Tectonic-scale geological phenomena are fundamentally controlled by nanoscale physiochemical mineral processes. Understanding these processes across multiple length scales is crucial for determining how mass and stress are transferred during tectonism. Minerals exhibit complex structure-property relationships that govern their mechanical and chemical behaviour, yet these relationships have been historically underexplored in Earth sciences. Advances in nanotechnology and instrumentation, including techniques such as high-resolution electron backscatter diffraction, electron channeling contrast imaging, transmission electron microscopy, and atom probe tomography, now enable unprecedented investigations of nanoscale features in geomaterials. The correlative approach has given rise to the emerging field of nanogeology, which helps bridge the gap between nanoscale and tectonic-scale processes. Recent nanoscale investigations have demonstrated the fundamental role of structural defects and element mobility in controlling the mechanical properties and deformation behaviour of minerals in the brittle-ductile regime. For example, detailed microanalyses of garnet reveal a novel precipitation hardening mechanism where Fe diffused along grain boundaries of recrystallized garnet, nucleating Fe-rich nanoclusters. These clusters act as barriers to dislocation migration, resulting in localized strain hardening. This process provides a potential mechanism for mechanical strengthening in the lower continental crustsubsequently influencing large scale geodynamic processes. Similar investigations of pyrite, a critical metal-bearing sulfide mineral, reveal nanoscale fluid inclusions that facilitate the diffusion of trace elements into crystalline defects, such as dislocations, inhibiting their movement, and leading to mineral hardening. Such findings are particularly significant, as the brittle-to-ductile behaviour of sulfides has been directly linked to the upgrading of critical metal deposits. These discoveries highlight the dynamic interplay between nanoscale element mobility and the rheology of minerals, and by consequence, larger mass transfer dynamics. Moreover, deformation-driven element redistribution raises questions about the reliability of deformed minerals as petrological tools. For instance, the deformation of zircon may compromise its use as a robust geochronometer, whereas the deformation of garnet may influence its reliability as a thermobarometer. A deeper understanding of element mobility in the presence of defects is essential for accurately interpreting geochemical data and reconstructing tectonic histories. Overall, these breakthroughs highlight the pivotal role of nanoscale processes in shaping tectonic phenomena, emphasizing the need for a multi-scale approach to understanding Earth's dynamic behaviour.

How to cite: Dubosq, R., Schneider, D., Rogowitz, A., and Gault, B.: Scaling up: Nanoscale insights into tectonic phenomena , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4567, https://doi.org/10.5194/egusphere-egu25-4567, 2025.

Understanding the physical processes and interactions that govern the dynamics of the seafloor is key to unraveling the fundamental mechanisms of the marine lithosphere and its boundaries. These processes not only shape the ocean floor but also influence a variety of geohazards that pose significant risks to coastal communities.

One of the greatest challenges in marine geosciences is to identify the locations and underlying causes of marine geohazards. To achieve this, we need a multidisciplinary approach and comprehensive, multi-sensor geophysical measurements that integrate observations of the seafloor conditions and the complex interactions of sub-seafloor processes that lead to these events. This includes a detailed understanding of the physical, chemical, and mechanical factors that drive natural hazards including earthquakes, tsunamis, and volcanic eruptions. Seafloor processes, from tectonic movements to crustal deformation to magmatic activity, are central to understanding how these phenomena occur.

Recent advancements in marine geophysical research, particularly in seafloor geodesy, ocean bottom array seismology, and micro-bathymetry have allowed for the quantification of seafloor processes and their dynamic changes under tectonic stress. The majority of large (magnitude Mw>8.5) earthquakes occur in subduction zones. The associated surface deformation is concentrated at the seafloor and is often coupled with the triggering of tsunamis. The seabed therefore harbors information about tectonic stress and elastic deformation. This information is crucial for early warning concepts and can be methodically analyzed using seafloor geodesy in conjunction with seismic and earthquake studies. The plate boundary offshore northern Chile is one of the seismically most active regions on the globe and is the site of comprehensive multi-sensor seafloor monitoring. The integrated data analysis revealed co-seismic stress changes and aftershock activation of extensional faulting of the upper continental plate, indicative of active subduction erosion during the co-seismic and post-seismic phase. One of the most striking findings is the correlation of the seismogenic up-dip limit with a pronounced decrease in plate boundary reflectivity. High-resolution in-situ strain measurements from seafloor geodetic arrays monitor the tectonic stress build-up across the subduction zone, which is characterized by very low rates during the interseismic phase. Tectonic stress build-up across a plate boundary was also monitored along the offshore segment of the North Anatolian Fault Zone in the Sea of Marmara, revealing a significant slip on this fully locked segment.   

Looking ahead, marine geophysical research is poised to expand significantly, addressing new challenges and utilizing new technologies and methods across disciplines. The next generation of seabed monitoring systems will use real-time data analysis and underwater acoustic communication in autonomous ‘smart’ networks for targeted monitoring. On a broader scope, the utilization of existing telecommunication systems has the potential to profoundly change solid earth monitoring. These observations may also elucidate potential preparatory phases of major subduction earthquakes, detect landslides on coastal slopes, and monitor largely unknown submarine volcanic activity. Operational real-time access will reduce earthquake and tsunami early warning delays significantly.

How to cite: Kopp, H.: The Dynamic Seafloor: Enhancing Our Knowledge of Seafloor Processes, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4841, https://doi.org/10.5194/egusphere-egu25-4841, 2025.

This study examines field-aligned currents (FACs) and polar electrojet (PEJ) characteristics during the extreme May 2024 geomagnetic storms across dawn, dusk, daytime, and nighttime in both hemispheres. FAC and PEJ intensities were up to 9 times greater than usual, with equatorward FACs reaching -44º Magnetic Latitude. Maximum FACs and PEJ are larger at dawn than dusk in the Northern Hemisphere but larger at dusk than at dawn in the Southern Hemisphere. Dawn and duskside FACs correlate best with Dst or solar wind dynamic pressure (Pd) in both hemispheres. On the dayside (nightside), most FACs in both hemispheres are primarily correlated with Pd (merging electric field, Em or Pd). The PEJs correlate largely with Dst and partly with Em and Pd. Duskside (nighttime) currents are located at lower latitudes than dawnside (daytime), and northern currents are positioned more poleward than southern currents. The latitudes of peak FACs are most strongly correlated with Dst or Pd in both hemispheres. However, in the northern daytime sector, they are primarily influenced by Em. The latitudes of peak PEJ show the strongest correlation with Dst or Pd in both hemispheres, except on the northern dawnside, where they are primarily influenced by Em. The qualitative relationships between peak current density, corresponding latitude, solar wind parameters, and the Dst index are derived.

How to cite: Wang, H., Lühr, H., and Cheng, Q.: Local Time and Hemispheric Asymmetries of Field-aligned Currents and Polar Electrojet During May 2024 Superstorm Periods, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-396, https://doi.org/10.5194/egusphere-egu25-396, 2025.

The collision of the Indian and Eurasian plates ~ 55 Ma formed the Himalaya, one of the youngest mountain belts. This convergence led to two significant metamorphic stages: M1, which occurs under high pressure and low temperature in a thick crust, and M2, resulting from crustal thinning in a high-temperature, low-pressure environment, evolved the gneissic domes. This study provides the first apatite fission track (AFT) and zircon fission track (ZFT) thermochronological record from one of such gneissic domes in the NW Himalaya viz., the Gianbul Dome (GD). The dome is bounded by two extensional shear zones, namely the South Tibetan Detachment System (STDS) dipping towards NE and the Khanjar Shear Zone (KSZ) dipping towards SW.  The AFT cooling ages range from 14.2 ± 1.2 to 5.7 ± 1.1 Ma, and ZFT ages range from 22.8 ± 2.2 to 14.6 ± 0.9 Ma. The ZFT ages remain almost constant across the dome, suggesting thermal relaxation during this period, while the AFT ages are young towards the extensional shear zones of KSZ and STDS. The fission-track data, in combination with the published Ar-Ar and (U-Th)/He cooling ages, has been modeled using a thermo-kinematic inverse and forward model to analyze the processes that led to the exhumation of the dome. Various scenarios like river incision, lithology, deformation along faults like Main Himalayan Thrust, Main Central Thrust, STDS, glacier control, and erosion control over exhumation have been tested. Our results suggest that the extension of normal fault is the primary mechanism for the exhumation of the GD. The extension happened in two phases: (a) during the initial normal sense movement along the STDS when the reverse sense of shear was switched to the usual sense of shear during the early Miocene, and (b) during the Late Miocene. The initial phase of extension is a well-recognized phenomenon in the Himalayan orogen that has been explained through models like channel flow or ductile wedge extrusion. However, the first report of extensional activity along the STDS during the Late Miocene allows us to test whether it is a local phenomenon or a regional event that happened in the brittle stage. Thus, we compiled all the published geochronological and thermochronological data of all the prevailing gneissic domes in the Himalayas from west to east and ran the 3D thermokinematic model to assess the exhumation path of the rocks and brittle stage deformation history. Our results suggest that two phases of extension happened in the entire arc of the Himalayan orogen. The first phase facilitated the southwest migration of ductile materials of rocks from mid-crustal depths accompanying the extension because of gravitation, favoring the channel flow concept. The second phase of extensional collapse happened during ~7-3 Ma ago in the brittle stage. We hypothesize that a drop in gravitational potential energy led to the reactivation of extensional faults along the Himalayan arc. Thus, we propose that extensional collapse in the collisional mountain belts is a cyclic phenomenon that happens to attain a stable, steady state of the orogens.

How to cite: Patel, R. K., Adlakha, V., Mukherjee, K., Pundir, S., Pradhan, P., and Patel, R. C.: Extensional collapse of the Himalayan orogen in the Late Miocene., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1368, https://doi.org/10.5194/egusphere-egu25-1368, 2025.

The Kaliguman Shear Zone (KSZ) in northwestern India marks the boundary between the South Delhi Fold Belt to the west and the Aravalli Fold Belt to the east. Structural analysis reveals a narrow, high-strain zone characterized by the development of mica schist along this boundary. The principal structural orientation trends in the NE-SW direction. Strain analysis indicates that the rocks in this zone formed under transpressional deformation conditions.

The metamorphic history of the KSZ is well-preserved in the mica schists, which predominantly contain garnet and staurolite. Petrological and textural studies have helped establish the relative crystallization sequence of mineral phases during the metamorphic events. Examination of garnet porphyroblasts reveals a complex deformation pattern, reflecting pre-, syn-, and post-tectonic events associated with fabric formation. Geothermobarometric analysis indicates that the mica schists underwent amphibolite facies metamorphism. Phase equilibria analysis, supported by PT pseudosections, shows peak metamorphic conditions at approximately 590±10 °C and 4.7 kbar. Garnet isopleth plots suggest increasing pressure and temperature during metamorphism, which is consistent with the inferred PT path. Variations in the modal abundance of index minerals further corroborate this evolutionary trajectory. These findings support a model of crustal thickening for the KSZ. The textural control monazite age data from the mica schists confirms that the shear zone was formed during the early Neoproterozoic. The study provides valuable insights into the tectonic evolution of the contact between the Delhi and Aravalli Fold Belts, highlighting the role of shear zones in accommodating deformation and facilitating metamorphic processes during Neoproterozoic orogenic events.

Keywords: Kaliguman Shear Zone, Aravalli, Delhi Fold Belt, Neoproterozoic, NW India

How to cite: Mondal, S., Roy, A., and Chatterjee, S.: Neoproterozoic Tectonics of the Kaliguman Shear Zone: Implications for the Delhi-Aravalli Fold Belt Contact, NW India, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1504, https://doi.org/10.5194/egusphere-egu25-1504, 2025.

       Bathymetry of marginal sea basins is commonly deeper than the half-space cooling prediction for large oceans, but what controls this pattern is poorly understood. Here, based on abundant seismic sections with increasingly available databases, we perform an enhanced approach that specifically corrects for post-spreading cooling to reassess thermal subsidence across the Southwest Subbasin (SWSB) and the broader South China Sea (SCS) basin. We attribute the current excessive subsidence of the SCS basin primarily as a response to the post-spreading cooling process, which has global applicability to other marginal sea basins and accounts for at least 86% of the observed depth anomaly. Additionally, the mode of magma supply during seafloor spreading plays a crucial role in shaping reconstructed shallower bathymetry of the SCS basin relative to predictions from the half-space cooling model. A stronger magma supply deriving from the regional subduction system can explain the relatively shallow depth developed during the opening of the SCS compared to large oceans. In contrast, a westward decayed magma supply, driven by localized rift propagation induced by the inherited pre-Cenozoic heterogeneous lithospheric structure of South China, attributes to subsidence discrepancies among sub-basins and within the SWSB.

        The sediment-corrected depth of most marginal seas is, on average, more than 500 m deeper than that of large oceans, with maximum anomalies ranging from -0.95 to -2.70 km (in 0.5° bins). The sediment-corrected depths exhibit statistically poor correlations with the spreading rate, indicating that the thermal evolution of marginal seas is not primarily controlled by the spreading rate, unlike large oceans. Neither can this anomaly be fully explained by dynamic topography driven by large-scale mantle convection or by localized variations in the degrees and patterns of subduction systems, although the latter may be an important factor influencing the bathymetry of still-active marginal seas. We interpret at least 44.5% of these anomalies as a result of long-term post-spreading thermal subsidence in inactive marginal seas, with magmatic processes influencing bathymetry during oceanic plate formation. We propose that the post-spreading secular cooling, together with the variable mode of magma supply and potential dynamic subsidence processes driven by subducting slabs, play pivotal roles in the formation of the topographic anomalies within the oceanic basins of marginal seas.

How to cite: Fang, P. and Ding, W.: Reconciling bathymetric anomalies of marginal sea basins through magmatic and cooling processes, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2072, https://doi.org/10.5194/egusphere-egu25-2072, 2025.

Slip characteristics of tectonic faults are highly correlated with earthquake risks. However, the stress conditions in-situ are not static, because tides and seismic waves produce dynamic stress disturbances. The effect of fluids also needs to be considered. The fault slip evolution considering both, stress perturbation and fluid pressure is poorly investigated in the laboratory.

We performed direct shear tests on saw-cut granite joints using a shear box device with external syringe pump. The lower part of the specimen was driven by constant load point velocity, and static/dynamic normal loads were applied to the upper part. LVDTs recorded horizontal and vertical movements: fault slip and vertical dilatancy, respectively. The impact of two factors are studied in the experiment: pore fluid pressure and applied normal stress oscillation amplitude.

In conclusion, static pore fluid pressure reduces effective normal stress and shear stiffness of the sheared fault. Under constant normal stress, the reduction in fault shear stiffness caused by fluids synchronously competes with the reduction in critical stiffness (K_c) as the effective normal stress decreases. The stick-slip events are most intensive under low fluid pressure and high normal stress. Under oscillating normal stress, as the normal stress oscillation amplitude increases, the overall fault shear strength weakens continuously. Frictional strengthening and aseismic slips always occur in the normal stress loading stage. Normal stress unloading leads to multi-step stick-slip behavior of the sheared fault. The fault normal deformation is controlled by both normal loading/unloading and asperity overriding. Increasing pore pressure and superimposed normal stress magnitudes lead to more dramatic shear stress changes, but the degree of seismic slip is reduced.

How to cite: Tao, K., Konietzky, H., and Dang, W.: Seismic fault slip affected by pore pressure and cyclic normal stress – deduced by lab investigations, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2323, https://doi.org/10.5194/egusphere-egu25-2323, 2025.

Surface rupture caused by a strong earthquake is extremely hazardous to the safety of people’s lives. Understanding the rupture evolution mechanism of co-seismic faults and assessing the influence of fault area propagation is essential for disaster prevention and resilience. Since 2000, Hualien and nearby areas in eastern Taiwan have experienced 33  earthquakes, which is a good area to study the evolution of fault rupture. In this study, we propose a dynamic discrete element model to explain fault rupture evolution and use it to analyze the rupture behavior of the 2024 Mw7.4 earthquake of Hualian. This earthquake occurred near the northern Longitudinal Valley Fault (LVF), where crustal movement can be seen from the Milun Fault (MF) to the north part of the LVF. We use ALOS-2 data to identify major faults and the Interferometric Synthetic Aperture Radar (InSAR) method to access the spatial displacement on the surface of the study area. In order to simulate the complex geometry and corresponding deformation of the co-seismic rupture surface under the compound influence of multiple faults, we set a rock biaxial simulation test to obtain effective model parameters. We then established a series of dynamic models with different bond types and strengths based on the discrete element method. The model demonstrates the deformation along the fault rupture surface, corresponding to the observation results. The simulation results cover the rupture behavior of the fault and the displacement of the shallow fault under long time series, which can provide a reference for the subsequent seismic hazard assessment and fault displacement analysis.

How to cite: Guo, X., Aoki, Y., and Li, J.: Discrete element modeling of earthquake-induced fault rupture evolution: The 2024 Mw7.4 Hualien Taiwan earthquake, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2827, https://doi.org/10.5194/egusphere-egu25-2827, 2025.

Meteorites that have impacted the Earth's surface in the past have created impact craters. Most of these craters have not been preserved in a form that allows for their contemporary identification, but some, especially in Central and Northern Europe, have been described and classified as geological structures formed by meteorite impacts. When a celestial body strikes the Earth's surface, it causes a temporary increase in temperature to several hundred degrees Celsius, sometimes exceeding the Curie temperature for ferromagnetic rocks and minerals that make up the near-surface layer. Magnetization is relatively stable from a geological time perspective. The magnetic record in magnetite is usually stable and is quite difficult to remagnetize (Fassbinder, 2015).

The impact leads to a change in the direction of magnetization in the minerals, which sometimes persists after the impact. This phenomenon is known as Thermoremanent Magnetization (TRM). It is characteristic of meteorite impact sites. This property is attributed to minerals cooled from high temperatures resulting from plutonic/volcanic processes or meteorite impacts. It is one of several types of remanent magnetization, but only this type will be present in impact structures (Fassbinder, 2015).

The project aims to conduct research in the field of applied geophysics and the magnetic properties of rock and mineral samples in the area of craters formed by meteorite impacts in the context of thermomagnetic anomalies.

As part of this project, proton magnetometer measurements have been conducted in the areas of the Morasko craters in Poland, the Dobele crater in Latvia, the Vepriai crater in Lithuania, and several craters in Estonia. Samples from the Estonian craters have been collected for paleomagnetic studies, which will soon be analyzed using a rotational magnetometer and a magnetic susceptibility instrument. The results of the magnetometric measurements are very promising and exhibit characteristic patterns of magnetic field anomalies typical of impact craters.

The project is funded under the 'Pearls of Science' program by the Ministry of Science and Higher Education of the Republic of Poland.

How to cite: Zawadzki, M., Godlewska, N., and Oryński, S.: Measurements of Earth's magnetic field anomalies caused by meteorite impacts, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2898, https://doi.org/10.5194/egusphere-egu25-2898, 2025.

The Chatree Region, located in central Thailand, holds significant potential for gold exploration, hosting substantial mineral resources. The complex geological setting of this region, with its diverse lithologies and intricate structural controls, poses both significant opportunities and challenges for successful mineral exploration. Given these challenges, this study utilizes a geophysical approach focusing on magnetic data interpretation to enhance the precision and efficiency of identifying potential gold prospects.

Enhancements to the magnetic data were achieved through the application of Downward Continuation (DWC) and Automatic Gain Correction (AGC), which amplified near-surface features and improved signal clarity. Subsequently, by employing the Centre for Exploration Targeting (CET) Grid Analysis, zones of structural complexity, indicative of epithermal gold deposits were detected, which generated two heat maps, including Contact Occurrence Density (COD) and Orientation Entropy (OE). These maps revealed six major and seven minor potential gold prospect zones, providing a critical dataset for subsequent geological analysis. Geological correlations and lineament interpretation were then conducted to validate and refine the magnetic interpretations by integrating several filtered magnetic images with existing geological knowledge of the region. The results of this integrated approach show that many of the identified magnetic anomalies and lineaments correlate with known geology, highlighting the critical role of synthesizing advanced magnetic data analysis with geological expertise. This integration provides a valuable foundation for future exploration in the region and establishes an applicable methodological approach to other mineral exploration efforts.

How to cite: Pinkaew, K.: Identifying Prospective Areas for the Chatree Epithermal Gold Region from Airborne Magnetic Data Using Advanced Analyzing Techniques, with Interpretative Correlation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3794, https://doi.org/10.5194/egusphere-egu25-3794, 2025.

The differences in the tectonic interpretation of ophiolite suites have become a major issue of the debate in the tectonic reconstruction of an ancient orogenic belt, especially when it comes to subduction polarity. In this regard, the Sanjiang Paleo-Tethyan Orogenic Belt in the southeastern Tibetan Plateau provides an excellent case study. The Sanjiang Paleo-Tethyan Orogenic Belt in the northern, eastern, and southeastern Tibet is bounded by the western Jinshajiang‒Garzê‒Litang suture to the north and the Shuanghu‒Changning‒Menglian suture to the south and west. The southern Jinshajiang Suture separates the Zhongzha Block to the east and the eastern Qiangtang Block to the west. The tectonic nature of the NNW-trending southern Jinshajiang ophiolitic mélange remaining controversial. A detailed linear traverse mapping was c across the southern Jinshajiang ophiolitic mélange, with a focus on pillow lavas and the structural relationship between the lavas and their country rock (Paleozoic sedimentary rocks). The results of a field study, in conjunction with new geochronological data and geochemical data, have enabled the identification of the Zhongdian continental back-arc basin. This basin was filled with a flysch succession and at least two horizons of pillow basalt from 267 to 254 Ma. The fining- and thinning-upward nature of the sedimentary succession, widespread syndepositional folds and syndepositional breccias, and submarine channel sediments, as well as intensive basaltic volcanism suggest that this back-arc basin generated in a typical extensional environment. The inversion of the back-arc basin was completed within a relatively short period of one million years (254~253 Ma), resulting in the development of overturned folds of the flysch succession and a latest Permian to Early Triassic back-arc foreland basin in front of the folded belt. Whole-rock geochemical data for the basalts and coeval gabbros suggest that the petrogenetic process of the basalts in the back-arc basin is likely comparable to that of basalt in a rift system as well, which is a lithospheric extension induced uplift of lithospheric mantle and asthenosphere and allowing decompression partial melting of the mantle peridotite. The late stage pillow basalts exhibit a stronger arc signature than the earlier massive basalt and diabase. The Zhongdian back-arc basin is considered to be an extinct continental and arc-type back-arc basins, which are characterized by thick crust, shallow bathymetry, and may not evolve into “normal” oceans. The formation and inversion of the Zhongdian back-arc basin are believed to have been caused by rollback and subsequent break-off of the subducted oceanic slab. During the inversion, the crust shortening occurred predominantly in the back-arc basin, while the southwestern shoulder of the back-arc basin, which was weakly deformed, was shifted north-eastward for ~30 km. The formation process of the Zhongdian back-arc basin is comparable to that of a typical continental rift system, the asymmetric architecture of which has mostly been inherited by the structures formed during the basin inversion. The southern Jinshajiang ophiolitic mélange is representative of the inverted Zhongdian back-arc basin, which is a short-lived, partially mature oceanic basin.

How to cite: Xin, D., Yang, T., Xue, C., Jiang, L., and Xiang, K.: Formation and inversion of a short-lived continental back-arc basin in Southeastern Tibet, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3894, https://doi.org/10.5194/egusphere-egu25-3894, 2025.

The development of continental rift basins is often accompanied by multiple episodes of volcanic activity. The impact of these volcanic eruptions on the sedimentary filling process of the basin is a geological problem worth considering. This relationship is not only the premise for reasonably explaining the binary filling characteristics and development of sequences of volcanic rocks and sedimentary rocks in rift basins but also the key geological basis for the prediction of volcanic and sedimentary rock reservoirs in rift basins. On the basis of a large amount of three-dimensional seismic data, logging data and lithology data, we estimated the volcanic eruption period, volcanic rock mass and spatial shape of the Changling faulted depression in the Songliao Basin. We then studied the spatial distribution characteristics of lithofacies and sedimentary facies in the basin. Finally, we assessed the influence of volcanic eruptions on the type of sedimentary filling, the distribution of sedimentary facies and the spatial stacking of sedimentary strata. This study revealed that during the rapid rifting stage (Yingcheng Formation depositional period), the Changling faulted depression developed mainly fan delta, braided river delta and lacustrine sedimentary systems and experienced four phases of volcanic eruptions. The lithology, scale and spatial distribution of volcanoes were directly related to the activity and location of the basement faults in this area, reflecting the control that basement fault activity had on the volcanic eruptions. Moreover, the stacking form and eruption scale of volcanic rocks played a substantial role in the paleogeomorphology of the basin, which in turn affected the form of the source channel of the basin, causing changes in the sedimentary facies type and spatial distribution and changes in the spatial overlapping pattern of the sedimentary sequence. Moreover, volcanic eruptions provided different sources of debris to the continental lake basin. The differences in location and delivery methods of these materials complicate the rock structure and reservoir properties of the basin sandstone.

How to cite: Wang, H., Zhang, H., and Liu, A.: Influence of volcanic eruptions on the sedimentary filling of a continental rift basin — A case study of the Yingcheng Formation in the Changling faulted depression in the Songliao Basin, China, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4049, https://doi.org/10.5194/egusphere-egu25-4049, 2025.

Electrical image logs can intuitively reflect the development status and characteristics of dissolution pores, which is of significant importance for the development of oil and gas resources. However, traditional methods for identifying pores in electrical image logs are not only cumbersome and labor-intensive but also incapable of distinguishing between different types of pores. Moreover, the strong heterogeneity and dissolution effects in carbonate reservoirs result in significant variations in pore size and complex, diverse pore morphologies, making it difficult to extract pore parameters. To address these issues and challenges, this paper proposes a semantic segmentation model, FILnet, designed using computer vision technology and deep learning frameworks. This model aims to achieve intelligent recognition and segmentation annotation of pores of different scales in the wellbore region of electrical image logs. The data selection process involved using a sliding window to choose electrical log images containing dissolution pores and caves. Image processing techniques were then applied to complete and augment the images, thereby enhancing data diversity. Furthermore, a dual-attribute dataset was created using dynamic and static images from electrical image logs to assist the model in learning the semantic features of pores. Finally, the proposed model was compared with traditional pore identification methods, such as threshold segmentation. The results showed that FILnet demonstrated significant performance advantages on the dual dataset, with a mean intersection over union (MIoU) of 85.42% and a pixel accuracy (PA) of 90.54%. Compared to traditional pore identification methods, the deep learning semantic segmentation approach not only achieves recognition of different types of pores but also improves identification accuracy. This indicates that the network model and data processing methods proposed in this paper are effective and can achieve intelligent recognition and accurate segmentation of pores in electrical image logs.

How to cite: Zhuolin, L., Guoyin, Z., and Yifan, G.: Intelligent Pore Recognition Method for Carbonate Rock Electrical Image Logs Based on Deep Learning, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4896, https://doi.org/10.5194/egusphere-egu25-4896, 2025.

In northwest India, the South Delhi Fold Belt (SDFB) is a NE-SW trending region of the Neoproterozoic age, consisting of poly-deformed and poly-metamorphosed rocks. To the west lies the Marwar Craton, and the boundary between them is defined by a crustal-scale shear zone, dated to 810 Ma, known as the Phulad Shear Zone (PSZ). The syn-tectonic Phulad Granite, which runs along the PSZ, played a key role in stitching together the Marwar Craton and the SDFB during the 810 Ma tectonic event. Approximately 30 km to the east of the PSZ, a quartz monzonite pluton, emplaced within the calc-silicates of the SDFB, is observed. This study focuses on the meso- and micro-structures, as well as the geochemistry of the quartz monzonite, to better understand its emplacement conditions and the tectonic processes at that time.

In the field, the quartz monzonite exhibits a saccharoidal texture with a crude foliation, defined by the alignment of feldspar grains. The foliation in the monzonite has a mean orientation of 14°/67° E. The quartz monzonite is primarily composed of k-feldspar and plagioclase feldspar, with minor amounts of quartz, amphibole, and titanite. Microstructural analysis reveals features indicative of sub-magmatic, high-temperature deformation, suggesting that the rock underwent solid-state deformation. These microstructural characteristics of the quartz monzonite suggest a syn-magmatic deformation event. The foliation in the monzonite is broadly parallel to the mylonitic foliation in PSZ, further supporting the idea of a syn-tectonic emplacement. The geochemical study of the quartz monzonite displays a syn-collisional granite-type geochemical signature with a distinctly negative REE pattern. The REE pattern features suggest that garnet played a significant role in the petrogenesis. By integrating micro and meso-structural analyses with geochemical data, we infer that the emplacement of the quartz monzonite coincided with the development of the PSZ and the intrusion of the Phulad Granite. Despite the temporal overlap, the quartz monzonite and the Phulad Granite display significant geochemical differences, denoting distinct petrogenetic processes. Based on the integration of all available data, we propose that the quartz monzonite was emplaced during the 810 Ma collisional event, resulting from the partial melting of garnet-bearing mafic crust. While both quartz monzonite and Phulad Granite likely share a common source, the depth of melting was different. The greater depth of melting in the eastern portion suggests an eastward subduction of the Marwar Craton during this tectonic event.

How to cite: Ghosh, D. D. and Chatterjee, S. M.: Neoproterozoic magmatism in NW India and its implication for crustal evolution, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5552, https://doi.org/10.5194/egusphere-egu25-5552, 2025.

In highly populated urban areas such as Tehran, with over 17 million inhabitants, identifying active faults is essential to hazard and risk management. Tehran is located in central Alborz within the Arabia-Eurasia collision zone. The region is the manifestation of interplay between structural systems of the western and eastern Alborz. The study area focuses on the Lavizan and Babaei fault-related fold structures as the main Quaternary features of the Tehran piedmont. Tehran's rapid urbanization in the past few decades has made it impossible to access fault traces and the associated geomorphic features in the field. This research is the first study which uses photogrammetric methods to extract detailed 3D data and digital terrain model (DTM) from archival imaging. Historical aerial photographs were acquired from 1955-1965, before the city's development. A DTM with a spatial resolution of about 86 centimeters, an orthophoto-mosaic, and a three-dimensional model were created employing photogrammetric methods. The geomorphic analysis of the model reveals lateral unidirectional eastwards growth of the Lavizan and Babaei structures during Pleistocene and Holocene. The presence of wind gaps developed from water gaps, and sharp fault scarps in the upper Pleistocene and Holocene geomorphic surfaces testify this lateral propagation. This study presents a typical example for a long-lasting tectonic activity and its Holocene continuation on the E-W fault-related fold structures, which directly affect urban areas in the Iranian metropolitan.

How to cite: Alizadeh, P., Shabanian, E., and Masoumi, Z.: Investigation of the Growth of Active Faults in the Tehran Metropolitan Employing Historical Aerial Photos and Photogrammetric Techniques, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6123, https://doi.org/10.5194/egusphere-egu25-6123, 2025.

In November 2023, the ESA Swarm constellation mission celebrated 10 years in orbit, offering one of the best-ever surveys of the topside ionosphere. Among its achievements, it has been recently demonstrated that Swarm data can be used to derive space-based geomagnetic activity indices, like the standard ground-based geomagnetic indices, monitoring magnetic storm and magnetospheric substorm activity. Given the fact that the official ground-based index for the substorm activity (i.e., the Auroral Electrojet – AE index) is constructed by data from 12 ground stations, solely in the northern hemisphere, it can be said that this index is predominantly northern, while the Swarm-derived AE index may be more representative of a global state, since it is based on measurements from both hemispheres. Recently, many novel concepts originated in time series analysis based on information theory have been developed, partly motivated by specific research questions linked to various domains of geosciences, including space physics. Here, we apply information theory approaches (i.e., Hurst exponent and a variety of entropy measures) to analyze the Swarm-derived magnetic indices around intense magnetic storms. We show the applicability of information theory to study the dynamical complexity of the upper atmosphere, through highlighting the temporal transition from the quiet-time to the storm-time magnetosphere around the May 2024 superstorm, which may prove significant for space weather studies. Our results suggest that the spaceborne indices have the capacity to capture the same dynamics and behaviors, with regards to their informational content, as the traditionally used ground-based ones. A few studies have addressed the question of whether the auroras are symmetric between the northern and southern hemispheres. Therefore, the possibility to have different Swarm-derived AE indices for the northern and southern hemispheres respectively, may provide, under appropriate time series analysis techniques based on information theoretic approaches, an opportunity to further confirm the recent findings on interhemispheric asymmetry. Here, we also provide evidence for interhemispheric energy asymmetry based on the analyses of Swarm-derived auroral indices AE North and AE South.

How to cite: Papadimitriou, C., Balasis, G., Boutsi, Z., and Giannakis, O.: Dynamical Complexity in Swarm-derived Storm and Substorm Indices Using Information Theory: Implications for Interhemispheric Asymmetry, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6137, https://doi.org/10.5194/egusphere-egu25-6137, 2025.

We present high-resolution ocean bottom node (OBN) seismic data of the Dongfang 1-1 structure in the Yinggehai Basin of the South China Sea, which hosts China's largest offshore gas reservoir. The OBN seismic data reveals more continuous and detailed reflections compared to conventional seismic data, highlighting the internal structure and formation mechanism of a diapir-like structure. The seismic images show a tapered conical structure characterized by a concentric distribution of fractures, with a significant increase in fracture intensity and connectivity towards the center. These fractures, particularly the sub-vertical ones, are interpreted as natural hydraulic fractures formed due to intense overpressurization in the Lower Miocene strata, with formation pressure coefficients up to 2.2. The fractures are believed to have originated from thermogenic hydrocarbon gas generation and inorganic CO₂ production. The throughgoing fractures that traverse the entire Neogene succession, including the thick Upper Miocene sealing mudrocks, provide crucial pathways for deep gas-bearing fluids to charge the Pliocene sandstone reservoir. The study underscores the importance of natural hydraulic fractures in bypassing thick sealing sequences and conduiting fluids in deep overpressured environments. Moreover, our results may provide guidance for accurate geological interpretations of mud diapir-like structures in conventional seismic images in many other sedimentary basins.

How to cite: Meng, Q., Yang, B., Guo, Z., and Hao, F.: Architecture of a mud diapir-like structure: insights from ocean-bottom-node seismic data, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6417, https://doi.org/10.5194/egusphere-egu25-6417, 2025.

This study explores the application of Fracture-Induced Electromagnetic Radiation (FEMR) for stress analysis in the Ramon Crater, a tectonically “stable” region in southern Israel. FEMR, an innovative geophysical method, detects electromagnetic pulses emitted during micro-fracturing events to infer stress orientations. Unlike traditional seismic techniques, FEMR is sensitive to subtle stress changes, making it suitable for regions with limited seismicity. Field measurements were conducted at nine locations using the ANGEL-M device, capturing high-sensitivity electromagnetic signals to determine the stress azimuth. The results revealed a dominant mean stress azimuth of 308°, aligning closely with the acute bisector of two principal joint sets in the region, WNW-ESE and NNW-SSE. These orientations correspond to historical compressional stress from the Syrian Arc Stress (SAS) regime and more recent extensional stress from the Dead Sea Stress (DSS) field. The superimposition of these regimes has created a complex tectonic environment, evidenced by features such as joint sets, fault planes, and basaltic dikes. FEMR measurements correlate with these geological indicators, confirming the technique’s ability to detect regional stress directions and their evolution over time. In the past decade, the method of FEMR has progressively gained impetus as a viable, non-invasive, cost-effective, real-time geophysical tool for stress analysis in various parts of the world. Its range lies in delineating tectonically active zones, landslide-prone weak slip planes, highlighting stress accumulation in mines and tunnels, etc. This study highlights FEMR’s viability for stress field analysis, especially in stable tectonic zones. Its ability to capture micro-crack activity and subtle stress shifts offers a detailed understanding of how tectonic forces shape regional geodynamics. While FEMR enhances stress detection capabilities, careful calibration with geological models is essential to differentiate transient stress changes from long-term tectonic trends. This research advances FEMR’s application in geophysical studies, particularly for monitoring stress fields in regions influenced by ancient and ongoing tectonic forces.

How to cite: Das, S. and Frid, V.: The Fracture Induced Electromagnetic Radiation (FEMR) technique as a tool for stress mapping: A case study of the Ramon Crater, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9651, https://doi.org/10.5194/egusphere-egu25-9651, 2025.

The slip behavior of the Altyn Tagh Fault (ATF) plays a key role in improving our understanding of the tectonic deformation processes shaping the Tibetan Plateau. In this investigation, a three-dimensional (3D) model representing the central segment of the ATF was constructed using discrete element numerical simulations to examine the main damage zones and stress distribution in the Akato Tagh Bend, AKsay Bend, and Xorkoli segments. The simulation results were then cross-referenced with fault orientation measurements from the northern Qaidam Basin and focal mechanism solutions (FMS) to assess their precision and reliability. The results indicate that the stress environment is stable in the linear strike-slip Xorkoli segment, whereas the stress distribution in the Akato Tagh Bend and AKsay Bend segments exhibits significant heterogeneity, with alternating regions of high and low stress. On the concave side of these bends, compressive stress accumulates, fostering the formation of local thrust faults or folds along the fault plane. Conversely, on the convex side, tensile stress dominates, promoting the development of normal faults or extensional fractures. In the restraining bend region, tensile stress remains horizontal, though its orientation shifts considerably as fault displacement increases. The bend segments also show significant variations in shear stress, which can lead to the creation of secondary fault features like Riedel shears. The intensity and distribution of shear stress are influenced by the curvature and bending angle of the fault, with larger bending angles in the Akato Tagh Bend producing more pronounced shear stress concentrations. Fractures are primarily concentrated at the fault tips, along fault intersections, and within the fault plane, with the fault damage zone being notably wider in the Akato Tagh Bend and AKsay Bend segments. As fault displacement increases, the width of the damage zone and fracture density initially increase rapidly before reaching a plateau. Moreover, the primary damage zone develops earlier in the restraining Akato Tagh Bend and AKsay Bend segments compared to the linear strike-slip Xorkoli segment, which absorbs more strain before the principal displacement zone forms. Therefore, the Akato Tagh Bend exhibits the highest fracture intensity, followed by the AKsay Bend and Xorkoli segment. These findings offer significant insights into the slip behavior and stress distribution along the ATF and enhance our understanding of the tectonic processes in the Tibetan Plateau.

How to cite: Chen, Z., Li, J., and Liu, G.: Stress Distribution and Fracture Development Along the Altyn Tagh Fault:Insights from 3D Discrete Element Modeling, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9784, https://doi.org/10.5194/egusphere-egu25-9784, 2025.

Monazite has the potential to place temporal constraints on the crustal melting in high-grade metamorphic rocks like granulites and migmatites. Melt loss in granulite-grade metamorphic rocks plays a key role in progressively depleting LREE in the residue and enhancing the dissolution of monazite during heating to the metamorphic peak. Newly formed monazite are therefore more abundant in leucosomes than the residue. Higher degree of partial melting and subsequent melt loss, therefore, poses a major hindrance to constraining the mobility of these elements in the micro-domain scale, particularly during the early stage of melting at amphibolite to granulite facies transition. To overcome such an issue and to understand the behavior of this mineral during the onset of granulite facies metamorphism, metamorphic rocks that have reached the P-T conditions culminating at the aforesaid transition should be targeted. Considering this, the present study has been carried out on charnockite from the northern Eastern Ghats Belt, India which underwent such transition (M₂) following crystallization during an earlier granulite facies metamorphic event (M₁). The rock is composed of plagioclase (Pl), K-feldspar (Kfs), quartz, orthopyroxene, biotite, and garnet with apatite, allanite, and monazite as accessory phases. The rock has well-developed gneissic foliation, demarcated by alternate biotite +garnet-rich and quartzofeldspathic layers. While both the feldspars show grain boundary migration recrystallization, quartz grains are deformed by sub-grain rotation recrystallization. Garnet is porphyroblastic and post-kinematic as it overgrows the matrix biotite. The former phase is closely associated with cuspate Kfs and quartz grains which developed as a result of incipient dehydration melting of moderately fluorine rich biotite during the aforesaid transition. Monazite grains are coarse (up to 200 µm across), mostly elliptical and either partially or completely replaced by the reaction rim of apatite+ thorite with an external corona of allanite in the biotite+garnet-rich layers. In case of partial replacement, the oscillatory-zoned relict monazite core is preserved. Th-rich patches are present in such cores. Interestingly, the coronitic assemblage overgrows the matrix biotite is always associated with porphyroblastic garnet. On the contrary, corona-free monazite grains are abundant in quartzofeldspathic layers. Spot dates from the oscillatory-zoned relict monazite core yield a weighted mean age of 960±6 Ma. Th-rich patches, showing prominent huttonite substitution, yield a weighted mean age of 938±7 Ma. Integrating monazite textural and age data, we interpret that the ca. 960 Ma represents the crystallization age of the charnockite magma which coincides with the M₁ metamorphic event of the Eastern Ghats Province (EGP). The ca. 938 Ma, additionally, corresponds to the age of the M₂ event when biotite dehydration melting occurred and porphyroblastic garnet was formed. Based on the textural evidence and mineral phase chemical data, we propose that the replacement of primary monazite occurred via coupled dissolution precipitation process in the presence of incipient melt originated during biotite dehydration melting. Such melt was fluorine rich and helped to mobilize REEs by forming REE-fluoride complexes and was incorporated in allanite corona. Monazite grains in quartzofeldspathic layers must have escaped the melting reaction and the melt-induced element mobility.   

How to cite: Ganguly, P., Banerjee, A., and Das, K.: Behavior of monazite during incipient dehydration melting of charnockite at the northern Eastern Ghats Belt, India: Insights on the mobility of REE at amphibolite-granulite facies transition, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11645, https://doi.org/10.5194/egusphere-egu25-11645, 2025.

Predicting earthquakes remains one of the most profound challenges in seismology and a long-standing aspiration for humanity. Among the array of potential precursors, changes in the Earth’s magnetic field have emerged as a promising yet contentious avenue of research (e.g., De Santis et al., 2015). With advancements in satellite technology, especially with the advent of the European Space Agency’s Swarm mission, we now have the unprecedented ability to measure the magnetic field with extraordinary precision, unlocking exciting opportunities for earthquake forecasting.

In this study, we leverage data from Swarm satellites to investigate whether magnetic anomalies can serve as reliable precursors to earthquakes. Our approach integrates two complementary methodologies: a) global statistical analysis: We applied superposed epoch and spatial techniques to several years of global earthquake data, correlating it with Swarm's magnetic field measurements (De Santis et al., 2019; Marchetti et al., 2022); b) tectonic case study: We focused on major earthquakes occurring from 2014 to 2023 within the tectonically active Alpine-Himalayan belt (Alimoradi et al., 2024).

To analyze these events, we employed an advanced automated algorithm (De Santis et al., 2017) to detect magnetic anomalies in satellite data recorded up to 90 days prior to global earthquakes and up to 10 days before events in the Alpine-Himalayan region. The findings revealed compelling evidence of clear magnetic anomalies preceding earthquakes. Notably, in the Alpine-Himalayan case study, we observed a striking correlation between earthquake magnitude and the duration and intensity of these anomalies: larger earthquakes were associated with stronger and more prolonged signals.

Our predictive framework demonstrated remarkable performance, achieving an accuracy of 79%, a precision of 88%, and a hit rate of 84%. These results underscore the transformative potential of satellite-based magnetic field analysis, paving the way for an operational earthquake prediction system. Such a system could serve as a powerful tool for mitigating the devastating impacts of earthquakes and safeguarding communities worldwide.

The work has been developed in the framework of the following projects: UNITARY- Pianeta Dinamico (funds from MUR), SPACE IT UP (PNRR), Limadou Scienza + (ASI) and FURTHER (INGV).

References

Alimoradi, H., Rahimi, H., De Santis, A. Successful Tests on Detecting Pre-Earthquake Magnetic Field Signals from Space, Remote Sensing, 16(16), 2985, 2024.

De Santis et al., Geospace perturbations induced by the Earth: the state of the art and future trends, Phys. & Chem. Earth, 85-86, 17-33, 2015.

De Santis A. et al., Potential earthquake precursory pattern from space: the 2015 Nepal event as seen by magnetic Swarm satellites, Earth and Planetary Science Letters, 461, 119-126, 2017.

De Santis A. et al. Precursory worldwide signatures of earthquake occurrences on Swarm satellite data, Scientific Reports, 9:20287, 2019.

Marchetti D., De Santis A., Campuzano S.A., et al. Worldwide Statistical Correlation of eight years of Swarm satellite data with M5.5+ earthquakes, Remote Sensing, 14 (11), 2649, 2022.

How to cite: De Santis, A., Campuzano, S. A., Cianchini, G., Alimoradi, H., Perrone, L., and Rahimi, H.: Harnessing Swarm Satellite Magnetic Data to Revolutionize Earthquake Prediction, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11991, https://doi.org/10.5194/egusphere-egu25-11991, 2025.

The Precambrian history of the Indian continent centers around several Archean cratonic nuclei, e.g. the Singhbhum Craton, the Bundelkhand Craton and the Aravalli Craton in the north; the Bastar Craton in the central-east, and the Dharwad Craton in the south. Apart from a group of less disturbed and unmetamorphosed Meso- to Neoproterozoic platformal sedimentary packages resting over deformed and metamorphosed Archean to Paleoproterozoic basement, several Neoproterozoic orogenic belts occur at the margins of these Archean cratonic blocks. These craton-margin orogenic belts are the areas of intense deformation and multiple phases of deep- to intermediate depth, and hence constitute the sites of major records of crustal-scale material recycling through plate movements. They occur on the east, south and west of the Archean cratonic clusters (conjugate north and south Indian cratonic blocks). Though major deep-crustal deformation and metamorphism in these craton-margin orogenic belts can be tracked mostly up to the earliest Neoproterozoic, exhumation-related reactivation seems to be more common in these belt around ca. 800–750 Ma. 

In this study, we shall highlight the east and west Indian marginal belts. We shall present the new data showing conditions of metamorphic pulses and their age from the rocks of the Mercara Shear Zone, marking the south-western boundary of the Archean (>3000 Ma) Dharwar craton. The results indicate at least four events; (1) ~2900 Ma; basin formation with supply from craton, (2) 2900–2700 Ma; age of prograde metamorphism, (3) 2700–2500 Ma, age of charnockite magmatism during Dharwar Orogeny with metamorphic peak, and (4) final reactivation at 830–730 Ma marking exhumation of deep crust during retrograde metamorphism along the crustal scale shear zone (stretching lineation and S-C fabric formation as last deformation event. We shall also review our group’s recent published data on the pressure-temperature-deformation-fluid-age histories during the orogenic reactivation of the western boundary, and the Chilka Domain of the northern Eastern Ghats Belt. Together we shall try to collate data showing the idea of a near-synchronous orogenic reactivation surrounding Indian cratonic cluster during middle to late Tonian Period with various preceding age gaps.

How to cite: Das, K., Bose, S., Ganguly, P., and Chatterjee, A.: Circum-Indian craton-margin orogenic reactivation during ca. 800-700 Ma: Tectonometamorphic characterization, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14725, https://doi.org/10.5194/egusphere-egu25-14725, 2025.

Efficient handling of climate change issues in order to mitigate its negative impact of the flora and fauna of the earth, or on the pace of industrialization, is a big challenge in every disposition around the world.  Amongst the many options available, geological storage of CO₂ in the basalt formations is proving to be a promising one due to its large and pervasive occurrence, to facilitate stable carbonation of the sequestered CO₂, and with ready access to the basalt deposits for operational requirements. Laboratory testing and a few field   implementations showed that carbon dioxide injected in basalts would form stable carbonate minerals, keeping the substance in place for thousands of years.

This work applies the machine learning applications aimed at the classification of different facies in basalts, particularly flow tops and flow interiors, towards the selection of a sequestration site based on their relevant petrophysical characteristics.

After the facies were identified, several rock physics models were run with an outlook of predicting the elastic properties of basalt. Based on our results, we found the Differential Effective Medium (DEM) model enables the most accurate prediction with the least error as compared to Self-Consistent Approximation and Kuster-Toksӧz model. This finding provides a foundation for using the DEM model to create an initial reservoir matrix, which can be applied to simulate geomechanical changes upon CO₂ injection in Basalt. Additionally, facies classification aids in delineating zone boundaries within basalt flows, allowing for the selection of optimal injection sites based on their petrophysical properties.

How to cite: Nagarkoti, N., Kumar, T., Panwar, N., and Sharma, R.: Determination of Lithofacies and Elastic Behavior Modeling in Columbian River Basalt Group (CRBG) Formations , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17658, https://doi.org/10.5194/egusphere-egu25-17658, 2025.

The South Delhi Fold Belt (SDFB) within the northwestern Indian Shield is a Proterozoic NE-SW trending fold belt. The western boundary of the SDFB is defined by the Phulad Shear Zone, formed during a transpression regime around 820-810Ma. Granite rocks of ~1Ga have been documented from the western part of the fold belt and are linked with the formation of the Rodinia Supercontinent. These granites are closely associated with gabbroic rocks. The present study focuses on the geochemistry of these granites and the mafic rocks, as well as their field structure and petrography. 
The granites and the mafic rocks are confined to a narrow linear belt along the western part of the fold belt. Detailed field studiesreveal that the foliations in the granites, mafic and mylonites within PSZ share a common stress regime and are broadly synchronous. Geochemically these granites are ferroan, calc-alkalic, metaluminous to weakly peraluminous and their classification in granite discrimination diagrams confirms A-type and within plate granite. The mafic rocks exhibit a compositional range fromtholeiitic to calc-alkaline, with atrace element ratio resembling enriched mid-oceanic ridge basalt (E-MORB) type magma. The tectonic discrimination diagram suggestsrift-relatedmagmatism. Geochemical analysis of these bimodal magmatic compositions in the SDFB, encompassing both mafic rocks and A-type granites are typically associated with areas experiencing extensional tectonics, particularly rift-related magmatism. Integrating field structures, petrography and geochemistry of these granite and the mafic rocks suggests that the ~1Ga granite and the associated mafic rocks formed in an extensional regime and are not directly linked to the collisional assembly of the Rodinia Supercontinent.

How to cite: Manna, A., Chatterjee, S. M., Roy, A., and Sarkar, A. K.: Geochemistry of ~1Ga granite and associated mafic rocks from the South Delhi Fold Belt, NW India, and its tectonic significance, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19572, https://doi.org/10.5194/egusphere-egu25-19572, 2025.

The Central High Atlas in Morocco is characterized by complex geological structures shaped by tectonic and magmatic processes. Gravimetry, a geophysical technique sensitive to subsurface density variations, plays a crucial role in exploring and understanding these features. This study provides a bibliometric analysis of global research on the application of gravimetry, with a specific focus on its use in the Central High Atlas.

The main objectives of this study are to identify global research trends and applications of gravimetry in the study of geological structures, analyze key contributors, scientific collaborations, and dominant themes in gravimetric research, and compare findings from studies conducted in the Central High Atlas with those from other regions worldwide.

A bibliometric analysis was conducted using data from Scopus and Web of Science databases. Keywords such as "gravimetry," "Central High Atlas," and "geological structure" were employed to extract relevant studies. The analysis utilized the R-bibliometrix package and VOSviewer software to map collaboration networks, visualize thematic clusters, and analyze global research trends over time.

The results reveal a significant increase in gravimetric studies over the last two decades, reflecting growing interest in its applications in mountainous regions like the Central High Atlas. The findings highlight deep-seated geological structures, active fault systems, and the relationship between gravimetric anomalies and tectonic processes. Moreover, a comparative analysis shows that studies in Morocco focus heavily on tectonic and magmatic processes, while research in other countries often emphasizes technological advancements and methodological innovations.

This bibliometric study underscores the importance of gravimetry as a tool for exploring complex geological structures in the Central High Atlas. It also highlights the need for stronger international collaborations and interdisciplinary research to advance gravimetric methodologies and foster knowledge exchange across regions.

How to cite: Assoussi, S., Hahou, Y., Khalifa, M., Saadaoui, F., and Oujane, B.: Gravimetric Investigation and Analysis of Tectonic Features and Mineralization Zones in the Central High Atlas (Morocco)., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20105, https://doi.org/10.5194/egusphere-egu25-20105, 2025.

This study evaluates the contributions of geophysics and remote sensing to structural mapping in the Central High Atlas region of Morocco. A bibliometric analysis was conducted using data collected from databases such as Scopus and Web of Science. Keywords related to geophysics, aeromagnetic, remote sensing, structural mapping, and the Central High Atlas were used to systematically identify relevant research articles. Analytical techniques, including citation analysis, co-authorship analysis, keyword analysis, and network analysis (using VOSviewer), were applied to explore research trends, collaborations, and key focus areas.

The findings highlight notable research trends in the application of geophysics and remote sensing, identifying key contributors, influential institutions, and pivotal publications in this domain. Research gaps and opportunities for further investigation were also uncovered. Visualization of research networks provided insights into collaboration patterns and thematic focus areas.

This study underscores the importance of geophysics and remote sensing in enhancing the understanding of the Central High Atlas's structural geology. It offers a foundation for future research, emphasizing the need for interdisciplinary collaboration and advanced methodologies to address existing research gaps and further explore the region's geological complexities.

How to cite: Saadaoui, F., Hahou, Y., Ousaid, L., Assoussi, S., and Oujane, B.: Geophysical and Remote Sensing Contributions to Understanding Geological Structures in the Central High Atlas (Morocco): A Review and Analytical Study., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20168, https://doi.org/10.5194/egusphere-egu25-20168, 2025.

The Congo craton is an early Archaean through Paleoproterozoic basement block in Central Africa. It consists of a vast heterogenous granulitic complex extending over 1200 km between the Lomami River (24°E) and the Atlantic coast in Angola. The well-exposed domains of the Congo craton are the Kasaï block, Tanzania block, West-Nile complex, and Ntem-Chailu complex. The latter represents the northwestern edge of the craton in southern Cameroon. The Memve'ele area belongs to the Ntem Complex, where recent investigations have highlighted various lithologies, including TTG gneiss and intensely sheared and folded charnockitic and granitic gneiss, pervasively intruded by younger monzogranite. This region provides a critical window into the complex tectonic evolution of one of Earth's oldest continental blocks. Both TTG and granitic gneiss are riddled with folded or sheared leucogranitic veins, suggesting a local origin through melting and dynamic recrystallization. This study presents a comprehensive investigation of the highly sheared Memve’ele mylonitic corridor. Through detailed field mapping, systematic kinematic analysis, and meticulous petrographic and microstructural studies, we aim to unravel the multiple deformation events that have shaped this region. U-Pb zircon geochronology was employed to precisely constrain the timing of these processes and to correlate them with regional tectonic events. The ultimate goal of this research is to better understand the broader geodynamic implications of these findings for the evolution of the Congo Craton. Initial results reveal that the Memve’ele area has undergone a complex polyphase deformation history, involving at least four distinct events. The early ductile deformation (D1) resulted in the development of a pervasive foliation and associated structures. Subsequent ductile-brittle deformation (D2) overprinted the earlier structures, while later brittle deformation events (D3 and D4) further modified the rock fabric. The studied mylonites yield Mesoarchean ages of 2927 ± 52 Ma. The presence of a sinistral shear zone within the area suggests that the region was subjected to significant shear stresses, likely related to regional tectonic processes such as continental collision or crustal extension. These findings have important implications for understanding the tectonic evolution of the Congo Craton and may provide insights into the potential for mineral exploration in the region.

How to cite: Takodjou Wambo, J. D., Ganno, S., Nzenti, J. P., and Asimow, P. D.: Archean shear tectonics in the Congo craton: insights from Petro-structural characterization and U-Pb geochronology of the Memve’ele mylonite, Southern Cameroon, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20667, https://doi.org/10.5194/egusphere-egu25-20667, 2025.

Pseudotachylytes, quenched melts from frictional heating, and ultracataclasites, comminution of host rock, are considered direct evidence of coseismic slip. In hydrated systems, fluid-rock interactions can influence the nucleation and propagation of these earthquake-induced structures by facilitating element mobility and fault zone weakening. We conducted 2D microstructural and geochemical analyses on a series of ultracataclastic veins hosted in a deformed granodiorite on Naxos, Greece, to investigate potential interactions between physical and chemical processes along rupture paths. Naxos is a classical Miocene Cycladic metamorphic core complex, defined by a central migmatite core, with fluids introduced during peak metamorphism and subsequent brittle deformation. An I-type granodiorite was syn-tectonically emplaced, cooling rapidly from crystallization (650-680°C) at c. 12 Ma to <60°C by c. 9 Ma. The extensional Naxos-Paros Detachment System, active between c.12-9 Ma, dissects the pluton, producing a strong N-S stretching lineation and SCC' fabric generating top-to-N kinematics. Host rock from the immediate footwall of the detachment is composed of a coarse-grained (50 μm-2 mm) matrix, primarily composed of albite (35%), quartz (25%), orthoclase (16%), and biotite (12%). Fine-grained (5-60 μm) anastomosing ultracataclastic veins of the same composition intersect the host rock, with the thickest veins (7 cm) occurring sub-parallel to host rock foliation. Electron backscatter diffraction (EBSD) mapping of albite, orthoclase, and quartz targeted foliation-subparallel veins tips and host porphyroclasts crosscut by the veins. Evidence for minor crystal plasticity is observed as continuous to heterogeneous lattice distortion with an average misorientation of 03° within the host clasts, increasing to 15° towards clast rims and microfractures. The localization of microfractures emanating from the vein tips coupled with the spatial relationship between lattice distortions and microfractures, indicates that strain accommodation via crystal plasticity is linked to brittle deformation. This suggests that cataclasis is the primary deformation mechanism related to the propagation of ultracataclastic veins, which is supported by EBSD orientation data of fine-grained (<60 μm) fragments surrounding clasts (80-120 μm) of the same phase. The fine-grained populations are randomly oriented, with low internal misorientations up to ~10°, and no crystallographic relationship to the host porphyroclasts. Scanning electron microscopy (SEM) imaging highlighted aggregates of fine-grained albite (2-35 μm) along the vein margins with patchy zonation near microfractures and grain rims. A cuspate phase boundary between the albite grains and bordering orthoclase host clasts (2 mm) is characteristic of a dissolution-precipitation reaction front. Electron microprobe mapping of the phase interface reveals a K-depleted rim, 3 μm wide, along orthoclase clast margins, decreasing from ~13.6 wt% to 9.5 wt%. Inclusions of albite grains and Na-enriched zones, increasing to 2.6 wt% from 0.4 wt%, related to microfractures within the orthoclase clasts are present up to 55 μm away from the interface. Based on these observations, we propose that the interplay between cataclasis and interface-coupled reactions localized weakening, creating a feedback loop that promoted fracture propagation and drove continued injection of cataclastic material within the granodiorite. Our results demonstrate the impact of fluid-rock interactions on fault zone evolution and rupture conditions.

How to cite: Rolfe, O., Dubosq, R., Schneider, D., and Grasemann, B.: Feedback between cataclasis and interface-coupled reactions in ultracataclastic veins: insights from the Naxos granodiorite, Greece, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-498, https://doi.org/10.5194/egusphere-egu25-498, 2025.

Dilational hydroshear veins are hybrid veins that involve slip along weak planes and simultaneous extension under local hydrofracturing conditions (sensu Fagereng et al. 2010). These structures are considered as a possible record of episodic tremors and slow slip events (ETS). Carpholite-bearing dilational hydroshear veins and cyclic brittle-ductile deformation have been suggested to represent possible markers of these phenomena occurring at depth >30 km in subduction zones (Giuntoli & Viola 2022). In the Western Alps, similar structures in the form of lawsonite/carpholite-bearing veins have recently been reported (Herviou et al. 2023).

In this study, we analyzed the Lago Nero Unit (Western Alps), representing a fragment of the Liguro-Piemontese oceanic lithosphere and the related metasedimentary cover, deformed at 300-350 °C and 0.8-1.3 GPa during the Alpine Orogeny (Agard, 2021). We performed a detailed meso and microstructural characterization of mylonitic marble lenses wrapped by weak metapelite, both deformed by sheath folds. Hybrid veins in mylonitic marbles occur with crack-seal textures, oriented both parallel and at high angles to the main metamorphic foliation. The regional stretching mineral lineation oriented NE-SW is both parallel to the carpholite fiber composing veins and to the sheath fold axes. A few carpholite veins are folded within mylonitic marble, attesting to cyclic switches between brittle and ductile deformation in the stability field of carpholite, i.e. under blueschist facies conditions.

We focus on veins parallel to the foliation mainly composed of Ca-carbonate (now calcite, formerly aragonite), quartz and Fe-Mg carpholite (0.32<XMg<0.43). Frequently, large quartz and carpholite fibers form shear boudins in a plastically deformed Ca-carbonate mylonitic and ultramylonitic matrix, with a top-to-SW shear sense. Therefore, elevated strain partitioning is visible between host mylonitic marbles and veins and within single veins. Optical cathodoluminescence analysis shows different carbonate generations: larger and more luminescent fibers surrounded by small equant less luminescent grains. Electron Backscattered Diffraction analyses highlight that large Ca-carbonate fibers (50-500µm) deformed preferentially by subgrain rotation recrystallization, with the most deformed domains composed of smaller equant grains (<10µm) deforming by diffusion creep and grain boundary sliding. Summarizing, Ca-carbonate grew as fibers in veins and subsequently was affected by local strong grain size reduction due to strain partitioning at the microscale that activated grain size sensitive creep mechanisms along bands of accelerated creep.  Strain partitioning was likely favored by differences in the initial carbonate grain size and/or crystallographic orientation and by the presence of stiffer quartz and carpholite. Paleopiezometry is underway to constrain differential stresses and strain rates responsible for the formation of the observed microstructures.

In conclusion, oceanic metasedimentary covers record evidence of transient and cyclic pore fluid pressure fluctuation, reaching sub-lithostatic values and elevated strain partitioning under transiently high strain rates. These structures likely reflect cyclic seismic and aseismic creep occurring at >30 km depth in the Alpine subduction zone. Our results may be compatible with the geophysical and geological data ascribed to deep ETS in subduction zone contexts.

How to cite: Casoli, L., Petroccia, A., Dobe, R., and Giuntoli, F.: Ultramylonitic carpholite-bearing veins as a proxy for deformation mechanisms from deeply subducted oceanic units (Liguro-Piemontese Zone, Western Alps), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-617, https://doi.org/10.5194/egusphere-egu25-617, 2025.

Phyllosilicates play a key role in controlling the rheology of shear zones, the style of deformation and the syndeformational fluid budget. The latter, including aqueous fluids released by metamorphic reactions, can transiently increase pore pressure and trigger cyclic switching between brittle and ductile deformation conditions. Unfortunately, it is still unclear how these processes act together in exhuming low-grade shear zones in a continental collisional framework.
To tackle this scientific question, we studied the top-to-the N/NE Hulw Shear Zone in the Saih Hatat Window of Oman. This shear zone is responsible for part of the exhumation of the subducted continental crust, but its pressure-temperature (P–T) and deformation behaviour remain largely unconstrained. Its footwall is mostly composed of metapelites, with a modal enrichment in K-rich white mica and pyrophyllite, matched by a progressive increase in the physical interconnectivity of phyllosilicates along its internal strain gradient. Similarly, marbles in the hanging wall evolve from mylonitic to ultramylonitic towards the core of the shear zone. 
In the Hulw Shear Zone coexist two opposite deformation behaviours, with ductile deformation accommodated preferentially along laterally continuous phyllosilicate-rich bands and brittle deformation in the form of hybrid/dilational hydroshear veins found regularly at the outcrop. To constrain the metamorphic conditions of dehydration reactions during the exhumation path, we integrated forward thermodynamic modelling with Raman Spectroscopy on Carbonaceous Material, and K-rich white mica multiequilibrium barometry on a representative mylonite from the shear zone footwall. The resulting metamorphic evolution of the Hulw Shear Zone started from peak conditions of 300-350 °C and 0.9-1.2 GPa, followed by the main shearing event at 350-420 °C and 0.6-0.9 GPa and ended with sustained shearing at low-P conditions (350 °C, 0.3-0.4 GPa). Therefore, the Hulw Shear Zone accommodated progressive shearing while exhuming its footwall from epidote blueschist to low-pressure greenschist facies conditions. 
Decompression-driven fluid-gain reactions facilitated the growth of synkinematic phyllosilicates, which created a pervasive and interconnected K-rich white mica and pyrophyllite network that promoted strain localisation, causing significant mechanical weakening as well as the potential for discrete and compartmentalised fluid cells within the mylonitic foliation. Brittle structures formed due to aqueous fluid release by metamorphic dehydration reactions close to peak-P conditions (e.g., kaolinite-out reaction) or along the exhumation trajectory, transiently increasing pore pressure and triggering brittle failure, resulting in coeval mylonitic foliation and crack-seal hybrid veins. 
Our findings support the idea that sustained shearing was promoted by synkinematic growth of K-rich white mica and pyrophyllite and by cyclic switching between brittle and ductile deformation conditions. Therefore, the studied structures might also represent a record of deep episodic tremors and slow slip events during exhumation-related tectonics in the accretionary wedge above the subduction interface of the Oman continental lithosphere.

How to cite: Petroccia, A., Giuntoli, F., Pilia, S., Viola, G., Sternai, P., and Callegari, I.: Evidence for strain localisation and episodic tremors and slow slip events in exhuming continental shear zones (Saih Hatat Window, Oman), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-618, https://doi.org/10.5194/egusphere-egu25-618, 2025.

Fractures are the manifestation of brittle deformation and act as vital conduits for fluid transport in upper crustal rocks. To measure rock strength and stability, infer deformation mechanisms, and reconstruct the stress condition under which they developed, a systematic examination of their geometrical characteristics is essential which also provide insights on how upper crustal rocks respond to stress. Since fractured rock bodies frequently consist of interconnected networks of different fracture sets, topological characterization aids in quantitative assessment of their connectivity, which directly affects comprehension of their permeability and, consequently, the history of fluid migration through the host rock body. Additionally, characterization of fracture networks has direct implications in recent applications like nuclear waste disposal and carbon sequestration which contribute significantly to environmental sustainability.

The present study examines the origin and characterizes subsequent networking of fractures developed within younger granites (~ 2.61 Ga) of the Chitradurga Schist Belt, an Archean age granite-greenstone belt from the Western Dharwar Craton of peninsular India integrating field-based observations with network topology and fractal analysis. We systematically document the geometrical attributes of fracture patterns developed within the granites across varying outcrop scales to understand their formation and characterize them topologically to assess their connectivity and record if fracturing patterns, intensity, density and connectivity vary across scales and also spatially along the areal extent of the granitic plutons. It is found, that although indicative of being formed by the activation of a Riedel shear system under the same tectonic stress regime, the networking patterns which the fractures have developed through their mutual interaction vary spatially in their geometrical, topological and fractal characters. Our study ventures upon the possible causes of this variation and highlights the role of ambient stress state, rheology, pre-existing mechanical anisotropy, orientation of pluton margin and its proximity to adjacent shear zone and superimposition of fractures behind the development of these spatially varying fracture network patterns across the areal extent of the granitic plutons.

How to cite: Biswas, S. K., Banik, B., Mondal, T. K., and Hossain, Md. S.: Spatial variations in Geometry, Topology and Fractal attributes of a Riedel shear induced Fracture Network system in Granites, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-866, https://doi.org/10.5194/egusphere-egu25-866, 2025.

This study investigates the relationships between eclogite-facies mineral assemblages and deformation microstructures in the Yuka terrane, part of the North Qaidam ultrahigh-pressure (UHP) metamorphic belt in NW China. The analysis focuses on understanding the mineralogical and microstructural evolution during subduction and exhumation processes. Eclogites from the study area were found to exhibit distinct mineral assemblages and deformation features, reflecting multiple stages of metamorphism.

During prograde metamorphism, garnet, omphacite, and phengite were predominantly deformed by intracrystalline plasticity, indicative of dislocation creep as the primary deformation mechanism. These minerals contributed to the development of well-defined foliations and lineations in the rock, shaped by the alignment of omphacite and phengite grains. Garnet grains often displayed concentric zoning with inclusion-rich cores and inclusion-free rims, recording growth under varying pressure-temperature conditions. Omphacite showed evidence of dynamic recrystallization, highlighting the mechanical and chemical adjustments during progressive subduction.

In contrast, amphibole, which formed through the topotactic replacement of omphacite under fluid-present conditions, exhibited features associated with diffusional flow, such as dissolution-precipitation creep. This retrograde mineral is thought to have crystallized during amphibolite-facies retrogression, marking the exhumation of the eclogites. The lack of significant deformation microstructures in amphibole, such as subgrain boundaries or undulose extinction, supports its formation during a late-stage metamorphic environment.

The Yuka eclogites contain a range of mineral assemblages, including garnet + omphacite, garnet + omphacite + phengite, and garnet + omphacite + phengite + amphibole, reflecting diverse pressure-temperature paths. The compositional variability of these assemblages is tied to the complex geodynamic history of the North Qaidam UHP belt, which underwent subduction, continental collision, and exhumation. This work highlights the significance of integrated petrographic and microstructural studies for deciphering the metamorphic and tectonic evolution of UHP terranes.

How to cite: Park, M.: Microstructures and deformation mechanisms of the Yuka eclogites in the North Qaidam UHP belt, NW China, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2428, https://doi.org/10.5194/egusphere-egu25-2428, 2025.

Many aspects of the evolution of boudinage are still poorly understood, and using boudins as rheology-gages is in its infancy. The aim of the study is to achieve a better understanding of the evolution of boudinage by numerical mechanical modeling integrated with three-dimensional characterization and analysis of natural boudinage structures. We use results from a 3D field study of boudins as a basis for high resolution numerical modeling.

We use the computationally expensive 3D Discrete Element method to model the boudinage process from loading to strain localization and post failure deformation in parametric studies using high resolution and a realistic representation of the coupled brittle and ductile deformation processes. This provides quantitative insight into the acting mechanisms and coupled processes during the formation of boudins to link the large variety of boudin geometries to specific boundary conditions. In particular we show that the transition from blocky torn boudins to drawn boudins can be modeled as a function of material strength and confining pressure. Furthermore, local heterogeneities can cause shear failure already before the critical stress is reached in the entire rock volume

The numerical simulations are augmented by studies on a world class example of boudinage structures on the island of Naxos, involving an extensive field study, detailed 3-dimensional reconstruction of boudinage structures and microstructural investigation of the underlying deformation processes.

Our ultimate goal is to pave towards a mechanically meaningful 3D boudinage classification scheme that allows for quantitative analysis of boudinage structures in order to invert boudin geometry to the kinematics and rheology of the rock during its deformation, as well as to its stress and strain history.

How to cite: von Hagke, C., Abe, S., Virgo, S., and Urai, J.: Failure mode transition in brittle Boudinage: effects of cohesion, mean stress and layer thickness in discrete element models and field examples, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4090, https://doi.org/10.5194/egusphere-egu25-4090, 2025.

Intermediate-depth earthquakes (IDEQs), which occur at depths of 50 to 300 km, are relatively poorly understood compared to shallow seismicity, and their source mechanisms and physical environment remain ignored. This scientific gap exists because obtaining data from these depths—whether through geophysical imaging or geological sampling—is exceptionally challenging. The dehydration of serpentinites, which can release up to 13 wt% of H2O at these depths, is thought to play a key role in driving deformation associated with IDEQs. However, the mechanical role of the fluids released during these metamorphic reactions remains unclear. To provide new insights into the physical habitat of IDEQs, we investigate olivine- and Ti-clinohumite-rich veins in the Zermatt-Saas meta-ophiolite, a natural laboratory that records dehydration and fluid flow processes under (ultra)high-pressure (UHP) conditions typical of IDEQ depths.

We conducted petro-structural analyses and identified three main vein types: dilational, hybrid dilational-shear, and highly strained sheared veins. Key observations include (i) foliated sheared veins; (ii) newly formed olivine and Ti-clinohumite aligned in mineral lineations within sheared veins and shear bands; (iii) olivine and Ti-clinohumite fibers sealing porphyroclasts; and (iv) mutual crosscutting relationships between dilational and shear features. These features indicate cyclic brittle fracturing and ductile shearing at 2.3–2.7 GPa and 520–650°C, reflecting transient shearing and dilational fracturing under conditions of elevated pore fluid pressures, potentially approaching or exceeding lithostatic levels. The observed structures suggest that fluid escape occurs through interface-parallel, fracture-controlled pathways localized in high-strain zones, particularly near ultramafic sliver boundaries.

Strain gradients reveal distinct deformation styles, with dilational veins prevalent in low-strain regions and sheared veins and shear bands dominating within high-strain zones. These findings highlight the role of local stress regimes during serpentinite dehydration. Cyclic brittle-ductile deformation and fracturing, potentially linked to seismic or sub-seismic strain rate bursts, may have facilitated fluid migration and strain localization along olivine-bearing vein networks. These results align with geophysical observations suggesting high pore fluid pressures within the intermediate-depth seismicity region, providing insights into the mechanisms linking dehydration, fluid flow, and seismicity at depth.

How to cite: Munoz, J., Angiboust, S., Minnaert, C., Ceccato, A., Morales, L., Gasc, J., and Behr, W.: Fluid Flow and Shear Instabilities in the Subducted Mantle at Intermediate-depths: insights from the Western Alps meta-ophiolites, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5197, https://doi.org/10.5194/egusphere-egu25-5197, 2025.

Eclogites compose the majority of the subducted oceanic crust at great depth, with garnet and clinopyroxene as major phases. High stress concentration could exist in the UHP eclogites, with a mechanical contrast between garnet and clinopyroxene that leads to complex microstructures, between brittle and ductile deformation. Coexistence of frictional and viscous regime in such two-phase aggregates raise the question of the competitivity between phases in leading deformation. The fracturation of garnet in natural rocks has been interpreted as related to seismicity in the lower crust and oceanic crust at the interface plate in subduction zones (Trepmann and Stöckhert, 2002, Angiboust et al., 2012, Hawemann et al., 2019), but the question remains if such features can also be produced at lower strain rates (Yamato et al., 2019, Rogowitz et al., 2023).

In order to understand the effect of hard- vs. weak mineral fraction on eclogite mechanical properties, stresses distribution and deformation mechanisms of synthetic eclogites were experimentally investigated under deep subduction zones conditions. Samples were deformed under ultrahigh pressures (3 to 5 GPa), high temperature (820°C) and constant strain rate (1 x 10^-5 – 2.5 x 10^-5 s^-1), using X-rays diffraction to measure in-situ stresses during deformation in each phase in garnet-clinopyroxene aggregates, with various garnet fraction. Back-scattered electron (BSE), electron backscatter diffraction (EBSD), scanning transmission electron microscopy (STEM) with automated crystal orientation mapping (ACOM) was used on the recovered samples, in order to determine deformation mechanisms from the micrometric to the nanometric scale.

In our experiments, deformation was accommodated by a mix of brittle and intracrystalline plastic mechanisms, as proposed or observed in previous studies at lower pressures (e.g. Yamato et al., 2019, Rogowitz et al., 2023). Cataclastic flow and dynamic recrystallization are observed. The distribution of stresses in the phases and variations in stress levels depend on garnet vs. pyroxene fraction in the samples. Differential stresses are greater in garnet than pyroxene and stresses increase with increasing % vol. garnet. Phase fraction impact the mechanical behavior, i.e. fracturation of each phase and deformation accommodation mechanisms vary. In this semi-brittle regime each phase is rheologically active and contributes to the deformation of the aggregate except at the lowest pyroxene fraction.

Our experiments together with last studies (e.g. Yamato et al., 2019, Rogowitz et al., 2023), indicate that frictional deformation of eclogites is not limited to seismic strain rate (i.e. > 1 s^-1) but can occur at strain rate around 10^-5 s^-1 and slower with a high amount of garnet. The grain size reduction mechanisms observed could allow a switch to grain size sensitive mechanisms like grain boundary sliding.  Questions still remain about the extrapolation of such mechanical distribution and fracturation in deep subduction zones.

How to cite: Molines, C., Hilairet, N., Chantel, J., Chardelin, M., Mandolini, T., Officer, T., Addad, A., and Fadel, A.: In-situ stresses distribution and deformation mechanisms in eclogites at ultrahigh pressure, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5696, https://doi.org/10.5194/egusphere-egu25-5696, 2025.

Water weakening of nominally anhydrous minerals of upper mantle is important for understanding the rheological structure of Earth’s interior. Enstatite is the 2nd dominant phase in the upper mantle, next to olivine. The partition coefficient for water between olivine and enstatite aggregates C^ol_OH/C^en_OH is ~0.5 at 3.8‒6.3 GPa and 1323‒1573 K (Zhang et al., 2017; JGR), indicating that water weakening of enstatite effectively proceeds in the olivine-enstatite system. The water weakening of enstatite would be accelerated at high pressures, since the amount of water dissolved into enstatite drastically increase with pressure.

We hereby experimentally evaluated the creep strength of wet orthoenstatite aggregates under pressure and temperature conditions at 1.9‒5.3 GPa and 1200‒1380 K using a deformation DIA apparatus combined with synchrotron X-ray radiation. At a constant strain rate ranging from 6.7 × 10^-6 to 9.4 × 10^-5 s^-1, steady-state creep strength of wet orthoenstatite followed the power-law flow law with the stress exponent of ~3, indicating deformation in the dislocation creep regime. Our results show dislocation creep rate of wet orthoenstatite is ~1 order of magnitude faster than dry orthoenstatite under the same P-T conditions. FTIR spectra from the recovered samples indicate that the amount of dissolved water in orthoenstatite is up to 1370 ppm wt.%. The dependence of strain rate on water fugacity was determined with the water fugacity exponent of ~1. Depending on the water content in the upper mantle, dislocation creep of wet orthoenstatite could lead to strain localization in the lithosphere.

How to cite: Tsubokawa, Y., Ohuchi, T., Higo, Y., Tange, Y., and Irifune, T.: Deformation experiments on orthoenstatite aggregate at upper mantle pressures and temperatures under hydrous conditions, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5763, https://doi.org/10.5194/egusphere-egu25-5763, 2025.

Pseudotachylytes are solidified frictional melts produced in silicatic rocks during an earthquake (Sibson, 1975). They form both as fault and injection veins, with thickness ranging from some millimeters to some centimeters. Still, exposures of meter-thick pseudotachylytes associated to seismic faulting have been documented (Musgrave Ranges, Australia; Lofoten Island, Norway; Ivrea-Verbano Zone, Italy).

In this study, we perform field (UAV, photogrammetry, structural geology, etc.) and microstructural/mineralogical (FESEM-BSE, EDS, micro-Raman, etc.) investigations of thin (mm-cm) and giant (up to 1 m thick) pseudotachylytes approaching the Canavese Line (strike ~NNE-SSW), the major tectonic lineament of the Western Alps (Ivrea-Verbano Zone, Italy; Techmer, 1992; Ueda et al., 2008; Ferrand et al., 2018). Though thin pseudotachylytes have been extensively investigated, in-depth studies of the giant pseudotachylytes are lacking. The aim is thus to determine (i) the ambient P-T conditions, the geodynamic setting, and the seismogenic environment (megathrust?) of the giant-pseudotachylytes, and, in the future, (ii) their mechanisms of formation.

We investigated for ~11 km the polished outcrops exposed along the ~E-W trending Valsesia river and other creeks in the area and selected three outcrops (I, II, and III) within ~2 km to the W and ~9 km to the E from the Canavese Line. We found pseudotachylytes only to the E of the Canavese Line. In detail:

Outcrop (I), < 500 m to the E from the Canavese Line (altered gabbro host rock) shows:

• multiple generations of pseudotachylyte-bearing faults, including giant-pseudotachylytes with breccia (suggesting a single melt pulse) overprinting microgabbro schlierens. The giant-pseudotachylytes are sub-parallel to the Canavese Line and include breccia clasts of the altered (greenschist facies) host rock;
• late quartz- and epidote-, and chlorite-bearing faults/fractures cutting the pseudotachylytes;
• matrix of the pseudotachylyte overprinted by greenschist facies minerals (epidote, chlorite and albite).

Outcrop (II), ~2 km to the E from the Canavese Line (Balmuccia peridotite) shows:

• multiple giant pseudotachylytes-bearing faults, sub-parallel to the Canavese Line, and associated with thin pseudotachylyte faults and veins;
• serpentine-bearing faults/fractures cutting and cut by the pseudotachylytes;
• giant pseudotachylytes with homogeneous matrix suggesting a single friction melt pulse. The matrix (altered into serpentinite) includes microlites of olivine and pyroxene plus vesicles;

Outcrop (III), ~9 km to the E from the Canavese Line (unaltered diorite) shows:

• only thin pseudotachylyte overprinting/associated with foliated cataclasite-bearing faults cutting and overprinting aplitic dykes;
• pristine matrix of the pseudotachylyte, with well-preserved microlites, chilled margins and flow structures.

In conclusion, the giant-pseudotachylyte-bearing faults are (i) made of a homogenous layer of pseudotachylyte, (ii) sub-parallel and found only near (< 2 km to the E) of the Canavese Line, (iii) overprint and cut dykes and ductile shear zones, (iv) cut and are cut by (sub-) greenschist facies cataclasite-bearing faults, and, (v) are cut by epidote- and chlorite-bearing fractures and veins. The giant pseudotachylytes could be generated by large in magnitude earthquakes, associated with the activity of the Canavese Line and thus of Alpine age.

How to cite: Aldrighetti, S., D'Ippolito, G., Pennacchioni, G., Gomila, R., Baccheschi, P., and Di Toro, G.: Field and microstructural characterization of Valsesia pseudotachylytes (Ivrea Zone, Italy), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5837, https://doi.org/10.5194/egusphere-egu25-5837, 2025.

Deep focus earthquakes offer insights into Earth’s mantle and supports plate tectonics theory. Because high pressures and temperatures hinder brittle failure, their mechanisms differ from shallow quakes. Dehydration embrittlement, proposed as dominant at 100-350 km depth, involves fluid release from minerals like serpentine, increasing pore pressure and triggering failure. However, serpentine dehydration has a net decrease in pressure, requiring low-permeability layers to trap fluids to enable seismic failure. Experiments also show that serpentine dehydration often leads to ductile weakening without acoustic emissions.

To better understand the micro mechanisms involved in the dehydration of serpentinite, especially in the incipient stage, we have performed high pressure-temperature experiments under isostatic and non-isostatic conditions. Cores of serpentinite with 2 mm diameter were mounted in cubic assemblies with 12 mm edge. Experiments were carried out with the 6-Ram multi anvil press at the Bayerisches Geoinstitute, at pressure of 5 GPa, to a maximum strain of 15% at strain rates between 1.67x10^-4 s^-1 to 2.91x10^-6 s^-1. Temperature during isostatic and non-isostatic conditions was kept constant. Isostatic experiments were conducted at 550 °C and 784°C. non-isostatic experiments were conducted at ~650 °C.

Results show that isostatic dehydration of antigorite at 5 GPa starts at ~ 550 °C and is completed at ~ 800°C. Between 550-650 °C incipient dehydration of antigorite is evidenced by the growth of olivine and phyllosilicate at antigorite grain boundaries.  At these conditions, no failure microstructure is observed. Pores are present between olivine and enstatite grains of fully dehydrated serpentine. When deformation is imposed at incipient dehydration conditions, olivine and phyllosilicate start to cluster and form microscopic shear bands oblique to the main stress direction. These results demonstrate that at microscopic level, dehydration and failure of serpentine is complex. Pre-existing microstructural heterogeneities may influence nucleation of olivine and phyllosilicates. Pore overpressure may not be the only mechanism involved in serpentinite failure. Further work is required to determine the importance of the strength of the dehydration products in leading to localized failure.

How to cite: Silva Souza, D., Thielmann, M., Frost, D., Heidelbach, F., and Gasc, J.: Microscale processes in experimental serpentine dehydration: implications for deep earthquake mechanisms, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6058, https://doi.org/10.5194/egusphere-egu25-6058, 2025.

The relationship between the long-term strength of the lithosphere and seismic hazard has remained a fundamental, yet open question in geosciences. The lithosphere's long-term rheology controls its deformation patterns, playing a crucial role in understanding the spatial and temporal distribution of seismicity in a given region. One of the primary factors influencing the rheological state of the lithosphere is its thermal regime, which is strongly affected by the heterogeneous properties of both the crust and the lithospheric mantle, as well as by the three-dimensional interactions between deeper and shallower domains.

To explore how long-term off-fault rheology influences the spatial distribution of seismicity, we leverage extensive geophysical data from Central and Southern California, a region where the San Andreas Fault represents a significant seismic hazard. Previous thermal models of the area have not converged on a consistent thermal structure for the lithosphere, resulting in uncertainties in the rheological models based on them.

Our 3D thermal model is built using a data-integrative approach that incorporates recent tomographic models and a detailed, heterogeneous crustal architecture drawn from prior community efforts. Furthermore, our model fits the general pattern of observed surface heat flow in the region.  The lower boundary condition in our 3D model -temperature at 70 km depth - is based on an integrated geophysical – petrological inversion within a self-consistent thermodynamic formalism of Rayleigh and Love surface-wave dispersion curves (0.5 x 0.5 degree lateral resolution), supplemented by other geophysical data and models: satellite data, surface heat flow and average temperature, topography, Moho depth, P-wave seismic crustal velocities, and sedimentary thickness.

Notably, our model is consistent with major regional tectonic features, such as the fossil Monterey microplate slab, which is responsible for the well-known high-velocity Isabella Anomaly. We discuss the implications of this anomaly, focusing on the dehydration of the slab and its potential role in seismogenesis, especially in the creeping section of the San Andreas Fault near Parkfield.

How to cite: Gómez García, Á. M., Jiménez-Munt, I., Cacace, M., Scheck-Wenderoth, M., Root, B., Clemente-Gómez, C., Fullea, J., Lebedev, S., Xu, Y., and Becker, T.: 3D Lithospheric-scale thermal model of central and southern California, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6407, https://doi.org/10.5194/egusphere-egu25-6407, 2025.

Metamorphic transformations are often associated with strain localization which can be observed in the field either as ductile zones, or brittle, and possibly seismogenic, structures. Deformation experiments in the laboratory not only replicate such features but also allow us to measure the associated weakening. In all these contexts, reaction overstepping and disequilibrium metamorphism appear to be the rule. Reaction rates are usually very fast once transformation initiates, in particular within highly stressed and strained volumes where the produced mechanical work is sufficient to overcome kinetic barriers. New very fine-grained and dense phases nucleate in conditions where mineral growth is impeded. Understanding how heterogeneous nucleation, along with changes in density and viscosity, affects the rock's strength during a metamorphic transformation appears therefore critical.

In that prospect, results of a 2D numerical study in which reaction products preferentially nucleate in areas of high strain energy are presented. Special attention is given to the weakening or hardening effects induced by these transformations, as well as to the deformation patterns within the model. Results of our numerical study are then discussed in the light of experimental data obtained at comparable pressure-temperature-strain rate conditions.

We show that rock weakening is not only linked to the strength of the reaction products. Indeed, (1) densification alone can generate sufficient stress to induce plastic yielding of the surrounding matrix, even when the nuclei are stronger, and (2) heterogeneous nucleation controlled by mechanical work has greater influence on the rock’s strength than the intrinsic properties of the reaction products. Weakening is primarily driven by the initiation and propagation of plastic shear bands between the closely spaced nuclei that generate significant stress concentration in their vicinity. This study highlights the importance of transformational weakening that results from fast heterogeneous nucleation in rocks close to their brittle-ductile transition.

How to cite: Baïsset, M., Yamato, P., Duretz, T., Labrousse, L., Gasc, J., and Schubnel, A.: Weakening induced by phase nucleation: from experiments to numerical models, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9848, https://doi.org/10.5194/egusphere-egu25-9848, 2025.

Understanding the deformation modes of the lower crust is crucial if we are to predict the rheological behaviour of the crust in space and time.  The Nusfjord locality (Lofoten, Norway) represents a natural laboratory to study the interplay between seismic and aseismic deformation in the Earth’s lower crust. The area exposes pseudotachylytes (quenched frictional melt produced during coseismic slip) within a network of ductile shear zones bounding strong low-strain domains of granulitic anorthosites. Pseudotachylytes formed within the low-strain domains, during ongoing viscous creep in the ductile shear zones, at a depth of 25-35 km. The ductile shear zones themselves contain several generations of mylonitized pseudotachylytes suggesting repeated switches from frictional to viscous deformation within shear zones. The underlying reasons and rheological consequence of mutual overprinting relationships between ductile shear zones (generally considered to be weak) and several generations of pseudotachylytes remains enigmatic.

Field investigations, photogrammetry, structural logs, and microstructural analysis reveal that (1) pseudotachylytes invariably nucleate within the low strain domains of the anorthosite host rock located between subparallel shear zones, and not along the shear zones themselves; and (2) that the rupture migrates along the material interface provided either by the shear zone/host rock boundary or by the shear zone foliation. The observed relationships suggest transient stress pulses that are supported by variations in the recrystallized grain size of quartz along individual shear zones.

We propose that repeated episodes of pseudotachylyte generation and associated host-rock fracturing  represent a mechanism of shear zone growth and thickening, because the pseudotachylyte veins are mylonitized and become part of the actively deforming shear zones, which in turn control the further development of pseudotachylytes in the adjacent rigid blocks (low-strain domains). Structural logs show that shear zone width depends on the initial spacing between subparallel shear zones: when shear zones are widely spaced (>1 m), the rigid block in between is essentially undeformed, it contains a low density of pseudotachylytes and the shear zones themselves are thin (<10 cm thick). In contrast, closely spaced shear zones are thicker (up to 1 m thick) and are separated by highly damaged rigid blocks that contain a greater density of pseudotachylytes. Thus,  pseudotachylytes overprinting ductile shear zones are not necessarily the result of frictional-viscous switches along individual structures but may rather represent seismic fractures that initiated at stress concentrations within adjacent rigid blocks, which then followed preexisting shear zones. Importantly, repeated production of pseudotachylytes will progressively transform the lower crust from dominantly rheologically stiff to weak. Such rheological weakening will have major consequences on the dynamics of lower-crustal regions.

How to cite: Clivet, F., Piazolo, S., Michalchuk, S. P., Zertani, S., and Menegon, L.: Shear zone growth by repeated generation of pseudotachylytes in the lower crust, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10795, https://doi.org/10.5194/egusphere-egu25-10795, 2025.

Deep crustal shear zones, fundamental to the dynamics of terrestrial plate tectonics, exhibit complex processes of initiation and evolution that are yet to be comprehensively quantified across both long and short temporal scales. Conventionally, thermo–mechanical models posit that crustal rock behaviour is dominated by monomineralic aggregates undergoing processes like intracrystalline plastic deformation by dislocation creep. However, high-pressure and temperature conditions in crustal rocks involve minerals with extremely strong mechanical properties, challenging strain localization theories.

Field studies reveal that mineral reactions are ubiquitous in viscous shear zones, while undeformed rocks can remain largely metastable despite significant changes in P–T and/or fluid conditions. Local dissolution and precipitation processes under deviatoric stresses have long been recognized to promote brittle and viscous strain localization by complex chemo–mechanical processes including pressure solution, diffusive mass transfer, fluid flowand nucleation of fine-grained aggregates. Yet, quantifying the nature and relative contribution of these processes remains hindered by the general lack of experimental investigations on crustal rheology at high – to very high – pressure conditions and thermodynamic disequilibrium.

Drawing on novel deformation experiments performed at eclogite-facies conditions and a compilation of characteristics of exhumed materials from fossil subduction zones worldwide, this presentation demonstrates that inception and progression of crustal shear zones are predominantly steered by local transient changes of rheology from dislocation creep to dissolution–precipitation creep (DPC). Strain accommodation and mass transfer are further accelerated by local transient fluid flow resulting from grain boundary movements, fracturing and densification reactions. Because intergranular fluid-assisted mass transfer is orders of magnitude faster than solid-state diffusion, DPC can indeed explain strain accommodation at relatively high strain rates and low magnitude of differential stress, regardless of the mineral plastic strength. Yet, DPC remains a transient process because both fluid depletion and completion of mineral reactions favor grain growth, reducing in turn the efficiency of intergranular mass transfer.

How to cite: Soret, M.: Deep crustal shear zones driven by reaction-induced weakening and fluid flow, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10948, https://doi.org/10.5194/egusphere-egu25-10948, 2025.

The Acauã Formation, located within the Estância Domain in the Sergipano Belt of the Borborema Province, comprises carbonates and slates that preserve evidence of multiple deformation regimes, including ductile, ductile-brittle, and brittle. Located along the western border of the Central Tucano Basin, this lithostratigraphic unit displays diverse structural and mineralogical characteristics that are crucial for understanding its tectonic evolution. The foliation dips gently to the NW and SE, forming regional open folds, and an anticline drag-fold related to the thrust fault propagation was formed at the contact of the massive and laminated carbonate facies. The massive facies are characterized by dark gray, very fine-grained dolostones lacking visible internal structures, while the laminated facies comprise light to medium gray dolostones with fine to silty grain size, well-defined laminations, and occasional "beef" structures (fibrous calcite veins). Petrographic analysis revealed a micritic dolomite matrix in both facies, with disseminated quartz, biotite and pyrite observed in the laminated facies. Cathodoluminescence analysis confirmed dolomite as the primary mineral phase in the matrix and identified two distinct vein generations: dolomitic and calcitic. These veins exhibit elongated crystal growth along their margins and blocky central fills, indicating a process of progressive dilation followed by abrupt opening. The veins acted as nucleation sites for faults, with their reactivation during deformation stages evidenced by the formation of normal and thrust faults, which are predominantly oriented NW-SE and NE-SW, respectively. U-Pb geochronology of carbonates provided constraints on the timing of deformation. The micritic dolostone matrix yielded an age of 601.5 ± 13.3 Ma, likely reflecting post-glacial carbonate deposition. A dolomitic vein near a thrust fault was dated at 508 ± 138 Ma, while slickenfibres on the fault surface yielded an age of 316 ± 83 Ma. Bed-parallel faults yielded a Lower Permian age of 291 ± 48 Ma. These results, although imprecise, suggest that the Acauã Formation carbonates formed during the Ediacaran, with vein formation initiated in the late Neoproterozoic and being reactivated during the Paleozoic era. The structural evolution highlights the significant role of mineralized veins played in fault nucleation and reactivation during regional tectonic events.

How to cite: Correia, O., Izídio, A., Miranda, T., barbosa, D., Roberts, N., Sanglard, J., Carvalho, B., Araújo, R., Laura, M., Pacheco, S., and Neumann, V.: MULTI-STAGE DEFORMATION AND U-Pb GEOCHRONOLOGY OF CARBONATES IN THE ACAUÃ FORMATION, SERGIPANO BELT, NE BRAZIL, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11789, https://doi.org/10.5194/egusphere-egu25-11789, 2025.

Availability of fluids and induced metamorphic reactions are primary factors controlling the rheological behaviour of rocks. During subduction, fluids enhance the kinetics of eclogitization reactions, playing a fundamental role in promoting strain localization and shear zone nucleation. In particular, reaction-induced grain-size reduction has long been considered one of the most effective strain weakening mechanisms. To investigate the relationship between fluid-rock interaction, metamorphism and deformation, we focus on pre-Alpine ultramafites and mafic granulites of the Austroalpine Mt. Emilius klippe (Western Alps) that underwent eclogite-facies metamorphism during Alpine subduction.

The Mt. Emilius ultramafites consist of enstatite, diopside, olivine and spinel websterites deformed along a hydrated mantle shear zone that developed a fine-grained (10 µm) ultramylonitic assemblage of enstatite, diopside, olivine, anorthite, kaersutite¹. During Alpine HP metamorphism, fine-grained (down to 2 µm) aggregates of jadeite, quartz, kyanite, clinozoisite (Czo) completely and statically replaced plagioclase, locally forming spatially continuous layers. Such fine-grained, hydrated aggregates did not promote any ductile eclogite-facies deformation.

The pre-Alpine mafic granulite consisted of assemblages of medium-grained garnet (Grt), diopside, plagioclase and subordinate hornblende that were replaced by Grt, omphacite (Omph), amphibole, phengite, chlorite and Czo during Alpine eclogite-facies metamorphism². Early Alpine deformation (D1A) developed a pervasive eclogitic foliation (S1A) parallel to the granulitic layering². This event was promoted by complete transformation and reaction-induced grain-size reduction (down to a few tens of µm) of plagioclase to Czo aggregates, together with replacement of hornblende by fine-grained chlorite-garnet-amphibole-epidote-Phe. A second eclogite-facies deformation event (D1B) is represented by localized ductile deformation closely linked to development of Czo, Omph, tremolite, Grt-filled veins and associated host-rock alteration haloes. Ductile shear is typically localized to the outer boundary of Omph-rich alteration haloes forming paired shear zones. A set of samples ranging from haloes with well-developed flanking shear zones to haloes free of shear localization was collected to investigate the role of fluid-rock interaction on shear zone nucleation and strain localization.

Preliminary data indicate that Omph-rich haloes surrounding Czo-Grt veins induced hardening in the host metagranulite (undeformed and foliated, S1A) associated with extensive replacement of the Czo aggregates (after sites of granulitic plagioclase) by Omph. However, this replacement did not always result in hardening and consequent strain localization at the outer boundary of the halo. In samples lacking shear localization, Omph accommodates deformation homogenously across the halo dominantly by diffusion creep (variable CPO, quasi-random distribution of misorientation angles, weaker SPO), with minor contribution of crystal plasticity (rare subgrains). The predominant contribution of diffusion was likely assisted by availability of fluids.

The processes driving frequent strain localization and formation of paired shear zones at the outer boundary of hardened haloes are still matter of ongoing study. Progressive advancement of the reaction front towards the host rock may form a compositional gradient across the halo, where chemical/mineralogical modifications may play a major role in determining the rheological behaviour.

[1] Benciolini, 1996. Memorie Scienze Geologiche, 48, 73-91.

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

How to cite: Cacciari, S., Pennacchioni, G., Toffol, G., Scambelluri, M., and Cannaò, E.: Strain localization at eclogite-facies conditions: interplay between fluids, metamorphism and deformation (Mt. Emilius klippe, Western Alps), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11959, https://doi.org/10.5194/egusphere-egu25-11959, 2025.

It is agreed that mylonitic shear zones are first-order fluid conduits in the crust, but we lack a systematic and comprehensive understanding of porosity and permeability generation in natural mylonitic shear zones. As a consequence, we are unable to predict their synkinematic fluid transport properties, which affects our assessments of fluid-mediated processes in shear zones. 

Here we present insights into the dynamic porosities in a quartzo-feldspathic layered ultramylonite from the Redbank Shear Zone (Australia) that formed during a stage of retrograde thrusting and hydration at lower amphibolite-facies conditions. In our analysis, we have combined non-invasive, high-quality x-ray microtomographic datasets from 5-mm-diameter core samples drilled orthogonally to the mylonitic foliation with high-resolution electron microscopy, electron backscatter diffraction and energy dispersive x-ray spectroscopy on the same samples. 

The sample is dominated by two fine-grained (<5 µm) microstructural domains, which differ by the relative proportions of Qz, Or and An, and the occurrence of Czo, respectively. Both deformed dominantly by grain-size-sensitive diffusion creep and grain boundary sliding. Newly grown Czo is thought to have resulted from the hydrothermal alteration of plagioclase at lower amphibolite-facies conditions during continued retrograde thrusting. Five types of synkinematic porosity were identified in the sample: pores at the boundaries, and dissolution pores inside of feldspar porphyroclasts, strain-shadow pores around Czo porphyroblasts, creep cavities, and pore sheets. These porosity types are the results of different mechanisms acting locally in the microstructure. On the sample scale, the porosity distribution is dependent largely on the distribution of porphyroclasts and porphyroblasts, and creep cavitation in the matrix. Porosity in the Qz-dominant layers, which lack Czo, is ‘localised’ around and inside shrinking feldspar porphyroclasts, whereas porosity in the fine-grained polyphase microfabric containing Czo porphyroblasts is more common and ‘distributed.’ The latter may allow more efficient but anisotropic fluid transfer. Creep cavities appear to have coalesced to form pore sheets along foliation boundaries or connecting strain-shadow pores. Our findings further corroborate the description of strain shadow porosity by Fusseis et al. (2023, Geology). We interpret that a feedback between clinozoisite growth and fluid ingress promoted further creep cavitation, and resulted in a greater potential for cavity coalescence to cause ductile failure in the fine-grained polyphase microfabric.

How to cite: McDowell, A., Gilgannon, J., Killian, R., and Fusseis, F.: Synkinematic porosity and ductile failure in mid-crustal ultramylonites from the Redbank Shear Zone, Central Australia, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12927, https://doi.org/10.5194/egusphere-egu25-12927, 2025.

Crustal strength has been estimated to become the largest at the brittle-ductile condition^[1]. Previous experiments have shown that water reduces the crustal strength not only at shallower depth regions where frictional slip becomes dominant^[2] but also at greater depth regions where viscous flow becomes dominant^[3]. However, the microphysical process of how water alters deformation mechanisms and reduces rock strength at the brittle-ductile transition zone remains unclear. To investigate the effect of water on controlling deformation mechanisms at the brittle-ductile transition, we perform a series of shear deformation experiments with a trace amount of water (either 0.2 wt % or 0.4 wt %). We deformed a quartz-albite mixture using a Griggs-type solid salt assembly. Each experiment uses ~ 0.1 g of the sample mixture. The shear strain rate is sequentially changed between ~ 10^-3 /s and 10^-4 /s to investigate the strength dependence on velocity. We further conducted microstructural observations using electron microscopes.

Here, we report a preliminary result of a series of water-added experiments conducted with 0.4 wt% water (i.e., 0.4 μL) at a confining pressure P_C of 760 MPa and a temperature T of 720 °C. Mechanical results show that the peak shear stress is 790 MPa at a shear strain of 1.4, followed by a strain weakening by 200 MPa towards a final shear strain of 4.9. This peak stress is much weaker than a previous result of a room-dry experiment performed at a similar experimental condition (P_C = 750 MPa and T = 720 °C)^[4]. In the dry experiment, the peak shear stress was 1280 MPa, followed by a strain weakening of 230 MPa^[4]. Microstructural analyses showed that the water-added sample is pervasively covered with microcracks. A transmission electron microscopy revealed that nano-grains as small as 50 nm are distributed in the areas between the microcracks. Meanwhile, a sample from the dry experiment exhibits fewer microcracks and contains nano-grains similar in dimensions to those in the sample of the wet experiment^[5].

Our results suggest that water enhances fracturing in the sample layer, and nano-grains are formed regardless of the addition of water. This indicates that the reduction in the peak stress of wet conditions is due to the fracturing promoted by water, while the strain weakening after peak stresses is controlled by nano-grain domains in both conditions. We propose that water reduce the crustal strength by fracturing, that is brittle deformation, accompanied with weakening mechanisms in nano-grain domains such as grain boundary sliding. Furthermore, this suggests that brittle deformation remains dominant even at a greater depth in wet conditions, compared with in dry conditions.

[1] Kohlstedt et al., 1995JGR. [2] Blanpied et al., 1995JGR. [3] Kronenberg & Tullis, 1984JGR. [4] Furukawa et al., 2023 WRI-17. [5] Furukawa et al., 2025 in preparation.

How to cite: Furukawa, M., Sawa, S., Nagahama, H., Plümper, O., and Muto, J.: Water-added experiments of simulated quartz-feldspar shear zone at brittle-ductile transitional condition, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14614, https://doi.org/10.5194/egusphere-egu25-14614, 2025.

The Yangsan Fault in southeastern Korea is a long-lived intracontinental fault system characterized by both seismic slip and aseismic creep. Despite its significance, the microstructural evidence that clarifies the fault’s deformation mechanisms remains incomplete. In this study, we present an analysis of the mechanical behaviors displayed by the Byeokgye section of the Yangsan Fault over seismic cycles. Our results are based on detailed microscopic observations of drillcore samples recovered from the Byeokgye section, using an electron backscattered diffraction (EBSD) technique. In injected calcite veins located close to the principal slip zone (PSZ) of < 2 cm in width, plastic deformation (including dynamic recrystallization by subgrain rotation and deformation twins) is concentrated in the blocky calcite grains. In a narrow microbrecciated slip zone (< 1 cm wide) within the granitic damage zone, we observed mechanical Dauphiné twins associated with fractures and microfaults in quartz, as well as intergranular pressure solution (IPS) in the quartz fragments. Given that dynamic recrystallization and IPS are indicative of mechanical behavior of aseismic creep, it is possible that aseismic creep occurs upon the fault during interseismic periods. Conversely, the presence of mechanical Dauphiné twins, coupled with the nature of the PSZ, gouge injections, and the blocky structure of calcite veins, suggests the exposure of the fault section to local seismic stresses during coseismic slip. In conclusion, various deformation processes have operated upon the Yangsan Fault at the studied section throughout multiple seismic cycles. Moreover, our results demonstrate the effectiveness of EBSD in elucidating the mechanical behavior within fault zones.

How to cite: Choi, S., Cheon, Y., Kim, C.-M., Jung, H., and Park, M.: Microstructural insights into the coseismic and aseismic behavior of fault rocks in the northern Yangsan Fault, SE Korea, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15187, https://doi.org/10.5194/egusphere-egu25-15187, 2025.

Serpentinites play a critical role in subduction zones due to their unique mechanical properties, which influence tectonic and seismic processes and facilitate deformation along the subduction interface. A long-standing question is the discrepancy between experimentally deformed serpentinites, which exhibit brittle/brittle-ductile microstructures, and naturally deformed serpentinites, which predominantly show ductile features. Additionally, there is a strong debate on whether deformation in antigorite bearing rocks is driven by crystal plasticity, dissolution-precipitation, or a combination of both. Moreover, studies on deformation in partially dehydrated or hydrated serpentinites (containing metamorphic olivine and clinopyroxene), subducted down to (ultra)high pressure conditions, remain scarce. To address these issues, we conducted a detailed microstructural study of serpentinites from a hectometer-scale strain gradient zone within the Zermatt-Saas meta-ophiolite, examining deformation mechanisms in antigorite and olivine at depths relevant to intermediate-depth earthquakes and subsequent exhumation across mantle wedge conditions.

In low-strain serpentinites, dehydration of brucite-antigorite produces coarse-grained olivine-diopside-clinohumite-magnetite veins (“olivine veins”), while the host antigorite displays mesh textures, weak crystallographic preferred orientations (CPOs), and evidence of twinning. Deformation begins to localize around olivine veins, where olivine exhibits a B-type CPO with [010] parallel to the pole of foliation and [001] parallel to the lineation but no internal deformation. With increasing strain, antigorite foliation becomes continuous and penetrative, accompanied by CPO strengthening, grain size reduction, and localized folding and boudinage of olivine, where the CPO strength also increases. High-strain domains exhibit mylonitic fabrics, intense antigorite foliation with (001) maxima aligned to the pole of foliation and (010) parallel to lineation, and transposed olivine vein folds reduced to isoclinal rootless folds. Additionally S-C’ foliations form locally, with fine-grained olivine fibers coating C’ planes, and pressure shadows around olivine porphyroclasts containing olivine-diopside mixtures forming mm-scale bands within antigorite foliations. The olivine grains in the pressure shadows also present a strong B-type olivine CPO.

Our findings highlight a progressive transition from brittle-ductile to ductile deformation in serpentinites in a fluid-rich environment. This deformation seems to be controlled by dissolution-precipitation processes and dislocation creep. Furthermore, this study provides one of the few datasets of deformation of metamorphic olivine in subduction zones. The conditions documented are not only relevant for the oceanic lithosphere but also for the mantle wedge near the subduction channel, offering critical insights into the interplay of deformation, metamorphism, and fluid-rock interactions in these tectonic settings.

How to cite: Morales, L. F. G., Muñoz-Montecinos, J., Ceccato, A., and Behr, W.: Microstructural Evolution of High- and Low-Strain Serpentinites from the Zermatt-Saas Meta-Ophiolite: Insights into Antigorite and Olivine Deformation at Intermediate-Depth Seismicity Depths, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15688, https://doi.org/10.5194/egusphere-egu25-15688, 2025.

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. Electron backscatter diffraction analyses suggest that large (several hundreds of µm) detrital quartz grains experienced grain size reduction by subgrain rotation recrystallization to form equant grains of less than 100 µm in size.

Hyperspectral cathodoluminescence highlights different luminescence for the larger detrital grains, producing a bright signal and containing yielded cracks, and smaller equant grains, darker in cathodoluminescence and devoid of cracks. Interconnected chains of small quartz grains are located in contact with the phyllosilicates, suggesting an interplay between pinning and grain growth from a fluid phase.

In pyrophyllite-muscovite intergrowths, Transmission Electron Microscope analyses highlight more defects and kinking in pyrophyllite than in muscovite, intergrowths at the submicron scale and crystallites as small as 2 µm with truncated boundaries likely reflecting dissolution and precipitation mechanisms.

Summarising, these results suggest that strain localization and weakening of this rock volume was achieved by an interplay of the following mechanisms: I) synkinematic nucleation of retrograde mineral phases along discrete C and C’ planes, forming an interconnected network of phyllosilicates, II) microcracking in larger quartz grains followed by subgrain rotation recrystallization leading to a finer grain size of quartz, III) pinning of the grain size and IV) dissolution and precipitation processes of phyllosilicates. Different types of phyllosilicates appear to differently accommodate strain by both plastic deformation and recovery by dissolution-reprecipitation.

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, P., Searle, M. P., Alsop, G. I., & Dubacq, B. (2010). Tectonics, 29(5). https://doi.org/10.1029/2010TC002669

Petroccia, A., Giuntoli, F., Pilia, S., Viola, G., Sternai, P., & Callegari, I. (2025). Journal of Structural Geology, 191. https://doi.org/10.1016/j.jsg.2024.105328

Ring, U., Glodny, J., Hansman, R., Scharf, A., Mattern, F., Callegari, I., van Hinsbergen, D. J. J., Willner, A., & Hong, Y. (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., and Raimbourg, H.: Phyllosilicates do their job: insights into their role in exhuming subducted continental units, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16287, https://doi.org/10.5194/egusphere-egu25-16287, 2025.

The processes that rule coupling/decoupling and rupture mechanisms along the plate interface in the deep portions of active subduction zones are largely inferred from geophysical observations. These observations highlight that a wide range of rupture and deformation mechanism may coexist, such as: aseismic slip, episodic non-volcanic tremor and slip (ETS), and regular earthquakes. Despite the high amount of data obtained through indirect approaches, our comprehension of the processes occurring along the plate interface is still limited. In particular, processes occurring at great depth along the subduction interface are difficult to interpret solely based on flow laws and rheological properties of rocks also due to the scarcity of direct geological observations.

Exhumed ultra-high pressure (UHP, > 90 km of depth) rocks represent a natural laboratory to investigate the interplay of metamorphic reactions and fluids, both affecting slab rheology, at great depth. The Lower Shear Zone (LSZ) from the Monviso massif (W Alps) represents a fossil plate interface accreted within the Western Alpine chain and constitutes the one-off example of an oceanic plate interface that reached coesite stability field at UHP depth., i.e., ca. 90-100 km, and was then exhumed^[1,2]. The LSZ preserves snapshots of the different stages of deformation and metamorphism along the subduction plate interface shear zone, testifying the coexistence of brittle (brecciation of rigid eclogite-facies gabbroic mylonites) and ductile behaviour (shearing along weak, serpentinite-rich shear zone) at eclogite-facies depth^[1,3].

Our new field, micro-structural and petrographic observations extend the existing record of brittle features along the LSZ and show that brecciated blocks of mylonitic eclogites are systematically traceable for almost 25 km, i.e. the entire length of the exposed LSZ. These blocks are embedded within a highly deformed serpentinitic matrix. The brecciated fabric is defined by a mosaic breccia texture with randomly distributed clasts cemented by a polyphasic omphacite-rich matrix. The matrix is locally brecciated and sealed again, highlighting a cyclic rupture and healing mechanism promoted by fluid pulses and consequent dehydration embrittlement. These features are comparable to the classical geological observations of structures attributed to ETS described from shallower region of the plate interface. The similarity suggests that ETS may transiently occur even at greater depths than those at which they are currently recorded by seismometers and GNSS stations. Our observations imply that decoupling at great depth along the plate interface could be favoured by embrittlement of the plate interface.

1: Angiboust, S., Agard, P., Yamato, P., Raimbourg, H., 2012. Eclogite breccias in a subducted ophiolite: A record of intermediate-depth earthquakes? Geology 40, 707-710.

2: Ghignone, S., Scaramuzzo, E., Bruno, M., Livio, F. A. 2023. A new UHP unit in the Western Alps: First occurrence of coesite from the Monviso Massif (Italy). American Mineralogist, 108(7), 1368-1375.

3: Locatelli, M., Verlaguet, A., Agard, P., Federico, L., Angiboust, S., 2018. Intermediate-depth brecciation along the subduction plate interface (Monviso eclogite, W. Alps). Lithos.

How to cite: Scaramuzzo, E., Ghignone, S., Toffol, G., Boero, F., Locatelli, M., Gilio, M., Livio, F., Bruno, M., Scambelluri, M., and Pennacchioni, G.: Multiple rupture and healing events along the Plate Interface at Ultra-High-pressure depth. Insights from the Lower Shear Zone, Monviso Massif, Italy , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16426, https://doi.org/10.5194/egusphere-egu25-16426, 2025.

The Menderes Massif in western Anatolia is a large metamorphic core complex that formed in the back arc of the Aegean subduction zone. Geological and geodetic studies show that extension has occurred almost uniformly since the cessation of continental collision at c. 30 Ma. In this study, we used 2D numerical modeling informed by measurements of abundances of radioactive heat producing elements in exhumed Menderes metamorphic rocks (gneiss, schist, migmatite, granite) to investigate the effect of variation in vertical distribution of crustal radioactivity on the style of extensional deformation during core complex evolution. We assumed four different scenarios with the same total crustal radioactive heat production but fractionated differently between the upper and lower crust: 0%, 25%, 50%, and 62.5% of the total crustal radioactivity located within the thickened lower crust. Our numerical experiments reveal that lower crustal radioactivity has a major effect on the temperature (T) of the lower crust and hence its geodynamic evolution. We observed significant partial melting and core complex development only in the scenarios with fractions of 50% or more. The results are nearly independent of upper crustal radioactivity. The elevated radioactivity levels and therefore T of the lower crust drives partial melting, which in turn results in lower viscosity and enhanced crustal flow. According to these results, the lower part of the thickened orogenic crust in western Anatolia must be highly radiogenic in order for the formation of the observed core complex structure.

How to cite: Erkan, K., Whitney, D. L., and Rey, P. F.: Effect of variation in the vertical distribution of crustal radioactivity in metamorphic core complex development (Menderes Massif, Türkiye), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16582, https://doi.org/10.5194/egusphere-egu25-16582, 2025.

 Strain hardening is a key feature observed in many rocks deformed in the so-called ``semi-brittle'' regime, where both crystal plastic and brittle deformation mechanisms operate. Experimental observations in calcite aggregate show a negative correlation between strain hardening rate and microcrack density. Strain hardening is typically caused by accumulation of unrelaxed elastic stresses, for instance due to dislocation storage or frictional sliding, but the role of tensile cracks in that process is not clear. Here, I will first summarise key experimental observations in calcite aggregates, documenting the co-evolution of microstructural features as a function of strain, and then propose a simple microphysical hardening model that couples tensile microcracking with dislocation storage. The model relies on viewing tensile cracks as free surfaces that absorb dislocations, thus reducing the dislocation storage rate and the hardening coefficient. The model captures important qualitative features observed in calcite marble deformation experiments: pressure-dependency of strength in the ductile regime, and a reduction in hardening linked to an increase in crack growth with decreasing confining pressure. Although very promising at a conceptual level, the model has limitations and needs to be tested more systematically before it can be used to make geological predictions of strength in the semi-brittle regime.

How to cite: Brantut, N.: Semi-brittle flow of rocks: Cracks, dislocations and strain hardening, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17888, https://doi.org/10.5194/egusphere-egu25-17888, 2025.

The collapse of orogenic belts is commonly thought to involve viscous flow in a mid-crustal channel, and manifests as extensional faulting in the upper crust. Recent observations in some orogenic belts have indicated a power-law relationship between local elevation and extensional strain rates. Simple mechanical considerations predict that the flow of the weak crustal layer beneath these belts is driven by topographic gradients, suggesting that the observed extension is linked to this flow. To test this hypothesis and examine the temporal evolution of collapsing orogenic belts, we developed a 2-D numerical model simulating how topography-driven viscous flow in the weak mid-lower crust induces, and is affected by, orogenic belt extension. Our results show that flow of a weak mid-lower crust triggers orogenic collapse via normal faulting, provided mountain height exceeds a critical threshold (h_min​). The simulated faults form within the highest regions of the orogen, where the weak crustal layer flow originates. Once the mountain collapses so much that its height falls below h_min, extension ceases, where h_min​ depends on both the thickness of the weak layer and the strength of the upper crust.  Additionally, we find that collapse rates increase with hotter and thicker weak channels, taller orogens, and weaker upper crustal faults, while stronger upper crust restricts fault distribution, concentrating deformation within smaller areas, leading to a core complex extension mode. Finally, a strong agreement between our numerical and analytical (detailed in companion abstract: Dawood et al. 2025 EGU General Assembly 2025) models demonstrates that orogenic collapse rates and their temporal evolution are jointly controlled by the brittle and ductile properties of the continental crust.

How to cite: Aharonov, E., Dawood, R., and Olive, J.: How Coupled Brittle-Ductile Deformation Controls the Rates and Temporal Evolution of Orogenic Collapse, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18427, https://doi.org/10.5194/egusphere-egu25-18427, 2025.

Rocks occupying the back-arc areas in subduction zones present a structural complexity resulting from subduction and exhumation processes, the latter contemporaneous with hydrothermal fluid circulation and ore deposition along crustal-scale shear zones. In many cases, the exhumation starts while rocks are situated in the middle crust, where ductile deformation prevails and ends when these rocks are exposed to the surface, juxtaposed against hanging wall rocks with contrasting mechanical properties and deformation history. The interplay between high- and low-grade rocks often results in complex patterns and puzzling structural inventories.

The Rhodope crystalline complex (north Greece) comprises high-grade ortho-and paragneisses that were subducted in HP-UHP in the Mesozoic and exhumed in the Oligo-Miocene, through a complex network of ductile shear zones and low-angle normal faults constituting the Kechros Detachment. The high-grade footwall rocks belong to the Lower and Intermediate Rhodope Terranes, juxtaposed against the low-grade carbonates and phyllites of Makri Unit and the late-Eocene-Oligocene supra-detachment sediments and volcanic rocks.

We have conducted a detailed mapping and structural study of the Kallintiri area (SW Byala Reka-Kechros Dome, Rhodope, northern Greece) to define the tectonostratigraphy of the area and discriminate between early ductile, subsequent brittle-ductile, and late brittle structures. Our results established a continuum of large-scale structures that brought the high-grade rocks from the middle crust to the surface, accompanied by corresponding fault rocks and structures, revealing the acting deformation mechanisms. During the exhumation process, the deformation was localized at the lower structural level of the Makri Unit due to the significant competence contrast between the structurally lower amphibolite-facies gneisses and the overlying lower-greenschist facies carbonates. As a result, the carbonate rocks from the hanging wall Makri Unit were mechanically coupled to the footwall and served as the main lithology that experienced mylonitic deformation.

How to cite: Soukis, K., Kanellopoulos, C., Voudouris, P., Mavrogonatos, C., Sboras, S., Lazos, I., Tarantola, A., Koehn, D., and Moritz, R.: From hanging wall to footwall: a story of crustal-scale piracy during the exhumation of the South Rhodope complex (northern Greece)., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20083, https://doi.org/10.5194/egusphere-egu25-20083, 2025.

Geological observations, seismic data as well as laboratory experiments have shown that faults lithify and recover their strength (heal) during interseismic periods. The mechanical-chemical process of fault healing is a key in understanding many aspects of fault behavior, such as earthquake recurrence and rupture dynamics. Such processes do not only play an important role in understanding unconventional seismicity, such as ‘slow and low frequency earthquakes’ as observed at active plate boundaries, but are also pivotal for the application of deep geothermal energy, CO₂ sequestration and the underground storage of radioactive waste. In this study we investigate the mechano-chemical recovery of fractures in carbonates at upper crustal conditions. In the upper crust, fractures are dominantly sealed through mineral precipitation from supersaturated fluids that are chemically out of equilibrium with the host rock. In order to simulate the healing process, we performed fluid percolation experiments on intact as well as pre-fractured carbonates with varying timescales representing different healing rates. In order to quantitatively document the healing process, the selected rock samples are analyzed by X-ray microtomography before and after the experiments. In addition, optical as well as scanning electron microscopy is applied to document the mechanical-chemical processes of healing. The role of the initial (micro)fracture network, the effect of the initial chemistry of the injected fluid and the effect of temperature on the healing process will be investigated. The experiments on both intact and pre-fractured rock are carried out with a percolation cell that allows the fluid-rock interaction to be reproduced at confining pressures up to 100 MPa, pore pressures up to 100 MPa and temperatures up to 250°C. This work will advance knowledge about the damage-recovery cycle in fractured carbonates through the investigation of healing processes active at different timescales using a unique experimental approach.

How to cite: Akker, I. V. and Fondriest, M.: Fracture healing processes in upper crustal carbonates – insights from fluid percolation experiments, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-140, https://doi.org/10.5194/egusphere-egu25-140, 2025.

Reactions involving variable exchange of latent heat are ubiquitous dynamic metamorphic processes: prograde dehydration and melting reactions cause an increase of the effective heat capacity by over an order of magnitude as they advance, while melt crystallization and retrograde hydration leads to transient heat production similar to radioactive heating in the continental crust. We show results of  thermokinematic models simulating the release and consumption of latent heat in an upward advecting rock pile to constrain thermal histories akin to exhumation in active tectonic settings. We show that hydration of dry gneisses leads to a gain of 20–40 g water per kg of rock and releases 80–160 kJ/kg latent heat. This transient thermal perturbation delays cooling and enhances thermally-activated processes such as diffusive loss of radiogenic Argon, which can rejuvenate apparent ⁴⁰Ar/³⁹Ar ages in white mica by up to ~10%. Biotite and feldspar display a similar distortion, even for large grains of ~1 mm in diameter (Schorn et al., 2024). In another case study we present multicomponent diffusion modeling of garnets in hydrated micaschist from the polymetamorphic Koralpe–Saualpe locality (Austria). We explore exhumation paths for varying hydration and latent heat production to constrain temperature–time histories, with a best-fit of modelled garnet zoning pattern achieved for ~120 kJ/kg released at 550°C and an exhumation rate of 4 mm/yr. As for melting reactions, we simulate periodic sill emplacement in 5-km wide ‘hot zone’ at 25 km depth, like magma injection in a subduction-related arc setting (e.g., Annen et al., 2006). Focusing on the thermal–temporal evolution of metapelitic source rocks at depth, we investigate the thermal retardation related to the endothermic melting of mica followed by the exothermic crystallization of leftover melt in comparison to the unbuffered case. This interplay leads to a clustering of temperatures around the conditions of melt-related thermal buffering and is consistent with the predominance of mineral assemblages related to focused biotite–sillimanite breakdown in metapelites (Schorn et al., 2018), as observed at the orogen-scale in large exhumed hot orogens such as the granulite-facies domain of the Namaqua–Natal Metamorphic Province in southern Africa (Diener & Macey, 2024).

References

Annen, C., Blundy, J. D., & Sparks, R. S. J. (2006). The genesis of intermediate and silicic magmas in deep crustal hot zones. Journal of Petrology, 47(3), 505-539.

Diener, J. F., & Macey, P. H. (2024). Orogen‐scale uniformity of recorded granulite facies conditions due to thermal buffering and melt retention. Journal of Metamorphic Geology.

Schorn, S., Diener, J. F., Powell, R., & Stüwe, K. (2018). Thermal buffering in the orogenic crust. Geology, 46(7), 643-646.

Schorn, S., Moulas, E., & Stüwe, K. (2024). Exothermic reactions and ³⁹Ar–⁴⁰Ar thermochronology: Hydration leads to younger apparent ages. Geology, 52(6), 458-462.

How to cite: Schorn, S. and Moulas, E.: Latent heat of metamorphic reactions: boosting diffusion – hampering cooling, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3797, https://doi.org/10.5194/egusphere-egu25-3797, 2025.

How to cite: Cateland, B., E.Beaudoin, N., Battani, A., Mouthereau, F., Caracausi, A., and Pujol, M.: Tracing mantle-crust fluid interactions in a lithospheric extension zone: Insights from the Betic Cordillera, Spain., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5693, https://doi.org/10.5194/egusphere-egu25-5693, 2025.

Fluid-rock interaction represents a common geological process that is highly dynamic and may cause substantial microscale petrophysical and geochemical changes both in a static and syn-deformational environment. Understanding how these local microscale dynamics occur is crucial to comprehend macroscale behaviour of the lithosphere, and for advancing critical subsurface engineering challenges, such as carbon capture and storage; a process that, together with hydrogen storage and geothermal energy recovery, is vital for the energy transition. With the advance of non-destructive imaging techniques (X-ray Computed Tomography - XCT), we can image the evolution of these microscale dynamics and understand how they drive changes in crustal dynamics and subsurface engineering. We present two applications, in which we use XCT to characterise the evolution of reservoir storage properties, such as porosity and permeability, and provide further insights into carbon sequestration. In the first application, we used XCT to investigate the precipitation history of an amygdaloidal basalt now partially filled by calcite as an analogue for CO₂ mineral trapping in a vesicular basalt. We quantified the evolution of basalt porosity and permeability during pore-filling calcite precipitation by applying novel numerical erosion techniques to “back-strip” the calcite from the amygdales and fracture networks. We found that once the precipitation is sufficient to close off all pores, permeability reaches values that are controlled by the micro-fracture network. These results prompt further studies to determine CO₂ mineral trapping mechanisms in amygdaloidal basalts as analogues for CO₂ injections in basalt formations. In the second application, we considered the combined effect of upstream corrosion of the carbon capture and storage (CCS) infrastructure and pre-existing reservoir rock compositions on the evolution of reservoir storage properties. Reactions from the corroded pipelines can change the chemistry of injected brine, which can then react with the adjacent rock formations reservoir, affecting reservoir porosity, permeability and caprock integrity. These are important parameters that determine the injectivity and storage capacities of deep geological sites for long term CO₂ storage. Reservoir rock samples are characterised before corrosion and after carbonation reactions using XCT and other micro-analytical techniques, to assess the changes in the rock storage capacity properties. Our preliminary results prompt further studies into the understanding of fluid-rock interactions for subsurface engineering challenges, with a particular focus to pre-existing microfractures and changes in the injected brine due to corrosion of the upstream pipelines and interaction between CO₂ brine and reservoir rocks.

How to cite: Macente, A., MacDonald, J., Dobson, K. J., Pessu, F., and Piazolo, S.: From pore-scale to macro-scale: Understanding fluid-rock interactions using X-ray Computed Tomography, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6019, https://doi.org/10.5194/egusphere-egu25-6019, 2025.

Fluid circulation in the shallow crust is modulated by faults, which can act as barriers, conduits, or combined systems. In fault zones, fluids may vary in temperature and composition, originating from meteoric, connate, or magmatic/hydrothermal sources, and leading to the precipitation of various minerals in fault-related rocks and veins (e.g., carbonates, silicates, sulphates, oxides/hydroxides, clay minerals). In limestone, stable isotopes analyses (C, O), clumped isotopes, microthermometry of fluid inclusions, and U-Pb dating on carbonate mineralizations (e.g., slickenfibers, veins) are generally applied to determine the temperature and the source of fluids circulating within the fault zone during deformation. Discriminating temperature and fluid origin in clay-rich fault zones is more challenging, due to the coexistence of detrital minerals derived from the mechanical comminution of the host rocks and authigenic/synkinematic minerals precipitated during transient frictional heating or by prolonged fluid circulation. The compositional and temperature variation of fluids over time is recorded by authigenic minerals, that may reflect mixing with external sources or deformation at different depths and structural levels. The extent of fluid interaction with detrital minerals also contributes to their isotopic signature, and the evaluation of fluid sources can be very tricky due to the various mineral-water fractionation factors for every mineral. Indeed, H and O isotopes studies in clay-rich fault zones are generally applied as long as fault rock samples are nearly mono-mineralic, leading to very low number of data to develop a reliable dataset. To solve this issue, we applied a multi-method approach based on X-ray diffraction analyses of clay minerals, paleotemperature evaluation, and H, O isotope studies of different grain size fractions (from <0.1 to 10 µm) combined with a new calculation that allows to evaluate the fractionation processes of every single mineral (detrital vs. authigenic). In addition, K-Ar ages on syn-kinematic K-bearing minerals allowed to determine the age of faulting and eventually build an evolutionary model of fluid composition and temperature. In this contribution, we investigated two regional-scale fault zones on Lemnos Island (Greece), the Kornos-Aghios Ioannis extensional fault and the Partenomythos extensional fault, that are affected by Si-rich hydrothermal alteration. Our findings show that authigenic clay minerals (illite-smectite) from the <0.1 fractions are not in isotopic equilibrium with the host-rock, suggesting a meteoric-derived component infiltrated during faulting and recorded by clay minerals as a progressive change in fluid composition through time. These results represent an important step forward for fluid characterization in clay-rich fault zones, improving our understanding on how temperature and fluid source control the formation of authigenic minerals and fractionation processes in fault rocks.

How to cite: Moretto, V., Berio, L. R., Dallai, L., Viola, G., Balsamo, F., Grathof, G., Warr, L. N., Xie, R., and Aldega, L.: A new approach for constraining temperature, fractionation process, and fluid evolution in clay-rich fault zones: a case study from Lemnos Island (Greece), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6490, https://doi.org/10.5194/egusphere-egu25-6490, 2025.

CO₂ degassing in orogenic settings, derived from either prograde metamorphic decarbonation or deep mantle sources, is commonly linked to deep and shallow seismicity along major deformation zones. However, the scarcity of natural fossil analogues of these deformation zones hosting CO₂-rich fluid flow limits our understanding of the causative mechanisms and hinders validation of proposed explanatory models and inferences from geophysical and geochemical observations.

We describe a set of shear zone-hosted, carbonate-bearing breccias from the basement units of the Gotthard nappe, Aar, and Mont Blanc massifs (Central and Western Alps), interpreted as evidence of regional-scale, fault zone-hosted flow of CO₂-rich metamorphic fluids during Alpine orogenesis. Field observations, microstructures and geochemical data suggest these breccias serve as fossil analogues of fault/shear zone networks controlling CO₂-rich fluid flow in active orogenic settings, providing new insights on crustal-scale transport of carbonic fluids and its relationship with tectonic deformation.

The breccias are localized on a pre-Alpine fault network within the crystalline basement rocks and are mainly composed of coarse-grained (mm-to-cm in crystal size) blocky calcite/dolomite forming a matrix that encloses angular host rock clasts. The large volumes of carbonates and the macro- and micro-textures indicates formation during transient, but repetitive (carbo-)hydraulic fracturing in the presence of CO₂-rich fluids, potentially at amphibolite/upper-greenschist facies conditions. C-O stable isotopes (-8.49‰ < d¹³C_VPDB < +0.73‰, +8.07‰ < d¹⁸O_VSMOW < +16.39‰) and the enrichment of (Heavy) REE elements, as well as the characteristic Y/Ho and La_N/Lu_N ratios, suggest the fluids potentially originated from high-grade metamorphic decarbonation during Alpine collision.

These breccias, together with previously reported H₂O-CO₂ flow examples in the Central-Western Alps (e.g., carbonate shear zones along the Glarus thrust; retrograde calcite-bearing Alpine clefts), point to orogen-scale flow of CO₂-rich fluids spanning prograde, peak, to retrograde metamorphism during Alpine collision. Although evidence for seismogenic deformation is limited, field and microscale structures show evidence for transient tectonic deformation, potentially aided by elevated pore fluid pressure. Tectonic stress drops associated with fluid pressure changes in these zones might have promoted (transient) H₂O/CO₂ phase immiscibility, leading to carbonate saturation and voluminous deposition. The resulting large carbonate volumes suggest that the shear zones intermittently acted as both conduits and reservoirs for CO₂ -rich fluids transported from depth toward the surface. This highlights their dual role in controlling orogenic CO₂ degassing and buffering emissions, with implications for understanding fluid-mediated tectonics and carbon cycling in collisional orogens.

How to cite: Ceccato, A., Tavazzani, L., Malaspina, N., Behr, W. M., and Bernasconi, S. M.: Shear zone-mediated transfer and buffering of CO2-rich fluid during orogenic degassing, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6533, https://doi.org/10.5194/egusphere-egu25-6533, 2025.

Earthquakes release vast quantities of energy over very short timescales. At shallow depths, a portion of this energy is used to fracture, crush, and grind fault hosting rocks, resulting in reduced particle size and mineral crystallinity; frictional heating; mass movement of pore-fluid; and overall extreme but transient conditions. These seismic  processes partially control mineral alteration reactions that often take place within fault gouges. The mineralogy and therefore mechanical and chemical properties of fault core material will influence the style of future slip on faults. Many studies have shown that mineralogical differences within fault cores result from inter-seismic alteration by pore fluid, but have neglected co-seismic processes. Here we highlight the role of co-seismic mechanically and mechanochemically influenced mineral reactions. These reactions enhance fluid driven alteration and affect the frictional properties of fault rocks.

Transient co-seismic conditions cannot be studied in the field, so earthquakes were simulated in the lab using a high velocity rotary shear apparatus and silicate based synthetic fault material to enable control of experimental inputs. We found significant frictional differences in reworked gouge after having experienced a high velocity (seismic) event, particularity in healing capabilities. Our investigations indicate this is due to generation of “shocked” material that has undergone dehydration and dehydroxylation of hydrated minerals, amorphisation, and  grain comminution; all equating to a more reactive gouge. In natural post-seismic settings, this ‘shocked’ material sits in contact with pore-fluid that is at least partially externally derived. Due to the increased reactivity of this gouge, it is more prone to rapid post-seismic fluid-rock alteration, producing clay abundant retrograde authigenic minerals and reducing fault strength.

Using experiments to simulate this process, we show that synthetic post-seismic gouge exhibited increased fluid-rock interaction and enhanced precipitation of authigenic material relative to unsheared gouge. This was traced by analysing pore-fluid chemistry after prolonged contact with the gouge, close examination of the clay sized fraction using SEM techniques, and detailed XRD of shear inputs and outputs. Our work highlights the key role that co-seismic processes play in 1) the initial post-seismic change of frictional properties; 2) accelerated retrograde mineral evolution due to increased gouge reactivity; 3) and the associated reduction of fault strength and friction coefficient of fault core material post alteration.

How to cite: Robertson, B., Menzies, C., De Paola, N., Nielsen, S., Craw, D., Boulton, C., and Niemeijer, A.: Earthquake driven mechanical alteration of fault core material and its effect on post-seismic fluid-rock interaction, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6845, https://doi.org/10.5194/egusphere-egu25-6845, 2025.

Rock petrophysical properties, including porosity and permeability, are fundamental factors in regulating fluid ingress, flow and fluid-rock interaction across various length scales and tectonic settings. The composition of fluids, and their modes of ingress into- and reaction with host rocks, in turn influence bulk rock properties. Therefore, fluids can significantly alter rock rheology, potentially modifying their density, long-term viscosity, porosity and permeability. Mineralizing fluids may also change the rock composition, heal fractures, and promote strain hardening within active fault systems, potentially impacting the seismic cycle.

To better understand these processes and the governing background conditions, it is particularly useful to investigate the pathways for fluid flow, especially in rocks with low primary porosity and permeability, which would normally hinder significant fluid circulation. In micritic, horizontally-bedded carbonates, for example, vertical fluid flow is generally limited far away from tectonic fractures, whereas lateral, bed-parallel circulation is favoured exploiting laterally continuous planar anisotropies (e.g., bed-bed interfaces), to progressively permeate the succession. Secondary anisotropies, such as pressure-solution seams (e.g., stylolites), may represent other interesting features for fluid flow, being at times very abundant in limestone. However, they are typically considered to reduce the permeability and porosity of host rocks due to (i) the cementation and precipitation of dissolved materials in pores within the immediately surrounding rock and (ii) the accumulation of insoluble, fine-grained and low-permeability material on their surfaces. Recent studies in carbonates challenged this assumption, demonstrating that burial stylolites can be preferential pathways for karst dissolution.

We present here new data on the petrophysical properties of Cretaceous micritic limestone from the Lessini Mountains, Veneto, Italian Southern Alps, where much of the exposed, generally sub-horizontal Mesozoic carbonate succession underwent pervasive dolomitization during Eocene extensional tectonics and the onset of the Venetian Volcanic Province magmatism. Petrographic analyses, Hg-porosimetry, and He-pycnometry were applied to assess the effects of Mg-rich fluids, probably connected with the volcanic environment, on micritic limestone. Preliminary results indicate that burial stylolites developed in low porosity limestone are selectively dolomitized, with dolomitization seams 1-10 mm thick, suggesting pervasive bedding-parallel fluid flow. Dolomitization occurred also along vertical fractures and in fracture meshes of mosaic breccias, suggesting across-bedding Mg-rich fluid circulation associated with normal faults accommodating.

Dolomitization significantly increased rock density from ~2.65 g/cm³ in the pristine micritic limestone to ~2.9 g/cm³ in the fully dolomitized rock. Additionally, pore size, porosity, and the capillary threshold pressure of Hg injection change gradually but substantially from the limestone to the dolomite, with median pore sizes increasing from ~0.012 μm to ~0.33 μm, porosity from ~3.2% to 21.8%, and capillary threshold values decreasing from ~5140 to ~35 PSI.

These results demonstrate that, under specific conditions, stylolites can actually serve as effective pathways for fluid ingress/migration and thus promote fluid-rock interaction in rocks characterized by overall low porosity and permeability (e.g., micritic limestone). Furthermore, we show that dolomitization significantly modified the petrophysical rock properties, further enhancing fluid ingress and likely promoting fracturing and brecciation by changing rheology, with consequences for deformation localization and partitioning during later tectonic activity.

How to cite: Zuccari, C., Vignaroli, G., Balsamo, F., Berio, L., and Viola, G.: Burial stylolites favouring Mg-rich fluid ingress and fluid-rock interaction: petrophysical variations during regional dolomitization in the Lessini Mountains (Southern Alps, Italy) , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7082, https://doi.org/10.5194/egusphere-egu25-7082, 2025.

Mylonitic shear zones funnel significant amounts of fluids through the crust. However, the physical mechanisms and pathways for mass transfer remain debated. Grain boundaries, creep cavities, and pores formed by mineral reactions involving volume change and mass transport are considered the most important fluid conduits. So far, the imaging of these pores was either limited to µm-resolution, or in the case of nm-scale resolution, to very small areas (e.g., TEM investigates regions in the µm²-range) with limited statistical power. Moreover, many pores involved in ductile fluid flow only remain open intermittently before they are closed again by mineral growth and plastic deformation. To find traces of closed pores and assess pore production and consumption due to mineral reactions, high-resolution microchemical maps are needed.  

Here, we present a study that addresses these challenges through applying scanning small-angle X-ray scattering (SAXS) and X-ray Fluorescence Microscopy (XFM) to samples of a mylonite transitioning into ultramylonite, derived from a granitic protolith. SAXS delivers maps of nanopores with apertures between 1 and 280 nm while XFM enables spatially resolved mass balance computations, geochemical fluid fingerprinting, and the correlation of nanoporosity with mineral phase and trace-element composition. Importantly, both techniques are applied to an entire thin section, providing a sound observational statistical base from nm- to cm-scale.

Some key observations include:

1) A substantial amount of nanoporosity is discovered, with the same magnitude as microporosity measured previously in granitoid mylonites.

2) The ultramylonite contains twice as much nanoporosity as the mylonite.

3) Nanoporosity is strongly mineral-specific and highly elevated in regions enriched in epidote and mica.

4) Nanoporosity is highly anisotropic and usually aligned with, or at low angle to, the foliation, enabling mass flux along the shear zone.

5) Pore sheets from the mylonite connect with those of the ultramylonite, providing pathways for fluid exchange between high-strain shear zone and host rock.

These results highlight the importance of nanoscale fluid conduits and synkinematic mineral reactions for mass transfer in ductile shear zones. The implications for models of coupled fluid flow in the ductile crust will be discussed.

How to cite: Schrank, C., Bishop, N., Jones, M., Berger, A., Herwegh, M., Paterson, D., Manni, L. S., and Kirby, N.: Nanopores enable fluid flux in mylonites and ultramylonites – novel insights from Scanning Small-angle X-ray Scattering and X-ray Fluorescence Microscopy, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7433, https://doi.org/10.5194/egusphere-egu25-7433, 2025.

Petrological and geochemical evidence demonstrates that mylonitic ductile shear zones transport significant amounts of fluids through the lithosphere. Because of the lack of percolating pore networks in exhumed mylonitic rocks, it has been hypothesised that most of the fluid pathways in ductile shear zones consist of transient pores with micro- to nanoscale aperture. These transient pores typically include grain boundaries, creep cavities, and pores due to mineral reactions, all of which open and close cyclically during plastic deformation. However, it is not known how much each of these features contributes to crustal fluid flow. Moreover, nano-scale pores are notoriously difficult to map with non-destructive imaging methods. These are the key problems addressed by this research.

We recently used X-ray ptychography to image midcrustal quartzo-feldspathic mylonites at the X-ray Fluorescence Microprobe beamline of the Australian Synchrotron. This transmission small-angle scattering method maps X-ray phase contrast non-destructively with nanometre resolution. In addition, high-resolution trace-element maps are acquired coevally with X-ray Fluorescence Microscopy (XFM). We systematically sampled transects from mylonite to ultramylonite to capture the strain-time evolution of nano- and microvoids and to study how transient porosity and trace-element composition change with deformation intensity, composition, grain size, and deformation mechanism. This dataset will provide novel insights into mass transfer in ductile shear zones.

How to cite: Bishop, N., Schrank, C., Jones, M., Kewish, C., Paterson, D., Berger, A., and Herwegh, M.: Mapping nano- and microporosity in ductile shear zones with X-ray ptychography, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7480, https://doi.org/10.5194/egusphere-egu25-7480, 2025.

he interaction between a large, dissolved Mn reservoir in the oxygen minimum zone (OMZ) and the deeper oxygenated water allows for Mn oxidation and precipitation at their interface. The current paradigm posits that the OMZ acts as a Mn²⁺ source necessary for ferromanganese encrustation, while the encrustation itself is not thought to occur within the OMZ, though this remains a subject of ongoing debate. Marine Fe-Mn crusts enrich metals including those with high affinity for Mn oxides (e.g., Ce), which can provide insights into the origin of Mn oxides. In this study, we identify heterogeneous Ce and δ¹⁴²Ce_SW profiles in seawater and Fe-Mn crusts across the OMZ in the Northwest Pacific Ocean, closely linked to the cycle of Mn. Moreover, we quantify a close association of Ce with Mn oxides in Fe-Mn crusts and associated Ce isotope fractionation between the crusts and ambient seawater, bridging the marine Mn and Ce cycles. The outcome reveals that continuous precipitation of Mn oxides could initiate within the OMZ and extend into the deep ocean (5,000–6,000 m seawater depth) in the Northwest Pacific Ocean.

How to cite: Li, W. and the Wenshuai Li: Cerium Stable Isotopes Unveil Ferromanganese Encrustation Across the Oxygen Minimum Zone, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8324, https://doi.org/10.5194/egusphere-egu25-8324, 2025.

Sheet silicates play an important role in shaping crustal rheology and causing strain localization at shallow depths through their low strength. However, their effect on crustal rheology at deeper levels (>3 km) remains unclear. We conducted hydrothermal ring shear experiments on three simulated gouges with comparable quartz content but varying mica types (biotite/muscovite) and contents (8-61 wt.%). Applied temperatures (T) ranged from 20-650°C, with sliding velocities (V) between 0.03-1 μm/s, and an effective normal stress and pore water pressure of 100 MPa. Shear strains up to 30 were attained.

At 1 μm/s and 20°C, granitoid gouge exhibits a higher friction coefficient (μ=0.81) than the muscovite-rich (μ=0.47) and biotite-rich gouges (μ=0.44). With increasing T and decreasing V, granitoid gouge firstly remains its strength, and then exhibits substantial weakening when T reaches 450°C and V is lower than 1 μm/s. In contrast, muscovite-rich gouge hardens and then levels off at μ=0.68 as T reaches 450°C across all V tested, and finally weakens once T reaches 650°C and V is lower than 0.1 μm/s. Biotite-rich gouge hardens and reaches μ=0.56 at 450°C, with little further changes as T and V continue to change. Overall, the two mica-rich gouges become stronger than granitoid gouge at ≤ 0.01 μm/s and at least T=650°C.

For all post-mortem gouges, mainly samples with substantial weakening exhibit both principal slip zones constituting <6% width of the entire layer, and mineral reactions. Microstructures within the principal slip zones are consistent with dissolution-precipitation creep, including truncated grain contacts, mineral precipitates, submicrometer grain size and low porosity. Mineral reactions are often observed at 650°C and 0.1 μm/s under FEG-SEM, including Ca-rich feldspar rims of albite grains in granitoids, and muscovite breakdown plus biotite formation in muscovite-rich one. By fitting the shear strain rate to shear stress obtained from tests run at 650°C, the apparent stress exponent for granitoids is 2.2 ± 1.8, and for muscovite-rich gouge is 6.8 ± 2.2. Our results imply that mica enrichment in crustal faults (mainly granitoid composition) can lead to a stronger crust at deep levels when temperatures are high and strain rates are low. Multiple similarities between experimental and natural microstructures suggest that the interpreted mechanisms dissolution-precipitation creep and mineral reactions may trigger a frictional-viscous transition at a depth range corresponding to greenschist metamorphic facies under natural conditions. 

How to cite: Zhan, W., Niemeijer, A., Berger, A., Spiers, C., Gfeller, F., and Herwegh, M.: The Effect of Micas on the Strength of Experimental Granitoid Fault Gouge, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8523, https://doi.org/10.5194/egusphere-egu25-8523, 2025.

The 2018 Hualien earthquake (Mw 6.4) resulted in the Milun fault rupture and caused hundreds of casualties. The last rupture of the Milun Fault occurred in 1951, implying a short recurrence interval for the Milun Fault. The outcropped Milun Fault has not been recognized in the field, and its fault architecture and the relevant processes triggered during the seismic cycle remain unknown. This represents a significant limitation in our understanding of fault mechanics and seismic hazard assessment.

The Milun Fault Drilling and All-inclusive Sensing project (MiDAS) was conducted in 2020, and the drilling borehole cores showed the presence of the Milun Fault. The Milun Fault Zone exhibits an asymmetric fault structure, displaying altered spotted schist and non-cohesive serpentinite as the damage zone, and foliated grey and black gouges as the fault core. The damage zone of the Milun Fault has been described as a product of fluid-rock interaction, although direct evidence remains limited.

Here, we conduct synchrotron X-ray diffraction (XRD) on altered spotted schist and non-cohesive serpentinite to investigate fluid-rock interaction during the inter-seismic period. Previous data on the outcropping spotted schist showed that the mineral assemblages are mainly composed of muscovite, feldspar, and quartz. For the outcropping cohesive serpentinite, the major minerals are antigorite and magnetite. Our XRD data show that the altered spotted schist mainly contains quartz, feldspar, and clay minerals such as illite, chlorite, and kaolinite. Non-cohesive serpentinite is composed of chrysotile, talc, chlorite, and actinolite. The altered spotted schist exhibits an anastomosing occurrence, suggesting the presence of fluid-relevant interaction along the fractures and resulting in the observed clay-rich mineral assemblages. The non-cohesive serpentinite shows the reactions of antigorite to chrysotile with some residual antigorite fragments, suggesting the process of severe alteration by low temperature (< 200°C) fluid. To further explore fluid-rock interaction processes, we will conduct X-ray Fluorescence (XRF) analysis to detect changes in chemical elements within the Milun Fault zone in this month. Our findings will help identify the source and composition of the fluids involved and provide insights into the structure and evolutionary history of the Milun Fault.

How to cite: Chiang, P.-C., Kuo, L.-W., Ma, K.-F., and Ling, Y. Y.: Fluid-rock interaction within the active Milun Fault: In the case of the Milun Fault Drilling and All-inclusive Sensing project (MiDAS), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9249, https://doi.org/10.5194/egusphere-egu25-9249, 2025.

The emplacement of basic to acid magmas into ultramafic rocks represents a specialized scenario where bimetasomatic exchanges commonly occur. In the Sierra Chica of Córdoba, near the city of Alta Gracia, remmants of the intensely serpentinized upper mantle -tectonically emplaced into the metasedimentary sequence during the Neoproterozoic (pre-Pampean)- are intruded by dykes of tonalitic to granitic compositions. This contribution deals with some of these dykes of trondhjemitic composition that exhibit tabular shapes and relatively small dimensions (< 50 m long and 2 m thick). They develop penetrative reaction zones symmetrically at the contacts between serpentinized ultramafic rocks and trondhjemitic dykes, with regular thicknesses between 0.10 and 1.0 meter. Three distint mineralogical zones were identified, progressing from the dykes towards the serpentinite:

1.- Magnesio-hornblende zone (Mg-Hbl ± Pl ± Ttn ± Chl),

2.- Chlorite zone (Chl ± Mg-Hbl ± Zrn ± Ap)

3.- Anthophyllite zone (Ath ± Tr ± Chr ± Tlc ± Chl).

According to textural and mineralogical evidence, the following sequence of formation is proposed: 1) Ath zone, 2) Mg-Hbl zone and 3) Chl-zone.

Zircon U-Pb dating of the trondhjemitic dykes yields a concordia age of 528 Ma for the crystallization and emplacement of these rocks. Zircons extracted from the Chl-zone share similar petrographical features and ages, evidencing that they represent xenocrysts derived from the trondhjemitic magmas later affected by chlorite-forming reactions. The age of 528 Ma place the emplacement of the trondhjemitic dykes during the high-grade metamorphism, anatexis and magmatic events of the Pampean orogeny, occurred at 0.65 - 0.85 GPa and 700º- 850ºC. Anthophyllite, typically formed at relatively high temperature (>600 °C), is interpreted to have formed close to the time of trondhjemitic dyke emplacement and crystallization, when the whole enclosing region was undergoing high amphibolite-to-granulite facies at the main events of the Pampean orogeny. The conspicuous presence of chlorite (clinochlore) in all zones crosscutting the formerly crystallized minerals (e.g., Ath and Mg-Hbl), temperature ranges of 273º - 418ºC estimated by Chl geothermometers, along with pervasive low temperature deformation textures (kink folding) evidence that Chl was formed at lower P-T conditions.

Field and petrographic evidence, geochemical and geochronological data obtained in this work indicate that the reaction zones were formed in two different periods. Anthophyllite and Mg-Hbl zones resulted from the bimetasomatic ionic diffusion between the ultramafic rock and trondhjemitic dykes during the Pampean orogeny at high P-T conditions. Conversely, Chl crystallization should have occurred at lower P-T conditions during the Famatinian orogeny that took place from ~500-440 Ma in the Sierras de Córdoba mainly as deformational events. The chlorite zone is therefore proposed to have formed as a result of fluid infiltration in the already altered contact zones between the ultramafic rocks and the trondhjemitic dykes.

How to cite: Muratori, M. E., López Morlhiere, S., Demartis, M., Coniglio, J. E., Boffadossi, M. A., D’Eramo, F. J., Pinotti, L. P., Coniglio, J., and Esteban, J. J.: Bimetasomatic reaction zones between ultramafic rocks and trondhjemitic dykes in the Sierra Chica, Córdoba, Argentina: temporal variations and geochemical features, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10967, https://doi.org/10.5194/egusphere-egu25-10967, 2025.

The mechanical and hydraulic behavior of faults in geothermal systems is strongly impacted by fluid-induced alteration. For instance, hydrolytic alteration of felsic volcanic rocks deeply affects the frictional and permeability properties of fault rocks, controlling the hydraulic behavior of faults. We investigated fault rocks from the caprock of a fossil hydrothermal system in the Northern Apennines, by combining field structural observations with mineralogical and microstructural analyses, friction experiments and permeability tests on fault rocks. Hydrolytic alteration promoted general weakening of fault rocks by enrichment of kaolinite-alunite-group minerals in the fault core, favoring strain localization. Enrichment of kaolinite along major faults induces a local decrease in permeability of three orders of magnitude (1.62x10^-19 m²) with respect to the unaltered protolith rocks (1.96x10^-16 m²) transforming faults from fluid conduits into barriers.

Alunite-kaolinite-rich rocks shows a velocity-strengthening frictional behavior, suggesting that hydrolytic alteration favors stable slip of faults at low temperatures (160-270°C). 

How to cite: Marchesini, B., Pozzi, G., Collettini, C., Carminati, E., and Tesei, T.: Structurally controlled genesis of caprock in volcanic hydrothermal systems, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11033, https://doi.org/10.5194/egusphere-egu25-11033, 2025.

In the last decades many studies focussed on carbon capture and storage (CCS) to find a possible remedy to reduce the large increase of anthropogenic carbon dioxide (CO₂). CCS can potentially sequester billions of tonnes of CO₂ per year using the Earth as the widest laboratory available for long-term storage. In geological reservoirs, CO₂ can be trapped by physical and chemical mechanisms. Among chemical mechanisms, mineral carbonation plays a crucial role in CCS, being almost irreversible, involving the chemical reaction in aqueous environment between CO₂ and Mg- and/or Ca-rich minerals, where CO₂ is converted into solid carbonates.

In nature, listvenite, a rock mainly composed of Mg-Ca-bearing carbonates, quartz and Cr-bearing mica (fuchsite), documents natural CO₂ sequestration. Indeed, listvenites are the result of the extensive alteration of ultramafic rocks by CO₂-bearing fluids, which involved the substitution of olivine, pyroxene and serpentine by Ca- and Mg-carbonates. To date, very little is known about the kinetics and rate of this reaction, spanning from weeks (serpentinites) to thousands of years (peridotites).

We studied carbonated serpentinites from the Zermatt-Saas Zone (Corno del Camoscio, Western Alps, Italy) which underwent fluid-mediated natural carbonation under hydrothermal conditions. Hydrothermal carbonation is spatially associated to Oligo-Miocenic brittle faults of the Aosta-Ranzola system (Bistacchi et al., 2001). Field structural surveys identified two main strike-slip fault sets (N-S and NW-SE striking) controlling fluid flow, with voluminous carbonation observed mainly along the NW-SE-striking set. We collected a series of structurally-controlled samples along a reaction front from serpentinite to listvenite close to a major fault zone, aiming to relate the CO₂-rich fluid/rock interaction with mega and meso-structures, along with detailed microstructural and chemical analyses.

The petrographic study, along with X-ray maps and microprobe chemical analyses, identify the following mineral associations, from serpentinite to listvenite: (i) serpentine + chlorite and minor quartz + fuchsite, talc, calcite and dolomite, (ii) serpentine + brucite + chlorite and minor quartz, talc, calcite and dolomite-siderite, (iii) dolomite, quartz, chlorite, serpentine and minor fuchsite associated with quartz-chlorite layers, (iv) quartz, dolomite and fuchsite with relict brucite. Interestingly, samples collected close to the serpentinite show microfolds where dolomite is stable, subsequently cut by brittle deformation related to the large-scale faults, suggesting a previous stage of fluid-mediated carbonation under a ductile deformation regime.

Qualitative and quantitative X-ray powder diffraction data enabled us to calculate a mass balance to model the rate of reaction and the composition of the original fluids. Preliminary results indicate a structural control on the fluid drainage and the role of brucite to dominate the carbonation reaction, as reported by experimental results of Campione et al. (2024), along with fuchsite.

Bistacchi, A., Dal Piaz, G., Massironi, M., Zattin, M., Balestrieri, M. (2001). The Aosta–Ranzola extensional fault system and Oligocene–Present evolution of the Austroalpine–Penninic wedge in the northwestern Alps. International Journal of Earth Sciences, 90, 654-667

Campione, M., Corti, M., D’Alessio, D., Capitani, G., Lucotti, A. Yivlialin, R., Tommasini. M., Bussetti, G., Malaspina, N. (2024). Microwave-driven carbonation of brucite. Journal of CO₂ Utilization, 80, 102700

How to cite: Pasquinucci, A. M. B., Malaspina, N., Ceccato, A., Giuntoli, F., D'Alessio, D., Campione, M., and Dal Piaz, G. V.: A natural laboratory for carbon capture and storage: listvenites along regional fault zones (Zermatt Saas Unit, Western Alps, Italy), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11801, https://doi.org/10.5194/egusphere-egu25-11801, 2025.

Stress gradients and non-hydrostatic stresses are to be expected in rocks in the lithosphere, even in the presence of fluids. This complexity challenges the reliability of existing hydrostatic thermodynamic models, and, currently, in the geological lterature there is still no accepted theory for evaluating the thermodynamic effect of non-hydrostatic stress on reactions [e.g. 1, 2].

Large-scale Molecular Dynamics (MD) simulations (i.e., >2e6 atoms) give us the opportunity to investigate reactions in deforming systems by directly bridging the scale between atomic-level processes and continuum deformation. With MD, the a-priori assumption of a specific thermodynamic potential is not required, which makes it a robust approach to test existing thermodynamic theories [3]. With MD simulations the energy of the system, the pressure of the fluid, the stress of the solid, as well as the overall dissolution and precipitation process can be monitored over time until the stressed system attains equilibrium conditions.

Our findings indicate that a solid under non-hydrostatic stress can be equilibrated with its pure fluid. However, for deformations at constant temperature, the non-hydrostatic equilibrium differs from the hydrostatic equilibrium in that the pressure of the fluid must increase to maintain equilibrium with the solid. At low differential stresses, such pressure deviations from the reference hydrostatic equilibrium are small, allowing phase equilibria predictions by considering the fluid pressure as a proxy for equilibration pressure, as suggested by previous experimental investigations.

In the presence of substantial non-hydrostatic stresses, the stressed system becomes unstable, leading ultimately to the precipitation of a quasi-hydrostatically stressed crystalline film on the surfaces of the initial highly stressed crystal. During crystallization, the total stress balance is preserved until the newly formed solid-film-fluid system reaches again a stable equilibrium. At the final equilibrium conditions only the low-stressed solid film is exposed to the fluid, bringing back the equilibrium fluid pressure close to the value expected for the equilibrium at homogeneous hydrostatic conditions. While our results agree qualitatively and quantitatively with previous theories of thermodynamics in deformed systems [4,5] and with experiments [6,7], they cannot be predicted by theories proposed to interpret reactions in deformed geological systems [e.g., 2,8].

References

1) Hobbs, B. E., & Ord, A. (2016). Earth-Science Reviews, 163, 190–233.

2) Wheeler, J. (2020). Contributions to Mineralogy and Petrology, 175(12), 116.

3) Mazzucchelli, M. L., Moulas, E., Kaus, B. J. P., & Speck, T. (2024). American Journal of Science, 324.

4) Gibbs, J. W. (1876). Transactions of the Connecticut Academy of Arts and Sciences, 3, 108–248.

5) Frolov, T., & Mishin, Y. (2010). Physical Review B, 82(17), 1–14.

6) Berréhar, J., Caroli, C., Lapersonne-Meyer, C., & Schott, M. (1992). Physical Review B, 46(20), 13487–13495.

7) Koehn, D., Dysthe, D. K., & Jamtveit, B. (2004). Geochimica et Cosmochimica Acta, 68(16), 3317–3325.

8) Paterson, M. S. (1973). Reviews of Geophysics, 11(2).

How to cite: Mazzucchelli, M. L., Moulas, E., Schmalholz, S. M., Kaus, B., and Speck, T.: Instability and equilibration of fluid-mineral systems under stress investigated through molecular dynamics, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12877, https://doi.org/10.5194/egusphere-egu25-12877, 2025.

Silicate melts exist as lava flows which form when molten or partially molten magma erupts, and when they cool, magmas evolve into solid rocks. Depending on the cooling rate, they can evolve into fully crystalline rocks, partially-crystallized rocks or even glassy rocks. The composition of the glass at the cooling interface with air or water may be different than in the bulk. Here we study how some major elements could be concentrated or depleted at the surface of a cooling basaltic melt. This may have effects on how glass will interact with water at the onset of weathering. To model silicate melts, we have trained a machine-learned interatomic potential for basaltic glass, which we use to run molecular dynamics simulations of molten basalt and a basalt surface at temperatures consistent with fresh deposits of basalt during eruption. We have studied the difference between bulk molten basalt and a free surface of molten basalt by comparing the diffusion coefficient, lifetime of species and the spatial distribution of atoms between the two domains. We show that the diffusion at the surface is higher than in the bulk, indicating a higher rearrangement of the surfaces compared to the bulk, and the coordination numbers are generally lower at the surface than in the bulk. When studying the composition of a surface and bulk, our results show that most of the cations on the surface are iron, magnesium and calcium, i.e. the cations that can react with CO2 to precipitate as carbonate minerals. These simulations are relevant for the initial weathering of silicate melt, and knowledge of the composition of the surface are relevant for the potential reactions with CO2 and carbon mineralization.

How to cite: Guren, M. G., Sveinsson, H. A., Caracas, R., Malthe-Sørenssen, A., and Renard, F.: Atomic structure at the surface of warm basaltic glasses, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12944, https://doi.org/10.5194/egusphere-egu25-12944, 2025.

Collisional orogens are characterized by complex contractional and extensional deformation, which significantly impact fluid migration and mineralization. These processes are crucial for understanding subsurface fluid flow dynamics, with implications for geothermal energy, hydrogen production, and CO2 storage. The Dinarides orogen in southeastern Europe, formed during the closure of the Neotethys Ocean and subsequent Adria-Europe collision, provides an excellent natural laboratory to investigate fluid-flow and fluid-rock interactions driven by orogenic deformation.

The Dinarides have experienced a sequence of tectonic events in their late evolution, including NE-EW Late Cretaceous-Eocene contraction, NE-SW Oligocene contraction, and bimodal NE-SW/NW-SE Miocene extension. These phases created fracture networks, tension gashes, and fault gouges, facilitating fluid migration and mineral precipitation within the orogen. Field investigations across five tectonic units in Montenegro (Dalmatian, Budva, High Karst, Pre Karst, and East Bosnian-Durmitor) documented structural features associated with these deformation phases.

Petrographic and geochemical analyses of vein-filling cements, including optical microscopy, cathodoluminescence, and stable isotope measurements, reveal that vein formation predominantly occurred under burial conditions with episodic transitions to meteoric environments. These results suggest that deformation-controlled fracture network acted as fluid pathways, driving localized dolomitization and calcite precipitation. The structural timing of these features correlates with major orogenic events, providing insights into the relationship between deformation and fluid flow.

Our findings contribute to understanding how fluid migration is driven by tectonic deformation in collisional orogens. By integrating field observations with petrographic and geochemical data, this study offers a framework for linking mineralization processes to tectonic evolution, with broader implications for fluid flow modelling in similar orogenic systems worldwide.

How to cite: Maleš, M., Nader, F. H., Stojadinović, U., Matenco, L., Krstekanić, N., Randjelovic, N., and Divies, R.: Exploring deformation-driven fluid-flow and fluid-rock interactions: insights from the Dinarides orogen, southeastern Europe, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12965, https://doi.org/10.5194/egusphere-egu25-12965, 2025.

Understanding the interaction of stress waves with fluid-filled rock joints is crucial for seismic hazard assessment, hydrocarbon extraction, geological CO₂ storage, geothermal energy exploration, and wastewater disposal. This study investigates dynamic mechanical behaviors (including elastic modulus and initial joint stiffness) and wave propagation characteristics (i.e., transmission and reflection coefficients, energy attenuation) of single fluid-filled rock joints under the normal incidence of high-intensity stress waves, with a focus on the effects of liquid content and viscosity.  Dynamic compression tests were conducted using the split Hopkinson pressure bar (SHPB) technique combined with high-speed photography on rock joints with varying liquid content and viscosity. The results demonstrate that higher liquid content and viscosity increase the dynamic elastic modulus and initial joint stiffness of the joints. Increasing joint stiffness leads to an increase in wave transmission but a decrease in wave reflection. Besides, the increasing liquid viscosity reduces both wave transmission and reflection but enhances wave attenuation by individual fluid-filled rock joints. High-speed imaging revealed a transition from turbulent to laminar jet behavior with increasing liquid viscosity. These findings advance the understanding of fluid-rock interaction under dynamic conditions, offering valuable insights for theoretical development and practical applications in geophysical and geomechanical engineering.

How to cite: Yang, H., Duan, H., and Zhu, J.: The role of fluid viscosity in the interaction between individual fluid-filled rock joints and high-intensity stress waves , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15332, https://doi.org/10.5194/egusphere-egu25-15332, 2025.

During the interseismic phase, faults regain frictional strength through a process commonly referred to as fault healing. Key mechanisms include contact welding by dissolution-precipitation creep and cementation by mineral precipitation in fluid-rich environments. While much research has focused on experimental investigations of silicate systems, e.g. in slide-hold-slide experiments, the complex interaction between mechanical and chemical processes, as well as recurring fault healing over multiple earthquake cycles remain understudied. Particularly in the case of natural fault systems, the database is scarce as processes of interest occur at depth and show a low preservation potential during exhumation.

Here, we present a combination of microstructural and microchemical observations from a carbonate-hosted fault zone located within the Helvetic nappe stack of the south-western Swiss Alps which was recently exposed due to glacial retreat, creating excellent outcrop conditions. The microstructural record allows us to distinguish three major healing episodes within the principal slip zone. These episodes follow brittle deformation at sub-seismic to seismic rates, forming veins along sets of discrete fault-parallel fractures. Due to continuous brittle deformation, individual veins experienced subsequent mechanical overprinting which led to the modification of the vein texture, an increase in the local porosity and the formation of new fluid-rock interaction faces. Additionally, we use the unique geochemical fingerprint of each set of veins, documented and analysed by high-resolution X-ray fluorescence mapping of trace elements, to differentiate and characterize individual fluid pulses and dynamic changes in the physio-chemical conditions of the fluid-rock system over time. While we interpret the principal slip zone to represent the youngest deformation event in our study, adjacent vein-derived domains that are deformed by aseismic viscous processes represent relatively older structures. Comparison of the isotopic composition of newly formed calcite crystals with relict grains and the country rock provides insight into possible isotope fluid-rock equilibria during tectonic processes and therefore fluid sources. Measured stable oxygen isotopes (δ¹⁸O) show a significant influence of meteoric water while clumped isotope thermometry indicates temperatures of 65-120°C, which are at least 100°C lower than in the country rock and literature values of T_max in the area.

Our results suggest that the observed microstructural record is representative of seismic deformation and associated fault healing caused by low-magnitude earthquakes at shallow crustal levels near the upper limit of the seismogenic zone. This interpretation is consistent with the depth distribution of current hypocenters within a seismically active structure that is located in the vicinity of our study area, the so-called Rawil Fault Zone. We, therefore, conclude that the processes identified in the exhumed tectonite samples can serve as proxies for active deformation and fluid flow at depth. In a wider context, this may offer valuable insights for geothermal exploration in southwest Switzerland.

How to cite: Schwichtenberg, B., Berger, A., Herwegh, M., Schrank, C., Jones, M. W., Bernasconi, S. M., Fleitmann, D., Kewish, C. M., and Neuenschwander, T.: Unravelling cyclic fault healing in carbonates: A natural example for the interaction of mechanical and fluid-mediated processes, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15657, https://doi.org/10.5194/egusphere-egu25-15657, 2025.

The dehydration of rocks, such as gypsum, is a critical process influencing plate tectonics and fault zone dynamics. Gypsum dehydration, serving as a model for serpentine dehydration, involves complex hydraulic, mechanical, and chemical (HMC) interactions that remain poorly understood under shear stress. Our study investigates how dehydration reactions and microstructural developments relate to macro-scale frictional responses, providing new insights into the conditions leading to mechanical instabilities and shear localisation.

To study the couplings between shear stress, strain and the microstructures formed during the dehydration of gypsum, we have conducted a series of direct shear experiments on Volterra Alabaster slabs and 99% pure gypsum powder. We performed the tests in a new direct shear setup of the x-ray transparent Heitt Mjölnir Cell (Freitas et al. 2024) at the ID19 beamline at the European Synchrotron Radiation Facility (ESRF, Grenoble, France). This new setup allows fast 4D microtomography (4DμCT) to record the time evolution of the microstructure. In several 4D operando experiments, the samples were loaded with 10 - 25 MPa confining pressure and 2 MPa fluid pressure while allowing initial thermal equilibration of the system at 60 °C. Following equilibration, temperature was increased to 115 - 125 °C to start the dehydration of the gypsum. Simultaneously, a constant axial displacement rate of 0.2 - 0.3 µm/s was applied, which produced shear strain within the sample. Pore pressure oscillations were applied to monitor changes in hydraulic permeability across the samples.

The 4DµCT datasets allow good discretization of the three phases of interest (gypsum, hemihydrate and pore space) on the relevant microscale. Our ongoing analyses of the various 4DµCT datasets focus on 1) digital volume correlation (DVC) to quantify the deformation in the sample on the grain scale, 2) the calculation of reaction rates for dehydration and 3) the quantification of grain-scale permeability during shearing and reaction. Initial analyses show well-resolved shear structures forming throughout the different tests (e.g., boundary shears, compaction bands, Riedel-shears). By quantifying the reactions and the deformation over time, we identify the minor and major processes controlling the development of the microstructure. These processes are then related to changes in friction and transport parameters.  In future experiments, we will focus on different lithologies to further understand the effects of fault gouge composition and grain geometry as well as the analysis of rate-and-state friction (RSF) for the quantification of sliding stability. Our data demonstrate the potential that 4D operando direct shear experiments hold for the study of friction processes in fault zones.

How to cite: Harpers, N., Ng, A., Gilgannon, J., Freitas, D., Eberhardt, L., Rizzo, R., Cordonnier, B., Butler, I., and Fusseis, F.: Faults inside out: 4DμCT on direct shear dehydrating gypsum experiments, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17178, https://doi.org/10.5194/egusphere-egu25-17178, 2025.

Reconstructing the tectonic and geomorphological history of geological terranes poses significant challenges, particularly when interpreting deeply buried and exhumed settings. The Rolvsnes granodiorite terrane exposed in the Bømlo archipelago on Norway’s southwestern coast, provide invaluable insights into these processes. This granodioritic to granitic intrusive body serves as an onshore counterpart to offshore basement reservoirs, such as those on the Utsira basement high, where altered basement rocks are overlain by Permian to Cretaceous sediments. These analogs enable researchers to link surface observations to subsurface conditions, offering a rare opportunity to understand complex fracture and alteration histories.

We provide new evidence from multidisciplinary investigations constraining the fracturing and alteration history of the Rolvsnes granodiorite. Multiple chemically altered bedrock outcrops associated with fracture zones were identified across Bømlo, with samples collected for geochemical, mineralogical, and isotopic analyses. We characterize secondary clay assemblages and constrain the timing of alteration processes. Additionally, three bedrock cores drilled through prominent fracture zones were logged and sampled to enhance the dataset with subsurface information.

K-Ar geochronology dates range from the Carboniferous to the Paleogene, suggesting multiple alteration events over extended periods. Geochemical and mineralogical data indicate significant leaching of alkali and alkaline-earth elements, with the formation of kaolinite, smectite, interstratified illite-smectite, illite, and lepidocrocite in the altered material. Scanning electron microscopy reveals small but significant differences between alteration zones. In some zones, K-feldspar is altered into a mixture of kaolinite, smectite, and illite, while plagioclase (particularly Na-rich laminae) and biotite remain relatively unaffected. In other zones, biotite transforms into vermiculite, illite, and iron (hydro-)oxides, while plagioclase alters into smectite and kaolinite, leaving K-feldspar relatively intact. These findings suggest alteration predominantly by low-temperature meteoric water. Many samples did not yield Kübler Index determinations due to poorly defined 10 Å peaks, but acquired illite-crystallinity data indicate fluid temperatures ranging from 120 to 295 °C. Thus, K-Ar ages should be interpreted cautiously, as multiple alteration events may occur along the same fracture zone at different times.

The data suggests that faulting and hydrothermal alteration initiated as early as the Carboniferous, continuing through the Permian. Late Triassic brittle faulting may have coincided with supergene weathering under humid, tropical conditions, resulting in saprolitic weathering of the crystalline basement along pre-existing fractures. During subsequent marine transgressions, most saprolitic material was eroded, leaving remnants buried beneath sedimentary cover, which was in turn largely removed during the Plio-Pleistocene. The Rolvsnes granodiorite appears to have experienced additional fracturing and alteration events beneath this sedimentary cover, as indicated by K-Ar dates extending into the Early Cretaceous and Paleogene. 

This study highlights the inherent difficulties of reconstructing complex tectonic and geomorphological histories in such terranes. The Bømlo archipelago offers a compelling case study for linking onshore observations to offshore settings, but challenges remain in disentangling overlapping alteration processes and correlating them to specific tectonic events.

How to cite: Margreth, A., Drivenes, K., Schönenberger, J., van der Lelij, R., Fredin, O., and Knies, J.: The complex and prolonged fracturing and chemical alteration history of the Rolvsnes granodiorite on the Bømlo archipelago in southwestern Norway, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18686, https://doi.org/10.5194/egusphere-egu25-18686, 2025.

Shallow crustal fault zones, particularly those within carbonate successions, are deformation zones that influence subsurface fluid migration, localization, and interaction. These zones, characterized by fault-controlled fracturing and brecciation, can act as important conduits for fluid flow in the Earth's crust. The interaction between fluids and the fractured carbonates rock matrix can induce significant changes in the mineralogical and mechanical properties of fault zones. Therefore, studying fluid-rock interactions in such environments is crucial not only for understanding the natural processes governing subsurface fluid dynamics, deformation, and metamorphism but also for addressing significant challenges in energy and mineral exploration as well as in underground engineering.

This study presents, for the first time in the southern Apennines (southern Italy), evidence of fault-driven hydrothermal dolomitization during the late Triassic rifting event in the western Adria plate. We investigated the fault-controlled saddle dolomite formation in Norian dolomites exposed in the western sector of the Matese Massif. The study focused on dolomite breccias associated with N-S and NNW-SSE striking normal faults. These structures include layers of mature cataclasites made of clasts with angular boundaries within a highly porous matrix, crossed by veins, mosaic and chaotic breccias. The breccias are composed of angular clasts of host rock dolomite, formed by early marine replacive dolomitization of shallow-water carbonates, surrounded by coarse saddle dolomite cement. The saddle dolomite cement is characterized by two distinct phases. The first phase (SD1) is yellow, inclusion-rich, and forms a rim around the clasts, while the second phase (SD2) is euhedral, exhibiting well-defined zoning with a transition from cloudy to limpid crystals. The saddle dolomite cement texture, coupled with decreasing δ¹⁸O and ⁸⁷Sr/⁸⁶Sr values, suggests that it precipitated at temperatures of 100-120°C from fluids that likely interacted with magmatic sources.

U-Pb dating of the dolomite cement provides late Triassic crystallization ages of approximately 206 ± 13 Ma and 217.0 ± 6.6 Ma. Additionally, the ferroan dolomite cement contains quartz and hydrothermal minerals, including fluorite and apatite, in minor quantities. These findings suggest that the brecciation and hydrothermal saddle dolomite precipitation were linked to normal fault activity during the breakup of Pangea, contributing to the separation of the SW sector of Eurasia from the western margin of the Adria Plate.

How to cite: Awais, M., Diamanti, R., Camanni, G., D'antonio, M., Della porta, G., Di renzo, V., Graziano, S. F., Iannace, A., and Kylander-clark, A.: Fault-induced saddle dolomitization during the Late Triassic rifting of Pangea in the southern Adria region (Southern Italy), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19182, https://doi.org/10.5194/egusphere-egu25-19182, 2025.

The Coulomb stress transfer analysis is based on the hypothesis that failure on a fault plane occurs when the Coulomb stress exceeds a certain threshold. Positive Coulomb stress changes are thought to bring faults closer to failure, whereas negative ones inhibit failure. The Colca Region in Central Andes, southern Peru, is prone to small- to moderate-sized (Mw ≤ 6.0) shallow (< 20 km) earthquakes associated with normal and strike-slip crustal faults within the overriding plate in the Nazca-South American subduction zone. Along with the activity of the Sabancaya volcano, which in recent years comprised mainly of ash and fumarole emissions, this region offers an opportunity to investigate the complex relationship between seismic and volcanic activity, their potential interplay, and triggering factors.

To explore these dynamics, we carried out a Coulomb stress transfer analysis examining the interactions between source faults of 28 significant recent earthquakes (1991-2022), as well as the impact of magmatic inflation (2013-2022) on seismic events. The results confirm a tectonic origin for most earthquakes, while the magmatic source appears to play a secondary role, primarily amplifying the effects of prior seismic activity. However, the Coulomb stress transfer does not seem to be the main factor impacting the seismicity of the Colca Region, as most of the analyzed source faults were not brought closer to failure due to a positive stress change. Preceding seismic activity induced positive Coulomb stress changes on source faults in 43% of the events, while negative stress changes potentially inhibited 25%. Magmatic inflation contributed to positive stress changes in 22% of cases but also induced negative changes in a similar proportion. Notably, no direct connection was identified regarding the significant increase in seismic activity in 2013, which appeared to be potentially correlated with the start of the fumarolic emissions (late 2012) by the Sabancaya volcano.

While the coseismic static Coulomb stress change does not fully account for the complexity of seismic and volcanic activity and their interplay in the Colca Region, it provides valuable insights into active geological processes and highlights open questions warranting further investigation.

This research was funded by the National Science Centre (Poland), Grant Number 2020/39/B/ST10/00042.

How to cite: Woszczycka, M., Gaidzik, K., Anccasi, R., Mendecki, M., and Benavente, C.: The Coulomb stress transfer and possible interactions between seismic and volcanic activity in the Colca Region (Central Andes), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-368, https://doi.org/10.5194/egusphere-egu25-368, 2025.

Paleostress reconstruction in fractured rocks is generally conducted through fault-slip inversion. Estimating spatio-temporal variation of paleostress directions from deformed rocks with prolonged deformation history is extremely challenging due to heterogeneity, non-coaxiality of deformation, ascertaining relative timing of formation of different fracture sets, genetic association with the larger structure, and probable reactivation among other factors.  Therefore, fault-slip-based kinematic studies from roof thrusts of duplexes are generally less common. In this context, we attempt to deduce the fault kinematics from the leading-edge exposure of the Ramgarh thrust (RT) sheet in Darjeeling-Himalaya, which hosts a thrust-related antiform. The RT also acts as the roof thrust of the Lesser Himalayan duplex (Bhattacharyya et al., 2015). We studied the slickenline data from near the footwall contact of the RT zone (part of the overturned forelimb of the antiform) up to ~3.4km into the RT sheet (backlimb). As part of an ongoing study (Ammu and Bhattacharyya, in revision), we have deciphered a first-order relative timing among the fracture sets along with fold-fracture relative timing using various factors, for example, spatial variation of shear fracture-bedding angles, offset, relative abundance at different locations, fold test, correlation between the dihedral angles of conjugate faults and depth. We used the PBT method (Huang and Charlesworth, 1989; Sperner et al., 1993) to invert the fault-slips and reconstruct the stress regimes. This multi-proxy workflow can be used to systematically reconstruct the fault kinematics from structurally complex settings.

The major fault, RT, is oriented ~72°, 304°, along a cross-section with a bearing of ~130-310° and has a top-to-the-south vergence. The shear fractures (n=208) record normal (~61%), inverse (~39%), sinistral (~54%), dextral (~46%), oblique- (~74%), dip- (~18%) and strike- (~8%) slip movement. Although the RT sheet records a heterogeneous fault-slip population, proximal to the RT zone, the shear fractures show dominantly inverse (~58%) and sinistral (~67%) sense, i.e., a similar sense of slip as that of the major fault. We divided the heterogeneous fault-slip data into fourteen homogeneous subsets, which were categorized into pre-, syn-, and post-folding stages. The RT sheet records eight post-folding fault-slip subsets with ~NNE-SSW compression, ~NW-SE, and ~NE-SW extension, and strike-slip regimes with ~NE-SW, ~E-W, and ~NW-SE compression. The post-folding ~NNE-SSW compressional regime conforms to the present-day orientation of the regional tectonic stress field. Most stress regimes exhibit an anticlockwise rotation of stress fields proximal to the fault as compared to the interior of the RT sheet. The rotation of stress fields is observed across the pre-, syn-, and post-folding stages. Thus, the RT caused stress perturbations and influenced fracture kinematics within the RT sheet across space and time.

How to cite: Jayalakshmi Krishnankutty, A. and Bhattacharyya, K.: Decoding fault kinematics from the roof thrust of the Lesser Himalayan duplex: Insights from paleostress reconstruction, Ramgarh thrust, Darjeeling-Himalaya, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-791, https://doi.org/10.5194/egusphere-egu25-791, 2025.

Reconstructing the spatiotemporal evolution of fault systems in rift basins is essential for characterizing reservoirs used in geothermal exploration and CO₂ or hydrogen storage projects. This study aims to elucidate the growth and reactivation of inverted normal faults in the West Netherlands Basin and their implications for subsurface renewable energy projects. With a complex tectonic history, the area experienced multiple rifting phases and basin inversion. Fault displacement-distance diagrams were produced by using an updated semi-automated workflow for nine major basin-scale faults, providing new insights into the lateral and vertical growth of faults in inverted rift basins.

This study demonstrates that the faults in the West Netherlands Basin developed their lateral lengths during the early stages of Triassic rifting. Subsequent Jurassic extensional phases caused reactivation, leading to a consequent increase of vertical displacement and creating accommodation space for the deposition of the study area’s main reservoir rock. Variations in the reactivation behaviour along the different fault segments were promoted by stress field rotations, which significantly influenced the distribution and extent of sediment deposition. This resulted in a complex reservoir architecture that is characterized by spatial heterogeneities in porosity and permeability.

We identified contractional features, such as pop-up structures and fault-propagation folds, formed by positive fault reactivation during Late Cretaceous basin inversion. The strength of inversion was influenced by the geometry and orientation of pre-existing faults and the thickness of the underlying sedimentary cover. Inversion-related structures further complicate the basin’s architecture, by compartmentalizing the reservoir rock and influencing sediment distribution patterns.

Our findings show an example of how fault dynamics can affect the geothermal reservoir quality and storage capacity of subsurface exploration targets. This study provides valuable insights for optimizing exploration strategies and storage site selection by integrating fault growth and reactivation analysis. This helps to further reduce geothermal exploration risks and enhances storage efficiency in rift basin settings.

How to cite: Weert, A., Camanni, G., Mercuri, M., Ogata, K., Vinci, F., and Tavani, S.: Fault growth and reactivation in the West Netherlands Basin: Implications for subsurface renewable energy projects, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1144, https://doi.org/10.5194/egusphere-egu25-1144, 2025.

        The Enping 17 sag (EP17) is located in the northern part of the Pearl River Mouth Basin, and the Cenozoic boundary faults with complex-trending have been developed. The genetic mechanism of the boundary faults is still unclear, which limits the understanding of the structural evolution of the sag in the study area. Using high-quality three-dimensional seismic data, through seismic interpretation, throw-distance (T-x) plots, physical experiment and tectonic evolution sections, the geometric characteristics, activity and genesis of boundary faults are analyzed, and the reactivation mechanism of pre-existing faults and the evolution process of sags are discussed. The results show that the EP17 boundary fault is curved, and there are differences between the north and south sides: the north is dominated by NE-trending faults, accompanied by nearly ENE-trending faults, with slope-flat type and four-stage rolling anticline on the seismic profile; the south is a near NS-trending fault with shovel-type and two-stage rolling anticline. The selective reactivation of NE-trending and near NS-trending pre-existing faults in the Cenozoic controlled the evolution of the sag. The Enping 17 sag has undergone multiphase extension, and the extension direction rotates from NW-SE to N-S clockwise. The early Eocene NE-trending and near NS-trending pre-existing faults are reactivated at the same time, forming a strong extension zone at the fault tips, resulting in a rapid link between the north and south faults. These group of faults continued to be active during the Middle Eocene, and a new ENE-trending fault was formed in the north. The NE-and ENE-trending faults in the northern part of the Late Eocene continued to move, controlling the migration of the sedimentary center to the northern sag.

How to cite: Zhang, Z., Liu, H., Peng, G., and Long, Z.: Reactivation mechanism of pre-existing faults in multiphase extensional setting: A case study of Enping 17 sag, Pearl River Mouth Basin, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2194, https://doi.org/10.5194/egusphere-egu25-2194, 2025.

Fault rocks are influenced by physical conditions such as frictional properties, temperature, effective normal stress, and differential stress. Their formation is examined with respect to energy distribution in fault zones, fault slip velocity, and etc. In this study, the fault-zone materials were retrieved from Hole-A of MiDAS project and were examined at 491.3 m in which the extremely fractured quartz was found in the vicinity of upper boundary of the active Milun Fault zone. The quartz was analyzed using optical microscopy, X-ray powder diffraction (XRD), and synchrotron XRD and Laue diffraction to understand their microstructures and potential deformation mechanisms. Microstructural observations showed angular, extremely fractured quartz grains with intragranular fracturing and no significant shear strain. XRD analyses showed a notable rightward peak shift in the 491.3 m quartz compared to quartz from other depths (389.1 m and 505.45 m), suggesting compressive stress-induced strain. Synchrotron-based XRD confirmed the absence of amorphous phases, indicating the quartz experienced rapid brittle deformation rather than prolonged shear. Laue diffraction demonstrated significant lattice distortion and high residual stress within the quartz, further supporting a mechanical origin. On ther other hand, triaxial compression tests on synthetic quartz were conducted to simulate deformation under semi-static deformation conditions. These tests revealed that strain localization are inconsistent with observations from the extremely fractured quartz. Based on these findings, thermal fracturing and comminution due to shear deformation were excluded as primary mechanisms. Instead, the results suggest that the pulverized quartz formed under extreme high strain rates likely associated with seismic rupture dynamics. This study provides a comprehensive microstructural characterization of pulverization, advancing our understanding of fault zone processes during earthquakes.

How to cite: Wu, W.-J., Lien, P., Huang, T.-H., Chiang, C.-Y., Kuo, L.-W., and Ma, K.-F.: Extremely fractured quartz within Milun fault zone: implications for pulverization, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5986, https://doi.org/10.5194/egusphere-egu25-5986, 2025.

The spatio-temporal evolution of seismicity is of paramount importance as it provides insights into the brittle deformation of the Earth’s crust. During a seismic sequence, the spatial distribution of aftershocks typically reveals the structural architecture of the fault zone activated during the mainshock. However, within the seismogenic crust, seismicity is not necessarily exclusive of the mainshock rupture plane. In the case of the Mw 5.9 and Mw 6.5 2016-2017 Visso-Norcia (Central Italy) seismic sequence, widespread distributed seismicity (Mw < 4.5) has been recorded down-dip in the hangingwall of the ruptured fault in the depth range of 4-9 km (down-dip hanging-wall seismicity, DHwS). This is the portion of the seismogenic crust where seismic reflection profiles identify the presence of large volumes of Triassic Evaporites, TE, a geological formation composed of anhydrites and dolostones. Field and laboratory observations show that, away from the major brittle faults, TE deformation consists of a background ductile deformation interspersed with brittle processes in the form of distributed failure and folding of the anhydrites associated with boudinage hydro-fracturing and faulting of dolostones. 

In this work we used the DHwS to highlight the seismological evidence of distributed deformation within a layer of the seismogenic crust affected by a background ductile deformation.

Through the construction of a series of seismological cross sections oriented perpendicularly to the strike of the mainshock rupture plane, we identified two main types of DHwS: 

• Diffuse, non-localized seismicity characterized by low variability of daily seismicity rate, and
• Localized seismicity featured by both swarm-like and Omori-like event decay.

In particular, localized seismicity is mostly located south of the mainshock, and illuminates kilometers-long structures with different orientations.

We interpret the occurrence of DHwS as the result of the embrittlement of the evaporitic layer induced by an increase of  strain rate following the Norcia mainshock. This is supported by recent laboratory experiments showing that an increase in strain rate promotes brittle failure and faulting in TE samples, rather than ductile deformation. Furthermore, Coulomb stress changes simulated after the Norcia mainshock suggest an increase in strain rate within rock volumes where DHwS is recorded.

Our results suggest that the aftershock distribution during a seismic sequence can be strongly controlled by the rheology of the lithologies contained within the seismogenic crust.

How to cite: Rossi, F., Guglielmi, G., Trippetta, F., and Collettini, C.: Rheological control on distributed aftershock activity: insights from the Mw 6.5 Norcia seismic sequence, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6171, https://doi.org/10.5194/egusphere-egu25-6171, 2025.

The Triassic Evaporites (TE), made of alternating dolostones and anhydrites, hosted the mainshocks and most of the aftershocks of the Mw 6.0 1997 Colfiorito and Mw 6.5 2016 Norcia earthquakes in central Italy. Their complex rheology features elasto-frictional behavior and ductile deformation. The latter is highlighted by the spatio-temporal evolution of the Norcia aftershock sequence showing widespread distributed seismicity within a large crustal volume, suggesting a rheological embrittlement of the whole evaporitic layer.

To understand the main factors causing rheology variations of TE, we performed triaxial compression experiments on TE borehole samples varying strain rate, confining pressure, lithology and fabric. We tested pure anhydrite, foliated anhydrite-dolostone and mixed-chaotic dolostone-rich specimens. Samples, subjected to confining pressures of 10 and 20 MPa, were loaded up to failure at strain rates of 10-4 and 10-5 s-1, and then reloaded after a holding time of 1000 s to evaluate fault reactivation characteristics.  We also performed friction experiments on TE gouge at normal stress of 20 MPa and load-point velocity of 10 μm/s, to capture fault structure development starting from randomly distributed anhydrite-dolostone particles. Mechanical measurements were coupled by microstructural analyses to elucidate the deformation processes operating at different boundary conditions.

In triaxial experiments, all samples exhibited brittle shear failure with associated stress drop. Upon rock failure, we observed a spectrum of fault slip behaviors ranging from slow (< 50 μm/s) to fast (> 600 μm/s) fault slip.  We observe that dynamic faulting occurred preferentially at 10 MPa, and at higher strain rate in dolostone-rich samples . Upon fault reactivation, we recorded fault slip instabilities, mainly for the same type of conditions: dolostone-rich samples and at confining pressure of 10 MPa. On the contrary, neither fabric nor textural heterogeneities appeared to influence rock failure properties or fault reactivation behavior. During friction experiments on gouge, we measured similar friction coefficients between anhydrite and dolostone (μ ∼ 0.65), detecting minor fault slip instabilities within dolostone-rich samples. Microstructural investigations revealed the enrichment of dolostone within the experimentally developed fault slip zones, characterized by grain size reduction and shear localization.

The analysis of mechanical data suggests that rheological embrittlement of TE is facilitated by low confining pressure, high strain rate and high dolostone content. The simultaneous occurrence of these conditions promotes dynamic faulting upon rock failure and fault slip instabilities during fault reactivation. Shear localization favors dolostone concentration along slip planes, implying that the shear strength of TE-hosted faults is primarily controlled by frictional properties of dolostone, which create favorable conditions for the development of slip instabilities. When upscaling laboratory results to the crustal scale, we can speculate that low effective pressure is given by pressurized fluids while mainshock-induced stress changes facilitated a strain rate increase. These processes together contribute to the embrittlement of the evaporitic layer, explaining the distributed seismicity observed after the Norcia mainshock.

How to cite: Guglielmi, G., Giorgetti, C., De Paola, N., Mauro, M., Collettini, C., and Trippetta, F.: Rheology and fault slip behavior of Triassic Evaporites: an experimental study, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6237, https://doi.org/10.5194/egusphere-egu25-6237, 2025.

After the devastating M_w 7.3 Nanbu-Kobe earthquake of 1995, a 750 m deep borehole was drilled by the Geological Survey of Japan to intersect the Nojima fault at depth. The Hirabayashi borehole intersected the fault core at 625 m in 1996, less than one year after the earthquake. Such a short span provides an exceptional opportunity to assess the co-seismic damage generated by this earthquake.

As part of the AlterAction project (https://anr-alteraction.osug.fr/), which assesses the interplay between the alteration and deformation of faults embedded in crystalline rocks under hydrothermal conditions, several core samples were collected at regular intervals along the Hirabyashi borehole. X-ray CT scans with a voxel size of ~50 μm  were systematically performed on the samples.

This allows to quantify the fracture pattern across the fault zone. The scans show a network of open fractures, whose density increases when approaching the principal slip zone at 625 m, suggesting that these fractures are related to fault activity. The damage appears symmetric extending ~70m above and below the fault (corresponding to an effective thickness of ~15m given the low angle between the borehole and the fault plane). The relative volume occupied by the open fractures strongly decreased in the vicinity of the principal slip zone. This is related to the sealing of the fracture network, which is assumed to have occurred during the short interval between the 1995 earthquake and the time of drilling.

Several samples exhibited a dense and diffuse fracture pattern, but with very moderate deformation. Such a damage pattern is reminiscent to the “pulverized rocks” found at the surface near active faults. This would provide the first direct evidence of pulverization at depth in the Nojima Fault and confirms that a high strain rate was achieved during the 1995 Nanbu-Kobe earthquake.

How to cite: Doan, M.-L., Iaquinta, R., and Nagy, C.: Co-seismic damage of the Nojima fault one year after the 1995 Nanbu-Kobe earthquake, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7316, https://doi.org/10.5194/egusphere-egu25-7316, 2025.

Layer-bound faults in fine-grained sediments, observed in sedimentary basins worldwide, are frequently interpreted as the result of polygonal faulting. In clay formations, these faults are linked to clay tectonics—post-depositional processes that generate intraformational fault systems with no predominant orientation. As part of subsurface investigations for future wind farm development in the Princess Elisabeth Zone (PEZ), offshore Belgium, we analyse faulting observed within the Eocene-aged Kortrijk Clay Formation, a lateral equivalent of the London Clay Formation.

Using ultra-high-resolution seismic and acoustic reflection data with dense grid spacing (40-60 m), we identified two distinct fault groups that transition sharply between the southern and northern PEZ. The first group, dominant in the southern PEZ, comprises ENE-WSW oriented faults (N75°E-N85°E) with steep dips of 60°-70°, narrow spacing of 50-160 m, and fault lengths typically under 0.5 km. In contrast, the second group, prominent in the northern PEZ, features NNE-SSW faults (N355°E–N25°E) with shallower dips of 35°–45°, wider spacing of 100–500 m, larger displacements of up to 5 m, and fault lengths extending to 1.7 km. This group also includes a major fault with a displacement of up to 15 m, aligned parallel to the other faults. Unlike the southern faults, some of these northern faults propagate deeper, potentially reaching the top of the Cretaceous. Additionally, folding structures are also observed, with fold axes oriented both parallel and oblique to the fault directions.

The distinct partitioning of fault orientations and distributions between the two areas raises critical questions about the origin and controls of the faulting process. The presence of dominant fault orientations contradicts the diagenetic-related polygonal faulting model, which typically predicts faults lacking a preferred orientation. These dominant orientations, along with the occurrence of major faulting—particularly in the northern PEZ—point to the potential influence of regional tectonic stresses. However, the controls on the spatial partitioning of these fault groups remain unclear, highlighting the need for deeper seismic data that would allow further investigation into the deeper regional structure to better understand the faulting processes within this clay formation.

How to cite: Andikagumi, H., Mestdagh, T., Saritas, H., Missiaen, T., de Batist, M., Stuyts, B., and Pirlet, H.: Faulting variability in the Kortrijk Clay Formation, Princess Elisabeth Zone, offshore Belgium: Implications for clay tectonics, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9308, https://doi.org/10.5194/egusphere-egu25-9308, 2025.

Drilling through fractured carbonate formations often presents significant geomechanical challenges, including borehole collapse, cavings, tight spots, stuck pipe and lost circulation issues. These instabilities arise due to complex interactions between in-situ stresses, natural fractures, and thermal effects, requiring a robust geomechanical model for risk mitigation.

The LTG-01 well, drilled in the Dutch subsurface, encountered severe wellbore instability issues while drilling the 8.5-inch section through the Dinantian carbonates. The well exhibited tight hole conditions, stuck pipe events, severe mud losses, wellbore breathing, along with washouts identified from caliper logs. Borehole resistivity imaging further revealed borehole breakouts and drilling-induced tensile fractures (DIFs) within the carbonate interval. To investigate these geomechanical challenges, a 1D Mechanical Earth Model (MEM) was constructed using well log data available from the NLOG public database.

Elastic properties such as Young’s modulus and Poisson’s ratio, along with strength parameters including Unconfined Compressive Strength (UCS), tensile strength, friction angle, and cohesion, were computed using empirical correlations. Pore pressure was estimated using Eaton’s method for the clastic overburden and a gradient-based approach in the carbonate section. Vertical stress was computed via density log integration, while horizontal stresses were derived from the poroelastic horizontal strain equation, constrained by LOT and FIT data.

A key finding was that the drilling-induced fractures in the carbonate interval could be linked to thermal stress effects, caused by the temperature contrast between the borehole fluid and formation temperatures which were in order of ~160-190°C. The breakdown gradient computed from the MEM approached the equivalent circulating density (ECD) in zones where DIFs were observed, suggesting that thermal stress significantly reduced the rock’s tensile strength, leading to DIF formation. Additionally, borehole washouts observed in calipers, along with vuggy and brecciated intervals, highlighted the presence of mechanically weak zones.

Furthermore, borehole breakouts appear to correlate more strongly with plane of weakness (PoW) shear failure rather than intact rock failure. In addition to Mohr-Coulomb intact rock failure, alternative shear failure mechanisms were assessed—one using bedding planes as failure surfaces, and another considering natural conductive fractures as weakness planes. A notable correlation between PoW shear failure gradients and breakout intervals suggests that pre-existing weak planes significantly influenced wellbore instability.

Despite these insights, some uncertainties remain, particularly regarding fracture connectivity and fluid interaction effects, which merit further investigation. Nonetheless, this study provides critical geomechanical insights for future drilling in fractured carbonate formations, emphasizing the need for thermal stress considerations and plane of weakness analysis in wellbore stability assessments.

How to cite: Konwar, D., Mishra, C., and Chatterjee, C.: Wellbore Stability Challenges in Fractured Carbonates: Analyzing Stresses, Planes of Weaknesses, and Thermal Effects , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10835, https://doi.org/10.5194/egusphere-egu25-10835, 2025.

The Borborema Province (NE Brazil) is a key region to investigate fluid-assisted brittle deformation related to the reactivation of continental-scale shear zones during South Atlantic rifting. The Cruzeiro do Nordeste shear zone (CNSZ), defines the northern boundary of the Jatobá Basin and displays brittle structures such as faults, veins, breccias and pseudotachylytes, which contain evidence of paleoseismic cycles. These structures formed during the brittle reactivation of pre-existing ductile and brittle-ductile fabrics. They are filled by multiple stages of mineralization, first epidote, followed by two phases of calcite. Calcitic carbonate veins appear as cement in tectonic breccias, or as late veins associated with quartz. Fault breccias are characterized by angular clasts with crackle textures, while hydrothermal-type breccias are distinguished by matrix-supported subangular to subrounded clasts, indicative of multiple fluid injection events. To determine the timing of these deformation events, five carbonate samples were dated using U-Pb geochronology. The samples include spatic calcite in hydrothermal breccias, late sub-horizontal veins, and spatic calcite that crosscuts epidote-filled veins. The textures of these carbonates range from coarse tabular crystals with mechanical twins to fine-grained, granular shapes. Cathodoluminescence imaging reveals two distinct calcite mineralization phases reflecting episodic fluid flow during deformation. The samples yielded ages from 140 to 114 Ma, spanning the Lower Cretaceous (Berriasian) to Early Cretaceous (Aptian), a timeframe associated with South Atlantic rifting and the development of the intraplate Brazilian sedimentary basins. The structural and chronological data suggest that reactivation of brittle-ductile structures played a crucial role in channelling carbonate-rich fluids during successive seismic cycles. The variations in orientations and ages of brittle structures indicates that deformation occurred episodically, driven by regional stress variations during rifting. These findings highlight the importance of brittle-ductile deformation in accommodating tectonic stresses and facilitating fluid circulation throughout rift evolution. This study enhances our understanding of the tectonic and fluid-flow processes associated with shear zone reactivation during South Atlantic opening. By integrating structural analysis with U-Pb dating of carbonate minerals, we provide a framework for reconstructing paleoseismic cycles and their role in shaping the geological record of intraplate deformation. These insights contribute to broader discussions on rifting dynamics and the evolution of continental margins.

How to cite: Miranda, T., Barbosa, D., Correia Filho, O., Silva, A., Viegas, L. G., Neves, S., Carvalho, B., Reis, M. L., Roberts, N., and Neumann, V.: TRACKING PALEOSEISMIC CYCLES TROUGH U-Pb CARBONATE DATING: EVIDENCE FROM THE BRITTLE DEFORMATION OF THE CRUZEIRO DO NORDESTE SHEAR ZONE, NE BRAZIL, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10920, https://doi.org/10.5194/egusphere-egu25-10920, 2025.

Faults can be studied at different scales and through different methods. Hence, every method provides some insight into our understanding of fault geometry and properties despite the limitations inherent to these methods. Among fault characteristics, fault geometric attributes such as fault plane roughness, displacement, length, and width can be studied using both outcrops and 3D reflection seismic data. Fault geometric attributes from different scales of study are then used within fault scaling laws to increase our knowledge of fault growth mechanism and mechanics.  In addition, fault rock properties from fault core and damage zone allow us to study the effect of faults on the fluid flow behavior of rocks as well as their mechanical integrity. In order to have a better knowledge of fault characteristics and mechanical behavior, it is important to integrate fault geometric studies with fault rock properties.  This needs an interdisciplinary approach, combining geomechanics, earthquake and exploration seismology with structural geology, hence, allowing us to study both active and non-active faults from different perspectives.

How to cite: Torabi, A.: Fault characteristics from outcrop and reflection seismic studies, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11403, https://doi.org/10.5194/egusphere-egu25-11403, 2025.

In 2011, the Tohoku subduction zone endured a Mw 9.0 large earthquake and a large tsunami with co-seismic displacement exceeding 40 meters at the trench. Such an unexpected tsunami earthquake is expected to have created intense damage across the plate boundary fault zone (PBFZ).

We propose to characterize the hydraulic damage pattern across the PBFZ of the Tohoku earthquake. To achieve this, we will use the rich dataset collected during IODP Expedition 405, during which the PBFZ was drilled directly several times between September and December 2024. In particular, we focus on drilling and logging data to estimate the flow entering the borehole. By carefully modelling the mud pressure, we can evaluate the hydraulic inflows and outflows to the borehole as drilling advances. The flow profile provides valuable insights into permeable and/or overpressurized intervals.

Using the Logging-While-Drilling data collected in Hole C0019H, we obtain a high-resolution and continuous fluid flow profile along the borehole.  The results show that above the plate boundary (~815 mbsf), we observe the incoming fluid flow, with strong flow pulses at 775 mbsf and 805 mbsf. These two pulses are associated with variations in mud temperature and some clear fracture zones identified by the electrical images. This suggests the hydraulic structure of the PBFZ has two components highlighted by the two types of flow: (1) A background damage, increasing progressively in the hanging wall when approaching the PBFZ, with a potential for fluid flow across the fault zone. (2) A fracture-supported flow, related to the major subfaults which sustain a more longitudinal flow.  

How to cite: Pei, P. and Doan, M.-L.: Hydraulic structure of the Tohoku plate boundary fault zone: insights and evidence from direct drilling (IODP expedition 405), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11611, https://doi.org/10.5194/egusphere-egu25-11611, 2025.

Granite emplacement is often assisted by shear zones. These shear zones may be synmagmatic and can lead to the development of various structural features, ranging from complex fabric to faults and their interactive networks. The Neoarchean Closepet Granite (2.56-2.51 Ga) of Eastern Dharwar Craton, India is replete with faults and fractures of various orientations, which are used to decipher the regional brittle tectonics. Detailed analysis of strike-slip and high-angle oblique-slip faults using the Right Dihedron and Rotational Optimization methods reveals an overall E–W oriented horizontal regional compressive stress. The area is further subdivided into smaller domains to observe variations in the local stress field. Results indicate a distinct deformation pattern across the region. The northern portion is influenced by ENE-WSW directed regional compression, while the southern portion is shaped by ESE-WNW directed regional compression. Fault orientations and their kinematics across the pluton indicate that the faults are associated with a large-scale Riedel shear along the pluton boundary. In the north, deformation is compatible with an NW-SE sinistral shear zone, while in the south, fault patterns are consistent with an NNE-SSW dextral shear zone. This variation in shear is attributed to the overall geometry of the pluton boundary. We also interpret that, during the emplacement and the post-emplacement period, the rheological boundary between the granite and the surrounding host rock, evolved into a shear zone, facilitating the development of faults across the Closepet pluton.

How to cite: Das, G. and Mondal, T. K.: Paleostress Field Reconstruction from the strike-slip faults of Neoarchean Closepet Granite (Eastern Dharwar Craton, South India): Its implications in understanding Precambrian Tectonics, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12263, https://doi.org/10.5194/egusphere-egu25-12263, 2025.

The geology of northern Oman is distinctive because of the emplacement of the massive Semail Ophiolite onto the stable Arabia platform in the late Cretaceous followed by the later development of the Jebal Akhdar and Saih Haitat domes.  East of Muscat, the Wadi Kabir Fault forms an important and long-lived structure at the northern edge of the Saih Hatat dome.  In the Bandar Jissah area, Triassic carbonates occur in the footwall of the NNE-dipping Wadi Kabir Fault while rocks of the Semail Ophiolite, newly discovered rocks of the metamorphic sole, and a sequence of Paleogene-Eocene sedimentary rocks crop out in the footwall.  Previous workers posit that regional extension commenced via early low-angle detachment faulting (‘Banurama detachment’) that was followed by higher-angle normal faulting along the Wadi Kabir and associated faults which developed as basin-bounding structures for the Paleocene Bandar Jissah rift basin.  Folds in the hanging wall cover sequence are interpreted as the product of rollover during extension and basin formation.

Our detailed mapping as well as structural and kinematic analysis illustrates that folds in the hanging wall are contractional structures that formed due to tectonic inversion along the Wadi Kabir and other faults.  The overall shortening is modest (~10%) and primarily confined to the hanging wall rocks, consistent with buttressing against mechanically rigid rocks in the footwall of the Wadi Kabir Fault.  The low-angle ‘Banurama detachment’, which places Paleogene-Eocene sedimentary rocks over Triassic carbonates, records south-directed thrust movement (not north-directed extensional slip) and with contractional slip past the null point.  This structure appears to be an emergent fault in which the reactivated cover strata above the Wadi Kabir fault were thrust southward over the ground surface and shed sediment from the growing hanging wall anticline.

These structures require an interval of shortening/transpression in northern Oman that post-dates rift basin formation and deposition of mid-Eocene marine sediments in the Seeb Formation. The Wadi Kabir Fault also has localized zones of listwaenite preserved in its damage zone that is derived from ophiolitic rocks in the hanging wall. Collectively, the Wadi Kabir Fault is a long-lived structure that’s experienced multiple episodes of both extensional and contractional slip since the Cretaceous.

How to cite: Bailey, C. and Andreas, S.: Cenozoic tectonic inversion, buttressing, and emergent faulting along the Wadi Kabir Fault, northern Oman, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14151, https://doi.org/10.5194/egusphere-egu25-14151, 2025.

Stylolite, a discontinuity between blocks of rock with complex mutual columnar interdigitation, is a pressure solution dehydration structure and is useful for estimating the paleostress. Stylolites are found in lithofacies such as limestones and evaporites, although only those in limestones have been used to estimate the paleostress. Stylolite formation could be a major contributor to the creation of sedimentary space after evaporite deposition in general. Therefore, the state of stylolite formation in evaporite is necessary to understand basin evolution. This study analyzes stylolites in evaporites collected by IODP Expedition 402 at Hole U1617B, located in the Tyrrhenian sea about 110 km southwest of the Italian peninsula, with the aim of estimating paleostress value at the time of stylolite formation.

Stylolites were photographed on the vertical section of cores perpendicular to the stylolites. Their traces were analyzed using the discrete Fourier transform method to estimate crossover-length L which separates two scaling regimes with different roughness exponents for small and large scales. Most stylolites show L as ~2 mm. Corresponding overburden stress σ_zz ≈ 9 MPa assuming gypsum physical properties of Young's modulus E = 50 GPa, solid-fluid interfacial energy γ = 47 mJ/m², and Poisson's ratio ν = 0.25. The corresponding depth z ≈ 330 m by assuming σ_zz = ρgz, with rock density ρ = 2.7 g/cm³ and gravitational acceleration g = 10 m/s². The water depth of the hole was 2822.33 m and analyzed stylolites were located at ~328 m below the sea floor. An estimated overburden stress value is not unnatural compared with the sampling depth, suggesting that stylolites in evaporite would also be useful for stress estimation. The gypsum-anhydrite transition is thought to occur at a burial depth of 500~1000 m. Therefore, stylolite formation in Hole U1617B would have occurred before the transition.

How to cite: Abe, N. and the IODP Exp. 402 Scientists: Paleostress value estimation using stylolite in evaporite of IODP core sample from Expedition 402 at the Tyrrhenian Sea, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14539, https://doi.org/10.5194/egusphere-egu25-14539, 2025.

Structural highs formed during the extensional tectonics of the Mesozoic rifting phase, represent important petroleum systems in Italy and hold potential for geothermal energy exploitation and geofluid storage. These structural highs, commonly exposed in the Southern Alps, Northern Apennines, or buried beneath the Po Plain foredeep, were generally overprinted by later tectonic phases. Specifically, the compressional tectonics associated with the Alpine orogeny since the Eocene often mask the original structure of mesozoic structural highs. The Gaggiano structure, located in the southwestern Po Plain, offers a rare opportunity to study a Mesozoic high only partially affected by subsequent tectonics.

This study investigates the geological and structural evolution of the Gaggiano area using a 3D seismic volume covering an area of approximately 180 km². Seven seismic horizons, spanning from the Lower/Middle Triassic to the Miocene, were interpreted with detailed mapping, using a spacing of 300 m and 250 m between adjacent inline (N-S) and crossline (E-W) sections, respectively. The interpreted fault network, combined with thickness and top maps of key horizons, allowed the reconstruction of the structural evolution of the area and its associated fault system.

Well data from the topographically most elevated portion of the Gaggiano high highlight a stratigraphic succession that includes continental to evaporitic rocks (Lower Triassic), overlain by organic-rich mudstone limestones and dolomitized intervals (Middle Triassic), followed by Jurassic and Cretaceous pelagic limestones. The succession is highly condensed on the structural high, with significant stratigraphic gaps, including the absence of Upper Triassic rocks, most of the Jurassic (~30 m preserved), and Lower Cretaceous deposits. From the Middle Eocene onward, siliciclastic sediments of the Po Plain foredeep were deposited.

During the Middle Triassic, the structural evolution of the Gaggiano high was controlled by a N-S trending, east-dipping domino-style extensional fault system, with associated E-W striking normal faults. During the Late Triassic to Early Jurassic, the N-S striking east-dipping faults dominated. The Gaggiano high formed at the footwall of an E-dipping fault which accommodates a maximum throw of ~700 m. By the Early Jurassic, pelagic carbonates were deposited unconformably over the structural high, progressively leveling the paleotopography during the deposition of the Scaglia Formation (Upper Cretaceous-Middle Eocene). During or shortly after the deposition of Scaglia Fm., the Gaggiano area was affected by extensional tectonics testified by NW-SE striking normal faults affecting the top of Scaglia Fm. and of older formations. The NW-SE striking faults locally reactivate the pre-existing fault system. Partial involvement in Miocene compressional tectonics is evident from gentle folds affecting the sedimentary succession near Mesozoic extensional faults, suggesting positive fault reactivation.

The findings provide key insights into the interplay of extensional and compressional tectonics in shaping the evolution of the Gaggiano area. This study contributes to a better understanding of Mesozoic reservoirs and their potential reuse in sustainable energy applications.

How to cite: Mercuri, M., Bigi, S., Doglioni, C., Trippetta, F., and Carminati, E.: Anatomy and Structural Evolution of a Mesozoic Structural High: An Example from the Western Po Plain (Italy), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15680, https://doi.org/10.5194/egusphere-egu25-15680, 2025.

Tectonic faults can slip with diverse behaviors: from aseismic creep to seismic slip. Such diverse slip behaviors of a fault are mainly controlled by the frictional stability of the rough fault interface, where a complex set of real contacts are established by numerous discrete asperities. However, understanding how these asperities precisely control the slip stability still remains elusive.

Here we develop a novel analog fault model to overcome the difficulty of imaging an exhaustive spatiotemporal variability of a natural fault interface at depth. Specifically, numerous identical rigid spherical PMMA (poly-methyl-methacrylate) beads, which are used to model the discrete frictional asperities, are embedded with height variations and random spatial distribution in a soft viscoelastic silicone block to establish numerous micro-contacts with a thick transparent rigid PMMA plate on the top. During the entire shear process of such a heterogeneous fault interface, not only the subtle motion of each local asperity can be directly measured by the high-resolution optical monitoring system, but also the seismic waves emitted from slip transients that occurred at local asperities can be captured by the acoustic monitoring system.

The synchronization of the local rapid slips at all asperities is responsible for the unstable system-size stick-slip of the macroscopic fault that generates large amplitude energetic acoustic event. It is interesting to observe that complex seismic activities initiate also early during the interseismic phases and are interpreted as the seismic signature of destabilizing transients that originate from spatiotemporal interactions of limited local asperities. We locate acoustic events at the asperity scale and correspond them with these slow transients. We further quantify the partitioning of the resolved slip taking place on the asperities as dynamic events to interpret the nature of the complex seismicity. Our results provide insights into a better understanding of the physical processes leading to the occurrence of foreshocks and complex seismic sequences.

How to cite: Shu, W., Lengliné, O., and Schmittubhl, J.: Complex seismic sequences originated from the collective behavior of asperities: an experimental approach, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16530, https://doi.org/10.5194/egusphere-egu25-16530, 2025.

Crystalline rocks can be highly fractured at shallow depths due to tectonic stresses and topographic effects. This fracturing largely controls the rheological properties of the rock in a way that depends on the properties of the fracture networks, including the distribution of fracture sizes and orientations, the distribution of sealing minerals in the fracture network, and the normal and shear stiffnesses. Better knowledge of these properties at the site scale would improve both mechanical and hydrological models, essential for risk assessment or resource management. To this end, we combine models of mechanical properties based on Discrete Fracture Network (DFN) properties (Davy et al., 2018), stress measurements, and strain deduced from large-scale lithosphere models. The consistency of this rheological equation provides a basis for discussing the hypothesis used to infer the mechanical properties of the rock mass.

We apply this methodology to the Forsmark site in Sweden, which is being studied as the future location for a deep nuclear waste repository, supported by a comprehensive database of fractures. Fracture network models have been developed based on core and outcrop observations. Fracture density decreases with depth, showing a significant reduction down to 300-400 meters, followed by a more gradual decline. Fracture stiffnesses and matrix elasticity have been extensively measured in the laboratory. As a rule for upscaling, we assume that openness and stiffness are influenced by fracture size and normal stress. The complete 3D compliance matrix of the effective properties is calculated, facilitating the identification of the primary anisotropy planes of the rock mass.

Our findings indicate that, under specific conditions, the closure of the crustal-scale rheological equation can be guaranteed, i.e. the combination of DFN-inferred mechanical properties and measured stresses gives reasonable deformations, compatible with most lithospheric models. Using additional glaciation models, we then infer paleostresses and paleostrains during the Last Glacial Maximum, 20,000-10,000 years ago.

References:
Davy, P., Darcel, C., Le Goc, R., Mas Ivars, D., 2018. Elastic Properties of Fractured Rock Masses With Frictional Properties and Power Law Fracture Size Distributions. JGR Solid Earth 123, 6521–6539. https://doi.org/10.1029/2017JB015329

How to cite: Vairé, E., Davy, P., Darcel, C., Steer, P., and Mas Ivars, D.: Evaluating a Crustal-Scale Rheological Equation Using Stress Measurements and Mechanical Property Estimations from Discrete Fracture Network Models: A Case Study of Fennoscandia, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16531, https://doi.org/10.5194/egusphere-egu25-16531, 2025.

The ratio of compressional to shear wave velocity (Vp/Vs) provides critical insights into rock composition, fluid saturation, porosity, and fault characteristics. However, resolving fine-scale Vp/Vs structures is challenging with conventional tomographic imaging techniques due to uneven ray path coverage and regularization constraints in inversion. Alternatively, high-resolution Vp/Vs imaging can be achieved through in-situ Vp/Vs ratio estimation by analyzing differential P-wave and S-wave arrival times (Δt_p,Δt_s) from closely located earthquake clusters, effectively circumventing the limitations of traditional tomography. Conventional in-situ Vp/Vs estimation method mitigates origin time difference errors by subtracting the mean Δt_p and Δt_s value for each event pair. However, this approach clusters data points near the coordinate origin, increasing estimation uncertainty. In this study, we present an improved in-situ Vp/Vs ratio estimation technique that corrects the origin time difference errors for each event pair within a cluster using spatially dense seismic arrays. This correction ensures that P-wave and S-wave time differences precisely represent the actual travel time differences, thereby enhancing the reliability of Vp/Vs estimation. Synthetic tests confirm the robustness of the method and its ability to reduce uncertainty. Applying the improved method to the 2021 Ms 6.4 Yangbi Yunnan aftershock sequence, we identify significant spatial variation in Vp/Vs ratio along the fault zone, with an increase in Vp/Vs at greater depth and a strong correlation with fault structures and their geometries (figure 1). We further investigate how mineral composition, stress state, and fluid content influence Vp/Vs in the Yangbi region. Our method, suitable for dense array observations, demonstrates strong potential for application in other seismically active regions.       

Figure1. Spatial Variations and uncertainties of in-situ Vp/Vs in the 2021 Ms 6.4 Yangbi Yunnan aftershock sequence. The red star is Yangbi earthquake. (a)In-situ Vp/Vs ratio of the Yanbi aftershock sequence. The black line indicates the fault trace, where F1 represents the Weixi-Qiaohou-Weishan Fault. (b)Uncertainties of the Yanbi aftershock sequence. The blue line represents the fault surface, and the gray triangles denote seismic stations. (c)Vp/Vs variation with depth.

How to cite: Zhang, Q., Yu, C., Meng, H., and Zeng, X.: Improved in-situ Vp/Vs Estimation Using Dense Seismic Array with Application to the 2021 Ms 6.4 Yangbi Yunnan Aftershock Sequence, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17081, https://doi.org/10.5194/egusphere-egu25-17081, 2025.

Studying megathrust shear zones is crucial for understanding the mechanics of subduction zone earthquakes, as these zones are recognized as weak interplate faults that localize deformation under low shear stresses. The low effective friction coefficient of megathrust faults, often due to fluid overpressures, facilitates deformation and is evidenced by the occurrence of mineral-filled fracture sets. These fracture networks, which form in response to cyclic stress states and fluid pressures, provide valuable insights into the palaeostress orientations and the characteristics of fault zone fluids over time.

The Sestola Vidiciatico Unit (SVU) in the Northern Apennines is a tectonic unit interpreted as the plate boundary shear zone between the Ligurian prism and the underthrusting Adria microplate during early-to-middle Miocene, active at temperatures up to 170°C. The SVU is 200 - 400 m thick, composed of kilometer-sized tectonically juxtaposed slices of marls, shales, sandstones and mud-rich deposits. The hanging wall, formed by slope sediments along with Ligurian Units incorporated at the toe of the prism, was overthrusted along a basal décollement onto the younger foredeep turbidites of Adria microplate. Here we present the results of a mesoscale structural analysis of a well exposed sector of the basal contact, where the SVU overthrusts foredeep turbidites along a thrust ramp dipping to the south. Early stages of deformation involved soft-sediment deformation, with polygonal normal faults accommodating boudinage and flattening, producing a pervasive flattening foliation and secondary shear surfaces in the hanging wall. As lithification progressed, shear localization occurred, transitioning from distributed shearing to focused slip on a few dominant thrusts lined by thin calcite shear veins, including the basal contact with the turbidites. Along the footwall ramp, irregular and unfavorably oriented shear surfaces were gradually abandoned as slip localized along a sharp, smooth, and planar slip surface, incorporating slices of the hanging wall to the footwall. The deformation within the footwall includes an oblique cleavage in the fine-grained horizons, minor bed parallel shear planes exploiting pelitic horizons, and a conjugated set of NNE-SSW left-lateral and N-S right-lateral subvertical transtensional faults. The latter either crosscut or are crosscut by the basal thrust of the SVU, and are mineralized by at least two carbonate phases, including an early-stage light gray carbonate rich in organic matter. These results highlight a marked contrast in deformation style and stress state between the soft hanging wall and (relatively) strong footwall. Subvertical dilatant shear fractures served as fluid conduits for vertical fluid flow, which is instead very limited in the hanging wall. This study highlights the potential of combining structural and geochemical analyses in megathrust shear zones in providing insights into the interplay between stress state and fluid circulation in both fossil and modern shallow subduction zones.

How to cite: Rocca, M., Mittempergher, S., Remitti, F., Molli, G., and Tesei, T.: Shear localization and deformation patterns of a regional scale overthrust: an example from the Sestola Vidiciatico Unit (Northern Apennines) , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17416, https://doi.org/10.5194/egusphere-egu25-17416, 2025.

Studying fault zone properties is crucial in addressing key challenges in subsurface exploration, resource management, and seismic risk evaluation. As global interest in geothermal energy, hydrogen, and carbon storage intensifies, the mechanical and structural characterization of faults, as well as their impact on fluid migration, reservoir integrity, and fault sealing analysis, is becoming increasingly important.

The presented study focuses on the structural and mechanical properties of superposed fault zones in dolostones, an issue that has been poorly investigated. In particular, we addressed how the architecture of an earlier, large-scale normal fault (F1) influences the geometry and deformation mechanisms of younger, superposed strike-slip faults (F2). The F1 fault consists of four sub-parallel fault rock units, each several tens of meters thick: (i) a cataclastic core (Cu), bounded in the hanging wall by (ii) cemented micro-mosaic breccia (MB), and in the footwall by (iii) high-strained (HS) and (iv) low-strained (LS) fault rocks. To achieve this aim, we performed some geotechnical and morphometric analyses. Uniaxial compressive strength (UCS) tests revealed mean values of 83 MPa and 120 MPa for MB and LS, respectively, whereas Cu and HS exhibit lower UCS values of 58 MPa and 62 MPa, respectively. MB and Cu exhibit heterogeneous particle size distributions (PSD) and porosities of 4.02% and 4.96%, respectively, while HS and LS show more homogeneous PSDs with porosities of 2.87% and 2.19%, respectively.

The F2 faults developed a spectrum of structural facies such as cataclastic shear bands (CSBs) in the LS and HS, and as compaction bands (CBs) in the MB and Cu. In the LS, cataclasis is highly localized within widely spaced, thick, tabular, and cemented CSBs. In the HS, deformation occurs through anastomosing CSBs, accommodating diffuse cataclasis and dissolution-precipitation mechanisms. In the Cu, anastomosing CBs develop through pore collapse and dissolution-precipitation processes. In the MB, compaction is driven by pore collapse and grain crushing, forming well-localized, widely spaced CBs.

Overall, the microstructural properties (PSD and porosity) and mechanical strength of the F1 fault rocks are key factors influencing the deformation mechanisms (cataclasis vs. compaction) and the geometry (localized vs. anastomosing) of the F2 faults. These findings contribute to the understanding of fault permeability and fluid flow dynamics in multi-faulted dolomite reservoirs.

How to cite: Vitale, S., Diamanti, R., Vitale, E., Russo, G., and Camanni, G.: Deformation mechanisms and geometries of superposed fault zones in dolostones, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18371, https://doi.org/10.5194/egusphere-egu25-18371, 2025.

Faults accommodate shear motion in the upper crust through brittle deformation such as cataclastic flow. Plenty of field evidence suggests that the width of the embrittled volume, the lateral extent of the fault plane, and the grain size distribution within the fault core scale with shear displacement, indicating fairly solid scaling laws. However, at the onset of brittle failure, faults commonly exploit pre-existing anisotropic structures that facilitate slip localization and affect the early fault geometry as well as the fabric of the cataclastic products. The Bedretto Underground Laboratory represents a unique chance to study the structure of immature faults, whose pristine brittle structures are hardly preserved elsewhere during exhumation and exposure to weathering. We present the case study of the Waterfault (WF), a brittle shear zone hosted in the Bedretto tunnel within the Rotondo granite, which exploits pre-existing Alpine mylonites.

On the tunnel wall, the brittle products of the WF are confined within a 50 cm thick volume, comprised between two boundaries defined by the local mylonitic foliation. The WF is characterized by a large water outflow, suggesting that the fault behaves as a major permeable conduit inside the granite host. Such high permeability is commonly related to the occurrence of intense but localised brittle damage. However, careful sampling across the fault and detailed microstructural investigations revealed that the brittle damage introduced by shear displacement is much less volumetrically widespread than expected. Cataclasis is in fact confined to a thin (5 mm) fine-grained gouge shear band developed at the boundary of the fault zone. The damaged rock surrounding this layer presents intense fracturing, dominated by oriented fractures at high angle to the shear plane, but no sensitive displacement down to the grain-scale. Damage distribution and geometry is further controlled by the anisotropic mylonitic fabric and its mineralogy, showing grain boundary cracking and shard-like fragmentation of quartz and feldspar, as well as micro-boudinage-like cracking of mica-grains. More than 50% in weight of the material recovered both within and in proximity of the gouge layer is finer than 125 µm. These observations are evidence of dynamic rock shattering, pointing to a seismic origin of the embrittlement. The seismic damage preferentially fractured along the grain boundaries of the mylonite, producing a fine-grained material that eased the onset of localised brittle shear.

Our microstructural observations suggest that the WF might represent an optimal model for the early onset of faulting in anisotropic rocks driven by initial seismic damage, which unlocked the cohesive shear zone to favour slip localization. The “shallow” estimated depths of this event (< 5 km) might also open an interpretative window for the small-magnitude natural and induced seismicity (M<2) recorded in the Rotondo massif.

How to cite: Pozzi, G., Ceccato, A., Aretusini, S., Spagnuolo, E., and Cocco, M. and the FEAR Team: Shattering and structural inheritance at the onset of faulting in the Rotondo granite, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18550, https://doi.org/10.5194/egusphere-egu25-18550, 2025.

Boundary element modelling is a well-established numerical method to predict fracture patterns in the subsurface (Comniou and Dundurs, 1975 and others).
We apply this technique to an orthorhombic E-W, N-S normal fault system in the Wisting field, near the Hoop Fault Complex, Barents Sea. The study area has undergone significant Cenozoic uplift and thus the influence of S_V has decreased over geological time. The faults are less than 400 m depth below the sea floor present day, and the regional stress field is such that S_H is N-S (Gölke and Brudy, 1996). The aim is to explore potential local stress perturbations among intersecting E-W and N-S faults.
At the reservoirs present shallow depth, the reduced influence of S_V may have affected sub-seismic fractures around faults such that they may now be critically stressed and by proxy an indicator of permeability.
Given the regional stress orientation, the presumption would be that the most hydraulically conductive faults trend N-S. However, based on our findings and supporting evidence we propose that in fact due to local stress perturbations, E-W faults may be hydraulically conductive despite being in-optimally oriented for reactivation (Sibson, 1990).
Further work will focus on expanding the study area to the east and west of the Wisting field, where the orthorhombic fault systems no longer predominate.

References
Comniou, M., Dundurs, J., 1975. The angular dislocation in a half-space. Journal of Elasticity 5, 203-216. https://doi.org/10.1007/BF00126985
Gölke, M., Brudy, M., 1996, Orientation of crustal stresses in the North Sea and Barents Sea inferred from borehole breakouts, Tectonophysics, Volume 266, Issues 1–4, Pages 25-32, https://doi.org/10.1016/S0040-1951(96)00181-3.
Sibson, Richard, H., 1990, Conditions for fault valve behaviour. Geological Society London Special Publications, Volume 54, Pages 15-28, https://doi.org/10.1144/GSL.SP.1990.054.01.02

How to cite: Butcher, J., Jagemann, S., and Cardozo, N.: Local Stress Perturbations Among Intersecting Faults in an Orthorombic Fault System: The Wisting Field, Barents Sea, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19312, https://doi.org/10.5194/egusphere-egu25-19312, 2025.

The Olympic Subduction Complex (OSC) in the central Olympic Mountains is a deeply exhumed continuation of the offshore modern accretionary wedge of the Cascadia subduction zone. Metamorphic grade and thermochronology reveal that the OSC’s central core likely accreted by underplating at seismogenic depths. Duplexes of underplated sediments bounded above and below by abandoned paleomegathrust interfaces could therefore be preserved during exhumation. We characterize a previously unknown ~500 m wide belt of block-in-matrix mélange containing an anastomosing system of 9 major fault strands, which in turn include mm to cm wide discrete principal slip surfaces consistent with brittle-frictional, likely coseismic, slip. The lithologies are turbiditic sandstones and mudstones; other elements of ocean plate stratigraphy are absent. Raman spectroscopy of carbonaceous material refines peak paleotemperatures to 260-280°C, consistent with the seismogenic zone. We interpret these intermingled structures as a composite fault zone that records both slow and fast slip of the seismic cycle through coeval coseismic brittle-frictional and interseismic viscousdeformation. We show that the mélange forms by cataclasis, pressure solution, and development and abandonment of localized shear surfaces, while the fault strands are dominated by concentrated cataclasis and brecciation. We calculate the degree of pressure solution experienced by the mélange at the thin section scale using scanning electron microscopy and compare the accumulated strain across the fault zone through anisotropy of magnetic susceptibility. We interpret this fault zone as an exhumed paleomegathrust interface, the first direct analog for the modern Cascadia subduction zone. The absence of basalt indicates that the megathrust fault was localized within the incoming plate stratigraphic sequence in the past, facilitating sediment underthrusting, similar to offshore structures observed via seismic reflection imaging in Cascadia and elsewhere today.

How to cite: Ledeczi, A., Tobin, H., Chen, T.-W., and Mulcahy, S.: Structure and properties of the Cascadia plate interface: evidence from a newly-described exhumed paleomegathrust in the Olympic subduction complex, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-301, https://doi.org/10.5194/egusphere-egu25-301, 2025.

To assess the metamorphic and deformational history preserved in apatite, we selected high-pressure granulite to lower-amphibolite facies rocks from the East Himalaya Syntaxis (EHS) in the Tibetan orogen for in situ apatite analysis. Through the cross-correlation of cathodoluminescence (CL) images, Electron Backscatter Diffraction (EBSD) analysis, and in situ trace element and U-Pb geochronology, we elucidate the evolution of apatite growth and deformation. In a high-pressure granulite facies sample, apatite has no crystallographic preferred orientation (CPO) but instead a strong shape preferred orientation (SPO) and intragranular deformation, indicating that the apatite experienced peak metamorphism/deformation. Apatite from medium-pressure granulite facies rocks exhibits obvious oscillatory zoning, grow over the main foliation, and have weak intragranular deformation, as well as SPO and CPO, suggesting crystallization after the main phase of metamorphism/deformation. Euhedral apatite grains from upper-amphibolite facies samples display strong SPO and CPO, but only weak intragranular plastic deformation, indicating apatite growth coeval with the main deformation phase. Apatite from lower-amphibolite facies samples have core-and-mantle microstructures and a CPO subparallel to the stretching lineation, with a weak SPO and intragranular deformation, suggesting multiple phases of growth induced by fluid activity during and after peak metamorphism. Almost all apatites in high-pressure granulite facies, medium-pressure granulite facies, and upper-amphibolite facies samples exhibit similar rare earth element (REE) characteristics within individual samples. Despite undergoing different growth and deformation processes, this similarity likely reflects either chemical reequilibration under high-temperature conditions or the retention of comparable initial chemical compositions during their formation. In contrast, apatites within lower-amphibolite facies samples display inconsistent REE characteristics, suggesting that chemical reequilibration did not occur after their formation. Based on microstructural analysis, in situ U-Pb ages reveal peak and retrograde ages in the EHS at ~21 Ma and <16 Ma, respectively, overlapping with published chronology results. Therefore, multiple generations of apatite can be identified through careful cross-correlation of CL, EBSD, and geochemical data, providing a robust record of a rock's P-T-t evolution.

How to cite: Zhao, Z.: Growth and deformation of apatite in different metamorphic scenarios, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2190, https://doi.org/10.5194/egusphere-egu25-2190, 2025.

The rheology of granite under geological conditions cannot be determined directly by experiment, because of the different flow parameters for the constituent minerals, and changes in grain-size and microstructure during deformation. I use a recent geologically calibrated dislocation creep flow law for quartz, experimentally determined flow laws for feldspars, and a grain-size sensitive pressure-solution creep flow law for quartz-feldspar-mica ultramylonite utilizing a new stress/grain-size relationship for feldspar derived from subgrain piezometry. These flow laws are combined using various rheological mixing laws depending on the evolving grain-size and fabric anisotropy to give bulk rheological parameters.  This allows prediction of the effective viscosity for granite as a function of stress, temperature, and strain as the rock evolves from a load-bearing framework to an interconnected weak layer microstructure. The results have implications for tectonic processes such as rates of crustal thickening during continental collision, crustal thinning during rifting, channel flow, and diapirism. 

How to cite: Platt, J.: Rheology of granite under middle and lower crustal conditions:  tectonic implications, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2947, https://doi.org/10.5194/egusphere-egu25-2947, 2025.

Deformed quartz veins next (1-1.5m) to an exhumed pseudotachylyte-bearing (i.e. anciently seismic) fault within the Schobergruppe (Austroalpine Crystalline Complex, Eastern Alps) contain intensely kinked quartz grains. In general, kinking requires the presence of a planar mechanical anisotropy, e.g. a multi-layered structure with a regular periodic alternation of thin weakly bounded layers (or of high viscosity layers interleaved with thin low viscosity ones) such as in minerals with a strong cleavage, e.g. micas or in industrial metallic nano-laminates. Since quartz does not commonly have a strong mechanical anisotropy, we address the question of why and how kinking of quartz may develop during the seismic cycle.

The monoclinic symmetry of kink bands is consistent with the slip sense of the fault. Cathodoluminescence images show a very high density of intragranular, sub-planar, lamellae accompanied by nanometre-scale fluid-related porosity visible in electron backscatter orientation contrast. Based on the oscillating orientation variation across subgrain boundaries (misorientation angle 1-9°) these lamellae (oriented (sub)parallel to a rhomb plane and spaced 4-10 µm apart) are identified as short-wavelength undulatory extinction microstructures (SWUE). Transmission electron microscopy reveals a high degree of recovery (low dislocation density) across the SWUE. Only grains with SWUE oriented parallel to the vein boundary are kinked. We infer following history for the kink evolution related to the seismic cycle: (I) Deformation lamellae formed during high differential stresses preceding the earthquake rupturing or associated with seismic rupture propagation. The initial high dislocation density within the deformation lamellae provided the mechanical anisotropy in quartz required for (II) the subsequent coseismic initiation of kinking. The lamellae acted as a geometric filter that only allowed r<a> slip of dislocations parallel to the lamellae. These athermal dislocations were able to glide fast over a relatively large distance before piling up and initiating kinking during the coseismic event. Progressive build-up of dislocations resulted in deformation bands which accumulated the final misorientation angle between host domain and kink domain. (III) During post-seismic deformation dislocations were dynamically re-arranged under residual stress into sub-parallel subgrain boundaries which now characterize the kink band boundary region. We suggest that kinking in quartz potentially indicates coseismic deformation and is an important mechanism for incipient strain accommodation during high strain rates.

How to cite: Bestmann, M., Grasemann, B., Kilian, R., Wheeler, J., Morales, L. F. G., Bezold, A., and Pennacchioni, G.: Seismically induced kinking in quartz, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3597, https://doi.org/10.5194/egusphere-egu25-3597, 2025.

The various extensional structures developed at the shallow surface are the coupled embodiment of the deep dynamics, which have played significant roles in the Himalayan tectonic evolution. The two sets of intersecting extensional structures in the Cona area of eastern Himalayan orogen are studied, this paper presents new detailed field investigations, microstructures, quartz [c] axis CPO patterns, kinematic vorticity, deformation temperatures, zircon U-Pb, and mica ⁴⁰Ar/³⁹Ar geochronology. The results suggest that the Cona Detachment (CD) is mainly in pure shear deformation and the ductile deformation temperature ranges from 280 to 517℃. It was active between 19 and 16 Ma, and ceased at 15 Ma. However, the Cona Rift (CR) is mainly in simple shear deformation and its top-down-to-the-E ductile deformation is recorded at temperatures from 500 to 608℃. It initiated at ~16 Ma, and moved until 10 Ma. Statistically, it was found that the cessation of STDS and the initiation of NSTR seem to follow a youthful trend from west to east along the Himalayan orogen, and there is an overlap in the period of activity between them. Combined with previous studies, we speculate that this process resulted from the Indian plate tearing and the asthenosphere upwelling. Ultimately, this tectonic event leads to the mid-Miocene regime transition. The shallow surface reflects the change from N-S to E-W extensional movement in the mid-Miocene, while the fluid content, heat source, and stress field conditions are also changed in response.

How to cite: Dong, H., Tang, Z., Fei, R., Song, Y., Zhao, L., Gao, L., Wang, Y., Yan, L., and Zeng, L.: Structural and geochronological studies of the Cona Detachment and Cona Rift: Implications for the Miocene evolution of the eastern Himalayan extensional structures, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4148, https://doi.org/10.5194/egusphere-egu25-4148, 2025.

In the last decades, titanite has gained popularity in the petro- and micro-structural community due to its advantageous compositional and microstructural properties that make it an important tracer of fluids, chemical reactions and deformation mechanisms in the Earth’s crust. Thanks to its crystal structure, titanite incorporates a wide range of minor and trace elements, including OH, the equivalent chemical component of water. Indeed, although titanite is considered nominally anhydrous, it can incorporate significant amounts of OH (up to 0.1 wt.%). Understandings of hydrogen concentration, which constitutes the water molecule, in titanite is important since it can affect the physical and chemical properties of minerals and rocks of the upper mantle/lower crust, such as deformation mechanisms, rheology and fluid flow. Moreover, these studies could be useful to understand the reservoirs and the water cycle in the Earth system. Nevertheless, advanced studies regarding water content in titanite are still lacking. However, thanks to recent improvements in both petrochronological and microstructural techniques, the investigation of the link between hydrogen mobility and deformation processes at the nanoscale is now possible.

In this study, we combined the Atom Probe Tomography (APT) to obtain 3D maps showing the distribution of OH with Photo-induced Force Microscopy (PiFM) to quantify the amounts of OH in selected deformed and undeformed domains of mylonitic titanite. In particular, we investigate OH variations between titanite domains showing different dislocation densities in rock layers with different composition (amphibolite vs calcsilicate). Preliminary results show interesting OH variations though the titanite grains from the different layers and a positive correlation between OH and dislocation densities, suggesting a heterogeneous distribution of water during deformation strongly dependent by the bulk rock composition. This study highlights the importance of a multi-disciplinary, multi-technique approach to advance our understanding the deformation/chemical processes occurring at the nanoscale ultimately governing the large scale rheological and chemical evolution of rocks during major tectonic events.

How to cite: Corvò, S., Chen, Y.-S., Holmes, N., Förster, M. W., Yaxley, G., Maino, M., Langone, A., Cairney, J., and Piazolo, S.: Deformation control on OH distribution at the nanoscale: a case of mylonitic titanite, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4781, https://doi.org/10.5194/egusphere-egu25-4781, 2025.

The strength and deformation behaviour of the Earth are necessary parameters to model and fundamentally understand crustal scale deformation. In the case of the Earth’s continental middle crust, so far, most models use extrapolated physical parameters from monomineralic deformation experiments, assuming pure quartz to represent the weakest rheological phase at mid-crustal conditions. This is an oversimplification as the Earth’s continental middle crust mostly consists of polymineralic granitoid rocks, the rheology of which is unknown so far.

We present the first experimental study investigating the viscous rheology of a natural, fine-grained, granitoid rock. To unravel the complex deformational behaviour, it is crucial to combine an in-depth microstructural analysis with thorough estimations of rheological parameters. Cylindrical natural granitoid ultramylonite samples, composed of qtz + ab + K-fsp + bt + ep, with grain sizes of 125-15 μm are deformed in a Griggs type apparatus at T=650°C, confining pressure=1.2 GPa, strain rates=10^-3 to 10^-5s^-1, and 0.2 wt% H₂O added. Mechanical data are combined with light microscope, SEM, TEM, and quantitative image analysis to connect microstructures with stress and strain evolution.

Only through this combination we can show that grain size sensitive deformation processes, namely dissolution precipitation creep (DPC) lead to extreme grain size reduction, and pinning prevents grain growth and produces a stable microstructure. These processes lead strain localization and overall very weak viscous behaviour – weaker than can be extrapolated from monomineralic quartz. We further can show through two different experimental setups how strain is localizing with and without preexisting fracture in an initially foliated rock. Verified with microstructural observations, we fit parameters into a constitutive equation for this DPC, based on an exponential diffusion creep flow law, to model our experiments and tackle the extrapolation to various natural conditions.

Our findings imply that the Earth’s granitic middle crust deforms faster in localized shear zones than previously modelled. This would result in faster stress buildup at the viscous to brittle transition, promoting seismic ruptures in the overlaying brittle crust than predicted so far. Thereby, our new insights can improve models investigating mechanics of extensions of earthquakes to the middle crust, where they supposedly nucleate and help understand seismic cycles or improve models of stress buildups and thermal flow below and influences on hydro/geothermal systems. We further highlight the importance to improve modelling of polymineralic systems.

How to cite: Nevskaya, N., Berger, A., Stünitz, H., Plümper, O., Zhan, W., Ohl, M., and Herwegh, M.: Weaker than quartz? – Strain localization mechanisms and rheology of fine-grained polymineralic rocks, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4855, https://doi.org/10.5194/egusphere-egu25-4855, 2025.

High-angle misorientations, including grain boundary domains within quartz aggregates, may exert significant control on how strain is accommodated in quartz-bearing domains of the continental lithosphere. Among the high-angle boundaries in quartz, Dauphiné twin boundaries (DTBs) are the most prominent type that displays coincident site lattice (CSL) relationships, a special grain boundary geometry commonly found in metals. In metals, it is known that CSL boundaries stabilize the microstructure by reducing the overall grain boundary surface energy, and thereby impart certain special properties to the material. The present study explores CSL relationships of DTBs and how they control strain accommodation and deformation mechanisms in quartz-rich rocks by combining optical microscopy, Scanning Electron Microscope-Electron Backscatter Diffraction (SEM-EBSD), and Atomic Force Microscopy (AFM). This investigation was carried out on thin sections of granulite facies quartzo-feldspathic gneisses prepared following a standard protocol, culminating with chemical mechanical polishing (CMP) using colloidal silica. The CMP procedure causes preferential material removal along less-compact random high-angle grain boundaries (RHAGBs) and forms indented channels that are prominent in high-resolution, nanoscale AFM images. The absence of corresponding channels along DTBs and the presence of 'bridge-like' structures at RHAGB-DTB intersections suggest greater compactness of DTBs in quartz, which is compatible with CSL relations. Importantly, there is a demonstrable change in misorientation across adjacent quartz grains between consecutive RHAGB-DTB intersections. Grains adjacent to these RHAGB segments have angles between their c-axes varying from 61-66° and 81–84° with parallel rhombohedral faces. These symmetries represent the Japan and Sardinian twin laws of quartz, indicating that the RHAGB segments are modified by DTBs into low-energy twin boundaries, thereby reducing the overall surface energy of the aggregate. Contextually, these quartzofeldspathic gneisses record multiple deformations of the previous ultra-high-temperature (UHT) fabric under granulite facies conditions, with grain boundary migration recrystallization (GBM) as the dominant dynamic recrystallization process. Owing to the GBM recrystallization, the interaction of RHAGBs with DTBs increases, thereby producing more intersection-induced twin boundaries. The relict vs. recrystallized quartz grain maps with DTBs indicate a higher frequency of DTBs in the relict grains. Crystallographic preferred orientations (CPOs) of these relict grains do not define any slip system pattern. In contrast, the recrystallized grains show the operation of prominent slip system patterns, suggesting differences in strain accommodation. Therefore, the formation of DTBs and DTB-RHAGB intersections in quartz can cause strain partitioning due to the higher compactness of DTBs, which will eventually control the response to the deformation of high-grade quartz aggregates. As a result, interpreting the timing of DTB formation becomes important while deciphering the deformation history of rocks based on quartz CPOs from a poly-deformed high-grade terrane.

How to cite: Dey, S., Chatterjee, S., Dobe, R., Mandal, S., and Gupta, S.: Grain boundary microstructures and their control on deformation mechanisms in high-grade quartz-rich rocks, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5264, https://doi.org/10.5194/egusphere-egu25-5264, 2025.

Fault zones are complex systems where mineralogy, fabric, and frictional properties interplay on fault strength and slip behavior. While prior investigations have focused on post-experimental microstructures to relate fault friction to deformation processes, the evolution of fault fabric coupled with Acoustic Emissions (AEs) during deformation remains elusive. In this study, we present experimental data integrating systematically microstructural, mineralogical, frictional, and AEs analysis coming from a suite of frictional experiments in a double direct shear configuration. These experiments aim to elucidate deformation micro-mechanisms and associated acoustic activity in heterogeneous fault systems. We performed a set of experiments in quartz-phyllosilicate mixtures and another set of tests where a quartz layer is sandwiched between two muscovite horizons in contact with the forcing blocks. The first set represents typical cataclastic rocks with random fabric while layered mixtures were designed to replicate the deformation behavior of block-and-matrix shear zones.

Pure quartz gouges exhibit cataclastic deformation (grain fragmentation and shear localization) which generates high AE rates and amplitudes. In contrast, muscovite-rich gouges deform through distributed sliding along anastomosed foliation and are characterized by low AE activity. In quartz-phyllosilicate mixtures, increasing muscovite content reduces friction, AE rate, and AE average amplitude inhibiting quartz grain interactions. Layered mixtures introduce additional complexity. While the two muscovite layers govern frictional strength and accommodate distributed deformation, cataclastic processes in the central quartz layer dominate AE activity. These layered systems combine the AE characteristics of quartz and muscovite, with high AE rates similar to pure quartz despite the overall weakening from muscovite. Microstructural observations support these findings, showing deformation concentrated at muscovite interfaces but also revealing localized shear bands in the quartz layer that significantly contribute to AE activity. Experiments performed at varying strain rates reveal that higher strain rates amplify AE rate and amplitude. In the layered mixtures, at high strain rate, AE rates and amplitudes are in the range of pure quartz gouge whereas at low strain rate, they are in the range of pure muscovite gouge.

The evolution of the frequency-magnitude distribution of AEs (b-values) with strain provides additional insights on micromechanical processes: quartz-dominated gouges show increasing b-values from yield to steady-state, suggesting a transition from distributed to localized deformation. In contrast, muscovite-dominated gouges maintain low b-values, reflecting consistent distributed deformation. Layered systems exhibit b-value evolution with strain similar to pure quartz, indicating the dominant role of quartz in the AE properties. Our findings emphasize that fault friction and acoustic behavior are controlled by mineralogical and structural heterogeneities and modulated by strain rate. Weak muscovite layers primarily control frictional strength, while strong quartz layers generate significant AEs, highlighting the potential for aseismic slip coupled with seismic activity in heterogeneous faults.

How to cite: Casas, N., Volpe, G., Collettini, C., and Scuderi, M. M.: How mineralogy and fault structure influence frictional and acoustic properties of quartz-phyllosilicate mixtures, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5757, https://doi.org/10.5194/egusphere-egu25-5757, 2025.

In situ garnet U-Pb geochronology by laser ablation-inductively coupled mass spectrometry (LA-ICPMS) is a powerful tool for rapid and high-spatial resolution dating of metamorphic pressure-temperature–time histories. However, ultra-low U and Pb contents in many metamorphic garnets pose a significant analytical challenge, including the risk of contamination by inclusions. Here we use split-stream analysis to simultaneously measure U, Th, and Pb isotopes and trace-element contents of garnet from eclogite-facies metamafic rocks (As Sifah, Oman). The data show strong linear correlations in U vs Zr contents, the slopes of which can only be explained by zircon contamination. Time-resolved U, Th, and Pb signals show some irregularities, but often lack sharp spikes typically diagnostic of inclusions. Abundant micro-zircon (<2 µm diameter) inclusions were observed by SEM in all five samples. Interestingly, garnet–zircon mixing lines in Tera-Wasserburg space project to well-defined concordia intercept dates of 94–89 Ma, whereas a sample with sufficient zircon-free analyses gave an intercept date of 71 ±7 Ma. The latter date overlaps within uncertainty of published Sm-Nd garnet–whole rock isochron ages (81–77 Ma) and U-Pb zircon ages (82–78 Ma), whereas ‘garnet’ dates dominated by the zircon-inclusion are 8 to 17 Myr older. The discrepancy between zircon inclusion and published zircon ages from the same samples likely arises from the fact that such small zircon size fractions are rarely dated and may represent a different age population from that of lager grains. We suggest that the <2 µm zircon fraction formed by diffusion-limited local nucleation from the expulsion of Zr from igneous precursor minerals during (sub-)greenschist facies metamorphism. Recrystallization and Ostwald ripening of matrix grains produced zircons that record peak metamorphic ages, whereas the micro-zircon grains remained isolated and were shielded by garnet. We therefore suggest that (1.) the ablation of inclusions may produce linear trends in Tera-Wasserburg space that may be easily misinterpreted as U-Pb garnet ages, and (2.) bulk inclusion dating of zircons by LA-ICPMS is a promising technique that requires further investigation. Finally, we establish more rigorous criteria for the screening of samples for inclusions prior to and during LA-ICPMS U-Pb garnet geochronology analysis.

How to cite: Walters, J., Garber, J., Beranoaguirre, A., Millonig, L., Gerdes, A., and Marschall, H.: What are we dating? The role of micro-inclusions for in situ garnet geochronology, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6118, https://doi.org/10.5194/egusphere-egu25-6118, 2025.

Subduction zone dynamics depend on the rheological behavior of shear zones defining the plate interface, which can accommodate a spectrum of slip from earthquakes to creep. In cold subduction zones, high-pressure, low-temperature metamorphism of the oceanic crust forms blueschist-facies rocks within the interface. Geologic observations demonstrate that deformation at the plate interface is often localized into blueschists with fabrics dominated by glaucophane, a sodic amphibole. Microstructural observations of exhumed blueschists suggest that glaucophane typically deforms by dislocation-accommodated mechanisms. Recent experiments have made progress in developing new flow laws for diffusion creep and dislocation creep. However, the rheological behavior of blueschists remains under-constrained due to challenges arising from phase stability that limits experimental temperatures. Therefore, we conducted experiments at very high pressures to suppress brittle deformation and favor dislocation-accommodated mechanisms to investigate low-temperature plasticity in glaucophane. We conducted cyclical loading experiments in the deformation-DIA on blueschist cores, both parallel and perpendicular to foliation, and cold-pressed glaucophane powders. Experimental conditions covered confining pressures of 6–8 GPa and temperatures of 20–800 °C at strain rates of ~10^–4 s^–1. We observe temperature-dependent yield and flow stresses and nearly temperature-independent backstresses. These mechanical results as well as the microstructures are characteristic of deformation dominated by dislocation glide. These experiments provide the foundation for a constitutive law describing low-temperature plasticity in glaucophane, which will serve as an input for geodynamic models that aim to understand the physics of strain localization, quantify steady-state interface strength, and explain the mechanics of slip transients.

How to cite: Seyler, C., Kotowski, A., James, K., Thom, C., Hansen, L., and Hein, D.: Dislocation-accommodated deformation in blueschists from high-pressure experiments, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7583, https://doi.org/10.5194/egusphere-egu25-7583, 2025.

We present detailed 1:10’000-scale geological maps of the eastern flank of the Lepontine Dome, focusing on the Tambo nappe. These maps, produced for the Grono, Mesocco, and Hinterrhein sheets of the Geological Atlas of Switzerland 1:25’000 (GA25), reveal the nappe’s complex internal deformation history, influenced by inherited rheological anomalies, localized fluid percolation, and varying metamorphic conditions. The study of basement nappes internal deformation provides crucial insights into contrasting exhumation mechanisms, a topic central to understanding orogenic dynamics. While nappes exhumed through channel flow mechanisms typically exhibit incoherent internal deformation, those exhumed via Stokes flow or within an accretionary wedge are more structurally coherent.

The Tambo nappe is composed of heterogeneous paragneisses, micaschists, amphibolites, and metagranitoids. This 20 km-long body was emplaced northward as part of the Alpine nappe stack derived from the Briançonnais paleogeographic domain since the Paleocene. The dominant structural elements (foliation, fold axes, and lineation) are predominantly eastward-oriented.

In the southern part of the nappe, the Truzzo meta-granitoid, a folded Permian batholith, acted as a competent, elongated body embedded within micaschists and paragneisses. Below the batholith, pervasive greenschist facies mineral assemblage (chlorite and phengite bearing), associated with top-to-the-east shearing, overprinted earlier amphibolite-facies metamorphism. Deformation subsequently localized along the Forcola Fault. Quartz veins supplied the fluids responsible for the greenschist-facies metamorphism during orogen-parallel shearing that postdated nappe emplacement.

In the northern part, approximately 2 km from the nappe’s front, the lithologies steepen into a cusp-like geometry oblique to the upper nappe boundary. This sector is characterized by sporadic metacarbonate lenses within metasomatized gneisses and schists. Over several hundred meters, syn-foliation veins were folded and boudinated at local peak metamorphic (lower amphibolite) conditions. Locally, some veins cut across the foliation, indicating fluid percolation during deformation. Unlike the southern part, deformation and fluid activity here occurred at peak metamorphic conditions, obliterating earlier textures. We postulate that metacarbonates at the cusp’s core may represent remnants of squeezed Permo-Mesozoic grabens or half-grabens.

These findings suggest that the Tambo nappe behaved as an incoherent body incorporating Mesozoic fragments during nappe emplacement. Subsequent orogen-parallel shearing further thinned the sequence. Field evidence highlights the crucial role of fluids in both deformation stages, influencing rheology and metamorphism.

How to cite: Schenker, F. L., Czerski, D., Scapozza, C., De Pedrini, A., Ambrosi, C., De Paoli, R., Bonazzi, O., Troger, M., and Gouffon, Y.: The Tambo nappe: insights into its internal deformation from geological mapping (Swiss-Italian Alps), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8302, https://doi.org/10.5194/egusphere-egu25-8302, 2025.

We investigated the microstructures in the pelitic schists of the high-P/low-T Sanbagawa metamorphic belt in the Shibukawa area, northwestern Shizuoka Prefecture, Japan, which are mostly classified as low-grade, non-spotted schists, and delineated the thermal structure and microstructure of the Sanbagawa pelitic schists in the vicinity of an ultramafic block using a Raman carbonaceous material (CM) geothermometer and SEM-EBSD crystallographic analysis. The pelitic schists consist of alternating mica-rich and quartzofeldspathic layers. The mica-rich layers consist mainly of muscovite, chlorite, and CM, whereas the quartzofeldspathic layers consist mainly of quartz and albite. The maximum experienced temperatures estimated from the carbonaceous material (CM) were 277–354°C, corresponding to the metamorphic temperature of the chlorite zone. In the quartzofeldspathic layers, both quartz and albite showed subgrain boundaries with undulatory extinctions, indicating plastic deformation. The crystallographic preferred orientations (CPOs) of the quartz grains within the pelitic schists show weak but distinct patterns somehow resembling a type-I cross girdle, whereas those of the albite locally show (100)[001] patterns. The average sizes of both quartz and albite grains were at around 10 µm for all samples and increased slightly with increasing modal composition, independent of the Raman CM temperature. This suggests that the duration time of the peak metamorphic temperature may not be long enough to mature metamorphic textures in the pelitic schists. As a consequence, the microstructures in the pelitic schists would result from the interaction between the grain growth due to heating and the grain size reduction due to deformation in the subduction zone.

How to cite: Katagiri, S., Kouketsu, Y., and Michibayashi, K.: Structural and thermal characteristics around an ultramafic body in the low-grade Sanbagawa pelitic schists, central Honshu Island, Japan, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8742, https://doi.org/10.5194/egusphere-egu25-8742, 2025.

Furtschaglschiefer is a thick layer of graphitic schists belonging to the Greiner unit in the western Tauern Window. The precise tectonostratigraphic assignment of these rocks is debated, but they are considered the post-Variscan cover of the Zillertal-Riffl nappe, which was metamorphosed only during the Alpine orogeny.

These schists display a peculiar structure, where biotite adds to garnet to form porphyroblasts up to 1 cm long, defining a marked lineation. The texturally well-equilibrated mineral assemblage of the schists comprises quartz, plagioclase, garnet, biotite, ilmenite, muscovite, graphite ± chlorite ± epidote ± staurolite.

Garnet occurs as euhedral rhombic dodecahedral porphyroblasts 2-3 mm in diameter. Biotite occurs in three textural generations, of which the most abundant and coarser is represented by microboudinaged porphyroblasts with cleavage at high angle to the foliation, showing evidence of repeated crystals opening at cleavage planes and sealing by new biotite and in places by quartz. The evidence of this process is given by the variable distribution of graphite inclusions within biotite. The abundant graphite is disseminated in layers defining the main foliation of the rock, probably an isoclinally folded sedimentary bedding. The resulting "accordion-like" biotite porphyroblasts display an average total strain of 175%. Like garnet and ilmenite, these porphyroblasts show symmetric synkinematic features (rotation of cleavage and internal foliations) and strain shadows. When observed, sinistral and dextral shear sense indicators are equally recorded. The 2D analysis performed on thin sections was complemented by a 3D µCT study on two samples. Microboudinaged biotites show a prolate strain, with the long axis parallel to the rock lineation. Many coin-shaped ilmenite porphyroblasts are oriented parallel to the main foliation. The microboudinaged biotite of the Furtschaglschiefer probably formed in a two-stage process including 1) random static growth followed by 2) pure shear, intense microboudinage and precipitation of new biotite (and quartz) with very little rotation.

Continuous biotite and garnet growth occurred during and after deformation and foliation development, under almost constant PT conditions as evidenced by the constant chemical composition of synkinematic phases like biotite.

Phase equilibrium modelling and thermometry based on Raman spectroscopy of carbonaceous matter and titanium in biotite converge to indicate metamorphic temperatures of c. 550 °C, in agreement with previous studies. Constraints on metamorphic pressure are still poor, and at present there is no evidence for significant decompression from higher-P conditions predating the development of the main Grt-Bt-Ilm±Chl assemblage.

Preliminary results of in situ U-Pb dating by LA-MC-ICPMS provide an age 24.0 ± 2.6 Ma for garnet and of 26.1 ± 3.3 Ma for ilmenite, confirming the Oligocene age of amphibolite-facies metamorphism in this part of the Tauern Window.

How to cite: Cesare, B., Bedon, S., Salvadori, L., Bartoli, O., Macente, A., Behr, W., and Beranoaguirre, A.: Microstructural, petrological and petrochronological study of the graphitic schists ("Furtschaglschiefer") of Passo di Vizze (SW Tauern Window, Eastern Alps, Italy), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9019, https://doi.org/10.5194/egusphere-egu25-9019, 2025.

The petrographic study and structural analysis of high-pressure rocks can bring to light some of the mechanisms involved in the subduction and subsequent exhumation of the crustal materials affected by these processes. In the Cabo Ortegal Complex (NW Spain) the emplacement, amalgamation and progressive deformation of still hot peridotites between the Chimparra gneisses and the Bacariza high-pressure granulites imposed a thermal gradient in the shear zones developed at the contact with the high-pressure gneissic formation. This gradient lead to a change of deformation mechanisms and operative intracrystalline slip systems in all the constituent minerals during initial stages of the exhumation of the complex in the subduction channel.

In external areas of the shear zone affecting the gneissic materials, large garnets accommodated strain by rigid rotation. However, close to the shear zones elongated garnets behaved more plastically assisted by 1/2<111>{110} and <100>{010} intracrystalline slip systems. Kyanite, in turn, shows kink bands, sigmoidal geometries and subgrain boundaries disposed subperpendicular to the X structural direction. This feature suggests the activation of [001](100) and [001](010) intracrystalline slip systems in outer sectors and glide on (100) planes along the [001] direction in areas closer to the contact. Although oligoclase does not show systematic orientation distribution patterns in distant sectors the lattice preferred orientation (LPO) obtained at the contact indicates clearly the activation of [100](001) intracrystalline slip systems which have been recognized in rocks deformed under medium- to high-grade conditions. This is in accord with the operation of prism-<a> slip systems in quartz for those samples closer to the ultramafic massif, indicative of temperature conditions up to 700º C. Away from the contact, the temperature decreases, leading to activation of basal- and rhomb-<a> systems in this phase suggesting lower deformation temperature or, more likely, higher strain rates along narrower sectors of the subduction channel accommodating deformation.

How to cite: Puelles, P., Abalos, B., and Esteban, J. J.: Constraining the thermal gradient in shear zones bounding the Chimparra high-pressure gneisses of the Cabo Ortegal Complex (NW Spain): an EBSD approach , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9048, https://doi.org/10.5194/egusphere-egu25-9048, 2025.

Pseudotachylytes (quenched frictional melts produced during coseismic slip) represent unambiguous evidence of fossil earthquakes. They are routinely studied to investigate the processes operating before, during, and immediately after seismicity. To place this knowledge into the relevant tectonic context, requires the capability of determining the age of seismicity. However, pseudotachylytes are thin (typically below 1 cm thick) and extremely fine grained, rendering the application of geochronological tools challenging. In the upper crust, the ⁴⁰Ar/³⁹Ar method has been successfully used to date pseudotachylytes, however, no successful attempts of dating lower-crustal pseudotachylytes have been reported until now.

We present results from in-situ apatite U-Pb geochronology applied to lower-crustal pseudotachylytes exposed in Lofoten, northern Norway. The sample suite includes a pristine pseudotachylyte, a mylonitized pseudotachylyte, and a mylonite with pseudotachylyte veins transposed into the foliation. The results are scrutinized by detailed microstructural investigations using electron backscatter diffraction and cathodoluminescence (CL) imaging. By doing so, we are able to identify apatite that deformed by crystal plasticity in response to the seconds-to-minutes-long thermal pulse generated by the earthquake. The corresponding apatite U-Pb data define a tight regression with a lower intercept at ~426 Ma; the age of lower-crustal seismicity in the Lofoten exposures.

Our analyses also yield an age population significantly younger than the age of seismicity. The single spot dates of this population are microstructurally controlled, i.e., spots within the same microstructural framework yield similar ages, and characterized by significant variations in CL intensity within single grains. The U-Pb data of this age population do not define a clear regression, but rather indicate several Pb-loss events. Patchy CL zoning in some of these apatites indicates modification by dissolution-precipitation, and thus the involvement of a fluid in partial resetting of the apatite U-Pb system. We interpret this age group to reflect protracted fluid percolation along the pathways provided by the pseudotachylytes and shear zones. In-situ apatite U-Pb dating coupled with microstructural investigations thus provides (1) the first robust age of a fossil lower-crustal earthquake, which indicates that seismicity occurred during the early stages of continental collision, as well as (2) evidence that such structures serve as long-lived fluid pathways long after seismicity occurred.

How to cite: Zertani, S., Menegon, L., Whitehouse, M. J., Jeon, H., and Jamtveit, B.: In-situ apatite U-Pb geochronology coupled with microstructural analysis reveals the age of lower-crustal seismicity, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10325, https://doi.org/10.5194/egusphere-egu25-10325, 2025.

Seismic activity often triggers liquefaction, a process where water-saturated sediments lose their strength due to an increase in pore water pressure. This process leads to the development of soft-sediment deformation structures (SSDS), such as load casts, clastic dykes, flame structures etc. However, distinguishing seismogenic SSDS from those triggered by other mechanisms (e.g., storms, overloading) can be challenging.

This study investigates the micromorphological changes in quartz grains derived from SSDS caused by liquefaction triggered by seismic shocks., Laboratory simulations mimicking seismic conditions revealed characteristic quartz alterations, including microcracks, edge corrosion, and grain fragmentation. These features were found to be closely linked to the duration of seismic exposure.

Particularly notable was the discovery of gold within quartz cracks, which serve as direct evidence of seismic events and underscore the role of seismicity in mineral redistribution. This novel finding highlights the potential of quartz grains as micro-scale markers for reconstructing past seismic activities. Geochemical factors, such as pH and redox potential, further influenced the behavior of liquefied sediments and the extent of quartz grain deformation, demonstrating the complex interplay between seismic forces and geochemical conditions in shaping sedimentary records.

How to cite: Świątek, S., Lewińska, K., Pisarska-Jamroży, M., and Günter, C.: Seismically-induced quartz grain alterations as indicators of past earthquake events, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12829, https://doi.org/10.5194/egusphere-egu25-12829, 2025.

Pristine microstructures preserved in a pseudotachylyte (coseismic-derived quenched frictional melt) are recordkeepers of the time-lapse processes associated with the dynamic rupture propagation and the earthquake slip. Unlocking these near-instantaneous processes from the microstructures in a pseudotachylyte provides insights into the chemo-mechanical processes operating during, and immediately following an earthquake.

Inclusion-rich, mutually intergrown garnet aggregates with a morphology akin to a cauliflower are a common product phase in lower-crustal pseudotachylytes. Pseudotachylyte veins in gabbroic rocks from Lofoten (Norway) formed at ambient temperatures of 650–700 °C and display pristine quenching microstructures in the matrix such as plagioclase microlites, dendritic clinopyroxene, cauliflower garnet aggregates, survivor lithoclasts of plagioclase, olivine, and orthopyroxene, and coronas of cauliflower garnet at the interphase boundary between orthopyroxene and the pseudotachylyte matrix. There is a lack of hydrous phases such as amphibole or biotite. Quantitative compositional maps across an entire vein show an irregular or patchy intercrystalline major element distribution in cauliflower garnet. Pyrope and spessartine contents are higher in garnet coronas around survivor lithoclasts comprised of orthopyroxene and olivine, while grossular is highest near plagioclase survivor lithoclasts and near the contact with the plagioclase-rich wall-rock. In some instances, trace elements maps show sharp compositional zoning within single garnet grains. In addition, electron backscattered diffraction data indicate that the garnet corona grains in contact with orthopyroxene along the pseudotachylyte vein boundary show evidence of crystal-lattice distortion through dislocation glide. Collectively, these microstructures indicate that the anhydrous pseudotachylyte melt quenched extremely quickly. The quenching of garnet proceeded faster than the chemical homogenisation in the frictional melt, freezing in and preserving local compositional variations without any later recrystallization at amphibolite-facies ambient conditions.

Using garnet-clinopyroxene geothermometry on cauliflower garnet cores and rims in contact with clinopyroxene inclusions and microlites, respectively, and using a cooling model for the pseudotachylyte vein, we estimate that garnet quenched from the frictional melt starting at ~1100–900 ºC, as recorded in garnet cores, and ceased growing upon reaching the ambient temperature of ~650–700 ºC in <1 hour. In the short duration of mineral growth, garnet captured the incipient distribution of major and minor elements from the frictional melt and was able to record the post-seismic stress relaxation that localized in the form of solid-state creep along the pseudotachylyte vein margin during quenching. High-T microstructures are preserved because dislocations in garnet are immobile at ambient lower-crustal temperatures, and anhydrous conditions inhibit recrystallisation, diffusion, and viscous deformation.

How to cite: Michalchuk, S. P., Zertani, S., Markmann, T., Hermann, J., Lanari, P., Rubatto, D., and Menegon, L.: High-T quenching microstructures in cauliflower garnet capture instantaneous post-seismic processes in a lower-crustal seismogenic fault, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12837, https://doi.org/10.5194/egusphere-egu25-12837, 2025.

In natural shear zones, micro-porosity is found to decorate the grain boundaries of rocks which have been deformed in viscous conditions. Whether porosity is formed during or after deformation is widely debated, and requires further investigation to test how micro-pores may be produced, particularly in monomineralic aggregates.

Using a new-generation Griggs-type apparatus, we performed two general shear experiments using a fine grained (∼ 3 μm) quartzite (white novaculite) with low to no primary porosity (< 1%). The experiments were performed at a temperature of 900 °C and pressures of 1.2 and 1.5 GPa, with bulk strain rates of ≅ 1.2×10^-4 and 2.3×10^-5 s^-1, respectively. In both, 1 wt% of water was added to the starting sample.

During deformation, both samples record a significant stress drop following a high peak of differential stress, after which a progressive strain weakening occurs over several gamma of shear strain. In the experiment at 1.2 GPa, the sample deformed above the Goetze criterion at peak stress, where σ₁-σ₃ > 1.2 GPa, which gave rise to a highly fractured sample. Within the sample, shear planes < 1 µm thick contain a material which is brighter than quartz in SEM-BSE, despite being composed of SiO₂. Microstructural observations of the same sample show the production of a penetrative secondary porosity, where most pores are < 1 µm in diameter. Pores decorate most grain boundaries, which are open and easily identifiable in the SEM.

In contrast, the experiment at 1.5 GPa did not experience any fracturing, and the maximum differential stress remained below the Goetze criterion at σ₁-σ₃ ≅ 0.8 GPa. In this experiment, a porosity of microns to tens of microns in size developed along apparent conjugate bands. Outside of these bands, there is no porosity nor open grain boundaries. Electron backscatter diffraction (EBSD) analyses reveal quartz which deformed viscously, both inside and outside of the porosity-decorated bands. However, quartz grains within the pore-decorated bands have a stronger intragrain misorientation and higher lattice curvature gradients, as well as slightly weaker lattice-preferred orientation than in the surrounding, non-decorated quartz. Finally, an interesting feature of EBSD maps is the lower indexation rate of quartz within the porosity-decorated bands, at 54% within compared to 83% outside. While the reason for this is unknown, the non-indexed area is considerably larger than the area of pores.

How to cite: Arbaret, L., McGill, G., Précigout, J., Prigent, C., and Airaghi, L.: Experimental deformation of natural monomineralic quartz to produce micro-porosity, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13040, https://doi.org/10.5194/egusphere-egu25-13040, 2025.

Dora Maira is one of the internal crystalline massifs of the Western European Alps, formed by basement nappes of the Penninic Domain. The massif is characterized by high-pressure (HP) and ultra-high-pressure (UHP) rocks and is famous for the presence of coesite-bearing whiteschists. These rocks can be employed as source of information for Earth’s subduction-exhumation cycle, as well as window into (U)HP mechanical processes. For these reasons, Dora Maira whiteschists have attracted the attention of petrologists for the last four decades. However, most of the previous studies focused on petrological aspects and little attention has been given to the peculiar microstructures of these intriguing rocks. This contribution shows the preliminary results of a microstructural and compositional study performed on the whiteschists, with a focus on garnet crystals.

These rocks are characterized by a spatially variable foliation, defined by the alignment of phengite and garnet crystals. Where the foliation is spaced, palisade quartz develops between phengite crystals with an orientation mostly at high angle to the main schistosity. Palisade quartz also formed within garnet crystals, surrounding or completely substituting pre-existing coesite inclusions.

Garnet grains are both elongated parallel to the rock’s foliation or rounded in shape. They show two sets of fractures: a parallel set developed at high angle to the rock schistosity, and radial fractures around coesite/palisade quartz inclusions. The formation of this second set of radial fractures is due to the large volumetric change involved in the coesite-quartz transition.

We adopted SEM-EDS, (HR-)EBSD techniques and performed microprobe analyses to study both microstructures and composition of garnet crystals. Their composition is ca. 88 up to 98% pyrope. However, they also show a distinctive chemical zoning around inclusions, which results in a higher grossular content.

These observations raise questions on the mechanical behaviour of garnets at UHP. In particular, the coesite-quartz transition provokes large volumetric changes which likely result in a mechanical modification of the host garnet. The question is whether the volumetric change of the phase transition and related fractures can trigger also a chemical redistribution. More investigations are still needed, however a strong influence of mechanics on garnet crystals’ behaviour in these rocks is undeniable. Only a meticulous microstructural and compositional analysis can shed light on the history written in the pyrope crystals.

How to cite: Tagliaferri, A., Tajčmanova, L., and Duretz, T.: A Garnet tale: chemical and mechanical responses in Dora Maira Whiteschists, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13122, https://doi.org/10.5194/egusphere-egu25-13122, 2025.

The Swiss deep geological repository for radioactive waste is to be sited in the easternmost part of the Jura fold-and-thrust belt that forms the external allochthonous units in the Western Alps. Although it is commonly accepted that the Jura is a thin-skinned fold- and-thrust belt, which is detached from the underlying autochthonous units along Middle Triassic evaporites, this is not so clear for Nagra’s siting region. There, the amount of shortening accommodated in the so-called Siglistorf Anticline is only some 150-220m (Jordan et al., 2015, Nagra Arbeitsbericht NAB 14‐105). The location of the anticline above the northern boundary fault of a crustal-scale Paleozoic graben gave rise to a dedicated discussion whether the calculated shortening is in fact related to a thin-skinned detachment or related to thick-skinned deformation involving the underlying basement (e.g., Schöpfer et al., 2023, Terra Nova, and references therein).

To test the competing thin- and thick-skinned models, we analyzed cores from four wells drilled by Nagra through the evaporitic detachment for structures which can be used for quantifying shear strain and, hence, the thrust displacement. Cores of the drilled anhydrite and halite succession are oriented allowing to determine the true orientation of structures. Variably oriented stretching lineations along with sigma clasts, winged inclusions, shear bands and asymmetrical boudins prove polyphase kinematics with different transport directions arguing against a continuous high-strain detachment. Recorded shear directions are top-NNW, top-NNE and top-W. The preservation of primary sedimentary and early diagenetic fabrics, shapes of winged inclusions, angles between S and C planes in shear bands, elongations calculated from boudinaged layers and the abundance and sizes of survivor grains are used to estimate the finite shear strain for the different lithotypes, which are categorized in undeformed (tangent of the shear angle γ=0), low shear strain (γ<2), medium shear strain (γ<7), and high shear strain (γ<15). The total maximum displacement, calculated from the sum of the thickness-shear strain products for each strain category, are 41m, 66m, 79m and 123m, for the four investigated boreholes.

Balanced cross-sections from the area of interest based on the thin-skinned deformation model state a shortening of approximately 150 to 220m which is accommodated in the Siglistorf Anticline north of the investigated boreholes. In this thin-skinned model the whole respective displacement must be accommodated by the regional evaporitic detachment, which, however, is considering the estimated total displacements from the respective core intervals not fully supported. We propose that shortening accomodated in the Siglistorf Anticline is also related to shortening involving the strata below the regional décollement level. It is concluded that deformation of the easternmost part of the Jura fold-and-thrust belt is in fact a combination of thin-skinned and thick-skinned tectonics.

How to cite: Mohideen, A., Decker, K., and Grasemann, B.: Fabric and shear strain of a potential halite detachment below the Swiss Eastern Tabular Jura, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13163, https://doi.org/10.5194/egusphere-egu25-13163, 2025.

The polymineralic nature of most rocks induce changes in deformation mechanisms with respect to those observed in monomineralic aggregates, and challenges our understanding of the feedbacks between microstructure and rheology. In-situ information from high pressure experiments, such as X-ray tomography and X-ray diffraction, offer the opportunity of quantifying the fabric and stresses, and their evolutions, in polymineralic rocks, under high pressures and high temperatures relevant for the deep earth. Here we present results relevant to the deformation of serpentinized peridotites. Interconnected weak layers (IWL) of serpentine can cause morphological anisotropy and strain localization in serpentinized peridotite, with important implications for the mechanical properties of the lithosphere. We quantify the morphological anisotropy, topology, and interconnectivity of serpentine, in serpentine + olivine aggregates, under torsion. We use in-situ X-ray absorption-contrast tomography at pressures of ca. 4 GPa and temperatures 300-400°C. At shear strains γ >= 4 and ~10 vol. % serpentine fraction, the topology of the serpentine clusters becomes simple, with few interconnections between long isolated serpentine clusters. Conversely, for ~20 vol. % serpentine, the clusters increase in length and topological complexity, resulting in large interconnected serpentine network for γ > 4. This study reveals how serpentinized peridotite with ~20 vol.% serpentine can develop a deformation-induced IWL of serpentine, where strain can preferentially localize. IWL of serpentine may not happen when the serpentine content is ~10 vol. % because of the formation of a serpentine disjoint network. The results will be put in perspective with published in-situ stresses distribution within various serpentine+olivine aggregates, deforming under similar pressures and temperatures. These experiments participate to set the ground for exploring the deformation distribution, stresses and fabric evolution in polymineralic rocks, using in-situ information.

How to cite: Hilairet, N., Mandolini, T., Chantel, J., Merkel, S., Le Godec, Y., Thielmann, M., Guignot, N., and King, A.: Layered or interconnected ? In-situ connectivity and topology from X-ray tomography on deformed serpentine+olivine aggregates, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13729, https://doi.org/10.5194/egusphere-egu25-13729, 2025.

Micro-scale porosity is a feature commonly found in viscously deformed quartz-rich mylonites. However, the processes which may form such porosity are actively debated, and whether or not pores are formed syn-kinematically to shear zone activity remains uncertain. Yet, the production of micro-pores during rock deformation may have several critical implications, such as affecting the rock strength, possibly through the brittle-ductile transition, and/or providing fluid pathways through active shear zones.

In this study we focus on quartz-rich shear bands produced during extensional deformation of a granitic pluton below the detachment of Ikaria (Cyclades, Greece). Nearby to the detachment, quartz aggregates are often decorated by micrometric and sub-micrometric pores, of which a large proportion adopt angular, crystallographically controlled shapes. Quartz in such decorated shear bands primarily deformed by crystal plasticity and underwent dynamic recrystallisation by subgrain rotation. Using a combination of standard and High-angular Resolution (HR) Electron Back-Scatter Diffraction (EBSD) analyses alongside Scanning and Transmission Electron Microscopy (SEM/TEM), we highlight that micro-pores decorate primarily grain boundaries, as well as some intragrain substructures including subgrain boundaries. EBSD analyses show that pore-decorated substructures are characterised by high (~4°) Kernel Average Misorientation (KAM), which describes the mean lattice misorientation of one EBSD pixel with respect to its closest neighbours. (HR)EBSD maps indicate a high lattice curvature gradient across these pore-decorated substructures, which can be seen by Geometrically Necessary Dislocation (GND) densities as high as 10¹⁵ per m².

TEM analyses of Focused Ion Beam (FIB) sections across grain and intragrain boundaries reveal that quartz contains free dislocation densities around 10¹³ per m², which matches our (HR)EBSD estimates for the interior of grains and subgrains. GND estimates of some porosity-decorated subgrain boundaries are between 10¹⁴ to 10¹⁵ dislocations per m², which are not visible in TEM. Instead, nm-scale layers of amorphous SiO₂ are seen, into which porosity is often partially or fully embedded.

Our results suggest that amorphous SiO₂ and porosity are formed from the same process, since pores are embedded into amorphous SiO₂. Furthermore, in the case of pore-decorated substructures where amorphous SiO₂ is present, a factor other than dislocation climb likely accounts for their quartz lattice distortion, possibly related to a stress concentration. Although the origin of quartz amorphization remains a matter of discussion, we hypothesise a stress concentration which caused quartz to amorphize, followed by subsequent pore formation through fluid exsolution while stress was released. If this is the case, it would strongly suggest that pore nucleation occurred syn-kinematically in Ikaria.

How to cite: McGill, G., Précigout, J., Prigent, C., Arbaret, L., Airaghi, L., and Cordier, P.: The origin of micro-porosity in quartz mylonites: insights from quartz-rich shear bands in the Ikaria granite, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17663, https://doi.org/10.5194/egusphere-egu25-17663, 2025.

Experimental and field-based research has shown that the presence of mica minerals drastically reduces the mechanical resistance of rocks during both brittle and viscous deformation, which allows the localization of deformation into narrow regions such as shear zones. Furthermore, experimental work has proved that the resistance of phlogopite-quartz assemblages with a mica abundance of 30% is as weak as a sample composed entirely of mica. This phenomenon occurs as a combination of several processes, including developing interconnected networks of mica domains and grain size decrease in quartz. Nevertheless, it remains unclear which other processes enhance weakening in these rocks, to what extent each process is significant, in which stage of deformation they take place, and how the presence of mica assists the deformation of quartz. In this work, we address these questions by analyzing the microstructure of six experimental shear zones carried on samples composed of 30% muscovite and 70% quartz with different amounts of strain. By combining microstructural and mechanical information, we aim to infer how and when different weakening processes occur.

Simple shear experiments were conducted using a Griggs-type apparatus to deform 0.1 g of a powder composed of 30% muscovite with an initial grain size of 62-125 µm and 70% quartz with an initial grain size of 10-20 µm. The samples reached different amounts of gamma strain ranging from 0 to 6.  The experiments were carried out at T=800°C, P=1 GPa, added H₂O=0.1%, and ė≈1x10^-5s^-1. Afterward, in the post-mortem samples, a detailed microstructural analysis was carried out comparing SEM-BSE images, cathodoluminescence in SEM (CL), and EBSD maps. Some microstructural parameters were acquired such as the interconnectivity of each phase, grain size distribution, and grain lattice misorientation, which were compared to the CL-signal.

As strain increases, the interconnectivity of mica grains does not increase or decrease significantly, but rather, mica grains decrease in size through breaking. Quartz grains located in mica-rich domains preserve their original size and shape while micas take most of the deformation. On the other hand, in quartz-rich domains, the grain size is intensely reduced as strain increases. Additionally, a blue material in the CL maps appears along grain boundaries and microcracks. This blue material becomes more and more abundant and interconnected as deformation increases, which is the main feature appearing as a consequence of progressive deformation. The correlation of CL and EBSD maps indicates that some of the newly formed grains (blue material) present low misorientation to the parental grain, some other grains preserve exactly the orientation of the parental grain, and others present a random misorientation to the parental grain. This suggests that the coupling between continuous dissolution-precipitation and crystal-plastic deformation of quartz is the most suitable mechanism behind the formation of the new material. However, the source of luminescence of the precipitated material in the CL spectra remains unclear.

How to cite: Serrano López, G., Airaghi, L., Raimbourg, H., Precigout, J., and Alaoui, K.: Weakening mechanisms on mica-bearing rocks: an experimental approach, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17810, https://doi.org/10.5194/egusphere-egu25-17810, 2025.

Strain localization and softening in metastable crustal rocks involve complex feedbacks between deformation mechanisms, metamorphic reactions and fluid circulation, as long pointed out for shear zones by previous authors. These feedbacks, however, have rarely been scrutinized and documented precisely at grain-scale. Furthermore, while recent studies have shown that high-grade metamorphic rocks (T°C>550°C) deform through a combination of dislocation creep (DC), diffusion creep and dissolution-precipitation creep (DPC), available creep laws only account for dislocation creep and/or solid-state diffusion processes. Deciphering the role and contribution of DPC to strain accommodation at grain-scale is therefore important to better understand the rheological behavior of rocks, as well as of plate boundaries (for example, deep mechanical coupling in subduction zones likely occurs when/where DC takes over fluid-assisted DPC).

This study investigates the evolution of deformation mechanisms and metamorphic reequilibration of the metamorphic sole of the Bay of Islands ophiolitic complex (BOIC, Newfoundland) and its associated overlying mantle, which jointly preserve evidence for deformation-reaction-fluid feedbacks leading to gradual strain localization at plate boundary scale. Detailed patterns and chronology of deformation-reaction-fluid interactions are constrained by structural, textural, chemical and microstructural data acquired in both the metamorphic sole and the basal mantle. A new method based on EPMA and EBSD maps overlay was also used to track and quantify grain-scale deformation mechanisms, as well as the interplay between grain size reduction, mineral reactions and material transfer. Results show progressive cooling of the mantle associated with increasing deformation (from protomylonitic to ultramylonitic stages) and fluid-related metasomatization of peridotites towards the contact. Fluids are interpreted as coming from the metamorphic sole in which activation of dissolution-precipitation processes are dominant.

How to cite: Mérit, L., Agard, P., Labrousse, L., Dubacq, B., Prigent, C., and Guilmette, C.: Metamorphic re-equilibration, deformation and rheology along a nascent plate boundary: the case study of the Bay of Islands Ophiolitic Complex, Newfoundland, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18133, https://doi.org/10.5194/egusphere-egu25-18133, 2025.

How to cite: Vass Payne, E., Butler, I., Gilgannon, J., Freitas, D., King, A., Rizzo, R. E., Eberhard, L., and Fusseis, F.: Imaging the textural evolution of gypsum through a metamorphic cycle, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18290, https://doi.org/10.5194/egusphere-egu25-18290, 2025.

The Sierra de Juárez Complex (SJC), located in Oaxaca, Mexico, comprises a N-S trending belt of deformed igneous and metamorphic rocks with a complex history of multistage deformation and metamorphism. The SJC exhibits penetrative mylonitic deformation attributed to Mesozoic tectonics; however, the timing and kinematics of this event remain poorly constrained.

New Rb-Sr geochronology on muscovite reveals ages ranging from ~164 to ~151 Ma, corresponding to a Middle to Late Jurassic event, and a younger age of ~67 Ma, associated with the Laramide Orogeny. Ongoing microstructural analyses, including traditional optical methods and Electron Backscatter Diffraction (EBSD), aim to characterize the kinematics and deformation style of the mylonitic belt.

The integration of geochronological and microstructural data will enhance our understanding of the role of the SJC in the tectonic evolution of southern Mexico during the Mesozoic, providing new insights into the broader tectonic framework of western equatorial Pangea.

How to cite: Monreal Roque, E., Weber, B., and Tajčmanová, L.: Combined microstructural and geochronological analysis of ductile deformation in the Sierra de Juarez Complex (Southern Mexico)., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18704, https://doi.org/10.5194/egusphere-egu25-18704, 2025.

Deformation of the oceanic and continental lithosphere induce by plates motion on Earth’s surface implies the existence of an oblique deformation component at plate boundaries. Oblique tectonic systems, including transpressional and transtensional regimes, represent complex oblique boundaries and in-between movements combining transcurrent with either convergent or divergent deformation. These oblique strain regimes could generate a wide variety of structures and strain patterns in the ductile domain of the middle to lower crust, that makes interpretation of fabrics challenging. Studied example of transtensional regimes in orogenic domains are still rare and deformation process in this context remains poorly understood. However, transtensional systems are expected to play a key role by accommodating tectonic forces within a rheologically and mechanically heterogeneous lithosphere, in particular during orogenic collapse and exhumation of high-grade metamorphic rocks by thinning the orogens.

Here we present a new multidisciplinary and multiscale study by combining (micro)structural, thermobarometric and geochronological analyses in the Variscan Tanneron massif to reconstruct its late P-T-t-D evolution. The Tanneron massif represent the most internal part of the Maures-Tanneron Variscan belt (SE France), which was mainly structured during the late stage Variscan orogeny. The aim of this study was to precise the nature, organisation and evolution of ductile deformation and associated structures inside a transtensional tectonic regime. Structural and microstructural analysis (AMS and 3D finite strain ellipsoids calculation) of the Tanneron massif indicates that the late tectonic event that structured the massif is a general transtensional regime characterised by a strong subhorizontal stretching that originated through two phases. The first phase is a pure shear- dominated transtension with the development of a subhorizontal constrictional flow associated with L>S tectonites and minority gently-dipping foliations. The second phase is a simple shear-dominated transtension characterised by a plane strain flow (S-L tectonites) and the development of vertical foliations and dextral shear zones. Thermobarometric modelling (X-Ray compositional maps and mineral composition using EPMA) and in-situ U-Th-Pb dating (monazite and xenotime, LA-ICPMS) of the migmatitic units allow us to precise the tectonic evolution of this transtensional deformation regime. This regime represents a tectonic continuum between ~ 325 and 300 Ma and defines a progressive deformation event synchronous with the exhumation of high-grade metamorphic units. Deformation was initiated at high-temperature conditions associated with partial melting of the crust between 7.2 - 10.0 Kbar and 730 - 770 °C, then progressed mainly under subsolidus conditions during the retrograde path of the migmatitic units until low greenschist metamorphic facies conditions reaching 4.1 Kbar and 370 °C. Linking structural, microtextural observations and thermobarometric data, we propose a rheologically driven strain path partitioning during the progressive exhumation of this deep crust. During the second phase of the transtensional regime, deformation was localised preferentially in the hydrated meta-sedimentary units, rather than in meta-igneous rocks.

How to cite: Gremmel, J., Duclaux, G., Corsini, M., Bosse, V., and Bascou, J.: Tectonic evolution in transtensional regimes: the example of the Variscan Tanneron massif (SE France), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20090, https://doi.org/10.5194/egusphere-egu25-20090, 2025.

On major plate-boundary fault zones, there is an expectation that large-magnitude earthquakes do not nucleate at shallow depths, but rather starting at depths of several km in the crust.  This depth dependence is generally understood  to be controlled by the frictional behavior of the sediments making up fault gouges, where velocity-strengthening friction and low effective stresses at shallow depths tends to produce stable fault slip.  The transition to velocity-weakening rocks at seismogenic depth has been suggested to be caused by a variety of diagenetic and low-grade metamorphic processes that lithify the sediments into more competent fault rocks.  Recent laboratory results on both natural fault rocks and desiccated clay-salt mixtures show that this is a viable mechanism, where velocity-weakening friction is seen in the lithified rocks having high cohesion and low porosity.  A remaining open question is whether the key ingredient for velocity-weakening friction is the porosity reduction, or the mechanical cementation.

Here, we test whether porosity reduction alone can induce velocity-weakening friction in powdered Rochester shale, an otherwise velocity-strengthening sediment.  We control the porosity by consolidating deionized water-saturated shale powders to a vertical stress of 86 MPa and shearing the samples under lower effective normal stresses of 0.1-10 MPa, for overconsolidation ratios (OCRs) of up to ~860.  We then measure the frictional properties of our overconsolidated samples with velocity-step tests from 10^-6–10^-5 m/s, repeated over long displacements to account for fading of the initial consolidation state with slip. 

We observe that overconsolidation induces an additional porosity reduction of 35-63%, relative to the porosity under normal consolidation. The velocity steps show predominantly velocity-strengthening friction; however, some scattered instances of velocity-weakening are observed for the highest tested OCR.  Analysis of the rate- and state-dependent friction parameters shows that the velocity steps in samples with the highest OCR have both larger values of “a” and large positive values of “b”, whereas the rest of the samples show predominantly negative values of “b”.  The observed pattern in “b” suggests that asperity contacts under shear, and therefore surface roughness is important.  This is supported qualitatively by photos of the shear surfaces, showing that for larger OCRs, a smaller proportion of the nominal surface area is in real contact during shear.  The results show that velocity-weakening friction begins to appear when the additional porosity reduction is ~55%, consistent with previous work.  However, the appearance of velocity-weakening friction due solely to porosity loss may require unrealistically large OCRs, and truly unstable sliding at depth likely requires cementation by mineralization.

How to cite: Ikari, M. and Hüpers, A.: Frictional slip behavior of highly overconsolidated fault gouge, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1722, https://doi.org/10.5194/egusphere-egu25-1722, 2025.

Fault geometric heterogeneities such as roughness, stepovers, or other irregularities are known to affect the spectra of radiated seismic waves that result from dynamic slip on a fault. To investigate the effect of normal stress heterogeneity on radiated spectra, we created a laboratory fault with a single, localized bump by utilizing the compliance and machinability of poly methyl methacrylate (PMMA). By varying the normal stress on the bump and the fault-average normal stress, we produced earthquake-like ruptures that ranged from smooth, continuous ruptures to complex ruptures with variable rupture propagation velocities, slip distributions, and mechanical stress drops. We used an array of eight piezoelectric sensors to measure vertical ground motions calibrated to determine source spectra and PGA for individual events. High prominence bumps produced complex events that radiated more high frequency energy, relative to low frequency energy, than continuous events without a bump. In complex ruptures, the radiated high frequency energy was spatially variable and correlated with local variations in peak slip rate and maximum mechanical stress drop caused by the bump. Continuous ruptures emitted spatially uniform bursts of high frequency energy as the rupture propagated along the fault. Near-field peak ground acceleration (PGA) measurements of complex ruptures show nearly an order-of-magnitude higher PGA near the bump than elsewhere. We propose that for natural faults, geometric heterogeneities may be a plausible explanation for commonly observed order-of-magnitude variations in near-fault PGA.

How to cite: Cebry, S. B. L. and McLaskey, G.: Heterogeneous high frequency seismic radiation from complex ruptures, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3342, https://doi.org/10.5194/egusphere-egu25-3342, 2025.

Natural faults are typically surrounded by a damage zone consisting of fractures of varying scales and geometries, which can significantly influence the dynamics of earthquake ruptures. Numerical studies have shown that damage can affect rupture parameters such as velocity, style, and extent. However, only a limited number of experimental studies have examined the effect of damage on the rupture process, focusing primarily on the rupture speed. Here, we present direct experimental observations on how damage geometry can affect earthquake rupture dynamics on a planar fault. We use polymethyl methacrylate (PMMA) specimens with a pre-cut planar fault and create damage near the fault with varying crack geometry and spacings using laser cutting technology. We generate spontaneously propagating shear ruptures along the main faults and image the rupture using an ultra-high-speed camera operating at one million frames per second. By employing Digital Image Correlation (DIC) techniques, we obtain detailed evolution maps of velocities, displacements, and strains associated with the propagation of the ruptures. Initial results demonstrate that damage zone characteristics can significantly influence the slip velocity, rupture style, and speed, including the transition from sub-Rayleigh to supershear velocities. Such results can offer valuable experimental observations on how damage zones influence earthquake rupture dynamics and deepen the understanding of the complex interplay between fault structures and rupture processes.

How to cite: Ghosh, S. and Tal, Y.: The Effect of Fault Damage Zone on the Dynamics of Earthquake Ruptures: Experimental Observations, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3648, https://doi.org/10.5194/egusphere-egu25-3648, 2025.

Cyclic fluid injection for industrial purposes within fault zones are commonly imposed, since they are observed to stabilize induced seismicity, often inducing aseismic slip along fault surfaces, without immediate seismic energy release (Zang et al., 2018; Noël et al., 2019; Ji et al., 2021a, 2021b, 2022). Even though dynamic variations of effective normal stress on fault zones due to both natural and anthropogenic causes are common (Chen et al., 2024), the impact of the perturbations and their frequencies on fault strength is less explored (Savage and Marone, 2007; Ferdowsi et al., 2015; Noël et al., 2019). The frequency of pore-pressure changes are  expected to impose a characteristic timescale, controlling the crossover from a drained to an undrained response, which in turn will promote markedly different deformation modes and rates (Passelègue et al., 2018).

In this work we present results from a coupled hydromechanical-discrete element model that simulates the response of a pre-stressed, fully saturated fault, filled with a granular fault gouge, subject to cyclic pore-pressure variations across frequencies of three orders of magnitude. For lower frequencies we see nucleation-arrest-nucleation dynamics within the granular rupture and for higher frequency we observe cyclic creep, both driven by pore-pressure perturbations. Within the frequency parameter space we see a crossover of the slip modes as we increase frequency, lower frequencies show unstable failure, while higher frequencies show creep. Our results might account for a) fluid induced slip stability in cyclic injection scenarios (higher frequencies) and b) low-frequency dynamic triggering of earthquakes.

How to cite: Parez, S., Sarma, P., and Aharonov, E.: Response of a fluid-saturated fault gouge to frequency varied cyclic pore-pressure variations, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4426, https://doi.org/10.5194/egusphere-egu25-4426, 2025.

Laboratory studies have demonstrated that faults undergo dynamic weakening during large displacements (>1 m) at seismic slip velocities (>0.1 m/s). However, the role of this weakening in small-displacement induced earthquakes (M 3–4), such as those in the Groningen Gas Field (the Netherlands), remains unclear. To address this, we conducted seismic slip-pulse experiments on Slochteren sandstone gouges (SSG), derived from the gas reservoir, using a rotary-shear apparatus to provide decimeter-scale constraints on the dynamic fault slip of quartz-rich gouges. Pre-sheared gouge layers, confined between ~1.5 mm thick sandstone host blocks, were subjected to slip pulses under initial effective normal stresses of 4.9–16.6 MPa and pore fluid pressures of 0.1 and 1 MPa under undrained conditions. The experiments achieved peak velocities of 1.8 m/s, accelerations up to 42 m/s², and displacements of 7.5–15 cm, using either dry Argon or water as pore fluid at ambient temperatures. Our results reveal that water-saturated gouges weaken rapidly from a peak friction of ~0.7 to ~0.3, accompanied by early fast dilatancy followed by slower ongoing dilation, with minimal dependence on normal stress, slip acceleration, or displacement. In contrast, Argon-filled samples exhibited only minor weakening. Microstructural analysis shows no systematic relationship between the width of the principal slip zone (PSZ) and frictional work or power input densities, indicating that wear or heat production alone does not control PSZ growth. Instead, our thermo-hydro-mechanical (THM) numerical modeling suggests that thermal pore fluid pressurization, potentially involving water phase transitions at asperity scales, drives weakening in short-displacement, induced seismic events. To extend these small-scale laboratory findings to reservoir-scale processes, ongoing research focuses on discrete element modeling (DEM) at particle and gouge scales coupled with THM solutions. This includes calibrating the THM-DEM models at both grain and gouge scales using laboratory data from Groningen sandstone-derived samples. The calibrated models will be validated with recent data generated under fast slip conditions with varying pore fluids.

How to cite: Hung, C.-C., André, N., Aretusini, S., Spagnuolo, E., Chen, J., and Hamers, M.: Thermo-hydro-mechanical mechanisms in sandstone-derived fault gouges during simulated small-magnitude earthquakes from experiments and models, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4873, https://doi.org/10.5194/egusphere-egu25-4873, 2025.

Compared to other kinds of fluid-related seismicity, reservoir-induced seismicity (RIS) is usually characterized by higher magnitudes. Seismic and water level monitoring and statistical modelling, however, do not provide comprehensive understanding of the RIS mechanism and controls. This study presents a novel finite element method-based 2D poro-visco-elasto-plastic fully dynamic earthquake model, specifically applicable to RIS simulations. In the first stage of the simulations, Drucker-Prager plasticity is used to generate a normal fault in the Earth’s upper crust with enhanced porosity, over a long time-scale of millions of years. In the second stage of the simulations, RIS is modelled under typical reservoir impoundment dynamics, producing four seismic sequences, triggered by pore pressure increase at the fault at shallow depth below the reservoir. This pressurization is released by aftershocks in every seismic cluster, accompanied by permeability hikes at the fault and associated with fault “valving” behaviour. A dynamic coseismic rupture phase driven by wave-mediated stress transfers coupled with rate-and-state dependent friction coefficient weakening is modelled, along with interseismic deformations. Coseismic crack opening in a dilatant regime, inducing porosity and permeability hikes especially emphasized at the fault, is implemented. The model component verifications demonstrate convincing agreement with theoretical predictions. The model allows investigation of spatio-temporal RIS characteristics and their controls. It may contribute to earthquake prediction in situ and facilitate earthquake mitigation policies.

How to cite: Katsman, R. and Ben-Avraham, Z.: A Dynamic Poro-Visco-Elasto-Plastic Earthquake Simulator with Spontaneous Dilatant Coseismic Rupture: Modelling Fault Deformations at Reservoir-Induced Seismicity, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5024, https://doi.org/10.5194/egusphere-egu25-5024, 2025.

Understanding earthquakes mechanisms still represents a challenge, motivated by the large consequences of the numerous earthquakes occurring each year. The complexity of fault zones and fault behaviour requests to make some simplifications and to down-scale the studied system. In our work we borrow from the tribological approach the pin-on-disk experiment so that the two rough surfaces in contact through a series of asperities fault concept is downscaled to a single asperity sliding on a rough surface. The single asperity response to shearing induced by sliding and the evolution of friction are studied closely to understand the behaviour of the down-scaled fault, especially when the velocity is changing. Moreover, mono-asperity experimental tests are an effective way to construct new friction laws for numerical simulations.

The original experimental apparatus consists in a centimetric pin with a hemispherical extremity representing the fault asperity while a large flat rotating disk stands for the opposite surface of the experimental fault. Both pieces are made in the same carbonate rock (Carrara white marble) with controlled roughness. Under co-seismic conditions (contact size, contact normal stress) and with a rough track with presence of granular gouge, the lab-fault is submitted to different velocities (from 0.001 m/s up to 1m/s). A number of high-sampling-rate sensors are used to constrain the observation of the asperity-rough track contact during the simulated seismic events. Moreover, complete post-mortem analyses of the contact surfaces with optical microscopy, SEM and roughness images allow to quantify the mechanisms and to reconstruct friction scenarios in accordance with the time-series acquired during tests. A quasi 2D numerical twin is also created with the elementary discrete method software MELODY, in order to compare the different features observed on the pin or on the track.

In this present work, we focus on the change of wear mechanisms in the lab-fault due to changes in sliding velocity. Independently of the normal load applied, the Carrara white marble asperity-track system experiences weakening velocity due to frictional heating. The friction coefficient evolution during the co-seismic events and the post-mortem analyses put in contrast two regimes. At low velocity, the carbonate rock lab fault wears off at a constant rate producing a large amount of granular gouge. At high velocity, the contact surfaces are covered by viscous sintered material, which can be called mirror surfaces.

How to cite: Clerc, A., Mollon, G., Ferrieux, A., Lafarge, L., and Saulot, A.: Velocity influence on the friction and wear of a single-asperity lab-fault, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6563, https://doi.org/10.5194/egusphere-egu25-6563, 2025.

Faults can slip in diverse modes, ranging from slow, aseismic creep to dynamic, earthquake-generating rupture. Geological observations reveal that many fault zones consist of localized slip zones surrounded by a broader damage zone, where microcracks and fractures interact in complex ways. Friction laws propose that whether a fault will host a slow slip event or a fast dynamic rupture depends on the relative stiffness of these slip zones and the surrounding material. Conversely, the local stress field imposes such structures' evolution. Although the evidence for these complex interactions indicates the opportunity to incorporate this knowledge in the theoretical framework, collecting data on the spatiotemporal evolution of elastic properties at seismogenic depth is inherently challenging, leaving this possibility mostly unexplored.

Laboratory experiments provide a controlled environment for studying the evolution of fault mechanics and elastic properties. Elastic waves are governed by the same equations that relate wave speeds to dynamic moduli. Therefore, they offer a pathway to link laboratory observations to natural fault processes.

This study investigates how microstructural reorganization during fault deformation and fault zone structure formation affects fault zone stiffness and slip behavior using synthetic quartz gouge layers sheared in double-direct shear (DDS) configuration.
Our DDS setup is instrumented with piezoelectric transducers designed to generate and record predominantly compressional (P) or shear (S) waves. By carefully characterizing the source time function, we ensure that early arrivals in each recorded signal represent a single wave mode with minimal mode conversion or side reflections.

We then apply Full Waveform Inversion (FWI) to these early arrivals to reconstruct velocity models for both P- and S-waves as deformation progresses. The inverted models reveal spatiotemporal variations in the bulk and shear moduli, which we interpret as signatures of contact-area changes, grain size reduction, and other micromechanical processes relevant to frictional stability. In particular, the evolving elastic properties allow us to gauge how the local stiffness of the gouge zone evolves relative to applied stress, linking the observed velocity changes to the constitutive laws underpinning rate-and-state friction (RSF). While RSF implicitly links frictional strength to contacts dynamic through a state variable, our results illustrate how ultrasonic waveform acquisition and modeling can provide hints toward the explicit rewriting of such laws in terms of the evolution of elastic properties, an intermediate level of description easier related to micromechanical processes.

This approach highlights the potential for ultrasonic measurements in earthquake laboratory experiments to probe fault zone mechanics and outline a framework for integrating seismic imaging with frictional mechanics to better understand fault behavior across scales.

How to cite: De Solda, M., Mauro, M., Guglielmi, G., Pignalberi, F., and Scuderi, M.: Probing Fault Zone Evolution with Ultrasonic Measurements: Seismic Imaging in Laboratory Experiments, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7148, https://doi.org/10.5194/egusphere-egu25-7148, 2025.

The geometrical roughness of faults results in significant stress heterogeneity across various length scales, thus affecting rupture and sliding behavior during earthquakes. The dilatancy behavior on rough joints also becomes much more complicated than on planar faults. Whether dilation or compaction will occur on rough faults, especially those with large asperity heights, during interseismic and seismic slips is still an open question. 

Here, we perform laboratory shear tests on rough faults with millimeter-scale asperity heights and analyze the four types of dilation or compaction behavior observed during stick-slip cycles. In the stick phases, dilatancy behavior inferred from the asperity contacts agrees well with the variation of normal displacement. The locations of acoustic emission (AE) events are also consistent with the potential surface damage regions estimated from the evolution of asperity contacts at various shearing displacements. Stick-slip events with compaction-dominant interseismic slip usually occur at large shear displacements on interlocking faults when overriding high asperities. In those stick-slip events, the proportion of large-magnitude AEs is lower, resulting in higher Gutenberg–Richter b values. A generalized schematic model is also proposed for the complex dilatancy behavior during stick-slip cycles.

The experimental results provide new insights into the effects of fault roughness on shear-induced dilatancy behavior and serve as valuable benchmarks for our numerical simulations considering visible contact evolutions during shear sliding on rough faults.

How to cite: Chai, S., Su, B., Zou, Y., and Zhao, Q.: Dilation or compaction? Laboratory insights into the role of fault roughness, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7873, https://doi.org/10.5194/egusphere-egu25-7873, 2025.

Fluid pressure plays a crucial role in controlling fault reactivation in both natural and induced seismicity. The effective normal stress is linearly reduced by an increase in fluid pressure (P_f) with σ_eff = σ_n  - S · P_f, which lowers the frictional strength of the fault, τ=µ·σ_eff, increasing the potential for fault reactivation and seismic slip. Upon reactivation, slip can occur quasi-statically or dynamically depending on the interplay between σ_n and P_f and mediated by the S parameter, (the hydromechanical coupling) and by the rate-and-state properties of the fault materials. In this context, the fault zone structure dictates the frictional properties as well as the fault hydro-mechanical coupling, particularly when S ≠ 1 and reactivation might occur at effective stresses different than predicted.

In Bedretto Underground Laboratory for Geosciences and Geoenergies (BULGG, Switzerland), a target fault zone chosen for fluid-induced fault stimulation is characterized by a fractured host rock surrounding a sub-centimetric fault core with fault gouge and bare-rock asperities.  Therefore, to define slip mode of the target fault it is important to characterized the frictional properties of both fault gouge and bare-rock asperities taking advantage of a laboratory controlled experimental environment.

Fault stimulation by fluid injection was simulated in laboratory by increasing the P_f following an injection protocol suitable for the BULGG fluid stimulation. Experiments were performed on both the fault gouge sampled from the target fault and on bare rock surfaces sampled in the surrounding host rock. We employed a rotary shear apparatus (SHIVA) to perform fluid injection experiments. First, we imposed the stress conditions measured at depth in the underground laboratory, halved due to apparatus limitations: 7.5 MPa σ_eff, 7.5 MPa confining pressure and 2.5 MPa P_f. Second, we imposed a slip rate of 10^-5 m/s for 0.01 m to develop a stable fault core structure. Third, we applied a constant shear stress of 2.7 MPa, considering the slip tendency measured on the target fault (0.35). We then increased stepwise the P_f by 0.1 MPa every 150 s. After fault slip initiation, the maximum allowed slip velocity was 0.1-1 m/s. Between each of the experimental stages, permeability and transmissivity were measured with the gradient (Darcy) or P_f oscillations methods.

We show that fault reactivation and slip behavior are different between gouge and bare rocks: in gouge creep and dilatancy precede reactivation, whereas in bare rock surfaces reactivation is sudden and not preceded by neither tertiary creep nor dilatancy indicating that dynamic reactivation is promoted in smooth bare-rock surfaces. Moreover, gouge displays a higher friction (0.58 vs 0.49) and a lower hydraulic transmissivity (i.e., 10^-19 vs 10^-17 m³) than bare rocks.

Here we proceed to test a suite of constitutive models against our data: rate and state friction, (Rudnicki, 2023; Cappa et al., 2022), and a fully coupled poromechanical model (Dal Zilio et al., in prep.), to understand what are the physical processes controlling the onset and style of fault activation in the two fault core structures.

How to cite: Aretusini, S., Cornelio, C., Spagnuolo, E., Volpe, G., Pozzi, G., Dal Zilio, L., Selvadurai, P., and Cocco, M.: Fault core structure and the fault slip behavior during fluid injection: insights from laboratory friction experiments, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7883, https://doi.org/10.5194/egusphere-egu25-7883, 2025.

Ophicalcites (carbonated ultramafic rocks) are commonly found when serpentinized mantle rocks are exposed to fluid-rock interaction in settings such as slip and damage zone of mid ocean ridges, transform faults and subduction zones. In this study, we analyze the frictional strength and healing properties of ophicalcites under hydrothermal conditions because of their possible role in the nucleation of “fast” (i.e., earthquakes) and slow slip events.

The ophicarbonates used in this study belong to the exhumed ophiolitic unit of Eastern Elba Island (Italy). The hand samples are black, red, and purple ophicarbonates and include veined breccias and cataclasites. The mineral assemblages, determined through optical microscopy and quantitative (Rietveld) X-ray powder diffraction, consist of serpentine (lizardite and chrysotile with rare relicts of pyroxene and olivine), talc and calcite veins. Hematite is only found in the red ophicalcite.

We conducted slide-hold-slide experiments with a rotary-shear apparatus coupled with a hydrothermal vessel (ROSA-HYDROS, Padua University, Italy) on a synthetic fault gouge derived from a red natural ophicalcite cataclasite with a composition of 26.9% lizardite and chrysotile, 12.3% talc, 57.6% calcite, 2.9% hematite and traces of smectite. The experiments were performed at an effective normal stress (σ_n^eff = σ_n - P_p) of 20 MPa, a fluid pressure (P_p) of 6 MPa, constant slip velocity of 10 µm/s between the holds (which lasted from 10 to 10.000 s) and temperatures ranging from 20 to 400°C. Our results show that, the “serpentinite” component dominates the bulk friction and the healing behaviour. For T≤ 200°C and in the presence of water in liquid state, the friction coefficient (µ = shear stress/σ_n^eff) is relatively low (µ = 0.35-0.45), poorly sensitive to fluid temperature and we observe creep.  Instead, for T ≥ 300°C and in the presence of water in vapor state, the µ is higher (= 0.80-0.90) and we observe stick-slip behaviour.

The experimental approach of this study aims to understand the nucleation of earthquakes or of slow slip events along mid ocean ridges and transform faults and to contribute to the assessment of seismic hazard associated with CO₂ storage by carbonation of serpentinite in deep reservoirs.

How to cite: Salvadori, L., Di Toro, G., and Tesei, T.: Frictional strength and healing behaviour of natural carbonated serpentinites at hydrothermal conditions, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8673, https://doi.org/10.5194/egusphere-egu25-8673, 2025.

The ability of faults to regain strength between seismic events (frictional healing) is crucial to understand the seismic cycle. In lithologically heterogeneous faults, differences in healing rates among various rock types can lead to the locking of specific fault patches. These locked segments may store significant amounts of elastic strain energy, which can be dynamically released during earthquakes. Anhydrite is a key component of the Triassic Evaporites, the seismogenic layer responsible for destructive earthquakes in Central Italy, e.g. 2016 Mw 6.5 Norcia mainshock. Its complex mechanical behavior, strongly influenced by boundary conditions, remains underexplored. Minor variations in effective pressure, humidity, loading rate, and temperature alter healing properties, rheology, and fault slip behavior of anhydrite. These mechanical characteristics are inherently tied to its elastic properties. Hence, the broad spectrum of mechanical behaviors observed should correspond to an equally wide variation in its elastic moduli.

We performed dry and wet friction experiments on anhydrite gouge using the BRAVA2 biaxial apparatus. These experiments include Slide-Hold-Slide (SHS) sequences to investigate the healing properties of anhydrite by varying temperature between 20 and 100 °C. To inform mechanical data with the microphysical evolution of the fault, we equipped the sample assembly with PZT sensors in transmission mode. These sensors record ultrasonic wave (UW) propagation through the sample during SHS tests. Finally, mechanical and ultrasonic measurements were accompanied by comprehensive microstructural analysis.

At room temperature, distinct mechanical features emerge between dry and wet samples. Wet experiments are characterized by lower friction (μ = 0.49) and higher healing rate (β = 0.015) with respect to dry ones (μ = 0.6 and β = 0.004). In both wet and dry tests fault healing follows a log-linear dependence with hold duration, however, in wet samples this relationship occurs only after a characteristic cut-off time, tc (s). We report a log-linear increase of UW amplitude with hold time. Microstructural analysis of wet samples reveals shear localization and grain size reduction within multiple Y, B, and R shear bands. Conversely, dry samples predominantly feature distributed deformation within R shear bands and local S-C structures.

The observed differences between dry and wet experiments suggest that water-activated processes play a major role in controlling shear strength and healing properties of anhydrite. This hypothesis is corroborated by the presence of a cut-off time for wet healing measurements. We interpret tc as a necessary time for water-activated deformation mechanisms to effectively operate by increasing the healing rate.
The evidence of a log-linear relationship between UW amplitude and hold duration testifies that fault restrengthening is intimately related to gouge porosity reduction and fault zone evolution.

Our approach aims to correlate reductions in shear modulus with shearing along specific slip planes, which are known to be active through microstructural analyses. To achieve this goal, the relationship between complex frictional healing and the elastic properties of anhydrite will be investigated via the extraction of elastic moduli from UW velocities.

How to cite: Mauro, M., Guglielmi, G., De Solda, M., Trippetta, F., Collettini, C., and Scuderi, M.: Dynamics of frictional healing of anhydrite bearing faults imaged by ultrasonic waves, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8830, https://doi.org/10.5194/egusphere-egu25-8830, 2025.

Recent observations of many large earthquakes suggest a pronounced interaction of seismic sequences and aseismic slip (Kato & Ben-Zion, 2020). Among them, the spatio-temporally clustered seismic sequences may be related to internal stress transfer through event-event triggering processes (Davidsen et al., 2021). These evolving stress correlations at different length scales may be a key component to earthquake nucleation. However, the coupling between aseismic deformation and seismic triggering remains poorly understood due to observational limitations. In this study, we performed a triaxial experiment at 50 MPa confining pressure on a Rotondo granite sample. The sample was pre-notched to induce localized stress concentrations. The deployment of distributed strain sensing (DSS), based on fiber-optic technology, captures the transient behavior of aseismic deformation leading to system-size failure. We show that, at peak stress (σ_p = 420 MPa), the strain first builds up and accelerates near the edges of notches. During a subsequent small stress drop (Δσ_d = 1.5 MPa), a sharp contrast between positive and negative strain rates appears. This asymmetric deformation reflects the local stress redistribution associated with the development of shear cracks emanating from the notches. Following this, a transient recovery is observed, as the concentrated strain rate transfers to the surrounding regions. This indicates that the shear crack is growing away from the notches towards the middle of sample. Acoustic emissions (AEs) recorded throughout the failure sequence were analyzed in conjunction with local strain rates, strain gradients, and the speed at which the strain concentrations propagate. This study provides novel insights into the spatio-temporal evolution of shear cracks, revealing the complex interplay between strain localization, aseismic transients, foreshock activity and stress redistribution during the preparatory phase preceding dynamic failure.

References:

Davidsen, J., Goebel, T., Kwiatek, G., Stanchits, S., Baró, J., & Dresen, G. (2021). What Controls the Presence and Characteristics of Aftershocks in Rock Fracture in the Lab? Journal of Geophysical Research: Solid Earth, 126(10), e2021JB022539. https://doi.org/10.1029/2021JB022539

Kato, A., & Ben-Zion, Y. (2020). The generation of large earthquakes. Nature Reviews Earth & Environment, 2(1), 26–39. https://doi.org/10.1038/s43017-020-00108-w

How to cite: Chen, H., Selvadurai, P. A., Michail, S., Salazar Vásquez, A. F., Madonna, C., and Wiemer, S.: Spatio-Temporal Organization of Earthquakes: Insights from Aseismic Transients and Seismic Triggering in Rock Fracture, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8940, https://doi.org/10.5194/egusphere-egu25-8940, 2025.

The mechanisms that lead a fault in the brittle crust to catastrophically fail during fluid injection remain poorly understood. In traditional rock mechanics, significant effort has been devoted to developing fluid injection experiments that reactivate small specimen surfaces over their whole area. However, in natural settings, only a portion of the fault may initially experience an increase in fluid pressure and reactivation by enlarging the area of slip.

Therefore, it is important to explore further the processes that lead to nucleation on a localized patch subjected to fluid injection to involve a larger portion of the fault that are not initially subjected to the elevated fluid pressure. To do so, we developed a new bi-axial apparatus specifically designed to investigate the nucleation phase of fluid injection-induced earthquakes on large enough sample.

This apparatus accommodates slabs of rock with a sliding surface of 15 x 17 cm and 1 cm thick. It is equipped with two servocontrolled hydraulic rams capable of delivering horizontal and vertical forces up to 450kN. The fluid pressure (both upstream and downstream) and both rams are controlled by adapted Nova-Swiss hand pumps that are independently managed by EUROTHERM process controllers, which operate MAXXON brushless electric motors via ESCON digital servo-controllers. This setup enables precise servo-control of each piston and upstream and downstream pressure in either displacement or pressure mode, using feedback from LVDTs and pressure transducers, respectively.

We designed a new double L-shaped assembly that ensures a uniform shear stress distribution on the sliding surface prior to reactivation, as shown by finite element analysis. The assembly is equipped with eight piezoelectric transducers to record acoustic emissions (AE) and to conduct active velocity surveys, as well as strain and displacement gauges placed across the sliding surface to monitor local strains and finite displacements. The slip surface can be machined to form an area with an initial stress concentration around its tip so that slip can be initiated entirely by raising the fluid pressure in the ‘crack’. The sample material chosen is Pennant sandstone, a tight sandstone with high cohesive strength and very low porosity and permeability.

Here, we present a suite of preliminary experiments ranging from conventional stable/unstable frictional sliding to fluid injection-induced fault reactivation. Acoustic emissions were utilized to monitor the evolution of the seismic b-value and to locate AE events throughout the experiments.

How to cite: Bigaroni, N., Mecklenburgh, J., Chandler, M., Paul, L., and Rutter, E.: BeeAx: A New Biaxial Apparatus to Investigate Shallow Brittle Rock Deformation and Frictional Sliding, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10995, https://doi.org/10.5194/egusphere-egu25-10995, 2025.

Fault geometry is increasingly regarded as a key parameter that affects all aspects of the earthquake cycle, yet the in-situ geometry of active faults remains poorly resolved, and they are often modeled as largely planar. On the other hand, recent advances in sensing and computational abilities enable measuring the co-seismic slip of large earthquakes in high resolution, both on the surface and at seismogenic depths. Previous analytical and numerical studies showed that the slip distribution of an earthquake is mechanically linked to the fault geometry through its curvature, though this link has not yet been verified. Here we show experimental verification of this link, by measuring at high precision the shear slip profiles along non-planar interfaces in laboratory earthquakes. We trigger dynamic shear ruptures that propagate along the interface between two loaded and matching PMMA plates and, using image correlation of ultrahigh-speed photography, resolve the propagating ruptures and the resulting displacements. The plates are pre-cut along a desired geometry, and we compare the slip calculated from the geometry and the experimental shear slip at each pixel along the interface. We show that, as predicted analytically, fault curvature is correlated to slip gradient when the plates are in contact and anti-correlated when there is opening. These relationships are clearly visible in our results at all measured scales regardless of the rupture complexity. Our results suggest that variations of the co-seismic slip along a fault can be highly indicative of the in-situ fault geometry and, alternatively, that slip distribution may be predicted along a fault with known geometry.

How to cite: Gabrieli, T., Romanet, P., Tal, Y., and Schuderi, M. M.: Validating the link between fault geometry and slip distribution, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12897, https://doi.org/10.5194/egusphere-egu25-12897, 2025.

Safe exploitation of high-enthalpy geothermal fields for energy production requires knowledge of the mechanical behavior of faults and the seismic cycle in the presence of hot, pressurized fluids. In geothermal reservoirs, fluids can exist in the liquid, liquid-vapor mixture, vapor, or supercritical fluid state. Here we investigate the frictional properties of simulated fault gouges derived from the main stratigraphic units present in Krafla Geothermal Field (Iceland) under realistic water temperature (T_f=100-400˚C) and pressure (P_f=10-30 MPa) conditions. These conditions correspond to water in the liquid, vapor, and supercritical state. 

Laboratory rotary shear, slide-hold-slide (SHS) experiments at a constant effective normal stress of 10 MPa are performed on gouges prepared from non-altered basalt (Krafla Fires eruptions, 1975-1984), chlorite-altered basalt (borehole KH-6, 708.5 m borehole depth), amphibole-altered basalt (borehole IDDP-01, 1686 m borehole depth), fine-grained basaltic dyke with scarce alteration (borehole IDDP-01, 1970 m borehole depth), and rhyolite (borehole KJ-39, 1637-1646 m borehole depth). All experiments are initiated with a 5 mm run-in slip at a loading point slip rate V of 10 μm/s followed by the SHS sequence with a hold time t_hold increased from 3 s to 10,000 s, separated by a slip interval of 1 mm.

The frictional strength μ_ss (friction coefficient during run-in) slightly increases with T_f in non-altered and amphibole-altered basalt and slightly decreases with T_f in chlorite-altered basalt, basaltic dyke, and rhyolite. However, for all rock types, μ_ss is higher (μ_ss__vap> μ_ss__sup and μ_ss__liq) when vapor is present (only exception, non-altered basalt with μ_ss__sup>μ_ss__vap> μ_ss__liq).

The frictional healing Δμ (frictional strength recovery during holds), is highest at T_f=400˚C in vapor and supercritical water. Still, the effect of T_f and the physical states of water on frictional healing rate (β=Δμ/log(1+t_hold/t_cutoff)) depends on rock type. In non-altered basalt, β increases with T_f but decreases in vapor water; in chlorite-altered basalt and basaltic dyke, β increases with T_f and is independent of the physical state of water; in amphibole-altered basalt and rhyolite, β is independent of both T_f and the physical state of water. However, systematic microanalysis of the deformed gouges is required to understand the underlying mechanisms. 

Lastly, for measured constant machine stiffness in this T_f-P_f range, all tested gouges show stable sliding (creep) at T_f=100˚C but become unstable (stick-slip) at T_f=200˚C (basaltic dyke) and T_f=300˚C (other rock types) regardless of the physical states of water. The highest stress drops during stick-slip are measured in supercritical water (non-altered basalt) or vapor (all the other gouges). Consistent with the seismological observations, our laboratory data show that the fault/fracture network in Krafla reservoir is less prone to nucleate earthquakes in the shallow hydrothermal system (T_f~170˚C, < 1 km depth), with most earthquakes located in the deep hydrothermal system (T_f≥300˚C, 1-2 km depth).

In conclusion, both temperature and the physical states of water should be considered when interpreting the seismicity in geothermal fields. 

How to cite: Wu, W.-H., Feng, W., Gomila, R., Tesei, T., Violay, M., Mortensen, A. K., and Di Toro, G.: The Effect of Temperature and Physical State of Water on the Frictional Properties of Gouges from Krafla Geothermal Field (Iceland), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13704, https://doi.org/10.5194/egusphere-egu25-13704, 2025.

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.

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.

How to cite: Bonnelye, A., Gouedar, A., and Faure-Catteloin, D.: PolystyQuakes : what can we learn from the use of polystyrene as analogue to earthquakes?, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16260, https://doi.org/10.5194/egusphere-egu25-16260, 2025.

Faults are heterogeneous at all scales. Crustal faults extend for tens or hundreds of kilometers across which they intersect many different lithologies. In the fault core, meter-scale blocks are embedded within a shear zone mélange and, at the grain-scale, fault segments comprise patches of weak and strong minerals. This structural heterogeneity may be  associated with an heterogeneous stress distribution on-fault and has therefore been invoked to explain observations of different slip behaviours occuring simultaneously and at the same location on individual faults.

Conceptually, the structural and mechanical heterogeneity along fault is often described in terms of rheological asperities that can either be competent, have a velocity-weakening frictional behavior and tend to slip unstably or be less competent, have a velocity-strengthening frictional behaviour and slip stably. To understand the spectrum of slow, intermediate, and fast slips behaviors observed in nature, the laboratory studies have investigated the effect of rheological asperities on fault stability and slip behaviour. Laboratory experiments have been performed using mixtures of gouge materials with different frictional properties mixed homogeneously in various proportions or using spatially heterogeneous gouges with predefined layering perpendicular or parallel to the shear direction. In gouge samples prepared using talc-calcite mixtures, a 20% fraction of talc – the weak phase – was shown to drastically change the frictional properties of calcite gouge. However, recent results for vertically segmented gouges prepared from claystone and sandstone showed that fault friction and its rate-dependence are not simply controlled by the weakest lithology nor by a homogeneous mixture of the juxtaposing lithologies.

We performed friction experiments at room temperature in a servo-controlled biaxial apparatus using homogeneous and heterogeneous gouge samples prepared from calcite and talc minerals in various proportions by weight. Calcite and talc were chosen for their well-known antagonist frictional behaviour, as the strong velocity-weakening lithology and weak velocity-strengthening lithology, respectively. Heterogeneous gouge samples consist of a cylindrical inclusion of talc/calcite embedded within calcite/talc. For each experiment, two identical layers of gouge were placed in between three grooved sliding blocks, in a double-direct shear configuration, and a constant normal stress of 40 MPa was applied. Samples were sheared at a sliding velocity of 10 µm.s-1 until a steady state was reached. Velocity-stepping and slide-hold-slide sequences were then performed, under fully dry conditions. 

Overall, we observe that the coefficient of friction decreases with increasing talc content. However, depending on the geometry of the slip interface – a weak inclusion in a stronger and continuous lithology or the opposite – the decrease in strength associated with the presence of phyllosilicate minerals varies non linearly. Velocity weakening during shearing is reduced in the case of a strong calcite inclusion embedded in a weaker talc continuum. Our results show that the rheological heterogeneity associated with the presence of weak or strong inclusions exerts a second order control on the frictional behaviour of our simulated gouges.

How to cite: Carbillet, L., Guérin-Marthe, S., Hofer Apostolidis, K., and Violay, M.: Effect of strong and weak inclusions on the frictional behaviour of fault gouges, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16382, https://doi.org/10.5194/egusphere-egu25-16382, 2025.

Subduction zone thrust fault systems accommodate deformation and guide the dynamics of converging tectonic plates. Investigating the frictional, healing, and deformation mechanisms at shallow depths within these zones is important for enhancing models of fault zone mechanics, slip behavior, and material transfer processes, as well as for understanding the nucleation and propagation of megathrust earthquakes. The Gova Fault Zone (GFZ), located within the basal portion of the Sestola-Vidiciatico Unit (SVU) in the Northern Apennines (Italy), offers a unique opportunity to examine the frictional properties, healing dynamics, and deformation processes within an exhumed erosive subduction channel.

The GFZ consists of multiple thrust surfaces that juxtapose highly folded and strained phyllosilicate-rich sediments derived from the upper plate (Marmoreto Marls, Fiumalbo Shales, FIU, and Civago Marls Fms., CIV) onto foredeep turbidites (Gova Sandstone Fm, GOV) of the lower plate.

We sampled the rocks in across the GFZ transect from the footwall GOV sandstones into the SVU shear zone, a sheared Civago marls and Fiumalbo shales.

Fault rocks were reduced to gouge and sheared in a rotary shear apparatus equipped with a hydrothermal vessel (RoSA+HYDROS, University of Padova) under an effective normal stress of 20 MPa and fluid pressure of 6 MPa, at both 25°C, and 200°C. We measured the friction and the time-dependent recovery of frictional strength during simulated interseismic periods (i.e., the healing).

The footwall GOV sandstone gouges are dominated by cataclastic processes, are frictionally strong and their friction increase from μ = 0.49 at 25°C to µ = 0.57 at 200° C. They are also characterized by positive healing and a transition from stable slip to stick-slip at 200° C.

The hangingwall rocks are frictionally weaker: the CIV marls show μ = 0.27 (25°C)-0.31 (200° C) and the FIU shales µ = 0.2 (25°C)-0.25 (200° C), display null/negative frictional healing rates. Both rocks maintained stable sliding, facilitated by phyllosilicate lamellae reorientation and buckling. The CAT, show intermediate friction µ = 0.37 (25°C)-0.42 (200° C) but negative healing rate slip behavior dominated by stable sliding.

The measured in the experiments of the footwall sandstones would, in nature, force strain to migrate into the weaker lithologies of the hangingwall. This “migration” of strain favors (1) the development of thrust surfaces within the hangingwall, such as the FIU on CIV contact, and contributes (2) to erosion, enabling the transfer of material from the upper plate to the lower plate. The positive frictional healing and stick-slip behavior at 200° C in the siliciclastic fault rocks (GOV) suggest a role of the footwall in favoring seismic slip nucleation. Conversely, the weakness and lack of frictional healing suggest a predominantly aseismic or slow slip behavior in the phyllosilicate-rich lithologies. Collectively, we found that frictional and microstructural heterogeneities between subducting sediments and the tectonic mélange at shallow depths may control the erosional vs. accretional character of subduction zones and be responsible for complex slip behavior within the frontal thrusts of subduction zones.

How to cite: Diamanti, R., Salvadori, L., Remitti, F., Mittempergher, S., Molli, G., Di Toro, G., and Tesei, T.: Frictional properties and healing in sedimentary subduction fault zones: insights from the Sestola-Vidiciatico unit, Northern Apennines., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17462, https://doi.org/10.5194/egusphere-egu25-17462, 2025.

We aim to determine the feedback between fault dynamics and fault gouge structures by examining gouge structures that formed during rupture and slip of initially intact granite under upper crustal conditions. Experiments were conducted under quasi-static (3·10^-5 mm/s), weakly dynamic (0.27 mm/s) and fully dynamic (≫1.5 mm/s) conditions, with or without fluids, and limited slip displacement (max. 4 mm). The extent in gouge amorphisation positively correlates with deformation rate, and we detected evidence of melting, e.g., magnetite nanograins, associated with the highest deformation rates. Gouge nanostructure is directly correlated to power dissipation rather than total energy input. The presence of amorphous material is shown to have no detectable impact on the strength evolution during rupture. We highlight that gouge textures, generally associated with large displacements and/or elevated pressure and temperature conditions, can form during small slip events (Mw <2) in the upper crust from initially intact materials.

How to cite: Incel, S., Ohl, M., Aben, F., Plümper, O., and Brantut, N.: Nanostructures as indicator for deformation dynamics, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17710, https://doi.org/10.5194/egusphere-egu25-17710, 2025.

Accurately modeling faulting in the so-called brittle crust remains a challenge due to limitations in numerical algorithms and problems in choosing realistic constitutive models. These challenges are reflected in difficulties linking observations across scales: from laboratory to outcrop and up to regional geology.

Here we present the Geotechnical Particle Finite Element Method (P-FEM), a large-deformation numerical tool developed to capture detailed progressive failure and fracturing using a non-local formulation.

A key advantage of P-FEM is its ability to simulate localized shear bands (fault zones with finite thickness) that naturally emerge independent of mesh discretization, both in thickness and orientation. Continuous remeshing further enables the modeling of large deformations within a Lagrangian framework, and techniques used to minimize numerical diffusion help producing realistic localized shear/fault zone patterns. Additionally, P-FEM benefits from its foundation in standard finite elements, allowing to use efficient and accurate solvers (tested through years by a large community of users) and a wide library of constitutive models to simulate various geo-materials, including non-cohesive soils and fault gouges, weak porous rocks (that could develop deformation/compaction bands), and “standard” brittle-frictional-plastic materials. Multiphysics implementations including fluid and heat flow are also available but will not be specifically discussed here.

These capabilities make P-FEM particularly suited for investigating fundamental tectonic processes such as (i) fault nucleation and growth in mechanically layered materials, (ii) the interplay between faulting and folding in thrust belts, and (iii) the development of fault damage and/or process zones in materials with heterogeneous mechanical properties. Our contribution will outline the P-FEM method and discuss its application to these tectonic problems.

How to cite: Bistacchi, A., Ciantia, M., Castellanza, R., Mittempergher, S., and Agliardi, F.: Applications of the Particle Finite Element Method (P-FEM) to faulting in the brittle crust, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17832, https://doi.org/10.5194/egusphere-egu25-17832, 2025.

Experimental observations on fault gouges suggest that shear localization is a prerequisite for the nucleation of instabilities. Synthetic gouges typically produce lab-quakes when the shear deformation is localized along sharp, knife-edge shear planes – a condition that satisfies the steady-state existence requirement of the rate-and-state friction framework. Similarly, exhumed fault cores exhibit shear planes so localized that they appear as mirror surfaces.

Yet, the scenario in nature is far more complex. Many faults contain multiple fault cores, multiple localized shear planes, cemented fault rocks, and evidence for fault core recycling. This suggests that deformation localized along a principal slip plane during an earthquake – unlike in controlled shear experiments – does not necessarily persist over the lifespan of a mature fault. While in laboratory gouge experiments the steady state corresponds to a microstructural fabric that does not evolve significantly over seismic cycles, fault rock fabrics in nature evolve significantly during the seismic cycle—remaining far from a simple localized steady-state fabric.

Here, we present a preliminary study aiming at reconciling these two perspectives: the laboratory-derived nucleation, which involves deviations from a pre-existing steady-state, and the field-derived nucleation, often occurring far away from any steady-state condition. To address this, we conducted double-direct shear experiments on quartz gouges at 30 MPa normal stress, reactivating faults via shear stress steps that allowed spontaneous fault acceleration or deceleration.

The novelty of our approach lies in (1) the different textural states imposed on the gouge before reactivation and (2) the integration of acoustic emissions monitored during fault acceleration with post-mortem microstructures. Specifically, we designed three textures: a pre-localized texture – with localized shear planes developed by prior shearing at constant velocity and normal stress of 30 MPa for a few millimeters; a homogeneous texture – compacted under a normal stress of 30 MPa without prior shearing before reactivation; and an overconsolidated homogeneous texture – compacted under normal stress of 60 MPa before reactivation at 30 MPa.

Preliminary results reveal clear correlations between acoustic emission rate, slip evolution, and the degree of localization in post-mortem microstructures. Pre-localized textures remain locked until a critical stress is reached, after which they abruptly accelerate. Homogeneous textures display slow, progressive acceleration, with increasing slip velocity at higher shear stresses. The overconsolidated texture exhibits intermediate behavior. The acoustic emission signature during low constant-velocity slip reflects the grain interactions typical of granular flow, while rapid acceleration produces impulsive lab-quake-like signals typical of localized rupture nucleation.

These preliminary observations suggest a feedback loop between the localization of deformation and instability growth. While this type of relationship between shear strain localization and slip rate is well-established for the co-seismic propagation phase, our observations indicate that it may play a role during the nucleation phase, challenging the steady-state assumption commonly derived from laboratory studies.

How to cite: Giorgetti, C., Pignalberi, F., Mastella, G., Scuderi, M. M., and Collettini, C.: Slip Localization versus Instability Nucleation Feedback Loop: A Laboratory Perspective from Gouge Deformation Experiments, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18843, https://doi.org/10.5194/egusphere-egu25-18843, 2025.

Flash heating is a critical phenomenon in fault gouges, occurring during rapid slip events that generate temperature surges at the grains or asperity contacts. These abrupt temperature increases trigger mechanisms that weaken the gouge material, reducing its shear strength. This process plays a key role in modulating fault dynamics and can significantly impact the propagation of earthquakes [1, 2]. Although many studies have considered microscale friction of uniform or narrowly graded grain sizes, natural fault zones commonly consist of a wide range of particle sizes [3].

In this work, we used a three-dimensional discrete element framework to simulate shear in a granular fault gouge to replicate the rotary-shear experiment. A local temperature-dependent friction law was implemented in which the friction angle of a grain decreases sharply once its temperature exceeds a specified cut-off threshold [4], quantifying how grain size ranges influence the onset of thermally-activated weakening by varying the PSD of our simulated fault gouge—from monodisperse (all grains have the same size) to highly polydisperse (very broad size ranges)—and determining how this range modulates the onset of flash heat weakening. Our numerical results demonstrate that monodisperse samples distribute contact forces more evenly among grains, and that heating and subsequent weakening occur more synchronously across the sample. This uniform distribution of contact forces and thermal effects results in a drop in the macroscopic friction coefficient in a relatively abrupt way. In contrast, samples with polydisperse particles have a wide range of grain sizes, which can give rise to a heterogeneous stress state and thereby strengthen local frictional heating in some areas of the sample, thus allowing a gradual and disordered decline in the macroscopic friction coefficient. Thus, larger grains in polydisperse assemblies can be considered as stress bridges, sustaining higher values of contact forces; on the other side, small grains have lower thermal capacities (via small masses), and they heat up and soften quicker than larger ones with the same frictional work rate. Such a mixture of these aspects explains a larger percentage of weakened particles in polydisperse assemblies and its lower residual friction compared to samples with narrower PSDs. These results emphasize the importance of considering the natural heterogeneity of grain sizes in models of, and observations of temperature-dependent weakening in fault gouges. The polydispersity degree can have a dramatic impact on the rate and extent to which flash heating can induce a transition from a strong (frictional) resisting to a weakened state. Knowledge of these PSD-dependent processes leads to a more realistic representation of dynamic fault weakening and enhances our understanding of earthquake rupture dynamics at naturally heterogeneous fault zones.

References

[1] J.R. Rice. Journal of Geophysical Research: Solid Earth, 111(5), 2006.

[2] B.P. Proctor, T.M. Mitchell, G. Hirth, D. Goldsby, F. Zorzi, J.D. Platt, and G. Di Toro. Journal of Geophysical Research, Solid Earth, 119(iv):3076–3095, 2014.

[3] C. Marone and C.H. Scholz. Journal of Structural Geology, 11(7):799–814, 1989.

[4] A. Taboada and M. Renouf. Geophysical Journal International, 233(2):1492–1514, 2023.

How to cite: Shahabi, H. and Rattez, H.: The Influence of Particle Size Distributions on Flash Heating and Thermally Induced Weakening in Fault Gouges, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-21574, https://doi.org/10.5194/egusphere-egu25-21574, 2025.

 A subgrain-size piezometer is intended to be free from subsidiary effects of recrystallisation, such as phase mixing and pinning, unlike the classical grain-size piezometers, which are best limited to monomineralic samples to avoid these effects. Previously calibrated subgrain-size piezometers have a wide range of uncertainty in stress for a given intercept length. The log-log linear regression fits contribute to the large and impractical error ranges in linear space. The reason behind this behaviour could be the method applied to measure the representative intercept length of the experimental samples. We reanalysed the same calibration datasets used in the existing subgrain-size piezometer and observed that the distributions of intercept lengths are not log-normal. Instead of taking the arithmetic mean of such datasets, we propose that the median may be a better statistic to represent the central tendency of the datasets. Additionally, we have considered subgrains having misorientation angles from 2–10°. Removing 1–2° subgrain boundaries strikes a balance between data loss and noise reduction. Moreover, we propose a method whereby the measurement of subgrain intercepts is free from grain-boundary intercepts, which usually contribute to the largest values in the datasets. Care is taken to minimise the noise in the electron backscatter diffraction datasets whilst preserving the subgrain boundaries by conservatively choosing the halfQuadratic filter parameters. In this updated subgrain-size piezometer, the error ranges in the linear space are reduced from hundreds of megapascals to a few tens of megapascals. We compared the new calibration with the classical grain-size piezometers in two recrystallised monomineralic quartz-bearing natural rock samples. One sample is from a deformed quartzite in a shear zone and the other is from a sheared silicic vein inside a craton. Misorientation axes of subgrain boundaries indicate that basal and prism slip occurred in the respective samples, implying that the deformation temperatures are different. Recrystallisation regimes are confined to certain temperature ranges, and we tested the subgrain-size piezometer in two separate regimes. The range of the differential stress estimated from our recalibrated piezometer is narrowest amongst the available piezometers, for both samples, even when postdeformation grain growth is observed in one of them.

How to cite: Sikdar, A., Misra, S., and Wallis, D.: Subgrain-Size Piezometer: A Recalibration and its Application in Natural Samples, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-907, https://doi.org/10.5194/egusphere-egu25-907, 2025.

The viscous deformation of glacier ice is governed by its temperature and the bulk ice crystal orientation fabric. Due to the mechanical anisotropy of ice crystals, the fabric’s influence on viscosity is directional: depending on the deformation direction, the ice becomes softer or harder. Representing the mechanical anisotropy in numerical ice sheet models is crucial for accurately predicting the future contributions of the Greenland and Antarctic ice sheets to global sea-level rise. However, the fabric strength, orientation, and its impact on viscosity are largely unexplored in fast-flowing ice streams and glaciers. Consequently, the fabric’s influence on ice dynamics is currently inadequately accounted for in ice sheet models. Advances in ground-based radar technologies and improved analysis methods enable the determination of depth profiles of the crystal orientation fabric. In this study, we investigate the fabric and its influence on the viscosity of the Rutford Ice Stream, Antarctica. We analyzed polarimetric measurements performed with an Autonomous phase-sensitive Radio Echo Sounder (ApRES) using a novel approach that allows the determination of fabric-depth profiles to significantly greater depths than previously possible. The results demonstrate a rapid increase in fabric strength within the upper 200 to 300 m depth, followed by a relatively stable fabric strength over depth. In the center of Rutford Ice Stream, our analysis revealed an average fabric strength ranging between 0.4 and 0.5 within the upper 1200 m and fabric rotation by 45° to the ice flow direction. Closer to the shear margin, the fabric strength increased up to 0.8, where the orientation is aligned with the ice flow direction. The findings indicate a substantial influence of the fabric on the effective viscosity, particularly near the shear margin where the ice is softened by a factor of three for horizontal-shear deformation. These findings contribute to a more comprehensive understanding of the distribution of fabric and its influence on the viscosity within ice streams and serve as validation for fabric evolution models.

How to cite: Zeising, O., Arenas-Pingarrón, Á., Brisbourne, A. M., and Martín, C.: Impact of fabric on viscosity of Rutford Ice Stream, Antarctica, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3075, https://doi.org/10.5194/egusphere-egu25-3075, 2025.

The exhumation of high-pressure metamorphic rocks from subduction zones involves dramatic pressure-temperature changes, triggering complex micromechanical responses at the grain-to-sub-grain scale. However, the mechanical aspects of these processes, particularly the origins and persistence of residual stresses within rock microstructures, remain poorly understood. To address this problem, we employed synchrotron-based three-dimensional X-ray diffraction to investigate residual strain, stress, and intra-grain misorientation in a garnet-quartz metamorphic rock from the Lago di Cignana ultra-high-pressure unit in the Western Alps. Our analysis reveals long-range residual stress heterogeneities spanning tens to hundreds of micrometers, with magnitudes reaching several hundred MPa. Significant intra-grain misorientations in both quartz and garnet provide insights into the interplay between plastic and elastic deformation processes.  These stress signatures are preserved in a sample lacking apparent macroscopic deformation, suggesting that subtle mechanisms—such as decompression-induced anisotropic expansion, grain interactions, and garnet compositional gradients—play a key role in stress retention. These findings highlight the potential of synchrotron X-ray diffraction for capturing the stress field within polycrystalline rocks. The ability to resolve three-dimensional strain and stress distributions across scales offers new opportunities to advance our understanding of micromechanical processes associated with rock deformation and metamorphism. 

How to cite: Jacob, J., van Schrojenstein Lantman, H., Cordonnier, B., Menegon, L., Wright, J., and Renard, F.: Exhumation-induced residual stress in undeformed, ultra-high-pressure metamorphic rock, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3540, https://doi.org/10.5194/egusphere-egu25-3540, 2025.

Transient creep of calcite controls the strength evolution of carbonate shear zones during postseismic deformation. However, a lack of information on the dominant microphysical mechanisms of transient creep of calcite hinders the development of constitutive equations. Specifically, for dislocation-mediated deformation, it is unclear whether strain hardening occurs primarily by short-range dislocation interactions and is therefore isotropic or by long-range elastic interactions and is therefore anisotropic. Here, I test whether mylonitic calcite marbles from the mid-crustal shear zone of the Karakoram Fault Zone, NW India, preserve residual stresses indicative of these long-range elastic interactions among dislocations. Previous work demonstrated that the mylonitic fault rocks experienced bulk stresses in the range 40–250 MPa as they were exhumed and cooled from approximately 480°C to 300°C. I analysed the microstructure and micromechanical state of three samples, including undeformed wall rock, protomylonite, and ultramylonite, using electron backscatter diffraction and high-angular resolution electron backscatter diffraction. The undeformed wall rock has a grain size of 130 µm, whereas the protomylonite and ultramylonite have grain sizes of 22 µm and 12 µm, respectively. Densities of geometrically necessary dislocations (GNDs) increase from the wall rock into protomylonite and ultramylonite. In the deformed lithologies, GND densities generally increase with proximity to grain boundaries over distances of 10–15 µm. Residual stresses in the wall rock are below the noise level of the HR-EBSD measurements, with a 99^th percentile of 54 MPa. However, significant heterogeneity in residual stress is present in the protomylonite and ultramylonite, with 99^th percentiles of 325 MPa and 742 MPa respectively. Both the spatial and probability distributions of the residual stresses reveal that they are imparted primarily by dislocations. Autocorrelation of the stress fields indicates that the typical length scale of stress heterogeneity increases from approximately 2 µm in the wall rock to 4 µm in the protomylonite and 7 µm in the ultramylonite. Collectively, these observations demonstrate that dislocations in calcite generate long-range internal stresses that cause elastic interactions. These elastic interactions are typically inferred to manifest as a backstress that counteracts the applied stress and generates a component of anisotropic kinematic hardening. The contribution of this mechanism of transient creep is missing from existing constitutive equations for calcite and should be represented by a backstress that is subtracted from the applied stress and can evolve with strain and time.

How to cite: Wallis, D.: The role of intragranular stress heterogeneity in transient dislocation-mediated deformation of calcite mylonites, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7403, https://doi.org/10.5194/egusphere-egu25-7403, 2025.

Although both ice and methane hydrates are hydrogen-bonded structures of water molecules, methane hydrates are orders of magnitude more creep resistant than ice. The power law scaling properties of this creep resistance was shown experimentally two decades ago, but a molecular-scale explanation for these exponents has still been lacking. Using molecular dynamics simulations over almost two orders of magnitude of stresses and three orders of magnitude of strain rates, we show that power law creep consistent with the creep experiments by Durham and coauthors in 2003 can emerge from a monatomic water model. A monatomic water model with an angular term resulting in tetrahedral ordering, a spherically symmetric methane model and the concept of a hydrate polycrystal are sufficient conditions for this behavior to emerge. We attribute a low-stress low-power relationship to shear of the amorphous layer on grain boundaries between hydrate grains, and show this by a separate set of simulations only containing amorphous hydrate. Higher power creep of polycrystalline hydrate at higher stresses scales with an exponent about twice that of the low-stress regime, but is slower than expected from the amorphous hydrate simulation results. We therefore attribute this creep to the degradation of hydrate corners that are carrying the compressional loading of the hydrate at stresses that cannot be carried by the grain boundaries.

How to cite: Sveinsson, H. A. and Cao, P.:  Distinct creep regimes of methane hydrates can be predicted by a monatomic water model , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8556, https://doi.org/10.5194/egusphere-egu25-8556, 2025.

Predictions of postseismic creep, glacial isostatic adjustment (GIA), and seismic-wave attenuation rely on a sound understanding of the microphysics of transient rheological behavior of olivine-rich rocks, the main constituent of the upper mantle. Recent work proposes that changes in dislocation density and dislocation interactions in olivine may explain the time-dependent evolution of the viscosity of the upper mantle as inferred from geodetic studies. We designed load-relaxation experiments to test whether this model (also known as the backstress model) can accurately predict the transient rheological behavior of polycrystalline olivine during load relaxations similar to those experienced by the upper mantle during postseismic creep and GIA.

We performed our experiments in a gas-medium apparatus at a confining pressure of 300 MPa and temperatures from 1100–1200℃ on dried and annealed Aheim dunite with a grain size of ~ 400 μm. In each experiment, we performed two load relaxations. The first relaxation was initiated after rapidly loading our annealed samples to a differential stress of ~ 200 MPa within 60 s, and the second relaxation was initiated after steady-state creep was reached at a similar, constant stress. 

During the first relaxation, we find that viscosities are initially 1–2 orders of magnitude lower than steady-state viscosities before converging to the steady-state creep flow law over the course of minutes to hours. Meanwhile, such an interval of transient rheological behavior is absent during load relaxations from steady state creep. Microstructural analysis of our starting materials and deformed samples indicates that the observed transient behavior cannot be attributed to changes in grain size or crystallographic preferred orientation. Instead, the transient behavior likely corresponds to changes in dislocation density, which systematically increased during deformation following a piezometric relationship.

We compare these observations to numerical predictions of the backstress model, taking into account the stress history preceding the relaxations, the grain size and the initial dislocation density of our samples. We find that the backstress model accurately predicts the viscosity reduction during the interval of transient rheological behavior, although it slightly underestimates the duration of the transient. In addition, the absence of transient behavior during relaxation subsequent to steady-state creep indicates that the magnitude of backstress during steady-state creep is similar to the applied stress, in agreement with the model. However, the backstress model tends to overestimate strain rates during steady-state creep and subsequent relaxation. Analysis of decorated dislocations in our deformed samples indicates that this discrepancy may be due to the overestimation of dislocation density during steady-state creep by the backstress model. We discuss potential modifications to improve the model involving the effects of temperature and internal stress heterogeneity on the transient behavior of olivine.

How to cite: Hein, D., Hansen, L., and Dillman, A.: Mimicking postseismic creep in the laboratory: Testing models for transient creep in the upper mantle, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8817, https://doi.org/10.5194/egusphere-egu25-8817, 2025.

Seismic failure of dry lower-crustal rocks requires very high differential stress on the gigapascal-level. Among the mechanisms proposed to generate such high stresses is the so-called jostling block model, in which stress is amplified in rigid blocks within lower-crustal shear zone networks, leading to seismic failure. The model is based on field observations from the Musgrave ranges, Australia and the Nusfjord ridge, Lofoten, northern Norway, where pseudotachylytes (quenched frictional melts produced by coseismic slip) occur within the aforementioned structural setting.

Here we present numerical models to test if stress can be amplified in jostling blocks to the levels necessary to fracture dry, intact, lower-crustal rocks, and on which timescales such stress amplification can be achieved. Our models are based on the geometries and material properties determined in the Nusfjord locality. We systematically test the influence of strain rate, viscosity, loading conditions (pure vs. simple shear), and geometry (shear zone thickness, spacing, angle) and find that the bulk strain rate has the most significant impact on both the magnitude and rate of stress amplification. At high to moderate strain rates of 10^-10-10^-12 s^-1 stress amplification to the required level is achieved in years to hundreds of years, while lower strain rates are insufficient to reach the required stress levels. Average long-term strain rates in the in the crust are on the order of 10^-13-10^-15 s^-1, and transiently high strain rates are reported from both field localities mentioned above. Our numerical results are thus well-supported by the rock record. Furthermore, we find that a high viscosity contrast in our models is necessary to reproduce the geometries observed in the field. A third notable contributor to the magnitude of stress amplification that can be reached in the jostling-block geometry is the loading conditions. Specifically, we find that the impact of pure shear on stress amplification is greater compared to simple shear. Shear zone angle and spacing typically have a minor effect. In contrast, increased shear zone width leads to a reduction of stress in the blocks as strain is accommodated fully by the viscous shear zones, and elastic loading of the rigid blocks is no longer necessary to accommodate bulk strain.

Our results clearly demonstrate that, geometric and material properties contribute to stress amplification in different ways, but that strain rate is the controlling factor. In fact, our results indicate that at moderate to high strain rates, stress amplification to levels necessary for failure of intact lower-crustal rocks in shear zone networks is not only plausible, but inevitable.

How to cite: Zertani, S., Thielmann, M., and Menegon, L.: Stress amplification in rigid blocks of lower-crustal shear zones is controlled by bulk strain rate, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9407, https://doi.org/10.5194/egusphere-egu25-9407, 2025.

Quantifying rock rheology is fundamental to understanding and modelling the lithosphere’s dynamics. However, although most rocks of the lithosphere deform at high (> 0.5 GPa) – to very high (> 3 GPa) – pressure over geodynamic events, available mechanical laws have been produced at low pressure (0.3 GPa) using gas-medium deformation apparatuses. To explore rock rheology at higher pressure – typically above 1 GPa – a solid-medium apparatus is required, which involves substantial friction-related stress overestimations while the sample is deforming within the confining medium. Here we provide a series of deformation experiments that aim to quantify such a stress overestimation in the new generation Griggs-type apparatus. The main goal is to better estimate how the friction “baseline” evolves with pressure, alongside defining the starting point of the strain-stress curve more accurately. To do so, we performed general shear experiments of Carrara marble at a confining pressure ranging from 0.3 to 1.5 GPa, while systematically applying a temperature of 650 °C and a displacement rate of 10^-4 s^-1. Using relaxation steps to highlight the friction baseline in a ‘force-displacement’ plot, we document a slope that increases linearly with pressure, from 0.1° to 1.5°. Moreover, none of the highlighted baselines crosses the conventional hit-point, which is the commonly used reference to define the “zero” point of strain-stress curves in the Griggs-type apparatus. Such a mismatch involves additional stress overestimations that we propose to correct by using a new “hit-point” at the intersection between the baseline and mechanical curve. Thanks to the latter and applying a “baseline” correction, we document stress measurements equivalent to the ones documented for Carrara marble using the gas-medium Paterson press.

How to cite: Précigout, J., McGill, G., and Arbaret, L.: Rheological perspective using the new generation Griggs-type apparatus: New constraints from general shear experiments of Carrara marble, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10412, https://doi.org/10.5194/egusphere-egu25-10412, 2025.

Ludox colloidal dispersions exhibit viscous, elastic, plastic and brittle rheological properties depending on their water content. This makes these dispersions a relevant model system to study a wide variety of phenomena, from drying paint to columnar joints. As for now, they are the only system that enables to generate one-sided subduction from convection in the laboratory. Rayleigh numbers, constraining the intensity of convection,  have a similar order of magnitude in the laboratory experiments and in the mantle. Prandtl numbers are much greater than 100, insuring negligible inertial effects. Ludox is thus a relevant analog system to study convection in planetary mantles, the water content playing the role of temperature in determining its rheological properties. 

We investigate here convective patterns  in a Ludox suspension (TM50) heated from below and dried and cooled from above, coupling neutron imaging (NeXT, ILL) and thermochromic liquid crystals (TLCs). Both imaging methods are complementary. Neutron imagery is used to estimate the local volume fraction of silica in the solution, which can be linked to the local rheological properties. TLCs  give us access to the temperature field. We therefore can follow in situ the development  of hot thermal plumes, and of a skin at the surface, that will eventually subduct spontaneously. 

In addition to the imagery, the evaporation rate, the surface, ambient and heating temperatures, and the ambient humidity rate are recorded. They are  used to estimate the heat and mass transfer at the surface and how the formation of a skin affects them compared to a case with an homogeneous newtonian solution. 

How to cite: Remise Charlot, H., Aubertin, A., Helfen, L., Pépin, M., Alba-Simionesco, C., and Davaille, A.: Coupling neutron imaging and thermochromic liquid crystals to investigate the properties of a laboratory-made subducting slab. , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11010, https://doi.org/10.5194/egusphere-egu25-11010, 2025.

During the gravity-driven flow of glaciers and ice sheets, polycrystalline ice tends to develop a strain-induced alignment of individual grains. This fabric development can act as a strain marker for understanding the recent-most deformation history, in addition to exerting significant rheological control on ice sheets compared to isotropic ice. We develop a new way to directly solve for depth-average fabric fields using satellite-derived velocities, assuming that velocities are approximately steady and that fabric evolution is dominated by lattice rotation, in a depth-averaged sense. We apply the method to the North East Greenland Ice Stream (NEGIS) and compare results to radar-derived observations of ice fabrics, suggesting the memory of past flow, stored in ice-stream fabrics, might be useful way to independently set bounds on the age of ice streams (assuming recrystallization is negligible in a depth-average sense). Source/sink flux terms for crystal orientations at the surface and basal boundary naturally appear in the problem as fabric-state-space attractors, and we discuss how the effect of ice—bed interactions on fabric evolution may be parameterized using such terms.

How to cite: Häußler, T., Rathman, N., and Grinsted, A.: What can modeling Steady-State Crystal Fabrics of Ice Streams tell Us about their Age?, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11108, https://doi.org/10.5194/egusphere-egu25-11108, 2025.

Olivine, the most abundant mineral in the Earth's upper mantle, affects seismic wave propagation through its crystallographic preferred orientation (CPO) developed during deformation. As a result, the seismic anisotropy of the crystals serves as a crucial tool for constraining large-scale geodynamic models, linking seismic observations to mantle flow processes via the orientation of olivine crystals.

Building on this link, we propose an optimization problem for inferring the crystal orientation fabrics of upper mantle olivine using oblique seismic data by adapting a method from ultrasound tomography, previously used to infer orientation fabrics of polycrystalline ice. The method relies on (i) a harmonic expansion of the grain orientation distribution function (unknown to be inferred), (ii) a fourth-order closure approximation of the distribution function (reducing the dimensionality of the problem), and (iii) a simple strain homogenization scheme (Voigt homogenization) over elastically orthotropic grains. We construct a one- and two-layer homogeneous slab model of olivine to demonstrate the feasibility of our method in idealized settings and discuss potential applications to regions where sufficient seismic data might exist for real-world application. We also discuss the limitations of our method and the caveats of the assumptions made, in particular the assumed orientation fabric symmetries assumed (hence the assumed mantle flow regime) and the well-posedness of our cost function approach.

How to cite: Hirche, L., Mosegaard, K., and Rathmann, N.: Inferring the Crystal Orientation Fabrics of Olivine from Oblique Seismic Data using a Spectral Fabric Representation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11179, https://doi.org/10.5194/egusphere-egu25-11179, 2025.

During the gravity-driven flow and spreading of ice shelves, polycrystalline ice tends to develop a strain-induced alignment of individual grains. This fabric development can exert significant rheological control on ice shelves, potentially softening or hardening anisotropic ice by several orders of magnitude compared to isotropic ice. We develop a new way to directly solve for depth-average fabric fields using satellite-derived velocities over ice shelves, assuming that velocities are approximately steady and that fabric evolution is dominated by lattice rotation, in a depth-averaged sense. We apply the method to Amery ice shelf, Antarctica, and compare results to previous observations of ice fabrics. Further, we calculate the equivalent isotropic enhancement-factor field using the “CAFFE” method, supposed to represent the first-order effect of fabric on ice viscosity. Because a significant fraction of the ice-shelf thickness on Amery is accreted marine ice, we explore how this may alter the depth-averaged estimate of fabric, and thus viscosity, by including an idealized source term to account for the sub-shelf flux of new grain orientations as ice accretes.

How to cite: Demuth, A., Rathmann, N., and Grinsted, A.: Fabric-induced flow enhancement of the Amery ice shelf inferred from satellite-derived surface velocities, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11382, https://doi.org/10.5194/egusphere-egu25-11382, 2025.

Due to the abundance of quartz in the continental crust, quartz rheology is fundamental to our understanding of many geodynamic processes. Quartz rheology is commonly characterized using a dislocation creep flow law with a stress exponent equal to 4; however, several recent studies indicate that the stress exponent for quartz aggregates can be as low as 2 at conditions where it has been proposed to deform by a combination of dislocation creep and grain boundary sliding (GBS), known as dislocation accommodated grain boundary sliding (disGBS). To address these differing hypotheses, we conducted axial compression load-stepping experiments in a Griggs apparatus at temperatures ranging from 800-950°C, 1.5 GPa, and differential stresses ranging from ~40 MPa to ~1430 MPa with water added. Quartz samples were prepared with different grain sizes of ~3, 5, 10, and 20 μm. For each experiment ~25 load steps were conducted during which the strain rate achieved a mechanical steady state. At the finest grain size, the mechanical data show a stress exponent of n = 1, which then transitions to n ~ 1.8 with increasing stress; for a given stress, strain rate increases with decreasing grain size in both regimes. For larger grain sizes over the same stress range, the stress exponent transitions from n ~ 4 to n ~ 1.8 to n ~ 3 with increasing stress, where only the intermediate stress regime (n ~1.8) shows a grain size sensitivity. We interpret the lowest stress and finest grain size mechanical data to represent grain boundary diffusion creep and assume a grain size exponent of 3. With increasing stress, the samples are interpreted to represent disGBS, where dislocation creep and GBS act in series, where GBS is determined to have a grain size sensitivity of 1. The highest stress data represents dislocation creep. Microstructurally, we observe minimal variation in the starting and final grain sizes, suggesting that the grain size was nominally constant throughout the experiments. Experiments quenched in the GBS regime show microstructures with straight grain boundaries consistent with observations from previous studies. Flow laws have been constrained for all four deformation mechanisms. Plotting a deformation mechanism map using our new flow laws extrapolated to geologic conditions, we show consistent relationships between our flow law estimates and c-axis fabric relationships with naturally deformed quartzites. These new mechanical relationships improve our understanding and constraints on grain-size sensitive rheologies in quartz as well as our ability to model quartz rheology over a wide range of geologic conditions.

How to cite: Tokle, L., Hirth, G., and Behr, W.: Characterizing quartz rheology through load-stepping experiments, from diffusion to dislocation creep, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12111, https://doi.org/10.5194/egusphere-egu25-12111, 2025.

Dynamic recrystallization plays a critical role in the texture evolution of polycrystalline materials undergoing high-temperature deformation, particularly in anisotropic materials such as ice. This study presents a novel, physically-based formulation to model texture evolution during dynamic recrystallization, leveraging detailed observations of ice microstructure under dislocation creep and recrystallization [1]. The formulation incorporates an orientation attractor that maximizes resolved shear stress on basal slip systems, coupled with an anisotropic viscoplastic law to capture mechanical responses. Implemented via finite-element methods in the R3iCe model [2], the approach successfully replicates experimental observations across diverse loading conditions, demonstrating its effectiveness in modeling texture-induced mechanical softening. While the model is validated for ice, it shows potential for application to other anisotropic materials such as olivine. Ongoing work is investigating the scalability and applicability of this formulation to large-scale models, such as glacial ice flow simulations, with a focus on addressing challenges related to computational efficiency and parameterization.

[1] Chauve, T., Montagnat, M., Dansereau, V., Saramito, P., Fourteau, K., & Tommasi, A. (2024). A physically-based formulation for texture evolution during dynamic recrystallization. A case study of ice. Comptes Rendus. Mécanique, 352(G1), 99-134. https://doi.org/10.5802/crmeca.243

[2] R3iCe repository : https://gricad-gitlab.univ-grenoble-alpes.fr/mecaiceige/tools/ice-polycrystal-models/rheolef_cti

How to cite: Chauve, T., Hilzheber, A., Montagnat, M., Dansereau, V., Saramito, P., Fourteau, K., and Tommasi, A.: A physically-based model for texture evolution during dynamic recrystallization: applicationsto ice and prospects for large-scale modeling, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12671, https://doi.org/10.5194/egusphere-egu25-12671, 2025.

Dynamic models of Earth's lithosphere and convecting mantle often simplify the rheological behavior of mantle rocks, for example by assuming constant grain size or considering limited changes in material properties with mineral assemblage. While these simplifications reduce computational requirements, they neglect key processes such as shear localization and transient rheological behaviour associated with phase transitions, which can profoundly impact mantle flow patterns. As incorporating the effect of an evolving grain size in dynamic models has garnered more interest in the geodynamics community, there is a growing need for accurate, scalable, and computationally efficient approaches to address this complexity.

Here, we present recent advancements in the finite-element code ASPECT that address this challenge. These include a higher-order particle method for tracking grain size evolution and the integration of the ARKode solver library, which offers adaptive time-stepping for solving the ordinary differential equation governing grain size evolution. Our implementation captures the simultaneous and competing effects of different mechanisms affecting grain size, such as dynamic recrystallization driven by dislocation creep, grain growth in multiphase assemblages, Zener pinning, and recrystallisation at phase transitions.

We showcase three applications that highlight the importance of grain size evolution—and its interaction with stress and strain rate—for mantle dynamics: (i) global-scale mantle flow, (ii) small-scale convection beneath lithospheric plates, and (iii) the collapse of passive margins. Our models reveal that grain size evolution induces viscosity variations spanning several orders of magnitude, promoting strain localization in all three settings. It therefore controls the shape of upwellings and downwellings as well as the onset time of instabilities. For instance, beneath oceanic plates, the development of large grain sizes before the onset of convection, when strain rates are low, can delay the initiation of cold downwellings. These initial downwellings, in turn, reduce both grain size and viscosity at the base of the lithosphere, allowing subsequent cold drips to form at younger plate ages. Grain damage can also facilitate the collapse of a passive margin through grain size reduction in the lower parts of the lithosphere—but only within a specific range of grain size evolution parameters. Furthermore, additional weakening mechanisms are required for breaking the upper ≥25 km of the plate for subduction initiation to occur. These applications illustrate the applicability of our method to large-scale 2D and 3D models of the convecting mantle and lithosphere and emphasize the critical role of grain-scale processes in shaping the dynamics of Earth’s interior. 

How to cite: Dannberg, J., Gassmöller, R., Myhill, R., Saxena, A., Fraters, M., and Li, R.: Modelling Grain Size Evolution and its Role in Mantle Dynamics: From Small-scale Convection to Passive Margin Collapse, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13052, https://doi.org/10.5194/egusphere-egu25-13052, 2025.

Glacial ice is formed as snow is compressed under its own weight, forming ice crystals with initially random orientations i.e. isotropic ice. Over time, under sustained accumulation and overburden stress, the ice crystals transition from a random arrangement to a more aligned structure, forming anisotropic ice. With continued stress, the ice starts flowing, further modifying the anisotropy. Unlike isotropic ice, which responds equally to stress in all directions, anisotropic ice can deform up to 10 times faster due to its aligned crystal structure. Widely used glacier flow laws, such as Glens flow law, assume the ice to be isotropic. Anisotropy significantly impacts flow dynamics and should therefore be included in ice sheet and glacier models. While enhancement factors are sometimes used to mimic anisotropy, they often do not accurately represent these effects.

In order to correctly represent anisotropy in ice flow, in-situ measurements of ice fabric are needed. However, obtaining such measurements is challenging, particularly in dynamic regions such as ice streams and outlet glaciers. Due to the evolving stress patterns they are subjected to over time, ice streams and outlet glaciers develop distinct anisotropic characteristics. This anisotropic signal contrasts with areas dominated by vertical compression, such as accumulation zones, where anisotropic measurements are typically conducted through ice cores. By applying the concept of seismic anisotropy, specifically shear wave splitting (SWS), we can effectively determine the ice fabric in these fast-flowing areas. This approach provides insights into ice anisotropy of ice streams and glaciers that is difficult to achieve with other methods.

Here, we present ice fabric measurements at Sermeq Kujalleq in Kangia (Jakobshavn Isbræ), Greenland's fastest flowing outlet glacier, with flow velocities reaching 30–40 m/d. By utilizing shear wave splitting observed using basal icequakes, measured directly within the main ice stream, we are able to make a first estimate of the ice anisotropy in such a fast-flowing ice stream.

How to cite: Nap, A., Hudson, T. S., Walter, F., Wehrlé, A., Kneib-Walter, A., Rousseau, H., and Lüthi, M. P.: Assessing ice anisotropy using basal icequakes at Sermeq Kujalleq in Kangia, Greenland, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13323, https://doi.org/10.5194/egusphere-egu25-13323, 2025.

The crystallographic preferred orientation (CPO) of polycrystalline olivine affects both the viscous and seismic anisotropy of Earth's upper mantle with wide geodynamical implications. In this methods contribution, we present a continuous field formulation of the popular directors method for modeling the strain‐induced evolution of olivine CPOs, assuming the activation of a single preferred crystal slip system. The formulation reduces the problem of CPO evolution to a linear matrix problem that can easily be integrated alongside large‐scale geodynamical flow models, and conveniently minimizes the degrees of freedom necessary to represent CPO fields. We validate the CPO model against existing deformation experiments and naturally deformed samples, as well as the popular discrete grain model D‐Rex. A numerical model of viscoplastic thermal convection is built to illustrate how flow and CPO evolution may be two‐way coupled, suggesting that CPO‐induced viscous anisotropy does not necessarily strongly affect convection time scales, boundary (lid) stresses, and seismic anisotropy, compared to isotropic viscoplastic rheologies. As a consequence, geodynamical modeling that relies on an isotropic rheology (one‐way coupling) might suffice for predicting seismic anisotropy under some circumstances. Finally, we discuss limitations and shortcomings of our method, such as representing D‐ and E‐type fabrics or modeling flows with mixed fabric types, and potential improvements such as accounting for the effect of dynamic recrystallization.

How to cite: Rathmann, N., Prior, D., Mosegaard, K., Bekkevold, I., and Lilien, D.: A Spectral Directors Method for Modeling the Coupled Evolution of Flow and CPO in Polycrystalline Olivine, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15461, https://doi.org/10.5194/egusphere-egu25-15461, 2025.

The rheological properties of Earth's lower mantle have a strong impact on global mantle dynamics. Previous studies have shown that the deformation of the ferropericlase-bridgmanite mixture may be strongly controlled by the morphology of the weaker ferropericlase. Due to elongation of weak ferropericlase clusters, the bulk viscosity of the two-phase mixture is significantly lowered and become anisotropic. As a result, this transient microstructural evolution may have a strong impact on the overall rheology of the lower mantle.

Existing numerical models of this process often do not consider that the elongation of ferropericlase during deformation may be counteracted by interfacial diffusion. This diffusion reduces the interfacial energy and may result in an increased rounding rate that reduces the deformation-induced elongation. However, it is unclear under which conditions this process has an impact on the overall dynamics and bulk rheology of a two-phase mixture. A scaling analysis of the governing equations reveals that the dynamics of the given system are mainly influenced by the ferropericlase-bridgmanite viscosity ratio and by the ratio of viscous to interfacial forces.

To explore the impact of these two properties on the dynamics and bulk rheology of the ferropericlase-bridgmanite mixture, we employ numerical models. In these models,  interfacial diffusion is approximated by adding a surface tension term to the governing equations and by directly resolving the ferropericlase-bridgmanite interface using body fitted meshes. The results show that for a range of model parameters, rounding due to surface tension may have a significant impact on the morphological evolution of the ferropericlase inclusions and may thus also exert some control over the rheology of the lower mantle.

How to cite: Thielmann, M. and Dabrowski, M.: Elongation inhibition in two-phase media due to surface tension effects, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16598, https://doi.org/10.5194/egusphere-egu25-16598, 2025.

The long-term fluid-like movements in the Earth’s mantle largely depend on the rheological behaviour of olivine, the main rock-forming mineral in the upper mantle. Although the average viscosity of the mantle can be estimated from post-glacial rebound or geoid anomalies, the micromechanical mechanisms that facilitate the deformation of the solid mantle have been identified from rock mechanics experiments. Dislocation creep emerges as the predominant deformation mechanism in the uppermost mantle, aligning olivine crystals into a crystallographic preferred orientation (CPO) parallel to the flow, while this alignment of crystals also results in anisotropic viscous behaviour. Thus, anisotropic viscosity and CPO evolve hand in hand, and this interaction may impact many geodynamic processes. For example, beneath tectonic plates CPO evolves parallel to the plate motion direction, weakening the asthenosphere in that direction. However, if the plate motion direction changes, the asthenosphere will resist this change, leading to smaller velocities, less deformation and therefore a slow evolution of the CPO towards the new plate motion direction. In the ANIMA project, we aimed to find an efficient way of modelling CPO evolution and the related anisotropic viscosity in a fully coupled way within a geodynamic simulation. We developed a method that tracks CPO evolution on advected particles based on the D-REX method and utilizes the eigenvalues of the mean CPO orientation matrices to predict the anisotropic viscous parameters. These parameters allow us to calculate a tensor form of the viscosity, which we then feed back into our model solution. This method can be applied in combination with other rheologies, although with a cost of having to represent the viscosity as a tensor in the entire model domain, regardless of the dominant deformation mechanism. Despite an estimated increase in computational cost by up to an order of magnitude, incorporating anisotropic viscosity coupled to CPO evolution stands feasible for regional geodynamic models. This development will facilitate the study of a broad new range of geodynamics problems that involve olivine texture and anisotropic viscosity.

How to cite: Király, Á., Wang, Y., Conrad, C. P., Dannberg, J., Fraters, M., Gassmöller, R., and Hansen, L.: ANIMA the journey: how we model olivine CPO-related anisotropic viscosity, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16902, https://doi.org/10.5194/egusphere-egu25-16902, 2025.

Olivine is the most abundant mineral in Earth’s mantle, and its rheological behaviour is likely to control upper-mantle deformation. While the rheological behaviour of olivine is widely studied, relatively little is known about the behaviour of individual olivine grain-boundaries. There is a pressing need to advance our understanding of their physical and chemical properties. Forsterite bicrystals, synthesized by direct bonding of highly polished single crystals at high temperature, were tested in a creep apparatus to investigate sliding along a single planar grain-boundary at high temperature (1300°C and 1400°C). Prior to deformation, the lateral surfaces of the bicrystals parallel to the shear direction were polished, and fiducial markers were scribed perpendicular to the grain-boundary trace to track grain-boundary sliding. Bicrystals were deformed in shear between two polycrystalline alumina pistons or two single crystal forsterite pistons, at 1 atm, with applied resolved shear stresses ranging from 1 to 30 MPa. Post-deformation microstructural analysis using a scanning electron microscope (SEM) shows discrete offsets of fiducial markers, which is the first direct evidence of grain-boundary sliding in olivine bicrystals. These results establish that the studied grain-boundaries are significantly weaker than crystal interiors, and that, crucially, grain-boundary sliding is controlled by the crystallography of crystal interiors and is favoured in a direction nearly parallel to the weakest slip direction in both crystals of the bicrystal.  The measured effective grain-boundary viscosities fit well theoretical models of a dislocation grain-boundary sliding mechanism and are higher than measurements inferred from attenuation. This evidence may highlight the important role of boundary dislocations in accommodating grain-boundary sliding in large grain sizes. These new results indicate that grain-boundary sliding in olivine could play a crucial role in the development of crystallographic preferred orientation and the resulting seismic anisotropy in the upper mantle and should therefore be accounted for in geodynamic models of Earth’s interior.

How to cite: Mecklenburgh, J., Singh, S., Mariani, E., Thom, C., Marquardt, K., Wheeler, J., and Hansen, L.: Direct Measurement of Grain-Boundary Sliding in Forsterite Bicrystals, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19836, https://doi.org/10.5194/egusphere-egu25-19836, 2025.

Ice streams are major contributors to ice sheet mass loss and critical regulators of sea level change. Despite their importance, standard viscous flow simulations of ice stream deformation and evolution have limited predictive power, mostly because our understanding of the involved processes is limited. This leads, for instance, to widely varying predictions of sea level rise during the next decades.

Here we report on a Distributed Acoustic Sensing experiment conducted in the borehole of the East Greenland Ice Core Project (EastGRIP) on the Northeast Greenland Ice Stream (NEGIS). For the first time, our observations reveal a brittle deformation mode that is incompatible with viscous flow over length scales similar to the resolution of modern ice sheet models: englacial ice quake cascades that are not being recorded at the surface. A comparison with ice core analyses shows that ice quakes preferentially nucleate near volcanism-related impurities, such as thin layers of tephra or sulfate anomalies. These are likely to promote grain boundary cracking, and appear as a macroscopic form of crystal-scale wild plasticity. A conservative estimate indicates that seismic cascades are likely to produce strain rates that are comparable in amplitude to those measured geodetically, thereby bridging the well-documented gap between current ice sheet models and observations. 

How to cite: Hofstede, C., Fichtner, A., Kennett, B., Svensson, A., Westhoff, J., Walter, F., Ampuero, J.-P., Cook, E., Zigone, D., Jansen, D., and Eisen, O.:  Brittle creep deformation observed in an ice stream from borehole distributed acoustic sensing , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20791, https://doi.org/10.5194/egusphere-egu25-20791, 2025.

The Saih Hatat Dome (SHD) in NE Oman forms a tectonic window revealing in an area of approximately 95 km by 50 km the par-autochthonous Neoproterozoic basement of the Arabian Plate and its Cambrian to Early Cretaceous cover. The SHD is surrounded by the allochthonous Samail Ophiolite and underlying nappes consisting of mostly sedimentary rocks from the Neo-Tethyan Hawasina Basin.

Within this dome, the Hulw Window exposes rocks that were subducted to depths of >30 km during the Late Cretaceous (Agard et al. 2010) and were subsequently exhumed and tectonically emplaced beneath low-grade metamorphic rocks, forming what is referred to as the "Lower Plate". The Hulw Window consists of marbles, metadolostones, and calcareous micaschists, with embedded mafic and felsic metavolcanic rocks. The entire Hulw unit underwent Late Cretaceous high-pressure/low-temperature metamorphism.

Earlier studies assumed Pre-Permian ages for the protolith for the metamorphic rocks of the Hulw unit (e.g. Miller at al. 2002). Newly obtained U-Pb zircon LA-ICP-MS data from felsic metavolcanic rocks yield ages of 283 ±2.9 Ma and 269 ±3.7 Ma, indicating an Early to Middle Permian volcanism.

Two blueschist-facies quartzites from the southern Hulw unit contain concordant detrital zircons, ranging in age between c. 530 and 2872 Ma with age clusters around 750 to 850 Ma and 1010 to 1164 Ma. The latter ages are not known from an Arabian source and might be derived from an Indian source. The maximum depositional age of the sediments is therefore Early Cambrian.

Field studies in the central part of the SHD revealed numerous mafic dykes, some reaching widths of up to 4 m. These dykes are oriented WNW-ESE and NE-SW. Zircons from one dolerite dyke yields an age of 267 ± 3.7 Ma, indicating that the mafic and felsic magmatism occurred simultaneously.

Whole-rock geochemical data of the mafic volcanic rocks demonstrate a significant partial melting trend, suggesting an increasing degree of upper mantle melting. The felsic metavolcanic rocks are classified as subalkaline to mildly alkaline rhyodacites, which are derived from crustal melting typical of early rift stages.

Overall, the SHD displays a progressive increase in Permian subvolcanic and volcanic rocks from the southeast toward the northwest, characteristic of rift-related crustal extension. This extension ultimately led to the opening of the Neotethys and the separation of the African/Arabian Plate from the Central Iranian/Qiantang blocks and the Indian Plate at a triple junction (Torsvik et al. 2014).

References

Agard, P., Searle, M.P., Alsop, G.I., Dubacq, B., 2010. Crustal stacking and expulsion tectonics during continental subduction: P-T deformation constraints from Oman. J. Struct. Geol. 26, 451-473.

Miller, J.M., Gray, D.R., Gregory, R.T., 2002. Geometry and significance of internal windows and regional isoclinal folds in northeast Saih Hatat, Sultanate of Oman. J. Struct. Geol. 24, 359-386.

Torsvik, T.H., van der Voo, R., Doubrovine, P.V., Burke, K., Steinberger, B., Ashwal, L.D., Trønnes, R.G., Webb, S.J., Bull, A.L. 2014. Deep mantle structure as a reference frame for movements in and on the Earth. Proc. Natl. Acad. Sci. USA 111, 8735–8740.

How to cite: Bauer, W., Qasim, M., Jacobs, J., Callegari, I., and Scharf, A.: Timing of Permian rifting in the Saih Hatat Dome (Sultanate of Oman), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-22, https://doi.org/10.5194/egusphere-egu25-22, 2025.

As the front edge of the continental collision zone, the Indo-Eurasian continental collision belt has great significance for studying the plate collision process, plateau uplifting mechanism and orogenic activities within the plateau. Several models have been proposed to explain north-south compression collision and east-west extension based on geological and geophysical observations. Among them, the distance and shape of subducted India's lower crust and its geometry under the southern Tibet rift are still controversial. To address these issues, we analyze arrival times of P- and S-wave from 35,193 local and regional earthquakes recorded by 575 permanent and temporary stations, and apply an improved double-difference tomography method to obtain high-resolution 3-D P- and S-wave velocity structures of the crust and upper mantle and the locations of the relocated events in the Indo-Eurasian continental collision zone. The east-west velocity profiles reveal that there exists a discrete high-velocity layer dipping eastward at depths of 40-60 km beneath the Longgar rift (LGR), Tingri-Nyima rift (TNR), Xianza-Dinggye rift (XDR), and Yadong-Gulu rift (YGR), which suggests that the subducted Indian lower crust had experienced tearing. On the basis of comprehensive analysis about seismicity, source mechanism of large earthquake in the mantle, and tomographic images, we propose a new dynamic model to present India-Eurasia collision and North-South rifts formation. The significant character of this model is that, the rifts do not cut through the crust vertically but obliquely.

How to cite: Pei, S. and Li, J.: Oblique Rifting in the Southern Tibetan Plateau Revealed From 3‐D High‐Resolution Seismic Travel‐Time Tomography Around the India–Eurasia Continental Collision Zone, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1935, https://doi.org/10.5194/egusphere-egu25-1935, 2025.

The Afar region provides a rare onshore glimpse into the dynamic processes of magmatic continental rifting and the progression towards continental break-up. This area features multiple active magmatic segments distinguished by varied morphologies, crustal thicknesses, rates of magma production, and magmatic-tectonic styles. In the Erta Ale Range rift segment, extension is accommodated magmatically, making it an ideal location to study the magmatic behavior of a mature rift segment. The Erta Ale Range includes sub-segments with magma compositions ranging from basalts to rhyolites, but only the Erta Ale Volcano (EAV) sub-segment is currently active, where only basaltic compositions have been reported so far. Our analyses of major and trace elements, along with isotopic studies of olivine crystals, interstitial glasses, and melt inclusions, combined with oxy-thermo-barometry and thermodynamic modeling, delineate the evolution of magma beneath EAV. We reveal extensive in-situ fractional crystallization within a shallow magmatic reservoir, evidenced by unique cognate gabbroic and microgabbroic blocks. These cognate samples uncover previously unknown mushy and evolved parts (up to 75 wt.% SiO2) of the EAV plumbing system. These findings highlight a sophisticated, transcrustal magmatic plumbing system that contrasts with typical oceanic rift systems, indicating a transitional phase in rift evolution. Our results suggest a magmatic plumbing system that extends up to 12 km in depth, accommodating the rift's extensional dynamics through both magmatic differentiation and tectonic processes. This system is indicative of a rift in an advanced stage of development yet not fully matured to oceanic spreading. Our findings contribute to refining the conceptual models of rift evolution by providing detailed insights into the magmatic and tectonic processes at a critical junction of the Afar rift system. The study emphasizes the complex nature of magmatic systems during the transitional phases of break-up and highlights the need for reconsidering the criteria used to determine the stages of continental break-up. We discuss this model within the geological contexts of the Erta Ale Range rift segment and the larger Afar region, and highlight contrasts with mature oceanic systems to argue that the region is not in the final stages of continental break-up.

Pin, Chazot, France, Abily, Gurenko, Bertrand, Loppin, 2024. Protracted magma evolution and transcrustal magmatic plumbing system architecture at Erta Ale volcano (Afar, Ethiopia). Journal of Petrology 65, egae118. https://doi.org/10.1093/petrology/egae118

How to cite: Pin, J., Chazot, G., France, L., Abily, B., Gurenko, A., Bertrand, H., and Loppin, A.: The magmatic plumbing systems during the continent-ocean transition: the example of the Erta Ale rift, in Afar, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2549, https://doi.org/10.5194/egusphere-egu25-2549, 2025.

Recent studies have highlighted the impact of thermal blanketing on the evolution of rifted margins. This has been achieved by employing 2D geodynamics models in conjunction with models of superficial processes, specifically erosion and sedimentation. The findings of Andrés‐Martínez et al. (2019) and Pérez‐Gussinyé et al. (2020) demonstrate how the sediment transport can influence the architecture over geologic time and how pure ductile deformation can be caused due higher fluvial coefficients. Although this approach is more realistic and can simulate how the mass is distributed along the rifting, with the erosion of uplifted regions deposited in the local basins, it complicates parametric analysis. The deposition is highly sensitive to the input parameters of the superficial dynamics, making it difficult to establish a direct correlation between the input parameters and the outputs. For these reasons, this study aims to establish a link between the response of the margins width and architecture to the basin depths, enabling a clearer connection between the thermal blanketing, sediments thickness and the resulting architecture in a parametric approach. To reach it, a 2D thermomechanical geodynamic model was used, varying the basin thickness (2-7 km) for fixed Moho depths (35-45 km). The effects of heat flow, mechanical and thermal subsidence, and crustal thickness in the basement were analyzed, and each scenario was compared to a control model in which no varied diffusivity was assumed (there was no blanketing effect) and to a model in which no pre/syn rift basin was present. The findings are in accordance with the results of previous studies, which indicate that crustal deformation is affected by larger sediment packages, resulting in greater extension (approximately 100 km) and slower rifting (approximately 4.5 million years) compared to control scenarios. In the models with thicker sedimentary packages, the results suggest a higher thermal flux in the break-up point, with a lower heat flux in proximal domains, accompanied by an increased subsidence in the distal margin and a lower uplift in the proximal domain. The subsidence observed in the central ridge was particularly pronounced in these models with great basins, with a notable reduction in uplift along the rift shoulders.

Funded by Petrobras Project 2022/00157-6 and Brazilian National Agency for Petroleum Project PHR43.1 (2024/10598-5).

References

Andrés‐Martínez, M., Pérez‐Gussinyé, M., Armitage, J., & Morgan, J. P. (2019). Thermomechanical Implications of Sediment Transport for the Architecture and Evolution of Continental Rifts and Margins. Tectonics, 38(2), 641–665. https://doi.org/10.1029/2018TC005346

Pérez‐Gussinyé, M., Andrés‐Martínez, M., Araújo, M., Xin, Y., Armitage, J., & Morgan, J. P. (2020). Lithospheric Strength and Rift Migration Controls on Synrift Stratigraphy and Breakup Unconformities at Rifted Margins: Examples From Numerical Models, the Atlantic and South China Sea Margins. Tectonics, 39(12). https://doi.org/10.1029/2020TC006255

How to cite: Bueno, J., Sacek, V., and Paes de Almeida, R.: The impact of thermal blanketing of pre-rift basins on rifted margins subsidence and basement heat flow: Insights from 2D thermomechanical modeling., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2915, https://doi.org/10.5194/egusphere-egu25-2915, 2025.

Northeastern Oman is located near a Late Paleozoic rift-rift-rift triple junction as part of the Pangea breakup. Above a major and plate-wide unconformity (“basal Saiq Unconformity” or “Hercynian Unconformity”), Late Permian shelf carbonates deposited in much of Arabia and northeastern Oman. In the southeastern Saih Hatat area of NE Oman, near Quriyat, a ~10-100-m-thick conglomerate to sandstone siliciclastic unit (basal Saiq) is sandwiched between the unconformity and the carbonates. We investigated 519 detrital zircons from 7 samples of different intervals within the ~80 m thick basal Saiq. The composite age distribution depicts Archean (2.998±0.007 Ga) to early Mesozoic ages (248±3 Ma). Minor age peaks are at ~2.3-2.6 Ga and 1.6-1.9 Ga. The majority of detrital zircons yield a Neoproterozoic to Paleozoic age (~0.3-1.0 Ga), with most of the ages between ~0.7-0.8 Ga. One sample from the middle part of the section contains zircon grains with a major age distribution of ~300-500 Ma and a peak at ~460-480 Ma. The same sample and a further sample from the lower part of the section contains a significant amount of zircon grains with ages at ~330-350 Ma. The youngest measured ages of 248±3 and 254±3 Ma are detected from two grains of two samples.

Our Precambrian detrital age distribution pattern is similar to patterns known from NW India and eastern Oman (comp. Gomez-Perez & Morton, in press). The Archean and Mesoproterozoic ages likely to have a Neoproterozoic Indian origin. Tonian to Cryogenian ages are the dominant ages, reflect crustal growth of the Omani crystalline basement, with identical U-Pb zircon ages from igneous basement rocks and with flysch-type rocks, formed in the surroundings of a volcanic arc outcropping at the surface in northeastern Oman (Bauer et al., 2025). Infra-Cambrian ages were produced during the final closure of the Mozambique Ocean, as part of the Angudan Orogeny (Gomez-Perez & Morton, in press). Ordovician ages of two samples reflect a regional to local alkaline magmatic event related to continental rifting. Abundant lower to mid-Carboniferous zircon ages (~330-350 Ma) within two samples documents for the first time that the Hercynian event in Oman produced magmatic rocks, beside known rock tilting. Finally, two Permian/Triassic zircon grains ages are derived from volcanic rocks during the Pangea rifting, overlapping in age with the depositional ages of the shallow-marine carbonate of the Saiq Formation. This suggests that the Pangea rifting produced minor acidic igneous rocks in NE Oman.

References

Bauer, W., Jacobs, J., Callegari, I., Scharf, A., Schmidt, J., Mattern, F., 2025. New constraints on the Neoproterozoic geological evolution of the SE corner of the Arabian Plate (NE Oman). In: Scharf, A., Al-Kindi, M. and Racey, A. (eds.) Geology, Tectonics and Natural Resources of Arabia and its Surroundings. Geological Society, London, Special Publication, 550(1), 49.

Gomez-Perez, I. & Morton, A. 2025. Neoproterozoic-Early Paleozoic tectonic evolution of Oman revisited: implications for the consolidation of Gondwana. In: Scharf, A., Al-Kindi, M. and Racey, A. (eds.) Geology, Tectonics and Natural Resources of Arabia and its Surroundings. Geological Society, London, Special Publications, 550(1).

How to cite: Scharf, A., Qasim, M., Callegari, I., and Bauer, W.: Detrital zircon U-Pb geochronology of the basal Saiq siliclastics – A complete magmatic record from the Archean to the Permian/Triassic of NE Sultanate of Oman, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3424, https://doi.org/10.5194/egusphere-egu25-3424, 2025.

Magma-rich continental rifts comprise en-echelon magmatic segments where magmatism and extension are localised, similar to slow and ultra-slow spreading centres. While rift segmentation is clear in mid-ocean ridges, it is less so in continental rifts like the Main Ethiopian Rift (MER). Faulting within the MER initiated at ~11Ma at the border faults which define the overall NE trend of the MER and are oblique (30°-45°) to the E-W extension direction. However, since ~2Ma extension has localised to the right-stepping Wonji Fault Belt (magmatic segments), in which small offset faults and alignments of volcanic features strike roughly orthogonal to the extension direction. Despite this general framework, there is a lack of quantitative analysis to understand rift segmentation and its relationship to volcanic systems, and how segments interact. It is unclear how the ratio of magmatic to tectonic processes varies along rift segments.

Using optical satellite imagery and SRTM digital elevation data with a resolution of 1 arc-second, we map fault traces, calderas, and volcanic craters in the central and northern MER at a scale of 1:100,000. We also map scoria cones in the same region using optical imagery at 1:20,000. This data is integrated with existing MER datasets, including previously mapped fault traces, digital elevation models, mafic intrusions derived from gravity data, InSAR-derived locations of magma bodies, and recent dyke intrusions between Fentale and Dofan to define the magmatic segments. We investigate characteristics and scales of MER magmatic segments by analysing fault trace patterns, along-segment displacement variations, elevation profiles, the distribution of volcanic activity, and shallow crustal density structures.

How to cite: Farrell, C., Keir, D., Corti, G., Sani, F., and Maestrelli, D.: New insights on segmentation of fault and magmatic systems in the Main Ethiopian Rift, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3745, https://doi.org/10.5194/egusphere-egu25-3745, 2025.

The Chukchi Basin, a sub-basin of the Amerasia Basin in the Arctic Ocean, remains enigmatic regarding its formation age and tectonic processes. Among the various hypotheses proposed, seafloor spreading or hyper-extended rifting during the Cretaceous are currently prominent, both supported by gravity and deep seismic survey data. Recent marine heat flow (MHF) observation efforts using the IBRV Araon from 2018 to 2024 have resulted in a comprehensive dataset covering the basin along and across the inferred N-S oriented spreading axis in the basin center. The formation age inferred from the newly observed MHF was the Early to Late Cretaceous, which is slightly older than the timing of Northwind Basin to the east. Notably, the MHF distribution revealed an asymmetric increase toward the eastern margin perpendicular to the axis and toward to southern margin parallel to the axis. Because MHF distribution often reflects deep tectonic structure such as the Moho depth or the lithosphere-asthenosphere-boundary, this asymmetric pattern suggests a difference in the depth of these boundaries within the basin. The observed discrepancy between the inferred spreading axis and the MHF distribution indicates that the Chukchi Basin may have undergone asymmetric rifting, challenging the conventional notion of symmetric rifting. Our future research will integrate gravity and magnetic anomaly data with numerical modeling to better constrain the deep structure and formation processes of the basin.

How to cite: Kim, Y.-G., Hong, J. K., Jin, Y. K., and So, B. D.: Asymmetric distribution of marine heat flow in the Chukchi Basin (Chukchi Abyssal Plain) as possible evidence for asymmetric rifting, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3906, https://doi.org/10.5194/egusphere-egu25-3906, 2025.

Dikes can contribute to rifting, but the space-time behavior and role of magma in young and slowly extending continental rifts is unclear. We use observations and modelling of InSAR and seismicity during the September to November 2024 Fentale intrusion in the Main Ethiopian rift (MER) to understand magma-assisted rifting at slow extension rates (5 mm/yr). From 2021 to mid-2024, the Fentale Volcanic Complex (FVC) uplifted up to 6 cm. From mid-September 2024, upper crustal diking started northwards along the rift, initially with subdued seismicity. From late-September to early November, dike opening increased to ~2m and propagated a total of ~14km north, causing increased seismicity from normal faulting. The dike made ~90% of the total geodetic moment, with the rest from faulting. The character of the event is similar to rifting episodes at mid-ocean ridges and demonstrates that episodic diking can occur in young, slow extending continent rifts but must be more infrequent. This marks the start of a major rifting episode.

How to cite: Keir, D., La Rosa, A., Pagli, C., Wang, H., Ayele, A., Lewi, E., Monterroso, F., and Raggiunti, M.: The September to November 2024 Fentale dike in the Ethiopian rift, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3968, https://doi.org/10.5194/egusphere-egu25-3968, 2025.

As one of the largest marginal seas in the Western Pacific, the structure and evolution of the South China Sea will provide important reference to the marginal sea research. In order to decode the continent-ocean transition and seafloor spreading process of the South China Sea, 3 normal and 1 extended IODP drilling expeditions were carried out from distal margin to the relict ridge of the South China Sea. However, large controversies still exist due to the lack of enough drill site-coordinated geophysical investigation to calibrate the drilling results. 30 active source OBSs were deployed along the 300 km long drilling transect and then a 3D network with 60 OBSs were deployed in the Continent-Ocean transition zone. The velocity models deduced from the OBSs suggest that thick and widespread magmatic underplating occurred below the northern continental margin, with the thickest underplating occurred below the continental slope where the crustal thickness is over 20 km. Correlated with the sedimentary history, the strong magmatic underplating is supposed to happen at late Eocene and caused strong uplift and erosion of early syn-rift sequences. Quantitative analysis suggests that up to 10 km thick magmatic underplating below the thick crust requires a highly attenuated if not fully devastated mantle lithosphere below the continental slope during Eocene. Therefore, the breakup of South China Sea is supposed to experience an earlier mantle breakup and then a crust breakup to generate the spreading ocean. In comparison with Atlantic, the mantle below the northern continental margin might be wetter to generate such large amount of syn-rift magmatic underplating. Forward mathematical modeling suggests that a pre-rift subduction may provide the mechanism of both unsteady lithospheric and more saturated mantle. This might explain why marginal sea basin usually has much wider underplating and more magma supply than the same spreading rate passive continental margin and ocean.

How to cite: Sun, Z. and Peng, T.: The structure and breakup mechanism of the South China Sea, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5129, https://doi.org/10.5194/egusphere-egu25-5129, 2025.

The Middle Proterozoic period (1800–800 Ma), often called the "Boring Billion", was characterized by a stable environment with low atmospheric oxygen levels and globally anoxic oceans. In East Asia, this period has been frequently linked to the breakup of the Columbia supercontinent at ca. 1400 Ma, as evidenced by widespread litho-stratigraphic evidence (e.g., Bayan Obo, Yanliao, Xionger rift systems) of rifting in the North China Craton. Similar Mesoproterozoic rift-related lithologies have been identified in the Hwanghae Rift Zone (HRZ) on the northern Korean Peninsula (Jon et al., 2011; Han et al., 2013), suggesting that the Korean Peninsula may have been a part of the global-scale rift system associated with the disruption of the Columbia Supercontinent.

From this point of view, this study examines the tectonic evolution of banded-iron formation (BIF)-bearing metamorphosed sedimentary and volcanic successions in the Western Gyeonggi Massif of the Korean Peninsula. The meta-sedimentary sequences consist of quartzite, biotite-muscovite schist, BIF, and marble, while the volcanic suite comprises amphibolite and meta-gabbro, occurring as clasts, boudins, and blocks within the marble beds. All the rock types exhibit amphibolite facies metamorphic alterations and deformations. Intercalation of quartzite with Algoma-type BIF suggests siliciclastic sedimentation concurrent hydrothermal Fe input from deep-seated faults in a matured continental shelf environment. The carbonate deposition indicates biological activities on the volcanic atoll in the calm marine environment, following active volcanism. The dismembered amphibolite blocks or lenses show massive, igneous textures, and sub-alkaline basaltic composition, with trace and rare earth element patterns resembling ocean island basalt (OIB) and enriched mid-ocean ridge basalt (E-MORB), indicative of rifting of continental landmass similar to modern-day Iceland driven by plume-ridge interactions. U-Pb zircon dating of dismembered amphibolite blocks or lenses reveals ca. 1419 Ma protolith age followed by ca. 251 Ma metamorphism. These findings represent the earliest Mesoproterozoic volcanism and sedimentation recorded in the central-western margin of the Korean Peninsula, which has been considered part of the Permo-Triassic collisional belt. We propose that the central-western margin of the Korean Peninsula witnessed rifting concurrently with its northwestern margin, coinciding with rifting in the North China Craton (e.g., Bayan Obo, Yanliao, Xionger rift systems) as part of the global rift system associated with the disruption of Columbia supercontinent during the "Boring Billion".

How to cite: Jang, Y., Samuel, V. O., Kwon, S., and Santosh, M. W.: Tectonic evolution of the proto-Korean Peninsula in the Boring Billion: Implication for the disruption of the Columbia Supercontinent, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5492, https://doi.org/10.5194/egusphere-egu25-5492, 2025.

While it is well documented that continental extension involves discrete tectonic or magmatic rifting events, little is known about how deformation accumulates between these events. Here we focus on strain localization across the Natron Basin, which is part of the eastern branch of the East African Rift, that experienced a major tectono-magmatic event in 2007.

A cross-rift profile of horizontal GNSS velocities (2013–2017) reveals a gradual transition between the rigid Tanzanian Craton and the Somalian Plate, with ~2 mm/yr of extension distributed across ~100 km (stretching zone). Such a pattern is commonly interpreted through the lens of dislocations in an elastic half-space. Here, an east-dipping border fault locked down to ~10 km may explain the observed width of the stretching zone, provided it extends to great depths, and creeps at a constant rate of ~3 mm/yr. The extent to which this is compatible with a hot lower crust riddled with magmatic intrusions is still debatable.

We thus explore an alternative model where the width of the stretching zone is entirely determined by the history of past, finite deformation, and the corresponding ambient stress state. We use a 2-D thermo-mechanical model to stretch a visco-elasto-viscoplastic brittle layer, first creating a major border fault that slips continuously, flexing its footwall and hanging wall. We then artificially “lock” this fault by instantaneously strengthening it, drastically reduce our computational time steps, and continue stretching the layer. While the system should behave as an homogeneous, elastic layer under far-field extension, i.e., produce a linear displacement profile, we obtain an arctangent-shaped profile with a characteristic stretching zone width. 
This suggests that strain localization is controlled by the heterogeneous distribution of pre-existing stresses. Specifically, regions of high stresses that accrued during flexure of the fault blocks are brought to failure first during inter-event stretching, prompting the localization of elasto-plastic strain in a wide zone centered on the border fault. This process explains the width of velocity gradients in rift zones without invoking a deep, continuously creeping fault. 

We therefore suggest that long-term stress buildup plays a key role in short-term strain localization, and discuss its implications for active deformation in magma-rich continental rift settings like the Natron Basin.

How to cite: Navarrete, I., Olive, J.-A., Calais, E., Dalaison, M., and de Monserrat, A.: Inter-event strain localization modulated by background stresses across the Natron Basin, East African Rift, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5777, https://doi.org/10.5194/egusphere-egu25-5777, 2025.

Low-angle normal faults (LANFs), characterized by dips of less than 30°, are frequently observed in rifted margins. Despite extensive research, the mechanical processes governing LANFs remain poorly constrained, raising critical questions about the angle at which they initiate, their evolution during extension, their three-dimensional geometry, and related deformation in the hanging-wall and footwall. Addressing these issues is essential for understanding extensional processes in such tectonic settings, including thinning of the continental crust and the exhumation of mantle material in rifted margins.

The Err and Bernina extensional detachment systems, within the lower Austroalpine nappes of the Central Alps, offer a rare natural laboratory for studying LANFs. Formed during the Jurassic rifting in the distal Adriatic rifted margin preceding the formation of the Alpine Tethys, these LANFs are exceptionally well-preserved despite the subsequent deformations from the Alpine orogeny.

This study presents results from extensive field campaigns conducted between 2022 and 2024, during which high-resolution data were collected over a ~100 km² area using Unmanned Aerial Vehicle (UAV) surveys supplemented by field mapping. Rigorous quality control and processing ensured the generation of 3D high-resolution digital outcrop models (DOMs) of the Err and Bernina extensional detachment systems, implementing differential positioning and SwissTopo terrain data for a resulting spatial error of less than 1 meter. The DOMs provide centimetre to decimetre-scale details that facilitate mapping of the spatial evolution of LANFs and the tectono-sedimentary architecture of the overlying allochthonous blocks. Detailed interpretations reveal their internal structure, including lithological changes, deformation patterns, and fault structures at various scales. Additionally, we characterized the sedimentary basins formed during the Jurassic extension, shedding light on their development and spatial relationships with the detachment systems. Comparison of our findings with seismic data across present-day low-angle normal fault systems bridges the scale-gap between detailed field-based analyses and large-scale seismic interpretations, providing crucial new insights to the evolution of LANFs.

How to cite: Morzelle, L., Mohn, G., Betlem, P., and Tugend, J.: High-resolution digital outcrop models of low-angle normal faulting: the  fossil distal Adriatic rifted margin (SE Switzerland), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6051, https://doi.org/10.5194/egusphere-egu25-6051, 2025.

The arrival of upwellings within the mantle from Earths deep interior are commonly observed worldwide, but their role in driving volcanism during continental breakup has long been debated. Given that only a small fraction of Earth’s upwellings are situated under continents and a limited number of them are associated with active continental rifting, our understanding of these processes remains incomplete.

Here, we investigate the interplay between continental breakup and mantle upwellings using the classic magma-rich continental rifting case study of the Afar triple junction in East Africa. Some studies previously proposed that the region is underlain by mantle upwelling(s), yet others argue for limited involvement of mantle plumes.  Several discrete segments of the rift have been studied in terms of magma petrogenesis. However, until now, a paucity of high-precision geochemical data across the broader region has hampered our ability to test the models and evaluate the spatial characteristics and structure of this upwelling in the recent geologic past.

Within this study, we present extensive new geochemical and isotopic data spanning the region and integrate these with existing geochemical and geophysical datasets shedding light on the spatial characteristics of the mantle beneath Afar.  By combining geophysics and geochemistry using statistical approaches, our multi-disciplinary approach shows that Afar is underlain by a single, asymmetric heterogeneous mantle upwelling. Our findings not only validate the heterogeneous characteristics of mantle upwellings, but demonstrates their susceptibility to the dynamics of the overriding plates. This integrated approach yields valuable insights into the spatial complexity of mantle upwellings.

How to cite: Watts, E. J., Rees, R., Jonathan, P., Keir, D., Taylor, R. N., Siegburg, M., Chambers, E. L., Pagli, C., Cooper, M. J., Michalik, A., Milton, J. A., Hinks, T. K., Gebru, E. F., Ayele, A., Abebe, B., and Gernon, T. M.: Afar triple junction fed by single asymmetric mantle upwelling, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6181, https://doi.org/10.5194/egusphere-egu25-6181, 2025.

The Dniepr-Donets Basin (DDB) represents a significant intracontinental rift system in Europe, whose formation remains an ongoing topic of research. Central to this investigation is whether the basin developed through passive rifting—driven by far-field tectonic stresses such as back-arc extension—or active rifting, which involves localized thermal anomalies from processes like mantle plume activity. This research seeks to address these competing models through integrated geological and geophysical analyses, contributing to our understanding of continental rift evolution.

This project involves interpretation of 23 regional seismic reflection and refraction profiles including “classical” seismic profiles: DOBRE’99 and Georift-2013. The seismic data will be calibrated by c. 4000 wells with stratigraphy. Seismic analysis will be focused on mapping of 14 key stratigraphic horizons covering the entire area of the DDB (~76,900 km²). The spatial orientation of structural elements will be resolved using potential field anomaly maps. Integration of the interpreted surfaces with the borehole stratigraphy will allow for determining the age of major unconformities and faulting. The evolution of the DDB will be quantitatively analysed using cross-section balancing technique along selected regional seismic profiles.

A key aspect of this work involves constructing a three-dimensional structural model of the DDB using borehole and seismic data. This model, still under development, aims to provide detailed insights into the basin’s geometry, sedimentary layer distribution, and fault system configuration. Particular emphasis is placed on identifying structural asymmetries, which could suggest the operation of simple-shear mechanisms often linked to passive rifting. By correlating surface geological features with deep crustal structures, this research is gradually building a comprehensive picture of the basin’s evolution.

Potential field data are also being analyzed to investigate mantle processes and their influence on rifting. Variations in gravity and magnetic fields are being studied for evidence of deep-seated magmatic intrusions and high-density bodies. This approach aims to evaluate whether mantle plume activity or crustal thinning contributed to the rifting mechanism, helping to distinguish between active and passive processes.

This ongoing research integrates data across crustal and mantle processes, with the goal of correlating mantle dynamics, surface volcanism, sedimentation patterns, and tectonic evolution. The findings aim to advance our understanding of intracontinental rifting and provide insights into the conditions under which rifting transitions to full continental break-up or remains an intracontinental feature, as in the case of the DDB.

How to cite: Nasiri, A., Stovba, S., Drachev, S., Stephenson, R., and Mazur, S.: Tectonic Evolution of the Pripyat-Dniepr-Donets-Donbas Basin: Insights into Intracontinental Rifting Mechanisms and Structural Dynamics, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6348, https://doi.org/10.5194/egusphere-egu25-6348, 2025.

Rifted margins mark the transition between a thick-crusted (35 ± 5 km) continental domain and a thinner-crusted (0–8 km) (proto-)oceanic domain. Yet, the mechanisms of crustal thinning during rifting are incompletely understood, especially the consequences and fingerprints of the so-called necking phase during which the continental crust is thinned from its initial thickness to ca. 10 km in only a few million years.

One major difficulty in studying necking arises from the necking phase being only transient in the timeframe of continental rifting and often followed by further extension and thermal relaxation. As a result, the structural, stratigraphic and thermal signatures of the necking process are partially dismembered and overprinted in present-day rifted margins. Hence, studying the necking process requires to identify and track its fingerprints in present-day rifted margins.

In this contribution, we synthesize data from the best calibrated necking domains worldwide to define general recognition criteria and hence clarify the definition of necking. We show that necking domains commonly display: (1) deformed (from cataclasites to black gouges) basement directly overlain by undeformed syn-rift sediments; (2) exhumation of deep continental crust; (3) syn-rift basement erosion and adjacent sandstone deposition; and (4) syn-rift and syn-tectonic shallow-water deposits rapidly followed by syn-rift but post-tectonic deep-water deposits. We argue that these fingerprints cannot be explained by high-angle normal faults by themselves and discuss the possible additional and/or alternative processes.

How to cite: Chenin, P. and Manatschal, G.: Fingerprints of necking domains at rifted margins: insights from the best documented examples worldwide, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6434, https://doi.org/10.5194/egusphere-egu25-6434, 2025.

The factors controlling the structure and morphology of oblique rifted margins remain enigmatic. Key features requiring explanation include: (1) long transform fault systems (>300 km) with transpression or transtension, (2) rift segments with varying asymmetry and obliquity, and (3) complex, variable drainage systems along the rift. We use large-scale 3D coupled thermo-mechanical and surface process models to explore how inherited transform weakness zones influence the structure and morphology of oblique rifted passive margins. Our results show that the orientation of inherited weaknesses determines the degree of transpression or transtension along transform faults, while the extent of over- or underlap among weaknesses controls segment obliquity and asymmetry, shaping fluvial drainage networks. These findings provide a conceptual framework to interpret the key structural and morphological characteristics of oblique rifted margins in the Equatorial Atlantic, North Atlantic/Arctic, and Mozambique regions.

How to cite: Theunissen, T., Huismans, R. S., Rouby, D., Wolf, S. G., and May, D. A.: Inherited transform weaknesses control structure and morphology of highly oblique rift-transform systems, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6786, https://doi.org/10.5194/egusphere-egu25-6786, 2025.

Post-cratonization rifting has emerged as a prominent research focus in structural geology due to its association with significant hydrocarbon accumulations. Such rift systems are extensively developed within and along the margins of cratonic regions during the Mesoproterozoic to Neoproterozoic, notably in areas such as the Siberian Craton, Australian Craton, and North American Craton. The genesis of these rift systems is typically attributed to extensional tectonic regimes that evolved during the post-orogenic reconfiguration of cratonic lithosphere. These systems represent critical tectono-sedimentary processes that influence crustal thinning, fault block development, and the formation of accommodation space, playing a key role in hydrocarbon source rock maturation, reservoir development, and trap formation. Recent advancements in natural gas exploration within the Ediacaran strata of the Sichuan Basin have revealed the substantial hydrocarbon resource potential of the Neoproterozoic sequences in the Upper Yangtze Craton. These exploration successes are intimately associated with the development of deep-seated extensional rift systems in the Yangtze Craton, which are interpreted as the result of rapid lithospheric extension following cratonization during the early Neoproterozoic. Despite these breakthroughs, a comprehensive understanding of the structural geometry, kinematic evolution, and petroleum systems of these rift systems remains limited, highlighting the need for further systematic investigation. This study integrates two-dimensional and three-dimensional seismic data with magnetotelluric data, deep borehole records, and field outcrop observations to construct, for the first time, a three-dimensional structural model of the Neoproterozoic rift systems in southwestern Sichuan Basin. The results reveal two distinct rifting phases during the Early to Middle Neoproterozoic, with rift dimensions ranging from 3-8 km in width and 7-23 km in length. The rift systems and associated fault networks predominantly display NE and NNE trends, with faults generally dipping northwestward. These faults governed the development of half-grabens during both rift phases, each accompanied by sedimentary deposits reaching thicknesses of 2–3 km. The stratigraphic sequences within the rifts exhibit strong correlations with the Neoproterozoic strata exposed along the western margin of the Yangtze Craton. Chronological evidence indicates that the first rift phase (800–720 Ma) was characterized by independently developed sub-rift basins. The second rift phase (720–635 Ma) inherited and expanded upon the earlier rifting, culminating in the development of a unified, large-scale half-graben that overlies the sub-rifts of the first phase. During the late syn-rift stage, significant compressional uplift along the western Yangtze Craton margin induced structural inversion of several pre-existing rift normal faults in southwestern Sichuan and the formation of pre-Ediacaran reverse faults. This compressional event eroded over 3 km of rift-related sequences. The Neoproterozoic rifting and subsequent compressional deformation along the western Yangtze Craton margin are closely tied to subduction and rollback dynamics of the Pan-Oceanic plate. This study emphasizes the excellent conditions for hydrocarbon source rock and reservoir formation in the Neoproterozoic of southwestern Sichuan, highlighting its vast potential as a target for future hydrocarbon exploration.

How to cite: Lu, G. and He, D.: 3D Structure, Evolution, and Geodynamic Model of the Neoproterozoic Rift Basins in Southwestern Sichuan Basin, South China, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7068, https://doi.org/10.5194/egusphere-egu25-7068, 2025.

The thermal and deformational history of a rift are directly correlated. Increased stretching, whether by faulting or by lower crustal flow, results in elevated heat flux,  which has significant implications for the asymmetrical evolution of the heat distribution in the basins (Lescoutre et al., 2019). Since the extensional rate also controls the amount of stretching, it also becomes an important parameter for understanding the thermal evolution. In natural rift systems, acceleration is a kinematic evolution inherent to all rifting processes (Brune et al., 2016). However, the role of the extensional rate in the evolution of the thermal flux is not clear. Ten thermo-mechanical numerical models were developed using a weak and decoupled rheology for the lithosphere. The models were run with extension rates varying from 1 to 5 cm/year with intervals of 0.5 cm/year, and one model with acceleration was simulated with values estimated by Araujo et al., 2022 for the Santos-Benguela conjugates, between Brazil and Africa. Results show that the heat flux values along the widest margin of the conjugated pair increases as the constant velocity rises. In contrast to the wide margins, the narrow margins show a simple thermal evolution. The thermal evolution of the wide margin cools from the necking zone to the end of the distal domain in velocities of 2 cm/year, following the rift migration evolution. In the models with 2.5 cm/year or higher, the thermal flux evolves similarly to the deformation process described in Souza et al., 2025 - where rift migration is not well established and two rifting sites are active simultaneously. In the acceleration model, thermal flux remains high throughout the distal domain of the widest margin, driven by rift migration. In all constant velocity cases, rifting time decreases with increasing velocity, as expected. However, the acceleration model yields a rifting duration consistent with that observed in the Santos region, where the extension rates were based.

Funded by Petrobras Project 2022/00157-6.

Araujo, M. N., Pérez-Gussinyé, M., & Muldashev, I. (2023). Oceanward rift migration during formation of Santos–Benguela ultra-wide rifted margins. J. Geol. Soc. London, Special Publications.

Brune, S., Williams, S. E., Butterworth, N. P., & Müller, R. D. (2016). Abrupt plate accelerations shape rifted continental margins. Nature, 536(7615), 201-204.

Lescoutre, R., Tugend, J., Brune, S., Masini, E., & Manatschal, G. (2019). Thermal evolution of asymmetric hyperextended magma‐poor rift systems: Results from numerical modeling and Pyrenean field observations. Geochemistry, Geophysics, Geosystems, 20(10), 4567-4587.

Souza, S. dos S., Salazar-Mora, C. A., Sacek, V., & de Araujo, M. N. C. (2025). Kinematic and rheological controls on ultra-wide asymmetric rifted margins evolution. Marine and Petroleum Geology, 171, 107171.

How to cite: dos Santos Souza, S., Salazar-Mora, C. A., de Souza Bueno, J. P., Sacek, V., and Neto Cavalcanti de Araujo, M.: The Interplay Between Extensional Rate and Heat Flux in Asymmetric Rift Systems, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7282, https://doi.org/10.5194/egusphere-egu25-7282, 2025.

Total continental lithosphere extension prior to breakup and sea-floor spreading in the South China Sea (SCS), a marginal ocean basin, ranges from approximately 360 km in the NE to 580 km in the SW. In contrast, total continental lithosphere extension prior to breakup for the Iberia-Newfoundland rifted margins is no more than 180km. SCS extension leading to continental breakup is between x2 and x3 greater than for the Atlantic margin type.

In the case of Atlantic type margins, lithosphere deformation transitions from initially wide rifting to more localised stretching and thinning, a process termed necking. The necking domain at rifted continental margins, so produced, typically has crustal thickness of 25 km proximally decreasing to 10 km distally. Further lithosphere stretching and thinning due to hyper-extension and the onset of decompression melting results in the rupture and separation of continental lithosphere, the creation of a divergent plate boundary, and the initiation of sea-floor spreading.

The SCS shows very wide domains of thinned continental crust with thicknesses between 25 and 10 km; widths of thinned crust much greater than those of Atlantic type margins. These wide regions of thinned crust on the SCS margin take the form of crustal boudinage with multiple sag basins underlain by highly thinned crust separated by basement highs underlain by less thinned crust.

The localisation of lithosphere deformation before breakup, during the formation of Atlantic type margins, is due to failure of the initially strong cold lithospheric mantle lid. The same mechanism of localisation cannot occur to generate necking in the SCS; the SCS was formed by rifting of volcanic arc lithosphere in which the lithospheric mantle was already hot.

We attribute the very wide regions of continental crust with thicknesses between 25 and 10 km in the SCS, very much wider than for Atlantic type margins, to a weak inherited lithosphere rheology which favours extensional boudinage of the continental crust rather than crustal rupture and separation, and distributed rather than focused decompression melting of wet mantle from the inherited volcanic arc setting.

How to cite: Zhang, C., Kusznir, N., Manatschal, G., Chenin, P., Taylor, B., Sun, Z., Li, S., Suo, Y., and Zhao, Z.: Lithosphere Extension Prior to Continental Breakup in the South China Sea: Comparison with the Atlantic Type Rifted Margin, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7891, https://doi.org/10.5194/egusphere-egu25-7891, 2025.

Syn-rift sequences, breakup unconformities and magnetic anomalies have been widely used to date rifting. However, it is generally accepted that rift systems are diachronous, both along dip and strike, and that the rifting processes are complex and difficult to date, in particular at magma-poor rifted margins. Therefore, new approaches need to be developed to date rifting. In our study we use the stratigraphic record of vertical movements to date a specific rift event and its propagation. In this work, we focus on two origins of uplift during rifting. First, the necking process, which corresponds to onset of localized deformation and significant differential crustal thinning over 4 to 14 my. Necking may result in a characteristic, fast and short-lived uplift limited to the future distal margin, followed by its fast subsidence (Chenin et al., 2018). Second, dynamic topography, which refers to a large wavelength (from 1,000 to 4,500 km) and fast (35 to 400 m.Ma^-1) uplift (Jones et al., 2012), due to convection/heterogeneities within the asthenospheric mantle, not necessarily linked to rifting.  In our study, we use the example of the widely studied Late Jurassic to Early Cretaceous southern North-Atlantic magma-poor rift system, forming the present-day West Iberian margin, its conjugate the Newfoundland margin, and the Bay of Biscay rifted margin. Thanks to the specific and characteristic fingerprints of each of the two types of vertical movements, they can be used to date rifting in an absolute and relative way. The necking signal dates a distinct event at a rift-segment scale, allowing to date the along strike diachronous evolution of the rift system. In contrast, the dynamic topography uplift occurs over a very wide area and is linked to simultaneous uplift and well-defined erosional unconformities that are time equivalent to a sudden increase in sedimentation rates offshore. Then, dynamic topography events occurring during rift propagation, could be considered as isochrons across a large area, allowing for along strike time correlations  

                Our preliminary results show a northward propagation of necking, which is consistent with the northward propagation of continental breakup already documented along the Iberian/Newfoundland conjugated margins. Secondly, we identify a dynamic topography event. Indeed, a Barremian to Aptian/Albian event can be defined by a large-scale uplift (e.g., Massif Central, Provence (France) and Southern England) that occurs at the same time of an increase in sedimentation rates and a change in seismic facies documented at the distal margins in the southern North Atlantic. The identification of these two types of events thanks to geological fingerprints and their relatively short duration, allows us to date rifting in the Iberian-Bay of Biscay system. While vertical movements associated with necking allow us to directly date the onset of crustal thinning and rift localisation, dynamic topography does not date a particular rift moment, but allows us to define an isochronous event that can be used for along strike time correlations and thus, for relative dating within propagating rift systems.

How to cite: Mathey, R., Autin, J., Manatschal, G., Sauter, D., Chenin, P., and Erratt, D.: How to date rifting thanks to vertical movements?, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8638, https://doi.org/10.5194/egusphere-egu25-8638, 2025.

Normal faulting in continental rifts creates pronounced relief which evolves over time. At the same time, many rifts are marked by 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. It is clear that magmatic intrusions and normal faulting interact in magmatic rifts such as the Kenya Rift, the Main Ethiopian Rift, the Afar triple junction, and in the Icelandic plate boundary. However, 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 remains difficult to isolate from observations.

Previous modeling studies of time-dependent magma-tectonic interactions in extensional tectonic settings fell into one of two categories: (1) simple models where diking is represented by a prescribed fixed rectangular zone of horizontal divergence (e.g., Buck et al.,  2005), (2) complex setups where magma ascent is represented by porous flow and fluid-driven fracture (e.g., Li et al. 2023). While the former approach can be applied to model of tens of millions of years of dike injection along spreading ridges, the simplicity prevents applications to continental rifts where magmatism manifests over broad areas. The latter approach allows to study the evolution of individual dikes, but its computational costs prevent application to lithospheric-scale rifts over geological times scales. 

Here, we propose a numerical workflow that can be categorized as a model of intermediate complexity. We nucleate the dikes at the brittle/ductile transition above magma-forming regions. The dikes 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 have approached this problem and how we implemented it in the open-source community geodynamics model ASPECT. We show how the generated dikes are being focused in specific regions, and how the 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., and Atnafu Muluneh, A.: Modeling dike-fault interactions in continental rifts on geological time scales, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8867, https://doi.org/10.5194/egusphere-egu25-8867, 2025.

The northeastern Brazilian rifted margin exhibits a diverse range of extensional structures, from failed onshore and offshore rifts and basins to South Atlantic seafloor spreading and continental breakup, making it an ideal natural laboratory for studying rifted margins.

Previous studies on the northeastern Brazilian rifted margin present conflicting interpretations of the basement structure in the Camamu, Almada, Jequitinhonha, Jacuípe, Sergipe, and Alagoas basins. Proposed models include: (a) hyperextended continental crust transitioning directly to oceanic crust; (b) hyperextended continental crust with exhumed lower crust and an immediate switch to oceanic crust; (c) hyperextended continental crust, exhumed mantle, and a direct transition to oceanic crust; and (d) hyperextended continental crust transitioning to proto-oceanic crust and then to normal oceanic crust. Additionally, there is ongoing debate about whether the Sergipe-Alagoas and Jequitinhonha-Almada-Camamu basins are magma-poor or more magmatic than previously thought.

The lithosphere in northeastern Brazil comprises diverse tectonic units, ranging from cratons to orogenic belts, which have undergone multiple orogenic deformations and metamorphic events. This structural and compositional heterogeneity likely exerted a first-order geologic control on the evolution of rifts, basin boundaries, and crustal structures during the opening of the South Atlantic. Analyses of basement rocks, structural trends (e.g., foliation, shear zones, and faults), and contact relationships between geologic units suggest significant geological influences on rift development.

To address these conflicting interpretations, this study adopts a thermo-mechanical approach using a newly developed numerical modeling technique, Kinedyn, which integrates seismic reflection profiles with geodynamic models. The results are expected to resolve discrepancies in previous studies and provide a more realistic reconstruction of rift evolution in the northeastern Brazilian rifted margin.

How to cite: Gün, E., Pérez-Gussinyé, M., García-Pintado, J., Gudipati, R. R., Mezri, L., and Araújo, M. N.: Geophysical, Geological, and Geodynamic Insights into the Northeastern Brazilian Rifted Margin, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8908, https://doi.org/10.5194/egusphere-egu25-8908, 2025.

The rifted margins of the NE Atlantic are among the most extensively studied regions in the world thanks to the extensive geological and geophysical data available for this area. Despite this extensive research, uncertainties remain regarding the timing and mechanisms of rifting. Key questions include the volume of magma, recognized as underplated layer in the lower crust, the precise position of the Jan Mayen Microcontinent, and the extent of rifting that preceded the final opening of the NE Atlantic in the Paleogene. These uncertainties have significant implications for plate reconstruction models.

In this contribution, we combine interpreted seismic stratigraphy with plate rotations to define a new plate reconstruction model of the study area, spanning from mid-Permian to early Eocene. Stretching and pre-drift extension for individual rifting events are derived from a set of conjugate crustal transects evenly distributed along the NE conjugate margins, allowing to identify “restored” position of the continent-ocean boundaries (COB) back in time. Using an optimization approach, we derive Euler Poles that best-fit fixed and rotated restored COBs of the Eurasian and North American plates. Our approach incorporates uncertainties in COB location and the amount of magma added to the lower crust.

First results indicate a tighter pre-break-up fit between Greenland and Eurasia than previously suggested, implying that earlier models underestimated stretching. Implementing the obtained Euler Poles to plate reconstruction software GPlates highlights the four distinct rifting events. Our new plate reconstruction model offers improved insights into passive margins affected by multiple rifting events and can inform further studies on paleogeography, rift dynamics and break-up kinematics in the NE Atlantic region.

How to cite: Haas, P., Abdelmalak, M. M., Shephard, G. E., Faleide, J. I., and Berndt, C.: Plate tectonic modeling of multi-rifting events in the NE Atlantic, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9069, https://doi.org/10.5194/egusphere-egu25-9069, 2025.

Lithospheric thinning initiates continental rifting and eventual break-up, driven by the interplay of tectonic, magmatic and surface processes. Recent findings from IODP expeditions and seismic surveys reveal that the northern South China Sea (SCS) margin exhibits distinctive features not typically alinged with classic magma-poor or magma-rich margins, including widespread detachment, syn-rift magmatism and a notably rapid transition from continental margin to seafloor spreading. However, the role of magmatism in the formation of detachments, which is key for elucidating the evolution of rifted margins, remains poorly understood. Here we use 2D numerical models to simulate the thermo-mechanical evolution of continental rifting, incorporating melt generation, emplacement and associated heat release. Our models reproduce the main observations from the northern SCS margin, including the hyper-extended crust, crustal boudinage, lower crust exhumation and dome structure. Particularly, we demonstrate that the thermal weakening related to the magmatism promotes the ductile lower crustal flow, which converges beneath a ‘rolling-hinge’ type detachment, facilitating the formation of core complex. Unlike magma-poor margins, the initial elevated lithospheric temperature by prior plate subduction and syn-rift magmatism from decompressing melting shape the ‘intermediate’ nature of the SCS margin. This work could provide valuable insights into how tectonic deformation and magmatism interact in continental rift systems around the globe.

How to cite: Yang, P., Pérez-Gussinyé, M., Liu, S., García-Pintado, J., and RaghuRam, G.: Magmatic controls on detachment fault formation at South China Sea rifted margin, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9481, https://doi.org/10.5194/egusphere-egu25-9481, 2025.

Rift systems play a crucial role in the Wilson cycle, where the extension and breakup of continental plates can lead to the formation of new oceans. Earth's rift systems exhibit various stages, from initiation to breakup, with the latter representing 'successful' rifting, as observed along the Atlantic margins. Whereas rifted margins can record successful extensional plate dynamics, deformation can also stop at earlier stages or shift to more favourable locations, resulting in 'failed' rifts, such as the North Sea or the Atlas rift. However, the mechanisms that control whether a rift fails or is successful are not very well known.

Understanding the dynamics of continental extension and tectonic processes in rift systems requires examining their initial conditions and subsequent evolution, with the latter influenced by both strengthening and weakening processes of the lithosphere. Here we numerically simulate rift evolution using geodynamic finite-element 2D ASPECT models incorporating shear zone (“fault”) dynamics and strain softening within a visco-plastic rheological framework. We use the landscape evolution model FastScape to simulate surface processes.

To understand which processes lead to the success or failure of a rift, we explore the role of strengthening and weakening processes. Our modelled strengthening processes comprise (1) lithospheric cooling, which enhances the strength of ductile domains via temperature-dependent viscosity, (2) gravitational potential energy gradients that impose a degree of compression outboard of high-elevation domains; and (3) fault healing, which strengthens frictionally weakened regions over time as a function of temperature. We also account for the following weakening processes: (1) frictional softening, which causes an increase in fault activity; (2) lithospheric necking, which thins and thereby heats the lithosphere beneath the rift centre; (3) erosion and sedimentation, as simulated by FastScape, which alters the distributions of surface loads in a way that increases fault longevity. Within the framework of these processes, we examine the effects of crustal thickness, extension rate, rheology, and friction angle, on the spatial and temporal occurrence of rift success and failure. To quantify the results, we analyse fault geometry and dynamics, as well as the forces required for continued extensional plate motion.

Preliminary results indicate the existence of a lower limit for the full extension velocity to achieve breakup. For models with typical continental lithosphere this limit is ~2 mm/yr. Lithosphere that is extending at a smaller velocity thins temporarily but strengthening mechanisms ultimately outweigh weakening processes resulting in relocalisation of deformation. Our analysis highlights the internal and external processes that influence rift systems at different evolutionary stages and provides criteria for understanding and predicting rift evolution.

How to cite: Neumann, T., Brune, S., Buiter, S., Neuharth, D., and Jackson, C.: Modelling of lithospheric weakening and strengthening processes and their impact on rift success and failure, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10113, https://doi.org/10.5194/egusphere-egu25-10113, 2025.

How to cite: Ding, X., Wang, Z., Brune, S., Dooley, T., Moscardelli, L., Neuharth, D., Glerum, A., Rouby, D., Fernandez, N., and Hudec, M.: Geodynamic modelling of salt tectonics and translation speed at rifted continental margins, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10665, https://doi.org/10.5194/egusphere-egu25-10665, 2025.

Volcanic rifted margins commonly form in association with the emplacement of Large Igneous Provinces. The intense associated volcanic activity coincides with shifts in the global carbon cycle and rapid climate change during several key geological periods and crises. The Mid-Norwegian and NE-Greenland conjugate rifted margins formed after late Paleocene to early Eocene continental breakup in association with the emplacement of the North Atlantic Igneous Province (NAIP). The NAIP and early opening of the North Atlantic occurred contemporaneous to a rapid 5-6 °C global warming episode known as the Paleocene Eocene Thermal Maximum (PETM). The rapid global warming documented during the PETM is hypothesized to result from the release of thermogenic gases into the atmosphere through thousands of hydrothermal vents. The gases were generated by contact metamorphism of carbon-rich sediments during the extensive sill emplacement from the NAIP. The potential climatic impact of these hydrothermally released greenhouse gases is dependent on the water depth at which they were released. Unless it is released in a shallow marine environment most methane, known for its significantly greater global warming potential compared to carbon dioxide, will be oxidized and dissolved in the ocean before it reaches the atmosphere.

First results of IODP Expedition 396 conducted on the Mid-Norwegian volcanic margin have documented the shallow marine to potentially sub-aerial setting of at least one of the hydrothermal vents (i.e. Modgunn vent). However, a comprehensive regional assessment of the water depth at which hydrothermal venting occurred remains necessary to validate the overall impact on paleoclimate and the PETM. To do so, we apply 3D flexural-backstripping and decompaction to remove the loading effects of sedimentary sequences and determine the sediment-corrected bathymetry down to the top Palaeocene surface at which most of the vents are mapped. Reverse subsidence cannot be directly modelled without knowing the detailed distribution of syn- and post-rift thermal subsidence from Cretaceous and Paleocene rifting as well as any mantle plume dynamic uplift during NAIP emplacement. Because these tectonic and geodynamic components of subsidence cannot be deterministically predicted at the required accuracy, we use local palaeobathymetric constraints from seismic observations and drilled biostratigraphic data, combined with our flexural backstripping and decompaction results to calibrate palaeobathymetric variations of the Paleocene venting surface at the time of the PETM.

Our results predict that hydrothermal venting occurred within a range of palaeo-water depths showing the complex palaeo-structure of the top Paleocene surface. Key post-Paleocene tectonic influences such as a well-documented Miocene doming episode influence the margin history, and hence, at this location, our palaeobathymetric results represent shallowest estimates and must be interpreted with caution. However, most of the vents (>80%) restore to bathymetries shallower than 500 meters, i.e., in sub-aerial to shallow marine conditions. Our work aims to confirm and extend initial results of IODP Expedition 396 from the Modgunn vent. Shallow water-depth hydrothermal venting most likely occurred during magma-rich continental breakup and NAIP emplacement; a large part of the released hydrogenic gas could have directly contributed to the global warming recorded by the PETM. 

How to cite: Tugend, J., Mohn, G., Kusznir, N. J., Planke, S., Berndt, C., Zastrozhnov, D., and Millett, J. M.: Paleo-depth of hydrothermal venting along the Mid-Norwegian volcanic margin during Paleogene continental breakup, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10746, https://doi.org/10.5194/egusphere-egu25-10746, 2025.

The margins bounding the Equatorial Atlantic were formed during the Cretaceous due to the breakup of Gondwana. Rifting led to the development of sedimentary basins between West Africa and South America. We have used a grid of ~10,000 km of 2D seismic data to investigate the crustal structure along ~600 km of the NE Brazilian margin, containing the eastern Ceará and Potiguar Basins. The dataset is provided by the Brazilian National Agency of Petroleum (ANP).

We have interpreted fault structure and sediment units and mapped key horizons (top synrift, top basement, and Moho), across the entire seismic grid to produce surface and thickness maps of the main units. The basement thickness, synrift thickness, and Moho structure maps revealed that the margin tectonic structure is divided into three main tectonic domains: the Southern, Central, and Northern segments. The Southern Segment is characterized by abrupt lateral basement thinning and steep faults forming a main fault system indicating strike-slip kinematics. In contrast, main extension in the Central and Northern Segments is associated with normal faulting kinematics. These two segments represent different styles of faulting because the focalization of the extensional deformation is decoupled and occurred farther outboard along the Central Segment. The Northern Segment displays a comparatively thinner basement and thicker synrift deposits across much of the margin, compared to the Central Segment. These differences appear to imply that crustal extension occurred at different rates.

The three segments are separated by tectonic boundaries defined in seismic images by abrupt lateral changes in basement structure. The main segments may also contain sub-segments where changes in structure are more subdued. The imaged segment boundaries form a consistent linear structure visible from under the continental shelf to the deep-water basin. Their geometry indicates the evolution over time of continental segmentation during rifting. Furthermore, the orientation of these boundaries is similar for all segments supporting that they approximately correspond to flow lines indicating the opening direction during rifting. Most segment boundaries during rifting spatially correlate with fracture zones on the oceanic plate, indicating a relationship between continental tectonic segmentation and oceanic magmatic segmentation. We propose that the tectonic segmentation of the margin appeared during Barremian-Aptian time as a lithospheric-scale response of the mode of deformation caused by a change in plate kinematics that imposed a change in opening direction.

How to cite: Fonseca, J., Ranero, C., Vannucchi, P., Iacopini, D., and Vital, H.: Tectonic Segmentation During Rifting of the Brazil Equatorial Margin, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11099, https://doi.org/10.5194/egusphere-egu25-11099, 2025.

Recent hydrocarbon discoveries in the Black Sea Basin (BSB) rekindled debate on whether the basin rifted open as one east-west oriented basin, or as two separate basins named Eastern and Western Black Sea Basins. Supporting the two-basin idea is the semi-parallel ridge and depression geometry of the BSB with NW-SE orientation in the eastern portion of the Black Sea Basin, and W-E orientation in the western portion of the Black Sea Basin. On the other hand, interpretations for a single basin configuration are supported by the regional structure of the BSB being consistent with  geodynamic models of rifting of the basin by slab roll-back about a hinge point located on the eastern edge of the basin.

To help resolve the tectonic uncertainty, we built a new structural framework for the BSB by reinterpreting 24 long-offset 2D seismic lines acquired by GWL in 2011. This in turn allowed us to develop  two sectioned 2D computational models representing the western and eastern parts of the BSB to model the variation in the kinematics of the basin formation. Our interpretations of continuous normal, inverted, and strike slip fault systems that define the ridge and depression geometry lead us to support a model in which the BSB opened as a single basin. The 2D sectioned models were extended to 3D to test whether the rifting occurred with increasing velocities towards west. We compare our findings with the structural elements that we interpreted on the seismic sections such as strike slip fault systems that have been active throughout the basin formation and the tectonic inversion of the Late Eocene era. Ultimately, this provides better insight of the timing of all the tectonic events of the BSB during the extensional and subsequent compressional stages of the basin’s evolution.

How to cite: Kaykun, A. and Pysklywec, R.: Structural Evolution of the Black Sea Basin Using 2D Sectioned and 3D Computational Models, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11717, https://doi.org/10.5194/egusphere-egu25-11717, 2025.

The sedimentary basins of the Brazilian Equatorial Margin (BEM) are considered a key frontier for petroleum exploration. The BEM is characterized by transform tectonics, featuring oblique and divergent brittle structures occurring on the Foz do Amazonas, Pará-Maranhão, Barreirinhas, Ceará, and Potiguar basins. This tectonic pattern is also recognized in the West African marginal basins (Ghana, Ivory Coast, and Liberia), including those of Cote d’Ivoire and Ghana. The central sector of the BEM, where the divergent segments of the Pará-Maranhão Basin meet the transform segment of the Barreirinhas Basin. To better understand the tectonic framework, a comprehensive dataset, including seismic data, in addition to well data (gamma-ray, density, sonic profiles, checkshots, and biostratigraphy), was analyzed across 80,000 km². These data, reinterpreted considering modern understanding of the BEM evolution, provided insights into the structural and stratigraphic characteristics of the margin. The basins were classified based on the obliquity of their segments relative to the rift extension direction. This obliquity, defined by the angle between the transform faults and segment direction, was used to delineate four distinct crustal domains: the continental thinning domain, the hyper-extended continental domain, the mantle exhumation domain, and the oceanic domain. Each domain reflects different geological processes contributing to crustal evolution. The Pará-Maranhão divergent segment, which connects with the Barreirinhas transform segment, is oriented NW-SE with a 53° obliquity. This segment has a wider continental thinning domain due to its higher obliquity. The sequence of crustal thinning progresses from continental to oceanic, marked by normal faults, horsts, and grabens, indicating tectonic extension. The sedimentation in this region is mainly controlled by thermal and tectonic subsidence, with distinct rift (syn-rift), post-rift, and continental shelf sequences. Fault blocks rotate, creating listric faults and rollover systems that affect sedimentation. In contrast, the West Barreirinhas segment, which is aligned with the Romanche Fracture Zone, has a 0° obliquity. This transform margin features a narrow continental crust neck, with differential subsidence and steep post-rift slopes. Listric faults and large negative flower structures are characteristic of this segment. Overall, the variation in obliquity across the margin segments significantly influences the width of the crustal thinning domain, with higher obliquities resulting in wider thinning zones. The presence of thinned continental crust and exhumed mantle in the deep-water region, prior to the first occurrence of oceanic crust, is similar to the analysis of the African conjugate margin, which is associated with a hydrocarbon system based on Upper Cretaceous turbiditic sandstone reservoirs. The same potential reservoirs are also found in the Brazilian counterpart.

How to cite: Costa de Melo, A. C., de Castro, D. L., and Oliveira, D. C.: Tectonic Architecture of the Equatorial Atlantic Margin: Insights from the Central Segment of Brazilian Counterpart, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12592, https://doi.org/10.5194/egusphere-egu25-12592, 2025.

Passive continental margins and sedimentary basins are key domains for understanding long-term geological processes driven by complex Earth dynamics, such as continental rifting, magmatism, and sub-lithospheric interactions. These processes shape regions and leave distinct, spatially variable imprints in the sedimentary record. Deciphering these records helps us understand the dynamic relationships between geological processes on passive margins and quantify the interplay among tectonic, magmatic, and sedimentary forces that influence basin architecture.

In this study, we model the thermal-kinematic history of the southern Vøring Basin, offshoreMid-Norway, along a regional 2-D transect, integrating basin- and lithosphere-scale processes through time-forward basin modeling and an automated inverse basin reconstruction approach. The results indicate that the evolution of the inner Vøring Margin can be explained by standard lithosphere extension models. However, these models fail to account for key observations at the outer volcanic province, such as regional uplift at breakup, excess magmatism, and higher geothermal gradients. These discrepancies suggest additional processes are involved. Excess magmatism and uplift may be linked to sub-lithospheric mantle processes, such as the arrival of the Icelandic mantle plume or small-scale convection. Melt retention in the asthenosphere, along with mantle phase transitions during extension, could enhance uplift.

The best-fit model must explain the following key observations at both the inner and outer margins: (1) observed stratigraphy and subsidence, (2) beta factors along the transect, (3) vitrinite reflectance, particularly the high %Ro values at the outer margin, (4) base Eocene paleobathymetry, with an emergent outer margin and structural highs, and (5) the interpreted magmatic underplate beneath the outer margin.

We test various tectono-thermal models that include or exclude these processes. Models incorporating a plume emplaced at Eocene time, accounting for magmatic processes like melt retention and underplating, successfully reproduce the observations at the outer volcanic margin. This supports the contribution of the hot Icelandic plume to the Vøring Margin's evolution.

How to cite: Abdelmalak, M. M., Faleide, J. I., Midtkandal, I., Druga, A., Aldinucci, M., Zastrozhnov, D., Tsikalas, F., and Gac, S.: Basin modelling of the complex multi-rift system on Southern Vøring Margin : mechanisms and implications, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13337, https://doi.org/10.5194/egusphere-egu25-13337, 2025.

Continental rifts and rifted margins are governed by the complex interplay of a range of factors: thermo-mechanical processes control deformation at depth modulated by the emplacement of melt, while erosion and sedimentation reshape surface topography. Understanding the intricate links between geodynamic, magmatic and surface processes is essential to unravelling how rifts evolve, how they interact with the Earth system and under which conditions georesources are generated.

This presentation highlights latest technical advances and insights into the interaction of rift processes. It uses a recently established framework in which the open-source geodynamic software ASPECT is bi-directionally coupled to the landscape evolution code FastScape. This approach captures the dynamic interaction between faulting, surface loading, isostasy, rift-shoulder erosion and intra-basin sedimentation from rift initiation to rifted margin formation. In addition, dikes are incorporated via a one-way coupling scheme using two approaches: (1) a post-processing technique that infers potential diking pathways based on the modelled tectonic stress field, or (2) via user-defined input where dikes are represented as thin vertical domains with prescribed horizontal dilation.

These models reproduce the common finding that melts often rise sub-vertically to the surface in the form of dikes. However, compressional domains associated with block rotation are surprisingly common features in our models that result in the deflection of ascending melt. This process could explain the formation of sills in sedimentary basins and basement rocks, as well as the horizontal offset between melting zones in crust and mantle: features observed in several magmatic rifts. Our models suggest a complex interaction between diking, faulting, and sedimentation, which are compared to selected regions in the eastern branch of the East African Rift. These results illustrate how advances in numerical modelling techniques, combined with multidisciplinary field data can lead to new insights into the process interactions that control the structure and evolution of individual rift segments.

How to cite: Brune, S.: Process interactions in continental rifts, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13571, https://doi.org/10.5194/egusphere-egu25-13571, 2025.

The Northeast Atlantic is a key region where advances in plate tectonics have been developed, tested, and refined. Final breakup and the onset of seafloor spreading started around magnetic Chron C24n (~55 Ma; earliest Eocene). However, prior to breakup, the Northeast Atlantic’s margins underwent at least four discrete phases of lithospheric-scale rifting and basin formation, extending back to mid-Permian times (ca. 264 Ma) following the Caledonian orogeny. The total amounts of extension are in the order of several hundred kilometers and therefore relevant to implement in regional and global plate tectonic reconstructions. Recently, deformable plate models using the GPlates software have emerged as a tool to capture such non-rigid domains. However, deformable models to-date have been largely constructed in an overall rigid plate framework, applying pre-existing Euler rotations from the surrounding plates to the intervening rift. Here we detail why, and how, a basin-to-plate scale approach should be considered in future regional and global refinements of deforming reconstructions, using the multi-phase Northeast Atlantic rifting as a focus site.

            We place basin-scale observations based on extensive seismic, stratigraphic and geophysical interpretations for the Norwegian margin and its Greenland conjugate (Abdelmalak et al. 2023) into new digital plate tectonic model (Shephard et al., in review). Central to our methodology is identification and restoration of rift basin hinges, and accounting for their along-margin variability. In this presentation we will detail the timing, location, amount and direction of extension across four discrete rift phases and their associated time-dependent rotations. A conjugate profile from the Foster and Northern Vøring margins (totalling 282 km of extension at average rates ranging between 0.13-0.58 cm/yr during rifting) yields the best fit accounting for along-margin heterogeneity whilst retaining the overall rigid framework requirements. We compare our results to previous regional models, including Barnett-Moore et al. (2018) and Müller et al (2019), and showcase some of the GPlates scalar field functionality including crustal stretching and tectonic subsidence. Finally, we have also developed an external routine for a backward-restored crustal thickness workflow which successively restores present-day thickness in conjunction with our deformable model.

How to cite: Shephard, G. E., Abdelmalak, M. M., Faleide, J. I., Clennett, E., Gac, S., Zahirovic, S., Haas, P., Gaina, C., and Torsvik, T. H.: A basin-to-plate deformable plate framework to capture the multi-phase rifting of the Northeast Atlantic, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13989, https://doi.org/10.5194/egusphere-egu25-13989, 2025.

We use a newly developed model formulation to explore the potential structural evolution of a spectrum of margins from Volcanic to Magma-poor. We assume that the melt is incompressible, and we simulate melt migration as magmatic intrusions and volcanic extrusions as volume change and stress change in the brittle and ductile crust. We also model heat transfer generated by melt migration, latent heat of recrystallization, melt production and hydrothermal circulation.

Based on our simulation and observations of passive margins, we propose models for the formation of volcanic and magma-poor margins. While magma-poor margins evolution follows well-known stages, we show that volcanic margins represent a wide spectrum of behavior from purely accretionary and volcanic to mixed extensional and volcanic. The nature and extent of seaward dipping reflectors (SDRs), the crustal composition and structure, the subsidence of the margins vary as a function of the mantle potential temperature in the asthenosphere and the initial geothermal signature of the lithosphere.

We can resume our main findings which diverge strongly from existing models for volcanic margins: (1) For mantle potential temperatures (T_p) greater than 1400^oC, we find that volcanic margins form through the accretion of intrusive magmatic and extrusive volcanic product of melt production in the asthenosphere. This system forms an accretionary center of thickness and width increasing with T_p. On both side of the accretionary axis, two symmetrical SDRs basins form. Subsidence of these basins increase with decreasing T_p. Increasing subsidence generated by far field extension leads to an increase in clastic sedimentation and controls SDRs composition. Decreasing T_p and increased subsidence leads to the formation of clastic rich SDRs while increasing T_p and decreased subsidence leads to formation of mainly volcanic/mafic SDRs. (2) The exhaustion of melt production leads to ridge jumps and the formation of eccentric accretionary center. When subsidence is more pronounced for a lower T_p we simulate periods of uplift and subsidence correlated with periods of higher and subdued melt production, respectively. This process may result in cyclical periods of mafic followed by clastic sedimentation. (3) For T_p lower than 1400^oC, intermediate margins form with both volcanic and extensional processes occurring concurrently. This processes eventually lead to the asymmetric propagation of volcanic centers which may lead to seafloor spreading.

How to cite: Lavier, L.: Magma-poor To Volcanic Margins: New Models, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14609, https://doi.org/10.5194/egusphere-egu25-14609, 2025.

A quadruple junction is a distinctive phenomenon in plate tectonics characterized by the convergence of four tectonic plate boundaries at a single geographic location. While such occurrences are infrequent within the realm of plate tectonics, they provide a valuable opportunity to explore the processes involved in the evolution of the solid Earth. In this context, we examine the Afro-Arabia plate boundary as a pertinent example of a quadruple junction. The establishment of the Makkah Madinah Transform Zone (MMTZ) as a significant tectonic boundary has profoundly influenced the geological framework of western Arabia, offering a fresh perspective on the geodynamics of the broader Red Sea area, particularly with the advent of the central Red Sea triple junction. The MMTZ is estimated to have an age ranging from 27 to 30 million years, inferred from the configuration of plate boundaries surrounding the southern Red Sea, Sirhan, eastern Mediterranean, and the Zagros orogenic zone. In our reconstruction of the Red Sea, we apply a rotation of 6.7 degrees for Arabia relative to Africa, utilizing the topographic alignment of both rift flanks to facilitate basin closure. We establish a connection between the MMTZ plate boundary and the Ader Ribad depression in Sudan, grounded in both spatial and temporal analyses. Chronological investigations of the Ader Ribad depression indicate an exhumation event occurring approximately 31 million years ago, coinciding with the timeline of the MMTZ. The coexistence of these two plate boundaries exemplifies a unique tectonic scenario of a quadruple junction. We present reconstructions of the Afro-Arabia plate and 3D thermo-mechanical numerical models with the code I3ELVIS of the Afro-Arabia plate boundary to substantiate our hypothesis. The code implements a marker-in-cell approach with finite differences  method. The model consists of upper and lower continental crust, lithospheric and sublithospheric mantle until 220 km depth. Multi-directional extension is simulated by imposing variable divergence velocities on the right and rear model sides. Extensional and transtensional deformation is initially localized along implemented rheological and thermal weaknesses.

How to cite: Aldaajani, T., Attila, B., Gerya, T., Ball, P., Almalki, K., and Abd El-Motaal, E.: Evolution of Quadruple Junction: Example from Afro-Arabia plate boundary, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14789, https://doi.org/10.5194/egusphere-egu25-14789, 2025.

Within Central Europe, remnants of the Variscan orogeny are found today at elevations exceeding 1000 m. Among these remnants are the Black Forest and Vosges Mountains that are separated by the N-NE-oriented Upper Rhine Graben. Subsidence of the Upper Rhine Graben began during the Eocene and was accompanied by the uplift of Variscan basement, which is now exposed in the Vosges Mountains and Black Forest at the western and eastern rift flanks, respectively. Overlying Mesozoic sediments have been extensively eroded, exposing the Variscan bedrock and confining the younger sediments to isolated, higher-elevation areas. The unloading of the lithosphere due to the erosion of 2 km of sediments amplifies the uplift due to flexural isostatic adjustment.

The Black Forest has been the focus of several low-temperature thermochronology studies, including zircon and apatite fission track analyses as well as apatite (U-Th)/He dating. In contrast, the Vosges Mountains have received significantly less attention, with no published apatite (U-Th)/He ages available. Results from previous fission track studies suggest a complex thermal history for the region, including a transient heating episode during the initial rifting phase, as well as recent hydrothermal events that have influenced the thermochronological measurements. However, the total amount of exhumation and the timing and extent of rock uplift remain so far unconstrained.

In this study, we aim to further constrain the thermal evolution of the region using more than 30 new apatite (U-Th)/He ages from two E-W profiles across the Upper Rhine Graben and its rift flanks. Samples were collected from outcrops previously dated using apatite fission tracks or, where unavailable, along new horizontal and vertical profiles. The southern profile spans the highest peaks, connecting the eastern edge of the Black Forest with the western edge of the Vosges Mountains. The second profile is located along the northern borders of the two mountain ranges.

How to cite: Dremel, F. C., Villamizar-Escalante, N., Heberer, B., Schönleber, L., Friedrichs, B., Robl, J., and von Hagke, C.: Constraining Exhumation and Rift Evolution in the Vosges and Black Forest Using Apatite (U-Th)/He Thermochronology, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15767, https://doi.org/10.5194/egusphere-egu25-15767, 2025.

The European Cenozoic Rift Intraplate System (ECRIS) is a deep crustal discontinuity. On the surface, its longest segment, the Rhine Basin, is a large scale asymmetric rift that has been largely studied by sedimentary and tectonic inquiries for its oil and geothermal potential. However, the mechanism behind its development is still under discussion. Different scenarios co-exist, among them an East-West Oligocene extension of unknown origin (Bergerat, 1985), a transtensive opening, associated with a North-South compression linked to the Pyrenean orogeny (Bourgeois et al., 2007) and an opening caused by the alpine slap pull (Merle and Michon, 2001).

This study focuses on the reinterpretation of 1500km of seismic lines and 330 boreholes in the Rhine Graben French part. Four evolutive isochrones and structural maps are proposed, showing the evolution of the fault activity and sedimentary deposition during the Cenozoic. They have been constructed through a seismic stratigraphy analysis that allowed to map five stratigraphic interpolated horizons within the Cenozoic sedimentary pile, including a newly interpolated intra-Chattian horizon. Furthermore, the 3D fault networks active during each period have been constructed, sorting the faults regarding their periods of activity and correlating their expression from one seismic profile to another, including their geometry, their measured throw values, and impact on the sedimentary filling of the Graben.

The first isochrone/structural map extends from the Lutetian to the end of Priabonian (Eocene), lasting 10Ma. It displays a North-South succession of small basins constrained by NS to N40° faults, except in the Erstein transfer zone, where a N70° Variscan suture marks the bedrock. Here, faults adopt a N150° trend. The major West border faults are segmented, alternating with onlap zones.

The second map is of Rupelian (Oligocene) age, lasting 2.9Ma. It displays three larger basins, the Strasbourg, Selestat and the Dannemarie basins, separated by EW thresholds of lower subsidence. In those basins, the three time faster subsidence indicate the climax of the rifting. Interestingly, intra-basin active faults are less numerous during this step and are only reactivated faults from the first step.

The third map points to a transition phase of Rupelian-Chattian age (Oligocene) lasting 4.4Ma. It is characterized by a global slowing down of the subsidence and the tectonic activity except for a small basin at the North-Eastern limit of our study area, constrained by a N10 fault.

The last map is of Chattian to Late Miocene age, lasting 21.1Ma. It is characterized by a new high subsidence in the North, lasting from Chattian to mid-Miocene, but also by the re-activation of the former faults and the development of newly formed normal or transtensive faults. This extensive event is followed by a transtpressive event (supposedly Late Miocene) illustrated by faults-flanked anticlines structures, interpreted as positive flower structures linked to the Alpine orogeny.

This study points to the complex structure of the Rhine basin, involving several sub-basins and fault kinematics evolving in space and time, and the major role of deep structural inheritances in governing the graben asymmetry and fault expression in the sedimentary cover.

How to cite: Ourliac, C., Homberg, C., Briais, J., Allanic, C., Schueller, S., Verlaguet, A., and Faure, A.: Deconvoluted evolution of the intra-plate Rhine Graben during the Cenozoic, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15994, https://doi.org/10.5194/egusphere-egu25-15994, 2025.

Numerical models are essential tools for investigating a variety of Earth phenomena, providing insights into the role of different surface to deep Earth processes. As with many laboratory approaches the effectiveness of the models can be assessed by comparing their results with natural case studies of the same phenomenon, which helps to constrain the large number of model parameters.

This presentation will take the example of the Pannonian Basin system having been formed within the Alpine–Carpathian–Dinaric orogenic belt, where geological data are abundant, and the temporal resolution of basin evolution including magmatic events are very good and in the range of the numerical modelling results.

We used 3D coupled thermo-mechanical and surface processes numerical models (I3ELVIS-FDSPM code) to simulate continental rifting and to shed light on the temporal evolution of the entire rift system. Namely, the extensional deformation starts than migrates from the (western) basin margins, from inherited lithospheric weakness zones towards the basin centre, but an early jump from the western margin toward the opposite basin part is also present in some experiments. This is followed by a second jump of basin formation toward the basin centre, between the first and second generations of basins. This is in good agreement with the compilation of the ages for the onset of basin subsidence and migration of activity of some major bounding faults including low-angle detachments of metamorphic core complexes. This migration is driven and supported by mantle flow and asthenospheric upwelling, eventually affected by thermal relaxation. Based on detailed geological and geophysical mapping, we point out the role of inherited weakness zone(s) – mostly former suture zones – within the crust and mantle lithosphere. Consequences are contrasting subsidence and uplift patterns and a variable heat flow evolution in different sub-basins.

The migration of basin formation shows remarkably similar migration of the magmatic activity. This started with granodioritic–dacitic products around 18.6 Ma along the western basin margin, then jumped toward the opposite basin part around 17.3–16.8 Ma and stepped back toward the basin centre around 15.3 Ma with a change toward andesitic volcanism. Geochemical characteristics indicate increasing mantle component in the melts during the continuing extension until ca. 14.4 Ma. The magma generation in the lower crust and mantle (by decompressional melting) is predicted by numerical models.

The evolution of basin formation and magmatism between ~14.9 and ~11.5 Ma is marked by the migration from the basin centre toward the eastern margin and is probably due to subduction roll-back, steepening of the slab and its detachment. This process is combined with self-consistent evolution of mantle processes deriving from the rifting of the overriding lithosphere.

The research was supported by the National Research, Development and Innovation Office project number K134873 granted to László Fodor and no. 145905 granted to Réka Lukács and MTA–HUN-REN CSFK Lendület "Momentum" PannonianVolcano Research Group

How to cite: Fodor, L., Balázs, A., Oravecz, É., Harangi, S., Cloetingh, S. A. P. L., Gerya, T., and Lukács, R.: Migration of deformation, basin subsidence, magmatism in extensional basins: comparative constraints from numerical models and observations (Pannonian Basin), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16095, https://doi.org/10.5194/egusphere-egu25-16095, 2025.

The rifting and continental breakup styles of Marginal Sea Basins is illustrated by well-constrained Western Pacific examples consisting of the South China Sea (SCS), the Coral Sea (CS) and the Woodlark Basin. In these examples, rifting directly followed an orogenic event which provided a strong thermal and structural inheritance as initial conditions to their formation. In the SCS and the CS especially, the rifting style is characterized by wide rifting forming a succession of sub-basins with thin continental crust, controlled by low-angle normal faults. The formation and development of extensional faults are enhanced by the reactivation of former thrust faults.

The final stages of rifting and continental breakup are contemporaneous with significant magmatic activity in the distalmost part of these margins with the emplacement of volcanoes, dykes and sills. Continent-Ocean transitions (COTs) are characterized by a sharp juxtaposition of the continental crust against igneous oceanic crust suggesting that a rapid shift from rifting to magmatic spreading occurred. High extension rates prevent conductive cooling allowing the focusing of volcanic activity into sharp COTs, quickly evolving to oceanic magmatic accretion.

The rifting style and mode of continental breakup during the formation of Marginal Sea Basins and their margins differs significantly from that of Atlantic-type margins. In the latter, these differences are influenced by transient high mantle temperatures, which lead to thick magmatic crust (i.e. magma-rich margins), or low-extension rates and mantle depletion, which result in subcontinental mantle exhumation (i.e. magma-poor margins). The evolution of Marginal Seas Basins is also controlled by the initial rheological conditions inherited from the previous orogenic event, where a combination of elevated geothermal gradients and rapid extension rates are driven by kinematic boundary conditions. These conditions are influenced by the presence of nearby subduction zones.

How to cite: Mohn, G., Ringenbach, J.-C., Tugend, J., Legeay, E., Kusznir, N., Vetel, W., and Sapin, F.: Rifting and Breakup during Marginal Sea Basin formation: Differences from Atlantic-type margins, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16548, https://doi.org/10.5194/egusphere-egu25-16548, 2025.

Normal faults and extensional detachments, their formation and migration are coupled to the formation of rifted margins, eventually leading to crustal break-up and the birth of new oceanic plates. Where and how this process occurs depends on the composition of the lithospheric layers and thus on different aspects of crustal and mantle elastic, plastic and viscous rheology. Among such indicators, the role of the shear modulus of the various lithospheric layers and thermal expansion, i.e. the relation between temperature related volume changes are not well understood. The latter, together with compressibility (i.e. the relative volume change due to pressure change), becomes particularly important during coseismic slip events, when the rock undergoes a sudden change in temperature and pressure. The influence of such parameters, under the assumption of elasticity, on continental break-up and subsequent formation of oceanic crust leading to a fully developed spreading center is still not well understood and requires further investigation.

 In our study, we aim to better understand the influence of different rheological parameters (such as shear modulus, compressibility or thermal expansion), assuming a visco-elastic-plastic rheology. A particular interest lies in the contribution of elastic, plastic and viscous deformation during break up and rifting. For this purpose, we perform a series of high-resolution pseudo-2D models (i.e., models based on a fully 3D code with a shortened third dimension) based on the petrological-thermomechanical model code i3ELVIS. These models include elasto-visco-plastic rheology with strain weakening, partial mantle melting, oceanic crustal growth, thermal contraction, and mantle grain size evolution.

How to cite: Ritter, S., Balázs, A., and Gerya, T.: Decoding rheological controls on rifting and continental break-up, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16706, https://doi.org/10.5194/egusphere-egu25-16706, 2025.

Continental passive margins are often defined by early salt-related rift systems buried beneath thick sedimentary successions, with structural and sedimentary architectures only directly observable when inverted in orogenic systems where primary salt structures are overprinted by compression. The Central High Atlas diapiric province (Morocco) is an inverted salt-related rift basin with active salt tectonics since early Mesozoic times that provides an exceptional view of early syn-rift sediments and structure. For the first time, regional balanced and restored cross-sections of the Central High Atlas showing the diapiric nature of the basin and the role of salt tectonics during its evolution are presented. The constructed cross-sections across the Central High Atlas include seven salt walls and six intervening elongated minibasins with associated halokinetic depositional sequences, providing evidence of diachronous diapiric growth from Early Jurassic to Cenozoic times. Several of these diapirs bifurcate or amalgamate along strike, so the number of major structures varies laterally. The comparison of the restored and balanced cross-sections allows estimating a shortening of about 38 km, 21 km accumulated in the Atlassic fold and thrust belt frontal domains, and 17 within the Jurassic rift basin.

During the Early Jurassic rifting, shallow water carbonate platforms nucleated both along the margins of the High Atlas Basin and around most salt walls (i.e., highs) within the basin, while intervening minibasins underwent higher subsidence rates and were filled with deeper-water limestones and marls. Subsequently, a longitudinal mixed clastic carbonate deltaic system prograded eastwards filling the minibasins between the long rising salt walls. During this stage, shallow marine shoals and reef patches developed attached to the diapiric walls, evidencing continuous diapir rise.

Throughout the whole rift basin, where local diapir uplift rate is similar to regional subsidence rate, shallow deposition environments or even local subaerial conditions occurred. Thus, platform development was enhanced and karstic processes could develop around salt structures in central parts of the basin. The lessons learnt in the Central High Atlas serve as a valuable analog and provide insights for understanding the early stages of rifting, salt tectonics, and the subsequent evolution of passive margins on a worldwide scale.

How to cite: Moragas, M., Saura, E., Martín-Martín, J. D., Vergés, J., Razin, P., Grélaud, C., Messager, G., and Hunt, D.: The Central High Atlas Jurassic diapiric province (Morocco): a field analogue for salt rift basins preceding continental break-up, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17309, https://doi.org/10.5194/egusphere-egu25-17309, 2025.

Countless studies have been conducted in order to determine the magmatic evolution and genetic heritage of extrusive magmatic rocks associated to continental intraplate magmatism, which in case of large igneous provinces (LIPs) is frequently linked to mantle plumes associated to continental breakup and rifting. By contrast, less attention is paid to the plumbing systems of LIPs, to magma transport, storage and differentiation en route to the surface, and to the volume and composition of the plutonic portion of intraplate magmatism. Studying the origin and magmatic evolution of LIP related plutonic rocks as counterparts of more evolved extrusive series, however, provides crucial knowledge about their volume and heat budget and will have direct implications on estimates about lithospheric strength.

Here we present mineral and whole-rock geochemical and petrological data from different types of gabbros from Western Namibia which are thought to represent a deeper crustal section of a plumbing system that fed the Paranja-Etendeka LIP ~132 Ma ago. Magmatism at this time broadly coincides with Gondwana breakup and opening of the South Atlantic. Intense differentiation and cooling of larger volumes of primary mafic magmas within the lithosphere and crust might have reduced lithospheric strength and thus might have supported or even triggered continental breakup.

Major- and trace element systematics and thermodynamic modelling suggest that the gabbro parental magma developed from a tholeiitic picritic melt with up to 18wt% MgO by >10% olivine fractionation. The picritic primary magma was formed by ~14% partial mantle melting. Liquidus temperatures have been as high as ~1525°C (3 GPa) and mantle potential temperatures in the order of 1455-1470°C, significantly higher than estimates for the convecting mantle (1280-1340°C; McKenzie & Bickle, 1988) but consistent with estimates assigned to the Tristan mantle plume head upon impacting the Gondwana lithosphere (Gibson et al., 2005). Clinopyroxene trace element data reveal that the REE concentration variation between the gabbro parental magmas was nearly an order of magnitude, inconsistent with gabbro formation by pure fractional crystallization from a common magma, but in support of substantial assimilation of Pan-African continental crust accompanied by high crystallization rates. These observations imply intense heat exchange between the plumbing system and ambient lithosphere, which possibly led to marked local heating and lithosphere weakening.

McKenzie, D., Bickle, M.J., 1988, The volume and composition of melt generated by extension of the lithosphere, J. Petrol., 29, 625-679.

Gibson, S.A., Thompson, R.N., Day, J.A., Humphries, S.E., Dickin, A.P., 2005, Melt-generation processes associated with the Tristan mantle plume: Constraints on the origin of EM-1.

How to cite: Pfänder, J. A., Holaschke, P., Klügel, A., Krause, J., Jung, S., and Nagel, T.: Magmatic evolution of Paranja-Etendeka related mafic intrusive rocks in Western Namibia - impact on lithosphere heating and weakening?, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17748, https://doi.org/10.5194/egusphere-egu25-17748, 2025.

Magmatism and volcanism are fundamental components of all tectonic environments on Earth, and play a particularly crucial role in the evolution of magma-assisted continental rifting. Magmatism alters the rheological behaviour of the lithosphere by building networks of intrusions, thereby modifying how plates accommodate tectonic extension. The geochemical footprint of the eruptive products is affected by both the architecture of magma ascent pathways and by the timescales of magma storage and ascent. Volcanism, the surface manifestation of magmatism, results in the construction of large volcanic edifices or distributed volcanic fields. Volcanism is observed to shift during the lifetime of rift systems, eventually focusing on the rift axis in mature rifts. Surface eruptive vents are fed through complex magma plumbing systems, which we can observe through geophysical imaging. 

Geodynamic modelling of the temporal evolution of lithospheric rheology and the magma evolution during ascent and storage demand for physics-based models of ascent pathways that incorporate the time scale of ascent and conditions for arrest. Such physics-based models would help better constrain the parameters of geodynamic codes by providing the tools to compare predicted magma pathways, magma evolution and distribution of volcanism with geological, geophysical and geochemical observations. However, this poses a challenge in linking the ductile deformation of the lithosphere and diking, which occur over vastly different spatial and temporal scales. The stress field has the dominant control on dike pathways and velocity: dikes open perpendicular to the axis of least compression to minimize work against the elastic stress field. Thus, an accurately calibrated stress field is fundamental for physically-consistent magma pathways. The stress field in the lithosphere evolves due to changing far-field stresses, new magmatic intrusions, growing surface loads, formation of basins, erosion and sedimentation; how can these be properly incorporated in geodynamic models? What rules do dikes follow when they propagate in a stressed medium? In this talk, I will present an evaluation of the dominant factors affecting the stress field, and propose guidelines for a physically consistent incorporation of magma pathways in geodynamic models.

How to cite: Rivalta, E.: Physically-consistent magma pathways in continental rifts, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17843, https://doi.org/10.5194/egusphere-egu25-17843, 2025.

The Hellenides in Continental Greece is a tertiary alpine belt with complex tectonic units distributed into two major crustal domains: the External Zones and the Internal Zones, whose geological histories diverged mainly during the late Jurassic, when the internal zones got loaded by the emplacement of large ophiolitic nappes. The Frontal Thrust of the Internal Zones, later partly reactivated as the Main Pelagonian Detachment, marks the boundary between these two major tectonic domains. Since the Miocene, the entire Greek territory has been affected by back-arc extension associated with the southward slab roll-back of the Ionian subduction (Africa Plate). This extension has led to the exhumation of core-complexes and by the formation of numerous extensional basins in the Aegean Sea and two major rifts on mainland Greece: the Corinth Rift from about 4 Ma, and the Sperchios – North Evia Gulf Rift considered to open since 3.5 Ma. The first one is located within the External Zones, while the later developed mainly within the Internal Zones. The Corinth Rift has been extensively studied through various techniques and datasets, whereas the Sperchios – Northern Evia Gulf Rift has been less well-investigated.

We present new crustal cross-sections through the Sperchios – North Evia Gulf Rift interpreted from the analysis of recently acquired seismic data and from filed-based tectonic analysis. These sections reveal (1) the location and variability of major normal faults, and associated depocenters, and (2) the presence of a magmatic chamber in the eastern part of the rift. On the basis of existing data and on new data from receiver functions, we propose an improved version of the Moho depth map in this area. This updated map shows significant latitudinal asymmetries within the rifts, along with longitudinal asymmetries across the entire region. We propose two new Moho depth cross-sections to account for these depth variations and asymmetries: one through the western parts of the rifts and another through the eastern portions. In the west, our results show crustal thickening beneath the western domains of both rifts and crustal thinning beneath some particular zones of the Hellenides, particularly beneath the highly elevated Parnassus zone. To the east, the crustal configuration differs, with a shallower Moho beneath the rifts and a slight crustal thickening between them, under the Kifissos Basin. Furthermore, within the Sperchios – North Evia Gulf Rift, depocenters and major faults are not localized along the same rift boundary. To the west, deformation is largely controlled by faults forming the southern boundary of the rift, whereas in the east, major faults and associated depocenters are located along the northern boundary. We propose that the crustal thickening and thinning observed are related to the presence of deep detachments beneath the Corinth Rift and the western part of the Sperchios – North Evia Gulf Rift, including the Main Pelagonian Detachment that seems particularly important to constrain the present crustal geometries.

How to cite: Chanier, F., Caroir, F., and Tiberi, C.: Crustal asymmetries within the Corinth and North Evia Gulf rifts (Greece): Moho depth variations and structural inheritances, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18056, https://doi.org/10.5194/egusphere-egu25-18056, 2025.

The Demerara Plateau is a submarine bathymetric high, 230 km-long and 170 km-wide, lying between 1000 and 3000 m-depth, and located north of French Guiana and Suriname shelves. On its northeastern border, the Bastille Plateau is a 16 km-long, 9 km-wide relief, at the intersection of the Cretaceous transform and divergent margins of the Demerara Plateau. It represents a crucial witness for understanding the early stages of the Equatorial Atlantic opening. Seismic profiles from GUYAPLAC^a (2003) and MARGATS^b (2016) cruises reveal that the Bastille Plateau is a continentward tilted block with a planar top surface culminating at bathymetric depths of 3650 m, 15 km from the continent-ocean boundary. In 2016, the DRADEM^c cruise dredged the rocks outcropping along the northern slope of the Bastille Plateau, retrieving mostly trachy-basalts and a single rudstone sample. During the DIADEM^d campaign (2023), a dredge on the southern slope and two Nautile submarine dives confirmed that the Bastille Plateau was almost entirely made up of magmatic material. Three pelagic carbonates were sampled during one Nautile dive and came directly from the top of the Bastille Plateau, between 3745 m and 3685 m-depth.

We combine petrology with absolute U-Pb dating on calcite for the rudstone, and biostratigraphic dating of the pelagic carbonates deposited at the top of the Bastille Plateau to constrain the chronology of the rifting of the Equatorial Atlantic along the Demerara Plateau. We interpret the rudstone as deposited on a subaerial unconformity surface, similar in seismic lines to the post-rift unconformity. U-Pb analyses on calcite date this post-rift unconformity as Mid-Albian and constrain a continental break-up at 106 ± 9 Ma. Unexpectedly, post-rift subsidence did not follow the break-up, with marine transgression occurring circa 103 Ma on the Demerara Plateau, but later than 98 ± 3 Ma on the Bastille Plateau, closer to the continent-ocean boundary, possibly in relation with the vicinity of the Sierra Leone hotspot. Biostratigraphic ages indicate that subsidence was rapid from the Cenomanian onward, resulting in the early establishment of a deep-sea current acceleration zone along the outer margin of the Demerara Plateau.

^a https://doi.org/10.17600/3010050

^b https://doi.org/10.17600/16001400

^c https://doi.org/10.17600/16001900

^d https://doi.org/10.17600/18000672

How to cite: Coudun, C., Bienveignant, D., Basile, C., Girault, I., Giraud, F., Vezinet, A., Loncke, L., Graindorge, D., Klingelhoefer, F., Léger, J., Menini, A., and Heuret, A.: Unexpected post-breakup altitude of the distal continental margin of the Demerara Plateau (French Guiana): New constraints from LA-ICP-MS U-Pb calcite dating, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18237, https://doi.org/10.5194/egusphere-egu25-18237, 2025.

The Eastern Continental Margin of India (ECMI) is a passive, magma-poor margin that formed during India’s separation from Antarctica in the Early Cretaceous period. The margin extends for approximately 2000 kilometres from the Cauvery Basin in the south to the Mahanadi Basin in the north, encompassing multiple river-fed sedimentary basins, including the Bengal, Mahanadi, Krishna–Godavari, Palar, and Cauvery basins. Geophysical methods, such as seismic, gravity, and magnetic surveys, have been instrumental in analysing the ECMI's nature and evolution. While detailed multichannel seismic imaging, particularly along the ION1000 profile in the Krishna-Godavari offshore basin, has enhanced our understanding of ECMI’s crustal structure, significant gaps remain in comprehending its rift architecture in relation to the surrounding geology. Additionally, critical thermal characteristics, such as Curie point depths and heat flow patterns have not been investigated in the ECMI till date.

In this study, we utilized magnetic, gravity, and multichannel seismic data to investigate the rifting processes, subsurface structure, and thermal characteristics of the Krishna-Godavari offshore basin and surrounding areas. In addition to reinterpreting the ION1000 profile, we conducted a thorough analysis along the ION1200 and ION1240 profiles using both qualitative and quantitative methods. This allowed us to refine the structural framework of the region. Radially averaged spectral analysis of magnetic data was used to estimate Curie depths, from which heat flow values were derived, providing insight into the geothermal framework of the margin. Gravity data analysis enabled us to estimate Moho depths through non-linear inversion, giving a more precise configuration of the crust-mantle boundary.

Qualitative analysis of the ION1200 and ION1240 profiles helped us to identify structural domains associated with rifting processes, while quantitative analysis revealed the patterns of tectonic subsidence and depth anomalies. These findings indicate significant subsidence and outer margin collapse in the exhumed domain, with no evidence of crustal thickening during rifting. Variations in heat flow values are attributed to substantial sediment accumulation that acts as a thermal insulator over old oceanic crust. This study presents a comprehensive model for the evolution of the ECMI, illustrating how deep crustal processes, mapped through seismic, gravity, and magnetic methods, influence the overlying sedimentary structure and thermal characteristics of the passive margin, ultimately shaping its geological and geothermal framework.

How to cite: Khan, S. A., Jacob, J., Desa, M. A., and Nelson, R.: Unveiling Crustal Dynamics and Thermal Structure in the Krishna-Godavari offshore Basin using Seismic, Magnetic, and Gravity Data, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-431, https://doi.org/10.5194/egusphere-egu25-431, 2025.

In recent years there have been diverse reports of material in the lithosphere under the oceans, away from continental shelves, that does not comprise classical oceanic crust underlain by progressively mantle lithosphere that thickens according to plate-cooling models. These reports come from multiple different research approaches including:

• Marine geophysical surveying and sampling, identifying microcontinents, e.g., the Jan Mayen Microcontinent Complex, and ”orogenic bridges”, e.g., the Davis Strait;
• Deep seismic profiling detecting continental material beneath surface basalts, e.g., the Alpha-Mendeleev Rise;
• Large-scale seismic imaging using teleseismic earthquakes, identifying lithosphere with continental characteristics, e.g., in the South Atlantic;
• Aeromagnetic surveying revealing the extent of seafloor-spreading-related anomalies, e.g., in the Fram Strait;
• Broad cross-disciplinary work identifying crust inconsistent with a purely basaltic composition, e.g. the Greenland-Iceland-Faroe Ridge;
• Direct observation, e.g., on the Rio Grande Rise;
• Geochemistry, e.g., on the South West Indian Ridge;
• Dating of zircons from igneous rocks in the oceans that pre-date the time of formation of the local oceanic crust, e.g., Mauritius.

Understanding the extent of continental material in the oceans, and acceptance that hybrid continental/oceanic crust may exist – a third kind of crust – is an emerging field. At present no systematic review has been done of potential continental or hybrid regions in the oceans away from continental margins. It is thus unknown how widespread it might be and there is no broad understanding of or how it got there and why. It is timely for a systematic review of the subject aimed at identifying key research targets for the future.

How to cite: Foulger, G., Koehl, J.-B., and Peace, A.: Continental Material in the Oceans, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3329, https://doi.org/10.5194/egusphere-egu25-3329, 2025.

“Oceanic” magnetic anomalies were “calibrated”, on Iceland, for just the last 4 million years. They extrapolated to 84 Ma (20x!) in the South Atlantic (Heirtzler et al., 1968), where there are 34 magnetic stripes west of the Mid Atlantic ridge. They extend back to 84 Ma and Cretaceous “Quiet Period” crust. This crust, however, is continental, not oceanic, and was not a 37 million year episode when Earth forgot to reverse its magnetic field.

There are 57 magnetic lineaments in the SE Pacific (140 Ma?), never discussed.

During the birth of Plate Tectonics Vine & Mathews (1963) famously related magnetic stripes to geomagnetic reversals. They noted, however, that alternations of ridges of high intensity and valleys of low intensity could result from presence of strongly magnetized material adjacent to weakly magnetized material. This qualification does not appear in subsequent literature.

In their paper Heirtzler et al., (op. cit.) qualified their work, writing: “the possible error in extrapolation cannot be overemphasized; if the Vine & Mathews (1963) theory is in error, the conclusions of this paper do not apply”. Thus, if the Vine & Mathews (op. cit.) qualification, above, is correct, extrapolation in the S Atlantic is incorrect.

What is “oceanic crust”? Karner (2008) described thinning of continental passive margin from 30 – 40 k to10 km, followed by rupture. Extension (100s percent) forms zones 100s km wide with organized magnetic anomalies from serpentinization. Correlatable magnetic anomalies do not unambiguously define “oceanic crust”.

Serpentinization involves reaction of peridotite with water at less than 500^oC. The reaction is exothermic and results in volume increase as much as 45%. Magnetite forms.

Southern Pacific Ocean magnetic striping is symmetric between extended and largely subsided continent Zealandia and South America.

Onshore, thick basinal prisms, elongated parallel to the Andes, have steep western (Liassic deep water shales) and gentle eastern (Upper Triassic shallow water carbonates) boundaries. The basins shallow up to carbonates, red beds and evaporites. The ages correspond to Pangaean breakup.

The asymmetric basins were uplifted from the Pacific via transpression along the N-S, dextral strike-slip plate boundary (Liquiñe Fault).

They came from the Pacific.

Triassic-Jurassic rifting marked initiation of Pangaean breakup along the NW margin of Colombia, also a zone dextral strike-slip faulting (Romeral suture: oceanic rocks to the west, continental rocks to the east). Transpression shortened the Jurassic-late Cretaceous passive margin into metasediments (graphitic schists and black marbles in the western and central Andean Cordillera).

Further northwards the striping pattern becomes complex. The spreading ridge approaches the Americas and evolves into the San Andreas dextral strike-slip fault.

Seismic data record seaward-dipping wedges in the eastern Pacific.

So, in view of all this, how much “oceanic” crust is there in the Southern Pacific?

How to cite: James, K.: How much oceanic crust in the Southern Pacific?, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3826, https://doi.org/10.5194/egusphere-egu25-3826, 2025.

Seamounts provide a unique record of volcanic processes in the oceans. In the Pacific Ocean, where seamounts are especially abundant, understanding their age and spatial distributions offers valuable insights into tectonic history, melt-extraction processes, and crustal provenance. However, detailed constraints on seamount formation history remain limited by sparse age data and age-dependent preservation, as older seamounts are progressively lost to subduction.

To address these challenges, we develop a data-driven approach to estimate seamount ages by analyzing relationships among multiple variables. Our analysis reveals that features such as crustal age, seamount height, and proximity to proposed “hotspots” illuminate the complex interactions between plate tectonics and magmatic processes. Using these relationships, we estimate ages for previously undated seamounts including uncertainty assessments. By adjusting volumetric measurements for ancient crustal area and subduction losses, we identify distinct phases in Pacific volcanism: (1) an Early Cretaceous period dominated by Large Igneous Provinces, (2) a Mid-Late Cretaceous transition marked by increasing non-hotspot seamount volcanism, and (3) a Cenozoic regime characterized by variable spreading rates and evolving ridge-seamount relationships.

This reconstruction provides new insights into the relative contributions of clearly plate-related- and other processes to Pacific volcanism through time, suggesting a more complex interplay between lithospheric and sub-lithospheric dynamics than previously recognized. Similar methods could be applied to other oceans, including the Atlantic and Indian Oceans, where they might also be adapted to discriminate crustal types.

How to cite: Zhao, Y., Riel, B., Zhang, J., and Foulger, G.: Data-driven Reconstruction of Pacific Seamount Ages: New Insights into Ocean Basin Volcanic Evolution, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3960, https://doi.org/10.5194/egusphere-egu25-3960, 2025.

Since the earliest debates on continental drift theory, the African-Arabian rift system and the Afar region have been used as typical examples for extensional processes and the rift-to-drift transition. New findings suggest that the classical evolution model proposing a linear evolution from continental rifting in the Main Ethiopian Rift to advanced rifting in the Afar and oceanic spreading in the Red Sea is an oversimplification. Instead, the style of rifting seems to have a more important control on the structure and composition of each region than the magnitude or the age of the extension. In particular, the Central Afar region shows important extension, but it is far from showing normal, Penrose-like oceanic spreading. As such, it is considered as a precursor of some types of oceanic plateaus, such as the Greenland-Iceland-Faroe Ridge. This suggests that some features found far offshore and usually considered as purely oceanic might represent an extreme type of passive margin, hyperextended and magma ultra-rich. Conversely, the Danakil Depression, adjacent to Central Afar, shows a typical magma-rich structure on seismic data with well-defined Seaward Dipping Reflector (SDR) packages. However, outcrop data shows that they chiefly consist of sediments with only a small volume of magmatic products. This questions the composition of other margins worldwide that were often assumed to be made of magmatic material solely based on the recognition of SDR.

These new findings suggest that the position, composition, and structure of the continent-ocean transitions might be more complex and diverse than previously assumed.

How to cite: Rime, V., Keir, D., Phethean, J., Kidane, T., and Foubert, A.: The complex rift-to-drift transition: surprises and lessons from the Afar region, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4330, https://doi.org/10.5194/egusphere-egu25-4330, 2025.