Plate boundary dynamics remain incompletely understood in the context of thermo-chemical convection. Strain-localization is affected by weakening in ductile shear zones, and a change from dislocation to diffusion creep caused by grain-size reduction is one of the mechanisms that has been discussed. However, the causes and consequences of strain localization remain debated, even though tectonic inheritance and strain localization appear to be critical features in plate tectonics.

   Frictional-plastic faults in nature and brittle shear zones in the lithosphere may be weakened by high transient, or static, fluid pressures, or mechanically by gouge, or mineral transformations. Weakening in ductile shear zones in the viscous domain may be governed by a change from dislocation to diffusion creep caused by grain-size reduction. In mechanical models, strain weakening and localization in the shallow parts of the lithosphere has mainly been modeled by an approximation of brittle behavior using a pseudo visco plastic rheology in combination with a linear decrease of the yield strength of the lithosphere with increasing deformation (plastic-strain (PSS) softening). Strain weakening in viscous shear zones, on the other hand, may be described by a linear dependence of the effective viscosity on the accumulated deformation (viscous-strain (VSS) softening). These different types of strain weakening are further explored and compared to the predictions from different laboratory-based models of grain-size evolution for a range of temperatures and a step-like variation of total strain rate with time. Such a parameterized, apparent-strain, or “damage”, dependent weakening (SDW) rheology (mainly PSS) can successfully mimic more complex weakening processes in global mantle convection computations. While we focus on GSS rheology to constrain the parameters of SDW, the analysis is not limited to grain-size evolution as the only possible microphysical mechanism.

   The SDW weakening rheology allows for memory of deformation, which weakens the fault zone as well as the lithosphere for a longer period and allows for a self-consistent formation and reactivation of inherited weak zones. In addition, the memory effect and weakening of the fault zone allows for a more frequent reactivation at smaller strain rates, depending on the strain-weakening parameter combination. Reactivation within the models occurs in two different ways: a), as a guide for laterally propagating convergent and divergent plate boundaries, and b), formation of a new subduction zone by reactivation of weak zones. A longer rheological memory results in a decrease in the dominant period of the reorganization of plates due to less frequently formed new plate boundaries. In addition, the low frequency content of velocity and heat transport spectra decreases with a decreasing dominant period. This indicates a more sluggish reorganization of plates due to the weaker and thus more persistent active plate boundaries. These results show the importance of a rheological memory for the reorganization of plates, potentially even for the Wilson cycle.

How to cite: Fuchs, L.: Plate-boundary maintenance – formation, preservation, and reactivation in global plate-like mantle convection models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9584, https://doi.org/10.5194/egusphere-egu22-9584, 2022.

Crystal defects such as vacancies, dislocations and grain boundaries are central in controlling the rheology of the Earth’s upper mantle. Their presence influences element diffusion, plastic deformation and grain growth, which are the main microphysical processes controlling mass transfer in the Earth’s lithosphere and asthenosphere. Although substantial information exists on these processes, there is a general lack of data on how these defects interact at conditions found in the Earth’s interior. A better understanding of processes occurring at the grain scale is necessary for increased confidence in extrapolating from laboratory length and time scales to those of the Earth. We examined the evolution of olivine grain boundaries during experimental deformation and their impact on deformation in the dislocation-accommodated grain- boundary sliding (disGBS) regime. This may be the main deformation mechanism for olivine in most of Earth’s upper mantle. Our results suggest that grain boundaries play a major role in moderating deformation in the disGBS regime. We present observational evidence that the rate of deformation is controlled by assimilation of dislocations into grain boundaries. We also demonstrate that the ability for dislocations to transmit across olivine grain boundaries evolves with increasing deformation. Lastly, we show that dynamic recrystallization of olivine creates specific grain boundaries, which are modified as deformation progresses. This might affect electrical conductivity and seismic attenuation in the upper mantle. The effective contribution of grain-boundary processes (such as disGBS) on the rheology of the upper mantle is correlated to the amount of grain boundaries in upper mantle rocks, that is, their grain-size distribution and evolution. The grain-size distribution in the Earth’s mantle is controlled by the balance between damage (recrystallization under a stress field) and healing (grain growth) processes. However, grain growth, one of the main processes controlling grain size, is still poorly constrained for olivine at conditions of the upper mantle. To evaluate the effects of pressure on grain growth of olivine, we performed grain growth experiments at pressures ranging from 1 to 12 GPa using piston-cylinder and multi-anvil apparatuses. We found that the olivine grain-growth rate is reduced as pressure increases. Our results suggest that grain-boundary diffusion is the main process of grain growth at high pressure. Based on extrapolation of our experimental results to geological time scales, we suggest that at deep upper-mantle conditions (depths of 200 to 410 km), the effect of pressure on inhibiting grain growth counteracts the effect of increasing temperature. We present estimations of viscosity as a function of depth considering the grain-size evolution predicted here. Our estimations suggest that viscosity is almost constant at the deep upper mantle, which corroborates postglacial-rebound observations.

How to cite: Ferreira, F., Thielmann, M., and Marquardt, K.: The role of grain boundaries for the deformation and grain growth of olivine at upper mantle conditions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10101, https://doi.org/10.5194/egusphere-egu22-10101, 2022.

The mechanics of olivine deformation play a key role in long-term planetary processes, such as the response of the lithosphere to tectonic loading or the response of the solid Earth to tidal forces, and in short-term processes, such as the evolution of roughness on oceanic fault surfaces or postseismic creep within the upper mantle. Many previous studies have emphasized the importance of grain-size effects in the deformation of olivine. However, most of our understanding of the role of grain boundaries in the deformation of olivine is inferred from comparison of experiments on single crystals to experiments on polycrystalline samples.

To directly observe and quantify the mechanical properties of olivine grain boundaries, we use high-precision mechanical testing of synthetic forsterite bicrystals with well characterised interfaces. We conduct in-situ micropillar compression tests at high-temperature (700°C) on low-angle (13° tilt about [100] on (015)) and high-angle (60° tilt about [100] on (011)) grain boundaries. In these experiments, the boundary is contained within the micropillar and oriented at 45° to the loading direction to promote shear along the boundary. In these in-situ tests, we observe differences in deformation style between the pillars containing the grain boundary and the pillars in the crystal interior. In-situ observations and analysis of the mechanical data indicate that pillars containing the grain boundary consistently support elastic loading to higher stresses than pillars without a grain boundary. Moreover, only the pillars without a grain boundary display evidence of sustained plasticity and slip-band formation. Post-deformation advanced microstructural characterization (STEM) confirms that under the conditions of these deformation experiments, sliding did not occur along the grain boundary. These observations support the hypothesis that grain boundaries are stronger than the crystal interior. 

These experiments on small deformation volumes allow us to qualitatively explore the differences between the crystal interior and regions containing grain boundaries. Overall, the variation in strain and temperature in our small scale experiments allows fundamental investigation of the response of well characterised forsterite grain boundaries to deformation. 

How to cite: Avadanii, D., Hansen, L., Darnbrough, E., Marquardt, K., Armstrong, D., and Wilkinson, A.: In-situ mechanical testing and characterization of olivine grain boundaries, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9765, https://doi.org/10.5194/egusphere-egu22-9765, 2022.

Olivine, the main rock-forming mineral of Earth's mantle, responds to tectonic stress by deforming viscously over millions of years. This deformation creates an anisotropic (direction-dependent) texture that typically aligns with the mantle flow direction. According to laboratory experiments on olivine, we expect this texture to also exhibit anisotropic viscosity (AV), with deformation occurring more easily when it is parallel to, rather than across, the texture. However, the direction dependency of lithospheric and asthenospheric viscosity is rarely addressed in geodynamic models.

 The open-source modeling package ASPECT can address AV in a 2D setting using the director method, where AV is present due to shape preferred orientation created by dike intrusions (Perry-Houts and Karlstrom, 2019). We have adapted this implementation for current versions of ASPECT and benchmarked it against similar Rayleigh-Taylor instability models by Lev and Hager (2008).

Unfortunately, a 2D method is inappropriate to address AV related to olivine crystallographic preferred orientation (CPO or texture), as, by default, olivine has three independent slip systems on which deformation can occur. Integrating anisotropic viscosity into 3D models would also allow us to use the actual laboratory-based parametrizations of the olivine slip system activities and texture parameters when describing the evolution of CPO and AV. One of the biggest challenges in addressing AV in a 3D setting is to find the full, rank 4, viscosity tensor, which can be done with a method similar to the one for the fluidity tensor in Király et al., (2021).

Here, we present the initial results of simple geodynamic setups (shear box, corner flow), where 3D olivine CPO develops, based on the D-Rex method (Fraters and Billen, 2021), and this CPO creates AV based on the micromechanical model described in Hansen et al., (2016).

Our goal is to create a tool within ASPECT that allows for CPO to develop and affect the asthenospheric or lithospheric mantle’s viscosity to improve modeling a wide range of geodynamic problems.

References listed:

Fraters, M.R.T., and Billen, M.I., 2021, On the Implementation and Usability of Crystal Preferred Orientation Evolution in Geodynamic Modeling: Geochemistry, Geophysics, Geosystems, doi:10.1029/2021GC009846.

Hansen, L.N., Conrad, C.P., Boneh, Y., Skemer, P., Warren, J.M., and Kohlstedt, D.L., 2016, Viscous anisotropy of textured olivine aggregates: 2. Micromechanical model: Journal of Geophysical Research: Solid Earth, doi:10.1002/2016JB013304.

Király, Á., Conrad, C.P., and Hansen, L.N., 2020, Evolving Viscous Anisotropy in the Upper Mantle and Its Geodynamic Implications: Geochemistry, Geophysics, Geosystems, v. 21, p. e2020GC009159, doi:10.1029/2020GC009159.

Lev, E., and Hager, B.H., 2008, Rayleigh-Taylor instabilities with anisotropic lithospheric viscosity: Geophysical Journal International, doi:10.1111/j.1365-246X.2008.03731.x.

Perry-Houts, J., and Karlstrom, L., 2019, Anisotropic viscosity and time-evolving lithospheric instabilities due to aligned igneous intrusions: Geophysical Journal International, doi:10.1093/gji/ggy466.

How to cite: Király, Á., Fraters, M., and Gassmoeller, R.: Implementing 3D anisotropic viscosity calculations into ASPECT, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5371, https://doi.org/10.5194/egusphere-egu22-5371, 2022.

One of the most discussed issues of whole-mantle geodynamic models is the need of an 'ad hoc' yield stress which is lower than any strength measurement of natural samples in the brittle or plastic regimes. It is commonly believed that grain size evolution, in particular grains size reduction due to dynamic recrystallization, may decrease the strength of the lithosphere and therefore aid the onset and persistence of the mobile-lid regime. In this work, we carry out an investigation of 2D whole-mantle annulus models with varying yield stress. We compare cases with different grain growth and grain reduction parameters to cases with constant grain size to make inferences on the feasibility of a plate-like convective regime as a function of the yield strength of the lithosphere.

Our results show that viscosity profiles of models with dynamic grain-size evolution are inherently different to those with constant grain size, and that those profiles vary little when changing grain-size evolution parameters. In this context, the lower mantle shows greater variations in viscosity than the upper mantle: with viscosity contrasts between upper and lower mantle and plume widths comparable to those of the Earth, in particular in models with enhanced grain growth. Furthermore, our models show that, while enhancing grain size reduction reduces episodicity and increases mobility up to some point, increasing grain growth favors mobile-lid convection even more. This is at odds with previous conceptions of the grain-size-evolution-induced mobile-lid regime, where grain groth should promote healing of the lithosphere and therefore inhibit subduction. We hypothesize that increased stiffness of the bottom of the lithosphere, together with a more viscous lower mantle, are the main reasons for the grain-grouth-favored mobile-lid regime.

How to cite: Manjón-Cabeza Córdoba, A., Rolf, T., and Arnould, M.: Feasibility of the mobile-lid regime controlled by grain size evolution, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3889, https://doi.org/10.5194/egusphere-egu22-3889, 2022.

Earth's mantle rocks are poly-aggregates where different mineral phases coexist. These rocks may often be approximated as two-phase composites with a dominant phase and less abundant one (e.g. bridgmanite-ferropericlase composites in the lower mantle). Severe shearing of these rocks leads to a non-homogeneous partitioning of the strain between the different phases, with the composite developing a laminar fabric of weak and thin material where strain localizes. The resulting bulk rock is a mechanically anisotropic media that is hardened against normal stress, while significantly weakened against fabric-parallel shear stress.

Due to the large scale difference between the laminar gran-scale fabrics and regional-to-global geological processes, Earth’s rocks are idealised as homogeneous materials instead of multi-phase bodies in numerical models. Thus, a characterization of the rheology evolution of the bulk composite is necessary to better understand large-scale geological processes in which anisotropy may play a fundamental role. Recent three-dimensional numerical (de Montserrat et al. 2021) studies have shown that the degree of lateral interconnectivity of the weak and thin layers is rather limited, thus estimating the rheology of a composite with laminar fabrics by the idealized Voigt and Reuss averages for fibres yield a strong underestimation of the strength of the composite. Instead, we use a combination of numerical results and micro-mechanics to develop an empirical framework to estimate the evolution of the (anisotropic) rheology of such composites.

We apply this rheology framework to study the effects of fabric-induced directional-weakening/hardening on global mantle convective patterns. First order effects of extrinsic anisotropy of lower mantle material observed in our two-dimensional models are a decrease of the wavelength of convective cells, and up to a ~50% increase in the average mantle flow velocity caused by the weakening of the flow-parallel component of the viscosity tensor. The latter is particularly evident in mantle plumes, where the ascent and transfer of hot lower mantle material to lower depths is enhanced by the near-alignment of the weak  fabrics with the plume channel.  

How to cite: de Montserrat, A., Faccenda, M., and Pennacchioni, G.: Shape Preferred Orientation at scale. From grain-scale aggregates to global mantle convection, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6124, https://doi.org/10.5194/egusphere-egu22-6124, 2022.

How to cite: Davaille, A., Chasse, T., Ribe, N., Carrez, P., and Cordier, P.: Influence of a yield stress on lower mantle dynamics: filtering and changing morphology of plumes and slabs, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10404, https://doi.org/10.5194/egusphere-egu22-10404, 2022.

The study of crustal stress examines the causes and consequences of in-situ stress in the Earth’s crust. Stress at any given point has several geological sources, including ‘short-term and local-scale’ and ‘long-term, ongoing and wide-scale’ source. In order to better characterise the crustal stress state, the analyses of both local- and wide-scale sources, and the consequences of their superposition are required. The global compilation of stress data in the World Stress Map database has increased significantly since its first release in 1992 and its analysis revealed large rotations of the stress tensor in several intraplate settings.

Large-scale stress analysis, so called first-order, (> 500 km) provides information on the key drivers of the stress state that result from large density contrasts and plate boundary forces. The analyses of stress at smaller-scales (< 500 km) have numerous applications in reservoir geomechanics, geo-storage sites, civil engineering and mining industry. To date, numerous studies have investigated the stress analysis from different perspectives. However, the stress, in geosciences, is still enigmatic because it is a scale-dependant parameter. It means, stress variations can be studied at both the ‘spatial-scale’ and ‘temporal-scale’. This paper aims to investigate the crustal stress pattern with a particular emphasis on the orientation of maximum horizontal stresses at various spatial-scales, ranging from continental scales down to basin, field and wellbore scales, to better evaluate the role of various stress sources and their applications in the Earth’s crust. The stress analyses conducted in this work shows that stress pattern at large-scales do not necessarily represent the in-situ stress pattern at smaller-scales. Similarly, analysis of just a couple of borehole measurements in one area might not yield a good representation of the regional stress pattern.

How to cite: Rajabi, M. and Heidbach, O.: Crustal stress across spatial scales, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12964, https://doi.org/10.5194/egusphere-egu22-12964, 2022.

Mafic rocks are a key constituent of the oceanic and lower continental crust. Strain localisation and fabric development in these rocks is controlled by the active deformation mechanisms. From studies of natural rocks it has been established that strain localisation and weakening in mafic rocks is directly related to fluid availability and resultant mineral reactions. Understanding the interplay between reactions, fluid availability, and deformation aids in quantifying the stresses and rates of deformation processes. We have conducted an experimental investigation to constrain the weakening mechanisms in gabbro. Shear experiments were performed in a Griggs-type apparatus at 800-900°C, and 1.2-1.5 GPa with a shear strain rate of 10⁻⁵s⁻¹. The starting material consists of mixed powders with <100 µm sized grains of plagioclase and clinopyroxene from an undeformed sample of the Kågen Gabbro in Northern Norway. Experiments have been conducted with ‘as is’ (dried at 110°C) starting material and with 0.1% added water. The experiments at 800°C are very strong with a peak shear stress ~0.8 GPa whilst the 900°C experiments are weaker, reaching peak stresses of ~0.35 GPa. The 800°C experiments show evidence of mineral reactions with newly formed phases making up 10-25% of the sample. In these reaction zones, plagioclase and clinopyroxene have reacted to produce amphibole and garnet. Additionally S-C’ mylonitic fabrics have developed in these samples. The 900°C samples show minimal evidence for mineral reactions (2-5% new material) or crystal-plastic deformation mechanisms. The lack of mineral reactions in the rheologically weak experiments (900°C) and abundance of reaction products in the mechanically strong experiments (800°C) is conflicting to our inferences of natural studies. However, if partial melting takes place in the higher temperature experiments, it may account for the pronounced strength decrease. We plan to conduct EBSD and TEM analysis to determine crystallographic properties and accurate grain size and shape parameters in the fine grained reaction zones. Future experiments will use fully dried natural starting material (dried at 700-800°C) and An60 and end-member diopside, these experiments will be compared with our current experiments and be used to determine the exact weakening properties from impurities in the natural starting material.

How to cite: Lee, A., Stünitz, H., Soret, M., and Précigout, J.: Constraining transformation weakening in plagioclase-pyroxene mixtures, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2627, https://doi.org/10.5194/egusphere-egu22-2627, 2022.

The rheology of mafic rocks buried at high to ultra-high-pressure conditions remains enigmatic. Minerals stable at these conditions are mechanically very strong (garnet, omphacite, glaucophane, zoisite, kyanite). In the laboratory, they show plastic deformation only at very high temperature (e.g. > 1000°C for pyroxene and garnet). Yet, viscous shear zones in mafic rocks metamorphosed at amphibolite and eclogite-facies conditions are commonly reported in fossil collisional and subduction zones. These shear zones localize and accommodate large amounts of strain by weakening of the host rocks. This weakening is interpreted as being induced by a transition from grain size insensitive to grain size sensitive creep, in particular with the activation of the dissolution–precipitation creep. However, the exact interplay between deformation, mineral reaction and fluid/mass transfer remains poorly-known.

We have conducted a first series of deformation experiments at eclogite-facies conditions on a 2-phase aggregate representative of mafic rocks. Shear experiments were performed in a new generation of Griggs-type apparatus (Univ. Orléans) at 850°C, and 2.1 GPa with a shear strain rate of 10⁻⁶ s⁻¹. The starting material consists of mixed powders with < 100 µm sized grains of plagioclase and clinopyroxene from an undeformed sample of the Kågen Gabbro in Northern Norway. Experiments have been conducted with ‘as is’ (dried at 110°C) starting material and with 0.2% added water.

The mechanical data indicate that the samples are first very strong with a peak differential stress at 1.4 GPa. Then, a significant weakening is observed with a stress decrease by 0.5 GPa. The high-strain sample is characterized by a strain gradient increasing toward the center of the shear zone. Metamorphic reactions occur throughout the sample, but the high-strain areas contain considerably more reaction products than the low-strain areas. The nucleation of new phases leads to a drastic grain size reduction and phase mixing, whose intensities are positively correlated with the strain intensity. The nature, distribution and fabric of the mineral products vary also progressively with the strain intensity.

• In the low-strain areas, dissolution-precipitation processes mainly occur along grain boundaries: plagioclase is rimmed by zoisite and a secondary plagioclase more albitic in composition while clinopyroxene is rimmed by amphibole.
• In the mid-strain areas, dissolution-precipitation processes are more pervasive: amphibole and a secondary more sodic clinopyroxene occurs in pressure shadows of primary clinopyroxene, while primary plagioclase is completely replaced by a fine-grained mixture of zoisite and quartz. Reaction products show a strong shape-preferred orientation parallel to the shear direction.
• In the high-strain areas, dissolution-precipitation leads to the nucleation of a fine-grained mixture of garnet and secondary clinopyroxene, quartz and kyanite. Most reaction products have subhedral shape with no clear preferred orientation. Hydrous minerals (amphibole and zoisite) are not observed.

Our preliminary results indicate that strain at eclogite-facies conditions is preferentially accommodated and localized by dissolution-precipitation processes. Further micro-structural and geochemical analyses are required to quantify the exact interplay between the physical and chemical processes controlling the dissolution-precipitation creep.

How to cite: Soret, M., Stünitz, H., Précigout, J., Lee, A., and Raimbourg, H.: Strain localization and weakening during eclogite-facies transformation in experimentally deformed plagioclase-pyroxene mixtures, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12327, https://doi.org/10.5194/egusphere-egu22-12327, 2022.

Blueschists are a major constituent rock type along the subduction zone interface and therefore critical to our understanding of subduction zone dynamics. Previous experimental work on natural blueschists focus on either seismic anisotropy or on the process of eclogization of a blueschist aggregate; however, little is known about the mechanical properties of blueschist rocks. We have conducted a suite of general shear deformation experiments in the Griggs apparatus to constrain the rheology of a blueschist aggregate. The sample material derives from a natural blueschist that was crushed into a powder. The powder consists of ~55% sodic amphibole, ~30% epidote, ~8% quartz, ~5% titanite, ~2% ilmenite, and <1% mica. Deformation experiments were conducted at 1.0 GPa confining pressure, temperatures of 650, 675, 700, and 750°C, and no water added. All of the deformation experiments were strain rate stepping experiments with either 4 or 5 strain rate steps per experiment with strain rates ranging from ~2.7e-5 to 5.2e-7 s-1. Based on the mechanical data we determine a stress exponent of 1.9 +/- 0.3. Microstructural and EDS analysis shows the initial Na-amphibole grains transform into a fine-grained aggregate of new Na-Ca-amphibole with lower Na and Si and higher Fe and Ca plus albite and ilmenite. The fine-grained aggregates accommodate the majority of the strain while epidote deforms by rigid body rotation or brittle deformation. Based on both the mechanical and microstructural observations, we interpret the fine-grained aggregates to be deforming by diffusion creep. Additional analyses will be conducted to constrain the grain size to develop flow law parameters to estimate the rheology of the subduction zone interface.

How to cite: Tokle, L., Hufford, L., Morales, L., Madonna, C., and Behr, W.: Diffusion creep of a Na-Ca-amphibole-bearing blueschist, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3268, https://doi.org/10.5194/egusphere-egu22-3268, 2022.

Subduction interface shear zones localize deformation and sustain plate-boundary weakness on million-year timescales, as well as host a variety of enigmatic seismicity and slow slip transients. A physical understanding of the steady-state and transient mechanics of subduction shear zones requires quantitative constraints of the plastic yield strength and deformation mechanism(s) of metamorphic rocks and minerals that occupy the plate interface. However, very little is known about the rheology of many hydrous minerals that occupy the plate interface, such as glaucophane (end-member sodic amphibole). This is partly because conventional deformation experiments meet technical challenges when trying to measure plasticity in the laboratory due to the stability field of glaucophane, the confining pressure needed to suppress fracture, and the limited range of trade-off between temperature and strain rate in experiments.

Here, we present preliminary results from room-temperature nanoindentation experiments on thin sections of glaucophane-rich rocks that produced crystal plasticity by dislocation glide under high-stress conditions. Nanoindentation produces in-situ confining pressure that typically inhibits brittle fracture during loading in favor of plastic deformation. Since the volume of deformation beneath the tips is very small compared to the grain size, each indent is essentially a single-grain mechanical test (i.e., effects of grain boundaries can be ignored). We convert load-depth data from two spheroconical tips of different radii to stress-strain curves to quantify the elastic-plastic transition and characterize post-yield behavior. We measure yield stress as a function of grain orientation. Both post-yield weakening and post-yield hardening occur, which likely reflect brittle fracture along micro-faults/cleavage planes, and dislocation bursts and pile-ups, respectively. Glaucophane hardness decreases with increasing length scale of deformation (i.e., indentation radius), capturing a “size effect” that may reflect an effective decrease in dislocation density as the volume of plastic deformation increases beneath the indent tip. This effect is well-constrained for many metals and some geologic materials, including olivine.

The mechanical tests provide a basis for interpreting microstructures of naturally-deformed blueschists, which suggest that glaucophane exhibits recovery-limited dislocation glide and dynamic recrystallization. Low-temperature plasticity may provide a micro-physical framework for long-term strain localization and transient brittle shear when meta-mafic rocks are deformed to high strain.

How to cite: Kotowski, A., Kirkpatrick, J., Thom, C. A., Alidokht, S. A., and Chromik, R.: Glaucophane plasticity and scale-dependent yield strength from nanoindentation experiments, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13371, https://doi.org/10.5194/egusphere-egu22-13371, 2022.

Quartz rheology in the presence of H₂O is crucial for modelling (numerical and geophysical) the deformation behavior of the continental crust and gives important insights into crustal strength. Experimental studies in the past have determined stress exponent (n) values for flow law between ≤ 2 to 4, while the values for activation energy (Q) vary from ~120 to 242 kJ/mol. Here, we investigated the quartz rheology under high-pressure and high-temperature conditions, using a new generation hydraulically-driven Griggs-type apparatus. In order to develop a robust flow law for quartzite, we performed constant-load coaxial deformation experiments of natural coarse-grained (~ 200 μm) high purity (> 99 % SiO₂) quartzite from the Tana quarry (Norway). Our creep tests were carried out at 750 to 900 °C on the as-is (no added H₂O) and 0.1 wt.% of H₂O added samples under 1 GPa of confining pressure. In contrast to earlier strain rate stepping experiments, the constant-load procedure needs lower strain at each step (≤1−2%) to achieve steady-state conditions. As a consequence, there is a very low amount of recrystallization. Importantly, we can determine the Q-value independently of the stress exponent (n). Microstructures from the deformed samples were characterized using polarized light microscopy (LM), SEM-cathodoluminescence (CL), and Electron backscatter diffraction (EBSD).

Our creep results for both the as-is and 0.1 wt.% H₂O-added samples yield Q = 110 kJ/mol and n = 2. Our microstructural analysis suggests that the bulk sample strain is accommodated by grain-scale crystal-plasticity, i.e., dislocation glide (dominantly in prism <a>) with minor recovery by sub-grain rotation, accompanied by grain boundary migration and micro-cracking. It is inferred that strain incompatibilities induced by dislocation glide are accommodated by grain boundary processes, including dissolution precipitation and grain boundary sliding. These intra-grain and grain-boundary processes together resulted in a lower n-value of 2 for the quartzite.

Our new flow law predicts strain rates that are in much better agreement with the inferred natural values than the earlier flow laws. It further suggests that the strength of the continental crust considering quartz rheology is significantly lower than previously predicted.  

How to cite: Ghosh, S., Stünitz, H., Raimbourg, H., and Précigout, J.: Constraining wet quartz rheology from constant-load experiments, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8484, https://doi.org/10.5194/egusphere-egu22-8484, 2022.

The effect of composition on microstructural development and mechanical strength was tested using mica-quartz-aggregates during deformation experiments.

This study used two chemically different biotite minerals mixed with quartz: (1) high F-phlogopite and (2) intermediate biotite in order to investigate the role of biotite-bearing systems for the development of shear zones and strain accommodation. Shear experiments (Griggs-type apparatus) were performed using mica (30 vol. %) and quartz (70 vol. %) assemblages at 750 and 800°C, 1000 MPa and a shear strain rate of ~10^-5 s^-1.

Mechanical results for the F-phlogopite-bearing assemblage indicate strong samples, approximately equivalent to pure quartz samples (Richter et al., 2018), deforming at differential stresses of 764-1097 MPa). F-phlogopite flakes are preferentially oriented parallel to the main shear direction, but poorly interconnected. Most of the strain is accommodated by quartz behaving as an interconnected network. Cathodoluminescence imaging reveals that quartz recrystallizes mainly by local pressure-solution and its strength controls the overall strain accommodation.

In contrast, intermediate biotite assemblages are significantly weaker and deform for lower differential stresses of 290-327 MPa, as expected for natural rocks. Biotite flakes form an interconnected network accommodating most of strain.

The interconnectivity of biotite grains thus plays a major role in weakening quartz-biotite assemblages. However, at similar P-T-strain and grain size conditions, the capacity of biotite grains to interconnect may also depend on its chemical composition, particularly considering the effect of trace elements incorporation (as fluorine) on the strength of the biotite interlayer bounds (Dahl et al., 1996, Figowy et al., 2021). This led us to conclude that different types of mica, behaving differently, strongly affect strength, deformation mechanism, and microstructure of the rock due to their structure, composition and stability fields.

How to cite: Alaoui, K., Airaghi, L., Stünitz, H., Raimbourg, H., and Précigout, J.: Different mechanical behavior at the same P-T conditions in biotite-quartz assemblage: interconnectivity and composition effect of experimentally deformed mica, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5108, https://doi.org/10.5194/egusphere-egu22-5108, 2022.

The production of micro-pores during viscous creep is a driving mechanism for fluid circulation in deep environments. However, strain-induced cracking in nature is nowadays attributed to grain boundary sliding (GBS), restricting this process to fine-grained ductile shear zones where rocks deform by diffusion creep. Here we give natural evidence of micro-cracking induced by dislocation creep, which is by far the dominant deformation mechanism in lithospheric rocks. Focusing on pure quartz shear bands across the Naxos western granite (Aegean Sea, Greece), we first document sub-micron pores that arise at grain and sub-grain boundaries. Their shape and location emphasize sub-grain rotation as a source of cracking. We then confirm that quartz is dominated by dislocation creep with evidence of a moderate to strong lattice preferred orientation (LPO) and many sub-grain boundaries, including at the margin of the pluton where the brittle/ductile transition was reached. These features coincide with (1) quartz grains located as inclusion into quartz porphyroclasts and (2) a dependency of the LPO strength on grain size. Our findings suggest that creeping cavities act as pumping sites for fluid to penetrate the crystal lattice and nucleate randomly oriented grains along sub-grain boundaries, accounting for (1) shear localization by enhancing hydrolytic weakening and (2) rock embrittlement through growth and interlinkage of cavities where phase nucleation is limited.

How to cite: Précigout, J., Ledoux, E., and Arbaret, L.: Cracking induced by dislocation creep in pure quartz shear bands of granitoids, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2816, https://doi.org/10.5194/egusphere-egu22-2816, 2022.

Rheological models of Earth’s granitoid mid- to upper crust are commonly based on the physico-chemical properties of the most abundant rock forming minerals quartz and feldspar. However, there is increasing field evidence that deformation in these rocks localizes in ultrafine-grained polymineralic shear zones, which are weaker than any of the end member minerals. Especially at the brittle to viscous transition, the localization and deformation mechanisms, i.e. the role of incipient brittle deformation vs. continuous viscous strain localization, is not yet fully understood.

To fill this gap in knowledge, ultramylonite samples with granitic composition from the Central Aar Granite (Aar Massif, Central Switzerland) were deformed using a Griggs type apparatus. The foliation of the ultramylonitic starting material was oriented 45° to the compression direction, to investigate the influence of grain size and composition on strain localization in the different mylonite bands. Two types of coaxial experiments were conducted at 650°C, and 1.2 GPa confining pressure: A) Discrete fractures were created before the shear deformation starts; B) No fractures were induced during an early stage of the experiment.

All experiments have in common that strain is accommodated in 20-100 µm wide viscous shear zones with elongated grains and minor grain size reduction. In these shear zones, most strain is further localized in 10-20 µm wide zones, showing dramatic grain size reduction down to few tens of nanometres. In the experimentally generated shear zones, both, brittle and viscous processes are active. In terms of overall rock strength, all newly formed ultrafine-grained shear zones are up to three times weaker than comparable experiments on pure quartz or coarser grained granites – which agrees well with field observations. Furthermore, pre-fractured type A) is up to two times weaker than the non-fractured type B), and the orientation and number of shear zones is also fundamentally different between the two experiment types.

This study confirms two weakening factors promoting different types of strain localization at the brittle to viscous transition: 1) The existence of fractures and their interconnectivity – facilitating highly-localized grain size reduction; 2) Initial sample heterogeneity by polymineralic composition and ultrafine grain size – generating grain size reduction along strain gradients by activating viscous processes. Further quantitative microstructural analyses will reveal the role of chemistry and the deformation mechanisms on the localization behaviour.

How to cite: Nevskaya, N., Zhan, W., Stünitz, H., Berger, A., and Herwegh, M.: Experimental strain localization in granitoid ultramylonites: Pre-fracturing vs. viscous strain localization, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5127, https://doi.org/10.5194/egusphere-egu22-5127, 2022.

Stylolites are ubiquitous structures generated by pressure solution primarily found in limestones. They and have been used as indicator for maximum stress a rock has suffered. This is commonly done by characterizing the fractal dimensions of stylolites. The current canon is the expectation from the theory that stylolites form through two physical pressure-driven regimes: low-frequency and higher-energetic - dominated by elastic forces and high-frequency lower-energetic dominated by surface tension. The so-called characteristic length separates both regimes, analytically marked by a kink in the power spectrum, which relates the energy contributions to the frequency.

However, determining this kink is not straightforward, and requires additional assumptions. We present a data set of stylolites recovered from a drill hole in the Alpine foreland basin. We mapped stylolites from different depths at sub-mm resolution semi-automatically and analyzed them using new methods of fractal analysis.

Excitingly, our preliminary studies did not identify the expected kink’s position from several different images of probes of drill cores, despite satisfactory reliability of laboratory preparation. Standard methods such as power spectral density, averaging wavelet coefficients, RMS, min/max, and rescaled range-based approaches revealed variations in their results, generally without evidence for a kink in the corresponding graphs. Implementing more recently developed methods such as adaptive fractal analysis could not improve the results. This finding challenges the classic interpretation of fractal characteristics of stylolites. 

How to cite: von Hagke, C., Hirländer, S., Frings, K., and Madritsch, H.: Revisiting stylolites as a gage of overburden pressure – insights from fractal analysis, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9842, https://doi.org/10.5194/egusphere-egu22-9842, 2022.

Many processes are at work when a sedimentary rock deforms. Quartz grains, for example, can rotate rigidly in the matrix, or on the contrary, undergo processes of dissolution and crystallization. Microtomography allows us to image the 3D geometry of minerals at the micron scale and quantify their fabric. Here, we use the quartz shape fabric extracted from microtomography data to evaluate the mechanisms active during burial and deformation of several sedimentary rocks systems.

Our first examples are of shales developing a slaty cleavage oblique to bedding. For shales that have undergone moderate burial (T_burial max ~200°C) (Sigues locality, Pyrenees), we show that the quartz grains rotate very little in the clay matrix. Even with the development of a slaty cleavage, a significant proportion of quartz grains remain parallel to the bedding plane. This surprising result implies that the rigid rotation of quartz grains, even in a ductile clay matrix, is not effective. 

In shales having undergone deeper burial and temperatures approaching the lower greenschist facies (T_burial max ~280°C) (Lehigh Gap locality, Appalachian mountains), we show that the average short-axis of the grains is orthogonal to the cleavage plane.  We suggest that this shape preferred orientation results from preferential dissolution of quartz faces oriented perpendicular to sigma 1, thus resulting in a shape preferred orientation without significant grain rotation.

Our last example concerns fine-grained sandstones, slightly deformed and buried at a shallow depth. If we refer to the example of shales with little burial, we would expect a very strong control of the bedding on the quartz fabric, since at these P-T conditions we expected dissolution-precipitation processes to be sluggish, and grain rotation to be ineffective.  However, surprisingly, the quartz in this rock is well oriented in the fabric which is oriented perpendicular to the bedding.

How the quartz grains were reoriented in the fine-grained sandstone suggests relations still not well understood with the deformation of a porous rock and the cementing processes of the rock. The microtomography approach in fine-grained rocks opens a door to this understanding of the behavior of quartz grains in sedimentary rocks.

How to cite: Aubourg, C., Saur, H., Moonen, P., and Stokes, R.: Quartz grain fabric in shales and sandstones: Some contrasting behaviors , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5979, https://doi.org/10.5194/egusphere-egu22-5979, 2022.

Understanding strain localization and development of shear fabrics within brittle fault zones at subseismic slip rates is crucial as they have critical implications for the mechanical strength and stability of faults and for earthquake physics. We performed direct shear experiments on ~1 mm thick layers of simulated quartz-rich fault gouge at an effective normal stress of 40 MPa, pore fluid pressure of 15 MPa, and temperature of 100°C. Microstructures were analyzed from strain hardening state (~1.3 mm displacement) to strain softening (~3.3 mm displacement) to steady-state (~5.6 mm) at different imposed shearing velocities of 1 µm/s, 30 µm/s, and 1 mm/s. We performed X-ray Computed Tomography (XCT) on sheared samples with a strain marker to analyze slip partitioning. To analyze and quantify localization from few hundreds to thousands of cross-section images, we used machine learning and developed an automatic boundary detection method to identify the type of shear fabrics and quantify the amount of them. Our results reveal that R₁ and Y (or boundary) shears are the two major localization features that developed in a repeatable manner. Slip on R₁ shears shows little dependency on both shear displacement and slip velocity and amounts to ~5 to ~30% of slip through the entire frictional sliding. On the other hand, Y and boundary shears show a strong correlation with displacement and velocity where more than 40% of strain was accommodated at steady-state for all velocities. However, Y and boundary shears become less prominent with increasing velocity, suggesting that velocity-weakening and the associated nucleation of unstable sliding are less likely to occur at higher slip rates as the overall friction behavior would be controlled by a thicker gouge layer. In other words, this suggests that Y shear development by grain size reduction is less efficient at high slip velocities which has important implications for the amount of heat generated during accelerating slip.

How to cite: Hung, C.-C. and Niemeijer, A.: Strain localization in quartz-rich fault gouge at subseismic slip rates, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4606, https://doi.org/10.5194/egusphere-egu22-4606, 2022.

Faults in the upper crust are considered major fluid pathways, raising the need for deformation experiments under wet conditions that focus on the nanoscale interaction between gouge material and pore fluid. Friction experiments on calcite at seismic slip velocities show strong dynamic weakening behaviour attributed to a combination of grain-size reduction and nanoscale diffusion. The resulting syn-deformational physico-chemical interactions between fluid and calcite are key in deciphering deformation mechanisms and rheological changes during and after (seismic) faulting in the presence of a fluid phase. We conducted rotary shear deformation experiments (1 m/s, σ_n = 2 and 4 MPa) on calcite gouge with water enriched in ¹⁸O (97 at%) as pore fluid to track and quantify potential fluid – mineral interaction processes. With our correlative, cross-platform workflow approach, we integrate Raman spectroscopy, nanoscale, and Helium-Ion secondary ion mass spectrometry (nanoSIMS, HIMSIMS), focused ion beam – scanning electron microscope (FIB-SEM) and transmission electron microscopy (TEM) to characterise the nanostructure and analyse isotope distribution. Raman analyses confirm the incorporation of ¹⁸O into the calcite crystal structure, as well as the presence of amorphous carbon. We identify three new band positions relating to the possible isotopologues of CO₃^2- (reflecting ¹⁶O substitution by ¹⁸O). In addition, we detect portlandite (Ca(OH)₂), pointing to a hydration reaction of lime (CaO) with water. Raman and NanoSIMS maps reveal that ¹⁸O is incorporated throughout the deformed volume, implying that calcite isotope exchange affected the entire fault gouge. Based on oxygen self-diffusion rates in calcite we conclude that solid-state ¹⁸O – isotope exchange cannot explain the observed incorporation of ¹⁸O into the calcite crystals during wet, seismic deformation. Hydration of portlandite and calcite containing ¹⁸O, implies breakdown and decarbonation of the starting calcite and the nucleation of new calcite grains. Our results question the state and nature of calcite gouges during seismic deformation and challenge our knowledge of the rheological properties of wet calcite fault gouges at high strain rates. The observations suggest that the physico-chemical changes are a crucial part of hydrous calcite deformation and have implications for the development of microphysical models that allow us to quantitatively predict crustal fault rheology.

How to cite: Ohl, M., King, H., Niemeijer, A., Chen, J., and Plümper, O.: Correlative, cross-platform microscopy application reveals deformation mechanisms during seismic slip along wet carbonate faults, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7406, https://doi.org/10.5194/egusphere-egu22-7406, 2022.

The Median Tectonic Line is a major east-west-trending arc-parallel fault that separates Sanbagawa metamorphic rock and Ryoke granite. We present the novel field observation of possibly pulverized rock and its evolution toward the fault cataclasite/breccia in the Ichinokawa antimony deposit in Central Shikoku. Ichinokawa was considered as largest stibnite mine in the world with a huge stibnite aggregate in which occurs in the brecciated-pelitic schist of the Sanbagawa belt. Based upon the texture in the outcrop and particle size distribution (PSD), this breccia is classified into two types. Breccia-1 (bx-1) is characterized by a centimeter-meter (up to 5m) angular breccia-clast with minimum to no shear displacement and rotational block. This bx-1 subsequently grows to be highly comminuted to produce breccia-2 (bx-2) which appear to have chaotic-polymict clast with matrix-supported texture within the fault zone with variable width and cut the bx-1 by recognizable breccia margin. Both of these breccia are cemented by reddish rock-flour matrix consist of dolomite, quartz, mica, ± pyrite. In addition, bx-2 has a more rounded shape with most of the clast size being less than 50mm and it shows orientation nearly parallel to the fault plane under a thin section. Based on this macro and micro-scale observation breccia in Ichinokawa is more likely to form under different mechanisms. Pulverization is plausible to rupture the pelitic schist and generate bx-1 without rotating the fragment. Hydrothermal activity in this area can’t be neglected which is responsible to create bx-2 as a result of fluid injection and transporting comminuted-fragment of bx-1 into the damage/fault zone. This breccia also underpins the formation of stibnite deposits that mark the latest fluid activity in this area where quartz-stibnite±pyrite±kaolinte vein truncate both pelitic schists of bx-1 as well as bx-2.

How to cite: Agroli, G., Uno, M., Okamoto, A., and Tsuchiya, N.: Pulverized rock and episodic hydrothermal brecciation along the Median Tectonic Line, Japan, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6926, https://doi.org/10.5194/egusphere-egu22-6926, 2022.

We report for the first time deformation features functionally described as deformation bands developed in low porosity rocks. This observation contradicts existing knowledge that deformation bands develop only in highly porous rocks. The studied formation is a bioclastic calcarenite of the Upper Cretaceous Maciños Unit in the Cotiella Massif. It is part of a megaflap adjacent to a salt diapir that has experienced extensional tectonics before the Pyrenean contraction. The bands present thickness variations, and in places they imitate the appearance of stylolites. This observation raises the question: how do deformation bands form in low porosity rocks?

To answer the question, we combine field observations with microstructural analysis to identify the occurring processes for the formation of deformation bands within low porosity rocks. After using optical microscopy and cathodoluminescence spectroscopy to conduct a petrographic study, we observe that the rock underwent multiple diagenetic cycles before the deformation stage, confirming that its porosity was significantly reduced before the deformation stage. Subsequently, we characterized the quartz grains inside the host rock and the dissolution-enabled deformation bands, using non-destructive imaging techniques. Three-dimensional image analysis from X-ray microtomography scans shows low grain size variations between the quartz grains in the host rock and in the bands, suggesting minor grain fracturing along the bands. However, grain reorientation has been reported for the quartz grains inside the bands, in relation to those in the host rock. Strain analysis was performed from Electron Backscattered Diffraction measurements, revealing higher strain along the quartz grain contacts inside the deformation band, compared to those in the host rock and stylolites. Our current data suggest that new porosity was created from local dissolution of the matrix, so the less soluble quartz grains were placed in contact. By wrapping-up the above observations, we propose a conceptual model that demonstrates the genesis and evolution of dissolution-enabled deformation bands in low porosity rocks, through local dissolution of the micritic matrix and transient porosity increase.

How to cite: Taxopoulou, M. E., Beaudoin, N. E., Aubourg, C., Charalampidou, E.-M., and Centrella, S.: Does porosity really matter? A first model for dissolution-enabled deformation bands in low porosity rocks based on microstructural analysis of calcarenite from Cotiella Basin, Spain. , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10153, https://doi.org/10.5194/egusphere-egu22-10153, 2022.

Carbonate rocks can be thick, mineralogically-homogeneous packages, which accomodate strain in orogenic belts. Despite its contribution to rock strength, the deformation of dolomite as a major rock forming mineral is understudied in comparison to calcite, quartz, and feldspar. We use field, petrographic, and electron back scatter diffraction (EBSD) analyses of dolomitic and calcitic marbles to investigate the response of these rocks to different degrees of strain under greenschist facies. Mt. Hymittos, Attica, Greece, preserves a pair of Miocene top-SSW ductile-then-brittle low-angle normal faults dividing a tripartite tectonostratigraphy. The bedrock of the massif comprises sub-greenschist facies phyllites and marbles in the uppermost hanging wall unit, and high-pressure greenschist facies schists and marbles of the Cycladic Blueschist Unit in the lower two packages. Ductile mylonites in the footwalls of both detachments grade into brittle-ductile mylonites and finally into cataclastic fault cores. The dolomitic and calcitic marbles of the lower units deformed under greenschist facies conditions and their fabrics reflect the relative differences in strengths between these two minerals. In the middle tectonostratigraphic unit, dolomitic rocks are brittlely deformed and calcitic marbles are mylonitic to ultramylonitic with recrystallized grain sizes ranging from 55 to 8 μm. Within the lower package, dolomitic and calcitic rocks are both mylonitic to ultramylonitic with previous P-T data suggesting metamorphism at ~470 °C and 0.8 GPa. EBSD analysis of six dolomitic marbles of the lower unit reveals a progressive fabric evolution from mylonites to ultramylonites reflecting the magnitude of strain and decreasing temperature of deformation. In mylonitic domains, average grain diameters range from 70 to 25 μm. The mylonitic dolomite exhibits low-angle grain boundaries, internal misorientation zones and textures suggestive of subgrain-rotation recrystallization. This mylonitic fabric is crosscut by ultramylonite bands of dolomite with grain diameters of 15 to 5 μm, which overlaps with the dominant grain size of the subgrains formed within the mylonitic domains. In samples closer to the fault core, the ultramylonite fabric is predominant though boudinaged veins, and relict mylonite zones with coarser grains may still be observed. Uniformly ultramylonitic dolomitic marbles exhibit grain diameters of 40 to 5 μm; the majority of grain diameters are less than 15 μm. The ultramylonite bands have low degrees of internal misorientation and an absence of low-angle grain boundaries that, along with correlated misorientation diagrams, suggest the ultramylonitic dolomite grains are randomly oriented and deforming via grain-boundary sliding. Interstitial calcite grains within these samples may reflect creep-cavitation processes interpreted to have occurred syn-kinematically with grain-boundary sliding. The change from subgrain-rotation recrystallization to grain-boundary sliding is interpreted to reflect the interplay of grain-size sensitive and insensitive processes. Following grain size reduction, subsequent deformation was dominantly accommodated by grain boundary sliding. The dolomitic marbles of the lower unit deformed on the retrograde path from the high-pressure, mid-temperature portion of the greenschist facies. The position of the dolomitic ultramylonites immediately below the cataclastic detachment fault suggest these ultramylonites were deforming very close to the brittle-ductile transition suggesting ductile deformation at lower temperatures than might be predicted by deformation experiments.

How to cite: Coleman, M., Grasemann, B., Schneider, D., Soukis, K., and Graziani, R.: Strain localization along a detachment system: Deformation of natural dolomitic and calcitic mylonites (Mt. Hymittos, Attica, Greece), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-407, https://doi.org/10.5194/egusphere-egu22-407, 2022.

The rheology and mechanisms of strain localisation in the middle and lower crust is yet to be fully constrained, but advances in analytical techniques mean we can revisit previously studied areas and build upon understanding already gained.

A strain profile across a Laxfordian-age (2300-1700 Ma) amphibolite-facies shear zone at Upper Badcall, NW Scotland, provides an excellent backdrop to investigate the hydration-strain-deformation mechanism relationship in the granulite-facies garnet-pyroxene quartzofeldspathic gneiss host rock and cross-cutting 25 m wide isotropic dolerite Scourie dyke. Both the granulite faces gneissic banding and mafic dyke are initially oriented at a high angle to the shear zone boundary. With increasing proximity to the shear zone centre the host rocks become progressively rotated, more deformed and hydrated. Increasing strain results in new foliation development, general grain size reduction and full or partial replacement of pre-existing pyroxene and hornblende by lower-temperature hornblende.

Tatham and Casey (2007) showed the 65 m wide shear zone has an estimated maximum shear strain of 15, which drops to ~7 towards the edge of the shear zone, and falls to < 1 at distances ≥ 40 m from the shear zone centre. We present data from four new transects, taken at 50-100 m intervals along the mafic dyke, which detail the change in deformation style and patterns of strain localisation and intensity. Localised anastomosing high strain zones envelop lenses of undeformed dolerite, with 65-70% of protolith undeformed in the dyke 350 and 230 m from shear zone centre. This decreases to 30 and 0% of undeformed protolith 100 m from and within the shear zone, respectively. Mylonite sensu stricto makes up 10% of dyke at distances ≥ 100 m from the shear zone, which increases to 70% within the shear zone, while the remaining dyke forms a weak fabric evidenced by the shape change of mafic grain aggregates.

Microstructural analyses show a switch in dominant deformation mechanisms from dynamic recrystallisation 350 m from the shear zone, to dissolution-precipitation creep inside the shear zone, identified by a change in crystallographic and shape preferred orientation, and distinct microstructural observations. An introduction of ~10 area % quartz and a loss of feldspar in the mafic dyke inside the shear zone accompanies this switch in dominant deformation mechanisms. We outline microstructural observations characteristic of dissolution-precipitation creep within the shear zone, and propose localised infiltration of quartz-rich fluid facilitates a switch from dislocation creep to pervasive dissolution-precipitation creep resulting in rheological weakening and local strain localisation. Our results suggest that strain localisation in the mid crust may be highly dependent on local fluid availability as fluid presence may trigger a switch in deformation mechanism and, with that, significant localised rheological weakening.

Tatham, D.J. and Casey, M., 2007. Inferences from shear zone geometry: an example from the Laxfordian shear zone at Upper Badcall, Lewisian Complex, NW Scotland. Geological Society, London, Special Publications, 272(1), pp.47-57.

How to cite: Carpenter, M., Piazolo, S., Craig, T., and Wright, T.: The link between water infiltration, deformation mechanisms and strain localisation in the mid crust – an example from the Badcall shear zone, NW Scotland, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13366, https://doi.org/10.5194/egusphere-egu22-13366, 2022.

Quaternary deformation in the northern Chile coastal forearc is mainly accommodated by ubiquitous upper-plate faults cataloged as weak fault zones, however, the deformation mechanisms and the internal structure of these reactivated faults remain poorly understood. To solve this problem, we selected seven study sites from reactivated upper-plate faults of the northern Chile forearc (23-25°S). These faults formed during the Early Cretaceous and reactivated during the Quaternary forming conspicuous fault-scarps. Here we present a new characterization of the internal structure at the outcrop and microscopic scale. Samples for thin-sections and XRD were collected in several cross-sections across faults. We define 4 units conforming the internal structure: (1) A decimetric well-defined principal slip zone, referred here as active fault core (AFC), consisting of a gouge layer subunit bounded by a fault breccia subunit, (2) a metric inactive fault core (IFC), surrounding the AFC, composed mostly of cataclasites and in some cases, mylonites, (3) a host-rock unit corresponds mainly to Jurassic-Cretaceous dioritic-granitic intrusives and Jurassic andesites, and (4) a decametric damage zone affecting both the IFC and the host rock. Near the topographic surface, the gouge layer subunit consists of a grey/green ultrafine matrix (40-80%) partially to completely replaced by massive iron oxides. In some sites, the gouge layers are partially foliated or/and exhibit millimetric bands of chaotic microbreccia. Porphyroclasts correspond mainly to (1) highly quartz and plagioclase intracracked individual crystals (<0.4mm), (2) larger fragments (<1mm) generally sigmoidal-like of the IFC (cataclasites) indicating different degrees of cataclastic-flow. Transgranular microfractures are densely propagated through the boundaries of larger porphyroclasts, breaking grains into ever-finer fragments (constrained communition) and generating chaotic microbreccia halos in the boundaries that grade into an ultrafine gouge matrix. (3) Another portion of large porphyroclasts (>1mm) grade from S-C cataclasite at its cores to S-C ultra-cataclasites at its boundaries. Frictional sliding is propagated through this S-C fabric formed by the ultracataclasite boundaries, generating well-defined and smoothened surfaces between large porphyroclasts and gouge layers. Microfractures -commonly filled with quartz>calcite>albite>chlorite-epidote veins- propagate mostly through the gouge layers, which are in turn displaced by microfaults affecting the entire subunit. The IFC composition changes markedly along-strike but multiple-fault cores are ubiquitous. In Jurassic andesites, the IFC is defined by protocataclasites with layers of red gouge, In Jurassic to Cretaceous diorite-metadiorite protoliths the IFC is defined by S-C cataclasites with microstructures showing undulating extinction, subgrains, and bulging recrystallization of quartz, and ultracataclasite bands and green gouge layers developed under low greenschist facies conditions. The IFC formed in mylonitic rocks derived from Jurassic to Cretaceous granitoid includes bands of hydrothermally-altered green and red mylonites. The complex overprinted microtextures indicate a progressive exhumation and shearing of the IFC. The microtexture analysis reflects the evolution of this unit from high temperature-low stain rates formed at deep structural levels to low temperature-high strain rates near-surface. We interpret the highly accumulated strain in S-C ultracataclastic bands and S-C gouge layers of the IFC (constrained communition) reduces the fault frictional strength and promote the frictional slip of the quaternary reactivations of the AFC.

How to cite: González, Y., Jensen, E., and González, G.: Internal Structure and Microtextures of a Quaternary Upper-plate Fault Zone: A Case Study from the Atacama Fault System, Northern Chile., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6742, https://doi.org/10.5194/egusphere-egu22-6742, 2022.