Displays

The dynamics of several geophysical phenomena, such as post-seismic deformation and post-glacial isostatic readjustment, are inferred to be controlled by the transient rheology of olivine in Earth’s mantle. However, the physical mechanism(s) that underlie(s) this behavior remain(s) relatively unknown, and most experimental studies focus on quantifying steady-state rheology. Recent studies have suggested that back stresses caused by long-range elastic interactions among dislocations could play a role in transient deformation of olivine. Wallis et al. (2017) identified an internal back stress in olivine single crystals deforming at 1573 K, which gave rise to anelastic transient deformation in stress dip experiments. Hansen et al. (2019) quantified the room-temperature strain hardening of olivine deforming by low-temperature plasticity and measured a back stress that gave rise to a Bauschinger effect, a well-known phenomenon in materials science wherein the yield stress is reduced upon reversing the sense of direction of the deformation.

To explore deformation at very high dislocation density, we have developed a novel nanoindentation load drop method to measure the back stress in a material at sub-micron length scales. Using a self-similar Berkovich tip, we measure back stresses in single crystals of olivine, quartz, and plagioclase feldspar at a range of indentation depths from 100–1700 nm, corresponding to geometrically necessary dislocation (GND) densities of order 10¹⁴–10¹⁵ m^-2. Our results reveal a power-law relationship between back stress and GND density with an exponent ranging from 0.44-0.55 for each material, with an average across all materials of 0.48. Normalizing back stress by the shear modulus measured during the indentation test results in a master curve with a power-law exponent of 0.44, in close agreement with the theoretical prediction (0.5) derived from the classical Taylor hardening equation (Taylor, 1934). For olivine, the extrapolation of our fit quantitatively agrees with other published data spanning over 5 orders of magnitude in GND density and temperatures ranging from 298-1573 K. This work provides the first experimental evidence in support of Taylor hardening in a geologic material, supports the assertion that strain hardening is an athermal process that can occur during high-temperature creep, and suggests that back stresses from long-range interactions among dislocations must be considered in rheological models of transient creep.

How to cite: Thom, C., Goldsby, D., Kumamoto, K., and Hansen, L.: When minerals fight back: The relationship between back stress and geometrically necessary dislocation density, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2773, https://doi.org/10.5194/egusphere-egu2020-2773, 2020.

Many regions of the Earth’s mantle deform in grain size-sensitive creep regimes. The grain size right below the transition zone is believed to be very small, and the grain size should in subsequent depths be mainly controlled by normal grain growth. The grain size evolution is commonly predicted using either existing grain growth laws in combination with grain boundary diffusion coefficients or by extrapolating empirically determined grain growth laws. Effects of Zenner pinning and different ratios of second phases have been studied, while the role of anisotropic grain boundary properties is currently neglected¹.  The grain boundary energy varies with the orientation of the grain boundary plane, as expressed through the typical crystal habitae (Wulff-shapes). Individual crystals in a polycrystalline material maintain a grain boundary energy anisotropy during grain growth. Here we study how grain boundary anisotropy impacts grain boundary migration and normal grain growth rates by three-dimensional phase-field simulations². We imply grain boundary energy minimization by faceting/varying the grain boundary plane to minimize the grain boundary energy. The ideal grain boundary energy anisotropy for the solid-solid interface is taken from experimentally investigated grain boundary plane distributions and grain boundary energy distributions on periclase (MgO). We compare the grain size evolution in simulations with isotropic and anisotropic grain boundary energy of cubic crystal symmetry. We found that the grain boundary energy anisotropy has a significant influence on grain boundary migration and grain growth kinetics².

<r>²- <r₀>²= kt

The change in grain size is given as variation between the initial and final average grain radius, r. The time is t, and a material-specific parameter that accounts for the grain boundary energy anisotropy is extracted from the simulations as

k = A·µ_gb·σ_gb

Where, the grain boundary energy, σ_gb varies with orientation, and the grain boundary mobility, µ_gb assumed to be isotropic. We found that the rate of grain growth for periclase, A is a factor of 3 smaller compared to an isotropic material.

These results are of three-fold importance:

A better prediction of grain size evolution will need to develop an anisotropic theory for grain growth, that address our lake in knowledge regarding pressure effects on both grain boundary energy anisotropy and diffusion.

1. Rohrer, G. S. Annu. Rev. Mater. Res. 2005

2. Salama et al. Acta Materialia 2020

How to cite: Salama, H., Marquardt, K., Kundin, J., Shchyglo, O., and Steinbach, I.: The role of grain boundary energy anisotropy on the grain size evolution during normal grain growth., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21694, https://doi.org/10.5194/egusphere-egu2020-21694, 2020.

A relocation of mica grains in marbles can be observed as trace of newly precipitated calcite material in cathodoluminescence microscopy analysis. Mica grains relocate by rotation and/or translation from foliation parallel to new irregular orientations. The mica grains can be either located at calcite grain boundaries or within large calcite grains.

The process erases deformed, inclusion-rich calcite material and creates undeformed and mostly inclusion-free grains and can therefore be regarded as postdeformational. Not every mica present relocates and the choice of whether a specific mica relocates cannot be related to a specific primary orientation. Furthermore, no significant difference in composition between relocated and non-relocated mica grains can be observed. Newly precipitated calcite has less Mg than the dissolved grain material.

The precipitation of new calcite material at the calcite-mica interface is supposed to be the initial trigger leading to dissolution of inclusion-rich, deformed calcite material at the opposite side of the mica grain. The newly precipitated calcite material inherits the already existing calcite grain‘s crystallographic orientation.

Assuming this process occurs to a larger extent in a material, it might modify a deformation-related microfabric. Therefore, an interpretation in terms of deformation conditions should be done carefully, considering postdeformational dissolution-precipitation processes.

How to cite: Kühn, R., Duschl, F., Leiss, B., and Schulze, T.: Relocated micas in marble – indicators for postdeformational microfabric modification, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13710, https://doi.org/10.5194/egusphere-egu2020-13710, 2020.

A grain boundary can move under stress by a mechanism called shear coupled grain boundary migration (SC GBM) and contribute to strain. SC GBM is considered to be a general property of all grain boundaries over a wide range of misorientation angles, although higher deformation temperatures favour grain boundary sliding. Apart from a structured boundary interface, SC also requires a critical shear stress. We examine evidence for SC GBM in ice. An extensive literature study showed that SC GBM of high angle boundaries does occur in ice bicrystals that were probably deformed under conditions close to those found in nature. We conclude that SC GBM is likely to be an important deformation mechanism for geological materials, where extensive GBM occurs and also in nano sized materials, such as fault gauges.

How to cite: Pennock, G. and Drury, M.: Shear coupled grain boundary migration as a deformation mechanism in minerals., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7681, https://doi.org/10.5194/egusphere-egu2020-7681, 2020.

How to cite: Stoll, N., Weikusat, I., Kerch, J., Eichler, J., Shigeyama, W., Homma, T., Jansen, D., Franke, S., Kuiper, E.-J., Wallis, D., Westhoff, J., Saruya, T., Faria, S. H., Kipfstuhl, S., Azuma, K. G., Azuma, N., and Dahl-Jensen, D.: The upper 2121 m at EastGRIP - Results from physical properties of NEGIS , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-871, https://doi.org/10.5194/egusphere-egu2020-871, 2020.

Small grain size and a well-mixed phase assemblage are key features of upper mantle (ultra)mylonitic layers. In those layers, Zener pinning inhibits grain growth, which could lead to diffusion creep. This increases the strain rate for a given stress significantly. Prerequisite is phase mixing which can occur by dynamic recrystallization (dynRXS) plus grain boundary sliding (GBS), metamorphic or melt/fluid-rock reactions, creep cavitation plus nucleation, or by a combination of those processes. In order to get insights into the interplay of phase mixing and dynRXS we investigate microfabrics (EBSD, optical microscopy) displaying the transition from clasts to mixed assemblages. Samples are taken from the Lanzo peridotite shear zone (Italy).

Olivine dynamically recrystallizes from protomylonitic to ultramylonitic samples. Its grain size varies systematically between monomineralic (~20µm) and polymineralic layers (~10µm). Olivine is the dominant mixing phase for both, dynamically recrystallizing orthopyroxene (ol~55vol.%) and clinopyroxene clasts (ol~45vol.%). In contrast, recrystallizing olivine clasts show little evidence of phase mixing. In phase mixtures, olivine neoblasts show weak (J-index ~1.8) C-Type and weak (J-Index ~1.5) B-type CPO’s. Both types suggest the presence of water during deformation.

Isolated, equiaxial orthopyroxene clasts are present in all samples. DynRXS of opx starts in mylonites. Some clasts and tips of extensively elongated opx bands (max. axial ratios 1:50) are bordered by fine-grained (min. ECD~5µm) mixtures of olivine, opx ± anorthite/ cpx/ pargasite. Mixing intensities seem to depend on the connection to the olivine-rich matrix. Clast grain boundaries are highly lobate with indentations of secondary phases (mostly olivine). Opx neoblasts have no internal deformation and show large misorientations close to their host clast (misorientation angle >45° at ~20µm distance). Their grain shape is either flat and elongated or equiaxial. Both shapes have lobate boundaries. Their CPO depends on the host clast orientation. In ultramylonites, opx bands disappeared completely.

Clinopyroxene porphyroclasts dynamically recrystallize in protomylonite to ultramylonite samples. Olivine is the dominant mixing phase (~45vol.%). Cpx mixed area grain sizes tend to be coarser (~10µm) than in corresponding opx areas (~6µm). Ultramylonitic cpx-ol assemblages have a higher mixing percentage (phase boundaries/grain boundaries ~70%) than mylonitic assemblages (~40%). In the mylonitic layers, clusters of cpx neoblasts form ‘walls’ parallel to their host grain borders. Olivine neoblasts between these clusters show no CPO. The overall cpx CPO varies from [001] perpendicular and [010] parallel to the foliation with (J -Index ~2.5) to [100] perpendicular and [001] parallel to the foliation (J-Index ~1.2).

Beside few thoroughly mixed areas, bands of cpx+ol and of opx+ol are still distinguishable in ultramylonitic layers. This suggests their origin to be dynamically recrystallized opx and cpx clasts. Therefore, phase mixing is assumed to occur simultaneously to clast recrystallization. Beside a small gradient of opx/cpx abundance depending on the distance from their host clast there is little evidence for phase mixing by dynRXS+GBS only. High abundances of olivine neoblasts at grain boundaries of recrystallizing clasts and their instant mixed assemblage with host phase neoblasts suggest phase mixing being strongly dependent on olivine nucleation during dynRXS of opx and cpx.

How to cite: Tholen, S. and Linckens, J.: Phase mixing in upper mantle shear zones: Olivine nucleation during dynamic recrystallization of orthopyroxene and clinopyroxene porphyroclasts, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4983, https://doi.org/10.5194/egusphere-egu2020-4983, 2020.

Diffusion creep and the wet low temperature version, pressure solution, are major deformation mechanisms in the Earth. Pressure solution operates in many metamorphosing systems in the crust and may contribute to slow creep on fault surfaces. Diffusion creep prevails in areas of the upper mantle deforming slowly, and possibly in most of the lower mantle. Both mechanisms contribute to localisation since small grain sizes can deform faster.

However, there has been limited attention paid to the evolution of microstructure during diffusion creep. In some experiments grains coarsen; in some but not all experiments grains remain rather equant. We have developed a grain-scale numerical model for diffusion creep, which indicates that those processes are very important in influencing evolving strength. Our models illustrate three behaviours.

1. Strain localises along slip surfaces formed by aligned grain boundaries on all scales. This affects overall strength.
2. Diffusion creep is predicted to produce elongate grains and then the overall aggregate has intense mechanical anisotropy. Thus strength during diffusion creep, and localisation on weak zones, is influenced not just by grain size but by other aspects of microstructure.
3. Grain coarsening increases grain size and strength. Our most recent work shows how it interacts with ongoing deformation. In particular grain growth can lead to particular grain shapes which are directly related to strain rate, and influence strength. Consequently, understanding localisation during diffusion creep must encompass the effects of diffusion itself, grain boundary sliding and grain coarsening.

How to cite: Wheeler, J., Evans, L., Gardner, R., and Piazolo, S.: Strain localisation during diffusion creep: influence of grain coarsening and grain boundary sliding, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9953, https://doi.org/10.5194/egusphere-egu2020-9953, 2020.

About 50 years ago, John Ramsay and colleagues established the thorough foundation for the field-scale observational and mathematical description of the structures, deformation, and kinematics in ductile shear zones. Since then, these probably most important instabilities of the ductile lithosphere enjoyed an almost explosive growth in scientific attention. It is perhaps fair to say that this tremendous research effort featured four main themes:

[1] The historic scientific nucleus – quantification of shear-zone geometry, strain and associated kinematic history from field observations

[2] Qualitative and quantitative analysis of microphysical deformation mechanisms in the field and the laboratory

[3] Shear-zone rheology

[4] The development of physically consistent mathematical models for shear zones, mainly using continuum mechanics.

In concert, these four cornerstones of shear-zone research enabled tremendous progress in our understanding of why and how ductile shear zones form. So, what are some of the outstanding problems?

A truly comprehensive model for ductile shear zones must account for the vast range of length and time scales involved, each easily covering ten orders of magnitude, as well as the associated intimate coupling between thermal, hydraulic, mechanical, and chemical processes. The multi-scale and multi-physics nature of ductile shear zones generates scientific challenges for all four research themes named above. This presentation is dedicated to highlighting exciting challenges in themes 2, and 3 and 4.

In the microanalytical arena [2], the nano-scale is an exciting new frontier, especially when it comes to the interplay between metamorphism and ductile deformation. The nano-frontier can be tackled with new synchrotron methods. I showcase some applications to fossil shear-zone samples and discuss opportunities for in-situ experiments. In the domain of rheology [3], I present some simple experiments with strain-softening materials and field observations that support the notion: transient rheological behaviour is very important for shear localisation. In the modelling domain [4], some recent examples for the intriguing physical consequences predicted by new multi-physics and cross-scale coupling terms in ductile localisation problems are illustrated.

How to cite: Schrank, C.: Ductile shear zones – future perspectives, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4273, https://doi.org/10.5194/egusphere-egu2020-4273, 2020.

We revisit large shear strain deformation experiments on Carrara marble and observe that anisotropic porous domains develop spontaneously during shearing. Specifically, as samples are deformed periodic porous sheets are documented to emerge and are found to transfer mass. These results imply that viscous shear zones may naturally partition fluids into highly anisotropic bands. As this hydro-mechanical anisotropy is produced by creep, each porous sheet is interpreted to represent a transient dynamic pathway for fluid transport. It is unclear how long each porous domain is uniquely sustained but it is clear that sheets are persistently present with increasing strain. Our results forward the idea that viscous shear zones have dynamic transport properties that are not related to fracturing or chemical reaction. We believe these new results provide experimental foundation for changing the paradigm of viscosity in rocks to include dynamic permeability. In our view making this change in perspective could alter many classical interpretations in natural banded mylonite zones, for example shear zone parallel syn-kinematic veining may be the result of pore sheet instability and ductile fracturing.

How to cite: Gilgannon, J., Waldvogel, M., Poulet, T., Fusseis, F., Berger, A., Barnhoorn, A., and Herwegh, M.: Evidence that viscous shear zones spontaneously establish hydro-mechanical anisotropy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5250, https://doi.org/10.5194/egusphere-egu2020-5250, 2020.

Present exposure of the ductile Caledonian retrowedge in northwestern Scotland records the evolution of a shear zone that was exhuming while actively deforming, providing a natural laboratory to study strain localization in a progressively cooling system. Examination of rocks from two detailed transects across this region consistently show a transition from microstructures that are dominated by interconnected phyllosilicate networks in a quartz-rich matrix with feldspar porphyroclasts, to interconnected fine-grained regions of mixed quartz + phyllosilicate + feldspar. These polyphase regions are demonstrably weaker than surrounding quartz layers and likely deform by grain-size sensitive mechanisms including diffusion-accommodated grain boundary sliding.

Microstructures characterized by a quartz-rich matrix and interconnected phyllosilicates undergo quartz recrystallization by high temperature grain boundary migration and are dominated by prism a slip. In contrast, fine-grained polyphase microstructures record quartz recrystallization dominated by subgrain rotation and activation of rhomb a and basal a slip systems. We propose transient hardening occurs in quartz-dominated regions as quartz with a strong Y-axis maximum undergoes the switch from prism a easy slip to basal a easy slip during cooling, and thus partitions strain into interconnected phyllosilicate layers. In response, interconnected phyllosilicate layers undergo mechanical comminution, becoming increasingly mixed by grain-size sensitive creep processes to form polyphase layers as they accommodate an increased proportion of strain. This transition from quartz-rich matrix with phyllosilicate interconnected weak layers to fine-grained, polyphase weak layers could be of first-order importance in strain localization within polyphase mylonitic and ultramylonitic rocks.

How to cite: Lusk, A. and Platt, J.: Development of interconnected fine-grained polyphase networks during progressive exhumation of a shear zone, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12715, https://doi.org/10.5194/egusphere-egu2020-12715, 2020.

The area of Cap de Creus, at the eastern termination of the Axial Zone of the Pyrenean Belt, exposes some of the most famous outcrops of ductile shear zones and shear zone networks (Carreras, 2001). Recent studies proposed that the nucleation and growth of such shear zones may have taken place by brittle processes (Fusseis et al., 2006; Fusseis and Handy, 2008).

The present study investigates the geometrical relationships between fracture systems and some shear zones, the deformation temperature of these shear zones, and the processes leading to the nucleation and growth of shear zones along fracture planes. We selected two areas of the Cap de Creus, the Cala d’Agulles, and the Punta de Cap de Creus, because they are most intensely dissected by subparallel sets of shear zones and fractures. The orientation of the average shear zone planes is sub-parallel to the orientation of the major set of fractures, and the great extent and close spacing of some shear zones that we characterized by aerial photos from a drone, is similar to the distribution and extent of the fracture planes. These observations, in addition to those of Fusseis et al. (2006) suggest that the shear zones nucleated on previous fracture planes. 

These fractures are surrounded by haloes of nearly 1 cm thickness affecting the fabric of the country rock, an amphibolite-facies, biotite-andalusite bearing schist. Microscopic observations show that the haloes correspond to the wide-spread presence of thin (less than 2µm thickness) phosphate seams coating the grain boundaries, preferentially those oriented at low angle to the fracture plane, and to the alteration of plagioclase to white mica and sericite, and to the growth of tourmaline, also related to grain boundaries and micro-fractures.

Deformation temperature in the shear zones is assessed by white mica thermometry and pseudosections. The calculated T of at least 350-400° C is consistent with qualitative observations showing the presence of stable biotite within very fine-grained (<< 10 µm) shear bands and the recrystallization of quartz by rotation of sub-grain boundaries.

In summary, fractures formed at high temperature, possibly associated with the intrusion of tourmaline-bearing pegmatites and fluids, which predate the ductile mylonitic event (Druguet, 2001; Van Lichtervelde et al., 2017). Fluids altered and weakened a volume of approximately 2 cm thickness all along the fracture planes, whose extent may reach > 100 m. The inferred, relatively high T of ca.  400° C indicates that fracturing is not due to the proximity of the brittle-ductile transition. In addition, no significant micro-fracturing of the mylonites is observed in thin sections. Therefore, fracturing precedes the ductile shear zones, which nucleate on some of the “inherited” sets of thin, planar, weakened structures, the large majority of which remains undeformed. These observations raise the question on whether nucleation and propagation of ductile shear zones is mechanically unrelated to brittle fracturing. Their weakening of planar structures would originate from fluid migration along fracture planes, but fracturing would no longer be active during ductile deformation.

How to cite: Rosenberg, C., Labrousse, L., Landry, N., Druguet, E., and Carreras, J.: On the causes of brittle nucleation of shear zones: the example of Cap de Creus, Eastern Pyrenees (Spain), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19297, https://doi.org/10.5194/egusphere-egu2020-19297, 2020.

Multi-layered stratigraphic sequences present ample opportunity for the study of strain localization and its complexities. By constraining mechanisms of crustal weakening, it is possible to gain a sounder understanding of the dynamic evolution of the Earth’s crust, especially when applied to realistic, field-based scenarios. One such mechanism is that of strain-related carbon ordering. This is the process whereby the amorphous nanostructure of fossilized organic matter contained within the rock is progressively organized towards a more sheet-like structure, similar to that of graphite. One common method of studying this process is through Raman spectroscopy. This is a non-destructive tool which makes use of the relative positions and intensities of two key spectral peaks, where one peak represents graphitic carbon and the other disordered (or amorphous) carbon. The intensity ratio between these two peaks suggests the degree to which the carbon has progressed from its original kerogen-like structure towards that of graphite. This progression can be due to increasing temperature or increasing strain, and until now, these two contributory factors have been difficult to separate, particularly in field examples.

Previous field-based studies have focused on carbon ordering on fault planes, while experimental studies have monitored the effects of strain-related ordering in organic carbon on both fault surfaces and more distributed shear zones. These studies confirmed the occurrence of strain-related ordering at seismic rates, particularly in the form of graphitization of carbon. However, these experiments showed the effects of strain-related ordering at aseismic rates to be limited when distributed shear zones were considered, in part due to the geological timescales required to emulate true conditions.

In this study, Raman spectroscopy is used to compare the relative nanostructural order of organic carbon within a recumbent isoclinal fold formed of interbedded limestones and marls. The central, overturned fold limb forms a 170m wide, 1km long aseismic shear zone, with evidence of increased strain recorded in calcite grains relative to the upper and lower limbs. Raman spectroscopy intensity ratios (I[d]/I[g]) are compared across the fold, showing a marked 23% decrease in the overturned limb. Such a decrease in I[d]/I[g] suggests increased carbon ordering within the overturned limb, which in combination with evidence for increased strain in calcite, suggests that the carbon ordering here is derived directly from strain-related ordering. This has important implications. We infer, from previous studies, that strain-related carbon ordering encourages further strain partitioning in carbonaceous material, and may enhance zones of weakness in the rock. This ordering in aseismic shear zones has so far been unreported in nature, and so our field-based results are significant in supporting previous experimental evidence for this phenomenon. Our results also have implications for understanding dynamic crustal evolution, and will play an important role in the development of Raman thermobarometry, especially since current methods do not distinguish between strain-related and temperature-related ordering.

How to cite: Kedar, L., Bond, C., and Muirhead, D.: Carbon ordering in an aseismic shear zone: implications for crustal weakening and Raman spectroscopy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9155, https://doi.org/10.5194/egusphere-egu2020-9155, 2020.

Magmatic flows are rarely, if ever, entirely free of crystals. If these crystals distribute in an approximately homogeneous way, their impact on flow can be captured by defining a suitable effective viscosity for the suspension. A spatially heterogeneous crystal distribution, however, can build up to the degree that the flow behavior of the crystal-bearing magma becomes substantially different from that of a pure melt. One example is the transition from flow to sliding, in which the deformation in the crystalline magma is concentrated almost entirely in a thin interfacial layer as opposed to being distributed in a typical flow profile throughout the domain. The transition is particularly consequential for the large-scale dynamics of the system, because it can be associated with transport rates increasing by orders of magnitudes.

Most conduit models associate the flow-to-sliding transition with a critical crystal fraction, often in the 60% range. Here, we hypothesize that the flow to sliding transition can occur at crystal fraction as low as a few percents under certain conditions. We test our hypothesis by numerically reproducing existing laboratory measurements of the effective viscosity of plagioclase-bearing basalt in a rotational viscometer. We utilize a direct numerical method to resolve the interactions between the crystals and the magmatic melt at the scale of individual interfaces in 2D. Our numerical approach only requires assumptions about the pure phase including the crystal fraction and crystal shape. All phase interactions and their aggregate effect on the flow emerge self-consistently from the simulation itself. 

Our simulations suggest that the behavior of multiphase suspensions at low fluid Reynolds number is highly variable and depends sensitively on the characteristics of the immersed phases and the geometry of the flow domain. We show that there is no meaningful dilute limit in which the phase interactions can be neglected or captured by adjusting the effective rheology of the suspension in a way that removes dependencies on the properties of the immersed phase. Since our models operate at the scale of individual crystals, our model results are testable in both field and laboratory settings. In fact, they suggest that observations of microstructure provide valuable constraints on the large scale flow dynamics at the time. Particularly important is the degree of preferential crystal alignment and the existence of force chains or crystal clusters.

How to cite: Qin, Z. and Suckale, J.: The flow-to-sliding transition in crystalline magma, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13110, https://doi.org/10.5194/egusphere-egu2020-13110, 2020.

Deformation localisation in rocks can lead to a variety of structures, such as shear zones and shear bands that can range from grain to crustal scale, from discrete and isolated zones to anastomosing networks. The heterogeneous strain field can furthermore result in a wide range of highly diverse fold geometries.

We present a series of numerical simulations of the simple-shear deformation of an intrinsically anisotropic non-linear viscous material with a single maximum crystal preferred orientation (CPO) in dextral simple shear. We use the Viscoplastic Full-Field Transform (VPFFT) crystal plasticity code (e.g. Lebensohn & Rollett, 2020) coupled with the modelling platform ELLE (http://elle.ws) to achieve very high strains. The VPFFT-approach simulates viscoplastic deformation by dislocation glide, taking into account the different available slip systems and their critical resolved shear stresses. The approach is well suited for strongly non-linear anisotropic materials (de Riese et al., 2019). We vary the anisotropic behaviour of the material from isotropic to highly anisotropic (according to the relative critical resolved shear stress required to activate the different slip systems), as well as the orientation of the initial single maximum orientation, which we vary from parallel to perpendicular to the shear plane. To visualize deformation structures, we use passive markers, for which we also systematically vary the initial orientation.

At relatively low strains the amount of strain rate localisation and resulting deformation structures highly depend on the initial single maximum orientation in the material in all anisotropic models. Three regimes can be recognised: distributed shear localisation, synthetic shear bands and antithetic shear bands. However, at very high strains localisation behaviour always tends to converge to a similar state, independent of the initial orientation of the anisotropy.

In rocks, shear localisation is often detected by the deflection and/or folding of layers, which may be parallel to the anisotropy (e.g. cleavage formed by aligned mica), or by deflection/deformation of passive layering, such as original sedimentary layers. The resulting fold patterns vary strongly, depending on the original orientation of layering relative to the deformation field. This can even result in misleading structures that seem to indicate the opposite sense of shear. Most distinct deformation structures tend to form when the layering is originally parallel to the shear plane.

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. J. Struct. Geol. 124, 81-90.

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

How to cite: de Riese, T., Bons, P. D., Gomez-Rivas, E., Griera, A., Llorens, M.-G., and Weikusat, I.: Influence of initial preferred orientations on strain localisation and fold patterns in non-linear viscous anisotropic materials , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13160, https://doi.org/10.5194/egusphere-egu2020-13160, 2020.

Cost-effective and environmentally sensitive shale gas production requires detailed knowledge of the petrophysical characteristics of the shale from which the gas is extracted. Parameters such as the kerogen fraction, pore size distributions, porosity, permeability, the frackability of the rock and the degree to which natural fracturing already occurs are required in order to be able to estimate potential gas reserves and how easily it can be extracted. Innovative imaging techniques, including Focused Ion Beam Scanning Electron Microscopy (FIB-SEM) and Nanoscale X-Ray Tomography (nano-CT), can be used to characterise the microstructural properties of shale. Here we report using FIB-SEM serial sectioning and nano-CT on approximately cubic samples of side length about 25 µm. The resolution of the FIB-SEM scanning is approximately 20 nm, while that of the nano-CT is about 50 nm, providing between 125 and 1953 million voxels per scan. These ultra-high resolution techniques have been shown to be effective methods for the analysis and imaging of shale microstructure. Each technique can provide data over a different and separate range of scales, and with different resolutions. Results analysed so far indicate that pores which seem to be unconnected when imaged on a micrometre scale by micro-CT scanning, are connected by thin pathways when imaged at these higher resolutions. This nano-scale connectivity is responsible for the small but non-zero permeability of gas shales to gas flow, which is typically measured in the range 5 nD – 200 nD. The volume, size, aspect ratios, surface area to volume ratio and orientations have all been calculated from the scanned data as a function of scale. These data indicate an extremely complex, heterogeneous, anisotropic and multimodal pore nanostructure and microstructure for the shales, with structure at all scales contributing to both gas storage and gas flow. Further work analysing the connectivity of the microscale and nanoscale pore spaces within the rock is underway. We believe that the combination of nano-CT with FIB-SEM on the same sample has the potential for providing an enhanced understanding of shale microstructure, which is necessary for modelling elastic behaviour, gas storage, gas desorption and gas flow in gas shales.

Keywords: Gas shale, FIB-SEM, nano-CT, porosity, permeability, Kerogen, pore volume, size distribution, pore aspect ratio and surface area to pore volume.

How to cite: Garum, M., Glover, P., Lorinczi, P., and Hassanpour, A.: Ultrahigh resolution 3D imaging and characterisation of nanoscale pore structure in shales and its control on gas transport , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-45, https://doi.org/10.5194/egusphere-egu2020-45, 2020.

Hydrocarbon exploration is extending from the shallowly buried to deeply buried strata with increasing demands for fossil fuels. The variable storage and percolation capacities that intrinsically depend on the pore geometry restrict the hydrocarbon recovery and displacement efficiency and trigger studies on the micro-scale pore structure, fluid flow capacity, and their controlling factors. Minerals within sandstone are the results of the coupling control of depositional factors and diagenetic alternations, which determine the microscopic pore geometry and subsequently affect the fluid flow capacity. In order to investigate the impacts of mineralogy on the pore structure and fluid flow capacity, integrated analyses including porosity and permeability measurements, casting thin section (CTS), scanning electron microscopy (SEM), pressure-controlled mercury porosimetry (PCP), rate-controlled mercury porosimetry (RCP), nuclear magnetic resonance (NMR), and X-ray diffraction (XRD) are conducted on the deeply buried sandstone samples in the Jurassic Sangonghe Formation of the Junggar Basin. Microscopic pore structure is characterized by the combination of SEM, CTS, PCP, and RCP and fractal theory. Fluid flow capacity is evaluated by the innovative application of film bound water model in NMR and mineralogy is quantitatively measured by XRD. The results indicate that the deeply buried sandstone is rich in quartz (54.2%), feldspar (25.1%), and clay (14.2%), with dominant kaolinite (5.04%) and chlorite (5.38%) cementation. The reservoir has a wide pore-throat diameter distribution with three peaks in the ranges 0.01–1, 10–80, and 200–1000 μm. Pores are tri-fractal and can be divided into micropores, mesopores, and macropores, with average porosity contributions of 50.11, 21.83, and 28.04%, respectively. The movable porosity of deeply buried sandstone ranges from 1.75 to 8.24%, primarily contributed by intergranular (avg. 2.34%) and intragranular pores (avg. 2.56%). Most of the fluids are movable in intergranular pores but are irreducible in intragranular pores. Correlation analyses between mineralogy and pore structure suggest that quartz provides preservation to intergranular porosity, which increases pore size and macropores porosity and reduces heterogeneity of the pore system. The influence of feldspar reverses and becomes poor owing to the simultaneous clay precipitation and complex roles of feldspar dissolution in microporosity. Chlorite, kaolinite, and illite, all act as destructions to intergranular porosity. They enhance the mesopores and micropores porosities, reduce the pore size, and increase the microscopic heterogeneities of the macropores, micropores, and whole pore system. The relationships between mineralogy and fluid flow capacity indicate that quartz is favorable for the fluid flow capacity, but feldspar and clay play negative roles. The reversed impacts of quartz and feldspar lay in their opposite controls on pore size. However, both pore size and hydrophilia should be taken into account when considering the effects of clay minerals. These negative effects are associated with types, contents, and hydrophilic degrees of clay minerals, in which I/S and illite exhibit the strongest negative impacts. The fluid flow in the intergranular and intragranular pores is generally enhanced by higher quartz content, but reduced by higher clay content. Irreducible fluids in the intergranular and intragranular pores are determined by chlorite and kaolinite contents, respectively.

How to cite: Qiao, J., Zeng, J., and Feng, X.: Impacts of mineralogy on micro-scale pore structure and fluid flow capacity of deeply buried sandstone reservoirs, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1516, https://doi.org/10.5194/egusphere-egu2020-1516, 2020.

The coupling between fluid transport, chemical reactions, and deformation constitutes one of the frontiers of geoscientific research. From an analytical perspective, a fundamental challenge is posed by the fact that sub-nanometre- to micrometre-scale structures play a vital role in the macroscopic (centimetre- to metre-scale) response of deforming, reacting, fluid-bearing rocks. Sample analysis with conventional laboratory techniques quickly becomes prohibitively expensive and laborious when more than four orders of magnitude in length scales need to be resolved. This issue is particularly challenging in very fine-grained rocks such as mylonites and shales.

Here, we investigate calcite-vein-bearing shales to illustrate how synchrotron X-ray fluorescence microscopy, ptychography, and small- and wide-angle transmission scattering can be used for the quantitative multi-scale analysis of micro- and nano-textures in rock. These analytical techniques are applied to thin sections or thin rock slabs on the centimetre-scale and provide information on length scales from hundreds of micrometres down to angstroms. Therefore, the considered array of synchrotron techniques covers up to eight orders of magnitude in length scale in terms of chemical and structural information. In our case study, we demonstrate how this suite of analytical techniques can be employed to reveal, for example, the relative timing of mineralisation events, trace-element chemistry, texture, and the structural width of fluid pathways such as grain boundaries.

How to cite: Schrank, C. E., Jones, M. W. M., Kewish, C. M., and van Riessen, G. A.: Synchrotron multi-modal, multi-scale chemical and structural imaging of vein-bearing shales, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6968, https://doi.org/10.5194/egusphere-egu2020-6968, 2020.

The Late Silurian Moine Thrust Zone (MTZ) of the NW Highlands of Scotland has long been fundamental to the understanding of the nature and processes that occur during thrust tectonics in the upper continental crust. This complex imbrication zone formed during final Scandian stages of the Caledonian orogeny when collision of Baltica and Laurentia led to WNW-ESE tectonic foreshortening of >100km. The MTZ juxtaposed greenschist to amphibolite facies Neoproterozoic metamorphic rocks of the Moine Supergroup over sequences of little metamorphosed Cambro-Ordovician and Neoproterozoic sedimentary rocks and their Neoarchean to Paleoproterozoic gneissic basement in a zone ranging from <1km to >20km wide.

The mechanical problems represented by thrust wedges being transported over such great distances without losing their internal cohesion has highlighted the role played by detachment structures and the need for mechanisms that create significant weakening along basal detachments that allow overthrusts to accommodate major horizontal displacements in the shallow crust. Field studies and use of section balancing techniques have highlighted that a substantial proportion of the displacement seems to be accommodated along detachments that follow specific stratigraphic levels.

Other than the Moine Thrust Mylonites and the mylonitised parts of the Cambrian Quartzites, relatively little is known about the grain scale deformation and potential weakening processes that have occurred in other parts of the MTZ. New lithological descriptions of the fault rocks and sedimentary protoliths observed in the Assynt, Durness and Eriboll areas are presented here and provide detailed microstructural evidence for the long-term weakening mechanisms that were operating at the time. These mechanisms are consistently related to the onset of grain size reduction, triggered by both chemical enhanced and geometric processes. These include feldspar alteration to fine phyllosilicates associated with cataclasis and dynamic recrystallization of quartz.

Pressure solution, evidenced by changes in the shape of minerals along cleavage surfaces and the presence of dissolution seams and caps, is widespread throughout the studied rock sequences. The profuse occurrence of this grain-scale mechanism makes it very likely that syn-deformational fluid-influx lead to the destruction of load bearing microstructural frameworks and the development of interconnected weak layers due to alteration, explaining the occurrence of detachments within impure layers of the predominantly quartzose Pipe Rock and Salterella Grit members. The progressive development of these interconnecting fine-grained weak layers resulting from incongruent diffusive mass transfer is enhanced in the more mineralogically heterogeneous units of the Cambro-Ordovician sedimentary sequence (in particular, Fucoid Beds dolomitic siltstones and Durness limestones) explaining the consistently observed slip localization in these horizons.

How to cite: Guida Barroso, A., Dempsey, E., Holdsworth, B., and De Paola, N.: Weakening mechanisms and the role of easy slip horizons in thrust belt development: a microstructural approach, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21109, https://doi.org/10.5194/egusphere-egu2020-21109, 2020.

Graphitic carbon exhibits a large range of structures and chemical compositions, from amorphous-like compounds to crystalline graphite. The graphitic carbon-bearing rocks are widely occurred in low- to high- grade metamorphic massif and fault zone. The carbonaceous material in the rock will gradually transform from an amorphous into an ordered crystalline structure by thermal metamorphism, which is called graphitization. The degree of graphitization is believed to be a reliable indicator of peak temperature conditions in the metamorphic rock. In many low-grade metamorphic rocks, graphitic carbon (e.g., soot, low-grade coal) is often associated with brittle fault gouge whereas in high-grade metamorphic rocks, graphitic carbon (crystalline granite) are most commonly seen in marble, schist or gneiss. In recent years, graphitic carbon-bearing rocks have been reported from natural fault zones (reviwers paper see Cao and Neubauer 2019 and references therein). The graphitic carbon grains in our samples tend to enrich in slip-surface or micro-shear zone with strain localization in fault, performed as dislocation glide of deformation. The graphite LPO shows slip system in the direction of basil <a> combined basil <a> slip and weak prism <a> slip systems, suggesting a low-temperature to a medium to high temperature deformation conditions, which is in consistent with the results of Raman Spectra of Carbonaceous material (RSCM) thermometry. We also proposed that the graphitic carbon formed in the rocks can significantly affect the mechanical properties of the fault during the process of faulting. This process can effectively cause reaction weakening and strain localization, which is thought to play an important role as solid lubrication in fault weakening.

How to cite: Cao, S., Neubauer, F., and Lv, M.: Graphitic material in fault zones: Implication for strain localization and rheological weakening, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9086, https://doi.org/10.5194/egusphere-egu2020-9086, 2020.

Development of brittle and brittle-ductile shear zones involve partitioning of large shear strains in bands, called C-shear bands (C-SB) nearly parallel to the shear zone boundaries. Our present work aims to provide a comprehensive understanding of the rheological factors in controlling such SB growth in meter scale natural brittle- ductile shear zones observed in in Singbhum and Chotonagpur mobile belts.  The shear zones show C- SB at an angle of 0°- 5° with the shear zone boundary. We used analogue models, based on Coulomb and Viscoplastic rheology to reproduce them in experimental conditions.

These models produce dominantly Riedel (R) shear bands. We show a transition from R-shearing in conjugate to single sets at angles of ~15^o by changing model materials. However, none of the analogue models produced C-SB, as observed in the field. To reconcile the experimental and field findings, numeral models have been used to better constrain the geometrical and rheological parameters. We simulate model shear zones replicating those observed in the field, which display two distinct zones: drag zone where the viscous strains dominate  and the core zone, where both viscous and plastic strains come into play.  Numerical model results suggest the formation of  C- SB for a specific rheological condition. We also show varying shear band patterns as a function of the thickness ratio between drag and core zones.

How to cite: Roy, A., Roy, N., Saha, P., and Mandal, N.: Development of C- shear bands in brittle-ductile shear zones: Insights from analogue and numerical models., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-960, https://doi.org/10.5194/egusphere-egu2020-960, 2020.

Deformation of lithospheric rocks regularly localizes into high-strain shear zones that include fine-grained ultramylonites. Occurring as quasi-straight layers of intimately mixed phases that often describe sharp transitions with the host rock, these structures may channelize fluid flow^[1,2] and could serve as precursors for deep earthquakes^[3]. However, although intensively documented, ultramylonites originate from still unknown processes. Here I focus on a mylonitic complex that includes numerous mantle ultramylonites in the Ronda peridotite (Spain). Among them, I was able to highlight one of their precursors that I better describe as a long and straight grain boundary, along which four-grain junctions are observed with randomly oriented grains of olivine and pyroxenes. This precursor starts from a pyroxene porphyroclast and extends to an incipient, weakly undulated ultramylonite, where intimate phase mixing arises with asymmetrical grain size distribution. While the finer grain size locates on one side, describing a sharp – but continuous – transition with the host rock, the grain size gradually increases towards the other side, giving rise to a smooth transition. All phases have a very weak lattice preferred orientation (LPO) in the ultramylonite, which strongly differs from the host rock where olivine is highly deformed with evidence of high dislocation densities and a strong LPO. Altogether, these features shed light on the origin of mantle ultramylonites that I attribute to a migrating grain boundary, the sliding of which continuously produces new grains by phase nucleation, probably at the favor of transient four-grain junctions. Nucleated grains then grow and progressively detach from the precursor as it keeps on migrating depending on the dislocation densities in the host rock. Although such an unusual grain boundary remains to be understood in terms of source mechanism, these findings provide new constraints on the appearing and development of ultramylonites.

[1] Fusseis, F., Regenauer-Lieb, K., Liu, J., Hough, R. M. & De Carlo, F. Creep cavitation can establish a dynamic granular fluid pump in ductile shear zones. Nature 459: 974–977 (2009)

[2] Précigout, J., Prigent, C., Palasse, L. & Pochon, A. Water pumping in mantle shear zones. Nat. commun. 8: 15736, https://doi.org/10.1038/ncomms15736 (2017)

[3] White, J. C. Paradoxical pseudotachylyte – Fault melt outside the seismogenic zone. J. Struct. Geol. 38: 11-20 (2012)

How to cite: Précigout, J.: On the Origin of Ultramylonites, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3938, https://doi.org/10.5194/egusphere-egu2020-3938, 2020.

Faults that are active at retrograde conditions tend to contain metastable fault rock assemblages that are prone to undergo fluid-consuming reactions. These reactions typically lead to growth of minerals that are viscously and frictionally weaker than the reactants. This is illustrated in the well-studied Outer Hebrides Fault Zone (OHFZ) of Scotland, and we add observations from the Kuckaus Mylonite Zone (KMZ), Namibia. In both locations, deformation is localised in anastomosing networks of phyllosilicates that developed during deformation of amphibolite and/or granulite assemblages at greenschist facies conditions. Microstructures of these phyllonites show generally well aligned phyllosilicates wrapping around fractured feldspars and quartz with features indicating dislocation creep.

In the KMZ, further localization occurred in ultramylonites within the mylonite zone. These are characterised by a similar phyllosilicate proportion to surrounding mylonites, but lack interconnected phyllosilicate networks. Instead, they contain a very fine-grained assemblage of quartz, feldspar, and phyllosilicate, where both quartz and feldspar lack a CPO. We interpret this assemblage as having deformed through grain-size sensitive creep, at lower shear stress than the surrounding mylonite. It is possible that the ultramylonites developed by dismembering an earlier interconnected weak phase microstructure with increasing finite strain, as has been suggested experimentally by Cross and Skemer (2017).

Whereas these exhumed fault zones deformed at greenschist facies conditions, continued activity would exhume similar fault rocks to shallower depth. We explored frictional properties and microstructure of greenschist facies fault rock at low temperature conditions by deforming chlorite-amphibole-epidote assemblages in single-direct shear at room temperature and 10 MPa normal stress under fluid saturated conditions. As inferred at greater depth, presence of chlorite weakens and promotes aseismic creep along these experimental faults. Presence of chlorite also correlates with the development of striations on fault surfaces. Lack of chlorite, on the other hand, leads to velocity-weakening behaviour and, in epidotite, a fault surface containing very fine grains that do not develop when ≥ 50 % chlorite is present. We suggest that chlorite supresses wear at contact asperities between stronger minerals, and therefore also supresses velocity-weakening behaviour.

Overall, we see that growth of retrograde phyllosilicates lead to profound weakening, strain localisation, and frictional stabilisation of major shear zones, from greenschist facies to near-surface conditions. These processes and properties are, however, reliant on external fluids to allow hydration reactions in otherwise relatively dry host rocks. From scattered syn-deformational quartz veins, in the KMZ, such fluids appear to be of surface origin, whereas in the OHFZ, fluids were likely of a deeper, metamorphic or magmatic origin. Ready incorporation of such fluids into retrograde minerals would prevent substantial or widespread fluid overpressures from developing. These fluid sources are similar to present-day inferred fluid regimes in the Alpine and San Andreas Faults, respectively. We speculate that the variable slip behaviour seen on active retrograde faults relate to their degree of retrogression, and the development of time and strain-dependent microstructures with specific strengths and behaviours.

How to cite: Fagereng, A., Stenvall, C., Ikari, M., Diener, J., and Harris, C.: Processes, properties, and microstructures in faults active at retrograde conditions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3508, https://doi.org/10.5194/egusphere-egu2020-3508, 2020.

Quartz veins in poly-metamorphic settings often accommodate the latest deformation state and therefore can provide important information. Identification of microfabric (microstructure and crystallographic preferred orientation, CPO) evolution of quartz during mylonitization, and especially of the grain-scale interplay between brittle and crystal-plastic processes, has different relevant implications: e.g., on understanding the efficiency of fluid mobility through deforming quartz that can dramatically influence the rheology and the degree of chemical exchange. However, in order to interpret the microstructure and the related deformation processes it is necessary to relate these especially to the deformation temperature. Particularly the CPO and the Ti-in-qtz geothermometry is used to constrain the deformation temperature. However, both methods have to be applied with great caution because even when many times used some fundamental processes are not fully understood yet.

Here we present results from deformed quartz veins from the Prijakt Nappe (Autroalpine Unit, Schober Mountains, Central Eastern Alps). These veins localized ductile shear and eventually seismic faulting (recorded by the occurrence of pseudotachylytes) within Eo-Alpine eclogite-facies shists. The veins formed shortly after the eclogitic peak, but the temperature of their deformation remains unconstrained. CL imaging reveals critical details for understanding the role of microfracturing and fluid-rock interaction during initial stages of shear localization, the onset of dynamic recrystallization and the resetting of the Ti-in-quartz geochemistry. Even when optical-light-microscopy and EBSD analysis indicate crystal plastic deformation by subgrain rotation CL and orientation contrast (OC) imaging gives evidence of brittle stage of deformation at least for some of the deformation microstructure. Microshear zones show a bulk dark-CL, but still bright tones in cores of new recrystallized grains similar to the CL signature of the host coarse quartz crystals. CL dark tones also match with the pattern of subgrain boundaries. This reflects fluid permeability pathways along subgrain and grain boundaries (identified by widespread fluid inclusions) and the associated partial resetting of Ti concentrations. The CPO of the new grains within the micro-shear zones rotate with the sense of shear around the kinematic Y-axis and cannot be related to the activity of specific slip systems. In contrast the partial single girdle of c-axis within the ultramylonite with its elongated substructured grains and its characteristic layered microstructure can be related to the activity of several slip systems. Misorientation axis analysis indicates that prism

How to cite: Bestmann, M., Huet, B., Grasemann, B., and Pennacchioni, G.: Microstructural evolution of amphibolite-eclogite facies quartz veins under low greenschist facies deformation condition, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7480, https://doi.org/10.5194/egusphere-egu2020-7480, 2020.

Stress and strain are different physical entities. Do the stress and strain determined from e-twins in a sample of polycrystalline calcite have similar principal orientations and similar shape ratios? Köpping et al. (2019) tackled this question by applying Turner’s (1953) classical method of paleostress analysis to natural data. However, despite the assumption of the method, the orientations of P- and T-axes of an e-twin lamella do not have a one-to-one correspondence with the principal orientations of the stress that formed the lamella. And, the method cannot determine a shape ratio. Another difficulty arises when one tackles the question: Natural calcite has usually been subjected to polyphase tectonics with different stress conditions. One has to separate stresses and to evaluate corresponding strains from a sample. Once lamellae are grouped according to the stresses, the strain achieved by the formation of a group of twin lamellae is easily evaluated by the method of Conel (1962) if the total strain represented by a group is small.

The present authors tackled the question by combining Conel’s strain analysis method with a novel method of paleostress analysis of mechanical twins, which clusters the directional data of e-twins by means of a statistical mixture model and determines stresses for each group of data. And, the appropriate number of stresses is determined by means of Bayesian information criterion. The method also determines the probabilities of each lamella to be formed by the stresses, which are called the memberships of the lamella. The strain achieved under a stress condition can be computed using the memberships. We applied this integrated stress-strain analysis method to Data Sets I and II from two calcite veins in a Miocene forearc basin deposit in central Japan. Since the sampling area was close to a triple-trench junction, the young formation has experienced polyphase tectonics.

As a result, we obtained the consistent stress and strains from both of the data sets. Three stresses were obtained from Data Set I, and the corresponding strains were 0.17, 0.25 and 0.13%. Two stresses were obtained from Data Set II, and the strains were 0.39 and 0.42%. The stress and strain determined from the data sets for each deformation phase were consistent with each other. That is, the principal axes had difference as small as < 20 degrees, and the shape ratios of stress and strain had also similar values. It is not straightforward to generalize this result, but both the stress and strain analyses seem to give appropriate results, providing that polyphase deformations are coped with.

How to cite: Wakamori, K. and Yamaji, A.: The integrated stress-strain analysis of calcite twins: Consistent stress and strain determined from natural data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12819, https://doi.org/10.5194/egusphere-egu2020-12819, 2020.

Strain localisation and fabric development in the lower crust is controlled by the active deformation mechanisms. Understanding the driving forces of such deformation aids in quantifying the stresses and rates of the deformation processes. Here we show that diffusion creep plays a major role in deformation of gabbro lenses at 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 (depths of ∼26-34 km). The Kågen gabbro on south Arnøya is made up of undeformed gabbro lenses with sheared margins wrapping around them. This contribution analyses the evolution of the microstructures and fabric of the low strain gabbro to high strain margins. Microstructural and textural data indicate that preferential crystal growth of amphibole grains in the extension direction has produced the deformation microstructure and the CPO. Dissolution precipitation creep is inferred to be 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: amphibole, garnet and zoisite. 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. Deformation and metamorphic reaction were both important transformation processes during diffusion creep deformation of the margins of the gabbro lenses. The weakening is directly connected to a transformation process that facilitates diffusion creep deformation of strong minerals (pyroxene, garnet, zoisite) at far lower stresses 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., Stunitz, H., Battisti, M. A., and Konopasek, J.: Dissolution precipitation creep as a process for the strain localisation in gabbro, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13523, https://doi.org/10.5194/egusphere-egu2020-13523, 2020.

Microinclusions of Fe-Ti-oxides in rock-forming plagioclase are protected from subsea-floor alterations by the silicate matrix and are stable carriers of the paleomagnetic record of oceanic gabbros. We studied plagioclase-hosted microinclusions in oceanic gabbro (“gabbro 1241” from the Vema Lithospheric section, Mid-Atlantic ridge, Pertsev et al., 2015) with a complex petrogenetic history. Important events in the gabbro evolution caused consecutive transformations of the micro-inclusions and presumably affected their paleomagnetic records.

The earliest generation of the microinclusions was present as ulvospinel and presumably titanomagnetite which were probably formed by sub-solidus oxidation exsolution of early-magmatic plagioclase (An#42-45). An increase of the anorthite component around the early generation of microinclusions to typical values of late-magmatic plagioclase (An#53) suggest involvement of the late-magmatic fluid accompanying residual melt at 800-900°С (Pertsev et al., 2015). Subsequently, the gabbro was locally affected by hydrothermal alteration at about 600°С as a result of interaction of the gabbro with reduced brine containing 20-21% NaCl. The reducing conditions of this process ensured a “non-oxidative” character and primarily cooling driven exsolution of the microinclusions and transformation of the titanomagnetite microinclusions into ulvospinel-magnetite (“Usp-Mt₁”) intergrowths at about 500°С, which is close or higher than the Curie temperature (Tc) of the exsolving titanomagnetite but lower than the Tc of the newly forming Mt_1, and the acquired magnetisation may be referred to chemical remanence. The further evolution of the micro-inclusions correlates with low-temperature hydrothermal alteration induced by inflow of seawater-derived fluids during tectonic unroofing of the lithospheric section (Pertsev et al., 2015). The homogeneous ulvospinel inclusions and ulvospinel of the “Usp-Mt₁”-inclusions were replaced by “Ilm-Mt₂” -aggregates under more oxidizing conditions: 3 Fe₂TiO₄(Ulv) + 0.5 O₂= 3 FeTiO₃(Ilm) + Fe₃O₄(Mt₂). The Mt₁ was more stable to increase of fO₂ that resulted in simultaneous presence of the “Ilm-Mt₂”-, “Mt₁/Ilm-Mt₂“- and “Usp-Mt₁” -inclusions with two generations of magnetic phases (magnetite) within a single plagioclase grains.

Thus, despite of protection by silicate matrix the microinclusions of Fe-Ti-oxides in rock-forming plagioclases evolve under the influence of petrogenetic processes. It is important to note that bulk-rock AF demagnetization of 14 specimens (extracted from “gabbro-1241“) in the interval 100-250 Oersted (Oe) and 300-700 Oe revealed both moderately-grouped (k=27.4) and variable (deviated with angle 40-104°) directional components of magnetisation, which may have resulted from the presence of different generations of magnetite. Further magnetic investigation of separates of plagioclase single grains will allow to evaluate capacities of plagioclase-hosted Fe-Ti-micro-inclusions to save initial and “newfound” paleomagnetic information and to serve as stable sources of paleomagnetic record in regions of mid-oceanic ridges.

Funding by RFBR project 18-55-14003 and FWF project I 3998-N29 is acknowledged.

Pertsev, A. N., Aranovich, L. Y., Prokofiev, V. Y., Bortnikov, N. S., Cipriani, A., Simakin, S. S., & Borisovskiy, S. E. (2015). Signatures of residual melts, magmatic and seawater-derived fluids in oceanic lower-crust gabbro from the Vema lithospheric section, Central Atlantic. Journal of Petrology, 56(6), 1069-1088.

How to cite: Ageeva, O., Pilipenko, O., Pertsev, A., and Abart, R.: Microstructure evolution of plagioclase-hosted Fe-Ti-oxide microinclusions in oceanic gabbro, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8851, https://doi.org/10.5194/egusphere-egu2020-8851, 2020.

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 affect 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). Initial results show that the dominant phases are plagioclase and clinopyroxene[MOU1] , and different samples may have different contents of olivine, enstatite, magnetite, ilmenite, chlorite and amphibole. 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 ~5% in the more isotropic gabbros with weak magmatic fabric to a maximum of ~10% in more mylonitic terms. A similar effect is observed for the S-waves. Destructive interference between plagioclase CPO vs. clinopyroxene/olivine reducing anisotropy is possibly observed. 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. 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 effect 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 for the seismic properties of slow spreading mid-ocean ridges gabbros, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5297, https://doi.org/10.5194/egusphere-egu2020-5297, 2020.

Detachment faults are large offset normal faults that exhume mantle-derived rocks on the seafloor at slow spreading ridges. They are assumed to root at the base of the brittle lithosphere. Magma, if present, can help localize strain at the base of the axial brittle lithosphere by its own presence or crystallization of weaker phases such as plagioclase. This is the case of the Mid-Atlantic Ridge, where ductile shear zones are preferentially formed in and next to magmatic veins (Boschi et al., 2006; Cannat, 1991; Cannat & Casey, 1995; Ceuleneer & Cannat, 1997; Dick et al., 2002; Hansen et al., 2013; Picazo et al., 2012, Schroeder & John, 2004).

Here we focus of a nearly amagmatic case, the eastern part of the Southwest Indian Ridge (SWIR). Partially serpentinized peridotites recovered from dredging on and off axis record variable degrees of a heterogeneous high stress deformation. Orthopyroxene is primarily brittle (kinks, fractures), while olivine displays a wide range of plastic to semi-brittle deformation, ranging from weak to strong with development of extensively recrystallized anastomosing microshear zones. These microshear zones, which are a few mm to cm wide, represent the highest strain localization recorded in these samples. No further evidence of high strain localization, such as high temperature mylonites, has been recovered in this area. The fine-grained microshear zones are preferentially located along orthopyroxene grains or around kinked olivines. Both represent stronger grains (rheological contrast at the grains scale) that produce stress concentrations. Rock-scale thermo-mechanical models using orthopyroxene and olivine flow laws reproduce the observations: ductile shear zones in olivine also initiate preferentially next to brittle orthopyroxene.

We propose that this deformation is linked to the rooting of the detachment faults at depth, where an anastomozed network of microshear zones localizes strain at the base of the lithosphere, allowing the exhumation of variably deformed rocks.

How to cite: Bickert, M., Cannat, M., Tommasi, A., Jammes, S., and Lavier, L.: Strain localization in abyssal peridotites from a magma-starved mid-ocean ridge: a microstructural study, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10312, https://doi.org/10.5194/egusphere-egu2020-10312, 2020.

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 fine-grained mylonitic garnet-amphibole peridotites (Obata and Morten, 1987). Pyroxenites veins and dikes, transposed along the peridotite foliation, show a similar evolution from coarse garnet-free websterites to fine-grained garnet + amphibole 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 2.8 GPa) within the mantle corner flow (Nimis and Morten, 2000).

The newly formed garnet occurs as exsolution within porphyroclastic, high-T pyroxenes, and crystallises along the pyroxenite and peridotite foliation.

Textural evidence and crystallographic orientation data indicate that the transition from spinel- to garnet-facies conditions was assisted by intense shearing and deformation. Pyroxene porphyroclasts in garnet clinopyroxenites show well-developed crystallographic preferred orientation (CPO), high frequency of low-angle misorientations, and non-random distribution of the low-angle misorientation axes. These features indicates that pyroxene porphyroclasts primarily deform by dislocation creep on the (100) [010] slip system. Dislocation creep is accompanied by subgrain rotation recrystallisation, which promotes the formation of new, smaller and equant pyroxene grains around porphyroclasts. The grain size reduction promotes a switch in the deformation mechanism from grain-size insensitive creep (i.e. dislocation creep) in the porphyroclasts to grain-size sensitive (GSS) creep in the small recrystallised grains. The switch from dislocation to GSS creep is accompanied not only by grain size reduction of pyroxenes, but also by the formation of garnet exsolutions in pyroxenes and garnet crystallisation along foliation. We suggest that garnet crystallisation triggers the pinning of the recrystallised matrix, stabilising the fine-grained microtexture for GSS creep process, and finally contributes to the rheological weakening of pyroxenites.

Pyroxenites and peridotites of Ulten Zone thus offer a unique opportunity to investigate the effects of mantle deformation and 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: Zanchetta, S., Menegon, L., Pellegrino, L., Tumiati, S., and Malaspina, N.: Reaction-induced strain localisation in garnet pyroxenites during mantle corner flow: an example from the Ulten Zone (Eastern Alps, N Italy), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19034, https://doi.org/10.5194/egusphere-egu2020-19034, 2020.

The convective motion of Earth’s upper mantle is controlled by two main deformation mechanisms: grain size-insensitive dislocation creep and grain size sensitive diffusion creep. Grain size thus plays a key role in upper mantle deformation, as it has a significant impact on the viscosity of the upper mantle. Moreover, grain size also affects seismic velocities as well as seismic attenuation.

Despite the importance of grain size and its evolution during deformation, there is still a lack of experimental data on grain growth of olivine at upper mantle pressures. For this reason, we here investigate olivine grain growth at pressures ranging from 1 GPa to 12 GPa and temperatures from 1200 to 1400ºC. The experiments were done using piston cylinder and multi-anvil apparatuses. We used as a starting material olivine aggregates with small amounts of pyroxene (<10%) produced via sol-gel method.

Our results indicate that grain growth is reduced at increasing pressures.  This suggests that the enhanced grain growth due to the temperature increase with depth may be offset, thus facilitating a change from dislocation to diffusion creep in the deep upper mantle. This might have an important impact on the dynamics of the upper mantle.

How to cite: Ferreira, F., Thielmann, M., and Marquardt, K.: Pressure dependence of olivine grain growth at upper mantle conditions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18346, https://doi.org/10.5194/egusphere-egu2020-18346, 2020.

Rock deformation experiments are used to compile mechanical data sets for minerals and rocks and to study microstructure and texture development.

The Griggs apparatus, a solid medium piston cylinder machine was designed about 60 years ago to investigate rock deformation mechanisms and rheology at elevated confining pressures. In a typical experiment today, the confining medium is NaCl, with confining pressures up to 3 GPa, temperatures up to 1100°C, and displacement rates between 10^-8 and 10^-2 ms^-1 (equivalent to strain rates of 10^-7 to 10^-3 s^-1). In axial tests, the cylindrical samples are 12 to 15 mm long with a diameter of 0.625 mm. In shearing test, split cylinder assemblies are used with 0.5 to 1 mm thick samples introduced along the 45° pre-cut. Reasonable total strains are limited to 30% axial shortening or shear strains of gamma 4. (Higher strains can be attained but are difficult to analyse mechanically. Unlike for gas rigs, torsion is not available for solid medium machines).

As of now, the operational fleet of solid medium deformation apparatus comprises worldwide over 20 machines in different labs (mainly in Europe, U.S.A. and Japan), providing the scientific community with an ever-growing rheological and microstructural data base.

In view of numerous developments in experimental design, as well as improvements of hardware and software for data acquisition and processing, the experimental community was recently invited to a two-day workshop, hosted by the experimental group of Orléans University. The main goal was to discuss the following points:

- how to further improve the apparatus, increase its scope and improve calibrations; 
- how to further improve data processing, and the precision and reliability of the results;
- how to maintain consistency among the labs and through time (backwards compatibility);
- how ensure compatibility of results from axial and shearing experiments;
- how to make the data available to the community.

The poster represents a condensed report of the meeting highlighting a few issues of special interest to the structural community. Visitors to the poster are invited to share their thoughts and to give us feed-back.

How to cite: Heilbronner, R., Stunitz, H., Précigout, J., and Raimbourg, H.: The solid medium deformation apparatus – reloaded, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9859, https://doi.org/10.5194/egusphere-egu2020-9859, 2020.

The ability of water to enhance plastic deformation of a quartz aggregate has been experimentally demonstrated during the sixties (e.g. Griggs and Blacic 1965), however the processes involved are still questioned. Notably the processes combining the effect of water and pressure during the deformation are still not completely understood. Pressure strongly influences the strength of fine-grained (3.6 - 4.9 µm) wet quartz aggregates (Kronenberg and Tullis 1984), where diffusion creep operates (Fukuda et al. 2018) but its effect on coarser-grained material expected to deform only by dislocation creep is not well constrained. To re-assess the effect of pressure on quartz crystal plastic deformation, natural wet quartzite samples from the Tana quarry in northern Norway (grain size ≈ 150 µm) have been deformed using a Griggs-type apparatus at varying confining pressures (from 0.6 to 2.0 GPa). All the samples with 0.1 wt. % H₂O added were shortened coaxially up to 30% strain at constant strain rate (≈10^-6 s^-1) and temperature (900°C).

All mechanical records show that quartzite flow stresses decrease systematically with increasing pressure. These results allow to determine the strength of quartzite as a function of water fugacity, such as introduced in the flow law by Kohlstedt et al. (1995) to account for both pressure and water effects. In our case, the fugacity coefficient is m≈1 when using a stress exponent of n=2.

Microstructure and image analyses of samples reveal that the bulk strain results mainly from crystal plastic deformation of original grains whereas the recrystallization processes are limited volumetrically (less than 5%) and restricted to the boundaries of original grains. Deformation is not strongly partitioned into recrystallized domains compared to flattened original grains. Optical and SEM-cathodoluminescence images revealed the presence of cracks in conjunction with recrystallization (even for high-pressure samples) and associated chemical/fluid interaction, but the cracks do not contribute significantly to the bulk strain of the samples.

In order to determine the amount of water used for the deformation and the redistribution of H₂O during deformation, the H₂O content of the quartzite has been calculated from FTIR (Fourier Transform InfraRed spectroscopy) measurements for both, grain interiors and grain boundaries. The H₂O concentrations decrease inside grains with the onset of deformation with respect to the starting material. H₂O is primarily stored in the grain boundary region. There is no systematic correlation with pressure. Thus, pressure dependence of H₂O weakening is not restricted to fine-grained materials at high pressure and temperature. Deformation redistributes water from the grain interiors to their grain boundaries.

References:
Fukuda, J., Holyoke III, C.W., and Kronenberg, A.K. (2018). J. Geophysical Res.: Solid Earth, 123(6), 4676-4696.
Griggs, D. T., and Blacic J. D. (1965). Science, 147(3655), 292‑295.
Kohlstedt, D. L., Evans B., and Mackwell S. J. (1995). J. Geophysical Res.: Solid Earth, 100(B9), 17587-17602.
Kronenberg, A. K., and Tullis J. (1984). J. Geophysical Res.: Solid Earth, 89(B6), 4281‑4297.

How to cite: Nègre, L., Stünitz, H., Raimbourg, H., Précigout, J., Jeřábek, P., and Pongrac, P.: Effect of confining pressure on the strength of wet quartzite revisited, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5595, https://doi.org/10.5194/egusphere-egu2020-5595, 2020.

Since quartz is among the most abundant minerals in continental crust and one of the first to show plasticity with increasing pressure and temperature, understanding its mechanical behavior is crucial for estimates on crustal strength and modeling of geodynamic processes. Since discovery of significantly lower mechanical strength of quartz as a consequence of H₂O presence in the crystal (Griggs & Blacic, 1965), remarkable amount of work has been done in order to improve knowledge about processes and mechanisms responsible for so called H₂O weakening effect. As the weakening effect depends on molecular H₂O, it is a disequilibrium weakening process that is difficult to incorporate into existing flaw laws.

In order to estimate mechanical behavior of quartz in presence of H₂O, we performed deformation experiments in solid-medium Griggs-type apparatus in coaxial setting under controlled laboratory conditions using very pure natural quartzite from Tana quarry in northern Norway. Behavior of as-is and 0.1 wt% H₂O added samples was studied in 1) eight shortening experiments at 900 °C, 1 GPa and constant strain rate of 10^-6 s^-1 reaching 5% and 30% strain, 2) six strain rate stepping experiments covering 10^-5, 10^-6 and 10^-7 s^-1, 3) two temperature stepping experiments covering 750, 850 and 950 °C and 4) one hot-pressing experiment maintaining the starting experimental conditions for 14 hours.

There is a negligible strength difference between the as-is and H₂O added samples. Both H₂O added and as-is strain rate steeping experiments had shown surprisingly low stress exponent, with the highest value of 2.26. Temperature stepping experiments gave activation energy values of 177 kJ/mol and 198 kJ/mol. In all studied samples, strain increases towards the sample centers exhibiting grain size decrease from initial 250 – 300 µm. Three principal deformation mechanisms contributing to the bulk strain were identified: 1) crystal plasticity of original grains manifested by flattening, undulatory extinction, and development of subgrains, 2) cracking of original grains demonstrated by fluid inclusion trails and minor grain offset and 3) dynamic recrystallization via subgrain rotation recrystallization indicated by misorientation analysis from EBSD data. FTIR spectroscopy was applied to evaluate H₂O speciation, quantity and distribution. Regardless of added H₂O, most of deformed original grains showed relative H₂O concentration between 0 and 400 H/10⁶Si, implying significant decrease of H₂O content from original 600 to 2000 H/10⁶Si measured in undeformed grains. Average H₂O concentration in grain boundaries showed 750 H/10⁶Si for as-is samples and 1300 H/10⁶Si for H₂O added. Plasticity is most visible in CL-images, as well as higher degree of grain fragmentation and crack density in samples with added H₂O. Ubiquitous presence of fluid along the grain boundaries, demonstrated by FTIR results, may have facilitated sliding along grain boundaries which, in turn, could explain the low stress exponent derived from strain rate stepping experiments.

REFERENCES:

Griggs, D. T. & Blacic, J. D. (1965): Quartz: Anomalous Weakness of Synthetic Crystals. Science 147(3655):292-95.

How to cite: Pongrac, P., Jeřábek, P., Stünitz, H., Raimbourg, H., Nègre, L., and Précigout, J.: Behaviour of wet quartzite: deformation experiments revisited, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13172, https://doi.org/10.5194/egusphere-egu2020-13172, 2020.

To investigate the role of strong and weak secondary phases on the recrystallized grain size of quartz, we performed grain size analyses on quenched samples from general shear experiments on quartz-garnet and quartz-muscovite mixtures. Six general shear experiments were conducted in the Griggs apparatus; three with mixtures of quartz-garnet (vol.% garnet 5, 15, 30) and three with mixtures of quartz-muscovite (vol.% muscovite 5, 10, 25). The starting powders for both set of experiments were synthetic mixtures of quartz-muscovite or quartz-garnet with 0.1 wt.% water added. The quartz-garnet experiments were conducted at 900°C, a pressure of 1.2 GPa, and a shear strain rate of ~10^-5 s^-1, while the quartz-muscovite experiments were conducted at 800°C, a pressure of 1.5 GPa, and a shear strain rate of ~10^-5 s^-1. At these deformation conditions quartz is stronger than muscovite and weaker than garnet. We observed that the bulk strength of the aggregate decreases with a greater volume percent of muscovite and increases with a greater volume percent of garnet. Garnet at these conditions does not deform plastically. The presence of secondary phases within the deforming aggregate causes stress concentrations and partitioning of strain rate between the different phases relative to the measured bulk stress and strain rate. The degree of partitioning is primarily related to the rheology and volume percent of the phases. Due to the piezometric relationship between recrystallized grain size and stress, we can use the quartz recrystallized grain size to determine the local stress of quartz in the experiments and compare it to the measured bulk stress. The results from these analyses will provide new insight into the effect of strain partitioning in general and of strong and weak secondary phases on quartz rheology.

How to cite: Tokle, L., Hirth, G., Morales, L., and Stunitz, H.: The effect of garnet and muscovite on the recrystallized grain size of quartz from general shear experiments , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8179, https://doi.org/10.5194/egusphere-egu2020-8179, 2020.

The quartz recrystallized grain size piezometer was determined by axial shortening experiments on Black Hills quartzite in Griggs type triaxial deformation apparatus (Stipp & Tullis, 2003). The analysis of general shear experiments on Black Hills quartzite (Heilbronner & Kilian, 2017) reveal a striking discrepancy between both experimental  setups; at a given differential stress, recrystallized grains are much larger in the general shear experiments than in axial shortening.
A major difference between both sets of experiments is, that the finite grain volume of quartz in the general shear experiments almost entirely consists of recrystallized grains while the here investigated axial shortening experiments have fractions of recrystallized grains in the range of 15 to 30% in the high stress experiments and up to 60% in the low stress experiments. Quartz in the general shear experiments developed a moderate to strong crystallographic preferred orientation (CPO) while, apart from a Dauphiné-induced ordering of poles to {10-11} and {01-11}, no overall significant CPO developed in the axial shortening experiments.
Based on the analysis of the EBSD data of Cross et al. (2017), we observed that the dispersion axes of large quartz grains in the axial shortening experiments correspond to the global kinematic reference frame. The dispersion axes of the fraction of small, recrystallized grains depend on the local kinematics between the porphyroclasts. Slip transparency indicates that boundaries between the largest grains are rather hard while recrystallized grains can accommodate strain induced by crystal plastic slip more effectively and homogeneously.
These results suggest that recrystallized grains in the axial shortening experiments, at least in those with low fractions of recrystallized grains, correspond to a material deforming at a rate higher than the imposed shortening rate while the axial load is predominantly supported by the porphyroclasts. In contrast, recrystallized grains in general shear experiments deform at the imposed (global) rate and derived stresses correspond on average to the deforming zone. Due to strain and strain rate inhomogeneities in the latter experiments, however, there is a systematic variation in recrystallized grain size across the general shear zone. We compare these local microstructural variations and discuss their significance for the recrystallized grain size piezometer calibration.

How to cite: Kilian, R. and Stipp, M.: Do local kinematics have an effect on the recrystallized grain size piezometer?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14067, https://doi.org/10.5194/egusphere-egu2020-14067, 2020.

Forming in oceanic and continental subduction zones during high to ultra-high-pressure metamorphism, eclogites play an important role in convergent settings. To improve our understanding of eclogite deformation behaviour with respect to its phase abundance of omphacite and garnet, synthetic eclogite samples containing 25, 50 and 75% volume fraction of garnet have been deformed in a modified Griggs-type solid-medium apparatus. Deformation conditions were set to a confining pressure of 2.5 GPa, 900 °C and 3*10^-6 s^-1. Detailed microstructure analysis via optical and electron microscope imaging (SE, BSE and EBSD) served for identification of the dominant active deformation mechanism.
All eclogite samples show a foliation which is defined by a shape preferred orientation (SPO) of omphacite and intercalated foliation sub-parallel garnet aggregates. Another common feature is a weak crystallographic preferred orientation (CPO) of omphacite which is present throughout all samples. In accordance to uniaxial shortening the CPO resembles an s-type texture with a point maximum of [001] axis parallel to the foliation and a maximum of [010] axis perpendicular to the foliation. The stereographic projection of garnet crystallographic orientation is almost distributed randomly. Nevertheless, both phases show an intracrystalline misorientation, indicating activation of crystal plastic processes. With increasing garnet content the grain average misorientation is increasing in both omphacite and garnet crystals. On the other hand, deformation twinning in omphacite is decreasing with increasing garnet content. Further, all samples show indication of brittle deformation of both garnet and omphacite, increasing with increasing garnet content. In samples with a 25% volume fraction of garnet micro-cracks are primarily orientated perpendicular to the foliation, getting more randomly distributed in samples with a 50 and 75% volume fraction of garnet.
In conclusion, all samples show similar microstructures and textures indicating that similar mechanism are active during deformation. However, the overall dominant deformation behaviour is switching from crystal plastic to brittle with increasing garnet content.

How to cite: Klackl, S., Kraus, K., Renner, J., Grasemann, B., and Rogowitz, A.: HP/HT deformation experiments of garnet-omphacite aggregates – influence of volume fractions on deformation mechanisms., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8629, https://doi.org/10.5194/egusphere-egu2020-8629, 2020.

During exhumation, metamorphic rocks change their rheological behavior from dominantly ductile to brittle. Especially at the “brittle-ductile transition” at the bottom of the brittle crust, which coincides roughly with the domain where most “shallow” earthquakes nucleate, rocks exhibit a close interplay between ductile flow and fracturing.

In the Neves area (Tauern window, Eastern Alps) the exhumation across the brittle-ductile transition of amphibolite-facies meta-granitoids during the Alpine cycle is recorded by the association of pseudotachylyte veins and localized low-grade mylonites (stage-2 deformation). The stage-2 structures exploited the precursor amphibolite-facies foliation within meter-thick mylonites (stage-1 deformation) and were in turn overprinted by epidote-chlorite-bearing shear fractures and veins (stage-3 deformation). The kinematics and orientation of stage-1 and stage-3 structures indicate a slight rotation of the regional shortening direction from 345° to about 360°. This implies that stage-2 mylonites and pseudotachylytes developed at a high angle to the shortening direction.

The syn-kinematic metamorphic assemblage of stage-2 mylonites includes quartz, oligoclase (Ab₇₅), biotite, epidote, and minor muscovite and K-feldspar; garnet was not stable. This assemblage constrains the deformation at upper greenschist facies condition and temperatures of around 400 °C. During mylonitization the coarse-grained (mm-sized) amphibolite-facies quartz recrystallized by subgrain rotation to ultra-fine (~ 3 µm average grain size determined from EBSD maps) aggregates. Such a small grain size yields differential stress > 200 MPa during stage-2 mylonitization, considering the piezometer of Cross et al., 2017 ¹.

Pseudotachylytes are in a close spatial association with stage-2 mylonites and share the same sense of shear. There is no evidence of a ductile overprint of pseudotachylytes. The stage-2 structures developed at a very high angle to the inferred shortening direction, which implies that the coseismic slip occurred on planes with a very low friction coefficient (estimated <0.3), contradicting the high differential stress estimated for the mylonites. We infer a genetic relationship between stage-2 mylonite and pseudotachylyte. Mylonites progressively formed the mica-rich foliation planes, continuous over large distances, that provided the weak mechanical anisotropy eventually leading to coseismic slip.

Reference:

1: Cross, et al., 2017, The recrystallized grain size piezometer for quartz: An EBSD‐based calibration. Geophys. Res. Lett., 44(13), 6667-6674.

How to cite: Toffol, G., Pennacchioni, G., Grafulha Morales, L. F., and Papa, S.: Exploitation of unsuitably oriented foliation by localized mylonites and pseudotachylytes (Tauern Window, Eastern Alps), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11145, https://doi.org/10.5194/egusphere-egu2020-11145, 2020.

Two samples of mylonitic-ultramylonitic ortogneisses collected along the Contín shear zone were investigated for crystal preferred orientation and seismic anisotropy. Neutron diffraction data obtained at the D1B beamline at ILL (Institute Laue-Langevin, Grenoble) were analyzed with the Rietveld method as implemented in the code MAUD, to obtain the orientation distribution functions (ODF) of the principal phases (quartz, K-feldspar, plagioclase, phlogopite, muscovite and riebeckite). Texture and microstructure are compatible with the plastic deformation of the aggregates under medium to low-temperature conditions. Kinematic analysis supports a top-to-the SE sense of shear, suggesting a thrust character. Using preferred orientation data and single crystal elastic tensors, P and S-waves velocities and elastic anisotropy have been calculated. We have explored the role of several factors controlling the elastic properties of rocks, particularly the role of strain state and mineral changes in a shear zone. Those factors have a direct impact on the medium impedance and consequently on the interphase reflectivity. P-wave velocities, S-wave splitting and anisotropy increase with muscovite content. Seismic anisotropy is linked with the texture symmetry, which can result in large deviations between actual anisotropy and that measured along Cartesian XYZ sample directions (lineation/foliation reference frame). This is significant for the prediction and interpretation of seismic data. (Research support CGL2016-78560-P)

How to cite: Durán Oreja, M., Malecki, J., and Gómez Barreiro, J.: Quantitative texture analysis by neutron diffraction in deformed crystalline aggregates: Contín Shear Zone, Morais Complex, N Portugal., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6714, https://doi.org/10.5194/egusphere-egu2020-6714, 2020.

We apply Quantitative Microstructural Analysis (QMA) to a selection of mylonitic rocks that originate from different protoliths, ranging from tonalite to skarn and passing through migmatitic paragneiss. These rocks, that at the end of Paleozoic originally belonged to the lower crustal portion of the southern European Variscan chain, were successively involved in deep-seated strike-slip kinematics of the western Mediterranean realm, created by the relative movement in the Paleocene of the African-European colliding plates. This geodynamics leads to the roto-translation of the Sardinia-Corsica block and the drifting of the kabilo-clabride microplate system (Cirrincione et al., 2015). Remnants of these high strain-rate strike-slip zones are characterized by rheological behaviours controlled by the selective activation of their specific interconnecting weakening phase, as well as by the rheology and abundance of porphyroclasts. QMA is generated by means of new semi-automated GIS-based tools that allow us to extrapolate statistically meaningful kinematic and rheological properties of this meso-Alpine strike-slip mylonitic shear zone (i.e. Palmi Shear Zone). This, in turn, constrains the Alpine evolutionary stages of southern Calabrian geodynamics (Ortolano et al., 2020). Semi-automated image analysis of mXRF maps, combined with high-resolution thin-section scans involving new GIS-based tools developed for structural analysis (e.g. Ortolano et al., 2018; Visalli, 2018), was performed on a selection of three different mylonitic rock-types. These tools permit the user to quantitatively extrapolate rock-fabric parameters such as grain size, aspect ratio and orientation, which allows the nature and relative percentage of the weakening vs. hardening layers, as well as their kinematics, to be derived. Our results allow us to distinguish the porphyroclastic domain levels constituted alternatively by feldspar, amphibole, pyroxene or scapolite, from the weakening phase ones dominated by quartz, biotite plus quartz, or by calcite when the weakening layer is controlled by skarns. Image analysis of porphyroclastic domains has been used to infer the dominant shear-type through Rigid Grain Analysis, revealing a pure shear component of 66 to 68 % for the mylonitic tonalites; 62 to 66 % for the mylonitic paragneisses; and 58 to 62 % for the mylonitic skarn. Image analysis conducted on quartz-rich domains allows an estimate of the shear strain rate, which ranges on average from 1.14*10^-12 (1/s) for mylonitic paragneiss to 5.91*10^-12 (1/s) for mylonitic tonalite, and is in accord with high strain zones in natural settings. Our results provide new insights into the kinematics and rheology of this exhumed relic of the deep-rooted early-Alpine strike-slip tectonics of the western Mediterranean.

References

Cirrincione R., Fazio E., Fiannacca P., Ortolano G. & Pezzino A., Punturo R. 2015. Period. Mineral., 84(3B), 701-749.

Ortolano G., Visalli R., Godard G. & Cirrincione R. 2018. Comput. Geosci., 115, 56-65.

Ortolano, G., Fazio, E., Visalli, R., Alsop, G.I., Pagano, M., Cirrincione, R. 2020. Journal of Structural Geology, 131, art. no. 103956.

Visalli R. 2018. Plinius, vol 44, DOI:10.19276/plinius.2018.01014.

How to cite: Ortolano, G., Fazio, E., Visalli, R., Alsop, I. G., Pagano, M., and CIrrincione, R.: Quantitative microstructural analysis of western Mediterranean strike-slip kinematics: the Palmi Shear Zone, southern Calabria, Italy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11703, https://doi.org/10.5194/egusphere-egu2020-11703, 2020.

The Naab area is situated on the western border of the Bohemian Massif, 60 km south of the KTB (Kontinentalen Tiefbohrung). The main super-deep borehole of the KTB reached a depth of 9,101 meters in the Earth's continental crust. The fission-track data for the KTB and the Naab area present contrasting signatures. The apatite fission-track ages in the upper section of the KTB borehole and surrounding area are in the range 50-70 Ma (Wagner et al., 1994; Wauschkuhn et al., 2015). The apatite fission-track ages of the Naab basement are older than those of the KTB area, and span a broader range: 120-200 Ma (Vercoutere, 1994). The distributions of the confined-track lengths range from unimodal over skewed and mixed to bimodal, with mean lengths in the range 11-13 µm. In broad terms, this can be interpreted as that the Naab samples contain both an older and younger (in particular pre- and post-late Cretaceous) fission-track population. The aim of our research is to investigate the applicability of lab-based models to geological data, using improved measurement techniques.

We studied eighteen samples dated by Vercoutere (1994) from the Palaeozoic basement and seven large rock samples from the Rotliegend strata north of the Luhe fault.  We intend to extend the confined-track length measurements of Vercoutere (1994), aiming to achieve higher resolution through methodological innovations made possible by computer-controlled motorized microscopes. Improved statistics increase the resolution of the modelled thermal histories, which permits to better distinguish systematic from statistical differences between the modelled palaeotemperatures and geological estimates. Experiments have shown that the rate of length increase permits to distinguish older from younger tracks (Jonckheere et al., 2017). This allows us to distinguish between tracks formed before and after the Late Cre­taceous to Palaeocene exhumation. The etch rate of a confined track is also an indicator of its individual thermal history, supplementing the information gleaned from its etchable length under fixed conditions. We compiled a comprehensive, high-resolution confined-track-length dataset. The Naab thermal histories were determined using modern modelling algorithms, implementing the most recent empirical equations.

References

Jonckheere R., Tamer M., Wauschkuhn F., Wauschkuhn B., Ratschbacher L., 2017. Single-track length measurements of step-etched fission tracks in Durango apatite: Vorsprung durch Technik.American Mineralogist 102, 987-996.

Vercoutere C., 1994. The thermotectonic history of the Brabant Massif (Belgium) and the Naab Basement (Germany):   an apatite fission track analysis. Ph. D. thesis, Universiteit Gent, pp. 191.

Wagner G.A., Hejl E., Van Den Haute P., 1994. The KTB fission-track project: Methodical aspects and geological implications. Radiation Measurements 23, 95-101.

Wauschkuhn B., Jonckheere R., Ratschbacher L., 2015. The KTB apatite fission-track profiles: building on a firm foundation? Geochimica et Cosmochimica Acta 167, 27-62.

How to cite: Novakova, L., Jonckheere, R., Wauschkuhn, B., and Ratchbacher, L.: Thermal history modelling of the western margin of the Bohemian Massif using high-resolution apatite fission-track thermochronology, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15152, https://doi.org/10.5194/egusphere-egu2020-15152, 2020.

The Precambrian shield in southern Sweden exposes a granitic bedrock segment that represents a part of an ancient eroded mountain belt and expose a gradual change in metamorphic grade from cold (<300°C), little affected by recrystallization, to hot (>800°C) and partially molten sections at the west coast (Möller and Andersson, 2018). This area – the Eastern Segment – offers a large scale study of the interdependence of metamorphism, deformation, partial melting, and functional properties of crushed rock aggregates.

In the petrological community, it is well-known that the evolution of a metamorphic unit (e.g. a high-pressure unit) with respect to pressure, temperature, time, and deformation holds key information on its tectonic history. It has rarely been emphasized, however, that the same factors determine the physical properties of the rock and thus, its technical properties. Basic research in metamorphic petrology thus contributes with a fundament to applied and technical science, e.g. by providing data that lead to quarrying of proper materials.

This study assesses the variations of technical properties with the metamorphic state, primarily metamorphic temperature and partial melting during metamorphism. Our first results show the correlation between the petrological characteristics and technical properties of felsic orthogneiss within a migmatized eclogite-bearing terrane and its high-pressure granulite-bearing footwall.

Measurements include the Los Angeles and Micro-Deval value tests. The Los Angeles value is a measure of the resistance to fragmentation (EN 1097-2, 2010). The Micro-Deval test measures the resistance to wear.

High values of the Los Angeles and Micro-Deval analyses for felsic orthogneiss in the eclogite-bearing domain reflect poor technical properties and are largely linked to that the rocks underwent partial melting. Orthogneisses in the footwall, which recrystallized under high-temperature, dry conditions, and without partial melting, tend to have lower values. This group includes high-quality rocks for the production of aggregates suitable for asphalt base courses and unbound road layers.

Micro-textures in the orthogneisses are linked with these metamorphic conditions. The clinopyroxene-bearing orthogneisses have complex grain boundaries and micro-perthitic feldspars, finer average grain size, lower biotite content, and absence of migmatitic segregation or penetrative veining. These textures in the footwall orthogneisses contrast with those in the migmatitic orthogneisses from the eclogite-bearing domain, which have a coarser average grain size, even-grained and granoblastic texture, and lack of perthitic texture in feldspars. Thus, these petrographic parameters govern the technical differences.

Our ongoing research addresses the relations between macro-fabric, micro-texture and technical properties of felsic orthogneiss and metagabbro, respectively, along 120 km profile across the metamorphic field gradient from greenschist- to high-pressure granulite-facies in the Eastern Segment.

How to cite: Urueña, C., Möller, C., Andersson, J., Göransson, M., Lindqvist, J. E., and Åkeson, U.: Metamorphic micro-textural variations and their bearing on rock technical properties. A case study of the transition from greenschist- to high-pressure granulite-facies in the Eastern Segment, Sveconorwegian orogen, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-111, https://doi.org/10.5194/egusphere-egu2020-111, 2020.

Clinoptilolite, a micro-porous natural zeolite comprising tetrahedra of silica and alumina that commonly occurs in volcanic tuffs through devitrification of natural glasses, has numerous uses in the manufacturing, agriculture and building industries; it also has applications in veterinary and human medicines. Field observations and microstructural investigations in the natural clinoptilolite-tuff from Nižný Hrabovec (Slovak Republic) – one of the world’s economically most important high-quality clinoptilolite deposits – show evidence of strain localization. Brittle faults formed along pre-existing joints with plumose structures that had acted as a pathway for local infiltration of iron-, manganese- and potassium-rich fluids. Fault displacement formed structures that are indicative of both velocity hardening, with dissolution precipitation creep (SC/SCC’ foliation), and velocity weakening, with several phases of ultra-cataclasites forming along principal slip surfaces. Rock-fluid interaction is characterized by a high-mobility of K, with K-feldspar decorating SC/SCC’ foliations, infiltrating fractures in fault damage zones and precipitating as idiomorphic crystals in open cavities and along fault surfaces. Microstructures such as polished slickensides, injection of fluidized cataclasites, clast cortex grains in cataclasites and truncated grains along principal slip surfaces suggest that seismic slip probably occurred along some of the faults.

How to cite: Hou, Z., Rice, A. H. N., Tschegg, C., Berger, T., and Grasemann, B.: Strain localization associated with brittle faulting in a natural clinoptilolite-tuff (open-pit mine Nižný Hrabovec, Slovak Republic), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5167, https://doi.org/10.5194/egusphere-egu2020-5167, 2020.

Marmara granitoid (47 Ma) is a representative example of the Eocene post-collisional magmatism which produced several granitic plutons in NW Anatolia, Turkey. It is a W-E trending sill-like magmatic body which was concordantly emplaced into the metamorphic basement rocks of Erdek Complex and Saraylar Marble. The granitoid is represented by deformed granodiorite which displays well-developed lineation and foliation in meso-scale defined by the elongation of mica and feldspar crystals and recrystallization of quartz however, in some places, magmatic textures are preserved. Deformed granodiorite is broadly cut by aplitic and pegmatitic dikes and contains mafic enclaves which display the same deformation indicators with the main granitoid.

Microstructural analysis shows that the solid-state deformation of the Marmara granitoid is classified as ductile deformation with high temperatures and ductile-to-brittle deformation with relatively lower temperatures. Evidence for the ductile deformation of the granitoid is represented by chessboard extinction of quartz, grain boundary migration (GBM) and subgrain rotation recrystallisation (SGR) which exhibits that the deformation temperature changed from 600 ^oC to 400^oC. Bulging recrystallization (BLG), grain size reduction of amphibole, biotite and plagioclases and microcracks on plagioclases were considered as overlying ductile-to-brittle deformation signatures which develop between 300-<250 ^oC temperatures.

All of these field and micro-structural data collectively suggest that the shear sense indicators such as micafish structures and δ type mantled porphyroclasts displayed stair-steppings pointing out to a right lateral movement, indicating that the structural evolution and deformation history of Marmara granitoid was controlled by a dextral shear zone.

How to cite: Bayrak, S. B., Güraslan, I. N., Ünal, A., Kamacı, Ö., Altunkaynak, Ş., and Yiğitbaş, E.: Deformation history of the Marmara Granitoid and implications for a dextral shear zone in NW Anatolia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9231, https://doi.org/10.5194/egusphere-egu2020-9231, 2020.

Shear zones in the high-grade terranes represent the tectonic- fossils of strain history. One such shear zones, namely Balaram-Jogdadi shear zones defining the terrane boundary of the Ambaji granulites of the South Delhi terrane Aravalli –Delhi Mobile belt, NW India, provide evidence for strain variation during exhumation of lower-middle crustal rocks. Compilation of field and microscopic analysis of various samples of mylonite from shear zones suggest that the part of shear zone contains high-grade mineral assemblages such as cordierite, sillimanite, spinel, garnet in quartzo-feldspathic mylonite rock and exhibit signature of thrusting in which garnet behaved as brittle phase and quartz and feldspar grain show ductile deformation. 2D and 3D strain analysis estimate a plane to flattening type of strain pattern. Principal strain planes are used to calculate the strain ratios for estimation of variation of strain along the shear zone. This study indicates high-grade mylonite accommodates high strain. The flow of rigid porphyroclasts estimates mean kinematic vorticity number varies from 0.47 to 0.68, which indicates the dominance of pure shear during shearing. Vorticity by the Rs/θ method in quartz grain estimates ranges from 0.7 to 0.95, suggesting a non-steady strain towards the end of deformation. High-grade mylonites were overprinted by low-temperature mylonitisation marked by minerals like quartz, feldspar, biotite in which feldspar porphyroclast shows brittle deformation and quartz, biotite show ductile deformation. Several shear kinematics indicate top-to-NW sinistral strike-slip shearing. Thus it has been interpreted that the shear zone had undergone non-steady strain. The initial thrusting phase was dominated by more pure shear component. The strike-slip shearing part was dominated by more simple shear component. Monazite geochronology sets the age of shearing at 834-778 Ma suggesting the exhumation was a transition event between Grenville to Pan-African orogeny.

Keywords: Shear zone, Deformation, Vorticity, 3D strain analysis, Monazite dating

How to cite: Saraswati, R. and Biswal, T. K.: Non-steady strain in the terrane boundary shear zone of the Ambaji Granulite, NW India: Implications for understanding towards the dynamics of emplacement of the lower-middle crustal rocks., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2897, https://doi.org/10.5194/egusphere-egu2020-2897, 2020.

The Neoproterozoic (834 – 778 Ma) Ambaji granulite witnessed four deformation phases (D₁- D₄), of which the D₂ deformation phase was most significant for the exhumation of granulites in the ductile regime. We performed a field study to investigate the tectonic evolution of the D₂ deformation phase and investigated the deformation evolution of the ductile extrusion of the Ambaji granulite by estimating the vorticity of flow (Wm) with the Rigid Grain Net and strain ratio/orientation techniques.

During the D₂ deformation phase, the S₁ fabric was folded by F₂ folds that are coaxial with the F₁ folds. The F₂ folds were produced in response to NW-SE compression. Because the large shear zones are oriented parallel to the axial plane of the F₂ folds, they likely formed simultaneously during the D₂ deformation phase. Compression during the D₂ deformation phase accommodated most of the exhumation of the granulite along the shear zones. D₂ shearing was constrained between 834 ± 7 to 778 ± 8 Ma (Monazite ages).

The shear zones evolved from a high temperature (>700 °C) thrust-slip shearing event in the lower-middle crust to a low temperature (450 °C) retrograde sinistral shearing event at the brittle-ductile-transition (BDT). The Wm estimates of 0.32–0.40 and 0.60 coincide with the high temperature event and suggests pure shear dominated deformation. The low temperature phase coincides with Wm estimates of 0.64–0.87 and ~1.0, implying two flow regimes. The shear zone was first affected by general non-coaxial deformation and gradually became dominated by simple shearing.

We interpreted that the high temperature event happened in a compressive tectonic regime, which led to horizontal shortening and vertical displacement of the granulite to the BDT. The low temperature event occurred in a transpressive tectonic setting that caused the lateral displacement of the granulite body at BDT depth. The Wm values indicate a non-steady strain during the exhumation of granulite. From the BDT to surface, the Ambaji granulite exhumed through the NW-SE directed extension for normal faults via brittle exhumation through crustal extension and thinning.

How to cite: Tiwari, S. K., Beniest, A., and Biswal, T. K.: The kinematic vorticity analysis of ductile shear zones of Ambaji Granulite, NW India and its tectonic implications, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7397, https://doi.org/10.5194/egusphere-egu2020-7397, 2020.