Development Of Lattice Preferred Orientation Research Articles

Lattice-preferred orientation development in experimental and natural fine-grained dolomite shear zones

We compare microstructures and lattice preferred orientations (LPO) in shear zones formed in two experimentally deformed and two naturally deformed dolomites. Experimental samples were deformed at 900 °C and laboratory strain rates, and naturally deformed dolomites were collected from the Pioneer Landing thrust and Town Knobs thrust of the Southern Appalachians, deformed at ~300 °C and ~240 °C, respectively, at tectonic strain rates. Coarse-grained host dolomite, in all samples, exhibits undulatory extinction, subgrains, recrystallized grains, and, in the naturally deformed dolomites, fractures. Two types of fine-grained shear zones are characterized in experimentally deformed dolomites: 1) a fine-grained zone of recrystallized dolomite generated during axial shortening of coarse-grained dolomite that initially deformed by dislocation creep, with strain localization initiated by grain-size sensitive deformation processes and 2) a synthetic, fine-grained dolomite layer placed between coarse-grained dolomite shear pistons. Microstructures and lattice preferred orientations of the experimentally sheared dolomite reflect the concomitant operation of dislocation creep, diffusion creep, and grain boundary sliding. Microstructures, including subgrains, recrystallized grains, and four-grain junctions, and lattice preferred orientations that consist of weak c-axis maxima in the natural shear zones are similar to the experimental shear zones, however, a slightly stronger LPO, more common subgrains and the lack of four-grain junctions indicate that dislocation creep dominates within dolomites deformed at lower temperatures (240 °C).

Age-independent seismic anisotropy under oceanic plates explained by strain history in the asthenosphere

The depth of the oceanic lithosphere–asthenosphere boundary (LAB), as inferred from shear wave velocities, increases with lithospheric age, in agreement with models of cooling oceanic lithosphere. On the other hand, the distribution of radial anisotropy under oceanic plates is almost age-independent. In particular, radial anisotropy shows a maximum positive gradient at a depth of ∼70 km, which, if used as a proxy, indicates an age-independent LAB depth. These contrasting observations have fueled a controversy on the seismological signature of the LAB. To better understand the discrepancy between these observations, we model the development of lattice preferred orientation (LPO) in upper mantle crystal aggregates and predict the seismic anisotropy produced by plate-driven mid-ocean ridge flows. The model accounts for the progressive cooling of the lithosphere with age and can incorporate both diffusion and dislocation creep deformation mechanisms. We find that an age-independent distribution of radial anisotropy is the natural consequence of these simple flows. The depth and strength of anisotropy is further controlled by the deformation regime – dislocation or diffusion creep – experienced by crystals during their ascent towards, and subsequent motion away from, the ridge axis. Comparison to surface wave tomography models yield constraints on rheological parameters such as the activation volume. Although not excluded, additional mechanisms proposed to explain some geophysical signatures of the LAB, such as the presence of partial melt or changes in water content, are not required to explain the radial anisotropy proxy. Our prediction, that the age-independent radial anisotropy proxy marks the transition to flow-induced asthenospheric anisotropy, provides a way to reconcile thermal, mechanical and seismological views of the LAB.

Direct shear of olivine single crystals

Knowledge of the strengths of the individual dislocation slip systems in olivine is fundamental to understanding the flow behavior and the development of lattice-preferred orientation in olivine-rich rocks. The most direct measurements of the strengths of individual slip systems are from triaxial compression experiments on olivine single crystals. However, such experiments only allow for determination of flow laws for two of the four dominant slip systems in olivine. In order to measure the strengths of the (001)[100] and (100)[001] slip systems independently, we performed deformation experiments on single crystals of San Carlos olivine in a direct shear geometry. Experiments were carried out at temperatures of 1000 ° to 1300 °C, a confining pressure of 300 MPa, shear stresses of 60 to 334 MPa, and resultant shear strain rates of 7.4 × 10−6 to 2.1 × 10−3 s−1. At high-temperature (≥1200 °C) and low-stress (≤200 MPa) conditions, the strain rate of crystals oriented for direct shear on either the (001)[100] or the (100)[001] slip system follows a power law relationship with stress, whereas at lower temperatures and higher stresses, strain rate depends exponentially on stress. The flow laws derived from the mechanical data in this study are consistent with a transition from the operation of a climb-controlled dislocation mechanism during power-law creep to the operation of a glide-controlled dislocation mechanism during exponential creep. In the climb-controlled regime, crystals oriented for shear on the (001)[100] slip system are weaker than crystals orientated for shear on the (100)[001] slip system. In contrast, in the glide-controlled regime the opposite is observed. Extrapolation of flow laws determined for crystals sheared in orientations favorable for slip on these two slip systems to upper mantle conditions reveals that the (001)[100] slip system is weaker at temperatures and stresses that are typical of the asthenospheric mantle, whereas the (100)[001] slip system is weaker at conditions typical of the lithospheric mantle. These observations demonstrate that the relative strength of the dislocation slip systems in olivine and, thus, the development of lattice-preferred orientation and anisotropic viscosity in olivine-rich rocks are strongly dependent upon temperature and stress.

Metamorphic P–T–t–d evolution of (U)HP metabasites from the South Tianshan accretionary complex (NW China) — Implications for rock deformation during exhumation in a subduction channel

The late Carboniferous accretionary system of the South Tianshan orogen (North-Western China) underwent complex structural and polymetamorphic evolution. Combined petrological, geochronological and microstructural analysis of (ultra)high-pressure (UHP) metabasites (eclogites and blueschists) enclosed in metapelites show a relict coarse-grained eclogitic fabric S2 surrounded by a dominant fine-grained eclogite and blueschist facies retrograde fabric S2. The S2 fabric is reworked by upright folds F3 that are responsible for a major shortening of the whole accretionary system. For both the eclogite and blueschist, peak and retrograde P–T conditions have been thermodynamically constrained at 25–26kbar and 425–500°C and 10–13kbar and 500−550°C respectively, suggesting a shared exhumation history. The garnet-whole rock-amphibole isochron in the blueschist yielded Lu–Hf and Sm–Nd ages of 326.0±2.9Ma and 318.4±3.9Ma respectively, interpreted to date the prograde to peak metamorphic assemblage. The retrograde path of the eclogite is characterized by heterogeneous omphacite recrystallization into a mylonitic fine-grained matrix and crystallization of blue amphibole. Microstructures in both pristine porphyroclastic and recrystallized fine-grained domains in the eclogite indicate a gradual evolution from constriction-dominated (L>S-type) to flattening-dominated (S>L-type) type of deformation, increase of fabric intensity reflected by gradually growing M-indexes and the development of lattice preferred orientation (LPO) typical for dislocation creep under slightly hydrated conditions. Recrystallization of the matrix in the blueschist is homogeneous, which indicates a matrix dominated channel flow during exhumation. These LPOs evolutions suggest a significant mechanical coupling with the upper plate concomitant with oroclinal bending of the Kazakh orocline. Lock up of Kazakh orocline is responsible for further stress increase resulting in horizontal shortening of South Tianshan accretionary wedge and development of D3 upright folding and steepening of the whole sequence.

Full-field predictions of ice dynamic recrystallisation under simple shear conditions

Understanding the flow of ice on the microstructural scale is essential for improving our knowledge of large-scale ice dynamics, and thus our ability to predict future changes of ice sheets. Polar ice behaves anisotropically during flow, which can lead to strain localisation. In order to study how dynamic recrystallisation affects to strain localisation in deep levels of polar ice sheets, we present a series of numerical simulations of ice polycrystals deformed under simple-shear conditions. The models explicitly simulate the evolution of microstructures using a full-field approach, based on the coupling of a viscoplastic deformation code (VPFFT) with dynamic recrystallisation codes. The simulations provide new insights into the distribution of stress, strain rate and lattice orientation fields with progressive strain, up to a shear strain of three. Our simulations show how the recrystallisation processes have a strong influence on the resulting microstructure (grain size and shape), while the development of lattice preferred orientations (LPO) appears to be less affected. Activation of non-basal slip systems is enhanced by recrystallisation and induces a strain hardening behaviour up to the onset of strain localisation and strain weakening behaviour. Simulations demonstrate that the strong intrinsic anisotropy of ice crystals is transferred to the polycrystalline scale and results in the development of strain localisation bands than can be masked by grain boundary migration. Therefore, the finite-strain history is non-directly reflected by the final microstructure. Masked strain localisation can be recognised in ice cores, such as the EDML, from the presence of stepped boundaries, microshear and grains with zig-zag geometries.

Mid-mantle anisotropy in subduction zones and deep water transport

The Earth’s transition zone has until recently been assumed to be seismi-cally isotropic. Increasingly, however, evidence suggests that ordering of material over seismic wavelengths occurs there, but it is unclear what causes this. We use the method of source-side shear wave splitting to examine the anisotropy surrounding earthquakes deeper than 200 km in slabs around the globe. We find significant amounts of splitting (<= 2.4 s), confirming that the transition zone is anisotropic here. However, there is no decrease in the amount of splitting with depth, as would be the case for a metastable tongue of olivine which thins with depth, suggesting this is not the cause. The amount of splitting does not appear to be consistent with processes in the ambient mantle, such as lattice preferred orientation development in wadsleyite, ringwoodite or MgSiO3-perovskite. We invert for the orientation of several mechanisms—subject to uncertainties in mineralogy and deformation—and the best fit is given by up-dip flattening in a style of anisotropy common to hydrous phases and layered inclusions. We suggest that highly anisotropic hydrous phases or hydrated layering is a likely cause of anisotropy within the slab, implying significant water transport from the surface down to at least 660 km depth.

Seismic anisotropy and shear wave splitting associated with mantle plume‐plate interaction

AbstractGeodynamic simulations of the development of lattice preferred orientation in the flowing mantle are used to characterize the seismic anisotropy and shear wave splitting (SWS) patterns expected for the interaction of mantle plumes and lithospheric plates. Models predict that in the deeper part of the plume layer ponding beneath the plate, olivine a axes tend to align perpendicular to the radially directed plume flow, forming a circular pattern reflecting circumferential stretching. In the shallower part of the plume layer, plate shear is more important and the a axes tend toward the direction of plate motion. Predicted SWS over intraplate plumes reflects the asymmetric influence of plate shear with fast S wave polarization directions forming a pattern of nested U shapes that open in the direction opposing both plate motion and the parabolic shape often used to describe the flow lines of the plume. Predictions explain SWS observations around the Eifel hot spot with an eastward, not westward, moving Eurasian plate, consistent with global studies that require relatively slow net (westward) rotation of all of the plates. SWS at the Hawaiian hot spot can be explained by the effects of plume‐plate interaction, combined with fossil anisotropy in the Pacific lithosphere. In ridge‐centered plume models, the fast polarization directions angle diagonally toward the ridge axis when the plume is simulated as having low viscosity beneath the thermal lithosphere. Such a model better explains SWS observations in northeast Iceland than a model that incorporates a high‐viscosity layer due to dehydration of the shallow‐most upper mantle.

Shear wave anisotropy in textured phase D and constraints on deep water recycling in subduction zones

Regions of low seismic velocity and high shear anisotropies in cold subducted slabs have often been related to anisotropic fabrics in hydrous phases mainly induced by slab deformation. The interpretation of these seismic anomalies in terms of hydration thus relies on a better knowledge of the elasticity and plastic deformation mechanisms of candidate hydrous phases. Here we investigate the development of lattice preferred orientations (LPO) in phase D [MgSi2H2O6, 10–18 wt% H2O], the ultimate water carrier in hydrous subducted peridotite. The samples were deformed non-hydrostatically up to 48 GPa in a diamond anvil cell and the texture and strength were obtained from analysis of the X-ray diffraction patterns collected in radial diffraction geometry. We find that at low strains the layered structure of phase D displays strong 0001 texture, where the stacking fault axis (c-axis) preferentially align parallel to the compression axis. A subsidiary 101¯0 texture develops at higher strains. Plasticity simulations in polycrystalline aggregates using a viscoplastic self-consistent model suggest that these LPO patterns are consistent with shape preferred orientation mechanism during the first compaction steps and, with dominant easy glide on basal planes and harder first order pyramidal slip, respectively, upon further compression. We find that phase D displays the lowest strength and the highest anisotropy among phases in hydrous peridotite in the uppermost lower mantle and might thus control the shear wave anisotropy generated in subducted slabs below the transition zone. We further evaluate the effect of textured phase D on the seismic velocity structure and shear wave anisotropy of deformed hydrous peridotite and compare the results to seismic observations in Tonga subduction. We show that 16 vol% of phase D in hydrous subducted peridotite is required to explain the negative velocity anomalies of 3%, the extent of shear wave splitting (0.9±0.3%) and the shear wave ray polarization geometry (VSH>VSV) observed in a detached fragment of the Tonga slab laying sub-horizontally below the transition zone. Seismic observations may thus place constraints on the degree of hydration of the Tonga slab beyond the transition zone, leading to the requirement that a minimum of 1.2 wt% H2O is retained in the slab by hydrous peridotite below 670 km depth.

Seismic anisotropy beneath the Chinese mainland

We investigated the upper mantle anisotropy beneath China by applying teleseismic shear wave splitting measurements at 119 seismic stations from CDSN and GSN/IRIS networks in China. The splitting observations are characterized by apparent diversity of anisotropy pattern in adjacent tectonic domains, including the Tianshan orogenic belt, Tibetan plateau, the Yangtze craton, the North China craton and northeastern region. In western China (Tianshan orogenic belt and Tibetan plateau), fast polarization directions of split SKS waves coincide strikingly well with the dominating trend of deformational crustal features and delay times range from 0.5 s to 1.6 s. While in eastern China, seismic anisotropy deduced from shear wave splitting reveals a homogeneous NW-SE trending structure, almost perpendicular to the strike of large-scale surface structures. The observed delay times of 1.5 s to more than 2 s favor consistent mantle flow over large mantle thicknesses. Based upon the straightforward relationships between seismic anisotropy and the development of lattice preferred orientation of mineral in upper mantle rocks, we interpret the splitting results in terms of tectonic fabric within the upper mantle. Since the lithosphere is less than 100 km thick beneath eastern China and the observed fast directions are subparallel to the trend of the absolute plate motion (APM) of Eurasian plate, we propose that the asthenosphere may mainly contribute to the anisotropic effects beneath eastern China. However, the upper mantle anisotropy beneath western China may have developed more recently in the subcrustal lithosphere because of rather small delay times and thick lithosphere. We also use the opportunity of the dense geodetic measurements available in China to discuss the coupling between the crust and mantle. In the Eurasia-fixed reference frame, GPS and shear wave splitting both depict a similar trend beneath eastern China, suggesting a lithospheric block “escaping” toward the east that could orient olivine [001] axis in the upper mantle. There is a strong coupling between the crust and the mantle in eastern China. A different behavior is observed in western China. The GPS vectors trend NS-NE in Tibet and NW in Tianshan, close to the regional compression direction, whereas the fast directions trend EW in Tibet and NE in Tianshan, suggesting a tectonic regime close to a mode of axial shortening, generating the development of EW-trending foliation in Tibet and NE-trending foliation in Tianshan at depth. The crust and mantle deform independently in western China.

Numerical simulations of texture development and associated rheological anisotropy in regions of complex mantle flow

The development of Lattice Preferred Orientations (LPO) within olivine aggregates under flow in the upper mantle leads to seismic and rheological (or viscoplastic) anisotropies. We compare predictions from different micromechanical models, applying several commonly used theoretical descriptions to an upwelling flow scenario representing a typical oceanic spreading center. Significant differences are obtained between models in terms of LPO and associated rheology, in particular in regions where the flow direction changes rapidly, with superior predictions for the recently proposed Second‐Order approach. This highlights the limitations of ad hoc formulations. LPO‐induced rheological anisotropy may have a large effect on actual flow patterns with 1–2 orders of magnitude variation in directional viscosities compared to the isotropic case.

Shear wave splitting in eastern and central China: Implications for upper mantle deformation beneath continental margin

Eastern China, located at the east margin of the Eurasia plate, is an ideal place to study the deformation pattern of upper mantle beneath continental margin. We investigated the upper mantle anisotropy beneath eastern China by applying teleseismic shear wave splitting measurements. We examined shear wave splitting measurements at 33 permanent stations from CDSN and GSN/IRIS networks in eastern and central China, as well as splitting parameters from a previous regional study. The splitting observations are characterized by apparent diversity of anisotropy pattern in adjacent tectonic domains, including the Southeast China Orogenic Belt and the Yangtze Craton, and the eastern and central North China Craton. Based upon the straightforward relationships between seismic anisotropy and the development of lattice preferred orientation of mineral in upper mantle rocks, we interpret the splitting results in terms of tectonic fabric within the upper mantle. Our interpretation implies that the upper mantle anisotropy beneath eastern China remained the effects of tectonic rejuvenation occurred during the Late Mesozoic to Cenozoic. We proposed that there are possibly two kinds of mechanisms involved in the upper mantle deformation beneath eastern China, depending on the roles of thick root of cratonic lithosphere. One mechanism is the deflecting the asthenospheric flow by the thick root of craton and the other one is the extensional asthenospheric flow causing the craton lost part of its thick root.

Development of lattice preferred orientation in clinoamphiboles deformed under low-pressure metamorphic conditions. A SEM/EBSD study of metabasites from the Aracena metamorphic belt (SW Spain)

The mechanical properties of the lower continental crust are intimately related to the rheology of calcic clinoamphibole. Naturally deformed clinoamphibole typically shows brittle behaviour under a wide range of deformation conditions, whereas crystal plasticity is rarely observed. In this study, the microfabric of clinoamphiboles from metabasites deformed under low-pressure/medium-to-high temperature metamorphic conditions is presented. Amphiboles from all samples have developed LPO that can be attributed to different deformation mechanisms depending on deformation temperature, fluid content, structure and phase-strength contrasts. Rigid body rotation of amphibole prisms within a weaker plagioclase matrix corresponds to metabasites deformed under medium deformation temperatures that show a weak layer structure and a high phase strength-contrast between plagioclase and clinoamphibole. Lower temperatures and the presence of fluids mean that one of the studied rocks was affected also by dissolution-precipitation creep and cataclastic flow. In the final sample, dislocation creep was accommodated by recovery and subgrain rotation dynamic recrystallization in monominerallic hornblende layers at higher temperatures, which could be representative of the rheology of amphiboles in a lower continental crust at high temperature conditions.

Midmantle deformation between the Australian continent and the Fiji‐Tonga subduction zone?

Shear wave splitting measurements are performed at temporary stations deployed across the Australian continent for direct S waves from events occurring in the neighboring subduction zones, especially Fiji‐Tonga. Unlike core refracted shear waves, direct S waves are not polarized at the core‐mantle boundary and are therefore subject to influences on the source side, on the receiver side, and along the ray path in between. Source side contamination due to the development of lattice preferred orientation of olivine can be minimized by considering deep events with an epicentral location below 410 km depth, the olivine‐spinel phase transition depth. Anisotropic contributions from the transition zone or lower mantle can only be reliably inferred in the case of high delays that cannot be generated solely by the upper mantle structure underneath a station. The analysis of a comprehensive data set for direct S waves from deep events leads to an average delay time of 0.9 s at stations deployed across the continent. Despite the limitations of such an arithmetic average, the result highlights the lack of evidence for midmantle deformation between the Fiji‐Tonga subduction zone and the Australian continent, unlike previously suggested in studies dealing with the permanent seismological stations only. A striking consistency between measurements performed across several stations belonging to an array in western Australia suggests a very coherent lithospheric structure underneath the Yilgarn craton.

Interpretation of seismic anisotropy in terms of mantle flow when melt is present

The use of seismic anisotropy to image upper mantle flow is usually based on the assumption that the direction of fast propagation of seismic waves is the same as the flow direction. Laboratory experiments showed that when melt is present, it is rather the direction of slow propagation that aligns with the flow direction. This paper presents a modeling of the effect of melt on the development of lattice preferred orientation (LPO) and its implications for the interpretation of anisotropy. Taking into account strain partitioning associated with melt segregation in the modeling yields a good agreement with experimental results. Implementing strain partitioning in a test flow shows that the effect of melt is mainly local and has a limited effect on seismic anisotropy. Water‐induced texture, melt‐filled cracks or channelized flow, might better account for seismic observations of direction of fast propagation normal to plate motion on a regional scale.

D-Rex, a program for calculation of seismic anisotropy due to crystal lattice preferred orientation in the convective upper mantle

Models of development of lattice preferred orientation (LPO) of crystals aggregates in convective flow are necessary to interpret the anisotropic seismic signature of the Earth′s upper mantle. For that purpose we previously developed a model of LPO evolution in olivine aggregates by plastic deformation and dynamic recrystallization by subgrain rotation and grain-boundary migration. This paper presents a refined version of that model, called D-Rex (for dynamic recrystallization-induced LPO), a public version of which is made available on our web site. The code displays two new features: (1) enstatite is incorporated in the aggregates and (2) grain-boundary sliding (GBS) of small grains is taken into account. Enstatite is incorporated on the assumption of no direct interaction with olivine. The fast (a-)axis of enstatite grains tend to be parallel to the slow (c-)axis of olivine, which dilutes the total anisotropy. Grain boundary sliding is included using a threshold dimensionless volume fraction ?, defined as the ratio of the initial size of the grains over the size for which GBS is the dominant mechanism of deformation. Grains with a dimensionless volume smaller than ? do not rotate by plastic deformation and their strain energy is set to zero. Comparison with torsion experiments at very large strain constrains the threshold dimensionless volume to 0.3 ± 0.1. The incorporation of grain-boundary sliding prevents the LPO from becoming singular at large strains and yields more realistic predictions. Our kinematic formalism and the model′s semi-analytical character insures that it is fast, robust and stable. It can be applied efficiently to arbitrary 3-D convective flows.

The influence of water on the development of lattice preferred orientation in olivine aggregates

The calculation of the lattice preferred orientation of crystals in mantle flow has become a major step in the interpretation of the upper mantle seismic anisotropy. Available models for such calculations are based on experimental data obtained on dry crystal aggregates. Recent simple shear experiments showed that the presence of water may radically change the fabrics produced, which casts doubt on the validity of the models in water rich environments like subduction zones. Water may change both the mechanisms of dynamic recrystallization and the mechanical properties of the crystals. This paper illustrates how a change of the critical resolved shear stress of the slip systems is sufficient to reproduce the new experimental results. In contrast, no changes are necessary in the theory of dynamic recrystallization. For subduction zones, first order calculations predict that trench parallel flow rather than water is more likely to be responsible for the anisotropic signature.

Development of anisotropic structure in the Earth's lower mantle by solid-state convection.

Seismological observations reveal highly anisotropic patches at the bottom of the Earth's lower mantle, whereas the bulk of the mantle has been observed to be largely isotropic. These patches have been interpreted to correspond to areas where subduction has taken place in the past or to areas where mantle plumes are upwelling, but the underlying cause for the anisotropy is unknown-both shape-preferred orientation of elastically heterogeneous materials and lattice-preferred orientation of a homogeneous material have been proposed. Both of these mechanisms imply that large-strain deformation occurs within the anisotropic regions, but the geodynamic implications of the mechanisms differ. Shape-preferred orientation would imply the presence of large elastic (and hence chemical) heterogeneity whereas lattice-preferred orientation requires deformation at high stresses. Here we show, on the basis of numerical modelling incorporating mineral physics of elasticity and development of lattice-preferred orientation, that slab deformation in the deep lower mantle can account for the presence of strong anisotropy in the circum-Pacific region. In this model-where development of the mineral fabric (the alignment of mineral grains) is caused solely by solid-state deformation of chemically homogeneous mantle material-anisotropy is caused by large-strain deformation at high stresses, due to the collision of subducted slabs with the core-mantle boundary.

Progressive development of lattice preferred orientations (LPOs) of naturally deformed quartz within a transpressional collision zone (Panafrican Orogen in the Eastern Desert of Egypt)

Lattice preferred orientations (LPOs) of quartz were used to establish differences in deformation geometry, finite strain, and temperature within a transpressional collision zone within the Panafrican Orogen in the Eastern Desert of Egypt. Metamorphic and/or magmatic core complexes in the area are bordered in the NW and SW by ductile sinistral NW-trending strike-slip zones and low angle normal faults (LANFs). Simultaneous activity of both fault systems suggests bulk W–E shortening coeval with orogen-parallel extension. Displacement partitioned into orogen-parallel sinistral strike-slip faults and LANFs. This study compares both quartz-LPOs in shear-zones and normal faults. From south to north, quartz c-axis data show a continuous evolution along orogen-parallel strike-slip faults from maxima in Y, with a slight tendency to oblique single girdles at the margins of the Wadi Beitan and Hafafit complexes, to asymmetric crossed girdles and oblique single girdles along the margins of the Sibai and Meatiq complexes. The NW-directed LANFs to the NW of the Hafafit and the SE-directed LANFs to the SE of the Sibai show maxima in Y. The SE-directed LANF at the SE margin of the Meatiq complex shows symmetric crossed girdles, indicating coaxial deformation geometry. Oblique single girdles and maxima in Y occur in the southern part of the orogen, whereas crossed girdle distributions dominate in the northern part. The variation in quartz c-axis patterns is explained in terms of decreasing metamorphic grade during deformation from the S (medium to high grade) to the N (low grade), and decreasing finite strain. This is in accordance with the general progression of transpressional tectonics and exhumation of core complexes from S to N.

Simple shear deformation of olivine aggregates

Simple shear deformation of synthetic olivine aggregates to high strains shows that dynamic recrystallisation has strong effects on the mechanical behaviour and the development of lattice preferred orientations. At 1473 K, a pronounced strain hardening is observed associated with grain elongation and limited dynamic recrystallisation along grain boundaries. The lattice preferred orientations of relict olivine grains generally follow the strain ellipsoid up to a shear strain of 1.1. Both [100] and [001] axes have peaks parallel to the maximum grain elongation direction. At 1573 K and shear strains >0.6, a moderate strain softening was associated with the development of significant dynamic recrystallisation. The lattice preferred orientation of relict olivine grains is characterised by point maxima with [100] axes parallel to the shear direction, [010] axes perpendicular to the shear plane, and [001] axes within the shear plane and perpendicular to the shear direction. The results suggest that at 1473 K both the b =[100] dislocations and b =[001] dislocations contribute to plastic deformation; at 1573 K dynamic recrystallisation relaxed constraints on deformation at grain boundaries, leading to a situation where a single slip system with b =[100] dislocations controls the rheology and the fabric. The lattice orientations of dynamically recrystallised olivine grains were measured using the electron backscatter diffraction technique. The measurements reveal a bimodal pattern of [100] axes: one parallel to the shear direction and the other perpendicular to the maximum principal compressive stress. Analysis of the results shows that the development of the stress-controlled orientations is closely associated with grain boundary migration processes during recrystallisation and growth. As a consequence, the direction of the fastest seismic velocity would not be parallel to the shear direction for olivine aggregates when grain boundary migration has a strong influence on the fabric.

The role of dynamic recrystallization in the development of lattice preferred orientations in experimentally deformed quartz aggregates

Experiments on coaxially deformed quartz aggregates show that when dynamic recrystallization involves grain size reduction, the lattice preferred orientation (LPO) of the recrystallized grains depends on the recrystallization mechanism. At low temperatures and fast strain rates, strain-induced grain boundary migration recrystallization favors growth of grains oriented poorly for slip on the basal and prism planes, resulting in an LPO of c-axes parallel to the θ 1 direction. At higher temperatures or slower strain rates, progressive subgrain rotation produces recrystallized grains which inherit the small-circle girdle LPO of their deformed host grains and which continue to re-orient by slip. At even higher temperatures and slower strain rates, progressive subgrain rotation is accompanied by rapid grain boundary migration, which does not favor grains of any particular orientation; thus the LPO is again similar to that of non-recrystallized grains. In contrast, when dynamic recrystallization involves grain growth driven by surface energy reduction, a strong LPO of c-axes parallel to θ 1 develops during grain growth but then evolves with increasing strain toward a small-circle girdle. The correlation of LPOs with recrystallization mechanisms has important implications for the assumption of homogeneous strain in theoretical models of LPOs and for interpreting natural LPOs in recrystallized rocks.