Strong Crystallographic Preferred Orientation Research Articles

Natural fabrics of a hornblende-rich amphibolite: Implications for hornblende crystallographic preferred orientation and seismic anisotropy of the lower crust

The relation of seismic anisotropy to the deformation and flow of the lower crust relies on the intrinsic properties of the minerals in the crust and on the formation of crystallographic preferred orientation (CPO). Hornblende is an important constituent of the continental and oceanic lower crust and often exhibits a strong CPO. However, the formation of CPO in hornblende is not well understood, and it is difficult to uniquely relate its texture and anisotropy to conditions of metamorphism-deformation. We present a petrofabric analysis, using electron backscatter diffraction, of two hornblende-rich amphibolites from the Mamonia complex (Cyprus) with peak pressure and temperature of 0.6 GPa and 610 ± 40 °C. We distinguish between microstructures with varied strains and show that, while the common hornblende CPO—in which the [001] is aligned with lineation—is independent of strain, its symmetry does depend on strain: it is hexagonal (axial-[001]) under low strains but becomes orthorhombic ([001] and (100) parallel to the lineation and foliation normal of the deformation, respectively) under increasing strains. This transition is correlated with a decrease in the shape-preferred orientation and an increase in the intragrain misorientation, whose rotation is consistent with the easy slip system (100)[001] of hornblende. Thus, we interpret the hexagonal CPO to represent a metamorphic fabric (a texture formed under quasi-static conditions) and the orthorhombic CPO to represent a solid-state deformation fabric (a texture formed by strain accommodated via dislocation-mediated deformation). We consider the seismic anisotropy of the natural hornblende textures and compare two tectonic scenarios of lower crust deformation - crust thickening and channel flow with marked differences in the expected seismic anisotropy between the two scenarios. Taken together, the dataset presented here can be used as a new framework for understanding and interpreting natural textures and seismic anisotropy from the lower crust.

“In-plane” site-specific FIB lamella extraction from deformed magnetite and the investigation of low angle grain boundaries under TEM

In this study a modus operandi to investigate site-specific nanostructures in thin films (lamellae) excavated “in-plane” across (sub)grain boundaries is presented. This is done by discussing the case of a magnetite grain hosted in a thin section of banded iron formation (Norway) that is prepared parallel to the kinematic reference frame (XZ section of the strain ellipsoid). SEM-EBSD analysis reveal that the magnetite grains do not develop a strong crystallographic preferred orientation, although individual grains are strained and show evidence of intracrystalline deformation in form of low angle grain boundaries (LAGB's). Two “in-plane” lamellae using focused ion beam (FIB) technique are excavated from a magnetite grain in the kinematic reference frame, and nanostructures are studied along three LAGB's using high resolution transmission electron microscopy imaging followed by Fourier transformation (FT), inverse FT and estimation of dislocation densities. Our data establish an empirical relationship for the studied LAGBs, namely, the smaller the angle between LAGB and X-direction, the larger are the shear strain and dislocation density. This relationship is validated from numerical simulations of viscoplastic deformation and dynamic recrystallisation of polycrystalline aggregates of halite, which is also a cubic mineral analogous to magnetite. In addition to the site-specific “in-plane” FIB lamella information, this study also shows that in a deformed mineral the different orientations of the LAGB compared to the principal strain axes show a different dislocation density. This approach of full tracking of the extension direction (X) from the macroscopic to the nano-scale could play an important role in forward modelling of microstructure evolution in future studies.

TitaniQ temperatures and textural analysis as a record of the deformation history in a major continental shear zone system, Borborema Province, Brazil

We combine quartz textural analysis with Titanium-in-quartz (TitaniQ) geothermometry to investigate the late stages of deformation in mylonitic rocks from the Sucuru dike swarm in the Borborema Province. The wide range in TitaniQ temperatures (>340 to 740 °C) closely related to grain sizes are attributed to inefficient Ti resetting during bulging (BLG) and subgrain rotation (SGR) recrystallization. The highest temperature values (>500–740 °C) occur in the core of quartz ribbons and porphyroclasts. They are interpreted as the record of early-stage plastic deformation, likely during the intrusion of magmatic bodies, and are progressively overprinted during a lower-temperature stage (>340–500 °C) associated with intense dynamic recrystallization and production of a large amount of fine-grained matrix. The dominantly weak crystallographic preferred orientations (CPOs) can be attributed to a large variety in crystallographic orientations of parent grains, which control to a large extent the CPO evolution of daughter grains, and to activation of multiple slip systems in the <a > direction (basal, rhomb and prism) due to synkinematic temperature decrease. The CPOs are consistent with strain localized by operation of dislocation creep-accommodated grain boundary sliding (DisGBS) during the low-temperature stage. DisGBS seems to be less important in coarse pure quartz domains, where moderate to strong CPOs indicate the dominance of dislocation creep. Several evidence of intracrystalline deformation in quartz ribbons and porphyroclasts point to dominant activation of dislocation creep at the early high-temperature stage of deformation, with activation of prism <c>. The dike orientations, geometry and evidence of solid-state deformation in their margins, fit well with progressive E-W dextral shearing, where they intrude and crystallize in a NW-SE direction and rotate clockwise during shearing. This implies that the dextral shear regime that dominated the Borborema province from ca. 600 Ma was still in effect into the Cambrian some 60–70 million years later.

Microstructural and textural modification of columnar calcite under increasing shear strain (Evia Island, Greece)

We studied the microstructural and textural modification of columnar calcite naturally deformed at increasing shear strain. The studied marbles were sampled from different structural levels of the Basal Unit of the Cycladic Massif on Evia Island, Greece. The marbles have experienced high pressure metamorphism at peak temperatures around 350 °C. The long axes of the columnar calcite grains and their c-axes were originally oriented normal to the foliation and were subsequently rotated during exhumation. The obliquity of the long axes of relict calcite grains with respect to the foliation was used to infer shear strain. Electron backscatter diffraction (EBSD) analysis was used to characterize the marble microstructure as well as the crystallographic preferred orientations (CPO) and misorientations in calcite. Recrystallization of calcite was active from very low shear strains (0.1) and mylonitization (∼60% area fraction of recrystallized grains) took place at shear strains >∼4. Subgrain rotation was the dominant recrystallization mechanism in calcite in all samples. Differential stress and strain rate did not show any systematic variation with increasing shear strain. A shear strain of up to ∼4 was not enough to produce a strong CPO in calcite.

Crustal anisotropy and deformation of the southeastern Tibetan Plateau revealed by seismic anisotropy of mylonitic amphibolites

The composition and deformation patterns in the deep crust of southeastern Tibet are remain controversial. In this study, we present microstructures, fabric and seismic properties of amphibolites from the Ailao Shan-Red River shear zone, western Yunnan, China. The amphibolites display a distinct mylonitic foliation, and the amphibole grains show low intracrystalline deformation, slight compositional zoning but strong crystallographic preferred orientations (CPOs). We explain these CPOs could have formed by rigid body rotation during deformation accommodated by diffusion creep and grain boundary sliding. The amphibolites show strong seismic anisotropies (AVp = 4.6–9.3% and Max. AVs = 3.2–9.0%). Rock recipe modelling results indicate the seismic anisotropies of the mylonitic amphibolites are dominated by the content of biotite and amphibole. The delay time (0.28–1.01 s, average of 0.58 s) calculated for a 40 km-thick amphibolite layer is consistent with the observed delay time (0.52 s) of Pms wave splitting in this region, which indicates a thick amphibolitic layer in the crust of the southeastern Tibetan Plateau. The observed Pms wave splitting, extensive fold deformation and low velocity zones (LVZs) in the Diancang Shan (DCS) massif indicate two anisotropic layers in the crust. The upper layer related to frozen fabrics of amphibolites, which deformed during compressional folding and strike-slip shear. The lower layer related to the alignment of amphibole and melt pocket induced by present-day channel flow. The observed Pms wave splitting in the Ailao Shan (ALS) massif is attributed to amphibolites with vertical foliation and horizontal lineation which are generated by compressional folding and strike-slip shearing.

Dissolution precipitation creep as a process for the strain localisation in mafic rocks

The lower crust is, on average, mafic in composition and composed of minerals that remain mechanically strong up to high temperatures. Here we show that dissolution-precipitation creep (as a type of diffusion creep) plays a major role in deformation of gabbro bodies at upper amphibolite facies conditions. The Kågen gabbro, N. Norway, is comprised of undeformed gabbro lenses enclosed by mylonitised margins that deformed at 690 ± 25 °C and 1.0–1.1 GPa. The evolution of the microstructures and fabric of the low strain gabbro to high strain margins were investigated. Original clinopyroxene and plagioclase dissolved during mineral reactions and precipitated as new minerals phases: new plagioclase and clinopyroxene (different compositions relative to the magmatic parents) and additional amphibole and garnet. Microstructural and crystallographic preferred orientation (CPO) data indicate that dissolution-precipitation creep is the dominant deformation mechanism. Amphibole shows a strong CPO that is primarily controlled by orientated growth in the stretching direction. The progression of mineral reactions and weakening is directly connected to a fluid-assisted transformation process that facilitates diffusion creep deformation of strong minerals at far lower stresses and temperatures than required by dislocation creep. Initially strong lithologies can become weak, provided that reactions proceed during deformation.

The Effect of Grain Boundaries on Plastic Deformation of Olivine

AbstractThe plastic deformation of olivine has been studied for decades. However, the precise role of grain boundaries during deformation in, for example, the dislocation‐accommodated grain‐boundary sliding regime, remains poorly understood. Specifically, we lack knowledge regarding the manner in which grain boundaries interact with other defects, such as dislocations, during deformation. To investigate the interaction of dislocations and grain boundaries, we analyzed the structure and distribution of grain boundaries in a polycrystalline aggregate of Fo50 deformed in torsion (Hansen, Zimmerman, Dillman, &amp; Kohlstedt, 2012, https://doi.org/10.1016/j.epsl.2012.04.016). We characterized the microstructure of the aggregate using electron‐backscatter diffraction and transmission electron microscopy in three perpendicular directions. An increase in plastic strain is associated with the development of a strong crystallographic preferred orientation and a grain‐boundary plane distribution that evolves from a uniform distribution to one dominated by (010)‐type planes. We use the m' factor, to evaluate the potential for transmission of dislocations across grain boundaries based on the relative orientations of slip systems between neighboring grains. With progressive deformation, our analysis indicates an increase in abundance of apparently slip‐transparent boundaries until moderate strains ( = 4) are reached. Based on these observations, we propose that specific types of grain boundaries are created by dislocation activity and that the input of dislocations into grain boundaries facilitates grain‐boundary sliding. Our results provide insight into the microphysics of olivine deformation and highlight the importance of the coupled study of inter‐ and intragranular mechanisms during rock deformation.

Cross‐Scale Seismic Anisotropy Analysis in Metamorphic Rocks From the COSC‐1 Borehole in the Scandinavian Caledonides

AbstractMetamorphic and deformed rocks in thrust zones show particularly high seismic anisotropy causing challenges for seismic imaging and interpretation. A good example is the Seve Nappe Complex in central Sweden, an old exhumed orogenic thrust zone that is characterized by a strong but incoherent seismic reflectivity and considerable seismic anisotropy. However, only little is known about their origin in relation to composition and structural influences on measurements at different seismic scales. Here, we present a new integrative study of cross‐scale seismic anisotropy analyses combining mineralogical composition, microstructural analyses, and seismic laboratory experiments from the COSC‐1 borehole, which sampled a 2.5‐km‐deep section of metamorphic rocks deformed in an orogenic root now preserved in the Lower Seve Nappe. While there is strong crystallographic preferred orientation in most samples in general, variations in anisotropy depend mostly on bulk mineral composition and dominant core lithology as shown by a strong correlation between these. This relationship enables to identify three distinct seismic anisotropy facies providing a continuous anisotropy profile along the borehole. Moreover, comparison of laboratory seismic measurements and electron‐backscatter diffraction data reveals a strong scale dependence, which is more pronounced in the highly deformed, heterogeneous samples. This highlights the need for comprehensive cross‐validation of microscale anisotropy analyses with additional lithological data when integrating seismic anisotropy over seismic scales.

Microstructure and seismic properties of amphibole-rich rocks from the deep crust in southern Tibet

The crust of southern Tibet shows strong azimuthal and radial seismic anisotropies. However, their origin has remained ambiguous. Here we characterized the composition, microstructure, and seismic properties of amphibole-rich deep-crust xenoliths and exhumed rocks from a deep-crust terrane in southern Tibet. The xenoliths show typical magmatic textures and have amphiboles with homogenous chemical composition, low intracrystalline deformation, but strong crystallographic preferred orientations (CPOs). We interpret these CPOs as products of rotation of non-equant amphibole crystals in a deforming magma. The exhumed amphibolites display evidence for amphibole deformation by dislocation creep. Plagioclase in all samples shows nearly random CPOs. The calculated seismic properties of the studied amphibolites indicate that amphibole is likely a major contributor to the deep-crustal seismic anisotropies observed in southern Tibet. However, these anisotropies do not necessarily record solid-state flow. They may also result from magmatic flow in large synkinematic mafic plutons.

Deformation Behavior and Inferred Seismic Properties of Tonalitic Migmatites at the Time of Pre‐melting, Partial Melting, and Post‐Solidification

AbstractAs seismic data from the lower crust becomes more readily available, it is important to link seismic properties to the ongoing processes within lower crustal evolution. This includes high temperature, pre‐ and post‐migmatization solid state deformation as well as melt‐present deformation. We selected two tonalitic migmatites with variable former melt content (one metatexite and one diatexite) from the lower crustal Daqingshan area, northern North China Craton to assess the link between seismic properties and rock structure and rheology. Field observation along with microstructural features suggest that the characteristics of hornblende and plagioclase within the residuum of the metatexite can be used to derive information on the pre‐melt deformation. Residuum's plagioclase CPO (crystallographic preferred orientations) is consistent with high temperature dislocation creep as the main deformation mechanism; similarly, hornblende shows a strong CPO related to dislocation creep. During syn‐melt (melt present) conditions, phenocrysts of plagioclase in the metatexite's neosome and K‐feldspar and peritectic hornblende in the diatexite's neosome are present. The rheology of the rock was dominated by melt; hence is inferred to follow Newtonian flow. After melt crystallization deformation is minor but again dominated by dislocation creep. The seismic properties (seismic velocity, anisotropy, Vp/Vs ratio, etc.) for pre‐ and post‐melt have similar values expected values for solid mafic rocks, whilst the syn‐melt seismic velocities are generally lower and Vp/Vs ratios and seismic anisotropies are higher.

Interplay between melt infiltration and deformation in the deep lithospheric mantle (External Liguride ophiolite, North Italy)

Mantle peridotites from the External Liguride Jurassic ophiolites (Northern Apennines, Italy) show diffuse occurrence of pyroxenite bands, recording melt migration and crystallization at the lithosphere-asthenosphere boundary, during old, pre-Jurassic stages of their subcontinental lithospheric mantle evolution. We present coupled microstructural and geochemical study of profiles across various types of pyroxenite-peridotite layering in these ophiolites, aiming to constrain the relative timing and potential interplay between melt infiltration and mantle deformation. The mantle sequence is composed of lherzolite and harzburgite, occasionally interleaved with dunite, crosscut by centimeter- to decimeter-wide pyroxenite layers. The peridotites have a porphyroclastic texture and show a penetrative tectonic foliation subparallel to the pyroxenite layering. Peridotite-pyroxenite contacts are irregular at the grain scale. Olivine and pyroxenes in both peridotites and pyroxenites record moderate to strong crystallographic preferred orientations (CPO) with alignment of olivine [100] and pyroxenes [001] axes subparallel to the stretching lineation marked by olivine and pyroxenes elongation. This is compatible with coeval deformation of olivine and pyroxenes during a high-temperature, spinel lherzolite-facies deformation event. The major and trace element compositions of peridotites record a metasomatic imprint that decreases with distance from the pyroxenite layers, whereas the strength of the olivine CPO decreases from the country peridotites towards the pyroxenite layers. The parallelism between pyroxenite layers and the peridotite foliation, their irregular contacts, as well as the spatial correlation between CPO and geochemical changes, are consistent with syn- to late-kinematic emplacement of the pyroxenites. These observations point to a strong interplay between deformation and melt transport processes in the mantle, characterized by melt focusing in conduits parallel to the foliation and changes in the mantle deformation processes due to the presence of melts. Exhumation of this mantle section in the Jurassic resulted in partial replacement of the spinel-facies assemblage by a plagioclase-bearing assemblage. Topotaxial relationship between plagioclase and precursor spinel-facies minerals suggests that this exhumation was not associated with pervasive deformation of the peridotite, but rather accommodated by deformation localized in discrete shear zones, not sampled in the present study.

Strain localization and fluid-assisted deformation in apatite and its influence on trace elements and U–Pb systematics

This paper presents electron backscatter diffraction (EBSD), trace element and U–Pb data of apatite grains from a granitic mylonite from the Taxaquara Shear Zone (SE Brazil). The mylonite recrystallized under upper-greenschist facies and presents two types of apatite with distinct microstructures. Type-1 apatite appears in quartz-rich layers and does not exhibit any microstructural, crystallographic, or chemical evidence of deformation/recrystallization, and resembles the original igneous apatite. Type-2 apatite appears in mica-rich layers and exhibits core-and-mantle microstructures, and intragranular subgrain development, suggesting that they have undergone dynamic recrystallization. Recrystallized tails of type-2 apatite grains exhibit a strong c-axis crystallographic preferred orientation parallel to the X-direction (stretching lineation), and lack evidence of dislocation density. This evidence from type-2 apatite grains, combined with REE depletion, high La and a negative Ce anomaly compared to type-1 grains, suggests that type-2 apatite tails underwent recrystallization via dissolution-precipitation creep, whereas parental grains underwent crystal-plastic deformation and subgrain formation through dynamic recrystallization. Phase-equilibrium modelling and quartz CPO opening-angle thermometry are consistent with recrystallization at ∼480 – 530°C and 2.2 – 5.0 kbar. We were not able to determine precise deformation ages from type-2 apatite because fluid-assisted recrystallization appears to have substantially decreased the U/Pb ratio. We find that preferential fluid flow along high-strain, biotite-rich layers in the mylonite caused type-2 apatite to recrystallise, whereas type-1 apatite in low strain layers was unaffected and retained the characteristics of the protolith.

Crystallographic preferred orientation of talc determined by an improved EBSD procedure for sheet silicates: Implications for anisotropy at the slab–mantle interface due to Si-metasomatism

AbstractTalc is widely distributed over the Earth's surface and is predicted to be formed in various tectonic settings. Talc is a very soft and anisotropic sheet silicate showing very low friction behavior. Therefore, the formation of talc is expected to weaken the strength of talc-bearing rocks and may be associated with the initiation of subduction, and with a decrease in the coupling coefficient resulting in aseismic movements along faults and shear zones within subduction zones. For these reasons, understanding the crystallographic preferred orientation (CPO) of talc is important to quantify the anisotropy and physical properties of the host rock. However, it is difficult to measure a significant number of talc crystal orientations and to evaluate the accuracy of the measurements using electron-backscattered diffraction (EBSD). Therefore, talc CPO has not been reported, and there is uncertainty regarding the estimation of the strength of deformed talc-bearing rocks. Using methods developed for antigorite, we report the first successful EBSD measurements of talc CPO from a talc schist formed due to Simetasomatism of ultramafic rocks by subduction zone fluids. We used a combination of W-SEM and FE-SEM measurements to examine domains of various grain sizes of talc. In addition, we used TEM measurements to evaluate the accuracy of the EBSD measurements and discuss the results of talc CPO analysis. Talc CPO in the present study shows a strong concentration of the pole to the (001) plane normal to the foliation. The strongest concentration of the [100] direction is parallel to the lineation. The talc schist produces similar S-wave splitting and P- and S-wave anisotropy as antigorite schist in deeper domains, thus identifying talc-rich layers in subduction zones may require a combination of geophysical surveys, seismic observations, and anisotropy modeling. The presence of strong talc CPO in rocks comprising the slab–mantle interface boundary may promote spatial expansion of the slip area during earthquakes along the base of the mantle wedge.

The effect of hydrous mineral content on competitive strain localization mechanisms in felsic granulites

This study investigates the grain-scale mechanisms that facilitated strain localization in the lower crust within the East Athabasca Mylonite Triangle of northern Saskatchewan, Canada. There, two distinct lithotectonic units, the Upper Deck and Northwestern Domain, are juxtaposed across a km-scale shear zone that has been previously interpreted to record thrust and/or normal-sense displacement. Our study reports new quartz c-axis opening angle, Ti-in-biotite and Ti-in-quartz geothermometry and amphibole-plagioclase geothermobarometry within syn-kinematic phases indicating that both units were deformed and metamorphosed at upper amphibolite to granulite facies (~725 °C and ~0.8 GPa). While sharing similar conditions, the dominant mechanisms of deformation across the two units are strongly linked to modal proportion of syn-kinematic hydrated mineral phases. Detailed microstructural and crystallographic preferred orientation (CPO) analyses reveals that specimens poor in hydrated minerals (<10%) deformed mainly by dislocation creep and dynamic recrystallization allowing it to develop a strong quartz CPO, while in specimens rich in hydrated minerals (>10%) pressure solution and (re)precipitation processes enhanced phase mixing and promoted grain boundary sliding resulting in weakly developed or absent quartz CPO. These results indicate a strong mineralogical and fluid control on deformation and, therefore, which strain softening process (es) may dominate in lower crustal shear zones.

Architecture of Anoteropora latirostris (Bryozoa, Cheilostomata) and implications for their biomineralization

Cheilostome Bryozoa Anoteropora latirostris, a colonial marine invertebrate, constructs its skeleton from calcite and aragonite. This study presents firstly correlated multi-scale electron microscopy, micro-computed tomography, electron backscatter diffraction and NanoSIMS mapping. We show that all primary, coarse-grained platy calcitic lateral walls are covered by fine-grained fibrous aragonite. Vertical lateral walls separating autozooid chambers have aragonite only on their distal side. This type of asymmetric mineralization of lateral walls results from the vertical arrangement of the zooids at the growth margins of the colony and represents a type of biomineralization previously unknown in cheilostome bryozoans. NanoSIMS mapping across the aragonite-calcite interface indicates an organic layer between both mineral phases, likely representing an organic template for biomineralization of aragonite on the calcite layer. Analysis of crystallographic orientations show a moderately strong crystallographic preferred orientation (CPO) for calcite (7.4 times random orientation) and an overall weaker CPO for aragonite (2.4 times random orientation) with a high degree of twinning (45%) of the aragonite grains. The calculated Young’s modulus for the CPO map shows a weak mechanical direction perpendicular to the colony’s upper surface facilitating this organism’s strategy of clonal reproduction by fragmentation along the vertical zooid walls.

Coseismic ultramylonites: An investigation of nanoscale viscous flow and fault weakening during seismic slip

Faults weaken during the propagation of earthquakes due to the onset of thermally-activated mechanisms, which vary depending on the rock type. Recent experimental work suggests that carbonate-hosted faults are lubricated by viscous flow in nano-granular aggregates having ultramylonitic textures. However, their frail nature has often hindered unbiased characterisation of the textures and deformation mechanisms operating at such extreme conditions (strain rates as high as 104), which remain so far poorly investigated and understood.We explore the formation, evolution and deformation mechanisms of coseismic ultramylonites in carbonate-hosted faults generated during high velocity (1.4 ms−1), displacement-controlled shear experiments in a rotary apparatus. Microstructures were analysed using integrated SEM and TEM imaging while detailed crystallographic fabrics were investigated using the electron back-scattered diffraction (EBSD) technique.Mechanical data show that the strength of the experimental fault decays dynamically with slip, according to a characteristic four stage evolution; each stage is associated with characteristic textures. Microstructural observations show that brittle processes dominate when the fault is strong (friction coefficients >0.6). Cataclasis, aided by twinning and crystal plasticity, operates forming an extremely comminuted shear band (mean grain size ∼200 nm). As the fault starts weakening, shear localises within a well-defined principal slip zone. Here, thermally-activated grain size sensitive (GSS) and insensitive (GSI) creep mechanisms compete with brittle processes in controlling fault strength. GSI mechanisms produce strong monoclinic crystallographic preferred orientations in the slip zone, while textures and crystallographic orientations in adjacent locations do not evolve from the previous deformation stage. By the end of the transient weakening stage, the slip zone has reached a steady state thickness (30 μm) and shows a nanogranular ultramylonitic texture. The intensity of the crystallographic preferred orientation in the coseismic ultramylonite is reduced compared to the previous stage, due to grainsize sensitive creep mechanisms becoming gradually more dominant. As the experimental fault re-strengthens, upon deceleration to arrest, the ultramylonite may be partially reworked by brittle deformation.Our findings show that the crystallographic orientations of transient microstructures are preserved in the slip zone of coseismic ultramylonites and in narrow, adjacent deactivated layers, where mirror-like surfaces are located. This shows that EBSD techniques can usefully be employed to determine the deformation mechanisms of coseismic ultramylonites and their evolution during earthquake slip in both experimental and, potentially, natural faults.

Effect of hot-dip siliconizing time on phase composition and microstructure of Mo–MoSi2 high temperature structural materials

Mo–MoSi2 high temperature structural materials were obtained by hot-dip siliconizing method. The results show that the silicon melt and molybdenum substrate have good wettability, molten silicon reacts with molybdenum substrate to form a molybdenum silicide phase with a columnar structure, and a strong MoSi2 crystallographic preferred orientation (CPO) on the (110) crystal face is characterized. While, the Mo–MoSi2 high temperature structural materials consist mainly of silicon rich layer, MoSi2 layer, interface layer and molybdenum substrate. The coating surfaces contain a high silicon concentration (55–65 wt%) than the MoSi2 layer (30–35 wt%). Moreover, the thicknesses and the average grain sizes of siliconized coatings increase sharply with increasing siliconizing time. The coatings thicknesses are 20, 25, 30, and 50 µm, respectively. And the average grain sizes are 6.9, 9.3, 11.7, and 11.8 µm, respectively.

Microstructural and textural evolution of calcite deformed to high shear strain by high-pressure torsion

Calcite powder aggregates were deformed to shear strains up to 80 by high-pressure torsion at temperatures of 235 and 450 °C and confining pressures of 1–4 GPa. At 235 °C the deformation occurs by twinning and limited dislocation creep as well as by brittle processes. In contrast, subgrain rotation recrystallization is the predominant deformation mechanism at 450 °C. The high confining pressures affect the microstructural evolution by inhibiting crack nucleation and propagation and by constraining the mobility of point defects. While at 1.2 GPa confining pressure transient core-mantle microstructures are produced for all strain-rates, the recrystallization is strain-rate dependent at 2–4 GPa confining pressure. Core-mantle microstructures are produced at high strain-rates, whereas the porphyroclasts deform more homogeneously at low strain rates. This change in microstructural evolution is ascribed to the phase transition from calcite to CaCO3-II at 1.6 GPa. Within the stability field of CaCO3-II an increase in the confining pressure leads to reduction of the recrystallized grain size. Irrespective of the confining pressure a strong crystallographic preferred orientation is developed during deformation at 450 °C, which is compatible with dislocation glide on the r{101¯4}〈2¯021〉 slip systems.

Microstructural evidence for the deep pulverization in a lower crustal meta‐anorthosite

AbstractWe report on the evidence of the pulverization in a deep‐seated meta‐anorthosite in the Eidsfjord shear zone, Vesterålen, northern Norway. Some plagioclase porphyroclasts comprise a few large relict clasts and many fine grains that preserve the outlines of the original grains. The fine‐grained plagioclase does not show any plastic‐deformation microstructures and has strong crystallographic preferred orientations, which are inherited from the twinned porphyroclast. Misorientation‐axis distributions indicate that the grains have rotated randomly, so that the misorientation axes are not aligned with either the crystallographic or kinematic axes. The observed grain‐size distribution has a fractal dimension, suggesting their fracturing/fragmentation origin. The microstructures are characterized by the fracturing/fragmentation with a very low shear strain, indicating that it may be associated with pulverization at ~20–25 km depth.

Mechanics and microstructure of deformed natural anisotropic ice

We deformed coarse-grained (∼10 mm) natural ice in axial compression at −30 °C, to an axial strain of 0.2, at three different angles to the existing strong crystallographic preferred orientation (CPO). We used three strain rates for each sample orientation. Cryo-electron backscatter diffraction (EBSD) maps show that after deformation there is a mixture of large (∼1–2 mm) relict grains, and finer (100–200 μm) recrystallised grains. The fine grains form a connected network in all samples. The large grains define a very strong CPO with equivalent but weaker CPOs in the recrystallised grains. The final CPO changed completely from its original orientation. Lattice distortion and subgrains equivalent in size to recrystallised grains suggest a subgrain rotation recrystallisation process has generated the recrystallised fraction. We suggest that strain rates were higher in the connected network of recrystallised grains because of a significant component of grain boundary sliding (GBS) that enables large grains to rotate by a combination of glide on the basal plane and rigid rotation, to define a very strong CPO. GBS weakens the CPO in the finer grained regions. The patterns of mechanical behaviour and the resultant microstructures do not bear an obvious relationship to original CPO.