Partial melting often occurs alongside sites of rapid deformation in the Earth’s mantle and crust. Microstructural molten and crystalline components align in response to deformation, leading to anisotropies in mechanical, transport, and seismic properties detectable by remote sensing. Here, we investigate the co-evolution of melt, shape, and crystallographic preferred orientations (MPOs, SPOs & CPOs) at early stages of experimental shear deformation, constraining their contribution to observable signatures. We characterized the microstructures of partially molten (2-4 wt% melt) olivine-basalt aggregates deformed in general shear at a temperature of 1250°C under a confining pressure of 300 MPa, at shear stresses of τ = 0-175 MPa and shear strains of γ = 0-2.3. We then used the Gassman poroelastic differential effective medium method to calculate resultant seismic anisotropy.

The grain-scale network of melt pockets developed a strong preferred orientation parallel to the maximum principal stress at γ < 0.4. At higher strains, the orientation of the grain-scale melt pockets remained parallel to the maximum principal stress, but incipient, sample-scale melt bands formed at ~25° antithetic to the direction of shear.  While the orientation of individual melt pockets evolved quickly, grain SPOs and CPOs required larger strains (γ > 2) to strengthen and change. A weak SPO and CPO were induced during sample preparation, with grain long axes oriented perpendicular to the direction of maximum principal stress and a- and c-axes girdled perpendicular to the long axis of the sample. At the highest explored shear strain, a strong SPO was established and the girdled a-axes of the CPO rotated to align nearly parallel to the shear plane, developing clusters parallel to the shearing direction.  

These results yield two key conclusions about the orientation of melt networks in deforming partially molten rocks. First, the grain-scale and sample-scale alignments of melt pockets are distinct. At the grain scale, melt pockets align approximately parallel to the maximum principal stress, but the en echelon arrangement of melt pockets yields a sample-scale MPO at ~25^o to the maximum principal stress (20^o to the shear plane). Second, the relative timescales of melt and solid microstructural evolution are different, and are directly reflected in changes to seismic anisotropy. The grain-scale MPO reacts to a change in the orientation of the maximum principal stress after only a small amount of strain; in contrast, CPOs and SPOs require much higher strains before responding to a change in stress conditions. Seismic anisotropy is greatest when olivine a-axes and the grain-scale orientation of melt pockets are in relatively close alignment, so anisotropy will quickly decrease with any change in the orientation of the stress field that results in a rotation of the MPO away from the orientation of olivine a-axes. Perturbations to a local stress field can thus be observed almost immediately due to a rapidly reorienting melt network, making MPOs a more valuable predictor of instantaneous change than CPO-driven anisotropies.

How to cite: Seltzer, C., Peč, M., Zimmerman, M., and Kohlstedt, D.: Co-evolution of melt and crystal phases in experimentally sheared partially molten rocks and generation of seismic anisotropy during rapid deformation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-539, https://doi.org/10.5194/egusphere-egu23-539, 2023.

Olivine is the most abundant mineral in Earth’s mantle, and its rheology is likely to control upper-mantle convection. While the rheology of olivine is widely studied, little is known about the rheology of olivine grain boundaries and their effect on deformation in the mantle. Forsterite bicrystals, synthesized by direct bonding of highly polished single-crystal plates, were tested in this study to investigate sliding along the single grain boundary at high temperature (1300^°C). Prior to deformation, the bicrystals were polished and scratch markers were scribed perpendicular to the grain boundary to track grain-boundary sliding. Bicrystals were deformed in shear loading between two alumina pistons in a uniaxial creep apparatus at 1 atm with applied axial stress ranging from 1 to 30 MPa. The specimen deformation was measured in real time using a high-resolution (~1 μm) linear variable differential transducer. Each test was carried out until attainment of a quasi-steady state deformation rate to determine the creep parameters. Post-deformation microstructural analysis was conducted using a scanning electron microscope (SEM) and electron backscattered diffraction. Our study established that the creep-rate law for bicrystals is different than single and polycrystalline forsterite. Bicrystals are weaker and shows up to 1 order of magnitude higher deformation rates. SEM microstructures reveal the sliding of scratch markers, which is direct evidence of grain-boundary sliding in forsterite. However, the strain geometry is complex, and further experiments are necessary to determine the overall strain distribution in the sample. Here we present the rationale of our research, and we compare our results on grain-boundary sliding in forsterite with the earlier literature.

How to cite: Singh, S. P., Thom, C., Hansen, L., Marquardt, K., Wheeler, J., Mariani, E., and Mecklenburgh, J.: Direct Observation of Grain Boundary Sliding in Forsterite Bicrystals, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15498, https://doi.org/10.5194/egusphere-egu23-15498, 2023.

Earthquakes are triggered by the sudden release of strain energy accumulated in the Earth’s crust and mantle. Around 55 earthquakes are located every day around the world, and 16 large earthquakes of magnitude greater than 7 are expected in any given year. These events are responsible for many deaths and for major natural disasters. The periodicity of rupture events is controlled by complex variables such as fault surface roughness, fault geometry, fluid-rock interactions, fluid pressure oscillations and the mechanics of the fault rock. Seismology, rock mechanics experiments and modelling have provided vital insights into the behaviour and frictional properties of faults, but the brittle-viscous response of rock to earthquake rupture and the passage of shock waves is currently not understood.

Natural olivine exposed at the Earth surface, but derived from Earth’s upper mantle, display microstructures where brittle, frictional and viscous deformation coexist. Here we study the microstructures and textures locked in the geological record of the Premosello peridotite to understand the transient brittle-viscous deformation mechanisms triggered during large earthquakes, and their contribution to energy dissipation and build-up during the earthquake cycle.

In this study we use electron backscatter diffraction (EBSD) in the SEM, as well as TEM analyses, to detail the micron to nanoscale structures of olivine in samples from the shallow upper mantle, collected near thick pseudotachylytes in proximity of the Mohorovičić discontinuity, which juxtaposes these peridotites with the lower crustal granulites of the Ivrea-Verbano Zone.

How to cite: Mariani, E., Bagshaw, H., Bilton, M., and Gardner, J.: The effect of earthquake rupture on the brittle-viscous flow of olivine, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15541, https://doi.org/10.5194/egusphere-egu23-15541, 2023.

Oceanic Transform Faults (OTFs) are major plate boundaries that regularly offset the axis of Mid-Oceanic Ridges and are able to generate Mw 7 earthquakes. Yet, very little is known on the evolution of fault slip mechanics at depth, mainly due to rare exposures of deep sections at the seafloor. The Atobá Ridge is one of the rare structures where the roots of an active transform fault are exposed and accessible. This transpressive ridge is part of the northern transform fault of the St. Paul transform system in the Equatorial Atlantic (Maia et al., 2016). There, ultramafic mylonites are tectonically exhumed along the inner thrust faults of a positive flower structure.

Here we study the deformation mechanisms in ultramafic mylonites and ultramylonites sampled at the Atobá ridge. We show that all samples experienced ductile deformation at 750-900°C in the spinel stability field, resulting in a pervasive grain size reduction. We propose fluid-assisted dissolution-precipitation creep as the main deformation mechanism, leading to dissolution of orthopyroxene and formation of lens-shaped olivine and interstitial minor phases’ neoblasts (pyroxenes, spinel, and amphibole). Orthopyroxene neoblasts formed by this mechanism mimic the Crystallographic Preferred Orientation of the olivine neoblasts. Fluid-assisted dissolution-precipitation creep allows deformation of stiff minerals at significant lower stresses and temperatures than dislocation creep, possibly leading to an intense strain localization. This mechanism, previously reported in ophiolites and orogenic contexts (Hidas et al., 2016; Prigent et al., 2018), is described for the first time in the oceanic transform environment. Similar microstructures have been observed in mylonites from other OTFs, suggesting that this mechanism could be more widespread and could even represent one of the main deformation law in the lower oceanic lithosphere, with important implications on the mechanics and structures of (oceanic) transform faults and long-lived detachments.

This work is supported by PRIN2017KY5ZX8.

REFERENCES

Maia, M., Sichel, S., Briais, A., Brunelli, D., Ligi, M., Ferreira, N., Campos, T., Mougel, B., Brehme, I., Hémond, C. and Motoki, A., 2016. Extreme mantle uplift and exhumation along a transpressive transform fault. Nature Geoscience, 9(8), pp.619-623.

Hidas, K., Tommasi, A., Garrido, C. J., Padrón-Navarta, J. A., Mainprice, D., Vauchez, A., Barou, F., & Marchesi, C. (2016). Fluid-assisted strain localization in the shallow subcontinental lithospheric mantle. Lithos, 262(October), 636–650. https://doi.org/10.1016/j.lithos.2016.07.038

Prigent, C., Guillot, S., Agard, P., & Ildefonse, B. (2018). Fluid-assisted deformation and strain localization in the cooling mantle wedge of a young subduction zone (Semail ophiolite). Journal of Geophysical Research: Solid Earth, 123. https://doi.org/10.1029/ 2018JB015492

How to cite: Bickert, M., Kaczmarek, M.-A., Maia, M., and Brunelli, D.: Fluid-assisted deformation processes at the roots of oceanic transform faults., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5111, https://doi.org/10.5194/egusphere-egu23-5111, 2023.

Mechanisms driving the long-term dynamics of plate interfaces remain poorly-constrained. To date, the rheology of the crust is considered to be controlled by solid-state diffusion processes such as crystal plastic deformation (dislocation creep). Yet, most minerals formed at high-pressure conditions are mechanically very strong (garnet, omphacite, glaucophane, zoisite, kyanite) and can only be deformed plastically at unrealistically high stresses or temperatures. A growing number of studies point to the crucial role of fluid-rock interactions and mineral transformations in the development of crustal shear zones of low viscosity. The rock weakening is interpreted as being induced by dissolution and precipitation processes at grains boundaries in chemical disequilibrium. Here, we tackle the eclogite rheology conundrum by performing the first deformation experiments at high-pressure conditions (> 2 GPa) on a two-phase aggregate representative of the lower crust.

Shear experiments were performed in a new generation of Griggs-type apparatus (Univ. Orléans) at 850°C, 2.1 GPa and a shear strain rate of 10⁻⁶ s⁻¹. The starting material consists of mixed powders of plagioclase and clinopyroxene separated from an undeformed gabbro (Kågen, Norway) and hot-pressed with a grain size lower than 100 µm. Experiments have been conducted with 0.2% added water.

Mechanical data indicate that the samples are first very strong with a peak differential stress between 1.0 and 1.4 GPa. Then, a significant weakening is observed with a stress decrease of 0.5 GPa. The high-strain samples are characterized by a strain gradient and a reaction gradient, both increasing toward the center of the shear zone. The nucleation of new phases leads to a drastic grain size reduction and phase mixing. The intensities of both are positively correlated with the strain intensity. The nature, distribution and fabric of the reaction products vary also progressively with strain intensity. At the peak stress, the reaction products are restricted to grain boundaries where they form corona structures, while in the high-strain samples, they occur throughout the sample replacing most of the starting material. The primary plagioclase and clinopyroxene grains show incipient dynamic recrystallization, whereas reaction products never do. The nano-porosity reported in the samples attests to the presence of free-fluid phase along the reactive grain boundaries, despite the high-pressure conditions. This nano-porosity requires grain boundary sliding (GBS) processes to form, as indicated by the spatially associated quadrupole junctions.

Our results show that strain at eclogite-facies conditions is preferentially localized by GBS-accommodated dissolution and precipitation creep in reactive zones. We suggest that this dominant deformation process take place in rock at chemical disequilibrium in the presence of a free-fluid phase. Therefore, deformation along deep plate interfaces should be initiated and governed by transient and local transformation weakening, allowing long-term deformation at far lower stresses than dislocation creep.

How to cite: Soret, M., Stünitz, H., Précigout, J., Osselin, F., Lee, A., and Raimbourg, H.: Deep crustal dynamics driven by local and transient transformation weakening, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6569, https://doi.org/10.5194/egusphere-egu23-6569, 2023.

In the dry lower crust, earthquake-induced fracturing can increase permeability for fluids to infiltrate and flow, thus facilitating fluid-rock interactions, and potentially altering the strength and rheology of fault systems. Understanding the mechanisms that create and reduce porosity requires a detailed microstructural analysis. Here, we analyze microstructures that have recorded primary and secondary porosity generated by the dynamic rupture propagation of a lower crustal earthquake, and that were subsequently reworked during post- and interseismic viscous creep.

An exhumed lower crustal section comprised largely of anhydrous anorthosites cross-cut by a coeval network of pseudotachylytes (solidified melts produced during seismic slip) and mylonitized pseudotachylytes (overprinted during the post- and interseismic viscous creep), is found at Nusfjord, Lofoten, Norway. We study the microstructures using synchrotron X-ray microtomography (SμCT), focused ion beam scanning electron microscopy (FIB-SEM) nanotomography, electron backscatter diffraction (EBSD) analysis, and SEM imaging.

SμCT data reveals that porosity is dispersed and poorly interconnected within a pseudotachylyte vein (0.16 vol% porosity overall), and noticeably increased along the grain boundaries of garnet grains (1.07 – 1.87 vol%). The increased porosity around garnet is formed due to a net negative volume change (-DV) during garnet growth, as there is a localized increase in density of ~1.00 g/cm³ when a recrystallizing garnet overgrows a pseudotachylyte matrix (plagioclase + amphibole). Efficient healing of the earthquake damage zone (0.03 vol% porosity) resulted in the preservation of only a few but relatively large interconnected primary pores along fractures in the anorthosite. Fractures were healed by the growth of plagioclase neoblasts nucleated from extremely comminuted fragments of the host anorthosite, and by the precipitation of barium-enriched K-feldspar filling intragranular pores. Fluid-rock interaction was so efficient at sealing the porosity that a FIB-SEM transect along one of these microfractures revealed a myrmekite intergrowth replacing K-feldspar.

Porosity is dramatically decreased in the mylonitized pseudotachylyte (0.03 vol% overall), and focused mainly within monomineralic domains of plagioclase (0.07 – 0.11 vol%). These are interpreted as recrystallized and sheared survivor clasts of wall-rock fragments, while the polymineralic domains are primarily derived from the overprint of the original pseudotachylyte veins. The plagioclase grains in both domains are more-or-less equant, very fine grained (< 25 μm), lack a crystallographic preferred orientation, grain boundaries occasionally aligned to form quadruple junctions, and are well-mixed amongst the hydrous phases (polymineralic domain), suggesting that both domains deformed primarily by grain-size sensitive diffusion creep and viscous grain boundary sliding. The polymineralic domain has the least porosity (~0.01 vol%), which reflects the efficient precipitation of phases (amphibole, biotite, and feldspars) into transient pores during creep cavitation.

A porosity reduction on the order of 90% from a pristine to a mylonitized pseudotachylyte may eventually result in shear zone hardening, and development of new pseudotachylytes overprinting the mylonites. Therefore, earthquake-induced rheological weakening of the lower crust is intermittent, occurs when a fluid can infiltrate a transiently permeable shear zone, and may stop when the porosity becomes clogged.

How to cite: Michalchuk, S. P., Zertani, S., Renard, F., Plümper, O., Chogani, A., and Menegon, L.: Dynamic evolution of porosity in lower crustal faults during the earthquake cycle, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5451, https://doi.org/10.5194/egusphere-egu23-5451, 2023.

The grain size of polycrystalline ice affects key parameters related to planetary evolution such as the rheological and dielectric properties of Earth's glaciers and ice sheets as well as the ice shells of ice satellites. Although past experiments have studied the grain growth of pure water ice as well as polycrystalline ice doped with air bubbles and insoluble particles, the effect of soluble ions, such as Cl^- and SO₄^2-, which are commonly found in glaciers, on the growth of polycrystalline ice is not clear. To investigate the effect of soluble impurities on the grain growth kinetics of polycrystalline ice, we conducted annealing experiments on polycrystalline ice samples doped with different concentrations of KCl (10^-2, 10^-3, 10^-4, 10^-5 mol/L) and MgSO₄ (10^-2, 10^-5 mol/L), respectively. Ice powders were obtained by spraying a fine mist of solution (ultra-pure water + KCl or MgSO₄) into liquid nitrogen. The powders were then dried and uniaxially pressed into a cylinder at 30 MPa and -30°C and then hydrostatically pressed into a cylindrical ice sample at 100 MPa and -30°C for 15 min. The samples were annealed for a maximum of 320 h at a hydrostatic pressure of 20 MPa (corresponding to about 2 km glacier depth) and different constant temperatures (-5, -10, -15, -20, -25°C). After each experiment, we took microscopic images of the polished sample surface using an optical microscope equipped with a cold stage. Machine learning methods combined with human quality check were utilized on the images to distinguish the grain boundaries. Then a grain size was measured as the average equivalent grain diameter with a geometry factor applied. For KCl, at -5°C (the eutectic point of KCl solution is -10.7°C), lowest-concentration-doped ice (10^-5 mol/L) grew slightly faster than higher-concentration-doped ice (10^-2, 10^-3, 10^-4 mol/L), and both are faster than pure ice; at -10°C and -15°C, there was no significant difference between the growth rates of both doped ice with different concentrations and pure ice; while at -20°C and -25°C (well below the eutectic point of ice and KCl), the growth rates of higher-concentration-doped ice (10^-2, 10^-3 mol/L) were slower than those of pure ice and lower-concentration-doped ice (10^-4, 10^-5 mol/L), which are similar to each other. For MgSO₄, at -5°C (the eutectic point of MgSO₄ solution is -3.6°C), the growth rates of doped ice with different concentrations are not significantly different from that of pure ice; while at -10°C and lower temperatures, the growth rates of doped ice are slower than that of pure ice. We propose that at temperatures well above the eutectic point, grain growth may be largely influenced by partial melts and temperatures well below the eutectic point, soluble impurities impede grain growth. These results help quantifying the contribution of different creep mechanisms, grain-size sensitive and insensitive, under natural conditions and will contribute to future estimation of the rheological strength of glaciers and ice shells.

How to cite: Wang, Q., Fan, S., and Qi, C.: Grain growth of polycrystalline ice doped with soluble impurities, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17008, https://doi.org/10.5194/egusphere-egu23-17008, 2023.

Grain size is a critical parameter for viscous deformation processes and controls the rheological behaviour (weakening and strain localizations in mylonites) of polycrystalline aggregates. Recently,  Ghosh et al., (2022) used Tana quartzite (~200 μm) to develop a new grain-size-insensitive flow law (stress exponent, n = 2; activation energy, Q = 110 kj/mol). The n = 2 is interpreted to indicate the serial operation of grain interior (dislocation glide) and grain boundary (dissolution-precipitation, DPC + Grain boundary sliding, GBS) accommodation processes. It is shown that all the previous results obtained from coarse-grained (∼100 - 200 μm) quartz aggregates (high total strain, significant recrystallization) plot within a factor of ∼5 times the strain rate predicted by this flow law. Moreover, a number of earlier studies with fine-grained (3.6 to 12 μm) quartz reported an n-value range of 1.7 to 2.5 (Kronenberg and Tullis 1984; Fukuda et al., 2018), similar to the values obtained from coarse-grained samples. A deviation from the grain-size-insensitive creep-dominated process is expected at those grain-size ranges and poses the question: how much weakening is possible by grain-size-reduction within the continental crust?

Using a Griggs-type apparatus, we deformed fine-grained (~ 3-4 μm) as-is and 0.1 wt.% H₂O-added novaculites, representing a fully recrystallized shear zone material, at the same condition as coarse-grained Tana quartzite (~200 μm). The as-is novaculite is ~3.5 times stronger than the H₂O-added novaculite. A similar strength difference (~4 times) was also observed between the as-is and H₂O-added Tana quartzite. The H₂O-added novaculite is ~4 times weaker than the H₂O-added Tana. The as-is novaculite is ~4.5 times weaker than the as-is Tana quartzite. Thus, the addition of 0.1 wt.% H₂O causes a similar order of weakening as the smaller grain size; the as-is novaculite has a similar strength as the data predicted from the H₂O-added Tana flow law. The maximum strength difference (more than an order of magnitude) arises between the end-member samples i.e., the as-is Tana and H₂O-added novaculite, which indicate the combined effect of grain size and H₂O. The n (1.91 to 2.19) and Q (118 kj/mol) values measured from novaculites are similar to the Tana and the Tana flow law can express all the novaculite data within the error associated with the Griggs-type apparatus, only by manipulating the A-value of the flow law. Therefore, the n and Q values are largely grain-size-insensitive, while the strength differences due to grain size and added-H₂O can be expressed by varying the A-value. Further, the similarity in the n and Q values indicates that the same deformation mechanisms (i.e., dislocation creep accommodated by grain boundary processes) operate in both materials. Microstructural analysis of the novaculite reveals the development of a core-shell structure of individual grains due to grain boundary migration (GBM). A combination of deformation mechanisms (dislocation glide, GBM, GBS) are mutually dependent and interact to achieve the bulk sample strain. The variations in A-value may reflect the efficiency of the GBS mechanism depending on grain size and H₂O content. Finally, the implications of these results on crustal strength will be discussed.

How to cite: Ghosh, S., Stünitz, H., Raimbourg, H., and Précigout, J.: Rheology and Deformation Processes of Fine-grained Quartz Aggregate, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15832, https://doi.org/10.5194/egusphere-egu23-15832, 2023.

While mineral plasticity is often seen as the most important process to deform rocks in the deep, viscous realm, but an increasing number of studies is highlighting the contribution of mineral reactions and chemical processes to bulk strain. This issue is especially acute at the brittle-viscous transition, where plasticity is hampered by the low temperature conditions. In this work, we studied the deformation processes operating in mica+quartz assemblages, representative of greenschist to amphibolite-facies mylonites, with a special focus on the respective contributions of plasticity and dissolution-precipitation processes.

Assemblages made of phlogopite (Phl) and quartz (Qz) were experimentally deformed in simple shear (Griggs-type apparatus) at 800°C, 10kbar, shear strain rate ~10^-5 s^-1 and 0.1 wt. % added H₂O.  For well-constrained grain sizes (63μm < Phl < 125μm, 10μm < Qz < 20μm), we varied the phase proportions of phlogopite (10, 20, 30, 50, 70 and 100% vol. Phl).

Mechanical results indicate at first order a decrease in strength as the proportion of mica is increased. Samples for 10% vol. Phl deform at differential stresses of ∼1200 and at 20% Phl at ~900MPa, while 100% vol. Phl sample deform for differential stresses as low as ~300MPa. However, the weakest behavior is observed for 30% vol. of mica. The strength of the latter sample lies well out of the iso-strain/iso-stress curves, pointing to the fact that the assemblage does not behave as a simple mechanical mixture of the two end-member mineral phases.

In all samples, an important grain size reduction is noticeable for mica flakes. Grain size reduction is also present for quartz grains, especially with decreasing proportion of mica in the assembly. Most of the strain is accommodated by quartz as an interconnected network in Phl-poor samples, while in Phl-rich samples, phlogopite grains are interconnected. In quartz, a weak intragrain deformation is observed for most samples with no evidence for dynamic recrystallization Quartz is largely reworked as attested by trace element variations, with the reworked quartz proportion decreasing with increasing abundance of phlogopite Only the 10% vol. Phl sample is characterized by incipient polygonal subgrains between parent quartz grains (Qz1). Quartz is reworked mainly by dissolution-precipitation, with newly formed product (Qz2) surrounding original, inherited quartz grains. Qz2 is characterized by a large microporosity, and is enriched in aluminum compared to Qz1. The contribution of dissolution-precipitation seems to be maximal in the weakest sample (30% vol. Phl). Therefore, our study shows that dissolution precipitation processes are instrumental to weaken the two-phase material and that bulk strength strongly deviates from composite flow laws in case of a purely mechanical mixing.

How to cite: Alaoui, L. K., Airaghi, L., Stünitz, H., Raimbourg, H., and Précigout, J.: Effect of phlogopite on the strength of mica-quartz assemblage and underlying chemical processes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14586, https://doi.org/10.5194/egusphere-egu23-14586, 2023.

Many metamorphic rocks have a fabric. What is often not clear is how much deformational or metamorphic processes contributed to the formation of these fabrics. Are foliations always the result of strain? When does intrinsic crystallographic anisotropy alone lead to the formation of structural elements? Understanding the relative contributions of deformation and metamorphism in rock fabrics is fundamentally important because it is foundational to understanding the role of stress in reacting and deforming rocks.

To this end, we make a major advance in our understanding of fabric development in reacting rocks by showing in time-resolved (4D) synchrotron microtomography (µCT) experiments that when a gypsum dehydration reaction occurs in a differentially stressed sample the reaction products develop orthogonally to the largest principal stress. This is an important finding because we can show with our µCT data that this preferred orientation forms early in the reaction and at very small strains (<1%). Using a simple kinematic model we can demonstrate that it cannot have formed because of reorientation during mechanical compaction. It remains to be established if it is nucleation or growth of bassanite that is being affected by the stress or both. Our experiments suggest that metamorphic transformations may be inherently anisotropic when reacting under the influence of a non-hydrostatic stress state. 

The consequences of this are many. For example, there will be cases in natural rocks where the interpretation of a lineation, foliation or crystallographic preferred orientation as formed by strain may be incorrect. Moreover, the physical properties (e.g. hydraulic and mechanics) of metamorphic rocks could also be significantly anisotropic from early in a transformation. Mass transport pathways might initialise as channelled or partitioned conduits which would have an impact during subduction and in thin-skinned tectonics. Our data reveal a critical new finding related to the very common geological occurrence of reacting rocks experiencing a differential stress.

How to cite: Gilgannon, J., Freitas, D., Rizzo, R., Wheeler, J., Butler, I., Seth, S., Marone, F., Schlepütz, C., McGill, G., Watt, I., Plümper, O., Eberhard, L., Amiri, H., Chogani, A., and Fusseis, F.: A non-hydrostatic stress state forms fabrics during metamorphic reactions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6483, https://doi.org/10.5194/egusphere-egu23-6483, 2023.

X-ray tomographic imaging has become a very valuable tool for the analysis of (rock) materials, both for visualising complex 3D microstructures and for imaging internal features such as damage, mineral reaction, and fluid/rock interactions quantitatively. The validity of the results derived from X-ray tomography, however, hinge on the  accuracy of the image segmentation. There are many methods for image segmentation (from simple manual thresholding to machine learning and deep learning approaches), which can produce a high range of variability in the segmentation results. Accuracy of segmentation results is seldom checked and thus calling the reproducibility of the results into question. In this contribution we show how metamorphic reactions themselves can be used to constrain accuracy and highlight the benefits of deep learning methods to extend this over many large datasets efficiently.

Here, we demonstrate a methodology that uses deep learning to achieve reliable segmentation of time-series volumetric images of gypsum dehydration reaction, on which standard segmentation approaches fail due to insufficient contrast. We implement 2D U-net architecture for segmentation, and, to overcome the limitations of training data obtained experimentally through imaging, we show how labelled data obtained via machine learning (i.e., Random Forest Classification) can be used as input data and enhance the neural network performances. The developed deep learning algorithm proves to be incredibly robust, as it is able to consistently segment volume phases within the whole suite of experiments. In addition, the trained neural network exhibits short run times (<7 minutes for ~250 MB of image volumes) on a local workstation equipped with a GPU card.  

To confirm the precision achieved by our workflow, we consider the theoretical and measured molar evolution of gypsum (CaSO4.2H2O) to bassanite (CaSO4.½H2O) during the dehydration. Within all time-series experiments, errors between the predicted theoretical and the segmented volumes fall within the 5% confidence intervals of the theoretical curves. Thus, the segmented CT images are very well suited for extracting quantitative information, such as mineral growth rate and pore size variations during the reaction. To our knowledge, this is the first time an internal standard is used to unequivocally measure the accuracy of a segmentation model.  Being able to accurately and unambiguously measure the volumetric evolution during a reaction enables high-level modelling and verification of the physical (hydraulic and mechanical) properties of rock materials involved in tectono-metamorphic processes.

How to cite: Rizzo, R. E., Freitas, D., Gilgannon, J., Seth, S., Butler, I. B., Wheeler, J., Marone, F., Schlepuetz, C., McGill, G., Plümper, O., and Fusseis, F.: Deep learning and chemical constraints allow accurate segmentation of µCT data from metamorphic rocks, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7120, https://doi.org/10.5194/egusphere-egu23-7120, 2023.

The relationship between Raman peak intensity and crystal orientation via knowledge of the: Raman tensor of a given vibrational mode, incident laser light and Raman scattered light polarisation vectors is well established. Thanks to Loudon’s work in 1964 the Raman tensor structure is known for all 32 crystal classes[1]. Many researchers have exploited this to determine Raman tensor coefficients to study the nature of semiconductor and covalently bonded materials e.g.[2,3] using polarised Raman microscopy with the aim to determine crystal orientation locally.

In 2019 Ilchenko et al. demonstrated the feasibility of quantitatively mapping crystallographic orientation of polycrystalline materials in 2D, and in 3D exploiting the confocal nature of a Raman microscope[4]. The novelty of this work overcomes the need to serially collect Raman spectra at each map pixel for multiple combinations incident and scattered polarization needed to compute the local crystal orientation. A new generation of this technology, quantitative Raman imaging of crystallographic orientation (qRICO), rapidly collects Raman spectra of up to 20 combinations of incident and scattered polarization in a simultaneous manner.

Development work using ideal semiconductor materials has demonstrated that qRICO delivers the ability to produce crystallographic images of sample microstructures with sample stage step / pixel sizes down to 0.5 µm, contiguous scanning areas on the order of 10 x 10 cm and crystal orientation accuracy better than 2°. Thus, qRICO provides access to a very wide range of the microstructure length scales seen in geological materials and is amenable to typical geological specimen dimensions and shapes. Fundamentally qRICO is not limited to polished planar sample surfaces and is not restricted to surface studies for transparent materials.

In this work, in addition to non-natural polycrystalline materials, we will present high resolution as well as large area map examples of orientation mapping results on natural diamonds containing defect structures, polycrystalline quartz particles and multiphase petrographic thin slices. These examples will be used to illustrate the potential of qRICO for understanding geological materials in terms of grain boundaries, phase boundaries, orientation gradients, and crystallographic orientations and texture in relation to the conventional information contained in the underlying Raman spectra such as chemical gradients and internal stress.

[1]  R. Loudon, The Raman effect in crystals, Adv. Phys. 13 (1964) 423–482. https://doi.org/10.1080/00018736400101051.
[2]  C. Kranert, C. Sturm, R. Schmidt-Grund, M. Grundmann, Raman tensor elements of β-Ga2O3, Sci. Rep. 6 (2016) 35964. https://doi.org/10.1038/srep35964.
[3]  X. Zhong, A. Loges, V. Roddatis, T. John, Measurement of crystallographic orientation of quartz crystal using Raman spectroscopy: application to entrapped inclusions, Contrib. Mineral. Petrol. 176 (2021) 89. https://doi.org/10.1007/s00410-021-01845-x.
[4] O. Ilchenko, Y. Pilgun, A. Kutsyk, F. Bachmann, R. Slipets, M. Todeschini, P.O. Okeyo, H.F. Poulsen, A. Boisen, Fast and quantitative 2D and 3D orientation mapping using Raman microscopy, Nat. Commun. 10 (2019) 5555. https://doi.org/10.1038/s41467-019-13504-8.

How to cite: Bowen, J., Heinig, M., Reischig, P., Bachmann, F., Ilchenko, O., Pilgun, Y., and Barbee, O.: Raman based crystallographic orientation mapping with qRICO: first applications for geological materials characterisation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16744, https://doi.org/10.5194/egusphere-egu23-16744, 2023.

Raman spectroscopy on carbonaceous material (RSCM) is a method widely used to infer the temperature of metamorphism in metasediments. Furthermore, anomalies in the Raman signature of carbonaceous particles contained in fault zones have been interpreted as reflecting short-lived heating events due to friction during earthquakes. These applications of RSCM rely all on the assumption that temperature is the main factor controlling the reorganization of carbonaceous material (CM) and its expression by Raman spectroscopy. Nonetheless, few examples of natural fault zones and shear zones recently raised questions about the possible role of strain in such reorganization.

In this work, we studied the influence of deformation on the Raman spectra of CM. We carried out experiments of deformation on low-grade shales in the Paterson and Griggs-type rigs, corresponding to low and high pressure conditions, for variable conditions of temperature and strain rates, simulating deformation at the brittle-ductile transition. In parallel, we examined in these experiments the evolution of CM using the intensity ratio (IR), defined as the intensity ratio of the Defect band over the Graphite band of the Raman spectrum.

Deformation proceeded as a combination of slip on discrete planes, corresponding to macroscopic stick-slip events, and of ductile shear in few hundreds of µm’s-thick zones. The effect of stick-slip events on RSCM signal is unclear, principally because the deformed layers are too thin to contain CM to be analyzed. In contrast, in zones of distributed strain we observed a systematic and significant increase in IR compared to undeformed domains, reflecting an enhancement in the structural organization of CM. On the basis of additional experiments of static heating we carried out in parallel, we show that the IR increase cannot be connected to local and transient increase in temperature during the experiment, and has therefore to be connected to strain itself.

The zones of concentrated ductile strain are characterized by comminution of the clasts of quartz and feldspar initially present, accompanied by the development of a porosity at the submicron-scale. Chemical analysis revealed that along with intense grain-size reduction, the chemical composition of the feldspar was modified. On the basis of these microstructural and chemical evidence, it appears that the elementary processes behind ductile deformation include intense mechanical fracturing and mass transfer along the grain boundaries.

As a conclusion, it appears that strain has an undisputable effect to increase IR of . When applied to nature, the relevant temperature and mechanical realm of these experiments is the brittle-ductile transition, for temperature of the order or below 300°C. These conditions are also the ones corresponding to seismic deformation. Therefore we show here that Raman spectra in fault zones reflect not only the temperature history (including coseismic flash-heating), but also, and to a large extent, the strain history of the rock.

How to cite: Raimbourg, H., Benjamin, M.-M., Romain, A., Abdeltif, L., and Rémi, C.: The record of ductile strain by Raman spectroscopy of carbonaceous material – deformation experiments and applications to the brittle-ductile régime, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2302, https://doi.org/10.5194/egusphere-egu23-2302, 2023.

The subduction interface is characterized by downdip changes in seismic and aseismic behavior that is controlled by metamorphic reactions, deformation conditions, rheological properties, and fluid sources and pathways. Recent seismological evidence shows that a mixture of brittle-ductile deformation at the downdip limit of the seismogenic zone is responsible for producing slow slip and very low-frequency earthquakes. One way to address the possible mechanics and causes behind this dual deformation behavior is by observing exhumed rocks that provide us with the opportunity to make direct geological observations for microstructure and deformation mechanism analysis. Our project aims to understand the deformation mechanism hosted in the dominant mineral phases of metacherts and blueschist from an exhumed paleomegathrust in the Franciscan Subduction Complex. 

The Mesozoic Franciscan Subduction Complex consists mostly of underplated clastic sediment-rich terranes that are metamorphosed from prehnite-pumpellyite to blueschist facies. Angel Island, in the San Francisco Bay, is composed of blueschist facies metasedimentary and metabasic rocks containing potential paleo-megathrust faults that correlate to the source depths of slow slip earthquakes. Subduction-related faults and shear zones crop out in sea cliffs around the perimeter of the island and enable us to observe deformation features in characteristic subduction zone lithologies. We focus on a proposed subduction-related shear zone in blueschist-facies metachert juxtaposed against mafic/ultramafic rocks. Centimetric-scale field maps show coeval brittle-ductile features such as mutually cross-cutting extension fractures, kink folds, veins, and mylonitic foliations hosted in a wide variety of rheologically distinct lithologies. These lithologies contain synkinematic sodic amphibole and stilpnomelane demonstrating that this fault was active at blueschist facies. The main deforming mineral phases are sodic amphiboles, phyllosilicates, and quartz. The structural fabric comprises of shear fabrics, kink folding, dismembered veins, flattened grains, solution seams, and compositional layering. Amphiboles and phyllosilicates define the foliation of these rocks whereas quartz exhibits evidence of dislocation creep. Using EBSD-generated phase maps, we try to determine the deformation mechanisms hosted within the metacherts and blueschists along the paleomegathrust. Paleostress during megathrust creep will be constrained using EBSD-generated grain size statistics. Our study highlights the complexity of deformation within these lithologies while presenting evidence for possible deformation mechanisms witnessed at slow slip depths.

How to cite: Das, M., Rowe, C., and Mookerjee, M.: Microstructural investigations along a blueschist facies paleomegathrust: Implications for deformation mechanisms for deep slow slip behavior, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10817, https://doi.org/10.5194/egusphere-egu23-10817, 2023.

The Muddy Mt. thrust, Nevada, U.S.A, is an iconic contractual fault juxtaposing Paleozoic carbonates onto the autochthonous Mesozoic Aztec sandstone.  The minimum thickness of the thrust sheet is estimated at 4-5 km.  In contrast to many thrusts that exhibit shale-carbonate interfaces, the Muddy Mt. thrust has a mineralogically simple bimaterial interface defined by the carbonate-silicate interface.  The deformation associated with the thrust exists as a variable cataclastic zone on the order of 10-100m. Previous work has documented the extensive evidence of brittle deformation at a range of scales.  Questions at hand during such studies have included the systematic variation in fracture intensity; the contribution of fluid pressure, if any, to the thrust mechanics; the nature of deformation and permeability changes in the footwall porous sandstone and measurement of temperature transients along discrete slip surfaces.

Structures within the deformation zone, and along the thrust interface provide evidence of both seismic and interseismic displacements.  The latter comprise simple fracture, brecciation, shattered dolomite, extensive and cyclic cataclasis and fragment reduction producing tectonic foliation with evidence of cannibalized fragment, gouge injection, discrete slip surfaces and, notably, intracrystalline (plastic) deformation of carbonate aggregates.  Given that macroscopic structures, such as those listed, ultimately depend on atom-to-grain scale processes, this contribution examines the fine-scale textural attributes of aggregates proximal to the thrust interface that in many cases have grain/particle sizes sizes less than a micrometre.  The objective is to extract micromechanical behaviour that can be relevant to the displacement record of crustal-scale faults.  The microstructures and microfabrics of the interface rocks have been examined by analytical STEM, EBSD of samples prepared by focussed ion-beam techniques, with longer range elemental variations determined by mXRF.

How to cite: White, J. C., Merrithew, S. L., and Phillips, N. J.: Micro- to nanostructural analysis of deformation along the Muddy Mt. thrust, Nevada, U.S.A., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9254, https://doi.org/10.5194/egusphere-egu23-9254, 2023.

How to cite: Guggisberg, A., Lebihain, M., Moore, J., and Violay, M.: Effect of stress biaxiality on fracture energy and microstructures of tensile cracks, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12383, https://doi.org/10.5194/egusphere-egu23-12383, 2023.

In the southern Apennines, Mesozoic carbonates are characterized by the superimposition of different brittle deformation structures formed during different phases of rifting, orogenic shortening and post-orogenic extension. In the western Matese area, fault zones developed in the Triassic dolostones commonly exhibit the complex juxtaposition of different fault rocks, wide damage zones and pulverized dolomite rocks. Significantly, pulverization causes a drastic change in the petrophysical behavior of deformed rock masses through dynamic shattering. For these reasons, a better description of the processes that steer the “dolomite flour” spatial distribution through fault zones represents a fundamental aspect of the mechanical stratigraphy assessment related to subsurface infrastructure building and of the fault zone hydraulic behavior.

In this work, we present a multi-scalar and multi-methodological approach to provide a possible structural evolutionary scenario for the Matese Triassic dolostones. We analyzed fault zones outcropping along the eastern side of the Telese plain in the Ailano and Alife area. The fault zones consist of mature damage zones where pulverized rocks occur in patches of variable size (10s 100s of meters) immediately adjacent to the fault core. The fault zones show complex arrays of slip surfaces, including a major N-S normal fault crosscut and reactivated by NE-SW and NW-SE strike-slip faults. Extensive powder bodies are heterogeneously distributed within the damage zone and adjacent to the fault cores of both systems of faults. Fault zone architecture is associated with different brittle structural facies: i) cataclastic to ultra-cataclastic facies; ii) wide volumes of pulverized “dolomite flour”; iii) foliated cataclasite; iv) cataclastic shear bends; v) hydrothermalized dolomite; vi) unstrained lithons.

Fault analysis suggests that the N-S normal fault systems accommodated extension during the Mesozoic passive margin extensional phases. The NE-SW and NW-SE strike-slip fault systems developed in a successive phase of orogenic shortening. The shortening-related strike-slip faults propagation overprint the inherited Mesozoic fault zone through extensive pulverization processes of dolostones with the formation of a wide volume of “dolomite flour”.

How to cite: Diamanti, R., Camanni, G., and Vitale, S.: Structural evolution of long-lasting fault zones developed in dolostones in the south-western Matese area (southern Apennines, Italy)., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15512, https://doi.org/10.5194/egusphere-egu23-15512, 2023.

Stylolites are common microstructures in rocks, where mineral dissolution in a fluid localises in more or less discrete seams. Stylolite formation strongly affects rock porosity, pore connectivity and thus fluid flows. However, details of porosity evolution associated with stylolite nucleation, propagation and growth remain unclear, leading to the debate whether stylolites are conduits or barriers for fluids.

In this contribution, we use an exceptionally large high-resolution SEM-BSE mosaic (102,600 × 18,239 pixels, 0.17µm/pixel) to investigate the detailed microstructures of stylolites from the Eilean Dubh limestone of the NW Highlands in Scotland. Advanced image analyses indicate that porosity self-organises systematically around stylolites suggesting a four-stage growth process for stylolites: In stage 1, primary pyrites in the rock matrix concentrate stress and dissolution, leading to the formation of porosity. In stage 2, the dissolution porosity self-organises, triggering a chemical-hydraulic-mechanical feedback loop that facilitates further dissolution. In stage 3, the porous zones enlarge and grow along the direction of the smallest principal stress (process zone), concentrating the synkinematic precipitation of pyrites from an external fluid in the core (core zone). In stage 4, the isolated domains connect forming a stylolite with a core zone in the center (highest porosity) surrounded on both sides by a process zones (higher-than-matrix porosity), suggesting a conduit phase during stylolite formation. Since the stylolite-hosting Eilean-Dubh limestones were episodically imbricated below the Ullapool thrust and the stylolites formed sub-parallel to the internal thrust planes, we speculate that synkinematic fluids during the stylolite formation were pumped episodically into the rock during activity of the Ullapool thrust. The detailed stylolite microstructures and the proposed process for stylolite growth suggest that stylolite in carbonate may act as a conduit for a fluid flow.

How to cite: Hou, Z., Fusseis, F., Schöpfer, M., and Grasemann, B.: Syn-kinematic porosity evolution during nucleation and growth of stylolites (Eilean-Dubh limestone, NW Scotland), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8015, https://doi.org/10.5194/egusphere-egu23-8015, 2023.