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.