TS1.5 | Bridging field and laboratory studies to understand the rheology of the Earth’s crust: from fast to slow motion, brittle to ductile deformation, and fluid-rock interactions
EDI
Bridging field and laboratory studies to understand the rheology of the Earth’s crust: from fast to slow motion, brittle to ductile deformation, and fluid-rock interactions
Convener: Giacomo Pozzi | Co-conveners: Sarah Incel, Giovanni Toffol, Matthew TarlingECSECS, Jesus Munoz, Alberto CeccatoECSECS, Sascha Zertani
Orals
| Thu, 18 Apr, 14:00–18:00 (CEST)
 
Room K1
Posters on site
| Attendance Fri, 19 Apr, 10:45–12:30 (CEST) | Display Fri, 19 Apr, 08:30–12:30
 
Hall X2
Posters virtual
| Attendance Fri, 19 Apr, 14:00–15:45 (CEST) | Display Fri, 19 Apr, 08:30–18:00
 
vHall X2
Orals |
Thu, 14:00
Fri, 10:45
Fri, 14:00
In the crust, brittle fracturing, viscous deformation, fluid-rock interaction, and metamorphic reactions exhibit a complex feedback resulting in complex fault and deformation structures. This interplay leads to a large variability in rheological behavior, observed from micro-scale mineral reactions and deformation up to crustal-scale brittle and/or ductile deformation, including major earthquakes. Hence, it is believed that deformation processes taking place at the grain scale (nanometer to millimeter) are shaped by the presence or absence of fluids and may control the overall behavior of faults and shear zones, and thus affect rock deformation at a crustal scale (meter to kilometer).
The factors outlined above result in a variety of fault slip modes from slow slip to fast and seismic, changes in bulk rheological behavior, and crustal physical properties. The study of these processes holds the clues to understand how these factors interact from grain-scale mineral reactions to the nucleation of major earthquakes.
To derive meaningful physical models, it is fundamental to bridge several scales of observations and to integrate structural geology, petrology, experimental rock deformation, microstructural investigation, geochemical analysis, and numerical modeling.
We invite scientists of any expertise to bring their contribution and particularly welcome work that integrates different approaches to explore the role that brittle-viscous deformation and fluid-rock interaction play in shaping the rheological and physical properties of the solid Earth.

Orals: Thu, 18 Apr | Room K1

Chairpersons: Giacomo Pozzi, Alberto Ceccato, Giovanni Toffol
14:00–14:05
14:05–14:25
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EGU24-5807
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solicited
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Highlight
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On-site presentation
Patricia Martínez-Garzón, Grzegorz Kwiatek, Piero Poli, Georg Dresen, and Marco Bohnhoff

A longstanding question in geoscience concerns whether earthquakes show a preparatory process and precursory seismic activity. Some models hold that in the intermediate-term (from months to years), seismicity and/or aseismic transients in fault slip and in other fault properties occur. During the last decades, improvements in earthquake monitoring, the integration of geodesy capturing slow deformation, and the incorporation of novel data analysis techniques including machine learning and artificial intelligence have improved our ability to better discern how earthquake sequences evolve before a mainshock. The few available observations of transient deformation preceding well-recorded earthquake sequences show a high variability, thus our potential for improving earthquake forecasting is still limited. The body of knowledge available from mechanical models, numerical simulations, experimental work and field observations highlighted a wealth of structural, tectonic and boundary conditions which may control the dynamics of earthquake sequences. These suggest that several processes can affect earthquake preparation on different temporal and spatial scales, ultimately yielding highly varying transient observations prior to mainshocks. These observations also highlight that existing theoretical and conceptual models of the preparation/nucleation process may not fully capture the governing physics.

We analyzed seismicity transients prior to the occurrence of the 2023, MW 7.8 Kahramanmaraş/Türkiye earthquake. We identified seismic precursory activity composed of a handful of isolated spatio-temporal clusters occurring in a complex fault network within 65 km of the future earthquake epicenter. Some of these clusters contributed to acceleration of seismicity rates in an area surrounding the future mainshock and starting ca. 8 months before the event. Within that area, we also observed a decrease in Gutenberg-Richter b-values. Comparable seismic transients were not observed in the region at least since 2014. The complex preparatory process differs significantly from the cascade of close (<200 m) foreshocks observed before the 1999 MW 7.6 Izmit/Türkiye earthquake rupturing a mature fault segment. This indicates that fault structure and heterogeneity expressed as roughness or segmentation exert a strong control on deformation transients before an earthquake. This bears strong similarities with laboratory studies on faults with varying roughness. Trends of seismic preparatory attributes observable in the field follow those documented in both laboratory stick-slip tests and numerical models of heterogeneous earthquake rupture affecting multiple fault segments. In the lab, rough faults before stick-slip tend to display prolonged phases of precursory slip including an interplay of (dominating) slow transients combined with high-frequency seismic deformation in stark contrast to smooth faults.

How to cite: Martínez-Garzón, P., Kwiatek, G., Poli, P., Dresen, G., and Bohnhoff, M.: Transient deformation leading to earthquakes: bridging observations from the lab and the field , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5807, https://doi.org/10.5194/egusphere-egu24-5807, 2024.

14:25–14:35
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EGU24-3417
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On-site presentation
Jay Fineberg

Frictional motion is mediated by rapidly propagating ruptures, akin to shear cracks, that detach the ensemble of contacts that form the interface between
contacting bodies. While fracture mechanics describe the rapid motion of these singular objects, the nucleation process that creates them is not currently understood. By extending fracture mechanics to explicitly incorporate finite interface widths, we fully describe the nucleation process. We show, experimentally and theoretically, that slow steady creep ensues at a stress threshold. Moreover, as creeping patches approach the interface width, a topological transition occurs where they smoothly transition to rapid fracture. This new picture of the nucleation dynamics of fracture (and friction) is directly relevant to earthquake nucleation dynamics and the transition from aseismic to seismic rupture in natural faults.

How to cite: Fineberg, J.: The nucleation of frictional ruptures, theory and experiment, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3417, https://doi.org/10.5194/egusphere-egu24-3417, 2024.

14:35–14:45
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EGU24-9877
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ECS
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On-site presentation
Federico Pignalberi, Carolina Giorgetti, Pierre Romanet, Elisa Tinti, Chris Marone, and Marco Scuderi

A critical aspect of studying earthquake mechanisms involves understanding why a single fault can exhibit various slip behaviors. Fault heterogeneity leads to different slip behaviors in different fault portions: some slip seismically, generating catastrophic earthquakes, while others slip a-seismically in a stable and silent manner. Additionally, some fault portions exhibit slow, intermittent slip that can persist for months. Unraveling the physical mechanism at the base of these different fault slip behaviors is crucial for understanding how fault portions that slip slowly interact with portions capable of producing earthquakes.

In a laboratory setting, we can replicate the entire spectrum of fault slip behaviors by changing the loading stiffness of our experimental apparatus. Tuning the loading stiffness, we are able to match the critical rheological stiffness of the fault (kc) and investigate conditions around the critical point where k/kc = 1. Moreover, monitoring acoustic emissions (AEs) during laboratory earthquakes allows us to capture the rupture processes throughout the seismic cycle.

To constrain the nucleation mechanisms and rupture processes of different slip behaviors, we conducted friction experiments using quartz powder (MinUSil, average grain size 10 µm) to simulate fault gouge. The experiments were carried out in a double direct shear configuration, using an array of calibrated piezoelectric sensors for continuous, high acquisition rate (6 MHz) AE recording. The experiments were conducted at a constant displacement rate of 10 µm/s. During each experiment we maintained a constant normal stress and changed three acrylic blocks of different areas to change the apparatus stiffness (k). This technique allows us to reproduce both fast (i.e., when the apparatus stiffness is lower than a critical stiffness, k<kc) and slow (i.e., k=kc) slip events under the same stress conditions and test if the same fault patch can host a variety of slip behaviors.

Continuous AE recording, that is a proxy for seismicity, allows us to relate mechanical and acoustic fault behaviors. Our results show that different slip behaviors produce distinct acoustic waveforms during slip, with impulsive (high amplitude, short duration) AEs for fast slip, and emergent (low amplitude, longer duration) and continuous acoustic signals for slow slip. The distribution of AEs throughout the seismic cycle is characterized by an accelerating phase with small emissions for slow slips. While, fast slips exhibit no clear pre-seismic activity, and only strong AE in the co-seismic phase produced by fault rupture. Analyzing the frequency content of the acoustic signals also provides insights into the size, duration and the evolution of the seismic source along the seismic cycle.

By changing the stiffness of the fault, and monitoring acoustic emissions, our experiments not only accurately show that the same fault patch can experience different slip behaviors under the same stress conditions but also gives important insights into the complex dynamics of fault slip and rupture processes.

How to cite: Pignalberi, F., Giorgetti, C., Romanet, P., Tinti, E., Marone, C., and Scuderi, M.: From slow to fast earthquakes: laboratory insights on acoustic and mechanical fault slip behavior, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9877, https://doi.org/10.5194/egusphere-egu24-9877, 2024.

14:45–14:55
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EGU24-7980
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ECS
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On-site presentation
Adriane Clerc, Guilhem Mollon, Amandine Ferrieux, Lionel Lafarge, and Aurélien Saulot

Understanding earthquakes mechanisms still represents a challenge, motivated by the large consequences of the numerous earthquakes occurring each year. A number of uncertainties remain concerning the complexity of the fault structure, the constitutive properties of materials or the fault rheology. To address those points, we borrow from the tribological approach the pin-on-disk experiment so that the two rough surfaces in contact through a series of asperities fault concept is downscaled to a single asperity sliding on a rough surface. The single asperity response to shearing induced by sliding and the evolution of friction are studied closely to understand the different stages undergoing by the asperity and the consequences on the fault behaviour during co-seismic events.

The original experimental apparatus consists in a centimetric pin with a hemispherical extremity representing the fault asperity while a large flat rotating disk stands for the opposite surface of the experimental fault. Both pieces are made in the same carbonate rock (Carrara white marble) with controlled roughness. The experimental downscaled fault is submitted to co-seismic conditions: contact size of 0.1-5 mm, contact normal stress of 10-200 MPa, sliding velocity of 0.01-1 m/s, and sliding distance of 10 - 60 m. A number of high-sampling-rate sensors are used to constrain the observation of the asperity contact during the simulated seismic events. Complete post-mortem analyses of the wear tracks with optical microscopy, SEM and roughness images allow to quantify the regime features and to reconstruct friction scenarios in accordance with the time-series acquired during tests.

Independently of the velocity and the normal load applied, the friction coefficient exhibits a clear transition between an idealized lab conditions regime and a mature interface with the formation of granular gouge, as a function of the sliding distance. Within the same regime (clean surface, intermediate, mature gouge), velocity weakening and hardening due to higher loading are pointed out. We propose to focus on the clean surface to mature gouge transition and on the stability of the mature gouge interface regime to address the fault rheology and the role of asperities in seismic weakening.

How to cite: Clerc, A., Mollon, G., Ferrieux, A., Lafarge, L., and Saulot, A.: Evolution from a clean surface to a mature gouge interface in a seismic fault – asperity system through the lens of pin-on-disk experiments, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7980, https://doi.org/10.5194/egusphere-egu24-7980, 2024.

14:55–15:05
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EGU24-19532
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ECS
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On-site presentation
Miriana Chinello, Andrea Schito, Stephen A. Bowden, Telemaco Tesei, Elena Spagnuolo, Stefano Aretusini, and Giulio Di Toro

Mirror-like Surfaces (MSs) are ultra-polished fault surfaces that reflect visible light, thanks to their nanometer-scale surface roughness. They are often found in seismogenic fault zones cutting limestones and dolostones. Both natural and experimentally-produced fault-related MSs have been described in spatial association with ultrafine matrix (grain size <10µm), nanograins (<100nm in size), amorphous carbon, decomposition products of calcite/dolomite (i.e., portlandite, periclase), and larger but “truncated” clasts. However, the formation mechanism of MSs is still debated. Experiments show that MSs can develop both under seismic (slip rate ≈1 m/s; Fondriest et al., 2013; Siman-Tov et al., 2015; Pozzi et al., 2018) and sub-seismic (slip rate ≈0.1-10 µm/s; Verberne et al., 2014; Tesei et al., 2017) deformation conditions, involving various physical-chemical processes operating over a broad range of P-T conditions, strain, and strain rates.

To evaluate whether the MSs formed during the co-seismic (possibly associated with frictional heat pulses) or the inter-seismic (no heat pulses) phases where temperature might serve as a distinguishing factor, we assessed the thermal maturity of “bitumen” using biomarkers. We acquired data for natural and artificial MSs hosted within bituminous dolostones. We collected natural samples from faults with slip displacement from a few millimeters to a few meters, located in the Italian Central Apennines (Monte Camicia Thrust Zone, past burial depths up to ~3 km). We obtained experimentally-produced MSs by deforming powdered bituminous dolostones in a rotary shear apparatus (SHIVA, INGV) at sub-seismic (V = 10-4 m/s) and seismic (V = 1-3 m/s) slip rates for 1-3 m of slip, under room temperature and humidity conditions, and 20 MPa of normal stress.

We extracted solid bitumen of pre-oil window thermal maturity from the MSs and from the underlying slip zone of natural and artificial samples and we analyzed the bitumen using Gas Chromatography–Mass Spectrometry. We identified Steranes and other biomarkers based on relative retention time and measured peak heights to obtain thermal maturity parameters. By comparing different samples, changes in thermal maturity could be measured across slip zones bounded by the MS and possibly associated with frictional heat pulses during co-seismic slip.

Biomarker thermal maturity parameters are consistent with the immaturity of the host rock, which recorded a maximum ambient T < 100°C during diagenesis. In the experimental MSs produced at seismic slip velocity, where frictional heat pulses reached T∼400°C, thermal maturity of bitumen is higher than that of the entire slip zone and undeformed gouge. Higher thermal maturities were measured also in natural MSs but were not detected in the experimental MSs produced at sub-seismic slip velocity.

Chinello et al. (2023) proposed that the microstructures found in these slip zones recorded the main phases of the seismic cycle, from rapid co-seismic slip to post/inter-seismic viscous flow and fault strength recovery. The results presented here (1) confirm this interpretation, (2) show that the frictional heat pulse associated with seismic slip can be recorded by biomarkers thermal maturity of bitumen trapped in the fault MSs, and (3) some natural MSs are associated with heat anomalies caused by seismic ruptures.

How to cite: Chinello, M., Schito, A., Bowden, S. A., Tesei, T., Spagnuolo, E., Aretusini, S., and Di Toro, G.: Seismic Mirror-like Surfaces in bituminous dolostones (Central Apennines, Italy), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19532, https://doi.org/10.5194/egusphere-egu24-19532, 2024.

15:05–15:15
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EGU24-10158
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On-site presentation
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Laura Federico, Michele Locatelli, Laura Crispini, Elisabetta Mariani, Giovanni Capponi, and Marco Scarsi

Faults and shear zones within the metamorphic ultramafic rocks of the Voltri Massif (Ligurian Alps, NW Italy) often exhibit significant or complete carbonation of the host rocks and are locally associated with gold mineralization hosted in quartz veins (e.g., in the Lavagnina Lakes area). Here, a specific fault zone part of a larger regional system (i.e., the Bisciarelle Fault Zone) displays distinct structural characteristics linked to a fluid-assisted multistage brittle deformation in serpentinized peridotites, possibly indicating paleoseismic activity. Within the fault rocks, cataclasite and breccias are present along with saponite-bearing gouge, featuring layers of coseismic spherulitic grains interspersed in silica/chalcedony veins and cement. Spherulites consistently crystallize as concentric bands of fibrous Fe-dolomite and display multiple layers of radial crystal growth regularly alternating with darker, oxides-rich concentric bands. The concentric growth of spherulites is evident from the microtextural relations between successive bands, which depart radially from the spherulite cores, made of a submillimetric nucleus of carbonates or single grain of the host rock (e.g., relicts of fragments of fault core).

In this study, we present a multiscale analysis of this fault zone, integrating field observations, microstructural examination, SEM-EDS investigation, and electron backscattered diffraction (EBSD). The primary focus is on the microstructures within the fault core and the significance of distinctive carbonate spherulite layers in conjunction with silica/chalcedony cement and veins.

Our findings reveal that these structures are indicative of the interaction between CO2-rich fluids released during both coseismic and interseismic phases of faulting. This interaction occurs during cycles involving fluid pressure build-ups, faulting events, fluid flushing, and the subsequent precipitation and sealing of minerals during seismic failure of the fault.

How to cite: Federico, L., Locatelli, M., Crispini, L., Mariani, E., Capponi, G., and Scarsi, M.: Seismic faulting and fluid interactions: a structural study from carbonated fault damage zones within ultramafic rocks, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10158, https://doi.org/10.5194/egusphere-egu24-10158, 2024.

15:15–15:25
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EGU24-4370
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ECS
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On-site presentation
Lisa Eberhard, Manuel D. Menzel, André R. Niemeijer, and Oliver Plümper

To assess the seismogenic potential of fault zones it is crucial to understand fluid-rock interactions in these zones, because alteration affects the fault strength and stability, as well as the deformation mechanisms.

The San Andreas fault (SAF) system is known for infrequent large magnitude (M≥7) earthquakes, whereas some segments lack such strong seismic events [1]. Here, strain is largely accommodated by creep motion. Aseismic creep can be enhanced by the presence of fluids, which may additionally drive mineral reactions. For example, fluid composition and magnesite deposits in the SAF segment between San Juan Bautista and Parkfield suggests carbonation due to infiltration of CO2-bearing fluids into the fault [2]. Carbonation of ultramafic rocks leads to the formation of talc, which is known to be frictionally weak and promotes creep when wet [3]. However, our thermodynamic fluid-infiltration calculations show that carbonation will not produce pure talc but lizardite-talc-magnesite (LTM) and talc-magnesite rocks (soapstone) and, with increasing extent of reactive fluid flow, talc-magnesite-quartz (TMQ) and magnesite-quartz rocks (listvenite). The strength and seismogenic potential of serpentinite fault zones undergoing carbonation thus may change dynamically as the mineral proportions and assemblages change, but the respective frictional behaviour of these assemblages is unknown.

We performed rotary-shear experiments on gouge layers with compositions ranging from lizardite-serpentinite to LTM, soapstone, TMQ and listvenite at pressure, temperature and pore fluid pressures corresponding to a depth of about 10 km (300 °C, 250 MPa normal stress and 100 MPa pore pressure). We measured the frictional strength within the velocity range of 0.002 µm/s to 10 µm/s.

Our data show that lizardite gouges are relatively strong and slightly velocity-weakening. The friction coefficient dropped from 0.45 at 0.002 µm/s to 0.42 at 10 µm/s. A similar velocity-dependence is observed for soapstone gouges, although at lower absolute friction coefficients of 0.3 to 0.28. Interestingly, listvenite gouges show the opposite behavior, with friction coefficients increasing from 0.25 at 0.002 µm/s to 0.48 at 10 µm/s. Stick-slips were only observed in serpentinite and soapstone gouges at low velocities. Increasing velocities and progressing carbonation causes stable slip behavior. Microtextural observations indicate strong grain-size reduction and basal cleavage in serpentinite gouges. On the contrary, soapstone and listvenite gouges show a fine-grained magnesite matrix surrounding the silicates.

Our results suggest that serpentinized fault zones have the potential to nucleate unstable slip. The results further confirm the strong weakening effect of carbonation. CO2-fluid-rock interaction in ultramafic fault gouges may effectively suppress the nucleation of earthquakes. Since also listvenite gouges deformed aseismic and are found to be frictionally weak at low velocities, we suggest that besides talc also magnesite plays an important role in the deformation behavior of carbonated ultramafic fault zones.

 

[1] Jolivet et al. 2015. Geophys. Res. Lett. doi:10.1002/2014GL062222.

[2] Klein et al. 2022. Geophys. Res. Lett. doi:10.1029/2022GL099185.

[3] Moore et al. 2008. Tectonophysics. doi:10.1016/j.tecto.2007.11.039

 

Funding

LE: NWO (VI.Vidi.193.030)

M.D.M: Junta de Andalucía (Postdoc_21_00791) and MCIU, Spain (PID2022-136471N-B-C22)

How to cite: Eberhard, L., Menzel, M. D., Niemeijer, A. R., and Plümper, O.: Fault weakening due to CO2-fluid-rock interaction – evidence from deformation experiments of carbonated serpentinites, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4370, https://doi.org/10.5194/egusphere-egu24-4370, 2024.

15:25–15:35
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EGU24-5618
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ECS
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On-site presentation
Stephen Paul Michalchuk, Nils B Gies, Mona Lüder, Markus Ohl, Kristina Dunkel, Jörg Hermann, Oliver Plümper, and Luca Menegon

In anhydrous, strong, and metastable lower-crustal rocks, coseismic fracturing is an effective mechanism for creating pathways for fluids to infiltrate and interact with the host rock, ultimately resulting in metamorphism and rheological weakening. In this study, we have characterized the damage zone flanking a lower-crustal pseudotachylyte (solidified frictional melt produced during seismic slip) to understand the fracture generating and fluid-assisted healing processes operating during and immediately after a seismic event.

The Nusfjord East shear zone (Lofoten, Norway) is a network of coeval pseudotachylytes and mylonitized pseudotachylytes that formed at lower-crustal conditions within anhydrous anorthosites. We present a micro- and nanostructural analysis focusing on plagioclase in the damage zone of a pseudotachylyte using focused ion beam (FIB) prepared scanning transmission electron microscopy (STEM), Fourier Transform Infrared (FTIR) Spectroscopy, electron backscatter diffraction (EBSD) analysis, electron microprobe analysis (EMPA), and SEM-cathodoluminescence (CL) imaging.

The damage zone of the host anorthosite is characterized by a network of comminuted primary plagioclase (plagioclase1) grains with minimal offset. Very fine (<15 mm) plagioclase1 grains and secondary plagioclase neoblasts (plagioclase2), differentiated from each other by SEM-CL and EBSD observations, fill the fractures along with a minor amount of K-feldspar. Plagioclase1 and plagioclase2 have the same major element compositions (average: An52) and are not zoned aside from a small increase in anorthite along the grain boundaries. Away from the pseudotachylyte margin, plagioclase2 grains filling the fractures show a host-controlled crystallographic preferred orientation (CPO) governed by plagioclase1 grains. With decreasing distance toward the vein margin, the CPO is weakened as a result of minor amount of solid-state deformation by grain-boundary sliding after the coseismic event. Plagioclase1 grains often exhibit a diffuse CL intensity zonation from bright grain cores to a dark grey in healed cracks, while plagioclase2 have a uniform mid-tone grey CL intensity with dark grain boundaries. CL zonation in the plagioclase1 does not correlate with EMPA major element maps nor EBSD misorientation maps. TEM foils across the dark CL grain boundaries reveal microfractures filled with nanograins of plagioclase2 containing few dislocations. FTIR maps transecting the thin section do show the presence of molecular water trapped along fractured plagioclase1 grain boundary regions. At the thin section scale, there is no measurable gradient of molecular water with increasing or decreasing distance toward the pseudotachylyte margin.

In summary, these observations suggest that (1) fracturing was in accordance to a pulverization-style fragmentation process, (2) water is from a local source; presumably coseismic fracturing released fluid inclusions enclosed within plagioclase1 and the frictional heating caused the melting of primary biotite, and (3) the little amount of molecular water freely available did not diffuse within the plagioclase grains, and did not promote hydrolytic weakening in the damage zone. Strain localization is primarily determined by repeated occurrences of extreme grain-size reduction and phase mixing, in addition to some amount of fluid wetting the grain boundaries. Therefore the “wet and weak” structure, preferential for further ductile deformation, is often the pseudotachylyte vein when present and not the surrounding damage zone.

How to cite: Michalchuk, S. P., Gies, N. B., Lüder, M., Ohl, M., Dunkel, K., Hermann, J., Plümper, O., and Menegon, L.: Deformation and healing processes in the damage zone of a lower-crustal seismogenic fault, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5618, https://doi.org/10.5194/egusphere-egu24-5618, 2024.

15:35–15:45
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EGU24-17335
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ECS
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On-site presentation
Hugo van Schrojenstein Lantman and Luca Menegon

Pseudotachylytes (frictional melts formed during seismic slip) in the metamorphosed anorthosites from Nusfjord (Lofoten, northern Norway) preserve a record of seismic rupture in the dry lower crust at 650–750 °C, 0.8 GPa. Field observations indicate that the Nusfjord pseudotachylytes represent single-earthquake events associated with large stress drops, on the order of hundreds of megapascal (MPa) to 1–2 gigapascal. Such large stress drops are interpreted to reflect the high strength of the intact anorthosite at the high confinement conditions of the lower crust. One important question is whether evidence of the high stresses necessary to initiate seismic rupture in the lower crust is preserved in the microstructure of the Nusfjord pseudotachylytes and of their damage zone.

Pyroxene deformation microstructures associated with preseismic loading and coseismic fragmentation reveal strongly localized transient stresses that presumably reached GPa-level magnitude. Here we use high-angular resolution electron backscatter diffraction (HR-EBSD) on diopside grains to obtain spatial datasets of residual stresses that are retained in the crystal lattice of diopside. We apply this method combined with microstructural analysis on diopside in a sample from a pseudotachylyte from Nusfjord to reconstruct the spatial heterogeneities of stress and link them to the earthquake cycle and associated coseismic thermal effects.

Diopside contains micro- to nanoscale deformation twins within 3 mm from the fault and in clasts in the pseudotachylyte. Strong lattice undulations are locally present in survivor clasts, indicating low-T plasticity at high stress. Residual stresses from the wall rock and in a survivor clast vary between ~600 and ~200 MPa and form a gradient of decreasing residual stress away from the pseudotachylyte, only elevated within 200 µm from the pseudotachylyte margin and with the highest values occurring within the clast. Microfaults crosscut the deformation twins, lattice undulations, and residual stress spatial heterogeneities within the clast. The latter appear strongly similar to the lattice undulations, in distribution and orientation.

The obtained stresses are lower than estimated stress drops for the locality and than stresses expected during rupture propagation (both >1 GPa). As alternative stress source, we investigated thermal stress introduced by coseismic frictional heating. Calculations demonstrate that this process is only significant over a distance of less than 100 µm in the wall rock for a stress drop of 100 MPa, and less than 10 µm for a stress drop of 1 GPa. Instead, because coseismic microfaults crosscut twinning, lattice undulations, and the spatial heterogeneities of residual stress, we interpret that these features correspond to the progressive build-up of stress during preseismic loading. An explanation for the discrepancy between the residual stresses and suggested stress drop is that the stress build-up in diopside was partially dissipated by the formation of twins. Additionally, the stress drop is estimated at the scale of the bulk fault, whereas the residual stresses are measured at the single grain scale and as such are likely to vary locally depending on the microstructure and on the different ability of different phases to dissipate the stress build-up via e.g. twinning and recovery of dislocations.

How to cite: van Schrojenstein Lantman, H. and Menegon, L.: Residual stress in diopside: insight into localized transient high stress in seismogenic faults in the lower crust, Lofoten, Norway, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17335, https://doi.org/10.5194/egusphere-egu24-17335, 2024.

Coffee break
Chairpersons: Sarah Incel, Sascha Zertani, Jesus Munoz
16:15–16:35
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EGU24-8130
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ECS
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solicited
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On-site presentation
Kristijan Rajič, Antonin Richard, Hugues Raimbourg, Tomáš Magna, Clément Herviou, Catherine Lerouge, and Romain Millot

Despite the recognition that fluids play an important role in subduction zone processes, the extent of fluid circulation and fluid-rock interactions within subduction and accretionary complexes is still not fully understood. Here, we examined Li elemental and isotopic systematics in fluid inclusions trapped within hydrothermal quartz veins in metasedimentary rocks from three paleo-accretionary complexes (Kodiak complex, Alaska; Shimanto Belt, Japan; Western Alps), which are contemporaneous with the burial and metamorphism at temperatures ranging from 250 to 400°C. To provide a fuller understanding, we investigated (i) fluid inclusions, (ii) host quartz, and (iii) wall-rocks of syn-subduction veins.

The δ7Li of fluid inclusion leachates range from −1.5‰ to +17.1‰ and are variable among three localities. Two important processes control the 7Li/6Li ratios of fluids from inclusions: (i) Li release/uptake from the host rock, and (ii) the reactive volume of the rock. Higher δ7Li values of fluids in Kodiak (+8.1‰ to +17.07‰) are interpreted as a result of closed-system behavior, with a small reactive volume of metasediments. Lithium has not been lost to the fluid, where 6Li is dominantly preserved in metamorphic chlorite and illite. In closed-system samples from the Western Alps, the fluids are buffered by the host rock, causing a shift in δ7Li values of pore fluids (from −1.5‰ to +9.5‰) towards the values of the protolith. Conversely to the samples from Kodiak, the reactive volume of rock is significantly greater, resulting in a complete fluid–rock equilibration. Equally low δ7Li values of pore fluids in Shimanto (+2.53‰ to +10.39‰) is attributed to the large flow of externally derived fluids and interpreted to result by Li leaching from illite and chlorite.

The δ7Li values of quartz are globally higher than those of paired leachates (+10.93‰ and +22.61‰) without temperature-dependent isotopic fractionation between quartz and fluid. This is explained by either (i) a significant drop in pore fluid pressure which, in turn, facilitates rapid crystallization of quartz, or (ii) post-entrapment re-equilibration between fluid inclusions and the host quartz.

By comparing the metamorphic fluids in the present study with seawater or pore water from deep sea sediments, elevated Li concentrations in leachates (up to 24 ppm) combined with relatively low δ7Li values indicate that Li is progressively leached from sediments during burial, and that the δ7Li value of fluids is consequently shifted towards the signature of the protolith. Similarities in Li concentrations and δ7Li values between leachates and fluids expulsed through mud volcanoes in modern examples of subduction zone forearcs further confirms the origin of mud volcano fluids dominantly from subducted sediments. Such similarities imply that fluid circulation across permeable zones may reach at least a 20 km-scale in the forearc region. This study further demonstrates the relevance of Li elemental and isotope systematics to efficiently trace fluids across large distances within subduction zone forearcs. 

How to cite: Rajič, K., Richard, A., Raimbourg, H., Magna, T., Herviou, C., Lerouge, C., and Millot, R.: Tracing the extent of fluid circulation in subduction zone forearcs using lithium isotopes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8130, https://doi.org/10.5194/egusphere-egu24-8130, 2024.

16:35–16:45
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EGU24-4680
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ECS
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On-site presentation
Leonie Strobl and Andrew Smye

Variations in pore fluid pressure modulate effective normal stresses along fault zones and the subducting interface. Fluid availability is controlled by the decomposition of hydrous mineral phases and the subsequent rate of drainage. Geophysical observations suggest that the plate interface is a fluid-enriched region under near-lithostatic pore fluid pressure that may result in slow slip events (SSE) and non-volcanic tremor (NVT). The potential for fluid redistribution depends on dynamic changes of the porosity and permeability of the host rock as a function of solid-bound fluid volume change and the total system volume change during dehydration. Understanding the mechanisms involved with the evolution of porosity and permeability below the seismic zone is critical to gain insight into the formation of fluid networks and their rheological implications.

Here we present a petrological and mechanical analysis of the evolution of a suite of eclogite-facies veins from an archetypal HP-LT terrain: the Eclogite Zone, Eastern Alps. We define two dominant compositional types of mafic eclogite: banded and metagabbroic, respectively. Prograde metamorphic evolutions are similar for the two types of eclogites and comprise garnet core growth at 2.1 ± 0.25 GPa, 585 ± 15°C and rim equilibration at 2.6 ± 0.2 kbar, and 630 ± 10 °C. Contemporaneously to prograde garnet growth, the mafic eclogites underwent dehydration via the breakdown of several volumetrically significant hydrous phases: lawsonite, Na-amphibole (glaucophane), and epidote. The decomposition of lawsonite and glaucophane released up to 8 wt. % H2O, resulting in the formation of a transient fluid filled porosity of ∼ 15 vol. %.

Phase equilibria calculations serve as a framework to constrain a mechanical model explaining the formation of both tensile fractures (type I) and vein segregates (type II) within the brittle-ductile transition zone. We propose a petrological-mechanical model for the formation of Type I tensile veins associated with periods of rapid dehydration and Type II dilatant structures in which rock deformation is outpaced by the reduction in pore fluid pressure, leading to a decrease in silica solubility and the precipitation of high-pressure mineral phases. Finally, this suggests that the rate of dehydration during the blueschist-eclogite transition plays a significant role in determining the dominant mode of deformation possibly affecting the fluid storage capacity of the subducting interface.

How to cite: Strobl, L. and Smye, A.: Pore-fluid pressure evolution across the blueschist-to-eclogite-facies transition:  constraints from the Eclogite Zone (Eastern Alps), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4680, https://doi.org/10.5194/egusphere-egu24-4680, 2024.

16:45–16:55
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EGU24-17707
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On-site presentation
Philippe Goncalves, Thomas Leydier, Julie Albaric, Pierre Trap, and Kevin Mahan

Geophysical observations have led to the conclusion that slow earthquakes (EQ) occur in the ductile realm at depths greater than ~20 km, in domains of low Vp and elevated Vp/Vs, consistent with high pore fluid pressure conditions and/or fluctuating fluid conditions. Furthermore, slow EQ are characterized by concomitant viscous aseismic slip and transient frictional slip responsible for tectonic tremors and low frequency earthquakes. Understanding the physics of slow earthquakes can be done by integrating geophysics, rock deformation experiments and numerical models with the observation and characterization of the possible rock record of slow earthquakes.

In this contribution, we contribute to the quest for geological records of slow EQ by adding a new example to the fast growing list of examples. Our approach is based on field observations, petrological and microtextural analysis carried out on exhumed shear zones in late Variscan volcanic rocks from the Suretta nappe (Central Alps, Switzerland). We propose that in continental collision settings, like the Alps, exhumed continental shear zones preserve geological evidence that may be related to paleo-slow earthquakes. We show that burial of continental units is characterized by concomitant frictional and viscous deformation in the ductile realm at temperature conditions above 450°C for a depth range between 18 and 25 km, which resembles those where slow earthquakes are expected. The finite geometry of the shear zone consists of a network of anastomosed mica-rich weak and high strain ductile shear zones of various size (m to km) bounding high strength and low strain domains, resulting in a “mélange” rheology. At a smaller scale, the localization of millimeter to centimeter wide ductile shear zones is controlled by the prior development of a damaged zone that has been detected by imaging volcanic quartz phenocrysts with cathodoluminescence. This damage zone is defined by a domain of high density healed cracks and fluid inclusion planes preserved only in volcanic quartz phenocrysts. These microcracks, follow a riedel-type geometry consistent with the ductile kinematics. The ductile shear zones are commonly crosscut by mono-mineralic quartz veins of a few millimeters in thickness parallel to the shear zone walls and localized in the middle of the shear zone. Quartz veins are characterized by a crack-seal texture with elongated blocky quartz grains perpendicular to the vein-wall interface suggesting that quartz was precipitated from a fluid in a dilatant fracture. These quartz veins are always overprinted by ductile deformation. Ductile deformation is characterized by dynamic recrystallization of the blocky quartz into small new grains formed by bulging recrystallization. When ductile deformation overprinting is high, quartz veins are sheared, isoclinally folded and almost entirely recrystallized into a fine grain aggregate.

How to cite: Goncalves, P., Leydier, T., Albaric, J., Trap, P., and Mahan, K.: Concomitant brittle-ductile deformation, fluid-flow and metamorphism during continental subduction : a slow earthquake rock record in the Suretta nappe (Central Alps, Switzerland) ?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17707, https://doi.org/10.5194/egusphere-egu24-17707, 2024.

16:55–17:05
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EGU24-639
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ECS
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On-site presentation
Ritabrata Dobe, Francesco Giuntoli, and Alberto Vitale Brovarone

Carbon recycling in subduction zones involves metamorphism and fluid-rock interactions that are responsible for dissolution or destabilization of carbon-bearing minerals. Such large-scale devolatilization of carbonates should impose profound alterations in the rheology of subducted lithologies, but this is an aspect that has received relatively scant attention so far.  

This work focuses on the mechanical behaviour of carbonates in subducted carbonated serpentinites that constitute a substantial fraction of carbon input into subduction zones. These investigations have been conducted on carbonated serpentinites from the Negru Shear Zone in Corsica (France). Petrographic and fluid inclusion analyses indicate that these rocks recorded partial carbonate reduction by infiltrating H2-rich fluids, as indicated by the conversion of carbonate to graphite and CH4(Peng et al., 2021; Vitale Brovarone et al., 2017). The first generation of carbonate occurs as mm-sized equigranular, subhedral dolomite, with sutured grain boundaries (hereafter referred to as Carb1) along which graphite is distributed as discontinuous seams. Carb1 is fractured and brecciated, with limited evidence for crystal plasticity. A second generation of carbonate (calcite; Carb2), is observed in sheared carbonate + serpentinite domains, wherein the proportion of graphite is substantially higher. Carb2 grains are anhedral, elongate and form S-C structures within localized (~200 microns thick) shear zones.  

Electron Backscatter Diffraction (EBSD) analyses on the different carbonate domains provide greater insights into the deformation of Carb1&2. The microstructures within the dolomite-rich domains are dominated by twinning, with a strong crystallographic preferred orientation manifested by an M-index of 0.61. Dolomite grains display limited low angle boundaries and dislocations, which imply minimal strain accommodation by crystal plasticity and recrystallization during deformation. The occurrence of extensive twinning in dolomite coupled with antigorite being the dominant serpentine mineral, constrains the brecciation of dolomite grains to temperatures higher than 380 °C during the high-pressure evolution of Alpine Corsica. On the other hand, calcite grains within the shear zones have a weaker preferred orientation (M-index of 0.086), abundant low angle boundaries and dislocations, and lesser twin boundaries compared to the dolomite grains. Our observations are relevant for an improved understanding of the deformation of carbonated lithologies in faults associated with subduction zones. If these lithologies are dominated by dolomite, brecciation, likely associated with seismicity, may be the dominant mechanism of deformation, as crystal plastic mechanisms within dolomite are non-operative at the temperatures (<400°C) and pressures (~1GPa) prevalent till at least intermediate depths within subduction zones. On the other hand, if calcite is the dominant carbonate mineral undergoing subduction, crystal plasticity may be the dominant mechanism that accommodates strain. The presence of graphite in association with both dolomite and calcite rules out the possibility of it having influenced these rheological variations. Our results provide novel insights into the role of chemistry in controlling the rheology of carbonated lithologies undergoing subduction, with implications on our understanding of the localization of seismicity in subduction settings.   

How to cite: Dobe, R., Giuntoli, F., and Vitale Brovarone, A.: Fractures versus flow: How variations in carbonate composition control the rheology of altered subducting rocks , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-639, https://doi.org/10.5194/egusphere-egu24-639, 2024.

17:05–17:15
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EGU24-13445
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Virtual presentation
Bjørn Eske Sørensen, Eric James Ryan, Rune Larsen, Stefanie Lode, Kristian Drivenes, and Alf Andre Orvik

This study demonstrates how the response of ultramafic lithologies to infiltrating H20-CO2 fluids depends on the primary mineralogy. This has major implications on fluid flow through the lower crust and upper mantle as mineral reactions control the permeability and rheology. The studied samples are from the hanging wall of a 2 kilometer-long transtensional shear zone within ultramafic-mafic rocks in the Reinfjord Ultramafic Complex (RUC), part of the Seiland Igneous Province (SIP) in Northern Norway.

Fluid-rock interaction surrounding shear zones with abundant pseudotachlylites is highly variable and depends on bulk rock compositions. Thermodynamic modelling demonstrates that mineral reactions involving hydration and carbonation differ between dunitic rocks and the pyroxenitic dykes which intersect them. Alteration of dunitic rocks results in the formation of dominantly magnesite-anthophyllite-talc and talc-magnesite assemblages causing approximately 12% volume expansion. This results in a sharp reaction front contacts with the host rock. When the alteration zones cross the dunite-pyroxenite boundary the associated alteration has a more gradual boundary towards the unaltered rock and the alteration zone widens by approximately 40%. In contrast to the simpler dunite alteration assemblage, the pyroxenetic dykes are altered to a complex mixture of cummingtonite-anthophyllite, magnetite and chlorite. Additionally, orthopyroxene is completely pseudomorphed by a mixture of cummingtonite and magnetite, whereas olivine xenocrysts are partly preserved and surrounded by a magnesite-anthophyllite assemblage. Other, open cavity-like areas are filled by chlorite, amphibole, and Mg-MgCa carbonates, indicating volume reduction during alteration of the pyroxene.

Accordingly, dunite alteration effectuates a significant volume expansion, and are therefore only altered locally during seismic creep events. The pyroxenites are near volume neutral throughout interaction with the same fluids, and are thus more homogeneously altered. The formation of chlorite in hybrid compositions, such as the dykes in the lower crust, may create weak permeable zones that are consequently exploited as pathways for fertile mantle fluids and will hence also be the locus of ore bearing fluids moving to the upper crust. Increased understanding of fluid mediated metamorphism increases our current knowledge on fluid flow and strain localization in the lower crust. We further suggest that the hydrothermal assemblages are closely related to deformation leading to the formation of grain size sensitive creep in olivine facilitated carbonation of olivine and clinopyroxene to form orthopyroxene and dolomite and associated pseudoctacylites in the peridotites (Sørensen et al., 2019) , commonly associated with volatile rich mafic dykes (Ryan et al., 2022). Either the ductile magnesite-chlorite-talc assemblages formed at the same time in a shear-related heat gradient or they formed during cooling and continued CO2 infiltration from depth through the shearzones.

 

Ryan E J, et al.  2022  Infiltration of volatile-rich mafic melt in lower crustal peridotites provokes deep earthquakes.  J. Struct. Geol. (https://doi.org/10.1016/j.jsg.2022.104708)

Sørensen, B.E., et al., 2019 In situ evidence of earthquakes near the crust mantle boundary initiated by mantle CO2 fluxing and reaction-driven strain softening. Earth and Planetary Science Letters (https://doi.org/10.1016/j.epsl.2019.115713 )

 

How to cite: Eske Sørensen, B., Ryan, E. J., Larsen, R., Lode, S., Drivenes, K., and Orvik, A. A.: Linkage between ductile deformation, pseudotachylites, strain softening and volume expanding carbonation reactions during mixed-volatile infiltration in ultramafic-mafic rocks from the Reinfjord lower crustal field laboratory , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13445, https://doi.org/10.5194/egusphere-egu24-13445, 2024.

17:15–17:25
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EGU24-13986
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ECS
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On-site presentation
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Diana Mindaleva, Masaoki Uno, and Noriyoshi Tsuchiya

Fluid flow in the crust induces fluid-rock reactions and contributes to earthquake triggering. However, there are limited numerical constraints on the fluid volumes with the available duration of fluid infiltration. There is also a gap in our knowledge of time-integrated fluid fluxes estimated from geological samples and their influence on controlling seismic/aseismic activity. Merging the timescales of fluid infiltration with the transport properties estimated from the geological samples such as metamorphic reaction zones is essential to understanding the fluid flux during crustal fracturing and its influence on controlling some characteristics of seismic/aseismic events.

This study focuses on fluid flow through a single fracture and the fluid-rock reaction zones and applies its results to low-magnitude fracturing events, such as tremors and low frequency earthquakes. Physical properties of fluid flow provide an opportunity to calculate the seismic moment and cumulative magnitude of the possibly triggered seismic/aseismic event.

Particularly, to examine the duration of fluid infiltration and time-integrated fluid fluxes we analyze amphibolite-facies fluid-rock reaction zones and then combine with estimates of possible associated seismicity and conclude that flow along a single fracture is compatible with seismicity of non-volcanic tremor and low frequency earthquakes. This study is based on evidence of rapid fluid infiltration (~10 h) caused by crustal fracturing and permeability evolution from low- to highly-permeable rocks (~10−9–10−8 m2).

Time-integrated fluid fluxes perpendicular to a given fracture and those through the fracture were estimated. Coupled methodology, including reactive-transport modeling and thermodynamic analyses, based on Si alteration processes within reaction zones is used to estimate fluid volumes involved in triggering seismic activity. Time-integrated fluid flux through the fracture results in 103-6 m3/m2. The lower range is similar to the fluxes through the upper crustal fracture zones (~103-4 m3/m2), while almost the whole range is comparable to the contact metamorphism zone (~102-5 m3/m2).

Fluid volumes transported through the fracture were compared with fluid injection experiment results. We also compare the durations of fluid infiltration to the durations of the slow slip events. There is no universal theory of slow slip phenomena from the perspectives of geological and geophysical properties. In terms of pressure and temperature, high-grade metamorphic rocks can be related to slow slip events. Our finding reveals that the transportation of voluminous fluid volumes through a fracture may be related to short seismic/aseismic events such as tremors and LFEs, as suggested from duration (~10 h) and cumulative magnitude, representing the maximum values as 2.0–3.8, the lower limit of the magnitude for a single fluid-driven seismic event as –0.6 to 0.2. Single fractures described in this study make it possible to transfer voluminous fluid flow. They could be an essential control on the generation of seismic activity above the tremor and slow slip events source regions in the lower–middle crust.

How to cite: Mindaleva, D., Uno, M., and Tsuchiya, N.: Dynamic and short-lived fluid flow in the high-grade metamorphic rocks related to seismic events in the middle crust., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13986, https://doi.org/10.5194/egusphere-egu24-13986, 2024.

17:25–17:35
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EGU24-14313
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ECS
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On-site presentation
Jo Moore, Sandra Piazolo, Andreas Beinlich, Håkon Austrheim, and Andrew Putnis

The initiation of shear commonly occurs spatially associated with fluid-rock reactions along brittle precursors. In many cases the relative timing of fracturing, fluid infiltration, reaction, and recrystallisation is unclear, making it difficult to disentangle mechanisms of shear zone formation from subsequent deformation and recrystallisation. Here we present the transition from an anhydrous and relatively undeformed precursor rock into a highly deformed and hydrated plagioclase-rich rock. The studied outcrop remarkably preserves both (1) the interface between the anhydrous granulite-facies parent lithology and a statically hydrated amphibolite-facies rock, and (2) a transition from statically hydrated amphibolite to the sheared amphibolite-facies lithologies. Detailed study of plagioclase chemistry and microstructures across these two interfaces using Electron Backscatter Diffraction (EBSD) and wavelength dispersive spectrometry (WDS) allow us to assess the degree of coupling between deformation and fluid-rock reaction across the outcrop. Plagioclase behaves dominantly in a brittle manner at the hydration interface and so the initial weakening of the rock is attributed to grain size reduction caused by fracture damage at conditions of ca. 720°C and 10-14 kbar. Extensive fracturing induced grain size reduction locally increases permeability and allows for continuing plagioclase and secondary mineral growth during shear, as evidenced by a general increase in the amount of hydration reaction products across the shear zone interface. Due to the apparent coupling of deformation and reaction, and the plagioclase microstructures such as, an inherited but dispersed crystallographic preferred orientation (CPO), fine grain size (5-150 µm), and truncation of chemical zoning, we conclude that deformation is dominantly facilitated by dissolution-precipitation creep in the shear zone.

How to cite: Moore, J., Piazolo, S., Beinlich, A., Austrheim, H., and Putnis, A.: Brittle initiation of dissolution-precipitation creep in plagioclase-rich rocks: Insights from the Bergen arcs, Norway, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14313, https://doi.org/10.5194/egusphere-egu24-14313, 2024.

17:35–17:45
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EGU24-9352
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On-site presentation
Luca Menegon, Oleksandra Valter, and Sven Dahlgren

The Fen Carbonatite Complex in Norway contains the largest deposit of Rare Earths Elements (REE) in Europe, with estimated resources in the range of 30 – 50 Mt of total Rare Earth Oxides. If Fen will be targeted as an exploitable mineral resource, the geological processes that formed it must be understood, with specific emphasis on what controls the location and composition of the REE resources.

The Fen complex formed at 580 Ma through different stages of carbonatite melt intrusions followed by hydrothermal alteration. Fluid- and melt-assisted deformation accompanied the intrusive and hydrothermal evolution of the Fen Complex extensively and resulted in the formation of shear zones and breccias. However, the mechanism and significance of carbonatite deformation are poorly understood, and so are the effects of post-crystallization deformation processes on the remobilization of trace elements in carbonatites.

This study investigates deformation processes and REE remobilization in shear zones in dolomite-carbonatites from Fen. The shear zones display a compositional banding defined by alternating dolomite- and apatite-rich layers, where apatite grains are variably elongated with aspect ratio ranging from 2 to 11 and grain length from 50 to 500 µm. SEM images reveal the presence of carbonatitic melt pseudomorphs in the form of intergranular beads, cusps, films, and pools, which are particularly evident in the apatite layers, where individual grains are locally entirely rimmed by melt films. The apatite grains appear zoned in cathodoluminescence (CL) images, with dark cores and bright rims that are thicker parallel to the foliation. In the most elongated grains, the dark core forms less than 20% of the grain area, which is otherwise dominated by the bright rim. On the contrary, more equidimensional grains are dominated by the dark core. Hyperspectral analysis of CL images indicates that the elongated rims of apatite are enriched in REE (particularly in Nd) compared to their core. Electron backscatter diffraction (EBSD) analysis demonstrates that (1) the elongated apatite grains are internally strain free, and (2) grain elongation occurs parallel to apatite c-axis.

Our data show that deformation of apatite occurred by melt-assisted dissolution-precipitation creep, which was responsible for grain elongation and remobilization of REE. Thus, post-crystallization deformation and melt-rock interaction played an important role in redistributing REE within the Fen Complex.

How to cite: Menegon, L., Valter, O., and Dahlgren, S.: Deformation-induced Rare Earth Elements (REE) redistribution in apatite from the Fen Carbonatite Complex (Norway), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9352, https://doi.org/10.5194/egusphere-egu24-9352, 2024.

17:45–17:55
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EGU24-17549
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ECS
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On-site presentation
Berit Schwichtenberg, Marco Herwegh, Alfons Berger, Christoph Schrank, Teo Neuenschwander, Sandro Truttmann, Michael W. Jones, Stefano M. Bernasconi, Dominik Fleitmann, and Cameron M. Kewish

Worldwide, fault zones in carbonates regularly host medium to large earthquakes including recent ones in the Mediterranean and Middle East. In addition to that, faults can control fluid flow by either acting as a conduit or seal for fluid pathways and should be considered in e.g., geothermal exploration. Hence, understanding the (micro-) structural evolution of these fault zones as well as fluid mediated geochemical processes involved in their dynamic deformation history allows to better address topics of societal and economic relevance ranging from seismic hazards to the exploitation of natural resources. Unfortunately, active in-situ deformation at depth is difficult to access, emphasising the need for investigations on suitable exhumed analogues.

This study focuses on the microstructural and geochemical record of a recently exposed seismogenic dextral strike-slip fault zone in the seismically active southwestern Swiss Alps. Due to excellent outcrop conditions on glacially polished rock surfaces and a wide range of preserved tectonites and associated deformation structures, this particular fault zone provides a valuable record of potential paleoseismicity in carbonates. We combined microstructural analyses with micro-chemical and isotope data in order to reconstruct the spatio-temporal evolution of high-strain domains at variable crustal levels throughout exhumation. While the microstructural record allows us to differentiate between rate-dependent brittle and viscous deformation phases, we use the geochemical fingerprint to distinguish and characterize individual fluid pulses.

Here, we present microstructural evidence of fast, possibly seismic, deformation along a principal slip zone. While injection structures containing fluidized material, suggest highest deformation rates as feasible for seismic events, repeated brittle deformation that was accompanied by the formation of cataclasites and calcite veins, hints towards fast seismic to sub-seismic rates.
We also found that newly formed calcite crystals, in veins and linkage zones, show significantly decreasing δ18OSMOWvalues, as low as 5 ‰ δ18OSMOW, implying an influence of meteoric water. Clumped isotope thermometry of such calcites resulted in temperatures of 65-95°C, which are approximately 100°C lower than Tmax in the area. This suggests that the analyzed material did not record any potential shear heating. Moreover, the investigated tectonites have most likely formed along a retrograde exhumation path. 

In combination with detailed observations on the m- to 10er-m-scale our observations provide a dataset that allows direct comparison of different deformation processes and correlation of paleo-seismicity to fluid flow in fault zones. Further, we contribute to the longstanding discussion of differentiating microstructural evidence for seismic slip from slow or aseismic slip in carbonate hosted fault zones.

How to cite: Schwichtenberg, B., Herwegh, M., Berger, A., Schrank, C., Neuenschwander, T., Truttmann, S., Jones, M. W., Bernasconi, S. M., Fleitmann, D., and Kewish, C. M.: Paleo-seismic and aseismic processes and the role of fluids recorded in an exhumed carbonate fault , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17549, https://doi.org/10.5194/egusphere-egu24-17549, 2024.

17:55–18:00

Posters on site: Fri, 19 Apr, 10:45–12:30 | Hall X2

Display time: Fri, 19 Apr, 08:30–Fri, 19 Apr, 12:30
Chairpersons: Giacomo Pozzi, Giovanni Toffol, Sarah Incel
X2.92
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EGU24-4143
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ECS
Shattered veins elucidate brittle creep and slip processes in deep subduction interfaces
(withdrawn after no-show)
Jesus Munoz, Samuel Angiboust, Antonio Garcia-Casco, and Tom Raimondo
X2.93
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EGU24-4702
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ECS
Jesus Munoz, Whitney Behr, Dominic Hildebrant, and Leif Tokle

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.

X2.94
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EGU24-6361
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ECS
Mattia Pizzati, Anita Torabi, Luca Aldega, Cristian Cavozzi, Fabrizio Storti, and Fabrizio Balsamo

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.

X2.95
|
EGU24-6392
|
ECS
Takeru Yoshimoto, Michael Manga, Sarah Beethe, Iona McIntosh, Adam Woodhouse, Shun Chiyonobu, Olga Koukousioura, Timothy Druitt, Steffen Kutterolf, and Thomas Ronge and the IODP Exp. 398 Scientists

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.

X2.96
|
EGU24-7188
Keishi Okazaki, Samuele Papeschi, Kenta Kawaguchi, and Takehiro Hirose

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.

X2.97
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EGU24-8060
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ECS
Patrick Bianchi, Paul Antony Selvadurai, Luca Dal Zilio, Antonio Salazar Vásquez, Claudio Madonna, Taras Gerya, and Stefan Wiemer

The initiation of unstable fault slip leading to earthquakes involves intricate physical processes and interactions. Understanding these mechanisms is crucial for advancing our knowledge in earthquake seismology. Investigations at both field and laboratory scales have highlighted the existence of spatio-temporal variations in seismic or aseismic observations near the epicenter of a major seismic event, such as a rise in frequency of precursory earthquakes (Kato and Ben-Zion, 2021)  or even strong fluctuations in seismic velocities (e.g., Campillo & Paul, 2003). These variations are often associated with the preparatory phase of major earthquakes believed to involve processes resulting from progressive localization of deformation around the eventual rupture zone that eventually accelerates leading up to failure. However, the time and spatial scales of this behavior are not well understood due to our lack of understanding into the physical mechanisms within the preparatory zones.

In this study, we combined innovative laboratory techniques and numerical modelling to investigate (a)seismic preparatory deformation during a triaxial failure test in the laboratory. Employing distributed strain sensing (DSS) with optical fibers, we closely monitored strain rates on the sample surface. This was supplemented by active ultrasonic surveys and passive acoustic emission (AE) monitoring to investigate changes in P-wave velocity and locate regions prone to AEs within the sample. Using a physics-based computational model, we investigated strain localization within the sample by monitoring rock regions exhibiting high dissipation of mechanical energy. Highly dissipative regions spatio-temporally correlated with the observed AE locations and with sample regions experiencing P-wave velocity reduction. By further tracking the dissipation field within the sample, we recognized a system of conjugate bands that first emerged and quickly merged into a single band growing from the center towards the sample surface. The latter was interpreted to be related to the preparation of a weak plane. Shortly prior to failure, the model showed an acceleration of deformation that was also observed during the laboratory test with the DSS measurements and correlated with an increase of the seismicity rate in a similar volume of the sample. The combination of increased deformation and seismic rates mimics observations of precursory seismicity in nature. By methodically segregating the laboratory experiments from the numerical modeling, this study provides a comprehensive analysis of the physical processes underlying earthquake nucleation. The integration of cutting-edge laboratory techniques with advanced numerical modeling offers a novel perspective on the (a)seismic preparatory deformation that sets the stage for major seismic events.

 

References:

Campillo M., Paul A. (2003) Science 299, 547-549.

Kato, A., Ben-Zion, Y. (2021) Nat Rev Earth Environ 2, 26–39.

How to cite: Bianchi, P., Selvadurai, P. A., Dal Zilio, L., Salazar Vásquez, A., Madonna, C., Gerya, T., and Wiemer, S.: Exploring fault preparation and earthquake nucleation from the laboratory, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8060, https://doi.org/10.5194/egusphere-egu24-8060, 2024.

X2.98
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EGU24-8597
Yoshitaka Hashimoto, Jinpei Mitani, Rüdiger Kilian, Rebecca Kühn, and Michale Stipp

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.

X2.99
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EGU24-8751
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ECS
Lefan zhan, Shuyun Cao, Yanlong Dong, Wenyuan Li, Christoph von Hagke, and Franz Neubauer

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.

X2.100
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EGU24-9295
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ECS
Giacomo Pozzi, Giuseppe Volpe, Roberta Ruggieri, Cristiano Collettini, Marco Scuderi, Telemaco Tesei, Chris Marone, and Massimo Cocco

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.

X2.101
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EGU24-10106
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ECS
Michele Mauro, Michele De Solda, Carolina Giorgetti, and Marco Scuderi

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.

X2.102
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EGU24-10481
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ECS
Serena Cacciari, Giorgio Pennacchioni, Enrico Cannaò, Marco Scambelluri, and Giovanni Toffol

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 literature2,4, 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.

X2.103
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EGU24-11900
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ECS
Leonardo Salvadori, Telemaco Tesei, and Giulio Di Toro

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.

X2.104
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EGU24-14788
Yaniv Edery, Shaimaa Sulieman, Martin Stolar, Ludmila Abezgauz, and Shouceng Tian

The drilling of geothermal energy, CO2 sequestration, and wastewater injection all involve the pressurized flow of fluids through porous rock, which can cause deformation and fracture of the material. Despite the widespread use of these industrial methods, there is a lack of experimental data on the connection between the pore pressure rise, the deformation and permeability changes in real rock. In order to address this gap in the literature, this study developed an artificial rock material that can be deformed and fractured at low pressures. By controlling the porosity, permeability, and strength of the material during the sintering process, it is possible to mimic various types of rock. The artificial rock was designed to accommodate radial flow and deformation, allowing for the tracking of deformation by monitoring the flux and driving pressure and thus calculating the permeability changes under various pressure conditions. The study was able to examine the impact of both ductile and brittle deformation on the permeability during pressurized flow, which were captured by two models that were adjusted to this scenario. This study provides a link between pressurized flow, rock formation permeability and ductile to brittle deformation, that can constrain risk assessment to geothermal energy and CO2 sequestration.

How to cite: Edery, Y., Sulieman, S., Stolar, M., Abezgauz, L., and Tian, S.: Investigating the Permeability Evolution of Artificial Rock During Ductile and Brittle Deformation Under Pressurized Flow, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14788, https://doi.org/10.5194/egusphere-egu24-14788, 2024.

X2.105
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EGU24-15776
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ECS
Roberto Emanuele Rizzo, Samuele Papeschi, Edoardo Baroncini, and Paola Vannucchi

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.

X2.106
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EGU24-16363
Giorgio Pennacchioni and Giovanni Toffol

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/veins1, 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 (stage3A and stage3B). Stage3A 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. Stage3B is represented by a pervasive system of vertical extensional chlorite-quartz-filled veins, epidote-filled hybrid fractures and faults, that crosscut and offset stage3A structures. The stage3B 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 Stage3B 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 (stage3A); and (ii) formation of new extensional and shear fractures, that disregard previous anisotropy, under fluid-present conditions and transient low differential stress (stage3B). 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.

X2.107
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EGU24-16478
Simon R. Wallis, Kazuhiko Ishii, Yukinojo Koyama, and Takayoshi Nagaya

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.

X2.108
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EGU24-17072
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ECS
Giovanni Guglielmi, Telemaco Tesei, and Giulio Di Toro

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.

X2.109
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EGU24-17448
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ECS
Weijia Zhan, Natalia Nevskaya, André Niemeijer, Alfons Berger, Christopher Spiers, and Marco Herwegh

In the granitoid crust, phyllosilicate-rich fault gouges are prevalent in mature fault zones undergoing hydrothermal alteration and often exhibit lower frictional strength compared to framework minerals (e.g., qtz, fds) under deformation at room temperature. However, the mechanical behavior and deformation mechanisms of altered gouges under hydrothermal conditions are not fully understood so far.

To investigate these effects, we conducted a series of experiments on three types of fault “gouge” material using a ring shear deformation apparatus. We used gouge mixtures obtained from (i) crushed granitoid ultramylonite, (ii) biotite- and (iii) muscovite-bearing gouges to represent quartzofeldspartic materials with (i) no alteration, (ii) high-temperature and (iii) low-temperature alteration, respectively (see Table 1 for the mineralogy). Deformation temperatures (T) ranged from 20-650°C, with a sliding velocity kept at 1 μm/s, and an imposed effective normal stress and pore fluid pressure at 100 MPa. At large shear strain (g ≈ 22-25) and T = 20-450°C, granitoid gouges consistently showed higher shear stresses (t = 73-81 MPa) than muscovite- (t = 47-69 MPa) and biotite-bearing gouges (t = 44-56 MPa). Granitoid gouges showed a decrease in t at T ≥ 450°C, while mica-rich gouges showed an increase in t with T at all tested conditions. Microstructurally, all gouges experienced strain localization into relatively fine-grained and dense principal slip zones (PSZs) at elevated T. The presence of newly percipitated minerals (e.g. bt, qtz) suggested the operation of dissolution-precipitation creep (DPC). However, the PSZs of granitoid and mica-rich gouges exhibited distinctive geometric features in their microstructure at 650°C. Granitoid gouges showed PSZs with ultrafine-grained (≤ 1 μm) relicts of porphyroclasts sparsely distributed within a dense matrix. In contrast, the PSZs of mica-rich gouges showed the anastomosing P-foliation of aligned micas with intervening shear band cleavages. Within these localized domains, quartz in mica-rich gouges exhibited larger grain sizes (1-4 μm) compared to those in granitoid gouges.  

Our observations indicate that in all tested gouges, frictional deformation gives way to grain-size sensitive creep mechanism as T rises, leading to the formation of fine-grained PSZs. We suggest that the ultrafine grain sizes in granitoid gouges promote DPC-accommodated viscous granular flow more efficiently, leading to the low shear stresses. In contrast, the strengthening of altered gouges with T was attributed to two factors: a less efficient DPC-assisted deformation due to generally larger grain sizes, and a less efficient viscous granular flow due to the development of foliation and shear bands inclined to the shear direction. Therefore, the mechanical behaviour of granitoids along the retrograde hydration-path depends not only on the evolving mineralogy, but also on microstructures and grain sizes.

Table 1. List of Samples Used in This Study and Their Mineralogy According to Quantitative XRD.

Sample

Composition (wt%)

Altreation type

Granitoid ultramylonite

37% qtz, 49% fds, 8% bt, 6% ep

No alteration

Biotite-bearing natural fault gouge

35% qtz, 4% fds, 37% phl, 21% mus, 3% smc

High-temperature

Muscovite-bearing natural fault gouge

39% qtz, 5% fds, 38% mus, 11% ser, 6% chl, 1% cal

Low-temperature

Qtz:quartz, fds: feldspar, bt: biotite, ep: epidote, phl: phlogopite, mus: muscovite, ser: sericite, smc: smectite, chl: chlorite, cal: calcite

How to cite: Zhan, W., Nevskaya, N., Niemeijer, A., Berger, A., Spiers, C., and Herwegh, M.: Hydrothermal Alteration-Induced Weakening in Experimentally Deformed Fault Gouges, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17448, https://doi.org/10.5194/egusphere-egu24-17448, 2024.

X2.110
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EGU24-17577
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ECS
Akbar Aydin Oglu Huseynov, Jeroen van der Lubbe, Suzan Verdegaal - Warmerdam, Onno Postma, Klaudia Kuiper, and Jan Wijbrans

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.

Posters virtual: Fri, 19 Apr, 14:00–15:45 | vHall X2

Display time: Fri, 19 Apr, 08:30–Fri, 19 Apr, 18:00
Chairpersons: Giacomo Pozzi, Giovanni Toffol, Sarah Incel
vX2.9
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EGU24-14574
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ECS
Shunya Okino, Atsushi Okamoto, Yukiko Kita, Sando Sawa, and Jun Muto

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.