EGU General Assembly 2024  

Large continental strike-slip earthquakes produce significant surface ruptures, tens to hundreds of kilometers long, with displacement that can reach several meters, depending on the magnitude of the earthquake. However, this information, and more specifically the detail of the surface ruptures has long been overlooked, as we missed efficient ways to document extensive ruptures. In the last decade, with the emergence of new remote sensing tools, our community came to realize that earthquake surface ruptures could bear significant information about fault structure and the way deformation is accommodated during earthquakes. Hence, through a systematic survey of surface ruptures, based on field observation and remote sensing data, we demonstrate that it is possible to define a fault segmentation that would not be arbitrary, but instead is related to some first-order characteristic of the brittle crust, the thickness of the brittle crust, also called the seismogenic thickness. This observation result has been tested and confirmed through a series of analogue and numerical experiments that allowed us to systematically study fault segmentation when varied the thickness of the brittle material. We also questioned the distribution of deformation along those fault segments during earthquakes, as the development of high-resolution image correlation technics gives us access to a more detailed picture of the ground deformation distribution around large strike-slip earthquake ruptures. More specifically, we have been able to show that in some places a significant part of the deformation is distributed off fault, up to 30%, accommodated by micro-cracks, or even in the bulk. This deformation is almost impossible to measure in the field and thus is not considered in classic models of earthquake source inversion. It might in fact be the reason why many models involve some shallow slip deficit, which is physically not understood, while it might only be an artefact of modeling. Eventually, the evolution through time of fault geometry and off-fault deformation is questioning the long-term evolution of fault geometry, a potential proxy for fault maturity. Here we show through analogue experiments that in fact, once a growing fault system has gone pass through a localization stage, where distributed deformation is drastically reduced, the fault system never simplifies further and keeps having some level of complexity associated with a steady amount of distributed deformation close to 20%, independently of the total amount of deformation accommodated by the fault system, suggesting that a fault system can probably be considered definitively mature almost immediately after its localization stage, for a small amount of total displacement.

How to cite: Klinger, Y.: Fault segmentation, off-fault deformation, and fault maturity, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16741, https://doi.org/10.5194/egusphere-egu24-16741, 2024.

The metamorphic basement of the Southern Alps occurs in the Brixen unit (Meran – Brixen – Timau, “Brixner Quarzphyllite”), the Valsugana Unit (Trient – Borgo Valsugana – Agordo) and the Recoaro Unit (Recoaro Terme – Schio). The associated Variscan P-T conditions correspond to a greenschist-facies metamorphic overprint, which exhibits a metamorphic gradient that extends from the lower greenschist-facies in the South to the amphibolite-facies in the North. The aim of this study was to provide mineralogical and mineral-chemical constraints of major mineral phases as well as accessories such as apatite and tourmaline on this gradient and obtain P-T conditions along a North-South profile

Quartzphyllite samples were collected along a traverse from Reccoaro in the South to Brixen in the North. Petrographic investigations revealed that the metapelites contain quite a complex polyphase mineral assemblage. The mineral assemblage in the South is represented by chlorite + muscovite + albite + quartz. Towards the center of the traverse, biotite occurs in the mineral assemblage, which has subsequently been replaced by chlorite. Samples in the vicinity of the Permian Cima d’Asta intrusion show petrographic evidence for contact metamorphism. In the North the mineral assemblage is chlorite + muscovite + plagioclase + quartz + garnet. Therefore, the metapelite zones of chlorite, biotite and garnet were observed along the traverse from South to North.

Mineral chemical investigations of samples without the contact metamorphic overprint reveal additional hidden traces of the polymetamorphic nature of some of the samples. Although the chemical compositions of muscovite, chlorite and plagioclase vary continuously with increasing P-T conditions from South to North, the chemical data also reveal that the southernmost sample shows for instance chemical evidence for a later T-accentuated overprint texturally not visible.

The chemical composition of apatite changes continuously from South to North with slightly increasing F and FeO and Y₂O₃ contents. Tourmaline shows an increase in Ca(X) from the biotite to the garnet zone. Reliable multi-equilibrium geothermobarometry yielded P-T conditions of 554 ± 11°C and 6.49 ± 1.3 kbar in the northernmost sample. In contrast, muscovite-chlorite-quartz geothermobarometry shows considerable scatter in the data due to pervasive later retrogression.

Additionally, we applied low temperature thermochronology to the samples to reveal the post Variscan to Neoalpine thermal history of the rocks. Zircon U/Th-He (ZHe) data suggest cooling of the Valsugana Unit in the upper Carboniferous below 160°C whereas cooling in the Brixen Unit occurs only at the border of Middle to Upper Triassic. The latter can be interpreted as cooling after a thermal event related to the Ladinian Volcanism, which also reset the Apatite Fission Track (AFT) system in the Brixen Unit. This Ladinian AFT reset does not occur in the quarzphyllites of the Valsugana Unit. AFT data and time-Temperature models suggest Permian and Triassic cooling to 70 ± 10°C before 240 Ma in the Valsugana Unit, and post-Ladinian cooling of the Brixen Unit before 70 Ma.

This study shows that quartzphyllites are able to record complex metamorphic histories hidden in petrographic, geochronological and mineral chemical data.

How to cite: Tropper, P., Klotz, T., Pomella, H., and Dunkl, I.: Visible and invisible complexities in low-to medium grade metamorphic rocks: mineralogical and petrological constraints on the Variscan metamorphic gradient in the Southalpine metamorphic basement (Brixen quartzphyllites, Northern Italy), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1731, https://doi.org/10.5194/egusphere-egu24-1731, 2024.

Recognition of seismically induced microstructures is important to unravel the different deformation processes during seismic cycles, especially at the base of the upper crust where many earthquakes nucleate. Deformed quartz veins related to a strike-slip shear zone within the Schobergruppe (Austroalpine Crystalline Complex, Eastern Alps) contain intense kinking in elongated quartz grains. The kink band boundaries are inclined into the general dextral sense of shear. Cathodoluminescence (CL) images reveal that the entire thin section contains a very high density of intragranular, sub-planar microstructures developed as thin dark CL lamellae accompanied with nanometre-scale fluid inclusions. Based on the oscillating orientation variation across low angle boundaries (misorientation angle 1-9°) these lamellar microstructures are referred as short-wavelength undulatory extinction microstructures - SWUE (Trepmann & Stöckhert, 2013). Only grains with SWUE, orientated parallel to the foliation, are kinked. In general, kinked microstructures mainly develop in strongly anisotropic material or minerals with a strong cleavage, e.g. micas. Deformation at high differential stresses e.g. during coseismic loading can produce a strong anisotropic microstructure in quartz by the development of deformation lamellae. Trepmann & Stöckhert (2013) showed in deformation experiments of quartz that SWUE preserve evidence of an earlier coseismic stress peak, even when overprinted during subsequent crystal plastic creep deformation at lower stress. The SWUE in the deformed Schober quartz veins are interpreted in a similar way. These microstructures were primary deformation lamellae developed during coseismic loading. TEM images reveal a high degree of recovery (low dislocation density) across the SWUE. Subsequent overprint by ongoing creep at lower stresses is recorded by vein quartz samples with mylonitic microstructures. The densely spaced sub-planar microstructures cause a high anisotropy of the quartz grains, which finally were kinked. Electron backscatter diffraction data give evidence of different slip systems that were active during the development of the deformation lamellae followed by recovery (SWUE), and during the subsequent kink band formation. The opposite direction of the Burges vectors (based on Weighted Burges Vector analysis, Wheeler et al., 2009) at the corresponding kink band boundaries is geometrical consistent with sinistral shearing within the kink domain along the anisotropic deformation lamellae/SWUE related to the dextral sheared kink band. Intensively kinked micas (muscovite and biotite) in the mica-rich host rock (in direct contact to the kinked quartz vein sample) point to seismic induced kinking, which is supported by the vicinity (1-1.5m) of a fault zone with pseudotachylytes.

How to cite: Bestmann, M., Grasemann, B., Pennacchioni, G., Kilian, R., Wheeler, J., Morales, L. F. G., and Bezold, A.: Seismic induced anisotropy and kinking in quartz, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2884, https://doi.org/10.5194/egusphere-egu24-2884, 2024.

The breakdown of omphacite is one of the first signs of eclogite retrogression, and typically results in the formation of vermicular intergrowths of sodic plagioclase and diopside (± quartz ± amphibole), termed clinopyroxene-plagioclase symplectites. Such symplectites occur in most, if not all, eclogite localities worldwide. The reaction is associated with a substantial grain size reduction, and may thus significantly impact bulk rock rheology during eclogite exhumation. We study a suite of natural clinopyroxene-plagioclase symplectites by electron backscatter diffraction (EBSD). The sample suite comprises symplectites in various stages of their evolution: the beginning stages of nucleation (vermicular symplectites), partially recrystallized symplectites, and completely recrystallized and strongly deformed symplectites. We determine crystallographic relationship between the parent omphacite and the reaction products and interphase misorientation relationships between the reaction products (plagioclase and diopside), to shed light on nucleation and deformation mechanism during eclogite retrogression. 
We find that the nucleation of diopside and plagioclase in the symplectites is strongly controlled by the crystallography of the parent omphacite, with the diopside copying the crystal lattice of the parent grain, and the plagioclase nucleating in special orientation relationships to the diopside, along planes with favorable interplanar spacing. Initially strong crystallographic relationships are weakened as deformation of the symplectites proceeds by fracturing transitioning into grain boundary sliding accommodated by diffusion creep, i.e., grain-size sensitive (GSS) creep.
The results indicate that the formation of clinopyroxene-plagioclase symplectites does not increase permeability in crustal rocks, initially, but that deformation by GSS creep leads to progressive hydration and weakening of eclogites during retrogression. The symplectites thus significantly impact bulk crustal rheology. 

How to cite: Zertani, S., Morales, L. F. G., and Menegon, L.: Mechanisms for the nucleation and deformation of symplectites during omphacite breakdown     , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3483, https://doi.org/10.5194/egusphere-egu24-3483, 2024.

The geodynamic evolution of the Earth is highly governed by the mechanical behavior of rocks at plate boundaries. In convergent settings, continental and/or oceanic mafic rocks are subducted to great depths where they experience high pressures and temperatures and transform to eclogite. Accompanied mineral transformations subsequently result in mechanical changes and in density variations. In the last decades, many field, experimental and numerical studies targeted eclogite and aimed at quantifying its mechanical behavior as well as characterizing strain weakening processes. Especially, experimental investigations have shown that eclogite and its main constituents omphacite and garnet are strong phases which are not expected to creep at differential stresses below 1 GPa for tectonically relevant strain rates. Nevertheless, highly localized shear zones and mylonitic fabrics are frequently observed in eclogite, raising the question how and why strain localization occurred.
To characterize processes causing strain localization in eclogite, we investigate an eclogite facies shear zone located at the Hohl locality (Koralpe, Eastern Alps, Austria). The shear zone bears rocks with two distinct eclogite facies mineral assemblages of which one is dominated by clinozoisite, amphibole and garnet. This lithology occurs as foliated sigmoidal lenses hosted by typical eclogite containing omphacite, garnet, clinozoisite, amphibole, quartz, kyanite and rutile. Both lithologies derived from NMORB gabbro which intruded during Permian rifting. Protolith assemblage calculations suggest that lenses have originally been plagioclase-rich cumulates within a clinopyroxene-plagioclase gabbro matrix. Modal-composition based viscosity estimates indicate that previous to the high-pressure metamorphic overprint the cumulate was less competent than the gabbro. However, the sigmoidal shape of lenses surrounded by ultramylonitic eclogite suggests that the lenses were stronger during shear zone development. Microstructural investigations reveal an ultramylonitic fabric dominated by euhedral clinopyroxene (aspect ratio ~1.7) within the host eclogite. Triple- and quadruple-junctions, open grain boundaries and lack of intracrystalline strain suggest that eclogite dominantly deformed by grain boundary sliding. On the other hand, the microstructure of lenses is dominated by elongated clinozoisite (aspect ratio ~4) and elongated sigmoidal amphibole aggregates (aspect ratio ~3). Amphibole aggregates are characterized by coarse-grained highly strained clasts and strain free slightly elongated crystals in strain shadows. These observations indicate that lenses deformed by combined dislocation and dissolution-reprecipitation creep.
Our data show how mineral replacement resulted in strength inversion with lenses, initially weaker than their host, becoming stronger than the surrounding eclogite after metamorphism at eclogite-facies conditions (720 ± 20 °C, 21 ± 3 kbar). The switch in strength caused stress concentration at the lithological contacts and subsequent strain localization in the weaker eclogitic mineral assemblage.

How to cite: Rogowitz, A., Schorn, S., Huet, B., Grasemann, B., and Menegon, L.: Metamorphism induced strength inversion at high-pressure conditions – Implications for strain localization in eclogite., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3884, https://doi.org/10.5194/egusphere-egu24-3884, 2024.

Garnet is an exceptionally useful mineral for reconstructing the evolution of metamorphic rocks that have experienced multiple tectonic or thermal events. Understanding how garnet crystallizes and its mechanical behaviour, is important for establishing a petrological and temporal record of metamorphism and deformation, and to recognize multiple geologic stages within the growth history of an individual crystal. In this study, we integrate fine-scale microstructural (EBSD) and microchemical (LA-ICP-MS mapping) data obtained on a polycyclic garnet-bearing micaschist from the Alpine belt. Results suggest that fragmentation of pre-Alpine garnet porphyroblasts occurred during the late pre-Alpine exhumation and/or the onset of the Alpine burial, such that the older pre-Alpine garnet fragments were transported/redistributed during Alpine deformation and acted as new nucleation sites for Alpine garnet growth. These processes produced a bimodal garnet size distribution (macro mm-sized and micro sub-mm-sized grains). Thermodynamic modelling indicate that Alpine garnet grew during the final stage of burial (from 1.9 GPa 480 °C to 2.0 GPa 520 °C) and early exhumation (down to 1.6 GPa 540 °C) forming continuous idioblastic rims on macro- and micro-grains, and sealing fractures preserved in pre-Alpine garnet porphyroblasts. We propose that fragmentation-overgrowth processes coupled with ductile deformation in polycyclic rocks may produce a bimodal garnet size distribution and form multistage crystals resembling neoblasts. This study highlights the importance of linking microstructural (EBSD) and microchemical (LA-ICP-MS mapping) data by providing valuable information about the dominant deformation mechanisms at a given site by identifying potential links between major/trace element mobility and crystal deformation.

How to cite: Manzotti, P., Regis, D., Petts, D., Graziani, R., and Polivchuk, M.: Formation of multistage garnet grains by fragmentation and overgrowth constrained by microstructural and microchemical mapping, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4410, https://doi.org/10.5194/egusphere-egu24-4410, 2024.

Mantle heterogeneity is closely related to the distribution and circulation of volatile components in the Earth’s interior, and the behavior of volatiles in the mantle strongly influences the rheological properties of silicate rocks. In mantle xenoliths, these physicochemical properties of the upper mantle can be recorded in the form of microstructures and fluid inclusions. In this paper, I summarized and reviewed the results of previous studies related to the characteristics of microstructures and fluid inclusions from peridotite xenoliths beneath the Rio Grande Rift (RGR) in order to understand the evolution and heterogeneity of upper mantle. In the RGR, the mantle peridotites are mainly reported in the rift axis (EB: Elephant Butte, KB: Kilbourne Hole) and rift flank (AD: Adam’s Diggings) regions. In the case of the former (EB and KB peridotites), the type-A lattice preferred orientation (LPO), formed under low-stress and low-water content, was reported. In the case of the latter (AD peridotites), the type-C LPO, formed under low-stress and high-water content, was reported. In particular, in the case of AD peridotites, at least two fluid infiltration events, such as early (type-1: CO₂-N₂) and late (type-2: CO₂-H₂O), have been recorded in orthopyroxene. The upper mantle heterogeneity recorded by these microstructures and fluid inclusions is considered to be due to the interaction between the North American plate and the Farallon plate.

How to cite: Park, M.: Upper Mantle Heterogeneity Recorded by Microstructures and Fluid Inclusions from Peridotite Xenoliths Beneath the Rio Grande Rift, USA, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5017, https://doi.org/10.5194/egusphere-egu24-5017, 2024.

In the realm of modern solid Earth research, a profound understanding of rocks' intricate microstructures is essential for unraveling geological history and addressing critical challenges in the energy transition. These microstructures—grain boundaries, preferred orientation, twinning, and porosity—play a pivotal role, influencing the physical strength, chemical reactivity, and fluid flow properties of rocks. Their direct impact on subsurface reservoirs used in geothermal energy, nuclear waste disposal, and hydrogen/carbon dioxide storage underscores the importance of comprehending their distribution for the stability and efficacy of subsurface activities.

However, addressing the need for statistical representativeness requires imaging numerous samples at high magnification. In response, our research introduces an innovative image enhancement process for scanning electron microscopy datasets, showcasing a substantial potential for resolution improvement through Deep-Learning-Enhanced Electron Microscopy (DLE-EM).

Our proposed workflow involves capturing one or more high-resolution (HR) regions within a low-resolution (LR) area. Precise image registration is achieved in two steps: first, determining the HR region's location within the LR region using a Fast Fourier Transform algorithm (Lewis, 2005), and second, refining image registration through iterative calculation of a deformation matrix. This matrix, utilizing Newton's optimization method, aims to minimize differences between both images (Tudisco et al., 2017). Subsequently, paired HR and LR images undergo processing in a Generative Adversarial Network (GAN), comprising a generator and a discriminator. This GAN learns to generate HR images from LR counterparts through joint training in an adversarial process.

We benchmark our workflow using four distinct rock types and demonstrate that this approach accelerates imaging processes up to a factor of 16 with minimal impact on quality, offering possibilities for real-time super-resolution imaging of unknown microstructures. Additionally, we show that a model trained on a specific geological material is able to generalize its learned features to new domains, reducing the need for extensive training data.

[1] Lewis, J. P. "Fast normalized cross-correlation, Industrial Light and Magic." unpublished (2005).

[2] Tudisco, Erika, et al. "An extension of digital volume correlation for multimodality image registration." Measurement Science and Technology 28.9 (2017): 095401

How to cite: van Melick, H. and Plümper, O.: Breaking Boundaries: Deep-Learning-Enhanced Electron Microscopy for Accelerated Super-Resolution Imaging in Solid Earth Research, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8470, https://doi.org/10.5194/egusphere-egu24-8470, 2024.

Texture formation in rocks is modeled using a quantitative phase-field model of eutectic growth with three thermodynamic phases (e.g. Diopside - Anorthite - Melt) in a quasi-binary magmatic system with unlimited quantity of crystals of different orientation and anisotropy. In nature, specific layering microstructures have been observed for which many possible explanations have been provided. Moreover, textural irregularities initially induced during nucleation, by fluctuation in crystal size or by other mechanisms continue to develop and sharpen over time. By phase-field modeling, we verify two models of igneous layering. The first mechanism is that, due to the formation of a layer with crystals of larger size, these crystals will grow faster at the expense of smaller crystals. The second mechanism is that a layer with an increasing fraction of the second phase is formed on top of the initial layer of the first phase to have crystallized. We have found that, in this case, without anisotropy of surface energies, no layering is produced. The boundary conditions and parameters of the models necessary for the formation of layers will be discussed.

How to cite: Kundin, J. and Chakraborty, S.: Phase-field modeling of texture evolution in magmatic rocks, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8497, https://doi.org/10.5194/egusphere-egu24-8497, 2024.

The earth's internal dynamic processes are closely related to the rheological behavior of its internal constituent minerals under high temperature and high pressure conditions. Feldspar and pyroxene are the main constituent minerals in granulite in the continental lower crust. High-temperature experimental research on them is one of the main ways to understand the rheology of the continental lower crust. This experiment uses the Paterson high temperature and high pressure rheology device, at a temperature of 1273K-1423K and a strain rate of 6×10-6 s ^-2×10-5s^-1, to test the hot-pressed feldspar and pyroxene aggregates with and without water respectively. Creep experiments were carried out under added water conditions to determine the rheological parameters of the two-phase aggregate under different water conditions. By collecting infrared spectra and microscopic pictures of the two-phase aggregate of hot-pressed feldspar and pyroxene, the water content in the samples before and after deformation was calculated and its microstructural characteristics were analyzed. The Q value of the sample without adding water is 797.87±184.7 KJ/mol, and the n value is about 3. The Q value of the water-added sample is 472.57±96.29KJ/mol. From 1273K to 1373K, the n value is about 2. At 1373K, the n value is about 4. This shows that water has a significant weakening effect on rocks.

How to cite: zhang, Q. and Zhou, Y.: Experimental study on high-temperature rheology of hot-pressed mafic granulite, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8853, https://doi.org/10.5194/egusphere-egu24-8853, 2024.

The Kapıdağ Shear Zone (KSZ) is located in the Kapıdağ Peninsula (NW Anatolia) and syn-kinematically intruded by Northern Kapıdağ Pluton (NKP) along the northern coastline of the peninsula. The NKP displays a granodioritic composition with a discernible progressive deformation from south to north. The southern part is characterized by an isotropic granodiorite with no trace of deformation. Towards the north, it gradually passes into a deformed granodiorite in which the development of ductile and brittle structures is widely observed. To comprehend the nature and origin of deformation within the KSZ, a thorough analysis of micro- and mesostructural features was undertaken, accompanied by a three-dimensional kinematic analysis of the NKP. The NKP exhibits a well-defined mylonitic foliation and stretching lineation, characterized by the shape-preferred alignment of feldspar, quartz, and biotite crystals. Various shear sense indicators, including S-C fabrics and "σ"-type rotated porphyroclasts, are extensively distributed throughout the NKP, pointing to a dextral sense of shear. Microstructures such as chessboard extinction and Grain-Boundary Migrations (GBM) in quartz, myrmekitic textures, and flame pertites in feldspar, as well as sub-grain rotations and bulging recrystallization of quartz, along with the presence of micro-faults and cracks collectively suggest continuous deformation of the NKP from temperatures starting at 600°C to those below 250°C.

Three-dimensional strain analysis was conducted on the Northern Kapıdağ Pluton (NKP) using quartz crystals as shear sense indicators, and various parameters including kinematic vorticity (Wk) numbers, Flinn k values, Lode’s ratio, and octahedral shear strains were computed. The outcomes reveal a range of Flinn k values from 1.1 to 5.32. On Flinn’s diagram, the majority of samples plot above the k=1 line, indicative of a transtensional regime. Lode’s ratios exhibit a variation from -0.64 to +0.13, with the Hsu diagram showing that the majority of samples fall within the general constrictional field. To discern the strain component of the NKP, kinematic vorticity numbers (Wk) were determined, ranging from 0.73 to 0.99. This suggests a dominance of simple shear in the deformation rather than pure shear components. The U-Pb zircon and ⁴⁰Ar/³⁹Ar biotite dating results show that this deformation has developed between 48-36 Ma. 

In summary, both micro/mesostructural data and three-dimensional strain analyses of the NKP collectively suggest that the Kapıdağ Shear Zone (KSZ) is characterized by a dextral transtensional shear zone dominated by simple shear. We hypothesize that the KSZ was likely formed during the Eocene period as a consequence of strain localization along the break-off of the Tethyan oceanic slab.

How to cite: Arık, T., Ünal, A., and Altunkaynak, Ş.: Three-dimensional kinematic analysis of Northern Kapıdağ Pluton: Implications for a transtensional deformation in NW Anatolia , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9288, https://doi.org/10.5194/egusphere-egu24-9288, 2024.

The interplay between H₂O and quartz deformation is a long-standing question since the discovery of the H₂O-weakening effect by Griggs and others in the 60’s. Some of the early works focused on single crystal experiments and on intra-crystalline processes, but a complete understanding of the phenomenon requires to consider quartz aggregates, where both intra- and intercrystalline processes contribute to bulk strain and strength.

We have carried out a series of deformation experiments on quartz polycrystals at high pressure (0.6 to 2 GPa) and high temperature (800°C), at strain rates of ~1.10^-6 to 2.10^-5s^-1, in a Griggs-type apparatus. The main set of experiments used a natural quartzite with a large starting grain size (~150-200µm) in coaxial geometry (~30% strain). A second series used synthetic mixtures of large (~100-200µm) and dry quartz clasts embedded in a matrix of fine-grained (~6-10µm) powder of natural quartz in a shear geometry, up to large strains (𝛄 ≈ 3-4). In both sets of experiments, 0.1 to 0.15 wt% H₂O was added to the assemblage. The H₂O content was measured by FTIR on thick (~100-200µm) plates after deformation, either as spot analyses on grain interiors or on regions containing grain boundaries.

Nearly all strain in the coarse grained quartzite was acquired by crystal-plastic deformation of quartz grains, determined by the shape change of original sand grains that constitute the quartzite (revealed by cathodoluminescence) before and after deformation. Crystal plastic deformation is accompanied by minor recrystallization along grain boundaries, where a mantle of small-sized (~3-5µm) grains developed around some porphyroclasts. While crystallographic fabrics remained weak because of the low strain, low-angle grain boundaries are abundant and indicate incipient recrystallization by subgrain rotation and dominant prism <a> slip. In addition to this classic pattern of intracrystalline plasticity and dynamic recrystallization, there is evidence for fracturing and dissolution-precipitation that have produced small grains around the original large grains.

In the starting material, H₂O was mostly contained in fluid inclusions and aggregates, characterized in FTIR by broad-band molecular H₂O, (typically ∼4500 H/10⁶Si). The H₂O content in quartz grains was strongly diminished by (i) the application of pressure and temperature and (ii) deformation, down to ∼1000 H/10⁶Si. Irrespective of the conditions of deformation, the H₂O content systematically remains higher in grain boundary regions  compared to grain interiors. The H₂O expelled during deformation concentrated in domains of fine recrystallized grains of euhedral shapes with large intergranular porosity. These domains are interpreted as pockets of excess H₂O (sometimes with partial melt) where the storage capacity of the grain boundary regions of the quartz aggregate is exceeded. The FTIR spectra show no significant variation with the pressure conditions of the experiments, except for the peak at 3585cm^-1, which increased with pressure. As the strength of the aggregates decreased with pressure, we tentatively correlate this peak with point defects in quartz responsible for the pressure-dependent weakening. 

How to cite: Raimbourg, H., Stünitz, H., Pongrac, P., Ghosh, S., Palazzin, G., Nègre, L., Heilbronner, R., Précigout, J., and Jeřábek, P.: Deformation of quartz aggregates : interplay between plasticity and grain boundary processes, and the role of water, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9588, https://doi.org/10.5194/egusphere-egu24-9588, 2024.

Extensive micro-porosity can be found in numerous examples of quartz-rich mylonites deformed in crustal shear zones, but whether or not deformation is involved in the production of such pores remains a matter of active debate. The occurrence of syn-kinematic micro-scale porosity could result in overall changes in rock strength, as well as potentially generating a deep permeability. This would have major implications for fluid-rock interactions, earthquake nucleation and ore deposits. In this study, we focus on micro-pores occurring in quartz-rich shear bands from mylonitic granitoids which outcrop in Ikaria (Cyclades, Greece). Related to the back-arc geodynamics of the Aegean domain during the Miocene, this granitic body intruded the Cycladic basement during active detachment faulting, which has led to heterogeneous deformation of the pluton. While the granite exhibits large-scale levels of strain, increasing with proximity to the detachment fault, quartz-rich shear bands develop as a result of viscous strain localisation, giving rise to S-C structures. Micro- (to nano-) pores occur in pure quartz aggregates of these shear bands, where micro-structural features indicate dominant crystal plasticity, mostly recovering by subgrain rotation.

Based on five samples collected at different distances from the detachment fault, we performed microprobe-based hyperspectral cathodoluminescence and electron backscatter diffraction (EBSD) to characterise the micro-pores in pure quartz aggregates. In cathodoluminescence maps, parent and recrystallized quartz grains produce blue (420 nm wavelength) and yellow (650 nm wavelength) signals respectively. Furthermore, both parent and recrystallised grains exhibit a distinct increase in luminescence at 650 nm, which appear visually as very bright yellow rims/halos at their grain boundaries and, to a minor extent, at their subgrain boundaries. Using high-resolution and standard EBSD, we highlight high (to very high) geometrically necessary dislocation densities that partly coincide with such rims/halos, particularly where micro-pores are described. Most of the dislocations that may contribute to these high densities are related to the main dislocation slip system of quartz (prism <a>), as deduced from from lattice preferred orientation and subgrain analyses. Our findings, therefore, suggest that highly luminescent "yellow" boundaries of quartz grains result from dislocation accumulation, and hence, from crystal plasticity, which can be linked to the production of micro-porosity in these rocks. 

How to cite: McGill, G., Précigout, J., Prigent, C., Arbaret, L., Airaghi, L., and Wallis, D.: Micro-porosity found in quartz shear bands from Ikaria, Greece: insights from Hyperspectral Cathodoluminescence and High-Resolution Electron Backscatter Diffraction, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9750, https://doi.org/10.5194/egusphere-egu24-9750, 2024.

Micropores are commonly observed in quartz-rich rocks that deformed at depths of the viscous, metamorphic continental crust. Although the presence of such porosity – often occurring with angular, pyramidal shapes – has major implications for fluid circulations and rock strength, whether or not they are produced by deformation remains unclear. Here we provide detailed documentations of pure quartz aggregates decorated by micropores in granitic shear bands from Naxos (Greece). Through estimations of geometrically necessary dislocation densities, we first document very high values (>> 10¹⁵ m^-2) along intragranular boundaries, several of them containing micropores. We then performed focused ion beam (FIB) cross-sectioning and transmission electron microscopy to image pore shapes along all types of quartz boundaries. Pores do not necessarily arise with angular shapes, but they are systematically embedded within amorphous SiO₂, i.e., silica glass, along both grain and intragranular boundaries. FIB volume reconstruction also revealed pyramid-like pits occurring with round-shape faceted pores, the shape of which challenges long-lasting hypotheses for pores to originate. Together with recent studies^[1,2], our findings support deformation to produce porosity through (1) mechanical amorphization where dislocations accumulate and (2) fluid exsolution from the resulting glass because of a pressure/stress drop, here attributed to grain boundary sliding.

[1] Idrissi, H., Carrez, P. & Cordier, P. On amorphization as a deformation mechanism under high stresses. Current Opinion in Solid State and Materials Science 26: 100976 (2022)

[2]Li, B. Y., Li, A. C., Zhao, S. & Meyers, M. A. Amorphization by mechanical deformation. Materials Science & Engineering R 149: 100673 (2022)

How to cite: Précigout, J., Prigent, C., McGill, G., Arbaret, L., Airaghi, L., and Wallis, D.: Quartz amorphization to produce porosity in crustal shear zones, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9882, https://doi.org/10.5194/egusphere-egu24-9882, 2024.

Transient creep of olivine in the upper mantle plays an important role in large-scale Earth processes such as glacial isostatic adjustment and postseismic creep, as well as (exo-)planetary tidal heating and orbital dynamics. Yet, an experimentally confirmed microphysical understanding of transient creep across all timescales relevant to Earth processes remains elusive. An increasing body of laboratory and geodetic work suggests that nonlinear, dislocation-based dissipation mechanisms may play a more important role than previously thought. In response, several dislocation-based transient creep mechanisms have been proposed to explain transient creep in the upper mantle, including intergranular plastic anisotropy and the build-up of backstresses arising from long-range dislocation interactions. 

The time-dependent dissipation of strain energy during transient creep manifests as attenuation, Q^-1, in the frequency domain. Therefore, the constitutive equations of the proposed mechanisms should be able to predict the attenuation in polycrystalline olivine subjected to forced oscillations, providing an independent test of their applicability. Here we present numerical investigation of the nonlinear constitutive equations of these models in the frequency domain and comparisons thereof to the mechanical results of a set of high-stress, forced-oscillation experiments on polycrystalline olivine performed in a deformation-DIA coupled with synchrotron analysis techniques. Key microstructural variables needed to inform these comparisons, such as grain size, plastic anisotropy, and dislocation density, were obtained from electron backscatter diffraction and dislocation decoration.

The experiments demonstrate amplitude-dependent attenuation, which is characteristic of dislocation-based dissipation. In addition, we find that Q^-1 depends on the maximum stress amplitude experienced by the sample. Dislocation-density piezometry indicates that this history effect can be linked to dislocation density evolution as post-experiment dislocation densities reflect the highest stresses obtained in the experiment rather than the stresses obtained near the end of the experiment. Numerical analysis of the constitutive equations yields high Q^-1 values, up to ~5, which is similar to the experimental observations. We find that the experimental observations are consistent with predictions from the backstress model for the grain sizes and dislocation densities of our samples. When extrapolated to lower stress amplitudes, the backstress mechanism produces approximately linear behavior and behaves as a Burgers model in frequency space, suggesting that dislocation interactions may contribute to seismic wave attenuation as well.

How to cite: Hein, D. and Hansen, L.: Experimental and numerical investigation of dislocation-based transient creep mechanisms in the upper mantle, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10734, https://doi.org/10.5194/egusphere-egu24-10734, 2024.

Carbonate rocks compose 15% of Earth’s ice-free continental surface and commonly consist of kilometer thick sequences that host complex crustal-scale fault zones, accommodating displacements on the order of tens of kilometers. These intricate fault networks significantly influence fluid migration, further controlling crustal mechanics. Understanding the deformation mechanisms of calcite and dolomite, two of the dominant carbonate forming minerals, is therefore essential for predicting the rheological properties of carbonate rocks. Herein, we conduct a microstructural analysis to investigate the interactions between brittle-ductile structures under greenschist facies conditions in a naturally occurring biphasic marble mylonite. The framework of the mylonite, which is exposed in a tectonic window north of the Greek Cyclades on the Attica peninsula, indicates deformation occurred at 300-350°C and 7-8 kbar during the late Oligocene. The mylonitization is overprinted by a weak, but pervasive axial plane cleavage. A second strong, and densely spaced axial plane cleavage, oriented perpendicular to and truncating the first set, creates a ‘pseudo-boudinage’ of the dolomite layers. Localized shear bands cross-cutt the second axial plane cleavage set, suggesting a fourth phase of deformation. Electron backscatter diffraction analysis of the mylonite reveals coarse (30-200 µm) calcite with evidence of crystal-plasticity in the form of low-angle (<15°) grain boundary development (LAGB) and linear to heterogeneous misorientation patterns. LAGB density and misorientation angles increase towards the clast rims, to maximum misorientations reaching 44° relative to the mean orientation of the grain. The coarse grains are surrounded by fine (<25 µm) calcite revealing little to no intracrystalline misorientation. Fine calcite is similar in size to subgrains defined by LAGBs within high misorientation domains of coarser grains, which is consistent with subgrain rotation recrystallization and lower greenschist facies conditions. Calcite in the mylonite record a grain-shape preferred orientation that is parallel to the main foliation and oblique to that of the cross-cutting ductile shear bands. These shear bands are characterized by fine grained (2-10 µm) inequigranular calcite with no internal misorientation and sparse, 20-30 µm anhedral calcite with weak heterogeneous misorientation patterns and a maximum misorientation of 8° in the panhandles of grains. Contrastingly, deformation in the dolomite bands is dominantly brittle as evinced from the brecciation of these layers. Clasts commonly display primary growth twinning characterized by a rotation of 180° around one of the [11̅2̅0] axes, 12-120 µm in diameter and show minor evidence for crystal-plasticity in the form of intragranular lattice distortions (maximum misorientation of 22° relative to the grain average orientation). Calcite infilling the space between dolomite fragments exhibits no grain-shape preferred orientation and consists of 5-25 µm diameter grains with minimal intracrystalline lattice distortion (0°-8°) and e-twins characterized by an 80° rotation about the [0̅2̅21] axes. The same twinning is observed in the coarse calcite with twin density increasing with proximity to dolomite ‘boudins’. Our study identifies the active deformation mechanisms in calcite and dolomite during four successive phases of deformation to clarify the feedback between brittle-ductile microstructures on strain localization, yielding insight into rheological evolution of carbonates.

How to cite: Bakowsky, C., Dubosq, R., Schneider, D., and Grasemann, B.: Microscale investigations of the evolution of deformation mechanisms in a low-temperature marble mylonite, NE Attica, Greece, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11323, https://doi.org/10.5194/egusphere-egu24-11323, 2024.

Orhaneli ophiolite is an Upper Cretaceous ophiolitic suite obducted over the Late Cretaceous high-pressure rocks. It covers approximately 43 km in length and 14 km in width. It is part of the Neotethyan ophiolite belt along the southern side of the Izmir-Ankara-Erzincan Suture. The lithological and structural mapping of the Orhaneli ophiolite revealed that the mantle rocks, the Moho Transition Zone (MTZ), and the ophiolitic lower crust are exposed along the region but the subvolcanic and volcanic sequences are missing. The study of the deformation mechanisms of available three units is our research.

The mantle rocks in the Orhaneli ophiolite comprise harzburgite (~50%), dunite (~40%), websterite, and clinopyroxenite (~10%). Harzburgite and dunite are coarse-grained and show well-developed L-S tectonic fabric. Websterite and clinopyroxenite are coarse/very coarse-grained with granular texture. The mantle tectonites (harzburgite and dunite) in the region are characterized by widespread high-temperature (1200-1250 °C) deformation partially overprinted by low-temperature (800-1000 °C) deformation. The grain boundary migration (GBM) and subgrain rotation (SGR) recrystallizations are the dominant mechanisms of the high-temperature deformation in this unit. The subsequent low-temperature deformation predominantly proceeded through subgrain rotation (SGR), and bulging (BLG) recrystallizations accompanied by kinking and twinning. Contrarily to the mantle tectonites, the pyroxenite (websterite and clinopyroxenite) predominantly shows low-temperature deformation structures. They are mainly deformed through the kinking of the pyroxene grains, however, high-temperature deformation structures also exist. A possible explanation is that the pyroxenite predominantly deformed through viscous flow under spreading center conditions.

The MTZ in the Orhaneli ophiolite is a ~1 km thick, strongly sheared zone between the mantle and lower crustal rocks. It mainly consists of serpentinite, layered gabbro, and mylonitic peridotite. The serpentinite, the most prevalent lithology in this zone, commonly shows anastomosing foliation. The layered gabbro mainly consists of orthopyroxene and plagioclase. It is characterized by thin and continuous layers of plagioclase and orthopyroxene. In some cases, these layers are transposed into isoclinal folds with detached limbs by continuous, layer-parallel, simple shearing. The stretching lineation is well-developed and defined by plagioclase and orthopyroxene. The mylonitic peridotite mainly consists of olivine and orthopyroxene. It is characterized by ribbons of orthopyroxene and elongated aggregates of olivine within a fine-grained olivine and pyroxene-rich matrix. The orthopyroxene ribbons indicate high-strain conditions within the MTZ. The aggregates of olivine suggest that the SGR recrystallization is an important mechanism of deformation within this zone.

The lower crust in the Orhaneli ophiolite comprises cumulates of gabbro, peridotite, pyroxenite, and anorthosite, in order of prevalence. Peridotite is more abundant in the stratigraphically lower sections of the crust and it diminishes stratigraphically upward. The serpentinization of peridotite is commonly over 90%. In general, the crustal section does not show evident plastic deformation. The gabbroic rocks commonly show magmatic foliation defined by the preferred orientation of undeformed plagioclase and pyroxene grains. This suggests that the crustal section is mainly deformed through viscous flow in the magmatic state.

How to cite: Paksoy, Y. C., Paksoy, N., and Natal'in, B. A.: The deformation mechanisms of Upper Cretaceous Neotethyan Orhaneli ophiolite, NW Turkey, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12851, https://doi.org/10.5194/egusphere-egu24-12851, 2024.

Semi-brittle deformation, which is characterized by the simultaneous occurrence of fracturing and crystal plasticity, plays a critical role in determining the mechanical properties of the middle crust. Laboratory experiments have identified semi-brittle deformation as ductile flow involving distributed microfracturing, an absence of localized macroscopic failure, and widespread plasticity. However, a constitutive law of semi-brittle deformation remains elusive, and a lack of quantitative microstructural analyses has hindered the development of micromechanical models for semi-brittle deformation.

This study aims to address these limitations by providing quantitative characterization of twins, lattice distortion, and intragranular fractures in Carrara marble that has undergone semi-brittle deformation. Three sets of samples were uniaxially shortened to varying strains up to 8% under a confining pressure of 400 MPa and different temperatures at 20, 200, and 350ºC. The tested samples were examined by forescattered electron imaging and electron backscattered diffraction mapping. The results reveal that, in the early stages of deformation (strain < 2%), deformation is primarily accommodated by twins. Lattice distortion, linked to geometrically necessary dislocations, becomes prominent in the later stages (strain > 4%). Intragranular fracture intensity shows a linear correlation with strain. Despite some nuanced variations, the qualitative development of each microstructure type remains similar at different temperatures. At the onset of semi-brittle deformation, microstructural evidence has shown that the nucleation of microfractures or lattice distortion is induced by strain incompatibility at granular scale. The local stress concentrations associated with such strain incompatibility are enhanced by irregularities of grain boundaries. These observations provide a foundational microstructural understanding, facilitating the development of a robust microphysical model for semi-brittle deformation in the lithosphere.

How to cite: Qu, T., Brantut, N., Wallis, D., and Harbord, C.: On the microstructural evolution of Carrara marble during semi-brittle deformation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13583, https://doi.org/10.5194/egusphere-egu24-13583, 2024.

This study presents the conditions of pressure, temperature, and breccia mechanisms that allow the migration of emerald-bearing hydrothermal fluids within the emerald belts of Colombia. Based on detailed petrographic analysis, three different hydrothermal events were determined, establishing a chronological framework that enhances our understanding of geological processes in these emerald belts. The first event is characterized by the high presence of albite, due to an albitization event that altered the rock. The other events are marked by an early stage of carbonatization (second) and a late carbonatization stage (third). After understanding the hydrothermal events, Fermi diad bands were obtained by Raman spectroscopy in fluid inclusions. This approach permits the calculation of the rock pressure using the vibrational modes of the CO2 and their relationship with its density. These analysis was performed in minerals associated with each hydrothermal event. The data obtained from these analyses provides the evolution of the pressure in the whole hydrothermal history. 

Finally, a fractal analysis applied to breccias from both (western and eastern) emerald belts was performed. This analytical approach aimed to understand the breccia mechanisms throughout the entire hydrothermal history, providing a view of the geological evolution of these emerald belts. Even though both belts present similar events, the western emerald belt reveals higher mechanical energy in comparison with the eastern emerald belt, in which the pressure is generally lower and breccia mechanisms show a higher-intensity corrosive event, the presence of fluidization breccias manifests that the migration of the fragments may be fluid assisted against Halokinesis

How to cite: Betancur Acevedo, C. A., Kammer, A., and Garcia Toloza, J.: Breccia Mechanisms and Hydrothermal Evolution from Both Colombian Emerald Belts, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15060, https://doi.org/10.5194/egusphere-egu24-15060, 2024.

The Strandja Massif, exposed along northwestern Turkey is an NW striking polymetamorphic belt. The massif is mainly composed of a Neoproterozoic-Paleozoic metamorphic sedimentary complex intruded by plutons of various ages, all of which are covered by Mesozoic metasedimentary units. In previous studies Natal’in and co-authors have shown crosscutting relations between bedding or intrusive contacts were well documented all above-mentioned rocks reveal uniform penetrative foliation, which is observed in stratigraphic units of various ages. Isotopic dating shows that the massif has undergone at least two metamorphic events during the Mesozoic and Paleozoic. Greenschist to lower amphibolite facies metamorphism and intense deformation occurred in the Middle Jurassic to Early Cretaceous times as is evident from Ar-Ar and Rb-Sr studies. However, the formation of migmatites in some rocks assigned to the Paleozoic and the heterogeneous distribution of metamorphic rock types contrary to the more or less regular behavior of fabric require additional attention to the reconstruction of P-T conditions of deformations and their variations in time. The goal of this research is to improve our understanding of the P-T conditions during Paleozoic metamorphism and deformation. For this purpose, we studied the northwest part of the Strandja Massif which mainly consists of migmatitic biotite gneiss, metagranite, migmatitic biotite garnet gneiss, amphibolite, and quartzo-feldspathic schist.

The metamorphic conditions of the Paleozoic metamorphism are restricted by the following criteria: (1) The units that have undergone the Paleozoic metamorphism show evidence of migmatization. The temperature should exceed ~650 °C for the beginning of partial melting. (2) The occurrence of amphibolite indicates that the Paleozoic metamorphism was within the amphibolite facies conditions. (3) The founding of partially preserved kyanite restricts the pressure conditions of the Paleozoic metamorphism. By these criteria, the peak metamorphic conditions of the Paleozoic metamorphism are restricted to 650-720 °C and 6-12 kbar.

The Paleozoic metamorphism is accompanied by highly ductile deformation compatible with the metamorphic conditions. The Paleozoic deformation is characterized by mesoscale intrafolial folds, macroscale sheath folds, and migmatitic foliation. The intrafolial folds have foliation parallel axial planes and can only be recognized through their hinges since their limps are commonly detached. The presence of melt, due to migmatization, possibly has a crucial role on the rheological properties during this deformation.

The microstructural imprints of the Paleozoic metamorphism are investigated within the framework of this study. The chessboard subgrain pattern of quartz within the leucosomes of the migmatitic gneiss provides evidence that peak metamorphism occurred during one of the episodes of a prolog structural history of the Strandja Massif. The absence of this subgrain pattern out of leucosomes supports that the Paleozoic metamorphism did not advance into the granulite facies. The grain boundary migration (GBM) and subgrain rotation (SGR) recrystallizations and the growth of deformation myrmekites along the high-stress sites of feldspar also require higher temperature conditions than the compared with those that are seen in rocks that are assigned to the Mesozoic. These structures could be formed or represent relicts of Paleozoic metamorphism.

How to cite: Paksoy, N., Paksoy, Y. C., and Natal'in, B. A.: Relicts of high-temperature fabric in the Strandja Massif, NW Turkey, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16284, https://doi.org/10.5194/egusphere-egu24-16284, 2024.

The Koralpe Complex of the Eastern Alps hosts a major crustal-scale shear zone within which the exhumation of Eo-Alpine eclogites and the formation of highly-strained, high-P-T (680-700°C, 12-13 kbar) Plattengneis mylonites was localised. Although no convincing kinematic indicators have been published and macro- and microscopic fabrics record an orthorhombic symmetry in the N-S section parallel to the prominent stretching lineation, a top-north shear sense has been inferred from published quartz EBSD analyses. In this contribution, we present a clear monoclinic fabric perpendicular to the stretching lineation revealing a top-west shear sense. Vorticity axes preserved in the crystal lattice of deformed quartz grains (constrained by EBSD data) are used as a quantitative solution for deciphering the strain history and to establish a newly informed constraint on the Eo-Alpine kinematics of the Koralpe.

Observations of macro- and micro-scale monoclinic fabrics (e.g. feldspar sigma-clasts and tourmaline and garnet delta-clasts) revealed an unequivocal and consistent top-west shear sense, localised around a north-south (N-S) striking vorticity axis (VA_Fsp), perpendicular to previous estimations of kinematics. To resolve the conflict between reported and observed shear sense, our investigation probed for crystalline vorticity axes preserved in quartz grains that experienced crystal plasticity and rotational distortion of the crystal lattice during deformation. The vorticity analysis of quartz EBSD data revealed a bulk crystalline vorticity axis (CVA_Q) striking east-west (E-W), inclined 60-70° to the west. The inclined orientation of CVA_Q is geometrically incompatible with the kinematic configuration of pure-shear dominated general shear necessary to produce the defining structural fabric of the Plattengneis (N-S stretching lineation, L_S; pervasive planar foliation, S₁). This incompatibility, along with the implication of an additional vorticity axis (VA_Fsp), indicates that CVA_Q resembles a compound vorticity axis re-orientated into an inclined position during a two-phase deformation history.

To resolve the two-phase transposition of vorticity axes, we modelled a theoretical solution: a horizontally inclined initial orientation of CVA_Q with subsequent rotation around VA_Fsp using mechanically compatible quartz slip-systems. The initial orientation of CVA_Q (E-W striking) is predicted to form during D₁ by dominant prism<a> slip under nearly plain strain pure-shear conditions. During D₂, CVA_Q is subsequently rotated c. 60-70° around the N-S striking VA_Fsp with a top-W shear sense. Based on the quartz EBSD dataset (CPO and misorientation axes of low angle boundaries (LAB)) we presume a dominance of prism<a> slip during the initial pure-shear deformation in D₁ (under upper amphibolite facies condition). In the second deformation, continuation of prism<a> slip is inhibited by sub-optimal orientation of quartz grains relative to the D₂ stress field; based on the heterogeneous distribution of LAB misorientation axes, we propose that the D₂ rotation of CVA_Q was accommodated by the interaction of multiple, non-dominant and geometrically-necessary slip-systems.

The crystal-scale kinematic analysis revealed a previously unknown poly-phase deformation during the formation of the Plattengneis shear zone with a top-west component in accord with the overall Eo-Alpine kinematics and demonstrated the vast potential of the crystalline vorticity axis analysis method for accurately resolving complex kinematics.

How to cite: Hill, L., Grasemann, B., and Bestmann, M.: Using EBSD crystalline vorticity axes to deduce complex kinematics during the Eo-Alpine deformation of the Plattengneis Shear Zone (Koralpe, SE Austria)., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16733, https://doi.org/10.5194/egusphere-egu24-16733, 2024.

Deformation of polymineralic rocks at elevated temperatures usually results in grain size refinement but also in a grain scale mixture of mineral phases. Phase mixtures may either be homogeneous or exhibit a compositional and microstructural layering. Depending on the host rock stability, mixtures may consist of combinations of redistributed or newly, synkinematically formed phases. In general either neighbour-switching or heterogeneous nucleation are envisaged as processes responsible for mixing during diffusion creep s.l. including grain boundary sliding and grain-scale transport processes. In order to quantify phase mixtures, neighbourhood relations can be analysed to differentiate random, clustered or anti-clustered distributions. Heterogeneous nucleation is usually considered to allow for anti-clustered distributions, while neighbour switching during grain boundary sliding potentially produces random distributions.

Here, phase mixing is explored based on contact densities as well as on centre-to-centre distances. In particular, the effect of directionality that is neighbour relations as a function of the relative position in a 2D section is considered. The direction of neighbours is considered by the normal of the boundary trace as well as alternatively, by the direction of the centre-to-centre join.

Given sufficiently large datasets and non-extreme mixtures (e.g. with 0.2 < phase proportion < 0.8) a confidence interval of the results can be defined. Large datasets of ultramylonites of different metamorphic grade, phase proportions, compositions (ultramafic, mafic, quartzo-feldspatic) and microstructures (layered, isotropic) are tested.

It is found that phase anti-clustering is generally more pronounced in a direction close to the stretching direction in either layered or homogeneous ultramylonites. In layered mylonites, layer-normal relations are frequently found to be random while intralayer relations are often anti-clustered.

In the different rock types, specific anti-clustered phases can be discriminated, e.g., orthopyroxene with respect to olivine, k-feldspar with respect to plagioclase and quartz, and hornblende with respect to plagioclase. Other phase assemblages e.g. quartz-plagioclase are frequently found to be distributed randomly, hinting at mineral specific roles during diffusion creep s.l. and generally at element mobilities in deforming metamorphic rocks.

How to cite: Kilian, R.: Phase mixtures in shear zones, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17253, https://doi.org/10.5194/egusphere-egu24-17253, 2024.

The shape of pebbles in metamorphosed conglomerates has been used as an indicator of strain during tectonic deformation. Here we analyze deformed quartzite pebbles from metaconglomerates across the Salamanca detachment shear zone (SDSZ),  with variations of strain and deformation temperature. This structure is related to the late Variscan gravitational collapse in the Iberian Massif, Central Spain and has significant control on mineral resources. Strong preferred orientation is documented with time-of-flight neutron diffraction measurements and EBSD. The c-axes are in an asymmetric maximum perpendicular to the pebble elongation direction and there is considerable variation between samples. The oblique c-axis maximum relative to the elongated shape axis can be explained as a result of extension, combined with simple shear and dominant basal and rhombohedral slip, based on polycrystal plasticity modeling. It may also be influenced by recrystallization resulting in orientation patterns that resemble single crystals. The role of inherited textures is discussed and seems to be dominant in lower temperature segments on the base of the SDSZ hanging-wall.

Funding: grant PID2020-117332GB-C21 funded by MCIN/ AEI /10.13039/501100011033; SA084P20 from the JCyL government, and TED2021-130440B-I00 funded by MCIN/AEI/10.13039/501100011033.

How to cite: Gómez-Barreiro, J., Wenk, H. R., Vogel, S., Palomeras, I., Ayarza, P., and Martínez Catalán, J. R.: Texture of quartzite pebbles in metaconglomerates: strain paths across an extensional detachment , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20305, https://doi.org/10.5194/egusphere-egu24-20305, 2024.

Faults act as sites for preferential failure in the continental crust when it is subjected to sequential tectonic events. To do so, they will typically have favourable orientation, geometry and/or be weaker than the adjacent crust and are therefore prone to slip. However, fluid flow that is focussed along faults has a role in modifying their mechanical strength, especially where different fluids, and resulting mineralisation, are partitioned along particular structures. The East-Quantoxhead Fault (EQHF) in the Bristol Channel Basin (BCB), SW England has been identified as a reactivated normal fault with multiple slip events, the cause and precise timing of which are unknown. Using U-Pb geochronology of calcite veins located in the fault core we show that the timing of mineralisation (as a proxy for fluid flow) along the EQHF spans from 151-35 Ma. Microstructural analysis of different vein generations within the fault core shows that the longevity of this structure is a result of progressive weakening of the fault core. Initially this is represented by crystal plastic deformation of early-stage calcite mineralisation in the fault core, compatible with protracted phases of normal-sense slip. This is most likely mediated by the flow of syn-kinematic fluids that are hotter than the ambient temperature of the wall-rocks. However, later weakening of the fault core results from the precipitation of relatively weak fibrous celestine (SrSO₄) along the margins of older calcite veins. Celestine shows evidence of reverse-sense S-C fabrics and was therefore a site of strain localisation and fault reactivation during regional contraction. This later fluid is sourced from deeper formations within the BCB which are only accessed by larger faults like the EQHF. Smaller normal faults in the BCB containing no celestine do not show a protracted fluid history, however crystal plastic textures (GBM, SGR) can also be seen within these faults. Understanding the role different fluids play in altering fault core composition, and strength via the analysis of vein textures plays a key role in understanding the partitioning and significance of fluid flow along fractures. 

How to cite: Anderson, M., Connolly, J., Mottram, C., Price, G., and Sanderson, D.: Fluid-mediated reactivation of brittle faults in the Bristol Channel Basin, UK, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20713, https://doi.org/10.5194/egusphere-egu24-20713, 2024.

Fracture and fault networks are characterised by complex spatial and dimensional properties that might affect the flow and accumulation of subsurface fluids. In geological applications, DFN models are commonly employed to compute the multiscale properties of fractured rock volumes in terms of porosity and equivalent permeability. In the present contribution, we focus on an outcrop-to-reservoir investigation of Mesozoic shallow-water carbonates exposed along the axial zone of the southern Apennines fold-and-thrust belt, FTB, Italy. The carbonates were originally deposited in lagoon-to-proximal ramp settings of the Paleo Apenninic Platform during Lower Jurassic–Upper Cretaceous times and include well-layered and massive associations of bed packages several m-thick. By integrating the results of both field and digital structural analyses, we build multiple DFNs to assess the hydraulic behaviour of geocellular volumes representative of the different scales of observation, and stochastically populated with high-angle fractures, small, and medium faults. Fractures are either strata-bound, SB, or non-strata-bound, NSB, and results compartmentalized within single-bed packages. Small faults crosscut multiple bed-packages and show a few cm-to-m throws, whereas medium faults displace multiple bed-package associations and have throws > 1m. Both small and medium faults exhibit high peaks of fracture density, P₂₀, and intensity, P₂₁, in correspondence with the releasing jogs disrupting mechanical interfaces such as bed package boundaries and pre-existing low-angle thrust faults. As data input for DFN modeling, the aperture values of the stochastic fractures are set as proportional to either fracture length (most favourable flow conditions) or to the square root of fracture length (least favourable conditions). At the outcrop scale, 5m-side DFN models show the highest values of fracture porosity among those considered in this work. These results are therefore consistent with both SB and NSB fractures forming the main repository for fluid accumulation. At larger scales, the 50m-side and 500m-side DFN models including small and medium faults, are characterized by higher values of equivalent permeability, which range between 10^-2 and 10^-1 mD. Considering the computed Kxx and Kyy values for the single geocellular volumes, near-isotropic horizontal conditions are assessed across all scales of investigation. Accordingly, altogether, high-angle SB and NSB fractures, small and medium faults form a system of well-connected network through the shallow-water carbonates. Interpreting these data in light of published values for platform carbonates in Italy, we interpret this multiscale horizontal permeability isotropy due to the severe exhumation (~ 4 to 5km) the studied carbonates went through during the Quaternary downfaulting of the southern Apennines FTB.

How to cite: Abdallah, I. B., Panza, E., Dastoli, S., Manniello, C., Prosser, G., and Agosta, F.: Fracture porosity and equivalent horizontal permeability computed for shallow-water carbonates of the southern Apennines fold-and-thrust belt, Italy, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-277, https://doi.org/10.5194/egusphere-egu24-277, 2024.

A wide range of deformation and diagenetic mechanisms have been observed within faulted carbonate lithofacies.  These processes are known to influence the permeability of a fault, and hence the flow properties.  The mechanisms active during faulting are influenced by a range of factors, including: lithofacies, host porosity, host permeability, juxtaposition type, depth of burial, depth at time of faulting and kinematics.  Although we can estimate the fundamental controls on resulting fault rock permeability, the ability to predict flow properties within and surrounding faults in carbonates remains highly uncertain.  This presentation will discuss conditions for when a fault may act as a conduit or a potential barrier to flow, along with the gaps in current data and knowledge.

Further, conditions for when a permeability anisotropy may be created within the fault core of carbonate lithofacies will be presented, along with the implications for fluid migration across or along the slip surface.  A permeability anisotropy is observed within carbonate fault cores, dependent on lithofacies and juxtaposition.  The significant heterogeneity created when different lithofacies are juxtaposed outweighs the resulting permeability anisotropy that is created, such that no systematic permeability anisotropy can be defined.  However, when self- or similar juxtapositions occur, a systematic permeability anisotropy is recorded, creating a permeability that can be as much as over 5 orders of magnitude lower normal to fault strike than parallel to fault strike.  The permeability anisotropy is formed from differing mechanisms dependent on lithofacies; the intersections of shears with fractures/veins in recrystallised lithofacies, and oriented pores in high porosity grainstones.  This is similar to previous crystalline and siliciclastic studies.  The permeability anisotropy can act to allow flow in one orientation but prevent it in another.

How to cite: Michie, E.: Can we ever predict how fluids may flow within and surrounding faults in carbonates?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-390, https://doi.org/10.5194/egusphere-egu24-390, 2024.

Diagenetic and tectonic processes taking place in platform carbonates produce significant textural and mineralogical modifications through time, controlling the pore space in terms of dimension, geometry, shape, and connectivity of single pores, and influencing both total and effective porosity. Focusing on the different types of solution surfaces, this study is conducted on Lower Jurassic, Cretaceous, and Eocene, carbonates exposed at the Viggiano Mt. and Raparo Mt., southern Apennines, Italy. Microscale analyses show that the primary porosity of these rocks was occluded by pervasive blocky cements, which precipitated during burial diagenesis of the carbonates. We aim to assess the role exerted by the roughness of bed-parallel and low-angle to bedding solution surfaces, on the pore properties and permeability values of a variety of carbonate lithofacies such as mudstones, packstones, grainstones and rudstones. Specifically, we show the results of Nuclear Magnetic Resonance (NMR), gas-porosimetry and water-permeability tests conducted on plugs cored either orthogonal or parallel to bedding interfaces. All the study plugs show an amount of effective porosity lower than 5%, with mean values of ca. 3%. Excluding larger microfractures, and sporadic intrafossil and intercrystal molds, among the various types of solution surfaces we document that the rough, seismogram-type stylolites localize secondary porosity, while smooth, wave-type stylolites do not. The seimogram type stylolites, due to the non-selective carbonate’s dissolution, form a poorly connected vuggy porosity, and the NMR results the pores are subspherical to tubular (r<3 µm), with low aspect ratios (stiff pores), differently from the pores associated to open fractures (soft pores). Connectivity in the seismogram-type stylolites-related pores is due mainly to small microfractures forming capillary porosity (pore throat ca. r=1 µm). The results of permeability measurements at room pressure indicate that the amount of bed-perpendicular permeability is generally low (10^-1 and 10^-3 mD). The results of permeability measured at increasing confining pressure show that the in stylolite-dominated the bed-parallel samples have slightly higher values with respect to the bed-orthogonal ones and, at increasing confining pressure conditions, the permeability decreases of less than one order of magnitude. Differently, the fracture-dominated plugs show a permeability decrease of two orders of magnitude. These results are therefore consistent with a pore connectivity affected by open fractures only at shallow depths (<25 MPa) and influenced by both stylolites and primary pores at depths. Results of ongoing X-ray tomography analyses will better clarify the pore distribution along stylolites and at the fracture-stylolite intersections.

How to cite: Manniello, C., La Bruna, V., Bezerra, H. F. R., Araùjo, R. E. B., Morais, X. M., Michie, E., Faulkner, D., Allen, M. J., Prosser, G., and Agosta, F.: Pore space properties of solution surfaces in shallow-water carbonates, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-536, https://doi.org/10.5194/egusphere-egu24-536, 2024.

On a global scale, the South Caspian is a basin where mud volcanoes are
most densely distributed. Numerous active volcanoes are registered here, both on
land and in the adjacent sea basin (Caspian Sea), which, as a result of their daily
activity, discharge water fluids to the Earth's surface.
In this work, using samples taken from coastal and island mud volcanoes, a
comparative study of the chemical compositions of the fluids expelled from the
salse and gryphon-type emissions, as well as their origin, including the
contribution of various stratigraphic units to the water formation processes.
According to our results, the contribution of the condensed waters of the
Caspian Sea was greater in the formation of Na-Cl type waters of mud volcanoes
in the South Caspian Basin. In addition, the origin of volcanic waters also formed
as a result of the contribution of shallow ‘low-mineralized’ pore fluids and deep-
seated ‘high-mineralized’ brines correlates well with the depths (1-5.5 km) of the
Productive Series strata.

How to cite: Bayramova, A.: A Comparative study of water fluids of coastal and island mud volcanoes: Acase study of the South Caspian Basin, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-569, https://doi.org/10.5194/egusphere-egu24-569, 2024.

The complexity of fault zone structure and faulting mechanisms significantly impact the permeability of fault zone rocks. Various factors, including the type and porosity of the host rock, the fault’s geometry, differential strain, the history of deformation, and the tectonic setting, can cause permeability to exhibit wide fluctuations. However, research into the permeability of complex fault zones is constrained by the scarcity of in situ measurements. Utilizing analytical relationships, laboratory tests, outcrop measurements, or numerical modeling often yields biased results, as they may not accurately represent real-world conditions. Moreover, the effects of fault branching and interconnections have not been thoroughly explored. This study compares the permeability of faulted carbonate rocks using a comprehensive database of in situ permeability tests. We investigate how a network of seven faults influenced permeability variations at different locations and depths by examining surface and subsurface data, including information from excavated tunnels. Our findings reveal that factors such as fault dips, length, fault structure, and rock characteristics can create diverse impacts on permeability. We observe permeability values in fault damage zones one to five orders of magnitude higher than those in the host rocks. The thickness and condition of damage zones shed light on the range of fault zone permeability. Furthermore, we find that faulted rocks with higher porosity and lower mechanical strength exhibit more substantial alterations in permeability. This study provides valuable insights into the behavior of faulted carbonate rocks.

How to cite: Akbariforouz, M., Zhao, Q., and Zheng, C.: Understanding the Heterogeneity and Anisotropy of Permeability in Carbonate Rocks Within a Fault Network, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1240, https://doi.org/10.5194/egusphere-egu24-1240, 2024.

Shallow-water carbonates include a variety of heterogeneities such as bed interfaces, laminations, stylolites, and pressure solution seams forming rock multilayers crosscut by high-angle strata-bound fractures. At a larger scale, bed package interfaces and other primary stratigraphic contacts exert a similar control compartmentalizing high-angle faults within discrete sedimentary units. The aforementioned heterogeneities commonly form within the depositional environments, and/or at specific diagenetic conditions under given burial depths. Mechanical compaction, dissolution, cement precipitation, and other physical/chemical processes hence alter the original framework of the carbonates, and contribute to the acquirement of their long lasting mechanical properties. To further investigate this topic, in other words to assess the time-dependent fracture stratigraphy of shallow-water carbonates, this presentation focuses on the influence exerted by tectonics on the mechanical layering of the Mesozoic platform carbonates of southern Italy. By analyzing outcrops lying along the axial zone of the southern Apennines fold-and-thrust belt, and within its forebulge area, published data are discussed altogether to decipher the control exerted by thrusting tectonics on the formation of mechanical interfaces within Lower Jurassic to Upper Cretaceous carbonates. Rocks exposed in the foreland domain include a number of high-angle fractures. These fractures are mainly bounded by bed interfaces, and their spacing values vary proportional to the bed thickness. Such a proportionality is exhibited by both mud- and grain-supported carbonate lithofacies, which show saturated to oversaturate conditions. Differently, carbonates lying in the axial zone of the southern Apennines belt are characterized by values of fracture density and intensity that do not vary proportionally with the bed thickness. In order to investigate the significance of the latter data, detailed microstructural analyses aimed at assessing the timing of pressure solution processes with respect to the diagenetic history and tectonic evolution of the carbonates are considered. Besides the effects of early embrittlement of the carbonate grainstone lithofacies, which occurred due to cement precipitation in phreatic marine environment that prevented the effects of localized dissolution at the grain-to-grain contacts, two main phases of pressure solution characterized the carbonates. The first one took place during Meso-Cenozoic sedimentary burial with formation of wave-like, bed-parallel surfaces. In cat, the continuous burial of the carbonates, down to depths of ca. 1.5 km, promoted the development of solution surfaces along the bed interfaces, and also within the single beds. Small, isolated, wave-like surfaces formed as isolated elements within the single carbonate beds. The second phase occurred during Upper Miocene thrusting tectonics, at depths of ca. 4 km, with formation of seismogram-like surfaces at low-angle to bedding. The latter surfaces consisted of both stylolites and slickolites with sub-vertical teeth, which cut across the pervasive blocky cements of the carbonates, dissolved the pre-existing veins, and formed laterally persistent surfaces throughout the carbonates. Accordingly, the combination of both pure shear (stylolites) and sub-simple shear (slickolites) strain caused formation of new mechanical interfaces in the carbonate beds, and therefore modified the thickness of single mechanical units throughout the Mesozoic carbonates.

How to cite: Agosta, F.: Evolving fracture stratigraphy properties of layered carbonates through time: examples from the southern Apennines fold-and-thrust belt, Italy, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2077, https://doi.org/10.5194/egusphere-egu24-2077, 2024.

Fluid extraction from geological formations for purposes of subsurface utilization leads to pore pressure drop in reservoirs and subsequent compaction and seismicity, especially in porous sandstones. Petrography controls the geomechanical properties of the reservoir, crucial for predicting a reservoir's response to fluid extraction and understanding its lateral variability. This study focuses on the Groningen gas field in the Netherlands, addressing its compaction-induced surface subsidence and seismic events resulting from gas depletion. Core samples were examined to delineate spatial petrographic trends and develop a microscale model of the Groningen gas field. Optical microscopy (OM) and scanning electron microscopy (SEM) were used to determine mineralogical compositions, textural relationships and diagenetic processes. Predominantly constituted of sublitharenites, the sandstones exhibit dolomite and quartz cement as primary authigenic cements. Variations in clay types—kaolinite, illite, and chlorite—were observed, influencing localized pore-filling cementation processes.  Across the field, mineral relations revealed notable trends: depth-related feldspar decrease, correlation between kaolinite and feldspar abundance, and elevated chlorite content towards the northern sector together with the presence of an early quartz cementation phase, which is also observed within aquifer cores. The dissolution of feldspar potentially impacts the structural integrity of the sandstones, while authigenic mineralization appears intricately linked to depositional facies and localized fault-related fluid movements. The timing and extent of these diagenetic processes emerged as pivotal factors dictating sandstone stability within the reservoir. This comprehensive analysis enhances our understanding of Groningen's reservoir heterogeneity, offering critical insights to predict and manage subsurface responses to extraction-induced pressure changes. By providing predictive models, this study facilitates the evaluation of reservoir behavior and aids in mitigating risks associated with compaction-induced subsidence and seismicity.

How to cite: Mulder, S., Bublik, D., and Miocic, J.: Petrographic Heterogeneity and Sandstone Evolution in the Groningen Gas Field: Implications for Reservoir Geomechanics, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2471, https://doi.org/10.5194/egusphere-egu24-2471, 2024.

Just how much do sorting, cementation and compaction control the porosity and permeability of rocks? In this work we start with making extremely accurate porosity and permeability measurements on binary grain mixtures, arriving at good agreement between them and theoretical results. These seemingly simple experiments are difficult to carry out with the degree of precision needed to test the models. We have developed a methodology allowing porosity and permeability to be measured to within ± 4.415% and ± 4.989% (at a flow rate of 5.13 cm³/s) of each value, respectively. The newly developed theoretical framework includes both the interstitiation mixing process and several replacement processes.

A major result of this work is that the theoretical models describing these two processes are independent of grain size and grain shape. The latter of these two findings infers that the models developed in this work are applicable to any shape of grain or type of packing, providing that a representative porosity of each size of grain pack is known independently, either experimentally or theoretically. Experimental validation has shown that the newly developed relationships for porosity described measurements of porosity for near-ideal binary mixtures extremely well, confirming that porosity is always reduced by binary mixing, and that the degree of reduction depends upon the size of the ratio between the two grain sizes.  

Calculation of permeability from the packing model has also been carried out. Six different permeability estimation methods have been used. It was found that the most accurate representations of the experimental permeability were obtained (1) when the exact RGPZ method was used with the porosity mixing models developed in this work, and (2) when the exact RGPZ method was used with the weighted geometric mean to calculate a representative grain size. For mixtures where there is a large difference in grain sizes, permeability (k) varies little for the ranges of mixtures from 28% to 100% of small grains (about 4*k), only increasing significantly as the fraction of small grains falls below 28% to zero (to 2 orders of magnitude in k) because the small grains then only partially occupy the space between the large grains. For mixtures of grains of similar sizes, the situation is remarkably different. The variation in permeability for small grain fractions between 100% and about 28% is much amplified (2 orders of magnitude in k), while the increase in permeability as the fraction of small grains falls from about 28% to zero is similar to the other case, and perhaps slightly less pronounced (about 1.5 orders of magnitude in k). This counterintuitive behaviour is important for the interpretation of how sorting affects permeability, implying a greater spread of permeabilities for rock composed of grains with a small difference in grain size.

How to cite: Luo, M., Glover, P., and Lorinczi, P.: Theory and modelling of the effects of grain sorting, compaction & cementation on porosity and permeability, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3571, https://doi.org/10.5194/egusphere-egu24-3571, 2024.

Multiple episodes of brittle deformation tend to increase the structural complexity of fault zones. This commonly results in the development of juxtaposed and non-coeval distinct Brittle Structural Facies (BSF) formed at different times, depths, and temperature. Indeed, these BSFs are characterized by an irregular distribution of inherited, syn-kinematic, and post-kinematic minerals, whose study provides useful information about the temperature conditions of (de)formation, and the origin of fluids circulating within faults. We combined X-ray diffraction (XRD) analyses and polytype determinations of whole-rock and several grain-size fractions (6-10 μm, 2-6 μm, 0.4-2 μm, 0.1-0.4 μm, and <0.1 μm), with H-isotope data of 76 fault rocks and 6 protoliths from two structurally and well-characterized fault zones with different kinematics: the Carboneras strike-slip fault zone (Betic Cordilleras, SE Spain) and the Kornos-Aghios Ioannis extensional fault zone on the Lemnos Island (North Aegean Trough, Greece). Mineralogical and geochemical data allowed us to (1) reconstruct the distributions of syn/post-kinematic minerals in distinct BSFs, (2) constrain their formation temperature, and (3) unravel the origin of fluids involved during faulting. In the Carboneras fault rocks, we distinguished a protolithic mineralogical assemblage consisting of quartz, carbonates, K/Na-micas (2M₁ polytype), chlorite, kaolinite, and Fe/Ti-oxides, a syn-kinematic assemblage composed of mixed layers chlorite-smectite and illite-smectite (1M_d polytype), and a post-kinematic assemblage made up of smectite, chlorides, and sulphates. The coexistence of randomly (R0), short-range (R1), and long-range (R3) ordered illite-smectite in different BSFs indicates contrasting formation temperatures. Bulk samples generally display δ²H values (V-SMOW) between -15‰ and -60‰ (with a few exceptions at -90‰), while their respective <2μm fractions show δ²H values between -10‰ and -60‰. The combination of mineralogical and geochemical data from the Carboneras fault zone depicts a complex history of multiple brittle events occurring at different temperature conditions, wherein parental fluids of mostly meteoric origin infiltrated into the fault zone and interacted with the host rocks at various degrees and depths. In the fault rocks of Lemnos Island, we identified a host-rock mineralogical assemblage composed of quartz, feldspars, carbonates, K-mica (2M₁ polytype), chlorite, R0 illite-smectite, anatase, and Fe-oxides and hydroxides, a syn-kinematic assemblage made up of R3 illite-smectite (1M_d polytype), and kaolinite, and a post-kinematic assemblage characterized by halite and gypsum. Bulk samples display δ²H values (V-SMOW) between -70‰ and -140‰, while their respective <2μm fractions show δ²H values between -60‰ and -85‰. Such results indicate that Kornos-Aghios Ioannis fault rocks formed during a deformation event with predominantly hydrothermal fluids circulating into the fault zone (>160°C). This multidisciplinary approach represents an innovative point of view for studying fluid circulation, mineral crystallization, and temperature evolution in complex fault zones, and it can be applied to both orogen scale faults and smaller fault systems.

How to cite: Moretto, V., Del Sole, L., Curzi, M., Dallai, L., Vignaroli, G., Viola, G., Balsamo, F., Berio, L. R., Grathoff, G., Warr, L. N., and Aldega, L.: Tracing the origin of parental fluids and paleo-temperature distribution in fault zones: case studies from the Carboneras Fault (Spain) and Lemnos Island (Greece), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4381, https://doi.org/10.5194/egusphere-egu24-4381, 2024.

Deformation may exert a positive or negative impact on rock strength, stiffness and porosity depending on the initial properties of the rock. In porous rocks, deformation bands (DBs) reduce both porosity and permeability, and potentially increase rock strength and stiffness. In low porosity (tight) rocks, deformation occurs via shear and opening-mode fractures, thus enhancing permeability and deteriorating strength and stiffness.

In this study we describe field observations and laboratory analyses to characterize sub-vertical deformation bands in the porous carbonates of the Calcarenite di Gravina Fm. exposed in Matera and Gravina in Puglia, Southern Italy. Matera and Gravina in Puglia are located at the boundary between the Apulian foreland and the foredeep of the Southern Apennines thrust belt. Both study areas consist of an asymmetrical horst structure involving the Cretaceous (Senonian) tight carbonates of the Apulian platform (Calcare di Altamura Fm.) unconformably overlain by Plio-Pleistocene shallow-marine coarse-grained lithic sandstone and grainstone (Calcarenite di Gravina Fm.). The Calcarenite di Gravina Fm. is dominated by pervasive DBs organized into 2 main sets dipping at high angle and striking N-S and NNE-SSW, except in some limited areas where the DBs form a complex network with the presence of a secondary set, striking NW-SE, showing mutual crosscutting relationships. The DBs have a positive relief, due to their relatively higher resistance to erosion, and appear whitish, tabular with some slight undulations, 10’s of meters long in map view, continuous and steeply inclined (from 75° to 90°). These structures have thicknesses from few mm up to a few cm depending on the facies of the calcarenite. At the outcrop scale the DBs don’t seem to accommodate significant shear offsets. A total of 53 oriented samples were collected for thin sectioning, petrophysical, and microstructural analysis. We acquired porosity measurements using a Hg-intrusion porosimeter, which showed that the DBs have an average porosity of 11%, while for the host rock is 38%. 29 Blue-impregnated thin sections of host rocks and DBs were analysed by standard microscopy, cathodoluminescence (CL), and a scanning electron microscope (SEM). The analysis of the CL images shows that the clast size distributions are similar in the DBs and host rock, and they are not crushed or fractured. Furthermore, while in the host rock the clasts are randomly oriented, in the DBs they are iso-oriented with the long axis describing low angles to the band.

Our work shows that these DBs are characterized by the absence of grain size reduction with a strong preferred orientation of long axes without significant fragmentation. These data may indicate that DBs formed in high porosity conditions, at very shallow depths. Moreover, given the strong difference between the petrophysical properties of the host rock and DBs and the apparent abundance and continuity of the DBs, they would provide a significant anisotropy in the flow pathways of the calcarenite.

How to cite: Freda, G., Mittempergher, S., Balsamo, F., Pizzati, M., Di Cuia, R., and Ricciato, A.: Microstructural characterization of deformation bands in shallow porous carbonates of Apulian Platform, Southern Italy, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5693, https://doi.org/10.5194/egusphere-egu24-5693, 2024.

Here we present the results of surface and subsurface (caves) geological mapping in the Lefka Ori Massif. The structural field data collection and the stratigraphy analysis were supported by the IGCP-715 project in collaboration of the Speleological Association of Crete (SPOK) and the Gourgouthakas 2023 exploration team. The overall structure of the Lefka Ori Massif consists of a roughly E-W trending open anticline. Regional uplift is related to compressional and extensional events during the Late Oligocene to present. The primary lithologies of the Lefka Ori Massif are comprised of the Plattenkalk Unit and the Trypali Unit. The Plattenkalk Unit contains (1) black brecciated karstified marbles/dolostones with interbedded stromatolites (Triassic), (2) white to gray marbles/dolostones (Triassic-Lias), and (3) thin bedded marbles (Dogger-Oligocene). Black dolostones and brecciated marbles (with stromatolite fragments) are building up the overlying Trypali Unit (Late Triassic-Lias).

The results from the structural analysis in the Lefka Ori Massif highlight 3 sets of faults, which are of (A) E-W striking thrust faults, (B) dextral NNW-SSE strike-slip faults and (C) E-W striking steep dip-slip faults. A regional thrust (hereafter "Pachnes Thrust") has a vertical displacement of 1000 m and duplicates the Plattenkalk Unit stratigraphy (sections 1 and 2) in the Lefka Ori Massif. The hanging wall consists of Plattenkalk units 1 and 2, while the footwall comprises the Plattenkalk unit 3. The Pachnes Thrust has an apparent horizontal displacement of approximately 12 km. The cross-cutting relationship of the thrust with the strike-slip faults indicate that the faults are coeval. The steep dip-slip faults are still of an unknown relative age but certainly most of them appear as normal, younger faults. The Pachnes Thrust is folded around a roughly E-W trending open fold axis. The Late Oligocene green and red, calcschist sediments in the footwall of the Pachnes Thrust indicates the maximum age of thrusting.

The third deepest cave in Lefka Ori Massif (Sternes Cave, depth -616 m) contains a system of galleries reaching a total length of 5.6 km. This karst system of subhorizontal galleries is well explained by the Pachnes Thrust as it parallels the thrust. Likewise, the Pachnes Thrust has been delineated in the deepest cave (Gourgouthakas) at a depth of -700 m. In addition, the accepted hydrology in the Lefka Ori Massif is defining an upper fast flowing reservoir and a lower slow flowing reservoir. The physical model for these two reservoirs is explained by the hanging wall 1 and 2 lithologies (fast flowing reservoir) and the footwall lithology 3 (slow flowing reservoir) of the Pachnes Thrust.

The Pachnes Thrust in Lefka Ori Massif probably correlates with the recorded southward thrusting and nappe stacking due to convergence between the African and Eurasian plates (Oligocene-Early Miocene). The deformed Pachnes Thrust plane is coeval with the southward thrusting, due to the uplift and exhumation phases during extension described in the literature. The present findings are offering new evidence of the tectonic evolution and the exhumation of the high pressure metamorphosed rocks in the Hellenic Subduction Channel.

How to cite: Moraetis, D., Leontaritis, A., Scharf, A., Fassoulas, C., Zacharias, S., Pavlopoulos, K., Mattern, F., Alnaqbi, A., Yu, X., Pennos, C., Adamopoulos, K., Hamdan, H., and Nikolaidis, N. P.: A new geological model for the Lefka Ori Massif in the accretionary wedge of the Hellenic Subduction Zone. , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9013, https://doi.org/10.5194/egusphere-egu24-9013, 2024.

Understanding the origin and distribution of damage within carbonate-hosted fault zones is crucial, yet it remains a complex challenge, which hampers the overall assessment of their mechanical and hydraulic structure. In carbonate-hosted fault zones, shattered to intensely brecciated non-cohesive rocks have been reported. Although their origin has been related to the propagation of multiple seismic ruptures, great uncertainties persist regarding their interpretation and distribution.

The NW-SE striking, approximately 15 km long Roccapreturo Fault, in the central Apennines of Italy, is an intriguing case study where non-cohesive fault rock domains occur within its footwall damage zone. These domains elongate in a NE-SW direction for ~200 meters from the main slip surface.

We employed a multiscale approach to better understand the distribution and origin of the non-cohesive fault rocks. The fault geometry and throw distribution along the main fault segments were characterized through fault-perpendicular geological cross-sections. Virtual outcrop models of key exposures, located in and around an abandoned quarry, were constructed using Structure from Motion-Multiview Stereo photogrammetry. These models utilized photos taken with a Mavic Mini 2 drone. The interpretation of virtual outcrop models, combined with classical fieldwork, allowed us to map the damage and minor fault strands.

The Roccapreturo Fault displaces Cretaceous rocks originally deposited in various depositional environments. Along its strike, from NW to SE, the fault intersects rocks from internal or restricted carbonate platform, margin, and proximal slope to basin depositional environments. Notably, non-cohesive fault rocks are exposed between the margin and proximal slope rocks. This area coincides with the maximum throw of the fault, which is ca. 600 meters, and with the intersection with a system of pre-existing NE-SW-striking steeply dipping faults.

At the outcrop scale, faults exhibit two preferred orientations, parallel and perpendicular to the main slip surfaces of the Roccapreturo Fault, respectively. The former ones show predominant dip-slip kinematics, while the latter ones show both dip-slip and strike-slip kinematics.

We interpret the distribution of non-cohesive fault rocks along the Roccapreturo Fault as influenced by its intersection with the NE-SW fault system, where most of the slip accumulated. Accordingly, the pre-existing NE-SW faults accommodated transtensional slip during latest extensional deformation and coeval rock exhumation from depth. The transition of the Cretaceous depositional environments, which was accommodated by the NE-SW-striking faults, therefore highlights the pivotal role of pre-existing anisotropies in dictating the distribution of damage, particularly of non-cohesive fault rocks, in carbonate hosted faults.

How to cite: Mercuri, M., Agosta, F., Fondriest, M., Smeraglia, L., Billi, A., Tavani, S., and Carminati, E.: Influence of pre-existing faults on damage distribution in carbonate fault zones: the case study of the Roccapreturo Fault, central Apennines, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10510, https://doi.org/10.5194/egusphere-egu24-10510, 2024.

In porous sandstone, fluids are guided by major features such as faults or lithologic discontinuities. At the local scale, deformation bands are common structures to baffle fluid flow in such rock. Potential flow-hindering structures less frequently reported of are veins in porous sandstone (Skurtveit et al., 2015, as a rare case), which may root in the circumstance that they do not appear very often and/or have simply been overlooked. 

We here present a case where calcite veins formed in the highly porous (up to 25 % porosity) and partially poorly lithified eaolian Lower Cretaceous Twyfelfontein Formation in NW Namibia. This sandstone was buried by the extrusion of voluminous Paraná-Etendeka flood basalts at around 130 Ma and was since then subject to exhumation. Calcite veins occur in roughly half of the visited outcrops of the Twyfelfontein Formation and their dominant parallel trend to the continental margin suggests a tectonic origin. As the host rock is void of carbonate framework material or cement, the veins must have formed through advective fluid circulation. An external source of the calcium may possibly be the alteration of overlying and intercalated basalt. The veins exhibit a remarkable multitude of textures ranging from blocky, colloform, to microcrystalline calcite generations, that have partially experienced brecciation. This argues for highly variable formation conditions, potentially spanning from normal fluid advection to boiling and injection (c.f., Moncada et al., 2012; Salomon et al., 2021). 

Preliminary clumped isotope data of the veins indicate a low temperature formation in the range of 19-61°C, which suggests overall shallow burial conditions. This is in agreement with the diagenetic paragenesis of the rock arguing for late stage vein formation, i.e. during exhumation of the rock. Upcoming U/Pb calcite dating is expected to bring greater clarity on this regard. A halo in the host rock surrounding the veins became calcite cemented due to the growth of calcite from the fractures into the sandstone body. This appearance demonstrates the following evolution: (1) fracturing of the sandstone, which enhances advective fluid flow in the rock body; (2) vein precipitation and near-vein host-rock cementation; and consequently (3) reduction of permeability in the fracture and adjacent wall rock. 

Due to their potential of forming effective barriers to fluid flow, we stress that their formation needs to be understood in greater detail. The variable vein textures indicate differing formation conditions, which sets the base for a more common occurrence of calcite veins in porous uncemented sandstone. 

References:

Moncada, D., et al. (2012). Journal of Geochemical Exploration 114, 20-35, doi:10.1016/j.gexplo.2011.12.001.

Salomon, E., et al. (2021). Journal of Structural Geology 153, 104463. doi:10.1016/j.jsg.2021.104463.

Skurtveit, E., et al., (2015). Petroleum Geoscience 21, 3-16, doi:10.1144/petgeo2014-031.

How to cite: Salomon, E., Meckler, A. N., and Stollhofen, H.: Calcite veins as local fluid flow barriers in reservoir rock? The odd occurrence of veins in highly porous aeolian sandstone in Namibia, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16378, https://doi.org/10.5194/egusphere-egu24-16378, 2024.

The shortening of sediments in accretionary prisms is accomplished by localized faulting as well as non-localized deformation. While faulting is often easily recognized from seismic sections, accessing the amount and extent of non-localized deformation is rather challenging. In order to address this challenge, we explore samples from the active accretionary prism offshore Gisbourne, NZ at the Hikurangi margin which contains accreted sediments of Pliocene to recent age. Drilling at Site U1318F of IODP Expedition 375 recovered non- to semi-lithified sediments from a major accretionary fault, the Papaku Fault, including its hanging wall and footwall. The crystallographic preferred orientation (CPO) of the clay minerals is a measure for their alignment and was determined in 66 sediment samples from the drill core (250-500 mbsf) using high energy X-rays. The results show that the CPO strength of the clay mineral basal planes (00l) is in general weak and no depth-related trend can be observed. In the hanging wall of the Papaku Fault, (00l) pole figures have non-rotationally symmetric, unimodal density distributions displaying incomplete girdles. In the footwall, most (00l) pole figures exhibit unimodal, rotationally symmetric to weak girdle density distributions, with most maxima pointing parallel or subparallel to the drill core axis. Fault zone samples also exhibit rotationally symmetric, unimodal (00l) distributions, with maxima perpendicular to the fault plane.

We assume that pre-shortening and pre-faulting, sediments had a weak initial CPO related to sedimentation and compaction with a rotationally symmetric, unimodal (00l) distribution. The girdle shape of the distribution in the hangingwall and to a minor extent in the footwall is introduced by non-localized deformation which results in grain-scale folding. Accordingly, diffuse shortening was larger in the present-day hanging wall than in the present-day footwall. Furthermore, we interpret the CPO in the Papaku fault itself to be a result of sediment shearing, overprinting any pre-existing CPO. The position of the Papaku fault is compatible with fault initiation where diffuse shortening was unable to propagate sufficiently towards the foreland.

While our results also confirm existing tectonic models from this part of the Hikurangi margin, more importantly they demonstrate implications for strain distribution in fault and thrust systems as well as the usefulness of clay mineral CPO for unravelling deformation and tectonic processes in accretionary prism sediments.

How to cite: Kühn, R., Kilian, R., and Stipp, M.: Crystallographic preferred orientation of clay minerals in sediments from the Hikurangi accretionary prism offshore New Zealand, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17144, https://doi.org/10.5194/egusphere-egu24-17144, 2024.

Networks of deformation bands in porous granular rocks represents potential low-permeable baffles to fluid flow in subsurface reservoirs. However, little work addresses the network properties of such networks, like spatial intensity, network connectivity and network geometry. Motivated by this, we here present an investigation of two-dimensional, horizontal exposures of deformation band networks within the Jurassic Entrada Sandstone in the San Rafael Desert (Utah). We analyse the geometry and topology (i.e. a network represented as nodes and branches) of the studied networks to: 1) characterise deformation band orientation, connectivity and areal intensity; 2) assess spatial topological variability; 3) elucidate large scale variation across the study area; 4) evaluate effective network permeabilities. Effective deformation band network permeability is calculated by incorporating a topological measure of network connectivity into the permeability calculations. Deformation band networks show distinct topological signatures, typically being dominated by Y-nodes, and IC- and CC-branches. Depending on the orientation of deformation bands and numbers of different sets of deformation bands within each studied network, both topology and areal intensity vary. Low proportion of isolated II-branches reflects the evolution of deformation bands through bifurcation and abutment, creating Y-nodes, to form interconnected networks. We document great spatial variability I network connectivity and topology within individual networks. Similarly, the effective permeability within well-connected (parts of) the studied deformation band networks (>1.5 connections per branch) significantly reduce effective permeabilities, whereas areas within the networks with low connectivity offer higher-permeable pathways for tortuous fluid flow.

How to cite: Heggernes, H., Rotevatn, A., Demurtas, M., Nixon, C. W., and Fossen, H.: Spatial variability in topology, connectivity and permeability within deformation band networks, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17174, https://doi.org/10.5194/egusphere-egu24-17174, 2024.

The study of deformation, fluid flow and diagenesis within porous granular rocks includes processes spanning from the sub-millimetric pore and grain scale, to 10s to 100s km-long fault systems that delineate entire sedimentary basins and rift systems. Based on outcrop examples globally, we here show how selective, structurally controlled diagenesis is manifested across scales, discuss some of the key aspects of the governing processes involved and how such understanding may be used in attempts to subsurface predictions. Grain scale observations are focused on deformation within siliciclastic, carbonates and volcaniclastic rocks, allowing us to investigate the role of material properties in controlling how deformation is localized and accommodated. We further discuss how grain-scale deformation affects permeability, fluid flow and structurally controlled fluid-rock interaction. At the opposite end of the spectrum, we discuss the relations between deformation, fluid flow and diagenesis at the scale of basin bounding fault systems in rift basins. Finally, we address the significance of understanding structures as elements of structural networks, and how network properties may hold the potential for a more quantitative understanding of structurally controlled fluid flow.

How to cite: Rotevatn, A., Dimmen, V., Heggernes, H., Demurtas, M., Fossen, H., Osagiede, E., and Cavailhes, T.: Deformation, fluid flow and diagenesis in deformed granular rocks across scales, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20887, https://doi.org/10.5194/egusphere-egu24-20887, 2024.

How to cite: Dobe, R., Giuntoli, F., and Vitale Brovarone, A.: Fractures versus flow: How variations in carbonate composition control the rheology of altered subducting rocks , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-639, https://doi.org/10.5194/egusphere-egu24-639, 2024.

Frictional motion is mediated by rapidly propagating ruptures, akin to shear cracks, that detach the ensemble of contacts that form the interface between
contacting bodies. While fracture mechanics describe the rapid motion of these singular objects, the nucleation process that creates them is not currently understood. By extending fracture mechanics to explicitly incorporate finite interface widths, we fully describe the nucleation process. We show, experimentally and theoretically, that slow steady creep ensues at a stress threshold. Moreover, as creeping patches approach the interface width, a topological transition occurs where they smoothly transition to rapid fracture. This new picture of the nucleation dynamics of fracture (and friction) is directly relevant to earthquake nucleation dynamics and the transition from aseismic to seismic rupture in natural faults.

How to cite: Fineberg, J.: The nucleation of frictional ruptures, theory and experiment, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3417, https://doi.org/10.5194/egusphere-egu24-3417, 2024.

To assess the seismogenic potential of fault zones it is crucial to understand fluid-rock interactions in these zones, because alteration affects the fault strength and stability, as well as the deformation mechanisms.

The San Andreas fault (SAF) system is known for infrequent large magnitude (M≥7) earthquakes, whereas some segments lack such strong seismic events [1]. Here, strain is largely accommodated by creep motion. Aseismic creep can be enhanced by the presence of fluids, which may additionally drive mineral reactions. For example, fluid composition and magnesite deposits in the SAF segment between San Juan Bautista and Parkfield suggests carbonation due to infiltration of CO₂-bearing fluids into the fault [2]. Carbonation of ultramafic rocks leads to the formation of talc, which is known to be frictionally weak and promotes creep when wet [3]. However, our thermodynamic fluid-infiltration calculations show that carbonation will not produce pure talc but lizardite-talc-magnesite (LTM) and talc-magnesite rocks (soapstone) and, with increasing extent of reactive fluid flow, talc-magnesite-quartz (TMQ) and magnesite-quartz rocks (listvenite). The strength and seismogenic potential of serpentinite fault zones undergoing carbonation thus may change dynamically as the mineral proportions and assemblages change, but the respective frictional behaviour of these assemblages is unknown.

We performed rotary-shear experiments on gouge layers with compositions ranging from lizardite-serpentinite to LTM, soapstone, TMQ and listvenite at pressure, temperature and pore fluid pressures corresponding to a depth of about 10 km (300 °C, 250 MPa normal stress and 100 MPa pore pressure). We measured the frictional strength within the velocity range of 0.002 µm/s to 10 µm/s.

Our data show that lizardite gouges are relatively strong and slightly velocity-weakening. The friction coefficient dropped from 0.45 at 0.002 µm/s to 0.42 at 10 µm/s. A similar velocity-dependence is observed for soapstone gouges, although at lower absolute friction coefficients of 0.3 to 0.28. Interestingly, listvenite gouges show the opposite behavior, with friction coefficients increasing from 0.25 at 0.002 µm/s to 0.48 at 10 µm/s. Stick-slips were only observed in serpentinite and soapstone gouges at low velocities. Increasing velocities and progressing carbonation causes stable slip behavior. Microtextural observations indicate strong grain-size reduction and basal cleavage in serpentinite gouges. On the contrary, soapstone and listvenite gouges show a fine-grained magnesite matrix surrounding the silicates.

Our results suggest that serpentinized fault zones have the potential to nucleate unstable slip. The results further confirm the strong weakening effect of carbonation. CO₂-fluid-rock interaction in ultramafic fault gouges may effectively suppress the nucleation of earthquakes. Since also listvenite gouges deformed aseismic and are found to be frictionally weak at low velocities, we suggest that besides talc also magnesite plays an important role in the deformation behavior of carbonated ultramafic fault zones.

[1] Jolivet et al. 2015. Geophys. Res. Lett. doi:10.1002/2014GL062222.

[2] Klein et al. 2022. Geophys. Res. Lett. doi:10.1029/2022GL099185.

[3] Moore et al. 2008. Tectonophysics. doi:10.1016/j.tecto.2007.11.039

Funding

LE: NWO (VI.Vidi.193.030)

M.D.M: Junta de Andalucía (Postdoc_21_00791) and MCIU, Spain (PID2022-136471N-B-C22)

How to cite: Eberhard, L., Menzel, M. D., Niemeijer, A. R., and Plümper, O.: Fault weakening due to CO2-fluid-rock interaction – evidence from deformation experiments of carbonated serpentinites, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4370, https://doi.org/10.5194/egusphere-egu24-4370, 2024.

Variations in pore fluid pressure modulate effective normal stresses along fault zones and the subducting interface. Fluid availability is controlled by the decomposition of hydrous mineral phases and the subsequent rate of drainage. Geophysical observations suggest that the plate interface is a fluid-enriched region under near-lithostatic pore fluid pressure that may result in slow slip events (SSE) and non-volcanic tremor (NVT). The potential for fluid redistribution depends on dynamic changes of the porosity and permeability of the host rock as a function of solid-bound fluid volume change and the total system volume change during dehydration. Understanding the mechanisms involved with the evolution of porosity and permeability below the seismic zone is critical to gain insight into the formation of fluid networks and their rheological implications.

Here we present a petrological and mechanical analysis of the evolution of a suite of eclogite-facies veins from an archetypal HP-LT terrain: the Eclogite Zone, Eastern Alps. We define two dominant compositional types of mafic eclogite: banded and metagabbroic, respectively. Prograde metamorphic evolutions are similar for the two types of eclogites and comprise garnet core growth at 2.1 ± 0.25 GPa, 585 ± 15°C and rim equilibration at 2.6 ± 0.2 kbar, and 630 ± 10 °C. Contemporaneously to prograde garnet growth, the mafic eclogites underwent dehydration via the breakdown of several volumetrically significant hydrous phases: lawsonite, Na-amphibole (glaucophane), and epidote. The decomposition of lawsonite and glaucophane released up to 8 wt. % H₂O, resulting in the formation of a transient fluid filled porosity of ∼ 15 vol. %.

Phase equilibria calculations serve as a framework to constrain a mechanical model explaining the formation of both tensile fractures (type I) and vein segregates (type II) within the brittle-ductile transition zone. We propose a petrological-mechanical model for the formation of Type I tensile veins associated with periods of rapid dehydration and Type II dilatant structures in which rock deformation is outpaced by the reduction in pore fluid pressure, leading to a decrease in silica solubility and the precipitation of high-pressure mineral phases. Finally, this suggests that the rate of dehydration during the blueschist-eclogite transition plays a significant role in determining the dominant mode of deformation possibly affecting the fluid storage capacity of the subducting interface.

How to cite: Strobl, L. and Smye, A.: Pore-fluid pressure evolution across the blueschist-to-eclogite-facies transition:  constraints from the Eclogite Zone (Eastern Alps), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4680, https://doi.org/10.5194/egusphere-egu24-4680, 2024.

Characterizing deep subduction dynamics is crucial for understanding processes of high-pressure-rock exhumation, fluid flow, seismicity and transient slip events. Metamorphic phase transformations at the blueschist-to-eclogite transition induce important rheological changes, commonly transitioning from a more brittle to a mixed brittle-viscous plate interface rheology. This shift may promote slip transients such as slow slip and tremor (SST) observed in modern subduction zones. Geophysical and geologic data as well as numerical models suggest that slow slip is likely accommodated along weak, fluid-rich shear zones and that accompanying tremor may represent km-scale brittle asperities embedded within localized slip zones. Here we use the geologic record exposed on Syros Island (Greece) to investigate the relationships between strain localization and fluid-rock interactions along the deep megathrust, and explore their implications for SST.

We used high-resolution drone surveys, along with microstructural, geochemical, and petrologic data, to examine a blueschist-to-eclogite facies subduction shear zone in the Kampos Belt near Grizzas locality on northern Syros. The estimated P-T conditions are comparable to the SST zone along active warm subduction margins such as Cascadia and Central Chile. Our approach involved mapping strain and lithologies, constructing a 3D geological model, and performing detailed analyses of localized shear zones and metasomatic rocks.

At the hectometre-scale, the Grizzas locality exposes a stack of progressively underplated oceanic and metasedimentary rocks. Individual slices include brittlely deformed metagabbros up to 200 m-thick, weakly-strained to undeformed igneous breccias up to 30 m-thick, and foliated quartz-mica schists. These slices are repeated along five localized shear zones composed of chlorite-tremolite and glaucophane schists that are less than 10 m-thick. Fine-scale characterization of one of these shear zones reveal several discrete intercalations of blueschists/glaucophanites, tremolite-chlorite schists and metasediments. Microstructural and petrologic analyses suggest that blueschist/glaucophanite layers formed through the transformation of a gabbro/blueschist breccia precursor, likely induced by along-dip fluid influx. This metasomatic process extensively replaced the precursor gabbro fabric with nearly pure glaucophane and also enhanced the development of high-strain zones. Geochemical analyses indicate the formation of tremolite-chlorite (+/- talc) schists through chemical exchange between metamafic and metaultramafic rocks or by the interaction with serpentinite-derived fluids. This is supported by the presence of partially-digested metagabbro pods which contain garnet and chlorite with anomalously high Cr₂O₃ contents (up to 1.2 and 2.1 Wt%, respectively) as well as omphacitites associated with glaucophane-phengite veins and glaucophane-bearing veins crosscutting the chlorite schists.

We suggest that metasomatism triggered localized deformation around gabbro blocks and permitted repeated down-slicing and underplating of subducting oceanic material on the deep subduction interface. The metasomatism likely exploited precursory features such as lithological contacts, fractures, and/or fabric heterogeneity, to transiently increase permeability and allow further fluid ingress eventually resulting in the development of major shear zones. The degree of localization in these major shear zones and the concentration of foliated phyllosilicates within them mean they may have been capable of hosting slow slip (to be explored further), and the up-to-km-scale of brittlely deformed metagabbro blocks embedded between the shear zones are compatible with tremor sources.

How to cite: Munoz, J., Behr, W., Hildebrant, D., and Tokle, L.: Feedbacks between metasomatism, rheological heterogeneities and strain localization in deep subduction interface shear zones, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4702, https://doi.org/10.5194/egusphere-egu24-4702, 2024.

In anhydrous, strong, and metastable lower-crustal rocks, coseismic fracturing is an effective mechanism for creating pathways for fluids to infiltrate and interact with the host rock, ultimately resulting in metamorphism and rheological weakening. In this study, we have characterized the damage zone flanking a lower-crustal pseudotachylyte (solidified frictional melt produced during seismic slip) to understand the fracture generating and fluid-assisted healing processes operating during and immediately after a seismic event.

The Nusfjord East shear zone (Lofoten, Norway) is a network of coeval pseudotachylytes and mylonitized pseudotachylytes that formed at lower-crustal conditions within anhydrous anorthosites. We present a micro- and nanostructural analysis focusing on plagioclase in the damage zone of a pseudotachylyte using focused ion beam (FIB) prepared scanning transmission electron microscopy (STEM), Fourier Transform Infrared (FTIR) Spectroscopy, electron backscatter diffraction (EBSD) analysis, electron microprobe analysis (EMPA), and SEM-cathodoluminescence (CL) imaging.

The damage zone of the host anorthosite is characterized by a network of comminuted primary plagioclase (plagioclase₁) grains with minimal offset. Very fine (<15 mm) plagioclase₁ grains and secondary plagioclase neoblasts (plagioclase₂), differentiated from each other by SEM-CL and EBSD observations, fill the fractures along with a minor amount of K-feldspar. Plagioclase₁ and plagioclase₂ have the same major element compositions (average: An₅₂) and are not zoned aside from a small increase in anorthite along the grain boundaries. Away from the pseudotachylyte margin, plagioclase₂ grains filling the fractures show a host-controlled crystallographic preferred orientation (CPO) governed by plagioclase₁ grains. With decreasing distance toward the vein margin, the CPO is weakened as a result of minor amount of solid-state deformation by grain-boundary sliding after the coseismic event. Plagioclase₁ grains often exhibit a diffuse CL intensity zonation from bright grain cores to a dark grey in healed cracks, while plagioclase₂ have a uniform mid-tone grey CL intensity with dark grain boundaries. CL zonation in the plagioclase₁ does not correlate with EMPA major element maps nor EBSD misorientation maps. TEM foils across the dark CL grain boundaries reveal microfractures filled with nanograins of plagioclase₂ containing few dislocations. FTIR maps transecting the thin section do show the presence of molecular water trapped along fractured plagioclase₁ grain boundary regions. At the thin section scale, there is no measurable gradient of molecular water with increasing or decreasing distance toward the pseudotachylyte margin.

In summary, these observations suggest that (1) fracturing was in accordance to a pulverization-style fragmentation process, (2) water is from a local source; presumably coseismic fracturing released fluid inclusions enclosed within plagioclase₁ and the frictional heating caused the melting of primary biotite, and (3) the little amount of molecular water freely available did not diffuse within the plagioclase grains, and did not promote hydrolytic weakening in the damage zone. Strain localization is primarily determined by repeated occurrences of extreme grain-size reduction and phase mixing, in addition to some amount of fluid wetting the grain boundaries. Therefore the “wet and weak” structure, preferential for further ductile deformation, is often the pseudotachylyte vein when present and not the surrounding damage zone.

How to cite: Michalchuk, S. P., Gies, N. B., Lüder, M., Ohl, M., Dunkel, K., Hermann, J., Plümper, O., and Menegon, L.: Deformation and healing processes in the damage zone of a lower-crustal seismogenic fault, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5618, https://doi.org/10.5194/egusphere-egu24-5618, 2024.

A longstanding question in geoscience concerns whether earthquakes show a preparatory process and precursory seismic activity. Some models hold that in the intermediate-term (from months to years), seismicity and/or aseismic transients in fault slip and in other fault properties occur. During the last decades, improvements in earthquake monitoring, the integration of geodesy capturing slow deformation, and the incorporation of novel data analysis techniques including machine learning and artificial intelligence have improved our ability to better discern how earthquake sequences evolve before a mainshock. The few available observations of transient deformation preceding well-recorded earthquake sequences show a high variability, thus our potential for improving earthquake forecasting is still limited. The body of knowledge available from mechanical models, numerical simulations, experimental work and field observations highlighted a wealth of structural, tectonic and boundary conditions which may control the dynamics of earthquake sequences. These suggest that several processes can affect earthquake preparation on different temporal and spatial scales, ultimately yielding highly varying transient observations prior to mainshocks. These observations also highlight that existing theoretical and conceptual models of the preparation/nucleation process may not fully capture the governing physics.

We analyzed seismicity transients prior to the occurrence of the 2023, M_W 7.8 Kahramanmaraş/Türkiye earthquake. We identified seismic precursory activity composed of a handful of isolated spatio-temporal clusters occurring in a complex fault network within 65 km of the future earthquake epicenter. Some of these clusters contributed to acceleration of seismicity rates in an area surrounding the future mainshock and starting ca. 8 months before the event. Within that area, we also observed a decrease in Gutenberg-Richter b-values. Comparable seismic transients were not observed in the region at least since 2014. The complex preparatory process differs significantly from the cascade of close (<200 m) foreshocks observed before the 1999 M_W 7.6 Izmit/Türkiye earthquake rupturing a mature fault segment. This indicates that fault structure and heterogeneity expressed as roughness or segmentation exert a strong control on deformation transients before an earthquake. This bears strong similarities with laboratory studies on faults with varying roughness. Trends of seismic preparatory attributes observable in the field follow those documented in both laboratory stick-slip tests and numerical models of heterogeneous earthquake rupture affecting multiple fault segments. In the lab, rough faults before stick-slip tend to display prolonged phases of precursory slip including an interplay of (dominating) slow transients combined with high-frequency seismic deformation in stark contrast to smooth faults.

How to cite: Martínez-Garzón, P., Kwiatek, G., Poli, P., Dresen, G., and Bohnhoff, M.: Transient deformation leading to earthquakes: bridging observations from the lab and the field , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5807, https://doi.org/10.5194/egusphere-egu24-5807, 2024.

In scientific literature, the seismogenic zone is defined as the portion of the Earth's upper crust where most hypocenters are located. According to seismological data collected in different geodynamic settings and under different kinematics, the depth interval of the unstable seismogenic zone is typically comprised between 5 and 35 km. However, worldwide earthquake distribution shows extensive occurrence of shallow seismicity with hypocentral depths < 5 km, shallower than the unstable seismogenic zone’s upper boundary. Such shallow seismic sources represent potential additional threats and deserve to be thoroughly investigated and included in current seismic hazard evaluations.

To shed light on this subject, we studied a Pleistocene-age fault system which affects poorly consolidated deltaic, sandstone-dominated sequence composing the Late Pliocene-Pleistocene infilling of the Crotone forearc Basin, in South Italy. We focused on an extensional fault zone exposed along the walls of the Vitravo Creek Canyon, displaying a maximum displacement of ~50 m, with a sharp master fault surface separating the fault blocks. The footwall block is composed of an 8-10 m-wide damage zone with extensive occurrence of deformation bands and subsidiary faults. Towards the master fault, a 1-1.5 m-wide mixing zone is located, characterized by tectonic mixing of sandstone strata with different textural features due to the presence of high-displacement boundary faults. Eventually, the fault core is composed of ~1 m-wide, tightly cemented, cataclastic volume with subsidiary slip surfaces and deformation bands. The hanging wall damage zone shows a wealth of thin deformation bands with diminishing frequency moving away from the master fault. The master fault, where most of the displacement is accommodated, is decorated with a 1-2 cm-thick dark gouge layer. The dark gouge can be traced along the entire fault exposure and maintains a straight pattern parallel to the master fault. Locally it appears to have been injected into the fractures affecting the underneath calcite-cemented fault core. Microstructural analysis allows to document a severe and asymmetric cataclastic grain size reduction, with the footwall side of the dark gouge being more comminuted than the hanging wall side. Grain size analysis reveals a strong mechanical comminution of particles in the 70-500 µm size interval. XRD analysis conducted on the < 2 µm grain-size fraction of the gouge layer displays short-ordered illite-smectite mixed layers which support deformation temperatures of 100-120°C. Conversely, XRD analysis performed on clay fraction of the fault core, at few cm distance from the dark gouge layer, indicates temperatures < 50°C, consistent with the expected shallow burial conditions (< 800 m). We link the localized temperature increase within the dark gouge with frictional heating during coseismic deformation. Combining the microstructural, grain size and mineralogical data could facilitate the study of coseismic deformation affecting high-porosity granular materials at near surface conditions. Such multidisciplinary study could be useful to enhance the earthquake risk and hazard evaluation in seismically active geodynamic settings.

How to cite: Pizzati, M., Torabi, A., Aldega, L., Cavozzi, C., Storti, F., and Balsamo, F.: Pleistocene near-surface earthquake events recorded in high-porosity fluvial sandstone sequence (Crotone forearc Basin, Italy), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6361, https://doi.org/10.5194/egusphere-egu24-6361, 2024.

An abnormal rapid accumulation of volcaniclastics is expected in sedimentary basins around submarine volcanoes. This phenomenon makes the sedimentary basin unstable because the drastic increase in overburden leads to generation of excess pore fluid pressure which prevent consolidation of sediments. Therefore, consolidation state of the sediments would be a crucial information for assessing the slope instability around volcanos.

IODP Expedition 398 cored marine sediments in the Christiana, Santorini, and Kolumbo (CSK) volcanic field in the Aegean Sea of Greece. In this study, we performed consolidation tests on mudstones and calcareous oozes just below the thick volcaniclastics in three basins (Anafi, Anydros, and Christiana Basin) oriented roughly NE-SW including Santorini caldera. Consolidation trends (void ratio vs. applied stress) show clear yield stress which indicate maximum consolidation stress of sediments. Some of ooze-dominated mudstones show the effects of cementation in their consolidation trends.

In the IODP site U1590 (Anydros Basin) and U1592 (Anafi Basin), consolidation yield stress of sediments was ~2 MPa lower than the overburden. It implies that the excess pore fluid pressure generates in the sediments and prevents the normal consolidation. The Anydros and Anafi basins represented underconsolidation state at 300-400 mbsf and ~300 mbsf, respectively. Both underconsolidated intervals are covered by >200 m thick volcaniclastics derived from the Santorini and the Kolumbo volcanos. Therefore, rapid sediment-supply (0.8-1.0 m/ky) from the submarine volcanos apparently makes the surrounding sedimentary basins unstable.

On the other hand, IODP site U1591 (i.e., Christiana Basin) and U1599 (i.e., Anafi Basin) represents the normal-consolidation state and the consolidation yield stress balances the overburden. There is relatively thin cover of volcaniclastics (~100 m) above the non-volcanic sediments and the sedimentation rate is moderate (0.1-0.4 m/ky).

In this presentation, we are going to compare the consolidation characteristics of three basins and discuss their spatial-temporal variation in relation to the sedimentation rate and physical properties of sediments.

How to cite: Yoshimoto, T., Manga, M., Beethe, S., McIntosh, I., Woodhouse, A., Chiyonobu, S., Koukousioura, O., Druitt, T., Kutterolf, S., and Ronge, T. and the IODP Exp. 398 Scientists: Consolidation characteristics of offshore sediments in the Christiana, Santorini, and Kolumbo volcanic field, Greece (IODP Expedition 398), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6392, https://doi.org/10.5194/egusphere-egu24-6392, 2024.

Fluids are generally thought to assume a key role in controlling fast and slow earthquakes, not only because they lower the effective stress, but also because they act as catalysers of mineral reactions, moving chemicals in the rock mass. Serpentinites are particularly prone to carbonation reactions, which cause bulk-rock and volume change. The feedback between CO₂ ingress in serpentinites, H₂O-release, and tectonic appears to be able to sustain cycles of fluid pressure build-up and stress release that may be compatible with slow slip and tremor. However, the carbonation of pure serpentine (Mg₃Si₂O₅(OH)₄) should run to completion over geologic time scales, bearing the question if carbonation can sustain slow earthquakes in the long term at the subduction interface. On the other hand, the presence of Al, which does not enter the structure of talc, should slow down carbonation reactions and the products of serpentinite carbonation, allowing the process to be sustainable over long time scales.

We, therefore, investigated natural samples of sheared carbonated serpentinite from a fossil shear zone in the Sanbagawa metamorphic belt exhumed from ~ 35–45 km and ~ 450–550 °C, corresponding to the present-day conditions of the source region of deep episodic tremor and slow slip in the nearby Nankai Trough. The shear zone preserves ‘intact’ antigorite-serpentinite, talc- and chlorite-bearing serpentinite breccia, and complex brittle/ductile shear zone consisting of quartz-bearing carbonate-chlorite-talc schists, talc - carbonate veins, and talc-rich mylonitic shear zones. We document that the presence of Al in antigorite and spinel causes the formation of abundant chlorite which inhibits carbonation reaction. We show that the formation of talc- and carbonate-rich domains is primarily related to the formation of veins crosscutting the carbonated rock fabric. Hence, the formation of talc mylonites is primarily associated with parts of the rock that became Si-rich, whereas Al-rich domains deform primarily by fracturing and veining. Finally, the presence of fractured sulphides in the rock documents multiple cycles of fracturing, sulphide precipitation, and healing, compatible with successive embrittlement, stress release, fluid infiltration, and fluid pressure drop events.

We suggest that the presence of Al in the protolith serpentinitic material, which is common for ultramafic rocks, (1) slowed-down carbonation reactions, (2) prevented the rapid formation of talc-rich domains, and (3) kept the fabric heterogeneous and the rheology mixed, overall preventing the formation of weak domains that should have localized aseismic creep and possibly hosting episodic tremor and slow slip.

How to cite: Okazaki, K., Papeschi, S., Kawaguchi, K., and Hirose, T.: Deformation of carbonated serpentinite controlled by Al- and Si-partitioning in phyllosilicates: a record of deep episodic tremor and slip?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7188, https://doi.org/10.5194/egusphere-egu24-7188, 2024.

Understanding earthquakes mechanisms still represents a challenge, motivated by the large consequences of the numerous earthquakes occurring each year. A number of uncertainties remain concerning the complexity of the fault structure, the constitutive properties of materials or the fault rheology. To address those points, 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 different stages undergoing by the asperity and the consequences on the fault behaviour during co-seismic events.

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. The experimental downscaled fault is submitted to co-seismic conditions: contact size of 0.1-5 mm, contact normal stress of 10-200 MPa, sliding velocity of 0.01-1 m/s, and sliding distance of 10 - 60 m. A number of high-sampling-rate sensors are used to constrain the observation of the asperity contact during the simulated seismic events. Complete post-mortem analyses of the wear tracks with optical microscopy, SEM and roughness images allow to quantify the regime features and to reconstruct friction scenarios in accordance with the time-series acquired during tests.

Independently of the velocity and the normal load applied, the friction coefficient exhibits a clear transition between an idealized lab conditions regime and a mature interface with the formation of granular gouge, as a function of the sliding distance. Within the same regime (clean surface, intermediate, mature gouge), velocity weakening and hardening due to higher loading are pointed out. We propose to focus on the clean surface to mature gouge transition and on the stability of the mature gouge interface regime to address the fault rheology and the role of asperities in seismic weakening.

How to cite: Clerc, A., Mollon, G., Ferrieux, A., Lafarge, L., and Saulot, A.: Evolution from a clean surface to a mature gouge interface in a seismic fault – asperity system through the lens of pin-on-disk experiments, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7980, https://doi.org/10.5194/egusphere-egu24-7980, 2024.

The initiation of unstable fault slip leading to earthquakes involves intricate physical processes and interactions. Understanding these mechanisms is crucial for advancing our knowledge in earthquake seismology. Investigations at both field and laboratory scales have highlighted the existence of spatio-temporal variations in seismic or aseismic observations near the epicenter of a major seismic event, such as a rise in frequency of precursory earthquakes (Kato and Ben-Zion, 2021)  or even strong fluctuations in seismic velocities (e.g., Campillo & Paul, 2003). These variations are often associated with the preparatory phase of major earthquakes believed to involve processes resulting from progressive localization of deformation around the eventual rupture zone that eventually accelerates leading up to failure. However, the time and spatial scales of this behavior are not well understood due to our lack of understanding into the physical mechanisms within the preparatory zones.

In this study, we combined innovative laboratory techniques and numerical modelling to investigate (a)seismic preparatory deformation during a triaxial failure test in the laboratory. Employing distributed strain sensing (DSS) with optical fibers, we closely monitored strain rates on the sample surface. This was supplemented by active ultrasonic surveys and passive acoustic emission (AE) monitoring to investigate changes in P-wave velocity and locate regions prone to AEs within the sample. Using a physics-based computational model, we investigated strain localization within the sample by monitoring rock regions exhibiting high dissipation of mechanical energy. Highly dissipative regions spatio-temporally correlated with the observed AE locations and with sample regions experiencing P-wave velocity reduction. By further tracking the dissipation field within the sample, we recognized a system of conjugate bands that first emerged and quickly merged into a single band growing from the center towards the sample surface. The latter was interpreted to be related to the preparation of a weak plane. Shortly prior to failure, the model showed an acceleration of deformation that was also observed during the laboratory test with the DSS measurements and correlated with an increase of the seismicity rate in a similar volume of the sample. The combination of increased deformation and seismic rates mimics observations of precursory seismicity in nature. By methodically segregating the laboratory experiments from the numerical modeling, this study provides a comprehensive analysis of the physical processes underlying earthquake nucleation. The integration of cutting-edge laboratory techniques with advanced numerical modeling offers a novel perspective on the (a)seismic preparatory deformation that sets the stage for major seismic events.

References:

Campillo M., Paul A. (2003) Science 299, 547-549.

Kato, A., Ben-Zion, Y. (2021) Nat Rev Earth Environ 2, 26–39.

How to cite: Bianchi, P., Selvadurai, P. A., Dal Zilio, L., Salazar Vásquez, A., Madonna, C., Gerya, T., and Wiemer, S.: Exploring fault preparation and earthquake nucleation from the laboratory, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8060, https://doi.org/10.5194/egusphere-egu24-8060, 2024.

Despite the recognition that fluids play an important role in subduction zone processes, the extent of fluid circulation and fluid-rock interactions within subduction and accretionary complexes is still not fully understood. Here, we examined Li elemental and isotopic systematics in fluid inclusions trapped within hydrothermal quartz veins in metasedimentary rocks from three paleo-accretionary complexes (Kodiak complex, Alaska; Shimanto Belt, Japan; Western Alps), which are contemporaneous with the burial and metamorphism at temperatures ranging from 250 to 400°C. To provide a fuller understanding, we investigated (i) fluid inclusions, (ii) host quartz, and (iii) wall-rocks of syn-subduction veins.

The δ⁷Li of fluid inclusion leachates range from −1.5‰ to +17.1‰ and are variable among three localities. Two important processes control the ⁷Li/⁶Li ratios of fluids from inclusions: (i) Li release/uptake from the host rock, and (ii) the reactive volume of the rock. Higher δ⁷Li values of fluids in Kodiak (+8.1‰ to +17.07‰) are interpreted as a result of closed-system behavior, with a small reactive volume of metasediments. Lithium has not been lost to the fluid, where ⁶Li is dominantly preserved in metamorphic chlorite and illite. In closed-system samples from the Western Alps, the fluids are buffered by the host rock, causing a shift in δ⁷Li values of pore fluids (from −1.5‰ to +9.5‰) towards the values of the protolith. Conversely to the samples from Kodiak, the reactive volume of rock is significantly greater, resulting in a complete fluid–rock equilibration. Equally low δ⁷Li values of pore fluids in Shimanto (+2.53‰ to +10.39‰) is attributed to the large flow of externally derived fluids and interpreted to result by Li leaching from illite and chlorite.

The δ⁷Li values of quartz are globally higher than those of paired leachates (+10.93‰ and +22.61‰) without temperature-dependent isotopic fractionation between quartz and fluid. This is explained by either (i) a significant drop in pore fluid pressure which, in turn, facilitates rapid crystallization of quartz, or (ii) post-entrapment re-equilibration between fluid inclusions and the host quartz.

By comparing the metamorphic fluids in the present study with seawater or pore water from deep sea sediments, elevated Li concentrations in leachates (up to 24 ppm) combined with relatively low δ⁷Li values indicate that Li is progressively leached from sediments during burial, and that the δ⁷Li value of fluids is consequently shifted towards the signature of the protolith. Similarities in Li concentrations and δ⁷Li values between leachates and fluids expulsed through mud volcanoes in modern examples of subduction zone forearcs further confirms the origin of mud volcano fluids dominantly from subducted sediments. Such similarities imply that fluid circulation across permeable zones may reach at least a 20 km-scale in the forearc region. This study further demonstrates the relevance of Li elemental and isotope systematics to efficiently trace fluids across large distances within subduction zone forearcs. 

How to cite: Rajič, K., Richard, A., Raimbourg, H., Magna, T., Herviou, C., Lerouge, C., and Millot, R.: Tracing the extent of fluid circulation in subduction zone forearcs using lithium isotopes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8130, https://doi.org/10.5194/egusphere-egu24-8130, 2024.

Microstructural evidence for slow earthquakes is a matter of debate as micromechanical processes are not fully understood and hence resulting deformation microstructures remain unclear. One of the best study areas to investigate the phenomena of seismic and aseismic deformation and also possible paleo-events of slow slip in an exhumed accretionary complex is the Shimanto Belt in SW-Japan, where lithologies, age, and pressure-temperature conditions are well-constrained. Our investigations focus therefore on the Yokonami mélange of the Cretaceous Shimanto Belt. The Goshikinohama fault is a fossil seismogenic fault at the northern margin of the Yokonami mélange. It contains several 20 cm thick cataclastic faults within which thin (less than 1 mm), discrete slip zones occur. Based on vitrinite reflectance the paleo-maximum temperature of the surrounding host rocks is about 250˚C. An exothermic event was identified in the cataclasite of up to 300-360˚C evidenced by paleomagnetic and rock-magnetic analyses [Uchida et al., 2024]. In order to access microstructures related to the seismic cycle as well as to explore whether this proposed thermal event resulted in characteristic changes in deformation mechanism, we conducted observations on the cataclastic shear zone using optical microscopy,electron microscopy,  electron backscatter diffraction (EBSD), energy dispersive X-ray spectroscopy (EDS) and cathodoluminescence (CL).

The studied cataclasite consists of mm- to cm-size fragmented quartz veins in a shale matrix with quartz and feldspar clasts. Quartz displays solution-seam contacts to the shale and various generations of subsequent fracturing and healing are recognized. Characteristic are (i) synkinematic fiber growth microstructures related to a crack-seal mechanism accommodating foliation-parallel stretching of quartz aggregates within the shale matrix as well as numerous generations of blocky veins and (ii) static shattering of quartz grains at the µm-scale and subsequent healing. The static nature of the shattering is interpreted from the lack of any offset or misorientation in the affected quartz grains. In addition, there is some undulous extinction and very minute and local indication of quartz dynamic recrystallization by grain boundary bulging. The shale matrix exhibits a compositional flow banding detected by EDS.

Veining, solution seams and the general clast-in-matrix structure are interpreted to relate to the interplay of brittle fracturing, cataclastic flow and dissolution-precipitation processes. Very few and local evidence for bulging recrystallization fits deformation conditions at the brittle to ductile to viscous transition in accordance with the temperature estimates given before. The origin of shattered quartz is hypothesized to relate to seismic wave-induced shock deformation.Mutual overprinting of brittle/cataclastic deformation and creep deformation as well as synkinematic and static vein growth might be an indication for the formation of these microstructures during the seismic cycle and possible transient creep or slow slip. However, if these processes produced heat to the extent of the proposed exothermic event is a matter of further investigations.

[Ref] Uchida,T., Hashimoto, Y., Yamamoto, Y. and Hatakeyama, T., 2024, Exothermic events in a fossil seismogenic fault acquiring thermoviscous remanent magnetization in an exhumed accretionary complex, Tectonophysics, V. 871,https://doi.org/10.1016/j.tecto.2023.230177. 

How to cite: Hashimoto, Y., Mitani, J., Kilian, R., Kühn, R., and Stipp, M.: Brittle/cataclastic deformation and dissolution-precipitation creep in the Goshikinohama fault (Shimanto Belt, SW-Japan): Indication of seismic cycling and possible slow slip?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8597, https://doi.org/10.5194/egusphere-egu24-8597, 2024.

The deformation behavior and mechanisms of mineral grains play a pivotal role in comprehending the solid-state rheological behavior of the lithospheric crust. However, the fluid present during the deformation processes of grains is often overlooked. The study presents a comprehensive analysis of water-induced superplastic deformation within deformed quartz veins exposed in the continental-scale exhumed Gaoligong shear zone by combining microstructure analysis with EBSD mapping and infrared spectroscopy. We observe fine-grained aggregates of quartz form micro-shear zones that are either localized at the rims or within the coarse clasts during deformation. The nucleation of these fine-grained zones is controlled by microcracks/fracturing, which are further associated with dynamic recrystallization. Numerous fluid inclusions are leaked and water is pumped into thicker fine-grained shear zones. The water migration plays a crucial role in accommodating boundary plasticity, with tiny water clusters being sealed within grain boundaries. The recycling of water is linked to a superplastic flow process, involving water influx, grain boundary sliding (GBS), accommodation of strain incompatibilities, and sealing of water. Our findings suggest that water migration into fine-grained aggregates within micro-shear zones not only restrict grain growth but also releases strain incompatibilities, enhancing grain boundary sliding. This process delays brittle fracturing of quartz, highlighting the significant role of water in influencing the deformation behavior of quartz.

How to cite: zhan, L., Cao, S., Dong, Y., Li, W., von Hagke, C., and Neubauer, F.: Water-induced superplastic deformation and its mechanism of quartz , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8751, https://doi.org/10.5194/egusphere-egu24-8751, 2024.

Faults accommodate most of the brittle deformation that occurs in the lithosphere  through a spectrum of fault slip behaviours including, but not limited to, seismic and aseismic slip. The rocks deforming inside the core of the faults are the main actors that control the modality of slip and thus their mechanical properties are a key subject of study that is carried out through experimental investigation. The most relevant characterization is that of friction, a property that commensurates the resistance to shear motion of the rocks. Nevertheless, friction is not an intrinsic constant feature of the investigated materials. It is instead modulated by several attributes and external factors. For instance, the rate and state constitutive framework describes the sensitivity of friction to the sliding velocity, proving a successful theory to quantify the potential of the onset of dynamic instabilities and seismic slip in natural faults. Several works have also demonstrated that the frictional properties of the same material can dramatically change as function of the fabric (textural, geometrical attributes of the deforming rock). It is therefore evident that brittle deformation of rocks cannot be assessed in isolation of the conditions at which the phenomenon is measured. To fully understand the complex bulk behaviour of a deforming fault zone material we must investigate the interaction of several scale-dependent mechanisms that are active from the grain-scale up to the entire fault zone thickness.

In this work we present the results of several case-studies that cover relevant lithotypes: anhydrite-dolomite, quartz-calcite-mica, lizardite-magnetite mixtures. These studies collect more than 60 friction experiments performed on BRAVA biaxial apparatus (INGV, Italy), presented here by associating the analysis of mechanical data with the analysis of rock microstructures. This joined investigation highlights the mechanisms that control rock friction: cataclasis, crystal plasticity, pressure-solution, grain-boundary sliding, cementation, and indentation. We also show the emergence of complex slip behaviours (experimental fault stability) as function of the coexistence of processes with different timescales and explained by the spatial arrangement of the mineral phases in the fault core.

Our results shed light on the origin of the macroscopic frictional properties of fault rocks, stressing the fact that they are not a characterising property but rather the observable of a complex, dynamic, and highly non-linear system.

How to cite: Pozzi, G., Volpe, G., Ruggieri, R., Collettini, C., Scuderi, M., Tesei, T., Marone, C., and Cocco, M.: Friction, Mineralogy, and Microstructures: How Complex is the Brittle Deformation of Faults?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9295, https://doi.org/10.5194/egusphere-egu24-9295, 2024.

The Fen Carbonatite Complex in Norway contains the largest deposit of Rare Earths Elements (REE) in Europe, with estimated resources in the range of 30 – 50 Mt of total Rare Earth Oxides. If Fen will be targeted as an exploitable mineral resource, the geological processes that formed it must be understood, with specific emphasis on what controls the location and composition of the REE resources.

The Fen complex formed at 580 Ma through different stages of carbonatite melt intrusions followed by hydrothermal alteration. Fluid- and melt-assisted deformation accompanied the intrusive and hydrothermal evolution of the Fen Complex extensively and resulted in the formation of shear zones and breccias. However, the mechanism and significance of carbonatite deformation are poorly understood, and so are the effects of post-crystallization deformation processes on the remobilization of trace elements in carbonatites.

This study investigates deformation processes and REE remobilization in shear zones in dolomite-carbonatites from Fen. The shear zones display a compositional banding defined by alternating dolomite- and apatite-rich layers, where apatite grains are variably elongated with aspect ratio ranging from 2 to 11 and grain length from 50 to 500 µm. SEM images reveal the presence of carbonatitic melt pseudomorphs in the form of intergranular beads, cusps, films, and pools, which are particularly evident in the apatite layers, where individual grains are locally entirely rimmed by melt films. The apatite grains appear zoned in cathodoluminescence (CL) images, with dark cores and bright rims that are thicker parallel to the foliation. In the most elongated grains, the dark core forms less than 20% of the grain area, which is otherwise dominated by the bright rim. On the contrary, more equidimensional grains are dominated by the dark core. Hyperspectral analysis of CL images indicates that the elongated rims of apatite are enriched in REE (particularly in Nd) compared to their core. Electron backscatter diffraction (EBSD) analysis demonstrates that (1) the elongated apatite grains are internally strain free, and (2) grain elongation occurs parallel to apatite c-axis.

Our data show that deformation of apatite occurred by melt-assisted dissolution-precipitation creep, which was responsible for grain elongation and remobilization of REE. Thus, post-crystallization deformation and melt-rock interaction played an important role in redistributing REE within the Fen Complex.

How to cite: Menegon, L., Valter, O., and Dahlgren, S.: Deformation-induced Rare Earth Elements (REE) redistribution in apatite from the Fen Carbonatite Complex (Norway), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9352, https://doi.org/10.5194/egusphere-egu24-9352, 2024.

A critical aspect of studying earthquake mechanisms involves understanding why a single fault can exhibit various slip behaviors. Fault heterogeneity leads to different slip behaviors in different fault portions: some slip seismically, generating catastrophic earthquakes, while others slip a-seismically in a stable and silent manner. Additionally, some fault portions exhibit slow, intermittent slip that can persist for months. Unraveling the physical mechanism at the base of these different fault slip behaviors is crucial for understanding how fault portions that slip slowly interact with portions capable of producing earthquakes.

In a laboratory setting, we can replicate the entire spectrum of fault slip behaviors by changing the loading stiffness of our experimental apparatus. Tuning the loading stiffness, we are able to match the critical rheological stiffness of the fault (kc) and investigate conditions around the critical point where k/kc = 1. Moreover, monitoring acoustic emissions (AEs) during laboratory earthquakes allows us to capture the rupture processes throughout the seismic cycle.

To constrain the nucleation mechanisms and rupture processes of different slip behaviors, we conducted friction experiments using quartz powder (MinUSil, average grain size 10 µm) to simulate fault gouge. The experiments were carried out in a double direct shear configuration, using an array of calibrated piezoelectric sensors for continuous, high acquisition rate (6 MHz) AE recording. The experiments were conducted at a constant displacement rate of 10 µm/s. During each experiment we maintained a constant normal stress and changed three acrylic blocks of different areas to change the apparatus stiffness (k). This technique allows us to reproduce both fast (i.e., when the apparatus stiffness is lower than a critical stiffness, k<kc) and slow (i.e., k=kc) slip events under the same stress conditions and test if the same fault patch can host a variety of slip behaviors.

Continuous AE recording, that is a proxy for seismicity, allows us to relate mechanical and acoustic fault behaviors. Our results show that different slip behaviors produce distinct acoustic waveforms during slip, with impulsive (high amplitude, short duration) AEs for fast slip, and emergent (low amplitude, longer duration) and continuous acoustic signals for slow slip. The distribution of AEs throughout the seismic cycle is characterized by an accelerating phase with small emissions for slow slips. While, fast slips exhibit no clear pre-seismic activity, and only strong AE in the co-seismic phase produced by fault rupture. Analyzing the frequency content of the acoustic signals also provides insights into the size, duration and the evolution of the seismic source along the seismic cycle.

By changing the stiffness of the fault, and monitoring acoustic emissions, our experiments not only accurately show that the same fault patch can experience different slip behaviors under the same stress conditions but also gives important insights into the complex dynamics of fault slip and rupture processes.

How to cite: Pignalberi, F., Giorgetti, C., Romanet, P., Tinti, E., Marone, C., and Scuderi, M.: From slow to fast earthquakes: laboratory insights on acoustic and mechanical fault slip behavior, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9877, https://doi.org/10.5194/egusphere-egu24-9877, 2024.

Geophysical and geological evidence highlighted that faults can slip in a wide spectrum of modes, ranging from stable aseismic creep to unstable dynamic slip. Rocks composition plays a key role among the multiple factors favoring a specific type of frictional sliding. 

In particular, phyllosilicates in fault zones can change the mechanical behavior of the rocks involved in deformation. A relevant example is the presence of smectites (hydrated phyllosilicates) in subduction zones that are thought to influence the updip limit of the seismogenic zone. This group of clay minerals exhibits remarkably low friction values due to their platy microstructure and the tendency to absorb water within their lattice, making faults particularly weak. Therefore, studying the mechanical properties of clay minerals, especially smectites, has become crucial to illuminate the dynamics leading to the generation/arrest of large earthquakes in subduction zones.

Frictional laboratory experiments make it possible to evaluate the stability of experimental faults using the Rate and State Friction (RSF) framework. However, upscaling these phenomena and laws formulated in the laboratory to natural cases is still challenging due to a fundamental lack of understanding of the microphysical processes governing friction, mainly due to the empirical nature of the laws.

Modern friction theories propose that the frictional forces holding the fault in place are controlled by small asperities defining the real contact area (RCA). In the laboratory, experimental faults can be probed with ultrasonic waves to investigate the mechanics and evolution of contacts under applied stress variations.

Here, we present preliminary results on the stability of experimental faults with varying percentages of montmorillonite gouge (a specific type of smectite). The experiments are conducted using the biaxial apparatus BRAVA2 in the Rock Mechanics and Earthquake Physics laboratory at Sapienza University of Rome. 

Velocity steps experiments are performed in Double Direct Shear (DDS) configuration to obtain RSF parameters under different normal stress conditions. The apparatus is equipped with a recently developed UW generation and acquisition system.

The system comprises longitudinal and transversal polarized piezoelectric transducers, where a well-characterized pulse and frequency response allow the exploitation of information contained in the entire waveforms. The variation of transmitted amplitude, compressional, and shear velocity is used to track the changes in elastic properties. 

The synchronization procedure between mechanical and ultrasonic measurements will allow inferring the physical processes leading to RCA evolution from the obtained data.

How to cite: Mauro, M., De Solda, M., Giorgetti, C., and Scuderi, M.: Probing the Micromechanics of Velocity Strengthening Laboratory Faults using Ultrasonic Waves , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10106, https://doi.org/10.5194/egusphere-egu24-10106, 2024.

Faults and shear zones within the metamorphic ultramafic rocks of the Voltri Massif (Ligurian Alps, NW Italy) often exhibit significant or complete carbonation of the host rocks and are locally associated with gold mineralization hosted in quartz veins (e.g., in the Lavagnina Lakes area). Here, a specific fault zone part of a larger regional system (i.e., the Bisciarelle Fault Zone) displays distinct structural characteristics linked to a fluid-assisted multistage brittle deformation in serpentinized peridotites, possibly indicating paleoseismic activity. Within the fault rocks, cataclasite and breccias are present along with saponite-bearing gouge, featuring layers of coseismic spherulitic grains interspersed in silica/chalcedony veins and cement. Spherulites consistently crystallize as concentric bands of fibrous Fe-dolomite and display multiple layers of radial crystal growth regularly alternating with darker, oxides-rich concentric bands. The concentric growth of spherulites is evident from the microtextural relations between successive bands, which depart radially from the spherulite cores, made of a submillimetric nucleus of carbonates or single grain of the host rock (e.g., relicts of fragments of fault core).

In this study, we present a multiscale analysis of this fault zone, integrating field observations, microstructural examination, SEM-EDS investigation, and electron backscattered diffraction (EBSD). The primary focus is on the microstructures within the fault core and the significance of distinctive carbonate spherulite layers in conjunction with silica/chalcedony cement and veins.

Our findings reveal that these structures are indicative of the interaction between CO2-rich fluids released during both coseismic and interseismic phases of faulting. This interaction occurs during cycles involving fluid pressure build-ups, faulting events, fluid flushing, and the subsequent precipitation and sealing of minerals during seismic failure of the fault.

How to cite: Federico, L., Locatelli, M., Crispini, L., Mariani, E., Capponi, G., and Scarsi, M.: Seismic faulting and fluid interactions: a structural study from carbonated fault damage zones within ultramafic rocks, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10158, https://doi.org/10.5194/egusphere-egu24-10158, 2024.

In subduction zones, fluids released by dehydration reactions strongly influence rock rheology and seismicity. In particular, the occurrence of deep Episodic Tremor and Slow Slip events (deep ETS) along the subduction interface, at 25-60 km depth¹, is likely fostered by the simultaneous presence of fluctuating fluid pressure and rheological heterogeneities, that allow for strain partitioning into low-strain domains radiating tremor, and high-strain domains accommodating slow slip events.

The Erro-Tobbio meta-peridotite (Ligurian Alps) records fluid-rock interactions and associated deformation that occurred within the deep ETS depth range. Heterogeneous serpentinization of the original mantle peridotite resulted in partitioning of the eclogite-facies deformation into high-strain domains of antigorite mylonites and low-strain domains of undeformed meta-peridotites. Both mylonites and meta-peridotites contain veins/reaction bands of metamorphic olivine (Ol₂) and Ti-clinohumite (Ti-chu), formed by breakdown of brucite (Brc) and antigorite (Atg) at estimated P-T of 1.5 GPa and 500 °C³. Ol₂ + Ti-chu reaction bands are arranged into two main sets, mutually oriented at ~50°: (i) Set₁, steeply-dipping around 320°, (ii) Set₂, trending N-S and parallel to the mylonites. The mylonites include: (i) type₁ mylonites, composed of a planar foliation marked by Set₂ reaction bands, and (ii) type₂ mylonites, displaying a chaotic structure.

Within the undeformed domains, hydration and dehydration events occurred statically. In such domains, Al-rich Atg (Atg₁) epitaxially replaced mantle olivine (Ol₁), and was in turn epitaxially overgrown by Ol₂, that crystallized in radial aggregates and along Set₁-Set₂ reaction bands. Along the mylonitic horizons, Atg₁ is affected by ductile deformation, and Set₂ reaction bands mark a foliation parallel to that of Atg₁. In this case, Ol₂ is rarely crystallographically related to Atg₁ and is mostly oriented with a-axis parallel to the reaction bands. Atg₁ and Brc relics are preserved along Set₁ and Set₂. The absence of Brc in the wall rock suggests that formation of Ol₂ localized along original Brc-rich layers. Later stage, Al-free serpentine locally extensively (up to 70% volume) replaces Ol₂ along a pervasive network of microcracks that exploited the previous Set₁-Set₂ structures. These observations suggest the occurrence of localized Brc ± Atg₁ dehydration to Ol₂ along specific planes, likely related to Brc distribution and Atg deformation, and subsequent Ol₂ hydration localized along serpentine-bearing microcracks.

In-situ LA-ICP-MS reveals an enrichment in fluid-mobile elements (As, Sb, Ba, W, Li, B) in prograde Ol₂ and retrograde Al-free serpentine. This information provides evidence of infiltration of external fluids, indicating open system conditions during eclogite-facies deformation, in agreement with the literature^2,4, and during retrogression.

References

1: Behr et al., 2021, What’s down there? The structures, materials and environment of deep-seated slow slip and tremor. Phil. Trans. R. Soc. A 379: 20200218.

2: Clarke et al., 2020, Metamorphic olivine records external fluid infiltration during serpentinite dehydration. Geochem. Persp. Let. 16, 25–29.

3: Hermann et al., 2000, The importance of serpentinite mylonites for subduction and exhumation of oceanic crust. Tectonophysics 327, 225±238.

4: Scambelluri et al., 2012, Boron isotope evidence for shallow fluid transfer across subduction zones by serpentinized mantle. Geology 40, 10,  907–910. 

How to cite: Cacciari, S., Pennacchioni, G., Cannaò, E., Scambelluri, M., and Toffol, G.: Fluid-rock interaction in eclogite-facies meta-peridotite (Erro-Tobbio Unit, Ligurian Alps, Italy), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10481, https://doi.org/10.5194/egusphere-egu24-10481, 2024.

Serpentinites are “weak” rocks that play a critical role in the nucleation and propagation of slow slip events, tremors and earthquakes due to their unique rheological properties that promote strain localization and are common in a variety of tectonic settings, from mid-ocean ridges, to transform faults and subduction zones. In this study we analyze the microstructures of natural and experimental faults made by low-grade serpentinites (chrysotile and lizardite ± magnetite) to infer the possible deformation mechanisms operating in nature at hydrothermal conditions.

The natural serpentinites pertain to the exhumed Monte Fico shear zone (Elba Island, Italy) that reached greenschist facies conditions during subduction related to the Apennine orogeny. The shear zone is made of dm-m scale lenses of massive and less deformed serpentines surrounded by foliated serpentinites and cut by brittle faults. Bulk deformation in the natural shear zones was accommodated by anastomosing and pervasive S/C foliation structures. Fault surfaces are covered with slickenfibers mostly composed of chrysotile and polygonal serpentine. The interpretation of the microstructural analysis indicates the coexistence of ductile pressure-solution within the massive lens with fracturing, veining and frictional slip along the faults bounding the lenses. This fault zone rock assemblage and microstructural association suggests that cycles of high fluid pressures are limited by dilatant slip along the faults.

To determine the frictional properties and deformation mechanisms of these serpentinite-bearing faults we performed experiments with a rotary shear apparatus equipped with an hydrothermal vessel (ROSA-HYDROS, Padua University, Italy). We conducted slide-hold-slide (SHS) experiments at an effective normal stress of 20 MPa, a fluid pressure of 6 MPa, constant sliding velocity of 10 µm/s and at four different temperatures (room, 100°C, 200°C and 400°C). Friction experiments allowed to determine the rheological difference between the massive lens and the bounding faults, which represents favorable sites for slip nucleation.  The frictional healing properties document how the strength of these heterogeneous brittle-ductile shear zones evolve during the interseismic period.

The combination of natural and experimental observation in our project aims at the understanding of the mechanical behaviour of such lithologically and geometrically complex fault zones and to elucidate slip processes during earthquakes and slow slip events.

How to cite: Salvadori, L., Tesei, T., and Di Toro, G.: Frictional strength, healing behaviour and deformation mechanism of low-grade serpentinites at hydrothermal conditions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11900, https://doi.org/10.5194/egusphere-egu24-11900, 2024.

This study demonstrates how the response of ultramafic lithologies to infiltrating H₂0-CO₂ fluids depends on the primary mineralogy. This has major implications on fluid flow through the lower crust and upper mantle as mineral reactions control the permeability and rheology. The studied samples are from the hanging wall of a 2 kilometer-long transtensional shear zone within ultramafic-mafic rocks in the Reinfjord Ultramafic Complex (RUC), part of the Seiland Igneous Province (SIP) in Northern Norway.

Fluid-rock interaction surrounding shear zones with abundant pseudotachlylites is highly variable and depends on bulk rock compositions. Thermodynamic modelling demonstrates that mineral reactions involving hydration and carbonation differ between dunitic rocks and the pyroxenitic dykes which intersect them. Alteration of dunitic rocks results in the formation of dominantly magnesite-anthophyllite-talc and talc-magnesite assemblages causing approximately 12% volume expansion. This results in a sharp reaction front contacts with the host rock. When the alteration zones cross the dunite-pyroxenite boundary the associated alteration has a more gradual boundary towards the unaltered rock and the alteration zone widens by approximately 40%. In contrast to the simpler dunite alteration assemblage, the pyroxenetic dykes are altered to a complex mixture of cummingtonite-anthophyllite, magnetite and chlorite. Additionally, orthopyroxene is completely pseudomorphed by a mixture of cummingtonite and magnetite, whereas olivine xenocrysts are partly preserved and surrounded by a magnesite-anthophyllite assemblage. Other, open cavity-like areas are filled by chlorite, amphibole, and Mg-MgCa carbonates, indicating volume reduction during alteration of the pyroxene.

Accordingly, dunite alteration effectuates a significant volume expansion, and are therefore only altered locally during seismic creep events. The pyroxenites are near volume neutral throughout interaction with the same fluids, and are thus more homogeneously altered. The formation of chlorite in hybrid compositions, such as the dykes in the lower crust, may create weak permeable zones that are consequently exploited as pathways for fertile mantle fluids and will hence also be the locus of ore bearing fluids moving to the upper crust. Increased understanding of fluid mediated metamorphism increases our current knowledge on fluid flow and strain localization in the lower crust. We further suggest that the hydrothermal assemblages are closely related to deformation leading to the formation of grain size sensitive creep in olivine facilitated carbonation of olivine and clinopyroxene to form orthopyroxene and dolomite and associated pseudoctacylites in the peridotites (Sørensen et al., 2019) , commonly associated with volatile rich mafic dykes (Ryan et al., 2022). Either the ductile magnesite-chlorite-talc assemblages formed at the same time in a shear-related heat gradient or they formed during cooling and continued CO₂ infiltration from depth through the shearzones.

Ryan E J, et al.  2022  Infiltration of volatile-rich mafic melt in lower crustal peridotites provokes deep earthquakes.  J. Struct. Geol. (https://doi.org/10.1016/j.jsg.2022.104708)

Sørensen, B.E., et al., 2019 In situ evidence of earthquakes near the crust mantle boundary initiated by mantle CO2 fluxing and reaction-driven strain softening. Earth and Planetary Science Letters (https://doi.org/10.1016/j.epsl.2019.115713 )

How to cite: Eske Sørensen, B., Ryan, E. J., Larsen, R., Lode, S., Drivenes, K., and Orvik, A. A.: Linkage between ductile deformation, pseudotachylites, strain softening and volume expanding carbonation reactions during mixed-volatile infiltration in ultramafic-mafic rocks from the Reinfjord lower crustal field laboratory , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13445, https://doi.org/10.5194/egusphere-egu24-13445, 2024.

Fluid flow in the crust induces fluid-rock reactions and contributes to earthquake triggering. However, there are limited numerical constraints on the fluid volumes with the available duration of fluid infiltration. There is also a gap in our knowledge of time-integrated fluid fluxes estimated from geological samples and their influence on controlling seismic/aseismic activity. Merging the timescales of fluid infiltration with the transport properties estimated from the geological samples such as metamorphic reaction zones is essential to understanding the fluid flux during crustal fracturing and its influence on controlling some characteristics of seismic/aseismic events.

This study focuses on fluid flow through a single fracture and the fluid-rock reaction zones and applies its results to low-magnitude fracturing events, such as tremors and low frequency earthquakes. Physical properties of fluid flow provide an opportunity to calculate the seismic moment and cumulative magnitude of the possibly triggered seismic/aseismic event.

Particularly, to examine the duration of fluid infiltration and time-integrated fluid fluxes we analyze amphibolite-facies fluid-rock reaction zones and then combine with estimates of possible associated seismicity and conclude that flow along a single fracture is compatible with seismicity of non-volcanic tremor and low frequency earthquakes. This study is based on evidence of rapid fluid infiltration (~10 h) caused by crustal fracturing and permeability evolution from low- to highly-permeable rocks (~10⁻⁹–10⁻⁸ m²).

Time-integrated fluid fluxes perpendicular to a given fracture and those through the fracture were estimated. Coupled methodology, including reactive-transport modeling and thermodynamic analyses, based on Si alteration processes within reaction zones is used to estimate fluid volumes involved in triggering seismic activity. Time-integrated fluid flux through the fracture results in 10^3-6 m³/m². The lower range is similar to the fluxes through the upper crustal fracture zones (~10^3-4 m³/m²), while almost the whole range is comparable to the contact metamorphism zone (~10^2-5 m³/m²).

Fluid volumes transported through the fracture were compared with fluid injection experiment results. We also compare the durations of fluid infiltration to the durations of the slow slip events. There is no universal theory of slow slip phenomena from the perspectives of geological and geophysical properties. In terms of pressure and temperature, high-grade metamorphic rocks can be related to slow slip events. Our finding reveals that the transportation of voluminous fluid volumes through a fracture may be related to short seismic/aseismic events such as tremors and LFEs, as suggested from duration (~10 h) and cumulative magnitude, representing the maximum values as 2.0–3.8, the lower limit of the magnitude for a single fluid-driven seismic event as –0.6 to 0.2. Single fractures described in this study make it possible to transfer voluminous fluid flow. They could be an essential control on the generation of seismic activity above the tremor and slow slip events source regions in the lower–middle crust.

How to cite: Mindaleva, D., Uno, M., and Tsuchiya, N.: Dynamic and short-lived fluid flow in the high-grade metamorphic rocks related to seismic events in the middle crust., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13986, https://doi.org/10.5194/egusphere-egu24-13986, 2024.

The initiation of shear commonly occurs spatially associated with fluid-rock reactions along brittle precursors. In many cases the relative timing of fracturing, fluid infiltration, reaction, and recrystallisation is unclear, making it difficult to disentangle mechanisms of shear zone formation from subsequent deformation and recrystallisation. Here we present the transition from an anhydrous and relatively undeformed precursor rock into a highly deformed and hydrated plagioclase-rich rock. The studied outcrop remarkably preserves both (1) the interface between the anhydrous granulite-facies parent lithology and a statically hydrated amphibolite-facies rock, and (2) a transition from statically hydrated amphibolite to the sheared amphibolite-facies lithologies. Detailed study of plagioclase chemistry and microstructures across these two interfaces using Electron Backscatter Diffraction (EBSD) and wavelength dispersive spectrometry (WDS) allow us to assess the degree of coupling between deformation and fluid-rock reaction across the outcrop. Plagioclase behaves dominantly in a brittle manner at the hydration interface and so the initial weakening of the rock is attributed to grain size reduction caused by fracture damage at conditions of ca. 720°C and 10-14 kbar. Extensive fracturing induced grain size reduction locally increases permeability and allows for continuing plagioclase and secondary mineral growth during shear, as evidenced by a general increase in the amount of hydration reaction products across the shear zone interface. Due to the apparent coupling of deformation and reaction, and the plagioclase microstructures such as, an inherited but dispersed crystallographic preferred orientation (CPO), fine grain size (5-150 µm), and truncation of chemical zoning, we conclude that deformation is dominantly facilitated by dissolution-precipitation creep in the shear zone.

How to cite: Moore, J., Piazolo, S., Beinlich, A., Austrheim, H., and Putnis, A.: Brittle initiation of dissolution-precipitation creep in plagioclase-rich rocks: Insights from the Bergen arcs, Norway, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14313, https://doi.org/10.5194/egusphere-egu24-14313, 2024.

It is thought that the supply of Si-rich fluid from subducting slab results in the formation of talc, a mantle mineral lowest frictional coefficient, at the slab-mantle interface. In contrast, the exhumed metamorphic belts often contain serpentinite bodies with extensive carbonate veins accompanying with talc. Recent experiments showed that the interaction of mantle rocks and CO₂ fluids rapidly produces the carbonate+quartz and carbonate+talc assemblage (Sieber et al., 2018). However, it is not well understood whether silica or CO₂ contributes more significantly to talc formation at the mantle wedge condition. In this study, we conducted a new type of experiments on the metasomatic reactions at slab-mantle interface at the mantle wedge condition to evaluate the role of CO₂ fluid relative to silica fluid on the formation of talc.

A Griggs-type piston cylinder apparatus was used for experiments on metasomatic reactions at the crust-mantle boundaries at 500°C, 1 GPa. We prepared the three layers of core samples; pelitic schist (the Sanbagawa belt, Japan) or quartzite was sandwiched between harzburgite (Horoman peridotite, dry mantle) and serpentinite (Mikabu belt, wet matle). Two types of fluids were introduced: pure H₂O fluid and H₂O-CO₂ fluid. The latter produced by the decomposition of Oxalic Acid Dihydrate (OAD). We maintained 4wt% H₂O and set the XCO₂ = 0.2 for the H₂O-CO₂ experiments.

In all conditions, the alteration more proceeded in the mantle rocks (harzburgite or serpentinite) than on the crust side. In the experiment with H₂O, talc was formed both in harzburgite and serpentinite at the contact with crustal rocks. In the pelitic schist at the contact with ultramafic rocks, albite was selectively replaced by Mg smectite, whereas in the quartzite, a small amount of talc was formed, indicating that counter diffusion of Si from crust to mantle, and Mg from mantle to crust. In the experiments with H₂O-CO₂ fluids, talc was formed with magnesite both in harzburgite and serpentinite with intense fracturing. The rough mass balance calculations reveal that the amount of talc in the ultramafic rocks can be explained solely by the reaction with CO₂-fluid, even if quartz-bearing rocks existed at the contact.

These experimental results suggest that talc formation at the slab-mantle interface is greatly enhanced by the infiltration of CO₂ fluids, at least, at the mantle wedge corner of the warm subduction zone, where the P-T conditions are similar to those of our experiments. In addition, not only silica but also other elements such as Mg and Al move significantly, which contributes to the various metasomatic reactions. Such heterogeneous metasomatic reactions could produce the rheological heterogeneities of the mantle wedge rheology at the slab-mantle interfaces, and may explain a wide spectrum of the slow slip events observed at the mantle wedge corner.

How to cite: Okino, S., Okamoto, A., Kita, Y., Sawa, S., and Muto, J.: Relative significance of CO2 and silica on talc formation at slab-mantle interface: Insights from experiments on metasomatic boundary, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14574, https://doi.org/10.5194/egusphere-egu24-14574, 2024.

Carbonation of serpentinites is a crucial factor in controlling the earthquake cycle in subduction zones. Serpentinites are commonly found within subduction zones, both in the mantle wedge above subducting slabs and on the incoming plate, formed from peridotites exposed directly on the ocean floor. Carbonation of these serpentinites often results from the “contamination” of CO₂-rich fluids derived from sediments involved in the subduction. This process leads to the formation of carbonate minerals within the serpentinite, which in turn influences the mechanical properties of the rock. A critical factor affecting the spatial and temporal progress of carbonation reactions - and thus their potential to trigger mechanical instabilities at the plate boundary - is the emergence of a permeable network of cracks and pores, which facilitates the interaction of CO₂-rich fluids with the serpentinite rocks.

We present a detailed three-dimensional (3D) characterization of variously carbonated serpentinite samples through computed microtomographic (µCT) imaging integrated with a machine learning algorithm (i.e. Random Forest classifier) to segment the different mineral phases. Machine learning offers a robust and accurate means of identifying and quantifying the carbonate phases, leveraging the Random Forest capacity for handling complex, multidimensional data. This allows for a comprehensive 3D examination of the alteration phases affecting the serpentinite samples and provides quantitative insights into the volumes and geometries of the carbonate vein networks. Our focus is on samples from an exhumed subduction channel separating  the fossil Cretaceous – Eocene accretionary prism (Ligurian Units) from the continent-derived nappes of the Northern Apennines. The subduction channel, part of the Norsi – Cavo Complex, is exposed over approximately 10 km along the N-S strike on the Island of Elba, consisting of oceanic sediments and ultramafic rocks detached from the prism base. Our analyses reveal that carbonation preferentially follows pre-existing serpentine veins, exploiting inherent anisotropies in the rock. In addition, the geometry of the vein network, as illuminated by the µCT 3D data, can help us to correlate with the carbonation timescale and fluid fluxes, as inferred from geochemical data.

The presence of carbonate-rich fluids can be responsible for increasing pore-fluid pressure, pushing the rock toward failure. The formation of extensive carbonate vein networks can also lead to a net volume increase in the original serpentinite, thus increasing instability. The observed preferential distribution of carbonates along pre-existing structures not only provides crucial insights into the mechanics of subduction zones but also offers valuable implications for CO₂storage models, highlighting potential fluid migration and reaction pathways.

How to cite: Rizzo, R. E., Papeschi, S., Baroncini, E., and Vannucchi, P.: Microcomputed Tomography Unravels CO2-fluid/rock Interaction in Elba's Carbonated Serpentinites, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15776, https://doi.org/10.5194/egusphere-egu24-15776, 2024.

The meta-granitoids of the Zillertal unit of the Tauern window (eastern Alps) record a sequence of Alpine deformations, developed during exhumation, ranging from ductile (at amphibolite-upper greenschist facies metamorphic conditions) to brittle (at conditions close to the base of the seismogenic crust).

In the core of the Zillertal unit, the high grade deformation (stage₁) is common and localized to steeply-dipping strike-slip shear zones, mainly striking around E-W and hierarchically organized in thick (up to several metres), km-long mylonitic major shear zones (MSZs), and small-scale (mm-dm-thick) shear zones (SSZs). SSZs are strictly associated with precursor tabular heterogeneities (e.g. dykes) and fractures/veins^{1, 2}. Stage₁ deformation occurred (i) in presence of fluids, recorded by cyclic vein formation and extensive alteration haloes surrounding fracture/veins, (ii) at low differential stress, and (iii) during shortening at 345° (i.e. at a high angle to the orientation of most shear zones)³. Stage₂ deformation is recorded by very discrete, local shear reactivation of the core of SSZs and of the mylonitic foliation of MSZs. Stage₂ shear zones have a similar strike-slip shear sense as the overprinted stage₁ shear zones, but developed (i) under fluid-deficient conditions, and (ii) high differential stress.

At lower temperature the meta-granitoids were involved into 2 stages of brittle deformation (stage_3A and stage_3B). Stage_3A is represented by thin (mm-thick) cataclasites and pseudotachylyte veins formed by slip along the mylonitic foliation of MSZs with the same strike-slip kinematics of the exploited stage₁ and stage₂ shear zones. Cataclasites are not associated with any significant alteration and pseudotachylytes do not show ductile reactivation. Stage_3B is represented by a pervasive system of vertical extensional chlorite-quartz-filled veins, epidote-filled hybrid fractures and faults, that crosscut and offset stage_3A structures. The stage_3B structures are surrounded by haloes of alteration of the host rock. The mineral filling of fractures (chlorite, epidote, albite) indicates conditions close to the base of the brittle crust. The orientation and kinematics of Stage_3B structures constrain shortening as horizontal, oriented ca. N-S³.

We interpret this structural sequence as the result of deformation at decreasing temperature and, basically, under constant orientation of tectonic shortening. At ductile/brittle transition conditions yielding occurred by (i) seismic slip along the highly misoriented planes of anisotropy provided by the persistent (km-scale) foliation of MSZs, under fluid-deficient conditions and high differential stress (stage_3A); and (ii) formation of new extensional and shear fractures, that disregard previous anisotropy, under fluid-present conditions and transient low differential stress (stage_3B). This indicates that the fluid availability dramatically modifies the rock strength and the type of mechanical response of anisotropic rock systems.  

¹Mancktelow, N.S., Pennacchioni, G., 2005. The control of precursor brittle fracture and fluid–rock interaction on the development of single and paired ductile shear zones. Journal of Structural Geology 27, 645–661.

²Pennacchioni, G., Mancktelow, N.S., 2007. Nucleation and initial growth of a shear zone network within compositionally and structurally heterogeneous granitoids under amphibolite facies conditions. Journal of Structural Geology 29, 1757-1780

³Pennacchioni, G., Mancktelow, N.S., 2018. Small-scale ductile shear zones: neither extending, nor thickening, nor narrowing. Earth-Science Reviews 184, 1-12.

How to cite: Pennacchioni, G. and Toffol, G.: Across the brittle-ductile transition: the role of fluids and anisotropy, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16363, https://doi.org/10.5194/egusphere-egu24-16363, 2024.

Deep slow earthquakes are commonly observed downdip from the seismogenic zone in relatively warm subduction zones. Most of these events occur close to the Moho depth of the overriding plate at depths of 30–40 km. Slow earthquakes show characteristics that can be related to both brittle and ductile behavior and their occurrence is thought to be closely related to the brittle-ductile transition. There is also good evidence that slow earthquakes develop in regions of high fluid pressure. The temperature of subduction zones is an important control on the location of the brittle-ductile transition and the release of fluid and healing of cracks along which fluids may move. However, temperature estimates along subduction zones are subject to considerable uncertainty. One of the main uncertainties is the amount of shear heating; many thermal models of subduction zones assume such shear heating is negligible. The Sanbagawa metamorphic belt of Southwest (SW) Japan formed along an ancient subduction boundary and now includes slivers of mantle wedge-derived serpentinite which are in direct contact with metasedimentary rocks derived from the subducted oceanic plate. These areas can be related to the ancient subduction plate interface. P-T paths from petrological studies combined with information on ancient plate reconstructions and thermal modelling suggest significant shear stresses developed along the subduction boundary and these strongly affect the thermal structure. Rocks originally located deep in subduction zones can record information about deformation processes, including shear stress. The estimated shear stress is likely to be representative of shear stress experienced over geological timescales and be suitable to use in subduction zone modelling over time scales of millions to tens of millions of years. Stress estimates based on quartz microstructure yield differential stresses of 30–80 MPa at depths close to the Moho of the overlying plate. Such stresses are compatible with the estimates from thermal modelling and imply shear heating needs to be considered when estimating the thermal structure in the domain of slow earthquakes.

How to cite: Wallis, S. R., Ishii, K., Koyama, Y., and Nagaya, T.: Shear heating along subduction zones and thermal structure in the domain of deep slow earthquakes: evidence from the exhumed subduction-type Sanbagawa metamorphic belt, SW Japan, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16478, https://doi.org/10.5194/egusphere-egu24-16478, 2024.

The stick-slip model for earthquakes consists of slip instabilities due to elastic strain energy storage followed by sudden stress drops along seismogenic faults, phenomenologically representing the seismic cycle. During the interseismic time, the fault regains strength (healing) and stores elastic energy that will be partially released in the following earthquake. Fault healing has been consistently documented by field observations, geophysical studies, and laboratory experiments.

Despite the large focus on laboratory experiments in addressing this topic, observations of stress drops and recurrence intervals in natural earthquakes generally showed more pronounced fault healing in comparison to laboratory measurements. This discrepancy may arise from the difficulty of reproducing the natural conditions in the laboratory in terms of time, stress, fluids, and temperature. In particular, fluid-rock interaction and thermally-driven processes are widely accepted as crucial for faulting at seismogenic depths. For instance, the presence of pressurized fluids, at temperatures at the onset of crystal plasticity could lead to chemically assisted healing processes such as compaction and cementation. Although this mechanism finds support in a multitude of field observations, there have been only few systematic laboratory studies reproducing and quantifying the occurrence of incipient cementation in the laboratory seismic cycle. In addition, frictional healing has usually been experimentally measured at relatively low shear strain, often overlooking the “strain history” of laboratory faults.

We present a suite of 15 friction experiments in which we performed Slide-Hold-Slide (SHS) tests to evaluate the healing of quartz gouges under hydrothermal conditions. The temperature range investigated spans from 23 to 400 °C, at different effective normal stresses and fluid pressures. We also documented the role of shear strain in controlling the evolution of frictional healing through systematic repetitions of SHS tests at different amounts of strain of the laboratory fault. Our results indicate that frictional healing is positively dependent on temperature, especially at temperatures corresponding to the onset of crystal plasticity for quartz (> 350 °C). Best fit lines of healing measurements at 400 °C deviate from the classical log-linear time-dependent Dieterich-type healing, following an exponential relationship between ∆μ (frictional healing) and the logarithm of hold time. This suggests that incipient cementation processes play a major role during quasi-stationary, interseismic periods, better reflecting the higher fault healing usually observed in natural environments. In addition, experimental results relative to high strain SHS tests revealed that the “strain history” of laboratory faults exerts a strong control on the evolution of friction during the experiments. These results improve our understanding of a critical healing mechanism, constraining the dependence of frictional healing on temperature and shear strain.

How to cite: Guglielmi, G., Tesei, T., and Di Toro, G.: Temperature and strain-dependent healing of quartz gouges at hydrothermal conditions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17072, https://doi.org/10.5194/egusphere-egu24-17072, 2024.

Pseudotachylytes (frictional melts formed during seismic slip) in the metamorphosed anorthosites from Nusfjord (Lofoten, northern Norway) preserve a record of seismic rupture in the dry lower crust at 650–750 °C, 0.8 GPa. Field observations indicate that the Nusfjord pseudotachylytes represent single-earthquake events associated with large stress drops, on the order of hundreds of megapascal (MPa) to 1–2 gigapascal. Such large stress drops are interpreted to reflect the high strength of the intact anorthosite at the high confinement conditions of the lower crust. One important question is whether evidence of the high stresses necessary to initiate seismic rupture in the lower crust is preserved in the microstructure of the Nusfjord pseudotachylytes and of their damage zone.

Pyroxene deformation microstructures associated with preseismic loading and coseismic fragmentation reveal strongly localized transient stresses that presumably reached GPa-level magnitude. Here we use high-angular resolution electron backscatter diffraction (HR-EBSD) on diopside grains to obtain spatial datasets of residual stresses that are retained in the crystal lattice of diopside. We apply this method combined with microstructural analysis on diopside in a sample from a pseudotachylyte from Nusfjord to reconstruct the spatial heterogeneities of stress and link them to the earthquake cycle and associated coseismic thermal effects.

Diopside contains micro- to nanoscale deformation twins within 3 mm from the fault and in clasts in the pseudotachylyte. Strong lattice undulations are locally present in survivor clasts, indicating low-T plasticity at high stress. Residual stresses from the wall rock and in a survivor clast vary between ~600 and ~200 MPa and form a gradient of decreasing residual stress away from the pseudotachylyte, only elevated within 200 µm from the pseudotachylyte margin and with the highest values occurring within the clast. Microfaults crosscut the deformation twins, lattice undulations, and residual stress spatial heterogeneities within the clast. The latter appear strongly similar to the lattice undulations, in distribution and orientation.

The obtained stresses are lower than estimated stress drops for the locality and than stresses expected during rupture propagation (both >1 GPa). As alternative stress source, we investigated thermal stress introduced by coseismic frictional heating. Calculations demonstrate that this process is only significant over a distance of less than 100 µm in the wall rock for a stress drop of 100 MPa, and less than 10 µm for a stress drop of 1 GPa. Instead, because coseismic microfaults crosscut twinning, lattice undulations, and the spatial heterogeneities of residual stress, we interpret that these features correspond to the progressive build-up of stress during preseismic loading. An explanation for the discrepancy between the residual stresses and suggested stress drop is that the stress build-up in diopside was partially dissipated by the formation of twins. Additionally, the stress drop is estimated at the scale of the bulk fault, whereas the residual stresses are measured at the single grain scale and as such are likely to vary locally depending on the microstructure and on the different ability of different phases to dissipate the stress build-up via e.g. twinning and recovery of dislocations.

How to cite: van Schrojenstein Lantman, H. and Menegon, L.: Residual stress in diopside: insight into localized transient high stress in seismogenic faults in the lower crust, Lofoten, Norway, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17335, https://doi.org/10.5194/egusphere-egu24-17335, 2024.

In the granitoid crust, phyllosilicate-rich fault gouges are prevalent in mature fault zones undergoing hydrothermal alteration and often exhibit lower frictional strength compared to framework minerals (e.g., qtz, fds) under deformation at room temperature. However, the mechanical behavior and deformation mechanisms of altered gouges under hydrothermal conditions are not fully understood so far.

To investigate these effects, we conducted a series of experiments on three types of fault “gouge” material using a ring shear deformation apparatus. We used gouge mixtures obtained from (i) crushed granitoid ultramylonite, (ii) biotite- and (iii) muscovite-bearing gouges to represent quartzofeldspartic materials with (i) no alteration, (ii) high-temperature and (iii) low-temperature alteration, respectively (see Table 1 for the mineralogy). Deformation temperatures (T) ranged from 20-650°C, with a sliding velocity kept at 1 μm/s, and an imposed effective normal stress and pore fluid pressure at 100 MPa. At large shear strain (g ≈ 22-25) and T = 20-450°C, granitoid gouges consistently showed higher shear stresses (t = 73-81 MPa) than muscovite- (t = 47-69 MPa) and biotite-bearing gouges (t = 44-56 MPa). Granitoid gouges showed a decrease in t at T ≥ 450°C, while mica-rich gouges showed an increase in t with T at all tested conditions. Microstructurally, all gouges experienced strain localization into relatively fine-grained and dense principal slip zones (PSZs) at elevated T. The presence of newly percipitated minerals (e.g. bt, qtz) suggested the operation of dissolution-precipitation creep (DPC). However, the PSZs of granitoid and mica-rich gouges exhibited distinctive geometric features in their microstructure at 650°C. Granitoid gouges showed PSZs with ultrafine-grained (≤ 1 μm) relicts of porphyroclasts sparsely distributed within a dense matrix. In contrast, the PSZs of mica-rich gouges showed the anastomosing P-foliation of aligned micas with intervening shear band cleavages. Within these localized domains, quartz in mica-rich gouges exhibited larger grain sizes (1-4 μm) compared to those in granitoid gouges.  

Our observations indicate that in all tested gouges, frictional deformation gives way to grain-size sensitive creep mechanism as T rises, leading to the formation of fine-grained PSZs. We suggest that the ultrafine grain sizes in granitoid gouges promote DPC-accommodated viscous granular flow more efficiently, leading to the low shear stresses. In contrast, the strengthening of altered gouges with T was attributed to two factors: a less efficient DPC-assisted deformation due to generally larger grain sizes, and a less efficient viscous granular flow due to the development of foliation and shear bands inclined to the shear direction. Therefore, the mechanical behaviour of granitoids along the retrograde hydration-path depends not only on the evolving mineralogy, but also on microstructures and grain sizes.

Table 1. List of Samples Used in This Study and Their Mineralogy According to Quantitative XRD.

Qtz:quartz, fds: feldspar, bt: biotite, ep: epidote, phl: phlogopite, mus: muscovite, ser: sericite, smc: smectite, chl: chlorite, cal: calcite

How to cite: Zhan, W., Nevskaya, N., Niemeijer, A., Berger, A., Spiers, C., and Herwegh, M.: Hydrothermal Alteration-Induced Weakening in Experimentally Deformed Fault Gouges, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17448, https://doi.org/10.5194/egusphere-egu24-17448, 2024.

Worldwide, fault zones in carbonates regularly host medium to large earthquakes including recent ones in the Mediterranean and Middle East. In addition to that, faults can control fluid flow by either acting as a conduit or seal for fluid pathways and should be considered in e.g., geothermal exploration. Hence, understanding the (micro-) structural evolution of these fault zones as well as fluid mediated geochemical processes involved in their dynamic deformation history allows to better address topics of societal and economic relevance ranging from seismic hazards to the exploitation of natural resources. Unfortunately, active in-situ deformation at depth is difficult to access, emphasising the need for investigations on suitable exhumed analogues.

This study focuses on the microstructural and geochemical record of a recently exposed seismogenic dextral strike-slip fault zone in the seismically active southwestern Swiss Alps. Due to excellent outcrop conditions on glacially polished rock surfaces and a wide range of preserved tectonites and associated deformation structures, this particular fault zone provides a valuable record of potential paleoseismicity in carbonates. We combined microstructural analyses with micro-chemical and isotope data in order to reconstruct the spatio-temporal evolution of high-strain domains at variable crustal levels throughout exhumation. While the microstructural record allows us to differentiate between rate-dependent brittle and viscous deformation phases, we use the geochemical fingerprint to distinguish and characterize individual fluid pulses.

Here, we present microstructural evidence of fast, possibly seismic, deformation along a principal slip zone. While injection structures containing fluidized material, suggest highest deformation rates as feasible for seismic events, repeated brittle deformation that was accompanied by the formation of cataclasites and calcite veins, hints towards fast seismic to sub-seismic rates.
We also found that newly formed calcite crystals, in veins and linkage zones, show significantly decreasing δ¹⁸O_SMOWvalues, as low as 5 ‰ δ¹⁸O_SMOW, implying an influence of meteoric water. Clumped isotope thermometry of such calcites resulted in temperatures of 65-95°C, which are approximately 100°C lower than T_max in the area. This suggests that the analyzed material did not record any potential shear heating. Moreover, the investigated tectonites have most likely formed along a retrograde exhumation path. 

In combination with detailed observations on the m- to 10er-m-scale our observations provide a dataset that allows direct comparison of different deformation processes and correlation of paleo-seismicity to fluid flow in fault zones. Further, we contribute to the longstanding discussion of differentiating microstructural evidence for seismic slip from slow or aseismic slip in carbonate hosted fault zones.

How to cite: Schwichtenberg, B., Herwegh, M., Berger, A., Schrank, C., Neuenschwander, T., Truttmann, S., Jones, M. W., Bernasconi, S. M., Fleitmann, D., and Kewish, C. M.: Paleo-seismic and aseismic processes and the role of fluids recorded in an exhumed carbonate fault , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17549, https://doi.org/10.5194/egusphere-egu24-17549, 2024.

The presence of fluid inclusions in quartz veins is crucial to reconstruct fluid migration pathways in the subsurface. In this study, we provide an innovative approach to analyse  the hydrogen and oxygen isotopic composition of water fluid inclusions using cavity ring-down spectroscopy (CRDS). The CRDS is connected to a mechanical crusher in order to release fluid inclusion water from the host mineral. The evaporated fluid inclusion water from the crushed sample is added to a moistened background of nitrogen gas. For this purpose, we designed a temperature-regulated evaporation unit at Earth Science Stable Isotope Laboratory at the Vrije Universiteit Amsterdam (VU) to ensure that the isotopic composition and concentration of the background water vapour remains constant. The isotopic compositions of the fluid inclusions are calculated by subtracting the isotopic and concentration of the ‘wet’ background.

This newly designed setup allows for reliable measurements of the oxygen and hydrogen isotopic compositions of fluid-inclusions in quartz minerals. The objective of this study is to analyse the isotopic compositions of fluid-inclusions in quartz veins from distinct regions in Europe (Germany and Portugal), which are both linked to the Variscan orogeny. The isotopic data align with the modern Global Meteoric Water Line, providing evidence for the presence of meteoric fluids in the examined fold-and-thrust belts of the Variscan orogeny. Complementary microthermometry data, isotopic signatures of silicon and oxygen of  the quartz host mineral further document the cooling of hydrothermal systems under the influence of meteoric water at various geological events. This interpretation concords with the ⁴⁰Ar/³⁹Ar dating fluid rich fraction of quartz vein minerals.

How to cite: Huseynov, A. A. O., van der Lubbe, J., Verdegaal - Warmerdam, S., Postma, O., Kuiper, K., and Wijbrans, J.: Isotopic signatures of fluid inclusions from quartz veins record sub-surface fluid-rock interaction associated with the Variscan orogeny, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17577, https://doi.org/10.5194/egusphere-egu24-17577, 2024.

Geophysical observations have led to the conclusion that slow earthquakes (EQ) occur in the ductile realm at depths greater than ~20 km, in domains of low Vp and elevated Vp/Vs, consistent with high pore fluid pressure conditions and/or fluctuating fluid conditions. Furthermore, slow EQ are characterized by concomitant viscous aseismic slip and transient frictional slip responsible for tectonic tremors and low frequency earthquakes. Understanding the physics of slow earthquakes can be done by integrating geophysics, rock deformation experiments and numerical models with the observation and characterization of the possible rock record of slow earthquakes.

In this contribution, we contribute to the quest for geological records of slow EQ by adding a new example to the fast growing list of examples. Our approach is based on field observations, petrological and microtextural analysis carried out on exhumed shear zones in late Variscan volcanic rocks from the Suretta nappe (Central Alps, Switzerland). We propose that in continental collision settings, like the Alps, exhumed continental shear zones preserve geological evidence that may be related to paleo-slow earthquakes. We show that burial of continental units is characterized by concomitant frictional and viscous deformation in the ductile realm at temperature conditions above 450°C for a depth range between 18 and 25 km, which resembles those where slow earthquakes are expected. The finite geometry of the shear zone consists of a network of anastomosed mica-rich weak and high strain ductile shear zones of various size (m to km) bounding high strength and low strain domains, resulting in a “mélange” rheology. At a smaller scale, the localization of millimeter to centimeter wide ductile shear zones is controlled by the prior development of a damaged zone that has been detected by imaging volcanic quartz phenocrysts with cathodoluminescence. This damage zone is defined by a domain of high density healed cracks and fluid inclusion planes preserved only in volcanic quartz phenocrysts. These microcracks, follow a riedel-type geometry consistent with the ductile kinematics. The ductile shear zones are commonly crosscut by mono-mineralic quartz veins of a few millimeters in thickness parallel to the shear zone walls and localized in the middle of the shear zone. Quartz veins are characterized by a crack-seal texture with elongated blocky quartz grains perpendicular to the vein-wall interface suggesting that quartz was precipitated from a fluid in a dilatant fracture. These quartz veins are always overprinted by ductile deformation. Ductile deformation is characterized by dynamic recrystallization of the blocky quartz into small new grains formed by bulging recrystallization. When ductile deformation overprinting is high, quartz veins are sheared, isoclinally folded and almost entirely recrystallized into a fine grain aggregate.

How to cite: Goncalves, P., Leydier, T., Albaric, J., Trap, P., and Mahan, K.: Concomitant brittle-ductile deformation, fluid-flow and metamorphism during continental subduction : a slow earthquake rock record in the Suretta nappe (Central Alps, Switzerland) ?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17707, https://doi.org/10.5194/egusphere-egu24-17707, 2024.

Mirror-like Surfaces (MSs) are ultra-polished fault surfaces that reflect visible light, thanks to their nanometer-scale surface roughness. They are often found in seismogenic fault zones cutting limestones and dolostones. Both natural and experimentally-produced fault-related MSs have been described in spatial association with ultrafine matrix (grain size <10µm), nanograins (<100nm in size), amorphous carbon, decomposition products of calcite/dolomite (i.e., portlandite, periclase), and larger but “truncated” clasts. However, the formation mechanism of MSs is still debated. Experiments show that MSs can develop both under seismic (slip rate ≈1 m/s; Fondriest et al., 2013; Siman-Tov et al., 2015; Pozzi et al., 2018) and sub-seismic (slip rate ≈0.1-10 µm/s; Verberne et al., 2014; Tesei et al., 2017) deformation conditions, involving various physical-chemical processes operating over a broad range of P-T conditions, strain, and strain rates.

To evaluate whether the MSs formed during the co-seismic (possibly associated with frictional heat pulses) or the inter-seismic (no heat pulses) phases where temperature might serve as a distinguishing factor, we assessed the thermal maturity of “bitumen” using biomarkers. We acquired data for natural and artificial MSs hosted within bituminous dolostones. We collected natural samples from faults with slip displacement from a few millimeters to a few meters, located in the Italian Central Apennines (Monte Camicia Thrust Zone, past burial depths up to ~3 km). We obtained experimentally-produced MSs by deforming powdered bituminous dolostones in a rotary shear apparatus (SHIVA, INGV) at sub-seismic (V = 10^-4 m/s) and seismic (V = 1-3 m/s) slip rates for 1-3 m of slip, under room temperature and humidity conditions, and 20 MPa of normal stress.

We extracted solid bitumen of pre-oil window thermal maturity from the MSs and from the underlying slip zone of natural and artificial samples and we analyzed the bitumen using Gas Chromatography–Mass Spectrometry. We identified Steranes and other biomarkers based on relative retention time and measured peak heights to obtain thermal maturity parameters. By comparing different samples, changes in thermal maturity could be measured across slip zones bounded by the MS and possibly associated with frictional heat pulses during co-seismic slip.

Biomarker thermal maturity parameters are consistent with the immaturity of the host rock, which recorded a maximum ambient T < 100°C during diagenesis. In the experimental MSs produced at seismic slip velocity, where frictional heat pulses reached T∼400°C, thermal maturity of bitumen is higher than that of the entire slip zone and undeformed gouge. Higher thermal maturities were measured also in natural MSs but were not detected in the experimental MSs produced at sub-seismic slip velocity.

Chinello et al. (2023) proposed that the microstructures found in these slip zones recorded the main phases of the seismic cycle, from rapid co-seismic slip to post/inter-seismic viscous flow and fault strength recovery. The results presented here (1) confirm this interpretation, (2) show that the frictional heat pulse associated with seismic slip can be recorded by biomarkers thermal maturity of bitumen trapped in the fault MSs, and (3) some natural MSs are associated with heat anomalies caused by seismic ruptures.

How to cite: Chinello, M., Schito, A., Bowden, S. A., Tesei, T., Spagnuolo, E., Aretusini, S., and Di Toro, G.: Seismic Mirror-like Surfaces in bituminous dolostones (Central Apennines, Italy), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19532, https://doi.org/10.5194/egusphere-egu24-19532, 2024.

The Mw 7.8 earthquake of 2023 ruptured the southern (main) branch of the East Anatolian Fault (EAF), followed by the Mw 7.6 earthquake on the northern (Çardak Fault) branch of the EAF nine hours later. In March, we installed ten carbon-rod extensometers across segments of the main and northern branches of the ruptured faults, where potential slip deficits were considered possible, to investigate if afterslip continues. It is important to measure afterslip to understand the behaviour of a fault, if any, resulting from stresses associated with local coseismic slip deficits.  Seven of these extensometers recorded less than a few millimetres of slip since March. In Göksun near the western end of the Çardak fault, we recorded more than a 25 mm of accelerating afterslip preceding local aftershocks of magnitude ≤Mw5.1.

The Mw 6.8 Elazığ-Sivrice earthquake of January 24, 2020, and the Mw 7.8 Kahramanmaraş earthquake of February 6, 2023, stopped in the Pütürge region, where it is named as Pütürge Gap. To understand why these two earthquakes terminated there, an array of five extensometers were ultimately deployed. One of the extensometers, which is 52 m long, shows that slip > 3.8 mm/yr continues at depth. Extensometers spaced 45 km apart recorded an eastward propagating subsurface creep event in September 2023. Four cGPS stations recording at 5 Hz were installed in an array to better investigate the subsurface evolution of aseismic slip on the Pütürge Fault in the village of Taşmış.

How to cite: Ayruk, E. T., Turğut, M., Farımaz, İ., Köküm, M., Bilham, R., and Doğan, U.: AFTERSLIP of 6 FEBRUARY 2023 KAHRAMANMARAS EARTHQUAKE SEQUENCE : PRELIMINARY RESULTS, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-428, https://doi.org/10.5194/egusphere-egu24-428, 2024.

Earthquakes are the result of propagation at ∽km s^-1 of a rupture and associated slip at ∽m s^-1 along a fault. The total energy involved in a seismic event is unknown, but qualitatively most of it is dissipated by rock fracturing and frictional heat. Seismic fracture energy G (J m^-2) is the energy dissipated in the rupture propagation and can be estimated by the inversion of seismic waves. However, its physical significance remains elusive. G may include the contributions of both rock fracturing (energy to form new rock surfaces U_S, J m^-2) and fault frictional heating (Q, J m^-2) per unit fault area. Here we determine both U_S and Q in natural and experimental pseudotachylyte-bearing faults, following the approach used by Pittarello et al. (2008). In fact, in pseudotachylytes, or solidified frictional melts produced during seismic slip, (i) U_S is proportional to the surface of new fragments produced in both the slip zone and in the wall rocks, and (ii) Q is proportional to the volume of frictional melt.

The selected natural pseudotachylytes belong to the east-west-striking, dextral, strike-slip Gole Larghe Fault Zone (Adamello, Italian Alps). To estimate U_S we employed Electron Back-Scatter Electrons (EBSD), High Resolution Mid Angle Back-Scattered Electrons (HRMABSD) and Cathodoluminescence-Field Emission Scanning Electron Microscopy (CL-FESEM). In particular, CL-FESEM imaging reveals a microfracture network in the wall rocks that cannot be detected with the other techniques. In the pseudotachylyte-bearing fault, the microstructural analysis reveals (i) a high degree of fragmentation of the wall rock adjacent to the pseudotachylyte fault vein (formed along the slip surface), with clast size down to <90 nm in diameter, and (ii) a systematic difference in fracture density and orientation of the microfractures in the two opposite wall rock sides of the fault. In fact, in the northern wall rock the fracture density is low and the microfractures are oriented preferentially east-west, while in the southern wall rock the fracture density is high and oriented preferentially north-south. Instead, this asymmetric microfracture pattern is absent in the experimental pseudotachylytes produced by shearing pre-cut cylinders of tonalite (the rock that hosts natural pseudotachylytes) in the absence of a propagating seismic rupture. Thus, the formation of the asymmetric microfracture pattern is associated with the propagation of the seismic rupture and, therefore, can be used to estimate U_S.

In natural pseudotachylytes, fracture density decreases exponentially from the pseudotachylyte-wall rock contact towards the wall rock. The rock volumes with highest coseismic damage at the contact with the pseudotachylytes were assumed to represent the host-rock damage preceding frictional melting along the slip zone. Based on this assumption, U_S was estimated in the range 0.008-1.35 MJ m^-2, while Q was estimated from the thickness of the pseudotachylyte vein to be ∽32 MJ m^-2. In the case of the Gole Larghe Fault, numerical modelling of seismic rupture propagation yields fracture energies G in the range 8-67 MJ m^-2 suggesting that U_S is a subordinate component of G and that most of the seismological fracture energy is heat.

How to cite: Aldrighetti, S., Di Toro, G., and Pennacchioni, G.: Estimate of seismic fracture surface energy from pseudotachylyte-bearing faults, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2299, https://doi.org/10.5194/egusphere-egu24-2299, 2024.

How to cite: Ju, F., Meng, H., Chen, X., and Yu, C.: Detection of Immediate Foreshocks Using Dense Seismic Array: A Case Study of the 2021 Ms 6.4 Yangbi aftershock sequence, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3922, https://doi.org/10.5194/egusphere-egu24-3922, 2024.

A general correlation between maximum co-seismic fault slip and fault length has been well constrained by observations from numerous earthquakes occurring on many crustal faults. In spite of this global consistency, there are notable examples of co-seismic fault slip magnitudes that far exceed the expected maxima for the fault dimensions.  Two instances of extreme fault slip that occurred on short-to-moderate length upper-plate faults in subduction systems, where the megathrust lies at shallow depths are the 2016 Kaikoura (New Zealand) earthquake, and the 1855 Wairarapa (New Zealand) earthquake. In both cases, co-seismic fault slip was 5 to 10 times greater than expected from the rupture length - fault slip scaling relationships. The typical crustal fault model  is a fault with a brittle-to-ductile transition at depth; in this scenario, co-seismic slip is inhibited by viscous resistance from the deeper, ductile component of the fault.  In contrast, the tectonic characteristics of faults that experience extreme co-seismic slip, involve upper plate faults that truncate against the megathrust at seismogenic depths (i.e. are fully frictionally coupled over their entire depth extent). During a megathrust earthquake, the plate interface unlocks and upper-plate faults that extend to the ruptured plate interface transiently experience free-slip boundary conditions on both their upper (surface) and lower (megathrust) ends. As a result, such upper-plate faults can potentially experience full strain release (and therefore maximum slip), independent of their length. For appropriately oriented faults, this effect may be enhanced by co-seismic stress changes associated with the megathrust earthquake. 

Geologic evidence of large displacement (and/or displacement rate) upper-plate faults in other subduction systems indicates this process may commonly occur. One example is a set of upper-plate faults along the Cascadia margin (near Newport, Oregon), that have strike-slip geologic slip rates, averaged over 10s of thousands of years, exceeding tens of mm/yr and approaching local plate convergence rates. These upper-plate Cascadia faults are also located where the plate interface is sufficiently shallow and seismogenic, indicating that these high-slip, upper-plate faults are likely frictionally locked over their entire depth range. In spite of the high overall slip rates of these upper-plate faults, because they are locked along their entire depth extent between earthquakes,  they may be unrecognized by inter-seismic geodetic observations.

How to cite: Furlong, K. P., Herman, M. W., and McKenzie, K. A.: Big Slip on Small Faults - How Does Extreme Fault Slip Occur on Short-to-Moderate Length Faults, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4427, https://doi.org/10.5194/egusphere-egu24-4427, 2024.

We use geodetic measurements to characterize aseismic deformation along the western boundary fault of the Dead Sea pull-apart basin, which is located at the southern part of the sinistral Dead Sea Fault. This research provides constraints on patterns and timescales of deformation and its dependence on regional tectonics and the rheology of the upper crust. We use creepmeter, GNSS, InSAR and airborne LiDAR observations and show transient aseismic slip on the western boundary fault of the Dead Sea basin. A biaxial creepmeter with a 30 s sampling interval was installed in early 2021 showing high extensional deformation (an average rate of ~8.6 mm/yr), which is consistent with the ~30 cm of subsidence recorded 2017-2019 differential LiDAR data. The data imply modulated slip on a 60° dipping normal fault with maximum slip rates of ~0.5 µm/hour starting in late August and varying close to zero in late April. We attribute these large movements to local tectonics, sediment compaction, thermo-elastic response and dissolution of subsurface salt responsible for the formation of sink-holes in the region. The creepmeter measurements also show some sinistral deformation with an average rate of ~2.1 mm/yr, comparable to the rate of 2.5±0.4 mm/yr that was observed for the Sedom Fault, the southernmost segment of the western boundary fault, using GNSS data. Several minor creep events were detected by the creepmeter. The 19 Feb 2022 creep event lasted more than an hour following heavy rain in this area with abrupt sinistral slip of ~2.5 mm preceding dilation and dip-slip by 20 minutes. Small Baseline Subset (SBAS) analysis of InSAR data reveals up to 7mm/yr of line-of-sight deformation across the western boundary fault, north of the creepmeter. It also reveals high subsidence rate (up to ~20 mm/yr) along the southern shores of the Dead Sea Lake that can be explained by high compaction rate of clay sediments and reduction of pore pressure along the lake shores. This high subsidence rate is also observed in our near shore GNSS stations. Our results indicate that deformation within the Dead Sea basin is not solely controlled by the active tectonics. The observed vertical deformation is apparently modulated by the response of sediments to seasonal variations of local conditions.

How to cite: Hamiel, Y. and Bilham, R.: Characterizing shallow creep along the Dead Sea pull-apart basin using geodetic observations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4550, https://doi.org/10.5194/egusphere-egu24-4550, 2024.

The interactions between aseismic slip and seismicity is mostly studied during postseismic afterslip associated with the generation of aftershocks following large earthquakes. However because of the large stress perturbations produced by the coseismic rupture, it remains difficult to distinguish the contribution of the coseismic stress perturbation and the effects of stresses induced by the afterslip on the triggering of aftershocks. Studying seismicity triggered by slow slip events enables us to understand the direct effect of aseismic slip on the generation of the seismicity. While most subduction slow slip events are deep-seated, at the down-dip edge of seismogenic zones accompanied by tectonic tremors, some are also observed at shallower depths associated with seismic swarms. Among them are the well-studied Boso slow slip events located on the Sagami trough, between 10 and 20 km depth. Recorded every ~4 years since 1996, they are always accompanied by swarms of Mw 1 to 5 earthquakes on their northern and western flanks. Being located right beneath the Boso Peninsula coastline, the kinematics of these slow slip events is particularly well imaged by dense GNSS and tiltmeter networks. In this study, we model the time dependent aseismic slip of the 2018 Boso slow slip event, with the largest moment released of all Boso slow slip events, by inverting time step by time step the slip on the fault with joint GNSS and tiltmeter data. We do not impose arbitrary temporal smoothing in the inversion and find that the well constrained fault slip is first growing and then migrating southwestward with a migration speed of ~ 2 km/day. In order to model the interaction with the seismicity, we compute the Coulomb stress change due to the transient slip on receiver faults located in 9 cubes centered in the seismicity swarm and parallel to the subduction interface. From June 2nd to June 18th, the seismicity is migrating up-dip at a rate of 1 km/day. This migration period coincides with the peak slip rate and with Coulomb stress produced by the slow slip migrating updip together with the seismicity, indicating a causal relationship. Adopting a rate and state friction formalism to explain the nucleation of the seismicity, we finally investigate the ensemble of parameters, in particular the constitutive parameter that relates changes in stress to logarithmic changes of slip velocity, the effective normal stress and the tectonic stressing rates, that can explain the seismicity rate. 

How to cite: Rousset, B., Inbal, A., Bürgmann, R., Uchida​​, N., Socquet, A., Marill, L., Matsuzawa, T., and Kimura, T.: Boso seismicity swarm propagation driven by slow slip stress change, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4561, https://doi.org/10.5194/egusphere-egu24-4561, 2024.

The shallowest region of subduction megathrust accommodates deformation by a spectrum of seismic modes including continuous aseismic creep and peculiar seismic phenomena as slow slip events. However, the mechanisms behind these phenomena remain enigmatic because they are not explained by conventional frictional models. This because the shallowest regions of subduction zones are characterized by unconsolidated, clay-rich lithologies that, nominally, cannot nucleate seismic events due to their frictionally weak and rate-strengthening attributes. Here we present laboratory friction experiments showing that clay-rich experimental faults with bulk rate strengthening behavior and low healing rate can contemporaneously creep and nucleate slow slip events. These instabilities are self-healing, slow ruptures propagating within a thin shear zone and driven by structural and stress heterogeneities. We propose that the bulk rate-strengthening frictional behavior promotes the observed long-term aseismic creep whereas local frictional mechanism causes slow rupture nucleation and propagation. Our results illustrate the complex behavior of clay-rich lithologies, providing a new paradigm for the interpretation of the genesis of slow slip as well as significant implications for seismic hazard.

How to cite: Volpe, G., Collettini, C., Taddeucci, J., Marone, C., and Pozzi, G.: Complex Frictional Behavior of Clay and Implications for Slow Slip , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5061, https://doi.org/10.5194/egusphere-egu24-5061, 2024.

The acoustic emissions (AEs) produced during the shearing of granular materials reflect the accumulation and release of stress, offering valuable insights into the failure mechanisms of seismic faults and stick-slip-controlled landslides. While various characteristics such as amplitude, energy, counts, and frequency of AE signals generated by stick-slip have been studied, the stress changes corresponding to different frequency AEs at various stages of the stick-slip process remain unclear. This knowledge gap hinders our understanding of the precursory signals leading to stick-slip failure. In order to enhance our comprehension of the physical mechanisms underlying granular stick-slip, we conducted monitoring of both mechanical and AE signals using high-frequency (2 MHz) synchronous acquisition. This was done during the constant-speed shearing of packs containing uniform glass beads of different sizes under varying normal stresses. Our findings revealed an accelerated release rate of AE energy in tandem with sample volume dilatation. Additionally, the stress drop during stick-slip increased with higher normal stress and particle size. This study identified three distinct events during a single cycle of stick-slip: main slip, minor slip, and microslip. We analyzed the AE frequency spectra associated with each of these events. Main slip and minor slip correlated with stress drop, generating high-frequency AEs (approximately several hundred kHz). In contrast, microslip produced lower AE frequencies (around tens of kHz) and exhibited stress strengthening. These characteristics, overlooked in prior studies due to low-frequency acquisition, suggest that microslip is primarily a result of sliding on grain contacts, while main slip and minor slip arise from the breakage and reformation of force chains. The low-frequency AEs from microslip may serve as a crucial precursor to seismic faults and landslides, providing a deeper understanding of the granular stick-slip phenomenon.

How to cite: Gou, H. and Hu, W.: Detection of Stick-Slip Nucleation and Failure in Homogeneous Glass Beads Using Acoustic Emissions in Ring-Shear Experiments: Implications for Recognizing Acoustic Signals of Earthquake Foreshocks, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5252, https://doi.org/10.5194/egusphere-egu24-5252, 2024.

This study investigates the dynamic interplay between fluids and fault slip transients in the portion of subduction zones subject to slow earthquakes. The permeable subduction interface in this region is believed to be saturated with fluids supplied by metamorphic dehydration reactions in the downgoing plate. Following Farge et al. (2021), we consider a model of a heterogeneous subduction channel filled with low-permeability plugs that behave as elementary fault-valves. Such a system is characterized by an intermittent fluid transport and rapid and localized pressure transients. Episodic rapid build-ups and releases of the fluid pressure affect the frictional strength on the fault and can result in transient slip accelerations. To study the possible effect of episodic fast fluid pressure variations on fault slip, we use numerical simulations in a 2D in-plane shear geometry. The fault is governed by rate-and-state friction, with velocity-strengthening steady-state properties, and is forced with time and spatially variable pore fluid pressure. In an initial set of tests, we show that periodic pore pressure oscillations can accelerate the fault slip akin to observed slow slip events. We then investigate how the fault slip responds to more complex and “realistic” pore pressure histories generated by the dynamic permeability model of Farge et al. (2021). Our results underscore the possible role of input fluid flux and permeability structure in determining the variations of fault slip and, in particular, in facilitating the slow slip events. 

How to cite: Gupta, A., Shapiro, N. M., Ampuero, J.-P., Farge, G., and Jaupart, C.: Interaction of fault slip with fast fluid pressure transients in subduction zones, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5921, https://doi.org/10.5194/egusphere-egu24-5921, 2024.

The present-day Eurasia-Africa plate convergence offshore SW Iberia gives rise to a diffuse plate boundary marked by deep lithospheric thrust and strike-slip faults. The Horseshoe Abyssal Plain Thrust (HAT) stands out as a key structure accommodating plate convergence, and it has been the site of deep (> 30 km depth) and large magnitude (Mw > 6) earthquakes. Additionally, the HAT has been proposed to be the source of the 1755 Lisbon earthquake (estimated Mw≥8.5), one of the most destructive earthquakes and tsunami in the history of Europe. The geometry of the fault and the physical properties of rocks surrounding it have been determined through tomographic models derived from controlled-source seismic data. Although large earthquakes along the HAT primarily occur at considerable depths within the peridotitic mantle (~40 km depth), the fault intersects a region of serpentinized mantle at shallower depths (10-20 km depth). In contrast to peridotite that undergoes seismic deformation, the frictional behaviour of serpentinized peridotite depends on factors such as pressure, water content, temperature, and slip velocity. Laboratory measurements indicate that serpentinite transitions from rate-strengthening behaviour at plate tectonic rates to rate-weakening at seismic slip rates. This dual nature suggests that large deep earthquakes, nucleated in pristine peridotite, could rupture seismically through the weaker serpentinized peridotite. While this mechanism has been proposed to explain the HAT's potential to generate large tsunamigenic earthquakes, it remains untested. In this study, we use dynamic rupture numerical simulations to investigate the role of serpentinized peridotite in the rupture process and the tsunamigenic potential of the HAT. In particular, we explore various frictional scenarios to determine the slip pattern necessary to account for the previously estimated tsunamigenic uplift associated with the 1755 Lisbon earthquake.

How to cite: Prada, M., Martínez-Loriente, S., B. Ruh, J., and Sallarès, V.: The role of serpentinized mantle on thrust-fault earthquake dynamics offshore SW Iberia, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7813, https://doi.org/10.5194/egusphere-egu24-7813, 2024.

Topographic features such as seamounts can influence the buoyancy of the slab and the short- and long-timescale mechanical properties of the subduction interface. How seamounts in the trench interact with the upper plate accretionary wedge during subduction— their stress field, their potential for ‘decapitation’, and their ability to host large megathrust earthquakes— is not fully understood. We utilize exhumed rocks to investigate seamount–upper plate interactions at shallow subduction interface conditions. 

We focus on a 250-m-thick cross-section of deformed, weakly metamorphosed basalt, limestone, chert, argillite and greywacke exposed in the inboard part of the Chugach accretionary complex near Grewingk glacier, southern Alaska. Temperatures from Raman spectroscopy on graphite yield ~260°C, suggesting deformation and metamorphism down to ~15-20 km depth. Detrital zircon data from greywacke lenses within and outside the shear zone overlap within error suggesting emplacement over less than ~1 m.y. at ~167 Ma. 

Basalts in the shear zone are dismembered into ~3 slices up to 35 m thick, all of which contain limestone patches suggesting the basalt is derived from the seamount’s very top (limited decapitation). The basalt slices are bounded by high-strain melange-like shear zones up to 25 m thick, interpreted to represent décollements along which the seamount slices were underplated. These mélange belts exhibit a block-and-matrix texture with a macroscopically ductile argillite and chert matrix, and pervasively disaggregated and brittlely deformed greywacke and basalt lenses. Both the matrix and the blocks show several generations of dilational and shear veins, suggesting high fluid pressures and low differential stresses. Features suggesting deformation at fast (potentially seismic) strain rates include fluidized cataclasites, but these do not extend along strike for more than 0.25 m and do not occur within the larger (m-to-dm-scale) basalt lenses, suggesting that large-magnitude earthquakes were limited during seamount underplating. Instead, the observed mix of brittle and macroscopically ductile deformation at high fluid pressures is more consistent with a potential record of shallow tremor and slow slip.

Our findings support geophysical observations and numerical models that suggest relatively weak mechanical and seismic coupling between seamounts and the overriding plate, and are consistent with recent suggestions (e.g. for the Hikurangi margin) that sediment envelopes around subducting seamounts are conducive to slow slip and tremor.

How to cite: Behr, W., Akker, I. V., and Rast, M.: Deformation processes during seamount dismemberment and underplating along the shallow subduction interface: a case study from the Chugach Complex, Alaska, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8244, https://doi.org/10.5194/egusphere-egu24-8244, 2024.

A large number of tourmaline fault mirrors are exposed in the north-south normal fault system in the southern part of the Tibetan Plateau. Microstructure analysis shows that the tourmaline fault mirror has the characteristics of co-seismic high speed friction sliding and high temperature plastic rheology. In order to reveal the mechanical process of friction-rheological strength and co-seismic slip of tourmaline fault, the frictional and rheological experiments were carried out on the gas-medium triaxial high temperature and high pressure experimental system using undeformed tourmaline in southern Tibet to determine the formation conditions of tourmaline fault mirror. The effective normal stress of frictional experiments is 100Mpa.The pore water pressure is 30MPa. The temperature is 25-500℃, and the shear slip rate is switched between 1μm·s^-1, 0.2μm·s^-1, 0.04μm·s^-1. The experimental results show that stick-slip occurs at 200-350℃, and the speed weakens at 400℃ and 500℃. The rheological experiment temperature is 850-950℃. The pressure is 300MPa, and the strain rate is switched between 2*10^-5s^-1, 1*10^-5s^-1, 5*10^-6s^-1, 7.5*10^-6s^-1. The experimental results show that the natural tourmaline sample is mainly fractured flow under the experimental conditions. The strength of hot-pressed dry tourmaline sample decreases with increasing temperature. The rheological strength of water samples synthesized by hot pressing was significantly reduced.

How to cite: Li, X., Zhou, Y., Cheng, L., and Li, J.: Implications of tourmaline frictional and rheological experiments on fault strength and sliding stability in southern Tibet, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8731, https://doi.org/10.5194/egusphere-egu24-8731, 2024.

Low-angle normal faults (i.e. with a dip < 30°) were assumed to have a very low seismic potential (Sibson et al., 1985). However, several observations have shown that earthquakes and aseismic slip can occur along such faults. For instance, the Alto Tiberina Fault (ATF), a 60-km long normal fault with a 15° low angle dip located in the active sector of the Northern Apennines (Italy), is seismically active as well as is actively accommodating part of the Apennines extensional strain. However, the relative contribution of seismic and aseismic slip on it is still unclear. The central and northern Apennines experienced several seismic sequences in the recent decades and a M_w ∼ 4.6 aseismic event accompanied by a seismic swarm of similar or smaller size was also recorded in 2013-2014 along two synthetic and antithetic fault in the hanging-wall of the ATF (Gualandi et al., 2017). The interactions between such minor conjugate faults and the ATF compose a system undergoing complex behavior making the area an ideal candidate to improve our understanding of interactions between different slipping modes. We benefit from data of the Alto Tiberina Near Fault Observatory (TABOO-NFO; Chiaraluce et al., 2014) looking for aseismic events on the ATF and its surrounding faults. The dense network of GNSS, seismometers and borehole strainmeters provides a rarely attained high spatial (inter-distance < 10km) and temporal (from 2009 to nowadays) resolution framework enabling the study of the ATF fault system slip history. We search for transients with a semi-automatic detection tool of slow slip events based on kinematic inversions of strainmeters time series. We also test if these events interact with larger seismic events of the region. We present the strain time series processed with the EarthScope Strain Tools (EarthScope Consortium) and the preliminary signals detected with our tool. The fine analysis of the ATF would help better constraining the behavior of faults and more generally large events. 

How to cite: Tissandier, R., Gualandi, A., Chiaraluce, L., Serpelloni, E., Gottlieb, M., Hanagan, C., and Marone, C.: A semi-automatic detection for transient events in northern Apennines using strainmeters and GNSS data, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8767, https://doi.org/10.5194/egusphere-egu24-8767, 2024.

In recent decades, there has been a proliferation of observations related to spatiotemporally intricate slip events occurring in fault systems. These events encompass a spectrum of transient energy releases, ranging from slow slip events to low-frequency earthquakes (LFEs) and tremors, in addition to the more familiar creep and fast ruptures. The prevailing focus in recent research has been to interpret these events by considering variations in frictional behavior along the fault plane.

However, it is crucial to acknowledge the inherent geometric complexity of fault systems across multiple scales. Recent studies have illuminated the significance of incorporating a fault volume or damage zone surrounding the fault in the analysis of slip dynamics. In the context of this study, we endeavor to investigate the influence of "realistic" fault geometry on the dynamics of slip events. To achieve this, we approach the problem from three interrelated perspectives:

• Forward Source Modeling: We employ forward source modeling techniques to simulate and understand the behavior of slip events.
• Bridging Source Modeling and Observations: We establish a connection between our source modeling and observed data by generating synthetic surface records that can be compared to actual observations.
• Energy Budget Analysis: We meticulously analyze the variations in the energy budget that occur throughout the earthquake cycles to gain insights into the mechanics of slip events.

Our primary objectives include deciphering how deformation within the volume is accommodated by both the off-fault damage zone and the primary fault. Specifically, we aim to determine the proportion of the supplied moment rate that is absorbed by off-fault fractures during an earthquake cycle. Additionally, we seek to unravel how the diverse sequences of complex behavior observed on the fault plane manifest in the signals recorded by seismic stations. This entails assessing the distinct contributions of the main fault and off-fault fractures to the radiated signals detected at the monitoring stations. Lastly, we delve into the evolution of the medium's energy budget throughout the earthquake cycles and evaluate the dissipative contribution of off-fault fractures to ascertain their energetic role in the context of earthquake cycles.

How to cite: Bhat, H., Almakari, M., Kheirdast, N., Villafuerte, C., and Thomas, M.: A mechanical insight into the continuous chatter of a fault volume, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9007, https://doi.org/10.5194/egusphere-egu24-9007, 2024.

Fluid induced seismicity represents a significant issue for numerous activities related to geo-energy production. Enhanced geothermal systems, enhanced oil recovery, disposal of wastewater and carbon dioxide capture and storage are associated with subsurface fluid injection that can change the state of stress within the crust and can induce or trigger earthquakes. In several regions, M>3 earthquakes occurred following fluid injection, whereas in others seismicity has been accompanied by slow slip events. Although several mechanisms have been proposed to explain slow slip associated with fluid injection, the conditions leading to the observed spectrum of fault slip behavior still remain elusive. Here we used a quasi-dynamic boundary element method, the QDYN earthquake cycle simulator, to model the response of a fault governed by rate-and-state friction to fluid injection within a reservoir. We imposed low long-term loading rates to simulate a fault located in an area of slow active deformation, leading to natural earthquake cycles with very long recurrence times. We then imposed fluid pressure perturbations (one-way coupling) at different stages of the seismic cycle, to evaluate pore-pressure effects on the triggering of the next event. 

Our results show that for injection at high fluid pressure, earthquakes are in general immediately triggered (during injection or soon after) irrespective of the stage (early or late) of the seismic cycle, whereas at lower fluid pressure fast triggering is observed only when injecting in the late stages of the seismic cycle. Our models produce a spectrum of fault slip behavior, from regular to slow earthquakes. The latter are observed for specific fluid pressure, flow rate and injection time relative to the seismic cycle. The physics underlying this complex slip behavior remain to be explained, and further studies are required to define the injection conditions that favor the occurrence of slow slip instead of regular earthquakes.

How to cite: Pardo, S., Tinti, E., Van den Ende, M., Ampuero, J.-P., and Collettini, C.: A spectrum of fault slip behaviors induced by fluid injection on a rate-and-state fault depending on time of injection relative to its natural fault cycle, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9169, https://doi.org/10.5194/egusphere-egu24-9169, 2024.

Aseismic slip has been recognized over the last 50 years as one of the modes of elastic stress accommodation by large tectonic faults. Long strike slip faults have been imaged with InSAR and scrutinized with GNSS networks and creepmeters, revealing the strikingly ubiquitous occurrence of aseismic slip globally. From the 600-km-long creeping section of the Chaman to the 70 km-long Ismetpasa creeping section along the North Anatolian Fault, geodetic imaging illustrates the rich behavior of such aseismic slip, from mm-scale transient episodes of slip to seemingly continuously sliding fault segments.

Most models explaining the occurrence of aseismic slip along continental faults rely entirely on an ad hoc parameterization of the frictional rheology of the fault. While the friction law governing slip along faults has been determined from laboratory experiments, the inference of the constitutive parameters of such friction law entirely derives from reproducing geodetic data in most cases. In particular, most creeping sections are interpreted as the signature of rate-strengthening material, diffusing stress through stable sliding. However, most rocks at seismogenic depths exhibit a rate-weakening behavior and some even show transient episodes of slip incompatible with purely strengthening properties. In addition, other mechanisms, including complex geometric configuration of faults or fluid circulation may offer the conditions for slow slip. Therefore, the direct inference of constitutive properties of a fault zone from kinematic observations may not be simple.

We propose here a model in which upwelling of fluids sourced in the upper mantle through a vertical fault zone leads to the conditions for slow slip, irrespective of the fault constitutive properties. We map aseismic slip along three different fault zones, including the North Anatolian Fault (Turkiye), the San Andreas Fault (USA) and the Leyte fault (Philippines) and find a systematic relationship between the effective locking depth and the occurrence of aseismic slip. Our model explains this modulation of locking depth along strike and the subsequent modulation of surface shear stressing rate with the along strike variation in the mantle fluid source. This model applied to fault segments with relatively high mantle fluid source leads to low effective normal stress, large nucleation size of a frictional instability, and predicts occurrences of shallow aseismic slip. We perform numerical modeling to show that the critical parameter is the flux of upwelling fluid through the fault zone, which increase leads to the widening of the near-surface region of aseismic slip and transition to full-fault aseismic slip at large enough flux. We finally discuss the potential sources of fluids explaining such behavior.

How to cite: Jolivet, R., Garagash, D., Pierre, D., and Jara, J.: Creeping sections on continental strike slip faults as the signature of deep fluid upwelling, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9437, https://doi.org/10.5194/egusphere-egu24-9437, 2024.

Deformation within the upper crust is mainly accommodated through slip on fault systems. These systems can accommodate slip via different modes, going from aseismic creep (i.e., stable motion) to dynamic earthquake (i.e., unstable motion). Notably, a single fault is not confined to a specific slip mode, as recent geodetic observations have indicated that a single fault can exhibit both stable and unstable motions. The distinct slip behaviours have been attributed to fault spatial heterogeneity of the frictional properties, rheological transitions, or geometrical fault complexities.

To comprehensively characterize the impact of frictional heterogeneities, we deformed heterogeneous fault samples in a triaxial apparatus, at confining pressure ranging from 30 to 90 MPa. The fault planes, sawcut at a 30° angle from the sample axis, consisted of two materials: granite and marble. Experiments were conducted for both marble asperities embedded in granite and vice versa, alongside homogeneous fault samples of single lithology. The selection of granite and marble was based on their different mechanical and frictional characteristics, with granite exhibiting seismic behaviour, while marble demonstrated aseismic behaviour across the pressure range tested.

Our findings reveal that the stress drops of seismic events are dependent on fault composition, with faults containing higher granite content exhibiting larger seismic events. In addition, by coupling the inversion of the kinematic slip from strain-gauge measurements and the records of acoustic activity during experiments, we demonstrate that the nucleation and propagation of seismic events are significantly influenced by lithological heterogeneity on the fault plane. In the case of homogeneous faults, the seismic event nucleation is relatively straightforward, initiating in the highest stressed region and propagating uniformly. Conversely, heterogeneous faults display more intricate nucleation patterns, often featuring multiple nucleation regions converging into a major dynamic event. The dynamic event propagation is expedited when traversing granite areas and more restrained within the marble. Remarkably, our experiments demonstrate that heterogeneities are required in order to induce earthquake afterslip. These results emphasize the crucial role of fault heterogeneity in earthquake nucleation and propagation, highlighting that even minor lithological heterogeneities are sufficient to complicate laboratory earthquake dynamics.

How to cite: Noël, C., Dublanchet, P., and Passelègue, F.: Exploring the impact of frictional heterogeneities on the seismic cycle: Insights from laboratory experiments, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9595, https://doi.org/10.5194/egusphere-egu24-9595, 2024.

How to cite: Bletery, Q. and Nocquet, J.-M.: Do earthquakes start with precursory slow aseismic slip?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9961, https://doi.org/10.5194/egusphere-egu24-9961, 2024.

Many previous studies have explored the role of granular media in controlling friction of faults. A gap exists though in understanding the failure process and sliding of a fluid-saturated fault gouge. Here we use a coupled 2D DEM-fluid code to simulate fault-gouge as a layer of grains, sheared by a constant stress boundary. We explore and compare two scenarios: 1) a dry granular layer, in which shear stress on the top wall is incrementally increased, or 2) a fluid-saturated granular layer, into which fluid is injected, so that fluid pressure is incrementally increased. Once the applied stress/pressure is high enough, the layer fails and starts accelerating, until it reaches a steady-state sliding rate (determined by the layers’ velocity-strengthening friction). We next incrementally step-down the shear stress or fluid pressure. Consequently, the slip-rate is observed to slow down linearly with decreasing stress/pore-pressure, until the layer finally stops, at a stress/pressure lower than that required to initiate the failure. Both the dry and fluid-saturated granular systems exhibit two main behaviors: 1) velocity-strengthening friction, following the mu(I) rheology, 2) a hysteresis effect between friction and velocity, porosity and grain coordination numbers. The hysteresis and strain-rate dependence agree with previous experimental, numerical and theoretical results in dry granular media, yet our work suggests these behaviors extend to fluid-filled granular media. We theoretically predict the transient and steady-state observations for dry and fluid-saturated layers, using the mu(I) friction rheology with an added component of hysteresis. Importantly, we show that fluid-filled faults exhibit a process which is absent in dry systems: fluid-injected layers may exhibit failure delay, with some time passing between pressure rise and failure. We link this delay to pre-failure creeping dilative strain, interspersed by small dilative slip events. Our numerical and analytical results may explain: (i) field measurements of fault creep triggered by fluid pressure rise (e.g. via injection), (ii) fault motion which is triggered by fluid-injection but continues even after fluid pressure returns to its pre-injection level. (iii) observed delay prior to failure in fluid-injection experiments.

How to cite: Aharonov, E., Sarma, P., Toussaint, R., and Parez, S.: Fluid-induced failure and sliding of a gouge-filled fault zone: Hysteresis, creep, delay and shear-strengthening., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10783, https://doi.org/10.5194/egusphere-egu24-10783, 2024.

A growing number of observations made using geodetic approaches have been able to detect large preparatory regions that experience accelerated deformation prior to and in close proximity to an earthquake’s hypocenter. An uptick in localized seismicity has also been observed in these regions and represents an opposite end-member of the spectra of deformation, in both space and time, from the opposite broad and slow process. If and how these preparatory observations are linked are not well understood. To study this, we conducted a triaxial experiment on a granitic rock sample instrumented with calibrated acoustic emission (AE) sensors and a distributed strain sensing (DSS) method using fibre optics. These two technologies were sensitive to seismic (100 kHz to 1 MHz) and aseismic (DC to 0.4 Hz) deformation at our sample scale and these were monitored as it was loaded and experienced brittle shear failure. DSS measurement allowed us to visualize the emergence of slow, heterogeneous strain fields that localized well before the failure of the sample. In the early stages of localized deformation, the regions exhibiting preferential damage were growing and doing so without producing seismicity. However, when approaching failure, these regions accommodating slow deformation began to accelerate and now produced clusters of seismicity. The cumulative seismic moment of the precursory seismicity was a fraction of the total anelastic deformation (< 0.1%) precluding the runaway dynamic failure. We also examined the clustering and frequency-magnitude distribution of the seismicity with respect to the localized strain field. In the later stages, moments prior to nucleation, the b-value begins to drop and becomes anti-correlated to the rapidly accelerating average volumetric strain rate measured using the DSS array. This observation better constrains the hypothesis that dilation of the relatively large preparation zone can host larger precursory earthquakes therein. These findings can help constrain models that better replicate the physics associated with the large spectrum of brittle deformation and will in turn help with our understanding of preparatory earthquake processes.

How to cite: Selvadurai, P. A., Salazar Vasquez, A. F., Bianchi, P., Madonna, C., Germanovich, L., Puzrin, A. M., Rabaiotti, C., and Wiemer, S.: Localizing slow deformation holds crucial information related to seismicity patterns precluding brittle failure in crystalline rocks, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10887, https://doi.org/10.5194/egusphere-egu24-10887, 2024.

How to cite: Thomas, M. Y. and S. Bhat, H.: Combined Effect of Brittle Off-Fault Damage and Fault Roughness on Earthquake Rupture Dynamics, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12095, https://doi.org/10.5194/egusphere-egu24-12095, 2024.

“Fault healing” is the ability of fault rocks to recover strength after rupture, due to a combination of several physical processes that include cementation, compaction, asperity growth etc. Healing is fundamental in the earthquake physics because it allows for the repeated accumulation of energy along faults over multiple seismic cycles. Fault healing is commonly studied in the laboratory, through Slide-Hold-Slide (SHS) tests and cementation experiments. However, laboratory measurements and the microstructures of experimental fault rocks are difficult to compare with natural rocks, due to the difference in kinetics of physical mechanisms and the small spatio-temporal scale of experiments.

Here, we review the field and microstructural evidence of various processes of fault healing along a carbonatic fault surface, taking advantage of an outstanding case study: the Pietrasanta Normal Fault (NW Tuscany, Italy). In the field, the most common evidence of fault healing is the occurrence of cohesive fault rocks (cataclasites) and veins, but other fault surface properties may influence the re-strengthening of fault surfaces: e.g. adhesion phenomena (sidewall ripouts and fault surface patches) and geometrical complexity.

We compare these observations with frictional healing experiments carried out on carbonatic fault rocks, in which both fault gouges and cohesive slip surfaces were used. We propose that a fault surface composed by “patches” of cohesive fault rocks bounded by anastomosing slip zones are the result of complex cycles of gouge formation and healing, which modulate the interplay of adhesion and localization along the fault surface.

How to cite: Tesei, T., Molli, G., Mittempergher, S., Pozzi, G., and Remitti, F.: Healing of fault surfaces: a field vs. experimental perspective, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12752, https://doi.org/10.5194/egusphere-egu24-12752, 2024.

In earthquake research, the discovery and ongoing investigation of interseismic transient processes has revealed that faults are non-steady between large earthquakes. These transients are typically identified in continuously operating tectonic GNSS stations, whereby an acceleration away from the average interseismic rates of displacement can be identified with a variety of time series analysis methods. However, the features of these transients can vary depending on the processing strategy employed to derive displacement time series from the raw GNSS observables.

In the processing strategy, the definition of the geodetic datum is necessary to determine global terrestrial reference frames (TRFs), providing an accurate and stable absolute reference of Earth's locations. It is essential for comprehending the dynamic changes in Earth's geometry driven by factors like tidal and non-tidal loading, plate tectonic seismic activity, and ongoing climate change. Therefore, just as geodesy aims for accuracy and stability in the TRF, the datum definition—i.e., the realization of the TRF-defining parameters origin, orientation, and scale—may emerge as a critical factor in processing GNSS networks for geodynamic purposes.

The purpose of this study is to assess up to what extent the transient velocities obtained from GNSS-derived displacement time series change under different regional and global datum definitions for the Cascadia subduction zone and Hikurangi margin; regions with very well-known catalog of interseismic transient tectonic events. In our study, we process data from Cascadia to produce network solutions both NNR (No-Net-Rotation) and NNR+NNT (No-Net-Translation) constraint for regional and global datum definition, respectively. We employed dual-frequency ionosphere-free linear combination observations from 125 GNSS stations for the time between 2015 and 2020. The same GNSS processing strategy is then followed for the Hikurangi subduction zone using a set of 72 stations from the GeoNet project as well as the same control stations used for Cascadia spanning from 2002 to 2010.

For the Cascadia displacement time series, we find variations in transient velocities under different datum definitions emphasizing the need for a comprehensive understanding of its impact on dynamic geophysical processes. Processing and analyses of the New Zealand data is ongoing and results will be presented, along with recommendations for both regions on how to reduce the occurrence of likely non-tectonic transients in the displacement time series. Ultimately, our results may have implications for improving the estimate of the slip budget at plate boundaries that is released aseismically.

How to cite: Garcia, C., Bedford, J., Männel, B., and Glaser, S.: Impact of datum definition on transient velocities from GNSS displacement time series in Networks mode: A Case Study of Cascadia Subduction Zone and Hikurangi margin., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12979, https://doi.org/10.5194/egusphere-egu24-12979, 2024.

It is believed that seismic failure conditions are sensitive to strain-softening behavior of nominal rock or fault gouge, and that precursors prior to a big earthquake (i.e., tectonic trains, water level changes, and V_p/V_s anomalies, etc) are provided by the acceleration of local slip. Previous studies of earthquake nucleation on laboratory faults show that the initiation of unstable fault slip is spatiotemporal dependent and consists of an interval of fault preslip (or creep) that localizes and accelerates to a dynamically propagating rupture. We pose that perturbation-type experiments can provide a natural condition to help analyze the potential mechanisms between instability events and the stress loading. In this study, we conducted three sets of double-direct shear experiments on a 300 mm long fault filled with gypsum-rich gouges, under normal stress of 10 MPa superimposed with perturbations of various amplitudes (i.e., 0-0.5 MPa) and a fixed frequency (0.1 Hz). The result showed that during each cycle of the stick slip behavior, the applied normal stress perturbations were redistributed along the fault zone as revealed by the along-fault strain measurement in the normal direction. As such, the fault can be divided into different zones characterized by varied coupling with respect to the applied perturbations. We found, coincidently, nucleation of the final instability, as revealed by the strain measurement in the shear direction, tended to occur at the boundary between the so-called strong and weak coupling zones (‘Transition Zone). Moreover, local normal stress near the nucleation zone also showed some weakening prior to the instability, which was similar to that seen in the local shear stress, and hereafter referred to as ‘normal failure’. Based on these observations, we proposed an empirical equation to fit the normal strain or stress data, giving the distribution of the coupling coefficient (c-value) and the anomaly (a-value) along the simulated fault. Finally, we applied the proposed equation to fit the water level data from 6 monitoring stations along the fault that hosted a nature earthquake (~M_L 4). The fitting results predicted a Transition Zone, which was close to the hypocenter. In the end, we propose that this approach can be tested widely to natural observations of various precursory signals, especially those considered to be sensitive to fault-normal deformation (“dilatation” or “compaction”), such as water level, soil gas, and V_p/V_s anomalies. Hopefully, the results can shed some lights on the location of the earthquake nucleation zone. 

How to cite: Zhang, L., Chen, J., Yu, B., and Zhang, M.: Discrepant stress distributions around instability regions: A new view for earthquake nucleation zones prediction, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13514, https://doi.org/10.5194/egusphere-egu24-13514, 2024.

A rich spectrum of slip behaviors, spanning from aseismic creep (mm/yr) to seismic slip (m/s), has been observed in many subduction zones and some strike-slip faults. Slow earthquakes, intermediate between these two end-member modes, exhibit transitional slip behaviors in fault sections adjacent to the seismogenic zone. Focusing on subduction zones, it is shown that they experience deformation not only along discrete fault planes but also over distributed frictional-viscous shear zones, the latter of which are thought to be responsible for the observed diverse slip behaviors. Here we employ a frictional-viscous mélange model consisting of brittle blocks surrounded by a viscous matrix to investigate its influence on slip behaviors. By varying the mélange's rheological and frictional properties, we observe a diverse range of slip behaviors. We also reproduce the source scaling relations observed in natural faults, including the relation between seismic moment and duration and that between moment magnitude and stress drop. Additionally, we find a close link between the modeled shear zone deformation patterns and the various geological structures observed in natural fault zones. Our study demonstrates that the interaction between the frictional and viscous compositions of the mélange is responsible for the resulting slip behaviors and their transitions under different compositional ratios. These results provide useful clues for constraining the environmental and rheological conditions of different subduction zone sections from the observed slip behaviors.

How to cite: Xie, J., Ding, X., and Xu, S.: Slip Behaviors Controlled by Rheological and Frictional Properties of A Two-Phase Mélange in Subduction Shear Zones, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13838, https://doi.org/10.5194/egusphere-egu24-13838, 2024.

The 5.5 magnitude (Mw) earthquake in Pohang, South Korea in 2017 was one of the largest triggered earthquakes at an enhanced geothermal system (EGS) site. Faults that ruptured in Pohang were not identified by preliminary geological investigations or geophysical surveys, and the subsequent study of the fault rocks at the Pohang EGS site was limited to depths of 3790–3816 m. In this study, we present new observations of fault rocks from drill cuttings retrieved from the Pohang EGS. The drill cuttings obtained from 3256 to 3911 m contained “mud balls,” which showed a clay matrix with foliation and a cataclastic texture, indicating a typical fault gouge or breccia. Furthermore, the mud ball samples retrieved from depths of 3256 m and 3260 m contained black fragments. Scanning and transmission electron microscopy revealed that the black fragments consisted of glass-like material, which is indicative of frictional melting during coseismic slip (Jung et al., 2023). The presence of these black fragments suggests that at least one seismic event had occurred at the Pohang EGS site prior to the hydraulic stimulation test.

Jung, S., J. -H. Kang, Y. Kil and H. Jung, 2023, Evidence of frictional melting in fault rock drill cuttings from the enhanced geothermal system site in Pohang, South Korea. Tectonophysics, 862, 229964.

How to cite: Jung, S., Kang, J.-H., Kil, Y., and Jung, H.: Deformation microstructure of the fault rock drill cuttings from the enhanced geothermal system site in Pohang, South Korea, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15197, https://doi.org/10.5194/egusphere-egu24-15197, 2024.

Faults in the brittle crust lie at any orientation to the far-field stress. However, laboratory experiments designed to investigate earthquake physics commonly simulate favorably oriented faults, potentially overlooking the complexity of natural fault behavior. Here, we assess the role of stress field orientation in fault reactivation and earthquake precursors by conducting triaxial saw-cut experiments with laboratory faults oriented at different angles to the maximum principal stress, ranging from 30° to 70°. The samples were instrumented with strain gauges and piezo-electric sensors. Laboratory well-oriented faults describe a rather simple system in which the elastic energy is stored via the deformation of the surrounding host rock during the inter-seismic period and released via on-fault slip during the co-seismic phase with associated precursor acoustic activity. Consistent with previous laboratory data, an abrupt increase in the on-fault acoustic emission rate occurs shortly before the laboratory earthquake. A more complex picture emerges when deforming laboratory misoriented faults. Particularly, acoustic emissions and strain gauge data indicate that when the fault is misoriented, off-fault permanent deformation occurs well before fault reactivation. The stress state in the host rock surrounding the fault is indeed far beyond the one required for the onset of inelastic deformation. In this case, acoustic activity distributed in the rock volume during the pre-seismic phase is associated with permanent deformation in the critically stressed host rock and is not a direct precursor to the following laboratory earthquake. Unlike well-oriented faults, laboratory mis-oriented faults lack detectable seismic precursors. The laboratory-observed increase in acoustic activity prior to, but not precursor of, mis-oriented fault reactivation impacts our understanding of earthquake precursors in natural faults.

How to cite: Giorgetti, C. and Brantut, N.: Fault orientation in earthquake seismic precursors: insights from the laboratory, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15878, https://doi.org/10.5194/egusphere-egu24-15878, 2024.

Laboratory stick-slip experiments are a simple analogue for the earthquake cycle. The acoustic emissions (AE) of these experiments have been shown to contain hidden patterns. Machine Learning (ML) can extract these patterns and information on the fault state can be inferred (e.g. shear stress and time to failure). Two different ML approaches have been used in the past: 1) ensemble tree models, which are relatively easy to evaluate why they made a certain prediction, but only look at a snapshot in time and 2) deep neural networks using Long Short-Term Memory (LSTM), which have the ability to find patterns in the temporal changes in the signal, but act more as a black-box model, so the final predictions are hard to evaluate. Here we introduce an additional step in the workflow that can be used to allow the ensemble tree models information about the temporal changes of the input features. Furthermore, it is able to quantify and visualize whether a pattern is repetitive or not. Like earlier studies we start by calculating (statistical) features using a rolling window on the AE. The features are not directly used as the input of the model, but are placed in a larger Hankel matrix, where the consecutive time windows are the rows of the matrix. Using Principal Component Analysis (PCA) and Uniform Manifold Approximation and Projection (UMAP) we create an embedded version of this array that holds temporal information of features calculated in the previous step. Visual inspection of these embeddings shows that some features map to very distinct patterns that are repetitive over the majority of the stick-slip cycles. The advantage of this method is that an inverse mapping is easily available, allowing for an interpretable embedding of the data.

How to cite: Elbertsen, R., Vasconcelos, I., and Niemeijer, A.: Interpretable Embedding of Laboratory Stick-Slip Acoustic Emission Time Series, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16581, https://doi.org/10.5194/egusphere-egu24-16581, 2024.

Seismic observations confirm that natural fault systems radiate waves across a continuum of frequency and amplitude. Within this spectrum, faulted systems exhibit a continuous range of slip rates, allowing them to irreversibly dissipate energy stored in rocks over a broad range of seismic moment. Despite advancements in observations and numerical modeling models, the question on how a given fault system can host such a wide range of ruptures, including slow ruptures, VLFEs, LFEs, and fast earthquakes needs a careful attention. Addressing this question requires a framework rooted in fracture mechanics, which explores the rate at which energy provided to a crack drives the rupture front forward and how this process radiates energy throughout the medium.

This work delves into the question of how frictional instability and mechanical interactions between faults and fractures, particularly concerning the geometrical distribution of off-fault damage, can generate observed rupture patterns in seismic catalogs. A model of a representative fault system is proposed, featuring a main fault embedded within a fractured zone where all fractures can slip independently. The length distribution of the off-fault fractures follows a power-law. The study then explores the fracture processes within the system, examining rupture speed from an energetic standpoint and exploring the impact of the damaged zone on the supply or reduction of energy to the process zone, ultimately influencing whether ruptures propagate rapidly or slowly.

The influence of this process is further examined by analyzing the amount of energy radiated away from the fault system. Moment-radiated energy and moment-fracture energy scaling relationships will be presented as mechanical quantities that both slow and fast earthquakes adhere to on a common curve. We will discuss radiation efficiency as a function of rupture speed to illustrate how a fault adjusts its rupture speed according to the energy provided to it and the amount of its breakdown work. The effect of damage on the process zone of the rupture will be discussed to examine how interactions between multiple fractures supply or detract energy to an active process zone, affecting its rupture speed and, consequently, the fast or slow advancement of the front.

How to cite: Kheirdast, N., Bhat, H., Almakari, M., Villafuerte, C., and Thomas, M.: Analyzing Earthquake Energy: Unveiling the Spectrum of Fault Behavior in Terms of Moment, Duration, and Rupture Speed, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18746, https://doi.org/10.5194/egusphere-egu24-18746, 2024.

How to cite: Essing, D., Cökerim, K., Bocchini, G. M., and Harrington, R. M.: Using small-magnitude earthquakes to investigate the interplay between seismic and aseismic deformation along the Hellenic Subduction System, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20753, https://doi.org/10.5194/egusphere-egu24-20753, 2024.

Double direct shear experiments serve as established methods for delving into the physics of laboratory earthquakes. Using a biaxial shearing apparatus with dual fault configurations, these friction experiments simulate real Earth faults' behaviors during loading and failure. 

Despite the presence of distinct layers, double direct shear experiments are commonly perceived as a unified fault system, where the evolution of fault zone properties captured through passive or active seismic imaging can be correlated with the instantaneous stress state affecting both layers uniformly. To further explore the physics of seismic cycles generated in this setup, we perform friction experiments aiming to independently monitor the behavior of each fault layer. In our experiments, we use granular quartz (medium grain size 40 µm) to simulate fault gouge, amd we vary the normal load and shear velocity, allowing us to modify the apparatus's loading stiffness, which relies on the critical fault rheologic stiffness (kc). In the Rate-and-State framework, increasing the normal load results in an  increase of kc, pushing the system towards instability, occurring when k/kc <1, where k is the fault stiffness. Under 50 MPa of  normal loads and 10 µm/s of loading rate, these conditions result in highly non-cyclic seismic cycles marked by significantly variable stress drops and recurrence times. This situation offers an exceptional opportunity to investigate stress partitioning between the two layers and understand their interactions. Experiments are monitored using high-frequency calibrated piezoelectric sensors with a sampling rate of 6 MHz, placed on each of the two forcing blocks. Such a sampling rate allows us to clearly distinguish the time delay between the Acoustic Emissions (AEs) generated from microslip events in different layers. Phase arrivals are detected using retrained, Deep Learning-based algorithms. By associating these phase arrivals using the DBSCAN clustering algorithm, we classify events as occurring on a single gouge layer or on both layers. Subsequently, we analyze the catalog of AEs,, and single seismic waveforms, in terms of general characteristics and frequency content, to look for differences in the physical sources generating them. Unsupervised clustering may help identify classes of AEs linked to specific stages within seismic cycles. By potentially using established supervised Machine Learning technique, it would be possible to verify the relation between AEs variance for each layer and macroscopic apparatus features, like instantaneous friction or time to failure. All of these techniques reveal differences in acoustic energy released before failure for each layer, observations that can be associated to changes in fault physical properties, asperity scale processes and/or  grains sliding or fracturing. In conclusion, our findings demonstrate that double-direct shear experiments can emulate a system of interacting double faults. In such a context, the continuous monitoring of AEs can provide insights into the stress partitioning between the two layers, a process that may guide the nucleation of major slip events as well as the long term behavior of the system. Additionally, our analysis may be helpful to investigate processes like fault interactions, faults synchronization, static, and dynamic stress triggering.

How to cite: Mastella, G., Pignalberi, F., Giorgetti, C., and Scuderi, M.: Double Direct Shear Experiments as an interacting two fault system:  insights from laboratory seismic cycles on fault interaction , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20908, https://doi.org/10.5194/egusphere-egu24-20908, 2024.

The Praid salt diapir is located in the Transylvanian Basin, Romania and it stands out as one of the earliest discordant salt structures to be described. The salt that forms this structure was deposited during the middle Miocene salinity crisis. Under geological conditions, rock salt exhibits plasticity, resembling a fluid, leading to highly intricate folding patterns in its deformation. Understanding the evolution of salt structures holds significant importance for a number of industries such as the hydrocarbon industry or hydrogen storage. To utilize such formations for storage purposes, a comprehensive understanding of the deformation processes, impurity distribution, and mineral composition becomes crucial. These factors wield considerable influence on the overall rock properties.

Certain diapiric salt formations within these areas hold potential as sites for hydrogen storage due to their substantial dimensions, reaching around ~3500m in size, and existing caverns within some of these formations. This investigation centers on analyzing the deformation of the Praid salt diapir. The site features a public-accessible salt mine and numerous surface salt exposures. Our study involves detailed mapping of both surface and subsurface areas, focusing on internal salt deformation, the nature and distribution of impurities, and exploring salt-sediment interaction where exposed.

In our research, we utilized surface and underground mapping within the accessible salt mine, coupled with photogrammetry and LiDAR technology, to construct detailed 3D models capturing complex large-scale deformation patterns. The majority of the salt layers exhibit steep to near-vertical inclinations, with diverse orientations that suggest curtain-fold-like structures. Certain areas notably display signs of refolding. Within the salt, various impurities of differing origins exist, predominantly large-scale blocks composed of siltstone to sandstone slabs that have undergone boudinage. This study is part of an ongoing initiative aimed at evaluating both the potential and associated risks of implementing hydrogen storage projects within these salt formations or similar structures.

Acknowledgements: The work of DD was financed through the Scientific Performance Scholarship, offered by Babes-Bolyai University, Cluj-Napoca.

How to cite: Dohan, D., Tamas, D. M., Tamas, A., and Tocariu, I. S. M.: The internal deformation of the Praid salt diapir and implications for potential storage applications, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-685, https://doi.org/10.5194/egusphere-egu24-685, 2024.

Geophysical methods based on the conductivity of electrical currents through the subsurface (IP, ERT, magnetotellurics) are commonly used to identify mineral deposits or aquifers, for which large conductivity contrasts exist between host rock and target. These methods may also provide insights about tectonic processes, such as when rock masses contain partial melt, or are affected by active deformation. However, further advances in electrical imaging of rocks at depth are hindered by the lack of understanding of the relative contributions of paragenesis, fabric, and active processes to electrical conductivity. The ICDP project DIVE (Drilling the Ivrea-Verbano ZonE) provides a unique opportunity to evaluate these parameters by combining samples and measurements from up to ~580 m depth with a wide range of geophysical surface surveys.

The DT‑1B drill core comprises lower crustal rocks consisting mostly of metapelite and amphibolite with embedded pegmatitic lenses. We took 25 samples from these main lithologies that cover the major variations in fabric (e.g., foliation strength, continuity and style, grain size), as well as intervals rich in micro fractures, hydrous alteration, or highly conductive phases (graphite, sulfide). We focus on two main research questions: (1) How do aspects of rock fabrics such as style and strength of foliation or mineral content and the connectivity of conductive phases affect the electrical properties, and (2) can these micro-scale, fabric-induced electrical properties be extrapolated to larger scales?

Fabric analysis and quantification of mineralogy are carried out at thin-section scale by optical and electron microscopy (EDX, EBSD). Computed tomography (CT) performed on small cylinders drilled from the same samples allows the microstructural data to be extended into the third dimension. The CT ably reveals the elongation and alignment of sulfides, the style, and continuity of the foliation which is defined by biotite, and in places graphite- or sulfide-decorated fracture systems. Measurement of electrical properties of the same cylinders under various fluid saturation conditions and a wide frequency range completes the comprehensive database, enabling us to detect and model electrical pathways in the lower crust.

To scale from the micro to the macro scale, these results will be compared to the data from electrical surveys carried out around the DT-1B drill hole. Results will expand the applicability of resistivity imaging for a wide range of future, structural applications.

How to cite: Tholen, S., Toy, V., Hawemann, F., and Mansouri, H.: Resisting the unknown: Enhancing resistivity imaging of the crust through a multidisciplinary approach from µ- to km-scale at the DIVE DT-1B drill site, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-709, https://doi.org/10.5194/egusphere-egu24-709, 2024.

Mylonites are characteristic rocks in ductile shear zones, and they contain two primary fabrics: i) C-fabrics, which are usually aligned parallel to the principal shear planes in the shear zones, ii) S-Foliations, which are oriented at angles to the shear plane, showing their vergence in the shear direction. Despite extensive studies of mylonite structures over several decades, the factors controlling the formation of S and C fabrics and their relative abundance in ductile shear zones are yet to be fully explored. This article investigates the competing development of S and C fabrics in ductile shear zones from two geological terrains of Eastern India. The shear zones offer macroscopic observations of various mylonitic rocks: i) C mylonites ii) S mylonites, and and iii) S-C mylonites.  Numerical simulations were performed to replicate them in model shear zones, considering a combination of transient visco-plastic rheology. The model study suggests that the growth of C- versus S- fabrics in mylonites depend on two fundamental non-dimensional parameters: imposed strain rate and bulk viscosity. It is observed that low bulk viscosity and strain rate conditions promoted the formation of S fabrics. With increase in bulk viscosity and strain rate, formation of C bands in the shear zones is facilitated leading to the localisation of strain in the form of narrow zones.

How to cite: Chatterjee, P., Roy, A., and Mandal, N.: Competing development of S and C foliations in mylonites, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1059, https://doi.org/10.5194/egusphere-egu24-1059, 2024.

Salt tectonics and salt structures are attractive targets for hydrocarbon exploration since salt-related deformation can form hydrocarbon traps and influence hydrocarbon migration. Salt diapirs are considered a suitable site for reserving natural oil/gas, landfills, and hazardous wastes. Determination of the origin and age of salt diapirs, the diapir formation and evolution model, and their impacts on the surrounding structures are very necessary for hydrocarbon exploration. To characterize the structural and tectono-sedimentary evolution of the salt diapir at the Qom Kuh, located in the Qom basin (Central Iran), this research describes the origin, timing, and evolution history of the salt structure through the tectono-sedimentary analysis and field study. Moreover, the effect of salt diapirism on the surrounding structures is investigated in the study area. The results obtained from the interpretation of seismic profiles and the investigation of the geometry of the sedimentary layers across the growth salt structure indicate that the salt extrusion occurred during two stages in the Qom Kuh. According to the structural evidence (e.g., hook and cusp) and the tectonic-sedimentary analysis, the first stage of salt extrusion happened in the Early Miocene concurrently with the extensional deformation event from the Eocene to the Early Miocene in the study area. The second stage of salt diapirism and its extrusion occurred during multi-stages of the Zagros orogenic compression from the Late Miocene to the present day. The final geometry of the salt diapir at the Qom Kuh formed along a releasing bend that was created by dextral transpressional strike-slip fault activity during the Zagros orogeny. Based on the lower thickness of evaporite units in the Upper Red Formation (Early Miocene) compared to the Lower Red Formation (Early Oligocene) and observing fragments of the Eocene volcanic rocks in the extruded salt on the ground surface, the Lower Red Formation salt along with the Upper Red Formation evaporites considered as the main source of salt diapirism in the Qom Kuh. Salt diapirism affected the surrounding structures of the Qom Kuh such as the Western Kuh-e-Namak and Western Alborz anticlines. The fold geometry and the hydrocarbon trap development in the Western Kuh-e-Namak and Western Alborz fields are controlled by salt tectonics and the occurrence of inversion tectonics. The results of this study could add data to worldwide examples of the impact of salt tectonics on the hydrocarbon trap development in collisional orogenic belts.

Keywords: Salt tectonics; Tectono-sedimentary analysis; Hydrocarbon traps; Qom Kuh; Central Iran

How to cite: Nikpoush, S. and Soleimany, B.: Control of salt tectonics on the hydrocarbon traps development: the surrounding structures of the Qom Kuh, Central Iran, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2464, https://doi.org/10.5194/egusphere-egu24-2464, 2024.

Full-field numerical modelling is a useful method to gain understanding of rock salt deformation at multiple scales, but it is quite challenging due to the anisotropy and complex plastic behavior of halite and other evaporite minerals at the single crystal level, together with dynamic recrystallization processes. We overcome these challenges and present novel results of full-field numerical simulation of dynamic recrystallization of halite polycrystalline aggregates during simple shear deformation, including subgrain rotation and grain boundary migration recrystallization processes. The results illustrate that the approach successfully reproduces the evolution of pure halite microstructures from laboratory torsion deformation experiments at 100-300℃ up to shear strain of four. Temperature determines the competition between (i) grain size reduction controlled by dislocation glide and subgrain rotation recrystallization (at low temperature) and (ii) grain growth associated with grain boundary migration (at higher temperature), while the resulting crystallographic preferred orientations are similar for all cases. The analysis of the misorientation reveals that the relationship between subgrain misorientation and strain follows a power law relationship with a general exponent of 2/3. However, with progressive deformation, dynamic recrystallization leads to a gradual deviation from this relationship. Therefore, predicting strain or temperature from microstructures necessitates careful calibration.

How to cite: Hao, B., Llorens, M.-G., Griera, A., Bons, P. D., Lebensohn, R. A., Yu, Y., and Gomez-Rivas, E.: Full-Field Numerical Simulation of Halite Dynamic Recrystallization From Subgrain Rotation to Grain Boundary Migration, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3225, https://doi.org/10.5194/egusphere-egu24-3225, 2024.

Salt tectonics on passive margins is driven by sediment loading and gliding with minimal influence from basement-involved tectonics and is associated with variable and complex salt structures such as minibasins and diapirs. A major enigma in salt tectonics is the origin of load-driven diapir-flanked minibasins, synclinal depocenters formed by localized subsidence of syn-kinematic sediments into salt. How can less-dense clastic sediments sink into the denser salt promoting diapirism at their flanks? We use 2D numerical modelling of lithospheric extension including syn- and post-rift sedimentation to understand the evolution of salt-tectonic minibasins along rifted passive margins. Our results show that these minibasins are driven by the deposition of dense early post-salt carbonates and then amplified during the progradation of less dense and compacting clastics. In contrast, basin-scale salt flow driven by clastic progradation alone, without deposition of early post-salt carbonates, does not produce minibasins as observed on salt-bearing passive margins.

How to cite: Muniz Pichel, L., Huismans, R., Gawthorpe, R., and Faleide, J. I.: The role of post-salt carbonates on salt tectonic minibasin formation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3623, https://doi.org/10.5194/egusphere-egu24-3623, 2024.

Published interpretations across the Basque Pyrenees vary significantly in their depictions of rifting and subsequent inversion. Major points of disagreement relate to: (i) the asymmetry of the margin, i.e., whether the major extensional and contractional detachment dipped toward the north or south; and (ii) the degree of decoupling between supra- and subsalt deformation and thus the amount of thin-skinned translation of the cover relative to basement. Here we use outcrop and subsurface data to analyze the salt structures along a regional transect in order to resolve this ongoing debate.

Several aspects of the salt-related geometries are diagnostic of thin-skinned deformation. First, Villasana de Mena diapir has significantly thicker synrift strata on its basinward (northern) flank, contains stringers of Paleozoic rocks, and was growing passively during crustal extension. Its origin was consequently related to an underlying basement fault, yet it is situated today above an unfaulted detachment, suggesting that the diapir was translated above the salt during rifting and/or shortening. Second, Salinas de Rosio diapir is located at the southern termination of landward-shifting synrift depocenters and developed as a postrift to synorogenic salt pillow and then diapir. Thus, thin-skinned translation over a fault-related ramp in the base salt created synrift ramp-syncline basins, with the basement fault subsequently localizing salt inflation due to differential loading and then contractional buttressing. Third, Poza de la Sal diapir is at the basinward end of another set of synrift ramp-syncline basins and along a thin-skinned fold and thrust structure that was active during the synrift. Fourth, the large-scale geometry from the Bilbao Anticlinorium to the south documents contractional translation above a continuous salt detachment with a ramp-flat geometry.

In summary, the salt and suprasalt geometries in a large part of the Basque Pyrenees demonstrate two phases of thin-skinned translation above a north-dipping salt detachment: (i) decoupled, basinward (northward) translation during rifting; and (ii) thin-skinned, southward-directed thrusting during inversion. The geometries are incompatible with thick-skinned inversion on a major south-dipping crustal detachment and smaller, basement-rooted faults that cut through the salt and its overburden.

How to cite: Rowan, M. G., Muñoz, J. A., Roca, E., Ferrer, O., Carola, E., and Garcia, I.: Using salt diapirs and minibasins to constrain interpretations of crustal rifting and inversion in the Basque Pyrenees, Spain, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4104, https://doi.org/10.5194/egusphere-egu24-4104, 2024.

The Kallianos Au-Ag-Te deposit, located on southern Evia island in the NW Aegean Sea, is a Cenozoic orogenic gold deposit hosted in carbonate-epidote-phlogopite schists and phlogopite marbles of the Cycladic Blueschist Unit (CBU). Fluids that generated the deposit were channelized by a crustal scale post-orogenic extensional structure, the North Cycladic Detachment System (NCDS), which facilitated Miocene exhumation of the CBU into the brittle crust. Whereas ore deposits in the Cyclades have been broadly related to late Miocene granitic intrusions, magmatism of this age is notably undocumented on Evia. Field observations illustrate the connection between the structural architecture that host mineralization and deformation associated with the post-orogenic structures, refining the paragenetic model for Cenozoic gold deposits in the Cyclades. Mineralized veins, alongside unmineralized tension gashes, faults, conjugate faults, and joints, occur in parallel sets that locally define en-echelon arrays. Younger sub-vertical tension gashes cross-cut older boudinage mineralized veins. The vein orientations of the Kallianos deposit strike NW-SE and NNW-SSE, which is generally orthogonal to the sub-horizontal ~NE stretching lineations related to crustal extension and thinning accommodated by the NCDS. Brittle-ductile kinematic indicators such as shear bands exhibit top-NE displacement, consistent with footwall deformation related to the NCDS documented elsewhere along strike of the detachment. The two populations of vein orientations are not evident based on structural data alone, but field observations show clear cross-cutting of the earlier NW-striking vein set by later NNW-striking veins. The mineralization is hosted in subvertical mm- to m-scale veins composed of quartz, calcite, albite, with minor titanite and epidote and notable sulfide mineralization including pyrite, galena, chalcopyrite, bornite and hematite concentrated in cm-scale veins. Obvious native Au and Ag are not observed in the veins. The NNW-striking vein sets contain significantly more albite and mineralization than the NW-striking veins and generally exhibit greater evidence of strain, with an abundance of sutured and bulging grain boundaries preserved in the quartz. Vein arrays developed within the cataclastic deformation zone below the exposed NCDS detachment plane are parallel to those observed deeper in its footwall. Our structural data strongly imply a connection between mineralized veins of the Kallianos Au-Ag-Te deposit and the regional strain field imposed by displacement along the NCDS. Despite structural evidence linking the architecture of this Cenozoic gold deposit to the crustal scale NCDS, an origin for the mineralizing fluids remains equivocal due to the local absence of magmatism, and the distribution of brittle-ductile strain to significant depths in the footwall may implicate devolatilization reactions coinciding with exhumation through the brittle-ductile transition as an important fluid source.

How to cite: Hamel, L., Ducharme, T., Schneider, D., and Grasemann, B.: Structural architecture of brittle-ductile mineralized veins in a Cenozoic orogenic gold deposit along the North Cycladic Detachment System, Greece, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4777, https://doi.org/10.5194/egusphere-egu24-4777, 2024.

Salt, an age-old resource, holds significance in human history and emerges as a potential solution in transitioning from fossil fuels to sustainable energy, due to its unique properties. Its geological fluidity results in formations like salt diapirs, influencing deformation in the past and present. Understanding the details and timing of such deformation is crucial for certain energy transition projects like hydrogen storage in salt caverns.

Salt tectonics in the Romanian Eastern Carpathians has long been studied. Initially, it was studied for its significance as a natural resource, but its implications for the hydrocarbon industry were later explored. Techniques such as seismic interpretation, well-log analysis, analogue and numerical modelling, and field observations are used to examine salt movement and interaction with sediment. In order to understand the Quaternary uplift rates of salt diapirs and the timing of salt movement in the Eastern Carpathians, we use radiocarbon and optically stimulated luminescence (OSL) dating.

This innovative combination of radiocarbon and OSL dating marks a breakthrough in understanding salt diapir uplift rates in the area of interest, shedding light on the historical dynamics of geological formations and erosion processes.

How to cite: Tamas, D. M., Tamas, A., Sava, G. O., Avram, A., and Timar-Gabor, A.: Timing and rates of salt movement in the Romanian Eastern Carpathians: insights from Radiocarbon and OSL dating, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5204, https://doi.org/10.5194/egusphere-egu24-5204, 2024.

General shear, wherein deformation incorporates elements of both coaxial and non-coaxial strain, is a prevalent strain regime in natural high-strain zones. In extensional tectonic settings, three-dimensional forms of general shear may enhance exhumation via additional crustal thinning (i.e., via pure shear or flattening strain components) or counteract it by inducing crustal thickening (i.e., via a constrictive strain component), without necessarily producing a conspicuous crustal-scale shear zone or fault. Schists and phyllonites demarcating a major tectonic boundary between thrust sheets on Evia in the NW Cyclades record structural evidence for general shear with a NE-directed non-coaxial component and contemporaneous flattening. The package of rock accommodating this strain is lithologically heterogeneous, comprising intercalations of carbonate-, quartz-, and phyllosilicate-dominated schist, as well as dispersed m- to hm-scale olistoliths and blocks of marble and metabasite. Flattening in these rocks is exemplified by foliation-oblique quartz ± calcite veins exhibiting pinch-and-swell or boudinage structure alongside dominant bidirectional dips perpendicular to the regional NE-SW stretching lineation. We combine in-situ ⁴⁰Ar/³⁹Ar and ⁸⁷Rb/⁸⁷Sr dating of white mica with quartz c-axis petrofabric analysis of the deformed quartz veins to elucidate the timing and styles of deformation recorded by these rocks. White mica provides mainly late Oligocene ⁴⁰Ar/³⁹Ar dates in samples with a single dominant foliation, whereas mica defining composite or crenulated foliations records late Eocene-early Oligocene dates, or age populations spanning the Oligocene. Some samples record dispersed Paleocene-Eocene dates older than the earliest proposed timing of metamorphism, although white mica from these rocks provides more geologically plausible early Oligocene ⁸⁷Rb/⁸⁷Sr dates. Vein quartz c-axis fabrics consist primarily of c-axis maxima or small-circle girdles centered about the Z-axis, with subordinate fabrics defining top-to-NE asymmetric type-I cross girdles or Y-axis maxima. Considered together with vein macro- and micro-structure, our data indicate that the deformed schists accommodated top-to-NE general shear at temperatures only slightly above 300°C, resulting in an oblate finite strain ellipsoid. Deformation over this interval produced differential transposition of earlier tectonic fabrics and structures into a sub-horizontal penetrative cleavage in the rheologically weak mica schists, whereas sections dominated by more quartzose- and carbonate-rich lithotypes display comparatively well-preserved older foliations and structures and a spaced secondary cleavage. The prevalence of late Oligocene ⁴⁰Ar/³⁹Ar dates in samples exhibiting a single, shallowly-dipping micaceous foliation implies that flattening general shear coincided with, and likely helped facilitate, exhumation. Our data indicate that unroofing may be partly facilitated by inconspicuous zones accommodating distributed inhomogeneous strain, a potentially important observation for exhumed subduction zones featuring prevalent block-in-matrix mélanges.

How to cite: Ducharme, T., Schneider, D., Grasemann, B., Camacho, A., Larson, K., and Scoging, V.: Strain partitioning during 3D general shear in a heterogeneous low-angle shear zone: some hold strong, while the schists fall flat, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6560, https://doi.org/10.5194/egusphere-egu24-6560, 2024.

We investigated the salt deposits found within the Altaussee salt mine, which represent the Permian to Triassic evaporitic Haselgebirge Formation situated in the Northern Calcerous Alps (Austria). The extensive deformation of the evaporite sequence spanning from the Middle Triassic to Neogene periods led to the formation of a tectonic mélange. The sediments commonly comprise fragments of anhydrite, polyhalite, sandstone and limestone embedded in the halite-rich matrix. The dimensions of these blocks can exceed 10 meters in diameter, while the bulk volume of halite content in these layers is ranging from approximately 30 to 65 volume percent.

Our investigation focuses on the internal structure within the evaporite sequence. In particular, we examine various outcrops in caverns, galleries, and corridors that illustrate the role of block shape and size on the deformation pattern. Particularly noteworthy are findings from a large, well-exposed salt cavern ceiling covering approximately 4000 square meters. Utilizing tailored photogrammetric approach, image post processing techniques and using lidar data as reference, we generated detailed ortophoto map of 1000 square meters of cavern ceiling with resolution of 1 mm/pixel. This reveals intricate patterns around rigid blocks and their interactions at different scales. Fine layering within the rock salt allowed to illustrate spectacular structures that developed around the rigid blocks and also interaction between the blocks. Significantly, observations of layer deflection beneath blocks hint at potential block-sinking dynamics, offering valuable insights into the complex geological processes at play.

How to cite: Adamuszek, M., Olkowicz, M., Dabrowski, M., Fiałkiewicz, M., Grochmal, B., Leitner, T., and Fernandez, O.: Deformation structures in the evaporitic melange. Case study from the Altaussee salt mine, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6744, https://doi.org/10.5194/egusphere-egu24-6744, 2024.

The Northern Calcareous Alps (NCA) in the Eastern Alps (Austria) developed during the Triassic as a Tethys-facing salt-rich passive margin. Extensionally-driven salt tectonics, mobilizing evaporites of Late Permian to Early Triassic age on the Tethyan passive margin, started as early as the early Middle Triassic and lasted through to the end of the Triassic. Here we document multiple examples of salt-related growth geometries in the central NCA, with specific emphasis on their dimensions and the implications these have on the balance between subsidence and carbonate production rates. The interaction between these two factors controlled the distribution of facies within the sedimentary growth wedges and the development of the transitions from shallow-water platforms into basinal domains >200 m in depth. The interplay between subsidence, carbonate production, and external factors (ocean currents) appear to have been critical for the persistence of long-lived intra-platform embayments, whose origin and development have long puzzled geologists.
The geometries and sedimentary architectures observed in the central NCA are directly comparable to those documented in other salt-rich basins such as the southern Atlantic passive margin or the (now inverted) Pyrenean rift. Excellent exposure and continuity over a large area render the Triassic of the central NCA an outstanding location for understanding the development of carbonate platforms above salt substrates.

How to cite: Fernandez, O., Sanders, D., Ortner, H., Grasemann, B., and Leitner, T.: Halokinetic growth wedges in platform carbonates: thoughts on subsidence and carbonate production rates, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8630, https://doi.org/10.5194/egusphere-egu24-8630, 2024.

The Valencia Trough is a NE-SW trending sector of attenuated crust located between the Iberian Peninsula and the Balearic Islands in the western Mediterranean, bordered by the Catalan Coastal Ranges to the northwest, the Iberian Chain to the west, and the Balearic fold and thrust belt to the south. It includes several kilometers of Jurassic-Cretaceous rocks deposited over Upper Triassic salt associated with rifting in the western Tethyan margin. The Mesozoic deposits are deeply eroded as a result of basin inversion and uplift in Oligocene time, followed by extension in latest Oligocene-Early Miocene time. Overlying Middle-Late Miocene foreland basin sediments are associated with the subduction and rollback of the Tethyan oceanic lithosphere. During the Pliocene and Quaternary, a prograding shelf complex was established on the eastern margin of Iberia reaching 3000m in thickness and affected by extensional faulting.

The inspection of the available surface (geological maps and structural data) and subsurface data (2D seismic profiles and exploratory wells) allowed us to document the major role of the Triassic evaporitic sequence on the tectonic style and the configuration of the Pliocene to present-day sedimentary infilling in the Valencia Trough. Our results indicate that large N-S trending extensional faults, which control the depocentres of the Plio-Quaternary prograding shelf complex and offset underlying Mesozoic-Cenozoic sequence, detach into the salt layer. Supra-salt extensional deformation appears to be decoupled from extension in the sub-salt basement.

Sequential backstripping restorations also illustrate the evolution of the deformation and depositional space associated with the flexing down of diapiric structures, which are nucleated over inherited basement faults, parallel to the supra-salt ones. These diapirs were developed in the basin margin during the Mesozoic and Miocene times. The salt expulsion is mainly triggered by the overburden deposition of the prograding clinoforms wedges sourced from the rivers located to the west (e.g., Júcar, Túria and Serpis). Salt diapirs recording a Plio-Quaternary activity can be encountered in the surroundings of the basin, synchronously to the development of the withdrawal salt depocenters.

Moreover, the determination of the two extensional faults systems, salt-detached versus basement-involved, has significant implications on evaluating the structures responsible for the instrumental and historical seismicity in the area.

How to cite: Ramos, A., de Ruig, M. J., Pedrera, A., Alfaro, P., and Martin-Rojas, I.: Evolution of a prograding shelf complex affected by salt tectonics, the case of the SW Valencia Trough, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8722, https://doi.org/10.5194/egusphere-egu24-8722, 2024.

The Tyrrhenian Sea salt is part of the Messinian Mediterranean salt giant. Its regional distribution was mapped during the early exploration of the Tyrrhenian back-arc basin. More recently, detailed studies have focused on the reconstruction of the salt setting in the Sardinian offshore. A regional overview of salt tectonics character in the Tyrrhenian Sea is thus missing. With this in mind, we present the first basin-scale interpretation of a combined data set of multibeam bathymetry and seismic lines. In the relatively flat, proximal areas of the Cornalia and the Campania Terraces, vertical rise of diapirs is the dominant style of salt movement. Whereas most of the diapirs are buried, some of them emerge at the seafloor and their circular form of salt stocks is apparent. Widespread extensional faulting of the overburden is indicative of reactive diapiric rise and control the location and evolution of local depocentres. However, large parts of the Tyrrhenian Sea consist of areas dipping seaward, towards the centre of the Tyrrhenian Sea. Here, salt gliding is the prevalent style of salt deformation. In the Sardinian margin a belt characterized by salt gliding spans a length of 230 km and is up to 130 km wide, reaching the Vavilov Basin in the centre of the back-arc system. In the Campanian margin  a more equant salt gliding area has a length of 106 km and a width of 80 km. Smaller areas with evidence of salt gliding are located at the foot of the base-of-slope escarpment in the northern Sicilian margin to the south of the Vavilov Basin. Salt gliding results in discrete lobes with complex pattern of deformation. Deformation in the overburden often originates polygonal networks of grabens, scalloped scarps, circular or elongate minibasins, growth anticlines and synclines. When the halokinetic structures are present at the seafloor, their relative importance in the different sectors of the main lobes is apparent. Discrete zones of deformation, and a highly 3-D style of salt gliding and overburden deformation are thus recognized. Belts dominated by strike-slip deformation separate the different sectors of the main lobes and are often associated with salt stocks or faults. They are indicative of the linkage between discrete salt gliding systems with different movement direction. A complex deformation style and movement is thus evident in the Tyrrhenian Sea and the deformation of the overburden indicate the recent or active character of salt flow. Our analysis illustrate the processes and elements that characterize salt tectonics in irregular continental slope, with divergent gliding, and where different system interact.   At the basin-scale, salt deformation style does not comply with the simple patterns often observed in passive margins, consisting, moving seaward, of three domains: extensional, translational and contractional. In the Tryrrhenian Sea a more complex pattern is evident and can be related to the complex trend of the peripheral zone of the gliding salt masses inherited from the rifting stage. The crustal evolution and magmatic history of the basin also influences the discrete, but kinematically linked, slat gliding domains.

How to cite: Gamberi, F. and Ferrante, V.: Salt tectonics and gliding in the Tyrrhenian Sea: a centripetal salt deformation system in a back-arc basin , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10801, https://doi.org/10.5194/egusphere-egu24-10801, 2024.

Janos Urai made major contributions to our understanding of rock deformation and the microstructural fingerprints that can be used to investigate it.

One such fingerprint is intracrystalline distortion. Crystals can be distorted due to deformation or growth but the distortion gives insights into processes in either case. Distortion is generally due to the presence of dislocations which give information on slip systems, stress levels, growth mechanisms etc. Electron backscatter diffraction (EBSD) allows detailed quantification of distorted crystals, and we summarise here a method for extracting information on dislocations from such data. The weighted Burgers vector (WBV) method calculates a vector at each point on an EBSD map, or an average over a region. The vector is a weighted average of the Burgers vectors of dislocation lines intersecting the map surface. It is weighted towards dislocation lines at a high angle to the map but that can be accounted for in interpretation. The method is fast and does not involve specific assumptions about dislocation types; it assumes only that elastic strains have little effect on the calculation. It can be used, with care, to analyse subgrain walls (sharp orientation changes) as well as gradational orientation changes within individual grains. It can complement established methods for subgrain wall analysis and frees us from some assumptions made in other methods.

We give examples of its use applied to olivine and plagioclase. The magnitude of the vector relates to dislocation density but, as a vector, we find its directional information particularly informative. Code to implement this approach is available from the first author (“Crystalscape”), from Oxford Instruments (a commercial version) and aspects are implemented in MTEX.

Urai, J. L., Means, W. D. & Lister, G. S. 1986. Dynamic recrystallisation of minerals. In: Mineral and Rock Deformation: Laboratory Studies (edited by Hobbs, B. E. & Heard, H. C.). Geophysical Monograph 36. AGU, Washington, D.C., 161-199.

Urai, J. L. & Spiers, C. J. 2007. The effect of grain boundary water on deformation mechanisms and rheology of rocksalt during long-term deformation. In: 6th Conference on the Mechanical Behavior of Salt. Proceedings and Monographs in Engineering Water and Earth Sciences, Fed Inst Geosci & Nat Resources, Hannover, 149-+.

Wheeler, J., Piazolo, S., Prior, D. J., Trimby, P. W. & Tielke, J. A. 2024. Using crystal lattice distortion data for geological investigations: the Weighted Burgers Vector method. Journal of Structural Geology 179, 105040.

How to cite: Wheeler, J., Sandra, P., Prior, D., Trimby, P., and Tielke, J.: Using crystal-lattice distortion data for geological investigations: the weighted Burgers vector method , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10987, https://doi.org/10.5194/egusphere-egu24-10987, 2024.

Ultracataclasites and pseudotachylytes often reflect localized deformation due to coseismic slip and the temporal evolution of seismogenic fault zones. Interaction of these structures and their mechanisms of nucleation and propagation into crustal rocks remain poorly constrained. Herein we conducted a microstructural analysis on a series of ultracataclasitic veins within a deformed granodiorite on Naxos, Greece. The island is a classical Miocene Cycladic metamorphic core complex, with migmatites and the granodiorite at its core. The Naxos detachment dissects the granodiorite, producing a strong N-S stretching lineation and SCC’ fabric indicating top-to-N kinematics. The granitoid cooled rapidly from crystallization (650-680°C) at c. 12 Ma to <60°C by c. 9 Ma. The investigated ultracataclastic veins are slightly anastomosing and oblique to the main foliation fabric in the granodiorite. Petrographic analysis of the granodiorite shows a coarse-grained (50 μm–2 mm) host rock matrix primarily composed of quartz, albite, orthoclase, hornblende and biotite, intersected by the fine-grained (5–60 μm) ultracataclasitic veins of the same composition. Quartz grains within the host rock occur as inequigranular, interlobate to amoeboid shaped grains exhibiting a shape preferred orientation that defines the foliation and appears to flow around larger feldspar porphyroclasts. Bulging and subgrains within the quartz grains are indicative of dynamic recrystallization. Albite occurs as subhedral porphyroclasts displaying undulose extinction, subgrains with fuzzy boundaries, tapered deformation twins, and bookshelf microfracturing. Orthoclase porphyroclasts within the host rock are subhedral to sigma-shaped, exhibiting undulose extinction with small subgrains (<50 μm) near the clast rims and vein margins. All feldspar porphyroclasts in the host rock are heavily fractured with increasing density proximal to the veins. Electron backscatter diffraction (EBSD) mapping of quartz, albite and orthoclase directly crosscut by the ultracataclastic veins reveals variations in relative phase deformation. Larger host rock quartz grains (50–300 μm) reveal internal lattice distortions (max. misorientations of ~20° relative to the grain average orientation) and low angle grain boundary (LAGB) development, with LAGB density and misorientation degree increasing towards grain edges. Smaller quartz grains (5–25 μm) display a moderate crystallographic preferred orientation and minimal misorientation (max. 8°). EBSD mapping of albite and orthoclase porphyroclasts (100 μm–1 mm) evinces crystal-plasticity in the form of a linear to heterogeneous misorientation pattern, with a maximum misorientations of ~38° and ~27°, respectively. Smaller grains of albite and orthoclase (<50 μm) with scattered orientations occur at clast rims and vein tips, and display a maximum misorientations of ~10° and ~15°, respectively. The localized subgrain structures observed in the feldspar are suggestive of dynamic recrystallization. The veins crosscut these recrystallized zones, suggesting propagation occurred after recrystallization of the feldspar. LAGB development is also observed in the feldspar clasts with increasing density towards the clast rims. The co-occurrence of fractures from dislocation and crystal-plastic microstructures in the feldspar porphyroclasts is indicative of fracture propagation in the brittle-ductile regime of feldspar (450–600°C). It remains equivocal whether the ultracataclastic material was injected into pre-existing fractures or the injection of the material induced fracturing within the host rock.

How to cite: Rolfe, O., Dubosq, R., Schneider, D., and Grasemann, B.: Poseidon’s seismic breadcrumbs: ultracataclasite vein evolution within a granodiorite along the Naxos Detachment System, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13072, https://doi.org/10.5194/egusphere-egu24-13072, 2024.

Salt diapirs are ubiquitous in the Zagros Mountains, but salt-flow dynamics in their extrusive parts and interaction with their caprocks are complex and poorly understood. For a better understanding of the interaction between salt dynamics and the caprock on the surface of the salt extrusions, knowledge of high-resolution spatiotemporal surface deformation and multispectral satellite imagery analysis is essential. However, the contemporary vertical surface deformation pattern across salt diapirs is difficult to detect and interpret along disciplinary boundaries. With the aid of high-resolution PSI measurements and multispectral imagery analysis we detected high-precision spatiotemporal deformation patterns of the surfaces of salt diapirs and their caprocks. Furthermore, time-series analysis helped to distinguish between salt-supply-driven domal uplift and vertical surface modification induced by precipitation, dissolution, and erosion.

In this study, we analysed Sentinel-1 PSI time-series, processed by the German Aerospace Center (DLR), to obtain the highest available spatiotemporal resolution of the vertical surface-deformation pattern across three diapirs – Karmostaj, Siah Taq, and Champeh – in the Zagros.

Furthermore, the Persistent Scatterers are correlated to their lithological composition based on multispectral analysis of Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) satellite images. Preliminary results indicate that the deformation pattern of the salt diapirs does not correlate with seasonal effects, such as precipitation and heat. The vertical surface deformation pattern on these three diapirs implies that these diapirs are active and that caprock influences the salt flow pattern.

Unnderstanding the activity of salt diapirs in general is also important, for example, in the feasibility studies of salt diapirs as strategic storage facilities for hydrocarbons, waste material, and CO₂ storage over longer time-scales worldwide.

How to cite: Rieger, S., Závada, P., Bruthans, J., Dusingizimana, M. W., Plattner, C., Kahle, B., and Friedrich, A.: Quantification of the surface deformation on salt diapirs using high-resolution Persistent Scatterer Interferometry (PSI) and Multi-Spectral satellite imagery, Zagros mountains, southern Iran, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15737, https://doi.org/10.5194/egusphere-egu24-15737, 2024.

The Kuh-e-Namak diapir consists of a dome and two glaciers and displays flow structures along its profile. Microstructures in salt dome and glaciers are studied for deformation and recrystallization mechanisms in terms of the grain size reduction, grain fabric and its influence on the flow dynamics of the salt system. Using reflected and transmitted light microscopy of gamma-irradiated rock salt thin sections, electron backscatter diffraction and quantitative analysis of digitized microstructures, we show the transition of dislocation creep followed by fluid-assisted recrystallization from the extrusive dome into the glacier where solution-precipitation creep dominates. Along the profile of the glacier, the degree of recrystallization increases, while the porphyroclasts content progressively decreases in favor of the fine-grained matrix. Fabric analysis support the decreasing amount of porphyroclasts and rectangular halite grains. Porphyroclasts in domal salt show the highest misorientation values at the grain boundaries and are consumed by almost misorientation-free, rectangular grains. Further, a development of shape preferred orientation (SPO) in glacier salt is inferred from alignment of the long axes of elongated halite grains visible in the fabric and their rose diagrams. The microstructures are interpreted in terms of combined dislocation creep and solution-precipitation creep. Grain analyses give a mean grain size ranging between 180 and 508 µm and show a moderate aspect ratio around 2, whereas fabric analyses indicate increasing values from dome to glacier salt of up to 4. Subgrain piezometry infers differential stresses of 1.9 to 6.1 MPa, reflecting the high stress in the cold diapir stem, whereas the shear stresses estimated for the glacier are much lower. Estimation for strain rates based on the combination of dislocation creep and solution-precipitation creep are in the orders of magnitude of x10^-10 to x 10^-09. Well-developed SPO is interpreted to support the hypothesis that solution-precipitation creep is the dominant recrystallization mechanism in glacier salt. Since solution-precipitation creep dominates in salt glaciers at low deviatoric stress, the fine-grained salt deforms much faster than predicted by dislocation creep, allowing salt glaciers to flow.

How to cite: Schmitz, J., Závada, P., and Urai, J. L.: Fabric and microstructural analyses of fine-grained glacier salt (Kuh-e-Namak, Dashti, southern Iran), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16570, https://doi.org/10.5194/egusphere-egu24-16570, 2024.

Due to the presence of low-viscosity rock-salt, evaporite sequences show a remarkable susceptibility to deformation across diverse geological settings. These sequences often exhibit intercalations of rock-salt with siliciclastic rocks, anhydrite, and sometimes various bittern salts like carnallite and bischofite. Their distinct layering serves as invaluable markers, facilitating a comprehensive analysis of internal salt deformation. The extensive deformation of the evaporites often gives rise to complex internal architectures within the salt body, characterized by commonly observed fold structures. The geometries of these structures are highly sensitive to the mechanical properties of the layers, thus offering profound insights into rock-salt rheology. Unravelling the rheological behaviour of rock-salt holds significant implications, particularly in salt mining, salt cavern operation, and advancing our understanding of salt tectonics.

In this project, we focus on specific outcrops within salt mines located in Romania, Austria, and Poland, where prominently exposed fold structures offer unique field laboratories. These sites hold significant potential for deciphering the mechanical behaviour of rocks during their long-term deformation. In our study, we combined field observations, detailed mapping and microstructural analysis of various single and multilayer folds complemented by numerical models of fold evolution. In our numerical simulations, we use the Carreau model for rock-salt, which captures two primary deformation mechanisms: pressure solution and dislocation creep. The mechanisms correspondingly result in the linear (Newtonian) and non-linear (power-law) rheological regimes, influenced by rock grain size and differential stress. Varying the rock-salt grain size enabled us to analyse fold evolution in both regimes as well as in the transitional domain. By systematically comparing our numerical analysis with field observations, we refine our understanding of the mechanical properties of evaporites, contributing to advancements in the study of rock deformation.

How to cite: Adamuszek, M., Barabasch, J., Urai, J. L., and Dabrowski, M.: Folds in evaporites. What can we learn about the rock-salt rheology?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17484, https://doi.org/10.5194/egusphere-egu24-17484, 2024.

Field analogue studies of fractured crystalline rocks are important for the clean energy transition and or better understanding the subsurface geothermal systems. In this contribution we present a workflow for multiscale quantitative analysis of fracture network and their connectivity in the monzogranitic pluton of Monte Capanne (Elba Island, Italy). Field structural analysis was integrated with Digital Outcrop Model (DOM) of a 1.5km-long outcrop and with microfracture analysis performed in thin section. The DOM was obtained from images acquired with UAV flights. Field analysis indicate the presence of three main fracture sets with different attributes and showing systematic crusscutting relationships. The quantitative analysis of the DOM was performed with QGIS software and allowed us to characterize the fracture length distributions, density (P20), intensity (P21), and topology (and their parameters). Data derived from field survey and DOM and analysis has been used to create a three-dimensional Discrete Fracture Network (DFN) using a DICE^® (https:// github.com/nicmenegoni/DICE) algorithm in MatLab^® to calculate the 3-dimensional fracture intensity (P32). In addition, we extended the two-dimensional topology concept in the third dimension. Thus, assuming circular fractures, new topology parameters have been calculated such as number of fracture intersection in volume and intersection fracture length in a volume, i.e., I30 and I31 respectively. Finally, based on the relative fracture chronology, we simulated the step-by-step evolution of 2- and 3-three-dimensional fracture density, fracture intensity and topology, describing the relationship between different fracture sets over time. The preliminary results show how fractures connectivity evolve over time. The ultimate goal of this work is to constrain the evolution of fracture porosity to enhance our ability for modelling fluid flow in crystalline rocks.  

How to cite: Porta, F., Berio, L. R., Cavozzi, C., Menegoni, N., and Balsamo, F.: Evolution of fracture intensity and topology in granitic rocks: insight from Mt. Capanne Pluton, Elba Island, Italy , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18140, https://doi.org/10.5194/egusphere-egu24-18140, 2024.

The megaflaps are halokinetic objects of multi-kilometer extension corresponding to vertical or overturned strata flanking salt walls or resulting welds. Although their geometry and mechanical behavior can be considered similar to detachment folds, their development mechanism and the consequences regarding strain record remain to be specified.

Megaflaps record strain before, during, and/or after tilting, and their development involves mechanisms specific to each megaflap (i.e. force folding with limb rotation or kink-band migration, passive folding), to their geological characteristics and the local and regional context. We highlight that the initial carapace, covering the evaporites and forming the future megaflap, constitutes a continuous mechanical unit allowing the transmission of an early homogeneous stress (either local or regional). Folding is then initiated by limb rotation, hinge migration or passive folding. During diapirism, the piercement of the salt structure generally accommodates the stress, that preserves the adjacent sediments from regional deformation. However, due to the evaporites loading, we observe a very localized compressive strain, which is perpendicular to the evaporite wall and results in the development of salt-related meso- and microstructures.

We compared the stress states determined on field analogs are with 2D uncoupled geomechanical models. The stress-strain distribution of two types of megaflaps were studied: a single development step megaflap, presenting a main mechanical unit (e.g. Karayün megaflap, Turkey), and a sequential development megaflap, presenting several mechanical units (e.g. Cotiella megaflap, Spain). Our models are based on well-defined geometries of known field analogs. In each model, the layers constituting the future megaflap record compressive strains, varying in extent and localized near the salt wall. Horizontal compression parallel to the layers, induced by salt push, is consistently observed and matches with field observations. Our models also depict layer folding, which can be accommodated through various mechanisms (extrados and intrados deformation, compressive mechanical guidance). Finally, it seems that the late stage tightening deformation would occur through an intensification of stress magnitude induced by complete welding. Our models show that welding significantly increases the maximum stress magnitude. The ratio between compressive stresses and background stresses is thus four to five after welding, compared to a ratio of two without welding. This matches our observations regarding the formation of new mesostructures during the late stage tightening within the Cotiella megaflap, induced synchronously in all layers (regardless their attitudes, already vertical and overturned, or gently dipping). In contrast, the preservation of salt bodies is correlated with the absence of late micro- and mesostructures perpendicular to the vertical layers as observed in other known field analogs (e.g., Sivas Basin, Paradox Basin, Witchelina diapir). Consequently, some megaflaps may remain unaffected by late-stage tightening even within a regional compressive system undergoing shortening.

How to cite: Lartigau, M., Callot, J.-P., and Gout, C.: Megaflaps flanking salt walls: strain and stress distribution from field observations to numerical modeling., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18516, https://doi.org/10.5194/egusphere-egu24-18516, 2024.

Janos Urai's contributions have significantly enhanced our understanding of salt deformation, particularly in predicting the long-term evolution of solution-mined caverns and radioactive-waste repositories in salt formations. His work delved into phenomena such as the weakening of rock salt by water during long-term creep at low differential stresses. Unlike most laboratory measurements, which are at higher differential stress, Urai's research considers dislocation creep and pressure solution (dissolution-precipitation creep), processes not commonly included in current engineering predictions.

Microstructural observations on Zechstein 2 (Z2) rock salt cores in the northern Netherlands reveal substantial grain-size-dependent differences in rock salt rheology. The study compares undeformed salt layers with strongly deformed diapiric ones, showcasing variations in megacrystals and fine-grained halite microstructures that point to different microphysical processes. The microstructural analysis, including optical microscopy of gamma-irradiated thin sections, recrystallized grain-size measurements, electron microscopy, and subgrain-size piezometry, indicates differential stresses between 0.5 and 2 MPa during deformation.

The findings highlight the importance of pressure solution creep at low differential stresses, demonstrating its significant impact on strain rate in rock salt. Integrating these results into constitutive flow laws reveals a four-order-of-magnitude difference in strain rates between halite types, emphasizing the role of different dominant deformation mechanisms. The study suggests that incorporating pressure solution creep and microstructural analysis can substantially enhance engineering and tectonic models of rock salt deformation in low-stress conditions.

How to cite: Barabasch, J., Schmatz, J., Klaver, J., Schwedt, A., and Urai, J. L.: The microstructure of naturally deformed gneissic Zechstein 2 rock salt (Kristallbrockensalz) from the northern Netherlands – a review, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19007, https://doi.org/10.5194/egusphere-egu24-19007, 2024.

On 27 March 2021 a three-months lasting seismic sequence struck the Central Adriatic Basin, part of Adria that is considered a relatively undeformed plate since recent times. Analyzing the waveform data acquired by the Italian and Croatian seismic networks, we computed the location parameters of 160 earthquakes and the focal mechanisms of the Mw5.2 mainshock and larger aftershocks. Most events align along a WNW-ESE, 30 km long, narrow belt. They form two clusters between 0-3 km and 4-14 km of depth, separated by a 1-2 km thick aseismic zone. Based on literature data, we suggest that such a seismic gap corresponds to a ductile salt layer, which constitutes the primary control factor for the evolution of the 2021 earthquake distribution. Moreover, the presence of a salt layer explains well the relatively high Vp/Vs ratio of 1.83 in the sediment rocks surrounding the salt bodies, as also observed in similar tectonic settings. We suggest that the seismogenic fault likely responsible for the 2021 events is an inherited SW-dipping normal fault, reactivated with prevalent reverse kinematics in response to the regional compressive stress. These results, and the recognition of a specific role of salt deposits in focusing deformation and seismogenesis represent a novel contribution to the long-standing problem of seismic hazard assessment of the Central Adriatic Basin, where moderate to large events could have devastating impacts along the highly populated coasts.

How to cite: Scognamiglio, L., Di Luccio, F., Palano, M., Marchetti, A., Dasović, I., Mustać, M., Magnoni, F., Artale Harris, P., Casarotti, E., Polonia, A., Gasperini, L., Dannowski, A., and Kopp, H.: The role of salt tectonics in the occurrence of the 2021 Central Adriatic seismic sequence, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20283, https://doi.org/10.5194/egusphere-egu24-20283, 2024.

Methane (CH₄) bubbles developed in shallow aquatic muds present a significant environmental risk. Macroscopic CH₄ gas content in the muds is accommodated in discrete bubbles that grow from below the pore scale size to the maximum size defined by muddy sediment mechanical properties. The bubbles force out the water within the pores and distort the structure of the muddy sediment by moving the grains apart at their growth above the pore scale. However, the interaction between growing bubbles was not understood. This study uses a mechanical/reaction-transport numerical model to simulate the interaction of competitive CH₄ bubbles paired with fracture-driven growth of varying initial sizes in aquatic muds. It reveals that mechanical and solute transport dynamics play a crucial role at different stages of bubble growth, particularly hindering the development of smaller bubble growth in competition. The stress from the larger bubble impacts the inner pressure and diffusive CH₄ flux to the smaller bubble, slowing its initial growth (at t < 40 s). Additionally, the larger bubble later diverts CH₄ from the smaller one, further inhibiting its growth expansion. This interaction may cause more horizontally oriented smaller bubbles and significant deformations in the larger bubble, especially as the distance between the bubble pair decreases. Such competitive bubble growth may explain the bubble size distributions observed in lab experiments and in situ, promoting CH₄ retention in muddy sediments and the formation of gas domes, which are precursors to pockmarks that can cause abrupt gas releases to the water and potentially the atmosphere. The study provides a foundation for upscaling to different models of gassy muddy sediment acoustic characteristics and models of gas retention evolution, while maintaining single bubble growth metrics. It contributes to better evaluating and potentially reducing long-persisting uncertainties around CH₄ emissions from shallow aquatic sediments.

How to cite: Zhou, X. and Katsman, R.: Competitive methane bubble growth in aquatic muds, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-249, https://doi.org/10.5194/egusphere-egu24-249, 2024.

Dykes are essentially magma filled fractures within the earth’s crust often formed by the pressure imparted by the intruding magma. Magnitude of the magma overpressure has been traditionally determined utilizing elastic properties of the host rock and the complete dimension i.e., full length and maximum width of the fractures. Full exposures of dykes (from tip to tip) are rare, however, as most of the dyke bodies encountered in the field are subject to erosion or disruption along its length as a result of geological time, making estimation of aspect ratios challenging.

We propose a new method of estimating total length and maximum width of dykes from their partial outcrops featuring at least one exposed tip. Taking into account the fact that majority of dykes form as dominantly opening mode fractures with an elliptical shape of opening, the method involves solving the equation of this ellipse using every conceivable combination of a pair of ground points recorded on the dyke margin considering the visible tip as the origin. Validity of the method has been checked using published data obtained from incomplete dyke outcrops exposed in the caldera walls of Miyake-jima volcano in Japan. The calculated estimates are in line with the results acquired through a previous published method. The present method has been effectively utilized to calculate the aspect ratios of partially exposed mafic dykes emplaced within the younger granite of the Chitradurga Schist Belt in the western Dharwar craton of peninsular India. We discuss the ranges of their magma overpressure and depths of origin as well as the stress intensity factors associated with the host granite.

How to cite: Mondal, T. K. and Biswas, S. K.: Dykes and their magma overpressure, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-392, https://doi.org/10.5194/egusphere-egu24-392, 2024.

Over the last two decades application of stylolite roughness inversion has become a common tool to reconstruct paleostress-fields, stress magnitudes and burial depth. While the orientation of the highest principal stress is free of any doubt, there are uncertainties coming along with tectonic stylolite inversion that require a differentiated debate. This includes data quality, rock physical parameters, timing of stylolite growth and burial depth.

We present results of statistical data analysis, showing that data quality depends on the number of samples as well as on the sample size. Thus, the dataset can be of very high quality and stable in outcrop scale. This is faced by field and microscopic observations and results of the stress inversion itself, that demonstrate that some common assumptions, i.e. that modelled paleodepth can be used for tectonic stylolite inversion, are not generally valid. We want to open the discussion towards the question of how we can refine such assumptions and parameters for our paleo stress models and better prediction of recent deformations.

How to cite: Köhler, S. and Koehn, D.: (Un)certainties in tectonic stylolite stress inversion, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-782, https://doi.org/10.5194/egusphere-egu24-782, 2024.

Observations of crustal stress orientation from the regional inversion of earthquake focal mechanisms often conflict with those from borehole breakouts. In particular, stress orientations from focal mechanism inversion tend to show little heterogeneity on length scales of kms to 10s of km, while borehole stress measurements often exhibit substantial short-length-scale heterogeneity.  Some of the difference may be because the two methods sample different locations within the crust, possibly indicating local stress heterogeneity, either laterally or with depth. We attempt to reconcile these two types of stress measurements, and investigate the implications for crustal stress heterogeneity. We compiled SHmax estimates from previous studies for 57 near-vertical boreholes with measured breakout azimuths across the Los Angeles region. We identified subsets of earthquake focal mechanisms from established earthquake catalogs centered around each borehole with various criteria for maximum depth and maximum lateral distance from the borehole. Each subset was independently inverted for 3-D stress orientation, and the SHmax direction compared with the corresponding borehole breakout-derived estimate. We find good agreement when both methods sample the basement stress (breakouts are close to the sediment-basement interface), or when both methods sample the mid- basin stress (sufficient earthquakes are present within a sedimentary basin). Along sedimentary basin margins, in contrast, we find acceptable agreement only when focal mechanisms are limited to shallow and close earthquakes, implying short-length-scale heterogeneity of <20 km. While the region as a whole shows evidence of both lateral and vertical stress orientation heterogeneity, we find a more homogeneous stress state within basement rock, over length scales of 1–35 km. These results reconcile the apparently conflicting observations of short-length-scale heterogeneity observed in boreholes, which sample primarily the basins, with the relative homogeneity of stress inferred from focal mechanisms, which sample primarily the basement.

How to cite: Hardebeck, J. and Luttrell, K.: A Unified Model of Crustal Stress Heterogeneity from Borehole Breakouts and Earthquake Focal Mechanisms , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3672, https://doi.org/10.5194/egusphere-egu24-3672, 2024.

Understanding the mechanisms controlling brittle rock failure at the grain to sub-grain scale is a fundamental challenge in geosciences. Recent advances in triaxial compression and dynamic shock experiments combined with dynamic X-ray microtomography provide unparalleled insights into the 3D strain field evolution within deforming rocks. However, these methods do not accurately predict the heterogeneous internal stress field prior to failure, which is crucial for predicting microfracture initiation and propagation, leading to macroscopic failure. In the past decade, efforts have focused on developing synchrotron X-ray diffraction techniques leveraging the high penetrative capacity of hard X-rays from the last generations of synchrotron light sources. These techniques offer spatially resolved information on crystal phase orientation and elastic strain within a 3D volume. The local orientation and elastic strain tensor is reconstructed grain-by-grain, with precision down to approximately 10^-3 radian for orientation and 10^-4 for strain. Stress is then calculated using Hooke's law for anisotropic materials and the elastic constants of the crystal phases. We employed 3D X-ray diffraction to investigate the internal stress field evolution in a rock core sample deformed under triaxial compression in the Hades apparatus. A 5mm-diameter core of Berea sandstone was subjected to axial step loading under constant radial stress of 10 MPa, reaching brittle failure at around 90 MPa differential stress. Elastic strain of individual quartz grains were measured at different load steps, and elastic stresses were calculated, providing maps of the internal strain and stress field in the sample. Results reveal progressive elastic shortening of quartz grains parallel to the compression axis and elongation in orthogonal directions due to the Poisson’s effect. Reorientation of principal stress components is also observed with increasing axial stress, which tend to align with the macroscopic stress field. Internal stresses distribution varies within a range of ca. 300 MPa, suggesting local stress amplifications occurred interpreted as force chains, potentially favoring crack nucleation. This experiment is among the first ones to characterize in-situ the stress distribution in a natural rock under compressive loading, and demonstrates the potential of synchrotron diffraction techniques for investigating strain and stress in geological materials.

How to cite: Jacob, J., Cordonnier, B., Wright, J., and Renard, F.: Mapping internal stress field in deforming rocks using synchrotron high-energy X-ray diffraction, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3863, https://doi.org/10.5194/egusphere-egu24-3863, 2024.

Abstract

The shale reservoir in the Lower Permian Fengcheng Formation of Mahu Sag, the Junggar Basin is prospective in hydrocarbon exploration and development. Due to the complex structures of the study area and the strong heterogeneity of shale reservoirs, the distribution of in-situ stress in the research area has always been poorly understood. Previous studies on in-situ stress are mostly limited to mechanical experiments, logging calculation and simple 2D numerical simulations. Nevertheless, this study combines multiple technical means to simulate the complex 3D in-situ stress in a more accurate and precise way. In this study, a detailed geological model was established by utilizing the method of ant tracking. Post-stack acoustic impedance, logging data and acoustic emission tests were used to jointly invert the accurate 3D geomechanical model. The orientation of the in-situ stress in the study area was determined by digesting the information from FMI (Formation MicroScanner Image) while the boundary condition was fixed by acoustic emission experiments. Finally, the in-situ stress distribution of the study area was clarified through finite element numerical simulation. As is shown by the simulation results, the in-situ stress modeling revealed that the complicated stress state, stress differences, and stress difference coefficients, all of which can provide valuable guidance for well deployment optimization and hydraulic fracturing in the study area, are closely related to burial depth, faults, and rock mechanics parameters. The stress regime in the research area is mainly reverse faulting type. However, as the burial depth increases, the stress regime will change accordingly, transforming from reverse faulting stress regime to strike-slip faulting stress regime. In the same time, the existence of faults will also affect the stress regime to a certain degree. In addition, most faults in the research area are stable and show little tendency of slippage, but there may be a higher risk of slippage in the deep strata. Therefore, it is advisable to avoid these areas as much as possible during geological exploration.

Keywords: 3D in-situ stress field, numerical simulation, shale reservoir, Mahu Sag

How to cite: Chen, P., Qiu, H., and Chen, X.: 3D numerical simulation of complex in-situ stress fields in shale reservoirs: A case study in the northwestern Junggar Basin of China, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4458, https://doi.org/10.5194/egusphere-egu24-4458, 2024.

Knowledge of the present‐day stress field in the Earth's crust is key for understanding the mechanical behaviour of rocks and structures under tectonic forces. The study of stress fields therefore remains a pivotal area for understanding the mechanical behaviour of rocks, fluid flow at depth and in revealing mechanisms that cause tectonic plates to creep, fail, or rupture. stress patterns in the Earth's crust appear on different scales: first order (plate scale), second order (regional scale), and third order (local scale). The latter is mainly controlled by basin geometry, topography, local inclusions, density contrasts, and active faults and can mask regional and plate stress patterns.

In this contribution, a couple of examples of stress states at the local, regional and large scale are presented using borehole breakouts as main stress indicator for the current stress field orientation.

In order to understand the influence of stress field evolution at local scale, a case study in Hawai´i concerns the effects of the two large overlapping shield volcanoes on the stress field at depth. The analysis reveals that the two-competing gravitational loads primary control the orientation of the present-day stress field, which deviates significantly from the plate and regional tectonic stress field. Therefore, knowledge of local and shallow stress fields can have a significant impact on future borehole planning. From a more regional point of view, an example of current stress orientation in Sweden is presented. The main objectives are to constrain the orientation of horizontal stresses using borehole data, and to discuss implications for geothermal exploration. Thus, obtaining detailed and accurate data on the stress state is of paramount significance to optimise the design of underground installations in order to maximise fluid flow and minimise the risks of wellbore instability. Finally, a large-scale study in Italy investigates the stress field at plate scale to reveal whether the orientation of horizontal stresses may change with depth or laterally indicating stress perturbations and heterogeneities related to areas with complex geo-tectonic setting.

In conclusion, this contribution aims to illustrate and emphasise the relevance of determining the horizontal stress orientation at depth in order to improve the understanding of subsurface stress fields and their applications in different fields of geosciences and different geological settings.

How to cite: Pierdominici, S.: Reconstruction of the state of stress in the upper crust by borehole breakouts stress indicators, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5744, https://doi.org/10.5194/egusphere-egu24-5744, 2024.

Transient stress perturbations caused by passing waves of distant earthquakes have been observed to activate fault slip. Observations of remotely triggered earthquakes at distances greater than ~2-3 mainshock fault lengths suggest that certain conditions promote fault activation, including large-amplitude shaking at periods below ~ 10 seconds within a geothermal setting and extensional and/or transtensional tectonics. Yet, it is still unclear if remote dynamic triggering is ubiquitous. An additional complication to determining the prevalence of triggering is that there are likely many small-magnitude earthquakes within sparsely instrumented regions that are uncataloged. Additionally, large mainshock signals can mask smaller local events that also are missing from the local catalogs. As a result, possible triggering mechanism(s) remain enigmatic. Bounding the necessary physical conditions for remote triggering, such as determining the upper or lower bounds of stress or strain amplitudes, the orientation of the seismic wave’s traversal with respect to the local stress field or fault geometry, or the geologic properties conducive to triggering can help provide clues about the physics of remote triggering. 

            The northern Chilean subduction margin provides an ideal setting to study remote dynamic triggering. Its dense instrumentation provides a long history of both seismic and aseismic deformation in both the subduction system and forearc faults, including the Atacama fault system. Our investigation combines a new, detailed regional earthquake catalog (2007-2021) from Sippl et al., (2023) and documented cases of triggered aseismic slip in the Atacama fault system (Victor et al., 2018). We use a twofold approach to determine the prevalence of earthquake triggering by candidate mainshocks that produce strains at our target location ranging from 1 to ~140 microstrain. The approach uses 1) a difference-of-means test of cataloged seismicity outside of the mainshock cluster (including foreshocks and aftershocks), and 2) a waveform-based approach to look for earthquake triggering at seismic stations located close to creepmeters that recorded triggered aseismic slip events.  We find a lack of evidence of persistent, statistically significant seismicity increases outside of the mainshock cluster associated with any of the candidate mainshocks.  Notably, there is an absence of significant seismicity changes outside of clustered foreshock or aftershock seismicity associated with the series of 11 M6.2-8.2 earthquakes that produced high-strain-rate events during the 2014 Iquique sequence. Seismic recordings of the 2011 M9.1 Japan earthquake at stations CX.PB01-CX.PB14 located near creep meter stations CAR3 and CH01 on the Mejillones peninsula near the Chomache fault reveal evidence of remote triggering. We observe local, uncataloged earthquakes that are only visible after applying a high pass filter that removes the mainshock signal that otherwise overprinted and swamped the local signals.  The uniformity of particle motions (circular or oblong) generated by local earthquakes on multiple stations (N=9) is lacking in most non-triggering mainshocks. This uniformity suggests that the orientation of transient stress perturbations imparted by the mainshock waveforms, in relation to the local fault orientations, may play a role in the triggering process. 

How to cite: Harrington, R. M., Kilb, D., Verdecchia, A., and Victor, P.: Putting faults into motion by remote dynamic triggering: local ground-motion orientations seem to eclipse strain in the northern Chilean subduction forearc crust, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6650, https://doi.org/10.5194/egusphere-egu24-6650, 2024.

The complex interplay between the Pacific and Australian plates in New Zealand offers a unique opportunity to investigate the present-day stress field in a tectonically active area. This study examines the present-day stress pattern of New Zealand through the analysis and compilation of data from 289 boreholes, 4291 earthquake focal mechanism solutions, and 72 neotectonic geological structures. Utilizing the Moho depth of New Zealand, we developed both crustal and mantle stress maps. Stress data above the Moho depth is categorized as the crustal stress map, while data below the Moho is classified as the mantle stress map.

The crustal stress map reveals a consistent ESE-WNW orientation of the maximum horizontal stress (S_Hmax) across much of the South Island, presenting a high angle to the strike of major active strike-slip faults. In the North Island, the crustal S_Hmax pattern is variable, highlighting the predominant role of the Pacific Plate subduction beneath the Australian Plate along the Hikurangi Margin. Within the Hikurangi Subduction Zone, both the crustal and mantle S_Hmax orientations are variable. However, the Taupo Rift Zone exhibits completely different stress pattern in mantle and crust, highlighting the Moho as a strong decoupling horizon in this region.

An examination of neotectonic stress regimes, derived from the neotectonic fault database, in comparison with the present-day stress regime from our stress database across 28 tectonic domains in New Zealand indicates a correlation between observed faults, faulting styles, and the stress field. Nevertheless, discrepancies emerge in certain domains, where the acting stress field diverges from the one expected according to the observed faults, suggesting non-optimal fault orientations.

How to cite: Rajabi, M., Ziegler, M., Heidbach, O., and Ziebarth, M.: Neotectonic stress characterization of New Zealand along the Australia–Pacific plate boundary , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6837, https://doi.org/10.5194/egusphere-egu24-6837, 2024.

The Sichuan-Yunnan region of the SE and E margin of the Tibetan Plateau, situated at the transitional nexus between the seismically-active intensely-deformed Tibetan Plateau and the tectonically stable Yangzi block with comparatively low seismicity, has experienced substantial geological transformations during the Quaternary. Given the pronounced seismicity, there is an escalating imperative for an accurate and refined distribution of the stress field in the region. To unravel the contemporary stress state within major active blocks and along active faults in the study area, an elaborate computation of their tectonic stress field is undertaken by comprehensive updated earthquake focal mechanisms catalog. The tectonic stress field in Sichuan-Yunnan region exhibits obvious lateral variations, with the principal compressive stress direction demonstrating a notable correlation with the azimuth of the P axis. The directions of the stress field show a variation from north to south at ~ 28°N. The directions of the maximum and minimum principal compressive stress in the north show nearly E-W compression and N-S tension, respectively. Conversely, in the south, there is a discernible clockwise rotation trend from east to west. Localized normal faulting stress regimes are observed in the middle section of Xianshuihe fault and southwest side of Litang fault. The extensional environment of the former may be attributed to the tectonic activities such as block translation, clockwise rotation and vertical uplift, as well as the clockwise rotation of the Xianshuihe fault from NW-SW to NNW-SSE. The latter may be related to the extensional structures, rift basins and the normal fault movements in the crust formed by the detachment of the plates and delamination of the mountain roots at the end of Triassic. We also found that the tectonic stress field under the large faults, such as Longmenshan fault, Red River fault, Xiaojiang fault and Lijiang-Xiaojinhe fault, show segmented variation. The findings yield invaluable insights into the intricate dynamics of tectonic deformation along the SE margin of the Tibetan Plateau [supported by NSFC Projects 42330311, 42074065 & 41730212].

How to cite: Tian, J., Gao, Y., and Li, Y.: Present-day stress field in Sichuan-Yunnan region based on comprehensive updated earthquake focal mechanisms catalog, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7176, https://doi.org/10.5194/egusphere-egu24-7176, 2024.

Short-term earthquake prediction remains one of the primary goals of seismotectonics. Here detailed observations of unusual fault kinematic behaviour and near-surface crustal stress variations are presented from before, during, and shortly after an earthquake series which culminated with two Mw 4.6 and 4.4 events near Breitenau, Vienna Basin, Austria, on 30 March 2021 and 19 April 2021, respectively. The oblique normal NNE-SSW trending Pitten Fault is exposed in Altaquelle Cave close to the southern margin of the Vienna Basin in the eastern Alps, which is known to have hosted several historical earthquakes of Mw = > 5. This cave has developed in Triassic marbles of the Central Alpine Permomesozoic. The observed branch of this active steeply dipping fault is associated with the seismogenic sinistral Vienna Basin Fault and the NE-SW trending Mur-Mürz Fault. To investigate the fault activity, TM71 moiré extensometers have been used to obtain precise three-dimensional records of fault kinematic behaviour at the micron scale while the recently developed SMB2018 protocol has been used to define the stress state associated with each fault reactivation event. The observations were then compared to the Copernicus European Ground Motion Service InSAR time series derived from Sentinel-1 data. From late 2018 to early 2021, the three-dimensional kinematic behaviour of the fault comprised a variety of different on-plane as well as out-of-plane hanging block displacements ranging in magnitude from 3 to 19 μm. Then, around the time of the earthquake series in 2021, four significant displacement events were recorded: (i) 0.186 mm along a vector of 186/-12° (i.e. upward) on 16 March; (ii) 0.615 mm along a vector of 177/-88° (upward) on 26 March; (iii) 0.066 mm along a vector of 013/26° (downward) on 30 March; and (iv) 0.022 mm along a vector of 308/54° (downward) on 11 May. The third of these events occurred on the same day as the largest earthquake. These events are all much larger than any other record of fault displacement recorded in the Eastern Alps since 2013. This contribution details this unusual fault displacement behaviour and compares the calculated stress states with both the focal solutions for each earthquake and InSAR maps of E-W and vertical ground motion. A comprehensive understanding of this important seismotectonic event helps to shed further light on potential earthquake precursory phenomena.

How to cite: Melichar, R., Baroň, I., Rowberry, M., Jelének, J., Sokol, Ľ., del Puy Papí Isaba, M., Freudenthaler, C., Hausmann, H., Plan, L., Grasemann, B., Stemberk, J., Schultz, R. A., and Bürgmann, R.: Unusual fault kinematic behaviour and near-surface crustal stress variations before and during an earthquake series in the Vienna Basin (Austria) in spring 2021, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7664, https://doi.org/10.5194/egusphere-egu24-7664, 2024.

Field studies established that seismicity in the lower crust is linked to brittle failure of dry, strong rocks. Failure of these strong rocks implies build-up of differential stresses to gigapascal (GPa) levels, but this requirement contrasts with current models of continental lithosphere deformation, which favour distributed flow of weak viscous lower crust. Although several mechanisms have been proposed to generate transiently high stresses, direct measurements are lacking. Recent advancements in microanalytical techniques (i.e., high-angular resolution electron backscatter diffraction, HR-EBSD) have proven successful at measuring the residual stress resulting from elastic strain retained in mineral grains.

We investigated with HR-EBSD the residual stresses in garnet and diopside from exhumed faults containing pseudotachylytes (quenched frictional melts produced during seismic slip). The samples come from Lofoten (Norway), Holsnøy (Bergen Arcs, Norway), and Musgrave Ranges (Central Australia). Pseudotachylytes from all three localities represent single earthquake events and formed at lower-crustal conditions (T = 500–720 °C, P = 0.5–1.0 GPa). Pseudotachylytes from Holsnøy show an asymmetric damage distribution, where host-rock garnet is pulverized nearby the fault on the side subjected to predominantly tensional stresses during rupture propagation, while garnet is intact on the other side. This asymmetry provides an opportunity to compare the residual stresses on both sides of the fault.

All samples preserve intragrain residual stress heterogeneities reaching 100s of MPa to GPa levels due to local high density of unrecovered lattice defects (dislocations). However, the timing of formation of lattice defects with respect to the seismic event differs. In samples from the Musgrave Ranges, the absence of any later deformation along with the sluggish mobility of dislocations in garnet at the ambient deformation conditions (500 °C, 0.5 GPa) allowed preservation of the high dislocation density produced during the earthquake rupture propagation, recording stress heterogeneities of as much as 6 GPa. In Holsnøy, residual stress heterogeneities of up to 1 GPa are only measured in pulverized grains and are also associated with unrelaxed dislocations generated during the earthquake rupture propagation. Intact garnet grains from the less damaged side of the fault show a limited range of intragrain stress heterogeneities, generally within 100 MPa, and a low density of dislocations. Residual stresses in diopside from Lofoten are only elevated (600 MPa) within 200 µm of the pseudotachylyte. Diopside recorded the progressive build-up of stress during interseismic loading, as suggested by the presence of coseismic cracks crosscutting lattice undulations that preserve the greatest stress heterogeneities. However, the ability of diopside to build up stress is limited, as stress is efficiently dissipated by the development of deformation twins.

In conclusion, great stress heterogeneities can be preserved in mineral grains that experienced the earthquake cycle in the lower crust. Different mineral phases can preserve stress heterogeneities to different extents, depending on the mobility of dislocations after their formation and on other relaxation mechanisms (e.g., twinning). Information on residual stress have important implications for the energy budget of an earthquake, the earthquake cycle deformation, and crustal rheology.

How to cite: Menegon, L., Toffol, G., van Schrojenstein Lantman, H. W., Wallis, D., Pennacchioni, G., and Jamtveit, B.: Earthquake induced residual stresses preserved in fault rocks exhumed from the lower crust, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9228, https://doi.org/10.5194/egusphere-egu24-9228, 2024.

The Armutlu Peninsula in north western Türkiye, a horst zone in an active transtensional pull-apart basin, is bounded by two major sub-branches of the North Anatolian Fault zone and host high rates of seismicity in the northern part. The ~25-station SMARTnet surface seismic network was installed in 2019-2020 with the purpose of augmenting permanent seismic stations in the northern part of the Armutlu Peninsula and to help increase the detection of small-magnitude earthquakes. Here, we employ a waveform-based clustering method that integrates detailed information from the seismicity and focal mechanism distribution enabled by the added station coverage to investigate the geometry and kinematics of the seismically active structures. We start by using an enhanced earthquake catalog of >4,000 double-difference-relocated events and >150 focal mechanisms obtained using P-wave polarities and amplitudes in the time period between January 2019 and February 2020. We perform a formal inversion of the stress field orientation from focal mechanisms to investigate the regional deviatoric stress field and its relation with activated fault structures. The stress-field inversion uses input data that combines the enhanced focal-mechanism catalog from background seismic events together with published focal mechanisms of M≥2.5 events that occurred between 1999 and 2019. Stress inversion results show an extensional stress regime for the broader northern Armutlu Peninsula and a transtensional stress regime for a narrow region of ~80 km², referred to as the Esenköy Seismic Zone (ESZ). Within the ESZ, the minimum principal stress (σ₃) is approximately horizontal and NE-trending, while the maximum (σ₁) and intermediate (σ₂) principal stresses are close in magnitude and vary between near vertical and near horizontal. We observe clusters of normal and strike-slip faulting events identified in the ESZ through waveform-based clustering analysis that are optimally oriented with respect to the stress field we derive for the area. The minimum principal stress in the ESZ is rotated clockwise by ~10-15° with respect to the minimum principal stress inferred for the broader Armutlu Peninsula and eastern Sea of Marmara. Based on the Mohr-Coulomb failure criterion, we quantify the relative and absolute magnitudes of the principal stresses, determine the local crustal stress and strength conditions, and will present a discussion of the implications for regional tectonic forces.

How to cite: Bocchini, G. M., Martínez-Garzón, P., Dielforder, A., Smeraglia, L., Harrington, R. M., and Bohnhoff, M.: Quantitative constraints on crustal stress and strength from seismological observations in the Armutlu Peninsula (northwestern Türkiye), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9630, https://doi.org/10.5194/egusphere-egu24-9630, 2024.

Electrical conductivity measurements on well-characterized materials in the laboratory allow accurate interpretations of electrical anomalies within the lithosphere and asthenosphere. But so far, most measurements have been performed statically, and hence, the effect of deformation and/or differential stress on electrical conductivity remains largely unknown. Here we report the first successful deformation experiments performed in a new-generation Griggs-type apparatus adapted for electrical conductivity measurements. The experiments were conducted on samples of Åheim dunites at a confining pressure of 1 GPa and temperatures of 500, 650 and 800°C. In silicate polycrystals, electrical charges are known to preferentially travel through grain boundaries, which act as high-diffusivity pathways. Our results show that stress and strain can significantly impact the electrical conductivity of peridotites by changing the thickness and the number of grain boundaries, respectively. At fixed P-T conditions, the electrical conductivity varies within an order of magnitude during deformation. This motivates to reappraise interpretations of electrical anomalies in mantle rocks, at least in tectonically active regions. The design presented in this study is fully stable at 1 GPa (≈ 30 km depth) and should be stable up to 2 GPa at least, and to average temperatures as high as 1000°C. Further developments should soon enable similar measurements at pressures up to 4 GPa (120 km depth). These experimental achievements open a new research field, which will help to understand electrical anomalies and strain localization processes in rocks under stress at depth, notably within the lower crust, the upper mantle, and subducting slabs.

How to cite: Ferrand, T. P., Précigout, J., Sifré, D., Savoie, F., Champallier, R., and Gaillard, F.: Electrical conductivity in a Griggs apparatus: a new experimental geophysical tool to investigate geological processes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10591, https://doi.org/10.5194/egusphere-egu24-10591, 2024.

Faults are normally thought to present shear fractures that develop at an angle to the main principal stresses, so that they have shear stresses active parallel to the fault plane and thus move. Here we present two “fault” features that deviate from this principle, they develop not due to stress but during kinematic movement, they are both oriented parallel to two of the main principle stresses and as such have no shear stresses in their planes. On the large tectonic plate scale one of these features are the well known transform faults between mid ocean ridges. The ridges themselves are extensional features with the lowest principle stress perpendicular to the ridge. Transform faults are oriented perpendicular to the ridges and show movement only or mainly between the ridges where the plates move in opposite directions. These are faults that do not develop due to shear stress, they develop only because of differential movement and are therefore only or mainly kinematic. On the small scale a very similar feature is the side of a stylolite tooth. Stylolites are dissolution features, they are thus in a way the opposite to mid ocean ridges and have the largest principal stress oriented perpendicular to the stylolite plane. Due to differential movement and growth of the stylolite roughness they develop steep teeth where the sides of teeth become oriented perpendicular to the stylolite plane. These are also movement surfaces or “faults” that have no shear stress.

What does this mean for stress inversion analysis? At least stylolite teeth show a quite pronounced set of striations on their sides and slikolites are also often developed on fault planes, at least in limestone. How do we separate a purely kinematic from a stress-related fault? This discussion and the potential consequences for stress inversion studies is not new, but remains to be very important and should be debated.

How to cite: Koehn, D., Hafermaas, D., and Koehler, S.: The development of kinematic shear-stress free faults , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10900, https://doi.org/10.5194/egusphere-egu24-10900, 2024.

Understanding the stress conditions of active subduction zones has been a longstanding hurdle with critical implications for natural disasters considering stress/strain orientations and magnitudes can control shallow earthquakes and tsunamigenesis. The Sestola-Vidiciatico Unit (SVU) in the Northern Apennines is an exhumed subduction channel with exposures of up to 9 km paleodepth, having reached up to 200°C. This unit experienced a relatively limited deformation history and serves as a rare analog to the shallowest portions of active subduction megathrusts. We use calcite twin data from shear veins along mineralized faults surrounding the exhumed subduction interface to reconstruct paleostress orientations through calcite twin stress inversion. Combining orientation data with calcite twin paleopiezometry and geothermometry, we are able to reconstruct the stress state of the SVU during peak subduction and subsequent exhumation.

During subduction, the maximum principal stress axis was oriented at a low angle to the subduction interface and the minimum principal stress axis oriented at a high angle, indicating N/NE directed compression. As subduction ceased and exhumation initiated, stress orientations inverted with the maximum principal stress axis becoming oriented at a high angle to the subduction interface and the minimum principal stress axis oriented at a low angle, indicating N/NE directed extension driven by primarily the weight of overburden material. These findings are consistent with theoretical orientations for both of these tectonic regimes and agree with previous studies interpreting subduction zone stress orientations. Calcite twin paleopiezometry and geothermometry suggests the rotation of principal stresses coincides with higher differential stresses during early exhumation. Based on the interpreted differential stresses and the reconstructed paleostress orientations, we model different possible explanations including contrasting mechanical strength between the contractional and extensional faults or changes in pore fluid pressure conditions between the two different tectonic regimes.

How to cite: Williams, S., French, M., and Rubin, C.: Determining the deformation temperatures and paleostress conditions of the Sestola-Vidiciatico Unit in the Northern Apennines, an exhumed shallow subduction zone, using calcite deformation twins, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14306, https://doi.org/10.5194/egusphere-egu24-14306, 2024.

During their activity, faults experience multiple seismic cycles, implying that faults recover strength between subsequent failures. Fault strengthening after failure (“fault healing”) occurs through processes having different space and time scales, including fault rock compaction, contact strengthening, contact area increase, fracture self-healing and precipitation of minerals in fractures (sealing). The efficiency of different processes varies depending on the geological setting, fault mechanics and availability and geochemistry of fluids. Here, we present preliminary data from a field-based study of the healing mechanisms of thrust faults inside the Sestola Vidiciatico Unit (SVU) in the Northern Apennines, a tectonic unit interpreted as the plate boundary shear zone between the Ligurian complex and the underthrusting Adria microplate during early-to-middle Miocene, active at temperatures up to 150°C.

The thrusts are sharp surfaces lined by calcite shear veins juxtaposing hectometer to kilometer-sized tectonic slices consisting of marls, shales, sandstones and mud-rich mass transport deposits. The marls and shales of the SVU bear a penetrative deformation pattern of fractures and incipient cleavage planes bounding oblate lithons, whose flattening planes define a foliation approximately parallel to the tectonic contacts. In the marls and shales adjacent to the main thrusts decimetric to metric-thick sheared domains may be observed showing an oblique foliation compatible with the sense of transport of the thrusts. Subvertical extensional calcite veins are common in the competent rock lithons. Multiple generations of normal faults lined by calcite shear veins crosscut the thrust faults, the oldest being rotated and deflected within the thrust-related shear zones. Calcite shear veins, in both thrusts and normal faults, display crack and seal domains and implosion breccias.

The lack of cataclastic rocks along faults indicates that the thrusts and normal faults were active at low differential stresses and high fluid pressures. Normal faults and subvertical extensional veins mutually crosscutting with thrusts are compatible with episodes of post-failure switching from reverse to normal fault stress regimes. Fault healing is dominated by calcite precipitation, which occurs both during fault slip (in crack and seal veins and implosion breccias), and after failure into subvertical extensional veins. Thrust compaction due to stress rotation is likely to be a factor promoting fault compaction and healing. Furter investigations will be conducted to constrain the origin of fluids involved in the vein cementation and the role of stress rotation in promoting different healing mechanisms.

How to cite: Mittempergher, S., Remitti, F., Tesei, T., and Molli, G.: Healing and strength cycling in a regional scale overthrust: insights from the Sestola Vidiciatico Unit in the Northern Apennines, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15071, https://doi.org/10.5194/egusphere-egu24-15071, 2024.

Paleostress inversion methods based on fault slip data (i.e. “fault slip inversion methods” or FSIMs) were formalised fifty years ago to become, shortly after, classical tools in structural geology and tectonics. The great popularity quickly gained by the methods was as remarkable as the enduring scepticism they prompted in the geological community. FSIMs belong to the rather narrow collection of methods, which allow for bridging traditional field observations and measurements (of fault planes and their respective slickenlines in the present case) to the stress tensor, a complex mathematical object. The latter statement highlights the originality of the approach and, perhaps, the roots of FSIM scepticism: stress are “observed” (or derived from observation of the nature) and not “measured” with the help of physical instrumentation, as it is traditionally done.

FSIMs are thus methods that involve mathematical processing of field data after adequate encoding of these. They rely primarily on the so-called “Wallace-Bott hypothesis”, which assumes parallelism between the measured fault stria and the computed maximum resolved shear stress, and on additional background conditions. The purpose of the present contribution is to discuss the limits of FSIMs in the light of their theoretical background and of realistic geological situations. The discussion will mostly focus on key-issues (e.g. is the stress restored by FSIMs in agreement with the formal definition of mechanical stress?) and will try to propose some future research tracks.

How to cite: Pascal, C.: Paleostress inversion of fault slip data: what is the problem?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15970, https://doi.org/10.5194/egusphere-egu24-15970, 2024.

Knowledge of the recent crustal stress state is crucial for a better understanding of crust stability. However, the amount of available stress data in Germany is low. Therefore, a reliable and comprehensive prediction of the complete stress tensor is not possible with these only. However, 3D geomechanical-numerical models, which represent the geometry of the subsurface and its mechanical properties and are calibrated to stress data, allow a continuum-mechanics based prediction of the complete stress tensor and its lateral and vertical variability. A new geomechanical-numerical model of Germany provides new insights into the recent crustal stress field. In contrast to previous models, an improved geological model with a significantly higher stratigraphic resolution is used, a high vertical resolution of ~40 m allows a better mechanical representation of individual units and mechanical inhomogeneities and new data records are used for calibration.

The results provide a comprehensive prediction of the complete stress tensor for Germany and can be used for a wide range of scientific questions and applications. Examples are the prediction of the fracture potential, the slip tendency of faults or as boundary conditions for small-scale models usable for example for engineering applications.

How to cite: Ahlers, S., Reiter, K., Henk, A., Hergert, T., Röckel, L., Morawietz, S., Heidbach, O., Ziegler, M., Müller, B., and Kuznetsova, V.: The recent crustal stress state of Germany - results of a new geomechanical–numerical model, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16586, https://doi.org/10.5194/egusphere-egu24-16586, 2024.

Seismicity studies on both fast- and slow-spreading ridge systems have found along-strike variations in mantle mechanical behavior on oceanic transform faults (OTFs), at pressure and temperature conditions above the long-term brittle-ductile transition of peridotites at ~700°C. Plate motion on some sections of the fault is accommodated by aseismic slip only (ductile deformation), whereas motion on other sections is by slip and deep swarms of microearthquakes (semi-brittle deformation) of mantle rocks (e.g., McGuire et al., 2012; Yu et al., 2021). To explore the mechanisms responsible for lateral variations in mantle mechanical behavior and the occurrence of this deep mantle microseismicity, we carried out an integrated study on peridotite mylonites dredged from two OTFs on the Southwest Indian Ridge that record deformation at 700-1000°C.

The samples show variable degrees of deformation, ranging from proto- to ultra-mylonitic textures. The most deformed zones of the mylonites are characterized by an increase in the proportion of fine grained (<10 micron) mylonitic shear bands compared to coarse grained (millimeter) porphyroclasts inherited from the protolith. These shear bands contain syn-deformation chlorine-rich amphibole indicating seawater-peridotite interaction during shear band formation.

Olivine and pyroxene porphyroclasts in protomylonites contain evidence for intense brittle deformation. The presence of subgrain walls, high aspect ratios, and internal misorientations crosscut by fractures imply that they deformed by low-temperature plasticity before brittle deformation. Fractures are sealed by the fine-grained shear bands present in the samples. In (ultra)mylonites, porphyroclasts also show evidence of fracturing after flowing through low-T plasticity. Fracturing was coeval with viscous flow of surrounding weak and hydrated mylonitic shear bands and triggered by hardening of larger grains due to dislocation accumulation. From existing flow laws, such brittle deformation of peridotite minerals necessitates high strain rate deformation, from 10^-9 to 10^-5s^-1, similar to strain rates associated with slow slip events.

From these results we propose that swarms of microseismicity on OTFs are triggered by deformation of a heterogeneous mantle. Seismic rupture occurs in lenses of coarse-grained peridotites, possibly driven by aseismic creep of surrounding hydrated mylonitic shear zones. Importantly, observations also suggest plastic flow of brittle (seismic) patches before rupture.

How to cite: Cécile, P. and Jessica, W.: Origin of brittle deformation and microseismicity in the ‘ductile’ mantle on oceanic transform faults, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16752, https://doi.org/10.5194/egusphere-egu24-16752, 2024.

Understanding the intricate dynamics of mantle flow and their influence on lithospheric stress patterns is critical for assessing reservoir responses to potential CO2 or nuclear waste storage, as well as for hazard and risk assessment. Stress patterns play a first-order control in the mechanical response within inherited tectonic structures, with varying stress sources governing different spatiotemporal scales. Our understanding of the present-day mantle flow state has much improved over the past decades, reflected in models that are consistent with first-order features. The World Stress Map (WSM) project serves as a primary observational dataset to validate our understanding of Earth's dynamics through a global compilation of crustal stress indicators.

Here we study mantle flow models as a simplified superposition of Couette and Poiseuille flow types, which have been useful in explaining sub-continental scale deformation in the lithosphere. We aim to understand the role of mantle flow as a stress driver by generating stress fields from an analytical representation of upper mantle flow, derived from the superposition of steady-state flow models. Our approach allows us to build first-order expectations and conduct fast hypothesis testing for upper mantle flow states.

How to cite: Hayek Valencia, J. N., Stotz, I. L., Bunge, H.-P., Carena, S., and Ghelichkhan, S.: First-order global stress patterns inferred from hierarchies of upper mantle flow models, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16807, https://doi.org/10.5194/egusphere-egu24-16807, 2024.

Strength-depth profiles for ductile portions of continental crust are derived from either extrapolation of flow laws from deformation experiments or paleopiezometric estimates in deformed and nominally hydrated plate margins. Lower continental crust in intracontinental settings, in contrast, is relatively dry and should be considerably stronger than the lower crust of hydrated plate margins. The relative strengths of dry quartz and feldspar are poorly constrained by experiments and paleopiezometric estimates from such rocks are sparse. As such, the strength of intracratonic lower crust is difficult to ascertain. Here, we use a recently calibrated subgrain-size piezometer to estimate paleostresses from feldspar and quartz deformed in relatively dry (<20 ppm H2O) lower continental crust of the Musgrave Ranges in central Australia. Neocrysts of plagioclase, K-feldspar, and quartz mantle partially recrystallized porphyroclasts, which is indicative of bulging and subgrain-rotation recrystallization. Using crystallographic preferred orientations and plotting misorientation axes of subgrain boundaries of each phase, we infer that dislocation creep involved the slip systems (010)[100] and (010)[001] for plagioclase, (010)[101] for K-feldspar, and (0001)<11-20> and {01-10}<0001> for quartz. Titanium in quartz and gradients in concentration of Ca and K in feldspars within neocrysts and along subgrain boundaries verify that subgrains in all three phases were formed at a temperature of ~650°C under dry, eclogite-facies conditions. Subgrain sizes of 10.6–18.1 µm in quartz, 11.5–16.9 µm in plagioclase, and 12.0–17.5 µm in K-feldspar correspond to differential paleostresses between 22–36 MPa and are consistent with a single mean paleostress of 28 MPa. Our results demonstrate that there is minimal stress partitioning between dry quartz, plagioclase and K-feldspar under typical crustal thermal gradients. Moreover, the differential stress accommodated by felsic rocks in the Davenport shear zone is lower than predicted by previous strength-depth profiles of lower cratonic crust.

How to cite: Osinchuk, A., Dyck, B., Wallis, D., and Camacho, A.: Subgrain-size piezometry of feldspar and quartz records a single paleostress from dry lower continental crust, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16849, https://doi.org/10.5194/egusphere-egu24-16849, 2024.

How to cite: Kundu, S., Mohanta, U., and Tiwari, S. K.: Paleostress reconstruction from fault slip data along the Purulia Shear Zone, Chotanagpur Gneissic Complex, India  , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16995, https://doi.org/10.5194/egusphere-egu24-16995, 2024.

Exhumed pseudotachylytes (quenched coseismic frictional melts) and their wall-rocks represent a source of information to investigate earthquake mechanics at hypocentre depth. Pseudotachylytes produced at eclogite-facies conditions in subducted oceanic rocks are of particular interest as they open a window into the elusive mechanics of intermediate-depth earthquakes¹.

Here we present observations from pseudotachylytes hosted in oceanic gabbros and peridotites from Moncuni (Lanzo Massif, W Alps). These pseudotachylytes record seismic faulting occurred at ca. 70 km of depth during subduction of oceanic lithosphere and have been explained as the result of brittle failure under high differential stress in dry rocks^2,3.

We focus on the pervasive damage surrounding pseudotachylytes within olivine-bearing gabbros. Brittle deformation comprises aseismic (cataclasite bands and foliated cataclasites) and coseismic (pulverized domains with shattering in-situ) features associated with the pseudotachylyte veins. Fluid-absent conditions promoted preservation of the pristine brittle features, including pseudotachylyte glass, throughout the exhumation path.

Pseudotachylyte veins and the associated sharp micro-faults are commonly bound by cataclastic domains. Locally, these domains develop an S-C fabric with ultracataclasites along the shear planes. This fabric shows a progressive localization of strain toward the core pseudotachylyte that cut through the S-C fabric, with the cataclastic aggregates proximal to the pseudotachylyte frequently impregnated by melt. Wall-rock olivine grains show evidence of low-temperature plasticity (deformation lamellae and undulatory extinction) and microfracturing. Both deformation lamellae and microfractures are oriented perpendicular to olivine c-axis. These deformation microstructures are also shown by olivine clasts within the cataclasites bounding the pseudotachylytes suggesting a temporal sequence of (i) crystal plastic deformation and (ii) shattering and pulverization. The small olivine clasts in contact with the sharp margin of the pseudotachylyte show substructures a few hundred nanometres in size and are characterized by absence of Kikuchi diffraction patterns. The lack of diffraction bands is interpreted as evidence of extremely high density of dislocations leading to amorphization of the material.

We interpret the low temperature plasticity of olivine and the progressive evolution of the S-C fabric to represent the precursory stage of stress localization predating the abrupt propagation of the seismic rupture, whose instantaneous high stress pulse is recorded by the shattered olivine clasts.

[1] Toffol et al., Earth and Planetary Science Letters, 2020, 578: 117289

[2] Scambelluri et al., Nature Geoscience, 2017, 10.12: 960-966

[3] Pennacchioni et al., Earth and Planetary Science Letters, 2020, 548: 116490

How to cite: Toffol, G., Pennacchioni, G., Scambelluri, M., and Grafulha Morales, L. F.: Record of high-stress deformation before and during an earthquake at intermediate-depth conditions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17092, https://doi.org/10.5194/egusphere-egu24-17092, 2024.

To comprehend the rheology of the Earth's crust and the relevant rock properties, one key approach is to deform rocks and minerals at elevated pressures and temperatures and then extrapolate the measured stress and strain rate values to natural conditions using constitutive equations. Laboratory experiments are mostly conducted on monomineralic rocks, with quartz being considered as the weakest constituent of the middle continental crust. However, field observations suggest that this is an oversimplification, and polymineralic fault rocks may be weaker than monomineralic quartz rocks. This study presents the first experiments on fine-grained, solid, natural rock samples, containing their natural homogeneities and inhomogeneities, demonstrating that granitoid rocks may be weaker than quartz at mid-crustal conditions. It also highlights the importance of pre-existing faults and polymineralic fine-grained zones for strain localisation and proposes values for extrapolation to natural conditions and their use in numerical models of the deformation of the granitoid crust.

Cylindrical granitoid ultramylonite samples, composed of qtz + ab + K-fsp + bt + ep, with grain sizes of 125-15 μm are deformed in a Grigg’s type apparatus at T=650°C, confining P=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. We show that polymineralic granitoid rocks deform through other mechanisms than monomineralic quartz aggregates at pressure and temperature conditions characteristic for the middle crust: Ultra-fine grain size reduction down to <50nm is developed by nucleation and growth of new grains in a polymineralic mixture. Grain size remains small because of pinning processes. We therefore refer to the deformation mechanism as pinning-controlled dissolution-precipitation creep (P-DPC).

Furthermore, we establish a new constitutive equation for this P-DPC, based on an exponential diffusion creep flow law, to model our experiments and tackle the extrapolation to various natural conditions. This flow law is supported by the microstructural evidence for the deformation mechanisms. Extrapolations show that the shear zones of the granitoid middle crust may be magnitudes weaker than extrapolated so far, and deformation may occur at magnitudes faster rates. The brittle to viscous transition may be shifted to shallower levels. This may have implications for the seismogenic zone and/or stress fields below geothermal reservoirs. Most importantly, we show the necessity to take polymineralic rocks into consideration for various numerical model applications.

How to cite: Nevskaya, N., Berger, A., Stünitz, H., Zhan, W., Plümper, O., Ohl, M., and Herwegh, M.: Implications for the strength of the Earth’s middle crust from novel experiments on natural fine-grained granitoid rocks , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17191, https://doi.org/10.5194/egusphere-egu24-17191, 2024.

Dehydration reactions can influence the occurrence of earthquakes at a range of depths, highlighting the importance of understanding these reactions in the study of seismic activity. We have performed several pilot experiments that included the construction of a controllable fast pressure drop unit attached to the piston-cylinder apparatus. This setup makes it possible to simulate conditions that represent a fast pressure drop during an earthquake event. We focused on serpentinite dehydration because 1/ it plays an important link between the deep geodynamic processes occurring in subduction zones and the seismic and volcanic activity and 2/ the interplay between serpentinite dehydration and deformation during the earthquake cycle is not yet fully understood. To test the experimental setting, we first performed a series of static experiments under the conditions that are already in the olivine stability field. After the static experiments at high pressure (1.1 GPa), we performed the controlled fast pressure drop experiments to 0.3 GPa as well as the ramping experiments, in which a series of pressure build-ups and drops were performed maintaining the high temperatures (570 to 640 °C) to simulate the earthquake cycle. In these experiments, olivine was an order of magnitude more abundant than in the one-hour low-pressure static experiment. The pressure drop occurs in seconds. The ramping experiment lasted only 10 mins before cooling down. The results may challenge conventional wisdom about the timescales of mineral reactions under extreme conditions, such as during earthquakes.

How to cite: Tajčmanová, L., Cionoiu, S., and Schuppenhauer, D.: Could earthquakes cause rapid dehydration of serpentinite?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19186, https://doi.org/10.5194/egusphere-egu24-19186, 2024.

Our goal is to constrain magnitudes and directions of forces that may explain present-day natural stresses within the Eurasian plate. Driving forces such as horizontal gravitational stresses (HGSs), mantle convective tractions including dynamic topography, and plate interaction tractions with bounding plates are considered. HGS resulting from lateral variations in gravitational potential energy are particularly relevant in the context of the Eurasian plate because there are no major slabs attached to it (i.e., no slab pull force). We illustrate that recently published models of lithospheric density including lateral variations in the lithosphere-asthenosphere boundary result in significantly different HGSs. Furthermore, we include observed major faults into a 2D spherical cap elastic model of the Eurasian plate. We present results of forward FEM calculations and compare them with observed stress directions from the world stress map. We propose different objective functions that determine the misfit of the modelled and observed stresses, fault slip directions, and magnitudes, the deviation of the net torque on the plate from zero, and the model representation error. Our analysis represents a first step towards a Bayesian inference workflow to constrain the dynamics of the Eurasian plate.

How to cite: Gutierrez Escobar, R. and Govers, R.: Eurasian plate-scale stress model considering driving and resistive forces, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19980, https://doi.org/10.5194/egusphere-egu24-19980, 2024.

Drilling-induced tensile fractures (DITFs) form due to stress concentrations around a wellbore and are in vertical wells typically parallel to the largest horizontal far-field stress and normal to the least horizontal far-field stress. The peak pressure in the wellbore exerted by the drilling mud that the wall rock can sustain is given by the so-called Hubbert-Willis (H-W) criterion, which predicts that wall rock failure takes place when the circumferential effective stress at the borehole wall reaches the tensile strength of the wall rock. However, even though the H-W criterion is a valuable fracture-initiation criterion, it cannot predict if and how an initiated fracture propagates. Linear elastic fracture mechanics (LEFM) can provide a solution to these questions under simple loading conditions, e.g. a vertical borehole in a rock mass that is under an Andersonian stress state. Predicting the initiation and propagation of DITFs in more complex settings, such as inclined boreholes or wellbores in mechanically layered sequences, however, necessitates three-dimensional numerical modelling.

Here we present results of three-dimensional numerical Rigid Body Spring Network (RBSN) lattice modelling. The model comprises so-called rigid blocks (tetrahedra in the present study), that interact with each other and can be bonded at their contacts; these bonds fail when the effective normal stress exceeds the bonds tensile strength, which corresponds to micro-fracture of the wall rock. Coalescence of these micro-cracks leads to the formation of macroscopic fractures. Wellbore failure is modelled by means of a hollow cylinder discretised by these bonded rigid blocks. The remote (tectonic) stress is applied to the hollow cylinder’s outer surface whilst the pressure exerted by the drilling mud is applied to its inner surface. Fractures connected to the wellbore receive the same internal pressure as the wellbore and quasi-static fracture propagation is achieved by gradually increasing the pressure on the borehole wall.

Validation of the numerical model under simple loading conditions illustrates that fracture lengths and associated aperture profiles as a function of wellbore pressure correspond well with LEFM predictions. Numerical models of moderately inclined boreholes exhibit stepping DITFs, where fracture stepping is most pronounced when the wellbore is inclined towards the least horizontal stress direction and no fracture stepping occurs when the wellbore is inclined towards the greatest horizontal stress direction. In mechanically layered sequences comprised of layers with different Young’s modulus, DITFs first nucleate in the stiff beds. Complex fracture geometries emerge in mechanically layered sequences, such as fracture stepping within individual beds or at layer boundaries. The detailed evolution of these more complex DITFs depends on several factors, such as the orientation of the remote principal stresses and layering relative to the wellbore axis. Our numerical modelling approach permits to systematically investigate the effects of these different factors on the geometry of DITFs and therefore offers a new tool that can assist in construing the mechanical genesis of fractures imaged in borehole logs and the current stress in the Earth’s crust.

How to cite: Schöpfer, M., Habermüller, M., Levi, N., and Decker, K.: Three-dimensional numerical modelling of drilling-induced tensile wall fractures, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20499, https://doi.org/10.5194/egusphere-egu24-20499, 2024.

The 1D Geomechanical Models (1DGM) were done for four vertical boreholes in the Early Paleozoic shale sequences of the Baltic Basin in Poland. The models assumed an elastic rock behaviour with anisotropy in VTI symmetry. The far-field horizontal stresses were calculated as the sum of two components: the vertical stress derivative acting on the horizontally constrained rock column and the effect of elastic tectonic strains. Local stresses in the borehole wall were assumed to induce breakouts (BBs) and drilling-induced tensile fractures (DITFs). Hydraulic fracturing tests additionally validated the stress modelling results.

Micro-resistivity images from XRMI logging revealed irregular BBs, usually confined to individual layers, often non-symmetrical, and with a tendency to encompass the entire borehole wall. Despite their irregularity, statistical analysis of their orientation provides a good quality and stable stress orientation.

The initial modelling results, balancing the cumulative length of the modelled (BB_M) and observed (BB_O), revealed a systematic misfit between BBm and BBo locations. A detailed comparison between the BBm and BBo intervals concluded that the artificial degradation of Young's modulus and Poisson's ratio is caused by the perturbation of the velocity of the acoustic wave from the dipole acoustic tool passing through the intervals with irregular BBs. This makes it impossible to model the BBs in the places where they are present. To deal with this, the initial stress models were recalculated to the final models in which the BBm were avoided in intervals where there were no BB_O.

The initial and final stress models differ significantly in terms of the tectonic strain values and the combined length of the BBm. Still, their stress profiles are similar due to the small contribution of tectonic strain to the far-field stresses. We concluded that irregular BBs developed due to small differential horizontal stresses, causing abrupt BB failure with rapidly growing angular width. The stress layering between lithostratigraphic units was obtained with a dominance of the normal faulting stress regime in the lower borehole sections and the reverse faulting present in the upper sections. The minor regional elastic tectonic strain value for the shale sequence was determined to be an order of magnitude lower than the strain in the crystalline basement, as determined from the satellite geodetic strain rate. We expect that this discrepancy could be explained by a higher rate of viscous relaxation in the shale sequence with > 60% of the clay mineral content. This suggests the need to implement the viscous relaxation into the 1DGM of sedimentary sequences.

How to cite: Jarosinski, M., Bobek, K., and Pachytel, R.: Geomechanical models of the shale sequence of the Baltic Basin (Poland): possible case of elastic properties degradation and viscous stress relaxation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20951, https://doi.org/10.5194/egusphere-egu24-20951, 2024.

Numerous studies have illustrated that mineral transformations have the capability to induce faulting at elevated pressure and temperature (PT), circumstances in which ductile flow would typically dominate. This mechanism, commonly known as transformational faulting, emerges as a plausible explanation for the puzzling phenomenon of deep-focus earthquakes occurring at depths up to 700 km. Currently, the debate partly revolves around determining why certain phase transformations lead to faulting while others do not. To better understand this phenomenon, we can compare different transformations taking place in similar experimental conditions and see how they do or do not cause strain localization and faulting. In this regard, we conducted a series of five deformation experiments in the large volume press at the PB61 beamline at DESY synchrotron. Two of these experiments involved deforming germanium-olivine samples as they transformed into ringwoodite (the high-pressure phase). The other three experiments were carried out on quartzite (novaculite) samples while they were transforming to coesite. Throughout the experiments, we collected X-ray diffraction patterns and images concurrently with the collection of Acoustic Emissions (AEs).

The results indicate, in both quartz and olivine experiments, the growth of the high-pressure phase at various rates depending on PT conditions and equilibrium overstep. Specifically, we observed rapid olivine-ringwoodite kinetics at elevated PT, far from equilibrium, while slower kinetics were noted for the quartz-coesite transformation. Thousands of AEs were collected in each experiment, and their locations reconstructed using arrival times on the six transducers used. Interestingly, the spatial distribution of these AEs revealed that for some quartz-coesite experiments, AEs originated from fault planes that formed within the initially intact rock cores. Furthermore, an analysis of the AE catalogues, focusing on the magnitude-frequency distribution, revealed a wide range of b-values influenced by varying PT conditions and transformation kinetics. This observation underscores the different underlying mechanisms since the obtained b-values are high when transformation and strain are distributed and lower when strain is localized (i.e., when a fault plane develops).

Our study supports the major role of mineral transformations in inducing faulting under high PT. These findings will help better quantify the intricate relationships between mineral transformations and faulting and in turn contribute to a better understanding of the fundamental geological processes behind deep and intermediate earthquakes.

How to cite: Mingardi, G., Gasc, J., Farla, R., and Schubnel, A.: Combining synchrotron and acoustic emission techniques to reveal the secrets of high PT faulting, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21427, https://doi.org/10.5194/egusphere-egu24-21427, 2024.

The Amerasia Basin, one of two major basins that comprise the Arctic Ocean, is thought to have a more complex formation history than its counterpart, the Eurasian Basin. Because the harsh conditions for marine expeditions last the entire year, there is a lack of observational data for constraining the tectonic history of the Chukchi Basin. Thus, there are multiple existing hypotheses for its tectonic history, with contrasting formation ages ranging from Mesozoic to Cenozoic and crustal types ranging from hyper-extended continental crust to oceanic crust. Recently, during the 2018 and 2021 Arctic expeditions of the Korean ice-breaking research vessel Araon, we obtained the new marine heat flow data along the east-west and northeast-southwest transect lines from the abyssal plain to the continental slope/shelf of the basin. These data may play an important role in constraining the formation age of the basin, as the extending axis among the hypotheses is likely oriented from north-south. Assuming an oceanic crust, the formation age can be inferred to be Late Cretaceous. This information concerning the formation age enhances our understanding of the underestimated complex tectonic history of the Amerasia Basin, because such inferred timing aligns with the formation age of the adjacent Chukchi Borderland.

How to cite: Kim, Y.-G., Hong, J. K., Jin, Y. K., and So, B. D.: Constraints on the Formation Age of the Chukchi Basin, Arctic Ocean, inferred from Marine Heat Flow Measurements, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1239, https://doi.org/10.5194/egusphere-egu24-1239, 2024.

Extensional detachments are commonly considered key structures in accommodating the exhumation of deeply buried or subducted crustal slivers, and in facilitating the syndeformation emplacement of plutons during the evolution of wide rift systems (i.e., Basin and Range type). In those settings, ductile shear zones and brittle faults may act for several million years to accommodate important vertical and horizontal displacements such that multiply reactivated and highly complex shear zones and faults may form. The analysis of these complexities, together with the possibility to constrain the age of strain and deformation localisation, is thus pivotal in reconstructing the onset and evolution of the processes that steer(ed) the crustal extension.

Aiming at better understanding these structural/chronological intricacies, we have studied the brittle Mykonos Detachment (MD), which is thought to have facilitated the emplacement of the Mykonos granite starting in the Middle Miocene (~14-9 Ma) and following the activation of the earlier (ductile) Livada Detachment (LD) that would have favoured the beginning of pluton cooling during the structuring of the Aegean rifting. The Mid. Miocene age of the MD is, however, only loosely constrained by the stratigraphic age of syn-tectonic siliciclastic deposits in the hanging wall of the fault. No absolute ages exist yet on the activation of the brittle MD or the ductile LD, and a detailed description of the internal architecture of the MD is still not available.

Aiming to fill this gap(s), we carried out a detailed study that couples a Brittle Structural Facies – based structural analysis with K-Ar dating on authigenic illite from fault gouge(s) that compose the MD fault core. Fault gouges normally rest on and are cut by the MD principal slip surface (PSS), which reasonably postdates the gouge formation and represents the effects of the latest fault activity. We have obtained a 7.1 ± 0.1 Ma K-Ar age from a fault gouge suggesting that the MD activation postdated the widely accepted ~14-9 Ma of the granite cooling, also considering that the PSS postdates the 7.1 Ma gouge, as indicated by field evidence. On this ground, together with published thermochronological data showing that the granite experienced a rapid cooling from ~14 to ~11 Ma before experiencing slow cooling until ~9 Ma, we can state that most of the granite exhumation cannot be ascribed to the MD, the activation of which postdates the late stage of the granite cooling.

These new geochronological data (which are soon to be implemented with new K-Ar dates) and the description of the architectural evolution of the MD fault zone, stress the role of the detachment during the unroofing of the Mykonos granite in the Aegean rifting context. In this perspective, the granite exhumed is mostly assisted by the ductile LD, which acted before the MD. The latter acted instead only at a later stage when it juxtaposed the Miocene siliciclastic against an already cooled and unroofed granite, which had reached a temperature of ~40°C about 2Ma before the latest Late Miocene activation of the MD, as shown by our preliminary age constraint.

How to cite: Zuccari, C., Mazzarini, F., Tavarnelli, E., Viola, G., Aldega, L., Van der Lelij, R., and Musumeci, G.: Evolution and activation of an orogen-scale shear zone in the northern Aegean Rift System: insights from the Mykonos Detachment, Cyclades, Greece, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1615, https://doi.org/10.5194/egusphere-egu24-1615, 2024.

A new onshore-offshore 3-D constrained gravity inversion methodology that incorporates onshore topography and laterally variable inversion mesh depths is used to determine the crustal density distributions, Moho depths, and crustal thicknesses of Iberia, Morocco, and their respective rifted continental margins. The results largely show an excellent correspondence with crustal characteristics determined from sparsely distributed controlled-source and passive seismic experiments, while also allowing the layered density structure of the region to be explored and analyzed in terms of upper, middle, and lower crustal layers. These detailed regional views as a function of depth can improve characterization of crustal types (continental versus oceanic versus transitional), and the resulting interpretations can be directly compared against equivalently derived crustal characteristics for onshore-offshore Atlantic Canada, which encapsulates both Iberia’s and Morocco’s conjugate rifted margins. Collectively, the conjugate 3-D crustal-scale density models allow for the extraction of mega-transects across both sides of the southern North Atlantic, joined together back through geological time using kinematic plate reconstructions.

How to cite: Welford, J. K.: Crustal structure of onshore-offshore Iberia, Morocco, and their rifted continental margins, from constrained 3-D gravity inversion using variable mesh depths, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3196, https://doi.org/10.5194/egusphere-egu24-3196, 2024.

Continental rifting is the Earth’s fundamental tectonic process that may result in a new plate boundary, i.e., mid-ocean ridges, with the accretion of new oceanic crust. At present, continental rifted margins are classified into two end-members based on the amount of magmatism that occurred during the rifting process: magma-rich and magma-poor. However, various factors influence the formation of these margins, such as the inheritance of segmentation, extension obliquity, syn-rift magmatism, and sedimentation. The Gulf of Aden represents a good example for understanding such spatial variations in the formation of rifted margins. It consists of an oblique rifting system, with young and segmented margins (34-17.6 Ma) and thin sediments. In addition, the Gulf of Aden exhibits magma-rich margins in the west, related to the Afar hotpot, and magma-poor margins in the east, with a possible zone of exhumed continental mantle.

In this study, we develop a 3-D P-wave velocity model across the north-eastern Gulf of Aden continental margin, using wide-angle seismic refraction data from a combined onshore-offshore survey with 35 ocean-bottom seismometers and 13 land seismometers. Approximately 187,000 P-wave first arrivals were picked and inverted in 3-D, with the modelling informed by constraints from previously published 2-D velocity models. Here, we present our preliminary tomographic results that illustrate the spatial variations in the crustal velocity structure of the continent, continent-to-ocean transition (COT), and oceanic domains, as well as the comparison between our 3-D and the published 2-D velocity structures of the north-eastern Gulf of Aden continental margin.

How to cite: Chen, J., Leroy, S., Watremez, L., and Robinson, A.: Three-dimensional crustal velocity structure of the north-eastern Gulf of Aden continental margin, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3979, https://doi.org/10.5194/egusphere-egu24-3979, 2024.

During the final stages of breakup at magma-poor rifted margins, mantle rocks are commonly exhumed and altered to serpentinite due to the ingress of ocean water. This mantle exhumation phase is followed by an increase in magmatism as new oceanic crust begins to form. However, the degree to which serpentinisation is focused at faults and whether the onset of magmatism is abrupt or gradual are both unclear. These processes are difficult to untangle with seismic data alone because the P wave velocities of mafic crustal rocks and partially serpentinised mantle rocks can be similar. However, serpentinised mantle rocks are generally more conductive, often by about an order of magnitude, than mafic crustal rocks, so controlled source electromagnetic (CSEM) and magnetotelluric (MT) techniques provide a promising route to resolve controversies around the structure of lithosphere formed during the onset of seafloor spreading.

To take advantage of the complementary information provided by seismic and electromagnetic data, in September 2023 we acquired a coincident and densely sampled wide-angle seismic, CSEM and MT datasets across the continent-ocean transition at Goban Spur, southwest of the UK. Our c. 200-km profile is coincident with a pre-existing high-quality seismic reflection profile. It extends from thinned continental crust, whose nature is confirmed by drilling, across a broad zone that is inferred on the basis of a previous wide-angle seismic experiment to be composed of exhumed and serpentinised mantle, and into oceanic crust, evidenced by the presence of the prominent seafloor-spreading magnetic anomaly A34. Along this profile, we deployed 49 seafloor instruments at c. 4-km spacing that were each capable of recording seismic, electric field and magnetometer data, plus an additional two instruments recording the inline electric field on 200-m dipoles. These instruments were on the seafloor for about two weeks. During this time we acquired two wide-angle seismic profiles: one using a 5200 cu. in. airgun array shot at 90-s intervals and a second using a 3900 cu. in. airgun array shot at 30-s intervals. We also acquired a frequency-domain  CSEM profile using a transmitter towed c. 100 m above the seabed that powered a 300-m electric dipole with a c. 100-A current at a fundamental frequency of 0.25 Hz. Preliminary data analysis showed that seismic signals were recorded to c. 90 km offset and CSEM signals to c. 8 km offset, while high-quality MT data were recorded at periods of 20-10000 s.

Thus we expect to recover coincident high-resolution images of the seismic velocity and resistivity structure of the upper few km of the basement, sufficient to image patterns of serpentinisation and mafic intrusion. We also expect to recover lower-resolution images of the resistivity to tens of km below the seabed and thus to distinguish continental mantle lithosphere from depleted oceanic lithosphere. We will present examples of the data acquired and the results of some preliminary analysis.    

How to cite: Minshull, T., Bayrakci, G., and Constable, S.: An integrated seismic, controlled source electromagnetic and magnetotelluric study of the continent-ocean transition southwest of the UK, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6227, https://doi.org/10.5194/egusphere-egu24-6227, 2024.

Volcanism in continental rifts is generally observed to shift over time from the inside of the graben to its flanks and back. These patterns are commonly observed across rifts from different tectonic contexts, different abundance of melt, and regardless of the rifts' specific complexities, suggesting a common control. However, despite recent advances, the mechanisms governing the spatio-temporal evolution of rift magmatism are still poorly understood. Here we test the hypothesis that the spatio-temporal evolution of rift volcanism is controlled by the crustal stresses produced during the development of the rift basin. To do so, we couple a gravitational unloading model of crustal stresses with a boundary element dike propagation code to investigate the effect of a deepening graben on the evolution of magma trajectories in rifts. We find that the progressive deepening of a graben rotates the direction of the principal stresses in the crust, deflecting ascending dikes. This causes a relatively sudden shift of volcanism from the inside of the graben to its flanks during the early stages of rifting. The intensification of this stress pattern, caused by further deepening of the basin, promotes the formation of lower crustal sill-like intrusions. These horizontal bodies can stack under the rift, shallowing the depth at which dikes nucleate, eventually causing a late stage of in-rift axial volcanism, which can alternatively be induced by compensation of graben unloading by sediment infill. Our model reproduces the general patterns of volcanism in rifts and provides a framework to explain their commonalities and account for possible differences. Given the agreement between our model results and observations, we conclude that the evolution of the stresses generated by a developing rift basin can account alone for the major aspects of the spatio-temporal evolution of rift magmatism.

How to cite: Ferrante, G., Rivalta, E., and Maccaferri, F.: How developing grabens dictate volcanism shifts in rifts, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6331, https://doi.org/10.5194/egusphere-egu24-6331, 2024.

Continental rifts accommodate shallow extensional stresses both by brittle deformation (normal faulting) and volcanism (i.e. dykes and lava flows). Lava flows, together with clastic sedimentation reshape the topography of the rift floor, forming fresh new layers of rock that cover ancient faults. Therefore, the influence of the inherited buried faults on the development of the new faults and the processes of linkage at depth between them remain difficult to investigate. Here we use analogue brittle-ductile modeling with orthogonal extension to elucidate fault growth and reactivation modes, and then compare the results with data from natural rift systems.

In our models, deformation is produced above an elastic band placed between a fixed and a moving wall controlled by a stepper motor. A  layer of viscous material distributes the deformation within the model. On top of the viscous material we use a  layer of sand mixture to simulate the brittle properties of the upper crust. A first phase of extension develops an entire normal fault network, which is then carefully buried under a variable thickness of sand, simulating a cover of sedimentary or volcanic deposits. A second phase of extension allows us to study the mode of reactivation of the inherited faults.The progress of the deformation is tracked using top view images and digital elevation models interpolated from perspective images. At the very end of the model, cross sections cut at regular intervals show the faults at depth by overlaying coloured brittle layers. The high quality of the images allow us to map and analyze the network semi-automatically. We derive displacement/length profiles to characterize the style of fault growth and propagation mode.

Model results show the development of normal faults creating systems of fault-bounded basins, horst-graben structures and conjugated faults. The setup creates a gradient of deformation from the moving wall, where the faults nucleate first near the fixed wall. We thus observe the coexistence of faults of slightly different ages on the same model, as would occur in nature over time. The cross-section shows an upward propagation and the propagation of faults from depth to surface. The preliminary results indicate different styles of reactivation depending on the stage of fault development: reactivation according to a propagating fault mode where faults still have space at tips to develop and a constant-length fault mode where the network is already fully developed. In addition, we find that the surface overlying the inherited structures first bends, then fractures (without observable vertical displacement), and finally develops from the fracture into a proper fault before it finally propagates to connect laterally within the network. This latter growth mode is consistent with the process observed in Iceland by Braham et al. (2021). Understanding the processes of fault network inheritance holds broader applications to many areas where lava or sediments cover faults, layer after layer, such as magma rich rifts like the Eastern Africa Rift or Iceland.

How to cite: Gayrin, P., Maestrelli, D., Corti, G., Brune, S., and Del Ventisette, C.: Reactivation of inherited faults in rift basins: insight from analogue modeling, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6401, https://doi.org/10.5194/egusphere-egu24-6401, 2024.

Rifting in back-arc basins is characterized by large extension rates, low-angle normal faults and metamorphic core complexes (MCC) displaying partially molten cores and granitic intrusions. The Aegean metamorphic core complexes (MCC) were exhumed underneath crustal-scale detachments accommodating large displacements of the order of 50-100 km and were intruded by Miocene syn-kinematic granites. A common finite geometry and kinematics of all these detachment/pluton systems is recognized with asymmetric intrusive bodies extracted from anatectic lower crust, whose internal structure is controlled by the large-scale dynamics, from the magmatic stage to mylonitization and final exhumation in brittle conditions. Detachments are organized in sets of structures working sequentially evolving from ductile to brittle, the successive branches of the detachment being progressively inactivated by emplacing plutonic batches. The Mykonos-Delos-Rheneia (MDR) MCC shows these interactions between lower crustal migmatites and different syn-kinematic plutons. Our new detailed map of Delos (1/5000) shows geometrical and kinematic relationships between the different magmatic venues during deformation. A strong internal orientation of granites is observed from the magmatic stage until the last ultramylonites below the upper detachments. The deepest magmatic batches are rich in high-grade rocks septae and mafic enclaves, also oriented parallel to regional stretching. Evidence for magma mixing and mingling further indicates interactions with mafic venues at the base of the crust from the mantle. Large high-grade rocks septae are intensely molten and the contact zone between host gneiss and plutons shows intense migmatitization with a foliation parallel to the granite magmatic foliation. Characteristic banded facies marking the contacts between the different intrusions result from high-temperature shearing at the magmatic stage. At all scales foliation and lineation in magmatic rocks and surrounding gneisses are parallel, suggesting a similar weak rheology. Delos shows the roots of these intrusions emplaced as a large-scale sheath-fold whose axis is parallel to the regional stretching direction. The quality of outcrops in Delos, Rheneia and Mykonos, as well as the links between magma emplacement and regional tectonics makes the MDR MCC a natural laboratory for studying the interactions between magmatic intrusions and crustal deformation in tectonically active and hot contexts. In such contexts magmatic and tectonic processes in the lower and middle crusts appear closely interconnected, working at a similar pace and interacting with mantle deformation and melting.

How to cite: Jolivet, L., Arbaret, L., and Augier, R.: Continental back-arc extension, molten lower crust and syn-kinematic granites: insights from Cycladic MCCs, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6489, https://doi.org/10.5194/egusphere-egu24-6489, 2024.

Continental rifting is one of the fundamental tectonics of the Earth evolution while our current knowledge on the dynamic mechanism of the strike-slip ones are seriously limited. Here, a systematically shear-wave splitting investigation has been performed in the typical strike-slip Dead Sea rift to illuminate the upper mantle azimuthal anisotropic status across a transform boundary. Totally, 1855 well-defined anisotropic measurements are observed from 102 stations with dominantly N-S fast orientation, which is parallel to the rift strike but deviate from the absolute plate motion direction, mainly result from the plate-driven mantle flow deflected by the thick lithosphere of the eastern Arabian plate. Additionally, the significant fluctuation patterns of splitting times are identified on both the rift-parallel and rift-orthogonal profiles, among which the relatively large splitting times are generally concentrated at the rift zone and attributed to additional coupling lithospheric deformation from the shearing-oriented melt pockets. The consistent rift-parallel fast orientations, combined with the other geoscientific evidences, rule out the role of mantle plume or edge-driven convection in the rift development and further infer the Dead Sea rift to evolve in a passive mode.

How to cite: Xu, H., Yu, Y., and Xi, J.: Upper mantle anisotropy under the strike-slip Dead Sea rift, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6912, https://doi.org/10.5194/egusphere-egu24-6912, 2024.

The evolution of continental rifts is influenced by the pre-rift rheology of the lithosphere and discrete lithospheric structures that segment the rift. The Great South Basin, offshore New Zealand, is a Cretaceous rift system that formed across heterogenous basement terranes which influence the rift architecture. Faults locally rotate or splay and segment along these terrane boundaries. While the impact of terrane boundaries on rift architecture is well understood, the temporal evolution of these rotated faults is poorly constrained. Here we use 3D reflection seismic data to investigate the timing and slip rate evolution of the rotated and segmented faults along two terrane boundaries. Our results show that these have a significant but variable impact on rift evolution and architecture: Faults in the Murihiku terrane show asymmetric throw-length profiles and are rotated along the terrane boundary to the Dun Mountain-Maitai terrane, as they detach into shallow crustal fabrics. Faults in the DMM terrane show less evidence of rotation and more symmetric throw-length profiles but are segmented along the DMM and Caples terrane boundary. The curving faults of the Murihiku terrane likely formed early on but remained as isolated segments only linking up during later stages of rifting when other faults became inactive. These results show the influence of the terrane boundaries was not only active early during initial segmentation but also during the linkage of curved fault segments in the later stages of rifting. These results may help understand the temporal evolution of lithospheric and crustal inheritance on rift evolution in other regions around the world like East Africa or North China.  

How to cite: Froemchen, M., McCaffrey, K., Phillips, T., Allen, M., and van Hunen, J.: A tale of two terrane boundaries – variable impact of terrane boundaries on rift geometry in the Great South Basin, New Zealand, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7884, https://doi.org/10.5194/egusphere-egu24-7884, 2024.

Lithospheric extension leads to rift formation and may continue to the point of breakup, with oceanic ridge initiation and the formation of two conjugate rifted margins. In some settings, extension can cease, and the rift may be abandoned. These so-called failed rifts archive snapshots of early phases of deformation, with geometries that may help better constrain the parameters that can prevent a rift from reaching breakup, such as lithospheric rheology, thermal state, rift opening direction and rate, inheritance.

This contribution summarizes a study of the Norwegian Continental Shelf which includes the North Sea Rift and the Møre and Vøring rifted margins. We proceeded to the interpretation of a new dataset of deep penetrating seismic reflection profiles and worked at the regional scale, deliberately ignoring local particularities, to focus on the large-scale structural picture. The aim is to list architectural similarities and differences between the failed rift and the successful rifted margins.

The mapping shows that the North Sea structural geometries and basement seismic facies are very similar to the observations listed for the adjacent Møre and Vøring rifted margins. Various types of tectonic structures are observed, from thick anastomosing shear zones possibly evolving into core-complex geometries, to composite large-scale detachment faults and standard high-angle normal faults. These are categorized into five classes and interpreted as exemplifying the rift tectonic evolution through distinct generations of deformation structures that can activate, de-activate and re-activate. Based on these observations, rift failure dynamics are discussed, and it is proposed that the North Sea rift abandonment may not be related to pre-rift local conditions but rather to the ability to initiate specific tectonic structures such as distal breakaway complexes.

How to cite: Peron-Pinvidic, G.: Structural observations of the northern North Sea: insights into rift failure dynamics, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7915, https://doi.org/10.5194/egusphere-egu24-7915, 2024.

The present-day surface morphology and fabric of the lithosphere of west-central Europe in the foreland of the Alpine orogen have been significantly affected by formation of a system of rifts and associated volcanic domains during the Oligocene and Neogene, known as the European Cenozoic Rift System (ECRIS). In order to better understand the geodynamic causes of formation of ECRIS and its volcanism, it is important to improve the knowledge of chronology of tectonic events in the entire ECRIS, and to test the validity of existing palaeostress interpretations. The Oligo-Miocene Eger Rift, so far the least-studied part of ECRIS, has the potential to bring new clues to some persisting controversies.

The axis of the Eger Rift roughly follows the trend of a major Variscan lithosphere-scale boundary, the Teplá-Barrandian/ Saxothuringian suture (TSS) formed during the collisional phases about 380-320 Ma. Following the Variscan collision, the lithosphere of the Bohemian Massif was affected by formation of a Late Paleozoic extensional basin system which in the western part of the Bohemian Massif largely follows the NE strike of the TSS. Another major structure in the basement underlying the Eger Rift is the WNW-striking Elbe Zone, with main periods of activity during the Paleozoic and Mesozoic through early Cenozoic.

We present a synthesis of presently available structural and stratigraphic data and a resulting first-order interpretation of tectonic evolution of central and eastern Eger Rift. The main data sources were borehole, outcrop, seismic reflection data, targeted field mapping, digital elevation models, and gravity data from both public and industry sources. Several stratigraphic levels (within the Neogene, Cretaceous, and top of Late Palaeozoic) were used as structural datums.

Analysis of fault populations in central and eastern Eger Rift shows that overall, the Late Paleozoic fracturation of the upper crust of the Bohemian Massif was key for localization of the main fault systems of the Eger Rift. This includes dextral shearing within the Elbe Zone that affected the basement structural grain responsible for segmentation of the Eger Rift during the Cenozoic. Changes between oblique and orthogonal extension modes are interpreted from the geometries and temporal relationships of key structures - both in time, likely due to a changing regional paleostress field, and in space, due to different orientations of basement structures between the rift segments.

This research has been supported by the Czech Science Foundation (GAČR) project 22-13980S.

How to cite: Ulicny, D., Cajz, V., Mach, K., Špičáková, L., Machek, M., Čech, S., Grygar, R., Mrlina, J., and Havlíček, F.: Structural inheritance and the evolution of an incipient rift: interaction between the Eger Graben and the Elbe Zone, Central Europe, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8883, https://doi.org/10.5194/egusphere-egu24-8883, 2024.

Ocean closure and collisional orogeny frequently enrich the lithospheric mantle in incompatible chemical elements. The most intensive enrichment usually occurs during the subduction of continent-derived sediments and continental crust. Radioactive isotopes of uranium and thorium are part of the HFSE (high-field-strength elements) group of incompatible elements and, therefore, can be also characterized by increased concentration within the post-orogenic lithospheric mantle in comparison to the common lithospheric mantle. The anomalously high content of uranium and thorium within the post-orogenic mantle lithosphere is reflected by the composition of potassic and ultrapotassic magmas, which are sometimes extremely enriched in these radioactive elements. This enrichment is well documented by numerous studies and, therefore, cannot be ignored during the numerical modelling of rifting processes.

According to pure conductive thermal modelling, the anomalously increased content of radioactive elements within the post-orogenic lithospheric mantle causes a time-dependent rise in temperature, providing favourable conditions for intracontinental rifting more than 20-100 million years after the closure of the ocean. A time gap between the orogeny and highly increased temperature within the lithosphere is controlled by two major factors: (1) the amount of thorium and uranium and (2) the size of the anomalous lithospheric mantle. According to numerical thermo-mechanic modelling, the post-orogenic increase in temperature not only weakens the lithosphere but also causes thermal expansion of the lithosphere which can be sufficient to initiate the first stage of intracontinental rifting without involving regional extensional forces.

Therefore, we propose a new concept of intracontinental rift initiation as a result of time-dependent temperature increase and thermal expansion of the post-orogenic mantle lithosphere due to the decay of radioactive elements. The described rather simple mechanism of rift formation provides a significant advance in our understanding of both local rift processes and global tectonic cycles on our planet.

How to cite: Maystrenko, Y. and Slagstad, T.: Post-orogenic radiogenic initiation of intracontinental rifting within the lithospheric mantle, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9325, https://doi.org/10.5194/egusphere-egu24-9325, 2024.

The present-day geological and morphological expression of the Cenozoic Eger Rift in central Europe is dominated by the faulted edge of the Krušné Hory (Erzgebirge) Mts., a plateau uplifted to c. 1 km above sea level following the mid-Miocene, resulting in partial deformation and erosion of parts of the Eger Rift sedimentary and volcanic infill. The main phase of the uplift is considered to have occurred in Plio-Quaternary times, but details of this process and its relation to the Eger Rift itself remain unclear.

The Oligo – Miocene Most Basin is the most extensive sedimentary basin preserved within the Eger Rift. The basin, bounded at the NW by the Krušné Hory uplift, is characterized by an economically important coal seam, up to 35 m thick. Previous research has shown that during the formation of the basinwide swamps in early Miocene the basin was hydrologically open, with at least one but probably more outlets draining its area toward the North and Northwest, across today’s Krušné Hory (Erzgebirge) Fault Zone (KHFZ). During the earliest Miocene times, most of the region of today’s Krušné Hory / Erzgebirge uplifted block was thus a generally low-relief area. Paleogeographic changes in the Most Basin suggest an increasing activity of its marginal faults, some of which were predecessors of the present-day KHFZ, still during the early to mid-Miocene. For understanding the formation of this major fault zone it is important to answer the question of the timing, magnitude and character of initial relief growth along the nw. edge of the Eger Rift.

The stratigraphic and structural record exposed recently at the KHFZ provides evidence of a small-scale relay ramp that formed between two overlapping normal faults of E-W general strike and breached later by a normal fault of NE strike. Debris-flows conglomerates were found interbedded with carbonaceous mudstones and lignite layers belonging to the early Miocene main coal seam, in the close vicinity of the lower bounding fault containing boulders from a tectonic breccia of the fault damage zone. This fact indicates the existence of a prominent fault scarp developed along the fault plane of the above fault and considered the source of coarse-grained clastics during the initial, coal-bearing, phase of the basin formation. The subsequent acceleration of the Most Basin subsidence that resulted in basin-wide expansion of the swamp environment, can be explained by linkage of the border faults accompanied by breaching and drowning of some relay ramps.

The studied sedimentary record provides evidence of faulted relief with an elevation of tens of metres that contributed to the supply of clastic material to the incipient rift during early Miocene time. The subsequent breakage and drowning of the relay ramp provide evidence for syn-sedimentary activity some of NE-SW segments of the KHFZ previously thought to be a manifestation of later, post-rift deformation.

This research has been supported by the Czech Science Foundation (GAČR) project 22-13980S. We acknowledge support by the Severní energetická, a.s., and Ing. Petr Šulcek in conducting research in the Důl ČSA Mine.

How to cite: Rajchl, M., Mach, K., Havlíček, F., Uličný, D., and Machek, M.: Early relief growth at the edge of an incipient rift – the Eger Graben, Bohemia, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9513, https://doi.org/10.5194/egusphere-egu24-9513, 2024.

Continental lithosphere undergoing the process of rifting has typically previously experienced a complex deformation history resulting in a highly heterogeneous mechanical structure. This structural inheritance can affect the developing continental rift across all scales, from rift localization and segmentation to individual fault geometries. Assessing the impact of such inherited structures on extensional basin geometries can be difficult, especially in the case of fossil rifts where uncertainties may arise about the orientation of regional stresses during extension. One such example is the Eger Rift which developed during the Oligocene to early Miocene as the easternmost branch of the European Cenozoic Rift System (ECRIS). Earlier interpretation proposed a two-phase extensional history for the rift.

We use a series of crustal-scale, brittle-viscous analogue models, based on the geometry of the central and eastern parts of the Eger Rift, to explore the development of a segmented rift in a multiphase setting with evolving extension direction. Our model crust rests on a basal velocity discontinuity (VD), a discrete boundary of a mobile base plate simulating a reactivated basement weakness localizing our model rift. The geometry of this weakness is a simplified representation of the geometry of older, mainly Upper Paleozoic basins, which are hypothesized to have greatly influenced the localization and geometry of principal fault systems and rift segments that they define. The VD thus consists of 3 segments oriented at various angles with respect to extension direction. The Model surface is imaged by stereoscopic cameras and analyzed by Particle Image Velocimetry (PIV) techniques to track surface deformation and topography evolution during the run.

Our results confirm that in a setting with an abrupt change in extension direction, the first extensional phase plays a key role in defining the final observed fault pattern with new second-phase faults generally being few in number and of limited length. This effect is enhanced above a segmented VD. If a larger portion of the VD is optimally oriented with respect to the first-phase extension, the final fault pattern is dominated by first-phase structures with the growth of second-phase faults being nearly inhibited. In a contrasting scenario, where most of the VD is initially oblique to extension direction, second-phase faults are more abundant, leading to a bimodal final fault pattern. By comparing our results with newly mapped fault populations in the Eger Rift we conclude that the proposed two-phase history for the rift is plausible with a major role of the initial phase of approximately N-S extension.

This research has been supported by the Czech Science Foundation (GAČR) project 22-13980S.

How to cite: Havlíček, F., Uličný, D., Krýza, O., Machek, M., Warsitzka, M., and Závada, P.: Modelling the effects of changing extension directions in a segmented rift: application to the Eger Rift, Central Europe, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9770, https://doi.org/10.5194/egusphere-egu24-9770, 2024.

Driven by discovery of contrasting structures of Continent to Ocean Transition (COT) discovered at rifted continental margins during the 90’s, several high-quality seismic datasets were acquired in these margins during the early 2000 to unravel the structure of unexplored regions. Despite the fact that some of these datasets are basically comparable to modern data in quality, the processing, imaging and modelling methodologies at the time of acquisition can be now refined and improved. Recent developments in parallel computing and novel geophysical approaches provide now the means to obtain a new look at the structure with enhanced resolution seismic models and a mathematically-robust analysis of the data uncertainty, that was formerly difficult, if not unfeasible, to achieve. 

We focused on the Newfoundland margin and applied up-to-date methodologies to the high-quality SCREECH dataset (2000). These data include three primary transects with coincident multichannel seismic (MCS) reflection data acquired with a 6-km streamer and wide-angle data recorded by short-period OBS and OBH spaced at ~15 km. We reprocessed the streamer data and also performed the join inversion of streamer and wide-angle OBS/OBH seismic data, using reflections and refraction arrivals, which significantly improved the resolution of the velocity model. We performed a statistical uncertainty analysis of the resulting model, supporting the reliability of the observed features. In particular the new velocity model provides a detailed definition of the top of the basement where the largest abrupt velocity change occurs. The comparatively high-resolution velocity model obtained from the joint tomography allowed to properly perform a Pre-Stack Depth Migration of the MCS. The improved velocity model and seismic images permit to characterize the different crustal domains of the margin with less uncertainty that previous attempts, and relate them to the tectonic structure.

The different domains reveal previously undetected crustal characteristics that change over short distances. The reprocessing of the MCS data allowed to a better understanding of the crustal structure, as the Moho is imaged for the first time under the slope domain.

Comparison of these new results on the Newfoundland margin with the most modern data on the West Iberian margin, acquired during FRAME (2018) and ATLANTIS (2022) cruises provides a new view of the evolution of the North Atlantic opening.

How to cite: Gómez de la Peña, L., R. Ranero, C., Prada, M., Shillington, D., and Sallarès, V.: New results on the crustal configuration of the Newfoundland margin: Implications for rifting, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9947, https://doi.org/10.5194/egusphere-egu24-9947, 2024.

It is common wisdom, based on many years of published simulations of continental rifting followed by spreading that in 2D when a mid-oceanic ridge form in a numerical simulation of continental rifting, extension stops and spreading take over the extension. This is generally due to the complete loss of strength of the mantle lithosphere that cannot transmit forces horizontally across the spreading zone anymore. Actually, in general even the onset of mantle lithosphere necking in a simulation can cause the end of the extension and for many years, I actually claimed very load in the past that two active necking system must be the signature of some obliquity causing 3D extensional conditions. However, recently, a whole series of 2D simulations produced systematically two spreading centers active at the same time. These results surprised me a lot. These simulations were very complex, including a lot of inheritance, the first easy conclusion could have been to say that inheritance causes multiple spreading… But we spent some time and effort to understand if this behavior was due to inheritance or something else. Simplifying our model set-up to the strict minimum, we found it was not inheritance, but a quite cold mantle temperature which permitted a larger shear coupling between the upper mantle dynamics and the mantle lithosphere.  A 50°C difference in mantle temperature radically change the results of the simulation and thanks to our failure, we have found the embryo of an alternative explanation to 3D interactions for the occurrence multiple active necking zones.

How to cite: Le Pourhiet, L. and Zhou, F.: The story of double spreading centers formed during continental rifting in 2D, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11403, https://doi.org/10.5194/egusphere-egu24-11403, 2024.

The FRAME geophysical cruise, conducted in 2018 onboard the Spanish R/V Sarmiento de Gamboa, acquired new multichannel seismic reflection (MCS) data on the SW Iberia and NW Moroccan margins. MCS data were acquired with a 6 km long solid-state digital streamer Sercel SENTINEL towed at 19 m water depth, and a 3920 c.i. source with two sub-arrays with 20 guns towed at 10 m depth. The system was designed to provide high penetration and map the entire crust and the upper mantle structure and retain enough resolution to image well the stratigraphy.

In this work we present a 220 km long seismic line acquired on the NW Moroccan margin, from the shallow continental shelf across the continental slope and extending across the deep abyssal plain of the Gulf of Cadiz. The NW Africa margin was selected because the region was the focus of several geophysical campaigns, and several DSDP drill sites that drilled into the synrift strata.  However, limited modern data has imaged the crustal-scale tectonic structure to unravel the late Triassic - early Jurassic rift history of the region.

We applied a seismic processing flow tailored to the attenuation of the multiple energy, signal designature and for the creation of a detailed macro-velocity model to image the lateral changes of the synrift tectonic structure and stratigraphy. The high-quality image of the structure of this rifted margin reveals a complex tectonic structure from the shelf to the deep-water basin where a deep basement containing salt bodies across the entire profile extension. These new results are of high importance for the understanding of the rifting and continent-ocean transition (COT) processes on the northern Central Atlantic and Neothetys domains, as well as for the subsequent compressive deformation processes in the Gulf of Cadiz related to the Africa-Eurasia plate collision.

This work was funded by the Portuguese Fundação para a Ciência e a Tecnologia (FCT) I.P./MCTES through national funds through the project LISA (https://doi.org/10.54499/PTDC/CTA-GEF/1666/2020).

How to cite: Foiada, S., Neres, M., Brito, P., Gomez de la Peña, L., Merino, I., and Ranero, C.: The seismic structure of the NW Moroccan margin, Gulf of Cadiz, from new high-quality multichannel seismic reflection data, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11494, https://doi.org/10.5194/egusphere-egu24-11494, 2024.

The wide rifting mode that preceded the opening of the South China Sea (SCS) in the Cenozoic generated a set of Paleogene rift basins presently buried under thick post-rift sedimentary infill. Much of the tectonostratigraphic evolution of the South China Sea is now relatively well-constrained (e.g., Pearl River Mouth Basin). However, the SCS's northeasternmost part (i.e., the Tainan margin sensu lato), which might represent the oldest passive margin segment, remains to be integrated into the framework of the rifting and opening of the SCS.

This work aims to review and revisit the tectonostratigraphic evolution of the Tainan margin. To do so, an integrative approach has been used combining the analysis of seismic reflection and gravity data. We use 3D gravity inversion to determine the distribution of Moho depth and crustal thickness within this margin segment. The gravity inversion scheme incorporates a lithosphere thermal gravity anomaly correction, which is critically important because of the elevated geothermal gradient within the young oceanic lithosphere of the South China Sea and its continental margins. In the Tainan margin, results show contrasted crustal domains from the continental shelf, to the distal margin and oceanic domain.

Only limited crustal thinning is observed over the continental shelf where a succession of rift basins is documented (i.e., Taihsi, Nanjihtao, and Penghu basins) that are part of the Northern Rift System. In contrast, the distal Tainan margin shows greater crustal thinning to less than 10 km thick under an aborted breakup basin, thereby forming the Southern Rift System. To the south, this basin is separated from the unambiguous oceanic domain (6 to 8 km thick) by a comparatively thicker crustal block (~ 10 to 15 km thick). This crustal block forms the Southern High where numerous volcanic edifices and magmatic intrusions are observed or inferred.

Half-grabens of the Northern Rift System are controlled by counter-regional faults and filled by Paleocene to Eocene syn-rift sediments. For the distal domain, no well calibration is available. There, we identified several seismic units bounded by regional unconformities. Our results show relatively thin syn-rift sediments locally controlled by a low-angle normal fault system in the Southern Rift System. In contrast, thick post-rift sequences are observed except over the Southern High.

Based on our results, we propose a review of structural style and age correlations from the continental shelf to the distal domains of the Tainan margin. To illustrate along-strike variations of the crustal structure and stratigraphic style, we build an array of regional geological cross-sections that are further compared with existing observations in the adjacent Pearl River Mouth Basin.

How to cite: Rodrigues de Vargas, M., Mohn, G., Tugend, J., Kusznir, N., and Lin, A.: Tectonostratigraphic evolution of the Tainan Margin (NE South China Sea): comparison with the Pearl River Mouth Basin, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11825, https://doi.org/10.5194/egusphere-egu24-11825, 2024.

Marginal Seas are extensional basins formed in a convergent setting near active subduction zones. They are characterized by a short life (<25 Ma), as well as unstable and changing directions of seafloor spreading. However, the processes related to their formation from rifting to seafloor spreading initiation remain debated (supra-subduction convection/extension, slab-pull). This problem is further compounded by the fact that our understanding of continental breakup used to be derived from the evolution of magma-poor and magma-rich Continent-Ocean Transitions (COT) of Atlantic margins.

Here, we describe and discuss the rifting style and the mode of continental breakup of three main Marginal Seas located in the Western Pacific, namely the South China Sea, the Coral Sea and the Woodlark Basin. All three examples formed under rapid extension rates and propagation of seafloor spreading.

In these three examples, continental extension is accommodated by a succession of hyper-extended basins controlled by low-angle normal faults that may form and be active at 30° (or less). These hyper-extended basins are filled by polyphase syn-rift sequences showing atypical geometries. These complex stratigraphic architectures result from the development of the low-angle normal faults interacting with antithetic faults, controlling the formation of extensional fishtails for example. The formation of such low-angle normal fault systems is enhanced by basement inheritance of the previous orogenic system.

Continental breakup and final extension are contemporaneous with an important magmatic activity emplaced in the distalmost part of these margins including 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 spreading occurred. High extension rate prevents conductive cooling allowing the focusing of volcanic activity in sharp COTs, quickly evolving to magmatic accretion.

In conclusion, the rifting style and mode continental breakup are most likely associated with initial rheological conditions with hot geotherm combined with fast extensions rates likely directed by kinematic boundary conditions directly or indirectly controlled by nearby subduction zones.

How to cite: Mohn, G., Ringenbach, J.-C., Legeay, E., Tugend, J., Vetel, W., and Sapin, F.: Rifting style and continental breakup of Marginal Seas, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11854, https://doi.org/10.5194/egusphere-egu24-11854, 2024.

The Mid-Norwegian volcanic rifted margin and its NE-Greenland conjugate formed in relation to continental breakup in the latest Palaeocene to earliest Eocene during the emplacement of the North Atlantic Igneous Province (NAIP). The development of the NAIP and opening of the North Atlantic occurred contemporaneous to the Paleocene Eocene Thermal Maximum (PETM) which corresponded to a rapid 5-6 °C global warming episode.

The cause of this rapid global warming, explored as part of IODP Expedition 396, is thought to relate to the thermogenic gases released to the atmosphere via thousands of hydrothermal vents. The thermogenic gases were produced by contact metamorphism of carbon-rich sediments during widespread sill emplacement from the NAIP. The potential of hydrothermally-released greenhouse gases to influence climate depends strongly on the water depth at which they get released. Unless it is released in a shallow marine environment most methane will be oxidized before it reaches the atmosphere.

Early results from IODP Expedition 396 have documented that at least one of the Mid-Norwegian hydrothermal vents was emplaced in shallow marine to potentially sub-aerial conditions. The aim of this contribution is to constrain further the paleo-water depth at which hydrothermal vents formed along the other parts of the mid-Norwegian volcanic rifted margin. This study focuses on an integrated workflow of quantitative geophysical and geodynamic analyses calibrated by new IODP drilling results and structural and stratigraphic observations. We use a 3D flexural-backstripping, decompaction and reverse thermal subsidence modelling to predict the palaeobathymetry and palaeostructure at keys stages of the syn- to post-breakup evolution that can be compared with palaeo-water depths estimated from biostratigraphic data.

Results provide new constraints on the paleobathymetry of hydrothermal vent complexes required to confirm whether the global warming recorded by the PETM was triggered by the magma-rich continental breakup leading to the opening of the northeast Atlantic Ocean. 

How to cite: Tugend, J., Mohn, G., Kusznir, N., Planke, S., Berndt, C., Manton, B., Zastrozhnov, D., and Millet, J. M.: Palaeobathymetry of the Mid-Norwegian volcanic margin during continental breakup and paleoclimate implications, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11873, https://doi.org/10.5194/egusphere-egu24-11873, 2024.

1D/2D data-based studies of active spreading centres brought the knowledge of extension ratedependent stretching-dominated v. buoyancy-dominated spreading. 3D reflection seismic data from the extinct centre of an initial oceanic corridor in the Caribbean allow us to see an along-strike transition between stretching- and buoyancy-dominated spreading where the spreading through detachment faulting is a precursor to the magma-assisted spreading. Studying progressively more evolved portions of the spreading centre, going from its end towards its centre, we see a progressively higher ascent of the asthenosphere, which heats the developing

core complex in the exhuming footwall of the initial stretching-dominated system. The asthenospheric ascent is associated with thermal weakening of the core complex, which eventually results in ductile deformation reaching the upper portion of the complex. Subsequently, the core complex is penetrated by the dyke located at the top of the asthenospheric body. The dyke, which subsequently evolves to a diapir-shaped body, reaches the sea floor

and establishes a magma-assisted steady-state seafloor spreading. These observations lead to a model explaining the initiation of the magma-assisted spreading in the initial oceanic corridor. Furthermore, they also improve our knowledge of multiple interacting mechanisms involved in the breakup of the last continental lithospheric layer, subsequent disorganized spreading and younger organized spreading.

How to cite: Nemcok, M. and Frost, B.: Along-strike magma-poor/magma-rich spreading transitions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11877, https://doi.org/10.5194/egusphere-egu24-11877, 2024.

The Porcupine Basin, situated in the North Atlantic, serves as a unique natural laboratory for investigating the temporal evolution of magma-poor rifts. Notably, the basin exhibits a progressive increase in the total degree of stretching from north to south, offering a valuable opportunity to interpret its structure in terms of the temporal evolution of magma-poor rifted margins. This study, as part of the broader PORO-CLIM project, focuses on Profile 2 to construct a whole-crustal seismic velocity model and integrate it with existing data to unravel the complete rifting history of the Porcupine Basin.

In the northern region, Reston et al. (2004) identified a detachment fault, the P-reflector, indicating substantial rifting  ^[1]. Recent analyses by Prada et al. (2017) extended this understanding to the central basin, revealing progressive crustal thinning and mantle serpentinization ^[2]. However, the southern sector remains largely unexplored. This project aims to capitalise on newly acquired Ocean Bottom Seismometer (OBS) data from PORO-CLIM Profile 2 to image the deep crustal structure and complement this with basement mapping of the southern Porcupine Basin using industry 2D seismic data.

Seismic refraction data from 20 OBS along a 226 km transect form the basis for constructing a comprehensive crustal velocity model. Utilising the RAYINV modelling package, a layer-by-layer forward modelling approach is employed to correlate calculated and observed travel times. Concurrently, structural mapping using long-offset 2D seismic reflection data assists in delineating major faults and regions of mantle unroofing, contributing to the understanding of the Porcupine Basin's subsurface. Preliminary findings reveal extreme crustal thinning and asymmetry, highlighting north-to-south crustal thinning and the emergence of the P-reflector in the southern region of the Porcupine Basin.

[1] Reston, T.J., Gaw, V., Pennell, J., Klaeschen, D., Stubenrauch, A. and Walker, I. (2004). Extreme crustal thinning in the south Porcupine Basin and the nature of the Porcupine Median High: implications for the formation of non-volcanic rifted margins. Journal of the Geological Society, [online] 161, pp.783–798.

[2] Prada, M., Watremez, L., Chen, C., O’Reilly, B.M., Minshull, T.A., Reston, T.J., Shannon, P.M., Klaeschen, D., Wagner, G. and Gaw, V. (2017). Crustal strain dependent serpentinisation in the Porcupine Basin, offshore Ireland. Earth and Planetary Science Letters, [online] 474, pp.148–159.

How to cite: Yusuf, I., M Jones, S., Reston, T., Funck, T., M O'Reilly, B., and R Hopper, J.: Structure and Dynamics of the Porcupine Magma-Poor Continental Margin from new Ocean Bottom Seismometer Data, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12130, https://doi.org/10.5194/egusphere-egu24-12130, 2024.

The Brazilian Equatorial Margin (BEM) is classically interpreted as a transform margin formed during the last phases of the Atlantic rifting of Gondwana. However, rift kinematics and subsequent continental break up has not been constrained.

We present a new model based on the interpretation of a 2D seismic grid acquired along the BEM. The datasets, provided by the Brazilian National Agency for Petroleum (ANP), expand for ~600 km of the margin and consist of approximately 10.000 km of crustal scale 2D seismic reflection profiles which have been calibrated with industry drillholes. The integration of crustal-scale tectonic structures and age and distribution of synrift sediment deposits allowed to determine the style and the timing of the different tectonic phases and to define the crustal thinning evolution of the entire rift system along the Potiguar and East Ceará Basins (NE Brazil).

Our findings indicate that: 1. rifting started ~140-136 My, 2. extension stopped earlier (late Aptian) in the shallow sector of the basin than in the deep-water (early Albian) domains. The shallow basin domains presents minor crustal thinning (~35 thick crust over ~100 km wide), whereas in the deep-water domains, about ~60 km wide, the crust is 4-8 km thick and it extended into the early Albian (116-110 My).

The distribution of deformation structures supports a model of rift evolution where: deformation is initially distributed while forming a shallow basin; it evolves by focusing the extension; finally, extension migrates toward the basin centre to form the deep-water domain. Constraints from seismic reflection data and drillholes help define an abrupt continent to ocean transition (COT), and breakup occurred during the early Albian. Basin sedimentation from its onset to the late Aptian is terrigenous, indicating an isolated environment disconnected from the Northern and Southern Atlantic oceans. Sedimentation changed during the late-most Aptian to the early Albian when marine facies deposited during a rapid ocean water infill of a previously endorheic basin.

The seismic images document that rifting across the margin is not dominated by transcurrent deformation, with strike-slip faulting limited to a relatively small sector, whereas most of the margin extended through normal faulting deformation during opening.

From the interpretation of the 2D seismic reflection grid it was possible to distinguish abrupt lateral changes in the architecture of the basement. These changes defined three distinct, first order segments along the margin named Southern, Central, and Northern segments. The different evolution of the three segments throughout the rifting process is defined by thickness map of the basement. The Northern segment is the only region that shows evidence of potential late synrift magmatism, likely formed during the COT emplacement, which defines second order segmentation. Our interpretation suggests a spatial correlation between first-order tectonic segmentation and second-order magmatic segmentation during the embryonic formation of the spreading center with the definition of fracture zone/transform faults. These findings suggest that most transform faults formed on the spreading centers may have originated from the pattern of continental segmentation during rifting.

How to cite: Fonseca, J., Ranero, C., Vannucchi, P., Iacopini, D., and Vital, H.: Tectonic structure and evolution of Brazilian Equatorial Margin , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12991, https://doi.org/10.5194/egusphere-egu24-12991, 2024.

During the Jurassic period, the Gondwana Continent progressively rifted from north to south along three huge transform faults (Davie Fracture Zone (DFZ), Mozambique Fracture Zone (MFZ) and Agulhas-Falkland Fracture Zone (AFFZ)), forming the northern, central and southern continental margins along Mozambique, producing a series of divergent and strike-slip margins. These margins are crucial areas for understanding the evolution of Gondwana as their crustal nature and geometry have strongly impacted the kinematic reconstruction of Gondwana. Especially, the debate about continental or oceanic crust for the Mozambique Coastal Plain (MCP) and North Natal Valley (NNV) at the southern Mozambique margin led to tens of kinematic reconstruction models of Gondwana. Based on the OBS and MCS data results of PAMELA MOZ3/5 Cruises, MCP and NNV were identified as continental crust. This has led the scientific community to reconsider the issue, for example, the opening time of the oceanic basin, the movement direction of rifting, and the intense magmatism during the rifting and break-up of Gondwana.

In June 2021, the Second China-Mozambique Joint Cruise was conducted onboard the R/V “Dayang hao”. Three wide-angle seismic OBS profiles were acquired where 70 four-component OBSs were deployed along profiles DZ02 and DZ04 oriented nearly W-E and DZ01 oriented nearly N-S. Four Bolt air guns with a total volume of 8000 in³ in total were towed at ~100 m behind the R/V “Dayang hao” at ~10 m below the sea surface. The shot interval was 200 m.

Here, we present the tomographic results of P-wave velocity along 442 km long profile DZ02, where 21 OBSs were deployed. It traverses through the Continent Ocean Transition (COT) and extends into the Mozambique ocean basin. Approximately 19,000 P-wave arrivals were manually picked, using the travel-time tomography inversion to get the velocity model. The tomographic result shows an apparent decrease in crust thickness from COT to the ocean basin, and the thickness of the oceanic crust is about 8 km. We also observe high-velocity anomalies up to 7.4 km/s in the lower crust above Moho, suggestive of more primitive melt. We will also present the S-wave velocity model for DZ02.  

How to cite: Wang, W., Singh, S., Wang, Z., Ruan, A., Tang, Y., Dyment, J., Leroy, S., Watremez, L., Wu, Z., Li, H., and Dong, C.: Crustal Structure of Continental Margin and Oceanic Basin at the Southern Mozambique Margin, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13269, https://doi.org/10.5194/egusphere-egu24-13269, 2024.

The tectonic formation of the Black Sea Basin (BSB) has been an ongoing debate: primarily, there is still not a consensus on whether the basin was rifted as one east-west oriented basin, or as two separate basins named Eastern and Western Black Sea Basins. These interpretations are based largely on deep-sea drilling projects and a growing dataset of seismic information (of variable access for academic use). 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 are supported by the regional structure of the BSB being aligned with the geodynamic models of the basins rifted as a result of slab roll-back. Complicating the understanding of the basin extension and development is the inferred tectonic inversion to shortening in the region starting in the Late Eocene.

To propose a model to answer ongoing debates, we interpreted 24 long-offset 2D seismic lines acquired by GWL in 2011 in a structural geology context. We focused on the structural elements such as big scale normal faults, reverse faults, and tectonic inversion features to create a basis for our 2D computational models for both east and west portions of the BSB. One important finding was to determine the null points on basin bounding faults where the extensional tectonic movements stopped, and the compressional tectonic movements started. Utilizing the ASPECT geodynamic code, we built 2D computational models parallel to the selected two 2D seismic profiles. We compared our findings in our seismic interpretations with the results to understand the timing and basin-wide distribution of structural highs and the compressional tectonic features that shaped the BSB.

How to cite: Kaykun, A. and Pysklywec, R.: Structural Evolution of the Black Sea Basin Using Sectioned Computational Models, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13432, https://doi.org/10.5194/egusphere-egu24-13432, 2024.

Many orogenic belts today preserve evidence of past crustal rifting. During the syn-rifting, the crust undergoes thinning, forming rift basins with thick sedimentary deposits. The upwelling of the mantle during this extensional phase increases the geothermal gradient within the basin, affecting the crust and sedimentary rocks.

In this study, we used numerical models to simulate the temperature changes in sedimentary rocks within rift basins during both the active rifting phase and the passive continental margin phase after rifting cessation. We found that under a stretching rate of 0.7 cm per year, after 20 million years of continuous stretching, the geothermal gradient within the basin can reach 50-60°C per kilometer, with sedimentary rocks reaching temperatures as high as 450-500°C. After 20 million years of cooling following the end of stretching, the temperatures of the sedimentary rocks decrease by nearly 100°C, and the geothermal gradient reduces to approximately 30°C per kilometer.

We believe that these phenomena can be correlated with the evolution of the Hsuehshan Range in Taiwan, which experienced a transition from rifting to a passive continental margin. During the rifting phase, the temperatures of the sedimentary rocks within the basin reached high metamorphic temperatures of 450-500°C, as indicated by carbonaceous material Raman spectroscopy (RSCM). As the region entered the passive continental margin phase, the rocks gradually cooled, with a temperature decrease of nearly 100°C prior to the onset of mountain building in Taiwan. Similar high-temperature metamorphic temperatures were obtained through RSCM analysis along the Central Cross-Island Highway and Northern Cross-Island Highway, exceeding the closure temperatures of zircon core tracks. However, some zircon core tracks in certain areas did not yield closure ages, suggesting that the high metamorphic temperatures obtained from RSCM analysis were inherited from previous stretching events rather than occurring during the Penglai orogeny.

How to cite: zheng, M., Lee, Y.-H., and Tan, E.: Numerical modelling of continental margin of the Eurasian Plate Rifting and Tectonic evolution.Causes of the highest metamorphic temperature in the Hsuehshan Range and Backbone Range , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14297, https://doi.org/10.5194/egusphere-egu24-14297, 2024.

Recent sub-ice topography compilations of East Antarctica have imaged a wide sector, spanning from 100° E to 160° E in longitude and from the Oates, George V and Adelie coastlines to 85° S in latitude, which contains numerous low-lying basins of variable size and uncertain origin. The sector shows a Basin and Range style tectonics comprising two major basins of continental proportions, the Wilkes Basin and the Aurora Basin complex, and many smaller basins such as the Adventure, Concordia, Aurora and Vostok trenches. The main longitudinal axes of the basins consistently point towards the South Pole and many exhibit intriguing distinct triangular shapes, sitting within an approximately 2000 x 2000 km fan-shaped physiographic region limited by a semi-circular coast line. We name this region as the East Antarctic Fan shaped Basin Province (EAFBP). To the West, this sector is limited by the intraplate Gamburtsev Mountains (GM) and to the East by the Transantarctic Mountains (TAM) constituting the uplifted shoulder of the Cenozoic West Antarctic Rift System (WARS).

Origins and inter-relationships between these four fundamental Antarctic tectonic units (WARS, TAM, EAFBP, GM) are still poorly understood and strongly debated. Very little is known about the mechanism generating the basins in the EAFBP, their formation time, whether they are all coeval and if and how they relate to Australia basins before Antarctica-Australia rifting. Present genetic hypotheses for some of the basins span from continental rifting to a purely flexural origin or a combination of the two. Also, post-tectonic erosional and depositional processes may have had a significant impact on the present-day topographic configuration.

Here we interpret the EAFBP as the result of a single genetic mechanism: a wide fan-shaped intra-continental extension around a near pivot point at about 135° E, 85° S that likely occurred at the Mesozoic-Cenozoic transition. We discuss evidence from the sub-ice topography and potential field airborne and satellite data. We have applied image segmentation techniques to the rebounded sub-ice topography to semi-automatically trace the first order shape of the sub-ice basins, that we assume to be fault controlled. Then we have fitted the edges of the basins by maximum circles and estimated the best Euler pole identified by their intersection. Potential field anomalies have been taken into account in order to enlighten major discontinuities not revealed by the sub-ice topography.

The reconnaissance of this large sector of East Antarctica as the result of rotational extension may have major implications on global and regional tectonics plate reconstructions, plate deformation assumptions and new tectonic evolutionary models of WARS, TAM, and GM.

How to cite: Armadillo, E., Rizzello, D., Balbi, P., Ghirotto, A., Scafidi, D., Paxman, G., Zunino, A., Ferraccioli, F., Crispini, L., Läufer, A., Lisker, F., Ruppel, A., Morelli, D., and Siegert, M.: Some evidence of a wide rotational extension in East Antarctica preceding Gondwana breakup, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14808, https://doi.org/10.5194/egusphere-egu24-14808, 2024.

A 15 km long dike formed rapidly in the Reykjanes Peninsula oblique rift on 10 November 2023 and propagated under the town of Grindavík.  From just before noon on 10 November until midnight, around 25 M_W≥4 earthquakes occurred, two of which were of M_W~5.2. Three-dimensional ground deformation is well resolved both temporally and spatially with dense Global Navigation Satellite System (GNSS) geodetic observations, which record cumulative displacements up to about 80 cm occurring mostly over 6 hours in the evening of 10 November and continuing at much reduced rates in the following days. Interferometric analysis of synthetic aperture radar images using Sentinel-1, COSMO-SkyMed, and ICEYE satellites records also well the dike deformation, which occurred simultaneously with deflation over the nearby central part of the Svartsengi volcanic system. Geodetic modelling, assuming uniform elastic host rock behavior, infers a dike volume of (130-139)×10⁶ m³, with up to ~8 m dike opening, as well as some strike-slip shear motion. Deflation at Svartsengi in our model is best fit using a spherical point source with a volume decrease of (76-82)×10⁶ m³up until 12 November. The temporal evolution of the dike opening was further modelled using hourly GNSS displacements, allowing better derivation of the temporal evolution of the flow rate into the dike and the contraction volume of the subsidence source. The maximum flow rate into the dike is inferred to be ~9500 m³/s, between 18:00 and 19:00 on November 10. We infer that the massive magma flow into the dike was established with only modest overpressure in the feeding magma body, a sufficiently large pathway opening at the boundary of the magma body, and pre-failure lowering of pressure along the pathway that had occurred through gradual build-up of high tensile stress over the previous eight centuries. This explains the unprecedented fast maximum magma flow rates that we infer. Such high flow rates provide insight into the formation of giant dike swarms under conditions of high tensile stress, and imply a high hazard potential for dike intrusions, considering their potential to transition into eruptions.

How to cite: Sigmundsson, F., Parks, M., Geirsson, H., Hooper, A., Drouin, V., Vogfjörð, K., Ófeigsson, B., Greiner, S. H. M., Yang, Y., Lanzi, C., De Pascale, G. P., Jónsdóttir, K., Hreinsdóttir, S., Tolpekin, V., Friðriksdóttir, H. M., Einarsson, P., and Barsotti, S.: The November 2023 Grindavik dike injection in Iceland:  Implications for continental rifting, dike formation in extensional tectonic settings, and giant dike swarms, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14829, https://doi.org/10.5194/egusphere-egu24-14829, 2024.

A new set of high-precision U-Pb data was acquired for two groups of phonolite bodies emplaced in the volcano-sedimentary sequence of the Cenozoic Eger Rift in Bohemian Massif. The phonolites are located in the western (5 bodies) and eastern (3 bodies) edge of this monogenetic volcanic field, stretched along the central part of the Eger Rift system. The selected phonolite bodies represent lava flows, cryptodomes or extrusive domes emplaced in phreatomagmatic maar-diatremes, remnants of dykes, and a laccolith. The U-Pb dates were acquired using the Laser Ablation Split Stream system at Santa Barbara University geochronology lab, which provides the coupled geochronology and also REE and selected major element geochemistry. Despite the great variety of internal zircon textures from oscillatory zoning to complex patchy patterns with a large range of cathodoluminescence intensity, the groups of spots gained coherent and surprisingly precise ages for each sample. The western group of phonolite bodies, namely the Bořeň, Želenický vrch, Špičák, Hněvín, and Ryzelský vrch display clusters of ages ranging between 33Ma and 36Ma, while zircons of the eastern group of the phonolites, Krompach, Mariánská hora and Luž (Lausche) indicate ages between 30Ma and 32Ma. Terra-Wasserburg diagrams for individual samples revealed remarkable precision marked by errors of only 90-180 thousand years (5 samples) and 300-650 thousand years (2 samples). The U-Pb zircon ages are interpreted to reflect primarily the high-temperature overprint of inherited (and possibly newly crystallized) zircons before emplacement of the phonolite bodies in the upper crust. In addition, titanite grains measured alongside the zircon grains (in another run) either overlap (Bořeň) with the zircon age error on Terra-Wasserburg diagrams (geochrone) or are 2 Ma years younger than corresponding zircon ages (Špičák phonolite body). REE binary diagrams revealed separate clusters of Sm/Nd and also Hf content of the zircons, which can be attributed to different degrees of partial melting of parental magma in the source upper mantle or the lower crust for both groups of sampled phonolites. In summary, the results suggest that U-Pb geochronology using the LASS system is a powerful tool with a great potential for deciphering the evolution of phonolites in the Cenozoic Rift system in Bohemian Massif and possibly other rift systems in the foreland of the Alpine orogeny.

This research has been supported by the Czech Science Foundation (GAČR) project 22-13980S.

How to cite: Závada, P., Cajz, V., Kylander-Clark, A., and Uličný, D.: High precision U-Pb geochronology of Cenozoic phonolite volcanic bodies in Cenozoic Eger rift basin (Bohemian Massif), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14830, https://doi.org/10.5194/egusphere-egu24-14830, 2024.

It is well known that triaxial deformation is a common feature of continental tectonics, and is accommodated by complex polymodal fault networks. Field investigations confirm that multiple phases involving time-dependent three-dimensional strain conditions (e.g. constriction, plane, and flattening strain) affect the spatial and temporal interaction of polymodal fault systems. However, a key question remains: How do changing strain conditions affect the reactivation of fault systems that formed during a previous deformation phase? Here, we conduct scaled analogue models with time-dependent boundary conditions to investigate how fault networks evolve under changing boundary conditions, including  reactivation and formation of new faults.

We have developed a setup in which a basal rubber sheet is stretched in one direction, so that longitudinal extension and layer thinning are accompanied by lateral shortening, hence producing triaxial deformation (Liu et al. in revision). According to previous brittle-viscous experiments with this set-up, an increase in longitudinal extension velocity results in a higher coupling between the rubber base and brittle layer, generating increasing transmission of lateral shortening from the base into the brittle layer. We thus induce constriction-to-plane strain conditions in the brittle layer as a function of longitudinal extension velocity by varying the magnitude of lateral contraction. In a new set of experiments, by varying extension velocity either stepwise or continuously, we realize time-dependent kinematic boundary conditions including deformation phases and secular changes, respectively. Digital image correlation (DIC) and photogrammetry (structure from motion, SFM) are employed to track the 3D kinematic surface and topography evolution, respectively.

Preliminary observations show both the formation of new faults and the reactivation of early phase faults through a change from plane to constriction strain. Conversely, a change from constriction to plane strain conditions results in the abandonment of the early phase fault network as it becomes overprinted by fault systems of the subsequent phase. Moreover, early-phase fault systems influence the propagation and linkage of fault populations in subsequent phases. Our analogue models highlight the impact of strain conditions on the overall plan-view geometry of fault populations, providing alternative explanations for complex fault patterns and interactions (e.g. the Jeanne d’Arc basin, the North Træna Basin, and the Beagle Platform).

How to cite: Liu, J., Rosenau, M., Kosari, E., Brune, S., Zwaan, F., and Oncken, O.: The evolution of fault networks during multiphase deformation: An analogue modeling approach, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15107, https://doi.org/10.5194/egusphere-egu24-15107, 2024.

The Most Basin is the largest and best-preserved of sedimentary basins formed within the Eger Rift, the easternmost part of the European Cenozoic Rift System. Previous work on the tectono-sedimentary history of the basin and its surroundings has led to an interpretation of two main extensional phases that governed the Oligo-Miocene rift initiation and subsequent evolution. That interpretation has been derived mainly from large-scale considerations of main fault geometries, while a satisfactory support by a large mesoscopic dataset from the basin infill was lacking.

Systematic acquisition of mesoscopic structural data in some of the open-cast coal mines operating in the Most Basin has been motivated by prevention of accidents of bucket wheel excavators, threatened by sliding of blocks of mainly clayey sediments. As a result, over 5 thousand mesoscopic measurements were acquired in the Bílina Mine alone and hundreds in other mines over the past 13 years. In the Most Basin, the main coal seam is located close to the base of the basin fill. Open-case mines thus expose a thick overburden and, locally, also the underlying basement. The structural measurements involved the superposition and evolution of mesoscopic structural features in geological time, from Variscan metamorphic rocks through Cretaceous sediments and Oligocene volcanics through the Miocene coals and clastics of the basin fill.

Structural analysis of the dataset and statistical comparison of specific regions focused on

spatial and stratigraphic distribution of fault directions and inclination arrays, resulting in interpretation of spatial and stratigraphic distribution of local paleostress. The principal results are as follows:

• the number of detected mesoscopic fault populations, as well as of interpreted deformation phases decreases upward through the stratigraphic column;
• orientation of faults generally changes from a dominant E-W and NW-SE strike of population in the pre-Miocene formations into dominant SW-NE up to WSW-ENE strike within the youngest Libkovice Member (Early Miocene);
• a trend of decreasing fault inclination from older, more consolidated formations to younger ones, most probably linked to rheological (stage of lithification) on brittle deformations;
• generally, data evaluation of inclination and direction of faults gave generally similar results for the Bílina and Libouš mines, in spite of the 60 km distance between them and their proximity to different leading fault systems (Bílina and Victoria faults in the former case and the Ahníkov and Kralupy faults in the latter);
• the large dataset of mesoscopic fault-slip data shows a generally more complex picture of possible paleostress evolution than the one derived from the geometries of the main bounding fault systems, due to the influence of local stress fields of normal and transtensional faults. The general picture, however, implies a plausible gradual evolution of extension vector from NNE-SSW to NW-SE orientations. throughout the early Miocene.

The Severočeské doly, a.s., supported the long-term acquisition of the structural dataset and its utilization for basic research purposes. This research has been supported by the Czech Science Foundation (GAČR) project 22-13980S.

How to cite: Grygar, R., Mach, K., Gramblička, R., and Novotný, T.: Insights into Miocene paleostress history of the Eger Rift from mining-related structural datasets: the Most Basin, Bohemia, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15188, https://doi.org/10.5194/egusphere-egu24-15188, 2024.

The East African rift overlies one or more mantle upwellings and it traverses heterogeneous Archaean-Paleozoic lithosphere rifted in Mesozoic and Cenozoic time. We re-analyze XKS shear wave splitting at publicly available stations to evaluate models for rifting above mantle plumes. We use consistent criteria to compare and contrast both splitting direction and strength, infilling critical gaps with new data from the Turkana Depression and North Tanzania Divergence sectors of the East African rift system. Our results show large spatial variations in the amount of splitting (0.1–2.5 s) but consistent orientations of the fast axes within rift zones: they are predominantly sub-parallel to the orientation of Cenozoic rifts underlain by thinned lithosphere with and without surface magmatism. The amount of splitting increases with lithospheric thinning and magmatic modification. Nowhere are fast axes perpendicular to the rift, arguing against the development of extensional strain fabrics. Thick cratons are characterized by small amounts of splitting (≤0.5 s) with a variety of orientations that may characterize mantle plume flow. Splitting rotates to rift parallel and increases in strength over short distances into rift zones, implying a shallow depth range for the anisotropy in some places. The shallow source and correlation between splitting direction and the shape of upper mantle thin zones suggests that the combination of channel flow and oriented melt pockets contribute > 1 s to the observed splitting delays. Enhanced flow, metasomatism, and melt intrusion at the lithosphere-asthenosphere boundary suggest that fluid infiltration to the base of the lithosphere may facilitate rifting of cratonic lithosphere. 

How to cite: Ebinger, C., Reiss, M., Bastow, I., and Karanja, M.: Shallow sources of upper mantle seismic anisotropy in East Africa , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15268, https://doi.org/10.5194/egusphere-egu24-15268, 2024.

Geophysical studies along the Main Ethiopian Rift and Eastern Rift in Kenya indicate that strain accommodation is dominated by magmatic intrusion rather than tectonic extension (e.g., Ebinger and Casey, 2001). However, it remains unclear how magmatic extension developed in the Turkana Depression, the low-lying, broadly rifted region separating the Ethiopian and East African plateaus. We investigate the rifting dynamics of the Turkana Depression with two-phase flow numerical models of melt transport through the ductile–brittle lithosphere. These models suggest that the pre-rift rheological structure of the lithosphere exerts a counter-intuitive control on melt extraction, which can explain the character of the Turkana region.

Recent seismic imaging shows that both the Turkana Depression and the uplifted plateaus are underlain by deep-seated, hot, partially-molten, buoyant mantle that ponds below a thinned plate (Kounoudis et al., 2021). Yet, Ogden et al. (2023) estimated the Moho is 10–20 km shallower throughout the Turkana Depression (~20–25 km) than surrounding regions (~35–40 km). Here, we hypothesise that variations in lithospheric strength across the Turkana Depression and the Ethiopian Plateau have influenced magma transport across the lithosphere and rift development (Morley, 1994). 

Our models of melt extraction through the ductile–brittle lithosphere incorporate a new poro-viscoelastic–viscoplastic theory with a free surface (Li et al., 2023), designed and validated as a consistent means to model dykes. We initialise models with a source of partial melt in the asthenosphere and investigate how rheology of the overlying lithosphere impacts melt migration to the surface. Experiments are performed for buoyancy-driven magma transport under no tectonic extension, and for low background tectonic extension rates typical to the Turkana Depression (4 mm/yr; e.g., Knappe et al., 2020). Results indicate that both the rheology of lithosphere and extension rate control the efficiency of magma extraction. Magma transport across a thick, elastic lithosphere is more efficient than across a thin, more ductile lithosphere, and increases with extension. Our results suggest that surface volcanism in the Ethiopian Plateau is more likely to occur compared with the Turkana Depression, and at earlier times. 

References

Ebinger and Casey (2001), Geology, DOI: 10.1130/0091-7613(2001)029<0527:cbimpa>2.0.co;2

Kounoudis et al. (2021), G-cubed, DOI: 10.1029/2021GC009782

Ogden et al. (2023), EPSL, DOI: 10.1016/j.epsl.2023.118088

Morley (1994), Tectonophys., DOI: 10.1016/0040- 1951(94)90170-8

Knappe et al. (2020), JGR: Solid Earth, DOI: 10.1029/2019JB018469

Li et al. (2023), GJI, DOI: 10.1093/gji/ggad173

How to cite: Pusok, A. E., Li, Y., Katz, R. F., Davis, T., and May, D. A.: Reduced magmatism in the Turkana Depression: a consequence of inefficient melt transport, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16081, https://doi.org/10.5194/egusphere-egu24-16081, 2024.

Constraining the evolution of the opening of the northernmost region of the Northeast Atlantic Ocean is of particular importance for understanding the diversity of ocean basin opening dynamics, including the development of oblique margins and shear zones. Accurately determining the timing and kinematics of the motion along the Senja Shear Zone and opening of the Fram Strait is of particular importance for climate research as this region forms the only deep-water gateway between the Northeast Atlantic Ocean and Arctic Ocean. This study combines new and legacy data and presents an analysis of the tectonic evolution of the northern Norwegian passive margin over the past 200 Ma, including integrating structural field observations and plate tectonics models.

Fieldwork took place on the islands of Senja and Kvaløya in Troms County of northern Norway. The field observations reveal four dominant brittle fault groups corresponding to four normal-oblique extension directions: E-W, NNW-SSE, NW-SE, NE-SW. In the Senja Shear Zone, the strike-slip faults are predominantly oriented NNW-SSE to NW-SE. Analysis of existing plate motion models for the region for 200 Ma to present day includes three prominent extension phases in chronological order: E-W, NNW-SSE, and NW-SE.

This study suggests that during the E-W oriented crustal thinning phase, normal faulting and minor strike-slip faulting dominated and gave way to basement-seated strike-slip faults during the NNW-SSE oriented extension phase. The presence of mid-upper crust faulting is argued by fault mineral striation assemblages and hydrothermal alteration. In the NW-SE oriented extensional phase, both normal faults and strike-slip faults were active. Comparisons to existing rigid plate tectonic models for the region suggest a revised deformable plate framework is required, and offers insights into the original thickness of the North-American and European plates and the role of mid-crustal tectonics in the breakup. The role of inheritance, including earlier shear zones and extensional phases will also be discussed. In addition, the present research encourages scientists to digitize analogue maps and data, preventing loss of knowledge during the analogue to digital transition.

How to cite: Distelbrink, A., Shephard, G. E., Koehl, J.-B. P., Bergh, S. G., and Beniest, A.: Insights into the tectonic evolution of the northern Norwegian passive margin: Integrating field observations and plate modeling over 200 million years., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16240, https://doi.org/10.5194/egusphere-egu24-16240, 2024.

As two tectonic plates drift away, the earlier movements, prior to oceanic crust formation, are ill-constrained. We are convinced that the kinematic models of plate movements could be significantly enhanced by focusing on the divergence before final lithospheric breakup. During this phase of transition several problems arise. Firstly, the classical interpretations of magnetic anomalies are not trustworthy (debated geometry and/or origin of anomalies). Secondly, the movements are more complex than in oceanic domain (polyphase deformation, obliquity, asymmetry). Those particularities occur especially if plate breakup happens in magma-poor conditions where mantle is exhumed at the surface (in about 50% of instances).

We focus on two pairs of conjugate magma-poor rifted margins: the Bay of Biscay and the Australia-Antarctica margins. In these areas, magnetic anomalies are controversial and seafloor formation started with large domains of hyperextended continental crust and exhumed mantle. Thus, the location and age of the LaLOC (landward limit of the oceanic crust) are uncertain. We aim to better define these domains in space, divergence direction and time through geophysical data and localized petrological observations and dating. This project is in its starting phase, it includes 2 PhD thesis. This presentation focuses on the general framework of the project and the preliminary results.

How to cite: Autin, J., Mathey, R., Suire, H., Ballay, M., Ulrich, M., Manatschal, G., Sauter, D., Petri, B., Schaming, M., and Somoza Losada, L.: The ANR project “FirstMove”: first movements of divergence between future tectonic plates, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17089, https://doi.org/10.5194/egusphere-egu24-17089, 2024.

Jieyang Depression is located between the Southern Depression of the Taixinan Basin and the Chaoshan Depression of the Pearl River Mouth Basin in northern South China Sea. A series of NE-SW striking half graben developed in this area from late Mesozoic to early Paleogene, and then another two stages of normal faulting happened during Neogene. The development of these stages of normal faults are separated by the breakup unconformity and a post-rift truncation. The main purposes of this study are to investigate the evolutionary sequences of the multi-stages of normal faults and the truncations during the multi-stages of extension, the spatial distribution of each phase of normal faults, and how the younger normal faults were affected by the pre-existing ones.

The normal faults in the Jieyang Depression can be divided into three types. Type 1 faults are related to the Paleogene half graben formation and only cut through the pre-rift and syn-rift strata. Type 2 normal faults only developed and cut through the post-rift strata. Type 3 normal faults cut through the syn-rift and post-rift strata. In this study, we further divide the Type 2 normal faults into two different kinds. Type 2-1 faults developed above the Paleogene half graben and cut off by the post-rift truncation. Type 2-2 faults developed after the truncation. Type 3 normal faults can be divided into two different kinds as well. Type 3-1 faults developed in the syn-rift stage and yet were reactivated and linked with the faults developing in the later extension. Type 3-2 faults are the Type 2 faults that developed continuously cutting downward into the syn-rift strata.

In terms of spatial distribution, Type 1 normal faults strike NE-SW, forming the half grabens. Most of the Type 2 normal faults are located far away from continental shelf. Type 3 normal faults mostly distribute on the northeast and northwest sides of the study area, close to the continental shelf, and most of them are cut by the post-rift truncation.

As a result, from the late Mesozoic to the early Paleogene, the northern slope of the South China Sea experienced a NW-SE extension. At the end of the Paleogene, the extension ceased, forming the breakup unconformity. In the Neogene, the Jieyang Depression experienced second extension. In this time the extension orientation was NNW-SSE, developing Type 2-1 and Type 3 faults before the post-rift truncation. The subsequent truncation cuts Type 2-1 faults and the upper part of the Type 3 faults. After accumulating new strata above truncation, the third stage of extension happened and Type 2-2 faults developed after the post-rift truncation.

Key words: South China Sea, Jieyang Depression, normal fault, multi-stage extension, truncation

How to cite: Wang, C.-H., Wu, P.-R., Yang, K.-M., Yang, C.-C., and Hsieh, B.-Z.: Structural Mode and Evolution of Multi-stage Normal Fault Development in Jieyang Depression, NE South China Sea, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17201, https://doi.org/10.5194/egusphere-egu24-17201, 2024.

The Danakil Depression is situated in the northern part of the Afar Depression in Ethiopia and Eritrea and is in an advanced phase of rifting close to continental breakup. It forms the equivalent of a magma-rich margin. As it is currently active and emerged, it offers a unique opportunity to study the processes of formation of these types of passive margins.

We combine seismic reflection data, field data, and remote sensing to constrain the structure and kinematics of this basin. Seismic data reveal the formation of Seaward Dipping Reflectors (SDRs). Surprisingly, field data show that these SDRs are dominated by clastic sediments and only contain relatively minor amount of magmatic material. Paleoshorelines and other proxies allow to quantify uplift and subsidence rates across the basin. These data highlight high spatial variability and allow to better understand the structure and evolution of older, deeply buried passive margins.

How to cite: Rime, V., Foubert, A., Keir, D., and Kidane, T.: Structure and kinematics of the Danakil Depression, Afar, Ethiopia: insights into the formation of a magma-rich margin, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17395, https://doi.org/10.5194/egusphere-egu24-17395, 2024.

Constraining the age of formation and movement along fault arrays in superimposed basins helps us to better unravel their kinematic history as well as the role of bounding faults or inherited structures in basin evolution. The Inner Moray Firth Basin (IMFB, western North Sea) comprises a series of superimposed basins overlying rocks of the Caledonian basement, the pre-existing Devonian-Carboniferous Orcadian Basin and a regionally developed Permo-Triassic North Sea basin system. The IMFB rifting occurred mainly in the Upper Jurassic – Lower Cretaceous after a long period of subsidence followed by localised uplift in its eastern parts due to thermal doming in the central North Sea (in the middle Jurassic). The rift basin later experienced further episodes of regional tilting, uplift and fault reactivation during Cenozoic.

New detailed field observations augmented by drone photography and creation of 3D digital outcrops, coupled with U-Pb geochronology of syn-faulting calcite-mineralised veins are used to constrain the absolute timing of fault movements and decipher the kinematic history of basin opening. It also helps to identify those deformation structures associated with earlier basin-forming events.

Five regional deformation events emerge: Devonian rifting associated with the older Orcadian Basin; Late Carboniferous inversion related to dextral Great Glen fault movements; Permian thermal subsidence with some evidence of minor fracturing; Late Jurassic – Early Cretaceous rifting and Cenozoic reactivation and local inversion. We were also able to isolate characteristic structures, fault kinematics, fault rock developments and associated mineralisation types related to many of these events.

How to cite: Tamas, A., Holdsworth, R. E., Tamas, D. M., Dempsey, E. D., Hardman, K., Bird, A., Underhill, J. R., McCarthy, D., McCaffrey, K. J. W., and Selby, D.: Using U-Pb geochronology of syn-faulting calcite-mineralised veins to track the evolution of superimposed rifting events: the Inner Moray Firth Basin., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17937, https://doi.org/10.5194/egusphere-egu24-17937, 2024.

The presentation is based on the results of the research work of the Arktic-2011, Arktic-2014 and Arktic-2022 expeditions and contains the results of analysis of the structure of the sedimentary cover of the Eurasian basin of the Arctic Ocean. For the first time, the entire array of seismic data, including Russian and foreign seismic profiles, was used for tectonic constructions. The results obtained make it possible to reconstruct extensive areas of continental lithosphere development in the Eurasian basin. Based on the analysis of the structure of the sedimentary cover of the Amundsen Basin, four stages of the geological history of the formation of the sedimentary system of the Eurasian basin of the Arctic Ocean are substantiated. During the first (Cretaceous-Paleocene) stage, extensive axis-symmetric epicontinental paleo-basins of the Amundsen and Nansen Basins were formed on the shoulders of the continental rift, which were subsequently separated by seafloor spreading. Evidence of similar riftogenic settings in the second half of the Cretaceous is recorded along the entire periphery of the Arctic basin from Greenland to the Chukchi Rise. The second (Eocene)-spreading stage was characterised by stage accretion of the oceanic crust in the Gakkel Ridge and was accompanied by a gradual expansion of the sedimentary basin up to the present-day boundaries of the Eurasian basin. The third stage (Oligocene-Miocene) of sedimentary flexure corresponded to the accumulation of a thick undisturbed sedimentary cover over the entire Eurasian basin, indicating the temporary cessation of spreading in the Gakkel Ridge and the establishment of a tectonic quiescence regime. Similar conditions at this stage are recorded throughout the periphery of the Arctic basin. The resumption of spreading processes occurred at the fourth (Pliocene-Quaternary) neotectonic stage. As the result of the intensification of spreading processes in the Norwegian-Greenland Basin, tectonic stresses penetrated intothe Eurasian Basin along the axis of the Gakkel Ridge. The distinct morphological division of the Gakkel Ridge into Siberian-Marine and Atlantic segments is explained by the jump-like transmission of tectonic stresses of the North Atlantic, which is also confirmed by the anomalously high tectonic, volcanic and hydrothermal activity of the Gakkel Ridge.

How to cite: Neevin, I., Rekant, P., and Budanov, L.: MODEL OF THE FORMATION OF THE SEDIMENTATION SYSTEM OF THE EURASIAN BASIN OF THE ARCTIC OCEAN AS A BASIS FOR RECONSTRUCTING Its TECTONIC HISTORY, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18255, https://doi.org/10.5194/egusphere-egu24-18255, 2024.

The spatio-temporal analysis of rifts and passive margin evolution is often done based on regional case studies, using non-standardized terminology and classification models to characterize crustal boundaries and basin infill. As example, the use of “continent-ocean boundary” to delineate crustal types or “syn-rift” as basin infill characterization has proven to be no longer adequate, given our evolved understanding of passive margins. In general, such local approaches do not lean themselves to aggregate data for global and large-scale comparative analysis and often struggle to reconcile the spatially varying magmatic/weakly magmatic margin architecture in a rift system context. They also do not allow efficient deployment of spatio-temporal data analytic models due to a lack of standardized data classification.

To overcome these limitations, we have designed a novel “data science-ready” data model for crustal architecture that is based on commonly accepted terminologies, can be used independent of input data heterogeneity and can be deployed globally across the whole spectrum of margin types and complex 3D margin geometries/microplate settings. We classify two key crustal boundaries, the oceanward limit of continental crust ("OLCC") and the landward limit of oceanic crust ("LaLOC"), along with several key crustal interfaces, such as the top basement and base crust which are further subdivided into sub-categories. This approach allows us to easily generate standardized data products on rift system scale, which quantitatively describe key parameters relevant to understand lithosphere extension dynamics, such as volumes, ratio, and distribution of continental and magmatic crust, crustal stretching factors, and amount of crustal embrittlement. Coupled with plate kinematic models, these data products allow to build reproducible, extensible, and quantitative models of rift and margin evolution through time and highlight the dynamics of stretching, localization of deformation, the basin infill response, and spatio-temporally varying patterns and types of magmatism.

Applying this data model, we have characterized the crustal architecture of the conjugate South Atlantic passive margins, interpreting more than 100k line-kilometers of 2D and 3D seismic reflection data. Our findings highlight substantial shortcomings of current plate models to reconcile the crustal type distributions in the southern South Atlantic with a tight pre-breakup fit, the temporal emplacement dynamics of SDRs and plume-related magmatism along the whole South Atlantic rift, as well as the localization of deformation and dynamics of basin infill.

How to cite: Heine, C., McDermott, K., Eldrett, J., Grant, C., and Thompson, P.: A generic crustal architecture data model for rift and passive margin analysis: Application to the conjugate South Atlantic margins, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19671, https://doi.org/10.5194/egusphere-egu24-19671, 2024.

The thermal history of margins controls the development of hydrothermal systems during rifting, diagenetic processes in the sediments and the generation and preservation of hydrocarbons. It also affects the depth of the oceanic gateways formed during continental break-up, thereby influencing ocean circulation and ultimately climate (Brune et al., 2023, Pérez-Gussinyé et al., 2023, Peron-Pinvidic et al., 2019). Observed heat-flow values however, do not always comply with classic rifting models. Here, we use 2D numerical models to investigate the relationship between rifting, sedimentation and thermal history of margins. We find that during the synrift, the basement heat flow and temperature are not only controlled by extension factor, but also by synrift sediment thickness and the evolution of deformation. As this progressively focuses oceanward, the proximal sectors thermally relax, while the distal sectors experience peak temperatures. This time lag is important for wide rifted margins. In the postrift, the lithosphere under the hyperextended margins does not return to its original state, at least for ~100 Myrs after breakup. Instead, it mimics that of the adjacent oceanic plate, which is thinner than that of the original continental plate. This results in heat flow values increasing oceanward at postrift stages where classic rifting theory predicts complete thermal relaxation. Our increased heat-flow estimations, may extend hydrocarbon plays into distal margin sectors and adjacent oceanic crust, previously discarded as immature. Finally, our models indicate that commonly used temperature approximations in basin analysis may strongly differ from those occurring in nature (Pérez-Gussinyé et al., 2024).

Brune, S., Kolawole, F., Olive, JA. et al. Geodynamics of continental rift initiation and evolution. Nat Rev Earth Environ 4, 235–253 (2023). https://doi.org/10.1038/s43017-023-00391-3

Pérez-Gussinyé, M., Collier, J., Armitage, J., Hopper, J. R., Sun, Z., and Ranero, C. R., Towards a process-based understanding of rifted continental margins, in Nature Reviews Earth and Environment, 2023, doi: 10.1038/s43017-022-00380-y

Marta Pérez-Gussinyé, Yanfang Xin, Tiago Cunha,  Raghu Ram, Miguel Andrés-Martínez, Dongdong Dong, Javier García-Pintado_,Synrift and postrift thermal evolution of rifted margins: a re-evaluation of classic models of extension, in press, Geol. Soc Spec. Publ., 2024

Peron-Pinvidic, G., Manatschal, G., eta al. Rifted Margins: State of the Art and Future Challenges, Front. Earth Sci., 22 August 2019, Sec. Structural Geology and Tectonics, Volume 7 - 2, https://doi.org/10.3389/feart.2019.00218.

How to cite: Pérez-Gussinyé, M., Xin, Y., Cunha, T., Ram, R., Andres-Martinez, M., Dong, D., and Garcia-Pintado, J.: Synrift and postrift thermal evolution of margins: a re-evaluation of classic models of rifting, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19853, https://doi.org/10.5194/egusphere-egu24-19853, 2024.

Multiple fault populations with different orientations and complex fault patterns can be observed during oblique rifting, conditions which result from a complex rift kinematics which combines dip-slip and strike-slip motion. Although analysis of different natural cases and analog or numerical modeling have shed light on the relations between rift obliquity and the related fault architecture, many aspects of the process remain poorly understood. One of these aspects is related to the existence of pre-existing fabrics in the upper crust, which may further complicate the fault pattern by forcing the development of faults with atypical geometries and orientation.

Here, we performed enhanced-gravity analog models of oblique narrow rifting to characterize the evolution and architecture of rift-related faults developing in a brittle upper crust characterized by inherited fabrics. The models reproduce a rift obliquity of 30° (angle between the rift trend and the orthogonal to the direction of extension), kept constant in all the experiments, and pre-existing vertical fabrics with variable orientation (from 0°, i.e. orthogonal to extension, to 90°, i.e. extension-parallel). Modeling results suggest that inherited fabrics have an important influence on rift-related faulting, with a significant correlation between the intensity of reactivation and their trend with respect to the extension direction. When the pre-existing fabrics trend perpendicular to the extension direction (obliquity 0°), they are strongly reactivated, localizing deformation and promoting the rapid development of faults and grabens perpendicular to the extensional direction. When the pre-existing fabrics trend at moderate obliquity (15°-45°), they are still reactivated and localize deformation causing the development of atypical fault trends and patterns. The degree of reactivation tends to gradually decrease with increasing obliquity; similarly, the influence of pre-existing structures decreases with progressive extension, and the fault pattern and evolution are progressively dominated by extension kinematics and crustal thinning. When the pre-existing fabrics trend at high obliquity (≥ 60°), they have almost no influence on the fault geometry and architecture.

This study has significant implications for explaining the fault geometry and evolution of some natural rift basins worldwide, such as basins of the East African Rift system, the North Sea Rift, and some offshore rift basins in eastern China.

How to cite: Zou, Y., Maestrelli, D., Corti, G., Del Ventisette, C., Wang, L., Wan, X., Gao, Y., and Shen, C.: Interactions between pre-existing fabrics and fault patterns during oblique rifting revealed by enhanced-gravity analog modeling, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19947, https://doi.org/10.5194/egusphere-egu24-19947, 2024.

The Afar depression at the northern end of the East African Rift system is presently experiencing the final stage of continental break-up and progressive onset of steady magmatic spreading. The Magmatic Rift Segments in Afar broadly analogous to those observed within the mid oceanic ridges, offer the opportunity to study both mantle and crustal processes. Investigating the crustal architecture of those magmatic segments represents a key aspect to decipher fundamental parameters that control focussing of magmatic and tectonic activity during the generation of magmatic crust. Here, we present the typical organization of a 32 km long subsegment of the Manda Hararo magmatic rift system, with fissural activities symmetrical to an apparent mid segment magmatic reservoir and establish geochronology of the last eruptive history. We combine field investigations, precise mapping of volcanological and tectonic features, cosmogenic ³⁶Cl exposure dating and geochemical analysis of lavas to constrain the temporal frame and the dynamics of magmatic processes. Our results show that the recent historical volcanic events (~ 500 to 2000 years) are sourced from calderas and fissures representing an alternating sequence of effusive and explosive (block fields) activities related to a coherent rifting episode along a single self-consistent magmatic sub-segment. Those recent fissural flows resurfaced a large portion of the segment and emplaced on older thick pahoehoe flows with a rather long lag-time of about 75 kyr separating the two episodes. Strongly contrasted geochemical signatures are also observed between those two volcanic episodes, with more differentiated and trace elements enriched basalts for the recent one, compared to the older one which are characterized by a unusual depleted signature. These new results for the Central Afar Manda Hararo rift have important implications for: (i) the local hazards along the segments, and (ii) the volcano-tectonic organization of the segment with coexistence of contrasted melt reservoirs on the underlying transcrustal plumbing system.

How to cite: Birhane, Y. G., Pik, R., Bellahsen, N., Schimmelpfennig, I., France, L., Flahaut, J., Ayalew, D., and Yirgu, G.:  The Last Fissural Eruptions of the Manda Hararo Magmatic Segment, Central Afar (Ethiopia), Constrained from New cosmogenic Ages, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20162, https://doi.org/10.5194/egusphere-egu24-20162, 2024.

Located at the southwestern terminus of the East African Rift System, the Okavango Rift System represents an opportunity to study the propagation of an active rift at its early stages (Gaudaré et al., in review). The Okavango Graben (northern Botswana) is an active half-graben of the Okavango Rift System, which shows normal to dextral strike-slip tectonic displacements of the order of 1 mm per year (Pastier et al., 2017). In addition to the impact of tectonics, large volumes of water (~10 km³ per year) brought in by the annual flood of the Okavango River generate seasonal subsidence of over 2 cm in the graben (Dauteuil et al., 2023). The prevalence of the hydrologic signal over the tectonic signal makes it challenging to provide clear interpretations of the Rift dynamics within the Okavango Graben. The previous studies are based on a network of GNSS stations, providing punctual data on displacements. To quantify the deformation field over the Okavango Graben, we analyze interferometric synthetic aperture radar (InSAR) data produced by the ForM@Ter LArge-scale multi-Temporal Sentinel-1 InterferoMetry service (FLATSIM, Thollard et al., 2021). FLATSIM automatically computes interferograms from Sentinel-1 synthetic aperture radar data and inverts them into displacement time series. The products span from April 2016 to April 2021 with a temporal resolution of 12 days, a spatial resolution of 115 x 115 m and cover the entire Okavango Rift System. We analyze and compare the seasonality of both the interferometric coherences and the InSAR displacement time series. Change detection in the interferometric coherence allows us to delineate flooded surfaces through time in the Okavango Graben, from which we deduce water loadings on the lithosphere and model the corresponding flexural response of the lithosphere. We then compare this response to the spatial distribution of annual vertical oscillations extracted from the displacement time series. Taking these seasonal signals into account, our objective is to estimate the rates of the tectonic subsidence in the Okavango Graben to better constrain the propagation of the East African Rift System at its southwestern end.

Dauteuil, O., Jolivet, M., Gaudaré, L., & Pastier, A.-M. (2023). Rainfall-induced ground deformation in southern Africa. Terra Nova, 00, 1–7. https://doi.org/10.1111/ter.12650

Gaudaré, L., Dauteuil, O., & Jolivet, M. Geomorphology of the Makgadikgadi Basin (Botswana): insight into the propagation of the East African Rift System. Tectonics, in review.

Pastier, A.-M., Dauteuil, O., Murray-Hudson, M., Moreau, F., Walpersdorf, A., & Makati, K. (2017). Is the Okavango Delta the terminus of the East African Rift System? Towards a new geodynamic model: Geodetic study and geophysical review. Tectonophysics 712–713, 469–481. https://doi.org/10.1016/j.tecto.2017.05.035

Thollard, F., Clesse, D., Doin, M.-P., Donadieu, J., Durand, P., Grandin, R., Lasserre, C., Laurent, C., Deschamps-Ostanciaux, E., Pathier, E., Pointal, E., Proy, C., & Specht, B. (2021). FLATSIM: The ForM@Ter LArge-Scale Multi-Temporal Sentinel-1 InterferoMetry Service. Remote Sensing, 13(18), 3734. https://doi.org/10.3390/rs13183734

How to cite: Gaudaré, L., Doubre, C., Jolivet, M., Dauteuil, O., Corgne, S., Grandin, R., Doin, M.-P., and Durand, P.: Unraveling Tectonic From Hydrological Subsidence Of The Okavango Graben (Botswana) Using FLATSIM InSAR Data., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20218, https://doi.org/10.5194/egusphere-egu24-20218, 2024.

The magma-poor passive rifted conjugate margins in the Southern Equatorial Atlantic, North Atlantic/Arctic oceans, and Northern Mozambique Channel display en-echelon extensional segments separated by long transform faults (>300 km), influenced by inherited weaknesses. Using advanced 3-D forward geodynamic modeling coupled with surface processes, we investigate the formation of oblique rifts and passive margins. Our focus is on pre-existing weaknesses parallel to the extension direction, exploring the system's sensitivity to various erodibility factors. Key findings include: (1) erodibility within a low to moderate range has limited influence on the morpho-structural evolution of the oblique continental rift, (2) pure-strike slip faults reactivating transform weaknesses result in reduced topography, (3) major catchments sink in the inner corner at the tip of each extensional segments, and (4) hinterland drainage network capture along extensional segments is absent, controlled by isostatic rebound during rift flank drainage divide migration. This study enhances our understanding of the complex interplay between inherited weaknesses, erodibility, and the evolving morphology of oblique rifted margins.

How to cite: Theunissen, T., Huismans, R. S., Rouby, D., Wolf, S., and May, D. A.: Interaction of tectonics and surface process during oblique rifted margin formation. Insights from 3-D forward coupled geodynamic-surface process modelling., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20223, https://doi.org/10.5194/egusphere-egu24-20223, 2024.

The basins composing the 1000-km wide West Antarctica Rift System (WARS), derived from extensional dynamics lasting from the Cretaceous to the Middle Neogene, bear evidence of a peculiar evolution through time: a transition from a diffuse to a localized thinning style and a migration of the focus of deformation, which likely progressed towards the cratonic domains of West Antarctica. Using the current observations, we aim at identifying which inherited starting conditions [1] result in outcomes compatible with the present-time structures and which do not allow so, unless other factors are accounted for.

To this aim, we turn to an extensive grid search in the parameter space, running a large number of forward numerical models to cover the possible permutations of parameters under test. We use the open source Underworld2 code [2] with a simplified scheme of starting conditions and kinematics boundaries, for lithospheric-scale 2-D thermomechanical models. We analyse the results obtained by changing a great number of parameters, including initial geometries of the crust and lithosphere, different rheologies, inherited structures, such as strain-weakening scars and thermal remnants of slabs.

We identify that a high crustal thickness (more than 45 km) is required to accommodate the first rifting phase (170 km ca. of cumulated extension, [3]) without producing crustal necking and eventual ocean formation. Parameters that favour a weaker strength profile, chiefly temperature (due to a thicker crust and/or a shallow lithosphere-asthenosphere boundary), are also required to avoid an early transition to localized deformation, in agreement with previous studies [4]. Smaller scale features, such as partition in multiple sub-basins, require additional factors, such as inherited weak-zone seeds (“scars”) in the crust and mantle, which are likely remnants of previous compressive phases [5].

[1] Perron, P., Le Pourhiet, L., Guiraud, M., Vennin, E., Moretti, I., Portier, É., & Konaté, M. (2021). Control of inherited accreted lithospheric heterogeneity on the architecture and the low, long-term subsidence rate of intracratonic basins. BSGF - Earth Sciences Bulletin, 192. https://doi.org/10.1051/bsgf/2020038

[2] Mansour, J., Giordani, J., Moresi, L., Beucher, R., Kaluza, O., Velic, M., Farrington, R., Quenette, S., & Beall, A. (2020). Underworld2: Python Geodynamics Modelling for Desktop, HPC and Cloud. Journal of Open Source Software, 5(47), 1797. https://doi.org/10.21105/joss.01797

[3] Brancolini, G., Busetti, M., Coren, F., De Cillia, C., Marchetti, M., De Santis, L., Zanolla, C., Cooper, A.K., Cochrane, G.R., Zayatz, I., Belyaev, V., Knyazev, M., Vinnikovskaya, O., Davey, F.J., Hinz, K., 1995. ANTOSTRAT Project, seismic stratigraphic atlas of the Ross Sea, Antarctica. In: Cooper, A.K., Barker, P.F., Brancolini, G., (Eds.), Geology and Seismic Stratigraphy of the Antarctic Margin. Antarctic Research Series, vol. 68, https://doi.org/10.1029/AR068

[4] Huerta, A. D., & Harry, D. L. (2007). The transition from diffuse to focused extension: Modeled evolution of the West Antarctic Rift system. Earth and Planetary Science Letters, 255(1–2), 133–147. https://doi.org/10.1016/j.epsl.2006.12.011

[5] Talarico, F., Ghezzo, C., & Kleinschmidt, G. (2022). The Antarctic Continent in Gondwana: a perspective from the Ross Embayment and Potential Research Targets for Future Investigations. In Antarctic Climate Evolution (pp. 219–296). Elsevier. https://doi.org/10.1016/B978-0-12-819109-5.00004-9

How to cite: Busetti, M., Pastorutti, A., Tesauro, M., Braitenberg, C., Colleoni, F., and De Santis, L.: The Ross Sea formation: enquiring the sensitivity of basin architecture to prior conditions, with numerical models and a parameter search, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20850, https://doi.org/10.5194/egusphere-egu24-20850, 2024.

The Sulaiman and Kirthar fold-and-thrust belts constitute the western boundary of the India-Asia collision zone. The Sulaiman fold-thrust belt is composed of a passive margin sedimentary sequence of Mesozoic to Cenozoic platform carbonates and clastic sedimentary rocks overlain by younger Himalayan foreland basin molasse sediments. This study uses newly acquired zircon (U‐Th)/He (ZHe) data and surface geology observations to document the structural evolution of the north Sulaiman fold-thrust belt. The hinterland zone includes the Zhob Valley Suture thrust, emplaced ophiolites, and folded Mesozoic strata, while the foreland is characterized by folded Eocene to Pliocene strata and hinterland-verging back thrusts. The initial deformation of the North Sulaiman Fold-thrust belt was driven by an east-west compressional event, giving rise to structures oriented in a north-south direction. The formation of fault propagation folds such as the Drazinda Syncline and Domanda Anticline in the foreland region resulted from east-vergent deformational events. Following this eastward deformation, the west-verging East Domanda Fault was activated, forming a roof thrust within the tectonic wedge beneath the Domanda Anticline.

ZHe dates were obtained from Jurassic to Pliocene sandstone samples collected along the Drazinda-Zhob transect, crossing the Sulaiman Fold-thrust belt. ZHe ages from the Middle Jurassic (Chiltan Formation) and late Jurassic to early Cretaceous (Sembar Formation) samples are completely reset. ZHe ages from the late Cretaceous (Mughal Kot and Pab formations) to early Paleocene (Ranikot Formation) samples are partially reset to unreset; Eocene to Pliocene sample ages are unreset. ZHe ages for the Chiltan Formation range from 3.9-16.2 Ma with an average of 7.6 Ma. The ZHe ages for the Sembar Formation range from 5.5-41.4 Ma with an average of 15.2 Ma. The fully to partially reset ZHe ages for the Mughal Kot Formation range from 4.5 to 64.2 Ma with an average of 31.6 Ma. The ZHe ages of the Pab Formation samples range in age from 25.1-58.6 with an average of 44.9 Ma. A single partially reset ZHe age from the Ranikot Formation is 38.5 Ma. Thick and rapid sedimentation in the Late Cretaceous, Early Eocene and Miocene-Pliocene in the region resulted in sufficient burial of the Chiltan and Sembar formations to reset the ZHe system. Most of the reset ages from the Chiltan and Sembar formations range between 3.9-7.5 Ma, indicating rapid late Miocene to Early Pliocene uplift of the north Sulaiman Fold-thrust belt. This uplift is associated with development of the Sulaiman anticline and the associated Dhana Sar backthrust. Located along the western boundary of the Indian Plate, this area exhibits transpressional tectonics. The dominant east-west compression component coupled with left-lateral wrenching plays a key role in this rapid and young uplift in the north Sulaiman Fold-thrust belt.

How to cite: Ali, N., Sobel, E. R., Ghani, H., and Bernhardt, A.: First constraints on the timing of structural evolution of the north Sulaiman Fold-thrust belt, Pakistan , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-665, https://doi.org/10.5194/egusphere-egu24-665, 2024.

Studying the complex subsurface structures in fold-and-thrust belts, such as the Eastern Carpathian Bend Zone, remains challenging due to poor seismic data quality caused by steep and intricate structures alongside multiple detachment levels.

To address this, we have conducted eight analogue modelling experiments at the Structural Modelling Laboratory of Babes-Bolyai University with mechanical stratigraphy scaled to the area of interest. We used coloured quartz sand, glass microspheres and silicone to simulate brittle rocks, ductile shales, and salt, respectively. The materials were layered in a 120 cm long deformation box, which was shortened at a constant rate of one centimetre per hour. The experiments were monitored using timelapse photography and particle image velocimetry. The models were consolidated, serially sectioned, and photographed. MOVE software (Petroleum Experts) was used for importing the section images and interpreting the 3D structural style for the models.

The structural style of the experiments varied based on the change in parameters, such as layer thickness, materials, shortening amounts, as well as the timing of erosion and salt deposition. The following description will refer to the most common deformation features observed in the experiments but will use the equivalent formation names and ages. The most noteworthy features are the post-salt lower Miocene's decoupling from the pre-salt section, the lower Miocene upper Kliwa formation decoupling from the Oligocene lower Kliwa formation along the lower Miocene Podu Morii shales, and the distinct wavelengths for the post-salt, upper Kliwa, and pre-upper Kliwa structures.

Considering well data, outcrop data, analogue modelling observations, and the 3D model, we performed seismic interpretation. This guided seismic data interpretation revealed short-wavelength upper Kliwa disharmonic folding, Oligocene–Cretaceous thrust-related hanging-wall anticlines, and large thrust fault displacements leading to hanging-wall erosion and out-of-sequence reactivation of the thrust faults. The results of these experiments proved to be effective tools for comprehending the controlling factors of deformation in the Eastern Carpathian Bend Zone and can aid in the interpretation of subsurface data, especially in areas with poor seismic data quality.

How to cite: Tocariu, I. S. M., Tamas, A., Tamas, D. M., Dohan, D., Lapadat, A., Schleder, Z., and Krezsek, C.: Analogue Modelling to Support Seismic Interpretation in the Eastern Carpathian Bend Zone, Romania, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-771, https://doi.org/10.5194/egusphere-egu24-771, 2024.

Many arcuate orogenic belts display complex patterns of fold structures. However, the kinematics of their development is poorly understood. This study aims to explore the origin of complex fold structures in the Proterozoic mobile belt of Cuddapah in peninsular India. This is a N-S trending thin-skinned fold-thrust belt, showing a spectacular crescent shape with westward convexity. The tectonic reconstructions suggest that the Cuddapah fold-thrust belts (CFTB) evolved through westward indentation of the westerly convex Nellore-Khammam schist belts (NKSB) during the collision between the Indian shield and the Antarctic block, interpreted as a consequence of the Rodinia Supercontinental event in Neoproterozoic time. The westward convergence resulted in the development of highly deformed Nallamalai fold belt (NFB) in the eastern part of CFTB, consisting of orogen-scale overturned folds with arcuate trends as well as higher order west-verging F₁ folds on a wide range of spatial scales. Inclined to recumbent F₁ folds are coaxially refolded by orogen-parallel F₂ folds in outcrop scale. However, the superposition of steeply inclined E-W trending cross-folds (F₃) that produced culminations and depressions on F₁ and F₂ indicates that the belt underwent orogen-parallel shortening in the course of its evolution. We performed laboratory experiments on a scaled representative CFTB model to understand such spatiotemporal evolution of the strain field in the CFTB. Two sets of experiments were run with a constant (2 cm/yr) and a temporally varying (2 cm/yr in Stage I to 1 cm/yr in Stage II) convergence velocity. The experimental results show that the CFTB during the initial periods of shortening undergoes mainly flattening deformations with maximum horizontal instantaneous stretching axes (ISA_Hmax), describing an arcuate trajectory in the eastern part that conforms to the indentor shape. In the later periods of the experimental runs (albeit at different times in the two experiments), the arcuate shape of the fixed wall causes a SW and NE-directed upper crustal flows from the elevated NE and SW parts of the CFTB, respectively. The flow convergence causes the strain field in the central part of the CFTB to develop a horizontal constriction. This kinematic transition leads to superposition of cross-folds on orogen-parallel folds, manifested in culmination (domes) and depression (basins) structures.

How to cite: Patsa, A. and Mandal, N.: Development of fold styles in the strongly arcuate Cuddapah fold-thrust belt, India: new insights from analogue models, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1068, https://doi.org/10.5194/egusphere-egu24-1068, 2024.

Fold-thrust belts are complex tectonic domains formed in response to near- or far-field compressional stress fields in the Earth’s crust. The complexity of structural style in theses belts is controlled by multiple factors. The presence of mechanically weak layers, defined by rocks that exhibit lower strength than their surroundings, play a significant role in the formation and evolution of fold-and-thrust belts. In this research, a two-dimensional numerical finite difference model incorporating a visco-elasto-plastic/brittle rheology is utilized to investigate the structural evolution of the Fars arc in the southeast of the Zagros fold-thrust belt. The modeling includes the tectonic inversion process, from Permian-Triassic rifting of the Neo-Tethys Ocean to the Oligocene-to-present continental collision between the Arabian and Eurasian plates. The Eulerian grid dimensions were considered as 500 km in length and 60 km in thickness, respectively, comprising 1101×121 nodes. Each cell contains 16 Lagrangian markers that carry the information and properties of each layer. According to the stratigraphic column of the Fars arc, there are 30 km of basement overlain by a 2-km-thick salt layer with a 3-kilometer-thick Palaeozoic sedimentary sequence above the salt layer. Furthermore, to replicate the interplay between Earth's internal and surface dynamics, a 25-kilometer-thick layer representing air is incorporated at the uppermost part of the model to approximate a free surface. The experiments include three inherited basement faults. The role of pre-existing weak zones in extensional tectonics is the shaping of rift basin geometry, especially the formation of half grabens. In the convergence phase, fold-thrust-belts with a basal detachment layer and pre-existing faults produce folding at two distinct scales. The faulting in the basement develops long wavelength folds in the sedimentary cover, while the presence of a salt layer shapes the smaller wavelength folds and thin-skinned thrust faults that extend from the detachment layer to the surface. The reactivated faults play an important role in stress transfer, leading to the emergence of new faults and seismic events. The results indicate that deformed listric faults show a meaningful correlation with the depth distribution of earthquakes throughout the Fars arc. The outcome associated with the presence of a thick basal salt layer and involvement of the basement exhibit a correlation with the structural style of the Fars arc.

How to cite: Gomar, F., Ruh, J. B., Najafi, M., and Sobouti, F.: Numerical modeling of basement inheritance and salt decoupling effects on the structural evolution of the Zagros Fold-Thrust Belt, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3163, https://doi.org/10.5194/egusphere-egu24-3163, 2024.

This presentation challenges the conventional theory that thrust systems necessarily grow by the upward propagation of tip-lines of faults away from a basal detachment. The model underpins mechanical considerations of thrust belt dynamics, explanations of thrust-related folding and commensurate forecasting of distributed deformation in the surrounding strata. Yet there have been few tests of the paradigm at the cross-section scale – not least because the quality of seismic imagery in most foreland thrust belts is insufficient to resolve structural geometries. Rather, the widespread application of the upward propagation (conventional) theory results from a variety of cognitive biases, especially restrictive citation of published literature.

Here we present interpretations of exceptional 3D seismic imagery from the eastern, lateral termination of the Jura fold-thrust belt of Switzerland. The area (the Baden-Irchel-Herdern Zone, referred to here as the BIHZ - broadly overlying the eponymous basement lineament) shows relatively little horizontal shortening and therefore preserves structures that would otherwise be overprinted by more extensive thrusting.  Contractional structures are developed in a multilayer of Mesozoic strata that comprise competent carbonates and incompetent mudrocks and marlstones. Interpretations based on 2D seismic profiles display thrusts sweeping through the Mesozoic strata as simple, continuous ramps, splaying from a basal detachment in Triassic evaporites. However, the higher-quality 3D seismic imagery does not support this type of interpretation. Within the BIHZ, statal reflectors are offset by small-offset (<50m) thrust segments. These do not map out as continuous fault surfaces, either in depth or in map-view. Some thrusts dip forelandward, others towards the hinterland. Collectively, these thrusts are layer-confined and, in plan-view, have highly sinuous forms. These geometries are indicative of having formed by linkage of originally distinct fault segments. Collectively this zone of segmented/low-displacement thrusting represents a “bead” of distributed deformation, restricted to the BIHZ. Presumably, if the area then evolved with greater contraction, part of the thrust array will become fully-linked and develop as a continuous structure – perhaps as elsewhere along the Jura arc. Those fault segments not incorporated into the now-continuous thrust would remain, forming a broad halo of distributed faulting in the surrounding rock. In this spatial context, the swathe of distributed deformation might be designated as a “damage zone” with respect to the main thrust but in fact, the distributed faulting would have no causative relationship to this main thrust.

Our interpretation of thrust system evolution derives from the “ramps first” model of Eisenstadt & DePaor (1987) – a structural concept that has received substantially less attention that the “conventional theory”. We do not wish to imply that there is a single mechanism by which thrusts nucleate and grow – the “conventional” theory may well apply in some situations. So too, may others. We do however emphasise the importance of considering a range of different behaviours and their resultant structural geometries when interpreting thrust systems. Some of these may yet to be described!

How to cite: Butler, R., Robledo, F., Sleath, P., and Bond, C.: Thrust system growth by segment linkage in sedimentary multilayers, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3200, https://doi.org/10.5194/egusphere-egu24-3200, 2024.

Deep earth geodynamic processes shape surface geology, topography, and the earth’s crust evolution with consequences on erosion, deposition, climate, and biogeography. This research investigates the early-stage growth of the NW Zagros belt in the Kurdistan region of Iraq after the early Oligocene Arabia-Eurasia collision and the geologic signatures of potential SE-ward propagating Neotethys slab tearing. We will test two end-member hypotheses: (i) the slab has already detached, causing subsidence before slab tearing, followed by rapid regional uplift in the vicinity of the suture zone afterward, or (ii) the slab is still attached, causing no pulse of significant hinterland uplift but continuous regional lower plate subsidence. In the NW Zagros belt, where the Arabian and Eurasian plates are sutured along the Main Zagros fault, allochthonous thrust sheets of ophiolitic and arc-related terranes were emplaced onto post-collisional autochthonous units of clastic and lower-middle Miocene shallow marine carbonate rocks, now at ~1-1.5 km elevation. The allochthonous thrust sheets host pre-collisional acidic and mafic intrusions that provide exhumation rate constraints throughout collision, whereas the post-collisional marine carbonate rocks attest to a significant suture zone subsidence and uplift. Thermal history modeling of bedrock samples using zircon and apatite (U-Th)/He thermochronometers suggests an early uplift during ~30-25 Ma, with most apatite (U-Th)/He samples being reset during ~15-10 Ma. To quantitatively determine the depositional age of the lower-middle Miocene marine carbonate rocks, ⁸⁷Sr/⁸⁶Sr isotope chronstratigraphy will be conducted. Furthermore, the isopach maps of the middle and upper Miocene foreland deposits show a notable shift in the depocenter axis and an enhanced subsidence toward the SE, concurrent with the arrival of the Afar plume to the suture zone in the NW. These preliminary results argue for the formation of the NW Zagros orogenic wedge as early as ~30-25 Ma, followed by suture zone subsidence during the early-middle Miocene and then uplift again during ~15-10 Ma, possibly due to initiation of Neotethys slab-tearing and its subsequent propagation along the suture zone from the NW to the SE. These findings have implications for investigating the role of the Arabia-Eurasia land bridge formation and deformation on vertebrate distribution and paleoclimate response to orographic barrier development since the late Miocene in the Middle East.

How to cite: Koshnaw, R., Kley, J., Dunkl, I., Ameen, F., and Vasilyan, D.: Tectonothermal constraints on the early-stage orogenic wedge formation along the Arabia-Eurasia suture zone in the NW Zagros orogenic belt, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3749, https://doi.org/10.5194/egusphere-egu24-3749, 2024.

The Pyrenees is a collisional orogen built by inversion of an immature rift system during convergence of the Iberian and European plates from Late Cretaceous to late Cenozoic. The full mountain belt consists of the pro-wedge and foreland of the southern Pyrenees and the retro-wedge and foreland of the northern Pyrenees, where the inverted lower Cretaceous rift system is mainly preserved. Due to low overall convergence and absence of oceanic subduction, this immature orogen preserves one of the best geological records of early orogenesis, the transition from early convergence to main collision and the transition from collision to post-convergence. During these transitional periods major changes in orogen behavior reflect evolving lithospheric processes and tectonic drivers. These records are best preserved in the North Pyrenean Zone (NPZ), the retrowedge thrust belt and its adjacent syn-orogenic basin. This paper reviews along strike variations in structural and stratigraphic characteristics of the NPZ and adjacent basin evolution and explores the changing role in space and time of rheological, thermal and structural Inheritance and local and plate scale drivers.

How to cite: Ford, M.: Lateral and temporal variations in thrust belt behaviour in a low convergence retrowedge, north Pyrenees. , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4040, https://doi.org/10.5194/egusphere-egu24-4040, 2024.

Fold-and-thrust belts are usually deformed by major transverse zones oriented orthogonally to the main thrust faults. These structural discontinuities represent ancestral fault zones originated mainly during rifting and subsequently reactivated during orogenic shortening (Zanchi et al., 2012). Sectors displaying thicknesses and sedimentary facies changes within the syn-rifting successions are juxtaposed along transverse zones (Thomas, 1990). This variability results in diverse deformation and shortening styles, as thrusts exhibiting flat (detachment levels) or ramp (more competent lithologies) geometry occur at different depths.

Limited data exist on the origin of fluids circulating along transverse zones and how they control fluid flow at the regional scale. In this study, we analyse transverse zones in the Lecco area of the Central Southern Alps (Lombardy, Italy). This belt features N-S trending, km-scale transverse zones initiated as normal faults during Ladinian, Norian-Rhaetian and Early Jurassic rifting phases and subsequently reactivated as strike-slip faults and/or lateral ramps during the Alpine orogeny (Schönborn, 1992). Two transverse zones are foci of this work: the Lecco Line to the west, running along the southeastern branch of Como Lake, and the Faggio Line to the east.

Geological mapping and mesostructural analysis were conducted and geological cross sections were built to constrain the geometry and kinematics of such transverse zones. Inorganic thermal indicators were used to constrain the eroded overburden and the exhumation depth of the analysed fault zones. U-Pb radiometric dating on syntectonic calcite mineralizations allowed us to constrain the age of tectonic activity. Syntectonic calcite veins precipitated either during the Early Jurassic rifting phase or younger Alpine orogenic phases, testifying the complex and long-lasting tectonic history of these transverse zones. C and O stable isotopes analysis allowed us to assess the origin of fluids circulating within transverse zones and their degree of interaction with host rocks. Two significant findings emerged from these analyses: 1) calcite mineralizations with variable δ¹³C and δ¹⁸O values characterize these fault zones, pointing out different degrees of fluid-rock interactions and/or different origin of fluids circulating during rifting and orogenic shortening; 2) transverse zones and related structures bound sectors where mineralizations occasionally exhibit values within narrow ranges of δ¹³C and δ¹⁸O; conversely, in other sectors, isotopic values display significantly wider ranges. In conclusion, transverse zones in the Southern Alps are structural features with a complex tectonic and fluid flow history, as testified by the large variability of age and C-O stable isotopes values. Furthermore, they possibly exert a partial compartmentalization of fluid flow at the regional scale.

References:

- Schönborn, G. (1992). Alpine tectonics and kinematic models of the central Southern Alps. Memorie di Scienze Geologiche, 44, 229–393.

- Thomas, W. A. (1990). Controls on locations of transverse zones in thrust belts. Eclogae Geologicae Helvetiae, 83(3), 727-744.

- Zanchi, A., D’Adda, P., Zanchetta, S., & Berra, F. (2012). Syn-thrust deformation across a transverse zone: the Grem–Vedra fault system (central Southern Alps, N Italy). Swiss Journal of Geosciences, 105(1), 19-38.

How to cite: Fiorini, A., Aldega, L., Dallai, L., Di Marcantonio, E., Rocca, M., Tavani, S., Zanchetta, S., Zanchi, A., and Carminati, E.: Multiple tectonic phases and fluids variability along transverse zones of the Central Southern Alps (Lombardy, Italy), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4663, https://doi.org/10.5194/egusphere-egu24-4663, 2024.

The Apennines fold-and-thrust belt builds part of the Africa-Eurasia convergent plate boundary. It developed due to the Neogene subduction of the Alpine Tethys underneath Europe and to the subsequent involvement of the Adria microplate rifted margin into the collisional process. Since the Miocene, eastward retreat of the slab caused extensional deformation of the thrust pile, eventually leading to the opening of the Tyrrhenian back-arc basin. Multiple schools of thought exist about the structural style of the Apennines, each of which proposes rather irreconcilable models. The amount of shortening, the involvement of the crystalline basement, the architecture of the inherited rifted system with its degree of reactivation during convergence, and the role played by compressive inheritance during back-arc extension, are still under discussion.

In this contribution we focus on the crustal structure of the central Apennines, along a transect that stretches from the Sardinian shelf (West) to the center of the Adriatic Sea (Est). Our aim is to critically evaluate prior models and contextually to illuminate the deeper part of the belt, which is still seldom considered in modern-day Central Apennines solutions. Based on the most recent ideas on rift inheritance and thrust tectonics, we present a brand-new crustal balanced cross-section. We employ well-constrained surficial geological data from available public maps, as well as the most recent deep-reaching geophysical data (especially, tomography and seismological data), which are supplemented by thermochronological, biostratigraphic, and Sr-isotope datings. On top of that, data are encompassed in a coherent geodynamic framework that is supported by a geometrically balanced and consistent kinematic model. This way, we provide a comprehensive explanation for the regionally observed coupled forward migration of compressional and extensional domains, which is related to the slab pull/trench retreat system.

Our results point to a predominantly thin-skinned style for the orogen with a secondary basement involvement in the later collisional stage. Inherited extensional faults, developed during the Mesozoic rifting of Adria, were partially reactivated during convergence, and influence spacing and geometry of the compressional features. The main thrusts of the area are characterized by significant displacements, ranging from 10 to >50 km, and sole out into a basal décollement located at the base of the post-Variscan sedimentary sequence. Finally, post-thrusting back-arc extension is accommodated by faults that either displace the compressional décollement levels or reactivate them with an opposing kinematics.

How to cite: Maresca, A., Granado, P., Muñoz, J. A., Manatschal, G., Ogata, K., and Tavani, S.: Rift inheritance in a fold-and-thrust belt evolution: Central Apennine crustal balanced cross-section, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5837, https://doi.org/10.5194/egusphere-egu24-5837, 2024.

Fault vergence in fold-and-thrust belts and accretionary prisms is characterized by mainly forward verging thrusts and pop-up structures, and only few examples exist where backthrusting dominates. However, backthrusting and triangle zones are known from most if not all fold-thrust belts in the world. The circumstances under which backthrusts instead of forethrusts form are still incompletely understood. Previous studies suggest that the strength and dip of the basal décollement of Coulomb wedges plays a key role. However, a systematic study of these parameters is still missing. We present numerical models of brittle-ductile wedges, varying dip and strength of the basal décollement systematically. We show that a new parameter, cumulative vorticity, is well-suited to characterize wedges based on dominant fault vergence. We find that backthrust-dominated wedges form in setups of very low basal dip (≤ 0.5°) and a weak décollement. Increasing décollement strength or basal dip results in the formation of pop-up-dominated wedges, before forethrust dominated wedges form. While our models corroborate the idea that décollement strength and basal dip may control thrust vergence, comparison with natural examples indicates this cannot be the only explanation for the formation of backthrusts. We probe into this using a compilation of structures along strike the fold-thrust belt of the entire Alpine-Carpathian Belt. Our results show that indeed triangle zones are associated with weak décollements, while additionally syn-tectonic sedimentation, rheological changes across strike, or structural inheritance may play a role. In some cases, interpretations of backthrusts at depth is challenging due to geometric uncertainty. We show uncertainty modeling of triangle zones using a well-known example from the boundary between Eastern and Central Alps. These uncertainty estimates may be combined with thermochronological data with the goal to distinguish the presence or absence of triangle zones from exhumation estimates. When present, triangle zones and backthrusts may serve as mechanical gages, providing tight constraints on fold-thrust belt mechanics.

How to cite: von Hagke, C., Bauville, A., Chudalla, N., Brisson, S., Wellmann, F., Tamas, D., Tamas, A., Krzywiec, P., and Malz, A.: Triangle zones as mechanical gages - results from numerical models and the Alpine Carpathian Belt, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6533, https://doi.org/10.5194/egusphere-egu24-6533, 2024.

The Fars Arc constitutes the southeastern segment of a tectonic recessions and salients array that characterizes the Zagros orogen. The delineation of the Fars Arc's western boundary is attributed to the prominent Kazerun fault system. However, further to the west, the southwestern periphery of the Arc (i.e., the Bushehr area) remains relatively unexplored, lacking documented evidence of tectonically active structures, as well as significant historical and instrumental seismic events. We combined tectonic geomorphology, remote sensing, and Quaternary geochronology to comprehensively study and identify tectonically active structures within the onshore area. Investigating the offshore area, we utilized 2D seismic-reflection data to interpret and image active subsurface structures in the Persian Gulf. We mapped and introduced several active faults in the Bushehr area, some of which are closely associated with active Quaternary anticlines, e.g., the Bushehr and Abtavil anticlines. Two generations of uplifted marine terraces, Terrace-I and Terrace-II across the Bushehr anticline, reveal a local uplift rate of approximately 0.8 mm/yr across the Bushehr Peninsula. The offshore 2D seismic-reflection data indicate ongoing deformation in the study area and show evidence of a recent sedimentary depocenter offshore of the Bushehr Peninsula. These observations define the present-day active deforming structures along the Zagros orogenic front in the Persian Gulf. The 2D seismic-reflection interpretations have characterized the active folding of the Bushehr anticline, indicating a minimum age of approximately 600 ka BP. The integration of on- and offshore geological analysis has revealed contemporaneous incipient deformation and syn-kinematic sedimentation, which significantly contributes to our understanding of geodynamics of the Zagros deformation front.

How to cite: Fathian, A., Koyi, H., Back, S., Nazari, H., H. Shugar, D., Shokri, M. A., and Reicherter, K.: Active tectonics within the southwestern edge of the Fars Arc, SW Iran, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6848, https://doi.org/10.5194/egusphere-egu24-6848, 2024.

The mechanisms of regional metamorphism in convergent orogens are still debated, with two perspectives on timing—either linked to convergence processes or pre-convergence rifting. The Hsuehshan Range in Taiwan experienced rifting from the Eocene to the Miocene, followed by a convergence since the late Miocene. In this study, we discuss the metamorphic mechanism of the Hsuehshan Range by combining Raman spectroscopy of carbonaceous material (RSCM) for the peak metamorphic temperatures and zircon fission track for the cooling ages, to constrain the burial temperature before the convergence.

In central Hsuehshan Range, RSCM temperatures peaked at 480°C, decreasing northward and southward. In the central-northern region, RSCM temperatures correlated positively with stratigraphic ages. However, in the southern region, east of the Tili Fault, temperatures increased eastward without matching stratigraphic ages.

In some areas, RSCM temperatures (260-300°C) indicate total reset during the late Cenozoic orogeny, while zircon fission-track ages show partial reset. Higher RSCM temperatures (>400°C) lack significant new biotite growth.

We propose that during the rifting stage, strata experienced peak metamorphism, followed by cooling before the late Cenozoic orogeny, thus explaining inconsistencies between RSCM temperatures and zircon fission track ages. The deposit thickness variations in the rifting basin that are contributed to the RSCM temperatures, show no apparent stratigraphic correlation.

How to cite: Hsieh, L.-C. and Lee, Y.-H.: Using Raman Spectroscopy of Carbonaceous Material to Explore the Metamorphic Temperature and its Tectonic Implications in the Southern Hsuehshan Range, Taiwan Orogen, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7239, https://doi.org/10.5194/egusphere-egu24-7239, 2024.

We present a new cross-section across the Jaca basin in the southern Pyrenees, aiming to analyze the temporal and spatial distribution of deformation in a fold-and-thrust belt. In this study, we have integrated all available subsurface data, including seismic sections and well data, with surface data to construct a balanced cross-section and partial restored cross-sections that illustrate the evolution of the south-Pyrenean fold-and-thrust belt.

Contractional deformation, initiating the development of the Pyrenean Mountain belt, started during Late Cretaceous times due to the reactivation of the Early Cretaceous hyperextended margin. Subsequently, deformation progressed during the Paleogene within the unstretched Iberian plate. In this region, the kinematics of the thrust wedge was controlled by inherited extensional faults and the unevenly distribution of Triassic evaporites above the basement.

Although deformation progressed toward the foreland, synchronous thrusting characterized the internal deformation of the thrust wedge. The localization and switching of deformation along the fold-and-thrust belt resulted from the interaction of unevenly distributed weak layers, including salt, and syntectonic sedimentation.

This work illustrates how detailed fieldwork, when combined with subsurface data and news concepts and methodologies (such as the use of drone imagery and analogue and numerical modelling), can significantly improve the understanding of the structure and evolution of fold-and-thrust belts.

How to cite: Muñoz, J., Izquierdo, E., Toro, R., Santolaria, P., Granado, P., Pueyo, E., and Casas, A.: Cover-basement relationships in a fold-and-thrust belt with inherited uneven weak horizons distribution: example from the southern Pyrenees across the Jaca basin, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7287, https://doi.org/10.5194/egusphere-egu24-7287, 2024.

The northwestern syntax of the Himalaya is a very rapidly deforming area at the edge of the India-Asia collision zone. Therefore, we quantify the current velocity field of the Potwar Plateau – Salt Range fold-and-thrust belt using GNSS horizontal surface velocities and Sentinel-1 interferometry line-of-sight velocities. From this velocity field which indicates a creep of the Potwar Plateau along the Main Himalayan Thrust, we infer a weak subhorizontal décollement level formed by a massive Precambrian salt layer. South of the Plateau, the Salt Range is uplifted along the Salt Range Thrust up to 5 mm/yr. The Kalabagh Fault, which forms the western boundary of the Salt Range and the Potwar Plateau, exhibits a creep rate along fault of 3.3 mm/yr. To characterize the slip distribution and coupling along the faults, we use numerical modelling with a set of dislocations in an elastic half-space. The preferred model shows the presence of a large asperity along the décollement level beneath the Potwar Plateau and several smaller asperities along the eastern basal thrust. These observations are consistent with the occurrence of the 2029 Mirpur earthquake of Mw 5.9 along the eastern part of the décollement level. Along the southern and superficial parts of the Salt Range Thrust, the model indicates a slip rate of 20 mm/yr which is greater than the 14 mm/yr slip rate along the Main Himalayan Thrust at depth. This observation suggests the existence of an internal southward flow of the massive salt layer along the upper part of the Salt Range Thrust. For the Kalabagh strike slip fault, an alternation of coupled and decoupled zones is observed, meaning that this fault can be characterised by creep and asperities where earthquakes and/or slow slip events can occur. Considering the lack of instrumental and historical large magnitude earthquakes in the area since the AD 25 Taxila earthquake, it can be concluded that the Main Himalayan Thrust and the Kalabagh Fault are likely to be affected by large magnitude earthquakes.

How to cite: Meyer, P., Jouanne, F., Doin, M.-P., Ahmed, A., Alam Awan, A., and Mugnier, J.-L.: Present-day quantification of seismic coupling along the décollement level beneath the Potwar Plateau region in Pakistan western Himalaya, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7329, https://doi.org/10.5194/egusphere-egu24-7329, 2024.

Tectonic mélanges, penetrative mixes of sediments and basalts, are often interpreted as fossil subduction plate interfaces. These formations, marked by intense deformation, contain witnesses of past earthquakes and commonly include remnants of the oceanic crust of the subducting plate. The original process that led to penetrative mixing of sediments and basalts is often controversial, and classically relies either on tectonic slicing of the downgoing plateduring subduction, or on pre-subduction, olistostrome-forming sedimentary mixing. In addition, magma emplacement in sediments of the downgoing plate may also explain the mixed lithologies of mélanges. This latter case should leave an aureole of contact metamorphism in sediments near basalts. In this work, we applied Raman Spectroscopy of Carbonaceous Material in the modern seafloor sediments where magmatism is reported, in order to evaluate the thermal influence from basalts onto carbonaceous material in the contacting sediments. Then, to check the potential contact metamorphism in mélanges, we employed the same methodology in several examples of sediment-basalt mélanges at (sub)-greenschist-facies conditions (Kodiak complex, Alaska; Shimanto Belt, Japan).

In modern ocean-floor settings, magmas intruding, and to a lesser extent, flowing onto sediments, resulted in higher crystallinity of carbonaceous material in a cm- to dm-thick contact aureole. In four of five studied mélanges, the crystallinity of carbonaceous material in metasediments increases toward basalts, indicating a ~1 cm-thick contact metamorphism aureole. Thus, we propose that for the studied mélanges the mixing likely occurred prior to subduction, with the preservation of contact metamorphism despite syn-subduction, low-temperature metamorphism.

As a consequence, the block-in-matrix structure observed in mélanges, as well as the occurrence of mafic bodies at seismogenic depths in accretionary prisms, is in many instances the result of pre-subduction structure, rather than tectonic slicing and step-down of the decollement into the oceanic crust. In particular, tectonic mélanges such as Mugi in Japan and Ghost Rocks in Alaska do not reflect simply the structure and thickness of the plate subduction interface, but a complex combination of pre-subduction geometry and tectonic processes during underplating and within the accretionary wedge.

Furthermore, in two studied paleo-accretionary complexes, deposition ages of the trench sediments forming the matrix of three examined mélanges overlap ages of magmatism within uncertainty. Considering that the basalts from these mélanges exhibit MORB signatures, this age overlap suggests that the mélanges possibly formed at the trench just before ridge subduction. We thus conclude that basalt-sediment mélanges stand as potential records documenting ancient ridge subduction events.

How to cite: Rajič, K., Raimbourg, H., Famin, V., and Moris-Muttoni, B.: The origin of tectonic mélanges and implication for the subduction interface processes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8453, https://doi.org/10.5194/egusphere-egu24-8453, 2024.

The Aragüés thrust system, in the Southern Pyrenees, is a superb and well-exposed sequence of thrust imbricates to study the deformation and development of fault-propagating folds. The mechanisms of folding and thrusting, including the importance of material contrast during the development of the thrust sequence, are well-studied in this area. However, insights into the distribution and magnitude of penetrative strain remain unknown. Therefore, analysis of anisotropy of magnetic susceptibility (AMS) is used to reveal penetrative deformation within the sedimentary rocks of the Aragüés thrust system. Rock samples from 66 locations are collected across the Aragüés thrust imbricates with a focus on the layered limestone unit. A total of 489 cylinders were retrieved from the oriented hand samples and their AMS signal was measured. Thermomagnetic and magnetic remanence measurements show that the magnetic susceptibility of the samples is dominated by paramagnetic minerals and the magnetic lineation (axis of maximum susceptibility) is mainly parallel to subparallel to bedding. However, the more strongly magnetic samples (kmean: 3-5 x 10-4SI) show a magnetic lineation parallel to the general NNE-SSW shortening direction at the back- and forelimbs of fault-propagation folds. In contrast, the samples with a lower magnetic susceptibility (kmean: 1-3 x 10-4 SI), which are also closer to fold hinges, reveal a magnetic lineation perpendicular to the main shortening direction. We interpret the differences in fabric alignment and magnetic susceptibility to mineral composition and structural evolution. For example, the magnetic lineation parallel to the shortening direction is a consequence of flexural slip and flow along bedding surfaces or incompetent beds of the layered limestone units in the fold limbs. Additionally, a relationship between the magnetic lineation and the development of cleavage and stylolites within the limbs can be identified, but is not consistent. The magnetic lineation at the fold hinge is perpendicular to the main shortening direction and needs further investigation for strain accommodation prior to folding, an effect of buckling and/or development as intersection lineation. These first interpretations of the magnetic fabric provide additional insights into the deformation of the Aragüés thrust system and the development of fault-propagating folds. Moreover, the AMS data adds new information on penetrative strain in this region of the Southern Pyrenees and reveals a general tectonic overprint on grain-scale within sedimentary rocks during deformation.

How to cite: Schöfisch, T., Koyi, H., Teixell, A., and Almqvist, B.: Penetrative strain of the Aragüés fault-propagating folds in the Southern Pyrenees revealed by magnetic fabric analysis, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9604, https://doi.org/10.5194/egusphere-egu24-9604, 2024.