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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

References

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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