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

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

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

EGU25-866

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

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

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

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

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

EGU25-4090

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

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

EGU25-16582

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

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

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

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

EGU25-20083

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

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

EGU25-11789

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

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

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

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

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

EGU25-14614

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

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

In the field, micaschist belonging to the footwall displays a strain gradient moving toward the contact with the hanging wall, corresponding to a development of a S-C-C’ fabric and a modal enrichment in K-rich white mica and pyrophyllite matched by a progressive increase in the physical interconnectivity of these phyllosilicates. Electron backscatter diffraction analyses suggest that large (several hundreds of µm) detrital quartz grains experienced grain size reduction by subgrain rotation recrystallization to form equant grains of less than 100 µm in size.

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

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

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

Concluding, this intimate and polyphase interplay between deformation and metamorphism is responsible for the formation and evolution of exhuming shear zones and the related structure of orogens.

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

References

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

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

Ring, U., Glodny, J., Hansman, R., Scharf, A., Mattern, F., Callegari, I., van Hinsbergen, D. J. J., Willner, A., & Hong, Y. (2024). Earth-Science Reviews, 250, 104711. https://doi.org/https://doi.org/10.1016/j.earscirev.2024.104711 

How to cite: Giuntoli, F., Petroccia, A., Airaghi, L., Précigout, J., and Raimbourg, H.: Phyllosilicates do their job: insights into their role in exhuming subducted continental units, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16287, https://doi.org/10.5194/egusphere-egu25-16287, 2025.

EGU25-16287

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

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

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

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

EGU25-12927

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

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

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

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

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

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

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

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

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

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

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

EGU25-5837

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

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

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

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

EGU25-10795

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

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

EGU25-15187

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

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

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

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

EGU25-6058

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

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

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

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

EGU25-15688

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

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

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

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

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

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

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

EGU25-16426

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

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

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

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

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

EGU25-2428

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

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

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

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

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

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

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

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

EGU25-11959

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

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

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

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

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

EGU25-5696

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

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

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

EGU25-5763

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