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EMRP1.17

The interplay between brittle and ductile deformation – the semi-brittle regime from Earth layers and laboratory experiments

This session aims to discuss recent advances in our understanding of the mechanical, structural, and seismic properties of the lower crust and upper mantle. Earth’s lithosphere is defined by its rheological and mechanical stability. Both shear localization and seismic instabilities are frequently observed through regional seismicity, laboratory experiments, numerical models,, and field observations, e.g., pseudotachylyte-bearing faults. In contrast to the shallow crust, dominated by brittle deformation, localized faulting, and frictional sliding, deformation observed in the mid- to lower- crust and upper mantle often displays localized faulting alongside more distributed flow and evidence for a variable mix of brittle and plastic deformation mechanisms. Because brittle and plastic deformation take place simultaneously over different time scales, the interplay between them is complex, and the resulting semi-brittle deformation style is still poorly understood. However, numerous enigmatic deformation phenomena, including slow-slip and lower crustal earthquakes, coincide with depths that either fall within, or mark the boundaries of, the semi-brittle field -- the brittle to ductile and the ductile to plastic transitions. We encourage geoscientists from across experimental geophysics, structural geology, seismology, and geodynamics with an interest in the interplay between different deformation mechanisms to contribute to this session.

Co-organized by TS2
Convener: Yuval BonehECSECS | Co-conveners: Sarah Incel, Keishi OkazakiECSECS, Lucy CampbellECSECS, Anna Rogowitz
Presentations
| Thu, 26 May, 17:00–18:30 (CEST)
 
Room -2.31

Thu, 26 May, 17:00–18:30

Chairpersons: Yuval Boneh, Sarah Incel, Anna Rogowitz

17:00–17:01
Introduction

17:01–17:07
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EGU22-5863
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ECS
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On-site presentation
Weijia Zhan et al.

Fault gouges of granitoid composition represent the principal non-cohesive tectonites within fault zones in the continental crust. Their velocity-dependent friction is crucial for understanding earthquake nucleation and the depth distribution of fault-related seismicity in granitoid shear zones (Wehrens et al. 2016; Blanpied et al. 1998). In the framework of rate-and-state friction laws (RSF), the friction parameter (a-b) is measured in sliding experiments to describe the velocity dependence of friction in fault gouges (Scholz, 1998). A velocity-strengthening system is frictionally stable, (a-b) >0, whereas a velocity-weakening system can be frictionally unstable, (a-b) <0. In earthquake mechanics, velocity weakening is prerequisite for stick-slip deformation, i.e. the nucleation of earthquakes. Although (a-b) values of granitoid gouge are sensitive to varying temperature conditions and sliding velocities, only a few studies have examined this velocity-dependence under hydrothermal conditions.

To address this issue, we conducted velocity stepping sliding experiments under hydrothermal conditions by using a ring shear apparatus. The powdered starting gouge was derived from a granitoid mylonite collected at the NAGRA Grimsel Test Site (Central Swiss Alps). The applied velocity steps were 1-3-10-30-100 μm/s. Pore fluid pressure and the effective normal stress were 100 MPa. Temperatures explored ranged from 20-650 °C. Values of (a-b) were obtained from RSF model inversions of the evolution of friction coefficients at mechanical steady state conditions. Our experiments showed pronounced changes in (a-b) values with across the full range of temperatures up to 650 °C and velocities investigated. At temperatures below ~100 °C and above ~400 °C, we observed mostly velocity strengthening with positive (a-b). In contrast, velocity weakening with negative (a-b) was observed between ~100 °C and ~400 °C. Samples deformed at a sliding velocity of 100 μm/s deviated slightly from this trend, as (a-b) values were negative between ~200 °C and ~400 °C.

The presented experimental study demonstrates a significant influence of temperature and sliding velocity on velocity-dependence during deformation of granitoid gouge. We suggest that the observed transitions in velocity dependence reflect an interplay of interactions. In terms of crustal faulting, our data suggest the existence of a seismogenic window that limits the depth distribution of earthquakes on faults in granitoid shear.

 

REFERENCES

Wehrens, P. C., Berger, A., Peters, M., Spillmann, T., Herwegh, M. 2016: Deformation at the frictional-viscous transition: Evidence for cycles of fluid-assisted embrittlement and ductile deformation in the granitoid crust, Tectonophysics, 693, 66-84.

Blanpied M. L., Tullis T. E., Weeks J. D. 1998: Effects of slip, slip rate, and shear heating on the friction of granite.

Scholz, C. H. 1998: Earthquakes and friction laws, Nature, 391, 37-42.

How to cite: Zhan, W., Niemeijer, A., Nevskaya, N., Berger, A., Spiers, C., and Herwegh, M.: Velocity-dependent friction of granitoid gouge under hydrothermal conditions: A contribution to understanding of fault zone seismicity, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5863, https://doi.org/10.5194/egusphere-egu22-5863, 2022.

17:07–17:13
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EGU22-3971
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ECS
Francesco Figura et al.

The majority of the seismic events in the Mediterranean region are hosted in carbonate-bearing rocks at depths representative of the semi-brittle regime. Within this regime, both brittle behavior (i.e. deformation is localized on the fractures and on the faults) and ductile one (i.e. deformation is distributed and accommodated in the rock core) coexist. The influence of this interplay on the nucleation and propagation of seismic events is poorly studied. Up to now, most experimental work has been conducted far from in-situ conditions, mostly at room temperature and low confining pressure.

Here we constrain the frictional behavior of faults in carbonate rocks under conditions relevant for their brittle-to-ductile transition. Velocity-step experiments are performed through the HighSTEPS (Strain, TEmperature, Pressure, Speed) biaxial apparatus installed at EPFL, investigating sliding velocities from 10-6 m/s to 10-2 m/s. Experiments are conducted under different values of confining pressure (Pc 15 MPa and Pc 50 MPa) and normal stress (σn 29 MPa and σn 95 MPa) on the experimental faults, keeping the ratio between them constant (around 2). The local strain field along the fault was measured with strain gauges. The collected data were modeled with rate-and-state friction laws (RSFLs) to define the rate and state parameters relate to the critical condition for fault stability. Moreover, microstructural observations of the post mortem sample were conducted at the SEM, to investigate the deformation mechanisms active during the experiments.

These results shed light on the evolution of rate-and-state frictional parameters with depth, as well as their dependence on the strain partitioning between on-fault slip and bulk-accommodated deformation with increasing depth.

How to cite: Figura, F., Giorgetti, C., Meyer, G., and Violay, M.: On the stability of carbonate-bearing faults at the brittle-to-ductile transition, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3971, https://doi.org/10.5194/egusphere-egu22-3971, 2022.

17:13–17:19
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EGU22-8365
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ECS
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Virtual presentation
Kristina G. Dunkel et al.

Earthquakes are often regarded as agents of rheological weakening of the dry and mechanically strong lower crust. The weakening is typically attributed to fluid infiltration and resulting fluid-mediated metamorphism along the seismic fault.

On Moskenesøya in SW Lofoten (Northern Norway), we observe lower-crustal pseudotachylytes (frozen frictional melts that record fossil earthquakes) that are unusually dry. This presents us with an exceptional opportunity to study the processes affecting the rocks during and after an earthquake:

  • We can observe the pristine microstructures of the pseudotachylytes, not overprinted by later metamorphism, to elucidate the earthquake-generating mechanism.
  • We can study the further development of these dry pseudotachylytes after the seismic event.

We have previously described the composition and microstructures of the pristine pseudotachylytes, and concluded that transient stress pulses caused by shallower earthquakes are the most likely explanation for the occurrence of fossil earthquakes in the analysed rocks from Lofoten, with no evidence of other mechanisms such as thermal runaway or dehydration embrittlement.

In this contribution, we focus on the evolution of the pseudotachylytes after their formation. We study their development from the initial, pristine pseudotachylytes, via pseudotachylytes with slightly mylonitized margins, to ultramylonites. We use compositional and microstructural analyses, including electron backscatter diffraction (EBSD), to understand the weakening mechanisms in this dry system.

In the mylonitized margins of the pseudotachylytes, a slight shape-preferred orientation is developed and the quenching microstructures, such as microlites, are lost. The mineralogical composition (dominantly feldspars and pyroxenes) stays the same as in the pristine pseudotachylytes. In the ultramylonite, quartz and amphibole appear as accessory minerals, which means that we cannot completely exclude the presence of minor amounts of hydrous fluids; however, feldspars and pyroxenes persist as the main components of the rock. The foliation of the ultramylonite is not defined by phyllosilicates, but by a compositional banding, which suggest a phase separation and aggregation during shearing. EBSD data indicate that the main constituent phases deformed dominantly by grain size sensitive creep.

Our preliminary results suggest that even in the absence of fluids, pseudotachylyte-bearing seismic faults represent weak zones in the lower crust that are localizing viscous shear during post- and interseismic deformation, presumably due to the intense grain size reduction that facilitates grain-size sensitive mechanisms. 

How to cite: Dunkel, K. G., Menegon, L., and Jamtveit, B.: Weakening mechanisms in dry, lower-crustal pseudotachylytes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8365, https://doi.org/10.5194/egusphere-egu22-8365, 2022.

17:19–17:25
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EGU22-13212
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ECS
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Charis Horn and Philip Skemer

Talc is a hydrous magnesium silicate with an extremely low coefficient of friction.  In recent years, the recognition that talc is present in many fault systems has led to the suggestion that talc strongly influences the strength of faults.  To understand the role of talc in the seismic cycle, we conducted high pressure and temperature torsional deformation experiments on specimens of natural talc at shear strain rates relevant to slow-slip earthquakes (~10-4 s-1).  Scanning transmission electron microscopy revealed decreasing talc grain sizes (from ~3-5 mm to <100 nm), alongside delamination and kinking of individual talc grains.  This microstructural evolution with progressive strain greatly increases the density of planar defects (including grain-boundaries), and is consistent both with observations of natural, talc-rich faults, and prior experimental work.  Nanoindentation tests at room temperature were performed on deformed specimens to assess precisely whether the observed microstructural changes also affect rheology. At these conditions, nanoindentation is assumed to produce deformation predominantly by intercrystalline frictional slip.  However, bulk hardness data determined from nanoindentation show that there is no change in indentation hardness with increasing strain or defect density, both for indents made parallel to and perpendicular to the shear plane.  Although the talc grains become increasingly damaged with strain, the overall strength of deformed talc does not change.  This suggests that accumulated slip on talc-bearing faults does not change their mechanical response or hazard potential.

How to cite: Horn, C. and Skemer, P.: Experimental deformation of talc at near-seismic deformation rates, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13212, https://doi.org/10.5194/egusphere-egu22-13212, 2022.

17:25–17:31
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EGU22-5029
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ECS
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On-site presentation
Gabriel Meyer et al.

With increasing depth, the rheology of rocks gradually transitions from brittle (localized, fractures) to ductile (homogeneous flow). Recently, it was demonstrated that, in the crust, the transitional zone might extend to shallower depth than previously thought (2km) with a zone where the deformation regime can be both localized and ductile (the LDT). In this regime, both extremely localized (fault slip) and distributed (cataclastic flow and/or plasticity) deformation may occur concurrently. This observation had great importance since the ductile regime is commonly thought to be aseismic and to mark the maximum depth of earthquake nucleation.

However, this observation was made experimentally in non-porous rocks; porous rocks on the other hand display an additional characteristic in that their ductile behaviour may consist in the formation of compactions bands which greatly impact the behaviour of porous reservoirs and systems (e.g., volcanoes). Moreover, ductile rocks are commonly believed to be aseismic, the potential coexistence of both ductile and localized regimes in reservoir rocks might therefore have great implications for induced seismicity mitigation.

Here, we present three conventional triaxial experiments on Volvic basalt (homogeneous, istropic, fine grain). We deformed cylindrical cores equipped with strain gages at 5MPa and room temperature until a sample-scale fracture nucleated and propagated. Subsequently, we increased confining pressure step wise, loading the sample every step until 0.2% irrecoverable strain was accumulated in the sample. In between confinement steps, the differential stress was unloaded. A pair of Linear Variable Differential Transformers (LVDTs) was used along with the strain gauges to accurately monitor the deformation behaviour of the samples.

We show that Volvic basalt transitions from being purely localized to being purely ductile over a rather narrow pressure range from 40 to 80 MPa. The transition initiates when the frictional strength of the fault equates the yield strength of the bulk and terminates when it becomes greater than the maximum strength of the bulk. In this pressure range, deformation is initially accommodated in the bulk (most likely by compaction bands) until strain hardening eventually leads to fault reactivation. Once both fault sliding and bulk flow are active, the partitioning of strain between the two can be described by the same empirical ratio as that already established for non-porous rocks, i.e. (σf - σy)/ (σflow - σy).

We conducted a second experiment at a faster strain rate (10-4 s-1) and show that faster deformation promotes brittle behaviour which pushes the LDT to greater confinement (i.e., greater depth).

Additionally, we conducted a similar experiment in the presence of water. In this case, the LDT occurs at lower confinement, showing that, fluids, by promoting ductile processes such as stress-corrosion, bring the LDT to shallower depth.

Our results are crucial for the understanding of reservoirs where ductile deformation (compaction bands) and induced earthquake mitigation have to be finely tuned.

How to cite: Meyer, G., Violay, M., and Heap, M.: The localized to ductile transition in porous rocks : experimental investigation on Volvic basalt., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5029, https://doi.org/10.5194/egusphere-egu22-5029, 2022.

17:31–17:37
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EGU22-5065
The Meran-Mauls segment of the Periadriatic Fault System, Italy: pure thrusting across the brittle-plastic transition
(withdrawn)
Andrea Zanchi et al.
17:37–17:47
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EGU22-8320
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ECS
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solicited
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On-site presentation
Christopher Harbord and Nicolas Brantut

Grain size is in an important microstructural parameter affecting both brittle and plastic deformation processes. In the low temperature brittle regime, larger grain size materials typically have lower strength, whereas in the plastic flow regime smaller grain size materials tend to be weaker. It is not clear how grain size impacts at intermediate conditions where deformation of rock is accommodated by coupled brittle and plastic deformation processes.

To investigate the role of grain size in the semi brittle regime we deformed three calcite-rich rocks, spanning 3 orders of magnitude in grainsize (0.006-2 mm). A gas medium triaxial apparatus was used at a range of confining pressures (200-800 MPa) and temperatures (20-400°C), and samples were loaded at a constant axial strain rate (1×10-5 s-1). Axial measurements of P-wave speed were performed during tests in order to infer the in-situ microstructural state of the sample.

Nearly all tests show strain hardening behaviour after yield, typical of semi-brittle deformation, which is quantified using the hardening modulus (h = ∂σ/∂ε). Grain size has a first order control on rock strength, with yield stress and h following a Hall-Petch type relationship at all P-T conditions. For a given temperature, h is low at low pressure (200 MPa) and accompanied by large decreases in wavespeed, and h increases at high pressure (>400 MPa) whereas velocity decreases by a smaller magnitude. This suggests that, at low temperature, strain hardening is relieved by microcracking. At constant pressure, wavespeed decreases significantly at 20°C with progressive deformation, but remains nearly constant at 400°C indicating a transition from dominatly brittle to fully plastic deformation with increasing temperature, in some cases with little change in the macroscopic strength.

Given that both strength and strain hardening behaviour depend on grain size, our data suggests that grain size dynamically impacts the long term rheology of the crust. Larger grain sizes will broaden the depth distribution of the brittle ductile transition and result in a weaker peak crustal strength.

How to cite: Harbord, C. and Brantut, N.: The effects of grain size on semi-brittle flow in calcite rich rocks, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8320, https://doi.org/10.5194/egusphere-egu22-8320, 2022.

17:47–17:53
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EGU22-162
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ECS
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On-site presentation
Arnab Roy et al.

Crustal deformations generally undergo a brittle-ductile transition with depth, producing fault-dominated structures at shallow depths, replaced by ductile shear zones at middle and lower crustal levels. One of the keys to shear zone modelling concerns the choice of rheological approximations that can successfully reproduce the characteristic features of natural ductile shear zones in the models.  With the help of 2D FE (finite element) simulations, this study shows viscoplastic rheology as a suitable rheological approximation to predict the competing growth and orientations of multiple sets of secondary shear bands in a ductile shear zone. The viscoplastic rheology is modelled by combining bulk viscous weakening of the shear zone material and plastic yielding (Drucker-Prager criterion) to replicate strain-softening behaviour, where the instantaneous viscosity decreases nonlinearly with increasing strain. The cohesive strength of the material is also assumed to reduce with progressive plastic strain. This rheological combination allows us to replicate the various shear band networks found in crustal-level ductile shear zones. It also addresses the conditions for fluid flow into ductile shear zones, which leads to metamorphic reactions, mineralisation, etc. We validate our model results with field observations of similar shear band structures from the Eastern Indian Precambrian craton. The present study finally leads us to conclude that a pressure-dependent viscoplastic rheology is an ideal rheological approximation to model ductile shear zones extensively found in this craton.

How to cite: Roy, A., Saha, P., and Mandal, N.: Viscoplastic Rheological Modelling- A Realistic Approach to Natural Ductile Shear Zones, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-162, https://doi.org/10.5194/egusphere-egu22-162, 2022.

17:53–17:59
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EGU22-5198
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ECS
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Virtual presentation
Rheology of plagioclase transforming under high pressure and temperature conditions: insights from deformation experiments
(withdrawn)
Marie Baisset et al.
17:59–18:05
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EGU22-7954
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ECS
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Virtual presentation
Lisa Katharina Mohrbach et al.

Previous experimental data and field observations demonstrate that fluids have a significant influence on rock strength. The relation between strength and hydration extent of the lower continental crust is still poorly constrained and thus a matter of an ongoing debate. We tested the impact of hydration extent on the strength of rocks representing the lower continental crust by performing deformation experiments on various plagioclase-epidote mixtures as well as on natural granulite samples in a Grigg's type deformation apparatus. In these samples, the plagioclase component represents rocks of the lower continental crust and epidote reflects hydration extent, because, alongside with quartz, kyanite and jadeite or albite, it forms as a decomposition product of plagioclase at high-pressure/ high-temperature conditions in the presence of even small amounts of fluids. To quantify the relation between strength and epidote content, we conducted the tests on plagioclase-epidote powders with a grain size of 90-135 mm and plagioclase-epidote ratios of 100:0, 99:1, 98:2, 95:5, 90:10, 85:15, and 0:100. The pre-dried powders were first hot-pressed at 550 °C and a confining pressure of 1 GPa for 3 h in the Griggs apparatus. Mixtures were subsequently deformed at 1 GPa and 550 to 650 °C at strain rates of 5·10-6 to 5·10-5 s-1. All stress-strain curves show pronounced maxima followed by strain softening towards a final strength. The deformation data yield an exponential decrease of the ultimate strength with increasing epidote content. Investigations of the microstructures of samples deformed at 550 °C and 5·10-5 s-1 using the SEM and polarized light microscopy reveal cataclastic flow by grain-scale fracturing of both epidote and plagioclase and the rotation and alignment of epidote grains at angles between 60° and 70° to the maximum principal stress  σ1. In addition, plagioclase grains show pronounced undulatory extinction but we found no evidence for deformation twinning. Some samples exhibit networks of conjugate bands of fine-grained plagioclase surrounding larger plagioclase grains oriented at an angle of around 50° towards σ1. These bands are mostly visible in samples without epidote.

How to cite: Mohrbach, L. K., Renner, J., and Incel, S.: Experimental study of the impact of hydration extent on the strength of the lower continental crust, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7954, https://doi.org/10.5194/egusphere-egu22-7954, 2022.

18:05–18:11
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EGU22-6966
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ECS
Yuval Boneh et al.

Amphibole is an important mineral in rocks of the lower crust and in subduction zones, forming as the product of metamorphic reactions and hydration of mafic rocks. As such, the textural and rheological properties of amphibole are of relevance for assessing the physical properties of these tectonic provinces. Aggregates containing amphibole grains often exhibit a strong texture, i.e., a crystallographic preferred orientation (CPO). Since amphibole possesses inherent anisotropic properties, the CPO will affect the bulk strength and elastic properties. However, amphibole’s rheological behavior is not well understood as its capability to deform purely via plastic deformation remains unresolved, previous studies suggesting numerous deformation mechanisms such as semi-brittle and cataclastic flow, dissolution precipitation, dislocation creep, recrystallization, micro-twinning, and diffusion assisted creep. Here, we use pre-textured natural samples cored at 60° to the foliation and lineation to investigate the deformation mechanism/s activated in a polycrystalline aggregate/rock of well-oriented amphibolite-rich hornblende. Samples from the Mamonia complex (Cyprus) with hornblende as the dominant mineral (> 70 % modal fraction) and strong initial alignment of the [001] axis were deformed using a Griggs-type solid-medium apparatus. Experiments were run at 1 GPa confining pressure, temperatures of 400 to 800 °C, and a strain rate of ~10-5 1/s. Samples show temperature-dependent differential stress that falls below the Goetze criteria (i.e., below the confining pressure, 1 GPa) - ~700, 500, and 200 MPa for samples deformed at 400, 600, and 800 °C, respectively. Microstructural analysis using Electron backscatter diffraction (EBSD) reveals folding and kink bands, accommodated by both plastic mechanisms, via dislocation glide on the hornblende easy slip system, and brittle mechanisms, via micro-fracturing along the crystal cleavage (110). We discuss the implications of the interplay and contribution of different deformation mechanisms for our ability to translate laboratory experiments to flow laws for the lower mantle and subduction zone interfaces.

How to cite: Boneh, Y., Sarah, I., and Renner, J.: Mechanism/s of deformation and strength of experimentally deformed hornblende-rich amphibolite with a strong pre-existing texture, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6966, https://doi.org/10.5194/egusphere-egu22-6966, 2022.

18:11–18:17
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EGU22-10023
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ECS
Maria Aurora Natale Castillo et al.

Rocks mechanical behaviour, and in particular, their transition from a brittle to a ductile deformation has been prevalently investigated through rheological experiments and numerical models. In conjunction with rocks mechanical studies, the analyses of seismic wave propagation can improve our knowledge of physical rocks behaviour and provide an alternative assessment of the brittle ductile transition (BDT).

In this study, we investigate the quantitative relationships between seismic attenuation and viscous rocks' rheology, especially across the BDT domain. For this purpose, we rely on the Burgers and Gassmann mechanical model to derive shear wave attenuation (1/Qs ), for several dry and wet crustal rheology, thermal conditions, and different strain rates values. This allows us to establish geothermal and mechanical conditions at which the BDT occurs and to cross-correlate this transition to computed shear seismic wave attenuation values. We observe that the variation with depth is related much more to the input strain rate than to the rock‘s rheology and thermal conditions, so that a fixed amount of Qs reduction can identify the average BDT depths for each strain rate used. Below the BDT depth, we observe a significant increase of the Qs reduction (up to 10-4 % of the surface value), depending also on rocks temperature and rheology. Since the greatest Qs reduction is estimated for the greatest input strain rate (10-13 s-1) and hot thermal conditions, the proposed method can find more applicability in tectonically active/geothermal areas.

We tested the obtained results by performing triaxial lab experiments, while monitoring ultrasonic P-waves, on a sample of Carrara marble, at ambient temperature and 180 MPa confining pressure. The transition from brittle to semi-brittle conditions is characterized by the increase of crack-density with a progressive rate reduction. At the same time, both the seismic velocity and energy significantly decrease during the first phase of deformation (brittle regime) and tend towards an asymptotic value, when the sample approaches the ductile deformation. We interpret the absence of an increase of energy loss at the BDT, as due to the persistent effect of the microfracturation. The last one usually accompanies the deformation mechanisms that occur at the BDT (e.g., pressure solution, twinning), masking the expected increase of attenuation at the beginning of the ductile conditions. This is a matter that still needs to be investigated.

How to cite: Natale Castillo, M. A., Tesauro, M., Cacace, M., Passelegue, F. X. T., Pimienta, L., and Violay, M.: Seismic attenuation across the brittle-ductile transition, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10023, https://doi.org/10.5194/egusphere-egu22-10023, 2022.

18:17–18:23
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EGU22-12465
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ECS
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Rhander Taufner et al.

Exhumation of oceanic core complexes occurs through large-scale extensional shear zones that expose parts of the deformed gabbroic lower crust. However, it is not well understood how these high-temperature shear zones nucleate and develop. Since diopside is traditionally described as a load bearing phase in deforming systems, its microstructures may record the deformation mechanisms involved in the progressive stages of shear zone development. In this study, we focus on the fabrics of diopside in both the host coarse-grained gabbro and the adjacent high-temperature shear zone from the Atlantis Bank (IODP Exp 360), in order to better constrain the role of diopside during strain localization in deep crustal detachment fault zones.

In the host rock directly in contact to the shear zone, diopside porphyroclasts display microfractures filled with fine-grained diopside (~ 65 µm) and minor amounts (~10%) of plagioclase, amphibole and Fe-Ti oxides with grain size ~ 30 µm that occur as interstitial phases. Diopside grains in the microfractures have little internal deformation and are interpreted as “new” grains. On the other hand, fragments of the host diopside within the fracture are distinguished by their larger diameters of ~200 µm and dominant cleavage planes that is systematically missing in the new grains. These microstructures indicate cataclastic deformation with later precipitation of plagioclase, amphibole and Fe-Ti oxides. Other diopside porphyroclasts in the host rock show undulatory extinction, low-angle grain boundaries and new grains with crystallographic orientations controlled by the host, indicating dislocation creep.

Diopside porphyroclasts within the shear zone show undulatory extinction as well as bent cleavage planes and exsolution lamellae. New grains of diopside (~35 µm) that occur rimming the porphyroclasts - concentrated at sites of strong undulatory extinction - have long axes correlating the orientation of the bent cleavages within the host. These new grains have a crystallographic orientation with poles of (100) planes close to the X-axis and [001] axes close to the Z-axis, and high angle boundaries (>140º) with misorientation axes clustered between [001] and [100]. We propose that these new grains are a result of dislocation glide and growth due to bending of the host diopside during the early stages of shear zone nucleation.

In the strain shadow of the porphyroclasts within the shear zone, new grains of diopside (~20µm) occur together with amphibole, plagioclase and Fe-Ti oxides. They are rounded, strain free, have random orientations and the amount of diopside decreases with distance from the host. These grains are interpreted to have precipitated from the pore fluid during ongoing deformation of the shear zone.

We suggest that diopside in the host rock was deformed by cataclasis associated with dislocation glide during nucleation of the shear zone at probably high stress, as indicated by the similar microfabric of diopside porphyroclasts in the shear zone compared to those in the host rock. Unlike, ongoing deformation localized within the shear zone is due to dissolution and precipitation, as indicated by the polyphase aggregates in the strain shadows and in the matrix. 

How to cite: Taufner, R., Trepmann, C., and Viegas, G.: Diopside microfabric development in lower-crust oceanic detachment fault zones, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12465, https://doi.org/10.5194/egusphere-egu22-12465, 2022.

18:23–18:29
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EGU22-10545
Bjørn Eske Sørensen et al.

We provide here in situ evidence from a network of well-preserved extensional shear zones cutting a rift related lower crustal Reinfjord Ultramafic Complex, Seiland Igneous Province, that formed in the late Ediacaran. Our results can explain seismic events well below the seimic zone of continental rifts and associated CO2 emissions. Processses leading to catastrophic failure of the weakened rocks led to extremely high strain rates and the formation of pseudotachylites can be traced from a netwok og mm-m scale steeply dipping transtensional shearzones associated with gabbronoritc dykes to a 2km long low angle extensional shearzone. Deformation, initiated through a priming of the dyke-host rock interface by magmatic fluids, exploits subgrains and microfractures in olivine, with reactive CO2-bearing fluids leading to volume expanding reactions such as olivine + diopside + CO2 = Dolomite + enstatite, enhancing olivine grain fracturing. Fragmentation of the olivine grains and addition of weaker phases facilitated strain localization and local increases in strain rate by two orders of magnitude. Catastrophic failure of the weakened rocks led to extremely high strain rates and the formation of pseudotachylites in several cyclic events. The frictional heat raised the temperature above the dolomite forming reaction, causing release of CO2 and H2O along the fault, but also in the surrounding mafic-ultramafic rocks, forming veins around the shearzone. Fluid-rock interaction surrounding shear zones 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, resultinig 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 pyroxenenitic 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.  We conclude that catastrophic failure along shear zones in lower crustal continental rifts is possible without remote stress events in the presence of pre-existing heterogeneities and volatiles. These zones also acted and transport conduits for volatiles from the lower crust to atmosphere.

How to cite: Sørensen, B. E., Ryan, E. J., Larsen, R., and Grant, T.: Infiltration of volatile-rich mafic melt in lower crustal peridotites provokes deep earthquakes, initiates km scale shearzones and volatile transfer from the lower crust to the atmosphere, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10545, https://doi.org/10.5194/egusphere-egu22-10545, 2022.

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