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.
Strain hardening is a key feature observed in many rocks deformed in the so-called ``semi-brittle'' regime, where both crystal plastic and brittle deformation mechanisms operate. Experimental observations in calcite aggregate show a negative correlation between strain hardening rate and microcrack density. Strain hardening is typically caused by accumulation of unrelaxed elastic stresses, for instance due to dislocation storage or frictional sliding, but the role of tensile cracks in that process is not clear. Here, I will first summarise key experimental observations in calcite aggregates, documenting the co-evolution of microstructural features as a function of strain, and then propose a simple microphysical hardening model that couples tensile microcracking with dislocation storage. The model relies on viewing tensile cracks as free surfaces that absorb dislocations, thus reducing the dislocation storage rate and the hardening coefficient. The model captures important qualitative features observed in calcite marble deformation experiments: pressure-dependency of strength in the ductile regime, and a reduction in hardening linked to an increase in crack growth with decreasing confining pressure. Although very promising at a conceptual level, the model has limitations and needs to be tested more systematically before it can be used to make geological predictions of strength in the semi-brittle regime.
How to cite: Brantut, N.: Semi-brittle flow of rocks: Cracks, dislocations and strain hardening, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17888, https://doi.org/10.5194/egusphere-egu25-17888, 2025.
Pseudotachylytes, quenched melts from frictional heating, and ultracataclasites, comminution of host rock, are considered direct evidence of coseismic slip. In hydrated systems, fluid-rock interactions can influence the nucleation and propagation of these earthquake-induced structures by facilitating element mobility and fault zone weakening. We conducted 2D microstructural and geochemical analyses on a series of ultracataclastic veins hosted in a deformed granodiorite on Naxos, Greece, to investigate potential interactions between physical and chemical processes along rupture paths. Naxos is a classical Miocene Cycladic metamorphic core complex, defined by a central migmatite core, with fluids introduced during peak metamorphism and subsequent brittle deformation. An I-type granodiorite was syn-tectonically emplaced, cooling rapidly from crystallization (650-680°C) at c. 12 Ma to <60°C by c. 9 Ma. The extensional Naxos-Paros Detachment System, active between c.12-9 Ma, dissects the pluton, producing a strong N-S stretching lineation and SCC' fabric generating top-to-N kinematics. Host rock from the immediate footwall of the detachment is composed of a coarse-grained (50 μm-2 mm) matrix, primarily composed of albite (35%), quartz (25%), orthoclase (16%), and biotite (12%). Fine-grained (5-60 μm) anastomosing ultracataclastic veins of the same composition intersect the host rock, with the thickest veins (7 cm) occurring sub-parallel to host rock foliation. Electron backscatter diffraction (EBSD) mapping of albite, orthoclase, and quartz targeted foliation-subparallel veins tips and host porphyroclasts crosscut by the veins. Evidence for minor crystal plasticity is observed as continuous to heterogeneous lattice distortion with an average misorientation of 03° within the host clasts, increasing to 15° towards clast rims and microfractures. The localization of microfractures emanating from the vein tips coupled with the spatial relationship between lattice distortions and microfractures, indicates that strain accommodation via crystal plasticity is linked to brittle deformation. This suggests that cataclasis is the primary deformation mechanism related to the propagation of ultracataclastic veins, which is supported by EBSD orientation data of fine-grained (<60 μm) fragments surrounding clasts (80-120 μm) of the same phase. The fine-grained populations are randomly oriented, with low internal misorientations up to ~10°, and no crystallographic relationship to the host porphyroclasts. Scanning electron microscopy (SEM) imaging highlighted aggregates of fine-grained albite (2-35 μm) along the vein margins with patchy zonation near microfractures and grain rims. A cuspate phase boundary between the albite grains and bordering orthoclase host clasts (2 mm) is characteristic of a dissolution-precipitation reaction front. Electron microprobe mapping of the phase interface reveals a K-depleted rim, 3 μm wide, along orthoclase clast margins, decreasing from ~13.6 wt% to 9.5 wt%. Inclusions of albite grains and Na-enriched zones, increasing to 2.6 wt% from 0.4 wt%, related to microfractures within the orthoclase clasts are present up to 55 μm away from the interface. Based on these observations, we propose that the interplay between cataclasis and interface-coupled reactions localized weakening, creating a feedback loop that promoted fracture propagation and drove continued injection of cataclastic material within the granodiorite. Our results demonstrate the impact of fluid-rock interactions on fault zone evolution and rupture conditions.
How to cite: Rolfe, O., Dubosq, R., Schneider, D., and Grasemann, B.: Feedback between cataclasis and interface-coupled reactions in ultracataclastic veins: insights from the Naxos granodiorite, Greece, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-498, https://doi.org/10.5194/egusphere-egu25-498, 2025.
Phyllosilicates play a key role in controlling the rheology of shear zones, the style of deformation and the syndeformational fluid budget. The latter, including aqueous fluids released by metamorphic reactions, can transiently increase pore pressure and trigger cyclic switching between brittle and ductile deformation conditions. Unfortunately, it is still unclear how these processes act together in exhuming low-grade shear zones in a continental collisional framework.
To tackle this scientific question, we studied the top-to-the N/NE Hulw Shear Zone in the Saih Hatat Window of Oman. This shear zone is responsible for part of the exhumation of the subducted continental crust, but its pressure-temperature (P–T) and deformation behaviour remain largely unconstrained. Its footwall is mostly composed of metapelites, with a modal enrichment in K-rich white mica and pyrophyllite, matched by a progressive increase in the physical interconnectivity of phyllosilicates along its internal strain gradient. Similarly, marbles in the hanging wall evolve from mylonitic to ultramylonitic towards the core of the shear zone.
In the Hulw Shear Zone coexist two opposite deformation behaviours, with ductile deformation accommodated preferentially along laterally continuous phyllosilicate-rich bands and brittle deformation in the form of hybrid/dilational hydroshear veins found regularly at the outcrop. To constrain the metamorphic conditions of dehydration reactions during the exhumation path, we integrated forward thermodynamic modelling with Raman Spectroscopy on Carbonaceous Material, and K-rich white mica multiequilibrium barometry on a representative mylonite from the shear zone footwall. The resulting metamorphic evolution of the Hulw Shear Zone started from peak conditions of 300-350 °C and 0.9-1.2 GPa, followed by the main shearing event at 350-420 °C and 0.6-0.9 GPa and ended with sustained shearing at low-P conditions (350 °C, 0.3-0.4 GPa). Therefore, the Hulw Shear Zone accommodated progressive shearing while exhuming its footwall from epidote blueschist to low-pressure greenschist facies conditions.
Decompression-driven fluid-gain reactions facilitated the growth of synkinematic phyllosilicates, which created a pervasive and interconnected K-rich white mica and pyrophyllite network that promoted strain localisation, causing significant mechanical weakening as well as the potential for discrete and compartmentalised fluid cells within the mylonitic foliation. Brittle structures formed due to aqueous fluid release by metamorphic dehydration reactions close to peak-P conditions (e.g., kaolinite-out reaction) or along the exhumation trajectory, transiently increasing pore pressure and triggering brittle failure, resulting in coeval mylonitic foliation and crack-seal hybrid veins.
Our findings support the idea that sustained shearing was promoted by synkinematic growth of K-rich white mica and pyrophyllite and by cyclic switching between brittle and ductile deformation conditions. Therefore, the studied structures might also represent a record of deep episodic tremors and slow slip events during exhumation-related tectonics in the accretionary wedge above the subduction interface of the Oman continental lithosphere.
How to cite: Petroccia, A., Giuntoli, F., Pilia, S., Viola, G., Sternai, P., and Callegari, I.: Evidence for strain localisation and episodic tremors and slow slip events in exhuming continental shear zones (Saih Hatat Window, Oman), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-618, https://doi.org/10.5194/egusphere-egu25-618, 2025.
The collapse of orogenic belts is commonly thought to involve viscous flow in a mid-crustal channel, and manifests as extensional faulting in the upper crust. Recent observations in some orogenic belts have indicated a power-law relationship between local elevation and extensional strain rates. Simple mechanical considerations predict that the flow of the weak crustal layer beneath these belts is driven by topographic gradients, suggesting that the observed extension is linked to this flow. To test this hypothesis and examine the temporal evolution of collapsing orogenic belts, we developed a 2-D numerical model simulating how topography-driven viscous flow in the weak mid-lower crust induces, and is affected by, orogenic belt extension. Our results show that flow of a weak mid-lower crust triggers orogenic collapse via normal faulting, provided mountain height exceeds a critical threshold (hmin). The simulated faults form within the highest regions of the orogen, where the weak crustal layer flow originates. Once the mountain collapses so much that its height falls below hmin, extension ceases, where hmin depends on both the thickness of the weak layer and the strength of the upper crust. Additionally, we find that collapse rates increase with hotter and thicker weak channels, taller orogens, and weaker upper crustal faults, while stronger upper crust restricts fault distribution, concentrating deformation within smaller areas, leading to a core complex extension mode. Finally, a strong agreement between our numerical and analytical (detailed in companion abstract: Dawood et al. 2025 EGU General Assembly 2025) models demonstrates that orogenic collapse rates and their temporal evolution are jointly controlled by the brittle and ductile properties of the continental crust.
How to cite: Aharonov, E., Dawood, R., and Olive, J.: How Coupled Brittle-Ductile Deformation Controls the Rates and Temporal Evolution of Orogenic Collapse, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18427, https://doi.org/10.5194/egusphere-egu25-18427, 2025.
The relationship between the long-term strength of the lithosphere and seismic hazard has remained a fundamental, yet open question in geosciences. The lithosphere's long-term rheology controls its deformation patterns, playing a crucial role in understanding the spatial and temporal distribution of seismicity in a given region. One of the primary factors influencing the rheological state of the lithosphere is its thermal regime, which is strongly affected by the heterogeneous properties of both the crust and the lithospheric mantle, as well as by the three-dimensional interactions between deeper and shallower domains.
To explore how long-term off-fault rheology influences the spatial distribution of seismicity, we leverage extensive geophysical data from Central and Southern California, a region where the San Andreas Fault represents a significant seismic hazard. Previous thermal models of the area have not converged on a consistent thermal structure for the lithosphere, resulting in uncertainties in the rheological models based on them.
Our 3D thermal model is built using a data-integrative approach that incorporates recent tomographic models and a detailed, heterogeneous crustal architecture drawn from prior community efforts. Furthermore, our model fits the general pattern of observed surface heat flow in the region. The lower boundary condition in our 3D model -temperature at 70 km depth - is based on an integrated geophysical – petrological inversion within a self-consistent thermodynamic formalism of Rayleigh and Love surface-wave dispersion curves (0.5 x 0.5 degree lateral resolution), supplemented by other geophysical data and models: satellite data, surface heat flow and average temperature, topography, Moho depth, P-wave seismic crustal velocities, and sedimentary thickness.
Notably, our model is consistent with major regional tectonic features, such as the fossil Monterey microplate slab, which is responsible for the well-known high-velocity Isabella Anomaly. We discuss the implications of this anomaly, focusing on the dehydration of the slab and its potential role in seismogenesis, especially in the creeping section of the San Andreas Fault near Parkfield.
How to cite: Gómez García, Á. M., Jiménez-Munt, I., Cacace, M., Scheck-Wenderoth, M., Root, B., Clemente-Gómez, C., Fullea, J., Lebedev, S., Xu, Y., and Becker, T.: 3D Lithospheric-scale thermal model of central and southern California, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6407, https://doi.org/10.5194/egusphere-egu25-6407, 2025.
Metamorphic transformations are often associated with strain localization which can be observed in the field either as ductile zones, or brittle, and possibly seismogenic, structures. Deformation experiments in the laboratory not only replicate such features but also allow us to measure the associated weakening. In all these contexts, reaction overstepping and disequilibrium metamorphism appear to be the rule. Reaction rates are usually very fast once transformation initiates, in particular within highly stressed and strained volumes where the produced mechanical work is sufficient to overcome kinetic barriers. New very fine-grained and dense phases nucleate in conditions where mineral growth is impeded. Understanding how heterogeneous nucleation, along with changes in density and viscosity, affects the rock's strength during a metamorphic transformation appears therefore critical.
In that prospect, results of a 2D numerical study in which reaction products preferentially nucleate in areas of high strain energy are presented. Special attention is given to the weakening or hardening effects induced by these transformations, as well as to the deformation patterns within the model. Results of our numerical study are then discussed in the light of experimental data obtained at comparable pressure-temperature-strain rate conditions.
We show that rock weakening is not only linked to the strength of the reaction products. Indeed, (1) densification alone can generate sufficient stress to induce plastic yielding of the surrounding matrix, even when the nuclei are stronger, and (2) heterogeneous nucleation controlled by mechanical work has greater influence on the rock’s strength than the intrinsic properties of the reaction products. Weakening is primarily driven by the initiation and propagation of plastic shear bands between the closely spaced nuclei that generate significant stress concentration in their vicinity. This study highlights the importance of transformational weakening that results from fast heterogeneous nucleation in rocks close to their brittle-ductile transition.
How to cite: Baïsset, M., Yamato, P., Duretz, T., Labrousse, L., Gasc, J., and Schubnel, A.: Weakening induced by phase nucleation: from experiments to numerical models, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9848, https://doi.org/10.5194/egusphere-egu25-9848, 2025.
Deep crustal shear zones, fundamental to the dynamics of terrestrial plate tectonics, exhibit complex processes of initiation and evolution that are yet to be comprehensively quantified across both long and short temporal scales. Conventionally, thermo–mechanical models posit that crustal rock behaviour is dominated by monomineralic aggregates undergoing processes like intracrystalline plastic deformation by dislocation creep. However, high-pressure and temperature conditions in crustal rocks involve minerals with extremely strong mechanical properties, challenging strain localization theories.
Field studies reveal that mineral reactions are ubiquitous in viscous shear zones, while undeformed rocks can remain largely metastable despite significant changes in P–T and/or fluid conditions. Local dissolution and precipitation processes under deviatoric stresses have long been recognized to promote brittle and viscous strain localization by complex chemo–mechanical processes including pressure solution, diffusive mass transfer, fluid flowand nucleation of fine-grained aggregates. Yet, quantifying the nature and relative contribution of these processes remains hindered by the general lack of experimental investigations on crustal rheology at high – to very high – pressure conditions and thermodynamic disequilibrium.
Drawing on novel deformation experiments performed at eclogite-facies conditions and a compilation of characteristics of exhumed materials from fossil subduction zones worldwide, this presentation demonstrates that inception and progression of crustal shear zones are predominantly steered by local transient changes of rheology from dislocation creep to dissolution–precipitation creep (DPC). Strain accommodation and mass transfer are further accelerated by local transient fluid flow resulting from grain boundary movements, fracturing and densification reactions. Because intergranular fluid-assisted mass transfer is orders of magnitude faster than solid-state diffusion, DPC can indeed explain strain accommodation at relatively high strain rates and low magnitude of differential stress, regardless of the mineral plastic strength. Yet, DPC remains a transient process because both fluid depletion and completion of mineral reactions favor grain growth, reducing in turn the efficiency of intergranular mass transfer.
How to cite: Soret, M.: Deep crustal shear zones driven by reaction-induced weakening and fluid flow, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10948, https://doi.org/10.5194/egusphere-egu25-10948, 2025.
Dilational hydroshear veins are hybrid veins that involve slip along weak planes and simultaneous extension under local hydrofracturing conditions (sensu Fagereng et al. 2010). These structures are considered as a possible record of episodic tremors and slow slip events (ETS). Carpholite-bearing dilational hydroshear veins and cyclic brittle-ductile deformation have been suggested to represent possible markers of these phenomena occurring at depth >30 km in subduction zones (Giuntoli & Viola 2022). In the Western Alps, similar structures in the form of lawsonite/carpholite-bearing veins have recently been reported (Herviou et al. 2023).
In this study, we analyzed the Lago Nero Unit (Western Alps), representing a fragment of the Liguro-Piemontese oceanic lithosphere and the related metasedimentary cover, deformed at 300-350 °C and 0.8-1.3 GPa during the Alpine Orogeny (Agard, 2021). We performed a detailed meso and microstructural characterization of mylonitic marble lenses wrapped by weak metapelite, both deformed by sheath folds. Hybrid veins in mylonitic marbles occur with crack-seal textures, oriented both parallel and at high angles to the main metamorphic foliation. The regional stretching mineral lineation oriented NE-SW is both parallel to the carpholite fiber composing veins and to the sheath fold axes. A few carpholite veins are folded within mylonitic marble, attesting to cyclic switches between brittle and ductile deformation in the stability field of carpholite, i.e. under blueschist facies conditions.
We focus on veins parallel to the foliation mainly composed of Ca-carbonate (now calcite, formerly aragonite), quartz and Fe-Mg carpholite (0.32<XMg<0.43). Frequently, large quartz and carpholite fibers form shear boudins in a plastically deformed Ca-carbonate mylonitic and ultramylonitic matrix, with a top-to-SW shear sense. Therefore, elevated strain partitioning is visible between host mylonitic marbles and veins and within single veins. Optical cathodoluminescence analysis shows different carbonate generations: larger and more luminescent fibers surrounded by small equant less luminescent grains. Electron Backscattered Diffraction analyses highlight that large Ca-carbonate fibers (50-500µm) deformed preferentially by subgrain rotation recrystallization, with the most deformed domains composed of smaller equant grains (<10µm) deforming by diffusion creep and grain boundary sliding. Summarizing, Ca-carbonate grew as fibers in veins and subsequently was affected by local strong grain size reduction due to strain partitioning at the microscale that activated grain size sensitive creep mechanisms along bands of accelerated creep. Strain partitioning was likely favored by differences in the initial carbonate grain size and/or crystallographic orientation and by the presence of stiffer quartz and carpholite. Paleopiezometry is underway to constrain differential stresses and strain rates responsible for the formation of the observed microstructures.
In conclusion, oceanic metasedimentary covers record evidence of transient and cyclic pore fluid pressure fluctuation, reaching sub-lithostatic values and elevated strain partitioning under transiently high strain rates. These structures likely reflect cyclic seismic and aseismic creep occurring at >30 km depth in the Alpine subduction zone. Our results may be compatible with the geophysical and geological data ascribed to deep ETS in subduction zone contexts.
How to cite: Casoli, L., Petroccia, A., Dobe, R., and Giuntoli, F.: Ultramylonitic carpholite-bearing veins as a proxy for deformation mechanisms from deeply subducted oceanic units (Liguro-Piemontese Zone, Western Alps), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-617, https://doi.org/10.5194/egusphere-egu25-617, 2025.
Intermediate-depth earthquakes (IDEQs), which occur at depths of 50 to 300 km, are relatively poorly understood compared to shallow seismicity, and their source mechanisms and physical environment remain ignored. This scientific gap exists because obtaining data from these depths—whether through geophysical imaging or geological sampling—is exceptionally challenging. The dehydration of serpentinites, which can release up to 13 wt% of H2O at these depths, is thought to play a key role in driving deformation associated with IDEQs. However, the mechanical role of the fluids released during these metamorphic reactions remains unclear. To provide new insights into the physical habitat of IDEQs, we investigate olivine- and Ti-clinohumite-rich veins in the Zermatt-Saas meta-ophiolite, a natural laboratory that records dehydration and fluid flow processes under (ultra)high-pressure (UHP) conditions typical of IDEQ depths.
We conducted petro-structural analyses and identified three main vein types: dilational, hybrid dilational-shear, and highly strained sheared veins. Key observations include (i) foliated sheared veins; (ii) newly formed olivine and Ti-clinohumite aligned in mineral lineations within sheared veins and shear bands; (iii) olivine and Ti-clinohumite fibers sealing porphyroclasts; and (iv) mutual crosscutting relationships between dilational and shear features. These features indicate cyclic brittle fracturing and ductile shearing at 2.3–2.7 GPa and 520–650°C, reflecting transient shearing and dilational fracturing under conditions of elevated pore fluid pressures, potentially approaching or exceeding lithostatic levels. The observed structures suggest that fluid escape occurs through interface-parallel, fracture-controlled pathways localized in high-strain zones, particularly near ultramafic sliver boundaries.
Strain gradients reveal distinct deformation styles, with dilational veins prevalent in low-strain regions and sheared veins and shear bands dominating within high-strain zones. These findings highlight the role of local stress regimes during serpentinite dehydration. Cyclic brittle-ductile deformation and fracturing, potentially linked to seismic or sub-seismic strain rate bursts, may have facilitated fluid migration and strain localization along olivine-bearing vein networks. These results align with geophysical observations suggesting high pore fluid pressures within the intermediate-depth seismicity region, providing insights into the mechanisms linking dehydration, fluid flow, and seismicity at depth.
How to cite: Munoz, J., Angiboust, S., Minnaert, C., Ceccato, A., Morales, L., Gasc, J., and Behr, W.: Fluid Flow and Shear Instabilities in the Subducted Mantle at Intermediate-depths: insights from the Western Alps meta-ophiolites, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5197, https://doi.org/10.5194/egusphere-egu25-5197, 2025.
Posters on site: Thu, 1 May, 10:45–12:30 | Hall X2
Fractures are the manifestation of brittle deformation and act as vital conduits for fluid transport in upper crustal rocks. To measure rock strength and stability, infer deformation mechanisms, and reconstruct the stress condition under which they developed, a systematic examination of their geometrical characteristics is essential which also provide insights on how upper crustal rocks respond to stress. Since fractured rock bodies frequently consist of interconnected networks of different fracture sets, topological characterization aids in quantitative assessment of their connectivity, which directly affects comprehension of their permeability and, consequently, the history of fluid migration through the host rock body. Additionally, characterization of fracture networks has direct implications in recent applications like nuclear waste disposal and carbon sequestration which contribute significantly to environmental sustainability.
The present study examines the origin and characterizes subsequent networking of fractures developed within younger granites (~ 2.61 Ga) of the Chitradurga Schist Belt, an Archean age granite-greenstone belt from the Western Dharwar Craton of peninsular India integrating field-based observations with network topology and fractal analysis. We systematically document the geometrical attributes of fracture patterns developed within the granites across varying outcrop scales to understand their formation and characterize them topologically to assess their connectivity and record if fracturing patterns, intensity, density and connectivity vary across scales and also spatially along the areal extent of the granitic plutons. It is found, that although indicative of being formed by the activation of a Riedel shear system under the same tectonic stress regime, the networking patterns which the fractures have developed through their mutual interaction vary spatially in their geometrical, topological and fractal characters. Our study ventures upon the possible causes of this variation and highlights the role of ambient stress state, rheology, pre-existing mechanical anisotropy, orientation of pluton margin and its proximity to adjacent shear zone and superimposition of fractures behind the development of these spatially varying fracture network patterns across the areal extent of the granitic plutons.
How to cite: Biswas, S. K., Banik, B., Mondal, T. K., and Hossain, Md. S.: Spatial variations in Geometry, Topology and Fractal attributes of a Riedel shear induced Fracture Network system in Granites, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-866, https://doi.org/10.5194/egusphere-egu25-866, 2025.
Many aspects of the evolution of boudinage are still poorly understood, and using boudins as rheology-gages is in its infancy. The aim of the study is to achieve a better understanding of the evolution of boudinage by numerical mechanical modeling integrated with three-dimensional characterization and analysis of natural boudinage structures. We use results from a 3D field study of boudins as a basis for high resolution numerical modeling.
We use the computationally expensive 3D Discrete Element method to model the boudinage process from loading to strain localization and post failure deformation in parametric studies using high resolution and a realistic representation of the coupled brittle and ductile deformation processes. This provides quantitative insight into the acting mechanisms and coupled processes during the formation of boudins to link the large variety of boudin geometries to specific boundary conditions. In particular we show that the transition from blocky torn boudins to drawn boudins can be modeled as a function of material strength and confining pressure. Furthermore, local heterogeneities can cause shear failure already before the critical stress is reached in the entire rock volume
The numerical simulations are augmented by studies on a world class example of boudinage structures on the island of Naxos, involving an extensive field study, detailed 3-dimensional reconstruction of boudinage structures and microstructural investigation of the underlying deformation processes.
Our ultimate goal is to pave towards a mechanically meaningful 3D boudinage classification scheme that allows for quantitative analysis of boudinage structures in order to invert boudin geometry to the kinematics and rheology of the rock during its deformation, as well as to its stress and strain history.
How to cite: von Hagke, C., Abe, S., Virgo, S., and Urai, J.: Failure mode transition in brittle Boudinage: effects of cohesion, mean stress and layer thickness in discrete element models and field examples, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4090, https://doi.org/10.5194/egusphere-egu25-4090, 2025.
The Menderes Massif in western Anatolia is a large metamorphic core complex that formed in the back arc of the Aegean subduction zone. Geological and geodetic studies show that extension has occurred almost uniformly since the cessation of continental collision at c. 30 Ma. In this study, we used 2D numerical modeling informed by measurements of abundances of radioactive heat producing elements in exhumed Menderes metamorphic rocks (gneiss, schist, migmatite, granite) to investigate the effect of variation in vertical distribution of crustal radioactivity on the style of extensional deformation during core complex evolution. We assumed four different scenarios with the same total crustal radioactive heat production but fractionated differently between the upper and lower crust: 0%, 25%, 50%, and 62.5% of the total crustal radioactivity located within the thickened lower crust. Our numerical experiments reveal that lower crustal radioactivity has a major effect on the temperature (T) of the lower crust and hence its geodynamic evolution. We observed significant partial melting and core complex development only in the scenarios with fractions of 50% or more. The results are nearly independent of upper crustal radioactivity. The elevated radioactivity levels and therefore T of the lower crust drives partial melting, which in turn results in lower viscosity and enhanced crustal flow. According to these results, the lower part of the thickened orogenic crust in western Anatolia must be highly radiogenic in order for the formation of the observed core complex structure.
How to cite: Erkan, K., Whitney, D. L., and Rey, P. F.: Effect of variation in the vertical distribution of crustal radioactivity in metamorphic core complex development (Menderes Massif, Türkiye), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16582, https://doi.org/10.5194/egusphere-egu25-16582, 2025.
Rocks occupying the back-arc areas in subduction zones present a structural complexity resulting from subduction and exhumation processes, the latter contemporaneous with hydrothermal fluid circulation and ore deposition along crustal-scale shear zones. In many cases, the exhumation starts while rocks are situated in the middle crust, where ductile deformation prevails and ends when these rocks are exposed to the surface, juxtaposed against hanging wall rocks with contrasting mechanical properties and deformation history. The interplay between high- and low-grade rocks often results in complex patterns and puzzling structural inventories.
The Rhodope crystalline complex (north Greece) comprises high-grade ortho-and paragneisses that were subducted in HP-UHP in the Mesozoic and exhumed in the Oligo-Miocene, through a complex network of ductile shear zones and low-angle normal faults constituting the Kechros Detachment. The high-grade footwall rocks belong to the Lower and Intermediate Rhodope Terranes, juxtaposed against the low-grade carbonates and phyllites of Makri Unit and the late-Eocene-Oligocene supra-detachment sediments and volcanic rocks.
We have conducted a detailed mapping and structural study of the Kallintiri area (SW Byala Reka-Kechros Dome, Rhodope, northern Greece) to define the tectonostratigraphy of the area and discriminate between early ductile, subsequent brittle-ductile, and late brittle structures. Our results established a continuum of large-scale structures that brought the high-grade rocks from the middle crust to the surface, accompanied by corresponding fault rocks and structures, revealing the acting deformation mechanisms. During the exhumation process, the deformation was localized at the lower structural level of the Makri Unit due to the significant competence contrast between the structurally lower amphibolite-facies gneisses and the overlying lower-greenschist facies carbonates. As a result, the carbonate rocks from the hanging wall Makri Unit were mechanically coupled to the footwall and served as the main lithology that experienced mylonitic deformation.
How to cite: Soukis, K., Kanellopoulos, C., Voudouris, P., Mavrogonatos, C., Sboras, S., Lazos, I., Tarantola, A., Koehn, D., and Moritz, R.: From hanging wall to footwall: a story of crustal-scale piracy during the exhumation of the South Rhodope complex (northern Greece)., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20083, https://doi.org/10.5194/egusphere-egu25-20083, 2025.
The Acauã Formation, located within the Estância Domain in the Sergipano Belt of the Borborema Province, comprises carbonates and slates that preserve evidence of multiple deformation regimes, including ductile, ductile-brittle, and brittle. Located along the western border of the Central Tucano Basin, this lithostratigraphic unit displays diverse structural and mineralogical characteristics that are crucial for understanding its tectonic evolution. The foliation dips gently to the NW and SE, forming regional open folds, and an anticline drag-fold related to the thrust fault propagation was formed at the contact of the massive and laminated carbonate facies. The massive facies are characterized by dark gray, very fine-grained dolostones lacking visible internal structures, while the laminated facies comprise light to medium gray dolostones with fine to silty grain size, well-defined laminations, and occasional "beef" structures (fibrous calcite veins). Petrographic analysis revealed a micritic dolomite matrix in both facies, with disseminated quartz, biotite and pyrite observed in the laminated facies. Cathodoluminescence analysis confirmed dolomite as the primary mineral phase in the matrix and identified two distinct vein generations: dolomitic and calcitic. These veins exhibit elongated crystal growth along their margins and blocky central fills, indicating a process of progressive dilation followed by abrupt opening. The veins acted as nucleation sites for faults, with their reactivation during deformation stages evidenced by the formation of normal and thrust faults, which are predominantly oriented NW-SE and NE-SW, respectively. U-Pb geochronology of carbonates provided constraints on the timing of deformation. The micritic dolostone matrix yielded an age of 601.5 ± 13.3 Ma, likely reflecting post-glacial carbonate deposition. A dolomitic vein near a thrust fault was dated at 508 ± 138 Ma, while slickenfibres on the fault surface yielded an age of 316 ± 83 Ma. Bed-parallel faults yielded a Lower Permian age of 291 ± 48 Ma. These results, although imprecise, suggest that the Acauã Formation carbonates formed during the Ediacaran, with vein formation initiated in the late Neoproterozoic and being reactivated during the Paleozoic era. The structural evolution highlights the significant role of mineralized veins played in fault nucleation and reactivation during regional tectonic events.
How to cite: Correia, O., Izídio, A., Miranda, T., barbosa, D., Roberts, N., Sanglard, J., Carvalho, B., Araújo, R., Laura, M., Pacheco, S., and Neumann, V.: MULTI-STAGE DEFORMATION AND U-Pb GEOCHRONOLOGY OF CARBONATES IN THE ACAUÃ FORMATION, SERGIPANO BELT, NE BRAZIL, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11789, https://doi.org/10.5194/egusphere-egu25-11789, 2025.
Crustal strength has been estimated to become the largest at the brittle-ductile condition[1]. Previous experiments have shown that water reduces the crustal strength not only at shallower depth regions where frictional slip becomes dominant[2] but also at greater depth regions where viscous flow becomes dominant[3]. However, the microphysical process of how water alters deformation mechanisms and reduces rock strength at the brittle-ductile transition zone remains unclear. To investigate the effect of water on controlling deformation mechanisms at the brittle-ductile transition, we perform a series of shear deformation experiments with a trace amount of water (either 0.2 wt % or 0.4 wt %). We deformed a quartz-albite mixture using a Griggs-type solid salt assembly. Each experiment uses ~ 0.1 g of the sample mixture. The shear strain rate is sequentially changed between ~ 10-3 /s and 10-4 /s to investigate the strength dependence on velocity. We further conducted microstructural observations using electron microscopes.
Here, we report a preliminary result of a series of water-added experiments conducted with 0.4 wt% water (i.e., 0.4 μL) at a confining pressure PC of 760 MPa and a temperature T of 720 °C. Mechanical results show that the peak shear stress is 790 MPa at a shear strain of 1.4, followed by a strain weakening by 200 MPa towards a final shear strain of 4.9. This peak stress is much weaker than a previous result of a room-dry experiment performed at a similar experimental condition (PC = 750 MPa and T = 720 °C)[4]. In the dry experiment, the peak shear stress was 1280 MPa, followed by a strain weakening of 230 MPa[4]. Microstructural analyses showed that the water-added sample is pervasively covered with microcracks. A transmission electron microscopy revealed that nano-grains as small as 50 nm are distributed in the areas between the microcracks. Meanwhile, a sample from the dry experiment exhibits fewer microcracks and contains nano-grains similar in dimensions to those in the sample of the wet experiment[5].
Our results suggest that water enhances fracturing in the sample layer, and nano-grains are formed regardless of the addition of water. This indicates that the reduction in the peak stress of wet conditions is due to the fracturing promoted by water, while the strain weakening after peak stresses is controlled by nano-grain domains in both conditions. We propose that water reduce the crustal strength by fracturing, that is brittle deformation, accompanied with weakening mechanisms in nano-grain domains such as grain boundary sliding. Furthermore, this suggests that brittle deformation remains dominant even at a greater depth in wet conditions, compared with in dry conditions.
[1] Kohlstedt et al., 1995JGR. [2] Blanpied et al., 1995JGR. [3] Kronenberg & Tullis, 1984JGR. [4] Furukawa et al., 2023 WRI-17. [5] Furukawa et al., 2025 in preparation.
How to cite: Furukawa, M., Sawa, S., Nagahama, H., Plümper, O., and Muto, J.: Water-added experiments of simulated quartz-feldspar shear zone at brittle-ductile transitional condition, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14614, https://doi.org/10.5194/egusphere-egu25-14614, 2025.
Exhuming shear zones are key structures in the dynamic evolution of orogens. Such shear zones accommodate most of the shear-related exhumation within relatively small rock-volumes. This is possible due to major strain partitioning occurring along weak rocks, frequently represented by phyllosilicate-rich rocks. Thus, the study of phyllosilicate-rich mylonites can provide fundamental insights into exhumation mechanisms responsible for the architecture of orogens.
The Hulw Shear Zone in the Saih Hatat Window of Oman (Agard et al., 2010) is one of these exhuming shear zones juxtaposing two subducted continental tectonic units. This tectonic contact experienced sustained shearing, accommodating a delta pressure of circa 0.8 GPa between 1.2 and 0.4 GPa at a relatively constant temperature of circa 400 °C (Petroccia et al., 2025) between 77 and 74 Ma (Ring et al., 2024).
In the field, micaschist belonging to the footwall displays a strain gradient moving toward the contact with the hanging wall, corresponding to a development of a S-C-C’ fabric and a modal enrichment in K-rich white mica and pyrophyllite matched by a progressive increase in the physical interconnectivity of these phyllosilicates. Electron backscatter diffraction analyses suggest that large (several hundreds of µm) detrital quartz grains experienced grain size reduction by subgrain rotation recrystallization to form equant grains of less than 100 µm in size.
Hyperspectral cathodoluminescence highlights different luminescence for the larger detrital grains, producing a bright signal and containing yielded cracks, and smaller equant grains, darker in cathodoluminescence and devoid of cracks. Interconnected chains of small quartz grains are located in contact with the phyllosilicates, suggesting an interplay between pinning and grain growth from a fluid phase.
In pyrophyllite-muscovite intergrowths, Transmission Electron Microscope analyses highlight more defects and kinking in pyrophyllite than in muscovite, intergrowths at the submicron scale and crystallites as small as 2 µm with truncated boundaries likely reflecting dissolution and precipitation mechanisms.
Summarising, these results suggest that strain localization and weakening of this rock volume was achieved by an interplay of the following mechanisms: I) synkinematic nucleation of retrograde mineral phases along discrete C and C’ planes, forming an interconnected network of phyllosilicates, II) microcracking in larger quartz grains followed by subgrain rotation recrystallization leading to a finer grain size of quartz, III) pinning of the grain size and IV) dissolution and precipitation processes of phyllosilicates. Different types of phyllosilicates appear to differently accommodate strain by both plastic deformation and recovery by dissolution-reprecipitation.
Concluding, this intimate and polyphase interplay between deformation and metamorphism is responsible for the formation and evolution of exhuming shear zones and the related structure of orogens.
Giuntoli acknowledges financial support of grant N° MUR 2022X88W2Y _002.
References
Agard, P., Searle, M. P., Alsop, G. I., & Dubacq, B. (2010). Tectonics, 29(5). https://doi.org/10.1029/2010TC002669
Petroccia, A., Giuntoli, F., Pilia, S., Viola, G., Sternai, P., & Callegari, I. (2025). Journal of Structural Geology, 191. https://doi.org/10.1016/j.jsg.2024.105328
Ring, U., Glodny, J., Hansman, R., Scharf, A., Mattern, F., Callegari, I., van Hinsbergen, D. J. J., Willner, A., & Hong, Y. (2024). Earth-Science Reviews, 250, 104711. https://doi.org/https://doi.org/10.1016/j.earscirev.2024.104711
How to cite: Giuntoli, F., Petroccia, A., Airaghi, L., Précigout, J., and Raimbourg, H.: Phyllosilicates do their job: insights into their role in exhuming subducted continental units, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16287, https://doi.org/10.5194/egusphere-egu25-16287, 2025.
It is agreed that mylonitic shear zones are first-order fluid conduits in the crust, but we lack a systematic and comprehensive understanding of porosity and permeability generation in natural mylonitic shear zones. As a consequence, we are unable to predict their synkinematic fluid transport properties, which affects our assessments of fluid-mediated processes in shear zones.
Here we present insights into the dynamic porosities in a quartzo-feldspathic layered ultramylonite from the Redbank Shear Zone (Australia) that formed during a stage of retrograde thrusting and hydration at lower amphibolite-facies conditions. In our analysis, we have combined non-invasive, high-quality x-ray microtomographic datasets from 5-mm-diameter core samples drilled orthogonally to the mylonitic foliation with high-resolution electron microscopy, electron backscatter diffraction and energy dispersive x-ray spectroscopy on the same samples.
The sample is dominated by two fine-grained (<5 µm) microstructural domains, which differ by the relative proportions of Qz, Or and An, and the occurrence of Czo, respectively. Both deformed dominantly by grain-size-sensitive diffusion creep and grain boundary sliding. Newly grown Czo is thought to have resulted from the hydrothermal alteration of plagioclase at lower amphibolite-facies conditions during continued retrograde thrusting. Five types of synkinematic porosity were identified in the sample: pores at the boundaries, and dissolution pores inside of feldspar porphyroclasts, strain-shadow pores around Czo porphyroblasts, creep cavities, and pore sheets. These porosity types are the results of different mechanisms acting locally in the microstructure. On the sample scale, the porosity distribution is dependent largely on the distribution of porphyroclasts and porphyroblasts, and creep cavitation in the matrix. Porosity in the Qz-dominant layers, which lack Czo, is ‘localised’ around and inside shrinking feldspar porphyroclasts, whereas porosity in the fine-grained polyphase microfabric containing Czo porphyroblasts is more common and ‘distributed.’ The latter may allow more efficient but anisotropic fluid transfer. Creep cavities appear to have coalesced to form pore sheets along foliation boundaries or connecting strain-shadow pores. Our findings further corroborate the description of strain shadow porosity by Fusseis et al. (2023, Geology). We interpret that a feedback between clinozoisite growth and fluid ingress promoted further creep cavitation, and resulted in a greater potential for cavity coalescence to cause ductile failure in the fine-grained polyphase microfabric.
How to cite: McDowell, A., Gilgannon, J., Killian, R., and Fusseis, F.: Synkinematic porosity and ductile failure in mid-crustal ultramylonites from the Redbank Shear Zone, Central Australia, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12927, https://doi.org/10.5194/egusphere-egu25-12927, 2025.
Pseudotachylytes are solidified frictional melts produced in silicatic rocks during an earthquake (Sibson, 1975). They form both as fault and injection veins, with thickness ranging from some millimeters to some centimeters. Still, exposures of meter-thick pseudotachylytes associated to seismic faulting have been documented (Musgrave Ranges, Australia; Lofoten Island, Norway; Ivrea-Verbano Zone, Italy).
In this study, we perform field (UAV, photogrammetry, structural geology, etc.) and microstructural/mineralogical (FESEM-BSE, EDS, micro-Raman, etc.) investigations of thin (mm-cm) and giant (up to 1 m thick) pseudotachylytes approaching the Canavese Line (strike ~NNE-SSW), the major tectonic lineament of the Western Alps (Ivrea-Verbano Zone, Italy; Techmer, 1992; Ueda et al., 2008; Ferrand et al., 2018). Though thin pseudotachylytes have been extensively investigated, in-depth studies of the giant pseudotachylytes are lacking. The aim is thus to determine (i) the ambient P-T conditions, the geodynamic setting, and the seismogenic environment (megathrust?) of the giant-pseudotachylytes, and, in the future, (ii) their mechanisms of formation.
We investigated for ~11 km the polished outcrops exposed along the ~E-W trending Valsesia river and other creeks in the area and selected three outcrops (I, II, and III) within ~2 km to the W and ~9 km to the E from the Canavese Line. We found pseudotachylytes only to the E of the Canavese Line. In detail:
Outcrop (I), < 500 m to the E from the Canavese Line (altered gabbro host rock) shows:
- multiple generations of pseudotachylyte-bearing faults, including giant-pseudotachylytes with breccia (suggesting a single melt pulse) overprinting microgabbro schlierens. The giant-pseudotachylytes are sub-parallel to the Canavese Line and include breccia clasts of the altered (greenschist facies) host rock;
- late quartz- and epidote-, and chlorite-bearing faults/fractures cutting the pseudotachylytes;
- matrix of the pseudotachylyte overprinted by greenschist facies minerals (epidote, chlorite and albite).
Outcrop (II), ~2 km to the E from the Canavese Line (Balmuccia peridotite) shows:
- multiple giant pseudotachylytes-bearing faults, sub-parallel to the Canavese Line, and associated with thin pseudotachylyte faults and veins;
- serpentine-bearing faults/fractures cutting and cut by the pseudotachylytes;
- giant pseudotachylytes with homogeneous matrix suggesting a single friction melt pulse. The matrix (altered into serpentinite) includes microlites of olivine and pyroxene plus vesicles;
Outcrop (III), ~9 km to the E from the Canavese Line (unaltered diorite) shows:
- only thin pseudotachylyte overprinting/associated with foliated cataclasite-bearing faults cutting and overprinting aplitic dykes;
- pristine matrix of the pseudotachylyte, with well-preserved microlites, chilled margins and flow structures.
In conclusion, the giant-pseudotachylyte-bearing faults are (i) made of a homogenous layer of pseudotachylyte, (ii) sub-parallel and found only near (< 2 km to the E) of the Canavese Line, (iii) overprint and cut dykes and ductile shear zones, (iv) cut and are cut by (sub-) greenschist facies cataclasite-bearing faults, and, (v) are cut by epidote- and chlorite-bearing fractures and veins. The giant pseudotachylytes could be generated by large in magnitude earthquakes, associated with the activity of the Canavese Line and thus of Alpine age.
How to cite: Aldrighetti, S., D'Ippolito, G., Pennacchioni, G., Gomila, R., Baccheschi, P., and Di Toro, G.: Field and microstructural characterization of Valsesia pseudotachylytes (Ivrea Zone, Italy), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5837, https://doi.org/10.5194/egusphere-egu25-5837, 2025.
Understanding the deformation modes of the lower crust is crucial if we are to predict the rheological behaviour of the crust in space and time. The Nusfjord locality (Lofoten, Norway) represents a natural laboratory to study the interplay between seismic and aseismic deformation in the Earth’s lower crust. The area exposes pseudotachylytes (quenched frictional melt produced during coseismic slip) within a network of ductile shear zones bounding strong low-strain domains of granulitic anorthosites. Pseudotachylytes formed within the low-strain domains, during ongoing viscous creep in the ductile shear zones, at a depth of 25-35 km. The ductile shear zones themselves contain several generations of mylonitized pseudotachylytes suggesting repeated switches from frictional to viscous deformation within shear zones. The underlying reasons and rheological consequence of mutual overprinting relationships between ductile shear zones (generally considered to be weak) and several generations of pseudotachylytes remains enigmatic.
Field investigations, photogrammetry, structural logs, and microstructural analysis reveal that (1) pseudotachylytes invariably nucleate within the low strain domains of the anorthosite host rock located between subparallel shear zones, and not along the shear zones themselves; and (2) that the rupture migrates along the material interface provided either by the shear zone/host rock boundary or by the shear zone foliation. The observed relationships suggest transient stress pulses that are supported by variations in the recrystallized grain size of quartz along individual shear zones.
We propose that repeated episodes of pseudotachylyte generation and associated host-rock fracturing represent a mechanism of shear zone growth and thickening, because the pseudotachylyte veins are mylonitized and become part of the actively deforming shear zones, which in turn control the further development of pseudotachylytes in the adjacent rigid blocks (low-strain domains). Structural logs show that shear zone width depends on the initial spacing between subparallel shear zones: when shear zones are widely spaced (>1 m), the rigid block in between is essentially undeformed, it contains a low density of pseudotachylytes and the shear zones themselves are thin (<10 cm thick). In contrast, closely spaced shear zones are thicker (up to 1 m thick) and are separated by highly damaged rigid blocks that contain a greater density of pseudotachylytes. Thus, pseudotachylytes overprinting ductile shear zones are not necessarily the result of frictional-viscous switches along individual structures but may rather represent seismic fractures that initiated at stress concentrations within adjacent rigid blocks, which then followed preexisting shear zones. Importantly, repeated production of pseudotachylytes will progressively transform the lower crust from dominantly rheologically stiff to weak. Such rheological weakening will have major consequences on the dynamics of lower-crustal regions.
How to cite: Clivet, F., Piazolo, S., Michalchuk, S. P., Zertani, S., and Menegon, L.: Shear zone growth by repeated generation of pseudotachylytes in the lower crust, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10795, https://doi.org/10.5194/egusphere-egu25-10795, 2025.
The Yangsan Fault in southeastern Korea is a long-lived intracontinental fault system characterized by both seismic slip and aseismic creep. Despite its significance, the microstructural evidence that clarifies the fault’s deformation mechanisms remains incomplete. In this study, we present an analysis of the mechanical behaviors displayed by the Byeokgye section of the Yangsan Fault over seismic cycles. Our results are based on detailed microscopic observations of drillcore samples recovered from the Byeokgye section, using an electron backscattered diffraction (EBSD) technique. In injected calcite veins located close to the principal slip zone (PSZ) of < 2 cm in width, plastic deformation (including dynamic recrystallization by subgrain rotation and deformation twins) is concentrated in the blocky calcite grains. In a narrow microbrecciated slip zone (< 1 cm wide) within the granitic damage zone, we observed mechanical Dauphiné twins associated with fractures and microfaults in quartz, as well as intergranular pressure solution (IPS) in the quartz fragments. Given that dynamic recrystallization and IPS are indicative of mechanical behavior of aseismic creep, it is possible that aseismic creep occurs upon the fault during interseismic periods. Conversely, the presence of mechanical Dauphiné twins, coupled with the nature of the PSZ, gouge injections, and the blocky structure of calcite veins, suggests the exposure of the fault section to local seismic stresses during coseismic slip. In conclusion, various deformation processes have operated upon the Yangsan Fault at the studied section throughout multiple seismic cycles. Moreover, our results demonstrate the effectiveness of EBSD in elucidating the mechanical behavior within fault zones.
How to cite: Choi, S., Cheon, Y., Kim, C.-M., Jung, H., and Park, M.: Microstructural insights into the coseismic and aseismic behavior of fault rocks in the northern Yangsan Fault, SE Korea, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15187, https://doi.org/10.5194/egusphere-egu25-15187, 2025.
Deep focus earthquakes offer insights into Earth’s mantle and supports plate tectonics theory. Because high pressures and temperatures hinder brittle failure, their mechanisms differ from shallow quakes. Dehydration embrittlement, proposed as dominant at 100-350 km depth, involves fluid release from minerals like serpentine, increasing pore pressure and triggering failure. However, serpentine dehydration has a net decrease in pressure, requiring low-permeability layers to trap fluids to enable seismic failure. Experiments also show that serpentine dehydration often leads to ductile weakening without acoustic emissions.
To better understand the micro mechanisms involved in the dehydration of serpentinite, especially in the incipient stage, we have performed high pressure-temperature experiments under isostatic and non-isostatic conditions. Cores of serpentinite with 2 mm diameter were mounted in cubic assemblies with 12 mm edge. Experiments were carried out with the 6-Ram multi anvil press at the Bayerisches Geoinstitute, at pressure of 5 GPa, to a maximum strain of 15% at strain rates between 1.67x10-4 s-1 to 2.91x10-6 s-1. Temperature during isostatic and non-isostatic conditions was kept constant. Isostatic experiments were conducted at 550 °C and 784°C. non-isostatic experiments were conducted at ~650 °C.
Results show that isostatic dehydration of antigorite at 5 GPa starts at ~ 550 °C and is completed at ~ 800°C. Between 550-650 °C incipient dehydration of antigorite is evidenced by the growth of olivine and phyllosilicate at antigorite grain boundaries. At these conditions, no failure microstructure is observed. Pores are present between olivine and enstatite grains of fully dehydrated serpentine. When deformation is imposed at incipient dehydration conditions, olivine and phyllosilicate start to cluster and form microscopic shear bands oblique to the main stress direction. These results demonstrate that at microscopic level, dehydration and failure of serpentine is complex. Pre-existing microstructural heterogeneities may influence nucleation of olivine and phyllosilicates. Pore overpressure may not be the only mechanism involved in serpentinite failure. Further work is required to determine the importance of the strength of the dehydration products in leading to localized failure.
How to cite: Silva Souza, D., Thielmann, M., Frost, D., Heidelbach, F., and Gasc, J.: Microscale processes in experimental serpentine dehydration: implications for deep earthquake mechanisms, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6058, https://doi.org/10.5194/egusphere-egu25-6058, 2025.
Serpentinites play a critical role in subduction zones due to their unique mechanical properties, which influence tectonic and seismic processes and facilitate deformation along the subduction interface. A long-standing question is the discrepancy between experimentally deformed serpentinites, which exhibit brittle/brittle-ductile microstructures, and naturally deformed serpentinites, which predominantly show ductile features. Additionally, there is a strong debate on whether deformation in antigorite bearing rocks is driven by crystal plasticity, dissolution-precipitation, or a combination of both. Moreover, studies on deformation in partially dehydrated or hydrated serpentinites (containing metamorphic olivine and clinopyroxene), subducted down to (ultra)high pressure conditions, remain scarce. To address these issues, we conducted a detailed microstructural study of serpentinites from a hectometer-scale strain gradient zone within the Zermatt-Saas meta-ophiolite, examining deformation mechanisms in antigorite and olivine at depths relevant to intermediate-depth earthquakes and subsequent exhumation across mantle wedge conditions.
In low-strain serpentinites, dehydration of brucite-antigorite produces coarse-grained olivine-diopside-clinohumite-magnetite veins (“olivine veins”), while the host antigorite displays mesh textures, weak crystallographic preferred orientations (CPOs), and evidence of twinning. Deformation begins to localize around olivine veins, where olivine exhibits a B-type CPO with [010] parallel to the pole of foliation and [001] parallel to the lineation but no internal deformation. With increasing strain, antigorite foliation becomes continuous and penetrative, accompanied by CPO strengthening, grain size reduction, and localized folding and boudinage of olivine, where the CPO strength also increases. High-strain domains exhibit mylonitic fabrics, intense antigorite foliation with (001) maxima aligned to the pole of foliation and (010) parallel to lineation, and transposed olivine vein folds reduced to isoclinal rootless folds. Additionally S-C’ foliations form locally, with fine-grained olivine fibers coating C’ planes, and pressure shadows around olivine porphyroclasts containing olivine-diopside mixtures forming mm-scale bands within antigorite foliations. The olivine grains in the pressure shadows also present a strong B-type olivine CPO.
Our findings highlight a progressive transition from brittle-ductile to ductile deformation in serpentinites in a fluid-rich environment. This deformation seems to be controlled by dissolution-precipitation processes and dislocation creep. Furthermore, this study provides one of the few datasets of deformation of metamorphic olivine in subduction zones. The conditions documented are not only relevant for the oceanic lithosphere but also for the mantle wedge near the subduction channel, offering critical insights into the interplay of deformation, metamorphism, and fluid-rock interactions in these tectonic settings.
How to cite: Morales, L. F. G., Muñoz-Montecinos, J., Ceccato, A., and Behr, W.: Microstructural Evolution of High- and Low-Strain Serpentinites from the Zermatt-Saas Meta-Ophiolite: Insights into Antigorite and Olivine Deformation at Intermediate-Depth Seismicity Depths, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15688, https://doi.org/10.5194/egusphere-egu25-15688, 2025.
The processes that rule coupling/decoupling and rupture mechanisms along the plate interface in the deep portions of active subduction zones are largely inferred from geophysical observations. These observations highlight that a wide range of rupture and deformation mechanism may coexist, such as: aseismic slip, episodic non-volcanic tremor and slip (ETS), and regular earthquakes. Despite the high amount of data obtained through indirect approaches, our comprehension of the processes occurring along the plate interface is still limited. In particular, processes occurring at great depth along the subduction interface are difficult to interpret solely based on flow laws and rheological properties of rocks also due to the scarcity of direct geological observations.
Exhumed ultra-high pressure (UHP, > 90 km of depth) rocks represent a natural laboratory to investigate the interplay of metamorphic reactions and fluids, both affecting slab rheology, at great depth. The Lower Shear Zone (LSZ) from the Monviso massif (W Alps) represents a fossil plate interface accreted within the Western Alpine chain and constitutes the one-off example of an oceanic plate interface that reached coesite stability field at UHP depth., i.e., ca. 90-100 km, and was then exhumed[1,2]. The LSZ preserves snapshots of the different stages of deformation and metamorphism along the subduction plate interface shear zone, testifying the coexistence of brittle (brecciation of rigid eclogite-facies gabbroic mylonites) and ductile behaviour (shearing along weak, serpentinite-rich shear zone) at eclogite-facies depth[1,3].
Our new field, micro-structural and petrographic observations extend the existing record of brittle features along the LSZ and show that brecciated blocks of mylonitic eclogites are systematically traceable for almost 25 km, i.e. the entire length of the exposed LSZ. These blocks are embedded within a highly deformed serpentinitic matrix. The brecciated fabric is defined by a mosaic breccia texture with randomly distributed clasts cemented by a polyphasic omphacite-rich matrix. The matrix is locally brecciated and sealed again, highlighting a cyclic rupture and healing mechanism promoted by fluid pulses and consequent dehydration embrittlement. These features are comparable to the classical geological observations of structures attributed to ETS described from shallower region of the plate interface. The similarity suggests that ETS may transiently occur even at greater depths than those at which they are currently recorded by seismometers and GNSS stations. Our observations imply that decoupling at great depth along the plate interface could be favoured by embrittlement of the plate interface.
1: Angiboust, S., Agard, P., Yamato, P., Raimbourg, H., 2012. Eclogite breccias in a subducted ophiolite: A record of intermediate-depth earthquakes? Geology 40, 707-710.
2: Ghignone, S., Scaramuzzo, E., Bruno, M., Livio, F. A. 2023. A new UHP unit in the Western Alps: First occurrence of coesite from the Monviso Massif (Italy). American Mineralogist, 108(7), 1368-1375.
3: Locatelli, M., Verlaguet, A., Agard, P., Federico, L., Angiboust, S., 2018. Intermediate-depth brecciation along the subduction plate interface (Monviso eclogite, W. Alps). Lithos.
How to cite: Scaramuzzo, E., Ghignone, S., Toffol, G., Boero, F., Locatelli, M., Gilio, M., Livio, F., Bruno, M., Scambelluri, M., and Pennacchioni, G.: Multiple rupture and healing events along the Plate Interface at Ultra-High-pressure depth. Insights from the Lower Shear Zone, Monviso Massif, Italy , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16426, https://doi.org/10.5194/egusphere-egu25-16426, 2025.
This study investigates the relationships between eclogite-facies mineral assemblages and deformation microstructures in the Yuka terrane, part of the North Qaidam ultrahigh-pressure (UHP) metamorphic belt in NW China. The analysis focuses on understanding the mineralogical and microstructural evolution during subduction and exhumation processes. Eclogites from the study area were found to exhibit distinct mineral assemblages and deformation features, reflecting multiple stages of metamorphism.
During prograde metamorphism, garnet, omphacite, and phengite were predominantly deformed by intracrystalline plasticity, indicative of dislocation creep as the primary deformation mechanism. These minerals contributed to the development of well-defined foliations and lineations in the rock, shaped by the alignment of omphacite and phengite grains. Garnet grains often displayed concentric zoning with inclusion-rich cores and inclusion-free rims, recording growth under varying pressure-temperature conditions. Omphacite showed evidence of dynamic recrystallization, highlighting the mechanical and chemical adjustments during progressive subduction.
In contrast, amphibole, which formed through the topotactic replacement of omphacite under fluid-present conditions, exhibited features associated with diffusional flow, such as dissolution-precipitation creep. This retrograde mineral is thought to have crystallized during amphibolite-facies retrogression, marking the exhumation of the eclogites. The lack of significant deformation microstructures in amphibole, such as subgrain boundaries or undulose extinction, supports its formation during a late-stage metamorphic environment.
The Yuka eclogites contain a range of mineral assemblages, including garnet + omphacite, garnet + omphacite + phengite, and garnet + omphacite + phengite + amphibole, reflecting diverse pressure-temperature paths. The compositional variability of these assemblages is tied to the complex geodynamic history of the North Qaidam UHP belt, which underwent subduction, continental collision, and exhumation. This work highlights the significance of integrated petrographic and microstructural studies for deciphering the metamorphic and tectonic evolution of UHP terranes.
How to cite: Park, M.: Microstructures and deformation mechanisms of the Yuka eclogites in the North Qaidam UHP belt, NW China, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2428, https://doi.org/10.5194/egusphere-egu25-2428, 2025.
Availability of fluids and induced metamorphic reactions are primary factors controlling the rheological behaviour of rocks. During subduction, fluids enhance the kinetics of eclogitization reactions, playing a fundamental role in promoting strain localization and shear zone nucleation. In particular, reaction-induced grain-size reduction has long been considered one of the most effective strain weakening mechanisms. To investigate the relationship between fluid-rock interaction, metamorphism and deformation, we focus on pre-Alpine ultramafites and mafic granulites of the Austroalpine Mt. Emilius klippe (Western Alps) that underwent eclogite-facies metamorphism during Alpine subduction.
The Mt. Emilius ultramafites consist of enstatite, diopside, olivine and spinel websterites deformed along a hydrated mantle shear zone that developed a fine-grained (10 µm) ultramylonitic assemblage of enstatite, diopside, olivine, anorthite, kaersutite1. During Alpine HP metamorphism, fine-grained (down to 2 µm) aggregates of jadeite, quartz, kyanite, clinozoisite (Czo) completely and statically replaced plagioclase, locally forming spatially continuous layers. Such fine-grained, hydrated aggregates did not promote any ductile eclogite-facies deformation.
The pre-Alpine mafic granulite consisted of assemblages of medium-grained garnet (Grt), diopside, plagioclase and subordinate hornblende that were replaced by Grt, omphacite (Omph), amphibole, phengite, chlorite and Czo during Alpine eclogite-facies metamorphism2. Early Alpine deformation (D1A) developed a pervasive eclogitic foliation (S1A) parallel to the granulitic layering2. This event was promoted by complete transformation and reaction-induced grain-size reduction (down to a few tens of µm) of plagioclase to Czo aggregates, together with replacement of hornblende by fine-grained chlorite-garnet-amphibole-epidote-Phe. A second eclogite-facies deformation event (D1B) is represented by localized ductile deformation closely linked to development of Czo, Omph, tremolite, Grt-filled veins and associated host-rock alteration haloes. Ductile shear is typically localized to the outer boundary of Omph-rich alteration haloes forming paired shear zones. A set of samples ranging from haloes with well-developed flanking shear zones to haloes free of shear localization was collected to investigate the role of fluid-rock interaction on shear zone nucleation and strain localization.
Preliminary data indicate that Omph-rich haloes surrounding Czo-Grt veins induced hardening in the host metagranulite (undeformed and foliated, S1A) associated with extensive replacement of the Czo aggregates (after sites of granulitic plagioclase) by Omph. However, this replacement did not always result in hardening and consequent strain localization at the outer boundary of the halo. In samples lacking shear localization, Omph accommodates deformation homogenously across the halo dominantly by diffusion creep (variable CPO, quasi-random distribution of misorientation angles, weaker SPO), with minor contribution of crystal plasticity (rare subgrains). The predominant contribution of diffusion was likely assisted by availability of fluids.
The processes driving frequent strain localization and formation of paired shear zones at the outer boundary of hardened haloes are still matter of ongoing study. Progressive advancement of the reaction front towards the host rock may form a compositional gradient across the halo, where chemical/mineralogical modifications may play a major role in determining the rheological behaviour.
[1] Benciolini, 1996. Memorie Scienze Geologiche, 48, 73-91.
[2] Pennacchioni, 1996. Journal of Structural Geology, 18, 549-561.
How to cite: Cacciari, S., Pennacchioni, G., Toffol, G., Scambelluri, M., and Cannaò, E.: Strain localization at eclogite-facies conditions: interplay between fluids, metamorphism and deformation (Mt. Emilius klippe, Western Alps), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11959, https://doi.org/10.5194/egusphere-egu25-11959, 2025.
Eclogites compose the majority of the subducted oceanic crust at great depth, with garnet and clinopyroxene as major phases. High stress concentration could exist in the UHP eclogites, with a mechanical contrast between garnet and clinopyroxene that leads to complex microstructures, between brittle and ductile deformation. Coexistence of frictional and viscous regime in such two-phase aggregates raise the question of the competitivity between phases in leading deformation. The fracturation of garnet in natural rocks has been interpreted as related to seismicity in the lower crust and oceanic crust at the interface plate in subduction zones (Trepmann and Stöckhert, 2002, Angiboust et al., 2012, Hawemann et al., 2019), but the question remains if such features can also be produced at lower strain rates (Yamato et al., 2019, Rogowitz et al., 2023).
In order to understand the effect of hard- vs. weak mineral fraction on eclogite mechanical properties, stresses distribution and deformation mechanisms of synthetic eclogites were experimentally investigated under deep subduction zones conditions. Samples were deformed under ultrahigh pressures (3 to 5 GPa), high temperature (820°C) and constant strain rate (1 x 10-5 – 2.5 x 10-5 s-1), using X-rays diffraction to measure in-situ stresses during deformation in each phase in garnet-clinopyroxene aggregates, with various garnet fraction. Back-scattered electron (BSE), electron backscatter diffraction (EBSD), scanning transmission electron microscopy (STEM) with automated crystal orientation mapping (ACOM) was used on the recovered samples, in order to determine deformation mechanisms from the micrometric to the nanometric scale.
In our experiments, deformation was accommodated by a mix of brittle and intracrystalline plastic mechanisms, as proposed or observed in previous studies at lower pressures (e.g. Yamato et al., 2019, Rogowitz et al., 2023). Cataclastic flow and dynamic recrystallization are observed. The distribution of stresses in the phases and variations in stress levels depend on garnet vs. pyroxene fraction in the samples. Differential stresses are greater in garnet than pyroxene and stresses increase with increasing % vol. garnet. Phase fraction impact the mechanical behavior, i.e. fracturation of each phase and deformation accommodation mechanisms vary. In this semi-brittle regime each phase is rheologically active and contributes to the deformation of the aggregate except at the lowest pyroxene fraction.
Our experiments together with last studies (e.g. Yamato et al., 2019, Rogowitz et al., 2023), indicate that frictional deformation of eclogites is not limited to seismic strain rate (i.e. > 1 s-1) but can occur at strain rate around 10-5 s-1 and slower with a high amount of garnet. The grain size reduction mechanisms observed could allow a switch to grain size sensitive mechanisms like grain boundary sliding. Questions still remain about the extrapolation of such mechanical distribution and fracturation in deep subduction zones.
How to cite: Molines, C., Hilairet, N., Chantel, J., Chardelin, M., Mandolini, T., Officer, T., Addad, A., and Fadel, A.: In-situ stresses distribution and deformation mechanisms in eclogites at ultrahigh pressure, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5696, https://doi.org/10.5194/egusphere-egu25-5696, 2025.
Water weakening of nominally anhydrous minerals of upper mantle is important for understanding the rheological structure of Earth’s interior. Enstatite is the 2nd dominant phase in the upper mantle, next to olivine. The partition coefficient for water between olivine and enstatite aggregates ColOH/CenOH is ~0.5 at 3.8‒6.3 GPa and 1323‒1573 K (Zhang et al., 2017; JGR), indicating that water weakening of enstatite effectively proceeds in the olivine-enstatite system. The water weakening of enstatite would be accelerated at high pressures, since the amount of water dissolved into enstatite drastically increase with pressure.
We hereby experimentally evaluated the creep strength of wet orthoenstatite aggregates under pressure and temperature conditions at 1.9‒5.3 GPa and 1200‒1380 K using a deformation DIA apparatus combined with synchrotron X-ray radiation. At a constant strain rate ranging from 6.7 × 10-6 to 9.4 × 10-5 s-1, steady-state creep strength of wet orthoenstatite followed the power-law flow law with the stress exponent of ~3, indicating deformation in the dislocation creep regime. Our results show dislocation creep rate of wet orthoenstatite is ~1 order of magnitude faster than dry orthoenstatite under the same P-T conditions. FTIR spectra from the recovered samples indicate that the amount of dissolved water in orthoenstatite is up to 1370 ppm wt.%. The dependence of strain rate on water fugacity was determined with the water fugacity exponent of ~1. Depending on the water content in the upper mantle, dislocation creep of wet orthoenstatite could lead to strain localization in the lithosphere.
How to cite: Tsubokawa, Y., Ohuchi, T., Higo, Y., Tange, Y., and Irifune, T.: Deformation experiments on orthoenstatite aggregate at upper mantle pressures and temperatures under hydrous conditions, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5763, https://doi.org/10.5194/egusphere-egu25-5763, 2025.
Posters virtual: Tue, 29 Apr, 14:00–15:45 | vPoster spot 2
EGU25-20667 | ECS | Posters virtual | VPS28
Archean shear tectonics in the Congo craton: insights from Petro-structural characterization and U-Pb geochronology of the Memve’ele mylonite, Southern CameroonTue, 29 Apr, 14:00–15:45 (CEST) | vP2.8
The Congo craton is an early Archaean through Paleoproterozoic basement block in Central Africa. It consists of a vast heterogenous granulitic complex extending over 1200 km between the Lomami River (24°E) and the Atlantic coast in Angola. The well-exposed domains of the Congo craton are the Kasaï block, Tanzania block, West-Nile complex, and Ntem-Chailu complex. The latter represents the northwestern edge of the craton in southern Cameroon. The Memve'ele area belongs to the Ntem Complex, where recent investigations have highlighted various lithologies, including TTG gneiss and intensely sheared and folded charnockitic and granitic gneiss, pervasively intruded by younger monzogranite. This region provides a critical window into the complex tectonic evolution of one of Earth's oldest continental blocks. Both TTG and granitic gneiss are riddled with folded or sheared leucogranitic veins, suggesting a local origin through melting and dynamic recrystallization. This study presents a comprehensive investigation of the highly sheared Memve’ele mylonitic corridor. Through detailed field mapping, systematic kinematic analysis, and meticulous petrographic and microstructural studies, we aim to unravel the multiple deformation events that have shaped this region. U-Pb zircon geochronology was employed to precisely constrain the timing of these processes and to correlate them with regional tectonic events. The ultimate goal of this research is to better understand the broader geodynamic implications of these findings for the evolution of the Congo Craton. Initial results reveal that the Memve’ele area has undergone a complex polyphase deformation history, involving at least four distinct events. The early ductile deformation (D1) resulted in the development of a pervasive foliation and associated structures. Subsequent ductile-brittle deformation (D2) overprinted the earlier structures, while later brittle deformation events (D3 and D4) further modified the rock fabric. The studied mylonites yield Mesoarchean ages of 2927 ± 52 Ma. The presence of a sinistral shear zone within the area suggests that the region was subjected to significant shear stresses, likely related to regional tectonic processes such as continental collision or crustal extension. These findings have important implications for understanding the tectonic evolution of the Congo Craton and may provide insights into the potential for mineral exploration in the region.
How to cite: Takodjou Wambo, J. D., Ganno, S., Nzenti, J. P., and Asimow, P. D.: Archean shear tectonics in the Congo craton: insights from Petro-structural characterization and U-Pb geochronology of the Memve’ele mylonite, Southern Cameroon, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20667, https://doi.org/10.5194/egusphere-egu25-20667, 2025.
