruction due to its discontinuous directional record. In contrast, samples from the lower part (~1000–3500 cal. BP) provided a continuous sequence suitable for paleosecular variation analysis. Although samples from this unit were not completely demagnetized at 100 mT, due to the presence of a high-coercivity component, magnetization directions consistently decayed toward the origin with high precision. Characteristic remanent magnetization (ChRM) directions were determined using principal component analysis, with maximum angular deviation (MAD) values below 2.5° for all non-turbidite samples. The resulting PSV record compares well with geomagnetic field models and other Patagonian paleomagnetic reconstructions. Inclination values range from −40° to −70°, displaying coherent directional variability over the last ~3500 years.

References:
Kastner, S., Enters, D., Ohlendorf, C., Haberzettl, T., Kuhn, G., Lücke, A., Mayr, C., Reyss, J.-L., Wastegård, S., & Zolitschka, B. (2010). Reconstructing 2000   years of hydrological variation derived from laminated proglacial sediments of Lago del Desierto at the eastern margin of the South Patagonian Ice Field, Argentina. Global and Planetary Change, 72(3), 201-214. https://doi.org/10.1016/j.gloplacha.2010.04.007

How to cite: Achaga, R. V., Gogorza, C. S. G., Irurzun, M. A., Ohlendorf, C., Haberzettl, T., and Zolitschka, B.: A Late Holocene Paleomagnetic Record from Lago del Desierto, Southern Patagonia (Argentina), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8141, https://doi.org/10.5194/egusphere-egu26-8141, 2026.

The source of compressional Pc3 magnetic pulsations has been considered to be the plasma process in the solar wind or in the magnetosphere. However, we found some strong inland earthquakes that occur on the dayside also excite Pc3s. The Lamb waves generated by the January 2022 Tongan undersea volcanic eruption are also believed to have excited large amplitude and short period compressional Pc3 geomagnetic pulsations in the dayside plasmasphere (Iyemori et al., 2025). Increases in power spectral density (PSD) in the Pc3 frequency band were observed 10-30 minutes after the origin time of large inland earthquakes (M>6.5) during the daytime (10-14 LT). During these large earthquakes, seismic waves with period of 10-30 seconds propagate far away (even more than several thousand km), causing slight fluctuations in the orientation of magnetometer sensors, resulting in apparent Pc3-like fluctuations. To avoid such sensor tremor effect, we analyzed the total force of magnetic field, or analyzed comparing with seismometer data. We also used the Swarm satellite observation. The PSD of Pc3s caused by earthquakes or by Lamb wave show many spectral peaks having interval of 3-5 mHz, and this is similar with the characteristic reported by, for example, Samson et al. (1995) for normal, i.e., solar wind origin Pc3s. In this paper, we will also show the commonality between the Pc3s caused by earthquakes or Lamb waves and those originated from the solar wind and discuss what the commonality means.

How to cite: Iyemori, T., Aoyama, T., and Yokoyama, Y.: Pc3 geomagnetic pulsations excited by earthquakes and their commonality with solar wind-originated Pc3, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8566, https://doi.org/10.5194/egusphere-egu26-8566, 2026.

Continental sedimentary sequences of alternating loess and palaeosol horizons preserve detailed records of past global climate changes during the Pleistocene. Obtaining deeper and genuine knowledge on the history of past climates using proxy data depends on interdisciplinary approaches, novel techniques and thinking “out-of-the-box”. The LOEs-CLIMBE team members gather around this concept and present here the first pilot magnetic data from the Kolobar loess-palaeosol section in NE Bulgaria. The 25 m thick section is exposed in an active quarry and was sampled at 2-cm-resolution, covering the Holocene soil, seven palaeosol units and loess horizons L1 to L7 of varying thicknesses. New high resolution magnetic susceptibility data, delineates palaeosol horizons with high values of mass specific magnetic susceptibility except the special case of fourth palaeosol S4, showing no magnetic enhancement as compared to the underlying thin loess. Such depletion of pedogenic magnetic enhancement in paleosol units from the Lower Danube area is rarely reported. This phenomenon will be further examined by detailed magnetic and colorimetric methods. The strongest pedogenic magnetic signal is observed in the three youngest palaeosol units S1, S2 and S3, tentatively related to the interglacial stages MIS 5, MIS 7 and MIS 9. The weakest magnetic susceptibility is typical for the younger part of the loess unit L2, punctuated by the signal of a tephra layer, which is a widespread chronostratigraphic marker in the region.  This research is carried out and financed within the framework of the second Swiss Contribution MAPS, LOEs-CLIMBE project № IZ11Z0_230102.

How to cite: Jordanova, D., Georgieva – Ishlyamska, B., Veres, D., Baykal, Y., Robu, M., Jordanova, N., Hambach, U., Ishlyamski, D., Dimov, D., Trott, A., and Wiesenberg, G.: High-resolution magnetic record of environmental changes during Middle – Late Pleistocene from a loess-palaeosol sequence in NE Bulgaria – pilot data from the LOEs-CLIMBE project, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8852, https://doi.org/10.5194/egusphere-egu26-8852, 2026.

The Earth's magnetic field is divided into internal and external sources, with the internal field including the main magnetic field, the crustal magnetic field, and the induced magnetic field. Among these, the main magnetic field accounts for approximately 95% of the Earth's total magnetic field. Data from paleomagnetic records in rocks, various geomagnetic observatories, and satellites indicate that the main magnetic field exhibits westward drift, polarity reversals, intensity decay, and brief geomagnetic excursions at the core-mantle boundary. To explain these phenomena, several models have been proposed in previous studies. The prevailing view is that the outer core is composed of liquid metal, and the Earth's main magnetic field is generated by the turbulent fluid motion of this liquid metal, influenced primarily by factors such as its composition and properties, thermal convection, Lorentz force, and Coriolis force. Considering the strong Coulomb forces between electrons and ions, previous research has usually treated the electrons and ions in the outer core's metallic fluid as a unified component, greatly simplifying the study and achieving satisfactory results. However, existing studies have not taken into account the differences in motion between ions and electrons under the dynamics of the outer core, the resulting spatial distribution differences of electrons and ions in the outer core, or the impact of these differential distributions on the Earth's main magnetic field. In view of this, This paper studies the effect of the factors generating outer core dynamics on the distribution of electrons and ions in the outer core. It examines the distribution of electrons and ions in the outer core space under equilibrium conditions and estimates their contribution to Earth's main magnetic field. Then, by changing parameter conditions (such as temperature gradients) and adding convective terms (non-equilibrium state), the calculations are redone. These results are used to explain changes in Earth's magnetic field.

How to cite: Wang, S., Li, Y., Zhu, B., Zhao, Y., Wang, Q., and Liu, H.: The influence of the heterogeneity (stratification) of the outer core fluid on the variation of the geomagnetic field, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9301, https://doi.org/10.5194/egusphere-egu26-9301, 2026.

The stability of critical infrastructure in Northwest Bulgaria (Western Moesian Platform) could be compromised by ground instability within Neogene sediments that cover the region. This is evidenced by the collapse of the I-1 national road near Dimovo town in 2006, which involved vertical displacements of 3–4 meters. The purpose of this study is to identify the underlying geological drivers of this failure and to evaluate the specific hazard in the area resulting from the interaction between the sediments and the local environmental conditions. We hypothesize that the instability is not merely a result of conventional failure mechanisms but is governed by an anomalous mineralogical composition, specifically by the presence of aragonite and gypsum layers, which could create a dual hazard.

To elucidate geological drivers, we employed a methodology that integrates field mapping and sampling with laboratory analyses. Samples from the Neogene sediments in the area of 2006 damage underwent mineralogical analyses using X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM) to determine phase composition and morphology. These analyses were coupled with X-ray fluorescence (XRF) for chemical profiling and standard geotechnical testing to determine grain size distribution, Atterberg limits, and Activity index according to Skempton’s classification.

The analysis reveals a heterogeneous sediment succession with the presence of inorganic clays with high plasticity. The XRD and SEM results identified a mineralogical anomaly where the high concentrations of metastable, acicular aragonite coexist with active swelling phyllosilicates (smectite/illite). Furthermore, various amounts of gypsum were detected in some of the samples, indicating an evaporitic paleoenvironment. Geotechnically, these materials exhibit extreme reactivity. Liquid limits range from 34.85% to 67.88%, and plasticity indices reach up to 47.39%. The Activity index peaks at 2.00, categorizing the sediments as "highly active" and prone to volume change driven by moisture variations.

The study concludes that ground failure is a direct consequence of a synergistic hydro-chemo-mechanical mechanism driven by the sediments' mineralogy. The specific aragonite fabric allows rapid water infiltration, triggering the hydration of smectites that could lead to loss of shear strength. Simultaneously, gypsum dissolution could create secondary porosity, reduce effective stress, and release sulfate ions, which could pose a potential chemical hazard to concrete foundations through sulfate attack. Furthermore, the high silt content facilitates internal erosion and possible piping through fracture networks, which could explain the sudden loss of support and large vertical displacements observed in the 2006 case. These findings imply that standard geotechnical data alone are insufficient for risk assessment in this region. Effective mitigation strategies must integrate mineralogical analysis to address both the physical swelling and the chemical durability risks.

Acknowledgements: This research was funded by the European Union-NextGenerationEU, through the National Recovery and Resilience Plan of the Republic of Bulgaria, Project No BG-RRP-2.004-0008-C01.

How to cite: Dotseva, Z., Vangelov, D., and Stanimirova, T.: Mineralogical Drivers of Ground Failure in Neogene Sediments: a Case Study from Northwest Bulgaria, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10911, https://doi.org/10.5194/egusphere-egu26-10911, 2026.

How to cite: Girgis, K., Arthus Schanner, M., Panovska, S., and Yoshikawa, A.: Effects of the Historical Geomagnetic Field on Earth's Energetic Particle Environment: Magnetic Anomalies and Auroral Regions , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14934, https://doi.org/10.5194/egusphere-egu26-14934, 2026.

The Swarm Data Fusion (SwarmDF) toolbox is designed as an easy-to-use Python module for analysing the local electrodynamics of the high-latitude ionosphere by combining measurements from ESA’s Swarm satellites with additional ionospheric and thermospheric datasets. Given a Swarm satellite ID, a regional grid, and a time interval, the toolbox automatically retrieves and combines available observations from SuperDARN, SuperMAG, Iridium/AMPERE, and Swarm electromagnetic field measurements. SwarmDF uses the local mapping of polar ionospheric electrodynamics (Lompe) technique to reconstruct two-dimensional maps of key electrodynamic parameters in the vicinity of the Swarm satellite tracks. To assess and quantify reconstruction performance, SwarmDF integrates the LompeOSSE Python module, which generates controlled synthetic electrodynamics datasets based on Gamera simulations and enables systematic comparisons with the toolbox outputs under different data availability and configuration scenarios. Featuring a user-friendly graphical interface, SwarmDF simplifies data handling and model setup for high-latitude ionospheric electrodynamic studies using Swarm observations.

How to cite: Decotte, M., Laundal, K. M., and Kebede, F. T.: SwarmDF: A toolbox for analysing high-latitude ionospheric electrodynamics, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18706, https://doi.org/10.5194/egusphere-egu26-18706, 2026.

The Iberian Pyrite Belt (IPB) hosts one of the largest concentrations of massive sulfide deposits in Europe, yet its lithosphere architecture remains incompletely understood. In this study, we employ magnetotelluric (MT) impedance tensor data to investigate the dimensionality and structural characteristics of the IPB crust. The analysis combines two complementary approaches: WAL invariants, computed from the MT impedance tensor using the Waldim code (Martí et al., 2013), which are scalar, rotation invariant quantities providing a robust, frequency dependent measure of three-dimensionality and highlighting anisotropic features in the conductivity distribution; and the Phase Tensor, following the methodology of Caldwell et al. (2004), which offers distortion free insights into the orientation and geometry of regional conductive structures. Integrating these methods enables a systematic dimensional analysis of the impedance tensor, revealing lateral heterogeneities, preferred orientations of conductive features, and depth dependent variations in lithospheric responses.

The results demonstrate that WAL invariants and Phase Tensor analysis together allow the separation of near surface distortions from deeper geoelectric structures, providing a robust framework for characterizing the lithospheric architecture of the IPB. This study highlights the enhanced resolution and robustness achieved by complementing the tensor based analysis of MT data with invariant derived quantities that provide rotationally independent measures of three-dimensionality and anisotropy.

Consequently, this dimensional and structural assessment constitutes a critical prerequisite for subsequent MT data inversion, as it provides essential constraints on model dimensionality, structural orientation, and the treatment of near surface distortion. By supporting the choice between 2D and 3D inversion strategies, the proposed framework enhances the stability of the inversion process, increases the reliability of the conductivity distributions, and ensures greater geological consistency of the resulting models.

Acknowledgment

This work is supported by FCT, I.P./MCTES through national funds (PIDDAC): LA/P/0068/2020, https://doi.org/10.54499/LA/P/0068/2020,UID/50019/2025,  https://doi.org/10.54499/UID/PRR/50019/2025, UID/PRR2/50019/2025

References

Caldwell, T.G., Bibby, H.M. and Brown, C. (2004). The magnetotelluric phase tensor. Geophysical Journal International, 158: 457-469. https://doi.org/10.1111/j.1365-246X.2004.02281.x

Castro, C., Hering, P., Junge, A. (2020). FFMT: a MATLAB-based toolbox for Magnetotellurics (MT). 10.13140/RG.2.2.12465.92007.

F. E. M. Lilley. (1998). Magnetotelluric tensor decomposition; Part, Theory for a basic procedure. Geophysics; 63 (6): 1885–1897. doi: https://doi.org/10.1190/1.1444481.

Martí, A., Queralt, P., Ledo, J., Farquharson, C. (2010). Dimensionality imprint of electrical anisotropy in magnetotelluric responses, Physics of the Earth and Planetary Interiors, Volume 182, Issues 3–4, 2010, Pages 139-151, ISSN 0031-9201. https://doi.org/10.1016/j.pepi.2010.07.007.

Martí, A., Queralt, P., Ledo, J. (2013). WALDIM: A code for the dimensionality analysis of magnetotelluric data using the rotational invariants of the magnetotelluric tensor. Computers & Geosciences. 2295-2303. 10.1016/j.cageo.2009.03.004

Miensopust, M. P. (2017). Application of 3-D electromagnetic inversion in practice: Challenges, pitfalls and solution approaches. Surveys in Geophysics, 38(5), 869–933. https://doi.org/10.1007/s10712-017-9435-1.

Vozoff, K. (1991). The magnetotelluric method: Electromagnetic methods. In M. N. Nabighian (Ed.), Applied Geophysics (pp. 641–712).

Kelbert, A., Meqbel, N., Egbert, G. D., & Tandon, K. (2014). ModEM: A modular system for inversion of electromagnetic geophysical data. Computers & Geosciences, 66, 40–53. https://doi.org/10.1016/j.cageo.2014.01.010.

How to cite: Baltazar-Soares, P., Martinéz-Moreno, F. J., Gonzaléz-Castillo, L., Galindo-Zaldívar, J., Monteiro Santos, F., Mateus, A., and Matias, L.: Dimensionality Analysis of the Iberian Pyrite Belt Lithosphere derived from the Magnetotelluric Impedance Tensor., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21147, https://doi.org/10.5194/egusphere-egu26-21147, 2026.

This study investigates the deep structure of the Ouarzazate Basin, located between the Central High Atlas and the Anti-Atlas, using gravity data analysis. Gravimetric methods were applied to map subsurface structural lineaments beneath the sedimentary cover. The resulting structural map reveals that the basin is mainly controlled by ENE-WSW oriented faults, with subordinate E-W and NE-SW trends related to Variscan deformation, Triassic-Jurassic rifting, and Atlas tectonic inversion. Positive gravity anomalies show preferential NE-SW and E-W orientations and are linked to the structural configuration of the Central High Atlas, which acted as a major source area for the basin. The identified fault systems and compressional structures in the Central High Atlas and Anti-Atlas are consistent with the regional geodynamic evolution. These results highlight the strong tectonic connection between the Ouarzazate Basin and adjacent Atlas basins, particularly the Central High Atlas, and provide new insights into the basin’s geodynamic development.

How to cite: Bouali, B., Tabayaoui, F.-Z., Sahbi, H., Manar, A., Nouayti, A., Boujamaoui, M., and Berkat, N. E.: Subsurface structural mapping using high-resolution gravity data and advanced processing techniques in the Ouarzazate Basin, Southern Morocco., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3508, https://doi.org/10.5194/egusphere-egu26-3508, 2026.

The Zahedan fault zone in Iran constitutes an active tectonic zone characterised by a complex network of strike-slip faults that dominate the local deformation pattern. This area is located within a large-scale transpressional shear zone accommodating relative motion between the Central East Iranian block and the Afghan Helmand block. The region provides a natural laboratory for investigating the relationship between strike-slip faulting and tectonic blocks rotated around vertical axes.

We present herein, based on high-resolution 2025 Airbus satellite imagery and cartographic and geophysical data, a new strike-slip fault pattern that facilitated the development of the rotated tectonic blocks.

Our observations show that the major strike-slip fault zones are accompanied by dense networks of second-order faults, including single sets of antithetic and synthetic strike-slip faults, conjugate strike-slip fault sets, restraining and releasing stepovers, and thrust faults. In several sectors along the major faults occur the zones of deformation consisting of the rotated tectonic blocks. The scale, orientation, and spatial organisation of the mapped structures indicate that block rotation is controlled by the interaction between major strike-slip faults and subsidiary fault networks.

The individual second-order antithetic faults display that these faults commonly accommodate small displacements, but the faults play a critical role in allowing internal deformation within blocks and facilitate the progressive block rotation. The sense of movements along the major fault and the antithetic strike-slip faults bounding the tectonic blocks allows us to consider the structures as the blocks rotated around vertical axes in a domino-like orientation. Recognised examples of structures show that some rotating blocks are rigid, with no evidence of significant internal deformation, while other rotating blocks exhibit strong internal deformation.

Understanding these spectra of behaviours and the determination of the relationships between them will improve our knowledge of fault interaction processes in eastern Iran and related patterns of seismicity, and it also has implications for seismic hazard assessment in active transpressional settings.

How to cite: Paktarmani, Z., Konon, A., and Mikołajczak, M.: Rotation of tectonic blocks controlled by strike-slip component along the Zahedan fault, Iran, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5809, https://doi.org/10.5194/egusphere-egu26-5809, 2026.

Charnockite is generally regarded as a product of high-temperature melting; however, its specific origin, generation, and preservation mechanisms, as well as its relationship to high-grade metamorphism or deep crustal reworking, remain poorly constrained. During the early Paleozoic, the South China Block underwent intense orogeny that resulted in significant crustal shortening and thickening, subsequently inducing widespread anatexis and extensive S-type granites. This study identifies ~431 Ma charnockites containing granulitic enclaves that were exposed in the Yunkai massif, providing key insights into the early Paleozoic crustal reworking and deep crustal melting behaviors in South China. The body displays A-type characteristics with crustal reworking zircon isotopic features (δ¹⁸O = 8.0–9.8 ‰; ε_Hf(t) = - 11.5 to - 3.4). The charnockite and its enclaves show identical mineral assemblages and comparable orthopyroxene chemical compositions. The two anhydrous minerals of orthopyroxene and garnet are identified as of peritectic and magmatic origins given their textural features and geochemical compositions. Moreover, petrographic observations and bulk geochemical data argue that the peritectic minerals were derived from the entrainment of their granulitic sources. Crystallization phase modeling indicates orthopyroxene would have been completely hydrated and formed biotite when water contents exceed ∼0.3 wt.% near the solidus. Water-in-zircon analysis and thermodynamic modeling indicate low magma water contents (∼0.15 wt.%; 135 ppm, zircon water medians) for the Gaozhou charnockite from early crystallization to final solidification. CO₂‐rich fluids flushed the charnockite reservoir further contributing to the stabilization of the orthopyroxene. Accordingly, the Yunkai charnockite reveals deep crustal melting processes involving anhydrous minerals entrained in a low-water environment. This low-water environment correlates with high-temperature melting of granulite-facies rocks in the lower crust and the presence of CO₂-rich fluids within the system. Regional magmatic-metamorphic-tectonic data indicate that the formation of the Yunkai A-type charnockite occurred within a post-orogenic extension regime, representing the peak of intracrustal reworking in South China.

How to cite: yang, H. and Yao, J.: Formation of A-type charnockite and constraints on deep crustal anatexis in early Paleozoic orogen, South China, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8520, https://doi.org/10.5194/egusphere-egu26-8520, 2026.

Unravelling the tectonic evolution of collisional orogens and the forces driving the exhumation of high-grade metamorphic rocks requires constraining the kinematics and timing of major fault systems. The Main Mantle Thrust (MMT) in the Swat Valley (northern Pakistan) marks the Eocene suture between the Indian Plate and the Kohistan–Ladakh Island Arc. Its later reactivation is critical yet debated, with models ranging from dominantly dextral strike-slip faulting with minor normal offset (typically less than a few meters) to significant normal faulting facilitating regional exhumation of high-grade metamorphic rocks. This research integrates structural geology with a new, multi-method low-temperature thermochronology dataset—including zircon and apatite (U–Th)/He and apatite fission-track ages—to contribute to this debate. Structural analysis of over 150 kinematic indicators collected along the MMT and its footwall shows that the MMT experienced semi-ductile to brittle reactivation dominated by top-to-the-north normal faulting. Structures formed under greenschist- to sub-greenschist facies conditions (e.g., C′ shear bands) in the footwall record progressive exhumation, with purely brittle cataclastic deformation marking the final stages. Our new thermochronometer dateset includes zircon (U–Th)/He (ZHe) single-grain ages (14.9 ± 1.2 to 24.4 ± 2.0 Ma), apatite (U–Th)/He (AHe) single-grain ages (7.6 ± 0.4 to 15.9 ± 1.0 Ma), and apatite fission-track (AFT) central ages (15.3 ± 2.4 to 16.4 ± 3.2 Ma). This dataset, alongside its thermal history modelling, reveals a consistent cooling signal across the Swat Valley. Following the Eocene collision and peak metamorphism, a major tectonic shift occurred in the Early Miocene. Our data indicate the onset of rapid cooling at ~20 Ma, with a slightly later initiation at ~18 Ma in the eastern Loe Sar Dome. This distinct phase of rapid cooling records the top-to-the-north normal reactivation of the MMT, which lasted until ~15–14 Ma. Collectively, our results provide structural and timing constraints supporting a model of protracted, normal-sense reactivation of the MMT between ~20 and 15 Ma. This event facilitated the final unroofing of high-grade metamorphic rocks in the Swat Valley. 

How to cite: Hernández Chaparro, D. R., Faccena, C., Olivetti, V., Fellin, G., and Ghani, H.: Constraining Early Miocene Reactivation of the Main Mantle Thrust (Swat Valley, Pakistan) through Integrated Structural and Thermochronologic Analysis, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12696, https://doi.org/10.5194/egusphere-egu26-12696, 2026.

The Kadavur Anorthosite Complex represents a distinctive structural domain within a folded high-grade crystalline terrain, where a massif-type anorthosite body occupies the core of a regional-scale fold and is surrounded by folded quartzite ridges. This study examines the relationship between lineament development and the pre-existing ductile fold architecture through integrated DEM–SRTM data analysis and quantitative lineament network characterisation using FracPaQ. The objective is to assess how fold geometry and lithological contrasts influence the spatial distribution and mechanical behaviour of brittle structures. DEM analysis reveals a coherent folded morpho-structural architecture characterised by a well-defined core, axial-plane domains, and limbs expressed as quartzite ridges. FracPaQ-derived results show that lineaments are non-randomly distributed and define multiple dominant orientation sets, reflecting systematic structural control rather than random patterns. Spatial variations in lineament density and lineament intensity show pronounced localisation within and adjacent to the fold core, whereas lineament attributes vary systematically between the anorthosite-dominated core and the surrounding folded quartzite limbs.

Slip tendency analysis indicates that brittle deformation is predominantly shear-controlled across the study area, while dilation tendency values are generally low to moderate, suggesting a subordinate role for opening-mode fracturing. Lineaments within the anorthosite core are comparatively longer, less densely spaced, and display lower orientation dispersion, reflecting brittle stress accommodation within a mechanically competent lithology. In contrast, lineaments developed in the folded quartzite ridges are shorter, more closely spaced, and strongly influenced by lithological layering and fold-related bending stresses.

Although comparable lineament orientation patterns occur across the fold core, axial planes, and limbs, their geometric characteristics, spatial distribution, and inferred mechanical roles differ significantly, indicating that brittle deformation was modulated by local fold geometry and lithological contrasts. The results indicate a structural association between ductile folding and later brittle deformation; however, the tectonic conditions responsible for anorthosite exhumation cannot be uniquely constrained from the present dataset. This study highlights the importance of domain-specific lineament analysis in folded crystalline terrains and emphasizes the role of inherited ductile architecture in controlling later brittle deformation.

How to cite: Prathapachandran, A., Manilal Girija, A., and Kumar, R. S.: Quantitative lineament network analysis of a folded crystalline terrain using FracPaQ: The Kadavur Anorthosite Complex, Southern Granulite Terrane, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16222, https://doi.org/10.5194/egusphere-egu26-16222, 2026.

With the continuous advancement of geological research, quantitative analysis of fracture networks has become a crucial research direction in geological exploration and resource development. To overcome the limitations of traditional methods in quantitatively analyzing fault data under complex geological conditions, we adopt a quantitative fracture characterization method based on geometric and topological theories. This study focuses on the overlay analysis of multi-period and multi-layer faults in a typical area of the western Junggar Basin, aiming to reveal their significant role in hydrocarbon exploration. By means of this method, we achieve multi-dimensional automatic quantification of geometric features (including fracture network length, orientation, and Pxy system), as well as node/branch types and topological parameters. Through the construction of a fracture topological network, we can quantitatively analyze the connectivity characteristics of each period and the vertically favorable conduction zones across multiple periods, thereby providing valuable guidance for hydrocarbon migration path prediction.

How to cite: Tao, Y., Cui, L., Niu, Y., Huang, Y., Bai, S., Zhao, G., Piluolan, P., and liu, Y.: Quantitative Characterization of Fracture Networks Based on Geometric-Topological Integration and Its Application in Hydrocarbon Migration Prediction in the Western Junggar Basin, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18367, https://doi.org/10.5194/egusphere-egu26-18367, 2026.

The Achankovil Shear zone (AKSZ), juxtaposing the Trivandrum Granulite Block (TB) and the Madurai Granulite Block (MB) of the Southern Granulite Terrane (SGT), represents a paleo-suture zone related to the late Neoproterozoic to early Cambrian Gondwana assembly (Rajesh et al., 1998; Praharaj et al., 2021). The polyphase deformational history of the Achankovil Shear Zone (AKSZ) reveals progressive transition from a ductile to brittle deformation regime concomitant with high-grade granulite facies metamorphism and subsequent cooling–exhumation of the granulites. Progressive evolution of the state of stress and the variation of crustal dynamics during the ductile to brittle deformation regime transition, and the genetic link between these two contrasting episodes, if any, have been evaluated from statistical analysis of solid-state fabric and kinematic analysis of brittle fractures.

Three ductile deformation phases, D₁, D₂, and D₃, and associated solid-state fabrics, i.e., S₁, S₂, and S₃, are discernible at the mesoscopic scale. S₁ fabric is gneissic in character and is only preserved in strain shadow regions. Elsewhere, S₁ is transposed along the later S₂ fabric, which is axial planar to fold on S₁. Prevalence of high-temperature deformation conditions during D₂ and D₃ deformation stages is manifested by the presence of S₂-parallel stromatic leucosomes and diatexite leucosome along dilatant S₃ fabric, developed parallel to the axial planes of fold on S₂. Significant simple shear component during D₃ deformation is evidenced from the asymmetric nature of S₂ folds, asymmetric porphyroclast tail and other shear sense markers. Eigen vector analysis reveals a change of maximum eigen vector, i.e., pole to the mean foliation, from NW-SE (D₁: 311⁰/62⁰ NE) to N-S (D₃: 187⁰/80⁰ W). The maximum eigen vector of D₂ (125⁰/53⁰ SW), though similar in trend with D1, shows a reversal of dip direction. Fabric shape analysis reveals a progressive change from girdle to a strong clustered distribution of solid-state fabric from D₁ to D₃ deformation regime, suitably accounting for intense ductile shearing and transposition of earlier fabric during the D₃ deformation stage.

Minor conjugate faults are ubiquitous at different locations along the AKSZ. Dihedral angle of ~60 or less for these faults suggests a shear or hybrid fracture origin, diagnostic of a compressive stress regime. Also, the observed slip of striations on slickensides suggests a consistent oblique reverse kinematics. Fault kinematic and paleo-stress analysis further reveals two distinct stress regimes with NW-SE and NE-SW directed maximum compressive stress (s₁). Stress ratios for these faults imply a compressional to transpressional tectonic regime. Superposition of the slip tendency of NW–SE directed stress tensor over NE–SW directed stress tensor and vice-versa suggests that the NW-SE stress tensor precedes the NE-SW stress tensor during a progressive brittle deformation regime. Summarily, the cooling and exhumation and the switch over from ductile to brittle deformation regimes of the granulites took place under a compressive stress field developed during terrane accretion along the AKSZ. The brittle faults seemingly result from the relaxation of the orogenic far-field stress.

How to cite: Manilal Girija, A. and Bhadra, S.: Ductile to brittle tectonic evolution of the Achankovil Shear Zone, Southern Granulite Terrane – Constraints from statistical analysis of fabric and paleostress inversion, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20066, https://doi.org/10.5194/egusphere-egu26-20066, 2026.

We investigate the heterogeneity of the Indian subcontinent using seismic recordings from the Mw 7.7 Myanmar earthquake that occurred on 28 March 2025. This event was recorded by broadband stations across India. These variations in waveforms at different stations highlight the influence of radiation pattern, crustal structure, wave-propagation paths, and local site conditions. Sedimentary basins, characterized by relatively soft sediments, are known to amplify seismic energy and modify ground motion characteristics, often resulting in enhanced shaking. Understanding these effects is essential for assessing seismic hazard.

We use time-series data from approximately 88 seismic broadband stations provided by the National Centre for Seismology (NCS), India. We apply frequency spectrum analysis, horizontal-to-vertical spectral ratio (HVSR) analysis, and surface wave dispersion analysis. The frequency spectrum helps identify frequency bands where seismic energy is amplified while HVSR analysis is used to estimate the site’s fundamental resonance frequency and the corresponding amplification factor. Surface wave dispersion analysis provides shear-wave velocity information, which is crucial for characterizing near-surface geological conditions.

Together, these analyses help us to understand the influence of local geological conditions at the receiver sites and contributes to a better analysis of regional seismic wave propagation and site-specific ground motion characteristics across the Indian subcontinent.

How to cite: Gupta, S., Silwal, V., and Bora, S. S.: Dynamic Triggering and Effects of Crust Heterogeneities on Propagating Waves due to the 2025 Mw 7.7 Myanmar Earthquake, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21372, https://doi.org/10.5194/egusphere-egu26-21372, 2026.

The northeastern margin of the Iberian Peninsula, extending from the Vallès-Penedès Graben to the Valencia Depression, constitutes a passive margin related to the opening of the Valencia Trough during the Neogene. It comprises a series of extensive basins bounded by several mountain ranges, where numerous NNE–SSW-oriented normal faults are present, commonly associated with mountain fronts. Previous studies show that some of these faults, such as the El Camp fault, remain active, although at very low slip rates, as low as 0.02 m/kyr for Late Pleistocene.
Recent analyses of high-resolution Digital Elevation Models (DEMs) have revealed several additional morphological escarpments crosscutting different Quaternary alluvial fan systems across the region. These scarps, found from the Sant Jordi Plain to the La Salzadella Basin, share the same orientation as the Neogene faults, suggesting a common tectonic origin. They are discontinuous, arranged in a right-stepped pattern, and locally display vertical offsets of up to 8 m. Furthermore, several geomorphic features indicate recent tectonic activity, including aligned fissures within the fans and entrenched channels developed on the upthrown block that fade out after crossing the escarpments. Each family of escarpments exceeds 10 km in length, with important implications for the seismic hazard of the region.
Geophysical surveys (ERT, GPR, SRT, HVSR, and MT), validated with borehole data, confirm the presence of faults beneath each analysed escarpment system. At the Vinaixarop escarpment, for instance, ERT profiles revealed a steeply dipping discontinuity plane with a vertical offset of approximately 40 m affecting upper Pliocene sediments.
The faulted alluvial fans are interpreted as Lower to Middle Pleistocene in age and are mainly composed of carbonate gravels. Paleoseismological trenches excavated at four sites (L’Ampolla, Vinaixarop, and two at Sant Rafael del Riu) on different scarps revealed consistent evidence of late Quaternary faulting, such as ruptured strata. Additionally, ground-based hyperspectral cameras (400–1700 nm) were deployed on trench walls as an ancillary tool to map faulted
stratigraphic layers and to detect subtle coseismic deformation. As sedimentation on the fans ceased by the Middle Pleistocene, subsequent activity has been primarily recorded through the deformation of pedogenic features (calcretes), commonly found in the study area. To constrain the timing of this activity, U–Th dating was performed on fault-related carbonates and deformed calcretes. Preliminary results indicate that tectonic activity along these faults persisted at least until the Late Pleistocene. However, a deformed colluvial wedge and the presence of open fissures observed on trench walls suggest Holocene activity.
Additionally, three cosmonuclide depth-profiles were sampled to date displaced geomorphic surfaces, and hence estimating the long-term slip rates of the faults. Ongoing analyses aim to demonstrate Holocene seismic activity and to further characterize paleoearthquakes and their seismic parameters.
In summary, studies of active tectonics in regions of very low strain are inherently challenging, especially when recent sedimentation is scarce and pedogenic processes are intense. Nevertheless, this work highlights that integrating multiple complementary techniques is the most effective approach to address such settings.

How to cite: Ollé-López, M., García-Mayordomo, J., Gallego-Montoya, J., Molins-Vigatà, J., Bellmunt, F., Gabàs, A., Andrés, J., Macau, A., Hasözbek, A., Martí, A., Figueiredo, P., Rodés, Á., García, D., Ortuño, M., and Masana, E.: Constraining Late Pleistocene to Holocene seismic fault activity in NE Iberia: The value of integrating complementary techniques in a low-strain region, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21385, https://doi.org/10.5194/egusphere-egu26-21385, 2026.

The evolution of rift-related fault systems is strongly influenced by pre-existing structural weaknesses, yet the role of their spatial arrangement in shaping fault patterns during multiphase non-coaxial extension remains unclear. To address this, we conducted a series of brittle–viscous analogue experiments to examine how left-stepping and right-stepping configurations of parallel weaknesses affect fault propagation, linkage, and orientation under two successive phases of orthogonal and oblique extension. We find that (1) fault orientation is jointly controlled by extension direction, weakness orientation, and the relative positioning of pre-existing weaknesses; (2) left-stepping and right-stepping systems, though geometrically identical, evolve differently under the same boundary conditions—left-stepping configurations develop greater fault linkage, strike diversity, and overall structural complexity; and (3) inherited weaknesses reduce the direct control of extension direction on fault strikes, implying that present-day fault patterns may not simply reflect paleostress orientations. Furthermore, our results suggest that apparent strike variability in multiphase rift systems can arise without local stress rotation, emerging instead from the interaction between regional stress and the spatial arrangement of inherited structures. Mechanistically, left-stepping configurations behave analogously to releasing steps in strike-slip systems, promoting more distributed deformation and strike-slip components, whereas right-stepping systems resemble restraining steps, producing simpler and more localized fault networks. Our findings provide new insights into how pre-existing structural configurations modulate rift fault evolution, highlighting the need to consider structural inheritance when reconstructing tectonic histories of multiphase extensional basins.

How to cite: Zhang, Y. and Huang, L.:  Fault System Evolution Controlled by Weakness Arrangement under Multiphase Non-Coaxial Extension：Analogue Modeling Insights, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-106, https://doi.org/10.5194/egusphere-egu26-106, 2026.

Shear zones are the areas of localised deformation that contain differential movement within the lithosphere. As such it plays a vital role in the tectonic evolution of continental margins and orogenic belts. The formation, geometry and kinematics of these zones are important for reconstructing the tectonic history. The Indian subcontinent exposes several crustal-scale shear zones, which are major zones of deformation that accommodate the movement of tectonic plates. Among these, the ~30 km wide east-west trending Palghat-Cauvery Shear Zone (PCSZ) forms one of the major transpressive dextral systems in the Southern Indian Granulite Terrain (SGT).

The PCSZ records D1–S1 fabrics that were overprinted by widespread dextral D2 mylonitisation (S2). This structural configuration is altered by brittle to brittle–ductile D3 structures, indicating significant structural heterogeneity in the area. On close examinations, the region is found to preserve evidence of conflicting nature of shears with brittle, brittle-ductile and ductile signatures. The structural complexity of the PCSZ is envisaged as a product of multiple deformation events, tectonic reworking and the overprinting of successive fabrics. The dextral and sinistral senses of shears include σ- and δ-type porphyroclasts, folds, minor faults, and fractures. The ductile dextral shears are characterised by well-developed S-C fabrics, σ- and δ-type porphyroclasts, and mica fish and folds that have oriented nearly in the E-W direction, while both the ductile and brittle sinistral shears are oriented mainly in the NNE to NNW direction. The younger brittle shears such as minor faults and fractures overlap the earlier ductile, which is oriented in the NW-SE direction. The structural analysis of the western PCSZ reveals that two principal stress regimes were in operation in this region. The early N-S compressive stress is associated with the collision of the Madurai and Madras blocks, producing E-W trending foliations, folds and σ- and δ-type porphyroclasts. The later E-W-oriented stress developed due to the transpressional movements leads to the development of conjugate brittle and brittle-ductile shear sets. Thus, the PCSZ form an ideal section to understand how alternating stress orientations and multiple deformations can form conjugate conflicting shear systems, exhibiting the interplay of ductile flow, strain partitioning, and brittle fracture in the deep crustal response to orogenic processes.

How to cite: Nandan Thayyilunnithiri, N. and Chettootty, S.: Geometry of conjugate shears with conflicting shear-sense indicators in the western Palghat–Cauvery Shear Zone, Southern India, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-999, https://doi.org/10.5194/egusphere-egu26-999, 2026.

The Central Indian Tectonic Zone (CITZ) records the Proterozoic collision between the Bundelkhand and Bastar cratons, with the adjacent Vindhyan Basin preserving evidence of deformation during the N-S convergence across CITZ. Previous interpretations associated the structural features in the Lower Vindhyan Group (LVG) to synsedimentary or seismic processes. To investigate the structural evolution of LVG and its relationship with CITZ, we conducted detailed litho-structural analysis of both the LVG and the Mahakoshal Supracrustal Belt (MSB) within the CITZ, specifically focusing on their mechanical coupling during Mesoproterozoic collisional deformation.

Our field investigations revealed polyphase deformation within the MSB, characterized by three distinct phases: (D1) E-W trending regional foliation (S1) and diversely oriented folds, (D2) E-W oriented steep folds associated with a crustal-scale shear zone along the Son-Narmada South Fault, and (D3) local cross-folds. In the LVG, we report, for the first time, characteristic fold-and-thrust belt features including buckle folds, kink bands, reverse faults, fault-related folds, and notably, 5-20 meters long outcrops of pop-up structures. The deformation style in the LVG was dominantly controlled by a mechanically weak detachment layer comprising the Kajrahat Limestone and Arangi Shale units, which enabled thin-skinned deformation within the overlying competent units of Porcellanite, Kheinjua Shale, and Chorhat Sandstone.

Based on geometric and kinematic analysis, we demonstrate that deformation in the LVG occurred between the D3 event in the MSB and the deposition of the Upper Vindhyan Group (1.5–1.2 Ga). Cross-sectional analysis reveals that the LVG deformation patterns closely mimic sandbox experiments of fold-and-thrust belt evolution, particularly in the sequential development of pop-up structures above a weak detachment horizon. We propose a tectonic model wherein the Vindhyan Basin initially developed as a peripheral foreland basin, followed by northward propagation of deformation through detachment folding mechanisms. The model involves initial northward subduction followed by polarity reversal to southward subduction, explaining both the basin formation and subsequent deformation patterns. Our findings also highlight the significance of thin-skinned tectonics in shaping structural architecture of Central India during the Mesoproterozoic period and reveal the far-field effects of cratonic collisions on basin evolution.

How to cite: Todkar, T., Saha, P., Dutta, D., and Misra, S.: Development of Mesoproterozoic Fold-and-Thrust Belt Structures in Central India: New Evidence from Detachment-Controlled Deformation in the Lower Vindhyan Basin, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1062, https://doi.org/10.5194/egusphere-egu26-1062, 2026.

Lamprophyre dykes are enigmatic volatile-rich, mantle-derived igneous melts that are often contemporary with lithospheric extension. Despite a rich literature on the petrology and geodynamic implications of lamprophyre intrusions into the continental crust, the emplacement mechanisms of these dykes (i.e. structural relationship with the host rock and structures, mode of intrusion, speed of magma ascent, etc) into the dry mid–lower crust is poorly constrained.

In the Jotun Nappe of south-central Norway, Proterozoic gabbro gneisses are overprinted locally by mutually crosscutting lamprophyre dykes, pseudotachylytes (coseismic-derived quenched frictional melts), and mylonitized pseudotachylytes. Mylonitized pseudotachylytes form networks of small-scale (cm- to dm-scale) ductile shear zones, orientated in roughly three sets of orientations, that separate relatively undeformed gabbro gneiss blocks, while pristine pseudotachylytes dissect these blocks and are bounded by the ductile shear zones – akin to observations from Lofoten, Norway (e.g. Jostling Block; Campbell et al., 2020). Pseudotachylytes and mylonitized pseudotachylytes have similar mineral assemblages containing plagioclase, K-feldspar, clinopyroxene, amphibole, Fe-Ti-oxides, with the mylonitized versions also containing garnet porphyroblasts and biotite in addition. Lamprophyre dykes (<1.5m wide), strike dominantly NW-SE, are either undeformed or are incorporated into viscous shear zones that are comprised primarily of mylonitized pseudotachylytes. Many of the undeformed lamprophyres show some amount of viscous shearing localized to <5 cm at the contact with the host rock, otherwise pristine undeformed dykes display primary igneous fabrics and textures. Injection veins of the dyke into the host rock are common, while dyke tips form sharp <45° indentations into the gabbro gneiss. The host rock around jogs is bleached and exhibits numerous small shear fractures filled with dyke material that can easily be misidentified for pseudotachylytes. Lamprophyres have a matrix composed of biotite, plagioclase, dolomite, orthopyroxene, amphibole, Fe-Ti-oxides, and apatite with xenocrysts of orthopyroxene surrounded by a corona of clinopyroxene, amphibole, biotite. Pristine pseudotachylytes crosscut the dykes, offsetting them by up to an apparent ~50 cm and dragging dyke material along the length of the pseudotachylyte surface.

Structural relationships between mylonitized pseudotachylytes and pseudotachylytes suggest that viscous creep along the shear zone network concentrated stresses towards the interior of the gabbro gneiss blocks, which resulted in failure of the blocks and the formation of pristine pseudotachylytes (Zertani et al., 2025). Because the dominant orientation of the lamprophyre dykes is orthogonal to the most dominant orientation of the ductile shear zones, we suggest the lamprophyres exploited transient crustal weaknesses caused by the stress drops during rupturing of the blocks, which created permeably fracture networks for the dykes to ascend through the gneiss. This study demonstrates through field and microstructural observations that lamprophyre intrusions are fundamentally linked to seismicity in the dry mid–lower crust.

Campbell et al., (2020). Nature Communications,  https://doi.org/10.1038/s41467-020-15150-x

Zertani et al., (2025). Geophysical Research Letters,  https://doi.org/10.1029/2024GL114350

How to cite: Michalchuk, S. P. and Augland, L. E.: Field and microstructural evidence demonstrating the interplay between seismicity and the emplacement of lamprophyres in the dry mid to lower crust, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1099, https://doi.org/10.5194/egusphere-egu26-1099, 2026.

Microstructural observations such as grain and subgrain sizes, pole figures (CPO), grain boundary irregularities, and thermometry (opening angles, Ti-in-Quartz) from the mineral quartz have become one of the most reliable proxies for deciphering the mechanical properties, including stress, strain rate, and deformation conditions (temperature) of the continental crust. These relationships were constrained from experimental studies on monomineralic quartz aggregates. Consequently, many field-based studies from continental shear zones focus on analyzing sporadically occurring quartzites and quartz veins. However, crustal rocks are predominantly polymineralic, yet, for simplification, most rheological models rely on homogeneous single-phase approximations. Interactions among multiple mineral phases can disrupt steady-state grain sizes, leading to violations of the piezometric relationships commonly applied to quartz mylonites. Experimental studies further show that polymineralic aggregates deform at significantly lower stresses than their monomineralic counterparts, implying that previous studies have likely overestimated the strength of the crust. In addition, experiments demonstrate that the presence of a secondary phase results in markedly different quartz CPO from that expected in single-phase quartzite. These observations raise an important question: to what extent can quantitative microstructural data from polymineralic rocks be used to infer realistic mechanical properties of the continental crust? Addressing this gap is crucial for developing rheological models that accurately reflect the deformation processes occurring in nature.

In this study, we focus on performing high-resolution EBSD analysis of quartz-bearing mylonites formed from metapelites and granites during thrust-sense shearing along the Main Central Thrust (MCT) shear zone (Western Himalaya, India), which runs along the entire Himalayan Mountain belt. From south to north, these samples record an increasing peak-metamorphic temperature and pressure condition; from 535 °C and 5.8 kbar to 683 °C and 11 kbar. Although strain is inhomogeneously distributed within the ~4 km thick shear zone, an overall increase in deformation intensity is recorded towards the north. The Crystallographic Vorticity Axis (CVA) analysis of quartz reveals monoclinic simple-shear flow kinematics consistent with earlier studies; however, the secondary phases (plagioclase) exhibit pure shear-dominated deformation. Depending on the proportion of the secondary phase (30 to 70%), quartz grains form either continuous layers (monophase domain) or isolated quartz aggregates (polyphase domain). Overall, the CPO pattern in the monophase domain exhibits a transition from a type-II crossed-girdle to an asymmetric type-I pattern, towards the north. The mixed polyphase domain exhibits a random CPO. Within the monophase domains, the fabric strength (M-index, B-index) is higher for the thicker domains (> 163μm). Thereafter, we segregate our analysis into two types of quartz grains: (i) quartz surrounded by quartz grains (Q-Q), and (ii) quartz surrounded by other phases (Q-S). Within a thin section, the Q-Q grains exhibit higher fabric strength, larger recrystallized grain sizes, but lower aspect ratios compared to the Q-S grains. The low-angle boundary density increases towards the north (0.0042 to 0.0095µm^-1), but the density is always higher for Q-Q grains than Q-S grains. Our study suggests Q-Q grains can be used for piezometry. We will discuss these results in terms of deformation mechanisms and strain partitioning between mono and polyphase domains.

How to cite: Ghosh, S. and Saha, S.: Quantifying the Rheology of Quartz-bearing Polyphase aggregates deformed under mid-crustal conditions: An EBSD-based Application, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1321, https://doi.org/10.5194/egusphere-egu26-1321, 2026.

Temperature-dependent rheological changes control the long-term brittle-ductile transition, but several other factors may control the transient and localized switching between brittle and ductile rheology in the continental crust during orogen build-up. Rheological transients are inferred from, for example, mutual overprinting relationship between localized ductile and brittle deformation features (faults, veins, foliations, and shear zones) in the field. These occurrences are commonly used as a starting point for developing models of the mechanisms controlling seismicity outside the upper-crustal schizosphere, including lower-crustal earthquakes, intermediate-depth seismicity, and slow seismic phenomena such as tremor and slow slip. Current geophysical/seismological investigations show indeed the occurrence of different types of seismicity potentially related to continental subduction; however, most recorded seismicity appears to be linked to collision and exhumation processes. Based on field observations from subducted and exhumed Alpine continental units (Corsica and the Central Alps), this contribution addresses key challenges in interpreting brittle–ductile transient rheology from the geological record, discussing how structural inheritance, metamorphic overprinting, and fluid composition complicate interpretations of seismic versus aseismic deformation.

During prograde subduction, increasing temperature and pressure should promote a progressive transition from brittle to ductile rheology. The blueschist-ecogitic facies continental units of Alpine Corsica, prime example of continental subduction, show indeed a general brittle-to-ductile (and potentially seismic-to-aseismic) evolution, with distinct deformation features developed across increasing metamorphic grades. However, the post-kinematic increment in metamorphic conditions may overprint brittle structures with higher-grade assemblages, precluding us to understand if these field occurrences are really representative of (seismic) rheological transients during deep subduction, or if they simply result from structural inheritance from the pre-orogenic stages. New field observations from the Crystalline Massifs of the Central Alps (Aar massif, Gotthard nappe) further demonstrate the role of inherited structures in steering the retrograde rheological evolution of the continental crust during Alpine collision and exhumation, challenging models for mid-crustal seismicity and strain localization. Rheological transients are commonly associated with fluid flow and fluid pressure fluctuations, manifested in the field as mineralised veins precipitating from metamorphic fluids. Yet, the polyphase nature of metamorphic fluids (e.g., CO₂-, CH₄-bearing fluids), and the resulting variability in chemo-physical properties are rarely considered in rheological models. CO₂-rich fluids and the resulting carbonate-bearing mineral veins might lead to transient rheological switches in both the brittle and ductile fields, as documented by sheared carbonate-bearing breccias in several Alpine crystalline units and plutons.

Together, these observations highlight that brittle–ductile transients inferred from the geological record require careful evaluation of inheritance, metamorphic overprinting, and fluid composition before being extrapolated to crustal rheology and seismicity models.

How to cite: Ceccato, A.: Brittle–ductile transients during continental subduction and exhumation: inheritance, fluids, and implications for seismicity, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4445, https://doi.org/10.5194/egusphere-egu26-4445, 2026.

Continental subduction is a fundamental stage in the tectonic evolution of convergent plate margins, accommodating the transition from oceanic subduction to continental collision and orogenic build up. Despite the elevated seismic hazard currently posed by destructive earthquakes in continental subduction settings (e.g., Taiwan 1999 Chi Chi M_W = 7.6, 2024 Hualien M_W = 7.4; Nepal 2015 Gorkha M_W = 7.8; Albania 2019 Durres M_W = 6.4), our understanding of the processes steering the seismogenic behaviour during prograde subduction of the buoyant, dry, quartzo-feldspathic continental lithosphere remains limited. With the Maria Skłodowska Curie Action SEISMI-COS, we aim at providing quantitative estimates of the stress state, rheology and petrophysical properties of natural deformation zones developed during progressive subduction of continental lithosphere. We will focus on fossil deformation zones exposed in the metamorphic units of Northern Corsica, where the former crystalline basement of the European continental margin has been coherently subducted to and exhumed from different depths during Eocene Alpine orogenesis. Different metamorphic units preserve pristine deformation structures developed at increasing subduction depth, making Northern Corsica the perfect natural laboratory to track the prograde rheological evolution and seismogenic behaviour of the subducted continental lithosphere from shallow seismogenic depths all the way down to conditions at intermediate-depth earthquakes are expected. Preliminary results show that units subducted at progressive depths show different structural features, from pseudotachylyte-bearing fault zones and brittle-ductile shear bands and veins in the outermost continental slices (Corte blueschist units), to high-pressure ductile shear zones (Tenda blueschist phyllonites) involving cycles of fluid pressure variation and veining (Centuri shear zones). This plethora of mesostructures represent the variable seismogenic behaviour during subduction of crystalline continental units subducted at different depths. Field, microstructural, and laboratory analyses will provide us with fundamental insights on how rheology and petrophysics control seismogenic deformation.

How to cite: Ceccato, A., Vannucchi, P., and Molli, G.: SEISMogenic behavIour of COntinental Subduction (SEISMI-COS): insights from rheology and petrophysics of Corsica blueschist and eclogite-facies continental units, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4448, https://doi.org/10.5194/egusphere-egu26-4448, 2026.

Pseudotachylytes are solidified frictional melts produced in silicatic rocks during an earthquake (Sibson, 1975). They usually form fault and injection veins with thicknesses ranging from few millimeters to several centimeters. Still, exposures of meter-thick pseudotachylyte breccias with evidence of a single melt pulse and associated with seismic faulting have been documented (i.e., Musgrave Ranges, Australia; Lofoten Islands, Norway; Outer Hebrides, Scotland; Greenland; Ivrea-Verbano Zone, Italy).

In the present study, we perform field, microstructural and mineralogical investigations of both “normal” (i.e., mm- to cm-scale) pseudotachylytes and “giant” pseudotachylyte breccias outcropping in the Ivrea-Verbano Zone moving away from the Canavese Line, a segment of the Insubric Line, the main tectonic lineament of the Alps. The giant pseudotachylyte breccias reach up to ~2 m in thickness, and up to 8 m in length, limited by the outcrop extension, and possibly fill pull-aparts. Despite pseudotachylytes in this area have already been studied in detail, giant pseudotachylyte breccias were somehow overlooked (Techmer et al., 1992; Ueda et al., 2008; Souquière and Fabbri, 2010; Ferrand et al., 2018). We aim to determine (i) the ambient P-T conditions of formation (discussed here), (ii) their geodynamic and seismogenic environment, and (iii) their formation mechanism.

We selected four main outcrops along the Sesia River for detailed field mapping and sampling, moving eastward from the Canavese Line for ~9 km. In fact, no giant pseudotachylyte breccias have been found to the west of the lineament. In detail:

Outcrop I, <500 m from the Canavese Line (altered greenschist facies gabbros) shows:

• multiple generations of pseudotachylyte-bearing faults, including giant pseudotachylyte breccias subparallel to the NNE-SSW striking Canavese Line, containing clasts of the altered host rock;
• matrix of the pseudotachylytes overprinted by greenschist facies minerals (epidote, chlorite, albite);
• late quartz-epidote- and chlorite-bearing faults cutting the pseudotachylyte-bearing faults and breccias.

Outcrop II, ~1 km from the Canavese Line (unaltered gabbros) shows:

• multiple generations of pseudotachylyte veins and giant breccias, the latter subparallel to the Canavese Line;
• cataclasite- and graphite-bearing faults cut by giant pseudotachylyte breccia;
• late quartz-epidote- and chlorite-bearing faults cutting the pseudotachylytes.

Outcrop III, ~2 km from the Canavese Line (Balmuccia peridotite) shows:

• multiple giant pseudotachylyte breccias cutting cataclasite-bearing faults;
• serpentine-bearing veins and pseudotachylytes mutually cross-cutting each other;
• giant pseudotachylyte breccias subparallel to the Canavese line; their matrix includes microlites of olivine, enstatite, and vesicles.

Outcrop IV, ~9 km from the Canavese Line (unaltered tonalite) shows:

• only thin pseudotachylytes overprinting foliated cataclasite-bearing faults;
• well-preserved matrix of the pseudotachylytes (microlites, chilled margins, flow structures).

In conclusion, giant pseudotachylyte breccias are (i) mostly subparallel and only outcropping close to the Canavese Line (<2 km), (ii) made of a relatively homogenous matrix, resulting from the solidification of a continuous melt layer, (iii) not reactivated by ductile deformation, (iv) cut and are cut by brittle faults and, (v) cut by quartz-epidote, chlorite-, serpentine- bearing faults and veins. Thus, they were possibly generated in a shallow (~15 km depth) and cold (<350°C) environment by individual earthquakes of large magnitude, associated with the activity of the Canavese Line.

How to cite: Aldrighetti, S., D'Ippolito, G., Pennacchioni, G., Gomila, R., Baccheschi, P., and Di Toro, G.: Giant pseudotachylyte breccias of Valsesia (Ivrea Zone, Western Italian Alps), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5415, https://doi.org/10.5194/egusphere-egu26-5415, 2026.

In the mafic lower crust, clinopyroxene is among the main rock-forming minerals. Based on experimental investigations, clinopyroxene is considered to be strong and to deform in a brittle manner at dry lower crustal conditions. However, field observations on Holsnøy, Norway, indicate ductile deformation of coarse-grained clinopyroxene in the mafic lower crust, reflected by bending of the granulitic foliation adjacent to eclogitic shear zones.

This study focusses on the strain accommodating processes of the granulitic clinopyroxene during incipient eclogitisation. Representative samples of deformed weakly eclogitised granulite were analysed via scanning electron microscopy, electron back-scattered diffraction mapping, electron probe micro analysis and Fourier-transform infrared spectroscopy.

Microstructural analysis reveals the formation of garnet lamellae along the {010} planes of the diopsidic clinopyroxene. Initial bending of this anisotropic clinopyroxene is accommodated by the development of en échelon microcracks at a high angle to the {010} planes. The micro-cracks are traced by garnet with similar composition as the lamellae, suggesting that both formed at similar pressure-temperature conditions. With ongoing strain, the cracks start to link and evolve into micro-shear zones, which systematically widen with strain and eventually connect forming networks. This widening is accompanied by the nucleation of amphibole and a second clinopyroxene with higher magnesium and lower aluminium concentration when compared to the host clinopyroxene, facilitating further macroscopic bending of the granulitic foliation. Increased intracrystalline misorientation and formation of subgrains adjacent to the micro-shear zone indicate that the diopsidic clinopyroxene host grain deforms by crystal plastic processes. In contrast, shape-preferred orientation and minor chemical zoning of the newly crystallised grains related to the micro-shear zone suggest that diffusion-related processes predominately accommodated the strain in the micro-shear zones.

In recent literature, low-permeable granulite has been described as dry. The observed deformation style as well as the formation of amphibole in the micro-shear zones indicate the presence of water, either in form of external fluids, infiltrating through en échelon microcracks, or as minor amounts of OH-groups occurring in the nominally anhydrous clinopyroxene. First Fourier-transform infrared spectroscopy results suggest that the nucleation of amphibole might be facilitated by the incorporated OH in the diopsidic clinopyroxene.

The observed microstructures and mineral compositions suggest that the micro-shear zones form at an early deformation stage throughout the eclogitisation process on Holsnøy. Our investigations show the complex interplay of brittle and ductile processes on a microscopic scale during macroscopically ductile flow.

How to cite: Lenz, L., Zertani, S., Grasemann, B., Stalder, R., Menegon, L., and Rogowitz, A.: Brittle-ductile deformation of granulitic clinopyroxene during incipient eclogitisation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5725, https://doi.org/10.5194/egusphere-egu26-5725, 2026.

A monocline fold forming in a sedimentary cover above a fault in a rigid basement is an example of fault-related folding and is often associated with fault propagation. We developed a custom implementation of finite element method model of folding in a viscous medium above a fault with an arbitrary dip (θ) and sense of slip. We explored three rheological scenarios: (1) a homogeneous isotropic cover, (2) a heterogeneous cover consisting of initially flat, alternating low- and high-viscosity (denoted by µ₁ and µ₂ respectively) isotropic layers, (3) an initially homogeneous anisotropic cover. Anisotropic fluids are characterised by shear and normal viscosity, i.e., viscosity under layer-parallel shearing and layer parallel shortening or extension, respectively. The model of an anisotropic fluid approximates the behaviour of a layered media in the limit of fine layering.

We performed systematic numerical experiments for fault dip angles ranging from θ=10° to θ=90°, number of layers n=8, 16, 32, 64, 128, viscosity ratios µ₂/ µ₁=10, 25, 50, 100 and shortening or extensional regime. Results demonstrated that an anisotropic viscous medium effectively approximates a finely layered sedimentary cover at both the onset of deformation and under large finite strain. However, the observations regarding the trends of structure evolution (e.g., fold amplitude growth rate) made at the onset or after a few initial time steps of deformation cannot be extrapolated for further stages of deformation. For sufficiently fine layering (e.g., n=64, 128), the simulated folds tend to be chevron-like. Two major geometrical types of folds can be described in the reverse fault case, i.e. in the shortening regime. A forelimb monocline alone forms above a basement fault with dip angles larger than θ=30-40°, but an additional pop-up anticline emerges in the case of a gentler dipping fault. In general, greater viscosity contrasts favour the amplification of the pop-up anticline. The anticline grows in time for most of studied cases, but its evolution is more complex for folds formed above a fault dipping close to the threshold value between two geometrical types. In these cases, the amplitude of the pop-up anticline decreases with progressive shortening at late stages of deformation. In the normal fault case (extensional regime), the covering layers tend to deform more or less parallel to the top basement boundary and fold geometries are rather similar regardless of the fault dip angle.

The work was supported by the National Science Centre, Poland, under research project “Numerical and field studies of anisotropic rocks under large strain: applying micro-POLAR mechanIcS in structural geology (POLARIS)”, no UMO-2020/39/I/ST10/00818.

How to cite: Mol, S. and Dabrowski, M.: Numerical modelling of viscous folding in a layered sedimentary cover above a basement fault, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6922, https://doi.org/10.5194/egusphere-egu26-6922, 2026.

Variations in rock deformation style across crustal levels are a fundamental topic in structural geology, yet the factors controlling strain localization in long-lived shear zones remain debated. Here we present a detailed field-scale and microstructural study of the >200 km-long Neoarchean Gadag–Mandya shear zone in the Dharwar Craton, southern India, based on a continuous along-transect observations.

The significance of this shear zone lies in its pronounced metamorphic gradient, from dominantly greenschist-facies assemblages in the northern Western Dharwar Craton to amphibolite- and granulite-facies assemblages in the south. This framework allows us to investigate variations in deformation mechanisms and the factors governing deformation style at different crustal levels, dominantly within granitic basement rocks. The shear zone has also been interpreted as a major tectonic boundary related to Neoarchean subduction, making its internal architecture critical for understanding the tectonic evolution of the Dharwar Craton.

Our results show that shear-zone width varies markedly along the transect, from centimetre- to metre-scale zones in the greenschist-facies domain to kilometre-scale zones near the amphibolite–granulite transition. The wider domains are characterized by (i) strong strain localization within granitic intrusions, (ii) the presence of pseudotachylytes, ultramylonites, and hydrous mineral assemblages, and (iii) pervasive overprinting relationships. EBSD data and quartz microstructural analyses indicate overprinting of earlier high-temperature deformation by lower-temperature deformation, particularly in the southern sector, where amphibolite-facies assemblages are locally retrogressed to chlorite–muscovite-bearing fabrics.

Beyond the amphibolite–granulite transition, marked by the appearance of clinopyroxene within the foliation, the main shear zone becomes difficult to trace as a single continuous structure. Instead, multiple metre-scale shear zones with variable orientations are observed, commonly spatially associated with melt-rich domains. These observations highlight the critical role of rheological heterogeneity, melt and fluid infiltration, and inherited thermal structure in controlling shear-zone width, strain localization, and deformation style in Neoarchean crustal-scale shear zones. Rather than recording a simple depth-controlled transition, the Gadag–Mandya shear zone preserves a composite record of spatially and temporally variable deformation processes, in which localized seismic slip, viscous flow, and melt-assisted deformation coexist and overprint each other. This integrated field–microstructural dataset provides new constraints on the mechanical behavior of long-lived lithospheric shear zones in Archean continental crust.

How to cite: Sreehari, L., Urakawa, M., Nakamura, Y., Satish-Kumar, M., and Sajeev, K.: Controls on Deformation Style in a Neoarchean Crustal-Scale Shear Zone: Evidence from the Dharwar Craton, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8935, https://doi.org/10.5194/egusphere-egu26-8935, 2026.

Microstructures within rocks and crystals describe their past deformation conditions, which may vary in time and space even within a single rock sample. This variation depends on pressure, temperature and strain rate, and determines whether minerals deform in a brittle or ductile manner. Investigating deformation mechanisms under extreme conditions, such as ultra-high-pressure (UHP), is particularly useful. Indeed, the analysis of well-preserved UHP rocks provides insights into crystal’s behavior over a wider range of pressures. For these reasons, we studied Dora Maira whiteschists, and specifically the pyrope crystals forming these rocks.

Dora Maira is one of the internal crystalline massifs of the European Western Alps, formed by HP and UHP units. The latter is famous for the presence of coesite-bearing whiteschists. These rocks are foliated, with a spatially variable foliation defined by the shape preferred orientation of phengite and garnet crystals. Garnet grains can be either rounded or elongated and show different sets of fractures. Moreover, garnet crystals are locally recrystallized.

The first set of garnet fractures is represented by parallel fractures oriented at high angle with respect to the main rock schistosity and affecting garnet crystals in the entire outcrop. These fractures are locally associated with another set developed at ca. 45°, formed together with small (µm-scale) rotating volumes of garnet. The parallel fractures are dislocated by the local recrystallization of some garnet grains and by radial fractures developed around coesite/palisade quartz inclusions. These radial fractures formed due to the large volumetric change happening at the coesite-quartz transition.

We analyzed the described microstructures using optical microscope and SEM in (HR)-EBSD mode. Additionally, we investigated garnet crystals’ composition with SEM-EDS and microprobe. In this contribution, we show the results of this combined analysis.

Our results provide new microstructural evidence that garnet can record alternating brittle and ductile deformation under UHP conditions. Besides, we document a correlation between deformation-related microstructures and major-element redistribution within garnet, highlighting the deep connection between these two aspects which was previously underestimated.

How to cite: Tagliaferri, A., Tajčmanová, L., and Duretz, T.: The memory of crystals: microstructures in UHP garnets from Dora Maira, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9776, https://doi.org/10.5194/egusphere-egu26-9776, 2026.

Fluids are widely recognized to weaken quartz and to be redistributed during deformation. However, integrated constraints that link water partitioning in natural quartz (fluid inclusions, grain boundaries, and crystal defects) to the evolution of dynamic recrystallization mechanisms (from SGR-dominated recrystallization, through increasing grain-boundary involvement, to GBM-dominated recrystallization) are still limited. Three types of quartz veins that are (sub-)parallel to foliation in the Xuelongshan metamorphic complex record a progressive shift in recrystallization style, providing an ideal natural laboratory to compare deformation mechanisms and fluid reservoirs.

We integrate field observations, microstructure, electron backscatter diffraction (EBSD), fluid inclusion (FI), laser Raman microspectroscopy (LRM), and Fourier-transform infrared spectroscopy (FTIR) to constrain coupled deformation-fluid evolution. All three quartz veins display widespread grain-size reduction and strong crystallographic fabrics. EBSD indicates dominant dislocation creep, with dynamic recrystallization evolving from subgrain rotation (SGR; Type I) through a transitional regime with enhanced grain boundary processes (Type II) toward grain boundary migration (GBM; Type III).

Fluid inclusions are mainly small, irregular, and are preferentially aligned along grain boundaries. Raman spectra from Types I and II quartz reveal a multicomponent fluid system including CO₂, SO₂, CH₄, and CO₃²⁻. FTIR spectra and spatial maps of bulk H₂O and Al-related OH demonstrate a systematic, mechanism-dependent redistribution of water among microstructural reservoirs. In SGR dominant quartz, water exist mainly as inclusion H₂O concentrated along (sub)grain boundaries, and inclusion deformation and rupture promote leakage so that recrystallized grains contain more bulk H₂O than porphyroclasts. Toward GBM, crystal defect OH increases significantly and the relative contribution of inclusion water decreases. In GBM dominant quartz, however, the proportion of defect water declines again as migrating boundaries efficiently sweep out dislocations and reduce the capacity for crystal defect H, despite continued high bulk H₂O.

Overall, our results suggest quartz deformed mechanism transitions are linked not only to the bulk water budget, but more critically to the redistribution of water among microstructural reservoirs (inclusions, grain boundaries, and defects), and to the evolving capacity of the microstructure to store mechanically effective water.

How to cite: Wang, S., Cao, S., von Hagke, C., and Zhan, L.: Coupled interaction between fluid and deformation mechanisms in quartz, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9812, https://doi.org/10.5194/egusphere-egu26-9812, 2026.

Metamorphic complexes preserve well-recorded evidence of multistage deformation, metamorphism, magmatism, and fluid-rock interactions. However, the structural relationships between microstructurally constrained ductile deformation and later brittle fault kinematics and stress fields remain insufficiently constrained. The Yuanmou Complex, located in the central–southern Kangdian region along the southwestern margin of the Yangtze Block, China, provides an ideal site to address these issues.  In this study, we combine field investigations with optical microscopy–cathodoluminescence (OM–CL), electron backscatter diffraction (EBSD) and paleostress inversion of fault-slip data to investigate the deformation process and stress fields of later brittle faulting in the Yuanmou Complex.

Microstructures and EBSD fabric results indicate that the Yuanmou Complex experienced multistage deformation, evolving from early high-temperature ductile deformation to low-temperature ductile deformation, followed by brittle deformation during exhumation to shallow crustal levels. EBSD fabric analyses of deformed quartz reveal a systematic transition in dominant slip systems, from high-temperature prism slip (>650 °C), through intermediate–high temperature prism

Paleostress inversion reveals the coexistence of compressional, extensional and strike-slip stress regimes. An early stress regime dominated by NNE–SSW-oriented compression is identified, whereas a later stage is characterized by a NW–NWW-oriented principal stress field, under which fault kinematics gradually evolved from thrusting to strike-slip–dominated deformation, accompanied by local extensional activity. Linking ductile deformation processes with subsequent brittle fault kinematics and stress fields, our results reveal their structural connection and reflect regional Cenozoic responses to eastward extrusion of the Tibetan Plateau and southeastward escape of the Sichuan-Yunnan rhombic block.

How to cite: Cheng, X., Cao, S., Jiang, S., and von Hagke, C.: Multiphase deformation and stress field evolution of the Yuanmou metamorphic complex, SW China, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9871, https://doi.org/10.5194/egusphere-egu26-9871, 2026.

The exhumation of lithospheric mantle at magma-poor rifted margins requires efficient strain localization and sustained weakening of ultramafic rocks. In the Tyrrhenian back-arc basin, recent IODP drilling by Expedition 402 has provided unprecedented access to mantle exhumed ≤4 Ma, revealing a complex interplay between deformation, magmatic intrusions, and hydrothermal fluid circulation.

We assess how hydrothermal fluids and syn-rift intrusions influence the mechanical evolution of the exhumed mantle in the Tyrrhenian Sea. Structural and microstructural observations from drilled mantle sections document a transition from high-temperature ductile deformation (mylonitization) to brittle faulting (e.g., brecciation). This evolution is accompanied by serpentinization and localized carbonation veins. These veins follow pre-existing lithological contacts such as felsic and mafic intrusions, which probably act as rheological barriers and as preferential pathways for fluid flow.

We integrate structural analysis, microstructural characterization and mineralogical constraints, and 3D tomography (synchrotron µCT) to evaluate how porosity distribution, connectivity of veins, reaction front, linked to fluid infiltration and fluid-driven mineral transformations, modify mantle rheology. Fluid-assisted weakening and reaction-induced volume changes may promote the development of localized shear zones and, ultimately, detachment faults. Preliminary observations indicate that magmatic intrusions (felsic and mafic) localize strain; subsequent serpentinization further reduces rock strength and facilitates the late stages of exhumation.

Our results suggest that mantle exhumation in the Tyrrhenian basin reflects complex coupled deformation-magmatism- fluid processes rather than  tectonic extension alone. This provides new constraints on strain localization mechanisms at magma-poor rifted margins and on the mechanical evolution of continent–ocean transitions.

How to cite: Vannucchi, P., Bickert, M., Poulaki, E. M., Montemagni, C., Baroncini, E., Rizzo, R. E., and Sanfilippo, A.: Deformation, fluid circulation, and strain localization during mantle exhumation in the Tyrrhenian back-arc basin, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10275, https://doi.org/10.5194/egusphere-egu26-10275, 2026.

Pseudotachylyte is a quenched coseismic frictional melt. As such, pseudotachylyte may provide invaluable information on the processes occurring on fault at hypocentre depths. Of particular interest are pseudotachylytes hosted in high-grade rocks, as they may record seismic ruptures propagated in the middle and lower crust. However, pseudotachylyte in high-grade rocks may also result from shallow deformation after uplift, thus constraining ambient conditions of faulting is crucial although not trivial.

The mineralogy of pseudotachylyte includes microlites crystallized during melt quenching, glass recrystallization products and, for deep-seated pseudotachylytes, minerals reflecting re-equilibration to the ambient metamorphic conditions. In absence of ductile deformation of pseudotachylyte promoting re-equilibration, the estimate of P–T conditions is typically based on the microlites. For example, the presence of microlitic ‘cauliflower’ garnet has been interpreted to reflect high-grade ambient conditions of faulting. However, Papa et al. (2023), described cauliflower-garnet-bearing pseudotachylytes hosted in granulite facies garnet-sillimanite-rich gneiss from Calabria and proposed shallow faulting conditions based on radiometric dating, suggesting that garnet can be transiently stable during quenching at shallow conditions.

Here we quenched at room conditions superheated (>2100 °C) melts produced by instantaneous laser-heating of the same peraluminous gneisses hosting the natural pseudotachylyte and compare the microlite population of the experimental glass with the microlites of the natural pseudotachylyte. Both the natural pseudotachylyte and the experimental glass contain: (i) acicular-shaped corundum microlites; (ii) sillimanite/mullite microlites overgrowing sillimanite clasts; (iii) skeletal-, dendritic-shaped spinel microlites, spatially associated with garnet, epitaxially nucleating on sillimanite/mullite and dispersed in the glass; (iv) microlitic cordierite, present in the natural pseudotachylyte as spherulitic aggregates and in the experimental glass as plumose microlites in melt-filled fractures of the wall-rock garnet; (v) newly formed euhedral rims of garnet epitaxial on garnet clasts and wall-rock garnet. The observed microlites crystallized during melt quenching following the same sequence, with slight differences due to the faster cooling rate of the experiments.

By comparing natural pseudotachylytes and experimentally produced analogues, we show that the mineralogy of natural microlites is essentially constituted by high-melting point phases and it is controlled by the local availability of chemical constituents and nucleation seeds (i.e. host-rock clasts). The experiments also prove that garnet can crystallize during quenching even at room conditions if seeds are available and the melt has the right composition. This observation calls for caution when using the mineralogy of pseudotachylytes, and in particular the presence of cauliflower garnet, to infer the depth of faulting. Finally, the melting experiments under static conditions highlight the relevance of thermal fracturing as deformation process aiding pseudotachylyte formation.

Papa et al. (2023), Lithos 460, 107375

How to cite: Pennacchioni, G., Toffol, G., Slupski, P., Pennacchioni, L., Wirth, R., Schreiber, A., and Cerwenka, G.: Microlites of natural and experimental peraluminous pseudotachylytes: a comparison, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10360, https://doi.org/10.5194/egusphere-egu26-10360, 2026.

Large continental shear zones play a fundamental role in crustal deformation, exhumation and lithosphere-scale tectonics, yet their duration of activity and the controls exerted by thermal regime on their geochronological record remain debated. Over the past decade, studies conducted in the Aegean domain, Menderes Massif and Alpine Corsica have generated a large and internally consistent set of geochronological data acquired across several major shear-zone systems. These datasets are dominated by ⁴⁰Ar/³⁹Ar ages complemented by U–Pb, Rb–Sr and low-temperature thermochronology.

Here we propose a synthesis of these datasets, integrating published results from different types of shear zones developed under contrasting P–T conditions, ranging from cold HP–LT subduction zone to Barrovian metamorphism in collisional environment and hot metamorphic core complex settings. We aim to compare age–distance relationships across shear zones, assess the temporal distribution and duration of deformation events recorded by argon systems, and place these observations in a broader tectono-thermal framework. Preliminary observations suggest systematic differences between cold and hot shear zones: cold systems tend to preserve a broad spectrum of argon ages spanning most of the deformation history, whereas hot shear zones commonly record shorter durations and younger ages biased toward the final stages of activity. These patterns appear to be robust across different tectonic settings and may reflect fundamental differences in deformation mechanisms, fluid circulation and argon mobility.

By combining shear-zone geochronology with independent constraints from magmatic intrusions, partial melting and tectono-metamorphic evolution, this synthesis identifies common timescales for shear-zone activity and clarifies how thermal regime controls both deformation processes and the geochronological record. Beyond regional implications for the dynamics of the Aegean and surrounding domains, this study provides first-order constraints on the mechanisms and longevity of continental shear zones and on the interpretation of geochronological datasets acquired in deformed rocks.

How to cite: Laurent, V., Roche, V., Jolivet, L., Augier, R., Raimbourg, H., Menant, A., Arbaret, L., and Labrousse, L.: Duration, thermal regime and argon behavior in continental shear zones: a synthesis from the Aegean domain, Menderes Massif and Alpine Corsica, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11126, https://doi.org/10.5194/egusphere-egu26-11126, 2026.

Subduction zones are uniquely direct pathways in which originally unconsolidated sediment is conveyed to great depths, all while experiencing continuous shear as it lithifies and metamorphoses. The largest earthquakes on our planet occur within these zones, along with other seismic and aseismic phenomena. The products of these processes are accreted mélanges which provide ‘windows’ into the otherwise inaccessible plate boundary interface at depth. The bulk physical behaviour of these subduction shear zones is controlled by the geometries of the blocks, the proportions of blocks to matrix, and the relative mechanical properties of blocks and matrix. Here we provide a structural and mechanical characterisation of the Chrystalls Beach Mélange, New Zealand, and trace its rheological evolution from the surface to the shallow seismogenic zone. We conducted a detailed 3D macro- and micro-structural investigation coupled with in-situ and laboratory-based rock mechanics to measure sub-block-scale heterogeneities and explain their origins.

The Chrystalls Beach Mélange formed within a Mesozoic Gondwanan–Pacific subduction zone, achieving maximum metamorphic conditions of <600 MPa/<300°C, within the shallow seismogenic zone and below the conditions required for quartz crystal-plasticity. This mélange is composed of subducted seafloor sediments that form decametre- to millimetre-sized blocks of sandstone and chert within a pelitic matrix, mixed with minor exotic blocks of altered basalt. These blocks display overprinting relationships showing a progression from ductile to brittle deformation as they transition from soft sediment to low-grade metamorphosed rock coincident with burial and pervasive shearing.

Four distinct rheological and tectonic regimes were responsible for the structural features we documented:

1) Layer-parallel shortening and fluidisation in the frontal toe of the subduction zone. Unconsolidated interbedded sand, mud, and siliceous ooze experienced ductile deformation producing isoclinal folds and injectites.

2) Layer-parallel extension of poorly consolidated ductile sediments resulted in boudinage and dismemberment in the shallowest subduction channel. This produced blocks with moderate – high aspect ratios, sharp tips, and asymmetric profiles.

3) Continued layer-parallel extension as the blocks lithified and embrittled. Internal stresses transferred from the matrix exceeded the yield stresses of the still-weak blocks, resulting in pervasive brecciation, followed by fragmentation as fluidised matrix injected into these fractures. This produced sub-rounded – sub-angular blocks with low – moderate aspect ratios, blunt tips, and irregular profiles. As blocks continued to indurate to the point that they could no longer be broken by stresses imparted by the matrix, they may still have been broken as they jostled and temporarily jammed the shear zone. At the same time, exotic blocks of basalt entered the mélange as rigid inclusions but underwent progressive weakening during subduction as they experienced brecciation and altered to clay minerals.

4) Localisation of strain previously distributed in the matrix towards more localised shear zones and veins in anastomosing networks.

In-situ Schmidt hammer strength tests show that block margins are systematically weaker than block cores across all lithologies. This is consistent with the increased fracture density towards block margins.  As such, mélange blocks within the shallow seismogenic zone display significant internal heterogeneity and should not be considered as two-phase mixtures.

How to cite: Clarke, A. P., Di Vincenzo, S., Weiler, M., Hawemann, F., Mitchell, T. M., and Toy, V. G.: The Other Ductile – Brittle Transition Zone: Syn-Deformational Lithification Within the Shallow Subduction Shear Zone and its Implications for Earthquake Nucleation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11270, https://doi.org/10.5194/egusphere-egu26-11270, 2026.

Semi-brittle deformation, characterized by the concurrent operation of cataclasis and crystal plasticity, plays a key role in constraining the strength of the middle crust. While the effects of temperature, pressure, fluid-abundance/pressure, and material properties (e.g., grain size) have been relatively well studied, the role of initial porosity in semi-brittle deformation remains poorly understood. Here, we performed a series of triaxial compression experiments on dry samples of Solnhofen limestone, which has an initial porosity of ~5.6% and an isotropic texture. Experiments were conducted at a range of confining pressures (P_c=30-300 MPa), temperatures (T=25 to 600 °C) and a constant strain rate of 1×10^-5 s^-1. Under these conditions, Solnhofen limestone mainly deforms in the semi-brittle regime associated with strain hardening, and brittle fracturing only occurs at low pressures (P_c≤50 MPa) and T <200 °C.

The strength, expressed as differential stress at a given strain, of Solnhofen limestone varies with imposed conditions. At 5% strain, the strength decreases with increasing temperature at all investigated pressures. In contrast, the pressure dependence of strength is temperature sensitive. At T =400 °C, the strength decreases significantly with increasing pressure from 30 to 300 MPa, in contrast to the positive pressure dependence observed for low porosity (~0.5%) Carrara Marble in the similar semi-brittle regime. This pressure sensitivity becomes less pronounced at 600 °C. Changes in porosity, determined from the pre- and post- deformation measurements, show that dilation and compaction are closely related to deformation mode. The extent in sample compaction correlates with the deformation ductility and becomes more marked with increasing temperature and pressure.

We speculate that the observed negative pressure dependence of strength during semi-brittle deformation arises from the presence of initial porosity and can be explained by the increasingly dominant role of plastic pore collapse. This hypothesis is supported by an additional experiment conducted at 400 °C, in which samples pre-compacted at 300 MPa for 3 h and subsequently deformed at 30 MPa exhibited higher strength than samples deformed directly at 30 MPa without a pre-compaction stage. Ongoing microstructural investigations will provide a basis for developing a microphysical model to better interpret deformation processes in rocks with intermediate porosity in the semi-brittle deformation regime.

How to cite: Feng, W. and Brantut, N.: Semi-brittle deformation of Solnhofen limestone: Initial porosity effects on strength, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12335, https://doi.org/10.5194/egusphere-egu26-12335, 2026.

Nanoindentation is a mechanical testing technique used to quantify material properties (e.g., hardness) and deformation behaviour (e.g., plasticity). By controlling the indenter tip with great precision in all dimensions, the range of available methods can be expanded to include rapid property mapping, constant-stiffness stress-strain curves, topography mapping, and scratch and frictional testing. We have set up a complete, affordable, and fast workflow centred around nanoindentation with a Bruker Hysitron TriboIndenter 990 and complemented by electron microscopy to tackle a variety of research questions concerning the behaviour of earth materials.

This contribution showcases this workflow as applied to several common rock-forming high-pressure metamorphic minerals. We begin with first-order sample characterisation of thin sections using optical microscopy and electron backscatter diffraction to quantify crystal orientations to determine which crystal axes are being indented. Transitioning to the mechanical testing phase, we use spherical tips to obtain stress-strain curves to analyse the transition from elastic deformation to low-temperature plasticity, and to quantify the yield hardness. Stress-strain curves can be calculated from regular constant loading rate indentation experiments, only valid within the elastic domain, or with constant stiffness measurements using tip oscillations to provide a full stress-strain curve including plastic behaviour. We image the residual indent sites with surface probe mapping, which measures surface topography with a vertical resolution down to 0.1 nm and thus produces 3D maps with which we can quantify the dimensions and geometries of indent pits.

The results of our case studies on glaucophane, omphacite, and garnet show that plastic yielding is controlled by the availability of nucleation points for dislocations, provided by pre-existing defects. The degree of this effect varies per mineral, and further depends on crystal orientation. Overall, we demonstrate an efficient workflow for mechanical and microstructural characterization of low-temperature plasticity with nanoindentation applicable to most silicates and other minerals. This workflow can also be adjusted to analyse and quantify many other aspects of the properties and behaviour of earth materials.

How to cite: van Schrojenstein Lantman, H. and Kotowski, A.: Deformation in great detail: A nanoindentation workflow for investigating low-temperature plasticity in silicate minerals, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12353, https://doi.org/10.5194/egusphere-egu26-12353, 2026.

Layer-parallel shortening of multilayer rocks results in the formation of folds. Using linear stability analysis, we obtain a growth rate curve. It allows us to determine the dominant wavelength during the initial stage of viscous folding. We derive an analytical expression for the growth rate curve of a single layer embedded in an anisotropic host, including confinement effects. The analytical results obtained for an anisotropic medium are compared to the growth rates obtained numerically for the corresponding cases of a finely laminated host. These cases split into two groups depending on whether a low- or high-viscosity layer borders perturbed interfaces of the central layer. However, in the limit of fine layering, their arithmetic mean tends to the results obtained for the anisotropic host. In search of an explanation, we calculate growth rates of the laminated host case analytically and show where the anisotropic approximation breaks down.

Next, we investigate an anisotropic rock medium under shortening along the anisotropy direction, with a locally perturbed axis of anisotropy orientation. It is a mean-field upscaled approximation to a multilayer system, which can tackle arbitrarily perturbed layer interfaces. In addition to the analytical approach, we use numerical simulations to study folding instability in such multilayer systems based on the direct (discretely layered medium) and upscaled (anisotropic medium) approaches. As a limiting case, we find the evolution of chevron fold amplitudes and study the convergence of the bilaminate dominant eigenmode to that obtained for the anisotropic medium.

Those results shed light on the limitations of the effective anisotropic models of layered rock systems, and provide a framework for more accurate mean-field approximations.

The work was supported by the National Science Centre, Poland, under research project “Numerical and field studies of anisotropic rocks under large strain: applying micro-POLAR mechanIcS in structural geology (POLARIS)”, no UMO-2020/39/I/ST10/00818.

How to cite: Gamdzyk, J. and Dąbrowski, M.: Viscous folding of multilayer rocks under layer-parallel shortening: discrete layering vs. anisotropic models, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15192, https://doi.org/10.5194/egusphere-egu26-15192, 2026.

Studies of multiple mantle exposures indicate that a fundamental shift occurs in polymineralic peridotites at ~850° C.  At these temperatures, there is a shift from dislocation creep (plus or minus dislocation accommodated grain boundary sliding) to reaction-facilitated grain-size sensitive creep.  This reaction results in a fine-grained matrix produced by neocrystallization.  The fine-grained shear zones that formed by dislocation creep dynamic recrystallization create increased grain-boundary surface area that localize the reaction-enhanced deformation.  Because the grains are formed by reaction, grain boundary pinning of the different mineral phases occurs.  Moreover, these fine-grain sizes are preserved during exhumation, because of the grain boundary pinning.  Thus, the fine-grain size – once it has been formed by reaction-facilitated deformation – continues to exist even if there is a change in temperature.  

This rheological behavior is not typically shown in deformation strength profiles, because monophase olivine does not show these effects.  Yet, the lithospheric mantle is polyphase, and we have observed evidence for reaction-facilitated deformation that occurred below ~850° C.  Once grain size reduction has occurred in a polyphase material, it is not expected to grow large grain sizes again, due to the role of grain boundary pinning.  Thus, once formed, a reaction-facilitated shear zone with smaller grain size relative to the surrounding mantle rocks would remain a lithospheric “scar”.  The fine-grain shear zones would preferentially reactivate because the zone can deform by grain-size sensitive creep at lower stress conditions that the surrounding mantle material can deform by dislocation creep.  This interpretation could explain the common reactivation of transform faults, and perhaps even extensional faults, in orogenic belts.  Reactivation of transform faults in the mantle may explain:  1) the Neoproterozoic transform faults of the eastern and western United States, which are reactivated by Pennsylvanian and Cretaceous deformation, respectively; and 2) the modern San Andreas System reactivating a Cretaceous – Paleogene proto San Andreas Fault. 

How to cite: Newman, J., Tikoff, B., and Chatzaras, V.: The microstructural legacy of mantle deformation during orogenic reactivation , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15465, https://doi.org/10.5194/egusphere-egu26-15465, 2026.

This study presents a paleostress reconstruction of the metavolcanic and granitoid rocks of the Hutti-Maski greentone belt, Eastern Dharwar Craton (EDC), southern India, aimed at evaluating progressive changes in the regional stress field at ca. 2.5 Ga. Paleostress was constrained using quartz vein orientations, Anisotropy of Magnetic Susceptibility (AMS) fabrics, and fault–slip data from metavolcanic and granitoid rocks.

Anisotropy of Magnetic Susceptibility (AMS) data from granitoids reveal a dominant NNW–SSE–striking magnetic fabric developed during earlier D1/D2 deformation. Paleostress analysis using vein orientations of dilational quartz veins in the granitoids yields an apparent NE–SW compressional stress field. However, kinematic analysis demonstrates that these veins in the granitoids were formed by dextral simple shear along the pre-existing NNW–SSE–oriented fabric under a regional N–S–directed D3 compression. From previous studies it is already well established that this regional N–S–directed D3 compression was responsible for D3 folds with E–W–striking axial planes found in different parts of EDC. N-S-oriented dilational quartz veins in the metavolcanic rocks of this greenstone belt were also formed due to this N-S oriented D3 compression. This interpretation is further supported by comparable stress ratio values obtained from three-dimensional Mohr circle analyses of vein populations in both lithologies.

Fault–slip analysis of displaced veins in granitoids reveals a late-stage NNE–SSW compressional stress field, indicating localised brittle deformation during the final stages of D3. This late brittle overprint is interpreted as resulting from late-D3 brittle deformation during the cratonization of the Dharwar Craton at approximately 2.5 Ga.

Therefore, this study demonstrates that there are pitfalls in the direct evaluation of paleostress using only vein orientations and that it is crucial to integrate kinematic constraints with vein orientation data during paleostress analysis of dilational veins.

How to cite: Goswami, S. and Mamtani, M. A.: Progressive evolution of paleostress in the Hutti-Maski Greenstone belt, Eastern Dharwar Craton, southern India, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17196, https://doi.org/10.5194/egusphere-egu26-17196, 2026.

Geophysical observations combined with detailed petro-structural analyses conducted in the field and in the laboratory indicate that « brittle » deformation occurs within subduction zones in rocks that are otherwise expected to deform in a « ductile » manner under the associated pressure–temperature conditions. These brittle events are most commonly localized in regions where metamorphic transformations are predicted to occur. Because such reactions may induce substantial changes in density and strength, they are frequently invoked as a primary mechanism driving the ductile-to-brittle switch in subducting rocks. However, the physical processes that link metamorphic transformations to changes in deformation style remain incompletely understood.

This contribution addresses this issue through the emblematic example of the granulite-to-eclogite transformation exposed at Holsnøy (Bergen Arcs, Norway). We combine field-based structural and petrological observations with numerical modeling developed over the past several years to investigate the mechanical and rheological consequences of this transformation.

We specifically examine whether eclogitization necessarily initiates along pre-existing brittle precursors or whether the reaction itself can trigger faulting, how the transformation propagates through the rock, and the extent to which the inherited granulitic foliation influences reaction localization. We further discuss the mechanisms leading to the formation of eclogitic shear zones as opposed to static eclogites (commonly referred to as eclogite fingers). Finally, we assess the relative roles of fluid availability and far field stress in controlling the spatial distribution and mechanical impact of the reaction.

By confronting field observations with numerical modeling, this presentation aims to show that the answers to these questions may not be unique, and that much remains to be done to fully understand the impact of metamorphic reactions on the rheological behavior of rocks.

How to cite: Yamato, P., Baïsset, M., Cochet, A., Duretz, T., Schmalholz, S., Podladchikov, Y., and Labrousse, L.: Linking metamorphic transformations and the brittle–ductile transition: Insights from numerical modeling of the granulite-to-eclogite transformation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17384, https://doi.org/10.5194/egusphere-egu26-17384, 2026.

Exhuming shear zones are key structures in the dynamic evolution of orogens. Such shear zones accommodate most of the shear-related exhumation within relatively small rock volumes. This is possible due to major strain partitioning occurring along weak rocks, frequently represented by phyllosilicate-rich rocks. Thus, the study of phyllosilicate-rich mylonites can provide fundamental insights into exhumation mechanisms responsible for the architecture of orogens.

The Hulw Shear Zone in the Saih Hatat Window of Oman (Agard et al., 2010) is one of these exhuming shear zones juxtaposing two subducted continental tectonic units. This tectonic contact experienced sustained shearing, accommodating a delta pressure of circa 0.8 GPa between 1.2 and 0.4 GPa at a relatively constant temperature of circa 400 °C (Petroccia et al., 2025) between 77 and 74 Ma (Ring et al., 2024).

In the field, micaschist belonging to the footwall displays a strain gradient moving toward the contact with the hanging wall, corresponding to a development of a S-C-C’ fabric and a modal enrichment in K-rich white mica and pyrophyllite matched by a progressive increase in the physical interconnectivity of these phyllosilicates. Microstructural analysis suggests that interconnected C planes were formed due to an interplay of fracturing allowing fluid to preferentially flow along the newly formed fractures and precipitating phyllosilicates, and preferential grain boundary sliding and glide of the quartz-phyllosilicate grain boundaries, with additional precipitation of new phyllosilicates in dilatant sites.

Hyperspectral cathodoluminescence highlights different luminescence for the larger (several hundreds of µm) detrital quartz grains, producing a bright signal and containing yielded cracks, and smaller equant quartz grains (less than 70 µm), darker in cathodoluminescence and devoid of cracks. Electron backscatter diffraction analyses suggest that large quartz grains experienced grain size reduction by subgrain rotation recrystallization to form smaller equant grains. Interconnected chains of small quartz grains are located in contact with the phyllosilicates, suggesting preferential recrystallization along these planes.

Transmission Electron Microscope analyses highlight pyrophyllite-muscovite intergrowths at the submicron scale as small as 300-500 nm, with truncated boundaries likely reflecting dissolution and precipitation mechanisms. Muscovite and pyrophyllite appear to deform differently, suggesting that strain partitioning occurred down to the submicron scale.

Summarising, these results suggest that strain localization and weakening of this rock volume was achieved by an interplay of the following mechanisms: I) diffuse microcracking and subgrain rotation recrystallization leading to a finer grain size of quartz, II) synkinematic nucleation of retrograde mineral phases along discrete C and C’ planes, III) preferential recrystallization along the shear planes and IV) dissolution and precipitation processes of phyllosilicates. Concluding, this intimate and polyphase interplay between deformation and metamorphism is responsible for the formation and evolution of exhuming shear zones and the related structure of orogens.

Giuntoli acknowledges financial support of grant N° MUR 2022X88W2Y _002.

References

Agard, et al., (2010). Tectonics, 29(5). https://doi.org/10.1029/2010TC002669

Petroccia, et al., (2025). Journal of Structural Geology, 191. https://doi.org/10.1016/j.jsg.2024.105328

Ring, et al., (2024). Earth-Science Reviews, 250, 104711. https://doi.org/https://doi.org/10.1016/j.earscirev.2024.104711 

How to cite: Giuntoli, F., Petroccia, A., Airaghi, L., Précigout, J., Raimbourg, H., and Kulhánek, J.: Insights into deformation mechanisms of exhuming brittle-ductile shear zones (Oman Mountains), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17819, https://doi.org/10.5194/egusphere-egu26-17819, 2026.

Eclogites are a major lithology of the subducting oceanic crust, and the strength contrasts between and with lithologies such as blueschists, serpentinites and peridotites, at depths, is likely what commands the timing and style of HP rocks exhumation, within subduction zones (Agard et al., 2016). These contrasts also influence the roughness and stress at the interface between the subducting slab and the overlying mantle wedge (Agard et al., 2018), and therefore may play a role in the stress relaxation and intermediate depths earthquakes sequences. Deformation mechanisms of the main minerals of eclogite, pyroxene and garnet, have been studied individually under high pressure and temperature. The rheology of eclogites themselves has received some attention using high pressure experiments (e.g. Zhang and Green, 2007, Farla et al., 2017, Rogowitz et al, 2023, Molines et al., EGU25-5696). These works and numerical models (e.g. Yamato et al, 2019, Angiboust et al, 2024) underline the importance of the interplay between brittle vs. ductile mechanisms in eclogites rheology at experimental strain rates. The garnet vs. pyroxene volume fractions are expected to have a major effect on brittleness and strength, since the spatial contiguity of the strongest component, or connectivity of the weakest component, may lead to transitions in the control of the deformation.

Until now the effect of shear strain on phases connectivities under GPa pressures has not been quantified, while it is one path to achieve connections between strong or weak domains. Here, we will present results from torsion experiments on two-phase aggregates of garnet and pyroxene as a proxy for eclogites, with garnet fractions from 15% vol. to 85% vol. We use in-situ absorption contrast tomography at the PSICHE beamline (synchrotron SOLEIL), under pressures of 2 to 5 GPa and temperatures of 850°C, to characterize quantitatively the fabric/microstructure of the aggregates under increasing shear strain (up to ca. 5).

We will discuss these microstructural quantifications with respect to 1) recent in-situ mechanical measurements in the same aggregates compositions, by Molines et al. (EGU25-5696), and 2) similar in situ characterizations during torsion experiments of serpentine+olivine aggregates – hence a different strength contrast between phases – by Mandolini et al. (e.g. EGU25-13729).

How to cite: Hilairet, N., Molines, C., Mandolini, T., Chantel, J., Addad, A., Fadel, A., Troadec, D., Le Godec, Y., Turpin, Z., and Guignot, N.:  Connectivity and fabric evolution with strain in eclogites : in-situ X-ray tomography under UHP conditions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18356, https://doi.org/10.5194/egusphere-egu26-18356, 2026.

Despite hornblende’s widespread occurrence in deformed rocks from exhumed crustal shear zones and metamorphic soles, its dominant deformation mechanism(s) and the respective microstructural fingerprints remain poorly constrained. Several deformation mechanisms have been documented in hornblende, including cataclastic flow, twinning, dissolution–precipitation, and dislocation-mediated deformation. Hornblende’s easy slip system, (100)[001], can be inferred from observations of intragrain misorientation axes (MOA) or crystallographic rotation about the [010] axis (Meher et al., 2026). Notably, even where some contribution from dislocation-mediated deformation is observed, hornblende is rarely deformed solely by dislocation creep. While crystallographic preferred orientation (CPO) and recrystallization suggest dislocation creep for most minerals (e.g., calcite, quartz, and olivine), in hornblende, these features seldom arise from alternative mechanisms.

We used electron backscatter diffraction (EBSD) to analyze microstructures in four natural hornblende-rich samples spanning a range of P-T conditions: (1) Mamonia complex, Cyprus (0.5 GPa, ~ 600 °C), comprising mm-scale conjugated kink bands. (2) Koralpe, Austrian Alps (~2.1 GPa, 750 °C), dominated by sigmoidal hornblende porphyroclasts surrounded by smaller, tabular grains. (3) Mayodiya, India (0.78–0.82 GPa, 770–820 °C), containing large grains with high intragrain misorientations and some twinning, and smaller needle-shaped grains with serrated boundaries between large grains. And (4) Koraput, India (0.76–0.84 GPa, 860–883 °C), which exhibits recrystallization of a centimeter-scale porphyroclast with smaller grains with lobate boundaries forming a core–mantle microstructure. By examining both CPO and MOA using detailed EBSD analysis, our goal is to (i) constrain the underlying deformation mechanism in these samples, and (ii) identify temperature-dependent transitions under natural conditions.

The Mamonia sample that experienced the lowest deformation temperatures exhibits deformation through fractures and kink bands, with no evidence of recrystallization. However, the MOA cluster is oriented toward [010], consistent with dislocation glide, suggesting semi-brittle deformation (e.g., Meher et al., 2026). The Koralpe sample exhibits a characteristic recrystallization microstructure, strain-free grains around large and highly strained porphyroclasts, and an MOA clustering around [101], which fits the orientation of (-101) twin planes and suggests twinning-driven recrystallization. The Mayodiya sample exhibits elongated recrystallized grains with MOA clustering around [001], while the porphyroclast exhibits MOA toward [010], again indicating twinning-driven recrystallization. The Koraput sample displays recrystallized grains that are slightly rotated compared to the parent porpyroclast with rotation around [010], consistent with hornblende’s easy slip system, (100)[001].

We infer that at low P-T conditions, hornblende deforms through semi-brittle deformation. At intermediate temperatures (Koralpe and Mayodiya), twinning-driven recrystallization dominates, activated via the (-101)[101] and (100)[001] twinning systems, respectively. At the highest temperatures (Koraput), hornblende undergoes grain-size reduction via dislocation-driven recrystallization. Together, those samples suggest a temperature-controlled transition from semi-brittle to dislocation creep mediated deformation between < 600 to > 850 °C.  

Meher, B., Incel, S., Renner, J. and Boneh, Y., 2026. Experimental deformation of textured amphibolites in the semi‐brittle regime: Microstructural signatures of dislocation‐mediated deformation. Journal of Geophysical Research: Solid Earth, 131(1), p.e2025JB031852.

How to cite: Meher, B., Incel, S., Renner, J., Rogowitz, A., and Boneh, Y.: Recrystallization and intracrystalline crystal-plastic deformation of naturally deformed hornblende, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18458, https://doi.org/10.5194/egusphere-egu26-18458, 2026.

Deformation microstructures in mylonites from the Plattengneis Shear Zone (PGSZ), Eastern Alps, provide new constraints on how mechanically anisotropic mid‑crustal rocks accommodate ductile strain. Although the PGSZ exhibits a strong Eo-Alpine N–S stretching fabric, it lacks many macroscopic structures typically associated with amphibolite facies deformation in anisotropic rocks. To determine where and how the high finite strains were localised, we investigate the microstructures of PGSZ mylonites with a focus on the polyphase ‘garnet–mica’ domains. Within these microstructural sites, locally elevated mechanical anisotropies form ideal conditions for nucleating and concentrating deformation structures. Importantly, this contrasts the relatively weaker mechanical strength contrasts observed in PGSZ quartz-feldspar domains, where localised deformation microstructures are scarce. Optical microscopy, back-scattered electron microscopy (SEM-BSE), and synchrotron microtomography (S‑µCT) were used to characterise both 2D microfabrics and the 3D architectures of garnet clusters. With this data, we present newly-described deformation microstructures in the PGSZ, discuss the importance of their spatial distributions, and consider the possible deformation processes involved.

SEM-BSE imaging uncovered a range of micro-scale shear bands, boudinage, and pinch‑and‑swell structures occurring exclusively within garnet–mica layers. Their restriction to these domains reflects the locally elevated mechanical strength contrast between competent garnet grains and weaker white-mica and biotite. Deformation is channelled into mica-rich areas, nucleating localised shear structures and rarely propagating further into quartz–feldspar domains. Garnet undergoes microcrack–induced fragmentation during producing synkinematic redistribution of garnet grains and fragments within the mica-rich matrix regions. This redistribution generates a range of (dis)aggregate cluster morphologies and biotite-infilled boudinage structures that align with the kinematic flow geometries predicted for the established D₁ + D₂ polyphase deformation history (Hill et al., in review). S‑µCT imaging resolved the 3D geometry of garnet clusters and revealed how fragmentation and redistribution record the cumulative kinematic evolution of the PGSZ. In more detail, the 3D data shows garnet forming complex clusters of both interconnected and disconnected grains elongated in the N-S direction, which are subsequently transposed in the E-W plane, in concordance with the D₁ and D₂ kinematic flow trajectories.

These results demonstrate that deformation in the PGSZ is highly localised within rheologically complex garnet–mica domains, where the elevated mechanical strength contrasts play a central role in the development of micro‑scale shear structures. Restricted development of shear bands exclusively within garnet-mica microstructural sites contributes to the apparent absence of larger-scale macrostructure development in the PGSZ, demonstrating the importance of a multi-scale approach to structural and kinematic analyses of ductile shear zones. Lastly, the (re)distribution of garnet in the PGSZ is proposed to be controlled by synkinematic growth and disaggregation during polyphase deformation, with the redistribution geometries potentially providing as a means of tracing strain histories in mechanically heterogeneous shear zones.

How to cite: Hill, L., Bestmann, M., Grasemann, B., Fusseis, F., and Dąbrowksi, M.: Mechanical-anisotropy controlled strain-localisation in garnet-mica domains of the Plattengneis Shear Zone (Koralpe, Eastern Alps), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20441, https://doi.org/10.5194/egusphere-egu26-20441, 2026.

The South-Central Zone of the Damara Belt records a history of intense, complex deformation resulting from the collision between the Congo and Kalahari cratons during the Pan-African Orogeny. Structural models have typically focused on multiphase deformation with inherent changes in the stress field and, to a lesser degree, on progressive deformation driven by a stress field with less variation. One example of the latter is a model that separates the crust in the South-Central Zone into two structural domains, a higher crustal level and a deeper crustal level. This allows the existence of orthogonal fabric domains resulting from different strain fields within the same orogenic zone, without the need for major changes in the regional tectonic stress orientation.

To date, geological maps and cross-sections have been used widely to graphically present the geological geometries of large areas in the Central Damara Belt. However, unlike 2D geological maps and sections, 3D models are more representative, providing additional insight to complex geometries and structural relationships. These complement and test traditional interpretations that often fail to account for the complexity and uncertainty of geological geometries.

This study provides the first large-scale 3D lithostructural modelling of the deeper structural levels of the South-Central Zone of the Damara Belt, south and east of the Rossing Dome. The different rock units in this area display kinematic and geometric features that support large scale constrictional-type strain characteristics and top-to-the-southwest displacement. In addition to field mapping data, digital elevation models, satellite imagery and published geological maps were used to delineate the regional geometry of folded lithological units. The resulting 3D model contributes to a better understanding of the deformation of the deeper crust during the collision of continental fragments and the development of large-scale fold geometries.

How to cite: Tuitz, C. and Uken, R.: Regional-scale 3D modelling of deep-crustal constrictional strain geometries within the Central Damara Belt, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21778, https://doi.org/10.5194/egusphere-egu26-21778, 2026.

Recrystallization of plagioclase is induced by deformation and/or chemical disequilibrium. It can be accomplished by several deformation mechanisms or their combination and it is typically accompanied by a change in plagioclase composition. The known agents facilitating the recrystallization are dislocations, mechanical twinning and cracking. In this contribution, we present incipient stages of dynamic recrystallization imposed on the magmatic plagioclase crystals in metagabbro from the Teplá-Barrandian Unit in the Bohemian Massif. The plagioclase crystals show chemically and mechanically heterogeneous internal structure related to its metamorphic-deformation transformation.

The chemical heterogeneity is manifested by decomposition of magmatic porphyroblasts of labradorite composition to the mixture of randomly oriented laths of bytownite and surrounding andesine. Crystallographically the laths are perfectly coherent with the rest of the crystal. The mechanical heterogenity is due to subsequent deformation that led to mechanical twinning followed by recrystallization. The initial low angle boundaries of the newly developing grains follow the network of bytownite laths while the later high angle boundaries are based on the original laths together with segmented twin boundaries. The resulting recrystallized microstructure shows small individual grains with andesine cores and bytownite rims. The misorientation analysis of the low angle boundaries indicate the geometry of till and twist boundaries resulting from dislocation glide and operation of (010)[001] slip system. Once the high angle boundaries are established they start to migrate and equillibrate, driven by chemical disequilibrium at the bytownite-andesine interfaces. The resulting fine-grained plagioclase shows evidence for grain size sensitive creep during subsequent deformation. Our findings indicate that crystal heterogeneity in feldspars may be an important parameter in the grain refinement process thus influencing the switch from dislocation creep to viscous flow.

How to cite: Jeřábek, P. and Racek, M.: Crystal heterogeneity controlling the grain size of dynamically recrystallized plagioclase, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21988, https://doi.org/10.5194/egusphere-egu26-21988, 2026.

In this study, we conducted rotary shear experiments to examine the frictional stability of Olivine and Quartz gouges over a range of temperatures (25–350 °C), slip velocities (100 μm s⁻¹ to 1 mm s⁻¹), and under a constant normal stress of 50 MPa. The two minerals exhibit contrasting stability behaviors: Olivine remains frictionally stable at room temperature but develops pronounced stick–slip instabilities at 350 °C. This unstable behavior persists at the velocity of 1 mm s⁻¹, although peak friction decreases slightly, indicating minor weakening. Quartz, by contrast, displays repeated stick–slip events at 25 °C, with stress drops that grow progressively larger with slip and are accompanied by continuous compaction, consistent with ongoing grain crushing. At 350 °C, Quartz behavior evolves from strong stick–slip at low velocities to stable sliding at higher velocities. These observations suggest that frictional stability is likely governed by a competition between the rate of tectonic loading, the specific healing kinetics, and the localization state of each mineral.

How to cite: Shahabi, H., Rattez, H., Tesei, T., Gomila, R., and Di Toro, G.: Contrasting frictional Stability of Olivine and Quartz: Rotary ShearExperiments under Hydrothermal Conditions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-23091, https://doi.org/10.5194/egusphere-egu26-23091, 2026.

Strain partitioning in quartzo-feldspathic rock is closely related to the degree of phase mixing. Both quartz and feldspar tend to form the load-bearing framework (LBF) in naturally deformed rocks, provided there are softer phases such as mica, which behaves as the interconnected weak layer (IWL). In cases where mica is absent, the scenario becomes complicated. Quartz in augen gneisses often behaves as the IWL and feldspar takes the role of the LBF. However, the relative degree of weakening in deformed quartz and feldspar depends on their respective deformation mechanisms. As the mechanisms are different, there is a possibility of dissimilar weaking, followed by a strength reversal.

We have studied a deformed quartzo-feldspathic vein from the Bundelkhand Craton in central India. Despite being Archean, this craton experienced long hiatuses between deformation events, which makes the delineation between different events simpler. The sample we collected from this craton is the result of the latest stage of deformation. A high-temperature fluid entered through fractures and softened the granitic country rock. The fluid, being syn-tectonic, allowed the granitic vein to facilitate different deformation mechanisms in quartz and feldspars.

We investigated the crystal-plastic behaviour of quartz and two feldspars in the deformed vein via electron backscatter diffraction. The quartz crystallographic preferred orientation (CPO) and misorientation index (M) is strongest when quartz grains are adjacent to each other. There is no significant difference in CPO strength in feldspars when the proportion of similar neighbouring phases changes. Additionally, a monomineralic quartz layer exhibits a class 3 buckling fold, implying a higher competency than the adjacent matrix, which contains recrystallised feldspar grains. However, the microstructural evidence suggests that the parent feldspar porphyroclasts are stronger than the recrystallised monomineralic quartz bands. From the inverse pole figure of low-angle (2–10°) misorientation axes in quartz, prism <a> activity is observed which is dominant in the temperature range of 500–650°C. Hence, we infer a deformation temperature of at most 650°C, although it can be lower depending upon the water weakening as such weakening activates prism <a> at lower temperatures. Randomised CPO in feldspar suggests strain accommodation via diffusion creep, followed by grain boundary sliding mechanism might have operated in feldspars. These processes could result in greater softening than that in quartz, which deformed by dislocation creep. Isolated quartz grains existing in the triple junctions of feldspars are not part of such pure dislocation creep; rather, it is more likely that they are byproducts of albitic transformation reactions. Hence, higher strength in quartz is limited to the monomineralic bands, which are purely affected by dislocation creep in the deformed quartzo-feldspathic vein of the Bundelkhand Craton.

How to cite: Sikdar, A., Wallis, D., and Misra, S.: Strength Reversal in Recrystallisation: an EBSD-based Study in a Naturally Deformed Granitic Vein, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1143, https://doi.org/10.5194/egusphere-egu26-1143, 2026.

Earthquake records preserved in rocks provide key insights into the processes that govern crustal deformation and seismic energy dissipation. This manuscript presents new approaches for identifying mineralogical signatures of paleoearthquakes using advanced microstructural analyses, including electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI). These techniques enable observations from the millimetre to nanometre scale of features associated with plastic deformation, including crystal reorientation and deformation twinning. Here, we investigate deformation microstructures in monazite, a key geochronometer, with the aim of assessing the impact of deformation on geochronological interpretations, as deformation-induced crystallographic defects can act as high-diffusivity pathways leading to Pb loss. Understanding the deformational behaviour of monazite is therefore critical for interpreting geochronological data. We examine monazite from an eclogite facies mylonite in the Musgrave Province (central Australia) to elucidate mechanisms of seismic deformation under dry (<0.002 wt% H₂O), lower-crustal conditions. The studied monazite grain is directly cross-cut by a pseudotachylite vein, indicating that the observed microstructures formed during the associated seismic event. EBSD and ECCI analyses reveal crystal-plastic deformation in the form of twinning with three distinct orientations: 180° <100>, 180° <001>, and 95° <201>. The latter is associated with dynamic recrystallization via subgrain boundary rotation. ECCI further reveals nanometre-scale (<15 nm) porosity within both parent grains and twins. These microstructures are consistent with those reported in monazite deformed during impact events. Recent studies of shocked monazite have shown that deformation by twinning can liberate Pb during rupture of rare-earth-element–oxygen (REE–O) bonds, enabling rapid diffusion along crystallographic defects and complete expulsion from the crystal, effectively resetting the geochronometer. The new insight provided by these microstructural focussed observations likely accounts for the disparity of electron probe microanalysis (EPMA)-based geochronology on the same monazite grain, which yielded ages of 1309 to 691 Ma. Seismicity in the Musgrave Province is primarily associated with the Petermann Orogeny (~550 Ma), suggesting that the younger EPMA ages were partially reset as a result of the twinning. Our results demonstrate the potential for monazite to record and date seismicity, opening new avenues for reconstructing paleoearthquake histories from deep crustal rocks.

How to cite: Dubosq, R., Camacho, A., and Britton, B.: Seismic twinning in monazite: Microstructural records of deep crustal earthquakes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4322, https://doi.org/10.5194/egusphere-egu26-4322, 2026.

Prograde metamorphic reactions that reduce solid volume are common in subduction zones and orogenic settings [1]. These reductions are often linked to irreversible deformation such as viscous compaction and deep mantle earthquakes [2]. Viscous compaction involves permanent closure of reaction-generated porosity and fluid release, making porosity transient property of metamorphic reactions [3]. The pore closure observation is often linked to the intuitive loss of the elastic strength of the rock leading to permanent strains [4, 5], but these assumptions are very rarely demonstrated experimentally. In most cases, the nature of field relationships and the design of experiments do not allow for such an assessment to be made. Time-resolved in situ experiments enable the observation of a sample volume undergoing metamorphic transformation to check such assumptions at every stage of the reaction (loading, heating, cooling and unloading).

In this contribution, we provide preliminary visual and quantitative strain mapping during the metamorphic reaction cycle of a rock sample at various stress states and reaction extent.

Using Mjolnir, an X-ray transparent miniature triaxial deformation rig, we performed a series of gypsum (Ca₂SO₄·2H₂O) dehydration experiments into bassanite (Ca₂SO₄·1/2H₂O) at constant confining pressure of 20 MPa, pore fluid pressure (5 MPa) and subjected to similar temperature paths (up to 125ºC).  We used a series of differential stresses (radial, hydrostatic and axial principal stresses) to explore how the rock volume responds mechanically while also displaying different reaction fabrics (see [6]). This dehydration reaction produces a solid volume reduction of ~30% [4] enabling the investigation of the evolution of elasticity by unloading fully and partially transformed samples.

Using synchrotron microtomography at the I13-2 beamline of the Diamond Light Source (MG34156), we performed high resolution imaging (1.625 microns/ voxel edge) during gradual unloading to observe and quantify the elastic behaviour both using the mechanical data from the Mjolnir rig [7], sample dimensions (using stitched images; [8]) and digital volume correlation (DVC) techniques (Avizo). 

Our results show the complexity of strain distribution and partial preservation in metamorphic rocks with:

• Significant elastic strain preservation during metamorphic reactions and its apparent minimisation during the ultimate stages of the reaction (textural “maturation” via pressure/solution).
• Complex strain distribution influenced by bassanite anisotropy, sample fabric, geometry, and stress state.

These experiments enable to visualise in 4D the grain-scale development of a complex porous network during the reaction. It opens pathways to document the emergence of poro-elasticity (initial solid has very low porosity) and then the release of the elastic strains. This dataset further demonstrates the importance and the complexity of elasticity in metamorphic systems, with complex displacement vector fields under relatively simple boundary conditions.

References:

[1] Brown & Johnson (2019). https://doi.org/ https://doi.org/10.2138/am-2019-6956

[2] van Keken & Wilson (2023). https://doi.org/10.1186/s40645-023-00573-z

[3] Putnis (2015). https://doi.org/10.2138/rmg.2015.80.01

[4] Leclère et al. (2018). https://doi.org/10.1016/j.epsl.2018.05.005

[5] Llana-Fúnez et al. (2012). https://doi.org/10.1007/s00410-012-0726-8

[6] Gilgannon et al. (2024). https://doi.org/10.1130/G51612.1

[7] Butler et al. (2020). https://doi.org/ 10.1107/S160057752001173X

[8] Turpin et al. in prep

How to cite: Freitas, D., Gilgannon, J., Duggins, D., Butler, I., Rizzo, R., Turpin, L., Chen, B., Reinhard, C., and Bourne, N.: The complex evolution of elasticity during metamorphic transformation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4517, https://doi.org/10.5194/egusphere-egu26-4517, 2026.

Deformation and metamorphism are fundamental processes that act synchronously throughout the Earth; however, their interaction remains unclear. Theoretical models predict that an applied tectonic stress has both a dramatic effect or no effect at all. While the small set of deformation experiments that document the interaction between reaction and deformation are either hard to compare or cannot provide the necessary time resolution to test the various theories. These disagreements of predictions and the gap in data invites new time-resolved experiments to be run that can probe details of model predictions and connect existing datasets from different materials deforming at a range of metamorphic conditions. To this end, we use state-of-the-art time-resolved synchrotron-based x-ray microtomography (4DSµCT) deformation experiments to map out the effect of a differential stress on the kinetics of the dehydration of polycrystalline gypsum samples. Our experiments are highly resolved in space (µm) and time (s), which allows us to track and contrast the emergence of the first small crystals (~100 µm3) and their growth through time in hydrostatic and differentially stressed conditions. We find that the kinetics of a metamorphic reaction are profoundly affected by the addition of deformational energy. Differentially stressed samples transform up to ~90% sooner than in the hydrostatic case, and reaction rates increase by a factor of ~5 with increasing differential stress. Importantly, our findings can be expanded to other published data for reactions occurring in the lower crust and the mantle to show that it is changes in the elastic strain energy that drive accelerated metamorphic kinetics. We find that, when we compare kinetic data from these different reactions and normalise the differential stress to each material’s yield strength, a trend emerges that shows stresses larger than the yield do not contribute to accelerating a reaction. Our results showcase the material-independent effect of a differential stress on metamorphic reactions and support theoretical models which place emphasis on the role of changes in stored energy. Current geodynamic models largely ignore the role of stored energy because it is assumed that it is not relevant at long time scales, our results show that its effect is important and should be accounted for when coupling deformation and metamorphism.

How to cite: Gilgannon, J., Vass Payne, E., Butler, I., Freitas, D., and Fusseis, F.: The material-independent effect of a differential stress on metamorphic kinetics, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6735, https://doi.org/10.5194/egusphere-egu26-6735, 2026.

The study of the evolution of petrophysical properties and alteration of host rock in an active fault system is essential for understanding the mechanisms of deformation localization. The distribution of alterations is closely linked to fluid flow paths, while the formation of new deformation structures depends on the mechanical contrasts induced by these alterations. Our study focuses on granodiorite samples from a borehole drilled in 1996 at Hirabayashi by the Geological Survey of Japan, one year after the Nanbu-Kobe earthquake. Crossing the active Nojima fault, this borehole intersects the fault core at 625 m. The analyses include imaging of samples using neutron and X-ray microtomography (ILL – NeXT) and thin sections, as well as mineralogical quantification by X-ray diffraction (XRD). X-ray diffractograms on oriented slides and Rietveld analyses of XRD data acquired on disoriented powders reveal the presence of secondary mineral phases (e.g., montmorillonite, kaolinite, laumontite, siderite, ankerite), representative of different fluid-rock interaction conditions during the exhumation of the massif. Their proportions, which increase as they approach the fault, reach more than 30% of the volume of a sample at 625 m. Whereas X-ray µ-tomography imaging allows us to observe density contrasts within the samples (e.g., mineral phases and fracture network). On the other hand, neutron imaging allows us to observe the distribution of hydrated mineral phases due to the high neutron absorption coefficient of hydrogen (e.g. for 25 meV neutrons, hydrogen attenuation is 3.44 vs 0.17 and 0.11 for oxygen and silicon, respectively). Neutron and X-ray image registration in the same reference frame allows us to perform joint image segmentation, using gaussian-mixture-model to quantify uncertainties, based on the neutron and X-ray coefficients of absorption of the pre-identified mineral phases. The volumes segmented in this way enable us to (i) quantify in a non-destructive way the volume of secondary mineral phases present in the samples along the fault damage profile and (ii) obtain their spatial distribution and assess the anisotropies of distribution in relation to the deformation structures. This work will subsequently enable us to understand the impact of both the distribution of secondary mineral phases and the network of microfractures on the evolution of the petrophysical and mechanical properties of a seismogenic fault.

How to cite: Jamet, M., Baron, F., Beaufort, D., Dazas, B., Patrier, P., Tengattini, A., Iaquinta, R., Doan, M.-L., Pezard, P., Gibert, B., and Luquot, L.: Neutron and X-ray µ-tomography-based 3D imaging of alteration phases in faulted granodiorite at Nojima (Japan), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7324, https://doi.org/10.5194/egusphere-egu26-7324, 2026.

During the International Ocean Discovery Program (IODP) Expedition 402 in the Tyrrhenian Sea, two of the six drilled sites, the U1613 and U1617, were located on the thinned continental crust of the Cornaglia and Campania terraces, where the deposition of evaporites during the Messinian Salinity Crisis (MSC) had been imaged with seismic data. Expedition 402 recovered Messinian evaporites beneath a relatively thin sedimentary cover at both drill sites. At Site U1613, the Messinian section is extremely thin (a few meters only). In contrast, at Site U1617, a complete 102 m-thick evaporitic sequence ranging from gypsum-enriched terrigenous sediments through anhydrite to halite layers was sampled. This scientific drilling site is the only one in the Mediterranean that penetrated the complete Messinian evaporitic sequence, providing a unique opportunity to study the properties of the so-called Upper, Mobile and Lower units. A series of physical property measurements was performed on these cores on board of the JOIDES Resolution drillship, including P-wave velocity, density, magnetic susceptibility, natural gamma ray and thermal conductivity. In addition, we collected representative discrete samples to measure P-wave velocity (Vp), bulk density, grain density and porosity. These data allowed us to analyze the sealing properties of the halite unit and its interaction with salt-induced tectonics. Furthermore, from Vp and density used as input to calculate reflection coefficients, we generated a 1D synthetic seismogram at Site U1617. We compared this synthetic seismogram with the multi-channel seismic data acquired across the drill site, namely the Medoc 6 line. These new data allowed us to compare the Messinian units recovered in situ with multichannel seismic data and thereby revise seismic interpretation of these units. Thanks to the unique opportunity offered by the IODP Expedition 402, we now have reliable data on the physical properties of Messinian evaporites and we are able to provide new constraints on the interpretation of Messinian facies.

How to cite: Loreto, M. F., Ligi, M., Filina, I. Y., Abe, N., Shuck, B. D., Pezard, P. A., Estes, E. R., Malinverno, A., Ranero, C. R., Yang, L., and Zitellini, N.: A new concept of Messinian Salinity Crisis based on physical properties from the IODP Exp.402 in the Tyrrhenian Sea, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7733, https://doi.org/10.5194/egusphere-egu26-7733, 2026.

The mechanisms of intraslab earthquakes at depths of > 40 km are fundamentally different from those of shallow earthquakes because the frictional strength of silicate rocks is proportional to the confining pressure. To understand the process triggering intraslab earthquakes, many experimental studies on faulting of slab-forming rocks have been conducted at upper mantle pressures. Previous studies have revealed that shear localization induced by dehydration of hydrous minerals (e.g., Okazaki & Hirth, 2016) or adiabatic shear heating (e.g., Kelemen & Hirth, 2007) is essential for the occurrence of faulting at high pressures. Although acoustic emission (AE) monitoring technique for D-DIA apparatuses enabled us to discuss the process of microcracking at high pressures, mechanical behavior at the onset of faulting is still unclear due to low time-resolution stress/strain measurements using synchrotron X-rays. The cause of bottleneck in stress/strain measurements is a long exposure time required for the acquisition of a two-dimensional X-ray diffraction pattern of minerals. Considering that the timescale of stress drop associating faulting is on the order of 0.01 sec (e.g., Okazaki & Katayama, 2015), a significant improvement for time resolution of stress/strain measurements is required. To improve the time resolution of stress/strain measurements, we installed a series of new devices at BL15XU, SPring-8.

We conducted in situ triaxial deformation experiments on olivine aggregates at pressures of 1-3 GPa and temperatures of 700-1250 K under nominally dry conditions using a D-DIA apparatus, installed at BL15XU, SPring-8. Two-dimensional radial X-ray diffraction patterns and radiographic images were alternately acquired by adjusting sizes of the incident slit and operating a flatpanel detector and a CCD camera using a high-flux pink beam (energy 100 keV) from an undulator source (0.2 s of exposure time for both ones). Pressure and differential stress were determined from the d-spacing of olivine. Strains of deforming samples were evaluated from the distance between platinum strain markers. AEs were recorded continuously on six sensors glued on the rear side of the 2^nd-stage anvils, and three-dimensional AE source location were determined.

Stress increased with strain at the beginning of sample deformation, and it reached the yielding point at strains of ~0.1 or less. AEs from the deforming sample were detected when stress exceeded ~1 GPa and the amplitude of AE is positively correlated with the magnitude of stress. At strains higher than 0.1 (i.e., beyond the yielding point), both softening (i.e., decrease in stress and/or increase in strain rate) and a decrease in AE rate were observed prior to the occurrence of faulting. Faulting was observed at 880-1150 K. Most of unstable slips proceeded within 1 s and associated a sudden stress drop (~0.5 GPa) and temporal radiation of large AEs. In contrast, neither stress drops nor AEs were associated with a few “aseismic” unstable slips. Differential stress continuously increased when stable slip proceeded and the stable slip was terminated by the occurrence of another unstable slip. Our observations suggest that unstable slips can be divided into two types (i.e., seismic and aseismic ones) under the P-T conditions of shallow subducting slabs.

How to cite: Ohuchi, T., Higo, Y., Tsujino, N., Kakizawa, S., Ohsumi, H., and Yabashi, M.: In situ observation of faulting in olivine at high pressures and high temperatures using high-flux synchrotron X-rays, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7751, https://doi.org/10.5194/egusphere-egu26-7751, 2026.

Evidence for the nature of fault slip across the seismic cycle is hard to decipher. Fault-related deformation near the fault surface develops over the seismic cycle, characterized by rapid coseismic slip and intense deformation, followed by slower interseismic slip and stress accumulation. While considerable focus has been placed on characterizing deformation through fracturing and mesoscale structures, the analysis of grain-scale plastic processes has been largely neglected. However, transient temperature increases due to frictional heating, combined with the ability of calcite-bearing rocks to deform plastically at relatively low temperatures, suggest that microstructural damage and subsequent recovery processes could leave diagnostic evidence in carbonate fault rocks. Indeed, Pozzi et al. (2019) demonstrated that shearing gouge at seismic rates (~1 m/s) develops a crystallographic preferred orientation (CPO), accompanied by grain growth and sintering. These observations, however, were confined to nanometer-scale grains, localized at the fault surface.

Here, we present a detailed microstructural analysis of carbonate samples from the North Anatolian Fault Zone. We used Electron Backscatter Diffraction to map the calcite grains' orientations and characterize intragrain deformation and grain-boundary morphologies. We identify three distinct layers extending from the fault surface to a distance of ~4 mm. Layer I, with a thickness of tens of µm to 0.5 mm, exhibits predominantly angular grains with grain sizes ranging from unresolved (<1 µm) to tens of µm. Layer II, with a thickness of 0-200 µm, is comprised of small equant grains (1-5 µm) and some larger grains (10-30 µm), characterized by wavy grain boundaries, suggesting active grain boundary migration. No CPO was observed in layers I and II. Layer III, with a thickness of ~2-3 mm, contains large grains (hundreds of µm) that can be divided into two populations of grains. Rounded grains with wavy grain boundaries indicate the progressive consumption of smaller grains. At the core of the layer, grains contain faceted boundaries and are elongated parallel to the fault surface. This layer is the only one to exhibit a distinct CPO with the c-axis oriented normal to oblique to the slip surface. Importantly, the large grains in layer III also comprise small, isolated ‘islands’ of finer grains.

We infer that deformation mechanisms vary systematically with distance from the fault surface. Layer I records cataclastic flow at the fault surface, whereas layer II, characterized by very small grain sizes, exhibits shearing by grain boundary sliding that resulted in grains with low intragrain misorientation and the absence of CPO. The most striking microstructural record is preserved in layer III, which initially shows strong evidence for recovery processes by abnormal grain growth. We propose that this latter process occurred during or immediately after coseismic frictional heating, resulting in the consumption of previously deformed grains, which maintains the CPO record of deformation and provides a microstructural record of the seismic cycle at millimeter-scale distances from the fault surface.

Pozzi, et al., 2019. Coseismic ultramylonites: An investigation of nanoscale viscous flow and fault weakening during seismic slip. Earth and Planetary Science Letters.

How to cite: Boneh, Y., Levi, T., Nuriel, P., and weinberger, R.: Abnormal grain growth in carbonate samples from the North Anatolian Fault: Microstructural evidence of the seismic cycle, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7796, https://doi.org/10.5194/egusphere-egu26-7796, 2026.

X-ray ptychography is a lensless, coherent-diffraction imaging technique developed over the last 20 years that affords 10-nm resolution for optically thick specimens¹. It reconstructs the optical transmission function (OTF) of a sample from raster-scanned overlapping 2D transmission diffraction patterns through iterative phase retrieval algorithms^1,2. The OTF projects the refractive index of the sample along the incident beam and thus quantifies the phase shift and amplitude attenuation of the transmitted beam², which in turn relate to the projected electron density of the specimen. X-ray ptychography is therefore an ultramicroscopy technique that is very well suited to mapping nano- and micron-sized objects with significant density differences relative to the bulk such as pores and dense accessory minerals.   

In this contribution, we present a primer for the application of X-ray ptychography to nano- and micro-scale studies of rocks. First, we illustrate the underlying physical principles that guide the data processing and interpretation of ptychographs. Then, we show exemplary applications to a wide range of rock samples (e.g., seismogenic brittle fault rocks, mylonites, veins, shale, and micrite) imaged at the XFM beamline of the Australian Synchrotron³ over the last 5 years^4,5. Application examples include the measurement of sample surface roughness, imaging of cracks and pores, 3D porosity measurements, and the detection of buried accessory phases.

References

1          Pfeiffer, F. X-ray ptychography. Nature Photonics 12, 9-17, doi:10.1038/s41566-017-0072-5 (2018).

2          Wittwer, F., Hagemann, J., Brückner, D., Flenner, S. & Schroer, C. G. Phase retrieval framework for direct reconstruction of the projected refractive index applied to ptychography and holography. Optica 9, 295-302, doi:10.1364/OPTICA.447021 (2022).

3          Howard, D. L. et al. The XFM beamline at the Australian Synchrotron. Journal of Synchrotron Radiation 27, 1447-1458, doi:doi:10.1107/S1600577520010152 (2020).

4          Jones, M. W. M. et al. High-speed free-run ptychography at the Australian Synchrotron. Journal of Synchrotron Radiation 29, 480-487, doi:https://doi.org/10.1107/S1600577521012856 (2022).

5          Schrank, C. E. et al. Micro-scale dissolution seams mobilise carbon in deep-sea limestones. Communications Earth & Environment 2, 174, doi:10.1038/s43247-021-00257-w (2021).

How to cite: Schrank, C. E., Jones, M. W. M., Kewish, C. M., van Riessen, G. A., Hinsley, G., Berger, A., Herwegh, M., Schwichtenberg, B., Bishop, N. D., Howard, D., Langendam, A. D., and Paterson, D. J.: Nano- and micro-scale imaging of rocks with X-ray ptychography, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8362, https://doi.org/10.5194/egusphere-egu26-8362, 2026.

Dehydration embrittlement is the dominant mechanism proposed to explain deep-focus earthquakes between 100–350 km in depth. Antigorite dehydration was extensively investigated in previous experimental studies, which demonstrated contrasting results regarding the seismic potential of antigorite dehydration. Additionally, microstructural aspects of antigorite dehydration and their implications for deep seismicity are scarce. Localized dehydration, on the other hand, might generate strain weakening, potentially leading to failure at depths relevant to deep earthquakes. Localized antigorite dehydration is demonstrated to occur in nature and the laboratory; however, it is not clear if this is a passive or dynamic process.

To better understand the micro-mechanisms of localized antigorite dehydration, we conducted high-pressure, high-temperature experiments under isostatic and non-isostatic conditions. Experiments were run at 3 GPa and temperatures within and above the antigorite stability field (530 °C–800 °C). Antigorite cores with 2 mm diameter were mounted in cubic assemblies and deformed in a 6-ram multi-anvil press at the Bayerisches Geoinstitute. Pure shear deformation was applied by inserting one pair of anvils while simultaneously removing the remaining two pairs orthogonal to it.

Results show that isostatic dehydration of antigorite at 3 GPa starts at ~530 °C and completes at ~800 °C. Localized dehydration occurs in isostatic and non-isostatic conditions within the antigorite stability field. It is enhanced during deformation experiments, resulting in the formation of nanocrystalline veins and networks containing olivine and pyroxene. These results demonstrate that localized dehydration might occur through passive and dynamic processes with the development of different microstructures.

How to cite: Silva Souza, D., Thielmann, M., Heidelbach, F., and Frost, D.: Localized dehydration of antigorite during experimental deformation at subduction zone conditions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9434, https://doi.org/10.5194/egusphere-egu26-9434, 2026.

U-Pb geochronology via Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) is a fast and reliable method for in-situ dating of calcite that is used across disciplines in earth science. However, the heterogeneous distribution of U (and Pb) in individual calcite crystals represents a yet unmitigated challenge and identifying zones of sufficiently high U concentrations that can provide precise constraints on timing of calcite precipitation is an inefficient “hit or miss” process. Moreover, it is challenging to confirm that targeted domains of a calcite crystal retain their pristine geochemical signature, given the range of post-crystallization dissolution-reprecipitation and solid diffusion processes that can affect this mineral. There is thus an urgent need to understand the spatial and temporal mechanisms of U incorporation and mobilization in calcite to ultimately improve this key geochronological tool. To determine where specific trace elements are located within calcite crystals, investigate how they are incorporated during crystal growth and how they are affected by post-crystallization fluid-assisted deformation processes, we applied Synchrotron X-Ray Fluorescence Microscopy (XFM) with emphasis on U mapping, Electron Backscatter Diffraction (EBSD), and LA-ICP-MS to a set of calcite veins. Samples were collected from drillcores through the Middle and Upper Jurassic carbonates and marls (max. 85°C) in the Neogene Molasse Basin in central northern Switzerland. By combining high-resolution trace element maps with information on the crystal lattice structure of calcite we show two main textural types of trace element distributions within syntaxial calcite veins: 1) oscillatory crystal growth zonations that reflect preferential incorporation of trace elements into structurally different growth steps and faces of growing calcite crystals during growth and, 2) complete overprint of the initial growth zonation upon potential secondary fluid infiltration and trace element replacement. The anti-correlation between Fe, Mn and Sr, U demonstrates the role of kinetic factors during trace element partitioning between fluid and calcite, pointing to the inhibition of Fe incorporation at higher growth rates. Where the Sr uptake during calcite growth is generally enhanced with growth rate. The results of this project give valuable insights in the complexity of fluid overprint during multi-staged deformation cycles in the modification of trace elements in calcite, with clear implications for the applicability and reliability of U-Pb geochronometer in calcite.

How to cite: Akker, I. V., Schrank, C. E., Jones, M. W. M., Howard, D., Tavazzani, L., and Morales, L.: Trace element mapping in vein calcite with synchrotron XFM: implications for U-Pb geochronology, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9614, https://doi.org/10.5194/egusphere-egu26-9614, 2026.

Salt caverns are formed in the subsurface during solution mining of salt. After the end of salt production, caverns need to be safely abandoned or may be repurposed for storage of energy carriers such as hydrogen. Salt caverns locally disturb subsurface stresses, leading to creep of the surrounding rock salt. Creep can cause cavern convergence at depth and may result in surface subsidence, with consequences for infrastructure and public safety. Accurate forecasting of cavern stability during abandonment or assessment of suitability for storage requires a deep understanding of the grain scale deformation mechanisms and processes controlling rock salt strength and creep rate. For rock salt, important deformation mechanisms are dislocation creep and pressure solution creep. Laboratory experiments have shown that dynamic recrystallization (DRX) associated with dislocation creep can be activated and contribute to mechanical weakening. However, the weakening effect of DRX is not included in engineering constitutive laws used in salt cavern numerical models. These laws are commonly based on low-strain laboratory experiments, where the influence of DRX is limited, and microstructural data are relatively rarely reported. The aim of this study is to experimentally investigate the dominant DRX process in deforming natural rock salt and its effect on the mechanical behaviour.

Lab experiments have been carried out on natural wet salt samples from the Zechstein formation. We conducted constant strain rate experiments using a triaxial compression apparatus. Experiments were performed at a confining pressure of 20 MPa and a temperature of 125 °C, using constant displacement rates corresponding to strain rates of approximately 5 × 10⁻⁵ s⁻¹ and 5 × 10⁻⁷ s⁻¹, up to 30–40% axial strain. After the experiment, all samples were studied using optical microscopy. Electron backscatter diffraction (EBSD) analysis was performed on the starting material and on two deformed samples, one from each strain-rate condition.

For all samples, we observed an initial transient creep stage followed by a quasi-steady state stage. The transition to quasi-steady occurred at a strain of about 10% for samples deformed at a strain rate of ~5 × 10^-7 s^-1. For samples deformed at the faster strain rate of ~5 × 10^-5 s^-1, continuous hardening occurred up to axial strains of 30%, with a gradually decreasing hardening rate approaching steady state. Light optical and EBSD microstructural analysis revealed grains with a dense substructure including subgrain walls, euhedral shape grains with low to no substructure, and grains with irregular shaped grain boundaries including bulges. We infer that the dominant deformation mechanism in the tested natural samples was dislocation creep, providing sufficient local differences in dislocation density to activate DRX dominated by grain boundary migration processes. DRX led to rheological weakening and quasi-steady deformation. We are working on robust understanding of the parameters controlling DRX as this is essential to evaluate the zones prone to weakening by DRX around salt caverns.

How to cite: Dialeismas, E., de Bresser, H., Hangx, S., and ter Heege, J.: An experimental investigation of dynamic recrystallisation processes and their influence on the mechanical properties of natural rock salt samples, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9678, https://doi.org/10.5194/egusphere-egu26-9678, 2026.

Bio-cementation is a new, environmentally-friendly soil-reinforcement process. It is used for civil engineering purposes, such as the fabrication of construction materials, as well as the preservation of monuments. This process uses bacterial activity, mainly that of Sporosarcina Pasteurii, that is capable of  hydrolysing the urea present in the medium, leading to the precipitation of CaCO₃ (calcite) crystals between sand grains, therefore binding them together, and reinforcing the soil. The macro and micro (contact scale) mechanical properties of bio-cememted sand have been extensively studied. However, the microstructure of the precipitated calcite crystals remains undiscovered, which induces mechanical differences under different conditions of cementation. The goal of this study is to investigate the microstructure of biogenic calcite, issued from bio-cementation of sand, and how it varies under different cementation conditions. For this, high resolution synchrotron diffraction imaging at the ESRF was performed, utilizing scanning 3DXRD (s-3DXRD) on ID11 and Dark-Field X-ray Microscopy (DFXM) on ID03. For this, the main experiment was performed on three samples that consist of 3D printed resin cells in which cementation was performed under different conditions, by varying the substrate on which the calcite was grown (between sand grains and glass beads), as well as varying the salinity of the medium. After each cementation cycle, and for each sample, layer measurements were acquired using s-3DXRD. A significant difference was observed between the sand and glass bead cases: the precipitated crystals on the glass beads were much smaller than those precipitated on the sand grains. DFXM measurements showed defects that are only present in the case of high concentration of NaCl in the medium, which could potentially alter the mechanical properties of the material. These two complementary techniques allowed for an in-depth study of the microstructure of the precipitated calcite crystals.

How to cite: Sarkis, M., Detlefs, C., La Bella, M., Naillon, A., Geindreau, C., Emeriault, F., Watier, Y., Ball, J. A. D., and Yildirim, C.: High-resolution microstructural study of calcite crystals precipitated through bio-cementation under different conditions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11242, https://doi.org/10.5194/egusphere-egu26-11242, 2026.

When deformed by dislocation creep the dominant slip (Burgers) vectors of olivine dislocations are parallel to [100] or [001]. Dislocations with an [010] Burgers vector component (b dislocations) have been recorded rarely. Here we show an experimentally deformed olivine sample has a substantial population (17%) of b dislocations. Electron Backscatter Diffraction maps of crystal orientations provided information on dislocations from the orientation gradients. Maps show the b dislocations form subgrain walls like those formed by other dislocation types and are interpreted to form similarly by glide and climb, so b dislocations are mobile. To confirm our approach, we used EBSD maps to select an area for Transmission Electron Microscopy imaging, down to an atomic scale image of a b dislocation. Our sample was deformed within range of subduction zone conditions; our approach can be used to investigate the scale and conditions of b slip in the mantle more widely.

How to cite: Wheeler, J., Hunt, S., Eggeman, A., Donoghue, J., Gholinia, A., Li, Y., Tillotson, E., and Haigh, S.: Olivine Deformation: to B Slip or not to B Slip, that is the Question, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11671, https://doi.org/10.5194/egusphere-egu26-11671, 2026.

The Cation Exchange Capacity (CEC) of clay minerals has been extensively studied in wide applications / purposes, using various imaging techniques to highlight changes of the clay sheet chargeability. Among the clay minerals, swelling clays such as smectite or vermiculite are particularly interesting regarding their adsorption-desorption properties strongly related to their high CEC (80-150 to 100-150 meq/100g respectively). To better monitor and predict cation exchange processes, the CEC has been investigated by different methodological approaches, including X-ray absorption spectroscopy (XAS) and geoelectrical methods. The Spectral Induced Polarization (SIP) is particularly well designed to quantify CEC because its complex conductivity measurements (in phase and quadrature) characterizes the electrical conduction of charge carriers (liquid) and the polarization phenomenon resulting from the local accumulation of electrical charge carriers in the porous medium (mineral interface).

The aim of this work is to investigate in situ how K cations are incorporated within the interlayer of a Ca-montmorillonite by coupling XAS and SIP experimental methods. This novel approach brings multi-scale information at the atomic and clay-sheet levels, providing new insights on enhancing the understanding of CEC mechanisms in terms of time and space and our ability to monitor it with SIP. The experiment was carried out on LUCIA (Soleil synchrotron), at the low energy of potassium K-edge with a microbeam size (2.5 x 2.5 µm²).

We used a 1.7 mm³ cell filled with 0.1 g of Ca-Montmorillonite isolated in a 0.8 µm sieve to avoid flushing of the clay sample during the experiment. The cell was subjected to a solution flux of a few cc per minute with 4 different KCl salinities (0.01, 0.05, 0.1, 1 M of KCl).  In situ SIP spectra are compared with XAS to conjointly monitor the CEC exchange at different scales. Preliminary results are shown to test-proof the methods as a new in situ / in operando cross-scale methods for CEC spatio-temporal characterization. Overall,this work is the first step of a technological development project, merging the approaches of geophysicists, mineralogists and physicists to monitor in real time the cation exchange processes of a Ca-montmorillonite by K in swelling clay minerals. 

How to cite: Courtin, A., Jougnot, D., Paineau, E., Roy, D., Vantelon, D., Dallaporta, A., and Léger, E.: Combining X-ray absorption and Induced Polarization Spectroscopies for in situ monitoring of Cation Exchange in clay materials, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11961, https://doi.org/10.5194/egusphere-egu26-11961, 2026.

The evolution of orogens is steered by complex deformation processes that act at several crustal levels, evolving over time from syn- to post-orogenic. Investigating how strain and deformation localize in the ductile domains of the deep crust and brittle domains of the shallow crust can improve our understanding of the processes ultimately controlling the exhumation of deeply seated rocks. Within this framework, the Sestri-Voltaggio Zone (SVZ) of the Italian Ligurian Alps provides a record of rocks and structures attesting to the complete subduction-exhumation cycle during the Europe-Adria convergence. The SVZ is a mature fault zone characterized by a polyphase tectonic evolution and a high lithological variability, which tectonically juxtaposes high-pressure (HP) metamorphic units to non-metamorphic rocks. It also represents an abrupt structural-metamorphic boundary between the Voltri Massif (an eclogitic domain defining a southern culmination of the Western Alps) to west, and the Northern Apennines units (anchi-metamorphic or non-metamorphic) to east. The exhumation processes that led to the current outcropping units of the SVZ occurred following a multi-stage progression from early ductile to later brittle conditions. However, open questions remain reflecting the generalized lack of systematic descriptions of structural fabrics formed during the exhumation-related events of the SVZ units. In this recently launched study we further explore the exhumation mechanisms of the SVZ by investigating how the pre-existing metamorphic fabrics helped localize the brittle deformation that occurred at later stages at shallow crustal levels. Preliminary field observations and structural analyses document N-S to NNE-SSW-striking brittle faults separating lenses of HP-mafic (metagabbros and metabasalts) and carbonate lithotypes from the enveloping phylladic schists and serpentinites. The enclosed lenses exhibit a pervasive internal schistosity that strikes either parallel or at high angle to the orientation of the main SVZ boundaries. By mapping the orientation of the rock fabric as a function of distance, perpendicular to the main tectonic boundaries, it is possible to identify systematic geometric trends between the metamorphic foliations and the bounding brittle faults. Within the matrix, the metamorphic schistosity wraps around the lenses, varying both in strike and dip. Brittle faults, with dominant oblique kinematics, are characterized by a double behavior: they truncate the metamorphic schistosity when approaching massive lenses; but they tend to rework the schistosity within the phylladic matrix. The overall structural record of the investigated units highlights the distribution of strain localization within the deeply exhumed units, suggesting a distinction between episodic vs. progressive transition from ductile to brittle during exhumation. In this sense, the SVG can be considered a useful example of the deformation history of the Western Alps-Northern Apennines tectonic junction, with noteworthy implications on the first-order mechanisms leading to the exhumation of deeply seated rocks.

How to cite: Generi, A., Viola, G., and Vignaroli, G.: Characterizing ductile-to-brittle exhumation of polymetamorphic units along the Sestri-Voltaggio Zone (Ligurian Alps, Italy)., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13049, https://doi.org/10.5194/egusphere-egu26-13049, 2026.

Microstructural investigations of halite are essential for understanding deformation mechanisms relevant to salt tectonics and underground storage applications, including radioactive waste disposal and salt caverns. However, the identification of subgrain boundaries, dislocation structures, acting creep mechanisms and fluid-related features remains challenging due to the optical transparency and inherently low defect contrast of halite. Gamma decoration provides a powerful solution by inducing radiation-related colour centers that selectively highlight lattice defects and deformation structures.

At the Forschungs-Neutronenquelle Heinz Maier-Leibnitz (FRM II, TUM), gamma decoration has been implemented since over a decade, recently we re-established and systematically optimized it using older spent fuel elements characterized by comparatively low dose rates. This contribution focuses on methodological developments and parametric studies that enable reliable gamma decoration under these conditions, extending the applicability of the technique beyond high-dose irradiation facilities.

We present results from controlled irradiation experiments on halite thin sections covering a wide range of total doses, irradiation times, and temperatures, combined with post-irradiation optical microscopy, spectroscopy, and digital colorimetry to quantify and optimize suitable optical contrast. Our experimental results from long-term irradiations are compared with theoretical models describing dose-rate-dependent radiation effects on defect formation in natural rock salt. This parametric approach allows identification of threshold conditions required for effective defect visualization, as well as optimization strategies to compensate for reduced dose rates, including extended irradiation times and temperature control.

These results establish gamma decoration at FRM II as a robust and versatile experimental method for salt-rock research, providing a valuable link between laboratory testing, microstructural analysis, and mechanical modelling, and ensuring continued applicability of this technique with ageing irradiation infrastructure.

How to cite: Hutanu, V., Li, X., and Schmatz, J.: Gamma decoration at FRM II: recent optimisations and parametric studies for microstructural investigations of halite, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13462, https://doi.org/10.5194/egusphere-egu26-13462, 2026.

Spatial phase distributions can be grouped into random, clustered or anticorrelated/distributed based on the probability to encounter a given nearest neighbour. This property can also be probed with respect to intervals of directions, frequently revealing an anisotropy in the spatial phase distribution. In this study, several high temperature ultramylonites with variable composition from felsic to ultramafic as well as coarse grained deformed rocks (e.g. eclogite from Münchberg, Germany) were investigated. Measurements of phase distribution anisotropy frequently manifest in a pronounced direction of phase clustering and one direction of anticorrelation. Especially in the investigated ultramylonites but also in deformed eclogites and amphibolites, those two directions are found not to be orthogonal and not to coincide with finite strain axes (as far as manifested by foliation and stretching lineation). Clustering phases (e.g. qtz, plg or grt, depending on the rock type) form stacks antithetically tilted against the sense of shear. These stacks are separated by phases such as kfs, bt or cpx. Below a certain volume threshold of the stack-forming phase, stacking is not observed.

In addition to the phase distribution, truncated chemical zonations and/or indications of directed growth are frequently observed. On the other hand, there is a lack of microstructures which can reasonabley be associated with steady state dislocation creep.

It is suggested that the observed microstructures in combination indicate deformation by a mechanism best described by dissolution-precipitation accommodated granular flow (or "diffusion creep" in the broadest sense). Stack-forming phases undergo mostly rigid-body rotation and translation temporarily forming transient force chains before being disintegrated again. Since these stacks can be observed in the rock record, the residence time in the force chain position must be greater than in a randomly distributed position, compatible with jamming of particles during granular flow.

The presence of this particular type of anisotropic spatial phase distribution may not only serve as a shear sense indicator but could in general be useful for the identification of deformation mechanisms.

How to cite: Kilian, R.: Spatial phase distribution and deformation processes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13669, https://doi.org/10.5194/egusphere-egu26-13669, 2026.

Serpentinites are key components in subduction zones, acting as primary carriers of water into the deep Earth and critically influencing seismic behavior. Several studies suggest that fluid-saturated deformation in serpentinized subduction channels may control a variety of processes associated with Intermediate-depth seismicity (~50 to 300 km depth) . A key problem in their rheology is the discrepancy between experimentally deformed serpentinites, which exhibit predominantly brittle behavior, and their naturally deformed counterparts, which show ductile fabrics. While the dominant deformation mechanism in subduction settings—whether crystal plasticity, dissolution-precipitation, or a combination— also remains poorly documented. Moreover, there is a lack of constraints on how serpentinites deform during and after partial dehydration at (ultra)high-pressure conditions and transformation to olivine and pyroxene-dominated assemblages. To address these issues, we present an integrated microstructural and geochemical study of serpentinites across a hectometer-scale strain gradient within the Zermatt-Saas meta-ophiolite, documenting the evolution of deformation and major element mobility during subduction and exhumation. In low-strain samples, dehydration forms coarse-grained olivine-diopside-clinohumite-magnetite veins. Host antigorite shows weak crystallographic preferred orientations (CPOs) and twinning. With increasing strain, deformation localizes around these veins, where olivine develops a weak B-type CPO, but with grains showing no evidence of intracrystalline deformation. Progressively, antigorite develops a strong, penetrative foliation with a (001) maximum normal to foliation and grain size reduction, while olivine veins are folded and boudinaged. Low angle grain boundaries are related to fracturing of olivine. In high-strain serpentinite mylonites, transposed olivine veins form isoclinal folds, and S-C' fabrics develop. Antigorite CPO strength increases considerably, something that is not observed for olivine. Whole thin section XRF mapping reveals an increase of Ni and S in the more deformed serpentinites, where pentlandite defines the C' fabric and wraps around olivine porphyroclasts. Antigorite mm thick bands show Cr depletion accompanied by grain size reduction, while Fe-Mn occur normally associated with each other. In the transposed olivine veins there is an increase of Fe content in comparison to the original olivine vein composition.  When present, Al-rich phases such as chlorite are mostly undeformed but can breakdown locally and transform into tremolite + magnetite in late shear bands. Our data document a fluid-assisted progression from localized brittle-ductile to distributed ductile deformation. Microstructural and chemical evidence indicate that deformation was primarily controlled by dissolution-precipitation processes, with limited crystal plasticity in antigorite and predominantly brittle olivine deformation. This study provides a rare dataset on metamorphic olivine deformation in subduction zones and highlights the fundamental coupling between element mobility, metamorphic reactions, and strain localization in the subduction interface and mantle wedge.

How to cite: Morales, L. F. G., Muñoz-Montecinos, J., Ceccato, A., Kilian, R., and Volante, S.: Dissolution-Precipitation dominated deformation in (ultra)high-pressure serpentinites from the Zermatt-Saas Meta-Ophiolite, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14234, https://doi.org/10.5194/egusphere-egu26-14234, 2026.

Olivine is a fundamental rock-forming mineral for which microstructures are closely tied to deformation conditions. However, visualization of olivine deformation has traditionally been limited to two-dimensional observations, ranging from petrographic microscopy at the millimetre– to micrometre scale to electron-based techniques probing crystallographic distortion and ordering at the micro- to nanometre scale (e.g., EBSD and TEM). Here, we introduce dark-field X-ray microscopy (DFXM) and present its first application to geological materials, conducted at beamline ID03 of the ESRF.

Using a focused line beam produced by compound refractive lenses, DFXM enables non- destructive, in-situ imaging with spatial resolution down to ~35 nm. By selectively illuminating a ~500 nm thick volume with the line beam, DFXM allows “slicing” through depth of the crystal volume. By translating the sample through the X-ray beam, the layers can be stacked and reconstructed into full 3D datasets.

In this work, we reconstruct the 3D microstructures of the mineral olivine across a range of deformation settings, spanning from hydrothermal single crystal olivine, to olivine in Åheim orogenic peridotite which experienced long-term dislocation creep, to olivine in heavily shock-metamorphosed martian basalt with relict crustal strain. We observe individual static dislocations and associated lattice strain field in the hydrothermal olivine single crystal, to arranged low-angle boundaries (LABs) formed by geometrically necessary dislocations (GNDs) in the Åheim peridotite, to chaotic dislocation networks connected by dense, short, and randomized LABs in shocked martian basalts.

By bridging conventional 2D crystallographic observations with volumetric 3D microstructural reconstructions, our work enables robust observations of microstructures developed in distinctive deformation conditions, providing a powerful and advanced 3D imaging technique for geological materials. Our study expands the application of DFXM to Earth and planetary materials and demonstrates the power of multi-scale, three-dimensional imaging for resolving complex deformation histories in geological systems.

How to cite: Li, Y., McCaulsand, P., Flemming, R., Yildirim, C., and Detlefts, C.:  Microstructure Across Deformation Regimes: 3D Imaging of Olivine by Dark-Field X-ray Microscopy, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15191, https://doi.org/10.5194/egusphere-egu26-15191, 2026.

The fault damage zone of the active Milun Fault in eastern Taiwan exhibits fractured and altered fault-rock textures, including spotted schist, serpentinite, and associated gouge. In the vicinity of the upper boundary of the damage zone, the recovered drill core hosts a non-cohesive, pulverized quartz body (~20-30 cm in length) within the fault rocks. The pulverized quartz is sandwiched between fractured schist and millimetre-scale laminae subparallel to the zone boundary. Microanalytical observations show that the quartz is shattered into a fine powder without an evident shear sense or preferred fracture orientation. No shear-induced amorphous phase is detected, whereas Laue diffraction indicates pronounced lattice distortion and elevated residual stress. The pulverized quartz displays a dense tensile fracture network, a feature commonly reported for seismically pulverized rocks along seismogenic faults, suggesting a dilatational, tensile-dominated fragmentation mechanism rather than progressive shear comminution. We propose that the quartz pulverization resulted from high strain rates associated with transient tensile stresses during coseismic rupture, potentially favoured by specific lithologic conditions.

How to cite: Wu, W.-J., Lien, P., Huang, T.-H., Chiu, W., Chiang, C.-Y., and Kuo, L.-W.: Microanalytic characteristics of extremely fractured quartz in fault damage zone and implications, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15835, https://doi.org/10.5194/egusphere-egu26-15835, 2026.

The interactions between fluid and solids in fault zones are governed by slip, slip rate, and constituent properties. These interactions are recorded by particle shape and size distributions, fracture patterns, and the geochemical composition of material within the deformation zone. The evolution of near-surface sediment microstructures and yielding behaviors under tectonic loading and at variable fluid saturation remains an open question. We collect undisturbed 10 x 40 mm cores from unconsolidated silt-sized sediments (fines) surrounding, and along, a fault strand that slipped while saturated, and likely experienced aseismic slip under variable saturation over the past 300 years. We use X-ray microtomography to analyze voids within the fines and found that they are ellipsoidal, have volume distributions that are best fit by a truncated power-law, orient sub-parallel to the fault strike, and sometimes merge into tabular or irregularly shaped fractures. The volume range for power-law scaling in the distributions separates a smaller population of voids with markedly different distributions in sphericity, tortuosity, aspect ratio, and minor/major axis lengths from a larger population of voids. The power-law truncation is likely due to the finite core size. We interpret the voids as initially small gas bubbles that nucleated where cavities existed within the fines and then grew via diffusion of immiscible gases when saturated, or via brittle/ductile yielding of the fines under variable saturation. Several fractures cross-cut or branch off some voids, indicating multiple deformation events and suggesting that the void boundaries are weak spots within the fines that accommodate tectonic strain. Similar growth mechanisms have been observed in magmatic systems, where ductile yielding of the melt occurs from the merging of bubbles that primarily orient at acute angles from the maximum extension direction. These findings suggest that, in addition to sands, pore structures in finer-grained sediments preserve a record of near-surface aseismic slip and may provide a relative estimate of near-surface strain. The findings further imply that a process akin to ductile yielding deformed the fines and, in turn, the pore voids. 

How to cite: Dasent, J., Chang, M., Su, K., Wright, V., and Manga, M.: Memory of brittle-ductile yielding within near surface fault zone sediments, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16278, https://doi.org/10.5194/egusphere-egu26-16278, 2026.

The mineralogical-geochemical composition and texture of salt rocks plays an important role for the site selection and construction of a repository for heat-generating, highly active radioactive waste. As the bromide content in halite depends on the degree of evaporation and subsequent processes, Br is frequently analyzed to estimate the genetic history of the rocks (Braitsch 1971). Typically, geochemical analytical methods are applied on powder samples (e.g. ICP-OES, XRF), and textural analyses (e.g. using EBSD) require extensive sample preparation. In this study, first results of diapiric Upper Permian rock salt samples are presented using non-destructive µXRF on polished rock samples.

The µXRF Bruker M4 Tornado Plus (Nikonow & Rammlmair 2016) was used to map and quantify element distributions in rock salt. For calibration, in a first step, a certified reference material (CGL), consisting of mostly halite (NaCl) with minor content of anhydrite (CaSO₄), and sylvite (KCl), was pressed into pellets of 2 g and 1 cm diameter. For representativity, three spot measurements and a mapping of the center (1 cm²) were chemically quantified. The µXRF measurements correlate with the certified values yielding an R² of 0.995. In a second step, pressed pellets with a range of defined concentrations of Br in halite and Rb in sylvite were prepared to estimate the concentration ranges measurable by µXRF. For Br, the concentrations range from 1 to 0.005 wt.% Br in halite, and for Rb the concentrations range from 0.4 to 0.005 wt.% Rb in sylvite. Both data sets show a good correlation with R² of 0.99 (n=21 for Br and n=17 for Rb). Therefore, µXRF seems suitable for quantification of Rb and Br in salt rocks.

Furthermore, naturally deformed halite samples were analyzed simultaneously for their geochemically and textural properties, which were previously analyzed using “conventional” methods (ICP-OES and EBSD, respectively; Mertineit et al. 2023). The bromide content in halite is ca. 200 µg/g and thus comparable to the known values. The textural results show misorientations of few degrees within single halite grains and pronounced misorientations at halite grain boundaries, indicating bending of the crystals, but no pronounced texture of the bulk rock.

Although the results are in good agreement with published data, further test should follow, especially on the textural analyses including the misorientation angle resolution and the indexing of the halite crystal axis. However, the application of µXRF on salt rocks offers a fast, non-destructive method providing reliable combined geochemical and textural information.

References

Braitsch 1971. Springer-Verlag, https://doi.org/10.1007/978-3-642-65083-3

Mertineit et al. 2023. Tectonophysics 847, https://doi.org/10.1016/j.tecto.2023.229703

Nikonow & Rammlmair. 2016. Spectrochim Acta B 125, https://doi.org/10.1016/j.sab.2016.09.018

How to cite: Nikonow, W., Mertineit, M., and Schramm, M.: Combined geochemical and textural analyses of halite: First results of non-destructive µXRF measurements, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17303, https://doi.org/10.5194/egusphere-egu26-17303, 2026.

Recent advances in microanalytical imaging and machine learning enable quantitative, multiscale characterization of geological materials with direct relevance for subsurface energy storage. This study presents an integrated workflow combining Broad Ion Beam (BIB) sample preparation, Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDX), and advanced machine learning to quantify pore structures, mineralogy, and their spatial relationships from the micrometre to nanometre scale (Klaver et al. 2021).

High-resolution secondary electron (SE2) and backscattered electron (BSE) imaging, complemented by low-resolution EDX data, provides multimodal datasets for automated analysis. Pore networks are segmented using a pre-trained U-Net deep learning model, enabling efficient and accurate porosity quantification. Mineralogical phases are identified and quantified through a semi-automatic, decision-tree–based segmentation approach. The alignment of SE2 and BSE datasets allows porosity to be directly correlated with specific mineral phases, establishing a robust link between microstructure, mineral composition, and petrophysical properties (Jiang et al, 2021).

The applicability of this technology-driven approach is demonstrated through two case studies. Case study 1 investigates geological hydrogen storage in underground salt caverns, focusing on the impact of biotic and abiotic reactions on anhydrite. Flow-cell experiments combined with cryogenic BIB-SEM analyses enable early detection of microstructural, mineralogical, and pore-space changes induced by hydrogen, hydrogen sulfide, and microbial sulfate reduction. Despite slow reaction kinetics, microstructural observations reveal the substantial onset of chemical alteration, biofilm formation, and evolving pore connectivity at the submicron scale, providing essential constraints for geochemical and hydraulic models (Berest et al., 2024).

Case study 2 examines fault sealing in mechanically layered limestone–marl successions. Oriented transfer samples from normal fault systems were analysed using multiscale microanalytical workflows to capture marl smearing, mechanical mixing, fracturing, and cementation processes. High-quality microstructural datasets serve as ground truth for training machine learning algorithms for efficient interpretation of 2D image data. The results show that fault cores are composed of recurrent structural building blocks whose distribution and sealing capacity are strongly controlled by the presence and properties of marly interbeds (Schmatz et al., 2022).

Overall, the integrated microscopy–machine learning framework provides a transferable, data-driven approach for quantifying coupled structural, hydraulic, and geochemical processes in complex geological systems.

References

Berest et al.,2024. Risk assessment of hydrogen storage in a conglomerate of salt caverns in the Netherlands. KEM-28 report. https://www.kemprogramma.nl/documenten/2024/04/03/kem-28-project-rapportfinal-report-kem-28-h2c3-240403_v2

Jiang et al., 2021.Workflow for high-resolution phase segmentation of cement clinker from combined BSE image and EDX spectral data. Journal of Microscopy, 1-7.

Klaver et al., 2021. Automated carbonate reservoir pore and fracture classification by multiscale imaging and deep learning. 82nd EAGE Annual Conference & Exhibition, Oct 2021, Volume 2021, p.1 – 5.

Schmatz et al., 2022. Prediction of Fault Rock Permeability With Deep Learning: Training Data from Transfer Samples of Fault Cores. 83rd EAGE Annual Conference & Exhibition, Jun 2022, Volume 2022, p.1 – 5.

How to cite: Schmatz, J., Jiang, M., and Schmitz, J.: Advanced Microscopy and Machine Learning for Multiscale Analysis of Porosity and Mineralogy, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17416, https://doi.org/10.5194/egusphere-egu26-17416, 2026.

The investigation of microstructural characteristics in concrete constitutes a fundamental basis for advancing its performance in civil engineering construction. Existing expertise in digital rock physics (DRP), developed for natural rock materials, is transferred and adapted for concrete. DRP utilizes non-destructive X-ray computed tomography (XRCT) to examine the internal microstructure of concrete, allowing for the visualization of features such as phase distributions, pore space, and microcracks. In this study, high-resolution digital concrete twins are created to capture and quantify internal microstructural changes induced by external mechanical loading. To overcome limitations in phase and microstructure identification caused by the restricted resolution of XRCT, these digital investigations are complemented by detailed microstructural analyses using standard polarization microscopy and scanning electron microscopy (SEM). The results show that externally applied stresses significantly influence the microstructural response of concrete and thus affect the accuracy of physical measurements conducted under high-pressure conditions.

XRCT datasets with varying spatial resolutions were acquired under in-situ confining pressures ranging from 0.1 MPa to 46 MPa. CT images of concrete in unloaded and mechanically loaded states were subsequently analyzed and compared to identify stress-induced microstructural changes, with particular emphasis on the segmentation workflow. Here, particular focus is on large and small concrete aggregates, grain/phase boundaries within the aggregates, (micro-)porosity, and especially the interfacial transition zone (ITZ), which represents a major source of uncertainty in phase assignment during segmentation.

Image quality was first assessed by identifying artifacts and evaluating grayscale histograms. Subsequently, global thresholding was applied for phase assignment and initial segmentation, which was iteratively refined using complementary microscopic analyses of thin sections, including SEM, as reference data. The resulting segmentation of the concrete subvolume (600x600x769) distinguishes large and small aggregates (<80 % quartz, ca. 20 % phyllosilicates), pore space, phyllosilicate-composed matrix, silica-composed matrix, and inclusions (mainly rutile, zircon, apatite, iron oxides). Small changes can be seen in the distribution of the individual phases at the different pressures. With increasing pressure, the porosity decreases, and partially areas with characteristic phase arrangements arise along the large aggregates, potentially indicating the influence of the ITZ.

However, the quantitative determination of the interfacial transition zone remains challenging using XRCT data, and microcracks are likewise difficult to reliably resolve and segment. Therefore, the high-resolution microstructural investigations are also required to adequately capture these features. Overall, the study highlights the necessity of detailed microstructural characterization for the reliable interpretation of XRCT data and the assessment of stress-induced changes in concrete.

How to cite: Beiers, L. M., Balcewicz, M., Lebedev, M., and Saenger, E. H.: Digital Concrete Physics – Microstructural Characterization of Concrete under Confining Pressure: Insights from X-ray Computed Tomography and Microscopy , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17609, https://doi.org/10.5194/egusphere-egu26-17609, 2026.

Ultramafic rocks exposed adjacent to mid-ocean ridges in the footwall to large slip oceanic detachment faults provide unique insight into deformation and reaction when transforming from peridotite to serpentinite. In contrast to orogenic serpentinites, oceanic serpentinites have not subjected to superposed metamorphic and/or tectonic overprinting. A suite of samples from mostly fresh peridotites (~20% alteration), with preserved olivine and pyroxene, to completely serpentinized rocks (100% alteration), dominated by serpentine (lizardite) and magnetite, were collected from a ~1.2 km long drill core from IODP Expedition 399 at the Atlantis Massif oceanic core complex.

A combined approach of synchrotron diffraction and electron backscatter diffraction in order to analyze the crystallographic preferred orientation (CPO), and micro X-ray fluorescence mapping and optical microscopy in order to image and analyze the microstructure, is used to explore the variable microstructures.

Magnetite forms polycrystalline aggregates defining a foliation, which ranges from anastomosing to highly parallel. In partially serpentinized, mylonitic peridotites showing olivine grain size reduction and CPO development; magnetite aggregates trace the preexisting mylonitic fabric. Lizardite and magnetite both have a variable CPO strength and different CPO types, suggesting that different processes and parameters influence the formation of these microstructures. Further, late stage deformation, is evident from microfaulting, sheared serpentine veins and dissolution features. The individual contributions of deformation and serpentinization reaction to the final microstructure will be evaluated and discussed.

How to cite: Kühn, R., Schlickum, L., Kilian, R., Morales, L., Parsons, A., John, B., and Deans, J.: Deformation and reaction in the microstructural record of oceanic serpentinites, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18276, https://doi.org/10.5194/egusphere-egu26-18276, 2026.

Lithium is a trace component, which is frequently observed in salt deposits and salt solutions collected in salt mines, respectively (Mertineit & Schramm 2019). So far, no naturally formed Li-bearing salt mineral is known, thus, the origin of Li in salt deposits must be related to other sources, e.g. to detrital phyllosilicates (Braitsch 1971). Detailed investigations on the Li content, the occurrence within a mine and the mineralogical composition of specific stratigraphic layers enable the reconstruction of rock-fluid interaction and fluid migration pathways. This is important for the construction, design and dimensions for a repository for radioactive waste in rock salt.

To verify which minerals are Li-hosts, diapiric Upper Permian (Zechstein) samples from the uppermost Staßfurt-Formation and the lower Leine-Formation were investigated for their mineralogical-geochemical composition. The succession contains salt clays, anhydrite and carbonate rocks as these rocks reveal the highest Li content (up to 159 µg/g bulk rock). The samples were previously investigated using ICP-OES, ICP-MS, XRD, SEM and thin section microscopy. Beside typical salt minerals in varying amounts (halite, anhydrite, magnesite, sylvite, carnallite), most samples consist of quartz, illite-muscovite, chlorite (clinochlore) and biotite, all of them with a grain size of ≤100 µm, often <20 µm. Only few samples contain traces of kaolinite, koenenite, hydrotalcite, anatase and tourmaline.

Additionally, µXRF and imaging LIBS (Laser Induced Breakdown Spectroscopy) analyses were performed at the same specimen to obtain detailed information of the element distribution including Li on thick section scale (Nikonow et al. 2019).

The clay containing rocks are intensively deformed by boudinage and subsequent brittle fracturing. The fractures are oriented in different directions and are filled with halite and/or carnallite and single grains of anhydrite and magnesite. Relics of bedding are present, but the phyllosilicates do not show a pronounced shape-preferred orientation. Shear strain is indicated by a slight rotation of single rock fragments. The spatial distribution of Li shows that Li is enriched in certain areas. Li accumulations are observed in single silicate grains, which are unequally distributed in a very fine-grained clay matrix. Furthermore, Li is enriched at the fracture rims, often associated with seams of Fe-bearing phases and probably organic matter.

Depending on the mineralogical composition of the investigated rocks, the Li content varies significantly. Li probably originates from illite-muscovite and a Li-bearing variety of a tourmaline (elbaite). Li was mobilized during brine-host rock interaction and precipitated in fracture infill, probably at reducing geochemical conditions. However, due to the limited spatial resolution of most used methods compared to the very small grain size of the rocks, a distinct relation of Li content to a specific mineral phase requires further analysis.

Braitsch 1971. Springer-Verlag, https://doi.org/10.1007/978-3-642-65083-3

Mertineit & Schramm 2019. Minerals 9, 766; doi:10.3390/min9120766.

Nikonow et al. 2019. Mineralogy & Petrology 113, https://doi.org/10.1007/s00710-019-00657-z

How to cite: Mertineit, M., Schramm, M., Nikonow, W., and Meima, J.: Lithium content and mineralogical composition of fractured salt clay (Upper Permian), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18322, https://doi.org/10.5194/egusphere-egu26-18322, 2026.

The properties of fault slip surfaces, gouge characteristics, and fluid-rock reactions are tightly coupled and control earthquake mechanics. To visualise and quantify the role of this coupling, we have developed a new operando imaging approach that allows the documentation of fast direct-shear deformation experiments in time-resolved 2- and 3-dimensional image data at low single-digit micrometer resolution. A direct-shear inset developed for the X-ray transparent Heitt Mjölnir triaxial deformation apparatus enables experiments at 20 MPa normal stress under fluid-pressurised conditions and allows real-time permeability measurements.

We apply this platform to three fault systems: 1) Slip surfaces in basaltic rocks, imaged while sliding at 1 mm.s^-1, reveal how asperities, phenocrysts, and surface roughness control stick-slip behavior and damage localization during fast slip. 2) Reactive quartz-gypsum gouges imaged during velocity stepping and healing experiments, enable the direct linking of evolving frictional properties to microphysical developments. 3) A shearing, dehydrating gypsum gouge provides insights into transient rheologies and the resulting strain distributions.

These datasets demonstrate that 4D imaging resolves coupled mechanical, chemical, and hydraulic fault evolution in real time. Our approach allows documenting microphysical processes underlying the frictional properties of faults and thereby constitutes a potent tool for studying faults in a variety of tectonic settings.

How to cite: Jayawickrama, E., Harpers, N., Schwichtenberg, B., Bell, A., Ng, A., Rizzi, R., Cordonnier, B., Herwegh, M., and Fusseis, F.: Recent Developments in 4D X-ray Tomography for Real-Time Observation of Fault Slip and Gouge Evolution, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18619, https://doi.org/10.5194/egusphere-egu26-18619, 2026.

The deformation and interaction of amphibole grains are crucial for comprehending the rheological behavior and physical properties of middle to lower crust. However, the mechanisms of strain accommodation and grain boundary processes in amphibolites are poorly studied. In this study, we analyzed a naturally deformed amphibolite from an exhumed continental strike-slip shear zone. The amphibole grains can be categorized into two distinct types: type I and type II, with the type II being embedded within type I. Type I amphibole grains exhibit typical plastic deformation behavior, distinguished by the presence of discernible dislocation arrays and formation of subgrains. In contrast, type II amphibole grains predominantly display microfractures in the middle of grains and voids occur in their elongated tails. Meanwhile, we identified three types of low-angle boundaries in amphibole grains with varying microstructural and nanoscale characteristics. Our findings indicate that low-angle boundaries in minerals are not exclusively associated with crystal-plastic deformation. Furthermore, the deformation characteristics in type II amphibole grains are related to grain boundary sliding (GBS) process. To relieve stress concentration during grain boundary sliding in type II amphibole grains, two accommodation mechanisms are proposed: (i) Grain boundary diffusion with elimination of grain boundary irregularities. (ii) Intragranular deformation of adjacent grains through either a brittle or a ductile process. Our findings hold significant implications for understanding the stress concentration and accommodation during deformation process in amphibolite

How to cite: Liu, J., Cao, S., and Cheng, X.: Development of low-angle boundaries in amphibole and their implications for accommodating grain boundary sliding in naturally deformed amphibolite, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20317, https://doi.org/10.5194/egusphere-egu26-20317, 2026.

The origin of the intense damage found in active fault cores is still a matter of debate. We investigated the potential co-seismic contribution to this damage by studying the Nojima fault, which ruptured during the 1995 Kobe earthquake (Mw 6.9). Drilled just a year after the event, the Hirabayashi borehole offers a snapshot of the fault zone’s state shortly after a major rupture.

Working within the French ANR AlterAction, we analyzed drill core samples using X-ray computed tomography (CT) at a resolution of ~50 μm. Instead of relying on complex segmentation of fracture geometries, we applied a 3D fractal analysis to the spatial distribution of voids (empty space) versus the rock matrix. This method allowed us to quantify damage intensity and organization using the fractal dimension D. This metric, ranging from 2 (highly clustered voids) to 3 (homogeneous distribution), tracks the transition from localized fracture networks to diffuse pulverization and correlates well with fracture porosity.

We observed a damage zone extending roughly 70 m on either side of the fault core. While open fracture density generally spikes toward the core, it drops sharply in the immediate vicinity, likely due to rapid post-seismic healing. Our analysis shows D values near 2 in clustered zones, rising toward 3 where damage becomes volumetric. Interestingly, some samples display intense micro-fracturing but lack significant macroscopic deformation, resembling the "pulverized rock" seen at other active faults. This texture suggests high strain-rate loading occurred during the earthquake.

To test the dynamic origin of this damage, we ran Split Hopkinson Pressure Bar (SHPB) experiments on intact borehole samples to reproduce pulverization in the lab. We found a linear link between strain rate and absorbed energy. When combined with the CT data, this relationship helps distinguish two modes of propagation: diffuse pulverization (matching near-fault observations) and sparse, poorly connected networks. Crucially, the fractal dimensions of the experimental samples confirm these contrasting morphologies.

These results suggest that the intense damage in the Nojima fault core likely stems from co-seismic processes, marked by specific fractal patterns associated with high strain rates. We conclude that 3D fractal analysis of void space offers a robust tool, independent of geometry, for identifying the dynamic origins of fault zone damage.

How to cite: Iaquinta, R., Doan, M.-L., and Donze, F. V.: 3D Fractal Analysis of Co-seismic Damage in the Nojima Fault Using X-Ray Tomography and SHPB Experiments, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21052, https://doi.org/10.5194/egusphere-egu26-21052, 2026.

Direct observations of geological processes are often limited by available observation time, the spatial resolution of imaging techniques or the accessibility of active sites. These limitations also apply to fault healing, whereby faults progressively regain strength throughout the interseismic phase of the earthquake cycle. Here, conventional approaches either capture a static snapshot of the final microstructure in exhumed natural fault rocks or focus on the bulk mechanical behaviour through slide–hold–slide or direct shear experiments. However, these approaches generally fail to resolve the dynamic evolution of the microstructural record, and the associated chemo-mechanical feedback that controls a rock’s hydraulic properties. To overcome these limitations and constrain the spatiotemporal coupling between mechanical, chemical, and hydraulic processes in healing fault gouges, we conducted a series of direct shear experiments on analogue fault gouges composed of a quartz–hemihydrate mixture. We then monitored their microstructural evolution using operando 4D synchrotron-based X-ray CT imaging.

Our experiments, performed at constant shear rates of 0.3–1 µm/s, were designed to mimic gouge-rich faults in the uppermost continental crust during the interseismic phase. In the presence of a reactive pore fluid, we simulated chemical fault healing through dissolution-reprecipitation and cementation, which are associated with the hydration reaction of CaSO₄ hemihydrate to gypsum. In our deforming samples, these time-dependent healing processes compete with mechanical weakening processes, such as frictional granular flow.

Our novel approach combines an innovative experimental setup [1, 2] with high-resolution 4D imaging and advanced image analysis techniques, including digital volume correlation (DVC). In this contribution we discuss the benefits of integrating micromechanical data with high-resolution 4D imaging by linking active deformation mechanisms to the evolving mechanical and hydraulic response of the simulated fault gouge. Further, we demonstrate a shear-rate-dependent competition between time-dependent healing processes and mechanical weakening.   

[1] Freitas, D. et al. (2024): Heitt Mjölnir: a heated miniature triaxial apparatus for 4D synchrotron microtomography. Journal of Synchrotron Radiation 31, 150-161. doi.org/10.1107/S1600577523009876 

[2] Jayawickrama, E. et al. (2026): Recent Developments in 4D X-ray Tomography for Real-Time Observation of Fault Slip and Gouge Evolution, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18619.

How to cite: Schwichtenberg, B., Fusseis, F., Jayawickrama, E., Cordonnier, B., Harpers, N., and Herwegh, M.: Probing active deformation - Fault healing through the lens of 4D operando imaging, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21418, https://doi.org/10.5194/egusphere-egu26-21418, 2026.

A common feature of many hydrothermal ore deposits is that they formed during tectonic switches between extension and shortening on plate boundaries. Several theories to explain this relationship have been proposed but evidence for a mechanism remains elusive. Many of these ore deposits occur within or adjacent to ductile shear zones that changed movement direction during the tectonic switch. Prior to tectonic switches, shear zone structures evolve to orientations optimised to accommodate deformation, which maximises strain rate and creates permeable pathways for fluid migration. But when a tectonic switch occurs the structures are misoriented and must reconfigure to accommodate the new shearing direction. Using numerical models of shear zone evolution, we determined that during tectonic switches the microstructural reconfiguration reduces the strain rate and mean stress, causing fluid influx into the shear zone. To further explore the effect of this microstructural reconfiguration on fluid migration we examined rocks of the Bergen Arc shear zone in Norway in a transition zone where sinistral shearing is progressively overprinted by dextral shearing. We find that during the tectonic switch, accretionary veins of quartz, ankerite and calcite formed in dilatational spaces that opened as the sinistral structures were reconfigured to accommodate dextral shearing. With increasing strain, fluid migration into the shear zone became more pervasive, evidenced by larger vein networks and hydrothermal breccias. Coincident with vein formation there is a statistically significant increase in the water content in quartz as determined by synchrotron FTIR. These data indicate that the microstructural reconfiguration in shear zones during tectonic switching causes fluid influx into shear zones. This process may be responsible for the introduction of ore fluids into the shear zone and the formation of hydrothermal ore deposits during tectonic switching.

How to cite: Finch, M., Knight, B., Tomkins, A., Gomez-Rivas, E., Bons, P., Ribeiro, B., and Olesch-Byrne, A.: Changes in fluid migration in ductile shear zones during tectonic switching may explain the formation of hydrothermal ore deposits, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2868, https://doi.org/10.5194/egusphere-egu26-2868, 2026.

How to cite: Tenczer, V., von Hagke, C., Hopfinger, M., Hörger, A. C., Topalović, E., and Strähler, I.: Alteration pathways in Cenozoic volcanic suites of the Styrian Basin: an integrated petrographic–geochemical approach , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5095, https://doi.org/10.5194/egusphere-egu26-5095, 2026.

Significant breakthroughs have been made recently in petroleum exploration within ultra-deep (burial depth > 6,000 m) carbonates in the Tarim Basin, northwestern China. The discovery of several large-scale oil and gas accumulation (e.g., Shunbei and Fuman) in these deeply buried, highly fractured and vuggy carbonates highlights the crucial role of strike-slip faults in reservoir development. However, the formation mechanisms of these ultra-deep, fault-controlled carbonate reservoirs remain poorly understood. It is therefore essential to conduct experimental simulations investigating the controlling factors and evolutionary trends governing the impact of deep CO₂-rich fluids on carbonate rocks.

For this reason, experiments were performed by using an ultra-deep, multi-tectonic-stage, high-temperature and high-pressure reservoir simulation system. This study focused on two key aspects, the dissolution mechanisms of dolomite in CO₂-saturated solutions, and the evolutionary trends of pore structure in carbonate rocks with different initial pore types during dissolution. Overall, two major findings were obtained. First, within temperature of 40–220 °C and pressure of 10–132 MPa, the saturated dissolution capacity of dolomite in CO₂-rich fluids exhibited an initial increase that was followed by a decrease, with the maximum dissolution occurred approximately at 60–110°C. This provides the theoretical basis for predicting favorable depth intervals where large-scale secondary pores may be formed in dolomite by deep CO₂-rich fluids. Second, influenced by the deep CO₂-rich fluid dissolution, both pore-dominated and fracture-dominated limestones tend to transform into fracture-vug reservoirs. Dissolution preferentially occurred along major fractures, gradually enhancing reservoir space and percolation capacity, ultimately becoming concentrated within these main fracture systems.

These results led to the construction of a genetic model for the development of fault-controlled, fracture-vug carbonate reservoirs. When deep CO₂-rich fluid activity coincides with fault development periods, fluids preferentially migrate into main faults, leading to dissolution-enlarged porosity along fault planes. When fluids migrate to fault intersections, they stagnate and induce dissolution and connectivity to form vugs. The fluids continue to expand along multiple sets of pre-existing faults, stagnating at new fault intersections to create more vugs. Such dissolution cycles are controlled by the episodic regional tectono-fluid activity. Ultimately, early-formed fracture-vug systems may become merged to formwell-connected fracture-vug reservoirs with superior reservoir performance. This model effectively explains the differences in dissolution and modification effects observed in different segments of strike-slip faults and clarifies the underlying mechanisms.

How to cite: She, M., Qiao, Z., and Liu, Y.: Influence of Deep CO₂-Charged Fluids on the Development of Carbonate Reservoirs in Fault-Controlled, Ultra-Deep burial setting: Insights from Water-Rock Interaction Experiment, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6340, https://doi.org/10.5194/egusphere-egu26-6340, 2026.

Deep fluid activities driven by strike-slip fault movement play important roles in the modification of carbonate and hydrocarbon accumulation. The complexities of deep fluid sources and temperature-pressure variations during strike-slip fault movement complicate fluid-rock reactions, diagenetic modification processes and the formation and evolution of reservoirs in deep to ultradeep carbonate strata. To understand the temporal-spatial coupling between strike-slip movement and deep fluid migration, we investigate the migration periods and sources of deep fluids along strike-slip fault belts in the Fuman Area of the Tarim Basin, considering the geometry of the strike-slip faults and analysing laser ablation U-Pb dating, clumped isotopes, REE, and fluid inclusions in diagenetic products such as calcite, chert, and quartz.

U-Pb age results indicate that vug-filling calcites were emplaced between 460.8 ± 3.4 and 448.6 ± 5.3 Ma, and at 335 ± 19 Ma during the early Hercynian orogeny, while the fracture-filling and megacrystalline calcites formed between 364 ± 53 and 282.9 ± 5.4 Ma, and during periods from 324 ± 23 to 300.9 ± 4.8 Ma and from 244.13 ± 13 to 240.5 ± 4.1 Ma, respectively. The latest fracture-filling calcites show a slightly younger U-Pb age of ca. 158 ± 17 Ma. In addition, the U-Pb ages for the chert and quartz in fractures (459 ± 57 Ma, 252 ± 56 Ma, and 174 ± 35 Ma) fall within the middle Caledonian, late Hercynian, and Yanshanian periods.

The combination of geochemical analyses on calcites, including clumped isotopes, d13C, d18O, and 87Sr/86Sr isotopes, REE, and fluid d18O calculation, suggests that these calcites were precipitated from formation fluids mostly of meteoric water origin with some input from hydrothermal fluids. Hydrothermal fluid flow resulted from strike-slip fault activity and volcanism, whereas meteoric water intruded from uplifted areas along the faults during tectonic quiescence. This study shows that the formation of fracture-related cavern reservoirs in the Fuman oil field is related to the early Hercynian, late Hercynian, and Yanshanian tectonic events and their associated fluid activity. The methodologies and outcomes of the present study may guide future hydrocarbon exploration in the Tarim Basin and be applied to other oil fields with similar tectonic backgrounds.

How to cite: Qiao, Z.: Fluid activities Controlled by Intra-craton Strike-slip Faults: A Case Study of Ordovician in Fuman Area in Tarim Basin, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6851, https://doi.org/10.5194/egusphere-egu26-6851, 2026.

Abstract

The volcanic rocks of the Mesoproterozoic Xiong’er Group (Changcheng System) in the Ordos Basin are pervasively albitized, a phenomenon mainly attributed to lava-seawater interaction. Understanding the mineralogical and textural imprints of this alteration is therefore essential.

This study focuses on deep core samples of the Xiong’er Group from the southwestern Ordos Basin. An integrated methodology was applied, combining petrographic observation with geochemical analyses (major and trace elements, TIMA automated mineralogy, C-O isotopes, and zircon U-Pb dating). Our results confirm intense lava-seawater alteration in the Xiong’er Group magmas and define the diagnostic mineral assemblages and textures produced by this process. Additionally, a comparative analysis with coeval volcanic rocks from the eastern Ordos Basin was conducted to reconstruct the tectonic environment during magma genesis.

Key findings are summarized as follows:

(1) Core and thin-section observations reveal volcanic rocks with vesicular-amygdaloidal textures, plagioclase-phyric porphyritic structures, and interbedded sedimentary layers. Distinct dark-reddish alteration zones occur along lithological contacts. Microscopically, the rocks show porphyritic texture with feldspar phenocrysts in a cryptocrystalline groundmass. Vesicles are commonly filled with calcite, quartz and chlorite.

(2) Geochemical data indicate that the Xiong’er Group volcanic rocks in the southwest basin are predominantly basaltic. They exhibit high alkalinity (σ = 4.6~10.6) alongside anomalous silica contents, classifying them as basic to intermediate igneous rocks. Rare earth element patterns are consistent with an intracontinental rift setting linked to mantle plume activity, with evidence of crustal contamination.

(3) TIMA automated mineralogical mapping shows that feldspar phenocrysts in the basalts are exclusively albite. The groundmass is pervasively altered by chloritization and argillization. Slilceous sediments occur widely, filling vesicles in basalts and appearing within sedimentary rocks at basaltic contacts.

(4) Marked petrological and geochemical differences exist between the Xiong’er Group volcanic rocks in the southwest and eastern Ordos Basin, reflecting contrasting tectono-magmatic environments-intracontinental rift versus continental arc settings.

The results advance the understanding of mineral alteration and element exchange during such interactions at the micro-scale and provide key mineralogical constraints on lava-seawater alteration processes.

Keywords

Xiong’er Group, Volcanic Rocks, Lava-Seawater Interaction, Alteration Mineralogy, Geochemistry, Ordos Basin

How to cite: Ren, Y. and Chen, Z.: Lava-Seawater Interaction of the Mesoproterozoic Xiong’er Group Volcanic Rocks in the Southwestern Ordos Basin: Insights from Alteration Mineralogy and Geochemistry, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7121, https://doi.org/10.5194/egusphere-egu26-7121, 2026.

Geological fluids play crucial roles in the stabilisation of minerals and in the mobilization and redistribution of elements during mid- to lower-crustal metamorphism, thereby influencing the chemical evolution of continental crust. The current study demonstrates extensive fluid-induced alterations and formation of a highly peraluminous (A/CNK~30), ferromagnesian yet silica-calcium and alkali-poor rock from the Makrohar granulite belt, part of the Proterozoic mobile belt, Central Indian Tectonic Zone. The study demonstrates a black, garnet-rich, massive rock composed of garnet, cordierite, sillimanite, quartz, and ilmenite, lacking gneissic banding and intruded by multiple veins in the outcrop. Thin microscopic veinlets consist of biotite (80% modal volume) with smaller proportions of quartz±cordierite and fibrolites. Late-stage veins of variable thickness, evident from the outcrop scale, contain coarse-grained sillimanite-quartz-garnet, with large sillimanite grains growing at high angles to the vein boundary, indicating a syntaxial growth. Vein garnets frequently grow inward from the vein wall, often growing on older garnet in the host. Phase equilibrium modelling, coupled with conventional thermobarometry, constrains the P-T conditions of the stabilization of the host rock at approximately 600ºC and 5 kbar. Slightly magnesian cores of the host garnet (X_Mg) yield marginally higher temperatures (~680ºC) than the rim (garnet isopleth yielding ~580ºC).

Garnet grains in the host rock display a distinct positive europium anomaly (Eu/Eu*), likely resulting from garnet growth in the absence of plagioclase. A moderate Gd/Dy ratio in the host garnet indicates stabilization at approximately 4 kbar, supporting low-pressure estimates from conventional barometry and phase-equilibria modelling. Rim-to-rim trace element profiles along host garnet grains show a uniform distribution of Sc, Y, and HREEs in the core, with oscillations and a sharp increase near the rim, suggesting that reverse zoning in HREEs was likely caused by homogenization by intragranular diffusion in the core but remained largely unaffected towards the rim. Whole-rock chemistry of the host, feldspar-free high-variance mineralogy, absence of leucosome and reverse zoning of Y-HREE, positive Eu/Eu* within garnet indicate potential metasomatic alteration of the host itself.

Garnets within the quartz-sillimanite veins exhibit distinct oscillations in trace element concentrations along wall-to-wall line scans, indicating minimal effects of diffusion and grain growth in the presence of vein-fluid. Ca and Mn zoning within vein garnet exactly replicate each other with gradual increase from vein-wall to vein-axis regions of the grains. Y and HREEs show resonating patterns with sharp central peaks in the mid-axis and oscillatory zoning within the vein-wall garnet portions. Sc and MREEs, i.e., Sm, Eu, Gd, Tb still show central peaks along with an annular maxima added with the oscillation within the vein-wall garnets. REE mobilization, at least in micro-scale, is further evident from large monazite clusters observed in sillimanite-quartz-garnet veins. The presence of large sillimanite grains further demonstrates the fluid's capacity to transport aluminium. The absence of any hydrous phases in the vein supports the prevalence of low-H₂O-bearing fluid. XCO2-µK2O and µK2O-µFeO topology further confirms that the intrusion of low-H₂O fluid presumably destabilized the host biotite, producing garnet and quartz in the vein.

How to cite: Chakrabarty, A., Anczkiewicz, R., and Sanyal, S.: Fluid-induced redistribution of REEs within alumino-silicate veins and peraluminous host rock in the Central Indian Tectonic Zone, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7449, https://doi.org/10.5194/egusphere-egu26-7449, 2026.

Interactions between minerals and reactive fluids in porous rocks and building materials often result in crystallization of new minerals. The forces exerted by minerals growing under confinement on the surrounding matrix can be large enough to cause fracturing. Fractures expose new reactive surfaces, leading to progressive disintegration of the material. These processes can result in severe damage to cultural heritage and modern infrastructure, as well as changes in the rheological properties and weathering of natural rocks. Understanding and controlling volume-expanding mineral replacement reactions in pore spaces is thus an important objective to address both societal and geological issues. While crystallization pressures have been measured at larger scales, nanoscale force evolution during confined mineral growth remains poorly constrained.

Here, we investigate volume-expansive mineral reactions in pores spaces by studying the hydration of MgO (periclase) in the Surface Forces Apparatus (SFA). Hydration of MgO to Mg(OH)­₂ (brucite) causes a volume increase to 220%, yielding high crystallization pressures. We use MgO thin films (~90 nm) prepared by atomic layer deposition as reactive surfaces in our experiments, which are performed at the Flow Laboratory, Njord Center, University of Oslo. With the SFA, we measure distance-resolved adhesive and repulsive forces acting between two MgO surfaces under variable external load and how these change over time as the reaction progresses. Preliminary results indicate evolution of forces from strongly adhesive to repulsive during the hydration reaction, likely due to the presence of amorphous, gel-like precursors in the early stages of the reaction. As a reference for the SFA experiments, we study the hydration of isolated MgO surfaces with Atomic Force Microscopy (AFM). This allows us to compare nucleation and growth rates, as well as microstructure and porosity of Mg(OH)­₂ grown in SFA and AFM.

How to cite: Trautner, V., Rogowska, M., Plümper, O., Nilsen, O., and Dziadkowiec, J.: How hard do crystals push when growing under confinement? Real-time measurements of surface forces during hydration of periclase to brucite, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7493, https://doi.org/10.5194/egusphere-egu26-7493, 2026.

We investigate infiltration of an aqueous fluid into granitic rocks by means of numerical models at the field scale. Our methodology is based on a finite difference approach for solving the transport problem in combination with lookup tables generated from precomputed thermodynamic equilibria covering the compositional space. We also compare results to an approach involving on-the-fly calculation of local equilibrium between fluid and rock. Porosity and density evolution is predicted based on mass conservation. The ability to predict porosity evolution is valuable to better understand applications such as enhanced geothermal systems (EGS). The prediction of reaction zone sequences is also helpful in the understanding of ore deposit formation. We demonstrate how sensitive the metasomatic zoning sequences are to varying rock and fluid composition. As an example, we model metasomatic zone sequences observed in topaz-greisen to show how metasomatic sequences comprising multiple lithologies can be formed in one event with constant incoming fluid composition as boundary condition. Lithological zones formed along fractures do not necessarily imply temporal changes in the fluid composition of the source.

How to cite: Vrijmoed, J. C. and John, T.: Numerical modelling of an aqueous F-Cl-Na-K-Al bearing fluid in local equilibrium with granitic rocks with relevance to enhanced geothermal systems and ore deposit formation., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7508, https://doi.org/10.5194/egusphere-egu26-7508, 2026.

Bedding-parallel fibrous calcite veins in organic-rich shale are commonly regarded as petrographic archives of abnormal pore-fluid overpressure and associated hydrocarbon expulsion and migration. Their formation reflects a dynamic cycle of fracture opening, fluid ingress and mineral precipitation, and subsequent re-opening. Constraining the evolutionary model of BPFVs and evaluating their influence on hydrocarbon accumulation are therefore of clear significance. In lacustrine shale systems, the sources of vein-forming fluids and the extent to which BPFV development couples with organic-matter maturation, overpressure generation, and hydrocarbon accumulation remain poorly constrained. This study investigates lacustrine shale of the second member of the Paleogene Funing Formation (E₁f₂) in the Qintong Sag, Subei Basin. Core observations, petrographic thin sections, and cathodoluminescence (CL) imaging were used to characterize vein petrography and constrain vein growth stages. Fluid inclusion petrography and microthermometry were conducted to define inclusion assemblages and homogenization-temperature (Th). Carbon–oxygen–strontium (C–O–Sr) isotopes and PAAS-normalized rare earth element (REE) patterns were integrated to diagnose vein forming fluid sources. These datasets were further evaluated against BasinMod-1D burial–thermal–hydrocarbon generation modeling to link Th stages with source-rock maturity and to assess the coupling between BPFVs and hydrocarbon accumulation. The results show that the BPFVs contain a well-defined median zone and symmetric antitaxial fibrous fabrics. Multiple internal growth records indicate repeated fracture opening and sealing. Oil inclusions commonly associated with aqueous inclusions, suggesting that hydrocarbons and formation water entered the bedding-parallel fractures during opening and were co-trapped during vein precipitation. Aqueous-inclusion Th values cluster into two populations (90–100°C and 120–130°C), matching the initial oil window and the main oil generation stage inferred from burial–thermal–hydrocarbon generation histories, and implying at least two vein filling episodes synchronous with source-rock thermal evolution. Geochemical data further show that vein calcite and host-rock carbonates share similar carbon sources and PAAS-normalized REE patterns, with no evidence for high temperature hydrothermal input. These observations indicate that vein forming fluids were dominated by basin-internal diagenetic pore waters, modified by fluids released during hydrocarbon generation and by sustained water–rock interaction. Based on these evidences, we propose a conceptual model for the development of BPFVs. Organic-matter thermal evolution elevates pore-fluid pressure and drives episodic opening of fractures along mechanically weak bedding planes. During opening, these fractures act as short-range pathways for hydrocarbon migration within the shale. Subsequent calcite precipitation partially to completely seals the fractures, preserving time-transgressive fluid properties and migration episodes in veins and fluid-inclusion assemblages. This framework provides key evidence for the dynamic coupling between the formation–evolution of bedding-parallel fractures and hydrocarbon accumulation in lacustrine shale, and offers a reference for reconstructing charging histories and timing in analogous lacustrine shale systems.

How to cite: Cao, S., Zeng, L., and Liu, G.: Fluid Sources of Bedding-Parallel Fibrous Veins in Lacustrine Shales and Their Implications for Hydrocarbon Accumulation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7664, https://doi.org/10.5194/egusphere-egu26-7664, 2026.

Many epithermal ore deposits form in fault-veins that channel large volumes of vertical fluid flow synkinematic with fault slip. A key genetic process is the interplay among fluid flow, fault activation, and mineral precipitation; however, significant questions remain regarding the mechanics of fault slip under high fluid-flux conditions and its impact on mineralisation. A fundamental question is whether ore deposition is coseismic, post-seismic, or interseismic—specifically, whether pressure drops during seismic rupture are the dominant trigger for mineral precipitation, or whether mineralisation occurs during longer-lived, aseismic creep and sealing cycles. A deeper understanding of these processes is essential for predicting ore grade and spatial distribution.

The Cretaceous Indiana deposit, located in the Coastal Cordillera of northern Chile, is a Cu–Au (Mo–Co) fault-vein system composed of several subvertical NW- or ENE-striking fault veins, with lengths ranging from 300 m to 2 km. Artisanal tunnels provide access to multiple structural levels in oxides and sulphides mineralization, offering exceptional three-dimensional exposure. NW-striking fault-veins host Au–Cu-Fe–Co-rich mineral assemblages associated with pyrite, chalcopyrite, magnetite, actinolite, albite, garnet, epidote, quartz, tourmaline, and late jarosite, clay and hematite. These are cross-cut by ENE-striking fault-veins containing Au–Cu–Mo-Co mineralisation in pyrite and chalcopyrite paragenetically associated with garnet, epidote, actinolite, quartz, and less sericite. High-grade ore shoots commonly occur in dilatational jogs and at the intersections between these two structural sets.

Fault-veins are 1–3 m wide and display complex internal structures. Fault zones of variable thickness occur along the vein margins and mainly record strike-slip motion, expressed as thin (<10 cm) clay-rich gouge bands or thicker (20–80 cm) foliated cataclasites. Ore-bearing veins commonly occur adjacent to these zones and display varied widths, textures, and mineral assemblages. Gold is hosted by quartz or amorphous silica, either free or in pyrite. Brecciated and banded veins record multiple mineralisation events, whereas comb quartz textures with 2–5 cm euhedral crystals indicate slow growth in open space.

Microstructural analyses document multiple episodes of quartz deposition in the form of subparallel and superposed veins that cross-cut clasts of the andesitic host rock. Some brecciated bands contain spherical clasts completely surrounded by concentric cement bands, forming cockade-like structures that suggest fluidised conditions in which cement precipitation occurred while clasts were suspended.

This preliminary evidence indicates the coexistence of long-lived mineralisation processes and cyclic, short-lived deposition events, likely linked to repeated fault activation. Ongoing analyses integrating microstructural observations with mineral chemistry aim to constrain the fault-slip mechanisms responsible for specific mineralisation styles.

How to cite: Stanton-Yonge, A., Fondriest, M., Pérez-Flores, P., Marquardt, M., Reinoso, F., and Cembrano, J.: Controls of fault mechanics on mineral precipitation in gold-bearing fault-veins, Indiana Deposit, Northern Chile, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8135, https://doi.org/10.5194/egusphere-egu26-8135, 2026.

The magnitude and distribution of stress in Earth’s crust is difficult to quantify, but impacts deformation behavior, phase stability, and metamorphic reactions. Stress is influenced by a variety of factors including compositional heterogeneities, volume changes during ongoing reactions, and the influence of far-field stresses. During metamorphic reactions the stress distribution may be modified, but prevailing stresses may also impact reaction kinetics, or which reactions take place. We studied one of the most impactful reactions within the continental crust; the fluid-induced breakdown of plagioclase at high-pressure conditions. The samples are from former lower-crustal granulites exposed on Holsnøy, western Norway. They preserve a reaction front along which the dry granulite is transformed into an eclogite. Reaction progress is intimately linked to fluid ingress and there is no microstructural evidence of deformation. This lack of deformation indicates that the studied microstructures are entirely related to fluid-induced metamorphic reactions. We measured the residual stress associated with plagioclase breakdown by high-angular resolution electron backscatter diffraction and contrasted the results with compositional variations (scanning electron microscope and electron probe micro analyzer). (Scanning) transmission electron microscopy was conducted on selected sites to link this information with the associated dislocation configuration. We find that intragrain residual stress associated with the breakdown of plagioclase is on the order of hundreds of megapascals, and dominantly caused by the elastic interactions of dislocations. Before the reaction plagioclase contains few, randomly oriented dislocations. Compositional modification of plagioclase during the reaction (increasing albite content) leads to dislocations occurring more frequently in the more albitic part of the plagioclase. In that case, dislocations have a preferred orientation, but no significant long-range increase in dislocation density, i.e., increased organization. Our results thus suggest that as plagioclase breaks down, dislocations are mobilized to accommodate the variations in lattice parameters associated with hundreds-of-megapascal stress variations on the grain scale.

How to cite: Zertani, S., van Schrojenstein Lantman, H. W., Kaatz, L., Chogani, A., Plümper, O., Menegon, L., and John, T.: Grain-scale residual stress distribution associated with fluid-induced plagioclase breakdown , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8177, https://doi.org/10.5194/egusphere-egu26-8177, 2026.

Fluid–rock interaction, and thus fluid flow plays a fundamental role in geological processes from the shallow crust to mantle depths. In the upper crust, fluid flow is predominantly controlled by brittle features and interconnected porosity. In contrast, at lower crustal conditions, elevated lithostatic pressures are commonly assumed to inhibit fracturing and major porosity, leaving unresolved how fluids migrate at depth. To investigate porosity development during high-pressure metamorphism of initially impermeable mafic rocks, we conducted a series of piston-cylinder experiments that varied reaction duration and fluid availability. Dolerite from the Kerforne dyke (Brittany, France) was used as starting material. Cylindrical samples (2.8 mm diameter, ~3.5 mm length) were characterized prior to experimentation using X-ray micro-computed microtomography (µCT; 3 µm Isovoxel), enabling three-dimensional quantification of mineral fabric, modal proportions, and initial porosity. The starting dolerite consists of ~50–65% plagioclase, 20–30% pyroxene, up to 10% ilmenite, and minor quartz, with an initial porosity of ~0.1%. The fabric is near-isotropic and dominated by randomly oriented plagioclase grains up to 300 µm in length.

Experiments were performed under quasi hydrostatic conditions at 700 °C and 2.4 GPa for varying durations. To evaluate the influence of fluid availability on reaction progress and porosity evolution, three experimental setups were employed: (1) nominally dry conditions without added fluid, (2) addition of paragonite as a source of fluid and sodium, and (3) addition of 5 vol% water (“wet” conditions). Wet experiments were conducted for durations of 1, 7, and 21 days to assess the temporal evolution of reactions.

Following experimentation, all samples were re-imaged using µCT, allowing three-dimensional mapping of reaction progress and porosity development. Largely unreactive Fe–Ti oxides served as internal markers, enabling accurate registration of pre- and post-experimental µCT datasets and direct comparison between the protolith and reaction products. Three-dimensional observations were complemented by high-resolution two-dimensional analyses using electron probe microanalysis and scanning electron microscopy.
Reaction progress increases systematically with fluid availability, from dry to paragonite-bearing to water-added conditions. Under nominally dry conditions, reactions are restricted to narrow zones along pyroxene–plagioclase interfaces and plagioclase grain boundaries, producing predominantly fine-grained zoisite needles (<5 µm). In paragonite-bearing experiments, reaction intensity increases within plagioclase, characterized by the growth of zoisite and phengite, while jadeite forms along pyroxene–plagioclase boundaries. In contrast, wet experiments result in complete replacement of plagioclase within 7 days by an assemblage of zoisite, phengite, amphibole, and minor omphacite and quartz. Pyroxene develops narrow reaction rims (<30 µm wide) marked by increasing Al and Na and decreasing Fe and Ca contents, while garnet occurs as idiomorphic grains in the fine-grained matrix or as coronae surrounding oxides.
Porosity development is closely coupled to reaction progress, and three distinct porosity types are identified: (1) micro- to nanopores within plagioclase reaction products, (2) nanopores within pyroxene reaction rims, and (3) microfractures. The first two porosity types are interpreted to result from volume reduction associated with density increases during metamorphic reactions, whereas microfractures likely form in response to stress concentrations and elevated pore-fluid pressures.  

How to cite: Rogowitz, A., Degenhart, G., Konzett, J., Huet, B., Stoiber, W., and Tropper, P.: High-pressure metamorphism induced porosity in mafic rocks – wet vs. dry, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9041, https://doi.org/10.5194/egusphere-egu26-9041, 2026.

The presence or absence of fluids strongly affects rock rheology. Lawsonite is a very hydrous mineral (~12 wt.% H₂O), characteristic of cold subduction zones. Its destabilization may generate fluid overpressure, reduce effective stress, and trigger brittle failure through dehydration embrittlement. On the other hand, its H₂O-consuming growth may deplete available fluids from the matrix and drive the rock dry. The quantity of lawsonite, the locus of maximum dehydration, and the amount of fluids produced/consumed depend on the pressure-temperature (P-T) path of the subducted crust. An accurate interpretation of P-T paths of natural blueschists is therefore crucial.

At Ile de Groix (Armorican Massif, France), garnet-bearing blueschists display cm-sized lawsonite pseudomorphs smoothly wrapped by an epidote- and glaucophane-bearing foliated matrix. Both garnet and pseudomorphed lawsonite porphyroblasts contain sigmoidal inclusion trails of fine-grained oriented epidote, glaucophane and titanite, continuous with the matrix schistosity. Garnet is zoned (rimward decrease of Mn and increase of Mg) and locally included in pseudomorphed lawsonite. Lawsonite pseudomorphs comprise coarse unoriented epidote, paragonite and chlorite. Textural analysis therefore suggests a prograde synkinematic growth of garnet and lawsonite in an epidote-bearing matrix. In the light of calculated phase diagrams, this points to a prograde P-T path dominated by a near-isothermal compression from LT epidote-blueschist facies toward peak pressure conditions in the epidote + lawsonite stability field, at ~19 kbar and ~550°C, consistent with garnet rim composition and modal proportions of major phases. 

Thermodynamic modeling further indicates that lawsonite growth in an epidote-bearing blueschist leads to the complete consumption of free fluid, resulting in a dry, fluid-absent rock near peak pressure conditions. However, dry rocks are commonly stronger than their wet equivalents. Our results thus suggest that, contrary to common expectations, hydration reactions may locally induce an increase in rock strength, as exemplified by lawsonite crystallization during the prograde transition from epidote- to lawsonite-blueschist subfacies. Such reactions could provide an explanation for earthquakes occurring within the lawsonite stability field, well prior to its destabilization.

How to cite: Pichouron, R., Pitra, P., and Yamato, P.: Prograde P-T path of lawsonite-bearing blueschists: insights from Ile de Groix and implications for fluid content and rheology of subducted oceanic crust, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9564, https://doi.org/10.5194/egusphere-egu26-9564, 2026.

Densification and deformation during eclogitization govern the strength and buoyancy of orogenic roots and the stability of mountain ranges over geological timespans. The breakdown of albite to jadeite + quartz represents a key end-member reaction that is associated with densification of about 20%; shear stresses induced by such volumetric changes may cause brittle failure and have been linked to intermediate-depth seismicity (Yamato et al., 2022). Eclogitization is a kinetically sluggish process that requires significant reaction overstepping and may proceed far beyond the equilibrium pressure–temperature conditions, and/or remain largely incomplete – particularly in fluid-deficient felsic crust as evidenced by field observations (Palin et al., 2017) and geophysical constraints (Hetényi et al., 2021). However, the kinetics of the albite = jadeite + quartz reaction is poorly constrained, especially regarding the roles of grain size, pressure–temperature overstepping, and reaction duration. To address this gap, we conducted high-pressure experiments using a piston-cylinder apparatus at the Institute of Geosciences, JGU Mainz. Natural albite crystals were crushed and sieved into grain size fractions between 50 and 500 µm, loaded into Au-capsules, and separated by Au-foils. A subset of experiments involved furnace-drying (~500°C) of the starting materials followed by hot-welding of the capsules to minimize atmospheric moisture contamination. In experimental stage I, pressure was initially set just below (~1 kbar) the albite = jadeite + quartz reaction boundary (Holland, 1980), followed by heating to target temperature. In stage II, pressure was increased at constant temperature to variable target pressures above the reaction to systematically explore the effect of reaction overstepping. Samples where quenched by power shutdown, and reaction progress was quantified using scanning electron microscopy (backscattered electron imaging and cathodoluminescence) based on the relative fractions of reactant albite and products jadeite–quartz. Preliminary results reveal highly variable degrees of reaction progress. Where present, jadeite–quartz occur as finely intergrown symplectites, typically decorating albite grains at the rims, as well as forming within larger albite grains. The latter textures indicate complications arising from fluid inclusions in the starting material. By combining constraints on P(T) overstep, grain size, and experimental run duration, we determine effective reaction rates for albite breakdown. These results provide end-member kinetic constraints on high-pressure transformation in fluid-deficient, coarse-grained felsic rocks, which constitute the bulk of many well-known (U)HP terranes such as the Western Gneiss Region (Norway) and the Dabie–Sulu belt (China).

References

Hetényi, G. et al. (2021). Metamorphic transformation rate over large spatial and temporal scales constrained by geophysical data and coupled modelling. Journal of Metamorphic Geology, 39(9), 1131–1143.

Holland, T. J. (1980). The reaction albite = jadeite + quartz determined experimentally in the range 600–1200°C. American Mineralogist, 65(1-2), 129–134.

Palin, R. et al. (2017). Subduction metamorphism in the Himalayan ultrahigh-pressure Tso Morari massif: an integrated geodynamic and petrological modelling approach. Earth and Planetary Science Letters, 467, 108–119.

Yamato, P. et al. (2022). Reaction-induced volume change triggers brittle failure at eclogite facies conditions. Earth and Planetary Science Letters, 584, 117520.

How to cite: Schorn, S., Yang, Z., Hawemann, F., Buhre, S., Botcharnikov, R., and Moulas, E.: Experimental constraints on the kinetics of the albite = jadeite + quartz reaction depending on grain size and reaction overstepping, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10026, https://doi.org/10.5194/egusphere-egu26-10026, 2026.

Megathrust shear zones are weak interplate faults that accommodate deformation under low effective stress in a fluid-rich environment. The evolution of stress and fluids during megathrusts activity can be reconstructed from syn- and post-tectonic mineral veins in exhumed settings.

The Sestola–Vidiciatico Unit (SVU) in the Northern Apennines is a fossil analogue of a shallow subduction megathrust mélange. It represents the plate boundary shear zone (200–400 m thick) between the Ligurian prism and the underthrusted Adria microplate active during the early–middle Miocene with peak temperatures of ~170 °C. The SVU is composed of kilometre-scale slices of marls, shales and sandstones derived from the Ligurian prism and its sedimentary cover, thrust along a basal décollement onto younger Adria-derived foredeep turbidites.

This study focuses on a well-exposed outcrop along a south-dipping thrust ramp of the basal décollement of the SVU. The footwall consists of sandstones and siltstones of Langhian age, overthrust by a slice of Aquitanian marls, and by an upper slice of Priabonian – Bartonian claystone. We performed structural mapping, microstructural and geochemical analyses (O-C stable isotopes, trace and major element geochemistry), and fluid inclusion studies on calcites filling shear and extensional veins and cementing tectonic breccias.

Marls and claystones within the SVU are bounded by sharp thrust surfaces decorated by multiple generations of shear veins. In the vicinity of the main thrusts, marls and claystone are crosscut by pervasive shear fractures, bounding flattened and elongated lithons defining a foliation at low angle to the thrusts. Deformation in the footwall includes oblique cleavage, bedding-parallel shear planes, and a conjugated set of NNE-SSW left-lateral and N-S trending right-lateral subvertical transtensional faults showing mutually crosscutting relationships with the basal thrust of the SVU. Calcite shear veins mark thrust surfaces, whereas transtensional faults in the footwall are marked by shear and extension veins, as well as calcite-cemented breccias. Two different calcite phases have been observed: an early-stage calcite, rich in host-rock inclusions and a later inclusion-free calcite.

Geochemical and thermometric results point to two groups of distinct mineralizing fluids circulating through the fracture network: (1) diluted seawater precipitating early-stage calcites at low temperatures (< 50 °C up to 70 °C); (2) an external low-salinity fluid precipitating later-stage calcites at higher temperatures (~80-100° C).

Our data suggest a transition from low temperature and low salinity fluids, possibly from mixing of seawater and fluids released from clay dehydration during progressive burial of the SVU, to the ingression of moderately hot fluids (up to 120 °C) external from the system. This indicates that the onset of fluid circulation by faulting is modulated by the embrittlement and seismic ruptures in subduction zones, favoured by a low-stressed environment.

How to cite: Rocca, M., Mittempergher, S., Remitti, F., Molli, G., Gasparrini, M., Hawemann, F., Diamanti, R., Preto, N., and Tesei, T.: Evolving fluid pathways in a shallow mega-thrust shear zone (Northern Apennines, Italy), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10373, https://doi.org/10.5194/egusphere-egu26-10373, 2026.

Diagenetic processes exert a strong control on reservoir potential, heavily impacting the exploitation of strategic fossil resources (oil and gas), preservation and management of aquifers, and underground storage of anthropogenic CO₂. Therefore, in high porosity media such as sandstones, the study of selective cementation is crucial to the quantification of reservoir properties and quality. The outcrop-based analysis of cementation types and patterns could unravel fossil fluid flow pathways affecting porous reservoir analogues.

This study is focused on the selective cementation of fluvio-deltaic, Lower to Middle Pliocene age, sandstone to conglomeratic bodies exposed in the Crotone forearc Basin, South Italy. The siliciclastic unit was deposited in a shallow marine setting and reaches a maximum thickness of ~200 m, unconformably overlying the Paleozoic Sila Massif metamorphic basement. The sandstone sequence is almost devoid of diagenetic cement thus preserving most of the original primary porosity. Sandstone beds show a gentle tilting towards SE, with mild brittle deformation in the form of deformation bands and low-displacement faults. Selective cementation of host sandstone can be traced as diagenetic concretions of different shapes and sizes. Concretion types span from tabular-lens shaped with lateral extension up to 10’s m, elongate blade-shaped from 10 cm up to several 10’s meter-long, asymmetric drop-shaped and nodular-spherical bodies. The elongation direction of concretions parallels the southeastward dip of bedding surfaces, while in the vicinity of deformation bands and faults, elongate concretions are parallel to their dip. Pervasive calcite precipitation was responsible for the dramatic porosity loss from 27-32% down to 2-3%, leading to an increase in sandstone cohesion and stiffness. The stiffness increase can be documented in tightly cemented bodies that host 2-3 sets of joints abutting at the concretion-host rock boundary. Cold cathodoluminescence revealed the ubiquitous presence of bright yellow, granular to poikilitic calcite cement in all concretions. Carbon and Oxygen stable isotopes of calcite cement suggest two fluid sources responsible for the selective cementation. The first source can be traced in weakly cemented lens-shaped bodies and along secondary faults and is made of mixed marine-meteoric fluids with contributions from soil percolation. Conversely, the second source can be detected in tightly cemented lens-shaped and nodular to elongate concretions and is given by a mix of marine fluids with contributions from biogenic methane likely related to biological-bacterial activity in a shallow marine setting. The evolution of fluids from meteoric to marine can be associated with a transgressive sea level rise and upward basin-boundary fault propagation that occurred during and after sandstone deposition. The source of methane could be traced in the thick evaporitic (gypsum and anhydrite) sequence underneath the studied sandstone formation, providing large volume of biogenic methane. Methane enriched fluids migrated vertically following major basin-boundary faults permeating the high porosity sandstone and mixed with meteoric to marine fluids providing bed-parallel fluid flow imparted by the hydraulic and topographic gradient.

How to cite: Pizzati, M., Berio, L. R., Cavozzi, C., Torabi, A., and Balsamo, F.: Carbonate concretions as proxy for methane-enriched fluid flow in high-porosity sandstone: example from Crotone forearc Basin, Italy, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11740, https://doi.org/10.5194/egusphere-egu26-11740, 2026.

Fluid transport, precipitation and accompanying mineral reactions along grain boundaries are among the most important processes impacting the rheology of the crust and the formation of mineral deposits. Porosity and permeability that constantly evolve during fluid flow govern major petrophysical properties of the rock. Most commonly, rocks are investigated in two-dimensional sections, where grain boundaries appear as thin lines and the three-dimensional structure cannot be captured. Computed tomography allows for a quantitative assessment of pore space but has a limited resolution. Additionally, it is difficult to assess the origin of pores, which may have been formed primarily in the crust or during near surface weathering or sampling.

In this study, we investigated grain boundaries directly using the “broken surface” technique: A cm-sized rock slice was broken and platinum-coated for scanning electron microscopy. In favorable cases, the rock slice broke along grain boundaries and pre-existing small-scale fractures, exposing these structures directly as true surfaces rather than sectioned traces. The samples investigated are from the ICDP-DIVE drilling project in the Ivrea Verbano Zone (Italy), an exhumed section of the lower continental crust, spanning the range of tens of meters to hundreds of meters of depth below surface; thus offer information about which grain boundary decorations can be clearly related to near-surface alteration. In addition, we compare samples from both amphibolite and granulite facies rocks to explore variations between supposedly fluid-rich and fluid-poor conditions.

Our observations contribute to the understanding of grain boundary processes through a catalogue of different features observed and interpreted, including, among other processes: formation of clays near the surface, sulfide precipitation, quartz recrystallisation and sericitization of feldspars.

How to cite: Linden, E., Hawemann, F., Venier, M., and Toy, V. and the DIVE science Team: Grain boundary processes from the deep continental crust to the surface (ICDP-DIVE, Drilling the Ivrea-Verbano Zone), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12642, https://doi.org/10.5194/egusphere-egu26-12642, 2026.

Crustal-to-lithospheric-scale strike-slip faults can act as major pathways for crustal and mantle fluids, with implications for natural resources (e.g., geothermal, ore deposits) as well as natural gas storage and migration. The Betic Cordillera (SE Spain) records a complex geodynamic evolution, from slab retreat, tearing and delamination to the later inversion of a thinned continental margin. This extreme crustal thinning (~15 km) associated with metamorphic dome exhumation during the Miocene was accommodated by crustal to lithospheric-scale faults that are still active today (e.g. the Mw 5.2 Lorca earthquake in 2011).

To determine the geochemical origin of fluids, their migration pathways, and fault-controlled permeability through time, we analyzed noble gases (He, Ne, Ar) as inert, non-reactive geochemical tracers in both paleo- and present-day fluids. Noble gases in paleo-fluids were analyzed in quartz and calcite minerals associated with faults (INGV, Palermo). We also analyzed noble gases dissolved in water discharged from thermal springs at ~20 to 53°C (Eawag/ETH Zürich).

In calcite and thermal water, low ³He/⁴He ratios (R/Ra ≤ 0.5) indicate mixing between a dominantly crustal component and a mantle contribution or a mixed crustal-atmospheric origins. Quartz samples show stronger atmospheric contamination than in calcite, although ⁴⁰Ar/³⁶Ar ratios may suggest deep input (mantle vs crust; ⁴⁰Ar/³⁶Ar values between 490 and 1215). He-Ne isotopes in paleo-fluids reveal two areas that show a likely mantle-derived noble gas signature: Sierra Elvira, with ~3% mantle contribution suggested, and the Carboneras Fault Zone, with ~6%. In contrast, present-day fluids could reflect a ~4% mantle contribution in the northeastern Betics at Mula and Archena. We infer that mantle-derived signatures detected in paleo-fluids are not preserved in the same locations in present-day fluids. For instance, along a single fault system (e.g., the Carboneras Fault), paleo-fluids display up to four times higher mantle contributions (Rc/Ra ≈ 0.5) than present-day fluids (Rc/Ra ≈ 0.1). This contrast opens new questions regarding potential changes in mantle fluid sources or migration pathways during the evolution of the Betic Cordillera, the impact of tectonic inversion on deep fault permeability, the residence time of fluids in the crust, and the role of fault geometry in controlling fluid pathways.

How to cite: Cateland, B., Battani, A., Mouthereau, F., Brennwald, M. S., Caracausi, A., Lefeuvre, B., and Pujol, M.: Migration pathways of crustal and mantle fluids during the formation of the Betic Cordillera (SE Spain)., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12738, https://doi.org/10.5194/egusphere-egu26-12738, 2026.

In deep sedimentary basins, the formation and evolution of fracture-vein systems are critical for understanding fluid migration and overpressure history. This study investigates antitaxial fibrous illite veins in the Upper Triassic Xujiahe Formation of the Wubaochang area, Sichuan Basin, to decipher the mechanisms of fluid overpressure and diagenetic evolution. A multi-proxy approach was employed, combining detailed petrography, SEM, and micro-XRD. Crucially, we applied Optical Photothermal Infrared Spectroscopy (O-PTIR)  to achieve sub-micron resolution mapping of organic functional groups within single illite fibers, alongside in-situ REE and C-O isotope analysis. Detailed petrography and SEM analysis reveal that these veins exhibit typical antitaxial growth characteristics, where mineral fibers grow from a median plane toward the host rock, recording the continuous opening and synchronous filling of fractures. In-situ rare earth element (REE) and carbon-oxygen (C-O) isotope analyses identify two distinct fluid evolution stages: Stage I reflects an external, deep-circulating basin fluid system driven by regional tectonic stress, characterized by significant water-rock interaction with host rocks. Conversely, Stage II represents localized diagenetic and hydrocarbon-generated fluids, where isotopic signatures shift toward organic-derived carbon sources, indicating a transition to hydrocarbon-generation-induced overpressure. To definitively address the timing of fluid injection, sub-micron resolution O-PTIR (Optical-Photothermal Infrared) analysis was conducted, revealing the simultaneous presence of organic acid (1720cm^-1), aromatic (1600cm^-1), and aliphatic (1450cm^-1) functional groups coexisting with the illite lattice vibration (1034 cm^-1) within single fibrous crystals. The high ratio of organic acids to mineral signals indicates that organic acids directly mediated water-rock reactions and mineral precipitation rather than being late-stage infiltrations. Our findings demonstrate that these fibrous veins are coupled products of tectonic-induced fracturing and organic-acid-mediated mineral growth. This study highlights the power of O-PTIR as a novel tracer for deciphering organic-inorganic interplays, offering new insights into the mechanisms of fluid overpressure and hydrocarbon expulsion in deep, complex basin systems.

How to cite: Wang, Z., Zeng, L., and Gasparrini, M.: Fluid overpressure and diagenetic evolution recorded by antitaxial fibrous illite veins in deep coal-bearing strata: Insights from sub-micron O-PTIR and in-situ geochemistry, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14497, https://doi.org/10.5194/egusphere-egu26-14497, 2026.

Large active faults tend to differ from younger, fresh faults. They are the loci of the greatest earthquakes, but they often creep. The damage zone thickness tends to saturate with large displacement. Deep scientific drilling in large active faults systematically reveals differentiated fault gouge surrounded by a halo of fracturing and alteration. Despite its importance for fault evolution, the process of alteration in large active faults remains poorly understood.

After the devastating M_w 6.9 Nanbu-Kobe earthquake of 1995, a 750 m deep borehole was drilled into the Nojima fault, reaching the fault core at 625 m. The Hirabayashi borehole was drilled ~1 year after the earthquake and provided an extensive dataset on the structure of the fault where the earthquake originated. Continuous coring and borehole geophysics conducted within the borehole showed that the granodiorite protolith experienced several stages of alteration, including fault-related alteration that produced for example laumontite (Ca-rich zeolite) and smectite at T>150°C.

The ANR project AlterAction is revisiting this data with a multidisciplinary team to better understand the interplay between alteration and fault deformation, with 3D imaging of the core samples (see presentations by Romain Iaquinta and Maxime Jamet) and systematic petrophysical characterization.

This presentation will focus on the reanalysis of geophysical logs. Sonic velocities, electrical resistivities, and lithodensities progressively decrease when approaching the fault, starting to deviate from the protolith rock. In the granitoid rocks composing the borehole, variations in gamma-ray may reflect changes in the protolith rather than alteration. This fault zone starts at 370 m (255 m above the fault core) in the hanging wall, with a more pronounced decrease below 540m (85 m above the fault core, corresponding to a zone of thickness ~8 m, given the well trajectory, which is almost tangent to the fault dip). In the footwall, the strong decrease extends down to 680 m (55 m long zone), with lower velocities, resistivities and densities between 625 and 635m, below the fault core. Crossplotting the logging dataset shows the same trends, whether in the footwall or the hanging wall, regardless of the distance to the fault. This suggests that most of the fault zone is affected by the same interplay between alteration and damage. The fault core (623-626m) is singular owing to its relatively large sonic velocities, suggesting that sealing was strongly localized and effective in the fault core one year after the earthquake.

How to cite: Doan, M.-L., Jamet, M., Iaquinta, R., Gibert, B., Patrier, P., and Lucas, Y.: Interplay between alteration and damage at the Nojima fault zone (Japan) revealed by borehole geophysics, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15103, https://doi.org/10.5194/egusphere-egu26-15103, 2026.

Ultramafic rocks are increasingly recognized as promising reservoirs for long-term carbon fixation through mineral carbonation. However, carbonation reactions are inherently self-limiting, as they involve solid volume increases of up to ~68%, which clog pore spaces, drastically reduce permeability, and inhibit further fluid infiltration. Geological processes capable of sustaining permeability during carbonation have therefore been invoked, including (1) continuous tectonic deformation that generates microfractures (Menzel et al. 2022, Nat. Commun.), and (2) metasomatic mass transfer from mantle rocks to the crust that reduces net solid volume (Okamoto et al. 2021, Commun. Earth Environ.). Despite their importance for both natural and engineered carbon storage, the dynamic coupling between metasomatic reactions, volumetric changes, and deformation remains poorly constrained by experiments.

Here we investigate reaction–deformation coupling at the slab–mantle interface using a series of hydrostatic and shear deformation experiments conducted at 500 °C and 1.0 GPa in a Griggs-type piston-cylinder apparatus. Experimental assemblies consisted of a three-layer configuration in which a crustal lithology (pelitic schist from the Sanbagawa belt or quartzite) was sandwiched between harzburgite (Horoman peridotite) and serpentinite (Mikabu belt). Hydrostatic experiments were performed with pure H₂O, whereas shear experiments employed H₂O–CO₂ fluids (XCO₂ = 0.2) generated in situ by thermal decomposition of oxalic acid dihydrate.

Hydrostatic experiments reveal that metasomatic reaction pathways and resulting textures are strongly controlled by the chemical composition of the adjacent crustal rock. In experiments involving pelitic schist, albite phenocrysts are preferentially replaced by Mg-rich saponite, while talc precipitates within dendritic fracture networks in the serpentinite. Mass balance calculations indicate that Mg absorption by Al-bearing minerals in the sedimentary rocks promotes progressive Mg extraction from mantle lithologies. Importantly, textural contrasts between lithologies indicate opposite volumetric responses: reaction-induced fracturing in serpentinite is associated with net solid volume reduction, whereas reactions in harzburgite proceed with solid volume expansion.

Shear deformation experiments conducted along quartzite–serpentinite interfaces exhibit a pronounced reaction-duration dependence on mechanical behavior. Short time reaction (6 h), friction coefficients are relatively high. In contrast, long reaction duration (68 h) results in stable sliding with exceptionally low friction coefficients. Microstructural observations show the development of a reaction zone dominated by extensive carbonation (listvenite formation: quartz + magnesite) localized at the lithological interface. Deformation is strongly localized within the carbonation products, which display laminar fabrics and magnesite-filled fractures containing nanoscale porosity.

Integrating hydrostatic and shear experiments, we suggest that metasomatic mass transfer is essential for sustaining carbonation reactions. Furthermore, the pronounced mechanical weakening observed in shear experiments may not be solely attributable to talc precipitation, but possibly also to the dehydration accompanying carbonation. Instead, the dynamic coupling between chemical reactions, solid volume changes, and deformation promotes fracture formation, permeability maintenance, and extreme rheological weakening. These processes provide a viable mechanism for overcoming reaction-induced pore clogging during long-term carbonation and have profound implications for carbon transport, storage efficiency, and the mechanical behavior of the slab–mantle interface.

How to cite: Okino, S., Okamoto, A., Oyanagi, R., Kita, Y., Sawa, S., and Muto, J.: Reaction-induced fracturing and rheological effects of carbonation at the slab–mantle interface: Constraints from hydrostatic and shear experiments, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15694, https://doi.org/10.5194/egusphere-egu26-15694, 2026.

During subduction, progressive heating and burial drive dehydration reactions that release H₂O-rich fluids from altered oceanic crust. Under sub-arc conditions (2–4 GPa), the transition from blueschist- to eclogite-facies mineral assemblages is accompanied by the release of substantial amounts of water (up to ~5 wt.%). If fluid transport occurs on timescales that are short relative to subduction, these fluids migrate through the overlying oceanic crust into the mantle wedge or along the slab–wedge interface. This process is critical for the generation of arc magmas and for their enrichment in fluid-mobile trace elements. Despite the importance of this process, relatively few direct constraints exist on the extent to which such fluids react with the eclogite-facies crust through which they migrate.

Here, we present field, petrological and geochemical observations from a pristine suite of transport veins preserved in mafic eclogites of the Tauern Window, Eastern Alps. The veins are dominated by high-variance, quartz-rich mineral assemblages and are surrounded by well-developed, omphacite-dominated selvages. Phase equilibrium modeling indicates that vein formation occurred at or near peak pressure–temperature conditions of ~2.5 GPa and ~600 °C. A striking feature of the fluid–rock interaction is the near-complete consumption of garnet by the reactive fluid. Trace-element zoning in partially reacted garnet porphyroblasts records a fluid-driven dissolution–precipitation mechanism that mobilized middle and heavy rare earth elements (MREE and HREE). Isocon analysis of the altered eclogite selvages reveals bulk gains in Na and Li, accompanied by losses of REE, Sr, K, Cu, Fe, Al, Y, Mn, Ba, and Cr, while Ni, Sc, and Ti appear to have been conserved.

Pure omphacite layers and seams are commonplace throughout the Eclogite Zone and are interpreted as sealed transport veins. The associated microstructures record embrittlement and fracturing following fluid–rock interaction. Collectively, these observations indicate that reactive fluid flow under eclogite-facies conditions may influence the rheology of subducting oceanic crust.

How to cite: Smye, A., Strobl, L., Forgeng, H., and Fisher, D.: Linking Reactive Fluid Flow to Rheology of Eclogite-Facies Oceanic Crust, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15970, https://doi.org/10.5194/egusphere-egu26-15970, 2026.

The extent to which ambient seawater permeates and interacts with submarine lithologies (e.g., sediments and seafloor basalt) is a critical constraint on the timing and rate of in situ subseafloor silicate weathering. Oxygen isotope (δ¹⁸O) values of low-temperature silicate minerals in the marine record often present seemingly inconsistent oceanographic and diagenetic histories. These uncertainties largely arise because neither formation temperature (e.g., burial or alteration) nor the modification of seawater δ¹⁸O values through water–rock interactions are well constrained. However, the triple oxygen isotope (Δ¹⁷O) approach provides additional constraints on the diagenetic temperature and seawater modification. 

Here we present two case studies of Δ¹⁷O variations in low-temperature silicate minerals from widely disparate marine settings that represent potential endmembers in subseafloor diagenetic environments and seawater modification through water–rock interactions. Marine sediment cores from the Ross Sea, Antarctica, collected during IODP Expedition 374 and the ANDRILL McMurdo Ice Shelf program, contain well-preserved biogenic opal (diatomite) of Pleistocene to middle Miocene age (~2.2–16.5 Ma). The mineral structure of opal from these sites (IODP U1521 and U1523; AND-1b and AND-2a) ranges from opal-A to chert, and the Δ¹⁷O values reflect isotopic equilibrium with a significantly modified seawater at a range of temperatures consistent with the geothermal gradient and burial depth. Measured Δ¹⁷O values for all opal samples fall below the seawater equilibrium curve and likely reflect equilibration with pore waters. The abundance of hydrous mineral phases (e.g., mirabilite, authigenic clays) in the Ross Sea cores suggest that water-rock interactions may have altered the pore water Δ¹⁷O values. Pore water δ¹⁸O values and chemistry at the ANDRILL sites suggest the presence of a cryogenic brine with a low δ¹⁸O value; however, in the IODP sites on the continental shelf and slope, pore water δ¹⁸O values are closer to that of modern Ross Sea Bottom Water. In a very different geologic setting in the North Atlantic, similarly modified seawater Δ¹⁷O values are recorded in alteration minerals (e.g., celadonite, saponite, and zeolite) in submarine basalt veins/vesicles from IODP Site U1564, which is located east of the Reykjanes Ridge in ~32.4 Ma crust. The alteration minerals Δ¹⁷O values appear to show a mixing relationship between seawater and unaltered basalt endmember with varying water–rock ratios and/or formation temperatures, which suggests fluid evolution or mixing of fluids with different Δ¹⁷O values. Observed Δ¹⁷O values in ancient geologic materials (e.g. Archean cherts) have been interpreted as reflecting primary oceanographic conditions or subsequent diagenetic alteration by meteoric waters. In the geologic settings described here, the Δ¹⁷O variability appears to record significant in situ subseafloor modification of seawater oxygen isotope values through low-temperature water–rock interactions. Constraining the timing and extent of water–rock interactions are, therefore, essential for refining models of geochemical reactions, fluid flow, global element cycling, and deep-biosphere microbial processes in the marine subseafloor environment.

How to cite: Dodd, J., Piccione, G., Ibarra, D., McNamara, D., Pasquet, G., Lindsay, M., Eason, D., Briais, A., Parnell-Turner, R., and LeVay, L.: Triple oxygen isotope evidence for modified seawater during low-temperature submarine silicate alteration and weathering, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16106, https://doi.org/10.5194/egusphere-egu26-16106, 2026.

Monazite from placer deposits along the southwestern coast of Taiwan was previously exploited as a source of rare earth elements (REEs). However, the formation mechanisms of the distinct monazite types remain debated and are commonly attributed to fluid-related processes. Due to the extremely high denudation rates of rivers in Taiwan, sediments undergo rapid transport, allowing the protolith characteristics of their provenance to be preserved. In this study, we examine river sediments from the Zengwen, Ailiao, and Laonong Rivers, which drain contrasting tectonostratigraphic units within each catchment. We characterize the occurrence and elemental distributions (e.g., La, Th, Nd) of monazite and examine the REE geochemical behavior of the bulk sediments. This study provides a comprehensive mineralogical and geochemical assessment of monazite associated with specific tectonostratigraphic units, offering constraints on sediment provenance.

Preliminary results summarize the variations in monazite occurrence and alteration across different tectonostratigraphic units. In the sediments of the Zengwen River, which primarily drains the Western Foothills, we observed a predominance of detrital monazite (<10 µm), as well as monazite associated with TiO₂, apatite, and clay minerals. These features suggest an origin primarily from the physical weathering of detritus or minor fluid precipitation, differing significantly from the occurrences of monazite found on the southwest coast.

Sediments from the Ailiao and Laonong rivers, which drain low-grade metamorphic rocks of the Central Range, exhibit evidence for variable degrees of low-temperature alteration affecting primary monazite. This includes inclusion-hosted, morphologically black monazite comparable to that observed along the southwestern coast of Taiwan. In the Laonong River, which additionally drains the Western Foothills, monazite occurs either within quartz grains or within the interlayers of clay minerals, similar to that observed in the Zengwen River. Furthermore, in rivers originating from the Slate Belt, in addition to monazite as a heavy mineral, we identified pyrite spherules comparable to those in the slate host rocks, as well as xenotime associated with thorite. Overall, these observations reveal distinct patterns in monazite occurrence and alteration among different tectonostratigraphic settings, with implications for sediment provenance in high-denudation river systems.

How to cite: Yeh, H. L., Wang, Y. T., Huang, C. C., and Chen, Y. H.: Monazite occurrence and low-temperature alteration in river sediments from contrasting tectonostratigraphic units in southwestern Taiwan, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16117, https://doi.org/10.5194/egusphere-egu26-16117, 2026.

Monazite is a phosphate mineral that contains Rare-Earth Elements (REEs), whose micro-textures and chemical compositions can serve as effective indicators of sediment provenance. In Taiwan, monazite in fluvial and coastal sediments may reflect contributions from both external sources and orogenic recycling; however, a systematic classification framework and provenance constraints for monazite in major river systems remain limited. This study focuses on the Zhuoshui River catchment and establishes a monazite classification scheme based on integrated microtextural and geochemical characteristics, which is then applied to assess the provenance of the monazite.

    Bedrock and riverbed sediment samples were collected from the upper reaches and along the Zhuoshui River system during both wet and dry seasons. Scanning Electron Microscope (SEM) backscattered electron (BSE) imaging and semi-quantitative Energy Dispersive Spectrometer (EDS) analyses were used to characterize grain morphology, inclusion features, and REE–Th–Y elemental systematics. The results show that monazite populations and La/Ce systematics are consistent between wet- and dry-season samples, indicating that the provenance signal is stable and not significantly affected by seasonal hydrological variability.

  Based on REE proportions, grain morphology, and inclusion characteristics, monazite grains can be classified into three types. Detrital monazite is generally larger, inclusion-free, relatively enriched in Th, and commonly displays rounded grain boundaries. Hydrothermal altered monazite is typically Th-depleted and LREE-dominated, commonly containing quartz–feldspar inclusions and occurring in association with hydrothermal minerals. Inclusion-hosted monazite shows distinct compositional boundaries and characteristic REE signatures, with relatively elevated middle-REE signals, suggesting early encapsulation rather than late-stage replacement. Similar micro textures and comparable La/Ce ratios observed in both upstream bedrock and downstream sediments support an orogen-derived provenance for monazite in the Zhuoshui River system. Two compositional clusters in La/Ce further imply at least two source regions, tentatively linked to metamorphic source rocks in the Central Range and the Hsuehshan Range. Ongoing U–Th–Pb geochronology using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) will provide independent temporal constraints. These data will strengthen the proposed classification and provenance interpretations, clarifying sediment transport pathways in the Zhuoshui River.

How to cite: Yang, P.-C. and Chen, Y.-H.: Classification and Provenance of Monazite in the Zhuoshui River System, Taiwan, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16249, https://doi.org/10.5194/egusphere-egu26-16249, 2026.

Gneiss domes are typical tectonic types related to deep crustal exhumation, and their polyphase superimposed deformation characteristics make them ideal natural laboratories for studying deep crustal exhumation processes. The Laojunshan gneiss dome in Yunnan, China, lies at the junction of the Tethyan and Circum-Pacific tectonic domains, as well as the boundary between the South China Block and Indochina Block. Its tectonic setting, deep crustal tectono-thermal evolution and exhumation history are closely linked to the kinematic evolution of the two blocks, making it a key site for investigating the Tethyan tectonic domain’s spatiotemporal evolution and inter-block interactions. Based on systematic field investigations combined with microstructural analysis, stress field inversion, electron backscatter diffraction (EBSD) analysis, geochemical and geochronological analyses, significant findings on the dome’s exhumation-related tectono-thermal evolution are obtained. The Laojunshan tectonic units comprise a core dominated by high-grade metamorphosed-deformed rocks and granites, an arcuate detachment fault system, and a sedimentary cover. Regional stress field inversion reveals two distinct regimes (compressional and extensional), with the latter predominant and radially distributed, reflecting late exhumation tectonics. EBSD analysis of major exposed minerals indicates the core underwent high-temperature plastic deformation (620–710 °C). Mylonite parameters (fractal dimension, differential stress) in the detachment fault zone reflect transitions between high and medium-high temperature deformation. Epidote EBSD constrains late exhumation P-T conditions to 350–500 °C, which, combined with geochemical data, divides late exhumation into three stages: deep compression, uplift transition and shallow extension. Geochronological data show the Caledonian (445–420 Ma) as the main formation period of granitic gneiss protolith (synchronous with coeval magmatism), core leucogranite emplacement at 416–411 Ma, and metamorphic zircons in plagioclase constrain Indosinian high-temperature metamorphism and shortening deformation to 241–220 Ma. An exhumation model is proposed: the dome initiated with early Caledonian (445–420 Ma) regional extension and magmatism, followed by 420–410 Ma compressional orogeny, crustal thickening and anatexis. Indosinian (241–230 Ma) compression induced thrusting, folding and detachment faults. Yanshanian (144–80 Ma) extension and magmatism accelerated exhumation, and Cenozoic (33–21 Ma) strike-slip faulting drove rapid exhumation to the surface.

How to cite: Tian, Y., Cao, S., Zhan, L., Liu, J., Zhou, D., Li, Q., and Tao, L.: Tectono-Thermal Evolution of the Laojunshan Gneiss Dome in Yunnan, China: Constraints from Multi-Mineral Deformation and Composition, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16463, https://doi.org/10.5194/egusphere-egu26-16463, 2026.

Shear zone nucleation in massive rocks commonly exploits pre-existing planar structures, whose presence and type control fluid availability and redistribution. While fluids are widely recognized as key triggers for metamorphic reactions and mineralogical transformations that influence rock rheology, the exact feedback processes between fluid-rock interactions, metamorphism and deformation remain enigmatic. In specific cases, traditional softening mechanisms (e.g. reaction-induced grainsize reduction, crystallization of weak minerals) do not apply or are insufficient to explain shear zone nucleation and strain localization, implying the existence of alternative processes.

Paired shear zones developed at the selvages of hydration haoles are a common product of fluid infiltration along hydrofractures, and are a source of information to investigate the rheological effects of the different reaction extents occurring during fluid percolation. Here, we investigate a suite of samples containing eclogitic clinozoisite-filled veins surrounded by omphacite-rich haloes. The sample set includes haloes (a) weakly affected by ductile deformation, preserving pristine metasomatic textures, and (b) displaying paired shear zones at their selvages (Pennacchioni, 1996). The eclogitic host rock foliation, consisting of garnet, clinozoisite, amphibole and omphacite, is only partially obliterated in the hydration halo by the metasomatic overprint, dominated by replacement of clinozoisite by omphacite. EDS major element and in-situ LA-ICP-MS trace element analysis suggests that fluid propagation caused recrystallization, changes in mineral proportions and (re)distribution of major and trace elements, forming a compositional gradient across the halo. Garnet and clinozoisite rims record the gradient with a progressive decrease in the Fe²⁺ content and a progressive increase in LREE and Fe³⁺ concentrations from the vein selvage towards the reaction front, respectively.

Electron backscatter diffraction (EBSD) maps provide evidence for (i) a constant omphacite grainsize across the haloes and at their boundaries in samples weakly affected by ductile deformation, suggesting that metasomatism does not produce textural gradients, (ii) development of very fine-grained monomineralic ribbons of omphacite along the shear zones, suggesting that omphacite is responsible for weakening and localized shearing, (iii) local orientation of these ribbons at 20-30° to the shear zone trace, defining C' bands, and (iii) random orientation of the fine grains. We interpret these observations as evidence for diffusion-assisted grain boundary sliding (GBS) and creep cavitation as the main deformation mechanism active along the shear zones, and for heterogeneous nucleation of very fine-grained omphacite within fluid-filled cavities formed during GBS.

We conclude that, when metasomatic reactions do not directly result in textural gradients (e.g. grainsize decrease) traditionally considered responsible for softening at the propagation front (i.e. halo boundary), shear zones may develop by heterogeneous nucleation of fine grains during fluid-assisted GBS, which further fosters grainsize-sensitive deformation sustaining strain localization within fluid-rich domains.

[1] Pennacchioni, 1996. Journal of Structural Geology, 18, 549-561

How to cite: Cacciari, S., Pennacchioni, G., Cannaò, E., Toffol, G., Scambelluri, M., and Hermann, J.: Shear zone nucleation by fluid-assisted heterogeneous nucleation recorded in texturally homogeneous eclogitized mafic granulites, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16837, https://doi.org/10.5194/egusphere-egu26-16837, 2026.

Dolomitization is a key diagenetic process that reorganizes porosity and permeability in carbonate rocks, yet the coupling between interface kinetics and transport heterogeneity remains poorly constrained. We performed time-resolved hydrothermal dolomitization experiments on oolitic limestone and chalk to test how carbonate texture impacts the dynamics of replacement fronts. Pseudo 4D micro-CT, along with SEM, EBSD and microprobe analyses reveal that the mechanism of dolomitization is depending on the texture and on the chemistry of the fluid. We studied oolitic limestone, chalk and carrara marble, three rocks with different porosity and permeability. In both oostone and chalk, dolomite rims propagate rapidly and produce wavy reaction fronts, witnessing a progressive replacement inward rather than along pore-connected pathways, unlike in the carrara marble. However, mass-balance calculation of the fluid rock interaction returns similar mass and volume loss independently of the texture, suggesting that the chemistry of the fluid controls the reaction. This is consistent with the produced dolomite grain size, who follow a similar distribution law regardless the texture, suggesting some self-organization during the replacement controlled by the fluids. Roughness characterization of the front shows that in oostone and carrara marble, the scaling law of the front follows a Brownian Motion (H=0.5), while it shows a persistent behaviour in the chalk (H=0.8 to 0.6). This suggests that the expression of the replacement process is governed by random distribution of heterogeneities in some cases, following the interface coupled dissolution precipitation model, but that there is a memory effect in other cases. In this study, the memory effect can be related mechanical processes such as local microfracturing, suggesting a potential role of local pressure on replacement. We propose that the shape of the front is governed by the rate of the front propagation, as if the latter is fast like in chalk, the mass transfer becomes less efficient to compensate the volume change, and some local overpressure may appear to drive the reaction propagation. This rate of front propagation appears to be affected by both initial grain size and pore size homogeneity.

How to cite: Beaudoin, N., Kularatne, K., Centrella, S., Lefeuvre, B., Sénéchal, P., Mascle, M., Youssef, S., and Nader, F. H.: Texture-controlled hydrothermal dolomitization experiments to investigate interface kinetics and pore-scale transport, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17135, https://doi.org/10.5194/egusphere-egu26-17135, 2026.

Serpentinization is a hydration process that forms distinct serpentine minerals depending on the pressure and temperature conditions prevailing during the fluid-rock reaction. The chemistry and petrology of serpentinite rocks provide constraints on the protolith composition and on the tectonic setting of serpentinization through insights of pressure and temperature conditions.

The Münchberg Massif is a stack of four tectonic nappes of different metamorphic grade, which were emplaced during the Variscan Orogeny. Within the lowermost Prasinit-Phyllit-Serie, several serpentinite bodies are intercalated. Understanding the formation of these serpentinites will add further insights into the tectonic development of the Münchberg Massif.

We present new petrological and chemical data of serpentinites from ten locations along the southeastern margin of the Münchberg Massif, as well as at two locations in the western region of the Massif. The samples are dominated by the serpentine minerals lizardite in the western region and antigorite along the southeastern margin. Furthermore, significant differences in the degree of serpentinization and tectonic strain were observed between the two regions. The petrological and chemical characteristics of the samples indicate distinct protolith material and serpentinization setting. We propose that the protoliths of the western and southeastern serpentinites originated from different structural positions within, or adjacent to, a subduction zone. These findings provide new constraints on the tectonic assembly and metamorphic evolution of the Münchberg Massif.

How to cite: Hasch, M., Klitzke, P., Bagge, M., Koglin, N., and Ostertag-Henning, C.: The formation of the Münchberg Massif: New insights from petrology and chemistry of its serpentinite occurrences., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18428, https://doi.org/10.5194/egusphere-egu26-18428, 2026.

The analysis of fault-related mineralization, with particular emphasis on fluid inclusions (FIs) trapped in syn-kinematic minerals, provides crucial insights into fluid circulation modality and fluid–rock interactions, so furnishing new tools to investigate the relationship between fluids and active tectonic. This study investigates the genesis, microstructural characteristics, and geochemical signatures of calcite veins associated with dip-slip faults in the Irpinia region (southern Apennines, Italy), a seismically active area located very close to the epicentral zone of the 1980 Mw 6.9 Irpinia earthquake. A comprehensive approach combining field observations, petrographic and microstructural analyses, fluid inclusion microthermometry, and geochemical profiling based on isotopic (δ¹³C and δ¹⁸O) and rare earth element (REE+Y) data reveals that the calcite veins precipitated from low-salinity H₂O–NaCl fluids, derived from the mixing of shallow and deep groundwater. These fluids, rich in CO₂ and coming from deep crustal reservoirs (8–12 km), migrated episodically through fault zones and were modified by mixing with post-depositional fluids produced during carbonate diagenesis, under varying thermal conditions (100–320 °C). Our study also proposes a computational model that reconstructs the isotopic evolution of the mineralizing fluids, capturing the sequential processes of fluid equilibration with dolostones, interaction with aquifer waters, and CO₂ degassing prior to calcite precipitation forming the mineralization. The good agreement between model predictions and measured isotopic data demonstrates the robustness of the model and highlights the dynamic fluid mixing processes within the fault zone. Furthermore, these findings highlight the role of episodic fluid migration, driven by fault-valve processes, in promoting calcite oversaturation and precipitation during seismic events. The integration of structural, geochemical, and modelling data refines our understanding of CO₂-rich fluid ascent, fault-related mineralization, and their link to fluid–rock interaction processes. This multidisciplinary approach offers new insights into fault mechanics and seismo-genesis, with implications for seismic hazard assessment and geochemical monitoring in active fault systems

How to cite: Zummo, F., Alvarez-Valero, A. M., Billi, A., Buttitta, D., Carnevale, G., Marchesini, B., Pibiri, I., Sinisi, R., Smeraglia, L., Caracausi, A., Agosta, F., and Paternoster, M.: Fault-related fluid circulation in the seismically active Irpinia region (southern Italy): insights from fluid inclusions and calcite veins, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19908, https://doi.org/10.5194/egusphere-egu26-19908, 2026.

The solidity and porosity of calcium carbonate rocks are of major interest for oil-reservoir
evaluation, groundwater flow studies, and civil engineering applications. Micritization—an
early diagenetic process that converts carbonate shells and skeletal grains into
microcrystalline carbonate—significantly affects these rock properties, yet its underlying
mechanisms remain poorly constrained. The coastal environments of Abu Dhabi provide
natural laboratories for studying micritization, as they are modern analogues of the low-angle
carbonate ramps that were widespread in epeiric seas throughout much of the geological past.
In this study, we investigate calcium carbonate muds and associated pore waters from a range
of depositional environments, including mangroves, tidal channels, sabkhas, and offshore
settings, to better understand the processes controlling micritization. We apply an integrated
approach combining sedimentological, mineralogical, and geochemical methods. Preliminary
results indicate that the carbonate mud is predominantly composed of aragonite, with minor
amounts of low magnesium calcite. Boring intensity increases with depth, particularly in
tidal-channel environments, and is closely associated with physical erosion by endolithic
fauna. In contrast, crystal morphologies observed in sabkha sediments suggest that chemical
precipitation processes are more dominant in these settings.
Trace element systematics reveal that micritization is accompanied by systematic changes in
Sr/Ca and Mg/Ca ratios. In all tested environments, grain size reduction (i.e., micritization) is
associated with a significant increase in Mg/Ca, while Sr/Ca is much less sensitive to the
same process. While both Sr/Ca and Mg/Ca are incorporated within the diagenetic aragonite
lattice according to their respective partition coefficients, Mg/Ca ratios are strongly increased
by adsorption during micritization-related grain size changes. The decoupling of Mg and Sr
during the micritization process may provide new constraints on the question of the
mechanism of micrite formation.

How to cite: Ash, A., Lazar, B., Torfstein, A., Antler, G., Cao, T., Rivlin, T., Alsuwaidi, M., Morad, S., and Stein, M.: Mechanisms of Micritization Revealedby Petrography, Mg/Ca and Sr/Ca Ratios of the Carbonate Sediments of theArabian (Persian) Gulf, Abu Dhabi, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22277, https://doi.org/10.5194/egusphere-egu26-22277, 2026.

Owing to the Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) and borehole observatories, slow slip events (SSEs) have been detected in the shallow extension of the source region of the 1944 Tonankai earthquake (DONET-1). However, a localized seafloor pressure anomaly—characterized by uplift and subsidence at two DONET-1 stations in 2013—has yet to be reasonably explained.

In this study, we explore possible source models for this pressure anomaly by assuming pore-water migration from compacted reservoirs, either arranged in layered formations or represented as swarms of small spheres, toward a dilated zone beneath the décollement. We also compile observations of seafloor crustal deformation driven by SSEs and oceanographic phenomena under baroclinic conditions to refine the spatio-temporal scaling relationship of seafloor pressure variations.

Our main findings are as follows. (i) The potential compacted pore-water reservoirs spatially overlap with the hypocenters of very low-frequency earthquakes (VLFEs), whereas the dilated zone lies in a region with normal-fault-type VLFE activity. (ii) A Kuroshio meander associated with an abrupt fluctuation in sea surface height (SSH) occurred around DONET-1 during the pressure event. (iii) Taken together, (i) and (ii) suggest that the local seafloor pressure change may be explained by pore-water migration destabilized by the Kuroshio current meander. (iv) As this is the first reported case in which a local seafloor pressure anomaly has been identified from only two observation points, the suggested causal link—namely, that the Kuroshio meander may have promoted pore-water migration—provides a strong scientific motivation for future geological surveys, particularly those monitoring seismic activity and seafloor crustal deformation before and after similar pore-water migration events.

How to cite: Ariyoshi, K., Nagano, A., Hasegawa, T., Nakano, M., Matsumoto, H., Aiken, C., Araki, E., Takahashi, N., and Hori, T.: A Local Seafloor Pressure Anomaly Potentially Triggered by Pore Water Migration during Ocean Current Meander, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-271, https://doi.org/10.5194/egusphere-egu26-271, 2026.

Shear zones are preferential fluid pathways during prograde and retrograde stages of subduction cycles, but the drainage and permeability of subduction interfaces are poorly quantified. Analyzing exhumed rocks for preserved signatures of fluid production and flow provides insights into fluid circulation during burial and exhumation.

Here, we investigated fluid flow processes recorded by garnets in quartz-schists from the As Sheik shear zone (Saih Hatat window, NE Oman) that records evidence for burial during subduction and local overprinting during exhumation. Garnet occurs as equant, oblate, and honeycomb (i.e., skeletal) shapes, which each documents distinct fluid-related growth stages from peak-pressure to early exhumation associated with a thermal excursion, both occurring at broadly eclogite facies conditions. We show with thermodynamic models and microstructures that garnet first nucleated at 2.0–2.2 GPa and 500–550°C after the chloritoid-out dehydration reaction, which promoted dissolution–precipitation processes. We infer a pseudomorphic replacement of peak-pressure chloritoid by garnet, and based on the absence of internal lattice strain, we suggest that elongate garnet morphology reflects reaction-controlled growth rather than plastic deformation. Our microstructures and models suggest that subsequent decompression and heating (1.5–1.3 GPa, 600–650°C) promoted further fluid release and a renewed stage of honeycomb garnet growth.

We present a conceptual model in which dissolution, transport, and precipitation rates primarily influenced whether garnets grew as oblate grains  (i.e., as pseudomorphs on peak-pressure chloritoid grains), or as newly nucleated equant grains. In addition, we argue that honeycomb garnet represents a snapshot of the permeability network that allowed the fluids to escape from the shear zone using grain boundaries and through reaction-forming pathways.

Using measured maximum mass fraction of fluid released from all the hydrous phases modelled by thermodynamic modelling on a representative rock-scale column of 1000 meters, we estimate the time-integrated fluid flux of the studied shear zone was ~34 m³ m^-2 at eclogite facies conditions for the entire duration of garnet growth. This volume represents a limited time-range of the shear zone lifetime during garnet growth, i.e., from peak-pressure to incipient exhumation still at eclogite facies conditions. Therefore, the full lifetime of the shear zone during prograde and retrograde conditions would indeed provide a higher fluid flux.

The different garnet morphologies analyzed all resulted from the chloritoid dehydration reaction, but reflect different rates of dissolution–precipitation and efficiency of dissolution. This study highlights garnet morphology as a tracer of transient fluid pathways during a burial-exhumation cycle of an eclogitic shear zone. The close connection between garnet morphology and fluids calls for a re-evaluation of similar microstructures in different tectonic settings.

How to cite: Petroccia, A., Giuntoli, F., Kotowski, A., Buono, G., Chogani, A., Hellebrand, E., Pappalardo, L., and Callegari, I.: Tracking dehydration reactions and fluid flow in exhuming shear zones using garnet microstructures, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-331, https://doi.org/10.5194/egusphere-egu26-331, 2026.

Absolute dating of remagnetization events remains the holy grail in paleomagnetism, with the potential to unlock thousands of rock units for new tectonic, metamorphic, and paleointensity studies. Constraining the timing of remagnetizations is especially crucial for understanding fluid flow and mineralogical transformations in orogenic systems.

As a first step toward dating fluid-flow–related remagnetizations, we investigate three Cambrian carbonate units from the northwestern Iberian Peninsula—Tamames, Láncara, and Vegadeo—remagnetized during the Carboniferous. Our goal is to identify, characterize, and ultimately constrain the age of these fluid-induced remagnetization events.

In this presentation we will show an integration of rock magnetism, paleomagnetism, mineralogical, and geochronology results. Magnetic characterization includes room-temperature and low-temperature hysteresis cycles, IRM acquisition curves, First‑Order Reversal Curve diagrams (FORC), thermomagnetic curves, thermal and AF demagnetization, and anisotropy of magnetic susceptibility (AMS). In addition, targeted mineral separation procedures were performed to obtain magnetic sulfide fractions for Re–Os geochronology. The identification and spatial distribution of magnetic phases were examined using scanning electron microscopy (SEM) and quantum diamond microscopy (QDM), allowing us to distinguish primary from secondary magnetic minerals and to evaluate textural evidence of fluid-rock interaction. Complementary U–Pb carbonate geochemistry provides independent age constraints to compare with paleomagnetic and Re–Os datasets.

Together, these results initiate the development of a robust framework for identifying, characterizing, and dating fluid-induced remagnetizations, offering new insights into the tectonic and mineralogical evolution of Iberia’s orogenic systems.

How to cite: Galan, C., Pastor-Galán, D., and Martín Hernández, F.: Towards Absolute dating of Fluid-Flow Remagnetizations: Initial results from Variscan Carbonates, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-492, https://doi.org/10.5194/egusphere-egu26-492, 2026.

White micas breakdown in down-going slabs of subduction zones implies consequent fluids release, inducing element transport into the overlying hanging-wall mantle. Phengite is the most common white mica occurring in HP / UHP metasedimentary rocks, carrying significant amounts of H₂O, LILE (K, Ba, Cs and Cr especially), and Li, B or N to the upper mantle. Here,  ²H/¹H (D/H) and ¹⁸O/¹⁶O ratios of 23 metapelites samples from the Devonian-Carboniferous Renge and Cretaceous Sambagawa belts are investigated to better understand the O and H isotope signatures of phengites in metapelites of the Pacific-type subduction zone. In addition, we try to constrain the stable isotopic compositions of metamorphic fluids equilibrated with phengites and see their behavior during continuous dehydration reactions.

The investigated pelitic blueschist-facies phengite samples presented non negligeable values of ∂D (∂D < -88‰). 14 of them belong to the Osayama serptentinite melange (central Chugoku Mountains, SW. Japan) of the Renge Belt and separated from lawsonite- and epidotes-grade. They presented a significantly negative ∂D composition, ranging from -113.2‰ to -88.3‰, and a ∂O composition ranging from 12.9‰ to 14.6‰ (∂D and ∂O values approximate SMOW). The 9 other samples are garnet-bearing metapelites of the Sarutagawa schists from Sambagawa Belt (central Shikoku, SW. Japan) and presented ∂D = -95.6‰ to -60.5‰ and ∂O = 12,3‰ to 14,4‰.

Fluids can be characterized as deep-sourced by looking at previous results on high-Si features and K-Ar ages of the investigated samples (Tsujimori & Itaya, 1999). The consequently low values of ∂D cannot be due to meteoric-hydrothermal alteration but by isotopic fractionation during prograde metamorphic dehydration of a plunging slab. Modelling on the obtained data and muscovite, H2O, H and O factors fractionation for nominal temperatures allowed to estimate an isotopic composition for metamorphic fluids equilibrated with phengites. We unveil through this study that slab-devolatilization derived fluids in Pacific-type subduction zone present low ∂D value, implying a non-negligeable role of the phengite breakdown on H isotope composition of nominally anhydrous minerals (NAMs) in deep mantle.

How to cite: Duringer, A., Pastor-Galán, D., Tsujimori, T., Yagi, K., and Álvarez-Valero, A.: Phengite breakdown and associated fluid flow in Pacific-type subduction zone: Investigating the nature of slab-derived fluids of blueschist-facies metapelites from the Renge and Sambagawa belts (SW. Japan)., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-559, https://doi.org/10.5194/egusphere-egu26-559, 2026.

Quartz slip systems are conventionally linked with their corresponding temperatures of activation, but fluid can affect them as well; and how the presence of syn-deformational fluid affects the slips system activation remains poorly constrained. Quartz textures are result of integrated effects of P-T-fluid-deformation, making it challenging to isolate the individual contribution of any single factor. The occurrence of metasomatic reaction zones (MRZs) due to fluid-rock interactions at the boundary between country rock, i.e., a pelitic garnet-mica schist and meta-mafic dykes (Dyke-I and -II) in the Northern Singhbhum Mobile Belt (NSMB) of eastern India, provides an opportunity to address this problem. This geometrically well-constrained system, whose P-T-reaction history has been petrologically and geochemically characterized, allows us to isolate and examine how fluids affect the quartz microstructure at same P-T-deformation conditions. We investigated samples from MRZs, using bulk-rock geochemistry, mineral chemistry, thermodynamic modelling and electron backscatter diffraction analysis.

Geochemical-reaction-path models show that MRZs (amphibole-epidote-plagioclase-quartz and chlorite) assemblage formed by Na-metasomatism at 2–3 kbar and 300–500°C after a post-peak condition (6–8.5 kbar and 550–600°C) of NSMB. The saline fluids reacted with the dykes, i.e., the source and then reacted with the pelite, facilitating the element-mass exchange between them. Our study covers two different scenarios, a fluid-abundant MRZs near Dyke-I (zone 1) and another fluid-limited near Dyke-II (zone 2). Zone 1 exhibits a plagioclase-quartz dominated polygonal mosaic matrix with complete removal of muscovite and garnet. The matrix is characterized by pervasive brown-colored anastomosing fluid networks along grain boundaries, fractures, and cleavages. Healed fractures containing Fe-oxide and fluid inclusion trails are abundant, and small epidote grains occur at grain boundaries and triple junctions. Zone 2 is more quartz-dominated with granoblastic texture subhedral grains showing straight to curved boundaries. Relict biotite and garnet are preserved. The matrix quartz shows isolated microfractures and trans-crystal fluid inclusion trails but lacks the extensive interconnected fluid-network architecture of Zone 1.

The slip system of quartz transitions from the country rock towards the two MRZs. Deformation in quartz of the country pelitic schist, was accommodated mainly via activation of prism <a> and <c> slips. They record abundant presence of <2° kernel average misorientation (KAM). Quartz in zone 1 shows deformation accommodation via rhomb <a> slip and near-complete absence of <2° KAM. The intensity of rhomb <a> slip increases towards its dyke-contact. Whereas in zone 2, quartz shows dominant prism <a> slip and abundant <2° KAM, just like the country rock. Near its corresponding dyke contact of zone 2, the quartz shows polygonization, emergence of rhomb <a> slip, and reduction in <2° KAM due to a relatively higher proportion of fluid presence at the contact.

This study demonstrates how fluid-rock-interaction intensity can play significant role in quartz deformations and display a preferred slip system activity under the same prevailing P-T condition. We propose that under fluid-abundant conditions, the quartz polygonised and rhomb <a> slips are activated in zone 1 due to complex reaction creep and hydrolytic weakening resulting from fluid-rock interactions at the same P-T-deformation conditions.

How to cite: Behera, S., Sikdar, A., Chakraborty, S., and Misra, S.: Fluid-Assisted Deformation: Rhomb Slip Preference in Quartz from Metasomatic Reaction Zones of a Mobile Belt in India, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-964, https://doi.org/10.5194/egusphere-egu26-964, 2026.

The Late Paleozoic Munster Basin of SW Ireland is predominantly composed of the non-marine siliciclastic-dominated fine-grained alluvial sediments of the Upper Old Red Sandstone magnafacies. Copper mineralisation in this sedimentary basin is important, either as sediment-hosted stratiform or locally abundant polymetallic vein-hosted copper. In the polymetallic extensional veins, the ore phases include chalcopyrite, tetrahedrite-tennantite, galena, and molybdenite, with gangue minerals commonly quartz, carbonates, chlorite, barite, and Fe-oxides. Recent Re-Os geochronology on molybdenite proved the latter veins opened ca. 367-366 Ma, during Upper Devonian basinal extension, and were deformed before ca. 316-312 Ma by the Variscan orogeny. However, the role of these two major geodynamic events on copper mineralisation was never studied in detail, such that the vein-hosted copper mineralisation and remobilisation processes are still poorly understood. A collection of mineralised vein samples from the western Munster Basin are characterised using reflected light microscopy, Raman spectrometry, and LA-ICPMS trace element analysis to better define the mineralised vein paragenesis. We have identified a pre-mineralisation chlorite veinlet generation. This generation appears to have been reopened by the quartz-rich polymetallic veins in a syntaxial manner, such that the chlorite rims the polymetallic veins. Both vein types show evidence of Variscan deformation (i.e., buckling, displacement). These new observations are critical as 1) the presence of chlorite may allow for precise geothermometry on the veins and 2) the veins appear to have used the same conduits, which may indicate important physicochemical variations (e.g., T, P, pH, fO2, etc.) and/or a pivotal switch in the fluid source(s).

How to cite: Bonin, F.-X., Meere, P., and Unitt, R.: Paragenesis of the Munster Basin Upper Devonian polymetallic veins, SW Ireland, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1188, https://doi.org/10.5194/egusphere-egu26-1188, 2026.

The Allihies region on the Beara Peninsula, SW Ireland possesses a mining history for vein-hosted Cu sulphide mineralisation. Structural and chronological control of the deposit has been studied extensively (Fletcher, 1969; Lang et al., 2020; Reilly, 1986; Sheridan, 1964). However, the spatial distribution of fluid alteration in the host rock and associated mineralogy remain unstudied. Several alteration minerals linked with the sulphide mineralisation have been recorded, such as chlorite, muscovite, siderite, calcite, dolomite, kaolinite, montmorillonite, and goethite (Fletcher, 1969).

Reflectance spectroscopy can be used for identifying alteration minerals. Hunt (1977) showed, due to the different electron and molecular structure of the compounds, most minerals absorb unique amounts of energy upon the incident of electromagnetic radiation, thus the reflected energy show characteristics absorption features in the spectra. Certain mineral groups exhibit unique features in the visible-near (400 – 900 nm) and short-wave infrared (900 – 2500 nm) wavelength ranges (Clark et al., 1990; Hunt, 1977). High-spectral resolution (hyperspectral) imaging (HSI) techniques provide a large amount of spectral information where each pixel contains hundreds of narrow, contiguous wavelength bands (Goetz et al., 1985; Lodhi et al., 2019). This gives the ability to identify wavelength positions of mineral absorptions and their subtle deviations that reveal the compositional variations.

Consequently, HSI can be used for analysing the host-rock alterations around the Mountain Mine, Allihies, which will reveal the spatial patterns. The target sulphide mineralisation/lodes are oriented in E-W and N-S (Reilly, 1986), and systematic sampling from the mineralized vein across the alteration zone  will help determine if the fluid alteration has a recognisable detectable spectral signature. Mineral groups such as chlorites, carbonates, and clays (Clark et al., 1990) possibly be differentiated of the existing propylitic and sericitic alteration phases (Fletcher, 1969) as  one moves away from the veins into the country rock.

The current study will use laboratory HS data from a rock scanner for initial analysis, followed by a HS drone survey for extending the spatial scale. Principal Component Analysis will be used for extracting the relevant spectral information (Burger & Gowen, 2011). Subsequently, Minimum wavelength mapper can be incorporated for further analysis of dominating mineral occurrences (Hecker et al., 2019), by studying unique absorption features and their feature depths, for mapping variations across the samples. Specifically, the wavelength range of 2100 - 2400 nm contains the diagnostic absorption features for phyllosilicates and carbonates that highlight the different alteration stages the region has undergone.

The research model has the potential to be further developed for identifying regions with similar spectral responses with mineral exploration potential.

How to cite: Kulugammana, M., Meere, P. A., and Unitt, R. P.: Characterizing the rock alteration associated with vein-hosted Cu sulphide mineralization using hyperspectral reflectance spectroscopy; A case study from the Allihies region, SW Ireland., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1223, https://doi.org/10.5194/egusphere-egu26-1223, 2026.

How to cite: Vogel, H., Unitt, R., and Meere, P.: Fluid migration, albitization, and metal concentration in the Munster Basin, SW Ireland, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1362, https://doi.org/10.5194/egusphere-egu26-1362, 2026.

The Kettara mining district (Central Jebilet, Morocco) hosts a pyrrhotite-rich massive sulfide lens enclosed within the low-grade metamorphosed Sarhlef volcano-sedimentary sequence and spatially associated with a syntectonic mafic–ultramafic intrusion. The deposit lies within a dextral strike-slip shear zone of the Variscan belt. The main objective of this study is to evaluate the structural role of the Kettara shear zone in the genesis, architecture, and redistribution of the massive sulfide lens, and to determine whether it represents a pre-existing sulfide accumulation subsequently remobilized during ductile deformation or a syntectonic sulfide formation linked to shear-zone activity.

Structural observations reveal an increasing deformation gradient from the volcano-sedimentary wall rocks toward the ore lens, with maximum strain at the ore–host interface. Deformation produced several structural generations: an early S1 foliation with a general N45 orientation associated with anisopachous P1 folds; a penetrative S2 foliation accompanied by tight isoclinal P2 folds; and late chevron P3 folds, observed exclusively within the ore body, which has been tectonically rotated and progressively steepened to a subvertical attitude in direct response to shear-zone deformation. Localized shear corridors exhibit well-developed C/S fabrics, indicating strain partitioning and a strong simple-shear component. These structures acted as preferential pathways for fluid flow, locally accommodating transient porosity through grain-size reduction and recrystallization.

Microscopic studies reveal a mineral paragenesis characterized by two distinct metallogenic stages. The first stage corresponds to a silica- and sulfur-rich fluid, dominated by massive pyrrhotite displaying textures indicative of syn-metamorphic remobilization and recrystallization, accompanied by subordinate pyrite, chalcopyrite, galena, and sphalerite, with chlorite as the main gangue phase. The second stage is characterized by fissuring of pre-existing sulfides and the infiltration of Cu–Zn–Fe-rich fluids, causing disseminated precipitation of pyrrhotite, chalcopyrite, galena, and quartz–carbonates, while reorganizing the minerals under the influence of ductile deformation and the preferential flow of fluids along the structural conduits of the shear zone. Collectively, these stages record the transition from an early Fe-rich massive sulfide accumulation to later fluid-mediated mineral precipitation.

These observations highlight the first-order structural control exerted by the Kettara dextral shear zone on hydrothermal fluid transfer. Although available data do not allow a definitive distinction between metamorphic remobilization of a pre-existing sulfide mineralization and the intervention of magmatic–hydrothermal fluids derived from the syntectonic intrusion, the structural control remains unequivocal. At all scales, the mineralization is strongly guided by the shear-zone architecture, forming anisotropic, high-permeability conduits that control fluid ingress, fluid–rock reactions, and the coupled chemical–mechanical evolution of the deforming rock mass.

Kettara thus represents a natural example of deformation-assisted fluid migration and shear-zone-controlled metallogenesis in an orogenic setting. Complementary petro-structural, geochronological, and isotope geochemistry investigations are needed to constrain the timing, sources, and physico-chemical conditions of the fluids involved.

Keywords: massive sulfides, C/S fabrics, ductile shear zones, fluid flow, remobilization, Kettara.

How to cite: Cisse, D. and Wafik, A.: Deformation-assisted fluid flow and massive sulfide evolution in a ductile shear zone: insights from the Kettara mining district (Central Jebilet, Morocco)., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1754, https://doi.org/10.5194/egusphere-egu26-1754, 2026.

Lithium (Li) isotopes have been widely used as powerful tracers of chemical weathering processes, providing insights into the coupling between climate and silicate weathering. Although Li isotope fractionation does not occur under equilibrium conditions but rather during kinetically controlled mineral dissolution, the relationship between incipient mineral weathering and Li isotope fractionation remains poorly constrained in natural weathering systems, particularly with respect to the direction and magnitude of fractionation. Here, we investigate elemental and Li isotope geochemistry in two types of biotite—oxidized biotite and hydrobiotite (a 1:1 regularly interstratified biotite–vermiculite)—collected from in situ granitoid weathering profiles. Both biotite types exhibit negative correlations between elemental concentrations and depth; however, Li shows the most pronounced depletion. Elemental loss reaches up to ~70% for Li, with more extensive depletion observed in hydrobiotite compared to oxidized biotite, despite the progressive transformation of biotite into secondary phases such as vermiculite and kaolinite. Lithium isotope analyses are currently underway. By integrating elemental geochemistry with Li isotope compositions, we aim to constrain Li isotope behavior during the initial stages of silicate weathering and to quantify potential Li isotope fractionation associated with distinct biotite alteration pathways. These results will provide new constraints on kinetic controls of Li isotope fractionation during incipient weathering and improve the interpretation of Li isotope signatures in natural weathering systems, including glacial and weathering-limited environments.

How to cite: Ryu, J.-S., Park, H., Yang, M., and Jeong, G. Y.: Variation in elemental and Li isotope geochemistry during the weathering of two types of biotite, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1973, https://doi.org/10.5194/egusphere-egu26-1973, 2026.

Shale oil is predominantly stored in nanoscale pores with ultra-low porosity and permeability, where conventional waterflooding commonly delivers poor recovery. While CO₂-enhanced oil recovery (CO₂-EOR) can improve production by inducing oil swelling, reducing viscosity, and promoting desorption, many existing evaluations still rely on bulk-phase properties and thus inadequately capture nano-confinement and mineral-specific surface effects, obscuring quantitative relationships among CO₂ fraction, desorption efficiency, and mobility. In this study, equilibrium and non-equilibrium molecular dynamics simulations are performed to quantify density layering, competitive adsorption, and rheological/slip behavior of shale oil–CO₂ mixtures confined in quartz and kaolinite nanopores. The simulations show that CO₂ preferentially enriches near pore walls, displaces adsorbed oil, and weakens oil–rock interactions, facilitating the release of interfacial hydrocarbons. Compared with bulk behavior, confinement increases apparent viscosity by about two- to threefold, and kaolinite exhibits pronounced boundary resistance manifested as adverse (negative) slip. As the CO₂ fraction increases to ~20–40%, viscosity decreases markedly and interfacial transport improves, shifting the displacement from unstable fingering toward a more coherent piston-like front. Building on these pore-scale insights, a multiscale coupling framework is developed by embedding MD-derived transport and interfacial parameters into reservoir numerical simulations to conduct 3D field-scale forecasts for the Gulong Sag. The resulting recovery factors that account for nano-confinement (~8–20%) better match field behavior, whereas bulk-parameter simulations substantially overestimate performance. Sensitivity analyses further indicate mineral-dependent economically favorable CO₂ windows (>20% for quartz-dominated pores and ~30–40% for kaolinite-rich pores), highlighting the need for differentiated injection strategies; overall, the proposed multiscale approach bridges microscopic interfacial physics and macroscopic development prediction, providing quantitative support for optimizing CO₂-EOR and enhancing CO₂ utilization and storage in unconventional reservoirs.

Keywords: Shale oil; Nano-confinement effects; Molecular dynamics simulations; Unconventional reservoirs

How to cite: Liu, H. and Xue, H.: Effect of CO2 Pre-Extraction on Water Flooding in Nanopores: Insights from Molecular Dynamics Simulations, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2439, https://doi.org/10.5194/egusphere-egu26-2439, 2026.

The presence of melt and associated fluids profoundly weakens the continental crust, promoting strain localization and establishing a close link between migmatites and ductile shear zones. Here we compare four migmatite case studies developed within major crustal-scale shear zones formed in contrasting tectonic settings, from collisional to extensional regimes: the Kinawa migmatite (Brazil), Opatica migmatite (Canada), Saint-Malo migmatite (France), and the Øksfjord Shear Zone (Norway). Our goal is to evaluate the connection between migmatites and shear zones, their impact on shear zone evolution, and the main macro- and microstructural features of migmatites in shear zones. We also examine the extent to which shear zones may serve as conduits for magma transport within the crust.

All migmatites formed at mid- to lower-crustal conditions (4–9 kbar; 650–820 °C) under both fluid-present and fluid-absent regimes. Macro- and microstructural observations reveal that the evolution of melt connectivity and permeability was strongly controlled by shear zone kinematics. In the Kinawa and Opatica examples, preservation of magmatic microstructures indicates that deformation ceased shortly after melt crystallization, suggesting limited post-melting deformation. In contrast, the Saint-Malo and Øksfjord shear zones record pervasive solid-state deformation overprinting magmatic fabrics, implying sustained deformation and continued microstructural reorganization after partial melting.

Across all examples, the spatial association between migmatites and shear zones highlights the role of deformation in enhancing melt segregation, extraction, and transient permeability. However, only some shear zones evolved into efficient pathways for melt migration. These and other case studies from the literature illustrate how ductile shear zones function as dynamic crustal domains in which deformation, partial melting, and fluid transport are tightly coupled, and where porosity and permeability evolve through time in response to changing rheology and strain.

How to cite: Carvalho, B. B. and Sawyer, E. W.: Interplay Between Migmatites and Deep Crustal Shear Zones, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3448, https://doi.org/10.5194/egusphere-egu26-3448, 2026.

Fluid-rock interactions drive critical lithospheric processes and industrial applications including CO₂ storage and geothermal energy extraction. In deep crystalline crust where static permeability is negligible and rocks do not deform, fluids primarily exploit transient pathways created through chemical reactions with minerals in disequilibrium. These reaction-induced pore networks dynamically alter rock permeability, yet their ephemeral nature makes direct characterization challenging.

We present an integrated methodology combining time-resolved synchrotron x-ray microtomographic imaging (4DSµCT) with generative artificial intelligence to quantify reaction-induced porosity evolution. Using 4DSµCT, we captured spatio-temporal pore network dynamics during KBr-KCl replacement, a well-established analogue for interface-coupled dissolution-precipitation processes. Advanced statistical microstructural descriptors and Minkowski functionals revealed intricate coupling between dissolution-precipitation mechanisms, transport regimes, and evolving connectivity governing transient permeability.

To extend insights beyond experimental limitations, particularly for high-temperature systems (>500°C) where direct imaging remains infeasible, we developed Pore-Edit GAN, a StyleGAN2-ADA framework trained on ~29,000 tomographic images. This model generates statistically realistic microstructures while enabling semantic editing of porosity and connectivity. We applied our approach to hydrothermally altered monzonite from the Oslo Rift, where feldspar replacement reactions at ~10 km depth created now-isolated pore networks. By navigating the GAN latent space along learned connectivity directions, we reconstructed plausible transient pore configurations, effectively reversing the porosity isolation that occurred as reactions ceased.

Voxel-based finite element simulations of incompressible Stokes flow through these AI-reconstructed networks yield permeabilities reaching 4.5×10⁻¹⁵ m², a two-order-of-magnitude enhancement upon pore reconnection, consistent with established transient crustal permeability-depth relations. This convergence of synchrotron capabilities, deep generative models, and computational fluid dynamics establishes a quantitative framework for predicting transport properties in reactive geological systems where direct observation remains challenging.

How to cite: Plümper, O., Amiri, H., and Fusseis, F.: Reaction-Induced Porosity During Fluid-Mineral Interaction: From 4D Synchrotron Imaging to AI-Driven Permeability Reconstruction, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3949, https://doi.org/10.5194/egusphere-egu26-3949, 2026.

Fluid flow through ductile shear zones is increasingly recognised as a key control on the localisation, upgrading, and redistribution of hydrothermal ore systems. We investigate how syn-deformational porosity evolves with increasing finite strain in a calcite-rich marble mylonite from the Western Mary Kathleen shear zone adjacent to the Mary Kathleen REE-U deposit (NW Queensland, Australia). Microstructural evolution and pore-network topology are tracked along a natural strain gradient using electron backscatter diffraction (EBSD) and synchrotron micro-computed tomography (3-D micro-CT). EBSD reveals a progressive transition from twin-rich, dislocation-dominated calcite fabrics at lower strain to uniformly fine-grained, foam-like mosaics at higher strain, where grain-size-sensitive deformation (diffusion creep and grain-boundary sliding) dominates and crystallographic preferred orientations weaken. In lower-strain mylonites, pores occur mainly as isolated to weakly connected cavities along subgrain and grain boundaries, concentrated at boundary junctions and locally associated with twin lamellae. With increasing strain and grain-size reduction, porosity reorganises into fewer but larger, high-aspect-ratio grain-boundary networks that link into laterally continuous pore sheets. Micro-CT-derived orientations show that the normals to these sheets cluster near the instantaneous shortening direction, indicating that connected pore sheets are commonly oblique to both the S- and C-planes rather than strictly foliation-parallel. These results demonstrate that finite-strain-driven grain-size reduction can generate transient, strongly anisotropic permeability by organising boundary-hosted porosity into interconnected, sheet-like conduits, providing a plausible microstructural mechanism for deformation-controlled fluid focusing and REE-U-bearing fluid redistribution in carbonate shear zones.

How to cite: Olesch-Byrne, A., Finch, M., and Vieira Ribeiro, B.: The evolution of syn-deformational porosity in a marble mylonite over increasing strain: Insights from EBSD and 3-D microcomputed tomography, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4662, https://doi.org/10.5194/egusphere-egu26-4662, 2026.

This study quantifies spatial variations in acoustic seepage intensity offshore southwestern Taiwan and assesses whether margin setting or conduit continuity better explains the observed differences. Seepage variability was characterized using volume backscattering strength (Sv), plume geometry, and subsurface structural features. A total of 21 plumes from 14 seep sites were characterized based on Sv and geometry derived from Simrad EK60/EK80 echosounder data. After applying transmission-loss correction, seepage sites on the passive margin (e.g., Horseshoe Ridge, Pointer Ridge, and Formosa Ridge) exhibit higher Sv and taller plumes than those on the active margin. Integration with multichannel seismic profiles and sediment-core records reveals extensive free gas beneath bottom-simulating reflectors (BSRs) and gas chimneys, indicating sustained fluid migration through persistent conduits. In contrast, relatively weak Sv at the mixed-origin G96 site suggests partial conduit infilling. These observations indicate that although tectonic deformation establishes the first-order structural framework for fluid migration, the continuity and evolutionary state of seep conduits exert the dominant control on seepage intensity. Potential tidal modulation was further evaluated by comparing Sv with the rate of tidal pressure change, but only weak correlations were observed, suggesting that tidal forcing plays a secondary role in controlling seepage variability.

Keywords: seepage intensity, margin setting, conduit continuity, volume backscattering strength (Sv), offshore southwestern Taiwan

How to cite: Ou, C.-Y., Chen, T.-T., Hsu, H.-H., and Wu, Y.-C.: Acoustic Characterization of Fluid Seepage Controlled by Tectonic Structures Offshore Southwestern Taiwan, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6268, https://doi.org/10.5194/egusphere-egu26-6268, 2026.

The Upper Rhine Graben (URG) is the central part of the European Cenozoic Rift System and holds a huge geothermal potential due to a reduced Moho depth and active hot brine convection cells. In addition to that appealing potential, hydrothermal brines of the URG show high Lithium concentrations. Yet, investors-relying deep geothermal energy companies face difficulties to predict fracture network permeability before drilling operations. This problem induces techno-economic risks, which frighten investments and in turn hinder the wide development of deep geothermal energy use. The goal of this work is to provide input data to develop modelling tools to help predict fracture network permeability before drilling operations. We will integrate data from both the French and German side of the URG, which is seldomly done in studies. Here we present a timeline that we’ll use as a data compilation base to reconstruct the URG setup and hydrothermal fracture clogging history. Once identified, the fracture clogging events will be deeper characterized to be implemented in a reconstruction model. A characterization of mineralized fractures all around the URG shoulders will help to complete the list of hydrothermal clogging events identified in the Black Forest (Permian, Jurassic-Cretaceous and Paleogene) and will allow to simulate the rate of precipitation in mineralized fractures and thus their clogging potential.

How to cite: Mazzinghi, L.: A timeline for the reconstruction of Upper Rhine Graben hydrothermal fracture mineralization events  , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7761, https://doi.org/10.5194/egusphere-egu26-7761, 2026.

Geological patterns in space and time are dependent on a number of processes that scale differently depending on whether or not they are linear or non-linear and on the involved constants (rate constants, diffusion constants). In order to predict geological processes and their occurrence in space and time one needs to understand at what spatio-temporal scales they are active. Quite often the slowest process is dominating the time scale of pattern evolution, therefore cross-over points in space and time are of special interest, where the dominance of one processes over another switches. When two processes are competing during the formation of a pattern, the cross-overs are critical points where the behavior of the system changes. Here we are exploring five important processes namely elastic wave propagation, fluid pressure diffusion, temperature diffusion, matter diffusion and reactions. While elastic wave propagation and reactions scale linearly, fluid pressure-, temperature-, and matter-diffusion have non-linear scaling behavior, which can be illustrated best in a log-log diagram of time versus space. In such a diagram the diffusion processes have a steeper slope than the two linear processes. Fluid pressure diffusion is 3 to 4 orders of magnitude faster than temperature diffusion, which itself is 3 orders of magnitude faster than matter diffusion (in a fluid). For example if a reactive fluid enters a fault, in a second the fluid pressure equilibrates on a m-scale, the temperature on a mm-scale and matter on the micro-meter scale. During fault slip that happens due to fluid overpressure, elastic wave propagation and fluid pressure diffusion act at the same time scale on micrometers but then diverge with fluid pressure diffusion equilibrating in seconds on the m-scale while elastic wave propagation reaches km-scale at the same time. We will discuss these scaling relations in details with examples from a variety of geological processes.

How to cite: Koehn, D. and Piazolo, S.: Geological process understanding in space and time, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7824, https://doi.org/10.5194/egusphere-egu26-7824, 2026.

Mid-crustal shear zones localize strain and control the migration of heat and fluids, making them central to understanding metallic ore-forming processes. Mica are widely used to date deformation in shear zones, however their radiometric ages can be difficult to interpret because hydrothermal alteration and strain-induced microstructural defects can promote recrystallization and disrupt isotopic retention. In Archean gold camps, mica ages commonly postdate mineralization events by >100 Myr, raising questions about whether these ages reflect primary mineralization, metamorphic or hydrothermal growth, or post-orogenic remobilization. Robust interpretation of these ages requires direct integration of geochemical and micro- to nanoscale structural analysis. We examined mica from the Sunday Lake Deformation Zone, a regional scale deformation zone controlling gold mineralization at Agnico Eagle Mines Ltd. giant Detour Lake Mine (DLM), in the northwestern Abitibi greenstone belt, Canada. The DLM orogenic gold deposit is characterized by c. 2734-2724 Ma volcanic rocks, comprising ultramafic-dominated lower units and mafic volcanic and volcaniclastic upper units, metamorphosed under greenschist to lower amphibolite facies conditions. Mafic host rocks are intruded by felsic to mafic sills and dikes. The main regional foliation is subvertical and axial-planar to west-trending, shallowly-plunging tight to isoclinal folds, which transposes the intrusive relationships. Gold mineralization occurred at c. 2670-2640 Ma in a syn-orogenic setting. Microstructural analyses were conducted on muscovite from felsic meta-intrusive rocks, collected from drill core, that are comprised of quartz, muscovite ± biotite, plagioclase, K-feldspar, chlorite, garnet, amphibole, carbonates and sulfides. Quartz microstructures record bulging recrystallization and nascent subgrain rotation, indicating deformation temperatures of ~300-400°C. Plagioclase display tapered deformation twins and brittle fracturing, consistent with low to moderate temperature deformation. Mica constitute between <5 and 30 vol% of the rock and occur as euhedral porphyroblasts/neoblasts to subhedral poikilitic, skeletal grains ranging in size from 15 x 50 μm to 250 x 650 μm. High-resolution electron channeling contrast imaging of the mica reveals weak undulatory orientation contrast patterns perpendicular to cleavage planes in ~20-50% of the grains. Such contrast patterns suggest deformation in the mica is accommodated primarily by dislocation glide. Backscatter electron imaging of the mica also revealed concentric chemical zoning, typically expressed as irregular and discontinuous rims along grain margins, which are weakly enriched in Fe and Al and depleted in Mg and Si relative to mica cores. Muscovite Ar-Ar analyses from unmineralized rocks at DLM yield single-crystal dates of 2600-2100 Ma, and complementary K-Ca and Rb-Sr dating will be conducted to assess the roles of Ar loss and element mobility in producing these younger and dispersed ages. The timing of metamorphism and deformation has important implications for understanding the nature and controls on mineralization at DLM, and whether the original geometry and mineralogy of the deposit has been modified through later stages of syn‐metamorphic deformation.

How to cite: Komendat, I., Schneider, D., and Dubosq, R.: Wrinkled clocks in the crust: dating deformation in Archean gold-bearing shear zones, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8033, https://doi.org/10.5194/egusphere-egu26-8033, 2026.

In the past decades, the extent of fluid-induced reaction halos in metabasaltic sills within the Argyll Group of the Dalradian Supergroup in the SW Scottish Highlands has been intensively used to constrain metamorphic fluid flow velocities (Skelton, 2011). However, recent findings revealed that reaction front propagation within numerous sills was primarily controlled by preferred fabric alignment at the margins during deformation events, as well as by mineralogical and chemical heterogeneities across the sills. Here, we revisit hydration and carbonation fronts in metabasaltic sills in the vicinity of major fluid pathways, i.e., the Loch Awe Syncline and Ardrishaig Anticline, to reevaluate fluid-induced reaction front propagation and constrain metamorphic fluid flow velocities.
This study integrates field observations, detailed petrological-textural analyses, and whole-rock geochemistry, including carbon and water contents as well as trace element data, along a transect across compositionally homogeneous metabasaltic sills. The aim is to constrain the mechanisms controlling fluid-induced reaction progress at the contact between metasedimentary rocks and metabasaltic sills.
The selected basaltic sills were metamorphosed under greenschist- and epidote-amphibolite-facies conditions and record at least four deformation events. In the sill margins, the rocks show increased calcite and chlorite contents and replacement of garnet, amphibole, and dark mica, reflecting localized retrogression. This retrograde overprint is also characterized by mobilization of large-ion lithophile elements (LILE; e.g., K, Na, Sr). In contrast, the sill interior preserves textural and mineralogical equilibrium among amphibole, dark mica, epidote, garnet, and titanite. Textural variations indicate a progressive decrease in hydration and carbonation toward the sill interior.
Carbon contents decrease systematically from 1.22-1.16 wt.% in the sill margins to 0.07-0.02 wt.% toward the sill interior. Similarly, water contents are highest in the sill margins (up to 1.95 wt.%) and lowest in the sill interior (0.52 wt.%). Petrographic observations further suggest that fluid infiltration and reaction are controlled by structural anisotropy inherited from earlier deformation during retrogression, rather than mineralogical heterogeneity. Fluid flow is preferentially channelized along lithological contacts and deformation-related weaknesses, such as foliation and mineral lineation, which are most developed near the sill margins.
The new dataset enables a recalculation of the true spatial extent of metamorphic fluid infiltration and allows time-integrated estimates of fluid fluxes based on carbonation and hydration reaction front geometries, as well as their relationships with trace element redistribution. Understanding the rates of CO₂ release and sequestration during orogenic processes provides new insights into the role of structural anisotropies and brittle-ductile processes in controlling the volume, pathways, and metal-enriching potential of metamorphic fluid flow.

How to cite: González-Ixta, Y., Kleine-Marshall, B., Skelton, A., and Koehn, D.: Carbon gradients as tracers of structurally controlled fluid flow in homogeneous metabasaltic sills (Loch Stornoway, Scottish Highlands)., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8377, https://doi.org/10.5194/egusphere-egu26-8377, 2026.

Abstract: The Beishan Orogenic Belt, located along the southern margin of the Central Asian Orogenic Belt, is one of the key mineral resource regions in northwestern China. The Hongjianbing fluorite deposit, located in the northern part of this belt, is a well-known quartz-vein-type fluorite deposit that has attracted considerable attention from researchers. Through field geological mapping, UAV and remote sensing measurements, and borehole structural recording, two east–west-trending strike-slip fault systems were identified in the study area, separated by a distance of 5 km, with nearly vertical dips. These faults exhibit multi-stage activity, with early deformation characterized by dextral strike-slip motion. The intervening blocks experienced ductile deformation, with S-C fabric development in the shear zones.39Ar-40Ar dating of biotite from the mylonite in the ductile shear zone yielded a plateau age of approximately 330 Ma, marking the timing of ductile deformation. Later, these two faults evolved into brittle left-lateral strike-slip faults, forming a Riedel shear system (R, R', T shears), and displaying fault breccia and fault gouge. K–Ar dating of authigenic illite from the fault gouge yielded an age of approximately 220 Ma, indicating a transition from ductile to brittle deformation over time.The host rocks of the fluorite deposit are mainly intermediate to acidic volcanic rocks from the Carboniferous Baishan Formation. Zircon U–Pb dating of these rocks yielded ages of approximately 330 Ma, suggesting that the host rocks may have formed during contemporaneous magmatic activity. All fluorite orebodies are located within the main damage zone of the southern brittle fault system, which exhibits left-lateral, right-stepping characteristics. The development of this brittle fault system provided the necessary space and conduits for ore-forming fluid migration, facilitating fluorite mineralization.The cataclastic texture of the ores further suggests that mineralization occurred after significant faulting, reflecting high fluid mobility within the fault damage zones. This high fluid mobility is also reflected in the characteristics of the fluorite ore bodies. After migrating along the brittle fault zones, mineralizing fluids precipitated fluorite as fine veinlets, which is the current form of mineralization observed in the deposit. A three-dimensional geological model of the ore body was constructed using GOCAD software, revealing the close temporal and spatial relationship between fluorite mineralization and fault activity.
Keywords: Structural control of mineralization; Fluorite deposit; Geochronology; Three-dimensional mineralization model;

How to cite: Bo, C., Wang, G., Zhang, P., and Zhang, J.: Strike-slip fault system and fluorite mineralization in the Hongjianbing area, Mazong Mountain, Gansu China, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9560, https://doi.org/10.5194/egusphere-egu26-9560, 2026.

Understanding how solid-state deformation and fluid flow interact is essential to constrain continental lithosphere evolution. The North-Pyrenean Zone, located in the Pyrenean retro-wedge, corresponds to an inverted Early Cretaceous rift that led to mantle exhumation. It comprises Mesozoic basins and Variscan basement massifs. Key rifting markers include: (i) thick Albian-Cenomanian detrital sequences, (ii) peridotite bodies reworked into pre-/syn-rift sediments, (iii) HT–LP metamorphic paragenesis in pre-/syn-rift series, and (iv) giant metasomatic stocks comprising talcschist and albitite. Although geochronological data show that metasomatic bodies are related to large-scale fluid circulation during Early Cretaceous rifting (130–90 Ma), the associated 3D–4D fluid circulation system remains poorly constrained.

Our study focuses on the eastern part of the Arize North-Pyrenean Massif, a syn-rift tilted block exposing, beneath a pre- and syn-rift halokinetic sedimentary cover, a complete Variscan metamorphic series from migmatites to the South, to low-grade Carboniferous pelites to the North. While foliation trajectories are homogeneously N100°-oriented across the western and central parts of the Arize massif, its eastern part is distinguished by a heterogeneous foliation pattern within a N140°E-oriented, transtensional folded and faulted zone. A pervasive metasomatic zone is developed within a 10 km² elliptical domain at the core of this structural system. It encompasses pure albitite stocks and results from a two-stage alteration process. The least metasomatized samples show minor plagioclase alteration and biotite destabilization, with newly crystallized titanite, apatite and epidote. Whole-rock data reveal a strong Ca-enrichment mainly hosted in the newly formed Ca-rich mineral assemblages. The most metasomatized samples exhibit quartz leaching and albitization of plagioclase associated to relatively limited net chemical change. Fluid inclusions trapped in metasomatized apatite contain H₂O–NaCl–CaCl₂ brines (≈ 16 wt.% NaCl eq.), recording trapping conditions of ~300 °C and ~205 MPa. Quartz generally shows no evidence of crystal-plastic deformation, despite its plasticity temperature being close to the inferred fluid trapping temperatures. Locally, a structural transition, marked by late low-angle normal faults associated with C-S structures and, in places, mylonites, documents increasing system temperature. This thermal increase is interpreted as resulting from progressive heating of the surrounding rocks by circulating fluids.

We interpret the Arize fossile hydrothermal fluid system as a transient reservoir of ascending hot fluids located above a transient brittle–ductile transition. In such a system, downwelling fluids are stored at the brittle–ductile transition, where subhorizontal anisotropy planes act as impermeable barriers. Locally, these fluids, heated to 300–400 °C, ascend into the upper crust along vertical anisotropy planes (tilted Variscan foliations) and brittle structures (faults). U–Pb dating of newly formed titanites at ca. 130 Ma supports the interpretation that metasomatism in the Arize upflow system occurred during the early stages of rifting. Finally, we demonstrate that, at the scale of the Pyrenean rift, the Arize hydrothermal system, located within an oblique transtensional zone, developed in a syn-rift linkage zone between N100°E-oriented rift segments. This integrated study underscores the role of fluid flow in linkage zones associated with continental crust stretching, with implications for hydrothermal and geothermal systems.

How to cite: Jansen, C., Denèle, Y., Estrade, G., Laurent, O., Leisen, M., and de Saint-Blanquat, M.: Fluid–rock interactions in a crustal-scale upflow system: syn-rift albitization of the North-Pyrenean Massifs, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10973, https://doi.org/10.5194/egusphere-egu26-10973, 2026.

Foreland basins often host important ore deposits (like MVT deposits; Bradley and Leach, 2003) which are associated with deep and shallow fluid circulation. Those fluids, expelled from the orogen, can have different origins like meteoric water, diagenetic fluid, metamorphic fluid or even deeper crustal origins (Oliver J., 1986). However, whether these fluids are expelled to the foreland as continuous flow or as series of rapid pulses remains largely unexplored. Here, we combine numerical modelling with geochemical data and petrographic observation of a sandstone and its associated veins and reaction halos to identify spatial and temporal fluid flow and its transport mechanism(s) in foreland basins.

Thin sections from a Rotliegend red arkose-sandstone formation (German Permian Variscan foreland) were investigated using microprobe analysis and BSE-EDS-SEM imaging. The arkose-sandstone exhibited tapering lighter reaction halos around veinlets, most likely produced through redox reactions upon fluid infiltration into the sandstone. The model Elle from Koehn et al., (2022) was subsequently applied to link fluid transport mechanism to the patterns and geometry observed in the samples. Pore pressure applied from a crack toward the host rock and a concentration gradient were used to create fluid flow in the sandstone from which a range of values for advection, diffusion and a reaction rate were deduced. In this way, the model allowed to mimic the same pattern/geometry as the sample on several scales and enabled a systematic assessment whether fluid flow may have been constant or pulsating.

Combining petrographic, geochemical and modelling investigations revealed that the reaction halos in the sandstone were in fact formed upon a single rapid fluid flow event, that presumably was fast and channelised in the vein, and pervasive and comparatively slow in the surrounding host rock. These preliminary results imply that fluid flow and transport in foreland basins may be of a more pulsating nature rather than continuous steady state fluid flow and transport mechanisms may thus be similar to what has been previously reported for subduction zone settings (e.g., Kleine et al., 2016).

Bradley, D. C., & Leach, D. L. (2003). Tectonic controls of Mississippi Valley-type lead–zinc mineralization in orogenic forelands. Mineralium deposita, 38(6), 652-667.

Kleine, B. I., Zhao, Z., & Skelton, A. D. (2016). Rapid fluid flow along fractures at greenschist facies conditions on Syros, Greece. American Journal of Science, 316(2), 169-201.

Koehn, D., Kelka, U., Toussaint, R., Siegel, C., Mullen, G., Boyce, A., & Piazolo, S. (2022). Outcrop scale mixing enhanced by permeability variations: the role of stationary and travelling waves of high saturation indices. Geological Magazine, 159(11-12), 2279-2292.

Oliver, J. (1986). Fluids expelled tectonically from orogenic belts: their role in hydrocarbon migration and other geologic phenomena. Geology, 14(2), 99-102.

How to cite: Lebrun, L., Kleine-Marshall, B., and Koehn, D.: Fluid flow in foreland basins: spatial and temporal scaling of their transport mechanisms, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11377, https://doi.org/10.5194/egusphere-egu26-11377, 2026.

Mobile shales strongly influence deformation, uplift, and fluid migration in compressional sedimentary basins, yet the mechanical pathway from "normal" shale to mobile shale is still debated. This study tests the idea that shear-induced dilation under high pore-fluid overpressure can trigger a positive feedback among shear localization, porosity–permeability increase, and fluid flow, thereby promoting long-lived, ductile-like shale mobility. We focus on the Zhongliao Tunnel area in southwestern Taiwan, where rapid uplift and sharp spatial gradients in vertical motion have been reported near major faults.

We develop a two-phase hydro-mechanical numerical model that couples a poro–visco–elasto–plastic solid with Darcy fluid flow. Porosity evolves through competing compaction and a strain-rate–dependent dilation term that is activated under elevated overpressure, allowing fault-related shear zones to dynamically transform into high-permeability conduits. In the reference experiment, a high-pressure layer sealed beneath a low-permeability cap sustains overpressure within mudstone. Once shear localizes, dilation increases porosity and permeability along damage zones, enhancing focused fluid discharge. The resulting seepage forces and reduced effective strength further intensify shear localization, producing sustained fault creep and pronounced uplift of the block bounded by the principal fault systems. The modeled uplift pattern reproduces key first-order observations: a sharp vertical-velocity contrast across the main fault and a more gradual decay of uplift away from it, with peak uplift rates reaching the order of centimeters per year.

Sensitivity tests demonstrate that overpressure alone generates only modest uplift without dilation-enabled conduit formation, while shear compaction suppresses localization and distributes deformation. Permeability exerts a non-monotonic control: very low permeability limits fluid flux and seepage forcing, whereas very high permeability drains overpressure too efficiently and weakens sustained creep. Overall, the results provide a mechanistic framework for how overpressured mudstone can evolve into mobile shale through coupled dilation and fluid flow, and offer testable criteria for identifying similar processes in other shale-dominated orogenic settings.

How to cite: Tan, E., Lin, C.-H., Wang, W.-H., Le Beon, M., and Gerya, T.: Hydro-Mechanical Modeling of Over-Pressured Mobile Shale: Insights into Shear Dilation Effects on the Uplift at Zhong Liao Tunnel, Taiwan, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11563, https://doi.org/10.5194/egusphere-egu26-11563, 2026.

Numerical study of phenomena in granular media is typically classified into two categories according to the scale of the domain: macro-scale modeling which relies primarily on continuum theories, such as the Finite Element Method (FEM), and micro-scale modeling, which is based on interparticle forces, commonly performed using the Discrete Element Method (DEM).

However, granular/porous media are inherently discontinuous due to their micro-structure, and traditional continuum-based approaches cannot accurately capture microstructure-induced anisotropy in these media. Particle-based approaches like DEM have therefore been widely used for the modeling of such discontinuous domains.

On the other hand, it is not computationally feasible to resolve the entire intricate microstructure of large domains using DEM. Thus, this work implements a multi-scale approach that combines the accuracy of DEM at the grain scale with the computational efficiency of FEM at the macro-scale.

The approach is called “Hierarchical FEM-DEM”, originally developed to study the mechanical response and strain localization (shear bands) in granular media [1]. It then has been extended to hydro-mechanical problems in saturated media [2]. In this framework DEM assemblies are treated as Representative Volume Elements (RVE) attached to Gauss (integration) points of a macroscopic FEM mesh. The DEM is used for the calculation of the homogenized effective stress corresponding to the interpolated strain field on each Gauss point, thereby eliminating the need for phenomenological constitutive assumptions for the solid skeleton, that are common in conventional nonlinear FEM analyses.

In this study we apply this method by implementing it in MATLAB to investigate the hydro-chemo-mechanics of granular media. The model is going to be used to study the effect of fluid flow and pore pressure on the solid skeleton deformation, and generation of shear bands, how micro-scale solute-related heterogeneities influence the macro-scale mechanical behavior, based on thin sections made from field sample collected by the authors.

We use periodic boundary conditions for DEM assemblies to satisfy the compatibility between the microscopic deformation and the macroscopic strain field imposed at the Gauss points, ensuring the condition for the satisfaction of Hill-Mandel micro-macro energy equivalence during homogenization. At the macroscopic level, the boundary conditions are prescribed to simulate the in-situ loading and hydraulic conditions, corresponding to the field sites from which the samples were extracted.

References

[1] Guo, N. and Zhao, J., 2014. A coupled FEM/DEM approach for hierarchical multiscale modelling of granular media. International Journal for Numerical Methods in Engineering, 99(11), pp.789-818.

[2] Guo, N. and Zhao, J., 2016. Parallel hierarchical multiscale modelling of hydro-mechanical problems for saturated granular soils. Computer Methods in Applied Mechanics and Engineering, 305, pp.37-61.

How to cite: Ahmadi Olyaei, E. and Koehn, D.: A multi-scale hierarchical FEM-DEM approach for hydro-chemo-mechanical modeling of granular media, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12850, https://doi.org/10.5194/egusphere-egu26-12850, 2026.

The particle migration phenomenon in porous media exhibits dual effects: it can both impede fluid flow and regulate the flow field. The temporary plugging effect induced by particle migration can delay the formation of preferential flow paths in waterflooding and enhance recovery efficiency. However, research on actively controlling the flow field through particle migration to improve recovery efficiency is still limited. This study aims to investigate the generation of a temporary plugging effect within the pores by controlling the particle size and concentration in the injected water, thereby regulating the distribution of the flow field and enhancing oil recovery. The research combines numerical simulation techniques with core flooding experiments, constructing numerical models with different micro-pore structure characteristics, such as moldic pores and intrafossil pores, and physical models with varying permeability gradients. Experimental results show that after the formation of preferential flow paths in waterflooding, continued water injection can no longer effectively displace the remaining oil in the porous media. At this point, the addition of suspended particles (median particle size: 5 μm, concentration: 200 mg L⁻¹) to the injected water further enhances displacement. The particles migrate with the water flow and preferentially accumulate in high-connectivity pores and throats, forming a temporary plugging effect. This alters the local flow path, expanding the sweep volume of waterflooding and effectively mobilizing oil in low-permeability pores.

When the particle size exceeds 10 μm or the concentration exceeds 400 mg L⁻¹, bridging or sealing effects are likely to occur at pore entrances, severely obstructing fluid flow. Conversely, when the particle size is too small (<2 μm) or the concentration too low (<10 mg L⁻¹), the particles fail to effectively retain and do not form a significant temporary plugging effect. After the particle-based flow regulation treatment, the final oil recovery efficiency of the model can be increased by approximately 10% to 20%.

How to cite: Penglei, Y.: Regulation of Fluid Flow Behavior in Porous Media Based on Particle Migration, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13057, https://doi.org/10.5194/egusphere-egu26-13057, 2026.

Dolomitization is among the most widespread processes affecting carbonate rocks and may significantly overprint carbonate successions during post-diagenetic and deformation-related fluid infilling and circulation. The process is generally hindered in low-porosity/low-permeability carbonates (e.g., micritic limestones). However, primary (e.g., bedding interfaces) and secondary (e.g., fractures) rock planar anisotropies might compensate for this low porosity/permeability, acting as potential pathways for fluid ingress and as loci for the initiation of fluid-rock interaction. Among these anisotropies, burial stylolites are particularly widespread in carbonate successions, forming through progressive chemo-mechanical dissolution-precipitation over time. Their large lateral continuity (>1 km) and potentially high vertical frequency make stylolites key features in governing the syn-to-post diagenetic evolution of sedimentary successions.

Although stylolites have traditionally been considered fluid barriers, recent studies challenge this paradigm, a view that we further stress here. We present petrophysical data from micritic limestones of the Lessini Mountains (Italian Southern Alps), where a large portion of the exposed carbonate Jurassic-Cretaceous succession (>700 m thick) is almost entirely overprinted by a regional dolomitization event, which produced large volumes of massive, sandy, crystalline dolostones. We studied preserved patches of micritic limestone where the progression of dolomitization from initiation to complete overprint is clearly visible. Hg-porosimetry, SEM imaging, μ-CT and cathodoluminescence were combined to constrain petrophysical variations associated with dolomitization. Results show that burial stylolites (Hg injection capillary threshold pressure – HgP c. 4 Psi) affecting the micritic limestones (HgP c. 5140 Psi) were systematically exploited by the dolomitizing Mg-rich fluids, transiently aided by fluid overpressure surges, which locally induced brecciation and further enhanced fluid-rock interaction.

Progressive dolomitization increased rock porosity and density from ~1% to ~20% and from 2.65 g/cm³ to 2.9 g/cm³, respectively (from micritic limestone to massive dolostone). Pore characteristics (pore-size, sphericity, 3D-φ-angle and 3D-Eulerian characteristic - all constrained by μ-CT data and Hg-μporosimetry) indicate a complex evolution characterised by (i) early diffuse dolomitization followed by (ii) localised dedolomitization triggered by the later ingress along the porous stylolites of a Mg-poor fluid, which selectively infiltrated the dolomitized succession and created significant rock porosity. Dedolomitization appears to have been more efficient (and is better preserved) along the dolomitized stylolites than within the massive dolostones, where fluid-rock interaction was inhibited by the larger rock volume.

Spatio-temporal porosity variations related to dolomitization and dedolomitization, guided by- and preserved within stylolites, have significant implications for (i) reservoir quality evaluation, and (ii) the mechanical behaviour of carbonate rock masses during post-fluid-infiltration deformation phases. In these settings, dolomitization and dedolomitization promote long-term fluid ingress and circulation, thus even modulating further deformation localisation.

How to cite: Zuccari, C., Vignaroli, G., Balsamo, F., Berio, L., Buono, G., Pappalardo, L., and Viola, G.: Stylolite-controlled dolomitization and dedolomitization in low-porosity carbonates (Lessini Mountains, Southern Alps, Italy), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14061, https://doi.org/10.5194/egusphere-egu26-14061, 2026.

The circulation of magmatic-hydrothermal fluids along crustal-scale fault systems plays a fundamental role in the formation of porphyry-type ore deposits, as these structures control magma emplacement, fluid pathways, and associated rock alteration. In the Rhodope magmatic-metallogenic belt of northern Greece, numerous Oligocene-Miocene porphyry-type ore deposits formed in an extensional back-arc environment. One example is the Maronia Cu-Mo±Re±Au porphyry deposit in the Mesozoic Circum-Rhodope metamorphic belt, where plutonic intrusion occurred during detachment fault activation. Despite this, the detailed sequence and timing of magmatic-hydrothermal fluid circulation related to ore formation remain poorly constrained.

In this study, we aim to unravel the relationship between magmatic and tectonic events to decipher the mechanisms of magmatic-hydrothermal fluid circulation along detachment faults associated with ore formation processes. A total of 20 rock samples were collected across the exposed detachment fault zone at Maronia, ranging from unaltered monzonite to the porphyry microgranite intrusion. We combined highly sensitive Anisotropy of Magnetic Susceptibility and Remanence (AMS and ARM) petrofabric tools with geochemical analyses (e.g., EPMA, LA-ICP-MS/MS). Petrofabric analyses identified multiple magnetic fabrics within individual samples, providing insights into magmatic intrusion emplacement, deformation, and fluid flow, as well as into whether magmatic and tectonic processes occurred concurrently or successively.

Preliminary geochemical and magnetic analyses of minerals and whole rocks constrain the genetic relationship between microgranite intruding the mylonitic rocks within the detachment fault. Petrofabric data are coaxial with observed field fabrics, whereas preliminary ARM results indicate that higher coercivity mineral phases deviate from both field observations and AMS results. Petrographic observations reveal the nature of mineralization and allow evaluation of textural changes related to fluid-rock interaction. We suggest that the combined dataset reflects a strain archive of multi-stage tectonomagnetic processes that drove fluid flow and possibly mineralisation in this sector of the Circum-Rhodope belt.

This study further demonstrates the potential of integrating petrofabric data with geochemical methods to better resolve fluid flow processes and tectono-magmatic evolution in detachment-controlled porphyry systems, providing new insights into the structural controls on mineralization at Maronia.

How to cite: Toivanen, E., McCarthy, W., Koehn, D., and Kleine-Marshall, B.: Tectono-magmatic controls on fluid flow in a detachment-related porphyry system: Insights from magnetic petrofabric analyses at the Maronia deposit, NE Greece, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14523, https://doi.org/10.5194/egusphere-egu26-14523, 2026.

The aims of this study is to understand how concurrent fluids and deformation influence the behaviour of the Rb/Sr geochronometer in micas. This study is conducted within a classical deformation framework, shear zones, key structures in the geodynamic evolution of rifts and orogens which can accommodate kilometer-scale displacements and facilitate significant mass transfer. While the last two decades have led to a better understanding of the processes of nucleation and widening of these structures, as illustrated by the world-famous case of Cap de Creus, the extreme localization of deformation they exhibit raises fundamental questions about the rheological evolution of the lithosphere.

By conducting a multiscale petro-tecto-geochronological study of the Cap de Creus shear zones, whose age is debated, we have underlined that these shear zones record a major event of fluid-induced strain softening during the formation of the Pyrenean prism. This event is characterized by concomitant muscovite blast neocrystallization and intense quartz dynamic recovery, whose contemporaneity can be uniquely demonstrated by evidence of Fe-rich surface-derived fluids that penetrated to depth. This event occurred between 60 and 50 Ma, as shown by in situ and in-context Rb/Sr dating of two shear zones. This study highlights the critical role of fluid-induced rheological softening in ductile reactivation of polycyclic basements and provides a context-dependent framework for interpreting the behaviour of the Rb/Sr geochronometer in muscovite during deformation.

The implications of these results for the three-dimensional formation and evolution of the Pyrenean orogenic prism, will be discussed, including the role of structural inheritance, fluid circulation, and their contribution to shear-zone reactivation processes.

How to cite: Denele, Y., Nicolas, C., Bosse, V., Merle, O., Gardés, E., and Lotout, C.: Fluid-induced strain softening during the formation of the Pyrenean orogenic prism: In situ Rb/Sr dating from the Cap de Creus shear zones (Eastern Pyrenees, Spain), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17657, https://doi.org/10.5194/egusphere-egu26-17657, 2026.

This study investigates the coupled roles of solid-state deformation, hydrothermal fluid flow, and critical metal enrichment mineralization. Integrating structural analysis, mineral microtextures, EBSD, fluid inclusions, and H-O isotopes, we show that the deposit experienced Neoproterozoic Nb pre-enrichment in alkaline volcanic rocks, later overprinted by Early Paleozoic tectonic-hydrothermal events. Ductile shear zones that characterized by foliation and lineatio enhanced permeability and channeled F⁻-Cl⁻-CO₂-rich fluids from deep sources, with fluid inclusion planes confirming foliation-parallel migration. Syn-tectonic breakdown of amphibole, pyroxene, and titanite released Nb, Y, and REEs into solution as soluble complexes.

Fluid evolution—driven by alkali consumption and CO₂ influx, lowered pH and increased Nb solubility. Nb enrichment occurred via water-rock interaction within shear zones, though complex stability initially inhibited precipitation. Localized Nb deposition took place at altered mineral margins, where Fe²⁺ spikes destabilized complexes. Ore bodies formed in brittle–ductile to brittle domains, governed by the interplay of deformation-controlled permeability and chemical feedbacks.

Strain regime shifts further enhanced permeability, enabling mixing between deep NbF₆⁻- YF₆³⁻-CO₂-rich fluids and external alkaline magmatic or Ca-rich fluids. This mixing triggered pH and redox changes that destabilized metal complexes, precipitating Nb minerals as magnetite/ilmenite-hosted inclusions or microveins, which direct evidence of strain and fluid pulsation. Fault-valve cycling induced fluid immiscibility and boiling, further disrupting complex stability. Our findings underscore that tectonically driven fluid migration is fundamental to Nb enrichment, providing a structural framework for exploring orogenic rare-metal deposits.

How to cite: Cao, S., Li, X., Zhan, L., Zhou, D., Liu, J., Cheng, X., Tao, L., Guo, J., and Hu, Z.: Tectonics and Fluid Coupling in Critical Metal Enrichment, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17856, https://doi.org/10.5194/egusphere-egu26-17856, 2026.

Anisotropy of Magnetic Susceptibility (AMS) and Anisotropy of Magnetic Remanence (AMR) are critical petrofabric tools commonly applied in investigating the evolution of volcano-magmatic, tectonic, and surface process systems. These highly sensitive techniques can distinguish multiple magnetic fabrics within individual samples, shown to be crucial in assessing archives of emplacement and deformation in intrusions where magmatic and tectonic processes occur concurrently or successively. They have also been used to understand magmatic processes within layered igneous complexes associated with the concentration of economic mineral phases. However, the application of AMS and AMR is hindered by the mineral phases that dominate magnetic properties and their susceptibility to hydrothermal alteration, potentially overprinting pre-existing petrofabrics. Despite the impacts of hydrothermal alteration being a well-known occurrence, the mechanisms and extent to which magnetic fabrics can be modified remains poorly constrained, raising concerns about the reliability of interpretations in studies involving hydrothermally altered rocks.

Our recent work assesses the significance of magnetic fabrics preserved in a hydrothermally altered fault zone that crosscuts a granitic pluton. Data were collected from unaltered granodiorite peripheral to the fault, the fault damage zone and the fault core to assess the impact of hydrothermal alteration on magnetic fabrics associated with magmatic and tectonic processes. Magnetic and hyperspectral data were used to characterise alteration distribution and intensity by quantifying changes in hydrous silicate and iron oxide phases. AMS and AMR fabrics were then measured and interpreted as either magma transport, tectonic, or hydrothermal alteration fabrics with context from field and petrographic data.

Our integrated hyperspectral-magnetic approach defines three alteration zones. Onset of hydrothermal alteration is identified from a subtle removal of white mica and low coercivity iron oxides (titanomagnetite) and the growth of new, high coercivity iron oxides (hematite) alongside chlorite and epidote. As alteration intensity increases, titanomagnetite and white mica are removed entirely, with hematite, epidote and chlorite becoming dominant in the system. In step with the changes in oxide and hydrous silicate mineralogy, we observe changes to AMS and AMR fabrics, with partial to complete destruction of tectonic and magmatic fabrics observed with increased alteration intensity. As these precursor fabrics are destroyed, they are replaced by a sub-vertical petrofabric defined by the alignment of hematite, interpreted as a product of hydrothermal fluid transport.

We demonstrate a threshold to alteration intensity, above which precursor petrofabrics are obliterated and replaced by fabrics associated with hydrothermal alteration. We envisage these results being highly informative in studies seeking to examine tectonic and mineralisation processes using rock magnetic methods.

How to cite: Latimer, B., McCarthy, W., Mattsson, T., and Reavy, J.: Investigating the Significance of Magnetic Fabrics Preserved in Hydrothermally Altered Rocks, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18788, https://doi.org/10.5194/egusphere-egu26-18788, 2026.

Rocks in the upper crust are generally subjected to true triaxial stress conditions, in which all three principal stresses are unequal (σ₁ > σ₂ > σ₃). Pore and fracture networks respond to anisotropic loading by opening in certain directions while closing in others, potentially inducing strong permeability anisotropy. However, most experimental constraints on stress-dependent permeability are derived from conventional triaxial tests, where two principal stresses are equal and permeability is measured in only one direction.

Here, we use a true triaxial apparatus equipped with a pore-fluid system to measure permeability parallel to all three principal stress axes in cubic samples of Etna basalt (EB) and Crab Orchard sandstone (COS) subjected to anisotropic loading.

For an initially isotropic EB sample, increasing stress along a single axis results in a sharp permeability decrease in the corresponding maximum stress direction, reaching up to two orders of magnitude (from ~10⁻¹⁶ to ~10⁻¹⁸ m²) at a differential stress (σ₁ − σ₃) of 215 MPa. In contrast, permeability parallel to σ₂ decreases mildly when stresses are increased up to ~75 MPa while permeability parallel to σ₃ remains largely unchanged. During unloading, permeability parallel to σ₁ recovers by approximately 1.5 orders of magnitude once σ₁ is reduced to 75 MPa.

Similarly, samples of the initially anisotropic COS also experience a decrease in permeability of two orders of magnitude (from ~10⁻¹⁷ to ~10⁻¹⁹ m²) along the maximum compressive stress at (σ₁ − σ₃) of just 100 MPa. Permeability along σ₁ recovers only partially after unloading, up to 10⁻¹⁸ m², indicating that some permanent compaction occurred along the maximum compression.

These results demonstrate that true triaxial stress conditions can induce pronounced permeability anisotropy through directional closure of pores and microcracks, with important implications for fluid transport in the upper crust, including fault zones, geothermal systems, and stressed reservoirs.

How to cite: Meredith, P., Stanton-Yonge, A., Mitchell, T., Browning, J., and Healy, D.: Permeability anisotropy under true triaxial stress states: strong flow reduction parallel to the maximum principal stress., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20074, https://doi.org/10.5194/egusphere-egu26-20074, 2026.

Graphite formation in deep crust during granulite facies metamorphism is documented in the Proterozoic gneisses of the Lofoten-Vesterålen Complex, northern Norway. Regionally distributed graphite zones are hosted in banded gneisses dominated by orthopyroxene-bearing quartzofeldspathic gneiss, including marble, calcsilicate rocks and amphibolite. The schist has major graphite, quartz, plagioclase, pyroxenes, biotite (Mg# = 0.67-0.91; Ti < 0.66 a.p.f.u.) and K-feldspar/perthite. Pyroxene is orthopyroxene (En_69-74) and/or clinopyroxene (En_33-53Fs_1-14Wo_44-53). Although graphite is usually described in pelitic rocks or as vein deposits in the granulite facies rocks, we document graphite in assemblage with metamorphic orthopyroxene.

Phase diagram modelling (plagioclase + orthopyroxene (Mg#-ratio = 0.74) + biotite + quartz + rutile + ilmenite + graphite-assemblage) constrains pressure-temperature conditions of 810-835 °C and 0.73-0.77 GPa; Zr-in-rutile thermometry 726-854°C. COH-fluids stabilise graphite and orthopyroxene; high Mg#-ratio of biotite and pyroxenes, and apatite Cl < 2 a.p.f.u. indicate importance of fluids during metamorphism.

Stable isotopic δ¹³C_graphite in the graphite schist is -38 to -17‰; δ¹³C_calcite of marbles +3‰ to +10‰. Samples with both graphite and calcite present give lighter values for δ¹³C_calcite = -8.7‰ to -9.5‰ and heavier values for δ¹³C_graphite = -11.5‰ to -8.9‰. δ¹⁸O_calcite for marble shows lighter values ranging -15.4‰ to -7.5‰ (Engvik et al. 2023).  We interpret the graphite origin as organic carbon accumulated in sediments contemporaneous with the Early Proterozoic global Lomagundi-Jatuli isotopic excursion, while an isotopic exchange between graphite and calcite reflects metamorphic and hydrothermal re-equilibration.

The high-ordered graphite (< modality 39%) and biotite with a strong-preferred orientation defines the well-developed foliation. Increased graphite content resulted in high-conductivity zones with a contrast to the host low-conductive crust (Rodinov et al. 2013; Engvik et al. 2021). Enrichment of graphite resulted in zones with strong schistosity and a sharp strain gradient towards host massive granulite gneiss. The presence of graphite causes strain localisation in the granulite facies crust, reducing crustal strength and may thereby influence continental architecture and evolution of collision zones.

References:

Engvik AK et al. (2023) Proterozoic Deep Carbon—Characterisation, Origin and the Role of Fluids during High-Grade Metamorphism of Graphite (Lofoten–Vesterålen Complex, Norway). Minerals 13(10), 1279

Engvik AK et al. (2021) The control of shear-zone development and electric conductivity by graphite in granulite: An example from the Proterozoic Lofoten-Vesterålen Complex of northern Norway. Terra Nova, https://doi.org/10.111/ter.12545

Rodinov A et al. (2013) Helicopter-borne magnetic, electromagnetic and radiometric geophysical survey at Langøya in Vesterålen, Nordland. NGU Report 2013.044

How to cite: Engvik, A. K., Gautneb, H., and Knežević Solberg, J.: Characterisation, origin, petrophysical properties and the role of fluids during high-grade metamorphism of graphite (Lofoten-Vesterålen Complex, Norway), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21791, https://doi.org/10.5194/egusphere-egu26-21791, 2026.

Volcanogenic massive sulfide (VMS) deposits are among the world’s most important sources of copper (Cu), zinc (Zn), lead (Pb), gold (Au), and silver (Ag), metals that are critical for modern infrastructure and energy technologies. These deposits are characterized by systematic hydrothermal alteration halos that preserve mineralogical and chemical gradients generated by spatial and temporal variations in temperature, redox conditions, and hydrothermal fluid composition. Such alteration zones provide important vectors for mineralization; however, their traditional characterization is commonly qualitative, reliant on subjective geological interpretation, and difficult to collolate at scale across exploration projects. This study investigates the Rävliden deposits of the Paleoproterozoic (1.89 Ga) volcanic–sedimentary sequence of the Skellefte district, Sweden, to evaluate whether integrated rock magnetic and VNIR–SWIR hyperspectral data can be used to objectively characterize hydrothermal alteration in VMS deposits. In this study, rock magnetic measurements are cross-referenced with hyperspectral data and supported by mineral chemistry and sulfur isotope analyses to develop a quantitative and reproducible framework for fingerprinting hydrothermal alteration in both metalliferous and barren VMS systems. The approach comprises three objectives: (1) defining diagnostic magnetic and hyperspectral signatures of alteration mineral assemblages to construct a reference dataset, which is then validated using Raman spectroscopy; (2) evaluating trace-element variations in magnetite and associated sulfide minerals to assess their influence on magnetic properties across alteration zones; and (3) using sulfur isotope compositions (δ³⁴S) of sulfide minerals across alteration zones and structural domains to constrain fluid sources and reconstruct the hydrothermal fluid evolution of the system. This workflow systematically links observable physical alteration patterns to their underlying mineral-chemical controls and fluid origins, providing a robust and scalable tool for hydrothermal alteration characterization in VMS exploration.

How to cite: Aryani, L. and McCarthy, W.: Integrated Rock Magnetic and VNIR–SWIR Hyperspectral Characterization: A Quantitative Classification Tool for VMS Alteration Systems, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22148, https://doi.org/10.5194/egusphere-egu26-22148, 2026.

Fluid flow in the Earth’s crust governs heat and mass transfer, critical metal mineralisation, rock rheology, and the development of deep, non-photosynthetic biospheres, yet its direction and mechanical drivers remain poorly constrained in natural systems. Conventional approaches infer fluid pathways from fractures, models, or geochemical tracers but rarely capture flow direction or mechanism directly. This PhD project develops a novel combination of fabric-based methods—integrating anisotropy of magnetic susceptibility and remanence (AMS/ARM), crystal preferred orientation analysis, and hyperspectral mineral mapping—to directly identify and quantify fluid-induced petrofabrics within the thermal aureoles of igneous intrusions, independent of fault kinematics. Similar integrated approaches have demonstrated their ability to track volatile-rich liquid migration through texturally layered intrusions, where permeability contrasts control fluid focusing and the development of REE-enriched horizons. Together, these methods provide new constraints on how fluids modify host-rock properties, localise permeability, and generate chemical enrichment, representing a step-change in our ability to observe and model crustal fluid flow.We present new petrofabric data from the Sherwood Sandstone Group, Northern Ireland, to assess fluid flow in permeable sandstones surrounding basaltic dykes. The study examines: (i) the geometry and extent of hydrothermal flow pathways, (ii) the interaction between thermally driven fluid circulation and pre-existing sedimentary anisotropy, and (iii) the impact of alteration on host-rock porosity and permeability. The Sherwood Sandstone Group forms the lowermost unit of the Triassic New Red Sandstone succession and is cross-cut by Palaeogene basaltic dykes related to North Atlantic rifting. Preliminary field observations and hyperspectral data identify laterally zoned alteration halos defined by systematic variations in clay, mica, and Fe-oxide mineralogy. AMS and ARM data reveal that primary sedimentary fabrics are preserved more than ~10 m from the dyke but are progressively overprinted toward the intrusion. Ongoing analyses test whether these overprinting fabrics record convective hydrothermal flow, with fluids ascending along dyke margins before dispersing laterally along bedding planes. We further evaluate the controls on stratigraphic fluid focusing and blockage, constraining the reciprocal relationship between fluid flow and evolving rock properties.

How to cite: Pelletier, M. and McCarthy, W.: Go with the Flow : Investigating Petrofabric Evidence for Hydrothermal Flow in Thermal Aureoles, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22152, https://doi.org/10.5194/egusphere-egu26-22152, 2026.

As the key mechanism of shallow earthquakes, the fault stick-slip behavior is usually explored under the assumption of constant normal stress. However, dynamic natural processes (tides, far-field earthquakes, etc.) and human activities (blasting, water injection, mining, etc.) generate periodic stress disturbances in the fault zone. So far the coupled results of fault seismic slips under variable normal stress are poorly understood.

We performed laboratory direct shear tests on saw-cut granite joints under constant and cyclic normal stress (σ_n), considering the role of load point velocity (V_lp), normal stress oscillation amplitude (ε) and normal stress oscillation frequency (f). Under constant normal stress, the joint exhibits a spontaneous stick-slip phenomenon for different V_lp. The shear stress drops and recurrence timespans of stick-slip events are reduced with faster V_lp. Under equivalent σ_n level, the cyclic σ_n weakens the frictional strength when V_lp is small and enhances the strength when V_lp is large. As ε grows, the joint slip style switches from regular stick-slip to chaotic slip, and eventually to compound stick-slip. The frictional strength is first increased and later weakened. In respect to effect of f: when f is small, one σ_n cycle can produce several stick-slip events. When f is medium, the period of the stick-slip event is equal to the cyclic σ_n period. For further increase of f , the recurrence period of stick-slip events becomes double the cyclic σ_n period. The frictional strength is decreasing or increasing at the critical point for frictional resonance.

The improved spring-block model equipped with rate-and-state friction framework matches the lab observations satisfactorily. Especially, the introduction of a stiffness response coefficient (Ψ) allows the model to reflect realistic fault frictional behavior, where shear stiffness varies with σ_n. A new parameter Θ is defined whose symbol (+ or -) directly determines the compression/relaxation status of the spring, and satisfactorily explains transitions in shear stress trends. Comparative analysis with the conventional Linker-Dieterich model highlights the improved physical consistency of our new approach, particularly in preserving the physical interpretation of the state variable, θ. The model also demonstrates that under large σ_n disturbances, a frictional system can effectively exhibit stick-slip behavior even in the velocity-strengthening scope. More importantly, the modeling implies that fast slip events greatly reduce the contact density of the fault interface. The contact state of the stick-slip joint/fault cannot be judged solely by σ_n. The contact area during the shear process is determined by both, the real-time σ_n level and the state variable θ.

How to cite: Tao, K. and Konietzky, H.: Experimental and modeling insights into fault stick-slip behavior under dynamic normal stress condition, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2318, https://doi.org/10.5194/egusphere-egu26-2318, 2026.

Pull-apart basins introduce extensional bends and strong along-strike heterogeneity into otherwise strike-slip systems, potentially altering slow slip, earthquake nucleation, and multi-segment rupture. The Princes Islands segment of the Main Marmara Fault (MMF) hosts a ~30-40 km pull-apart basin within the Marmara seismic gap south of Istanbul, where geodetic and geological observations suggest partial unlocking and complex rupture behavior. Yet the mechanical role of this extensional geometry in controlling fault slip and rupture remains poorly constrained. We perform three-dimensional quasi-dynamic simulations using PyQuake3D (Tang et al., 2025) to quantify the impact of the Princes Islands pull-apart basin on interseismic loading, slow-slip transients, dynamic rupture propagation, and earthquake recurrence along the MMF. We model a continuous fault surface with variable dip, bends, and segmentation, and prescribe depth-dependent effective normal stress, spatial frictional contrasts, and stress heterogeneity representative of extensional basin environments, guided by published geophysical constraints. Our results show that the basin exerts a first-order control on slip style and rupture outcomes through the competition between geometric unclamping, frictional heterogeneity, and stress structure. Extensional bends favor localized unlocking and recurrent aseismic or slow-slip episodes, which in turn modulate where dynamic ruptures nucleate. At the event scale, the basin can behave either as a rupture barrier or a rupture accelerator: in many realizations, ruptures do not continue smoothly across the basin but instead produce triggered seismicity via static stress transfer and re-nucleation near segment boundaries. Stress concentrations at geometric transitions primarily govern nucleation locations, while frictional contrasts regulate rupture persistence and arrest. These findings highlight that explicitly representing extensional fault structures is critical for assessing multi-segment rupture potential and time-dependent seismic hazard on the MMF near Istanbul.

Tang, R., Gan, L., Li, F., & Dal Zilio, L. (2025). PyQuake3D: A Python tool for 3‐D earthquake sequence simulations of seismic and aseismic slip. Journal of Geophysical Research: Machine Learning and Computation, 2(4), e2025JH000871.

How to cite: Osei-Tutu, D., Sopaci, E., and Dal Zilio, L.: How an extensional pull-apart basin modulates fault slip and earthquake rupture on the Main Marmara Fault, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3420, https://doi.org/10.5194/egusphere-egu26-3420, 2026.

Rock deformation experiments play a key role in our understanding of earthquake physics and friction constitutive laws. These laws commonly describe the response of analogue laboratory faults as a simple and homogeneous system, without accounting for the spatial-temporal evolution of structures in the sample. However, increasing experimental evidence suggests that slip instability is closely tied to heterogeneity, complex rheologies, and inhomogeneous boundary conditions. To address this, we designed a novel transparent setup to observe real-time deformation, track the spatial-temporal evolution of shear fabric, and document unstable slip in experimental faults. Our video documentation reveals that the progressive development of fault fabrics results in heterogeneous but not random stress redistribution. We show that stress and structural heterogeneities play a key role in the nucleation, propagation, and arrest of slip instabilities, raising questions about the robustness of scaling laboratory frictional laws to nature.

How to cite: Pozzi, G., Volpe, G., Taddeucci, J., Cocco, M., Marone, C., and Collettini, C.: Spontaneous complexity in the dynamics of slow laboratory earthquakes., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3472, https://doi.org/10.5194/egusphere-egu26-3472, 2026.

Elucidating fault zone processes during long-term seismic cycles is critical for mitigating earthquake hazards in intraplate regions. We investigated the hydro-mechanical evolution of a strike-slip branch of the Yangsan Fault, SE Korea, which bounds Triassic and Jurassic granites. By integrating multiscale observation with high-velocity rotary shear experiments and XRD, we characterized the fault architecture, which consists of a <35 m thick damage zone surrounding a <1 m thick core. The core contains breccia and foliated gouge rich in clay minerals (43 wt.%), specifically dominated by illite (21.2 wt.%) and smectite (13.3 wt.%). Shear experiments on the foliated gouge revealed a consistently low friction coefficient (μ_ss<0.17). Notably, instantaneous flash dilation of the mixed smectite/illite gouge was observed at seismic slip rates (1.3 m/s) when total displacement exceeded ~5 m. Microstructural cross-cutting relationships indicate a distinct sequence of events: (1) vigorous injection of pressurized fluids from wall rocks into the densely packed, low-permeability gouge directly; (2) precipitation of fibrous calcite veins along foliation planes and perpendicular to the Y-shear direction; and (3) subsequent injection of fluidized gouge material into the damaged wall rock. These observations suggest that cyclic coseismic and aseismic faulting occurred following the low-temperature formation of expanding clay minerals. We conclude that the dynamic interplay between fluid pressurization and the fluidization properties of clay gouge acts as a primary driver of mechanical instability, playing a key role in the long-term seismic evolution of intraplate granitic fault zones.

How to cite: Kim, C.-M., Woo, S., Lee, J., and Choi, J.: Fluid-Driven Injection and Pressurization of Clay-Rich Gouge in the Yangsan Fault: Implications for the Long-Term Seismic Cycle, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4626, https://doi.org/10.5194/egusphere-egu26-4626, 2026.

To address the limitations of traditional convolutional neural networks (CNNs) in seismic fault identification—such as restricted local receptive fields, limited capability for modeling long-range structural correlations, and low sensitivity to small or subtle faults—this study proposes a seismic fault identification framework based on a Vision Transformer (ViT) architecture combined with self-supervised pretraining and transfer learning. Self-supervised pretraining is first conducted on large volumes of unlabeled three-dimensional seismic data to learn general representations of geological structures, thereby reducing the dependence on manually labeled samples. The pretrained ViT model is subsequently transferred to the fault identification task and systematically compared with a conventional U-Net architecture. Experiments on a publicly available synthetic seismic dataset show that the ViT-based model achieves improved fault localization accuracy, spatial continuity, and robustness to noise compared to U-Net. Application to real 3D seismic data from an oilfield further demonstrates that the proposed method is capable of detecting a larger number of small-scale faults with enhanced structural continuity, highlighting its applicability in structurally complex settings. The results suggest that Transformer-based global modeling provides an effective alternative for automated seismic fault interpretation.

How to cite: Pei, H. and Zhang, G.: Seismic Fault Identification Based on Vision Transformer, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4788, https://doi.org/10.5194/egusphere-egu26-4788, 2026.

Slow-slip events (SSEs) are a well-known mode of aseismic deformation in subduction zones. Seismological and geological studies further suggest that SSEs enhance fault-zone permeability, enabling fluid migration from overpressured oceanic crust into the plate interface. However, it remains unclear whether the resulting pore-pressure changes dominate stress transfer and promote the commonly observed increase in seismicity during SSEs and, although less commonly, the occurrence of larger megathrust earthquakes. Here, we investigate the impact of an SSE that occurred three days before the 2017 Mw 6.9 Valparaíso earthquake in central Chile. We use a forward 4D hydromechanical (poroelastic) model and compare the resulting spatial stress changes with a high-resolution seismicity catalog of the foreshock sequence.

We simulate the SSE by prescribing a geodetically inferred slip distribution on the fault interface and assume an overpressured oceanic crust, together with transient permeability enhancement due to SSE-induced local fracturing of the plate interface. We compute stress transfer driven by these pore-pressure changes along the plate interface and compare the results with widely used models that consider elastic stress changes only. Our results show that fluid migration into the plate-interface zone generates stress changes of ~1–10 MPa, overwhelmingly dominated by pore-pressure variations. The largest stress (and pore-pressure) changes spatially correlate with zones of increased seismicity, repeating earthquakes, and the mainshock. In contrast, the elastic-only scenario produces stress changes that are two to three orders of magnitude smaller and shows a much weaker spatial correspondence with the observed seismicity.

Our modeling results indicate that transient permeability enhancement during SSEs enables fluid redistribution that fundamentally controls stress transfer along the plate interface. We conclude that pore-pressure changes exert first-order control on earthquake precursors in subduction zones, offering a physical explanation for foreshock clustering and the triggering of large earthquakes during SSEs. These findings highlight the importance of incorporating fluid–rock interactions in models of seismic hazard and earthquake nucleation.

How to cite: Peña, C., Cabrera, L., Muñoz-Montecinos, J., Ruiz, S., and Heidbach, O.: Hydraulic control of the foreshocks and mainshock of the 2017 Valparaiso earthquake in central Chile, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6156, https://doi.org/10.5194/egusphere-egu26-6156, 2026.

Fluid-induced fault reactivation and associated seismicity is a critical process in reservoir exploitation and emerging geo-energy activities such as Carbon Capture and Storage (CCS), Enhanced Geothermal Systems (EGS) and wastewater disposal. During fluid injection, the fault stress state progressively approaches the failure criterion τ = (σₙ - Pf) * µ + C , where τ is shear stress, σₙ normal stress, Pf fluid pressure, µ friction, and C cohesion. Once the stress state reaches the failure envelope, faults may reactivate either seismically or aseismically. However, the mechanisms governing aseismic versus seismic fault reactivation during fluid injection remain debated.

Previous laboratory studies suggest that this seismic vs. aseismic deformation may be influenced by fault frictional properties influenced by mineralogy, fault zone structure, stress state, and injection rate, yet the relative contribution of these factors remains unclear. To address this issue, we present an experimental study on binary and ternary fault gouges with variable fractions of quartz, calcite, and illite. These are minerals found along faults zones and within reservoir rocks commonly exploited for geo-energy applications.

For each mineralogical composition, two experimental datasets were acquired. In the first dataset, we performed slide–hold–slide and velocity-step tests to measure friction, frictional healing and the velocity dependence of friction. In the second dataset, we investigated fault slip behavior during fluid pressure-induced reactivation at three different stress states.

The frictional properties reveal a pronounced contrast between granular and platy phyllosilicate-rich gouges. Granular materials exhibit high friction (µ ≈ 0.6), positive frictional healing, and low a–b values, indicating velocity-weakening and potentially seismogenic behavior. In contrast, illite-rich gouges (illite > 40%) display low friction (0.28 < µ < 0.4), low to negative healing, and strongly positive a–b values, indicative of velocity-strengthening and aseismic behavior. Duringfluid injection induced-reactivation, granular-rich gouges reactivate through an exponential increase in slip velocity, mimicking seismic-like instability. Conversely, illite-rich gouges reactivate through aseismic but accelerated creep that does not evolve into dynamic failure.

Notably, reactivation in granular gouges is abrupt and occurs at stress states well above the predicted failure envelope, whereas in illite-rich gouges reactivation is gradual and occurs at or before the predicted failure envelope. In addition, at constant illite content, quartz-rich gouges reactivate faster than calcite-rich fault gouges.

The integration of these results suggests a conceptual framework in which fluid-induced fault reactivation is governed by the interplay between frictional healing and rate dependence, with mineralogy exerting a first-order control. In granular gouges, strong healing dominates the the fluid induced reactivation process, leading to delayed but abrupt fault reactivation that can overcome the stabilizing slight rate-strengthening effect, promoting an exponential acceleration under fluid pressurization. In contrast, in phyllosilicate-rich gouges, weak or negative healing combined with a marked rate strengthening behavior stabilizes slip, favoring continuous aseismic creep.

This framework demonstrates that the balance between healing and rate dependence, strongly linked to fault mineralogy, governs whether fluid-induced fault reactivation produces seismic slip or aseismic creep.

How to cite: Coppola, L., Volpe, G., Giorgetti, C., Pozzi, G., Wibberley, C., Bourgeois, F., and Collettini, C.: Fluid induced fault slip behavior: frictional healing vs velocity dependence of friction, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9583, https://doi.org/10.5194/egusphere-egu26-9583, 2026.

At the northern Hikurangi margin, Aotearoa New Zealand, slow slip events (SSEs) recur every 6-24 months to ~30 km depth. While shallow SSEs (0-10 km) are well-studied offshore, the deeper portion (10-30 km) remains poorly understood, limiting insight into SSE initiation. In Woodward et al. 2026 we investigate the relationships between newly resolved SSEs and seismicity. We combine passive seismological, geodetic, geochemical and seismic reflection data to analyse the relationships between seismicity and slow slip events, and the mechanisms that invoke them. Using time-dependent inversions, we resolve two small SSEs (M_W 6.2 and 6.4), one of which extends unusually deeply from 15 to 30 km depth. Using data from a dense onshore seismograph network, deployed directly above this deeper portion from December 2017 to October 2018, we construct a catalog of 3,071 high-quality earthquakes with hypocentral uncertainties ≤5 km, located with a 3-D velocity model and our new 1-D model. Focal mechanisms reveal numerous normal-faulting earthquakes, including some within the slab mantle. Seismicity distributions and normal-faulting earthquakes occur along vertically aligned pathways that link the subducting slab mantle to surface seeps, where fluids show mantle-derived signatures. We infer that normal faults form due to slab bending and localized uplift of subducting seamounts, which enhance plate interface roughness, damage the upper plate, and promote fluid migration. Landward of ~100 km from the trench, both surface seeps and normal-faulting mechanisms cease, coinciding with the downdip limit of shallow SSEs. Together, these results suggest that the Hikurangi margin’s rough subducting plate interface exerts strong control on forearc dewatering and SSE genesis.

How to cite: Woodward, A., Bastow, I., Bell, R., Wallace, L., Jacobs, K., Henrys, S., Fry, B., Merry, T., Lane, V., Ville, L., Houldsworth-Bianek, P., and Broadley, L.: The role of seamounts, fluids, and normal faults in slow slip regions: seismological insights from the northern Hikurangi margin, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9842, https://doi.org/10.5194/egusphere-egu26-9842, 2026.

The faulting caused by dyke intrusions provide a novel opportunity to study the way natural fault systems respond to time-varying changes to the stress field. The recent 2024-2025 Fentale-Dofen dyking episode in the northern Main Ethiopian Rift (NMER) offers a rare opportunity to investigate these processes, as the surface deformation was captured in unusually high spatial and temporal resolution by satellite radar. 

In our study, we combine Interferometric Synthetic Aperture Radar (InSAR) data from the COSMO-SkyMed satellite, high resolution Digital Elevation Model (DEM), with a catalogue of >150 relocated moderated-sized earthquakes (M4.5-6) to study the spatio-temporal evolution of seismic and aseismic fault slip linked to dyking in the NMER. We focus on an area ~15 km north of the tip of the dyke, where we find fault patches showing both repeated seismic and aseismic slip occurring in close proximity, associated with surface deformation of <13 cm in 3 months. We consider three possible mechanisms for the observed fault behaviour: (1) that this is normal mainshock-aftershock sequence on faults governed by rate-and-state friction, (2) that elastic stress perturbations from the ongoing dyke intrusions reloaded the fault patches, or (3) that elevated pore-fluid pressure caused transient reductions in effective normal stress on the faults. Using slip and stress modelling, we will test these hypotheses and quantify how much seismic/aseismic strain is accommodated by pre-existing and newly formed faults, as well as the relative contribution of seismic/aseismic strain to accommodating shallow crustal extension during the dyking episode.

These findings provide new constraints on fault mechanics and the interaction with magmatic processes in rifting environments, improving our understanding of dyke-induced seismicity and the evolving nature of faults with repeated earthquakes. 

How to cite: Orrego, S., Biggs, J., Wimpenny, S., Zheng, W., Way, L., Vallée, M., Grandin, R., and Lewi, E.: Investigating seismic and aseismic fault motion caused by the 2024-2025 Fentale dyke intrusions in Ethiopia, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12718, https://doi.org/10.5194/egusphere-egu26-12718, 2026.

Aseismic slip plays a key role in earthquake dynamics, but we currently do not fully understand why some faults slide aseismically. Aseismic slip is largely influenced by the fault zone's frictional behaviour, by its material composition, the presence of fluids, the geometry of the fault, and the fault zone fabric. Recent research has focused on the material composition, and more specifically on the evolution of resistance to slip with slip speed for different types of rocks. Generally, faults with rate-weakening behavior tend to host earthquakes, while faults with rate-strengthening behavior accommodate stress through aseismic slip. However, even in rate-weakening materials, low effective normal stress, induced by high pore fluid pressure, makes it unlikely for a slip instability to reach the critical size needed to nucleate regular earthquakes. Hence, the presence of high pressure fluid within fault zones may explain the presence of shallow aseismic slip along faults. However, we lack direct evidence of the presence of fluids along various faults where aseismic slip has been identified.

We use data from a dense network of seismometers along the North Anatolian Fault Zone SEISMENET¹ to investigate spatial changes in seismic velocity along the section hosting aseismic slip. This section slips aseismically since at least 1944 and is the epicentral region of the last two large earthquakes that have struck the area, namely the 1944 M7.3 Bolu-Gerede and the 1943 M7.6 Tosia-Ladik earthquakes. Using local earthquake tomography, we test for a possible presence of fluids in the fault zone and a damage zone surrounding the epicentral region of the 1943 and 1944 earthquakes.

Our network includes 5 broadband seismometers and 10 geophones deployed around the creeping section along a narrow swath paralleling the fault trace. In addition to data from these 15 temporary stations, seismic data from 5 permanent broadband stations were collected. The final dataset includes 24,756 P arrivals and 21,311 S arrivals from 2,272 earthquakes, with magnitudes ranging from Mw0 to Mw4. We use the simul2023 code² to simultaneously determine the 3D structure of the shallow crust and relocate the earthquake hypocenters.

We find kilometer-scale shallow high vp/vs anomalies (values in range of 1.8 up to 1.95), consistent with a damage asymmetry aligned with the observed rupture directions of historical earthquakes, indicating long-term preferred rupture directions along this segment of the NAF. Additionally, the creeping section of the North Anatolian Fault is shown to correspond to a 30-km-long zone of vp/vs above 1.8, consistent with the presence of high pore fluid pressure within the fault zone. The findings provide compelling evidence that fluid processes, rather than fault zone rheology alone, significantly influence aseismic slip behavior along the NAF. Together, these results suggest a dynamic interplay between structural damage, rupture history, and fluid migration in controlling fault zone mechanics, with implications for improving seismic hazard assessment in creeping fault segments.

¹https://geofon.gfz.de/waveform/archive/network.php?ncode=1O&year=2022

²https://doi.org/10.5281/zenodo.10695070

How to cite: Szrek, J., Jolivet, R., Schurr, B., Becker, D., Martínez-Garzón, P., Jara, J., and Çakir, Z.: Fluids and Creeping Faults: Insights From Local Earthquake Tomography of the Creeping Section of the North Anatolian Fault, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12855, https://doi.org/10.5194/egusphere-egu26-12855, 2026.

Understanding the physical mechanisms governing megathrust seismicity and the geodynamic feedback between the megathrust and the overriding accretionary wedge remains critical in subduction zone geophysics. The structural complexity of accretionary wedges—characterized by heterogeneous porosity, permeability, and fault networks—critically influences the configuration of pore fluid pressure and frictional properties along the megathrust interface. To investigate these interactions, we employ a fully coupled hydro-mechanical numerical model (Gerya, 2019) that simulates two distinct timescales within a single, consistent rheological framework. Our approach incorporates temperature-dependent dehydration reactions, including smectite-to-illite and zeolite-to-greenschist transitions, to evaluate how fluid production and migration evolve during both subduction and seismic processes. Additionally, we implement a dynamic fault-valving mechanism where reference permeability evolves transiently to mimic fracture-induced permeability enhancement during fast slip. The simulation follows a two-stage workflow: first, we conduct long-term wedge accretion modeling with adaptive time steps (10–500 years) using a higher stress tolerance to construct realistic wedge architectures. Subsequently, we switch to a rupture simulation mode by reducing the stress tolerance, allowing the adaptive time-stepping scheme to automatically resolve short-term seismic cycles (from days to years). This methodology successfully introduces the structural and hydrological complexity inherited from long-term geological evolution into the analysis of short-term megathrust slip behaviors. Results indicate that fast slip events preferentially initiate at the transition zones between low and high overpressure regions, whereas domains characterized by high pore fluid pressure ratios () predominantly host slow slip events. Furthermore, we find that hydraulic properties control the spatiotemporal stability of rupture nucleation: higher permeability promotes significant temporal pore pressure variability, resulting in scattered initiation depths, while lower permeability maintains stable pressure configurations, leading to spatially consistent rupture nucleation. We conclude that the long-term hydro-mechanical evolution of the wedge governs megathrust nucleation and slip segmentation. 

How to cite: Lin, C.-H. and Tan, E.: Hydro-Mechanical Modeling of Fluid-Regulated Deformation in Accretionary Wedges and Its Implications for Megathrust Slip, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12927, https://doi.org/10.5194/egusphere-egu26-12927, 2026.

Independent lines of seismic evidence suggest that pore fluid pressure at the depth range of episodic slow slip events (SSEs) may undergo periodic fluctuations synced with the SSE slip cycles.  Here we develop a numerical simulation framework that integrates the SSE model governed by the rate- and state-dependent friction with Bayesian data assimilation to optimize time-variable fault friction parameters, using constraints from the northern Cascadia GNSS time series.  We first conduct synthetic experiments to calculate surface displacement time series from the rate-state SSE model generated fault slip history with imposed Gaussian noise. Both frictional parameters, effective normal stress (normal stress minus pore pressure) and characteristic slip distance, converge to their true values in 5-10 iterations from the initial guesses that are 10-20% off from the true values, demonstrating the feasibility of the data assimilation framework. We then apply this framework to 2009-2020 GNSS time series that encompasses SSE cycles recorded at ~ 30 stations along the northern Cascadia subduction zone.  We use a GNSS time series of 1000 days (~3 SSE cycles) in each inversion run to fully resolve the temporal changes in stress or friction; longer time series will cause inversion convergence issues due to the system nonlinearity. Within an inversion run, we choose a sliding time window of 9 months for each optimization epoch, which is a trade-off that on one hand includes sufficient information for the prediction of fault slip in the next time step and on the other hand allows temporal distinctions between the inter- versus intra-SSE time periods. Our inversion results show clear cyclic fluctuations in the optimized characteristic distance and effective normal stress values during SSE cycles. Specifically, effective normal stress increases (pore pressure drops) during the intra-SSE period; effective normal stress decreases (pore pressure increases) during the inter-SSE period.  The pore pressure oscillation pattern is independent of whether the characteristic slip distance is time-invariant during data assimilation, but the converse does not hold. Our results are thus consistent with the proposed pore pressure build-up and release processes, i.e., fault-valve model, at the SSE depth range. 

How to cite: Liu, Y. and Zhang, W.: Fault-valve behavior during slow slip cycles constrained using Bayesian data assimilation for a Cascadia subduction fault slip model, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13013, https://doi.org/10.5194/egusphere-egu26-13013, 2026.

Given the significant risk that earthquakes pose to society, understanding the spatiotemporal evolution of slip rates on natural faults has been a central research objective over recent decades. Geological and geophysical observations indicate that fault slip is accommodated by multiple deformation mechanisms operating both on the fault plane and within the surrounding damage zone. At the outcrop scale, structures formed by seismic and aseismic slip commonly coexist within the same fault system, implying that fault displacement involves a combination of deformation processes controlled by mineral-specific rheology. At larger scales, however, these processes may be masked by the limited spatial and temporal resolution of geophysical and geodetic observations.

From a theoretical and experimental perspective, rate-and-state friction (RSF) laws have been widely used to explain unstable fault slip through velocity-weakening behaviour, in which fault strength decreases with increasing slip rate. In this framework, fault friction is governed by slip velocity and a single state parameter that evolves with time and slip, commonly expressed either through an aging law, where fault healing occurs primarily during stationary contact, or a slip law, where state evolution is driven by slip-dependent renewal of contacts. Because both formulations are typically expressed as local, slip-rate-dependent laws, the explicit role of cumulative slip history in controlling the fault state remains implicit.

To investigate the effect of cumulative slip history, we perform a suite of 3D quasi-dynamic simulations assuming a homogeneous distribution of rate-weakening frictional properties, in which fault slip is governed by a classical RSF formulation. By systematically decreasing effective normal stress from 50 to 10 MPa and explicitly rewriting the aging law in a slip-dependent, event-integrated form, we show that the well-documented transition from characteristic earthquake behaviour to deterministic chaotic slip (e.g., Rubin, 2008; Cattania, 2019; Barbot, 2019) is accompanied by a change in the role of the state variable. Specifically, state evolution becomes increasingly governed by cumulative slip history and slip-filtered healing, which inhibits convergence toward a unique healed state and results in incomplete state recovery between successive events. Importantly, this behaviour arises without prescribing any spatial heterogeneity in frictional properties or fault-zone structure.

These results have direct implications for the interpretation of fault kinematics. While regions where slip rates remain below the prescribed background velocity may persist constant over interseismic periods, the shear stress within those regions in models of low effective normal stress need not be stationary. Instead, shear stress can evolve significantly because of incomplete state recovery driven by cumulative slip history and slip-limited healing, leading to temporally heterogeneous mechanical behaviour despite stable kinematic expression.

References

Rubin, A. M. (2008). Episodic slow slip events and rate‐and‐state friction. Journal of Geophysical Research: Solid Earth, 113(B11).

Cattania, C. (2019). Complex earthquake sequences on simple faults. Geophysical Research Letters, 46(17-18), 10384-10393.

Barbot, S. (2019). Slow-slip, slow earthquakes, period-two cycles, full and partial ruptures, and deterministic chaos in a single asperity fault. Tectonophysics, 768, 228171.

How to cite: Julve Lillo, J., Fagereng, A., Ampuero, J.-P., van den Ende, M., and Toffol, G.: Fault state evolution governed by cumulative slip history., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14122, https://doi.org/10.5194/egusphere-egu26-14122, 2026.

Laboratory investigations into the behavior of fault zones have been a significant focus in experimental rock mechanics over the past decades. Various approaches have been developed, ranging from analog models to testing natural samples in triaxial cells. The primary goal of the latter is to infer the physical mechanisms responsible for failure under realistic conditions encountered in natural settings, albeit on small sample sizes (e.g., centimeter scale). In contrast, analog modeling aims to replicate similar mechanical behavior by applying scaling laws to geometry and material properties. In the present study we aim at combining large scale experiments (meter scale) with small scale experiments (cm scale) in order to highlight the underlying physical mechanisms preceding the slip.

To address the spatial scale limitations of classical rock mechanics, we developed new experiments that bridge the gap between traditional rock mechanics and analog experiments. These experiments utilize the unique capabilities of the DIMITRI setup, a giant true-triaxial apparatus (1.5m × 1.5m × 1m). Due to the size of this experimental device, the maximum stress it can apply is limited to 2 MPa per principal stress. Consequently, we chose polystyrene as an analog for rocks. The low elastic properties of polystyrene slow down physical processes, enabling comprehensive observation of rupture phenomena, from initiation to failure arrest. Our objective is to investigate the interplay between different types of slip occurring along the interface.

In this study, we conducted stick-slip experiments on large-scale polystyrene blocks with a pre-cut surface area of 1.5 m². We applied shortening rates ranging from 1 to 10 mm/min. Our experiments successfully reproduced stick-slip behavior, allowing us to observe variations in frictional behavior along the interface and identify different types of slip, from slow slip to dynamic slip.

In parallel, we performed small scale experiments uniaxial stick slip experiments, under the same conditions than the previous, that we monitored with 2D X-ray radiography at high frequency (12Hz). Preliminary observations highlight density contrasts in the bulk material around the fault plane, offering insight into potential precursory signs of slip.

Therefore, this study including two scales of observation demonstrates the relevance of our material to study the physical mechanisms controlling various slip types occuring along the seismic cycle.

How to cite: Bonnelye, A., Walter, B., Gouedar, A., and Faure-Catteloin, D.: PolystyQuakes : what can we learn from the use of polystyrene as analogue to earthquakes? From small to large scale and vice versa., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14191, https://doi.org/10.5194/egusphere-egu26-14191, 2026.

Aseismic slip is increasingly recognized as a fundamental driver of earthquake nucleation, affecting the spatio-temporal evolution of seismicity, yet its direct observation remains rare due to limited strain measurements close to a natural fault system. Here, we present results from the 'FEAR1' experiment conducted at the Bedretto Underground Laboratory for Geosciences and Geoenergies (Switzerland), where we used fluid injections to activate a natural fault and fracture network in crystalline rock under in-situ stress conditions at ~1 km depth. This experimental setting is particularly well suited to investigate induced seismicity and the role of aseismic processes in fault activation, thanks to dense near- and on-fault strain, pressure, and seismic monitoring.

During several injections performed in FEAR1, we observed the activation of a steeply dipping, highly permeable fracture zone, which intersects a densely instrumented borehole. Hydraulic stimulations triggered seismicity (−4.9 < Mw < −2.3) that organized along a plane whose orientation is consistent with geological observations in boreholes cores, logs and on the laboratory tunnel wall. Simultaneously, high-resolution Fiber Bragg Grating strain measurements revealed progressive, irreversible tensile deformation localized near the fracture intersection with the monitoring borehole, reaching nearly 1000 µε over the course of the experiment.

Static elastic modeling demonstrates that the cumulative strain produced by the recorded earthquakes accounts for less than 1% of the observed deformation, indicating that fault slip was dominantly aseismic. The spatial and temporal evolution of seismicity shows systematic up-dip migration toward the strain concentration zone and the emergence of families of repeating earthquakes. The recurrence rate and cumulative slip of these repeaters correlate with the measured strain rate and strain, suggesting a scenario where seismic asperities are embedded within a creeping fault segment sustained by pore pressure stress perturbations.

Inversions of irreversible strain for simplified slip sources indicate a predominantly strike-slip mechanism consistent with the estimated local stress field, although trade-offs between source location, source dimension and slip direction highlight the limits of 1D strain observations. Our results provide direct experimental evidence for fluid-driven aseismic slip on a natural fault and demonstrate how microseismicity and repeaters can serve as indirect proxies for underlying slow deformation.

How to cite: Lambiase, A., Meier, M.-A., Spagnuolo, E., Nikkhoo, M., Marsan, D., Rinaldi, A. P., Gischig, V., Selvadurai, P., Cocco, M., Giardini, D., and Wiemer, S.: Fluid-induced aseismic slip and seismicity on a natural fault: insights from the FEAR1 experiment at BedrettoLab, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14725, https://doi.org/10.5194/egusphere-egu26-14725, 2026.

Slip instabilities leading to earthquakes, landslides, and glacier surges may be triggered by high fluid pressures. On the other hand, high fluid pressures also suppress instability because of large nucleation length-scales in overpressured systems. We review geological, glaciological, and rock mechanical observations and highlight two key scales that control pore pressure induced frictional instabilities: (1) the length scale over which pore fluid overpressure is maintained, and (2) a time scale defined by relative rates of deformation propagation and pore fluid transport. These scales are also dependent on rheological regime, and we find three end-member regimes: (1) shallow and/or low temperature deformation where ambient stress is low, faults are close to frictional failure, and shear is easily delocalised; (2) deformation in a brittle and frictionally unstable regime (such as the crustal seismogenic zone), where planes are close to frictional failure and slip tends to localise; and (3) environments where viscous deformation is preferred over frictional, and hence bulk stress is low, frictional strength is high, and delocalisation dominant. In regimes 1 and 3, fluid-driven instabilities tend to be confined to local areas of overpressure, because deformation delocalises in the bulk and dilatant hardening prevents further propagation. In regime 2, however, slip tends to localise and it is potentially favourable for fluid-induced instabilities to grow, provided slip surfaces are sufficiently close to failure. These regimes also apply to glaciers, where viscous flow of ice competes with frictional sliding on the glacier base - here, interconnected overpressured water at the glacier base is a commonly invoked mechanism that promotes frictional instability. These concepts imply that fluid-driven frictional instabilities are only as large as the areas where fluid overpressured patches can be interconnected, and therefore highlight the key role of fluid pressure heterogeneity in determining whether fluid-induced instabilities can propagate.

How to cite: Fagereng, A., Zhu, W., Gagliardini, O., V. Schuler, T., and Renard, F.: Pore Fluid Pressure Effects on Friction and Fracture, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14991, https://doi.org/10.5194/egusphere-egu26-14991, 2026.

In subduction zones, the depth-dependent release of fluids from compaction and metamorphic dehydration reactions in hydrous lithologies plays a key role in modulating pore fluid pressure, fault strength, and slip behavior along the megathrust. The depth-distribution of fluid release is also the primary control on volatile fluxes through the forearc, and on the residual volatile content of the subducting plate. Here, we investigate the inventory and release of fluids from altered oceanic crust by low-grade dehydration reactions (~50-350 °C) at the Northern Hikurangi subduction zone, where slip on the outer (shallow) megathrust is accommodated almost entirely in frequent, large shallow slow slip events (SSEs).

Regional geophysical surveys and drilling during International Ocean Discovery Program (IODP) Expedition 375 show that the incoming plate of the Hikurangi Plateau carries a thick (>1.5 km) and extensively altered volcaniclastic sediment blanket characterized by an abundance of phyllosilicates (primarily Mg-smectite) and zeolite, and mineral-bound water contents as high as 14-16 wt.%, into the SSE source region. We quantify the distribution of fluid release from this sediment package by combining compaction trends to assess compactive water loss and thermodynamic phase equilibria models using sediment drill-core compositions to compute water release from dehydration reactions.

We find that: (1) compactive dewatering dominates in the outermost 15-20 km of the forearc, where temperatures remain too low (<100 °C) to drive dehydration reactions; and (2) a large volume (~5-8 wt.%) of mineral-bound water is released step-wise over the region spanning from ~30-90 km from the trench (corresponding to depths of 5-15 km below seafloor and temperatures of 150-260 °C), primarily from decomposition of zeolite and phllyosilicate phases. This contrasts with the behavior of Ca- and Na-smectites typically found in detrital marine sediments and altered volcanic ash, which undergo dehydration between 80-150 °C.

Because the majority of compactive dewatering precedes dehydration, mineral-bound water is released where porosity, permeability, and compressibility are reduced, maximizing the potential for excess pore pressure generation along and beneath the megathrust. The broad region of low-temperature metamorphic fluid release directly overlaps the slip zone of recurring SSEs, supporting the idea that dehydration - and associated elevated pore pressures and low effective normal stress - favor SSE as the prevailing mode of strain release on the plate interface. The presence of thick extensively hydrated oceanic crust and persistence of fluid production from clay dehydration to ~260 °C contrasts with other subduction zones, where low-T metamorphism is dominated by the transformation of Ca- and Na-smectites to illite by 120-150 °C. We speculate that this difference may offer an underlying explanation for the lack of a locked seismogenic zone at the Northern Hikurangi margin, whereas at other subduction margins, a lack of significant fluid production from dehydration in the 150-350 °C window may lead to a better-drained megathrust and promote stick-slip behavior.

How to cite: Saffer, D. and Smye, A.: Links between Low-T Dehydration and Recurring Shallow Slow Slip Events in the Northern Hikurangi Subduction Zone, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15863, https://doi.org/10.5194/egusphere-egu26-15863, 2026.

After an initial phase of circular expansion, very large earthquakes primarily grow horizontally, with their vertical extent limited by the seismogenic width of the Earth’s crust. This geometric evolution is accompanied by a transition in rupture dynamics from crack-like to pulse-like propagation. Such events are commonly referred to as elongated ruptures.

While classical models (f.e., Linear Elastic Fracture Mechanics (Freund, 1998)) successfully describe small to moderate earthquakes, they fail to capture the dynamics of large events. Recent theoretical and numerical work by Weng and Ampuero (2019) introduced a physical framework for elongated ruptures, which, although supported by numerical validation and natural observations, has yet to be experimentally validated.

To address this, we conducted 2D rupture experiments in a biaxial direct shear apparatus under unbounded and bounded conditions. The unbounded case corresponds to a uniform velocity-weakening interface, while the bounded case consists of an elongated velocity-weakening region adjacent to a wide velocity-strengthening zone, mimicking a seismogenic layer whose width is bounded by deep aseismic regions. This experimental model successfully reproduces confined elongated ruptures and reveals distinct propagation styles: crack-like ruptures under unbounded conditions and pulse-like ruptures under bounded conditions. This transition is also reflected in the temporal evolution of seismic moment: during the initial phase of propagation, seismic moment scales cubically with rupture duration, while after saturation of the seismogenic width, it transitions to a linear scaling, as expected for pulse-like ruptures.

Together, these observations highlight the role of the seismogenic layer in controlling rupture style and provide experimental support for the proposed theory of elongated ruptures.

How to cite: Paglialunga, F., Ampuero, J. P., and Passèlegue, F.: Seismogenic width control on the dynamics and scaling of laboratory elongated ruptures, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16488, https://doi.org/10.5194/egusphere-egu26-16488, 2026.

Fluid pressure variations within fault zones impact fault strength and have the potential to produce detectable geophysical signals that can help characterise fault dynamics. One key process impacting fluid pressure is pore volume variations (dilation or compaction) due to stress changes and inelastic deformation. Slip-induced dilation and compaction have been thoroughly documented in laboratory experiments, but their impact on pore pressure has not. In nature, we expect slip to be associated with stress variations, and there might be cumulated effects of poroelastic and inelastic pore pressure changes. In order to document such effects, we conducted laboratory rock friction experiments where fluid pressure was monitored in situ during sequences of quasi-static loading followed by dynamic slip event. The simulated fault was a 30 degrees saw-cut in a Westerly granite cylinder, saturated with water, tested under triaxial conditions. The low hydraulic diffusivity of the rock made the fault and wall rock transiently undrained during deformation. During quasi-static loading with no fault slip, we observed pore pressure rises that we interpret as poroelastic closure of the fault. During dynamic slip events, pore pressure systematically dropped, approximately in proportion to the drop in normal stress. A large contribution to the pore pressure drop is interpreted as poroelastic opening of the fault. Prior to stick-slip events, we detected systematic pore pressure decreases by up to around 1 MPa, correlated to the occurrence of inhomogeneous slip along the fault. Slip nucleation, inferred by kinematic inversion of local strain gauge data, is linked to local slip magnitudes of the order of 1 to 10 µm, and appears to lead to inelastic dilation. A stability analysis of fault slip including dilatant and poroelastic effects shows that poroelastic coupling tends to compensate normal stress variations, leading to faults operating under mostly constant effective normal stress if conditions are undrained.

How to cite: Brantut, N., Passelègue, F., and Dublanchet, P.: Pore pressure change during nucleation and slip along experimental faults, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17727, https://doi.org/10.5194/egusphere-egu26-17727, 2026.

The Main Marmara Fault lies at the western termination of the North Anatolian Fault. While the North Anatolian Fault ruptured from its eastern termination to the Izmit segment over the 20th century through a westward‐propagating sequence of Mw ≥ 7 earthquakes, the underwater Marmara segment has not experienced a large earthquake in recent times. However, due to its proximity to the megacity of Istanbul, this segment represents one of the most hazardous fault systems in the Middle East. In particular, no historical earthquake has been identified on the Central Basin segment since at least 1766, potentially making it a major seismic gap.

To better assess seismic hazard along the Main Marmara Fault, we estimate the slip rate by jointly using a dense GNSS velocity field and four Sentinel-1 InSAR tracks. The GNSS velocity field consists of 111 measurements, including newly acquired densified sites along the northern shore of the Marmara Sea. The InSAR velocity field was processed automatically within the framework of the FLATSIM project, covering the period from October 2016 to April 2021. InSAR velocities are referenced to a Eurasia-fixed plate using the GNSS velocity field. We then perform a joint Bayesian inversion of slip rates using both datasets, allowing us to quantify uncertainties on the estimated slip rates.

Our results indicate that the Main Marmara Fault is predominantly creeping between longitudes 27.5 and 28.6, implying that the Central Basin segment is largely aseismic. Uncertainty estimates and forward modeling demonstrate that our datasets are capable of resolving slip behavior on this segment with good accuracy. However, the shallow portion of the Central Basin segment is still accumulating up to ~10 mm/yr of slip deficit, which could permit earthquakes of up to Mw 6.0 every few decades, similar to the 2025 sequence. West of longitude 27.5 and east of longitude 28.6, including the Prince Islands segment, the fault appears to be mostly locked down to 12 km depth. On the Prince Islands segment, close to Istanbul, the accumulated strain has the potential to generate an earthquake with Mw > 7.

How to cite: Klein, É., Neyrinck, E., Rousset, B., Masson, F., Ozkan, A., Yavasoglu, H. H., Ulrich, P., Jolivet, R., Doubre, C., Durand, P., and Doin, M.-P.: Slip Rates on the Main Marmara Fault from Bayesian Inversion of Dense GNSS and InSAR Velocity Fields, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18025, https://doi.org/10.5194/egusphere-egu26-18025, 2026.

The Upper Tiber Valley, in the Northern Apennines of Italy, is a key natural laboratory for investigating how continental extension is partitioned between seismic and aseismic deformation. Extension in this sector of the Apennines has been primarily accommodated by the Altotiberina Fault (ATF), a low-angle (~15°) normal fault that is mechanically unfavorable for elastic shear failure, and by a network of high-angle synthetic and antithetic faults in its hanging wall. While the ATF is characterized by persistent background micro-seismicity, the high-angle faults host larger historical earthquakes and frequent seismic swarms, likely induced by fluid circulation and elevated pore pressure. Since the study of Anderlini et al. (2016), the local GNSS network has been significantly densified within the framework of the Alto Tiberina Near Fault Observatory (TABOO-NFO). The updated dataset now better resolves a sharp ~3 mm/yr chain-normal interseismic velocity gradient across the Upper Tiber Valley, providing unprecedented constraints on how ongoing extension is distributed across the fault system. We use the new GNSS velocity field to reassess the relative contribution of low-angle versus high-angle faults to crustal deformation and to quantify the partitioning between seismic and aseismic slip. We apply a block-modeling approach that jointly estimates rigid block rotations and spatially variable interseismic coupling through a newly developed iterative inversion strategy. The model includes 3D geometries, discretized in triangular dislocation elements, of both the ATF and its antithetic structures, permitting assessment of distributed slip rates across the fault system. Preliminary results show that shallow locking on high-angle syn- and antithetic faults plays a first-order role in explaining the observed velocity gradient, whereas the ATF accommodates a significant fraction of extension through aseismic creep. These findings refine earlier interpretations and provide new insight into how low-angle normal faults can interact with steeper faults during the earthquake cycle.

How to cite: Serpelloni, E., Nucci, R., Poggiali, G., Buttinelli, M., Anderlini, L., Marone, C., and Chiaraluce, L.: Seismic vs aseismic deformation in the Northern Apennines constrained from dense GNSS velocities, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20370, https://doi.org/10.5194/egusphere-egu26-20370, 2026.

Shallow slow slip events have long been observed along the strike slip faults of the San Andreas fault system and are now increasingly observed on many other faults on Earth. Creep events are thought to episodically release a portion of the fault’s interseismic stress budget that has accumulated over the earthquake cycle. However, it is not known what portion of strain these events release, and what residual strain remains available to drive earthquake occurrence. Near-field surface creep measurements, like alignment arrays and creepmeters, are unable to constrain the depth of creep, meaning that is difficult to constrain the release of strain or stress in each creep event without additional assumptions. In this study, we use radar data from InSAR platforms to resolve the depth of creep during creep events along the Superstition Hills fault in Southern California. We mitigate atmospheric noise by stacking co-event interferograms and by using empirically derived covariance matrices in the modeling. We apply a new nonlinear dislocation modeling method that constrains the slip distribution to be elliptical at each point along the fault and uses field and creepmeter data as lower bounds on surface slip. Using this model, we compute the strain drop throughout the rupture. We apply this technique to the 2006, 2010, 2017, and 2023 aseismic ruptures in Envisat, UAVSAR, and Sentinel-1 data. Lastly, we compare the resulting strain drops to strain accumulation rates calculated from backslip, testing the hypothesis that shallowly released strain is equal to the strain applied from deep dislocations in the crust. Using only the creep events in the instrumental record, we find that interseismic slip rates on the SHF must be above 10 mm/yr to explain the observations, a result consistent with regional-scale block modeling. Our results have implications for the strength of faults, the expected modes of seismic moment release in the shallow crust, and for seismic hazard analyses near creeping faults.

How to cite: Materna, K. and Bilham, R.: Shallow Aseismic Slip and Stress/Strain Budgets on the Creeping Faults in the Imperial Valley, California, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21901, https://doi.org/10.5194/egusphere-egu26-21901, 2026.

Strike-slip faults critically control hydrocarbon migration in the Mesozoic clastic reservoirs of the Tahe Oilfield, NW China. However, their identification is challenged by weak seismic responses due to subtle impedance contrasts, steep dips, and small throws. This study conducts a systematic, multi-method comparison to optimize fault detection, evaluating both conventional seismic attributes and a novel deep learning (DL) approach.

We first applied structure-oriented filtering to enhance data continuity. Subsequently, key conventional attributes were computed: coherence and curvature to delineate major structural discontinuities and flexures, ant tracking to highlight fault pathways, and likelihood to map fault lineaments. The core of our DL approach involved a ResU-Net model, pre-trained on extensive datasets and refined via transfer learning using 65 manually interpreted fault traces from the target area. This process generated a high-resolution fault probability volume.

Results from the key T34 horizon demonstrate a clear performance hierarchy. While coherence and curvature effectively image major faults, they lack resolution for secondary networks. Ant tracking and likelihood show sensitivity to small-scale features but suffer from poor continuity and noise. In stark contrast, the AI probability volume integrates the strengths of these methods, simultaneously providing superior boundary clarity for major faults and enhanced detection of subtle, secondary strike-slip faults crucial for hydrocarbon migration. It presents a more continuous, spatially coherent, and geologically plausible 3D fault system.

This work underscores the significant advantage of an AI-driven, integrated workflow over individual conventional attributes. It provides a robust, scalable template for multi-scale fracture characterization in complex reservoirs, effectively bridging the gap between geophysical data analysis and geological interpretation.

How to cite: Jiang, Y. and Han, C.: Characterizing Mesozoic Strike-Slip Faults in China's Tahe Oilfield: A Multi-Method Comparison from Traditional Seismic Attributes to AI, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2170, https://doi.org/10.5194/egusphere-egu26-2170, 2026.

To clarify the migration characteristics of the X Sag trough in the Zhuyi depression of the Pearl River Mouth Basin and understand the distribution patterns of hydrocarbon source rocks, this study employs newly processed high-resolution 3D seismic data and integrates techniques such as fault activity period determination, fault displacement-distance curve analysis, and balanced section methods. It systematically investigates the deformation mechanism of the X Sag Fault in the Zhuyi Depression and its control over hydrocarbon source rock distribution. The study reveals: ① The X Sag is a north-dip, south-lobe scoop-shaped half-graben controlled by the NE-NEE-NWW multi-trend arc-shaped F1 fault, with three sets of secondary faults (NE, NEE, EW) developing within the depression. Based on the segmentation characteristics of the main boundary fault F1 and its combination patterns with secondary faults, the study area is divided into eastern and western sub-basins. ② The X Sag underwent multiple phases of tectonic evolution under the influence of multi-phase, multi-directional stress fields, primarily comprising four stages: Fracture Stage I, Fracture Stage II, Fracture Stage III, and the Tilting Stage. Based on the long-term activity characteristics of the main boundary fault F1 and the activity features of different phases of the secondary faults within the sag, five sets of fault systems were delineated: faults active only during the Early Wenchang Period, faults active during the Early Wenchang-Enping Period, faults active during the Late Wenchang-Enping Period, faults active only during the Enping Period, and long-term active faults. ③ The secondary faults within the X Sag are collectively controlled by a pre-existing arc-shaped NE-NEE-NWW-trending reverse-transform fault system. During basin formation, the western sub-sag underwent extensional deformation along pre-existing NE-NEE-trending faults, forming a series of secondary faults aligned with the main boundary fault strike. These appear in cross-section as structures reverse-cutting the main boundary fault. Conversely, the eastern sub-sag underwent extensional-torsional deformation along pre-existing NWW-trending strike-slip faults, generating a series of near-EW-trending secondary faults. During deformation, a “V”-shaped structural pattern formed in the profile. ④ The segmented growth and differential activity characteristics of different control-depression faults within the basin governed the migration of the depression trough sedimentary center from northwest to southeast and from the basin margin toward the basin interior, thereby influencing the distribution of hydrocarbon source rocks during the Early and Late Wenchang periods.

How to cite: Sun, X., Sun, Y., and Chen, F.: Deformation Mechanism and Its Depression Controlling-Source Controlling Effect of X Sag Fault System in Pearl River Mouth Basin, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2947, https://doi.org/10.5194/egusphere-egu26-2947, 2026.

Tectonic fractures refer to a series of discontinuities formed in crustal rocks under the action of tectonic stress, serving as a key factor governing numerous geological processes and resource exploitation. In the field of oil and gas exploration, especially for the tight sandstone reservoirs in the Kuqa Depression of the Tarim Basin, the tectonic fracture system acts as the primary seepage pathway and reservoir space, directly determining the distribution of reservoir "sweet spots" and single-well productivity. To achieve quantitative characterization of reservoir fractures and accurate prediction of their spatial distribution, we innovatively introduced the principle of minimum energy dissipation and the principle of least action, which reflect the essential laws of nature. By fully integrating the complex tectonic evolution process with classical mechanics theory, we completed the quantitative prediction research on fractures during complex tectonic evolution based on four-dimensional (4D) dynamic stress field simulation. Based on the analysis of tectonic evolution history in the Keshen 8 area of the Kuqa Depression, combined with extensive field, seismic, core and logging data, as well as rock mechanics experiments and acoustic emission experiments, a reasonable paleotectonic geomechanical model was established. From a novel perspective, we introduced the principle of minimum energy dissipation and the principle of least action, and further combined them with classical mechanics theory and the variational principle of continuum media. A time-domain dynamic rock failure criterion and a fracture parameter characterization model were constructed, building a "bridge" between stress and fracture parameters. By selecting an optimal elastoplastic finite element simulation platform and setting appropriate time steps, we completed the time-domain 4D tectonic stress field simulation. On this basis, we implanted Python programs into the finite element simulation platform, realizing the quantitative prediction of the spatial distribution of reservoir fractures in the Keshen 8 area of the Kuqa Depression. The prediction results indicate that folding is the primary controlling factor for fracture development in the Keshen 8 gas reservoir. On the plane view, the linear fracture density in the structural high parts of the east-west anticlines is slightly higher than that in the saddle parts and both limbs, and the linear fracture density in the core of the eastern anticline is higher than that of the western anticline. The fracture dip angle gradually decreases from the structural high points to the two limbs of the anticlines. The prediction results are in high agreement with the actual well-point measurement data and production performance data. High-yield wells are basically located in fracture-developed zones with high linear density and near-vertical dip angles.

How to cite: Liu, S., Wang, G., and Feng, J.: Quantitative Prediction of Tectonic Fractures Coupled with Minimum Energy Dissipation and Least Action Principles: A Case Study of Keshen 8 Area, Kuqa Depression, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3308, https://doi.org/10.5194/egusphere-egu26-3308, 2026.

Crystalline rocks are typically  low in porosity, but they often contain fractures, which provide critical pathways for fluid flow and influence groundwater storage, resource estimation, and safety assessments for nuclear waste repositories. Despite their importance, effective fracture porosity in crystalline rocks remains poorly constrained due to limited and regionally biased measurements. In this study, we used global permeability datasets and modified an existing equation to estimate porosity from permeability, incorporating fracture roughness and aperture. This allowed us to calculate nearly 28,000 porosity values across a wide range of depths and geological settings. The resulting porosity distributions are highly right-skewed and show an exponential decrease with depth. Our findings indicate that porosity values in crystalline rocks are generally lower than previously assumed. Median porosity values in the upper 100 meters are several orders of magnitude lower than the commonly assumed 1% porosity, highlighting a significant discrepancy between our estimates and traditional assumptions. We quantified uncertainty using Monte Carlo simulations, which show that natural variability in porosity dominates over parameter uncertainty, underscoring the robustness of our global trends. These findings imply that groundwater storage in crystalline rocks is far smaller than previously estimated, and groundwater velocities may be higher than predicted by models assuming larger porosity, with implications for contaminant transport and nuclear waste safety.

How to cite: van den Berg, J. and Luijendijk, E.: Effective fracture porosity in crystalline rock, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3360, https://doi.org/10.5194/egusphere-egu26-3360, 2026.

Quantitative characterisation of the geometrical properties of discontinuities in fractured rock masses is fundamental for understanding their mechanical behaviour, structural characterisation, and for performing reliable rockfall susceptibility assessments. Discontinuity abundance parameters, such as intensity and density, play a key role in rock mass classification and hazard analysis. Yet, accurately estimating them remains challenging due to limited accessibility, scale effects, and censoring bias in conventional field surveys.

Recent advances in remote sensing techniques, particularly UAV-based digital photogrammetry, enable the acquisition of high-resolution three-dimensional point clouds and ortho-view images, commonly referred to as Digital Outcrop Models (DOMs). These datasets significantly improve access to steep or unstable rock faces and enable detailed, reproducible discontinuity mapping. However, standardised, open-source tools for the quantitative analysis of discontinuity abundance from 2D ortho-view images remain limited.

Here, we present a new toolbox within the open-source MATLAB application QDC-2D (Quantitative Discontinuity Characterization, 2D) (Loiotine et al., 2021), focused on the calculation and spatial visualization of discontinuity abundance parameters. The toolbox computes commonly used linear (P10) and areal (P20, P21) discontinuity intensity and density metrics using two approaches. It uses well-established Mauldon estimators (Mauldon et al., 2001) and introduces a circular scan window approach that improves fracture intensity and density estimation through direct calculation of discontinuity trace segment lengths and number within the circular scan window. The toolbox further allows user-defined regions of interest (ROI) and cluster-based abundance calculation to capture spatial variability in discontinuity density and intensity. This approach enables the detection of high-fracturing zones with high certainty.

The toolbox's capabilities have been thoroughly tested and validated using multiple synthetic discontinuity datasets, demonstrating robust, reliable performance. This extension toolbox for QDC-2D provides a reproducible, accessible framework for quantitative discontinuity analysis, thereby supporting improved structural characterisation of fractured rock masses.

References:

Mauldon, M., Dunne, W. M., & Rohrbaugh, M. B., Jr. (2001). Circular scanlines and circular windows: New tools for characterizing the geometry of fracture traces. Journal of Structural Geology, 23(2–3), 247–258.

Loiotine, L., Wolff, C., Wyser, E., Andriani, G. F., Derron, M.-H., Jaboyedoff, M., & Parise, M. (2021). QDC-2D: A Semi-Automatic Tool for 2D Analysis of Discontinuities for Rock Mass Characterization. Remote Sensing, 13(24), 5086. https://doi.org/10.3390/rs13245086

How to cite: Lukačić, H., Wolff, C., Krkač, M., and Jaboyedoff, M.: New Integrated QDC-2D Toolbox for 2D Discontinuity Abundance Calculation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3968, https://doi.org/10.5194/egusphere-egu26-3968, 2026.

The geometry and kinematics of small faults within relay ramps are affected by local stress perturbations due to coeval propagation of laterally overstepping normal faults with negligible separations. Independent of their stepping sense and relative fault slip rates, previous studies documented the high structural complexity of carbonate relay ramps cropping out in central Italy, where Mesozoic carbonates of the Lazio-Abruzzi Platform are crosscut by an active extensional fault system. Notably examples include the ~400 m-wide, ~900 m-long relay ramp within the Tre Monti fault dissected by small faults with variable attitudes and kinematics showing values of fault and fracture density peaks in its middle portion. Similarly, the ~3 km-wide, ~7km-long relay ramp bounded by the Venere-Gioia dei Marsi and Pescina-Parasano normal faults exhibits the highest amount of extensional strain within its central portion.

In order to gain new insights on the possible role of surface geometry of the main slipping planes on the spatial distribution of fault-related damage, we focus on the ca. 8 km-long, 110 m-offset, NW-SE striking and SW-dipping Monte Capo di Serre fault. This fault displaces Mesozoic-Tertiary platform carbonates and Pleistocene slope debris, and it is continuously exposed along a ~800 m-long along-strike outcrop. Studying a ~60 m-long and 32 m-wide relay ramp bounded by 100’s m- long fault segments forming a sinistral overstep, and at smaller scale a 9 m-long, 5 m-wide relay ramp bounded by 10’s of m-long dextral overstepping slip surfaces we first conduct field and digital structural analyses and then fault roughness analysis.

Results show that the slickenside attitude and kinematics are controlled by overstep geometry. In fact, dextral oversteps are associated with NNW-SSE to N-S striking high-angle slickensides recording pure-dip slip extension, whereas sinistral oversteps are characterized by ESE-WNW to E-W striking, moderate-angle slickensides recording left-lateral transtension. Independently of the overstep geometry, results of spectral analysis of the outcropping slickensides indicate they are significantly rougher (root mean square roughness, R_q≈25-68 mm) within relay ramps than along the main slip surfaces (R_q≈1 mm). Integration with microstructural observations suggests that the relay ramps localized diffuse post-seismic deformation and aftershock-related fracturing, as recorded by diffuse host rock brecciation and widespread fracturing. Conversely, the main slip surfaces predominantly accommodated seismic slip, as shown by truncated clasts and multiple generation of cataclasite and ultracataclasite layers. We argue that these results support the interpretation that fault-surface roughness within carbonate relay ramps might exert a primary control on local stress perturbations, thereby contribution to their complex structural and kinematics complexities.

How to cite: Agosta, F., Dastoli, S., Taddeo, C., Abdallah, I., Curzi, M., Mercuri, M., and Corradetti, A.: Structural complexity and fault roughness properties of carbonate relay ramps, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4008, https://doi.org/10.5194/egusphere-egu26-4008, 2026.

Along the downfaulted axial zone of the southern Apennines fold-and-thrust belt of Italy, ongoing work focuses on field survey of high-angle extensional fault zones, and integrated microstructural, mineralogical, and stable isotope analyses of fault-related calcite veins. Two study areas are investigated. The first one lies in the southern portion of the seismically active Irpinia region, the second one along the southern flanks of the Raparo Mt., Basilicata. There, we study Mesozoic shallow-water carbonates that first underwent thrusting tectonics, and then extension and exhumation from shallow crustal depths. Within the fault zones, we select the high-angle Slip Parallel veins (SP-veins) and low-angle Comb veins (C-veins), respectively oriented parallel and perpendicular to the fault dip.

In the Irpinia region, results of microstructural analysis of the vein assemblage indicate that the high-angle faults are characterized by veins containing blocky to elongated and fibrous calcite. Blocky calcite minerals show type I and II twinning. Furthermore, inclusion bands associated with crack-and-seal processes are also present. In line with established microstructural interpretations, blocky calcite is interpreted as post-kinematics, whereas elongated and fibrous calcite is regarded as syn-kinematics. Occurrence of type I and II calcite twinning suggests that the intracrystalline deformation temperatures in these regions falls within ca. 150^o C - 300^o C.

At the Raparo Mt., microstructural data are consistent with blocky, elongate-blocky calcite textures of both SP- and C-veins. The former veins are dominated by blocky calcite with established presence of Type I and II calcite twinning, while the latter veins occasionally show blocky calcite. This area also shows widespread occurrence of both high- and low-angle veins with microcrystalline textures, which suggest that rapid cooling of the mineralizing fluids and precipitation took place in their formation process. Common tar-rich mineralization is also observed along the low-angle veins.

Aiming at deciphering the relative timing of formation and paleo stress regimes, present work is dedicated to the detailed microscale documentation of the crosscutting/abutting relations among the different vein sets. At the same time, extraction of powder samples is taking place for subsequent geochemical analyses. Results will be key to determine the source/s of the mineralizing fluids, determination of isotopic fractionation, and amount of fluid-rock interaction. These analyses will enable formulation of valuable hypotheses regarding the modalities of ingression of the mineralizing fault fluids within the study fault zones.

How to cite: Kyari, A., Zummo, F., Abdallah, I., Paternoster, M., Caracausi, A., and Agosta, F.: Integrated field and laboratory analyses of vein assemblages from the downfaulted southern Apennines fold-and-thrust belt, Italy., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4297, https://doi.org/10.5194/egusphere-egu26-4297, 2026.

LA-ICP-MS U–Pb geochronology of syn-kinematic calcite in faults and fractures provides a direct means of dating brittle deformation. We present U–Pb calcite geochronological data from deformation features across a range of scales—stylolites, veins, minor faults, and major thrusts—within the Provence fold-and-thrust belt. Whether in thrust-related cover folds (Mirabeau and Bimont) or in the more complex Nerthe Massif, the results illustrate how calcite geochronology can enhance or challenge our understanding of fracture pattern development in reservoirs.

Calcite geochronology validates the sequence of fracture development during layer-parallel shortening, fold growth, and late-stage fold tightening, previously inferred from structural orientations and cross-cutting relationships, regardless of fold type. Dating of calcite jogs formed at the tips of sedimentary or tectonic stylolites further constrains the timing of deformation stages. Geochronology also helps differentiate local from regional deformation by defining a more precise chronological framework where other geological markers are absent.

Across all investigated structures, deformation features show remarkable age consistency and slight overlaps between stages, providing a continuous and detailed record of fracture development. The age overlaps may indicate that deformation lasted less than the analytical uncertainty or that fracturing was more continuous throughout folding and thrusting than previously assumed. The consistency of ages across structural scales suggests either coeval deformation or events too close in time to be distinguished by U–Pb dating. This observation supports the syn-kinematic nature of calcite mineralization in small tectonic veins, even where infills display blocky, non-stretched textures. While precipitation may lag slightly behind fracture opening in individual veins, at the vein-set scale, both processes remain coeval within dating resolution. This broadens the applicability of U–Pb calcite geochronology to diverse mesoscale structures.

The dataset reveals the multi-phase development of similarly oriented fractures, which possibly initiated during burial and were reopened or densified during subsequent tectonic episodes. Geochronology provides a robust way to test whether fractures grouped by orientation, deformation mode, and relative chronology (‘fracture sets’), as well as classical associations of veins, stylolites, and conjugate faults defined by kinematic and mechanical compatibility, truly reflect the same deformation event. Veins with up to 60° strike variation sometimes yield indistinguishable ages (within a few Myr), challenging conventional definitions of fracture sets and implying local stress variations. This questions the presumed stability of the stress field in tectonic reconstructions.

Regionally, clusters of U–Pb calcite ages, if not reflecting sampling bias, hint towards variations in fluid activity, redox conditions, and/or uranium mobility, or distinct pulses of brittle rock damage and fluid flow. The latter interpretation suggests two deformation phases—late Cretaceous (81–67 Ma) and late Paleocene–Eocene (59–34 Ma)—separated by a Paleocene tectonic quiescence, matching the two already recognized Pyrenean shortening phases and indicating a likely, though not systematic, link between regional tectonic activity, brittle rock damage, fluid circulation, and calcite mineralization.

These examples demonstrate how U–Pb calcite geochronology not only constrains the timing and duration of brittle deformation but also helps reassess models of fracture development and fold–fracture relationships.

How to cite: Lacombe, O., Beaudoin, N., Zeboudj, A., Callot, J.-P., Lamarche, J., Hoareau, G., Guihou, A., and Deschamps, P.: Refining the picture of fracture development in folded carbonate reservoirs : Insights from U-Pb geochronology of syn-kinematic calcite mineralizations in the Provence fold-and-thrust belt, France , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4842, https://doi.org/10.5194/egusphere-egu26-4842, 2026.

The Tuscan marbles, primarily exposed in the Alpi Apuane Metamorphic Complex and the Montagnola Senese ridge, record a protracted deformation history spanning the rheological spectrum from ductile flow to brittle fracturing. While the syn-metamorphic ductile evolution of these units has been extensively studied, the subsequent brittle deformation—specifically post-metamorphic faulting and fracturing—remains poorly constrained. These fracture networks are not only uplift-related features; they record a polyphase brittle history with direct implications for fluid migration, quarry slope stability, and Neogene–Quaternary stress field reconstruction.

In this work, we characterize brittle structures within marble from the Montagnola Senese, located along the Mid-Tuscan Ridge in the Northern Apennines. This marble has been quarried since Roman times, making rock mass characterization relevant for both scientific and practical purposes. We adopt a multidisciplinary approach, integrating classical field surveys with 3D digital outcrop models obtained by photogrammetry. Data were collected at the outcrop scale and subsequently extrapolated to define the fracture pattern across the entire Montagnola Senese ridge.

The detected fractures and faults cut the marble schistosity, therefore post-dating the last metamorphism event (middle Miocene). Our results reveal at least two brittle deformation phases: (I) a first, left-lateral strike-slip system, followed by (II) extensional structures, which crosscut or reuse the previous ones. Fracture attributes, such as fracture intensity and density, within the non-faulted rock mass were compared to those associated with fault damage zones. These data provide constraints on both quarrying operations and fluid circulation models, whilst contributing to the definition of the tectonic setting of this sector of the Mid-Tuscan Ridge from the middle Miocene to the present day.

How to cite: Risaliti, G., Rizzo, R. E., Tavani, S., Coli, M., Nesi, J., and Vannucchi, P.: Analysis of post-metamorphism brittle deformation in marbles: insights from Montagnola Senese, Northern Apennine, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5002, https://doi.org/10.5194/egusphere-egu26-5002, 2026.

This study investigates how local stress state governs permeability magnitude in fractured carbonate aquifers. By using outcrop-constrained Discrete Fracture Network (DFN) modelling from Mt. Viggiano of the southern Apennines, Italy, we investigate the control exerted by 500 m-depth tri-axial local stress state on computed horizontal permeability anisotropy. Fractured carbonate systems commonly exhibit strong permeability anisotropies that evolve with depth as fractures respond to changes in both normal and shear stresses. Accurately capturing this behaviour remains challenging due to the combined effects of fracture geometry and connectivity, as well as primary depositional architecture and stress-dependent aperture modification.
Field-derived fracture datasets from four carbonate outcrops representing two contrasting paleo depositional settings are used to construct three-dimensional DFN models at the bed-package scale. Two DFN-based modelling workflows are employed to explore how different representations of fracture connectivity and flow influence predicted permeability. One approach estimates bulk permeability from fracture population statistics within distinct geocellular volumes. Differently, the other one explicitly simulates steady-state fluid flow through hydraulically connected fracture networks within a fully meshed computational domain. This integrated strategy allows evaluation of how modelling assumptions related to connectivity, aperture scaling, and flow representation affect permeability predictions without implying a preferred modelling tool.
The results of this study show that increasing normal stress generally reduces horizontal permeability anisotropy, although local increases in permeability occur where favourably oriented fractures undergo shear induced dilation. Result are also consistent with the permeability response varying systematically with depositional architectures: (i) massive, high-energy carbonates dominated by non-strata bound fractures exhibit vertically persistent but weakly connected networks; (ii) on the contrary, layered, low-energy carbonates containing abundant strata-bound fractures display enhanced lateral connectivity and higher hydraulic effective transmissivity.
The main outcomes of this work demonstrate that permeability anisotropy in fractured carbonates evolves with stress through its interaction with fracture orientation, connectivity, and stratigraphic architecture. Incorporating stress dependent behaviour and explicit connectivity into DFN workflows therefore improves predictions of subsurface fluid flow relevant to groundwater resources, CO₂ storage, and geothermal systems.

How to cite: Abdallah, I. B., Healy, D., Hyman, J., Prosser, G., and Agosta, F.: Permeability Anisotropy in Fractured Mesozoic Platform Carbonates under Variable Triaxial Stress Conditions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5148, https://doi.org/10.5194/egusphere-egu26-5148, 2026.

The stiff Ypresian clays of the Kortrijk Formation occur extensively throughout the subsurface of the Princess Elizabeth Zone (PEZ), North Sea. The formation is pervasively deformed with large-scale polygonal fault networks, so-called Clay Tectonic Features (CTF; e.g. Verschuren, 2019, Marine Geology). Moreover, recently acquired samples from the Kortrijk Formation in the PEZ suggest a heavily fissured internal structure of these clays at the centimeter scale. The presence of faults and fissures in this formation have strong implications for its geotechnical properties, such as strength and stiffness, which may pose challenges for the foundations of the planned offshore wind energy farms.

With this in mind, we study the physical, mineralogical and chemical properties of the Kortrijk Formation in high-resolution using a multi-methodological approach including X-ray CT scanning, organic and inorganic geochemical analyses (LOI, organic material, calcimetry, pH, stable carbon isotopes, pXRF, XRD and ICP-OES) and sedimentological investigations (grain size, thin sections). The first samples were collected from a 20m deep borehole with alternating rotary coring and hydraulic push sampling in Rumbeke (from a section that is considered stratigraphically equivalent to the PEZ) and multiple drilling campaigns at other locations are planned.

Initial X-Ray CT scans of these samples reveal a heterogenous internal architecture containing four main feature types: bioturbation, concretions, fissures, and faults. Bioturbation occurs throughout the cores, often appearing as millimeter-thick, centimeters-long, high-density features, likely reflecting the presence of precipitated minerals such as pyrite, following microbially-mediated sulfate reduction. In contrast, concretions (siderite-fluorapatite) are rare in the core sections, consistent with their observed scattered presence in land-based observations. Fissures are recognized as low CT-density features which do not occur throughout all the core sections but are concentrated in localized zones, leaving intervening volumes of clay intact. The observed cm-scale normal faulting structures point to a local extensional regime. The geometry, pattern, and textures of the observed fissures and fractures are tested against established criteria (e.g. radial and axisymmetry, bending near the core rim, etc.) to conclusively differentiate natural features from coring-induced artifacts (Adriaens et al., 2024, Geoenergy). To quantitatively analyze all features, the X-ray CT data are processed using a comprehensive workflow involving filtering, segmentation, and grouping of features based on multi-ROI analysis using 3D connectivity. Following isolation, we perform a detailed analysis of the morphological characteristics (e.g., volume, surface area) and the three-dimensional orientation of the segmented features. The high-resolution 3D model of the features in the clay derived from CT scanning will be used to inform numerical models which will test the stiffness and long-term mechanical stability of the Kortrijk formation clays under different geotechnical loading scenarios.

By combining detailed sedimentological, mineralogical and geochemical characterization with the high-resolution CT-based structural analysis, we aim to establish the origin of the fissures and faults in the Kortrijk formation, thereby providing the geological context for their impact on geotechnical stability.

How to cite: Piret, L., Van Daele, M., Stuyts, B., De Batist, M., Dewaele, S., Kheffache, A., and Payan, M.: Cracks in our foundations: The nature and origin of fissures in the Kortrijk Formation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5516, https://doi.org/10.5194/egusphere-egu26-5516, 2026.

The structural architecture of the Eastern Province of Saudi Arabia is defined by a complex interplay between localized halokinesis and regional compressional stresses. This study provides a comprehensive geological investigation into the mechanical and tectonic linkages between the heavily fractured Late Miocene-Pliocene Hofuf Formation and the Eocene Rus Formation situated at the apex of the Dammam Dome. Historically, these two units have been studied as distinct stratigraphic entities; however, this analysis integrates field observations from Jabal Al-Shuʿba with regional geophysical data to demonstrate a shared deformation history. The Dammam Dome, an oval-shaped structure covering approximately 500 km, is cored by the Infracambrian Hormuz Salt. Its diapiric rise, occurring at rates of up to 7.5 m/Ma during the Neogene, induced a systematic fracture network in the Rus Formation characterized by radial and concentric Mode I tension joints. Concurrently, the Arabian Plate's collision with Eurasia, the Zagros Orogeny, transmitted far-field intraplate stresses that reactivated these older structural grains. Field data from the Hofuf Formation at Jabal Al-Shuʿba reveal a high-intensity, multidirectional fracture system within alternating sandstone and mudstone beds. Unlike the uniform patterns observed at the Dammam Dome apex, the Hofuf fractures exhibit bimodal and conjugate orientations (NNE-NE and NW-SE) with apertures reaching 15 cm. This disparity is attributed to mechanical stratigraphy; the bed-bounded nature of fracturing in the clastic Hofuf Formation prevents the stress relief found in the massive Eocene carbonates, leading to increased fracture density. Furthermore, the identification of a soft-sediment detachment within the Rus Formation suggests that the Dammam Dome served as a sensitive stress sensor for the initial stages of the Zagros collision. By establishing a structural bridge between the Eocene and the Neogene, this study explains how salt-induced uplift and plate-scale compression have combined to create the heavily fractured landscape of Al-Ahsa. These findings offer critical insights for reservoir characterization, groundwater flow modeling, and urban geomechanical stability in the region.

How to cite: Osman, M.: Integrative Structural Evolution of the Eastern Arabian Platform: Decoupling Salt-induced Halokinesis and Zagros-related Compression in the Fractured Neogene and Paleogene Successions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5893, https://doi.org/10.5194/egusphere-egu26-5893, 2026.

The Einstein Telescope (ET), a proposed third-generation underground gravitational-waves observatory requires an acoustically quiet subsurface environment to minimize the effect of seismic ambient noise. A detailed site characterization is currently underway in the Euregion Meuse-Rhine (Germany, Belgium, Netherlands), one of the potential locations for the ET. The site is wedged between the northern margin of the Variscan deformation front and the subsiding Lower Rhine Graben. Subsurface fluid flow, including natural groundwater circulation and drainage constitute sources of induced seismic and gravity-gradient variations. Pre-existing faults and fractures act as preferential flow paths and, critically control potential water inflow into the infrastructure and influence related water management strategies. Consequently, characterizing the relationship between the tectonic stress field and hydraulic characteristics of the host-rock formations is essential for a resilient ET design. In this study, we investigate the evolution of the tectonic stress field (i.e., from paleo- to recent in-situ stresses) and its control on fracture permeability, using an integrated dataset of boreholes drilled in the study area from 2021 to 2025 and down to 250 to 420 m. Paleo-stress conditions are reconstructed from fracture orientations and kinematic indicators observed on drill core material and borehole-televiewer data. The present-day stress-state is evaluated using hydraulic fracturing and hydraulic testing of pre-existing fractures tests. Fracture architectures are characterized using televiewer imagery and core samples, while their hydraulic relevance is assessed through in-situ methods such as impeller flow-meter measurements, temperature logs, and hydraulic packer tests. Slip versus dilation tendency analysis is applied to evaluate deformation modes and associated permeability anisotropies. These results are compared to independent hydraulic indicators to distinguish between hydraulically active and inactive discontinuities. Our findings demonstrate how the complex tectonic history governs the present-day fracture network and associated groundwater pathways, providing key constraints on groundwater management and suitability assessments for the site selection of the ET project in naturally fractured sedimentary host-rocks.

How to cite: Burchartz, R., Kruszewski, M., Vis, G.-J., Claes, H., Müller, A., VanBrabant, Y., Deckers, J., Orban, P., Drimmer, D., Scheltens, M., Vink, B., Kiehn, M., Amann, F., and Spychala, Y.: Implications of stress field evolution on groundwater flow at the northern Variscan front and its relevance for the Einstein Telescope site selection (Euregion Meuse-Rhine), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6648, https://doi.org/10.5194/egusphere-egu26-6648, 2026.

Exfoliation fractures are a defining feature of Yosemite’s granitic landscapes, yet the relative roles of lithology, glacial history, inherited structural fabrics, and near-surface thermal processes in controlling their geometry remain incompletely constrained. To evaluate these controls, we collected detailed field measurements across a suite of granitic domes spanning glaciated (Pothole Dome, Puppy Dome, Lembert Dome, Turtleback Dome, Olmsted Point) and non-glaciated (Half Dome, Sentinel Dome, North Dome) settings. Field investigations included systematic measurements of exfoliation sheet thickness and length, fracture orientation data, photographic documentation, and GPS surveys to assess spatial distributions of exfoliation fractures on individual domes. These data are integrated with lidar-derived topographic measurements to provide surface context and support geometry-based analyses.

Preliminary results indicate that exfoliation sheet thickness varies between domes, with glaciated domes tending to display thicker sheets and broader thickness distributions than non-glaciated domes, although substantial overlap exists between groups. Non-glaciated domes commonly exhibit thinner sheets and more variable geometries, potentially due to longer near-surface exposure and progressive weathering accumulation. Across all sites, exfoliation sheet length shows weak to moderate scaling with thickness; however, prevalent scatter in the data suggests that sheet geometry may not be influenced by thickness alone, but also by pre-existing joint sets and cross-cutting structural features that may limit lateral fracture propagation. Spatial context from GPS transects demonstrates that measurements were collected across broad surface positions on individual domes, with transects capturing tens to nearly 300 m of relief, reducing sampling bias and supporting dome-scale interpretation. Prior monitoring studies and field observations of rockfalls and active surface cracking in Yosemite suggest diurnal and seasonal thermal fluctuations contribute to ongoing subcritical crack growth, implicating thermal stresses as an active modern process superimposed onto background stresses (e.g., inherited structural features, removal of overburden, and tectonic and topographic stress). By comparing exfoliation characteristics across contrasting geomorphic settings, this study better constrains how factors such as lithology, glaciation history, inherited structures, and thermal forcing interact to shape near-surface fracture development in granitic terrains, with implications for rockfall hazard assessment and climate-sensitive rock damage processes.

How to cite: Reynolds, A. N., Stock, G. M., and Collins, B. D.: Investigating controls on exfoliation fracture geometry across glaciated and non-glaciated granitic domes, Yosemite National Park, USA, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6896, https://doi.org/10.5194/egusphere-egu26-6896, 2026.

Understanding the processes that govern fracture development in upper crustal rocks is crucial for characterizing the mechanical response of the Earth’s crust. While conventional failure criteria capture many aspects of fracturing observed in laboratory experiments, they fall short in explaining how system-spanning fractures emerge from the interaction and coalescence of microcracks distributed throughout a deforming rock mass. Additionally, empirical rupture models rarely distinguish the relative roles of tensile and shear mechanisms in macroscopic failure. In this study, we explore the influence of the geometrical arrangement of pre-existing microcracks on fracture formation by analyzing the elastic stress perturbations they generate, employing two-dimensional Finite Element Method (FEM) simulations. This approach allows us to quantify how cracks with different orientations modify the surrounding stress field, producing localized zones of elevated tensile and/or shear stress that may act as favorable pathways for fracture propagation. By systematically varying microcrack orientation and distribution, we can map how stress concentration patterns interact, providing a framework for understanding fracture coalescence in heterogeneous rock materials. Our results reveal that the orientation and spatial arrangement of pre-existing microcracks dictate the directions and magnitudes of stress perturbations, creating preferential trajectories for system-spanning fractures. In particular, regions of high tensile and shear stress develop between interacting cracks, offering a physical explanation for the formation of interconnected fracture networks, including en echelon fracture systems, under varying geometrical configurations. These findings indicate that macroscopic shear fractures may originate not only from the coalescence of tensile cracks formed during early deformation stages but also from the interaction of pre-existing cracks with different orientations, especially where tensile stress is concentrated at crack tips. The study demonstrates that the geometry of pre-existing microcracks is a primary factor controlling the spatial organization of resulting fracture networks. Fractures accommodating shear deformation, typically oriented at approximately ±30° to the axis of maximum compression, can arise from the coalescence of mode I cracks due to localized tensile stress concentration, rather than requiring shear-dominated initial conditions. This insight bridges a gap between classical fracture mechanics and observations of natural and experimental rock fracture systems, highlighting the interplay between tensile and shear mechanisms in shaping macroscopic failure patterns. Overall, our work emphasizes the importance of microstructural geometry in governing fracture evolution, offering a quantitative, framework which integrates LEFM analytical results with FEM-based models to predict the emergence of complex fracture networks from initial microcrack distributions. By linking local stress perturbations to large-scale fracture patterns, this study provides a more comprehensive understanding of the conditions leading to system-spanning fractures in the upper crust.

How to cite: Manna, L., Maino, M., Casini, L., and Dabrowski, M.: The role of pre-existing microcrack geometry in fracture initiation and propagation during elastic deformation: integrating LEFM analysis with FEM modeling, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11582, https://doi.org/10.5194/egusphere-egu26-11582, 2026.

Fault damage zones are regions surrounding a fault core where secondary fractures are intensely developed due to distributed deformation. Previous studies on small-scale faults (consisting of one or two geometric segments) have predicted that the characteristics of the damage zone vary depending on the position along the fault. However, applying these models to large-scale fault zones is challenging due to the lack of continuous exposure and their inherent structural complexity.

This study aims to analyze the spatial variability of damage zone width in relation to fault geometry, focusing on medium-scale strike-slip faults (comprising three or more geometric segments), which offer a balance between structural complexity and observable continuity. The study area, Geoje Island, consists of hornfelsic Cretaceous lacustrine sedimentary rocks. The extensive wave-cut platforms along the coast provide excellent exposure for characterizing the geometry of the master fault and associated damage zones.

Fracture density (P₂₁) was systematically quantified across fault segments and boundaries using circular scanlines arranged along strike-perpendicular traverses. The width of the damage zone along each traverse scanlines was determined by analyzing the changes in fracture density relative to the distance from the Principal Displacement Zone (PDZ). The results indicate that the width of the damage zone is highly variable and exhibits significant asymmetry in certain sections. Specifically, in linking damage zones, a widespread distribution of damage is observed beyond the extensional overlap zones, contrasting with patterns typically seen in small-scale faults. Furthermore, strong asymmetry is prominent in regions where the fault strike changes. However, such widespread damage distribution and asymmetry are well consistent with the characteristics of tip damage zones observed in small-scale faults.

These observations indicate that geometric complexity at these locations contributed to arresting rupture propagation during reactivation. Although individual rupture mechanics are similar across scales, the cumulative effect of geometric barriers in medium-scale faults appears to dictate the spatial evolution of the damage zone.  These findings are expected to provide valuable insights for predicting and understanding the architectural evolution of large-scale fault zones.

This research was supported by a grant (2022-MOIS62-001(RS-2022-ND640011)) of National Disaster Risk Analysis and Management Technology in Earthquake funded by Ministry of Interior and Safety (MOIS, Korea).

How to cite: So, J., Seo, Y., Park, K., Bae, S., and Kim, Y.-S.: Variability of Fault Damage Zone Width in Strike−Slip Faults: A Case Study from the Coast of Geoje Island, Korea, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12013, https://doi.org/10.5194/egusphere-egu26-12013, 2026.

Early fracture networks in carbonate reservoirs may constitute long-lasting fluid flow conduits and their characterisation is pivotal in reservoir modelling. These fractures are, however, often overlooked in reservoir characterisation due to a lack of predictive workflows. Yet, syn-depositional fracture networks significantly affect reservoir quality and performance due to (1) providing an early and effective fluid flow network, (2) are prone to reactivation during later tectonic events and (3) are pathways for early dolomitising fluids.

Understanding the processes of fracture formation is vital for predicting their spatial distribution. Syn-depositional fracture modelling is, however, challenging as they form without tectonic drivers, influenced instead by intrinsic stresses due to the internal geometries of carbonate platforms. Early lithification of carbonates further aids fracture formation due to internal weaknesses and rapid sediment progradation. In reef-rimmed platforms, fractures generally align perpendicular to the platform’s trajectory, while internal patterns vary without a predominant orientation.

Traditional Discrete Fracture Network (DFN) models are inadequate for predicting these networks, as fractures result from complex geometries rather than regional stress fields. Process-based stratigraphic modelling offers a powerful workflow to model carbonate internal geometries and their lithofacies zones, linking progradation/aggradation patterns to fracture intensity and spacing.

We developed a novel approach for syn-depositional fracture characterization, combining stratigraphic and fracture forward modelling to improve reservoir quality predictions and well placements. Outcrop analogue studies are used for ground-truthing and to validate the findings against digital outcrop models. This innovative workflow has the potential to significantly improve flow performance assessment for carbonate geothermal reservoirs.

How to cite: Winterleitner, G., Maerz, S., Meier, N., and Niederau, J.: Syn-depositional fracture network prediction in carbonates through process-based forward modelling., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12608, https://doi.org/10.5194/egusphere-egu26-12608, 2026.

The Tuscaloosa Marine Shale (TMS) is an unconventional play in southwestern Mississippi and southeastern Louisiana characterized by a well-developed network of natural fractures that strongly influences reservoir behavior and hydraulic fracturing performance. The play is significant to the energy industry due to its substantial hydrocarbon resources—estimated at approximately 1.5 billion barrels of oil and 4.6 TCF of gas—and its proximity to existing infrastructure. Although more than 80 wells have been hydraulically fractured in the formation, producing a total of 13.82 million barrels of oil and 9.04 BCF of gas, development remains challenging due to the shale’s high clay content, complex mineralogy, and the poorly constrained impact of natural fractures on production.

This study employs an integrated workflow to characterize natural fractures in the Tuscaloosa Marine Shale using electrical borehole image logs, shear-wave splitting data, and core descriptions from seven wells distributed across the play. The analysis indicates that the natural fractures are predominantly vertical to subvertical extension fractures, commonly fully mineralized, with heights ranging from 1 to 3 feet. These fractures preferentially trend east–west, are associated with calcite-rich intervals, and are capable of transecting the entire borehole. Smaller fractures often terminate at lithological boundaries but commonly reactivate along parallel planes.

The proposed methodology provides critical insight for optimizing hydraulic fracturing design by identifying stress orientation and optimal lateral placement relative to natural fracture distribution. In one lateral well alone, approximately 500 closed fractures were identified. Furthermore, the maximum horizontal stress orientation is shown to be consistent across the formation and aligned with the regional stress regime of the Gulf Coast Basin.

How to cite: Ruse, C. M. and Mokhtari, M.: Natural Fractures of the Tuscaloosa Marine Shale, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13667, https://doi.org/10.5194/egusphere-egu26-13667, 2026.

A NE-SW fault system described in the eastern sector of the Trans-Mexican Volcanic Belt (TMVB) has been active since the Pliocene and continued up to the Holocene with dip-slip kinematics. In a broad view, these NE-SW faults can be correlated to the Tenochtitlan fault system that extends from the southwest coast of Mexico to central Mexico, into the TMVB. The length of the faults, the damage zone width, and more than one slickensides on the fault planes suggests a complex deformation history. In this study, we investigate the geometry and kinematics of the NE-SW faults in the Miocene-Pleistocene rocks in the eastern sector of the TMVB to determine the kinematics of these faults during the Late Miocene to Holocene, for which it is unknown. Our results suggest that the Miocene rocks record two deformation events, one of which is related to crustal shortening that produced a strike-slip activity in the NE-SW faults during the Late Miocene. The second one is associated with crustal extension and the activity of the NE-SW faults with dip-slip kinematics. This extensional event was active during the Pliocene-Holocene. The reactivation analysis and our field observations suggest that the NE-SW normal faults are related to the reactivation of previous NE-SW strike-slip faults. The change in the kinematics of the NE-SW faults explains the complex geometry of the damage zones of the kilometric NE-SW faults and the highly fractured Miocene rocks.

Based on the fault system orientation, it is clear that these faults are incompatible with the field stress recorded in the eastern sector of the TMVB. This fact suggests that the NE-SW fault system is probably related to reactivated basement structures within a three-dimensional deformation with a complex deformation history. The activity type of the NE-SW faults is probably related to the dynamics of the subduction process in the southwest of Mexico, associated with the change in the dip (decrease) and the convergence velocity of the Cocos plate.

How to cite: Vasquez Serrano, A., Rangel Granados, E., and Arce Saldaña, J. L.: NE-SW fault system in the eastern sector of the Trans-Mexican Volcanic Belt: Origin, deformation, and reactivation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13737, https://doi.org/10.5194/egusphere-egu26-13737, 2026.

Strike-slip fault zones often play a key role in controlling brittle deformation patterns in fold-and-thrust belts. Yet, the distribution of associated fractures and their interactions with fold-related structures remain insufficiently understood across scales and as part of the deformation history. In the Jura Mountains, previous work in the Central Internal Jura, notably at Creux-du-Van, proposed a multi-scale hierarchy of strike-slip faults and associated fracture networks based on sub-seismic structures spanning meter- to kilometer-scale. This study extends the multi-scale structural analysis to a regionally mappable ~12 km long N–S striking sinistral fault zone, the Suchet Fault. The fault is exposed along the E–W-oriented Gorges de l’Orbe, where incision into Upper Malm limestones enables largely continuous outcrop-scale access to fault-related damage zones.

To address the multi-scale character of this system, we combine field-based structural mapping at 1:5,000 scale and three-dimensional structural interpretation of publicly available aerial LiDAR data (SwissALTI3D) with systematic fracture data acquisition along four scanlines totaling 157 m. Scanlines are positioned at varying distances from the fault core, defined by the presence of fault breccia and gouge lenses, and across western and eastern structural compartments, spanning fault-proximal to fault-distal domains. This configuration enables comparison between fracture populations associated with fault-zone architecture and those interpreted as fold-related or background fracturing.

The Suchet Fault separates two contrasting structural domains. The western compartment is characterized by NW–SE striking fold trains, whereas the eastern compartment exhibits a comparatively flatter structural geometry. Within the vicinity of the fault trace, bedding orientations rotate progressively toward the N–S fault trend, with gentle eastward dips (~15°). Locally, near the fault core, bedding dips steepen and may reach up to 70°, indicating increased strain localization within the damage zone. LiDAR-based structural interpretation identifies three dominant fracture populations, with the NW–SE striking set displaying comparatively longer fractures than the N–S and NE–SW sets.

Fault-slip indicators show dominant subvertical conjugate strike-slip pairs at outcrop scale, comprising sinistral N–S to NNE-SSW striking faults and dextral NW–SE striking faults. Preliminary paleostress inversion analysis indicates a strike-slip regime characterized by a subhorizontal NW-directed maximum principal stress and a subvertical intermediate stress, consistent with results from other sectors of the Central Internal Jura. Fracture density (P10) increases toward the fault core, with values close to the brecciated core notably higher than those measured beyond 100 m.

This study emphasizes the need for robust fracture set definition and sequencing as a basis for structural analysis, including paleostress orientations and spatial variations in fracture intensity and anisotropy. This study evaluates whether structural patterns and paleostress behaviors identified at Creux-du-Van are comparable to those observed in the Gorges de l’Orbe area, at the scale of larger strike-slip fault zones. It also considers the potential regional implications of these structural features for fracture-controlled fluid flow in the Jura Mountains and potentially downstream, in the geothermal reservoirs beneath the Molasse Basin.

How to cite: Caldeira, J., Samsu, A., Tanaka, A., and Bazalgette, L.: Strike-slip fault zone architecture in folded Upper Malm limestones at Gorges de l’Orbe, Central Internal Jura, Switzerland, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14987, https://doi.org/10.5194/egusphere-egu26-14987, 2026.

China hosts substantial lacustrine shale-oil resources and represents a key strategic replacement for sustaining reserves growth and production. Natural fractures are critical for enhancing the flow capacity of low-porosity, low-permeability, strongly heterogeneous lacustrine shale reservoirs and also exert fundamental controls on shale-oil accumulation and preservation. In the second member of the Funing Formation (E₁f²) in the Qintong Sag, Subei Basin (eastern China), fractures are abundant and diverse, yet their development characteristics remain insufficiently constrained and a systematic evaluation of controlling factors has not been fully conducted. This study integrates core-based fracture description, thin-section petrography, and borehole image logs. Fractures are classified according to geological origin, mechanical mechanism, and geometric relationships with bedding, and their development characteristics are quantitatively documented. For multiple geological attributes—including distance to faults, mechanical layer thickness, TOC, and XRD-derived mineral contents—we employ Theil–Sen estimators to conduct a “score–confidence interval” ranking of effect strength and thereby delineate the hierarchy of controlling factors.Results indicate that bedding-parallel fractures, intra-layer shear fractures, and cross-layer shear fractures are dominant, whereas intra-layer tensile fractures and bedding-parallel shear fractures are subordinate. Fractures are predominantly high-angle, with apparent fracture height on core surfaces generally <15 cm. Fracture strikes comprise multiple sets, with a dominant NNE–SSW orientation. Fractures exhibit an overall moderate degree of infilling, and calcite is the principal cement. Distance to faults is negatively correlated with structural-fracture density and is identified as the primary control, whereas mechanical layer thickness and clay-mineral content are secondary factors and also show negative correlations with structural-fracture density. In contrast, higher TOC and greater lamination density promote the development of bedding-parallel fractures and constitute the primary controls, whereas higher clay-mineral content and greater mechanical layer thickness act as secondary factors that are unfavorable for bedding-parallel fracture development. These results clarify fracture distribution patterns in E1f2 and provide geological constraints for shale-oil exploration and development in eastern China, while also offering a transferable framework for the quantitative evaluation and ranking of fracture-controlling factors.

How to cite: Li, S., Lyu, W., Zeng, L., Shen, B., Ma, X., and Li, P.: Development characteristics and controlling factors of natural fractures in lacustrine shale oil reservoirs: A case study of the second member of the Funing Formation (E1f2), Qintong Sag, Subei Basin, eastern China, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15124, https://doi.org/10.5194/egusphere-egu26-15124, 2026.

 Abstract:

Fracture networks play a critical role in controlling rock mechanics, fluid flow, and crustal deformation. However, many conventional analytical approaches do not adequately account for the spatial anisotropy of fracture nodes. This study introduces a wavelet-based angular variance method to quantify multiscale anisotropy in fracture network nodes, including I-, Y-, X-, and X + Y-nodes, as well as barycenters, using both synthetic and natural datasets.

Synthetic experiments demonstrate that isotropic fracture systems produce spatially random node distributions, whereas anisotropic systems generate distinct directional clustering, such as cross-shaped patterns aligned along NE–SW and NW–SE orientations. Application of the method to field data reveals strong correspondence between node anisotropy and underlying structural features. In the Jabal Akhdar dataset, X- and X + Y-nodes show pronounced elongation along an ENE–WSW direction, I-nodes exhibit weaker lobation in the same orientation, and barycenters remain largely isotropic. In contrast, the Getaberget dataset displays significant anisotropy across barycenters and multiple node types (Y, X, and X + Y), with dominant N–S to NNW trends consistent with NE–SW and NW–SE fracture sets.

These results demonstrate that wavelet-based node analysis is capable of detecting subtle, scale-dependent anisotropy in fracture systems. The proposed approach provides a sensitive, continuous, and scalable framework for quantifying fracture network organization, offering valuable insights for reservoir characterization, geothermal resource assessment, and the analysis of fracture-controlled fluid flow in geological systems.

 Keywords: Fracture network; Nodes; Spatial analysis; Point anisotropy; Wavelet analysis

 Acknowledgement

PG acknowledges the Indian Institute of Technology Roorkee and the Ministry of Human Resource Development (MHRD), Government of India, for support through a PhD fellowship. SB acknowledges financial support from the Department of Science and Technology (DST), Government of India (Project No: SRG/2021/001903), and from FIG (Grant No: FIG-100886-ESD), Indian Institute of Technology Roorkee, India.

How to cite: Gairola, P. and Bhatt, S.: Anisotropy of fracture nodes using wavelet analysis, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18303, https://doi.org/10.5194/egusphere-egu26-18303, 2026.

Rockfalls are among the most critical phenomena in geomechanics due to the significant risk they pose to human lives and infrastructure. Rockfall risk is defined by the interplay between hazard and the potential impact on exposed elements. Specifically, hazard assessment relies on the propensity for detachment (estimated frequency), event magnitude (volume), and intensity (kinetic energy).
Detachment propensity is governed by predisposing structural conditions and analyzed by the probability of failure modes, such as planar sliding, wedge sliding, or toppling, in relation to the main discontinuity sets. Conversely, magnitude and intensity depend on the probable volume of the unstable block and its potential propagation path.
Traditional geo-structural surveys, based on direct acquisition using standard instruments (e.g., geological compass and measuring tape), characterize, among other parameters, discontinuities in terms of orientation (dip angle/dip direction), spacing, and persistence. While the orientation of discontinuities, combined with mechanical properties, allows for the evaluation of the propensity to detachment, the definition of spacing and persistence is crucial for estimating block volume. However, this deterministic approach is often difficult to generalize to an entire slope, making accurate volume definition a persistent challenge.
To address this limitation and avoid the risks associated with direct data acquisition in hazardous areas, indirect remote sensing approaches have gained prominence. This study addresses the rockfall hazard characterization of a critical slope in the Palermo Mountain System (Sicily, southern Italy), where frequent rockfalls have disrupted vehicular traffic. Utilizing a Terrestrial Laser Scanner (TLS), the authors applied an indirect characterization method. Multitemporal acquisitions enabled a high-resolution 3D Point Cloud-based analysis, allowing for a more accurate and safe definition of hazard parameters in this complex environment.

How to cite: Mineo, G., Rosone, M., Martinello, C., Mercurio, C., Rotigliano, E., and Cappadonia, C.: Indirect Rock Mass Characterization Using High-Resolution 3D Point Clouds Applied in Hazardous Rock Slopes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18351, https://doi.org/10.5194/egusphere-egu26-18351, 2026.

Fracture networks characterization is fundamental for assessing the stability of engineered slopes; although fractures are primary drivers of rock mass behavior, capturing their complexity across scales remains a significant challenge. This study presents a multidisciplinary workflow that integrates field-based geological interpretation with advanced remote sensing and numerical modeling to characterize fractured rock slopes. While recent progress in Remotely Piloted Aircraft Systems (RPAS) and Structure from Motion (SfM) has optimized 3D data acquisition, a gap persists in standardizing the transition from Digital Outcrop Models (DOMs) to representative geomechanical models, such as the rock block volume (Vb).

To bridge this gap, we propose an integrated workflow that compares and integrates results from field surveys, DOM and Discrete Fracture Networks (DFNs). By moving beyond traditional analyses (e.g., Markland Test and ISRM suggested approaches), which often oversimplify spatial complexity, our approach leverages high-resolution 3D data to improve the identification and prioritization of critical structural features. This framework was applied to the Molassana quarry (Genoa, Italy) as part of the SkyMetro project, demonstrating how us a multi-methodological workflow provides a more robust, data-driven assessment for large-scale engineered fracture slopes.

How to cite: Foletti, M., Menegoni, N., Poggi, E., Benedetti, G., Comedini, M., Elmo, D., and Maino, M.: A multi-methodological workflow for fractured rock slope stability and block volume estimation , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20377, https://doi.org/10.5194/egusphere-egu26-20377, 2026.

Successful pilot projects, e.g., CarbFix (Iceland) and Wallula (USA), where CO₂ has been injected into subsurface basaltic rocks, have demonstrated the potential and advantages of mafic and ultramafic rocks (unconventional reservoirs) for long-term, safe CO₂ storage by mineral trapping. Despite the advantages, the CO₂ storage potential in unconventional reservoirs is relatively underexplored. Consequently, the key factors (and/or their interplay) that impact secondary mineralisation and the storage capacity of basaltic lava flows in the subsurface are less well understood. In this study, we integrate field-based geological investigation with whole-rock geochemical-, mineralogical-, and SEM- analysis, to characterise Miocene age Kaldakvísl basaltic lava flows, and associated deformation structures exposed along the western and eastern coastline of the Husavík–Tjörnes Peninsula, northern Iceland. Our results reveal at least two phases of fault activities and vein development in the western coast, associated with deformation along a major normal-dextral strike-slip fault, the Husavik-Flatey Fault Zone (HFFZ), whereas the eastern coast is far less deformed. Overall, the lava flows were largely characterized by tabular, sheet-like geometry, variable thicknesses ranging from c. 1 – 7 m, and sometimes interbedded with thin volcaniclastics and paleosols. Individual lava flows exhibited large variability in intra-flow vesicle morphology, intensity, connectivity, and mineral fill that allows us to subdivide each flow sequence into three distinct units: vesicular base-, massive core-, and vesicular top- of flow. Field observations and petrological analysis of lava flow sequences from the western coast show that the flow tops have been subjected to intense and higher degrees of hydrothermal alteration and secondary mineralization (e.g., zeolites and minor carbonates) compared to the flow base and core. Conversely, lava flow sequences from the eastern coast are generally less altered, preserving the primary composition and open vesicles of the lava flows. This suggests a strong correlation between the degree of deformation and tectonic fracturing, and the degree of hydrothermal alteration and secondary mineralization, underpinning the control the former has on the latter. Furthermore, the results of XRD analysis and optical microscopy identified zeolite minerals that formed both at lower temperatures (55-110 °C), such as chabazite and heulandite, and higher temperatures (70 °C up to 300 °C), such as stilbite and analcime. We propose that these zeolite minerals form from distinct hydrothermal events, reflecting a multi-stage rather than a continuous mineralization and alteration process. Our observations suggest that the multi-stage alteration process was most likely driven by the multiple phases of fault activities and vein formation associated with the HFFZ and subsidiary faults, which provided pathways for hydrothermal fluids. This study improves our understanding of the factors that influence hydrothermal alteration and secondary mineralization in basaltic rocks and has implications for evaluating the potential role of fractures in CO₂ storage in unconventional reservoirs.

How to cite: Osagiede, E. E., Brechan, C. A., Bjørnsen, T. W., Nixon, C., and Rotevatn, A.: Fracture-controlled multi-stage secondary mineralization and alteration in Kaldakvísl basaltic lava group, Husavik–Tjörnes Peninsula, northern Iceland: implications for subsurface CO2 storage via carbon mineralization, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20981, https://doi.org/10.5194/egusphere-egu26-20981, 2026.

Shale oil reservoirs are typically characterized by ultra-low porosity and permeability, in which natural fractures provide key pathways for hydrocarbon migration from the matrix to the wellbore. These fractures significantly influence production performance. In the Mahu Sag of the Junggar Basin (NW China), the Permian Fengcheng Formation comprises saline lake–facies mixed shales with limited primary porosity, where natural fractures and dissolution pores dominate the available storage space. The sustained high single-well production (exceeding 100 t/day in parts of the sag) underscores the importance of understanding fracture occurrence and effectiveness for efficient reservoir development. In this study, we interpret Full-bore Micro-scanner Imager (FMI) borehole image logs using Schlumberger Techlog to identify and quantify both drilling-induced and natural fractures. The results show that drilling-induced fractures, which appear as short vertical features with symmetric “feather” or en-echelon patterns, are used to infer the orientation of the current maximum horizontal stress (SHmax). SHmax varies across structural domains: it trends near E–W to ENE–WSW adjacent to the Wuxia fault belt, shifts locally toward NE–SW at the junction of the Wuxia and Kebai fault belts, and transitions back to ENE–WSW to E–W toward the southwestern and southernmost regions. Natural fractures are abundant, predominantly striking NE–SW and near E–W (40°–100° and 220°–280°, accounting for 51% of fractures), with a secondary set trending NNW–SSE (140°–160° and 320°–350°, accounting for 22%). These orientations largely align with major fault trends. Fracture dip distributions vary significantly between wells and are primarily controlled by bedding attitude, with the apparent dip deflection closely mirroring the formation dip. In proximity to faults, tectonic fractures tend to exhibit lower dips. Aperture statistics reveal that fracture effectiveness is strongly stress-dependent: fractures more closely aligned with SHmax exhibit larger apertures and higher inferred effectiveness, while aperture size decreases with increasing misalignment angle. In a representative well, ENE–WSW fractures exhibit the largest mean apertures (tens of micrometers) compared to other fracture sets. Overall, SHmax) in the Fengcheng Formation shale is predominantly oriented E–W to ENE–WSW, and natural-fracture trends broadly match the strikes of major faults. Fracture dip angles are largely governed by bedding attitude, whereas fracture effectiveness is strongly stress-dependent. These results provide a direct basis for sweet-spot evaluation (targeting intervals with larger apertures under more favorable stress conditions) and for optimizing stimulation orientation and treatment design.

How to cite: Du, X. and Zeng, L.: Present-day stress control on natural fracture effectiveness: quantitative evidence from borehole image logs in the Fengcheng Formation shales, Mahu Sag, Junggar Basin, NW China, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21984, https://doi.org/10.5194/egusphere-egu26-21984, 2026.

Continental rifting gives rise to margins with variable magmatic budgets, producing endmembers that range from magma-poor to magma-rich. At some magma-poor rifted margins like Australia-Antarctica conjugate margins and fossil margins seafloor preserved in Western Alps, portions of the lithospheric mantle were exhumed to the surface during the late phases of rifting. However, the key factors controlling this exhumation remain poorly constrained. From thermomechanical numerical scenarios, we investigated the controlling factors of the mantle exhumation process during rifting by varying crustal thickness, lithospheric mantle structure and rifting velocity. The results show that lower crustal strength and consequent lithospheric coupling drive the formation of exhumed mantle domains at magma-poor rifted margins. The exhumation process distributes different portions of lithospheric mantle along the rifted margins where at the most distal regions corresponding to initially deeper portions of lithospheric mantle. Factor as crustal thickness and mantle lithospheric structure affected the width of exhumed mantle domains. We observe that the stretching processes can exhume mantle particles from different lithospheric depths, sampling both shallow particles near the base of the crust and deeper portions of the lithosphere, especially in scenarios with an initially high degree of coupling between crust and lithospheric mantle. We also tracked the P-T-t paths of lithospheric mantle particles and our results agree with P-T-t paths from Iberian Margin, Diamantina Zone at SW Australian Margin and also from fossil rifted margins of the Western Tethys in the Alps and P-T estimation data for exhumed mantle samples from Newfoundland Margin and Terre Adélie seamount B at Antarctic Margin showing the potential of numerical models to explore the exhumation process in the context of magma-poor rifted margins.

How to cite: Macedo Silva, J. P., Sacek, V., Manatschal, G., and Ganade, C. E.: Reconstruction of exhumation history along magma-poor rifted margins - Insights from numerical models, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-156, https://doi.org/10.5194/egusphere-egu26-156, 2026.

Flat Moho is a characteristic feature beneath extended continental lithosphere and orogenic plateaus; however, the physical processes that govern their formation remain poorly understood. In particular, the mechanical conditions required for lower crustal flow to effectively suppress Moho deflection are still debated. It has been proposed that lower crustal flow may facilitate lateral mass redistribution, thereby limiting Moho deflection and Moho relief during extension. Here, we compare seismological (receiver function) and gravity data and geodynamic models to identify the controls of Moho variation across various tectonic regions. Namely, we perform two suites of two-dimensional visco-plastic numerical models using the finite element code ASPECT with systematically vary (1) the minimum effective viscosity of the lower crust, and (2) its brittle strength, represented by cohesion. Each model simulates the extension of a 50 km-thick crust overlying a previously thinned lithospheric mantle, allowing us to isolate the rheological controls on Moho geometry and crustal deformation. Our results show that the primary factor governing Moho topography is the viscosity of the lower crust. When the lower crust is weak (≤ 10¹⁸ Pa·s), the viscous flow efficiently redistributes the material, leading to diffuse deformation and flat Moho (ΔMoho < 5 km). In contrast, high-viscosity models (≥ 10²¹ Pa·s) exhibit localized crustal thinning and pronounced Moho deflection, with relief up to 50 km and slopes exceeding 0.04 km/km. Varying the cohesion of the upper crust influences the distribution of brittle strain, but has a limited effect on Moho morphology. We conclude that flat Moho geometries arise from the integrated mechanical response of the crustal column where a sufficiently weak lower crust accommodates crust-mantle decoupling. These findings provide a quantitative framework to interpret observed flat Moho patterns in extensional settings such as the western Anatolia, the Basin and Range Province, and Tibetan Plateau.

How to cite: Bodur, Ö., Göğüş, O. H., Çavdar, E. N., and Çalınak, G.: Flat Moho beneath orogens and extensional regions: What controls it?, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-979, https://doi.org/10.5194/egusphere-egu26-979, 2026.

The Central Kenya RIft (CKR) is one of the fastest deforming sections of the Eastern African Rift System (EARS). Extensive tectonic research has been performed on the rift in northern and southern Kenya, but the modern tectonic geomorphology of the CKR remains understudied. Existing fault maps show a change in the orientation of the EARS within the CKR, although faults have not been mapped in detail with modern techniques. Despite the numerous fault scarps that offset the rift floor, few large earthquakes have been recorded in the recent past, with the exception of a M_S 6.9 event in 1928. Maturing rifts demonstrate a shift from border fault seismicity to increased aseismic deformation dsitributed along intra-rift faults. This study aims to map active fault scarps within the CKR to better understand the modern tectonics, which may give insights into seismic hazard for an area with a high population growth rate. Rigorous examination of the high resolution TanDEM-X Digital Elevation Model (DEM) was used to formulate a digital fault database, which includes attributes about individual fault lengths and orientations. The NNW-SSE orientated CKR represents an intersection between NNE-SSW orientated EARS rifts to the north and south, and older NNW-SSE orientated structural fabrics. While the CKR itself shows a traditional mature rift morphology containing a developed inner graben with recent volcanism, the junction between the CKR and Northern Kenya Rift appears to be less mature. The 1928 earthquake, which occurred along a border fault in this junction, challenges the theory of axial strain concentration in an aging rift. Calculations on the balance of extension accommodated by larger border faults vs younger intra-rift grid faults allows for the possibility of continued border fault slip. The lack of large earthquakes in the CKR itself suggests an aseismic model to describe deformation, while seismic hazard appears to be greater in the junctions between rift segments of alternate orientations. 

How to cite: Botha, D., Sloan, A., Kübler, S., and Kahle, B.: Neotectonics of the Central Kenya Rift, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1035, https://doi.org/10.5194/egusphere-egu26-1035, 2026.

Rifted margins result from the complex interaction between tectonic, magmatic and sedimentary processes. Conceptual models explaining their evolution have changed considerably over the last few decades, moving from simple stretching models to more complex polyphase rift models that distinguish between structural domains (proximal, necking, distal) and distinct rift modes. Advances in dynamic numerical modelling have made it possible to not only reproduce the predicted sequential evolution of rift modes and the related rift domains, but also to create complex 2D and even 3D computer-generated simulations, which must be compared with real world examples. While increasingly sophisticated 2D and 3D seismic images of rifted margins allow theoretically to rigorously test and calibrate the models, the problem resides that their geological interpretations are none unique. It is therefore more important than ever to develop a ‘protocol’ which allows for objective, verifiable, consistent and reproducible geological interpretations of seismic data.

Rifted margins present, indeed, first- and second-order diagnostic geometries and seismic facies that can be mapped on seismic reflection profiles. Our contribution aims to synthesise current knowledge on margin architecture and present a systematic approach to seismic interpretation, supported by representative “champion” seismic lines. For each domain, we describe the main structural and stratigraphic characteristics and provide diagnostic criteria commonly observable on seismic reflection profiles. Rather than revisiting the mechanisms of margin formation, we assess whether first- and second-order observational features capture the full range of architectures between existing endmember models. While using the magma-rich/magma-poor dichotomy aids communication, natural rift systems span a continuum of intermediate and hybrid configurations. Our approach accommodates this variability and promotes standardized, reproducible interpretations, allowing to close the loop between increasingly sophisticated modelling and imaging techniques and their testable, reproducible, across-scales coherent geological interpretation.

How to cite: Manatschal, G. and Peron-Pinvidic, G.: Diagnostic criteria for mapping rifted margin architecture using seismic reflection profiles, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1953, https://doi.org/10.5194/egusphere-egu26-1953, 2026.

Many rift-related triple-junctions from various geological periods have been previously investigated worldwide. Some of these large extensional lithospheric phenomena even involved complex circular configurations composed of numerous omnidirectionally spaced radial and arcuate elements, thereby having a general multi-junction character. Although pointing to close association of horizontal plate kinematics with centrally directed vertical updoming as two conjugated significant geodynamic phenomena, such annular dichotomy of features penetrating and segmenting the surrounding upper Earth's mass has yet been addressed only poorly.

Based on spatial analysis of regional topography and/or hydrography, an extensive Cenozoic circular structure (> 6,000 km in diameter) even including recently active tectonic elements appears to have developed in whole Europe and some adjacent areas of Africa and Asia centred at a common intersection point of the Upper Rhine Graben, Lower Rhine grabens (with significant Roer structure), Hessian grabens (involving Leine structure), and more distant Eger Graben current axes. The pervasive surface fracturing of both higher / lower topographic levels was taken into account (numerous concentric boundaries between mountain summit blocks visualized by closed contours / ubiquitous multi-arc- and fan-shaped geometries within piedmonts and lowlands indicated in continental river network, less along important block-bounding slopes, and locally on sea floor). The fairly regular annular lithospheric fragmentation is expressed by a wide-scale spectrum of features from general mountain or basin belt orientations through trends of circumferential, centrifugal, or centripetal river sections and corresponding valleys to consistent sets of sharp stream bends.

Using a similar research approach, several analogous circular phenomena were detected within the Red Sea rift system. Despite possible links to various known geometrically consistent geological structures including magma plumbings or mantle plumes, it is yet hard to determine the main evolutionary processes and the closer time constraints of the circular systems. Their role should be considered and discussed on a broad disciplinary basis, among others, because similar surface configurations seem to exist in different tectonic settings such as large uplifting basement massifs or arcuate orogenic belts and intermontane basins. An attempt to invoke related communication is made also by means of this contribution.

How to cite: Roštínský, P.: Rhine Graben rift system-related multi-junction and other analogs: Large-scale circular lithospheric segmentation indicated in regional topographic and hydrographic data, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2582, https://doi.org/10.5194/egusphere-egu26-2582, 2026.

The massive Orphan Basin, offshore Newfoundland, preserves evidence of a complex, multiphase rift history influenced by structural inheritance tied to the Appalachian-Caledonian orogen. Despite recent advances in plate kinematic reconstructions of the Southern North Atlantic, the tectonic evolution of the Orphan Basin remains poorly constrained, largely due to limited seismic and well coverage. As a result, the contributions of structural constraints on deformation have been oversimplified, leading to the misrepresentation of their influence on continental breakup.

This study prioritizes the interpretation of recently available 2D seismic reflection datasets acquired by TGS/PGS and ION Geophysical, developing stratigraphic and structural constraints to inform plate kinematic modelling. An analysis of the spatial distribution of Jurassic to Early Cretaceous syn-rift sediments and the geometries of major fault systems provide new insights into rift migration and the temporal variability of strain localization in the Orphan Basin during continental breakup.

Seismic interpretation and fault analysis identify two temporally distinct hyperextended rift basins separated by a region of thick crust, highlighting the importance of mechanically rigid blocks, such as the Orphan Knoll, in focusing strain, controlling basin development, and influencing the timing and geometry of rift propagation. While previous reconstructions have represented extension within the Orphan Basin as continuous and uniform, our analysis indicates that strain was instead focused within discrete extensional corridors controlled by large detachment faults.

Using GPlates, these seismic constraints are integrated into a deformable plate tectonic reconstruction, refining the kinematic plate model of the Southern North Atlantic while improving its geological accuracy and reducing the reliance on uniform crustal stretching assumptions. The updated reconstruction aims to provide a significant step towards a reproducible analogue model for hyperextended rift basins during magma-poor continental breakup. 

How to cite: Nickson, T. and Welford, J. K.: Integrating Seismic Interpretation of the Orphan Basin, Offshore Newfoundland, with Deformable Plate Tectonic Modelling of the Southern North Atlantic, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3101, https://doi.org/10.5194/egusphere-egu26-3101, 2026.

Stretching of the crust, seafloor spreading, and volcanism commonly affect the overriding plate above retreating slabs in subduction settings. The Vavilov Basin (Tyrrhenian Sea) is a Pliocene–Quaternary back-arc basin formed in response to the eastward rollback of the Apennine–Tyrrhenian subduction system. The basin has a roughly triangular shape and it is bounded by major escarpments (e.g. the Selli Line) separating it from the continental margins (Cornaglia Terrace, De Marchi Seamount and Flavio Gioia Seamount). Its western sector is characterized by N–S–oriented ridges interpreted as the surface expression of basaltic magma injections during, or shortly after, mantle exhumation (e.g. the Gortani and the D’Ancona Ridges).

Near the centre of the basin, the Vavilov Volcano (VAV), a large volcanic edifice ~60 km long and ~32 km wide, rises from ~3600 m below sea level (b.s.l.) to a minimum depth of ~795 m b.s.l. The VAV consists of three main volcanic units: (i) west-tilted pillow lava flows below ~1500 m b.s.l., (ii) radial lava flows between ~1500 and 1000 m b.s.l., and (iii) scoriaceous lava flows from ~1000 m b.s.l. to the summit. K–Ar dating of pillow lavas sampled along the eastern flank at ~1000 m depth yields Pleistocene ages of 0.37 and 0.09 Ma, consistent with the observed magnetic pattern. Magnetic data show a positive anomaly over the shallow part of the volcano related to the Brunhes geomagnetic chron, and contrasting with negative anomalies on the outer flanks and surrounding basin.

Here we present an integrated magnetic and morphologic analysis of VAV aimed at constraining its internal plumbing system and the relationship with surface volcanic and tectonic structures. We develop an inverse magnetic model that images subsurface structural elements related to both an early spreading ridge and a later central volcanic system. Our results indicate that intervening intrusive ridges in small back-arc basins may evolve following a polyphasic evolution with a transition from fissural to central-type volcanism and developing a multi-level plumbing system. The VAV morphological asymmetry reflects an eastward migration of volcanic activity through time, possibly associated with asymmetric basin opening. The shallow plumbing system comprises: (a) an early NNE–SSW–elongated dike sheet feeding fissural volcanism along the summit ridge, and (b) a younger central magma reservoir beneath the summit feeding central vents. A NW–SE–oriented apophysis extending southeastward from the central reservoir likely supplied volcanic cones on the eastern flank.

How to cite: Cocchi, L., Muccini, F., Palmiotto, C., and Ventura, G.: Reconstructing the plumbing system of the Vavilov Seamount (southern Tyrrhenian Sea): insights into the transition from fissural to central-type volcanism back-arcs , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3400, https://doi.org/10.5194/egusphere-egu26-3400, 2026.

Located in the northern margin of the southern Okinawa Trough, the Mienhua Submarine Volcano (MSV) is probably formed during the post-collision of the former Taiwan orogeny. The MSV is accompanied by vigorous hydrothermal activities. To understand the related tectonic faults, volcanic intrusions, and hydrothermal activity of the MSV, we have collected several high-resolution sparker seismic profiles surrounding the MSV. Our results show that the east and west sides of the MSV show different features. In the east side, we have found unconformities, high-amplitude seismic reflectors, and acoustic blanking zones. The acoustic blanking zones indicate that hydrothermal fluid has penetrated the strata and migrated upwards and laterally. Many hydrothermal plumes are also found in the water column. In other words, hydrothermal activity is active in the eastern region. In contrast, in the west side of MSV, few unconformities or hydrothermal activities were found. Besides, large-scale mass-transport deposits (MTDs) are formed, possibly due to submarine landslides.

How to cite: Li, H.-W., Hsu, S.-K., Lin, L.-K., and Tsai, C.-H.: Geological structure related to the Mienhua Submarine Volcano in southern Okinawa Trough from High-Resolution Sparker Seismic profiles , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4342, https://doi.org/10.5194/egusphere-egu26-4342, 2026.

We present a new first-approach methodology, applicable to thermally re-equilibrated rifted margins, to determine margin crustal architecture and magmatic type from the TWTT of top basement of time-domain seismic reflection data. The method invokes Warner’s 10s Moho rule (Warner, 1987) to give the TWTT crustal basement thickness from top basement TWTT from which we determine crustal basement thickness. It does not require the Moho to be seismically imaged or sediment thickness to be known.

Determining rifted margin crustal thickness and assessing whether a margin is magma-normal, magma-starved or magma-rich is fundamental to understanding margin structure and formation processes. This is often a difficult task compounded by the absence of clear or unambiguous seismic Moho.

Warner observed that, for a thermally re-equilibrated margin, the Moho seismic reflection is approximately flat at ~10s TWTT and is constant irrespective of the complexity of geology above. Moho TWTT is at 10s for unthinned continental crust, oceanic crust, and for crust in between, and applies equally to magma-rich, magma-starved and magma-normal rifted margins.

We apply the new methodology using Warner’s 10s Moho rule to map crustal basement thickness for the Campos and Santos rifted margins offshore Brazil from TWTT of top basement observed on seismic reflection data. We show that the resulting map of crustal thickness determined from top basement TWTT shows a good correlation with that determined using gravity inversion.

Modelling shows that different magmatic-margin types have distinct shapes of top basement TWTT that is independent of sediment thickness. The lateral transition from downward-sloping to flat top basement TWTT corresponds to the oceanward taper of thinned continental crust to boxed-shaped oceanic crust, providing an estimate of the landward-limit of oceanic crust (LaLOC). Magma-starved margins show a step-up of top basement TWTT onto oceanic crust. For margins with magma, lateral inflections in the TWTT of base sediment provide information of the onset of magmatic-volcanic addition and the formation of hybrid crust consisting of thinned continental crust plus new magmatic crust. For magma-normal margins this lateral inflections of TWTT corresponds to the start of deep-water volcanics (SDRs) at 6-7s TWTT. For magma-rich margins (with sub-aerially erupted volcanic SDRs) this TWTT inflection occurs at 2-3s.

We interpret the top basement TWTT profiles on the Southern Campos Margin to indicate a slightly magma-poor margin. The thinnest crust occurs between thinned continental crust and normal-thickness oceanic crust, consistent with a simple isostatic model where maximum decompression melting to form oceanic crust does not occur until after continental crust separation.

On the SW Santos Margin, we interpret the top basement TWTT profiles to indicate a slightly magma-rich margin. A broad region separates the end of the crustal thinning taper and the LaLOC. A simple isostatic model can generate this top basement TWTT shape as a broad region of hybrid crust or thicker-than-normal early oceanic crust.

Top basement TWTT cannot reliably identify the margin domain transition between the necking zone and hyperextended crust. This transition coincides with the onset of normal decompression melting and the start of hybrid crust.

How to cite: Graça, M., Kusznir, N., and Manatschal, G.: Rifted Margin Crustal Architecture and Magmatic Type from Time-Domain Seismic Reflection Data Using the Warner 10 second Moho TWTT Rule: A New First-Approach Methodology , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4961, https://doi.org/10.5194/egusphere-egu26-4961, 2026.

The Newfoundland rifted margin (NLRM) exhibits complex lithospheric features, including failed rifts, continental ribbons, transfer zones, and along-strike segmentation. Although the spatial variability of these tectonic features is central to understanding the region’s tectonic evolution, their interactions and broader implications remain debated. In this study, drawing on an unprecedented deep multichannel seismic dataset, we interpret a grid of margin-scale seismic reflection profiles to examine the variability of crustal necking and rift domain architectures along the NLRM and the associated Orphan Basin–Flemish Pass failed rift. Our interpretation reveals asymmetrical crustal necking on the conjugate sides of the failed rift, consistent with published numerical modelling studies, which suggest that asymmetric rifting is an early-stage process, potentially occurring before the necking phase. We observe more gradual crustal necking in regions of thinned and inferred weaker crust. In contrast, more abrupt crustal necking is associated with areas of thicker, inferred stronger crust, where transcrustal faults extending to depths greater than 20 km are imaged. Mantle serpentinization interpreted beneath both the NLRM and the failed rift zone indicates that serpentinization is not contingent on rift success or failure but is primarily governed by rheology and the availability of transcrustal faults. For magma-poor rifted margins, in contrast to magma-assisted rifting, transcrustal faulting linked with mantle serpentinization appears to facilitate continental breakup. Our systematic mapping reveals pronounced across-strike and along-strike variations in rift domain distributions, predominantly controlled by inherited transfer zones that segment the margin and that range from localized to diffuse, accommodating extension and giving rise to alternating strong and weak margin segments.  

How to cite: Alehegn, W. N. and Welford, J. K.: Nature of Crustal Necking and Rift Domain Architecture Along the Newfoundland Margin, Eastern Canada: Improved Seismic Perspectives and Interpretational Uncertainties, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5349, https://doi.org/10.5194/egusphere-egu26-5349, 2026.

Tectonic inheritance often plays a significant role in the evolution from continental rifts to passive rifted margins in extensional settings. Continental ribbons, which constitute intact continental fragments that remain tethered to their parent plates within rift systems, can form through interacting propagating rifts in pristine lithosphere but can also represent the lasting manifestation of pre-rift lithospheric heterogeneity. In the southern North Atlantic rift system, which transects the vestiges of the older Paleozoic Appalachian-Caledonian orogen, large continental ribbons are plentiful, arguably more so than anywhere else in the entire Atlantic Ocean. The spatial distribution of these ribbons, wrenched away from the North American, European, and Iberian plates during Mesozoic rifting and breakup of the Pangean supercontinent, provides insights into the pre-rift orogenic architecture of the lithosphere. This complex inheritance would go on to influence strain partitioning and sedimentary basin evolution during subsequent rifting and extensional reactivation. Studying these key components of rift systems and their consequences is often complicated by sparse seismic coverage due to their limited resource potential and their more distal locations. Yet, the characterization of continental ribbons at the lithospheric scale is necessary for their faithful incorporation into basin and plate reconstructions. To that end, alternate and complementary geophysical methodologies, such as potential field analysis, are needed to infill sparse seismic constraints and properly capture the physical characteristics of these impactful features. In this presentation, I will discuss the continental ribbons of the southern North Atlantic, the methods used to characterize their attributes, their likely tectonic origins, and how this information can be used to improve and quantify their contribution to reconstructions of the region.

How to cite: Welford, J. K.: Continental ribbons within the southern North Atlantic rift system: attributes, origins, and consequences, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5429, https://doi.org/10.5194/egusphere-egu26-5429, 2026.

The interaction between mantle plumes and continental lithosphere results in a complex spectrum of rifting outcomes, ranging from magma-rich breakups to failed rifts. Current research in the Turkana Depression posits a "Refractory Paradox," suggesting that failed rifts like the Anza Graben remain "dead zones" because prior melting events extracted volatiles, leaving behind a mechanically strong, dried-out lithosphere resistant to modification. However, it remains unclear if this "baked-dry" signature is a global requirement for rift failure or a local anomaly. We investigate this hypothesis by mapping the subtle architectural differences—specifically Moho sharpness and seismic lid preservation—that distinguish magma-poor regions from their magma-rich counterparts. To overcome the limitations of standard receiver function (RF) analysis, which is often degraded by noise and reverberations, we apply a rigorous, high-resolution workflow. We first denoise seismic data using the CRISP-RF algorithm, employing sparsity-promoting Radon transforms to suppress incoherent noise while preserving full-wavefield phases. These clean data are then inverted alongside surface wave dispersion measurements using a transdimensional probabilistic Bayesian  framework. This approach allows us to quantify non-uniqueness and robustly constrain multi-layered crustal properties (Vp/Vs ratios) and lithospheric velocity structure without placing limiting assumptions on elastic properties. By integrating these refined seismic constraints with common-conversion-point (CCP) stacking, we resolve the trade-off between magmatic underplating (gradational Moho, Vp/Vs > 1.8) and tectonic thinning (sharp Moho, Vp/Vs ~1.74). Finally, we pair these structural observations with thermo-chemical modeling (WINTERC-G/PerPleX) to convert velocities into temperature and composition. This study aims to determine if the lithospheric strength beneath the African Rift is governed by volatile depletion or alternative weakening mechanisms, such as anisotropy or eclogitization.

How to cite: Maenner, M., Legre, J.-J., Stamps, D. S., Adams, A., and Olugboji, T.: Unified Mapping of the African Rift System: Lithospheric Strength and Magmatic Evolution, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5886, https://doi.org/10.5194/egusphere-egu26-5886, 2026.

The Continent-Ocean Transition (COT) in the young Tyrrhenian basin documents mantle exhumation punctuated with multiple episodes of discrete oceanic crust formation. This observation challenges prevailing models of magma-poor COTs, which typically describe mantle exhumation preceding the emplacement of oceanic crust. Notably, this COT developed without the conventional conditions associated with magma-poor rifted margins, such as slow rifting velocities and chemically depleted mantle sources. A key observation is the low shear-wave velocity observed in the uppermost mantle of the Tyrrhenian basin and its adjacent onshore regions correlates with subduction-related volcanism, suggesting the presence of a hydrated mantle wedge with low rheological strength. Here we show that, based on 3D magmatic-thermomechanical numerical modeling, the episodic formation of oceanic crust within the Tyrrhenian basin’s COT results from the mechanical weakness of the mantle. The lithospheric mantle is exhumed to the surface through exhumation channels initiated within the weak mantle zone. The subsequent flow of partially molten mantle toward these channels leads to the development of multiple short-lived spreading centers. Our findings shed light on characteristics and mechanisms shaping the COT of marginal basins, where their opening is influenced by subduction processes.

How to cite: Su, H. and Leng, W.: Weak mantle wedge causes mantle exhumation punctuated with discrete oceanic crust in the Tyrrhenian basin, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7005, https://doi.org/10.5194/egusphere-egu26-7005, 2026.

In many areas of active faulting, the continuity of normal faults with a short or incomplete historical earthquake record and subtle topographic expression is not fully understood: as a result the seismic potential of these faults is often underestimated. The Southern Gulf of Evia rift, Greece is an example of a poorly explored normal fault bounded system, where the location and spatiotemporal evolution of the major basin bounding faults is not well constrained. We integrate geomorphic and structural field data, topographic analyses and geodetic data to constrain the locations, footwall geometries and structural evolution of 8 major extensional structures bounding the Southern margin of the South Gulf of Evia. We propose that this fault system comprises two isolated fault groups containing both partially and fully linked segments. These fault linkage scenarios suggest that the eastern fault group may have a total linked length of ca. 40 km with a maximum credible earthquake size of Mw 7.0. Further, we reconcile new analysis of vintage sparker seismic reflection data previously acquired and interpreted in the 1980s, with onshore geomorphic indicators of tectonic uplift to provide new constraints on the continuity of active normal faults offshore, including the major normal fault zones bounding the northern margin of the rift. By comparing our reconstructions of footwall relief with the seismic reflection and Ocean Bottom Seismometer (OBS) data, we suggest footwall uplift to hanging wall subsidence ratios of 1:2-1:3 and total slip rates in the order of 2-3 mm/yr. Finally, based on the correlation of seismic stratigraphy with a global eustatic sea level curve and a comparison of estimated sediment fluxes into the Gulf with measured sediment volumes in the South Gulf, we propose updated Pleistocene-Holocene ages for the basin stratigraphy and suggest possible timescales for fault evolution and linkage along the rift margins.

How to cite: Coveney, S., Whittaker, A., Bell, R., Wood, J., Kranis, H., and Ganas, A.: New constraints on active normal faulting in the South Gulf of Evia, Greece, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7229, https://doi.org/10.5194/egusphere-egu26-7229, 2026.

The North Gulf of Evia is a young, active continental rift system located in Central Greece. Extension of 2-4 mm/yr is accommodated by large normal fault systems, such as the onshore Coastal Faults near Kammena Vourla, but slip rates and timing of initiation of these structures and intrabasinal offshore faults are poorly constrained. Extension is also coupled with strong rotational and strike-slip influence from the westward-propagating North Anatolian Fault, providing contrast to the nearby, orthogonal rifting in the Gulf of Corinth. The geodynamic setting of the rift has resulted in a complex configuration of normal, oblique and strike-slip faults across the North Gulf of Evia rift system. Detailed, high resolution study of faulting processes (initiation, linkage and migration) and the temporal evolution of such systems requires a high-resolution age model of syn-kinematic sedimentation. To date, no pre-Holocene sedimentary correlation has been proposed for the North Gulf of Evia, restricting the temporal scope of evolutionary studies.

We aim to unlock the temporal evolution of late-Quaternary (0-~325 ka) sedimentation and faulting in the North Gulf of Evia through the development of a syn-tectonic depositional age model for the Western Basin of the Gulf. To do this, we exploit a high resolution, high density 2D seismic reflection dataset (WATER I and II) to identify three key mappable horizons across the semi-enclosed basin using seismic stratigraphic principles including reflection terminations and onlap relationships. Based on observed late-Pleistocene deltaic clinoform packages, ages of ~12 ka (MIS 2), ~130 ka (MIS 6) and ~325 ka (MIS 9) are attributed to these horizons within our sequence stratigraphic model. The age model is applied across the Western Basin alongside a new network of offshore faults to determine the major structural components, depocentres and evolutionary history of the rift system for the first time.

We resolve the major modern structural controls on the basin to be the Kalypso Fault at the southern margin of the rift and the axial Central Graben. Holocene throw on the extensional Kalypso Fault is ~3.75 mm/yr with faults of the Central Graben deforming at throw rates of ~0.9 - 1.7 mm/yr. We show that the Kalypso Fault is linked to the western part of the onshore Coastal Fault System, widely considered the most active fault zone of the North Gulf of Evia and uplifts the hanging wall of the active Arkitsa Fault, where a sequence of uplifted Pleistocene marine terraces is preserved. Initiation of the Kalypso Fault is temporally constrained to ~325 ka from thickening relationships of syn-kinematic sediment packages following a strain migration event from the Arkitsa Fault. This migration event occurs across non-parallel structures with evolving strike of >20°, likely reflecting the regional rotational influence of the North Anatolian Fault on Central Greece. The Kalypso Fault represents the most active resolved normal fault in the Western North Gulf of Evia and presents significant, previously unrecognised seismic hazard.

How to cite: Wood, J., Bell, R., Whittaker, A., Coveney, S., Chanier, F., Caroir, F., Kranis, H., and Ganas, A.: Normal fault migration and basin evolution in complex rift settings: insights from the North Gulf of Evia, Central Greece, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7348, https://doi.org/10.5194/egusphere-egu26-7348, 2026.

The Mid-Norwegian margin and the North Sea rift are among the most extensively studied regions in the world, owing to their abundant geological and geophysical datasets. Their basement architecture is complex, having been shaped by the Silurian Caledonian orogeny and subsequent gravitational collapse during the Devonian. This was followed by multiple rifting episodes, separated by periods of tectonic quiescence. While the North Sea subsequently entered a post-rift phase dominated by thermal subsidence, rifting along the Mid-Norwegian margin persisted until continental breakup in the early Eocene.

Despite these studies, the mechanisms by which remnants of the Caledonian orogeny influenced later rifting stages remain unclear. For many years, seismic imaging could not penetrate to the depths required to investigate the complete basement architecture. Recent advances in seismic reflection imaging, however, have enabled the acquisition of long-offset, deep, high-resolution profiles extending up to 16 seconds two-way travel time (s-TWTT). The GeoexMCG Regional Deep Imaging (RDI) dataset thus provides an unprecedented opportunity to study the entire basement architecture, including the lower crust and lithospheric mantle.

This contribution summarizes the first results of a PhD study focused on a large-scale interpretation of the RDI dataset, supported by offshore-onshore geological correlations and gravity and magnetic modelling. Units with distinct seismic facies -i.e., zones of consistent reflectivity characterized by amplitude, frequency, and continuity - were defined in Petrel after multiple mapping iterations. Based on these results, the aim of the PhD study is to explore how inherited basement structures influence continental rifting and the formation of rifted margins at large scales.

How to cite: Castagné, C., Péron-Pinvidic, G., and Manatschal, G.: Basement Inheritance and Its Influence on Rift Evolution and Rifted Margin Architecture: The North Sea and Mid-Norwegian Margin., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7459, https://doi.org/10.5194/egusphere-egu26-7459, 2026.

Plate motion directions, and the orientations of rift zones and oceanic spreading ridges, and of transform faults and fracture zones that are perpendicular to these ridges, are generally controlled by tectonic forces such as slab pull, mantle convection, and mantle plumes. Here, it is hypothesized that within the confines of these general orientations, the exact orientations of these structures, and therefore plate motion directions, are partially controlled by suitably oriented sets of steep continental lithospheric discontinuities (CLDs), which work in concert with these larger tectonic forces.

Previously, the observation has been made that oceanic fracture zones are contiguous with CLDs, such as suture zones and other lithospheric fault zones. Based on high-resolution bathymetry, geological and geophysical data, it is demonstrated here that continents have multiple sets of lineaments parallel to such CLDs, or contiguous with CLDs where they occur farther inland and do not reach the ocean. Published analog experiments suggest that the orientations of transform faults and fracture zones are controlled by these CLDs if the angle between the spreading direction and the CLDs is no more than ~45°. Spreading ridge segments evolve in an orientation perpendicular to these transform faults and fracture zones, so that the spreading direction becomes parallel to the transform faults and fracture zones. The implication is that the exact plate motion directions are controlled by CLDs, if a set of CLDs is orientated at low angle with the spreading direction. When plate motion directions need to change due to tectonic forces, the new hypothesis predicts that the exact directions may be controlled by a different set of suitably orientated CLDs. During later stages of oceanic spreading, the larger tectonic forces such as slab pull, mantle convection, and mantle plumes become increasingly dominant and plate motion directions may no longer be controlled by the CLDs.

While the hypothesis needs further testing, it has potentially far-reaching implications. For example, Euler pole reconstructions are commonly based on small circle patterns formed by fracture zones and transform faults in the oceanic lithosphere. Oceanic crust older than ~200 Ma is typically destroyed by subduction, and pre-Mesozoic Euler poles can therefore not be reconstructed based on that method. If the hypothesis presented above is correct, the orientations of CLDs and associated lineament sets may be used as proxies for orientations of past transform faults and fracture zones, at least during early oceanic spreading. The locations of past Euler poles may thus be better estimated based on these CLDs and lineaments, and pre-Mesozoic plate tectonic reconstructions may be much improved in deep geologic time.

How to cite: Kuiper, Y.: Do continental lithospheric discontinuities exert control on tectonic plate motion directions?, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8469, https://doi.org/10.5194/egusphere-egu26-8469, 2026.

The Meso-Neoproterozoic period is sometimes referred to as the “Boring Billion” or “Earth’s Middle Age,” spanning the time between the formation of the Columbia supercontinent and the Rodinia supercontinent. This period records a key transition in the supercontinent cycle, shaping the global tectonic regime and paleogeographic pattern. In this study, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U-Pb geochronological analysis was conducted on five sandstone samples from the Ordos Rift Zone in the western North China Craton to constrain the regional tectonic evolution and basin development processes. The detrital zircon ages can be divided into multiple age groups, with zircon grains older than 1.8 Ga derived from the basement of the North China Craton, while the younger zircon populations (< 1.8 Ga) are associated with Mesoproterozoic magmatic events. Through an integrated approach combining zircon geochronology, major and trace element analysis, and sandstone modal analysis, the tectonic setting and parent rock properties of the provenance area were identified. The tripartite sedimentary cycle of volcanic rocks, continental-margin clastic rocks, and marine carbonate rocks in the Changcheng Period of the Ordos Rift Zone was finely delineated, and the response times (2.0  Ga, 1.8  Ga, and 1.6  Ga) of the assembly, consolidation, and breakup processes of the Columbia supercontinent in the western North China Craton were calibrated, respectively. The results show that the vertical sedimentary sequence of the Changcheng System in the Ordos Rift Zone corresponds to the rift evolution stages, forming a tripartite evolutionary cycle of igneous rocks–continental-margin clastic rocks–marine carbonate rocks, which records the transition process of tectonic activity from intense to stable. Three distinct stages of basin evolution during 1.8–1.4 Ga were defined: the initial rift stage and the rift expansion stage correspond to the disintegration of the Columbia supercontinent (1.8–1.6 Ga), and the passive continental margin stage coincides with a slowdown of the late supercontinent breakup rate (1.6–1.4 Ga). The detailed characterization of the regional tectonic evolution and rift zone sedimentary filling process during the Changcheng Period in the Ordos Basin reveals the source‑to‑sink spatiotemporal sedimentary pattern controlled by the rift system, providing key constraints for the evolution of the western margin of the North China Craton during the Precambrian supercontinent transition and offering new insights into the response of the North China Craton to global-scale geodynamic processes.

How to cite: Liu, G.: Detrital Zircon Records and Tectono-Sedimentary Evolution of the Mesoproterozoic Changcheng Period Strata in the Ordos Rift Zone, Western North China Craton, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8574, https://doi.org/10.5194/egusphere-egu26-8574, 2026.

From 5 to ca. 2 million years ago, faulting in the Corinth Rift, in central Greece, was concentrated onshore, to the south of the present-day Gulf of Corinth. Between 2 to 1.8 Ma the active fault network migrated northward, accompanied by footwall uplift, which led to active faulting and the rift being localized offshore in the present-day Gulf of Corinth. The factors controlling this fault migration remain unknown. Overall rift evolution is controlled by tectonics, but climate-driven surface processes affect rift topography, the development and longevity of normal faults, and overall rift evolution. A simple yet effective method for assessing strain distribution within a fractured region is the Kuiper’s test, which quantifies how much a line sampled through a faulted area deviates from a uniform distribution. By calculating the cumulative extension of faults distributed along a line, it is possible to infer if the strain in this section is distributed homogeneously throughout the fractures (values close to the uniform distribution) or if the strain is localized in few large faults (large departure of the uniform distribution), and whether this variation is statistically significant. We use the finite element thermo‐mechanical numerical model Fantom-2D coupled with the landscape evolution model FastScape to investigate how inheritance and surface processes control rift faulting and progressive localization during the early stages of continental rift evolution. We test different values of crustal strength and of frictional-plastic strain weakening to evaluate the response of the models. We tested each model without surface processes, and with different aggradation and progradation rates. We evaluated fault distribution, depocenter migration and rift localization through time and compared them to high resolution datasets from the present-day Corinth Rift and central Greece. The degree of localization obtained through the Kuiper’s test for five regions in the Corinth Rift were used to further validate the models. Using datasets of a rift system with a relatively simple extension history such as the Corinth Rift helps to better constrain numerical modelling parameters and improve rift evolution models.

How to cite: Barbosa, I., Huismans, R., Nixon, C., Gawthorpe, R., and Rouby, D.: Effects of inheritance and surface processes on strain localization during the early stages of the Corinth Rift system development , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8746, https://doi.org/10.5194/egusphere-egu26-8746, 2026.

As recent hydrocarbon discoveries rekindle exploration activities in the Black Sea Basin (BSB), efforts to understand the geodynamic processes that led to the formation and evolution of the basin have started to play a significant role in understanding the structural trends formed during rifting. The debate on whether the basin rifted open as one east-west oriented basin, or as two separate basins named the Eastern and Western Black Sea Basins, has been discussed in numerous models. Evidence for the two-basin hypothesis focuses on the basin's semi-parallel ridge and depression architecture, which trends NW-SE in the east and W-E in the west. Conversely, the single-basin model is supported by the correspondence between the regional structure and geodynamic rifting models, specifically those involving an asymmetrical rift pivoting on an eastern hinge caused by slab roll-back of the subducting plate located in the south of the basin.
To address existing tectonic uncertainties, we established a new structural framework for the BSB by reinterpreting 24 long-offset 2D seismic lines. These structural constraints enabled the development of two 2D computational models, allowing us to simulate the distinct kinematic evolution of the basin's western and eastern sections. Our 2D sectioned models show that rift velocities vary significantly in the east-west direction. This contradicts previous analog models showing that the formation of the BSB was related to a simple asymmetrical rift with constantly increasing velocities towards the west from a hinge point located at the eastern margin of the basin. The complex velocity changes throughout the rift axis suggest an uneven movement throughout the subduction zone that drives the back-arc rift. Ultimately, proposing a new complex kinematic history during the evolution of the rift and alternating rift velocities throughout the rift axis, provide a better understanding of the timing of all tectonic events and the final ridge depression geometry observed throughout the BSB.

How to cite: Kaykun, A. and Pysklywec, R.: A New Approach to Rift Kinematics During the Formation of the Black Sea Basin, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9303, https://doi.org/10.5194/egusphere-egu26-9303, 2026.

The kinematic of the southern North Atlantic is still debated and new kinematic markers are needed to improve our knowledge of the earliest movements. In this frame, we focus on the location of the first evidence of steady-state oceanic spreading offshore Galicia Bank. Such marker is a spatial criterion that can be used to propose refined new kinematic models. Galicia Bank is part of the magma-poor rifted margins of the southern North Atlantic. The margin is located west of Iberia and is conjugated to the southeastern margin of Flemish Cap. These plate corners are key for understanding the kinematics of the Iberia plate, as they are suspected to act as microplates with complex movements during the Late Jurassic – Early Cretaceous. Studies already proposed domains of exhumed continental and oceanic mantle along a seismic reflection and wide-angle profile offshore Galicia Bank (Dean et al., 2015; Davy et al., 2016) but this boundary is poorly defined on a large scale along the margin. As rift phases occurred during the ‘Cretaceous Quiet Zone’ (118–83 Ma), it is not possible to identify the first oceanic crust using Earth's magnetic field reversal. We propose to interpret several E/W to NW/SE oriented seismic reflection profiles from the BREOGHAM-2005 cruise (P.I. Luis Somoza) to better constrain these areas of exhumed mantle. We based our interpretation method on previous studies of the eastern part of the Southwest Indian Ridge (SWIR) that described a domain of exhumed mantle with successive detachment faults on either side of the ridge axis occurring over the last 11 million years (e.g. Sauter et al., 2013; Reston et al., 2018). In addition, recent seismic reflection data allowed the definition of new criteria for characterising ultra-slow nearly amagmatic spreading ridges. We therefore map these criteria in order to locate this domain along the West Iberia margin. We provide new spatial observations of landward-dipping reflectors and exhumed mantle ridges. They are interpreted as seismic indicators of the presence of flipping detachments. A new boundary is thus proposed along the West Iberia margin separating continental mantle exhumation from steady-state ultra-slow oceanic spreading, which could serve as a constraint in kinematic constructions. The indicators of early steady-state oceanic spreading may be applied to other magma-poor rifted margins. This study may indeed be supported by the presence of the same flip-flop structures in symmetry offshore the Flemish Cap southeast margin.

How to cite: Etcheverry, L., Autin, J., and Somoza, L.: Localisation of steady-state ultra-slow oceanic spreading along magma-poor rifted margins: Case example offshore Galicia Bank (West Iberia)., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9482, https://doi.org/10.5194/egusphere-egu26-9482, 2026.

The Dniepr–Donets Basin (DDB) is one of the largest and best-preserved intracontinental rift systems in Europe, yet the geodynamic processes responsible for its formation remain uncertain. There are two end-member models possible: (1) passive rifting driven by far-field tectonic stresses transmitted through the lithosphere, such as back-arc extension or plate boundary forces, and (2) active rifting associated with localized thermal anomalies in the mantle, potentially linked to plume-like upwellings. Distinguishing between these mechanisms is important for understanding why some continental rifts evolve toward oceanic break-up, whereas others, such as the DDB, remain confined within continental interiors.

This study aims to reassess the tectonic evolution of the DDB by integrating regional-scale seismic, borehole, gravity, and magnetic datasets into a coherent crustal and lithospheric framework. The core of the analysis is based on the interpretation of approximately 40 regional seismic reflection and refraction profiles, including classical and widely used datasets such as DOBRE’99 and Georift-2013. These seismic data are calibrated using stratigraphic, lithological, and velocity information from nearly 1,900 boreholes distributed across the basin. Fourteen key stratigraphic horizons are mapped consistently throughout the DDB, covering an area of ~76,900 km² and spanning the pre-rift, syn-rift, and post-rift sedimentary sequences.

Seismic interpretation is complemented by gravity and magnetic anomaly data, which are used to refine the geometry and continuity of major fault systems and crustal domains. The combined datasets allow the timing and kinematics of major faulting episodes and regional unconformities to be constrained with improved confidence. Balanced cross-section analysis along selected regional profiles provides quantitative estimates of crustal extension, fault displacement, and basin asymmetry, offering direct tests of competing rift models.

A three-dimensional structural model of the DDB that integrates seismic surfaces with borehole stratigraphy and velocity data is a key outcome of the work. Although still under development, this model reveals the three-dimensional architecture of the basin, including variations in sediment thickness, fault segmentation, and structural asymmetry along strike. Particular attention is paid to identifying systematic asymmetries in fault geometry and basin fill, which may indicate simple-shear deformation and lithospheric-scale detachment processes commonly associated with passive rifting. Linking shallow geological observations with deep crustal reflectivity patterns enables a more robust reconstruction of the basin’s long-term evolution.

Potential field data further provide constraints on the role of mantle processes during rifting. Spatial variations in gravity and magnetic anomalies are analyzed to detect possible mafic intrusions, high-density lower-crustal bodies, or anomalous mantle domains. These observations are used to evaluate whether thermal weakening of the lithosphere and magmatic underplating played a primary role, or whether rifting was dominated by mechanical stretching of a relatively cold lithosphere.

Overall, this ongoing research integrates crustal- and mantle-scale observations to explore the interplay between mantle dynamics, faulting, sedimentation, and basin subsidence. The results are expected to refine models of intracontinental rifting and clarify the conditions under which continental rifts either progress toward break-up or remain long-lived but abortive systems, as exemplified by the Dniepr–Donets Basin.

How to cite: Nasiri, A., Stephenson, R., Stovba, S., Drachev, S., Słonka, Ł., and Mazur, S.: Integrated Seismic–Potential Field Constraints on the Evolution of the Dniepr–Donets Rift Basin, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9582, https://doi.org/10.5194/egusphere-egu26-9582, 2026.

The complexity in the relationship between mantle and lithosphere processes may be most directly exemplified in the coupling between upwelling plumes and extending lithosphere at rifted margins, and the distribution of excess melting across these regions through space and time. Rifted margins are often described in two end-member classes; magma-rich and magma-poor, typified by the emplacement of seaward dipping reflector sequences (SDRs) and high velocity lower crustal bodies (HVLC) or the exhumation of serpentinised mantle with little extrusive melt, respectively. Previous studies have linked margin architecture and magmatic budget to extension velocity, lithosphere thickness, and rheology. The role of mantle plumes remains poorly constrained, with plumes associated with both magma-poor and magma-rich margins, implying that their influence on excess melt production is not straightforward. Our study aims to better constrain the relationship between mantle plumes and excess melting at rifted margins by exploring the interaction of plumes originating from the mantle transition zone and rifting.

We present two-dimensional numerical simulations to investigate how mantle plumes interact with lithosphere extension during continental rifting. Rifting is simulation using the ALE finite-element code FANTOM, incorporating a thermal anomaly at the base of the upper mantle to represent a stalled plume source. We systematically vary velocity, plume temperature anomaly, and plume position relative to the rift axis to explore how these parameters control the timing, magnitude, and spatial distribution of excess melting during breakup.

Our results indicate that excess melting associated with mantle plumes is both transient and spatially distributed. The timing, magnitude and lateral distribution of excess melting depends non-linearly on the interaction between plume buoyancy and lithospheric extension rate, with the strongest plume influence occurring at intermediate extension velocities. Plumes residing directly beneath the rift axis focus melt, producing temporally concentrated, focussed melt zones that promote earlier rift breakup whereas plumes which lie adjacent to the rift axis produce spatially offset and temporally delayed melt focussing, resulting in narrower but less efficiently coupled melt zones. These results demonstrate that plume-driven excess melting may be highly time-dependent with an evolving spatial distribution that reflects the efficiency of melt focussing relative to the thinning lithosphere.

How to cite: Plimmer, A., Huismans, R., and Wolf, S.: Controls on the spatio-temporal distribution of plume-related excess melting during continental rifting, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9953, https://doi.org/10.5194/egusphere-egu26-9953, 2026.

The Gulf of Argos, Greece, is a post-Miocene basin at the north-western extremity of the Cretan Sea (southern Aegean). Its formation is attributed to NE-SW-oriented back-arc extension, induced by rollback of the subducting slab in the Hellenic arc.

The western margin of the Gulf of Argos is marked by the almost linear coastline of the eastern Peloponnese, and is related to a c.100 km long, NNW-SSE normal fault system, stretching from Kiveri to Ariana. Despite it being a recognizable structure, there are few, if any, constraints related to its degree of activity, possible segmentation, and seismic hazard potential. The immediate footwall to this fault system, which we name Western Argos Fault System (WAFS), hosts several similarly striking high-angle normal faults, whose Quaternary degree of activity is also poorly understood.

To better understand fault activity and evolution in the Gulf of Argos, we study the mid- to long-term (several kyr to a few Myr) development of the footwall of the West Argos Fault System. Our study focuses on how drainage river long profiles and footwall relief have responded dynamically to tectonic activity. We estimate an uplift rate for each footwall catchment along the WAFS from knickpoint analysis and estimates of bedrock erodibility. We then compare these results with vertical motion data collected in the field and topographical data along the western margin of the Gulf of Argos.

We propose a throw rate of 0.9-2.4 mm/yr along the WAFS, which comprises at least four segments and an overall southward migration of fault activity, as the northernmost segments appear to be significantly less active than the southern ones.

How to cite: Viger, A., Kranis, H., Whittaker, A., Bell, R., and Ganas, A.: Fault activity along the western margin of the Argos Gulf (Peloponnese, Greece) revealed by tectonic geomorphology analysis, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10165, https://doi.org/10.5194/egusphere-egu26-10165, 2026.

                Deciphering the dynamics of continental breakup is fundamental to understanding how oceanic basins initiate, segment, propagate and connect to the global oceanic system. However, constraining the spatial and temporal evolution of continental rupture is challenging as it precedes the establishment of continuous oceanic spreading and reliable kinematic markers such as marine magnetic anomalies. Here we focus on the earliest stage of Pangea breakup, with the aim of constraining basin segmentation during the initial opening of the Central Atlantic Ocean (CAO), prior to its connection with the main Panthalassa Ocean through the Western Tethys.

                The CAO corresponds to the earliest opened branch of the Atlantic.  The timing of its continental breakup and onset of oceanic spreading remains debated, with proposed breakup ages ranging from 195 Ma to 175 Ma. This uncertainty leads to major ambiguities in the geodynamic context of continental rupture, with consequences for the interpretation of rifted and nascent oceanic basins segmentation, connectivity, and associated depositional environments. It also affects the interpretation of major Jurassic magnetic anomalies identified across the CAO: the East Coast Magnetic Anomaly (ECMA) and Blake Spur Magnetic Anomaly (BSMA), which are commonly used as kinematic markers in early Atlantic reconstruction.

                To address these issues, we have compiled a regional database to integrate major rift structures and basins, Upper Triassic salt distribution, and variations in the nature of the ocean-continent-transition and magmatic type. We present interpretations of seismic reflection data along the Central Atlantic rifted margins, calibrated using available drilling results. These data allow us to constrain rift basin age and architecture, fault system development and the distribution of rift-related salt provinces. In parallel, regional crustal thickness maps derived from gravity inversion are used to investigate along-strike variations in magmatic budget during continental breakup and the early stages of oceanic accretion, relation with the spatial distribution of the ECMA and BSMA.

                Our first results confirm pronounced along-strike variations in magmatic volumes emplaced during continental breakup and the initial phases of oceanic spreading. The newly compiled database will provide key constraints for paleogeographic reconstructions, with the aim of clarifying the duration of oceanic basin isolation, the timing of basin connectivity through the Western Tethys and sedimentation pathways associated with the early Atlantic evolution.

How to cite: Heudes, B., Tugend, J., Mohn, G., and Kusznir, N.: Early opening of the Central Atlantic and its connection to the Western Tethys, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10868, https://doi.org/10.5194/egusphere-egu26-10868, 2026.

The Natron Basin is located within the eastern branch of the East African Rift, a segment characterized by greater magmatic activity compared to the western branch. This activity has been shown to play a key role in accommodating deformation in the eastern rift, alongside crustal-scale faulting. The Natron Basin represents a particularly suitable natural laboratory to investigate the interaction between active tectonic and magmatic deformation, as previous studies have documented magmatic intrusion events associated with active rifting episodes in the region.

In this study, we use new geodetic observations acquired during a GNSS campaign conducted in the Natron Basin in summer 2025, and started in 2013, to investigate present-day deformation patterns. Campaign-derived horizontal (and vertical) velocities are used to estimate regional strain rates and to derive geodetic moment rates under standard mechanical assumptions. These geodetic estimates provide an integrated measure of ongoing extension across the basin.

To assess how this deformation is released seismically, we compare geodetic moment rates with seismic moment rates inferred from global earthquake catalogs, including NEIC and ISC; over comparable spatial and temporal scales. This comparison allows us to place bounds on the seismic coupling coefficient of rift normal faults.

The observed mismatch between geodetic and seismic moment rates suggest that a significant fraction of present-day deformation in the Natron Basin is accommodated though aseismic processes. These may include distributed crustal deformation and contributions from magma intrusions, which are known to influence rift evolution in magma-rich segments of the East African Rift. These observations illustrate the potential of combined geodetic and seismic analyses to investigate strain partitioning in magma-rich segments of continental rifts.

How to cite: Navarrete, I., Olive, J.-A., Calais, E., Keir, D., and Dalaison, M.: Strain partitioning in the Natron Basin, East African Rift: Insights from geodetic and seismic moment rates., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11130, https://doi.org/10.5194/egusphere-egu26-11130, 2026.

Several types of magmatism are typically associated with continental stretching and rift formation. The South Atlantic Rifted Margin is a particularly well-studied system that exhibits: (1) Thousands-km-long massive dike swarms – likely linked to the Tristan Plume/hotspot; (2) the Parana-Etendeka continental flood basalt (CFB); and (3) the formation of extensive seaward dipping reflector sequences (SDRs) along the southern portion of this rifted margin. Here we review the distribution, timing, and volumes of these different modes of rift-related magmatism in relation to rift evolution.

Great dike swarms formed prior to, during, and soon after the Parana-Etendeka flood basalt event at 136.5-135.5 Ma. Although comparable in spatial extent and volume to the well-known Proterozoic Mackenzie dike swarm that similarly extended from a continental flood basalt, summed dike volumes appear to only be ~10% (0.15e6 km^3) of the Parana CFB magmatism (~1.5e6) and ~2% of total magmatism (~6e6) associated with South Atlantic Rifting including SDR provinces.

The defining characteristic of the CFB event is that it occurred very rapidly, which appears most consistent with a sudden lithospheric thinning event (e.g. lower lithospheric delamination) in the presence of hot plume material. A plume-head rising under thick continental lithosphere simply could not create this sudden burst of volcanic activity, thus an abrupt lithospheric thinning event appears needed to explain this melting anomaly. Note that there is seismic evidence consistent with such a delamination event both in the thinned lower lithosphere beneath Parana and the presence of a delaminated lithospheric fragment in the transition zone near the site of the modern Tristan Plume.

Finally, the largest volcanism associated with South Atlantic rifting is linked to the SDR province including associated underplated magmas offshore the southern margins of South American and Africa. This post-CFB magmatic activity can be quantitatively explained by more extensive melting of southward flowing Tristan Plume material after extensive rifting has thinned the extending lithosphere to <~80km. The later timing of this activity (~130-125 Ma) relative to the CFB (136.5-135.5 Ma) suggests that it, too, was not linked to the arrival of a plume head, but rather the persistent ‘tail’ of the Tristan Plume.  We will also briefly discuss potential implications for the epeirogeny linked to plume-rift evolution.

How to cite: Morgan, J. P. and Ranero, C. R.: Reassessing modes of Plume-Rift Magmatism, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11281, https://doi.org/10.5194/egusphere-egu26-11281, 2026.

Many rifted margins develop in regions that previously experienced oceanic subduction and continent–continent collision. This implies that continental rifting commonly occurs in a lithosphere that contains significant inherited features, rather than in a homogeneous medium. Such inheritance can be broadly classified into three categories – structural, rheological, and thermal – which typically coexist. Inherited features may strongly influence rift evolution and resulting margin architecture.

In this study, we use 2D thermo-mechanical numerical models to investigate how complex inheritance, featuring structural, rheological and thermal components, affects subsequent phases of continental rifting. Our models simulate rifting following orogenesis that occurs through oceanic subduction, microcontinent accretion, and continental collision. By varying the size and complexity of the pre-rift orogen, we evaluate the relative importance of different types of inheritance in the development of rifted margins. We compare the resulting margin architectures with natural examples.

We find that a dynamic interplay exists between structural, rheological, and thermal inheritance, strongly influencing the resulting rifted margin architectures. In small, cold orogens, structural inheritance is predominant, whereas in large, warm orogens, thermal and rheological inheritance play more significant roles. The relative importance of thermal and rheological inheritance is particularly challenging to assess, but we propose that the former plays the more prominent role. To illustrate these contrasts, we compare conjugate rifted margin architectures of two end-member models with natural examples from the opening of the North and South Atlantic Oceans. Our experiments reproduce a diverse array of features observed in the natural examples, including the formation of continental fragments and allochthons. They illustrate the complex deformation pathways through which rifted margin structures may have been achieved. Our results thus highlight the critical role of deformation history in shaping the evolution of continental rifting.

How to cite: Erdős, Z., Peron-Pinvidic, G., Buiter, S., and Tetreault, J.: Styles of extensional reactivation in rifted margins – comparing numerical modeling results to nature, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11555, https://doi.org/10.5194/egusphere-egu26-11555, 2026.

Continental rifting often induces decompression melting and the ascent of magma that intrudes into the brittle crust in the form of dikes and sills and that extrudes along volcanic fields. At the same time, continental rifts experience stress from topographic loading due to rift flank uplift. It is clear that these two processes interact in magmatic rifts such as the Kenya Rift, the Main Ethiopian Rift, the Afar triple junction, and at the Icelandic plate boundary. However, separating the interplay between tectonic and magmatic processes, the evolving topography and the rift-related stress field, as well as the impact of these processes on dike-fault interactions from field observations alone remains difficult.

Previous modeling studies of time-dependent magma-tectonic interactions in extensional tectonic settings generally fall into two categories: (1) Simple models (e.g. Buck et al., 2005) represent diking by a prescribed fixed rectangular zone of horizontal divergence. While this approach can be applied to model tens of millions of years of dike injection along spreading ridges, its simplicity prevents applications to continental rifts where magmatism manifests over broad areas. (2) More complex setups simulating magma ascent via porous flow and fluid-driven fracture (e.g., Li et al. 2023). This approach allows to study the evolution of individual dikes, but its computational costs prevent application to lithosphere-scale rifts over geological time scales. 

Here, we present a numerical workflow that can be categorized as a model of intermediate complexity. The dikes are nucleated at the brittle-ductile transition above zones of partial melt. They are then propagated perpendicular to the minimum compressive stress, similar to the approach of Maccaferri et al. (2014), until they reach their freezing depth or the surface. In this presentation, we show how we  approach this problem and how we implement it in the open-source community geodynamics model ASPECT. We demonstrate that the generated dikes are being focused in specific regions, and how the directional dilation and heat injection during magma intrusion through dikes influence the long-term rifting evolution. 

References:

Buck, W. Roger, Luc L. Lavier, and Alexei N. B. Poliakov. “Modes of Faulting at Mid-Ocean Ridges.” Nature 434, no. 7034 (April 2005): 719–23. https://doi.org/10.1038/nature03358.

Li, Yuan, Adina E Pusok, Timothy Davis, Dave A May, and Richard F Katz. “Continuum Approximation of Dyking with a Theory for Poro-Viscoelastic–Viscoplastic Deformation.” Geophysical Journal International 234, no. 3 (September 1, 2023): 2007–31. https://doi.org/10.1093/gji/ggad173.

Maccaferri, Francesco, Eleonora Rivalta, Derek Keir, and Valerio Acocella. “Off-Rift Volcanism in Rift Zones Determined by Crustal Unloading.” Nature Geoscience 7, no. 4 (April 2014): 297–300. https://doi.org/10.1038/ngeo2110.

How to cite: Fraters, M., Brune, S., Rivalta, E., Gassmöller, R., Liu, S., Muluneh, A. A., and Thieulot, C.: Intermediate-complexity modeling of magma–tectonic interaction in continental rifts, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12670, https://doi.org/10.5194/egusphere-egu26-12670, 2026.

The presence of pre- to early synrift salt leads to varying degrees of decoupling between supra- and subsalt deformation during rifting. Decoupling is favored by thick salt or small fault displacement. This has been examined in detail in low-𝛽settings such as the southern and central North Sea and is applicable to the proximal domains of rifted margins. In addition, the role of late syn-rift salt on margins has been extensively studied. But the behavior of pre- to early synrift salt in the high-𝛽 necking, hyperextended, and exhumed mantle domains remains poorly understood.

A common suprasalt geometry in the necking and hyperextended domains of the western Iberian margin is that of strata that dip and thicken basinward. These might be mistaken for growth strata adjacent to a landward-dipping fault bounding a horst or for salt evacuation structures in a half graben, with both interpretations invoking low-𝛽, high-angle normal faults. However, they more likely record extension associated with large-offset detachment faults, but with thickening onto the top of the hanging wall instead of the fault. Slip ceases on the low-angle, basinward-dipping fault between the hanging- and footwall cutoffs of the salt, with continued extension on the deeper part of the fault transferred to slip on the steeper, landward-dipping hanging-wall salt in a zig-zag pattern like that of fish-tail thrusts. This simple concept can guide interpretations in areas with inadequate imaging.

The same idea also explains the presence of significant volumes of pre- to early synrift salt in the exhumed mantle domain, as seen in the Mauléon Basin of the NW Pyrenees. This relationship is enigmatic because mantle represents new real estate that formed after salt deposition and, moreover, any salt should be highly attenuated. The solution is that as mantle is exhumed from beneath the upper plate, extension on the landward-dipping exhumation detachment is transferred to the basinward-dipping salt detachment on that upper plate, thereby generating a zig-zag fault geometry. Effectively, the upper plate moves out from between both detachments, which merge at the hanging-wall cutoff of the upper plate such that salt and suprasalt strata end up juxtaposed above the footwall of the exhumation detachment. That part of the detachment becomes locked and the salt above the mantle does not get attenuated by further extension.

How to cite: Rowan, M., Chenin, P., and Manatschal, G.: Using stratal geometries above prerift to early synrift salt to constrain crustal fault interpretations in the distal domains of magma-poor rifted margins, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12780, https://doi.org/10.5194/egusphere-egu26-12780, 2026.