The multitude of possible factors determining the deformation style in the lithosphere make a comprehensive understanding of the deformation behavior of Earth’s lithosphere challenging. In this session we aim to tackle the complex topic of lithospheric deformation by combining observations from natural rocks with those from experimental and numerical studies.
A monocline fold forming in a sedimentary cover above a fault in a rigid basement is an example of fault-related folding and is often associated with fault propagation. We developed a custom implementation of finite element method model of folding in a viscous medium above a fault with an arbitrary dip (θ) and sense of slip. We explored three rheological scenarios: (1) a homogeneous isotropic cover, (2) a heterogeneous cover consisting of initially flat, alternating low- and high-viscosity (denoted by µ1 and µ2 respectively) isotropic layers, (3) an initially homogeneous anisotropic cover. Anisotropic fluids are characterised by shear and normal viscosity, i.e., viscosity under layer-parallel shearing and layer parallel shortening or extension, respectively. The model of an anisotropic fluid approximates the behaviour of a layered media in the limit of fine layering.
We performed systematic numerical experiments for fault dip angles ranging from θ=10° to θ=90°, number of layers n=8, 16, 32, 64, 128, viscosity ratios µ2/ µ1=10, 25, 50, 100 and shortening or extensional regime. Results demonstrated that an anisotropic viscous medium effectively approximates a finely layered sedimentary cover at both the onset of deformation and under large finite strain. However, the observations regarding the trends of structure evolution (e.g., fold amplitude growth rate) made at the onset or after a few initial time steps of deformation cannot be extrapolated for further stages of deformation. For sufficiently fine layering (e.g., n=64, 128), the simulated folds tend to be chevron-like. Two major geometrical types of folds can be described in the reverse fault case, i.e. in the shortening regime. A forelimb monocline alone forms above a basement fault with dip angles larger than θ=30-40°, but an additional pop-up anticline emerges in the case of a gentler dipping fault. In general, greater viscosity contrasts favour the amplification of the pop-up anticline. The anticline grows in time for most of studied cases, but its evolution is more complex for folds formed above a fault dipping close to the threshold value between two geometrical types. In these cases, the amplitude of the pop-up anticline decreases with progressive shortening at late stages of deformation. In the normal fault case (extensional regime), the covering layers tend to deform more or less parallel to the top basement boundary and fold geometries are rather similar regardless of the fault dip angle.
The work was supported by the National Science Centre, Poland, under research project “Numerical and field studies of anisotropic rocks under large strain: applying micro-POLAR mechanIcS in structural geology (POLARIS)”, no UMO-2020/39/I/ST10/00818.
How to cite: Mol, S. and Dabrowski, M.: Numerical modelling of viscous folding in a layered sedimentary cover above a basement fault, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6922, https://doi.org/10.5194/egusphere-egu26-6922, 2026.
Metamorphic complexes preserve well-recorded evidence of multistage deformation, metamorphism, magmatism, and fluid-rock interactions. However, the structural relationships between microstructurally constrained ductile deformation and later brittle fault kinematics and stress fields remain insufficiently constrained. The Yuanmou Complex, located in the central–southern Kangdian region along the southwestern margin of the Yangtze Block, China, provides an ideal site to address these issues. In this study, we combine field investigations with optical microscopy–cathodoluminescence (OM–CL), electron backscatter diffraction (EBSD) and paleostress inversion of fault-slip data to investigate the deformation process and stress fields of later brittle faulting in the Yuanmou Complex.
Microstructures and EBSD fabric results indicate that the Yuanmou Complex experienced multistage deformation, evolving from early high-temperature ductile deformation to low-temperature ductile deformation, followed by brittle deformation during exhumation to shallow crustal levels. EBSD fabric analyses of deformed quartz reveal a systematic transition in dominant slip systems, from high-temperature prism slip (>650 °C), through intermediate–high temperature prism
Paleostress inversion reveals the coexistence of compressional, extensional and strike-slip stress regimes. An early stress regime dominated by NNE–SSW-oriented compression is identified, whereas a later stage is characterized by a NW–NWW-oriented principal stress field, under which fault kinematics gradually evolved from thrusting to strike-slip–dominated deformation, accompanied by local extensional activity. Linking ductile deformation processes with subsequent brittle fault kinematics and stress fields, our results reveal their structural connection and reflect regional Cenozoic responses to eastward extrusion of the Tibetan Plateau and southeastward escape of the Sichuan-Yunnan rhombic block.
How to cite: Cheng, X., Cao, S., Jiang, S., and von Hagke, C.: Multiphase deformation and stress field evolution of the Yuanmou metamorphic complex, SW China, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9871, https://doi.org/10.5194/egusphere-egu26-9871, 2026.
Pseudotachylytes are solidified frictional melts produced in silicatic rocks during an earthquake (Sibson, 1975). They usually form fault and injection veins with thicknesses ranging from few millimeters to several centimeters. Still, exposures of meter-thick pseudotachylyte breccias with evidence of a single melt pulse and associated with seismic faulting have been documented (i.e., Musgrave Ranges, Australia; Lofoten Islands, Norway; Outer Hebrides, Scotland; Greenland; Ivrea-Verbano Zone, Italy).
In the present study, we perform field, microstructural and mineralogical investigations of both “normal” (i.e., mm- to cm-scale) pseudotachylytes and “giant” pseudotachylyte breccias outcropping in the Ivrea-Verbano Zone moving away from the Canavese Line, a segment of the Insubric Line, the main tectonic lineament of the Alps. The giant pseudotachylyte breccias reach up to ~2 m in thickness, and up to 8 m in length, limited by the outcrop extension, and possibly fill pull-aparts. Despite pseudotachylytes in this area have already been studied in detail, giant pseudotachylyte breccias were somehow overlooked (Techmer et al., 1992; Ueda et al., 2008; Souquière and Fabbri, 2010; Ferrand et al., 2018). We aim to determine (i) the ambient P-T conditions of formation (discussed here), (ii) their geodynamic and seismogenic environment, and (iii) their formation mechanism.
We selected four main outcrops along the Sesia River for detailed field mapping and sampling, moving eastward from the Canavese Line for ~9 km. In fact, no giant pseudotachylyte breccias have been found to the west of the lineament. In detail:
Outcrop I, <500 m from the Canavese Line (altered greenschist facies gabbros) shows:
- multiple generations of pseudotachylyte-bearing faults, including giant pseudotachylyte breccias subparallel to the NNE-SSW striking Canavese Line, containing clasts of the altered host rock;
- matrix of the pseudotachylytes overprinted by greenschist facies minerals (epidote, chlorite, albite);
- late quartz-epidote- and chlorite-bearing faults cutting the pseudotachylyte-bearing faults and breccias.
Outcrop II, ~1 km from the Canavese Line (unaltered gabbros) shows:
- multiple generations of pseudotachylyte veins and giant breccias, the latter subparallel to the Canavese Line;
- cataclasite- and graphite-bearing faults cut by giant pseudotachylyte breccia;
- late quartz-epidote- and chlorite-bearing faults cutting the pseudotachylytes.
Outcrop III, ~2 km from the Canavese Line (Balmuccia peridotite) shows:
- multiple giant pseudotachylyte breccias cutting cataclasite-bearing faults;
- serpentine-bearing veins and pseudotachylytes mutually cross-cutting each other;
- giant pseudotachylyte breccias subparallel to the Canavese line; their matrix includes microlites of olivine, enstatite, and vesicles.
Outcrop IV, ~9 km from the Canavese Line (unaltered tonalite) shows:
- only thin pseudotachylytes overprinting foliated cataclasite-bearing faults;
- well-preserved matrix of the pseudotachylytes (microlites, chilled margins, flow structures).
In conclusion, giant pseudotachylyte breccias are (i) mostly subparallel and only outcropping close to the Canavese Line (<2 km), (ii) made of a relatively homogenous matrix, resulting from the solidification of a continuous melt layer, (iii) not reactivated by ductile deformation, (iv) cut and are cut by brittle faults and, (v) cut by quartz-epidote, chlorite-, serpentine- bearing faults and veins. Thus, they were possibly generated in a shallow (~15 km depth) and cold (<350°C) environment by individual earthquakes of large magnitude, associated with the activity of the Canavese Line.
How to cite: Aldrighetti, S., D'Ippolito, G., Pennacchioni, G., Gomila, R., Baccheschi, P., and Di Toro, G.: Giant pseudotachylyte breccias of Valsesia (Ivrea Zone, Western Italian Alps), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5415, https://doi.org/10.5194/egusphere-egu26-5415, 2026.
Temperature-dependent rheological changes control the long-term brittle-ductile transition, but several other factors may control the transient and localized switching between brittle and ductile rheology in the continental crust during orogen build-up. Rheological transients are inferred from, for example, mutual overprinting relationship between localized ductile and brittle deformation features (faults, veins, foliations, and shear zones) in the field. These occurrences are commonly used as a starting point for developing models of the mechanisms controlling seismicity outside the upper-crustal schizosphere, including lower-crustal earthquakes, intermediate-depth seismicity, and slow seismic phenomena such as tremor and slow slip. Current geophysical/seismological investigations show indeed the occurrence of different types of seismicity potentially related to continental subduction; however, most recorded seismicity appears to be linked to collision and exhumation processes. Based on field observations from subducted and exhumed Alpine continental units (Corsica and the Central Alps), this contribution addresses key challenges in interpreting brittle–ductile transient rheology from the geological record, discussing how structural inheritance, metamorphic overprinting, and fluid composition complicate interpretations of seismic versus aseismic deformation.
During prograde subduction, increasing temperature and pressure should promote a progressive transition from brittle to ductile rheology. The blueschist-ecogitic facies continental units of Alpine Corsica, prime example of continental subduction, show indeed a general brittle-to-ductile (and potentially seismic-to-aseismic) evolution, with distinct deformation features developed across increasing metamorphic grades. However, the post-kinematic increment in metamorphic conditions may overprint brittle structures with higher-grade assemblages, precluding us to understand if these field occurrences are really representative of (seismic) rheological transients during deep subduction, or if they simply result from structural inheritance from the pre-orogenic stages. New field observations from the Crystalline Massifs of the Central Alps (Aar massif, Gotthard nappe) further demonstrate the role of inherited structures in steering the retrograde rheological evolution of the continental crust during Alpine collision and exhumation, challenging models for mid-crustal seismicity and strain localization. Rheological transients are commonly associated with fluid flow and fluid pressure fluctuations, manifested in the field as mineralised veins precipitating from metamorphic fluids. Yet, the polyphase nature of metamorphic fluids (e.g., CO2-, CH4-bearing fluids), and the resulting variability in chemo-physical properties are rarely considered in rheological models. CO2-rich fluids and the resulting carbonate-bearing mineral veins might lead to transient rheological switches in both the brittle and ductile fields, as documented by sheared carbonate-bearing breccias in several Alpine crystalline units and plutons.
Together, these observations highlight that brittle–ductile transients inferred from the geological record require careful evaluation of inheritance, metamorphic overprinting, and fluid composition before being extrapolated to crustal rheology and seismicity models.
How to cite: Ceccato, A.: Brittle–ductile transients during continental subduction and exhumation: inheritance, fluids, and implications for seismicity, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4445, https://doi.org/10.5194/egusphere-egu26-4445, 2026.
Large continental shear zones play a fundamental role in crustal deformation, exhumation and lithosphere-scale tectonics, yet their duration of activity and the controls exerted by thermal regime on their geochronological record remain debated. Over the past decade, studies conducted in the Aegean domain, Menderes Massif and Alpine Corsica have generated a large and internally consistent set of geochronological data acquired across several major shear-zone systems. These datasets are dominated by 40Ar/39Ar ages complemented by U–Pb, Rb–Sr and low-temperature thermochronology.
Here we propose a synthesis of these datasets, integrating published results from different types of shear zones developed under contrasting P–T conditions, ranging from cold HP–LT subduction zone to Barrovian metamorphism in collisional environment and hot metamorphic core complex settings. We aim to compare age–distance relationships across shear zones, assess the temporal distribution and duration of deformation events recorded by argon systems, and place these observations in a broader tectono-thermal framework. Preliminary observations suggest systematic differences between cold and hot shear zones: cold systems tend to preserve a broad spectrum of argon ages spanning most of the deformation history, whereas hot shear zones commonly record shorter durations and younger ages biased toward the final stages of activity. These patterns appear to be robust across different tectonic settings and may reflect fundamental differences in deformation mechanisms, fluid circulation and argon mobility.
By combining shear-zone geochronology with independent constraints from magmatic intrusions, partial melting and tectono-metamorphic evolution, this synthesis identifies common timescales for shear-zone activity and clarifies how thermal regime controls both deformation processes and the geochronological record. Beyond regional implications for the dynamics of the Aegean and surrounding domains, this study provides first-order constraints on the mechanisms and longevity of continental shear zones and on the interpretation of geochronological datasets acquired in deformed rocks.
How to cite: Laurent, V., Roche, V., Jolivet, L., Augier, R., Raimbourg, H., Menant, A., Arbaret, L., and Labrousse, L.: Duration, thermal regime and argon behavior in continental shear zones: a synthesis from the Aegean domain, Menderes Massif and Alpine Corsica, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11126, https://doi.org/10.5194/egusphere-egu26-11126, 2026.
The exhumation of lithospheric mantle at magma-poor rifted margins requires efficient strain localization and sustained weakening of ultramafic rocks. In the Tyrrhenian back-arc basin, recent IODP drilling by Expedition 402 has provided unprecedented access to mantle exhumed ≤4 Ma, revealing a complex interplay between deformation, magmatic intrusions, and hydrothermal fluid circulation.
We assess how hydrothermal fluids and syn-rift intrusions influence the mechanical evolution of the exhumed mantle in the Tyrrhenian Sea. Structural and microstructural observations from drilled mantle sections document a transition from high-temperature ductile deformation (mylonitization) to brittle faulting (e.g., brecciation). This evolution is accompanied by serpentinization and localized carbonation veins. These veins follow pre-existing lithological contacts such as felsic and mafic intrusions, which probably act as rheological barriers and as preferential pathways for fluid flow.
We integrate structural analysis, microstructural characterization and mineralogical constraints, and 3D tomography (synchrotron µCT) to evaluate how porosity distribution, connectivity of veins, reaction front, linked to fluid infiltration and fluid-driven mineral transformations, modify mantle rheology. Fluid-assisted weakening and reaction-induced volume changes may promote the development of localized shear zones and, ultimately, detachment faults. Preliminary observations indicate that magmatic intrusions (felsic and mafic) localize strain; subsequent serpentinization further reduces rock strength and facilitates the late stages of exhumation.
Our results suggest that mantle exhumation in the Tyrrhenian basin reflects complex coupled deformation-magmatism- fluid processes rather than tectonic extension alone. This provides new constraints on strain localization mechanisms at magma-poor rifted margins and on the mechanical evolution of continent–ocean transitions.
How to cite: Vannucchi, P., Bickert, M., Poulaki, E. M., Montemagni, C., Baroncini, E., Rizzo, R. E., and Sanfilippo, A.: Deformation, fluid circulation, and strain localization during mantle exhumation in the Tyrrhenian back-arc basin, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10275, https://doi.org/10.5194/egusphere-egu26-10275, 2026.
Recrystallization of plagioclase is induced by deformation and/or chemical disequilibrium. It can be accomplished by several deformation mechanisms or their combination and it is typically accompanied by a change in plagioclase composition. The known agents facilitating the recrystallization are dislocations, mechanical twinning and cracking. In this contribution, we present incipient stages of dynamic recrystallization imposed on the magmatic plagioclase crystals in metagabbro from the Teplá-Barrandian Unit in the Bohemian Massif. The plagioclase crystals show chemically and mechanically heterogeneous internal structure related to its metamorphic-deformation transformation.
The chemical heterogeneity is manifested by decomposition of magmatic porphyroblasts of labradorite composition to the mixture of randomly oriented laths of bytownite and surrounding andesine. Crystallographically the laths are perfectly coherent with the rest of the crystal. The mechanical heterogenity is due to subsequent deformation that led to mechanical twinning followed by recrystallization. The initial low angle boundaries of the newly developing grains follow the network of bytownite laths while the later high angle boundaries are based on the original laths together with segmented twin boundaries. The resulting recrystallized microstructure shows small individual grains with andesine cores and bytownite rims. The misorientation analysis of the low angle boundaries indicate the geometry of till and twist boundaries resulting from dislocation glide and operation of (010)[001] slip system. Once the high angle boundaries are established they start to migrate and equillibrate, driven by chemical disequilibrium at the bytownite-andesine interfaces. The resulting fine-grained plagioclase shows evidence for grain size sensitive creep during subsequent deformation. Our findings indicate that crystal heterogeneity in feldspars may be an important parameter in the grain refinement process thus influencing the switch from dislocation creep to viscous flow.
How to cite: Jeřábek, P. and Racek, M.: Crystal heterogeneity controlling the grain size of dynamically recrystallized plagioclase, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21988, https://doi.org/10.5194/egusphere-egu26-21988, 2026.
Microstructures within rocks and crystals describe their past deformation conditions, which may vary in time and space even within a single rock sample. This variation depends on pressure, temperature and strain rate, and determines whether minerals deform in a brittle or ductile manner. Investigating deformation mechanisms under extreme conditions, such as ultra-high-pressure (UHP), is particularly useful. Indeed, the analysis of well-preserved UHP rocks provides insights into crystal’s behavior over a wider range of pressures. For these reasons, we studied Dora Maira whiteschists, and specifically the pyrope crystals forming these rocks.
Dora Maira is one of the internal crystalline massifs of the European Western Alps, formed by HP and UHP units. The latter is famous for the presence of coesite-bearing whiteschists. These rocks are foliated, with a spatially variable foliation defined by the shape preferred orientation of phengite and garnet crystals. Garnet grains can be either rounded or elongated and show different sets of fractures. Moreover, garnet crystals are locally recrystallized.
The first set of garnet fractures is represented by parallel fractures oriented at high angle with respect to the main rock schistosity and affecting garnet crystals in the entire outcrop. These fractures are locally associated with another set developed at ca. 45°, formed together with small (µm-scale) rotating volumes of garnet. The parallel fractures are dislocated by the local recrystallization of some garnet grains and by radial fractures developed around coesite/palisade quartz inclusions. These radial fractures formed due to the large volumetric change happening at the coesite-quartz transition.
We analyzed the described microstructures using optical microscope and SEM in (HR)-EBSD mode. Additionally, we investigated garnet crystals’ composition with SEM-EDS and microprobe. In this contribution, we show the results of this combined analysis.
Our results provide new microstructural evidence that garnet can record alternating brittle and ductile deformation under UHP conditions. Besides, we document a correlation between deformation-related microstructures and major-element redistribution within garnet, highlighting the deep connection between these two aspects which was previously underestimated.
How to cite: Tagliaferri, A., Tajčmanová, L., and Duretz, T.: The memory of crystals: microstructures in UHP garnets from Dora Maira, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9776, https://doi.org/10.5194/egusphere-egu26-9776, 2026.
Studies of multiple mantle exposures indicate that a fundamental shift occurs in polymineralic peridotites at ~850° C. At these temperatures, there is a shift from dislocation creep (plus or minus dislocation accommodated grain boundary sliding) to reaction-facilitated grain-size sensitive creep. This reaction results in a fine-grained matrix produced by neocrystallization. The fine-grained shear zones that formed by dislocation creep dynamic recrystallization create increased grain-boundary surface area that localize the reaction-enhanced deformation. Because the grains are formed by reaction, grain boundary pinning of the different mineral phases occurs. Moreover, these fine-grain sizes are preserved during exhumation, because of the grain boundary pinning. Thus, the fine-grain size – once it has been formed by reaction-facilitated deformation – continues to exist even if there is a change in temperature.
This rheological behavior is not typically shown in deformation strength profiles, because monophase olivine does not show these effects. Yet, the lithospheric mantle is polyphase, and we have observed evidence for reaction-facilitated deformation that occurred below ~850° C. Once grain size reduction has occurred in a polyphase material, it is not expected to grow large grain sizes again, due to the role of grain boundary pinning. Thus, once formed, a reaction-facilitated shear zone with smaller grain size relative to the surrounding mantle rocks would remain a lithospheric “scar”. The fine-grain shear zones would preferentially reactivate because the zone can deform by grain-size sensitive creep at lower stress conditions that the surrounding mantle material can deform by dislocation creep. This interpretation could explain the common reactivation of transform faults, and perhaps even extensional faults, in orogenic belts. Reactivation of transform faults in the mantle may explain: 1) the Neoproterozoic transform faults of the eastern and western United States, which are reactivated by Pennsylvanian and Cretaceous deformation, respectively; and 2) the modern San Andreas System reactivating a Cretaceous – Paleogene proto San Andreas Fault.
How to cite: Newman, J., Tikoff, B., and Chatzaras, V.: The microstructural legacy of mantle deformation during orogenic reactivation , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15465, https://doi.org/10.5194/egusphere-egu26-15465, 2026.
Nanoindentation is a mechanical testing technique used to quantify material properties (e.g., hardness) and deformation behaviour (e.g., plasticity). By controlling the indenter tip with great precision in all dimensions, the range of available methods can be expanded to include rapid property mapping, constant-stiffness stress-strain curves, topography mapping, and scratch and frictional testing. We have set up a complete, affordable, and fast workflow centred around nanoindentation with a Bruker Hysitron TriboIndenter 990 and complemented by electron microscopy to tackle a variety of research questions concerning the behaviour of earth materials.
This contribution showcases this workflow as applied to several common rock-forming high-pressure metamorphic minerals. We begin with first-order sample characterisation of thin sections using optical microscopy and electron backscatter diffraction to quantify crystal orientations to determine which crystal axes are being indented. Transitioning to the mechanical testing phase, we use spherical tips to obtain stress-strain curves to analyse the transition from elastic deformation to low-temperature plasticity, and to quantify the yield hardness. Stress-strain curves can be calculated from regular constant loading rate indentation experiments, only valid within the elastic domain, or with constant stiffness measurements using tip oscillations to provide a full stress-strain curve including plastic behaviour. We image the residual indent sites with surface probe mapping, which measures surface topography with a vertical resolution down to 0.1 nm and thus produces 3D maps with which we can quantify the dimensions and geometries of indent pits.
The results of our case studies on glaucophane, omphacite, and garnet show that plastic yielding is controlled by the availability of nucleation points for dislocations, provided by pre-existing defects. The degree of this effect varies per mineral, and further depends on crystal orientation. Overall, we demonstrate an efficient workflow for mechanical and microstructural characterization of low-temperature plasticity with nanoindentation applicable to most silicates and other minerals. This workflow can also be adjusted to analyse and quantify many other aspects of the properties and behaviour of earth materials.
How to cite: van Schrojenstein Lantman, H. and Kotowski, A.: Deformation in great detail: A nanoindentation workflow for investigating low-temperature plasticity in silicate minerals, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12353, https://doi.org/10.5194/egusphere-egu26-12353, 2026.
In the mafic lower crust, clinopyroxene is among the main rock-forming minerals. Based on experimental investigations, clinopyroxene is considered to be strong and to deform in a brittle manner at dry lower crustal conditions. However, field observations on Holsnøy, Norway, indicate ductile deformation of coarse-grained clinopyroxene in the mafic lower crust, reflected by bending of the granulitic foliation adjacent to eclogitic shear zones.
This study focusses on the strain accommodating processes of the granulitic clinopyroxene during incipient eclogitisation. Representative samples of deformed weakly eclogitised granulite were analysed via scanning electron microscopy, electron back-scattered diffraction mapping, electron probe micro analysis and Fourier-transform infrared spectroscopy.
Microstructural analysis reveals the formation of garnet lamellae along the {010} planes of the diopsidic clinopyroxene. Initial bending of this anisotropic clinopyroxene is accommodated by the development of en échelon microcracks at a high angle to the {010} planes. The micro-cracks are traced by garnet with similar composition as the lamellae, suggesting that both formed at similar pressure-temperature conditions. With ongoing strain, the cracks start to link and evolve into micro-shear zones, which systematically widen with strain and eventually connect forming networks. This widening is accompanied by the nucleation of amphibole and a second clinopyroxene with higher magnesium and lower aluminium concentration when compared to the host clinopyroxene, facilitating further macroscopic bending of the granulitic foliation. Increased intracrystalline misorientation and formation of subgrains adjacent to the micro-shear zone indicate that the diopsidic clinopyroxene host grain deforms by crystal plastic processes. In contrast, shape-preferred orientation and minor chemical zoning of the newly crystallised grains related to the micro-shear zone suggest that diffusion-related processes predominately accommodated the strain in the micro-shear zones.
In recent literature, low-permeable granulite has been described as dry. The observed deformation style as well as the formation of amphibole in the micro-shear zones indicate the presence of water, either in form of external fluids, infiltrating through en échelon microcracks, or as minor amounts of OH-groups occurring in the nominally anhydrous clinopyroxene. First Fourier-transform infrared spectroscopy results suggest that the nucleation of amphibole might be facilitated by the incorporated OH in the diopsidic clinopyroxene.
The observed microstructures and mineral compositions suggest that the micro-shear zones form at an early deformation stage throughout the eclogitisation process on Holsnøy. Our investigations show the complex interplay of brittle and ductile processes on a microscopic scale during macroscopically ductile flow.
How to cite: Lenz, L., Zertani, S., Grasemann, B., Stalder, R., Menegon, L., and Rogowitz, A.: Brittle-ductile deformation of granulitic clinopyroxene during incipient eclogitisation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5725, https://doi.org/10.5194/egusphere-egu26-5725, 2026.
Despite hornblende’s widespread occurrence in deformed rocks from exhumed crustal shear zones and metamorphic soles, its dominant deformation mechanism(s) and the respective microstructural fingerprints remain poorly constrained. Several deformation mechanisms have been documented in hornblende, including cataclastic flow, twinning, dissolution–precipitation, and dislocation-mediated deformation. Hornblende’s easy slip system, (100)[001], can be inferred from observations of intragrain misorientation axes (MOA) or crystallographic rotation about the [010] axis (Meher et al., 2026). Notably, even where some contribution from dislocation-mediated deformation is observed, hornblende is rarely deformed solely by dislocation creep. While crystallographic preferred orientation (CPO) and recrystallization suggest dislocation creep for most minerals (e.g., calcite, quartz, and olivine), in hornblende, these features seldom arise from alternative mechanisms.
We used electron backscatter diffraction (EBSD) to analyze microstructures in four natural hornblende-rich samples spanning a range of P-T conditions: (1) Mamonia complex, Cyprus (0.5 GPa, ~ 600 °C), comprising mm-scale conjugated kink bands. (2) Koralpe, Austrian Alps (~2.1 GPa, 750 °C), dominated by sigmoidal hornblende porphyroclasts surrounded by smaller, tabular grains. (3) Mayodiya, India (0.78–0.82 GPa, 770–820 °C), containing large grains with high intragrain misorientations and some twinning, and smaller needle-shaped grains with serrated boundaries between large grains. And (4) Koraput, India (0.76–0.84 GPa, 860–883 °C), which exhibits recrystallization of a centimeter-scale porphyroclast with smaller grains with lobate boundaries forming a core–mantle microstructure. By examining both CPO and MOA using detailed EBSD analysis, our goal is to (i) constrain the underlying deformation mechanism in these samples, and (ii) identify temperature-dependent transitions under natural conditions.
The Mamonia sample that experienced the lowest deformation temperatures exhibits deformation through fractures and kink bands, with no evidence of recrystallization. However, the MOA cluster is oriented toward [010], consistent with dislocation glide, suggesting semi-brittle deformation (e.g., Meher et al., 2026). The Koralpe sample exhibits a characteristic recrystallization microstructure, strain-free grains around large and highly strained porphyroclasts, and an MOA clustering around [101], which fits the orientation of (-101) twin planes and suggests twinning-driven recrystallization. The Mayodiya sample exhibits elongated recrystallized grains with MOA clustering around [001], while the porphyroclast exhibits MOA toward [010], again indicating twinning-driven recrystallization. The Koraput sample displays recrystallized grains that are slightly rotated compared to the parent porpyroclast with rotation around [010], consistent with hornblende’s easy slip system, (100)[001].
We infer that at low P-T conditions, hornblende deforms through semi-brittle deformation. At intermediate temperatures (Koralpe and Mayodiya), twinning-driven recrystallization dominates, activated via the (-101)[101] and (100)[001] twinning systems, respectively. At the highest temperatures (Koraput), hornblende undergoes grain-size reduction via dislocation-driven recrystallization. Together, those samples suggest a temperature-controlled transition from semi-brittle to dislocation creep mediated deformation between < 600 to > 850 °C.
Meher, B., Incel, S., Renner, J. and Boneh, Y., 2026. Experimental deformation of textured amphibolites in the semi‐brittle regime: Microstructural signatures of dislocation‐mediated deformation. Journal of Geophysical Research: Solid Earth, 131(1), p.e2025JB031852.
How to cite: Meher, B., Incel, S., Renner, J., Rogowitz, A., and Boneh, Y.: Recrystallization and intracrystalline crystal-plastic deformation of naturally deformed hornblende, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18458, https://doi.org/10.5194/egusphere-egu26-18458, 2026.
Fluids are widely recognized to weaken quartz and to be redistributed during deformation. However, integrated constraints that link water partitioning in natural quartz (fluid inclusions, grain boundaries, and crystal defects) to the evolution of dynamic recrystallization mechanisms (from SGR-dominated recrystallization, through increasing grain-boundary involvement, to GBM-dominated recrystallization) are still limited. Three types of quartz veins that are (sub-)parallel to foliation in the Xuelongshan metamorphic complex record a progressive shift in recrystallization style, providing an ideal natural laboratory to compare deformation mechanisms and fluid reservoirs.
We integrate field observations, microstructure, electron backscatter diffraction (EBSD), fluid inclusion (FI), laser Raman microspectroscopy (LRM), and Fourier-transform infrared spectroscopy (FTIR) to constrain coupled deformation-fluid evolution. All three quartz veins display widespread grain-size reduction and strong crystallographic fabrics. EBSD indicates dominant dislocation creep, with dynamic recrystallization evolving from subgrain rotation (SGR; Type I) through a transitional regime with enhanced grain boundary processes (Type II) toward grain boundary migration (GBM; Type III).
Fluid inclusions are mainly small, irregular, and are preferentially aligned along grain boundaries. Raman spectra from Types I and II quartz reveal a multicomponent fluid system including CO₂, SO₂, CH₄, and CO₃²⁻. FTIR spectra and spatial maps of bulk H₂O and Al-related OH demonstrate a systematic, mechanism-dependent redistribution of water among microstructural reservoirs. In SGR dominant quartz, water exist mainly as inclusion H₂O concentrated along (sub)grain boundaries, and inclusion deformation and rupture promote leakage so that recrystallized grains contain more bulk H₂O than porphyroclasts. Toward GBM, crystal defect OH increases significantly and the relative contribution of inclusion water decreases. In GBM dominant quartz, however, the proportion of defect water declines again as migrating boundaries efficiently sweep out dislocations and reduce the capacity for crystal defect H, despite continued high bulk H₂O.
Overall, our results suggest quartz deformed mechanism transitions are linked not only to the bulk water budget, but more critically to the redistribution of water among microstructural reservoirs (inclusions, grain boundaries, and defects), and to the evolving capacity of the microstructure to store mechanically effective water.
How to cite: Wang, S., Cao, S., von Hagke, C., and Zhan, L.: Coupled interaction between fluid and deformation mechanisms in quartz, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9812, https://doi.org/10.5194/egusphere-egu26-9812, 2026.
Microstructural observations such as grain and subgrain sizes, pole figures (CPO), grain boundary irregularities, and thermometry (opening angles, Ti-in-Quartz) from the mineral quartz have become one of the most reliable proxies for deciphering the mechanical properties, including stress, strain rate, and deformation conditions (temperature) of the continental crust. These relationships were constrained from experimental studies on monomineralic quartz aggregates. Consequently, many field-based studies from continental shear zones focus on analyzing sporadically occurring quartzites and quartz veins. However, crustal rocks are predominantly polymineralic, yet, for simplification, most rheological models rely on homogeneous single-phase approximations. Interactions among multiple mineral phases can disrupt steady-state grain sizes, leading to violations of the piezometric relationships commonly applied to quartz mylonites. Experimental studies further show that polymineralic aggregates deform at significantly lower stresses than their monomineralic counterparts, implying that previous studies have likely overestimated the strength of the crust. In addition, experiments demonstrate that the presence of a secondary phase results in markedly different quartz CPO from that expected in single-phase quartzite. These observations raise an important question: to what extent can quantitative microstructural data from polymineralic rocks be used to infer realistic mechanical properties of the continental crust? Addressing this gap is crucial for developing rheological models that accurately reflect the deformation processes occurring in nature.
In this study, we focus on performing high-resolution EBSD analysis of quartz-bearing mylonites formed from metapelites and granites during thrust-sense shearing along the Main Central Thrust (MCT) shear zone (Western Himalaya, India), which runs along the entire Himalayan Mountain belt. From south to north, these samples record an increasing peak-metamorphic temperature and pressure condition; from 535 °C and 5.8 kbar to 683 °C and 11 kbar. Although strain is inhomogeneously distributed within the ~4 km thick shear zone, an overall increase in deformation intensity is recorded towards the north. The Crystallographic Vorticity Axis (CVA) analysis of quartz reveals monoclinic simple-shear flow kinematics consistent with earlier studies; however, the secondary phases (plagioclase) exhibit pure shear-dominated deformation. Depending on the proportion of the secondary phase (30 to 70%), quartz grains form either continuous layers (monophase domain) or isolated quartz aggregates (polyphase domain). Overall, the CPO pattern in the monophase domain exhibits a transition from a type-II crossed-girdle to an asymmetric type-I pattern, towards the north. The mixed polyphase domain exhibits a random CPO. Within the monophase domains, the fabric strength (M-index, B-index) is higher for the thicker domains (> 163μm). Thereafter, we segregate our analysis into two types of quartz grains: (i) quartz surrounded by quartz grains (Q-Q), and (ii) quartz surrounded by other phases (Q-S). Within a thin section, the Q-Q grains exhibit higher fabric strength, larger recrystallized grain sizes, but lower aspect ratios compared to the Q-S grains. The low-angle boundary density increases towards the north (0.0042 to 0.0095µm-1), but the density is always higher for Q-Q grains than Q-S grains. Our study suggests Q-Q grains can be used for piezometry. We will discuss these results in terms of deformation mechanisms and strain partitioning between mono and polyphase domains.
How to cite: Ghosh, S. and Saha, S.: Quantifying the Rheology of Quartz-bearing Polyphase aggregates deformed under mid-crustal conditions: An EBSD-based Application, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1321, https://doi.org/10.5194/egusphere-egu26-1321, 2026.
Deformation microstructures in mylonites from the Plattengneis Shear Zone (PGSZ), Eastern Alps, provide new constraints on how mechanically anisotropic mid‑crustal rocks accommodate ductile strain. Although the PGSZ exhibits a strong Eo-Alpine N–S stretching fabric, it lacks many macroscopic structures typically associated with amphibolite facies deformation in anisotropic rocks. To determine where and how the high finite strains were localised, we investigate the microstructures of PGSZ mylonites with a focus on the polyphase ‘garnet–mica’ domains. Within these microstructural sites, locally elevated mechanical anisotropies form ideal conditions for nucleating and concentrating deformation structures. Importantly, this contrasts the relatively weaker mechanical strength contrasts observed in PGSZ quartz-feldspar domains, where localised deformation microstructures are scarce. Optical microscopy, back-scattered electron microscopy (SEM-BSE), and synchrotron microtomography (S‑µCT) were used to characterise both 2D microfabrics and the 3D architectures of garnet clusters. With this data, we present newly-described deformation microstructures in the PGSZ, discuss the importance of their spatial distributions, and consider the possible deformation processes involved.
SEM-BSE imaging uncovered a range of micro-scale shear bands, boudinage, and pinch‑and‑swell structures occurring exclusively within garnet–mica layers. Their restriction to these domains reflects the locally elevated mechanical strength contrast between competent garnet grains and weaker white-mica and biotite. Deformation is channelled into mica-rich areas, nucleating localised shear structures and rarely propagating further into quartz–feldspar domains. Garnet undergoes microcrack–induced fragmentation during producing synkinematic redistribution of garnet grains and fragments within the mica-rich matrix regions. This redistribution generates a range of (dis)aggregate cluster morphologies and biotite-infilled boudinage structures that align with the kinematic flow geometries predicted for the established D1 + D2 polyphase deformation history (Hill et al., in review). S‑µCT imaging resolved the 3D geometry of garnet clusters and revealed how fragmentation and redistribution record the cumulative kinematic evolution of the PGSZ. In more detail, the 3D data shows garnet forming complex clusters of both interconnected and disconnected grains elongated in the N-S direction, which are subsequently transposed in the E-W plane, in concordance with the D1 and D2 kinematic flow trajectories.
These results demonstrate that deformation in the PGSZ is highly localised within rheologically complex garnet–mica domains, where the elevated mechanical strength contrasts play a central role in the development of micro‑scale shear structures. Restricted development of shear bands exclusively within garnet-mica microstructural sites contributes to the apparent absence of larger-scale macrostructure development in the PGSZ, demonstrating the importance of a multi-scale approach to structural and kinematic analyses of ductile shear zones. Lastly, the (re)distribution of garnet in the PGSZ is proposed to be controlled by synkinematic growth and disaggregation during polyphase deformation, with the redistribution geometries potentially providing as a means of tracing strain histories in mechanically heterogeneous shear zones.
How to cite: Hill, L., Bestmann, M., Grasemann, B., Fusseis, F., and Dąbrowksi, M.: Mechanical-anisotropy controlled strain-localisation in garnet-mica domains of the Plattengneis Shear Zone (Koralpe, Eastern Alps), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20441, https://doi.org/10.5194/egusphere-egu26-20441, 2026.
Subduction zones are uniquely direct pathways in which originally unconsolidated sediment is conveyed to great depths, all while experiencing continuous shear as it lithifies and metamorphoses. The largest earthquakes on our planet occur within these zones, along with other seismic and aseismic phenomena. The products of these processes are accreted mélanges which provide ‘windows’ into the otherwise inaccessible plate boundary interface at depth. The bulk physical behaviour of these subduction shear zones is controlled by the geometries of the blocks, the proportions of blocks to matrix, and the relative mechanical properties of blocks and matrix. Here we provide a structural and mechanical characterisation of the Chrystalls Beach Mélange, New Zealand, and trace its rheological evolution from the surface to the shallow seismogenic zone. We conducted a detailed 3D macro- and micro-structural investigation coupled with in-situ and laboratory-based rock mechanics to measure sub-block-scale heterogeneities and explain their origins.
The Chrystalls Beach Mélange formed within a Mesozoic Gondwanan–Pacific subduction zone, achieving maximum metamorphic conditions of <600 MPa/<300°C, within the shallow seismogenic zone and below the conditions required for quartz crystal-plasticity. This mélange is composed of subducted seafloor sediments that form decametre- to millimetre-sized blocks of sandstone and chert within a pelitic matrix, mixed with minor exotic blocks of altered basalt. These blocks display overprinting relationships showing a progression from ductile to brittle deformation as they transition from soft sediment to low-grade metamorphosed rock coincident with burial and pervasive shearing.
Four distinct rheological and tectonic regimes were responsible for the structural features we documented:
1) Layer-parallel shortening and fluidisation in the frontal toe of the subduction zone. Unconsolidated interbedded sand, mud, and siliceous ooze experienced ductile deformation producing isoclinal folds and injectites.
2) Layer-parallel extension of poorly consolidated ductile sediments resulted in boudinage and dismemberment in the shallowest subduction channel. This produced blocks with moderate – high aspect ratios, sharp tips, and asymmetric profiles.
3) Continued layer-parallel extension as the blocks lithified and embrittled. Internal stresses transferred from the matrix exceeded the yield stresses of the still-weak blocks, resulting in pervasive brecciation, followed by fragmentation as fluidised matrix injected into these fractures. This produced sub-rounded – sub-angular blocks with low – moderate aspect ratios, blunt tips, and irregular profiles. As blocks continued to indurate to the point that they could no longer be broken by stresses imparted by the matrix, they may still have been broken as they jostled and temporarily jammed the shear zone. At the same time, exotic blocks of basalt entered the mélange as rigid inclusions but underwent progressive weakening during subduction as they experienced brecciation and altered to clay minerals.
4) Localisation of strain previously distributed in the matrix towards more localised shear zones and veins in anastomosing networks.
In-situ Schmidt hammer strength tests show that block margins are systematically weaker than block cores across all lithologies. This is consistent with the increased fracture density towards block margins. As such, mélange blocks within the shallow seismogenic zone display significant internal heterogeneity and should not be considered as two-phase mixtures.
How to cite: Clarke, A. P., Di Vincenzo, S., Weiler, M., Hawemann, F., Mitchell, T. M., and Toy, V. G.: The Other Ductile – Brittle Transition Zone: Syn-Deformational Lithification Within the Shallow Subduction Shear Zone and its Implications for Earthquake Nucleation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11270, https://doi.org/10.5194/egusphere-egu26-11270, 2026.
Lamprophyre dykes are enigmatic volatile-rich, mantle-derived igneous melts that are often contemporary with lithospheric extension. Despite a rich literature on the petrology and geodynamic implications of lamprophyre intrusions into the continental crust, the emplacement mechanisms of these dykes (i.e. structural relationship with the host rock and structures, mode of intrusion, speed of magma ascent, etc) into the dry mid–lower crust is poorly constrained.
In the Jotun Nappe of south-central Norway, Proterozoic gabbro gneisses are overprinted locally by mutually crosscutting lamprophyre dykes, pseudotachylytes (coseismic-derived quenched frictional melts), and mylonitized pseudotachylytes. Mylonitized pseudotachylytes form networks of small-scale (cm- to dm-scale) ductile shear zones, orientated in roughly three sets of orientations, that separate relatively undeformed gabbro gneiss blocks, while pristine pseudotachylytes dissect these blocks and are bounded by the ductile shear zones – akin to observations from Lofoten, Norway (e.g. Jostling Block; Campbell et al., 2020). Pseudotachylytes and mylonitized pseudotachylytes have similar mineral assemblages containing plagioclase, K-feldspar, clinopyroxene, amphibole, Fe-Ti-oxides, with the mylonitized versions also containing garnet porphyroblasts and biotite in addition. Lamprophyre dykes (<1.5m wide), strike dominantly NW-SE, are either undeformed or are incorporated into viscous shear zones that are comprised primarily of mylonitized pseudotachylytes. Many of the undeformed lamprophyres show some amount of viscous shearing localized to <5 cm at the contact with the host rock, otherwise pristine undeformed dykes display primary igneous fabrics and textures. Injection veins of the dyke into the host rock are common, while dyke tips form sharp <45° indentations into the gabbro gneiss. The host rock around jogs is bleached and exhibits numerous small shear fractures filled with dyke material that can easily be misidentified for pseudotachylytes. Lamprophyres have a matrix composed of biotite, plagioclase, dolomite, orthopyroxene, amphibole, Fe-Ti-oxides, and apatite with xenocrysts of orthopyroxene surrounded by a corona of clinopyroxene, amphibole, biotite. Pristine pseudotachylytes crosscut the dykes, offsetting them by up to an apparent ~50 cm and dragging dyke material along the length of the pseudotachylyte surface.
Structural relationships between mylonitized pseudotachylytes and pseudotachylytes suggest that viscous creep along the shear zone network concentrated stresses towards the interior of the gabbro gneiss blocks, which resulted in failure of the blocks and the formation of pristine pseudotachylytes (Zertani et al., 2025). Because the dominant orientation of the lamprophyre dykes is orthogonal to the most dominant orientation of the ductile shear zones, we suggest the lamprophyres exploited transient crustal weaknesses caused by the stress drops during rupturing of the blocks, which created permeably fracture networks for the dykes to ascend through the gneiss. This study demonstrates through field and microstructural observations that lamprophyre intrusions are fundamentally linked to seismicity in the dry mid–lower crust.
Campbell et al., (2020). Nature Communications, https://doi.org/10.1038/s41467-020-15150-x
Zertani et al., (2025). Geophysical Research Letters, https://doi.org/10.1029/2024GL114350
How to cite: Michalchuk, S. P. and Augland, L. E.: Field and microstructural evidence demonstrating the interplay between seismicity and the emplacement of lamprophyres in the dry mid to lower crust, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1099, https://doi.org/10.5194/egusphere-egu26-1099, 2026.
Exhuming shear zones are key structures in the dynamic evolution of orogens. Such shear zones accommodate most of the shear-related exhumation within relatively small rock volumes. This is possible due to major strain partitioning occurring along weak rocks, frequently represented by phyllosilicate-rich rocks. Thus, the study of phyllosilicate-rich mylonites can provide fundamental insights into exhumation mechanisms responsible for the architecture of orogens.
The Hulw Shear Zone in the Saih Hatat Window of Oman (Agard et al., 2010) is one of these exhuming shear zones juxtaposing two subducted continental tectonic units. This tectonic contact experienced sustained shearing, accommodating a delta pressure of circa 0.8 GPa between 1.2 and 0.4 GPa at a relatively constant temperature of circa 400 °C (Petroccia et al., 2025) between 77 and 74 Ma (Ring et al., 2024).
In the field, micaschist belonging to the footwall displays a strain gradient moving toward the contact with the hanging wall, corresponding to a development of a S-C-C’ fabric and a modal enrichment in K-rich white mica and pyrophyllite matched by a progressive increase in the physical interconnectivity of these phyllosilicates. Microstructural analysis suggests that interconnected C planes were formed due to an interplay of fracturing allowing fluid to preferentially flow along the newly formed fractures and precipitating phyllosilicates, and preferential grain boundary sliding and glide of the quartz-phyllosilicate grain boundaries, with additional precipitation of new phyllosilicates in dilatant sites.
Hyperspectral cathodoluminescence highlights different luminescence for the larger (several hundreds of µm) detrital quartz grains, producing a bright signal and containing yielded cracks, and smaller equant quartz grains (less than 70 µm), darker in cathodoluminescence and devoid of cracks. Electron backscatter diffraction analyses suggest that large quartz grains experienced grain size reduction by subgrain rotation recrystallization to form smaller equant grains. Interconnected chains of small quartz grains are located in contact with the phyllosilicates, suggesting preferential recrystallization along these planes.
Transmission Electron Microscope analyses highlight pyrophyllite-muscovite intergrowths at the submicron scale as small as 300-500 nm, with truncated boundaries likely reflecting dissolution and precipitation mechanisms. Muscovite and pyrophyllite appear to deform differently, suggesting that strain partitioning occurred down to the submicron scale.
Summarising, these results suggest that strain localization and weakening of this rock volume was achieved by an interplay of the following mechanisms: I) diffuse microcracking and subgrain rotation recrystallization leading to a finer grain size of quartz, II) synkinematic nucleation of retrograde mineral phases along discrete C and C’ planes, III) preferential recrystallization along the shear planes and IV) dissolution and precipitation processes of phyllosilicates. Concluding, this intimate and polyphase interplay between deformation and metamorphism is responsible for the formation and evolution of exhuming shear zones and the related structure of orogens.
Giuntoli acknowledges financial support of grant N° MUR 2022X88W2Y _002.
References
Agard, et al., (2010). Tectonics, 29(5). https://doi.org/10.1029/2010TC002669
Petroccia, et al., (2025). Journal of Structural Geology, 191. https://doi.org/10.1016/j.jsg.2024.105328
Ring, et al., (2024). Earth-Science Reviews, 250, 104711. https://doi.org/https://doi.org/10.1016/j.earscirev.2024.104711
How to cite: Giuntoli, F., Petroccia, A., Airaghi, L., Précigout, J., Raimbourg, H., and Kulhánek, J.: Insights into deformation mechanisms of exhuming brittle-ductile shear zones (Oman Mountains), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17819, https://doi.org/10.5194/egusphere-egu26-17819, 2026.
Continental subduction is a fundamental stage in the tectonic evolution of convergent plate margins, accommodating the transition from oceanic subduction to continental collision and orogenic build up. Despite the elevated seismic hazard currently posed by destructive earthquakes in continental subduction settings (e.g., Taiwan 1999 Chi Chi MW = 7.6, 2024 Hualien MW = 7.4; Nepal 2015 Gorkha MW = 7.8; Albania 2019 Durres MW = 6.4), our understanding of the processes steering the seismogenic behaviour during prograde subduction of the buoyant, dry, quartzo-feldspathic continental lithosphere remains limited. With the Maria Skłodowska Curie Action SEISMI-COS, we aim at providing quantitative estimates of the stress state, rheology and petrophysical properties of natural deformation zones developed during progressive subduction of continental lithosphere. We will focus on fossil deformation zones exposed in the metamorphic units of Northern Corsica, where the former crystalline basement of the European continental margin has been coherently subducted to and exhumed from different depths during Eocene Alpine orogenesis. Different metamorphic units preserve pristine deformation structures developed at increasing subduction depth, making Northern Corsica the perfect natural laboratory to track the prograde rheological evolution and seismogenic behaviour of the subducted continental lithosphere from shallow seismogenic depths all the way down to conditions at intermediate-depth earthquakes are expected. Preliminary results show that units subducted at progressive depths show different structural features, from pseudotachylyte-bearing fault zones and brittle-ductile shear bands and veins in the outermost continental slices (Corte blueschist units), to high-pressure ductile shear zones (Tenda blueschist phyllonites) involving cycles of fluid pressure variation and veining (Centuri shear zones). This plethora of mesostructures represent the variable seismogenic behaviour during subduction of crystalline continental units subducted at different depths. Field, microstructural, and laboratory analyses will provide us with fundamental insights on how rheology and petrophysics control seismogenic deformation.
How to cite: Ceccato, A., Vannucchi, P., and Molli, G.: SEISMogenic behavIour of COntinental Subduction (SEISMI-COS): insights from rheology and petrophysics of Corsica blueschist and eclogite-facies continental units, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4448, https://doi.org/10.5194/egusphere-egu26-4448, 2026.
The Central Indian Tectonic Zone (CITZ) records the Proterozoic collision between the Bundelkhand and Bastar cratons, with the adjacent Vindhyan Basin preserving evidence of deformation during the N-S convergence across CITZ. Previous interpretations associated the structural features in the Lower Vindhyan Group (LVG) to synsedimentary or seismic processes. To investigate the structural evolution of LVG and its relationship with CITZ, we conducted detailed litho-structural analysis of both the LVG and the Mahakoshal Supracrustal Belt (MSB) within the CITZ, specifically focusing on their mechanical coupling during Mesoproterozoic collisional deformation.
Our field investigations revealed polyphase deformation within the MSB, characterized by three distinct phases: (D1) E-W trending regional foliation (S1) and diversely oriented folds, (D2) E-W oriented steep folds associated with a crustal-scale shear zone along the Son-Narmada South Fault, and (D3) local cross-folds. In the LVG, we report, for the first time, characteristic fold-and-thrust belt features including buckle folds, kink bands, reverse faults, fault-related folds, and notably, 5-20 meters long outcrops of pop-up structures. The deformation style in the LVG was dominantly controlled by a mechanically weak detachment layer comprising the Kajrahat Limestone and Arangi Shale units, which enabled thin-skinned deformation within the overlying competent units of Porcellanite, Kheinjua Shale, and Chorhat Sandstone.
Based on geometric and kinematic analysis, we demonstrate that deformation in the LVG occurred between the D3 event in the MSB and the deposition of the Upper Vindhyan Group (1.5–1.2 Ga). Cross-sectional analysis reveals that the LVG deformation patterns closely mimic sandbox experiments of fold-and-thrust belt evolution, particularly in the sequential development of pop-up structures above a weak detachment horizon. We propose a tectonic model wherein the Vindhyan Basin initially developed as a peripheral foreland basin, followed by northward propagation of deformation through detachment folding mechanisms. The model involves initial northward subduction followed by polarity reversal to southward subduction, explaining both the basin formation and subsequent deformation patterns. Our findings also highlight the significance of thin-skinned tectonics in shaping structural architecture of Central India during the Mesoproterozoic period and reveal the far-field effects of cratonic collisions on basin evolution.
How to cite: Todkar, T., Saha, P., Dutta, D., and Misra, S.: Development of Mesoproterozoic Fold-and-Thrust Belt Structures in Central India: New Evidence from Detachment-Controlled Deformation in the Lower Vindhyan Basin, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1062, https://doi.org/10.5194/egusphere-egu26-1062, 2026.
Shear zones are the areas of localised deformation that contain differential movement within the lithosphere. As such it plays a vital role in the tectonic evolution of continental margins and orogenic belts. The formation, geometry and kinematics of these zones are important for reconstructing the tectonic history. The Indian subcontinent exposes several crustal-scale shear zones, which are major zones of deformation that accommodate the movement of tectonic plates. Among these, the ~30 km wide east-west trending Palghat-Cauvery Shear Zone (PCSZ) forms one of the major transpressive dextral systems in the Southern Indian Granulite Terrain (SGT).
The PCSZ records D1–S1 fabrics that were overprinted by widespread dextral D2 mylonitisation (S2). This structural configuration is altered by brittle to brittle–ductile D3 structures, indicating significant structural heterogeneity in the area. On close examinations, the region is found to preserve evidence of conflicting nature of shears with brittle, brittle-ductile and ductile signatures. The structural complexity of the PCSZ is envisaged as a product of multiple deformation events, tectonic reworking and the overprinting of successive fabrics. The dextral and sinistral senses of shears include σ- and δ-type porphyroclasts, folds, minor faults, and fractures. The ductile dextral shears are characterised by well-developed S-C fabrics, σ- and δ-type porphyroclasts, and mica fish and folds that have oriented nearly in the E-W direction, while both the ductile and brittle sinistral shears are oriented mainly in the NNE to NNW direction. The younger brittle shears such as minor faults and fractures overlap the earlier ductile, which is oriented in the NW-SE direction. The structural analysis of the western PCSZ reveals that two principal stress regimes were in operation in this region. The early N-S compressive stress is associated with the collision of the Madurai and Madras blocks, producing E-W trending foliations, folds and σ- and δ-type porphyroclasts. The later E-W-oriented stress developed due to the transpressional movements leads to the development of conjugate brittle and brittle-ductile shear sets. Thus, the PCSZ form an ideal section to understand how alternating stress orientations and multiple deformations can form conjugate conflicting shear systems, exhibiting the interplay of ductile flow, strain partitioning, and brittle fracture in the deep crustal response to orogenic processes.
How to cite: Nandan Thayyilunnithiri, N. and Chettootty, S.: Geometry of conjugate shears with conflicting shear-sense indicators in the western Palghat–Cauvery Shear Zone, Southern India, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-999, https://doi.org/10.5194/egusphere-egu26-999, 2026.
Variations in rock deformation style across crustal levels are a fundamental topic in structural geology, yet the factors controlling strain localization in long-lived shear zones remain debated. Here we present a detailed field-scale and microstructural study of the >200 km-long Neoarchean Gadag–Mandya shear zone in the Dharwar Craton, southern India, based on a continuous along-transect observations.
The significance of this shear zone lies in its pronounced metamorphic gradient, from dominantly greenschist-facies assemblages in the northern Western Dharwar Craton to amphibolite- and granulite-facies assemblages in the south. This framework allows us to investigate variations in deformation mechanisms and the factors governing deformation style at different crustal levels, dominantly within granitic basement rocks. The shear zone has also been interpreted as a major tectonic boundary related to Neoarchean subduction, making its internal architecture critical for understanding the tectonic evolution of the Dharwar Craton.
Our results show that shear-zone width varies markedly along the transect, from centimetre- to metre-scale zones in the greenschist-facies domain to kilometre-scale zones near the amphibolite–granulite transition. The wider domains are characterized by (i) strong strain localization within granitic intrusions, (ii) the presence of pseudotachylytes, ultramylonites, and hydrous mineral assemblages, and (iii) pervasive overprinting relationships. EBSD data and quartz microstructural analyses indicate overprinting of earlier high-temperature deformation by lower-temperature deformation, particularly in the southern sector, where amphibolite-facies assemblages are locally retrogressed to chlorite–muscovite-bearing fabrics.
Beyond the amphibolite–granulite transition, marked by the appearance of clinopyroxene within the foliation, the main shear zone becomes difficult to trace as a single continuous structure. Instead, multiple metre-scale shear zones with variable orientations are observed, commonly spatially associated with melt-rich domains. These observations highlight the critical role of rheological heterogeneity, melt and fluid infiltration, and inherited thermal structure in controlling shear-zone width, strain localization, and deformation style in Neoarchean crustal-scale shear zones. Rather than recording a simple depth-controlled transition, the Gadag–Mandya shear zone preserves a composite record of spatially and temporally variable deformation processes, in which localized seismic slip, viscous flow, and melt-assisted deformation coexist and overprint each other. This integrated field–microstructural dataset provides new constraints on the mechanical behavior of long-lived lithospheric shear zones in Archean continental crust.
How to cite: Sreehari, L., Urakawa, M., Nakamura, Y., Satish-Kumar, M., and Sajeev, K.: Controls on Deformation Style in a Neoarchean Crustal-Scale Shear Zone: Evidence from the Dharwar Craton, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8935, https://doi.org/10.5194/egusphere-egu26-8935, 2026.
Semi-brittle deformation, characterized by the concurrent operation of cataclasis and crystal plasticity, plays a key role in constraining the strength of the middle crust. While the effects of temperature, pressure, fluid-abundance/pressure, and material properties (e.g., grain size) have been relatively well studied, the role of initial porosity in semi-brittle deformation remains poorly understood. Here, we performed a series of triaxial compression experiments on dry samples of Solnhofen limestone, which has an initial porosity of ~5.6% and an isotropic texture. Experiments were conducted at a range of confining pressures (Pc=30-300 MPa), temperatures (T=25 to 600 °C) and a constant strain rate of 1×10-5 s-1. Under these conditions, Solnhofen limestone mainly deforms in the semi-brittle regime associated with strain hardening, and brittle fracturing only occurs at low pressures (Pc≤50 MPa) and T <200 °C.
The strength, expressed as differential stress at a given strain, of Solnhofen limestone varies with imposed conditions. At 5% strain, the strength decreases with increasing temperature at all investigated pressures. In contrast, the pressure dependence of strength is temperature sensitive. At T =400 °C, the strength decreases significantly with increasing pressure from 30 to 300 MPa, in contrast to the positive pressure dependence observed for low porosity (~0.5%) Carrara Marble in the similar semi-brittle regime. This pressure sensitivity becomes less pronounced at 600 °C. Changes in porosity, determined from the pre- and post- deformation measurements, show that dilation and compaction are closely related to deformation mode. The extent in sample compaction correlates with the deformation ductility and becomes more marked with increasing temperature and pressure.
We speculate that the observed negative pressure dependence of strength during semi-brittle deformation arises from the presence of initial porosity and can be explained by the increasingly dominant role of plastic pore collapse. This hypothesis is supported by an additional experiment conducted at 400 °C, in which samples pre-compacted at 300 MPa for 3 h and subsequently deformed at 30 MPa exhibited higher strength than samples deformed directly at 30 MPa without a pre-compaction stage. Ongoing microstructural investigations will provide a basis for developing a microphysical model to better interpret deformation processes in rocks with intermediate porosity in the semi-brittle deformation regime.
How to cite: Feng, W. and Brantut, N.: Semi-brittle deformation of Solnhofen limestone: Initial porosity effects on strength, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12335, https://doi.org/10.5194/egusphere-egu26-12335, 2026.
Pseudotachylyte is a quenched coseismic frictional melt. As such, pseudotachylyte may provide invaluable information on the processes occurring on fault at hypocentre depths. Of particular interest are pseudotachylytes hosted in high-grade rocks, as they may record seismic ruptures propagated in the middle and lower crust. However, pseudotachylyte in high-grade rocks may also result from shallow deformation after uplift, thus constraining ambient conditions of faulting is crucial although not trivial.
The mineralogy of pseudotachylyte includes microlites crystallized during melt quenching, glass recrystallization products and, for deep-seated pseudotachylytes, minerals reflecting re-equilibration to the ambient metamorphic conditions. In absence of ductile deformation of pseudotachylyte promoting re-equilibration, the estimate of P–T conditions is typically based on the microlites. For example, the presence of microlitic ‘cauliflower’ garnet has been interpreted to reflect high-grade ambient conditions of faulting. However, Papa et al. (2023), described cauliflower-garnet-bearing pseudotachylytes hosted in granulite facies garnet-sillimanite-rich gneiss from Calabria and proposed shallow faulting conditions based on radiometric dating, suggesting that garnet can be transiently stable during quenching at shallow conditions.
Here we quenched at room conditions superheated (>2100 °C) melts produced by instantaneous laser-heating of the same peraluminous gneisses hosting the natural pseudotachylyte and compare the microlite population of the experimental glass with the microlites of the natural pseudotachylyte. Both the natural pseudotachylyte and the experimental glass contain: (i) acicular-shaped corundum microlites; (ii) sillimanite/mullite microlites overgrowing sillimanite clasts; (iii) skeletal-, dendritic-shaped spinel microlites, spatially associated with garnet, epitaxially nucleating on sillimanite/mullite and dispersed in the glass; (iv) microlitic cordierite, present in the natural pseudotachylyte as spherulitic aggregates and in the experimental glass as plumose microlites in melt-filled fractures of the wall-rock garnet; (v) newly formed euhedral rims of garnet epitaxial on garnet clasts and wall-rock garnet. The observed microlites crystallized during melt quenching following the same sequence, with slight differences due to the faster cooling rate of the experiments.
By comparing natural pseudotachylytes and experimentally produced analogues, we show that the mineralogy of natural microlites is essentially constituted by high-melting point phases and it is controlled by the local availability of chemical constituents and nucleation seeds (i.e. host-rock clasts). The experiments also prove that garnet can crystallize during quenching even at room conditions if seeds are available and the melt has the right composition. This observation calls for caution when using the mineralogy of pseudotachylytes, and in particular the presence of cauliflower garnet, to infer the depth of faulting. Finally, the melting experiments under static conditions highlight the relevance of thermal fracturing as deformation process aiding pseudotachylyte formation.
Papa et al. (2023), Lithos 460, 107375
How to cite: Pennacchioni, G., Toffol, G., Slupski, P., Pennacchioni, L., Wirth, R., Schreiber, A., and Cerwenka, G.: Microlites of natural and experimental peraluminous pseudotachylytes: a comparison, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10360, https://doi.org/10.5194/egusphere-egu26-10360, 2026.
In this study, we conducted rotary shear experiments to examine the frictional stability of Olivine and Quartz gouges over a range of temperatures (25–350 °C), slip velocities (100 μm s−1 to 1 mm s−1), and under a constant normal stress of 50 MPa. The two minerals exhibit contrasting stability behaviors: Olivine remains frictionally stable at room temperature but develops pronounced stick–slip instabilities at 350 °C. This unstable behavior persists at the velocity of 1 mm s−1, although peak friction decreases slightly, indicating minor weakening. Quartz, by contrast, displays repeated stick–slip events at 25 °C, with stress drops that grow progressively larger with slip and are accompanied by continuous compaction, consistent with ongoing grain crushing. At 350 °C, Quartz behavior evolves from strong stick–slip at low velocities to stable sliding at higher velocities. These observations suggest that frictional stability is likely governed by a competition between the rate of tectonic loading, the specific healing kinetics, and the localization state of each mineral.
How to cite: Shahabi, H., Rattez, H., Tesei, T., Gomila, R., and Di Toro, G.: Contrasting frictional Stability of Olivine and Quartz: Rotary ShearExperiments under Hydrothermal Conditions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-23091, https://doi.org/10.5194/egusphere-egu26-23091, 2026.
Eclogites are a major lithology of the subducting oceanic crust, and the strength contrasts between and with lithologies such as blueschists, serpentinites and peridotites, at depths, is likely what commands the timing and style of HP rocks exhumation, within subduction zones (Agard et al., 2016). These contrasts also influence the roughness and stress at the interface between the subducting slab and the overlying mantle wedge (Agard et al., 2018), and therefore may play a role in the stress relaxation and intermediate depths earthquakes sequences. Deformation mechanisms of the main minerals of eclogite, pyroxene and garnet, have been studied individually under high pressure and temperature. The rheology of eclogites themselves has received some attention using high pressure experiments (e.g. Zhang and Green, 2007, Farla et al., 2017, Rogowitz et al, 2023, Molines et al., EGU25-5696). These works and numerical models (e.g. Yamato et al, 2019, Angiboust et al, 2024) underline the importance of the interplay between brittle vs. ductile mechanisms in eclogites rheology at experimental strain rates. The garnet vs. pyroxene volume fractions are expected to have a major effect on brittleness and strength, since the spatial contiguity of the strongest component, or connectivity of the weakest component, may lead to transitions in the control of the deformation.
Until now the effect of shear strain on phases connectivities under GPa pressures has not been quantified, while it is one path to achieve connections between strong or weak domains. Here, we will present results from torsion experiments on two-phase aggregates of garnet and pyroxene as a proxy for eclogites, with garnet fractions from 15% vol. to 85% vol. We use in-situ absorption contrast tomography at the PSICHE beamline (synchrotron SOLEIL), under pressures of 2 to 5 GPa and temperatures of 850°C, to characterize quantitatively the fabric/microstructure of the aggregates under increasing shear strain (up to ca. 5).
We will discuss these microstructural quantifications with respect to 1) recent in-situ mechanical measurements in the same aggregates compositions, by Molines et al. (EGU25-5696), and 2) similar in situ characterizations during torsion experiments of serpentine+olivine aggregates – hence a different strength contrast between phases – by Mandolini et al. (e.g. EGU25-13729).
How to cite: Hilairet, N., Molines, C., Mandolini, T., Chantel, J., Addad, A., Fadel, A., Troadec, D., Le Godec, Y., Turpin, Z., and Guignot, N.: Connectivity and fabric evolution with strain in eclogites : in-situ X-ray tomography under UHP conditions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18356, https://doi.org/10.5194/egusphere-egu26-18356, 2026.
Layer-parallel shortening of multilayer rocks results in the formation of folds. Using linear stability analysis, we obtain a growth rate curve. It allows us to determine the dominant wavelength during the initial stage of viscous folding. We derive an analytical expression for the growth rate curve of a single layer embedded in an anisotropic host, including confinement effects. The analytical results obtained for an anisotropic medium are compared to the growth rates obtained numerically for the corresponding cases of a finely laminated host. These cases split into two groups depending on whether a low- or high-viscosity layer borders perturbed interfaces of the central layer. However, in the limit of fine layering, their arithmetic mean tends to the results obtained for the anisotropic host. In search of an explanation, we calculate growth rates of the laminated host case analytically and show where the anisotropic approximation breaks down.
Next, we investigate an anisotropic rock medium under shortening along the anisotropy direction, with a locally perturbed axis of anisotropy orientation. It is a mean-field upscaled approximation to a multilayer system, which can tackle arbitrarily perturbed layer interfaces. In addition to the analytical approach, we use numerical simulations to study folding instability in such multilayer systems based on the direct (discretely layered medium) and upscaled (anisotropic medium) approaches. As a limiting case, we find the evolution of chevron fold amplitudes and study the convergence of the bilaminate dominant eigenmode to that obtained for the anisotropic medium.
Those results shed light on the limitations of the effective anisotropic models of layered rock systems, and provide a framework for more accurate mean-field approximations.
The work was supported by the National Science Centre, Poland, under research project “Numerical and field studies of anisotropic rocks under large strain: applying micro-POLAR mechanIcS in structural geology (POLARIS)”, no UMO-2020/39/I/ST10/00818.
How to cite: Gamdzyk, J. and Dąbrowski, M.: Viscous folding of multilayer rocks under layer-parallel shortening: discrete layering vs. anisotropic models, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15192, https://doi.org/10.5194/egusphere-egu26-15192, 2026.
The South-Central Zone of the Damara Belt records a history of intense, complex deformation resulting from the collision between the Congo and Kalahari cratons during the Pan-African Orogeny. Structural models have typically focused on multiphase deformation with inherent changes in the stress field and, to a lesser degree, on progressive deformation driven by a stress field with less variation. One example of the latter is a model that separates the crust in the South-Central Zone into two structural domains, a higher crustal level and a deeper crustal level. This allows the existence of orthogonal fabric domains resulting from different strain fields within the same orogenic zone, without the need for major changes in the regional tectonic stress orientation.
To date, geological maps and cross-sections have been used widely to graphically present the geological geometries of large areas in the Central Damara Belt. However, unlike 2D geological maps and sections, 3D models are more representative, providing additional insight to complex geometries and structural relationships. These complement and test traditional interpretations that often fail to account for the complexity and uncertainty of geological geometries.
This study provides the first large-scale 3D lithostructural modelling of the deeper structural levels of the South-Central Zone of the Damara Belt, south and east of the Rossing Dome. The different rock units in this area display kinematic and geometric features that support large scale constrictional-type strain characteristics and top-to-the-southwest displacement. In addition to field mapping data, digital elevation models, satellite imagery and published geological maps were used to delineate the regional geometry of folded lithological units. The resulting 3D model contributes to a better understanding of the deformation of the deeper crust during the collision of continental fragments and the development of large-scale fold geometries.
How to cite: Tuitz, C. and Uken, R.: Regional-scale 3D modelling of deep-crustal constrictional strain geometries within the Central Damara Belt, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21778, https://doi.org/10.5194/egusphere-egu26-21778, 2026.
Geophysical observations combined with detailed petro-structural analyses conducted in the field and in the laboratory indicate that « brittle » deformation occurs within subduction zones in rocks that are otherwise expected to deform in a « ductile » manner under the associated pressure–temperature conditions. These brittle events are most commonly localized in regions where metamorphic transformations are predicted to occur. Because such reactions may induce substantial changes in density and strength, they are frequently invoked as a primary mechanism driving the ductile-to-brittle switch in subducting rocks. However, the physical processes that link metamorphic transformations to changes in deformation style remain incompletely understood.
This contribution addresses this issue through the emblematic example of the granulite-to-eclogite transformation exposed at Holsnøy (Bergen Arcs, Norway). We combine field-based structural and petrological observations with numerical modeling developed over the past several years to investigate the mechanical and rheological consequences of this transformation.
We specifically examine whether eclogitization necessarily initiates along pre-existing brittle precursors or whether the reaction itself can trigger faulting, how the transformation propagates through the rock, and the extent to which the inherited granulitic foliation influences reaction localization. We further discuss the mechanisms leading to the formation of eclogitic shear zones as opposed to static eclogites (commonly referred to as eclogite fingers). Finally, we assess the relative roles of fluid availability and far field stress in controlling the spatial distribution and mechanical impact of the reaction.
By confronting field observations with numerical modeling, this presentation aims to show that the answers to these questions may not be unique, and that much remains to be done to fully understand the impact of metamorphic reactions on the rheological behavior of rocks.
How to cite: Yamato, P., Baïsset, M., Cochet, A., Duretz, T., Schmalholz, S., Podladchikov, Y., and Labrousse, L.: Linking metamorphic transformations and the brittle–ductile transition: Insights from numerical modeling of the granulite-to-eclogite transformation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17384, https://doi.org/10.5194/egusphere-egu26-17384, 2026.
The evolution of rift-related fault systems is strongly influenced by pre-existing structural weaknesses, yet the role of their spatial arrangement in shaping fault patterns during multiphase non-coaxial extension remains unclear. To address this, we conducted a series of brittle–viscous analogue experiments to examine how left-stepping and right-stepping configurations of parallel weaknesses affect fault propagation, linkage, and orientation under two successive phases of orthogonal and oblique extension. We find that (1) fault orientation is jointly controlled by extension direction, weakness orientation, and the relative positioning of pre-existing weaknesses; (2) left-stepping and right-stepping systems, though geometrically identical, evolve differently under the same boundary conditions—left-stepping configurations develop greater fault linkage, strike diversity, and overall structural complexity; and (3) inherited weaknesses reduce the direct control of extension direction on fault strikes, implying that present-day fault patterns may not simply reflect paleostress orientations. Furthermore, our results suggest that apparent strike variability in multiphase rift systems can arise without local stress rotation, emerging instead from the interaction between regional stress and the spatial arrangement of inherited structures. Mechanistically, left-stepping configurations behave analogously to releasing steps in strike-slip systems, promoting more distributed deformation and strike-slip components, whereas right-stepping systems resemble restraining steps, producing simpler and more localized fault networks. Our findings provide new insights into how pre-existing structural configurations modulate rift fault evolution, highlighting the need to consider structural inheritance when reconstructing tectonic histories of multiphase extensional basins.
How to cite: Zhang, Y. and Huang, L.: Fault System Evolution Controlled by Weakness Arrangement under Multiphase Non-Coaxial Extension：Analogue Modeling Insights, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-106, https://doi.org/10.5194/egusphere-egu26-106, 2026.
Posters virtual: Wed, 6 May, 14:00–18:00 | vPoster spot 1a
EGU26-20066 | ECS | Posters virtual | VPS30
Ductile to brittle tectonic evolution of the Achankovil Shear Zone, Southern Granulite Terrane – Constraints from statistical analysis of fabric and paleostress inversionWed, 06 May, 14:00–14:03 (CEST) vPoster spot 1a
The Achankovil Shear zone (AKSZ), juxtaposing the Trivandrum Granulite Block (TB) and the Madurai Granulite Block (MB) of the Southern Granulite Terrane (SGT), represents a paleo-suture zone related to the late Neoproterozoic to early Cambrian Gondwana assembly (Rajesh et al., 1998; Praharaj et al., 2021). The polyphase deformational history of the Achankovil Shear Zone (AKSZ) reveals progressive transition from a ductile to brittle deformation regime concomitant with high-grade granulite facies metamorphism and subsequent cooling–exhumation of the granulites. Progressive evolution of the state of stress and the variation of crustal dynamics during the ductile to brittle deformation regime transition, and the genetic link between these two contrasting episodes, if any, have been evaluated from statistical analysis of solid-state fabric and kinematic analysis of brittle fractures.
Three ductile deformation phases, D1, D2, and D3, and associated solid-state fabrics, i.e., S1, S2, and S3, are discernible at the mesoscopic scale. S1 fabric is gneissic in character and is only preserved in strain shadow regions. Elsewhere, S1 is transposed along the later S2 fabric, which is axial planar to fold on S1. Prevalence of high-temperature deformation conditions during D2 and D3 deformation stages is manifested by the presence of S2-parallel stromatic leucosomes and diatexite leucosome along dilatant S3 fabric, developed parallel to the axial planes of fold on S2. Significant simple shear component during D3 deformation is evidenced from the asymmetric nature of S2 folds, asymmetric porphyroclast tail and other shear sense markers. Eigen vector analysis reveals a change of maximum eigen vector, i.e., pole to the mean foliation, from NW-SE (D1: 311⁰/62⁰ NE) to N-S (D3: 187⁰/80⁰ W). The maximum eigen vector of D2 (125⁰/53⁰ SW), though similar in trend with D1, shows a reversal of dip direction. Fabric shape analysis reveals a progressive change from girdle to a strong clustered distribution of solid-state fabric from D1 to D3 deformation regime, suitably accounting for intense ductile shearing and transposition of earlier fabric during the D3 deformation stage.
Minor conjugate faults are ubiquitous at different locations along the AKSZ. Dihedral angle of ~60 or less for these faults suggests a shear or hybrid fracture origin, diagnostic of a compressive stress regime. Also, the observed slip of striations on slickensides suggests a consistent oblique reverse kinematics. Fault kinematic and paleo-stress analysis further reveals two distinct stress regimes with NW-SE and NE-SW directed maximum compressive stress (s1). Stress ratios for these faults imply a compressional to transpressional tectonic regime. Superposition of the slip tendency of NW–SE directed stress tensor over NE–SW directed stress tensor and vice-versa suggests that the NW-SE stress tensor precedes the NE-SW stress tensor during a progressive brittle deformation regime. Summarily, the cooling and exhumation and the switch over from ductile to brittle deformation regimes of the granulites took place under a compressive stress field developed during terrane accretion along the AKSZ. The brittle faults seemingly result from the relaxation of the orogenic far-field stress.
How to cite: Manilal Girija, A. and Bhadra, S.: Ductile to brittle tectonic evolution of the Achankovil Shear Zone, Southern Granulite Terrane – Constraints from statistical analysis of fabric and paleostress inversion, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20066, https://doi.org/10.5194/egusphere-egu26-20066, 2026.
EGU26-17196 | ECS | Posters virtual | VPS30
Progressive evolution of paleostress in the Hutti-Maski Greenstone belt, Eastern Dharwar Craton, southern IndiaWed, 06 May, 14:33–14:36 (CEST) vPoster spot 1a
This study presents a paleostress reconstruction of the metavolcanic and granitoid rocks of the Hutti-Maski greentone belt, Eastern Dharwar Craton (EDC), southern India, aimed at evaluating progressive changes in the regional stress field at ca. 2.5 Ga. Paleostress was constrained using quartz vein orientations, Anisotropy of Magnetic Susceptibility (AMS) fabrics, and fault–slip data from metavolcanic and granitoid rocks.
Anisotropy of Magnetic Susceptibility (AMS) data from granitoids reveal a dominant NNW–SSE–striking magnetic fabric developed during earlier D1/D2 deformation. Paleostress analysis using vein orientations of dilational quartz veins in the granitoids yields an apparent NE–SW compressional stress field. However, kinematic analysis demonstrates that these veins in the granitoids were formed by dextral simple shear along the pre-existing NNW–SSE–oriented fabric under a regional N–S–directed D3 compression. From previous studies it is already well established that this regional N–S–directed D3 compression was responsible for D3 folds with E–W–striking axial planes found in different parts of EDC. N-S-oriented dilational quartz veins in the metavolcanic rocks of this greenstone belt were also formed due to this N-S oriented D3 compression. This interpretation is further supported by comparable stress ratio values obtained from three-dimensional Mohr circle analyses of vein populations in both lithologies.
Fault–slip analysis of displaced veins in granitoids reveals a late-stage NNE–SSW compressional stress field, indicating localised brittle deformation during the final stages of D3. This late brittle overprint is interpreted as resulting from late-D3 brittle deformation during the cratonization of the Dharwar Craton at approximately 2.5 Ga.
Therefore, this study demonstrates that there are pitfalls in the direct evaluation of paleostress using only vein orientations and that it is crucial to integrate kinematic constraints with vein orientation data during paleostress analysis of dilational veins.
How to cite: Goswami, S. and Mamtani, M. A.: Progressive evolution of paleostress in the Hutti-Maski Greenstone belt, Eastern Dharwar Craton, southern India, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17196, https://doi.org/10.5194/egusphere-egu26-17196, 2026.
