Deformation mechanisms and microstructures 

Deformation microstructures (e.g. fabrics, textures, grain sizes, shapes, cracks etc) give insight into the conditions and processes of brittle failure and ductile flow of geomaterials. Microstructures and textures are a key tool in unraveling deformation histories and processes, kinematics and conditions in deformed rocks or ice. Processes such as grain-size reduction, phase changes, and development of crystallographic preferred orientations modify the rheological, elastic, and thermal properties of these rocks, providing key information on the evolution and dynamics of the litho- and cryosphere. In this session, we invite contributions based on microstructure and texture analysis from field observations, laboratory experiments, and numerical modelling that aim to constrain deformation mechanisms, physical and mechanical properties of geomaterials using well established or novel techniques.

Co-organized by CR5/EMRP1/GMPV7
Convener: Amicia LeeECSECS | Co-conveners: Rüdiger Kilian, Renée Heilbronner, Damien FreitasECSECS, John BedfordECSECS, John Wheeler
vPICO presentations
| Mon, 26 Apr, 09:00–12:30 (CEST), 13:30–15:00 (CEST)

Session assets

Session materials

vPICO presentations: Mon, 26 Apr

Chairpersons: Amicia Lee, Damien Freitas
Textures and mechanical properties of mafic rocks
Rhander Taufner, Gustavo Viegas, Frederico Faleiros, Paulo Castellan, and Raylline Silva

Detachment faulting has been hypothesized as the main process of tectonic spreading in mid-ocean ridges. The ongoing faulting leads to exhumation of oceanic core complexes (OCC) through large-scale normal faults, exposing heterogeneous sectors of the mylonitic lower crust, locally interlayered with pristine upper-mantle rocks. However, the mechanisms involved in this process – and the interplay between magmatism, deformation and fluid-rock interaction – are still debatable. To address these issues, we performed a quantitative microstructural analysis and thermodynamic modelling on mafic shear zones that occur in the lower section (≥ 600 meters below sea-floor) of Site U1473A (Atlantis Bank OCC, SW Indian Ridge), the target of IODP Expedition 360, to constrain deformation conditions and strain localization mechanisms during detachment faulting. The gabbroic shear zones consist of large (up to 5 mm in size) porphyroclasts of clinopyroxene, orthopyroxene, plagioclase and olivine embedded in a fine-grained (≤ 30 µm), polyphase matrix composed of plagioclase, clinopyroxene, orthopyroxene, amphibole, ilmenite, magnetite and olivine. Plagioclase-rich layers (~ 80 µm) are in abrupt contact with the fine-grained mixture, which define the mylonitic foliation. The porphyroclasts have undulose extinction, subgrains and are surrounded by fine-grained recrystallized grains (core-mantle structure) showing internal lattice distortion. Microfractures are common in orthopyroxene porphyroclasts. Amphibole replaces clinopyroxene and orthopyroxene porphyroclasts at their margins and fills cleavage planes. The plagioclase-rich layers show undulose extinction and subgrain boundaries in the larger grains within the layers. Mechanical twin lamellae occur in some grains regardless of grain size. Plagioclase grains show a weak shape preferred orientation with their long axes parallel to the main planar fabric of the shear zone. The grains in the polyphase matrix are mostly strain free. EBSD data in clinopyroxene clasts indicate activation of (010)[001] slip system and twinning along (001)[100]. Plagioclase-rich layers deforms by slip along the (010)[100] system. The polyphase matrix has a very weak but non-random CPO pattern. #Mg and Al content in the recrystallized clinopyroxene and orthopyroxene grains are lower compared to the porphyroclasts. Plagioclase has similar An content in both porphyroclasts and recrystallized grains. Amphibole has low concentrations of Cl and high content of F. The content of #Mg, Al and Si is similar in amphibole grains replacing pyroxene and in the polyphase matrix. Thermodynamic modelling indicates that the gabbroic shear zones formed at 820-870 °C and 2.0-2.8 kbar. Our results suggest that deformation in the porphyroclasts was accommodated by combined mechanical fragmentation and intracrystalline plasticity, which resulted in fractured grains of orthopyroxene, and clasts rimmed by recrystallized neoblasts. Plagioclase-rich layers formed mainly through dislocation creep. Phase mixing and weak CPO in the polyphase matrix point to oriented-growth during diffusion-assisted grain boundary sliding, mainly in the presence of melt, as evidenced by amphibole formed at the expense of pyroxene. Magmatic fluids are the possible source of reactant amphibole. Such mechanisms effectively resulted in strain localization in fine-grained, polyphase shear zones that contributed to the weakening of the ocean crust during detachment faulting and subsequent exhumation of the Atlantis Bank OCC.

How to cite: Taufner, R., Viegas, G., Faleiros, F., Castellan, P., and Silva, R.: Microstructures reveal brittle and viscous flow during exhumation of the high-temperature lower oceanic crust from Site U1473A, Atlantis Bank, Southwest Indian Ridge (IODP Expedition 360), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1905, https://doi.org/10.5194/egusphere-egu21-1905, 2021.

Maël Allard, Benoît Ildefonse, and Émilien Oliot

The crustal architecture of slow-spread ocean crust results from complex interactions between magmatism, hydrothermalism, and tectonics. IODP Hole U1473A (809 m depth) was drilled during IODP Expeditions 360 and 362T at the summit of the Atlantis Bank, a gabbroic massif exhumed at the Southwest Indian Ridge (SWIR). In this study, we identify and quantify plastic deformation processes in oceanic gabbros and active slip-systems in plagioclase from 112 thin sections sampled throughout Hole U1473A.

We describe deformed zones using petrographic observations and modern Electron Backscattered Diffraction (EBSD) analyses made all along the core. Ductile deformation is widespread and is sometimes strongly localized. It initiated during accretion under magmatic conditions and continued until late brittle conditions. Porphyroclastic microstructures testify to post-magmatic, solid-state, high-temperature (HT) deformation. Plagioclase represents ~60% of rock’s volume and is the dominant phase accommodating deformation in the gabbro. It shows strong dynamic recrystallization accommodated by dislocation creep, forming a fine-grained matrix. Strain localizes in mylonitic and ultramylonitic zones, and these shear zones are often overprinted by lower temperature deformation.

EBSD analyses reveal weak to moderate crystallographic preferred orientations (CPO) of plagioclase first developed during early magmatic flow, that has produced a primary fabric with a (010) foliation plane and a [100] lineation axis. This CPO is persistent during subsequent plastic deformation and strain localization and is observed in almost all samples. However, a detailed investigation of internal misorientations measured at subgrains reveals the activity of at least 4 to 5 slip systems in plagioclase grains: , and maybe . The strength of CPO is first increasing from slightly foliated gabbros to mylonites before decreasing significantly in ultramylonites, which could be explained by orientation scattering after subgrain rotation recrystallization and grain boundary processes (e.g., nucleation, grain boundary sliding).

How to cite: Allard, M., Ildefonse, B., and Oliot, É.: Plastic Deformation of Plagioclase in a Gabbro Pluton at a Slow-Spreading Ridge (IODP Hole U1473A, Atlantis Bank, Southwest Indian ridge), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4089, https://doi.org/10.5194/egusphere-egu21-4089, 2021.

Luiz F. G. Morales, Mael Allard, and Benoit Ildefonse

Gabbros are the main component of the oceanic crust and represent ~2/3 of the total magmatic crustal thickness. At the interface between magmatic, tectonic and hydrothermal processes, gabbros from slow spreading ridges may have a complex mineralogy and microstructural evolution. This includes structures that vary from purely magmatic fabrics, with layering and magmatic alignment of minerals, to rocks deformed from subsolidus temperatures to the lower-T brittle-ductile conditions. Such a variation is normally accompanied with changes in mineralogy, microstructures and crystallographic preferred orientations (CPO) of the main phases of these rocks, which in turn a!ect their seismic properties. Here we present a database of the CPO-derived seismic properties of 70 samples collected during the IODP Expedition 360 (site U1473). The dominant phases are plagioclase and clinopyronexe, with variable contents of olivine, enstatite, magnetite, ilmenite, chlorite and amphibole.  Velocities of compressional and shear waves decrease drastically with increasing of plagioclase content, increase strongly with increasing of ilmenite content, but increase only slightly with clinopyroxene, while variations in olivine and enstatite content seem to be less important. Maximum velocities can be either parallel to the strongest concentration of (010) poles of plagioclase or olivine/clinopyroxene [001], depending on the proportions between these phases. Anisotropy of P waves vary from ~2% in the more isotropic gabbros with weak magmatic fabric to a maximum of ~9% in more mylonitic terms. A similar effect is observed for the S-waves. Destructive interference between plagioclase CPO vs. clinopyroxene/olivine reducing anisotropy observed in some samples. This is because the maximum Vp in a foliated gabbro is parallel to the maximum concentration of poles to (010), and perpendicular to olivine and clinopyroxene. As the lineation in our gabbros is generally marked by olivine and clinopyroxene [001] (instead of the fast direction [100]), this possibly cause anisotropy reduction. When present in the more mylonitized gabbros, amphibole has strong CPOs and help to increase the general anisotropy of P and S waves, but the increase is not drastic. An increase of Vp and Vs anisotropy is also observed with stronger plagioclase CPOs, which is not observed in the case of clinopyroxene. The elastic constants calculated from these aggregates will be used as input for more physically robust calculations using differential effective medium approaches to better understand the e!ect of melt inclusions in these rocks by the time of their deformation in the lower crust.

How to cite: Morales, L. F. G., Allard, M., and Ildefonse, B.: A database of gabbro seismic properties from an ultraslow spreading ridge (IODP Hole U1473A, Southwest Indian Ridge), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14607, https://doi.org/10.5194/egusphere-egu21-14607, 2021.

Rebecca Kühn, Jan Behrmann, Rüdiger Kilian, Bernd Leiss, and Michael Stipp

Physical properties of rocks are mainly controlled by the modal composition, crystallographic preferred orientation (CPO) and microstructure of a rock. One of the most relevant physical properties related to the interpretation of seismic data are the elastic properties of a mineral aggregate. Changes of elastic properties - and hence changes in our interpretation of the tectonic architecture of certain regions - can be related to mineral reactions and deformation.

In order to explore the impact of mineral reaction and deformation on elastic anisotropy, we study oceanic serpentinites formed at low-grade metamorphic conditions by hydration of peridotites. Samples are obtained from the Atlantis Massif, which is an Oceanic Core Complex located at 30°N, Mid-Atlantic Ridge. During IODP Expedition 357, oceanic serpentinites were recovered from drill cores along the southern wall of the Massif. Fully serpentinized samples displaying variable microstructures were analyzed regarding the influence of microstructure and CPO on the overall elastic anisotropy. Microstructure analysis was based on optical microscopy and large area micro X-ray fluorescence mapping. For CPO analysis synchrotron high energy X-ray diffraction in combination with the Rietveld method was applied and the derived CPO was used to compute seismic properties.

Serpentinites with a typical mesh microstructure are interpreted to represent undeformed samples and show a close to uniform CPO. The increase in fabric anisotropy of vein-like magnetite aggregates is interpreted as an increase in deformation. Samples show a single c-axis-maximum and enhanced CPO. Calculated seismic anisotropies show up to >5% anisotropy for compressional waves (Vp) and shear wave splitting up to 0.15 km/s in the deformed samples. Hence, such an anisotropy can be used to differentiate deformed from undeformed zones in seismic data sets using the elastic anisotropy data.

How to cite: Kühn, R., Behrmann, J., Kilian, R., Leiss, B., and Stipp, M.: Elastic anisotropy of oceanic serpentinites – influence of CPO and microstructure, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12660, https://doi.org/10.5194/egusphere-egu21-12660, 2021.

Manuel D. Menzel, Janos L. Urai, Peter B. Kelemen, Greg Hirth, Alexander Schwedt, and Andras Kovacs

Carbonated serpentinites record carbon fluxes in subduction zones and are a possible natural analogue for carbon capture and storage via mineralization, but the processes by which the reaction of serpentinite to listvenite (magnesite-quartz rocks) goes to completion are not well understood. Large-scale hydration and carbonation of peridotite in the Oman Ophiolite produced massive listvenites, which have been drilled by the ICDP Oman Drilling Project (OmDP, site BT1) [1]. Here we report evidence for localized ductile deformation during serpentinite carbonation in core BT1B, based on observations from optical microscopy, cathodoluminescence microscopy, SEM, electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) in segments of the core that lack a brittle overprint after listvenite formation [2].

Microstructural analysis of the serpentinized peridotite protolith shows a range of microstructures common in serpentinite with local ductile deformation manifested by a shape and crystallographic preferred orientation and kinking of lizardite. Listvenites with ductile deformation microstructures contain a penetrative foliation due to a shape preferred alignment of magnesite spheroids and/or dendritic magnesite, bending around Cr-spinel porphyroclasts. Locally the foliation can be due to aligned dendritic overgrowths on euhedral magnesite grains. Magnesite grains have a weak but consistent crystallographic preferred orientation with the c-axis perpendicular to the foliation, and show high internal misorientations. Locally, the microcrystalline quartz matrix also shows a crystallographic preferred orientation with the c-axes preferentially oriented parallel to the foliation. Folding and ductile transposition of early magnesite veins indicates that carbonation initiated before the ductile deformation stage recorded in listvenites with penetrative foliation. On the other hand, dendritic magnesite overgrowths on folded veins and truncated vein tips suggest that folding likely occurred before complete carbonation, when some serpentine was still present. TEM analysis of magnesite revealed that subgrain boundaries oriented at high angle to the foliation can consist of nano-cracks sealed by inclusion-free magnesite precipitates. High dislocation densities are not evident suggesting that dislocation creep was minor or negligible, in agreement with very low predicted strain rates for magnesite dislocation creep at the low temperatures (100 – 200 °C) of serpentinite carbonation. This points to dissolution-precipitation, possibly in addition to grain boundary sliding, as the main mechanism for the formation of the shape preferred orientation of magnesite. The weak magnesite crystallographic preferred orientation may be explained by a combination of initial growth competition in an anisotropic (sheared) serpentine medium with subsequent preferred dissolution of smaller, less favorably oriented grains. We infer that transient lithostatic pore pressures during listvenite formation promoted ductile deformation in the reacting medium through grain boundary sliding accommodated by dilatant granular flow and dissolution-precipitation. Because the reaction product listvenite is stronger than the reacting mass, deformation may be preferentially partitioned in the reacting mass, locally enhancing transient fluid flow and, thus, the carbonation reaction progress.

[1] Kelemen et al., 2020. Site BT1: fluid and mass exchange on a subduction zone plate boundary. In: Proceedings of the Oman Drilling Project: College Station, TX

[2] Menzel et al., 2020, JGR Solid Earth 125(10)

How to cite: Menzel, M. D., Urai, J. L., Kelemen, P. B., Hirth, G., Schwedt, A., and Kovacs, A.: Reaction-enhanced ductile deformation during carbonation of serpentinized peridotite, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10718, https://doi.org/10.5194/egusphere-egu21-10718, 2021.

Bob Bamberg, Richard Walker, and Marc Reichow

Faults constitute the major source for mechanical and permeability heterogeneity in basaltic sequences, yet their architecture, and mechanical and physical properties remain poorly understood. These are however critical as basaltic reservoirs are becoming increasingly important for geothermal applications and CO2 storage. Here we present a detailed microstructural- to outcrop-scale characterisation of mature (decametre-hectometre displacement) fault zones in layered basalts, in the Faroe Islands. Outcrop scale structures and fault rock distribution within the fault zone were mapped in the field to build 3D virtual outcrop models, with detailed characterisation of fault rock microstructure and petrology obtained from optical and SE-microscopy.

The fault zones exhibit evidence for cyclic activity controlled by fault internal fluid pressure variation. Deformation mechanisms in the core alternate between shear-compaction, evidenced by foliated cataclasite and gouge development, and dilatation through fluid overpressure, leading to hydrofracture and vein formation. Generally, a decametre-wide damage zone of Riedel faults is centrally transected by the fault core. The fault core is organised around a principal slip surface (PSS) hosted in a decimetre-wide principal slip zone (PSZ). The PSS and PSZ are dominantly composed of (ultra-) cataclasites, while the remaining core comprises anastomosing cataclastic bands bounding lenticular zones of various brecciated fault rocks. Further, PSS-proximal zones show significant late-stage dilatation by hydrothermal breccias or tabular veins with up to decimetre apertures, filled with early syntaxial to blocky zeolite and/or late coarse (≤ 1 cm) blocky calcite. The structures in the fault core are mutually overprinting, evidencing pulsed fault activity and PSS migration. The native plagioclase-pyroxene assemblage of the host rock is almost completely altered to zeolites and red-brown smectites in the fault core and along surrounding damage of mature faults, while lower displacement faults preserve the host rock mineralogy even in gouge. We infer that fluid flow along initial damage promotes alteration and the associated chemical weakening localises strain into a narrow PSZ. Here, fault activity is governed by alternating deformation styles – shear‑compaction and dilatation – suggesting changes in deformation mechanism linked to transient permeability decrease within the PSZ, followed by fluid overpressure and hydrofracture. Overall rock mechanical properties are thus governed by the combined effects of permanent chemical weakening and transient fluid-mediated mechanical weakening, alternating with cementation and healing, and will be explored by direct shear deformation experiments in the future.

How to cite: Bamberg, B., Walker, R., and Reichow, M.: Periodic Fluid-mediated Weakening and Cementation Drives Cyclic Reorganisation of Shallow Basaltic Fault Zones, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15655, https://doi.org/10.5194/egusphere-egu21-15655, 2021.

Effects of microstructures and texture on rheology
Martina Kirilova, Virginia Toy, Katrina Sauer, Francois Renard, Klaus Gessner, Richard Wirth, Xianghui Xiao, Risa Matsumura, David Prior, Francesco Cappuccio, and Soltice Morrison

Feedback between deformation mechanisms, fluid-rock interactions and porosity evolution in fault zone rocks has a crucial impact on their bulk rheology. Porosity formation within mid-crustal fault rocks (typically mylonites) can facilitate fluid flow, formation of mineral and geothermal resources, and can promote strain localization. On the contrary, porosity reduction in rocks from a brittle fault core (typically cataclasites) can cause elevated pore fluid pressures, and consequently influence the recurrence time of earthquakes.

We characterized the porosity distribution within the New Zealand’s Alpine Fault zone in cataclasite samples recovered during the first phase of the Deep Fault Drilling Project and outcropping mylonitic rocks collected at Stoney Creek, New Zealand. Synchrotron X-ray microtomography-derived analyses of open pore spaces show total microscale porosity values in the range of 0.1-0.24% within the cataclasites and up to 0.44% in the mylonites. Synchrotron nanotomography datasets reveal additional 0.03 to 0.19% pore volumes within the mylonites. In all samples, pores are very small, not connected, with mainly non-spherical, elongated, flat shapes and show subtle bipolar orientation. Scanning and transmission electron microscopy reveal the samples’ microstructural organization, where nanoscale pores ornament grain boundaries of the constituent minerals. Pores are mostly associated with (often newly formed) clay minerals in the cataclasite samples, suggesting the orientation of clays controls the shape and orientation of the associated pores. In the mylonitic samples, pores are sub-parallel to the foliation, and often associated with C’-type shear bands, indicating formation during creep cavitation.

Our observations imply that porosity within the Alpine Fault core was reduced due to pressure solution processes and the associated mineral precipitation. Simultaneously propagation of fluids triggered by cavity formation in the ductile regime is likely to cause further mineral precipitation in fluid filled pores within the fault zone. Such precipitation can affect the mechanical behavior of the Alpine Fault by decreasing the already critically low total porosity of the fault core, causing elevated pore fluid pressures, and/or introducing weak mineral phases, and thus lowering the overall fault frictional strength. We conclude that the current state of porosity in the Alpine Fault zone is likely to play a key role in the initiation of the next fault rupture.

How to cite: Kirilova, M., Toy, V., Sauer, K., Renard, F., Gessner, K., Wirth, R., Xiao, X., Matsumura, R., Prior, D., Cappuccio, F., and Morrison, S.: Porosity evolution within the active Alpine Fault zone, New Zealand. Implications for fault zone rheology., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9399, https://doi.org/10.5194/egusphere-egu21-9399, 2021.

Kamal Nanda, Santanu Misra, and Arghya Das

Permeability evolution of low permeable rocks is of critical importance during the flow of gases in processes like, enhanced reservoir recovery and CO2 sequestration. Permeability measurement depends on the geometric structure of flow path (hydraulic radius, connectivity, tortuosity), the stress regimes surrounding the rock (isotropic, deviatoric) and the characteristic of the fluid (viscosity, compressibility, pore pressure).  For the case of gas permeability within Knudsen diffusion regime (0.001 < Kn< 0.1), the effect of slippage is prominently observed.

Laboratory scale permeability experiments on an Indian sandstone having connected porosity ~10%, are performed under hydrostatic condition. Nitrogen gas is selected as pore fluid, to avoid adsorption phenomenon. Transient technique of pore-pressure-pulse decay is used for permeability measurement as it is faster and accurate to measure pressure, than the steady state method. Pore pressures and confining pressures are varied in the study to understand the relative effect of matrix compressibility and fluid compressibility on the permeability. Micro-CT analysis of sample is also performed to quantify the geometric attributes of sample.

Apparent gas permeability ranging from 0.1 to 1 micro-Darcy is obtained from the experiments. The permeability is found to be decreasing with simple effective stress (σii-p) for constant pore pressures. But a counter intuitive decrease in permeability with increasing pore-pressure at constant confining pressure is also evident and can be attributed to stress dependent Biot’s coefficient (λ).  Slippage corrected permeability is further analysed theoretically and numerically to formulate nonlinear permeability evolution equation in the functional form, f(σii-λp)  to support experimental outcomes.

How to cite: Nanda, K., Misra, S., and Das, A.: Effect of pore pressure on the permeability evolution in low porous Indian sandstone, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16128, https://doi.org/10.5194/egusphere-egu21-16128, 2021.

Hien Nho Gia Nguyen, Luc Scholtès, Yves Guglielmi, Frédéric Victor Donzé, Zady Ouraga, and Mountaka Souley

Fluid pressurization of critically stressed sheared zones can trigger slip mechanisms at the origin of many geological rupture processes such as earthquakes and landslides. It is now well assumed that the reduction of effective stress induced by fluid pressurization can lead to the reactivation of shear zones. However, the micromechanisms that govern this reactivation remain poorly understood. By using discrete element modeling, we simulate pore-pressure-step creep test experiments on a sheared granular layer at a sub-critical stress state in order to investigate the micromechanical processes at stake during fluid induced reactivation. The simulated responses are consistent with both laboratory and in situ experiments, confirming the scale independent nature of fluid induced slip. The progressive increase of pore pressure promotes slow steady slip at sub-critical stress states and fast accelerated dynamic slip once the critical strength is overcome. The analyses of both global and local quantities show that these two emergent slip behaviors correlate to characteristic deformation modes: diffuse deformation for slow slip and highly localized deformation for fast slip. Our results suggest that, besides the control of the fabric of shear zones on their emergent slip behavior, failure is associated to grain rotations resulting from unlocking of interparticle contacts mostly located within the shear band, which, as a consequence, acts as a roller bearing for the surrounding bulk.

How to cite: Nguyen, H. N. G., Scholtès, L., Guglielmi, Y., Donzé, F. V., Ouraga, Z., and Souley, M.: Grain scale investigation of shear reactivation by fluid pressurization, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7855, https://doi.org/10.5194/egusphere-egu21-7855, 2021.

Elliot Rice-Birchall, Daniel Faulkner, and John Bedford

As sandstone reservoirs are depleted, the pore pressure reduction can sometimes result in pore collapse and the formation of compaction bands. These are localised features which can significantly reduce the bulk permeability of the reservoir and are therefore problematic in the oil, water, geothermal, and CO2 sequestration industries. However, the influence that grain size, grain shape and sorting have on compaction band formation in sandstone is still poorly understood, due to the fact that finding natural sandstones with specific properties is challenging. Consequently, a method of forming synthetic sandstones has been developed, in order to produce a suite of sandstone specimens with controlled grain size and porosity characteristics. During production of the synthetic sandstones, amorphous quartz cement and sodium chloride are precipitated between sand grains as a product of the reaction between sodium silicate and hydrochloric acid. The salt can then be dissolved, resulting in synthetic sandstones that have very comparable physical properties to their natural counterparts. In this study, triaxial experiments were performed on synthetic sandstone cores with four different grain size ranges of 250-300, 425-500, 600-710 and 850-1000 microns, at three different starting porosities of 27%, 32% and 37%. The samples were each axially loaded from a point along their hydrostat corresponding to 85% of their hydrostatic yield point, P*, values. These conditions mean that failure will occur within the shear-enhanced compaction regime so as to try and produce localised compaction structures. All samples were taken to 5% axial strain. The microstructural results indicate that localisation of deformation within the samples did occur and was favoured in the low starting porosity, small grain size samples. Localisation of deformation was most easily recognised by grain size reduction through grain crushing. This was weakly correlated to a change in porosity but recognition of the localisation of deformation was difficult to make using variations in porosity alone. Porosity reduction was not necessarily associated with a reduction in grain size. With increasing grain size and starting porosity, the deformation becomes more distributed in the samples with the highest starting porosity samples (37%) exhibiting more widely distributed grain crushing which was less intense overall. The results indicate a significant grain size and starting porosity influence on localisation, but also that compaction can occur by two mechanisms; one involving mostly grain rearrangement and the other primarily by grain fracturing. Consequently, the localisation of deformation is most evident in grain size reduction and is only weakly shown by porosity reduction.

How to cite: Rice-Birchall, E., Faulkner, D., and Bedford, J.: The effect of grain size and porosity on compaction localisation in high-porosity sandstones, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11020, https://doi.org/10.5194/egusphere-egu21-11020, 2021.

Enes Zengin and Zeynal Abiddin Erguler

The cracking phenomenon of the brittle rock and rock-like materials (concrete, gypsum) have been widely researched. Such long-standing intensive research requirement is due to the fact that crack initiation, propagation and coalescence are some of the most important parameters for evaluating the rock failure behavior and strength properties. Especially defining the crack initiation stress is a fundamental part of crack propagation that leads to the rock material's final failure. However, due to the nature of rocks, they may have complex inherit structures containing various gaps and void with different sizes and numbers. Rocks mostly tend to have circular and ellipsoidal voids as a result of long and complex geological processes. Owing to this limitation, it is always hard to understand and assess the crack initiation stress comprehensively. Especially for a couple of decades, with the help of developing computer science and technology, numerical models were used on this subject. In this study, various two-dimensional numerical rock models created using Distinct Element Method (DEM) based Particle Flow Code (PFC) were used to understand the effect of different gap geometries over crack initiation stress values of rock materials under uniaxial loading conditions. A base numerical model was calibrated using laboratory test results belonging to basalt rocks. In order to calibrate the numerical model, uniaxial, conventional triaxial and in-direct tensile test results were used. A flat-jointed contact model was chosen to create bonded material during the calibration process. Seven different numerical models were used to investigate the gap geometry effect on crack initiation stress under uniaxial conditions. The base model has a circular gap with 5.40 mm diameter. The other models created to understand the effect of geometry on crack initiation stress have different ellipsoidal geometry depending on the initial circular gap, 1.5 (8.10 mm), 2.5 (13.50 mm) and 3.5 (18.20 mm) times the diameter in the vertical and horizontal direction, respectively. The results of numerical models reveal that the crack initiation stress value decreases with the increase of the gap's vertical length while the width of gaps remains constant. Based on numerical models' results, the crack initiation stress value decreases with the increase of the gap's vertical length while the diameter of gaps remains constant.

How to cite: Zengin, E. and Erguler, Z. A.: Effect of Gap Geometries on the Crack Initiation Stress of Synthetic Rock Material, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9409, https://doi.org/10.5194/egusphere-egu21-9409, 2021.

Léo Petit, Jean-Arthur Olive, Harsha S. Bhat, Laetitia Le Pourhiet, and Alexandre Schubnel

The Earth’s brittle upper crust is commonly modeled as a non-associated Mohr-Coulomb (MC) elasto-plastic continuum. This framework enables the localization of shear strain through a process referred to as structural softening: dilatancy related to the build-up of plastic strain inside a shear band can elastically unload the surrounding material as principal stresses rotate inside the band. The strains required to weaken the material and corresponding stress drops are compatible with experimental observations, and provide useful theoretical insights into strain softening parameterizations used in numerical geodynamics. This model however does not account for time-dependent behavior documented in rock deformation experiments, such as the loading rate dependence of the peak strength, and sample failure under a fixed applied stress in brittle creep tests. It also relies on macroscopic properties (e.g., dilatancy angle) which are not straightforwardly related to micro-mechanical and micro-structural rock properties. The MC model thus inherently carries an empirical parameterization which can be an obstacle to a deeper understanding of brittle inelastic deformation. 

On the other hand, models that account for time-dependent brittle behavior typically invoke the development of tensile microcracks around shear defects, and derive macroscopic constitutive laws from the micro-mechanics of fracture growth and interaction through a damage state variable. To investigate whether this class of models can account for the time-dependence of strain localization, we perform post-bifurcation analysis on the damage rheology constructed by Ashby & Sammis (1990), coupled with a stress corrosion law for crack growth kinetics. We calculate the co-evolution of stress and 2-D plane strain at a point located within an incipient damage shear band, and at a nearby point in the surrounding rock where damage cannot accumulate. We prescribe a constant shear strain rate within the band, enforce strain compatibility and stress continuity across the shear band boundary, and integrate the incremental constitutive relationships through time.

Dilatancy related to tensile crack growth in the band enables elastic unloading of the surrounding medium. In our simulations, this manifests as a sudden drop in shear stress coincident with a sharp increase in band damage. We characterize the localization phenomenon through the magnitude of both this stress drop and damage increase, and assess their sensitivity to macroscopic parameters such as shear strain rate, shear band orientation, confining pressure, as well as micro-mechanical parameters such as the orientation of shear defects, the stress exponent of the crack growth law, and the initial damage. This type of work may pave the way toward micromechanics-based parameterizations of brittle deformation in long-term tectonic models.

How to cite: Petit, L., Olive, J.-A., S. Bhat, H., Le Pourhiet, L., and Schubnel, A.: Shear banding in a micromechanics-based brittle damage model, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12673, https://doi.org/10.5194/egusphere-egu21-12673, 2021.

Matej Pec and Saleh Al Nasser

Relative motion of tectonic plates is accommodated along lithosphere-scale shear zones. The strength and stability of these shear zones control large scale tectonics and the location of earthquakes. It is widely accepted that rocks undergo a “brittle-to-viscous” transition as depth increases, however the details of how this transition is achieved are a topic of active research.

To study this transition in polymineralic rocks, we sheared bi-mineralic aggregates with varying ratio (30:70, 50:50 & 70:30 vol%) of quartz (Qtz) and potassium feldspar (Kfs) at temperature, T = 750˚C and pressure, Pc = 800 MPa under either constant displacement rate or constant load boundary conditions. Under constant displacement rate, samples reach high shear stress (τ ≈ 0.4 - 1 GPa, depending on mineral ratio) and then weaken. Under constant load, the strain rate shows low sensitivity to stress below shear stresses of 400 MPa, followed by a high stress sensitivity at higher stresses irrespective of mineral ratio (stress exponent, n = 9 - 13, assuming that strain rate ∝ stress n).

Strain is localized along "slip zones" in a C and C’ orientation in all experiments irrespective of mineral ratio. These zones delimit larger cataclastic lenses, which develop a weak foliation. Quartz in the lenses shows pervasive Dauphiné twinning that leads to clear CPO patterns in the {r} and {z} rhomb planes. The {r} maxima (and {z} minima) are sub-parallel to the loading direction and rotate synthetically with increasing finite strain suggesting that they track the local σ1 direction. The material in the slip zones shows extreme grain size reduction, no porosity and flow features. At peak strength, 1-2 vol% of the sample is composed of slip zones that are straight and short. With increasing strain, the slip zones become anastomosing and branching and occupy up to 9 vol%; this development is concomitant with strain-weakening of the sample. The best developed slip zones are observed in samples with high Kfs contents (70 & 50 vol%). We infer that the material in the slip zones is formed of nanocrystalline to partly amorphous material (PAM) that is predominantly derived from Kfs. By compiling literature data on PAM development, we show that the volume of PAM increases with increasing homologous temperature and work done (stress x strain per unit volume) on the sample in rocks containing feldspars.

Our results suggest that strain localization leads to microstructural transformation of the rocks from a crystalline solid to an amorphous, fluid-like material in the slip zones. This material forms over a broad range of P-T, stress and strain conditions suggesting that it should form readily in nature. The measured rheological response is a combination of viscous flow in the slip zones and cataclastic flow in coarser-grained lenses and can be modeled as a frictional slider coupled in parallel with a viscous dashpot.

How to cite: Pec, M. and Al Nasser, S.: Formation of Amorphous Materials Causes Parallel Brittle-viscous Flow of Crustal Rocks, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-499, https://doi.org/10.5194/egusphere-egu21-499, 2021.

Takeshi Miyamoto, Tetsuro Hirono, Akari Fuke, Kiyokazu Oohashi, and Satoshi Yukawa

Many experimental works have previously performed to understand frictional properties of various kinds of rocks and minerals by using friction apparatus at various orders of sliding velocities ranging from nm/s to m/s together with microscopic observation. However, friction experiments at wide range of velocities on a single type of rock or mineral have been rarely reported. Here we conducted friction experiments using powdered pyroclastic samples at velocities ranging from 0.0002 m/s to 1 m/s, 1.5–3.0 MPa normal stress, 10 m slip distance and dry and wet conditions. We also performed numerical simulation by using discrete element method (DEM) that focused on the changes of distances to adjacent particles (referred as CAP) and forces particles experiencing during frictional slip. At higher velocities, the sample showed relatively drastic decrease of friction coefficient and boundary-parallel Y shears. In contrast, R1 shears, oblique to shear direction, were observed in the samples at lower velocities. Numerical simulations at higher velocities of 0.1 and 1 m/s resulted in slip weakening and development of larger CAP lines parallel to boundary. At lower velocities, larger forces and CAPs were concentrated locally. These results could imply that the development of composite planar fabrics has a dependency on slip velocity. Now we are investigating the relationship using synthetic quartz powders, and will show the preliminary results of re-experiments, numerical simulations, and microscopic observations.

How to cite: Miyamoto, T., Hirono, T., Fuke, A., Oohashi, K., and Yukawa, S.: Experimental and numerical demonstrations for development of composite planar fabrics in fault zones, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5534, https://doi.org/10.5194/egusphere-egu21-5534, 2021.

Caleb Holyoke and Casey Braccia

Much of our understanding of the strength of the continental crust is based on flow laws derived from homogeneous mono-mineralic aggregates (quartzites).  However, crystal plastic deformation of rocks in the middle to lower continental crust during orogenic events forms foliations, lineations and lattice preferred orientations (LPOs) which produce physical and viscous anisotropies in rocks.  In some of these orogenic events, such as in the Appalachian mountains, multiple deformation events form different, cross-cutting foliations and overprint existing LPOs.  In order to determine the effects foliation/lineation and preexisting LPO have on the strength of rocks in the middle crust, we deformed a natural quartzite with a cross-girdle LPO from the Moine Thrust in Scotland with the compressive stress at six different primary orientations relative to the foliation and lineation. This quartzite has aligned but distributed fine-grained muscovite which defines a foliation and lineation.  The cores were deformed at the same temperature (800°C), pressure (1500 MPa) and strain rate (1.6*10-6/s) to similar strains (50-58%), leaving the foliation/lineation orientation as the only difference between experiments.  Peak stresses occur at strains of 10-20% and are lowest for the sample with foliation at 45o to the compression direction (400 MPa, the weak orientation).  All other cores (hard orientations) have peak strengths of 600 to 1100 MPa and highest for the cores with lineation perpendicular to the compression direction (1100 MPa). These cores in hard orientations all strain weaken to a similar stress (~500 MPa), but are still ~100 MPa stronger than the core with both foliation and lineation initially oriented at 45 degrees to the compression direction.  Optical microstructures include undulatory extinction, deformation lamellae, and at high strain (58%), the quartzite is more than 50% recrystallized. Scanning electron microscope electron backscatter diffraction analyses indicate that recrystallized grains in all cores reflect the deformation conditions of the experiment and original grains retain their initial LPO.  Strength anisotropy at low strains is due to placing the foliation and lineation at non-ideal (hard) orientations relative to the compression direction and is greatest in cores with the lineation perpendicular to the compression direction.  The evolution to a similar strength at high strains indicates that dynamic recrystallization creates new grains oriented for easy slip in the second (experimental) deformation event. These results suggest that differences in lineation and foliation orientations and a pre-existing LPO may cause strength anisotropy in rocks in the mid to lower continental crust, but this anisotropy may be transient and unlikely to exist to high strains.

How to cite: Holyoke, C. and Braccia, C.: Transient effects of a pre-existing lattice preferred orientation on the strength of foliated quartzite, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1860, https://doi.org/10.5194/egusphere-egu21-1860, 2021.

He Feng, Christopher Gerbi, and Scott Johnson

Strain localization occurs throughout the crust, in both the brittle and viscous regimes. The causes of strain localization remain under discussion. However, realistic rock records indicate that variations of material properties (e.g. active deformation mechanisms, crystallographic orientation, phase distribution, grain shapes, etc.) are likely to be the dominant factor for weakening. Determining the cause(s) of localization requires investigation of the earliest stages of strain concentration in different P-T conditions. Our study focuses on two rocks that experienced low macroscale strain at amphibolite and/or granulite facies conditions yet exhibit localization on the millimeter and smaller scale. We combine optical and electron beam petrography with chemical mapping and electron backscatter diffraction to characterize these rheologically important domains. Morphologically, these localized zones appear to mechanically link rheologically weak phases or domains. These “bridge” zones typically comprise a band of relatively fine grains with weak crystallographic preferred orientation. The major element compositions of like phases inside and outside the bridge zone are similar, but the modal mineralogy and trace elements vary somewhat. Bridge zones result from not only in-situ grain size reduction (due to, for example, nucleation, recrystallization, or cataclasis), but also chemical processes resulting in phase mixing or element mobility on a short spatial scale. Their spatial distribution suggests that the small modal fraction of microstructural change represented by the bridge zones can lead to a high degree of bulk weakening.

How to cite: Feng, H., Gerbi, C., and Johnson, S.: Rheological Bridge Zone: Initialization of Localization, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7917, https://doi.org/10.5194/egusphere-egu21-7917, 2021.

Chairpersons: Renée Heilbronner, Amicia Lee, Damien Freitas
Modelling of macrostructural features
Yuanbang Hu, Tamara de Riese, Paul Bons, Shugen Liu, Albert Griera, Maria-Gema Llorens, Enrique Gomez-Rivas, and Melanie Finch

Deformation of foliated rocks commonly leads to crenulation or micro-folding, with the development of cleavage domains and microlithons. We here consider the effect of mechanical anisotropy due to a crystallographic preferred orientation (CPO) that defines the foliation, for example by of alignment of micas. Mechanical anisotropy enhances shear localisation (Ran, et al., 2018; de Riese et al., 2019), resulting in low-strain domains (microlithons) and high-strain shear bands or cleavage domains. We investigate the crenulation patterns that result from moderate strain simple shear deformation, varying the initial orientation of the mechanical anisotropy relative to the shear plane.  

We use the Viscoplastic Full-Field Transform (VPFFT) crystal plasticity code coupled with the modelling platform ELLE (http://www.elle.ws; Llorens et al., 2017) to simulate the deformation of anisotropic single-phase material with an initial given CPO in dextral simple shear in low to medium strain. Deformation is assumed to be accommodated by glide along the basal, prismatic and pyramidal slip systems of a hexagonal model mineral. An approximately transverse anisotropy is achieved by assigning a small critical resolved shear stress to the basal plane. An initially point-maximum CPO at variable angles to the shear plane defines the initial straight foliation at different angles to the shear plane, limiting ourselves to orientations in which the foliation is in the stretching field. The resulting crenulation geometries strongly depend on the orientation of the foliation and we observe four types of localisation behaviour: (1) synthetic shear bands, (2) antithetic shear bands, (3) initial formation of antithetic shear bands and subsequent development of synthetic shear bands, and (4) distributed, approximately shear-margin parallel strain localisation, but no distinct shear bands.

The numerical simulations not only show the evolving strain-rate field, but also the predicted finite strain pattern of existing visible foliations. We show the results for layers parallel to the foliation, but also cases where the visible layering is at an angle to the mechanical anisotropy (e.g. in case of distinct sedimentary layers and a cleavage that controls the mechanical anisotropy). A wide range of crenulation types form as a function of the initial orientation of the visible layering and mechanical anisotropy (comparable to C, C' and C'' shear bands and compressional crenulation cleavage). Most importantly, some of may be highly misleading and may easily be interpreted as indicating the opposite sense of shear.


de Riese, T., et al. (2019). Shear localisation in anisotropic, non-linear viscous materials that develop a CPO: A numerical study. Journal of Structural Geology, 124, 81-90. DOI: 10.1016/j.jsg.2019.03.006

Llorens, M.-G., et al. (2017). Dynamic recrystallisation during deformation of polycrystalline ice: insights from numerical simulations. Philosophical Transactions of the Royal Society A, Special Issue on Microdynamics of Ice, 375: 20150346. DOI: 10.1098/rsta.2015.0346.

Ran, H., et al. (2018). Time for anisotropy: The significance of mechanical anisotropy for the development of deformation structures. Journal of Structural Geology, 125, 41-47. DOI: 10.1016/j.jsg.2018.04.019

How to cite: Hu, Y., de Riese, T., Bons, P., Liu, S., Griera, A., Llorens, M.-G., Gomez-Rivas, E., and Finch, M.: New sights in crenulation geometry developed in anisotropic materials undergoing simple shear deformation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13151, https://doi.org/10.5194/egusphere-egu21-13151, 2021.

Melanie Finch, Paul Bons, Florian Steinbach, Albert Griera, Maria-Gema Llorens, Enrique Gomez-Rivas, Hao Ran, and Tamara de Riese

C' shear bands are common structures in ductile shear zones but their development is poorly understood. They occur in rocks with a high mechanical strength contrast so we used numerical models of viscoplastic deformation to study the effect of the proportion of weak phase and the phase strength contrast on C' shear band development. We employed simple shear to a finite strain of 18 in 900 steps and recorded the microstructure, stress and strain distribution at each step. We found that C' shear bands form in models with ≥5% weak phase when there is a moderate or high phase strength contrast, and they occur in all models with weak phase proportions ≥15%. Contrary to previous research, we find that C' shear bands form when layers of weak phase parallel to the shear zone boundary rotate forwards. This occurs due to mechanical instabilities that are a result of heterogeneous distributions of stress and strain rate. C' shear bands form on planes of low strain rate and stress, not in sites of maximum strain rate as has previously been suggested. C' shear bands are ephemeral and they either rotate backwards to the C plane once they are inactive or rotate into the field of shortening and thicken to form X- and triangle- shaped structures.

How to cite: Finch, M., Bons, P., Steinbach, F., Griera, A., Llorens, M.-G., Gomez-Rivas, E., Ran, H., and de Riese, T.: The ephemeral development of C' shear bands, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-752, https://doi.org/10.5194/egusphere-egu21-752, 2021.

Lawrence Hongliang Wang, Viktoriya Yarushina, and Yury Podladchikov

The coupling between fluid flow and solid deformation plays important roles in earth dynamics at different timescales and length-scales. Related processes include, magma migration and focusing in the Mid-Ocean Ridges, fluid migration after slab dehydration in the subduction zone, channelized fluid flow observed as seismic chimney in the continental margin, as so on. Here we study how localized fluid channels can develop through asymmetric compaction and decompaction processes of the solid matrix by solving coupled two-phase equations with viscoplastic rheology. Previous studies produced fluid channels with decompaction weakening, while negative effective pressure (Pt-Pf) is inevitable due to the simplified rheology formulation. We develop a viscoplastic rheology formulation that considers the effects of shear stress and plastic failure on the volumetric deformation, consistent with experimental data.

Our model results show that this new rheology can produce channelized fluid flow without negative effective pressure in the model. Our numerical results also clarified that it is the flow instability of the coupled two-phase system that cause the formation of fluid channels. The ratio between shear viscosity and bulk viscosity determines how fast the flow instability develops and manifests. The geometry of the Reservoir, on the other hand, can affect where the channels form. We further study the effects of different background and reservoir porosity, different rock layer, permeability exponents, decompaction weakening factor, and so on. These results provide a better understanding of the two-phase system and its potential applications in geological environments.   


How to cite: Wang, L. H., Yarushina, V., and Podladchikov, Y.: Modelling coupled fluid flow and solid deformation with the viscoplastic rheology, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12021, https://doi.org/10.5194/egusphere-egu21-12021, 2021.

Markus Rast and Jonas Ruh

Strain weakening is a prerequisite for localization of strain and therefore crucial for the understanding of shear zone evolution. In the context of progressive deformation of multi-phase aggregates, it is unclear whether the change in geometry and orientation of the involved phases leads to structural or geometric strain weakening and thus may control strain localization. Consequently, the question arises how the ductile flow of two-phase rocks can be described or determined. To contribute to a better understanding of the knowledge gaps outlined above, two-dimensional numerical shear experiments of quartz-biotite aggregates were conducted at varying temperatures, background strain rates and fluid pressure ratios. Textural variations after a shear strain of γ ≈ 10 appear to be dependent on the viscosity contrast between the minerals involved. To estimate whether a numerical experiment is undergoing strain weakening or strain hardening (or both), the temporal evolution of the mean second invariant of the deviatoric stress tensor was tracked. The results suggest that strain weakening occurs if biotite-inclusions are distinctly isolated and that it is more effective under conditions with larger viscosity contrasts between matrix and inclusions. However, the stress drops in numerical experiments with purely structural / textural strain weakening are rather low (−1.1 to −6.4%) compared to other strain weakening processes. It appears that phase rearrangement and change in phase geometry with evolving strain is of minor importance for the occurrence of strain weakening. Based on the numerical experiments and assuming a power-law relationship between stress and strain, the flow-law parameters of quartz-biotite aggregates with different biotite contents were determined. The results are in the range of existing experimental and analytical mixed-aggregates flow-laws. However, the variations between the different flow-laws show that further research is required, for which numerical models as used in the present study could serve as basis.

How to cite: Rast, M. and Ruh, J.: Numerical modelling of quartz-biotite aggregates: Insights on strain weakening and two-phase flow laws, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10167, https://doi.org/10.5194/egusphere-egu21-10167, 2021.

Tamara de Riese, Paul D. Bons, Enrique Gomez-Rivas, Albert Griera, Maria-Gema Llorens, and Ilka Weikusat

Ice 1h shows a strong viscoplastic anisotropy, as the resistance to activate dislocation glide on basal planes is at least one order of magnitude smaller than on the other slip planes. During flow the viscoplastic anisotropy leads to the development of a crystallographic preferred orientation (CPO). The anisotropic behaviour of flowing ice can lead to strain localisation. Only when the ice is layered (e.g. due to cloudy bands) it may be possible to identify localisation structures, as ice otherwise has no readily recognisable strain markers.

We use the Viscoplastic Full-Field Transform (VPFFT; Lebensohn and Rollett, 2020) crystal plasticity code coupled with the modelling platform ELLE (http://www.elle.ws; Piazolo et al., 2019) to simulate the deformation of intrinsically anisotropic ice 1h with an initial single maximum CPO in dextral simple shear up to very high strains. The VPFFT-approach simulates deformation by dislocation glide, taking into account the different available slip systems and their critical resolved shear stresses. We use an anisotropy similar to that of ice 1h, systematically vary the orientation of the initial CPO, and use passive markers/layers to visualise deformation structures.

The localisation behaviour strongly depends on the initial CPO, but reaches a consistent steady state after very high shear strains of about 30. The fabric and stress evolution reach a steady-state situation as well. The orientation of the CPO controls the style of deformation, which varies from (1) synthetic shear zones with a stable shear-direction parallel orientation and that widen with ongoing strain to unstable, (2) rotating antithetic shear bands, (3) initial formation of antithetic shear bands and subsequent development of synthetic shear bands and (4) distributed localisation. Furthermore, evolving visual structures depend on the presence and orientation of a visual layering in the material. However, at very high strains, the material is almost always strongly mixed and any original layering would be destroyed.

Our results highlight the challenge to identify strain localisation in ice, yet they can help the ice community to identify and interpret deformation structures in large ice masses (e.g. the Greenland ice sheet). As strain localisation in anisotropic materials behaves scale independent (de Riese et al., 2019), large-scale equivalents may occur of the observed small-scale structures (Jansen et al., 2016).


de Riese, T., Evans, L., Gomez-Rivas, E., Griera, A., Lebensohn, R.A., Llorens, M.G., Ran, H., Sachau, T., Weikusat, I., Bons, P.D. 2019. Shear localisation in anisotropic, non-linear viscous materials that develop a CPO: A numerical study. Journal of Structural Geology, 124, 81-90.

Jansen, D., Llorens, M.-G, Westhoff, J., Steinbach, F., Kipfstuhl, S., Bons, P.D., Griera, A., Weikusat, I. 2016. Small-scale disturbances in the stratigraphy of the NEEM ice core: observations and numerical model simulations. The Cryosphere 10, 359-370.

Lebensohn, R.A., Rollett, A.D. 2020. Spectral methods for full-field micromechanical modelling of polycrystalline materials. Computational Materials Science, 173, 109336.

Piazolo, S., Bons, P.D., Griera, A., Llorens, M.G., Gomez-Rivas, E., Koehn, D., ... Jessell, M.W. 2019. A review of numerical modelling of the dynamics of microstructural development in rocks and ice: Past, present and future. Journal of Structural Geology, 125, 111-123.

How to cite: de Riese, T., Bons, P. D., Gomez-Rivas, E., Griera, A., Llorens, M.-G., and Weikusat, I.: Deformation structures resulting from anisotropy during high-strain deformation of ice 1h with initial CPO, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10209, https://doi.org/10.5194/egusphere-egu21-10209, 2021.

Daniel Richards, Sam Pegler, Sandra Piazolo, and Oliver Harlen
Antarctic ice flow shows deviation from the deformation regimes of pure and simple shear. By analysing the vorticity number from surface velocity data it is found that approximately 80% of the flow is outside these regimes. These deformations are both between pure and simple shear, as well as highly rotational, highlighting the need for fabric predictions away from the commonly studied regimes of pure and simple shear. 
We use the numerical scheme SpecCAF, which has been shown to accurately reproduce experimentally observed fabrics with no free parameters, to study ice fabrics in such general deformations. By exploring the parameter space of temperature and vorticity number, we present a definitive classification of fabrics patterns which arise, and construct a universal regime diagram for ice fabrics under general two-dimensional deformation. We find that intermediate deformations see a smooth transition between a cone-shape fabric and a secondary cluster. We present the first investigation of the fabrics produced in highly rotational deformations, which produce a weak girdle fabric with the axis aligned to the vorticity axis. We also show that across deformation and temperature space fabrics only reach a true steady-state above strains of 200%, and there is significant variation in this across the parameter space.  

How to cite: Richards, D., Pegler, S., Piazolo, S., and Harlen, O.: Ice fabrics in natural flows: moving away from pure and simple shear, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12078, https://doi.org/10.5194/egusphere-egu21-12078, 2021.

Process-focussed studies on microstructures and textures
Florian Fusseis and Craig Allsop

Shear zones are important conduits that facilitate the bidirectional migration of fluids and dissolved solids across the middle crust. It is a relatively recent revelation that mylonitic deformation in such shear zones can result in the formation of synkinematic pores that are potentially utilised in long-range fluid migration. The pores definitely influence a shear zone’s hydraulic transport properties on the grain scale, facilitating synkinematic fluid-rock interactions and mass transfer. Our understanding of how exactly various forms of synkinematic porosity integrate with the kinematics and dynamics of shear zones is still growing. Here we show a previously undescribed form of synkinematic porosity in an unweathered, greenschist-facies psammitic ultramylonite from the Cap de Creus Northern Shear Belt (Spain). The sizeable, open pores with volumes > 50k µm3 appear exclusively next to albitic feldspar porphyroclasts, which themselves float in a fine-grained, polymineralic ultramylonitic matrix that likely deformed by grain size-sensitive creep and viscous grain boundary sliding. The pores wrap around their host clasts, occupying asymmetric strain shadows and tailing off into the mylonitic foliation. A detailed analysis using high-resolution backscatter electron imaging and non-invasive synchrotron-based x-ray microtomography confirms that the pores are isolated from each other. We found no evidence for weathering of the samples, or any significant post-mylonitic overprint, unequivocally supporting a synkinematic origin of the pores. 

We propose that this strain shadow porosity formed through the rotations of the Ab porphyroclasts, which was governed by the clasts’ shapes and elongation. The ultramylonitic matrix was critical in enabling the formation of pores in the clast’s strain shadows. In the matrix, the individual grains were displaced mostly parallel to the shear direction. As a consequence of clast rotation it can be expected that, in the strain shadows, matrix grains followed diverging movement vectors. As a result, phase boundaries in the YZ plane experienced tensile forces, leading to the opening of pores. We infer that this tensile decoupling among matrix grains established a hydraulic gradient that drained the matrix locally and filled the pores with fluid. The fact that the strain shadow pores remained open in our samples suggests a chemical equilibrium with the fluid. Pore shape and volume will have been subject to continuous modification during ongoing matrix deformation and clast rotation.

This form of synkinematic porosity constitutes a puzzling, yet obvious way to maintain surprisingly large pores in ultramylonites whose transport properties are otherwise likely determined by creep cavitation and the granular fluid pump (Fusseis et al., 2009). We envisage that the strain shadow megapores worked in sync with the granular fluid pump in the ultramylonitic matrix and, while the overall porosity of ultramylonites may be small, locally, substantial fluid reservoirs were available to service fluid-rock interaction and fluid-mediated mass transfer. Our findings add another puzzle piece to our evolving understanding of synkinematic transport properties of mid-crustal ultramylonites and fluid-rock interaction in shear zones at the brittle-to-ductile transition.

How to cite: Fusseis, F. and Allsop, C.: Strain shadow “megapores” in mid-crustal ultramylonites - local, transient reservoirs servicing the granular fluid pump?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10343, https://doi.org/10.5194/egusphere-egu21-10343, 2021.

Joanna Dziadkowiec, Bahareh Zareeipolgardani, Hsiu-Wei Cheng, Dag Kristian Dysthe, Anja Røyne, and Markus Valtiner

Adhesive and repulsive, nm-range surface forces acting between mineral grains control colloidal stability and mineral aggregation but less is known about how these forces are affected by surface reactivity and to what extent these grain-scale forces can influence various deformation processes in rocks. In this experimental work, we explore and quantify the surface forces acting between dynamic mineral surfaces that undergo recrystallization or are chemically reactive in contact with water or aqueous salt solutions. Our experimental setup consists of the surface forces apparatus (SFA) coupled with the multiple beam interferometry (MBI). This setup can excellently reproduce a typical grain contact geometry with nanometer-thin water films confined between contacting mineral grains over relatively large, micron-sized contact areas. Owing to the use of MBI, both surface growth or dissolution processes can be monitored during force measurements in real-time. As such, SFA can provide information about the links between surface reactivity and adhesive or repulsive surface forces. Using the examples of force measurements between recrystallizing or chemically reactive mineral surfaces such as carbonates, hydroxides, and silicates, we comment on the relationship between the measured surface forces and surface reactivity. We link our findings with the observed changes in mineral phases, surface topographies, or surface roughness. We also comment on how the micron-scale confinement in the SFA affects the growth and dissolution processes in contrast to less confined regions. The magnitude of the forces associated with dynamic mineral surfaces and the potential significance of these forces to macroscopic deformation processes and cohesion in rocks are discussed.

How to cite: Dziadkowiec, J., Zareeipolgardani, B., Cheng, H.-W., Dysthe, D. K., Røyne, A., and Valtiner, M.: Forces between reactive surfaces, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-362, https://doi.org/10.5194/egusphere-egu21-362, 2021.

Christoph Schrank, Oliver Gaede, Tomasz Blach, Katherine Gioseffi, Stephen Mudie, Nigel Kirby, Klaus Regenauer-Lieb, and Andrzej Radlinski

The potential role of differential stress for mineral stability and the kinetics of mineral replacement reactions remains a matter of hot debate. We present a series of unique in-situ laboratory experiments on the dehydration of polycrystalline natural gypsum to hemihydrate, which were designed to test if the application of small differential stresses affects the mineral transformation rate. The dehydration experiments were conducted in a purpose-built loading cell suitable for in-situ monitoring with synchrotron transmission small- and wide-angle X-ray scattering (SAXS/WAXS). The time-resolved SAXS/WAXS data provide measurements of the transformation kinetics and the evolution of nano-pores of the dehydrating samples.

In our experiments, the kinetic effects of two principal variables were examined: dehydration temperature and axial confinement of the sample discs. In contrast to most previous dehydration experiments conducted in triaxial deformation apparatus, we applied different axial pre-stresses to the radially unconfined sample discs, which were well below the uniaxial compressive strength of the test material. This loading condition corresponds to constant-displacement rather than constant-stress boundary conditions. We find that in natural gypsum alabaster with randomly oriented grains an increase in axial pre-stress leads to a significant acceleration of the dehydration rate. Simple estimates of the energy budget suggest that the acceleration of the dehydration rate due to elastic straining is significantly cheaper energetically than due to heating. We hypothesise that the observed strong effect of differential stress on dehydration kinetics can be explained by geometry-energy interactions in the granular sample microstructure.

How to cite: Schrank, C., Gaede, O., Blach, T., Gioseffi, K., Mudie, S., Kirby, N., Regenauer-Lieb, K., and Radlinski, A.: On the stress sensitivity of the dehydration kinetics of gypsum: insights from fast in-situ synchrotron X-ray scattering  , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8567, https://doi.org/10.5194/egusphere-egu21-8567, 2021.

Anna Rogowitz, Renelle Dubosq, David Schneider, Kevin Schweinar, and Baptiste Gault

The interaction of trace elements, fluids and crystal defects plays a vital role in a crystalline material’s response to an applied stress. For example, dislocations can be arrested by the strain field of immobile defects (i.e., particles or precipitates) or by the accumulation of mobile solutes in their cores, which can lead to strain hardening. The rheology of minerals is also strongly influenced by interactions with fluids, which are typically known to facilitate ductile deformation in geomaterials (i.e., hydrolytic weakening, dissolution creep). Investigation of these nanometer scale processes however, requires a correlative approach combining high-spatial resolution analytical techniques. In recent years, increasing developments in microscopy and microanalysis have allowed for the compositional measurements and spatial imaging of materials at the near-atomic scale. Herein, we have combined electron backscatter diffraction (EBSD) mapping, electron channeling contrast imaging (ECCI), scanning transmission electron microscopy (STEM) and atom probe tomography (APT) on a naturally deformed polycrystalline pyrite aggregate from the Abitibi Subprovince in Canada to investigate the role of fluid inclusions on mineral rheology. The combined EBSD and ECCI data reveal minor crystal misorientation and low-angle grain boundary development in the vicinity and at the tip of microfractures indicating a dominantly brittle regime with minor strain accommodation via crystal-plasticity where dislocations are mostly emitted by the propagating fracture. These interpretations are consistent with the peak temperature conditions of the sample estimated at 302 ± 27°C, which falls within the lower range of the brittle to crystal-plastic behaviour of pyrite (260–450°C). Nanoscale structural and chemical data reveal nanoscale fluid inclusions enriched in As, O, Na and K that are linked by As-enriched dislocations. Based on these results, we propose a model of fluid hardening whereby dislocations get pinned at fluid inclusions during crystal-plastic deformation, initiating pipe diffusion of trace elements from the fluid inclusions into dislocations that leads to their stabilization and local hardening. Although additional experiments are required on other mineral phases, our initial efforts advance the understanding of the interplay between nanostructures and impurities and its impact on the rheology of geomaterials during relatively low temperature deformation.


How to cite: Rogowitz, A., Dubosq, R., Schneider, D., Schweinar, K., and Gault, B.: Fluid inclusion hardening: Nanoscale evidence from naturally deformed pyrite, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-894, https://doi.org/10.5194/egusphere-egu21-894, 2021.

Meghana Ranganathan, Brent Minchew, Colin Meyer, and Matej Pec

The initiation and propagation of fractures in floating regions of Antarctica has the potential to destabilize large regions of the ice sheet, leading to significant sea-level rise. While observations have shown rapid, localized deformation and damage in the margins of fast-flowing glaciers, there remain gaps in our understanding of how rapid deformation affects the creep and toughness of ice. Here we derive a model for dynamic recrystallization in ice and other rocks that includes a novel representation of migration recrystallization, which is absent from existing models but is likely to be dominant in warm areas undergoing rapid deformation within the ice sheet. We show that, in regions of elevated strain rate, grain sizes in ice may be larger than expected (~15 mm) due to migration recrystallization, a significant deviation from solid earth studies which find fine-grained rock in shear zones. This may imply that ice in shear margins deforms primarily by dislocation creep, suggesting a flow-law exponent of n=4 in these regions. Further, we find from existing models that this increase in grain size results in a decrease in tensile strength of ice by ~75% in the margins of glaciers. Thus, we expect that this increase in grain size makes the margins of fast-flowing glaciers less viscous and more vulnerable to fracture than we may suppose from standard model parameters.

How to cite: Ranganathan, M., Minchew, B., Meyer, C., and Pec, M.: Recrystallization of ice enhances the creep and vulnerability to fracture of ice shelves , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3266, https://doi.org/10.5194/egusphere-egu21-3266, 2021.

Fabian Dellefant, Claudia Trepmann, Stuart Gilder, Iuliia Sleptsova, Melanie Kaliwoda, and Benjamin Weiss

Shocked gneiss (~8 GPa) from the Vredefort impact structure (South Africa) contain planar fractures in quartz decorated by magnetite and ilmenite, which are commonly attributed to the impact event. However, the surface at Vredefort is riddled by lightning strikes, which also produce rapid pressure-temperature pulses that can modify the microstructure and the magnetic properties of the rocks. To understand the differences between lightning and impact-related shock effects, we investigated samples from two, 10 m-deep drill cores by Raman spectroscopy, polarized light microscopy/U-stage and electron microscopy/electron backscatter diffraction techniques. Magnetite and ilmenite within planar fractures in quartz occur at all depths, and are therefore intrinsic to the impact event, independent of lightning. Primary iron-bearing minerals were locally heated by the generation of shear fractures in neighboring quartz, leading to small volumes (micrometer scales) of melt intruding into nearby fractures. Frictional heating and rapid quenching of feldspar and quartz is indicated by localized, fine-grained aggregates along intragranular planar fractures as well as transgranular pseudotachylytic veins. On the other hand, altered ilmenite grains with exsolved magnetite occur only in gneisses from the uppermost 80 cm of both drill cores. When in contact with biotite, the ilmenite-magnetite boundaries are altered to chlorite, and the ilmenite is partly transformed to anatase. These alteration products contain fine-grained magnetite. It appears that lightning strikes altered the existing ilmenite-magnetite in the Vredefort samples to produce smaller, more single-domain like magnetite grains, consistent with the observed magnetic properties of the samples

How to cite: Dellefant, F., Trepmann, C., Gilder, S., Sleptsova, I., Kaliwoda, M., and Weiss, B.: Distinguishing shock-related microstructures in gneisses from the Vredefort impact structure, South Africa, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2494, https://doi.org/10.5194/egusphere-egu21-2494, 2021.

Lina Seybold, Claudia A. Trepmann, Stefan Hölzl, and Melanie Kaliwoda

Approximately one third of the worlds known impact structures are formed in carbonate-bearing target rocks. However, the response of their main constituent mineral, calcite, upon shock loading and unloading is still not well understood. Mechanical twins in calcite are described from natural impactites and shock experiments, yet, reliable indicators to distinguish these shock effects from the very common calcite twins generated in tectonites are missing. Here, we present scanning electron microscopic investigations of twinned calcite within calcite cemented brecciated gneisses from the Ries impact structure.

Calcite cemented brecciated gneisses occur at several outcrops of the Ries impact structure as well as in samples recovered from depth as low as 977 m in the research drilling 1973. At Maihingen, the polymict impact breccias contain shocked gneiss fragments and various generations of calcite in veins. The occurrence of rhombohedral PDFs in quartz from the gneiss fragments indicates shock conditions of >10 GPa. Coarse calcite grains, representing an early generation of calcite in the veins, show exceptionally fine-lamellar twins, indicating high stress and strain rates. The calcite twins show widths of < 0.5 µm, a high density of up to a few hundred lamellae per mm, and appear to crosscut each other, which has been suggested as a criterion for shock-induced twinning. Furthermore, a high density of sets of planar features occur associated and parallel to these twins, but along which no twin domains were resolved in the scanning electron microscope. Twin systems detected by EBSD measurements include e-twins, common also in calcite from tectonites, and another more rarely occurring twin system, characterized by a rotation axes parallel to <-2110> and a rotation angle of ca. 35°. A second generation of calcite without twins is represented by elongate palisade calcite, fine-grained aggregates and rims forming sutured grain boundaries surrounding twinned coarse calcite grains. EDS measurements show that these calcite grains contain up to 2.5 % Fe as well as traces of Mn, Mg, Si, Na and Al. In contrast, the coarse twinned calcite is almost pure CaCO3. Whereas the fine-grained aggregates and sutured grain boundaries indicate recrystallization, the palisade grains indicate precipitation from the pore fluid.

The twinned coarse calcite grains within the polymict impact breccias are interpreted to be shock induced. As coarse calcitic sedimentary target rocks are not known from the Ries area, they can either represent pre-shock calcite veins within the gneisses or possibly marbles that were brecciated together with shocked gneisses during impact cratering. The second generation of calcite represents post-shock recrystallization and precipitation from a fluid.

How to cite: Seybold, L., Trepmann, C. A., Hölzl, S., and Kaliwoda, M.: Twinned calcite within polymict impact breccias from the Nördlinger Ries impact structure, Germany – shock effects and post-shock annealing, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15559, https://doi.org/10.5194/egusphere-egu21-15559, 2021.

Michel Bestmann, Giorgio Pennacchioni, Bernhard Grasemann, and Christoph Schrank

In the last 10 years, many attempts have been mad to use the titanium-in-quartz geothermobarometer (TitaniQ) to constrain the ambient conditions during mylonitization of quartz in metamorphic rocks. However, most of the studies have shown that the TitaniQ is not readily applicable. First, the application of the TitaniQ calibrations1-2 is possible if two of the relevant variables (temperature, pressure and Ti activity) can be fixed. But the results of both calibrations can deviate by >100°C. Secondly, several studies have shown that deformation/recrystallization processes, the availability of aqueous fluids, the amount of strain and the duration of deformation result in microstructures with a heterogeneous distribution of Ti concentrations [Ti]. Therefore, in most cases, homogenous and complete equilibration of the [Ti] at the ambient conditions of deformation does not occur. In quartz mylonites, the microstructure is commonly complex as result of strain partitioning and total accumulated strain. For such a complex rock the challenge for applying TitaniQ is to identify those domains where Ti re-equilibration to the syn-kinematic ambient conditions, did possibly occur. Identifying such domains requires the strict integration of correlated high-resolution analysis by optical microscopy, SEM-CL, EBSD and Ti-in-qtz analysis using secondary ion mass spectrometry (SIMS). This integrated information especially provides a robust interpretative tool for the interplay between grain-scale deformation, fluid-rock interaction, geochemical exchange and the evolution of the crystallographic preferred orientation during progressive strain.

We present the study of the deformation microstructures of quartz veins (Schober Group, Eastern Alps) as key example of such an integrated data collection to unravel characteristic deformation processes responsible for the partial or complete resetting of the Ti-in-quartz system under retrograde conditions. The Schober quartz veins developed at amphibolite facies conditions (510-590 °C, 0.5-0.6 GPa) and were overprinted by deformation at lower greenschist facies. Subgrain rotation (SGR) recrystallization was the dominant recrystallization mechanism during mylonitization. During deformation complete resetting of the initial [Ti] of 3-4 ppm down to 0.2-0.6 ppm occurred in domains (e.g. pressure shadows) where sufficient fluids were available and could percolate through the microstructures. High strain and pervasive quartz dynamic recrystallization did not necessarily result in homogeneous and complete re-equilibration of the [Ti]. Our study reveals that subgrain boundaries were locally pathways for partial [Ti] reset.

Using the example of mylonitized quartz veins from the Schober Group in the Austroalpine domain of the Eastern Alps, we aim at showing that the initial Ti-in-qtz and corresponding CL signature of the quartz vein is reset to different degrees even at high strains and pervasive dynamic recrystallization, depending on the availability of fluids and its repartitioning.


(1) Huang, R., Audétat, A., 2012. The titanium-in-quartz (TitaniQ) thermobarometer: a critical examination and re-calibration. Geochim. Cosmochim. Acta 84, 75–89.

(2) Thomas, J.B., Watson, E.B., Spear, F.S., Shemella, P.T., Nayak, S.K., Lanzirozzi, A., 2010. TitaniQ under pressure: the effect of pressure and temperature on the solubility of Ti in quartz. Contrib. Mineral. Petrol. 160, 743–759.


How to cite: Bestmann, M., Pennacchioni, G., Grasemann, B., and Schrank, C.: Influence of deformation and fluids on the Ti exchange in quartz, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1359, https://doi.org/10.5194/egusphere-egu21-1359, 2021.

Luca Menegon, Alberto Ceccato, and Lars N. Hansen

The strength of experimentally deformed natural and synthetic quartz is strongly affected by the intracrystalline water content. Water–related defects weaken quartz by either decreasing the resistance to dislocation motion (Peierls stress) or by enhancing the nucleation of dislocations, during what is commonly referred to as hydrolytic weakening. However, hydrolytic weakening has been observed predominantly in synthetic quartz grains, with water contents higher than 20–30 wt ppm H2O and at high-homologous temperatures, for which the activation of dislocation climb and recovery processes is enhanced.

In the low-temperature plasticity (LTP) regime, at low-homologous temperatures and high stress conditions, quartz plasticity is mainly controlled by dislocation glide. At these conditions, the possible effect of intracrystalline water on quartz strength is still a matter of debate.

In order to analyse the effects of intracrystalline water content on the plastic yield and hardness of quartz in the LTP regime, natural samples from recrystallized quartz domains of a granulite-facies migmatitic gneiss, presenting different water contents and microstructures, have been investigated through a series of spherical and Berkovich nanoindentation tests at room conditions. Nanoindentation tests have been integrated with measurements of intracrystalline water contents of the indented grains with secondary ion-mass spectrometry (SIMS), and with electron backscatter diffraction (EBSD) measurements of the crystallographic orientation of the indented grains.

Water content of indented quartz grains ranges between 2 and 104 wt ppm H2O. Samples and related nanoindentation tests were thus classified as either “dry” (DQ, for water contents < 20 wt ppm H2O) or “wet” (WQ, for water content > 20 wt ppm H2O). Spherical nanoindentation tests revealed comparable yield stresses (ranging between 3.5 and 8.8 GPa, depending on the crystal orientation) for DQ and WQ grains. In addition, significant strain hardening was observed in both DQ and WQ grains. Berkovich nanoindentation tests also resulted in comparable hardness (ranging from 8.0 to 13.5 GPa) in both DQ and WQ grains. The hardness also increases with indentation depth, which is consistent with the “size-effect” on mineral strength during LTP.

These results suggest that, for the investigated range of water contents, the yield strength and flow stress of quartz in the LTP regime is not affected by the intracrystalline water content of the indented grain. Both the dry and wet quartz experienced significant crystal plastic deformation prior to the nanoindentation tests, as evidenced by the occurrence of undulatory extinction, misorientation bands, subgrains, and recrystallized grains. This pre-indentation strain history may have had a major role in generating the dislocation density, which then controlled the yield stresses during low-temperature plasticity in our experiments. Hence, inherited strain history, crystallographic orientation, and grain size may play a more important role than water in controlling the strength of the continental crust at the brittle–ductile transition, where LTP is dominant and quartz is the most abundant phase.

How to cite: Menegon, L., Ceccato, A., and Hansen, L. N.: Strength of dry and wet quartz in the low–temperature plasticity regime: insights from nanoindentation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14625, https://doi.org/10.5194/egusphere-egu21-14625, 2021.

John Wheeler, Sandra Piazolo, David Prior, Jake Tielke, and Pat Trimby

In many parts of the Earth rocks deform by dislocation creep. There is therefore a need to understand which slip systems operated in nature and in experimental products. Knowing the conditions of experiments may then allow natural conditions and strain rates to be characterised. Dislocation creep typically gives lattice preferred orientations (LPOs), since activity on particular slip systems leads to lattice rotations and alignment. For decades LPOs, measured first optically and since the 1990s by EBSD, have been used to infer slip systems. This is a valuable technique but the link between slip sytem activity and LPO is complicated, especially if recrystallisation and/or grain boundary sliding have been involved.

Here we present a more direct method to deduce “geometrically necessary” dislocations (GNDs) from the distortions within crystals. Distortions may be optically visible (e.g. undulose extinction in quartz) but EBSD has revealed how common distortions are, and allowed them to be quantified. The method does not give the complete picture of GNDs but allows hypotheses to be tested about possible slip systems. We illustrate this “Weighted Burgers Vector” method with a number of examples. In olivine the method distinguishes slip parallel to a and c, and in plastically deformed plagioclase it reveals a variety of slip systems which would be difficuilt to deduce from LPOs alone. GNDs may not necessarily reflect the full slip system activity, since many dislocations will have passed through crystals and merged with grain boundaries leaving no signature. Neverthless the method highlights what dislocations are present “stranded” in the microstructure. In many case these will have been produced by deformation although the method can also characterise growth defects.

Wheeler et al. 2009. The weighted Burgers vector: a new quantity for constraining dislocation densities and types using electron backscatter diffraction on 2D sections through crystalline materials. DOI: 10.1111/j.1365-2818.2009.03136.x

How to cite: Wheeler, J., Piazolo, S., Prior, D., Tielke, J., and Trimby, P.: Analysing crystal distortions to deduce dislocation slip systems, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2316, https://doi.org/10.5194/egusphere-egu21-2316, 2021.

Rüdiger Kilian, Luiz Morales, Geoffrey Lloyd, and Michael Stipp

Quartz c-axis pole figures are hugely popular for the estimation of various deformation conditions, such as strain state, slip system interpretation or deformation temperature. Most of these relations are purely empirical. Here we present quantitative results of the relation between microstructure and quartz c-axis pole figure data to add to the insights between deformation processes and texture development. We analyze EBSD data of experimentally sheared quartzite (kinematic vorticity number Wk = 0.9, experiments of Heilbronner & Tullis, 2006), a mylonitic quartzite from Eriboll (Wk = 0.5, Lloyd’s pers. collection) and a deformed quartz vein from the Tonale line (Wk = 0.4, Stipp & Kunze, 2008). All samples are composed of deformed old grains and recrystallized (by bulging and/or subgrain rotation) and deformed grains in variable proportions.

C-axis pole figures can be decomposed into several components (girdles and point maxima) which occupy distinct positions. These components can be related to two simple microstructural parameters, aspect ratio and long axis direction of grains. While the grain shape evolution in each of the samples differ in detail, they have several features in common:
1) c-axes of equiaxed grains occupy a position close to the inferred instantaneous shortening direction,
2) c-axes of grains with higher aspect ratios contribute to single girdle distributions,
3) the girdle position depends on the grain long axis direction,
4) grains with long axes parallel to the foliation (inferred XY plane of finite strain) provide highest c-axis concentrations in the center of the pole figure,
5) grains contributing to an oblique grain shaped foliation (“freshly” recrystallized, deformed grains) show elongated, peripheral maxima grading into single girdles inclined with the sense of shear and
6) grain shapes which relate to antithetic flow (in the low Wk samples), relations 3-5 hold, with the exception that the resulting peripheral maximum or girdle is also inclined against the sense of shear.

We interpret the individual c-axis pole figure components to reflect contributions from different processes which relate to oriented nucleation or growth (in the case of bulging recrystallization), as well as to a grains’ strain history. This strain history depends on the ratio of how fast a grain is straining (by glide) to how fast it is recrystallizing. The final c-axis pole figure of a polycrystalline aggregate simply reflects the weighted mixture of these components based on the synchronous contribution of each process.

The individual contribution of each process depends on several parameters (e.g., stress as a driving force for local grain boundary migration, grain boundary mobility, or rate of deformation among others). Since many of these parameters are also temperature-dependent, we suggest, for instance, that the variability of the c-axis opening angle with temperature is merely the result of the temperature different dependencies of the contributing processes. Hence, it is unsurprising that the so-called c-axis opening angle cannot be universally applied as a thermometer and is a good example of unrelated cause and correlation and may be expected to give arbitrary results.


Heilbronner, R., Tullis J., 2006 https://doi.org/10.1029/2005JB004194, 2006.
Stipp, M. and Kunze, K., 2008  https://doi.org/10.1016/j.tecto.2007.11.041, 2008.

How to cite: Kilian, R., Morales, L., Lloyd, G., and Stipp, M.: Relation of quartz c-axis pole figures to deformation processes and flow, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14508, https://doi.org/10.5194/egusphere-egu21-14508, 2021.

Melanie Finch, Roberto Weinberg, and Nicholas Hunter

Hydrolytic weakening has been suggested as a major process facilitating strain localisation, consistent with many studies that have found a positive correlation between water content and intensity of deformation. We examine the role of water in an unusually thick shear zone: the 1 km-thick ultramylonitic layer of the El Pichao shear zone, NW Argentina. We used Fourier Transform Infrared Spectroscopy to measure water content in quartz and feldspar, comparing ultramylonitic rocks to mylonites and weakly-deformed rocks. We found that quartz and feldspar in ultramylonites contained half the amount of water of weakly-deformed rocks, contrary to findings in previous studies. We propose that the kilometre-thick ultramylonite formed in three stages: (1) localised deformation and recrystallisation caused release of intracrystalline water to grain boundaries, which promoted grain-boundary sliding, forming the ultramylonite, (2) high pressure in the shear zone continuously expelled intercrystalline water to the surroundings, drying the boundaries and leading to strain hardening, (3) water migrated to neighbouring, less-deformed rocks causing hydrolytic weakening, repeating the cycle, and widening the ultramylonite.

How to cite: Finch, M., Weinberg, R., and Hunter, N.: Water loss and the origin of thick ultramylonites, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3456, https://doi.org/10.5194/egusphere-egu21-3456, 2021.

Orsolya Gelencsér, Csaba Szabó, Márta Berkesi, Alexandru Szakács, Ágnes Gál, Ábel Szabó, Tünde Tóth, and György Falus

Middle Miocene salinity crisis in the Central Paratethys resulted in significant amounts of marine evaporite deposits in the Transylvanian Basin (TB), Romania. The thickness of salt at Praid area is potentially suitable for underground storage of radioactive waste or gases. One of the main factors that determines the potential usage of this voluminous salt body for storage or disposal of various materials is the microstructural characteristics of the salt rock.

Praid is located at the eastern margin of the TB as part of the eastern diapir alignment. The underground salt mine at Praid has been operating there continuously for centuries.  It is an ideal place for sampling the internal part of a salt diapir body, where 20 representative samples were collected. The aim of this study is to extend our understanding of the deformation mechanism in the Praid salt rock.

Primary and secondary structural features were observed and distinguished through detailed petrographic observation. Two types of salt rock were identified: 1/ massive grey salt with large, elongated halite crystals, containing primary fluid inclusions (FIp), accompanied by submicrometer sized grains of halite and clay matrix, and 2/ layered salt with more uniform grainsize distribution showing alternation of greyish (clay rich) and white (clear halite) layers. The layered rock type has mosaic-like structure with a large number of secondary fluid inclusions (FIs). Beside halite, authigenic anhydrite and dolomite are present subordinately (~ 1 vol. %). Secondary fluid inclusions, composed of nitrogen and methane, are indicators of fluid migration pathways throughout the salt body.

Electron Backscatter Diffraction (EBSD) mapping was performed both in the massive and layered salt samples to shed light on the microstructure of the salt rocks. Gamma irradiation was carried as a complementary method of EBSD mapping. Comparing the subgrain diameters obtained from the two techniques, the values are fairly overlapping. The detailed microstructural observations allowed to recognize both dislocation creep and pressure solution processes, which acted concurrently in the Praid salt rock. The differential stress calculations on the salt rock samples indicate a maximum differential stress less than 2 MPa for the massive salt and less than 1.8 MPa for the layered salt. The strain rate calculations (total strain rate between 7.3*e-11 s-1 and 1.8*e-10 s-1) are in good agreement with the observed features in the salt mine, where one of the ~260-year-old salt extraction chambers suffered at least 10 % compressional deformation.

The microstructural characters of the salt body reveal a complex deformation history where fluids have played an important role. The results of this project will be useful and comparable with the regional geological knowledge, to better understand the evolution of this Middle Miocene salt body.

The project is supported by the Cooperative Doctoral Programme of the Ministry for Innovation and Technology (ITM).

How to cite: Gelencsér, O., Szabó, C., Berkesi, M., Szakács, A., Gál, Á., Szabó, Á., Tóth, T., and Falus, G.: Insight into a salt diapir: microstructural study of Praid (Transylvanian Basin, Romania) salt rocks, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15287, https://doi.org/10.5194/egusphere-egu21-15287, 2021.

Dripta Dutta, Santanu Misra, and David Mainprice

Partial melting of metapelites at high-P and high-T conditions typical of lower crustal levels is a well-known phenomenon. Its role in strain localization ­– both at micro- and regional scale – and subsequent rheological weakening of rocks have been widely investigated. Previous researchers have also explored the influence of such melt networks on the variation of the dominant phases' active deformation processes – especially quartz – over the entire range of tectono-thermal evolution of the rock. However, mechanisms driving the deformation of the phases crystallized in-situ from the melt has so far been largely overlooked.

In this work, we focus on the deformation behavior of the in-situ crystallized phases with increasing shear strain. In that pursuit, we took a quartz-muscovite mixture (dry) that was initially cold pressed at 200 MPa, followed by hot pressing at 160 MPa and 580 °C to obtain an analogue of pelite. The cylindrical sample was then experimentally deformed in a Patterson-type apparatus under a finite shear strain (γ) of 15.0 at 750 °C, a confining pressure of 300 MPa, and constant shear strain rate ( 3 × 10-4s-1). Subsequently, a longitudinal axial section was cut and was examined using electron backscattered diffraction (EBSD).

The initial minerals, quartz (Qtz) and muscovite (Ms), underwent deformation and reacted to produce K-Feldspar (Kfs), Mullite (Mul), Cordierite (Crd), Ilmenite (Ilm), and Biotite (Bt). The Qtz grains show limited evidence of dynamic recrystallization. Ms, on the other hand, exhibit strong crystal preferred orientations (CPO). The J-Indices of both Qtz and Ms increase with shear strain (from the center to the edge of the cylinder). Among the reaction products, Kfs (maximum in volume) show weak CPO throughout, similar to Qtz. The maxima of [001] plot near parallel to the shear direction in the pole figures for all values of γ. The rest of the phases show strong CPOs. The J-Index of Crd and Mul increase with shear strain, whereas that of Ms and Kfs increase till γ = 7 and fall at higher strains. Neighbor-pair misorientation axes for Crd, Ilm, and Kfs, corresponding to the high-angle boundaries (HAGBs), are randomly oriented, implying ‘rigid grain rotation,’ which could also be responsible for the lower [001] pole figure intensities. Overall, with increasing shear strain, the number of HAGBs decreases. The corresponding misorientation axes exhibit stronger preferred alignment, probably signifying restricted rotation with progressive melt crystallization. Although the area-equivalent diameters for all the melt-crystallized phases are nearly close (RMS: 1.5 – 2 µm), the Kfs CPOs are considerably weaker than the rest. This possibly affirms the dominance of fluid/melt in triggering diffusion creep and grain boundary sliding over grain size.

How to cite: Dutta, D., Misra, S., and Mainprice, D.: Understanding deformation behavior of lower crustal rocks from experimentally deformed metapelitic aggregate: an EBSD-based approach, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14101, https://doi.org/10.5194/egusphere-egu21-14101, 2021.

Edgar Berrezueta, José Cuervas-Mons, Cynthia Gallego-Ruiz, Berta Ordóñez-Casado, Manuel Ignacio de Paz-Álvarez, Juan Luis Alonso, and Sergio LLana-Fúnez

The size and shape of rock constituent particles can provide substantial information about the environment in which rocks are formed and also about their evolution during their geological history. There are several geological processes that generate specific particle shapes. We focus on three processes and their effects on particles as end members: sediment transport in water producing sub-rounded particles, tectonic fracturing producing angular fragments and chemical corrosion at grain boundaries increasing their rugosity. In this work we test several shape morphological parameters in natural rock specimens with the ultimate goal of quantifying the proportion of different typologies of particles in a rock, all of which can be related to specific geological processes. The main aim of this work is to distinguish different typologies of quartz particles according to the quantitative and qualitative evaluation of shape parameters by using several shape parameters in grains and/or particles.

The procedure followed includes: i) the petrographic characterization of rock specimens in thin section, visually establishing the different typologies of quartz grains present, ii) the acquisition and segmentation of outlines of quartz particles and iii) the quantification of size and shape parameters such as area (A), perimeter (P), fractal dimension (FD), solidity (So), normalized perimeter-area (PoA), Wadell roundness (Rw), Drevin roundness (RD), Pg/Pe roundness (RP), sphericity (S) and a regularity indicator (RBC). A total of 293 particles were studied by means of ImagePro-Plus, ImageJ and Roussillon Toolbox software.

We have used two rock specimens from the base of the Esla nappe, a thrust sheet emplaced in the foreland fold and thrust belt of the Variscan orogen in NW Iberia (Cantabrian Zone). The first phase of this work was to identify the petrographic characteristics of the samples. One specimen was sampled from a quartz sand injection at the base of the thrust sheet. The other is from a sandstone in the footwall, the likely source for the quartz grains injected in the hanging wall. There are some evidence of fracturing and corrosion of the injected quartz grains during the injection process at the base of the Esla nappe. In summary, the first sample contains quartz grains with distinctive shapes that can be directly related to very specific geological processes affecting particle shape in a rock.

The result of the analysis completed allowed the definition of: i) the parameters that best represent the grain shape variations and ii) the range of values for each parameter that are characteristic of each process, thus allowing the classification of the grain shapes. Furthermore, the analysis allowed distinguishing sub-rounded quartz grains of detrital sedimentary origin from grains that have been partially or totally fractured. However, the used shape parameters do not allow a univocal identification of grains corroded by fluids.

Acknowledgments: The Spanish National Plan (CGL2017-86487-P PETROCANTABRICA Project) funded this research.

How to cite: Berrezueta, E., Cuervas-Mons, J., Gallego-Ruiz, C., Ordóñez-Casado, B., de Paz-Álvarez, M. I., Alonso, J. L., and LLana-Fúnez, S.: Quantification and assessment of quartz-particle 2D size and shape using digital image analysis., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11091, https://doi.org/10.5194/egusphere-egu21-11091, 2021.

Lunch break
Chairpersons: Rüdiger Kilian, John Wheeler
Reaction related deformation of mafic rocks
Sara De Caroli, Ake Fagereng, Kohtaro Ujiie, and Francesca Meneghini

The geological properties of the subduction interface, such as stable metamorphic assemblages and the rheology of shear zone rocks, change with depth. Studies based on seismic and geodetic observations suggest that these changes can be accompanied by differences in seismic styles. In this realm, slow slip events (SSEs) and related tremor signals, grouped as episodic tremor and slip (ETS) events, have been detected down-dip of the subduction megathrust seismogenic zone. A wide range of mechanisms, some invoking rheological heterogeneity, has been proposed to explain ETS occurrence. Given that ETS events accommodate most of the plate interface displacement in a depth range below the seismogenic zone, it is of great interest to understand the rheology of the rock lithologies that are likely to host ETS along the deep subduction interface.

Here, we present data from an exhumed subduction complex in Ishigaki Island, Ryukyu Arc. In particular, we analyse the Triassic high pressure-low temperature Tomuru metamorphic rocks, which comprise blueschist and greenschist facies metabasites that underwent subduction-related deformation. These rocks offer an important natural laboratory in which to study the characteristics of blueschist deformation structures to infer rheology and, in particular, the role played by heterogeneities in an environment comparable to modern ETS down-dip of the seismogenic zone.

Through multiscale and multidisciplinary, field- and laboratory-based studies, including quantitative microstructural and image analyses, we focus on two main topics. Firstly, we aim to understand blueschist rheology, by documenting the deformation mechanisms active in blueschist rocks through electron backscatter diffraction (EBSD), in order to quantify intracrystalline deformation and lattice preferred orientation (LPO) development. Secondly, we study the effect of grain size on blueschist foliation development and, ultimately, on blueschist deformation.  Through these analyses, we hope to constrain both subduction interface strength and dominant mineral- scale deformation mechanisms at blueschist conditions.

How to cite: De Caroli, S., Fagereng, A., Ujiie, K., and Meneghini, F.: Types of heterogeneities and deformation mechanisms in blueschist rocks: an example from an exhumed subduction complex in Ishigaki Island, Ryukyu Arc, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10990, https://doi.org/10.5194/egusphere-egu21-10990, 2021.

Leif Tokle, Lonnie Hufford, Luiz Morales, and Whitney Behr

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. We have begun an experimental investigation to constrain the rheology and mechanical anisotropy of a naturally foliated blueschist from the Condrey Mountain Window, CA, USA. General shear experiments were performed in a Griggs apparatus using cores of the natural blueschist at 700°C, 1 GPa, and a shear strain rate of ~10-5 s-1. The starting material consists of ~55% glaucophane, ~40% epidote, ~5% titanite, and <5% quartz where both glaucophane and epidote have strong crystallographic fabrics and shape-preferred orientations that define the foliation. Three types of experiments were performed: 1) with the foliation parallel to the shear plane, 2) with the foliation parallel to the sigma1 direction, and 3) where the starting material was crushed into a powder representing no foliation. Both of the foliated experiments achieve similar peak shear stresses of ~250 MPa; however, the sample with the foliation parallel to the shear plane shows strain weakening while the sample with the foliation parallel to the sigma1 direction shows no strain weakening. We also observe several stress drops of ~20-30 MPa in the sample with the foliation parallel to the sigma1 direction prior to peak stress conditions. Microstructures from both of the foliated samples show evidence for brittle deformation processes, while kinking is also commonly observed in glaucophane. The sample with no foliation has a lower shear stress of ~130 MPa and shows no evidence for brittle deformation processes but rather shows development of a S-C-C’ mylonitic fabric. Additional experiments will be performed at different temperatures and strain rate conditions. A detailed microstructural analysis will accompany the mechanical results.

How to cite: Tokle, L., Hufford, L., Morales, L., and Behr, W.: The rheology and mechanical anisotropy of a foliated blueschist, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10686, https://doi.org/10.5194/egusphere-egu21-10686, 2021.

Lonnie Hufford, Leif Tokle, and Whitney Behr

Constraining the rheological properties of glaucophane is critical to understanding subduction zone rheology. Based on the rock record, glaucophane is a major constituent mineral associated with subducted mafic oceanic crust at blueschist metamorphic facies. No flow law describing the crystal-plastic deformation of this mineral has been developed. Previous experimental work involving glaucophane focused on the deformation of natural polyphase rocks with an emphasis on seismic anisotropy. Here we focus on experiments intended to activate crystal-plastic deformation mechanisms in glaucophane using a monophase aggregate powder separated from natural samples from Syros Island, Greece. We are conducting general shear and axial compression experiments in a Griggs apparatus using temperatures of 600-800°C, pressures of 1 GPa and shear strain rates between 10-5-10-6. Our first experiment was in a general shear orientation at 700°C, 1 GPa, and a shear strain rate of 1.18x10-5. This experiment had a ~80% modal abundance of glaucophane and appears to have been dominated by brittle deformation. After the first experiment, we decided to produce a purer glaucophane aggregate powder containing ~95% glaucophane with ~5% other phases and are finishing mineral separation at the time of submission. We will present early mechanical and microstructural data from experiments with the aim of developing a glaucophane flow law. Our results will also be compared to ongoing experiments focused on the viscous properties of experimentally deformed natural aggregates (see abstract in this conference by Tokle et al.).

How to cite: Hufford, L., Tokle, L., and Behr, W.: Experimental investigation of glaucophane rheology through shear and axial compression Griggs apparatus experiments on hot-pressed aggregates  , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14300, https://doi.org/10.5194/egusphere-egu21-14300, 2021.

Amir Topaz, Yuval Boneh, and Tzahi Golan

Amphibole’s ubiquitous occurrence in the lower crust and subduction zones together with its anisotropic elastic and rheological properties makes its texture evolution essential for assessing the past and current tectonic regimes. Amphibole often display a typical crystallographic preferred orientation (CPO) where the crystals [001] axes align with lineation and the [100] axes align with the normal to the foliation plane. However, this common CPO was attributed to numerous different deformation mechanisms, such as rigid body rotation, dislocation creep, or dissolution precipitation, and there yet to be found a distinct relation between amphibole CPO attributes and the prevailing deformation mechanism. Here, we present a microstructural analysis using electron backscatter diffraction (EBSD) of a highly deformed amphibolite from the metamorphic sole of Mamonia complex in Cyprus in order to investigate texture evolution in amphibole-rich samples. Samples from two localities ~40 km from each other were analyzed: ‘Agia Varvara’ (AV), and ‘Bath of Aphrodite’ (BOA). The two amphibolites show well-foliated microstructure, comprised mainly of hornblende (50-70%), and plagioclase (20-30%) grains under similar calculated P-T conditions of ~600 °C and 6 kbar. Despite the similar compositions and conditions, there are significant differences in the overall texture between the two samples. Samples from AV show strongly clustered amphibole CPO, with the [001] axis forming a strong point maximum parallel to the lineation (X-axis) and the [100] axis aligned perpendicular to foliation (Z-axis). In addition, amphiboles are aligned with the lineation with relatively curved boundaries and moderate aspect ratio (~2). For samples from BOA, amphiboles grains show two distinct CPO types: axial [001], where the [001] is aligned parallel to the shear direction while [100] and [010] oriented along the Y-Z plane, and orthorhombic, where the [001] and [100] are aligned with the lineation and normal to foliation, respectively. In addition, amphibole are tabular-shaped, elongated grains with distinctively straight boundaries and high aspect ratio of ~3.5. Comparison between the AV and BOA grains with average misorientation spread of >1° shows higher fraction for AV (35%) than BOA (13%). We interpret the textural and microstructural analysis of the amphibolites to reflect different deformation mechanisms for AV and BOA. The lack of compositional zoning within hornblende grains suggests no significant deformation by dissolution precipitation for both AV or BOA. For AV, the strong CPO, curved grains boundaries, and high ratio of grains with intragrain misorientations suggest deformation through dislocation creep. Differently, in BOA, the observations of tabular-shaped amphibole grains, the low amount of intra-grain misorientations, along with shape and crystal orientations that vary together with [001] as the rotation angle suggest deformation by rigid body rotation.

How to cite: Topaz, A., Boneh, Y., and Golan, T.: Texture Evolution of Amphiboles - a Case Study from the Mamonia Complex, Cyprus, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11935, https://doi.org/10.5194/egusphere-egu21-11935, 2021.

Lisa Marie Brückner and Claudia A. Trepmann

Pseudotachylyte-bearing amphibole-rich gneisses with concordant quartz-rich layers from the base of the Silvretta nappe, Austria, are analyzed by polarized light microscopy, scanning electron microscopy and electron back scattered diffraction. Amphibole grains show microfractures, undulatory extinction, deformation lamellae, kink bands, mechanical twins and locally recrystallized grains restricted to sites of high strain, e.g. along microshear zones and twin boundaries. The twins are characterized by a twin plane parallel to (-101), a rotation axis parallel to [101] and a misorientation angle of 178°. The (-101) amphibole twins document the high differential stresses during crystal plasticity coeval with pseudotachylyte formation, given their high critical resolved shear stress of 200 MPa. Directly at the contact to twinned amphibole within the gneisses, quartz grains commonly show subbasal deformation lamellae, short-wavelength undulatory extinction and cleavage cracks mostly parallel to {10-11} rhombohedral planes that are decorated by recrystallized grains with a diameter of < 10 µm. The small recrystallized grains show a crystallographic preferred orientation (CPO) that is controlled by the orientation of the host grains. This quartz microstructure consistently indicates high-stress crystal plasticity of quartz concurrent with high-stress crystal plasticity of amphibole and pseudotachylyte formation.

Quartz-rich layers (>90% quartz) concordant to the foliation of the gneisses commonly show evidence of dynamic recrystallization in the regime of dislocation creep. The recrystallized grain microstructure is mostly homogenous without a gradient towards the lithological contact to the amphibole-rich gneisses. Locally, however, a gradient of decreasing strain towards the contact can be observed as indicated by a decreasing number of recrystallized grains. Close to the contact, quartz grains are coarse with long axes of a few mm. A core-and-mantle structure, where recrystallized grains surround a few hundred µm wide and mm-long porphyroclasts, is occurring in transition towards an almost completely recrystallized microstructure. The recrystallized grains show a CPO indicating rhombohedral <a> dislocation glide. Recrystallized grains are isometric and subgrains in porphyroclasts are of similar shape and size, indicating dynamic subgrain rotation recrystallization. Stresses on the order of hundred MPa are suggested by the diameter of recrystallized grains of in average about 10 µm. Locally, the recrystallized quartz aggregate is affected by subsequent low-temperature plasticity, as indicated by shear fractures offsetting the recrystallized microstructure. The missing or decreasing strain gradients of dislocation creep within the quartz-rich layers towards the amphibole-rich gneisses indicate that dislocation creep in the quartz-rich layers cannot be responsible for transferring high stresses required for high-stress crystal-plasticity of quartz and amphibole as well as pseudotachylyte-formation and suggest that dislocation creep of quartz represents an independent earlier stage of deformation.

How to cite: Brückner, L. M. and Trepmann, C. A.: High-stress crystal plasticity of amphibole and quartz in pseudotachylyte-bearing gneisses and dislocation creep in concordant quartz-rich layers from the Silvretta basal thrust (Austria), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12346, https://doi.org/10.5194/egusphere-egu21-12346, 2021.

Yong Park, Sejin Jung, and Haemyeong Jung

To understand the crystallographic preferred orientation (CPO) of glaucophane and epidote and deformation microstructures at the top of a subducting slab in a warm subduction zone, deformation experiments of epidote blueschist were conducted in simple shear by using a modified Griggs apparatus. Deformation experiments were performed under high pressure (0.9–1.5 GPa), temperature (400–500 °C), shear strain (γ) in the range of 0.4–4.5, and shear strain rate of 1.5×10-5–1.8×10-4 s-1. After experiments, CPO of minerals were determined by electron back-scattered diffraction (EBSD) technique, and microstructures of deformed minerals were observed by transmission electron microscopy (TEM). At low shear strain (γ ≤ 1), the [001] axes of glaucophane were in subparallel alignment to shear direction, and the (010) poles were sub-normally aligned to the shear plane. At high shear strain (γ > 2), the [001] axes of glaucophane were in subparallel alignment to shear direction, and the [100] axes were sub-normally aligned to the shear plane. At a shear strain between 2 < γ < 4, the (010) poles of epidote were in subparallel alignment to shear direction, and the [100] axes were sub-normally aligned to the shear plane. At a high shear strain where γ > 4, the alignment of the (010) epidote poles had altered from subparallel to subnormal to the shear plane, while the [001] axes were in subparallel alignment to the shear direction. TEM observations and EBSD mapping revealed that the CPO of glaucophane was developed by dislocation creep, somewhat affected by the cataclastic flow at high shear strain. On the other hand, the CPO development of epidote is considered to have been affected by dislocation creep under a shear strain of 2 < γ < 4 but is highly affected by cataclastic flow with rigid body rotation under a high shear strain (γ > 4). Our experimental results indicate that the magnitude of shear strain and rheological contrast between component minerals plays an important role on the formation of CPOs of glaucophane and epidote.

How to cite: Park, Y., Jung, S., and Jung, H.: Experimental study on the deformation microstructures and crystallographic preferred orientation of glaucophane and epidote in deformed epidote blueschist at high pressure, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3789, https://doi.org/10.5194/egusphere-egu21-3789, 2021.

Marina Galindos Alfarache, Holger Stünitz, Jiří Konopásek, and Amicia Lee

Deformation of natural mafic rocks by viscous deformation mechanisms can occur even at low temperature conditions. In such instances, crystal plastic mechanisms are not operative, as their activity is restricted to very high temperatures for amphiboles, pyroxenes, and plagioclase. Instead, simultaneous mineral reactions may facilitate deformation at low temperature conditions. The gabbro from the Lyngen Magmatic Complex (LMC) constitutes a good example of such processes, because it has experienced deformation at low temperatures of greenschist to lower amphibolite-facies conditions, and the rock has been transformed from gabbro to greenschist. This study focuses on detailed analysis of deformation processes, metamorphic reactions and fabric development in the LMC gabbro. Most samples are overprinted by epidote amphibolite and greenschist-facies mineral assemblages. Preliminary observations distinguish two different types of amphiboles, which have been interpreted as different generations. The predominant type defines the stretching lineation and shows long prismatic habits whereas the less abundant type crystallized in a sub- to anhedral manner. The metamorphic conditions of growth for each amphibole type is yet not well constrained. However, we initially interpret the former to grow during epidote amphibolite- or greenschist facies-conditions, whereas the latter could represent relict grains from the original magmatic assemblage or products generated at amphibolite- or epidote amphibolite-facies conditions. Further analysis will determine the orientation, geochemistry and metamorphic conditions during growth for both amphibole types. A recent model proposed for eclogites suggests that simultaneous mineral growth and deformation can result in new products growing in a preferred direction. Such preferential growth can generate a shape preferred orientation parallel to the lineation, which results in the formation of crystal preferred orientations (CPO). We aim to test if similar microstructural observations can be translated to the amphiboles of the LMC gabbro. In such case, amphibole CPO’s would not be the product of crystal plasticity but of preferential growth. The large scale deformation of the LMC emphasizes the relevance of these results, as it would demonstrate that the interaction between mineral reactions and deformation can play a major role on regional deformation of large mafic bodies, such as the ocean floor.

How to cite: Galindos Alfarache, M., Stünitz, H., Konopásek, J., and Lee, A.: Amphibole CPO in retrograded gabbro from the Lyngen Magmatic Complex (Northern Scandinavian Caledonides, Norway)., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1303, https://doi.org/10.5194/egusphere-egu21-1303, 2021.

Luca Pellegrino, Luca Menegon, Stefano Zanchetta, Falko Langenhorst, Kilian Pollok, Simone Tumiati, and Nadia Malaspina

Pyroxenites are common compositional heterogeneities in the upper mantle and represent key lithologies in mantle deformation processes, as the local presence of pyroxene-rich compositions can weaken the mantle strength. Pyroxenites occur ubiquitously as variably deformed layers in most of oceanic and orogenic peridotite massifs, and thus can be used as a proxy to investigate the rheological behavior of the mantle in different geodynamic settings, including subduction zones.  
In the Ulten Zone (Tonale nappe, Eastern Alps, N Italy), numerous peridotite bodies occur within high-grade crustal rocks. Peridotites show a transition from coarse protogranular spinel lherzolites to finer-grained amphibole + garnet peridotites (Obata and Morten, 1987). Pyroxenites veins and dikes, transposed along the peridotite foliation, show a similar evolution from coarse garnet-free websterites to finer-grained garnet clinopyroxenites (Morten and Obata, 1983). This evolution has been interpreted to reflect cooling and pressure increase of pyroxenites and host peridotites from spinel- (1200 °C, 1.3-1.6 GPa) to garnet-facies conditions (850 °C and 2.7 GPa) within the mantle corner flow (Nimis and Morten, 2000). This results in the consequent formation of garnet at the expense of spinel. In particular, garnet initially formed as coronas around spinel and as exsolution lamellae in high-T pyroxenes, and later as neoblasts along the foliation of pyroxenites and host peridotites. 
Microstructures and crystallographic orientation data indicate that the transition from spinel- to garnet-facies conditions occurred in a deformation regime. Pyroxene porphyroclasts in garnet clinopyroxenites show well-developed crystallographic preferred orientation, high frequency of low-angle misorientations, and non-random distribution of the low-angle misorientation axes. These features indicate that pyroxene porphyroclasts primarily deformed by grain size insensitive (GSI) creep. Core-and-mantle microstructures in pyroxene porphyroclasts also suggest that GSI creep was aided by subgrain rotation (SGR) during recrystallization, leading the formation of smaller, neoblastic, and strain-free pyroxene grains around porphyroclasts. These recrystallized grains have been interpreted to deform by grain boundary sliding, i.e. a grain size sensitive (GSS) creep mechanism, as indicated by the occurrence of quadruple junctions between straight grain boundaries. Our rheological models also suggest that GSS creep of neoblastic pyroxenes occurred at differential stress of 40 MPa and strain rates of 10-18-10-15 s-1
The transition from GSI creep in the porphyroclasts to GSS creep in the neoblasts was accompanied not only by a reduction of the grain size of pyroxenes, but also by the crystallization of garnet along the pyroxenite foliation which facilitated pinning by second phase in the recrystallized matrix. This stabilized the fine-grained microtexture produced by the GSS creep process, and finally contributed to the rheological weakening of pyroxenites. 
Pyroxenites of Ulten Zone thus offer a unique opportunity to investigate the effects of mantle weakening on the processes that control the material exchange between crust and mantle at subduction zones.

Morten, L., & Obata, M. (1983). Bulletin de Minéralogie, 106(6), 775-780.
Nimis, P. & Morten, L. (2000). Journal of Geodynamics, 30(1-2), 93-115
Obata, M., & Morten, L. (1987). Journal of Petrology, 28(3), 599-623.

How to cite: Pellegrino, L., Menegon, L., Zanchetta, S., Langenhorst, F., Pollok, K., Tumiati, S., and Malaspina, N.: Reaction-induced rheological weakening in the supra-subduction mantle: an example from garnet pyroxenites of Ulten zone (Eastern Alps, N Italy), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2900, https://doi.org/10.5194/egusphere-egu21-2900, 2021.

Amicia Lee, Holger Stünitz, Mathieu Soret, Matheus Ariel Battisti, and Jiří Konopásek

Mafic rocks consist of strong minerals (e.g. clinopyroxene, plagioclase) that can only be deformed by crystal plastic mechanisms at high temperatures (>800°C). Yet, mafic rocks do show extensive deformation by non-brittle mechanisms when they have only reached lower temperatures (~650°C). In many of such cases, the deformation is accommodated by an interaction of deformation with simultaneous mineral reactions. Here we show that dissolution-precipitation creep plays a major role in deformation of gabbro lenses at mid and upper amphibolite facies conditions.

The Kågen gabbro in the North Norwegian Caledonides intruded the Vaddas Nappe at 439 Ma at pressures of 7-9 kbar, temperatures of 650-900°C, and depths of ∼26-34 km. The Kågen gabbro on south Arnøya is comprised of undeformed gabbro lenses with sheared margins wrapping around them. This contribution analyses the evolution of the microstructures and metamorphism from the low strain gabbro lenses to high strain mylonites at margins of the lenses. Microstructural and textural data indicate that dissolution-precipitation creep is the dominant deformation mechanism, where dissolution of the gabbro took place in reacting phases of clinopyroxene and plagioclase, and precipitation took place in the form of new minerals: new plagioclase and clinopyroxene, amphibole, and garnet. Amphibole shows a strong CPO that is primarily controlled by its preferential growth in the extension direction. Synchronous deformation and mineral reactions of clinopyroxene suggests mafic rocks can become mechanically weak during the general transformation weakening process, i.e. the interaction of mineral reaction and deformation by diffusion creep. The weakening is directly connected to a fluid-assisted transformation process that facilitates diffusion creep deformation of strong minerals at far lower stresses and temperatures than dislocation creep. Initially strong lithologies can become weak, provided that reactions can proceed during deformation, the transformation process itself is an important weakening mechanism in mafic (and other) rocks, facilitating deformation at low differential stresses.

How to cite: Lee, A., Stünitz, H., Soret, M., Battisti, M. A., and Konopásek, J.: Creeping gabbro: dissolution-precipitation creep facilitating deformation in mafic rocks, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12360, https://doi.org/10.5194/egusphere-egu21-12360, 2021.

Regional topics
Hiroaki Yokoyama, Jun Muto, and Hiroyuki Nagahama

  Microstructural analysis is essential for estimating the deformation conditions of plastically deformed rocks. In this study, we analyze the microstructures of carbonate mylonites and deformation conditions in natural shear zone to reconstruct tectonics. Carbonate mylonites originated from late Carboniferous Tateishi Formation and mylonitized in middle Cretaceous by the strike-slip motion of Shajigami shear zone in the eastern margin of the Abukuma Mountain, Northeastern Japan.
  Microstructural analysis was carried out by optical microscope and electron backscattered diffraction (EBSD) mapping to determine grain size, aspect ratio, shape preferred orientation (SPO) and crystallographic preferred orientation (CPO) of calcite aggregates.
  Pervasive deformation twins and dynamically recrystallized grains are observed. Although most porphyroclasts show symmetric structure, some show asymmetric structure that indicates dextral shear sense. Mean dynamically recrystallized grain size is 16-67 µm, and it decreases close to the shear zone. CPOs show that c-axes concentrate normal to the shear plane or slightly rotate to the shear sense. The strong CPOs suggest that the dominant deformation mechanism is dislocation creep. SPOs show the foliation which is slightly oblique or almost parallel to the shear plane. However, we observed the SPOs parallel to the shear plane at the location 150 m away from the shear zone.  The 3D dynamically recrystallized grain shapes are between plane-strain ellipsoid and oblate ellipsoid. The grain shapes tend to be relatively polygonal close to the shear zone, while more elongated further away from the shear zone. The distribution of the carbonate mylonite originated from same Tateishi Formation is known to be about 5 km apart from the Shajigami shear zone (Tateishi location). However, based on many aspects of differences in microstructures among both locations such as SPOs of recrystallized grains, we infer that the deformation of Shajigami shear zone was not related to one at Tateishi location. The pervasive dynamic recrystallization suggests that the deformation temperature was at least 200°C. Observed type Ⅱ and type Ⅲ twin morphologies (Burkhard, 1993) of calcite grains suggest deformation temperature below 300°C. 
  These results indicate that the deformation of the Shajigami shear zone was in the range from 200 to 300℃ and deformation was stronger near the shear zone. In addition, the polygonal grain shape close to the shear zone suggests that the deformation temperature is higher close to the shear zone. Furthermore, SPOs show that pure shear component is larger than simple shear component in terms of SPOs that almost parallel to the shear plane away from the shear zone. This study including several additional results will provide the microstructural development of carbonate mylonites in natural strike-slip shear zones deformed near the brittle-ductile condition of the upper crust.

How to cite: Yokoyama, H., Muto, J., and Nagahama, H.: Microstructures and deformation temperature of carbonate mylonites in Shajigami shear zone at eastern margin of Abukuma Mountains, Northeastern Japan, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14195, https://doi.org/10.5194/egusphere-egu21-14195, 2021.

Mark Coleman, Bernhard Grasemann, David Schneider, Konstantinos Soukis, and Riccardo Graziani

Microstructures may be used to determine the processes, conditions and kinematics under which deformation occurred. For a given set of these variables, different microstructures are observed in various materials due to the material’s physical properties. Dolomite is a major rock forming mineral, yet the mechanics of dolomite are understudied compared to other ubiquitous minerals such as quartz, feldspar, and calcite. Our new study uses petrographic, structural and electron back scatter diffraction analyses on a series of dolomitic and calcitic mylonites to document differences in deformation styles under similar metamorphic conditions. The Attic-Cycladic Crystalline Complex, Greece, comprises a series of core complexes wherein Miocene low-angle detachment systems offset and juxtapose a footwall of high-pressure metamorphosed rocks against a low-grade hanging wall. This recent tectonic history renders the region an excellent natural laboratory for studying the interplay of the processes that accommodate deformation. The bedrock of Mt. Hymittos, Attica, preserves a pair of ductile-then-brittle normal faults dividing a tripartite tectonostratigraphy. Field observations, mineral assemblages and observable microstructures suggests the tectonic packages decrease in metamorphic grade from upper greenschist facies (~470 °C at 0.8 GPa) in the stratigraphically lowest package to sub-greenschist facies in the stratigraphically highest package. Both low-angle normal faults exhibit cataclastic fault cores that grade into the schists and marbles of their respective hanging walls. The middle and lower tectonostratigraphic packages exhibit dolomitic and calcitic marbles that experienced similar geologic histories of subduction and exhumation. The mineralogically distinct units (calcite vs. dolomite) of the middle package deformed via different mechanisms under the same conditions within the same package and may be contrasted with mineralogically similar units that deformed under higher pressure and temperature conditions in the lower package. In the middle unit, dolomitic rocks are brittlely deformed. Middle unit calcitic marble are mylonitic to ultramylonitic with average grain sizes ranging from 30 to 8 μm. These mylonites evince grain-boundary migration and grain size reduction facilitated by subgrain rotation. Within the lower package, dolomitic and calcitic rocks are both mylonitic to ultramylonitic with grain sizes ranging from 28 to 5 μm and preserve clear crystallographic preferred orientation fabrics. Calcitic mylonites exhibit deformation microstructures similar to those of the middle unit. Distinctively, the dolomitic mylonites of the lower unit reveal ultramylonite bands cross-cutting and overprinting an older coarser mylonitic fabric. Correlated missorientation angles suggest these ultramylonites show evidence for grain size reduction accommodated by microfracturing and subgrain rotation. In other samples the dolomitic ultramylonite is the dominant fabric and is overprinting and causing boudinage of veins and relict coarse mylonite zones. Isolated interstitial calcite grains within dolomite ultramylonites are signatures of localized creep-cavitation processes. Following grain size reduction, grain boundary sliding dominantly accommodated further deformation in the ultramylonitic portions of the samples as indicated by randomly distributed correlated misorientation angles. This study finds that natural deformation of dolomitic rocks may occur by different mechanisms than those identified by published experiments; notably that grain-boundary migration and subgrain rotation may be active in dolomite at much lower temperatures than previously suggested.

How to cite: Coleman, M., Grasemann, B., Schneider, D., Soukis, K., and Graziani, R.: Deformation mechanisms in natural dolomite mylonites from a detachment system (Mt. Hymittos, Greece), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3600, https://doi.org/10.5194/egusphere-egu21-3600, 2021.

Samuele Papeschi, Giovanni Musumeci, Omar Bartoli, Bernardo Cesare, Hans-Joachim Massonne, and Francesco Mazzarini

The Calamita Schists in the aurole of the Late Miocene Porto Azzurro pluton underwent partial melting and HT metamorphism at P < 0.2 – 0.3 GPa and T > 650 – 700 °C, coeval with regional deformation. Deformation produced a network of shear zones that evolved from melt-present conditions to the brittle-ductile transition. Shearing at high temperature in the presence of melt allowed deformation to remain relatively distributed in wide high-strain zones. As the thermal pulse associated with the intrusion progressively faded away, deformation localized into anastomosing, mylonitic greenschist-facies shear zones surrounding lozenges of high-grade migmatitic schist. Mylonitic shear zones formed at low-angle with respect to the well-established high grade foliation preserved as a relic, oblique foliation. We show that such an extreme strain localization was determined by strain hardening of the no longer melt-bearing quartz-feldspar schist, localized embrittlement on precursory shear bands, and fluid-enhanced reaction softening that caused the breakdown of Al-silicates and the development of phyllosilicate-rich mylonitic bands. Consequently, tectonic structures with different orientation developed under the same kinematic regime, as a result of the changing physical and mechanical properties of the cooling rock volume.

How to cite: Papeschi, S., Musumeci, G., Bartoli, O., Cesare, B., Massonne, H.-J., and Mazzarini, F.: Evolution of melt-bearing shear zones during cooling within an upper crustal aureole: the Calamita Schists (Island of Elba, Italy), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14906, https://doi.org/10.5194/egusphere-egu21-14906, 2021.

Jeremie Malecki, Juan Gómez Barreiro, Manuela Durán Oreja, José Ramón Martínez Catalán, Magdalena Tettamanti, Santos Barrios Sánchez, Juan Morales Sánchez Migallón, and Inés Puente Orench

The NW Iberian Massif represents a segment of the Variscan Belt, where several allochthonous complexes crop out: : Cabo Ortegal, Ordenes and Malpica-Tuy, in Spain, and Bragança and Morais in Portugal. These allochthonous complexes comprise allochthonous units, overthrusting parautochthonous and autochthonous units. The suture zone of the Variscan orogeny in the NW Iberia preserves the testimony of the collisional dynamics between Gondwana and Laurussia during the Carboniferous. The stacking of allochthonous units into an accretion wedge, and their subsequent incorporation by thrusts into the continental margin of Gondwana, resulted in polyphasic tectonothermal evolution. Different units record valuable information about the deformation mechanisms, rheological behaviour and the configuration of plates during the Palaeozoic.

The kinematic and deformational evolution of major tectonic boundaries of the Variscan Allochthonous units, as well as their mutual relationship in Iberia is critical, in order to constrain their regional meaning and correlation with similar units along the European Variscan Belt. In shear-zones, plastic deformation of polycrystalline aggregates result into microstructural and textural fingerprints that need to be interpreted. Quantitative analyses of fabrics has been crucial in untangling complex tectonothermal evolutions. In this case neutron diffraction experiments have been conducted in transmission mode in the Institute Laue-Langevin (ILL) (France), to characterize mylonites from the basal shear zone of the Lower Allochthon in Morais Complex. Two different experimental sets have been tested in D1B and D20 beamlines, comparing textural standards and new vanadium sample holders in order to optimize the procedure. Diffraction data were refined with Rietveld software MAUD to obtain quantitative texture information and orientation distribution functions (ODF) for main phases. Afterward, pole figures of relevant planes were interpreted in terms of slip-system activity to understand deformation conditions. Overall, microstructural data and fabric analysis points to a top-to-the SE shearing with a pure-shear component in the mylonitic flow.

Keywords: Shear zones, texture analysis, neutron diffraction, Rietveld method, Variscan orogeny, Morais complex.

How to cite: Malecki, J., Gómez Barreiro, J., Durán Oreja, M., Martínez Catalán, J. R., Tettamanti, M., Barrios Sánchez, S., Sánchez Migallón, J. M., and Puente Orench, I.: Basal Shear-Zone of the Lower allochthon in the Morais complex (Portugal): microstructural and neutron diffraction constraints., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9986, https://doi.org/10.5194/egusphere-egu21-9986, 2021.

Xuemei Cheng and Shuyun Cao

Within orogenic zone and continental extensional area, it often developed metamorphic complex or metamorphic gneiss dome that widely exposed continental mid-lower crustal rocks, which is an ideal place to study exhumation processes of deep-seated metamorphic complex and rheology. The Yuanmou metamorphic complex is located in the south-central part of the "Kangdian Axis" in the western margin of Qiangtang Block and Yangtze Block, which is a part of the anticline of the Sichuan-Yunnan platform. Many research works mainly focus on the discussion of intrusion ages, aeromagnetic anomalies, and polymetallic deposits. However, the exhumation process and mechanism of the Yuanmou metamorphic complex are rarely discussed and still unclear. This study, based on detailed field geological observations, optical microscopy (OM), cathodoluminescence (CL), electron backscatter diffraction (EBSD) and electron probe (EMPA) were performed to illustrate the geological structure features, deformation-metamorphic evolution process and its tectonic significance of Yuanmou metamorphic complex during the exhumation process. All these analysis results indicate that the Yuanmou metamorphic complex generally exhibits a dome structure with deep metamorphic rocks and deformed rocks of varying degrees widely developed. Mylonitic gneiss and granitic intrusions are located in the footwall of the Yuanmou, which have suffered high-temperature shearing. The mylonitic fabrics and mineral stretching lineations in the deformed rock are strongly developed, forming typical S-L or L-shaped structural features. The high-temperature ductile deformation-metamorphism environment is high amphibolite facies, that is, the temperature range is between 620 ~ 690 ℃ and the pressure is between 0.8 ~ 0.95 Gpa. In the deformed rocks closed to the detachment fault, some of the mylonite fabric features are retained, but most of them have experienced a strongly overprinted retrogression metamorphism and deformation. At the top of the detachment fault zone, it is mainly composed of cataclasites and fault gouge. The comprehensive macro- and microstructural characteristics, geometry, kinematics, and mineral (amphibole, quartz and calcite) EBSD textures indicate that the Yuanmou metamorphic complex has undergone a progressive exhumation process during regional extension, obvious high-temperature plastic deformation-metamorphism in the early stage, and superimposed of low-temperature plastic-brittle and brittle deformation in the subsequent stage, which is also accompanied by strong fluid activities during the exhumation process.

How to cite: Cheng, X. and Cao, S.: Structural deformation-metamorphism and exhumation processes of Yuanmou metamorphic complex, Yunnan, China, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10483, https://doi.org/10.5194/egusphere-egu21-10483, 2021.

Nania Laura, Montomoli Chiara, Iaccarino Salvatore, Leiss Bernd, and Carosi Rodolfo

A challenge in tectonic studies concerns the attempt to relate deformation features at the microscale and the crystalline lattice scale of rock-forming minerals up to the regional scale. The South Tibetan Detachment System (STDS) in Himalaya is a natural laboratory for such correlations, being a prime example of regional-scale low-angle ductile extensional fault/shear zone systems within collisional settings, with a top-down-to-the-north sense of shear. The STDS shearing involves, with a thickness of c. 1-2 km, the uppermost part of the metamorphic core of the belt, the Greater Himalayan Sequence (GHS), and the basal part of the Tethyan Himalayan Sequence (THS), developing a mylonitic foliation and a nearly constant strike. Recurrent ideas on the STDS architecture and rheological behavior come from the clearly and well exposed 3D outcrops around the Everest area (Eastern Nepal), where it mostly developed in quartz-bearing lithologies with a lower ductile shear zone and an upper brittle fault. Vice versa, the location of the exact shear zone boundaries and structural evolution of the STDS are still under controversial discussions in Central-Western Nepal, where few kinematic indicators occur in the carbonate-bearing lithologies of both GHS and THS.

In this contribution, we examine a suite of over 20 field-oriented marble samples affected by the STDS, comparing the deformation recorded by calcite in two different areas in central Himalaya, where essentially only the ductile shear zone has been clearly identified. Calcite microstructures (e.g., grain size and shape) and crystallographic preferred orientations (textures) of impure marbles from the Lower Dolpo region and pure marbles from the Manaslu area (Western Nepal), coupled with petrographic observations, allowed us to conclude on temperature, paleo stress, strain rates, and kinematic of the flow. Our results support the idea of a complex history of the STDS in regard to different thermal and lithospheric stress regimes during deformation. Decreasing temperatures from an early-stage of shearing (at HT-MT condition) to a late-stage of shearing (LT conditions) are coupled with increasing differential stress recorded at comparable strain rates and decreasing simple shear conditions. We propose a progressive exhumation of the STDS towards shallower structural levels, with a temporal (rather than spatial) lowering of kinematic vorticity (“decelerating strain path”), in which progressively more general shear replaced high-temperature simple shear flow during cooling, strain hardening, and narrowing of the shear zone. Microstructural and texture analysis of pure and impure marble proved to be a useful approach to characterize the STDS location and architecture, supporting that, when the upper-brittle fault is not well developed, the ductile shearing proceeded at high structural levels.

How to cite: Laura, N., Chiara, M., Salvatore, I., Bernd, L., and Rodolfo, C.: Kinematics, non-coaxial flow and rheological constraints of the South Tibetan Detachment System in central Himalaya, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10947, https://doi.org/10.5194/egusphere-egu21-10947, 2021.

Salim Birkan Bayrak, Alp Ünal, Işıl Nur Güraslan, Ömer Kamacı, Erdinç Yiğitbaş, and Şafak Altunkaynak

Marmara Granitoid (MG) is an E-W trending sill-like magmatic body exposed in the center of the Marmara Island, NW Anatolia, Turkey. MG is lower Eocene in age and was concordantly emplaced into metamorphic basement rocks of Saraylar Marble and Erdek Complex. It is represented by a deformed granodiorite which widely displays protomylonitic-mylonitic textures with prominent foliation and lineation. Foliation planes display a mean dip direction-angle of 335/29 and mineral stretching lineations show mean trend-plunge of 286/34. Mica-fishes, rotated porphyroclasts and micro-faults are commonly observed and serve as shear gauges pointing out to a dextral movement. Mineral deformation thermometers such as myrmekite development, chessboard extinction, grain boundary migration (GBM), sub-grain rotation recrystallization (SGR), and bulging recrystallization (BLG) in quartz crystals indicate that solid-state deformation of the MG has experienced a high-temperature ductile deformation and superimposed ductile to brittle deformation.

Three-dimensional strain ellipsoid measurements are investigated on the MG in order to determine the relative amounts of pure shear and simple shear deformation and the mean kinematic vorticity number (Wm). The image processing of quartz grains is used as strain markers to obtain the three-dimensional best-fit ellipsoids. The results show that, Lode’s ratio (ν) of the samples change between -0.010 and -0.650 and Flinn’s k-values range from 1.026 to 11.157 indicating to a general constrictional (prolate) deformation. The calculated kinematic vorticity numbers change between 0.429 and 0.958 which show that shear deformation of MG is mostly dominated by simple shear. All of these micro and meso structural properties and three-dimensional strain and kinematic analyses collectively suggest that MG has experienced a dextral transtensional deformation.

How to cite: Bayrak, S. B., Ünal, A., Güraslan, I. N., Kamacı, Ö., Yiğitbaş, E., and Altunkaynak, Ş.: Insights into a dextral transtensional shear zone in NW Anatolia, Turkey: Preliminary results from the three dimensional strain and kinematic analyses of the Marmara Granitoid. , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12884, https://doi.org/10.5194/egusphere-egu21-12884, 2021.

Christoph Kettler, Katarina Pichler, Daniel Smirzka, Thomas Vandyk, and Daniel Le Heron

Neoproterozoic rocks from the Kingston Peak Formation (KPF) in the Valjean Hills (USA) expose a succession of diamictites associated with major glacial events taking place during the Cryogenian, approximately 700 million years ago. Within any glacial period, diamictites are widespread and in addition, their mechanics of deposition are highly variable. Some are massive in appearance at outcrop or in hand specimen, and apparently lacking any information that allows their mode of emplacement to be elucidated. Yet the correct interpretation for deep time successions in this area is especially important, since it is debated whether the diamictites have a tectonically driven, gravitational (Mrofka & Kennedy, 2011) or direct (sub)glacial origin (Le Heron et al. 2016).

In this contribution we determine the origin of the diamictites based on its internal microfabric and associated microstructures. We base our method on the technique of Philips et al. (2011) for Quaternary sediments, by mapping the apparent longest axes of skeleton grains (ranging from fine-grained sand to fine-grained pebbles) in oriented thin sections and reconstructing their fabric in a 3D space, we could identify a bimodal signal in the orientation of the longest axes. Contrary to gravitational deposition, clasts in subglacial diamictites tend to align themselves to a stress field, induced by the movement of the glacier. Macroscopic observations (Fig. 1A), microtexture- and structures (Fig. 1B) as well as the reconstructed microfabric domains (Fig. 1C) suggests a subglacial origin. These circumstances suggest temperate glacial conditions with wet based ice sheets during the deposition of the KPF. Moreover, the quantitative data allow confident flow directions to be extracted from seemingly chaotic diamictites.


Figure 1: (A) Valjean Hills Diamictite (label is 5x5 cm), (B) Rotational structure around bigger skeleton grain, (C) traced long axes of clasts (white lines) and interpreted  microfabric domains (blue, orange)



Le Heron, D.P., Tofaif, S., Vandyk, T. & Ali, D.O. (2017): https://doi.org/10.1130/G38460.1

Mrofka, D., Kennedy, M., (2011):  https://doi.org/10.1144/M36.40

Phillips, E. et al., (2011): https://doi.org/10.1016/j.quascirev.2011.04.024

How to cite: Kettler, C., Pichler, K., Smirzka, D., Vandyk, T., and Le Heron, D.: 3D-Microfabric reconstruction of Neoproterozoic diamictites from the Valjean Hills, California (USA), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13064, https://doi.org/10.5194/egusphere-egu21-13064, 2021.

Meixia Lyu and Shuyun Cao


Graphitic carbon-bearing rocks can occur in low- to high-grade metamorphic units. In low-grade matamorphic rocks, graphitic carbon is often associated with brittle fault gouge whereas in middle- to high-grade metamorphic rocks, graphitic carbon commonly occurs in marble, schist or paragneiss. Previous studies showed that carbonaceous material gradually ordered from the amorphous stage, e.g. graphitization, is mainly controlled by increasing thermal metamorphism and has a good correlation with the metamorphic temperature. Besides, this ordered process is irreversible and the resulting structure is not affected by late metamorphism. Subsequently, the degree of graphitization is believed to be a reliable indicator of peak temperature conditions in the metamorphic rock. In this contribution, based on detailed field observations, the variably deformed and metamorphosed graphitic gneisses to phyllites, located within the footwall and hanging-walls unit of the Cenozoic Ailaoshan-Red River strike-slip shear zone are studied. According to lithological features and temperature determined by Raman spectra of carbonaceous material, these graphitic rocks and deformation fabrics are divided into three types. Type I is represented by medium–grade metamorphism and strongly deformed rocks with an average temperature of 509 °C and a maximum temperature of 604 °C. Type II is affected by low-grade metamorphism and deformed rocks with an average temperature of 420 °C. Type III is affected by lower–grade metamorphism and occurs in weakly deformed/undeformed rocks with an average temperature of 350 °C. Slip–localized micro–shear zone and laterally continuous or discontinuous slip planes constituted by graphitic carbon aggregates are developed in Types I and II. The electron back–scattered diffraction (EBSD) lattice preferred orientation (LPO) patterns of graphitic carbon grains were firstly observed in comparison with LPO patterns of quartz and switch from basal <a>, rhomb <a> to prism <a> slip systems, which indicate increasing deformation temperatures. According to the graphitic slip–planes, micro–shear zones and mylonitic foliation constituted by graphitic carbon minerals, we also propose that the development of fine–grained amorphous carbon plays an important role in rheological weakening of the whole rock during progressive ductile shearing.

Key Words: graphitic carbon, strain localization, graphitic thermometry, slip–localized micro–shear zone, rheological weakening

How to cite: Lyu, M. and Cao, S.: Strain localization mechanism of graphitic carbon-bearing rocks: Constraints from microstructure, texture and graphite geothermometry, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-808, https://doi.org/10.5194/egusphere-egu21-808, 2021.