Subgrain Rotation Recrystallization Research Articles

Subgrain Rotation Recrystallization During Shearing: Insights From Full‐Field Numerical Simulations of Halite Polycrystals

AbstractWe present, for the first time, results of full‐field numerical simulations of subgrain rotation recrystallization of halite polycrystals during simple shear deformation. The series of simulations show how microstructures are controlled by the competition between (i) grain size reduction by creep by dislocation glide and (ii) intracrystalline recovery encompassing subgrain coarsening by coalescence through rotation and alignment of the lattices of neighboring subgrains. A strong grain size reduction develops in models without intracrystalline recovery, as a result of the formation of high‐angle grain boundaries when local misorientations exceed 15°. The activation of subgrain coarsening associated with recovery decreases the stored strain energy and results in grains with low intracrystalline heterogeneities. However, this type of recrystallization does not significantly modify crystal preferred orientations. Lattice orientation and grain boundary maps reveal that this full‐field modeling approach is able to successfully reproduce the evolution of dry halite microstructures from laboratory deformation experiments, thus opening new opportunities in this field of research. We demonstrate how the mean subgrain boundary misorientations can be used to estimate the strain accommodated by dislocation glide using a universal scaling exponent of about 2/3, as predicted by theoretical models. In addition, this strain gauge can be potentially applied to estimate the intensity of intracrystalline recovery, associated with temperature, using quantitative crystallographic analyses in areas with strain gradients.

Crystallographic control and texture inheritance during mylonitization of coarse grained quartz veins

Quartz veins within Rieserferner pluton underwent deformation during post-magmatic cooling at temperature around 450°C. Different crystallographic orientations of cm-sized quartz vein crystals conditioned the evolution of microstructures and crystallographic preferred orientations (CPO) during vein-parallel simple shear up to high shear strains (γ≈10). For γ<2, crystals stretched to ribbons of variable aspect ratios. The highest aspect ratios resulted from {m}<a> glide in ribbons with c-axis sub-parallel to the shear zone vorticity Y-axis. Ribbons with c-axis orthogonal to Y (XZ-type ribbons) were stronger and hardened more quickly: they show lower aspect ratios and fine (grain size ~10–20μm) recrystallization along sets of microshear zones (μSZs) exploiting crystallographic planes. Distortion of XZ-type ribbons and recrystallization preferentially exploited the slip systems with misorientation axis close to Y. New grains of μSZs initiated by subgrain rotation recrystallization (SGR) and thereupon achieved high angle misorientations by a concurrent process of heterogeneous rigid grain rotation around Y associated with the confined shear within the μSZ. Dauphiné twinning occurred pervasively, but did not play a dominant role on μSZ nucleation. Recrystallization became widespread at γ>2 and pervasive at γ≈10. Ultramylonitic quartz veins are fine grained (~10μm, similar to new grains of μSZ) and show a CPO banding resulting in a bulk c-axis CPO with a Y-maximum, as part of a single girdle about orthogonal to the foliation, and orientations at the pole figure periphery at moderate to high angle to the foliation. This bulk CPO derives from steady-state SGR associated with preferential activity, in the different CPO bands, of slip systems generating subgrain boundaries with misorientation axes close to Y. The CPO of individual recrystallized bands is largely inherited from the original crystallographic orientation of the ribbons (and therefore vein crystals) from which they derived. High strain and pervasive recrystallization were not enough to reset the initial crystallographic heterogeneity and this CPO memory is explained by the dominance of SGR. This contrast with experimental observation of a rapid erasure of a pristine CPO by cannibalism from grains with the most favourably oriented slip system under dominant grain boundary migration recrystallization.

Plastic deformation and post-deformation annealing in chromite: Mechanisms and implications

Plastic deformation in chromite is not frequently reported in literature. We present a detailed microstructural analysis of this mineral from the massive chromitite of the Neoarchaean Sittampundi Complex, southern India. The study reveals intracrystalline plasticity is dominantly active in this mineral, and it produces distinctive features corresponding to at least two different microstructural regimes. The Regime 1 is deformation-related and it commenced with recovery of strained grains and formation of new grains, corresponding to subgrain rotation recrystallization. This was followed by nucleation of strain-free new grains in regions of high strain. The Regime 2 appears to be post-deformational and dominantly temperature-controlled, producing distinctive features of static annealing of already deformed grains. This regime corresponds to grain boundary migration recrystallization resulting coarsening of strain-free facetted, new grains at the expense of high-dislocation density subgrains. The resultant micro-features resemble closely what is known as “abnormal grain growth,” not yet documented for chromites. These coarse grains, in places, also feature accommodation of deformation by displaying very low-angle subgrain boundaries. The movement of high-angle grain boundaries in Regime 2 through precursor strained grains provided high diffusivity paths for the rapid exchange of components, producing compositional heterogeneity between grains dominated by deformation features and facetted new grains. These microstructural observations coupled with chemical heterogeneity provide new directions in interpreting the deformation mechanism of chromite and its annealing history at the post-deformation stage.

Internal structure of the Supragetic Unit basement in the Serbian Carpathians and its significance for the late Early Cretaceous nappe-stacking

Fault-related folds and hanging-wall structures reflect the geometry of the main thrusts in foldthrust belts. The results of the structural analysis of the Supragetic Unit metamorphic basement in eastern Serbia at map-, outcrop- and thin-section scale, and its importance for the late Early Cretaceous nappe-stacking are presented in this paper. The Supragetic Unit metamorphic basement includes various volcano-sedimentary rocks of Ordovician-Silurian protolith age. They were metamorphosed to the low greenschist facies with temperatures reaching 300-350?C and pressure reaching 0.3-0.5 GPa. The microscale studies show that quartz and albite demonstrate dominantly bulging and locally subgrain rotation recrystallisation, while chlorite, sericite and muscovite define spaced to continuous foliation recognised both at the outcrop- and the thin-section-scale. The statistical analysis based on the available map data shows low- to high-angle west-dipping foliation which is interpreted as an indicator of flat-ramp geometry of the Supragetic thrust, rather than east-vergent tight to isoclinal folding. At the thin-section scale ductile to semi-ductile C?-S structures indicate top to ESE thrusting. Subsequent kinking, recognised both at the outcrop- and the thin-section-scale, deform the older foliation. Those kink bands are the result of WNW-ESE to NW-SE compression and could represent the later stage of a continuous deformation event during which C?-S structures were formed. The youngest, brittle deformation is represented by subvertical joints with no offset recognised in thin-sections. The structural characteristics of the Supragetic Unit low-grade metamorphic basement in the studied areas, combined with tectonothermal events recognised elsewhere in Dacia mega-unit, could imply a possible initiation of the late Early Cretaceous nappe-stacking in the ductile to semi-ductile/semi-brittle domain.

Rio Maria granodiorite and associated rocks of Ourilândia do Norte – Carajás province: Petrography, geochemistry and implications for sanukitoid petrogenesis

Ourilândia do Norte rocks are located near Rio Maria-Carajás domains boundary and are associated to Rio Maria Mesoarchean sanukitoid. Two groups were defined on the basis of geochemistry: (i) quartz-monzodiorites and granitoids (tonalite and granodiorite) which match to the sanukitoid (stricto sensu); and (ii) enclaves and (quartz) diorites that do not. Despite this, these rocks are spatially and temporally associated and they develop microstructures under three dynamic recrystallization regimes: (i) bulging recrystallization (300–700 °C); (ii) subgrain rotation recrystallization (<700 °C); (iii) grain boundary migration recrystallization (>600 °C). Furthermore, they belong to medium to high potassium calc-alkaline series, being magnesian and mainly metaluminous. The Mg#, Cr and Ni abundances point out an ultramafic mantle source. Variation of LILE, Nb/Y, (La/Yb)N and Sr/Y contents indicate a metasomatized mantle by two independent agents, namely TTG-like melt and aqueous fluids, at different depths. In this way, quartz-monzodiorites and granitoids were derived from fractionation of a mafic magma, whose source is a TTG-metasomatized mantle, at greater depths, while enclaves and (quartz) diorites were generated from a fluid-metasomatized mantle, at shallower depths. The close spatial and temporal association of rocks with sanukitoid (quartz-monzodiorites and granitoids) and BADR (enclaves and (quartz) diorites) signature suggest that Ourilândia do Norte granitoids were generated in close spatial and temporal association with active subduction.

Microstructural evidence for the transition from dislocation creep to dislocation-accommodated grain boundary sliding in naturally deformed plagioclase

We use quantitative microstructural analysis including misorientation analysis based on electron backscatter diffraction (EBSD) data to investigate deformation mechanisms of naturally deformed plagioclase in an amphibolite gabbro mylonite. The sample is from lower oceanic crust exposed near the Southwest Indian Ridge, and it has a high ratio of recrystallized matrix grains to porphyroclasts. Microstructures preserved in porphyroclasts suggest that early deformation was achieved principally by dislocation creep with subgrain rotation recrystallization; recrystallized grain (average diameter ∼8 μm) microstructures indicate that subsequent grain boundary sliding (GBS) was active in the continued deformation of the recrystallized matrix. The recrystallized matrix shows four-grain junctions, randomized misorientation axes, and a shift towards higher angles for neighbor-pair misorientations, all indicative of GBS. The matrix grains also exhibit a shape preferred orientation, a weak lattice preferred orientation consistent with slip on multiple slip systems, and intragrain microstructures indicative of dislocation movement. The combination of these microstructures suggest deformation by dislocation-accommodated GBS (DisGBS). Strain localization within the recrystallized matrix was promoted by a transition from grain size insensitive dislocation creep to grain size sensitive GBS, and sustained by the maintenance of a small grain size during superplasticity.

An experimental study on the brittle–plastic transition during deformation of granite

Naturally and experimentally deformed samples of granite show that the deformation mechanisms of plagioclase and K-feldspar are different. To investigate these mechanisms and the brittle–plastic transition that takes place in granitic rocks composed of quartz, plagioclase, and K-feldspar, five leucosome granite samples were deformed with a constant strain rate of 10−5s−1 at different temperatures from 850°C to 1050°C and confining pressure (CP) of 300MPa using a Paterson-type gas deformation apparatus. To consider pressure effects, two more samples were deformed at 950°C but with different CPs, one with CP=100kPa and the other with CP=100MPa. In addition, an eighth sample was deformed under torsion shear at 950°C with CP=400MPa. Microstructures of an undeformed sample and experimentally deformed samples were analyzed using an optical microscope and a scanning electron microscope. The granite is composed of about 36 volume percent quartz, ∼26% plagioclase, ∼34% microcline, ∼3% muscovite, and ∼1% chlorite.The stress–strain curves for all but two samples display weakening. The two exceptions are the sample that deformed with steady-state creep under a CP of 100MPa and the sample that displayed brittle fracture under a CP of 100kPa. For the other samples, peak strengths decreased with increasing temperature or lower CP. Microstructures show that samples underwent a brittle–plastic transition with increasing temperature. Samples fractured by cataclastic flow at 850°C with CP=300MPa and at 950°C with CP=100kPa. Microcline deformed by cataclastic flow at 900–1050°C with CP=100–400MPa accompanied by dislocation glide at temperatures of 1000°C and 1050°C. At 900–1050°C with CP=100–400MPa, plagioclase displayed bulging recrystallization and grain boundary migration recrystallization and quartz deformed by subgrain rotation recrystallization. Diffusion rims were observed between quartz, plagioclase, and microcline grain boundaries at 900–1050°C with CP=100–400MPa. Partial melting appeared around plagioclase, microcline, and quartz grains at 1000°C and 1050°C. Exsolution of albite from microcline took place in a sample deformed at 950°C and CP=100MPa. Electron backscatter diffraction analysis showed that quartz has a lattice preferred orientation in both undeformed and experimentally deformed samples. The basal 〈a〉 slip of the quartz c-axis was found in undeformed samples. The c-axis of quartz closes to the rhomb 〈a〉 slip with a secondary in the basal 〈a〉 slip in samples deformed at 900°C and 950°C with CP’s of 300MPa and 100MPa respectively. The c-axis of quartz changes to prism 〈a〉 slip in samples deformed at 950–1050°C with CP=300MPa and does the same for a sample deformed with torsion shear at 950°C with CP=400MPa.Microstructures show that plagioclase and microcline deform differently. Microcline is a strong phase compared with quartz and plagioclase. Therefore, microcline is important for the brittle–plastic transition in granitic rocks and the depth of that transition in the middle crust will be greater where the granitic rocks in the crust contain large amounts of microcline.

Experimental deformation of coarse‐grained rock salt to high strain

AbstractThe processes and deformation mechanisms (e.g., dislocation creep, pressure solution, grain boundary sliding, and recrystallization) of rock salt are still a matter of debate. In order to fill this gap, high strain constriction experiments at 345°C, atmospheric pressure and a strain rate of ~10−7 s−1 have been conducted on natural halite cuboids (60 × 60 × 45 mm) from the Morsleben mine of Northern Germany. Most samples were almost single crystals and contain a small amount of smaller grains (10–26%). The grain boundaries are decorated with fluid inclusions. The experiments were stopped at different final strains (εy = z of ~10, 20, 30, and 40%) corresponding to a maximum strain (εx) range of 20–170%. The halite is deformed by dislocation creep, and the size of developed subgrains corresponds to the applied stress. The combined Schmid factor and subgrain boundary analysis indicate that slip was largely accommodated by the {110} &lt; 110 &gt; slip systems, with possible minor contribution by slip on the {100} &lt; 110 &gt; slip systems. Some of the deformed samples show evidence of grain boundary migration. In addition, subgrains with small misorientations form that result in large cumulative misorientations within a single grain (&gt;40°). However, no subgrain rotation recrystallization is observed (i.e., misorientation angles are &lt;10°). All the experiments show strain hardening, suggesting that recrystallization by grain boundary migration was not extensive and did not reset the microstructure. The experiments show that high finite strain in coarse‐grained relatively dry rock salt can be accommodated by dislocation creep, without extensive dynamic recrystallization.

Syntectonic fluids redistribution and circulation coupled to quartz recrystallization in the ductile crust (Naxos Island, Cyclades, Greece)

The presence of external fluids in metamorphic rocks has been shown to have a profound impact on rock rheology as high fluid pressure processes promote embrittlement and favor ductile deformation by recrystallization. Moreover, it has been proposed that brittle deformation guides fluid circulation and that intracrystalline deformation is responsible for fluid redistribution at the grain scale. Nevertheless, the amount of fluid present in the metamorphic ductile crust is debated and the nature of the interaction between fluids and recrystallization processes are not clearly identified. The aim of this study is to document the spatial distribution of fluid inclusions relative to microstructures in quartz grains and aggregates from veins sampled in amphibolite facies metamorphic rocks, exposed in the island of Naxos in the center of the Attic-Cycladic Metamorphic Complex in Greece. The veins, ranging from discordant structures with sharp contacts to totally transposed structures into the metamorphic foliation, display a large variety of microstructures and fluid evidences interpreted as recording exhumation processes through the ductile/brittle transition: (i) remnants of primary quartz grains contain abundant CO2-H2O fluid inclusions, decrepitated for the most part, distributed in clusters and in fluid inclusion trails, (ii) fluid inclusions with a similar composition are less abundant in recrystallized zones and in subgrains but are concentrated along grain boundaries indicating that grain boundary migration is responsible for redistribution of CO2-H2O fluids, (iii) subgrains of the last generation are almost devoid of fluid inclusions and are characterized by thick grain boundaries with abundant metamorphic fluids locally forming a continuous film. CO2-H2O fluid inclusions aligned in parallel, regularly spaced intragranular trails, locally rooted into grain boundaries, are interpreted as reflecting the spatial redistribution of these fluids in quartz slip planes owing to the increase of fluid pressure in grain boundaries. This proposition is corroborated by the parallelism between slip planes and fluid inclusion trails. Perpendicular sets of fluid inclusion trails formed at the junctions of subgrains suggest that redistribution of fluids along quartz slip planes contributes to the initiation of subgrain rotation recrystallization. Transgranular fluid inclusion planes oriented perpendicular to the regional mineral and stretching lineations of the host metamorphic rocks mark a transition from grain to rock scale brittle behavior associated with the infiltration of H2O fluid, mixing to various degrees with the initial CO2-H2O fluid already present in the metamorphic rocks. These features indicate that, in the ductile-metamorphic crust, fluid redistribution is intimately linked to recrystallization mechanisms allowing fluids circulation under lithostatic pressure.

Recrystallization fabrics of sheared quartz veins with a strong pre-existing crystallographic preferred orientation from a seismogenic shear zone

Microstructural investigations were carried out on quartz veins in schist, protomylonite, and mylonite samples from an ancient seismogenic strike-slip shear zone (Sandhill Corner shear zone, Norumbega fault system, Maine, USA). We interpret complexities in the microstructural record to show that: (1) pre-existing crystallographic preferred orientations (CPO) in the host rock may persist in the new CPO patterns of the shear zone and (2) the inner and outer parts of the shear zone followed diverging paths of fabric development.The host rocks bounding the shear zone contain asymmetrically-folded quartz veins with a strong CPO. These veins are increasingly deformed and recrystallized with proximity to the shear zone core. Matrix-accommodated rotation and recrystallization may position an inherited c-axis maximum in an orientation coincident with rhomb <a> or basal <a> slip. This inherited CPO likely persists in the shear zone fabric as a higher concentration of poles in one hemisphere of the c-axis pole figure, leading to asymmetric crossed girdle or paired maxima c-axis patterns about the foliation plane.Three observed quartz grain types indicate a general trend of localization with decreasing temperature: (1) large (>100μm), low aspect ratio (<~5) and (2) high aspect ratio (~5–20) grains overprinted by (3) smaller (<~80μm), low aspect ratio (<~4) grains through subgrain rotation-dominated recrystallization. In the outer shear zone, subgrain rotation recrystallization led to a well-developed c-axis crossed girdle pattern. In the inner shear zone, the larger grains are completely overprinted by smaller grains, but the CPO patterns are relatively poorly developed and are associated with distinctively different misorientation angle histogram profiles (“flat” neighbor-pair profile with similar number fraction for angles from 10 to 90°). This may reflect the preferential activation of grain size sensitive deformation processes in the inner-most part of the shear zone where transient deformation associated with the seismic cycle may have localized.

Strain localization in ductile rocks: A comparison of natural and simulated pinch-and-swell structures

We study pinch-and-swell structures in order to uncover the onset of strain localization and the change of deformation mechanisms in layered ductile rocks. To this end, boudinaged monomineralic veins embedded in an ultramylonitic matrix are analyzed quantitatively. The swells are built up by relatively undeformed original calcite grains, showing twinning and minor subgrain rotation recrystallization (SGR). Combined with progressive formation of high-angle misorientations between grains, indicative of SGR, severe grain size reduction defines the transition to the pinches. Accordingly, dynamically recrystallized grains have a strong crystallographic preferred orientation (CPO). Toward the necks, further grain size reduction, increasingly random misorientations, nucleation of new grains, and a loss of the CPO occur. We postulate that this microstructure marks the transition from dislocation to diffusion creep induced by strain localization. We confirm that the development of boudins is insensitive to original grain sizes and single-crystal orientations. In order to test these microstructural interpretations, a self-consistent numerical grain size evolution is implemented, based on thermo-mechanical principles, end-member flow laws and microphysical processes. Applying constant velocity and isothermal boundary conditions to a 3-layer finite element pure shear box, pinch-and-swell structures emerge out of the homogeneous layer through grain size softening at a critical state. Viscosity weakening due to elevated strain rates and dissipated heat from grain size reduction promotes strain rate weakening until a critical grain size is reached. At this point, a switch from dislocation to diffusion creep occurs. This state locks in at local steady states and is microstructurally expressed in pinches and swells, respectively. Thus, boudinage is identified as an energy attractor, identifying the high-energy steady state of an extending layered structure. We conclude from the similarity between natural observations and numerical results that critical deformation conditions of ductile creep can be derived for the surrounding highly-strained host rock matrix.

Dislocation creep of dry quartz

AbstractSmall‐scale shear zones within the Permian Truzzo meta‐granite developed during the Alpine orogeny at amphibolite facies conditions. In these shear zones magmatic quartz deformed by dislocation creep and recrystallized dynamically by grain boundary migration with minor subgrain rotation recrystallization to a grain size of around 250–750 µm, consistent with flow at low differential stresses. Fourier transform infrared (FTIR) spectroscopy reveals very low water contents in the interior of recrystallized grains (in the form of discrete OH peaks, ~20 H/106Si and very little broad band absorption, &lt;100 H/106Si). The spectral characteristics are comparable to those of dry Brazil quartz. In FTIR spectra, magmatic quartz grains show a broad absorption band related with high water concentrations only in those areas where fluid inclusions are present while other areas are dry. Drainage of fluid inclusions and synkinematic growth of hydrous minerals indicates that a hydrous fluid has been available during deformation. Loss of intragranular water during grain boundary migration recrystallization did not result in a microstructure indicative of hardening. These FTIR measurements provide the first evidence that quartz with extremely low intragranular water contents can deform in nature by dislocation creep at low differential stresses. Low intragranular water contents in naturally deformed quartz may not be necessarily indicative of a high strength, and the results are contrary to implications taken from deformation experiments where very high water contents are required to allow dislocation creep in quartz. It is suggested that dislocation creep of quartz in the Truzzo meta‐granite is possible to occur at low differential stresses because sufficient amounts of intergranular water ensure a high recovery rate by grain boundary migration while the absence of significant amounts of intragranular water is not crucial at natural conditions.

Characterization of transpressive deformation in shear zones of the Archean North Caribou greenstone belt (NW Superior Province) and the relationship with regional metamorphism

The 2.7–3.0Ga North Caribou greenstone belt (NCGB), host to the Musselwhite BIF-hosted gold deposit, possesses abundant shear zones on its northern margins, which appear to have formed under amphibolite facies conditions. Protracted deformation and regional metamorphism are coeval with widespread magmatism and accretion events during crustal amalgamation of the Western Superior Province, and are responsible for folding the ore-hosting BIF and channeling fluids. The importance of shear zones in behaving as conduits for fluids during the tectonic evolution of the NCGB is not well known and their relationship with metamorphism is equivocal, yet higher-grade, syn- to post-tectonic metamorphic minerals seem to correlate with loci of higher strain. Structural analyses support oblique transpressive collision that produced steeply-dipping planar and shallowly-plunging linear fabrics with dominant dextral kinematics, that trend broadly parallel to the doubly arcuate shape of the belt. Electron backscatter diffraction analyses were conducted on strategic samples across one shear zone in order to characterize crustal conditions during transpressive deformation. The Dinnick Lake shear zone cuts through mafic metavolcanics and at its core is an L-tectonite granite composed of recrystallized quartz. Whole rock geochemistry shows little variation in Ca, Na, Mg and K (often used as indicators of hydrothermal alteration) from surrounding less deformed units, suggesting deformation in a dry environment. Microstructural analysis indicates subgrain rotation recrystallization and deformation by prism a- and c-slip in quartz, as well as aligned hornblende that suggest deformation temperatures above 500°C. Quartz in mafic rocks along the margins of the shear zone also exhibits a basal a-slip component, indicating a slight decrease in strain or temperature. Although the NCGB exhibits some first-order evidence of vertical tectonism (dome and keel geometries), the dominant strain record within shear zones is that of horizontal (oblique transpressive) displacement. This is in agreement with other greenstone belts in the Western Superior Province where vertical tectonism and horizontal tectonism were coeval.

Brittle grain-size reduction of feldspar, phase mixing and strain localization in granitoids at mid-crustal conditions (Pernambuco shear zone, NE Brazil)

Abstract. The Pernambuco shear zone (northeastern Brazil) is a large-scale strike-slip fault that, in its eastern segment, deforms granitoids at mid-crustal conditions. Initially coarse-grained (&gt; 50 µm) feldspar porphyroclasts are intensively fractured and reduced to an ultrafine-grained mixture consisting of plagioclase and K-feldspar grains (&lt; 15 µm) localized in C' shear bands. Detailed microstructural observations and electron backscatter diffraction (EBSD) analysis do not show evidence of intracrystalline plasticity in feldspar porphyroclasts and/or fluid-assisted replacement reactions. Quartz occurs either as thick (∼ 1–2 mm) monomineralic veins transposed along the shear zone foliation or as thin ribbons ( ≤ 25 µm width) dispersed in the feldspathic mixture. The microstructure and c axis crystallographic-preferred orientation are similar in the thick monomineralic veins and in the thin ribbons, and they suggest dominant subgrain rotation recrystallization and activity of prism &lt; a &gt; and rhomb &lt; a &gt; slip systems. However, the grain size in monophase recrystallized domains decreases when moving from the quartz monomineralic veins to the thin ribbons embedded in the feldspathic C' bands (14 µm vs. 5 µm respectively). The fine-grained feldspar mixture has a weak crystallographic-preferred orientation interpreted as the result of shear zone parallel-oriented growth during diffusion creep, as well as the same composition as the fractured porphyroclasts, suggesting that it generated by mechanical fragmentation of rigid porphyroclasts with a negligible role of chemical disequilibrium. Once C' shear bands were generated and underwent viscous deformation at constant stress conditions, the polyphase feldspathic aggregate would have deformed at a strain rate 1 order of magnitude faster than the monophase quartz monomineralic veins, as evidenced by applying experimentally and theoretically calibrated flow laws for dislocation creep in quartz and diffusion creep in feldspar. Overall, our data set indicates that feldspar underwent a brittle-viscous transition while quartz was deforming via crystal plasticity. The resulting rock microstructure consists of a two-phase rheological mixture (fine-grained feldspars and recrystallized quartz) in which the polyphase feldspathic material localized much of the strain. Extensive grain-size reduction and weakening of feldspars is attained in the East Pernambuco mylonites mainly via fracturing which would trigger a switch to diffusion creep and strain localization without a prominent role of metamorphic reactions.

Dynamic recrystallization mechanisms and their transition in the Daling Thrust (DT) zone, Darjeeling–Sikkim Himalaya

The Daling Thrust (DT) delineates a zone of intense shear localization in the Lesser Himalayan Sequence (LHS) of the Darjeeling–Sikkim Himalaya. From microstructural studies of deformed quartzite samples, we show a transition in the dynamic recrystallization mechanism with increasing distance from the DT, dominated by grain boundary bulging (BLG) recrystallization closest to the DT, and progressively replaced by sub-grain rotation (SGR) recrystallization away from the thrust. The transition is marked by a characteristic variation in the fractal dimension (D) of grain boundaries, estimated from the area–perimeter method. For the BLG regime, D≈1.046, which decreases significantly to a value as low as 1.025 for the SGR regime. Using the available thermal data for BLG and SGR recrystallization, we infer increasing deformation temperatures away from the DT in the hanging wall. Based on the quartz piezometer our estimates reveal strong variations in the flow stress (59.00MPa to 16.00MPa) over a distance of ~1.2km from the DT. Deformation mechanism maps constructed for different temperatures indicate that the strain rates (10−12S−1 to 10−14S−1) comply with the geologically possible range. Finally, we present a mechanical model to provide a possible explanation for the cause of stress intensification along the DT.

Major softening at brittle‐ductile transition due to interplay between chemical and deformation processes: An insight from evolution of shear bands in the South Armorican Shear Zone

AbstractThe formation of S‐C/C′ fabrics in the South Armorican Shear Zone has been evaluated by detailed microstructural study where the focus was given to initiation and early evolution of the C/C′ fabric shear bands. Our observations suggest that the S‐C/C′ fabrics formed at distinct temperature conditions indicating &gt;550°C for the S fabric and 300–350°C at 100–400 MPa for the C/C′ fabric shear bands. The evolving microstructure within shear bands documents switches in deformation mechanisms related to positive feedbacks between deformation and chemical processes and imposes mechanical constraints on the evolution of the brittle‐ductile transition in the continental transform fault domains. Three stages of shear band evolution have been identified. Stage I corresponds to initiation of shear bands via formation of microcracks with possible yielding differential stress of up to 250 MPa. Stage II is associated with subgrain rotation recrystallization and dislocation creep of quartz and coeval dissolution‐precipitation creep of microcline. Recrystallized quartz grains show continual increase in size and decrease in stress and strain rates from 94 MPa to 17–26 MPa and 3.8 × 10−12 s−1–8 × 10−14 s−1 associated with deformation partitioning into weaker microcline layer and shear band widening. The quartz mechanical data allowed us to set some constrains for coeval dissolution‐precipitation of microcline which at our estimated pressure‐temperature conditions suggests creep at 17–26 MPa differential stress and 3.8 × 10−13 s−1 strain rate. Stage III is characterized by localized slip along white mica bands accommodated by dislocation creep at strain rate 3.8 × 10−12 s−1 and stress 9.36 MPa. Our mechanical data point to dynamic evolution of the studied brittle‐ductile transition characterized by major weakening to strengths ~10 MPa. Such nonsteady state evolution may be common in crustal shear zones especially when phase transformations are involved.

Deformation inside a paleosubduction channel – Insights from microstructures and crystallographic preferred orientations of eclogites and metasediments from the Tauern Window, Austria

The Eclogite Zone, of the Tauern Window is an exhumed subduction channel comprising eclogites with different grades of retrogression in a matrix of high-pressure metasediments. The rocks were exposed to 600 °C and 20–25 kbars, and then retrogressed during their exhumation, first under blueschist facies and later under amphibolite facies metamorphism. To gain insights into the deformation within the subduction channel during subduction and exhumation, both fresh and retrogressed eclogites, as well as the surrounding metasediments were investigated with respect to their deformation microstructures and crystallographic preferred orientations (CPOs). Pristine and retrogressed eclogites show grain boundary migration and subgrain rotation recrystallization microstructures in omphacite. A misorientation axes analysis reveals the activity of complementary deformation mechanisms including grain boundary sliding and dislocation creep. The omphacite CPOs of the eclogites correspond to dominant SL-fabrics characteristic of plane strain deformation, though there are local variations towards flattening or constriction within the paleosubduction channel. The glaucophane CPOs in retrogressed eclogites match those of omphacite, suggesting that a constant strain geometry persisted during exhumation at blueschist facies conditions. Plastic deformation of the host high-pressure metasediments outlasted that of the eclogites, as indicated by white mica fabrics and quartz CPO. The latter is consistently asymmetric, pointing to the operation of non-coaxial deformation. The microstructures and CPO data indicate a continuous plastic deformation cycle with eclogite and blueschist facies metamorphism related to subduction and exhumation of the different rock units.

Modeling the effect of subgrain rotation recrystallization on the evolution of olivine crystal preferred orientations in simple shear

Homogenization models are widely used to predict the evolution of texture (crystal preferred orientations) and resulting anisotropy of physical properties in metals, rocks, and ice. They fail, however, in predicting two main features of texture evolution in simple shear (the dominant deformation regime on Earth) for highly anisotropic crystals, like olivine: (1) the fast rotation of the CPO towards a stable position characterized by parallelism of the dominant slip system and the macroscopic shear and (2) the asymptotical evolution towards a constant intensity. To better predict CPO-induced anisotropy in the mantle, but limiting computational costs and use of poorly-constrained physical parameters, we modified a viscoplastic self-consistent code to simulate the effects of subgrain rotation recrystallization. To each crystal is associated a finite number of fragments (possible subgrains). Formation of a subgrain corresponds to introduction of a disorientation (relative to the parent) and resetting of the fragment strain and internal energy. The probability of formation of a subgrain is controlled by comparison between the local internal energy and the average value in the polycrystal. A two-level mechanical interaction scheme is applied for simulating the intracrystalline strain heterogeneity allowed by the formation of low-angle grain boundaries. Within a crystal, interactions between subgrains follow a constant stress scheme. The interactions between grains are simulated by a tangent viscoplastic self-consistent approach. This two-level approach better reproduces the evolution of olivine CPO in simple shear in experiments and nature. It also predicts a marked weakening at low shear strains, consistently with experimental data.

Fluid-present deformation aids chemical modification of chromite: Insights from chromites from Golyamo Kamenyane, SE Bulgaria

Chemical signatures of chromitites are commonly used to track the evolution of the Earth's mantle. However, chemical modification during deformation may have important implications for the interpretation of chromites' signatures. Here, we describe the details of how deformation promotes chemical modification in chromite. Physicochemical characteristics of the chromites were quantified by measuring crystallographic orientation relationships using Electron Back-Scattered Diffraction (EBSD) and electron microprobe analysis (EMP). Chromites show porphyroclastic textures with coarse-grained porphyroclasts (ca. 0.2–5mm) and fine-grained neoblasts (<200μm). Coarse-grained chromites are chemically zoned in terms of major elements from core to rim, preserving this initial igneous feature in the cores, while the outer rims reveal a metamorphic signature. Large chromite grains are characterized by local crystal-plastic deformation, exhibiting distinct inter-crystalline deformation including continuous crystal bending and subgrain boundaries as well as chemical modification in their outer, deformed parts. Two types of fine-grained chromite, F1 and F2, are present. While F1 exhibits a well-developed polygonal texture, straight grain boundaries and low intercrystal misorientation (<1°), F2 shows low-angle boundaries and significant intercrystalline misorientation (2–8°). Both F1 and F2 have higher Fe3+ and Cr and lower Mg# values than the cores of large grains. We interpret F1 and F2 to represent chromite recrystallized by heterogeneous nucleation and subgrain rotation recrystallization, respectively. Crystallographic preferred orientation (CPO) and misorientation data on the well-developed low-angle (subgrain) boundaries in coarse grains and F2 grains indicate that deformation in chromite was accommodated mainly by dislocation creep with the dominant activation of the {111}<100> slip system. The retrograde P–T exhumation path predicted by thermodynamic and chemical modeling suggests that these fine-grained chromites were produced when the initial chromitites reacted with oxidizing fluids during retrograde metamorphism (~1.0GPa and 500–700°C). Our results show that deformation in the dislocation-creep regime in a chemically open system has induced chemical modification and homogenization within chromite aggregates as well as strain localization. This close physicochemical link offers new avenues of interpreting the chemical signatures of chromites, utilizing their microstructurally controlled variation or lack thereof.

The influence of phase and grain size distribution on the dynamics of strain localization in polymineralic rocks

Deformation microstructures of a quartzo-feldspathic pegmatite deformed at mid-crustal levels allow the study of the dynamics of strain localization in polymineralic rocks. Strain localization results from (i) difference in grain sizes between phases, both original and obtained during fluid present reactions and (ii) initial compositional banding. Due to original difference in grain size stress concentrates in the initially finer-grained phases resulting in their intense grain size reduction via subgrain rotation recrystallization (SGR). When the grain size is sufficiently reduced through either deformation or interphase coupled dissolution–precipitation replacement of the coarse grained feldspar, aggregates start to deform by dominantly diffusion accommodated grain boundary sliding (GBS). Phase mixing inhibits grain growth and sustains a grain size allowing GBS. Consequently, discontinuous microscale shear zones form locally within initially coarse grained areas. At the same time difference in strain rate between feldspar-rich and quartz-rich domains needs to be accommodated at domain boundaries. This results in the formation of continuous mesoscale shear zones deformed by GBS. Once these are formed, deformation in the coarse grained parts is arrested and strain is mainly accommodated in the mesoscale shear zones resulting in “superplastic” behaviour consistent with diffusion creep.