Quartz Lattice Preferred Orientation Research Articles

Lattice preferred orientation of quartz in granitic gneisses from Tso Morari Crystalline Complex, Eastern Ladakh, trans-Himalaya: evaluating effect of Dauphiné twin in dynamic recrystallization during exhumation

Abstract The Tso Morari Crystalline complex (TMCC) of eastern Ladakh, India, is part of the north Indian continental margin and is characterized by eclogitic enclaves embedded within ortho- and paragneisses known as the Puga Gneiss. Two fault zones bound the TMCC: the Karzok fault to the southwest and the Zildat fault to the northeast. In the present study, we carried out Electron Backscatter Diffraction study of quartz of 10 samples collected from the Puga Gneiss. The relict and recrystallized quartz grains were treated separately to understand the deformation conditions of the Puga Gneiss during early and late deformation stages related to UHP metamorphism and final stage of exhumation during retrogression, respectively. Microstructural observations suggest dynamic recrystallization in quartz and plagioclase at different temperature ranges. Misorientation analysis of both relict and recrystallized quartz grains reveals presence of Dauphiné Twins. Lattice preferred Orientation (LPO) of <c> axis of relict quartz grains generally shows more than one point maxima indicating that the relict grains preserve LPO developed during different stages of metamorphism/deformation. On the other hand, LPO of <c> axis of recrystallized grains from Karzok and Zildat fault zones shows asymmetric single girdle either normal or at an angle to the foliation plane, which suggests simple shear. We conclude that grain size reduction and recrystallization of the Puga Gneiss was greatly influenced by Dauphiné Twin and the final exhumation of the TMCC took place in a simple shear environment aided by activity along its two binding fault zones.

Dating Strike‐Slip Ductile Shear Through Combined Zircon‐, Titanite‐ and Apatite U–Pb Geochronology Along the Southern Tan‐Lu Fault Zone, East China

AbstractDating the timing of ductile shear deformation is critical to the understanding of complex, multi‐episode kinematics that commonly occur within the study of continental tectonics. The Tan‐Lu fault zone (TLFZ) in East China is a lithospheric discontinuity that records multistage sinistral strike‐slip shearing during Mesozoic time. Herein, based on field mapping, pressure‐temperature estimates via microstructures, quartz lattice preferred orientations, and geothermobarometer analysis, and U–Pb geochronology, we present a detailed examination of a portion of the southern TLFZ known as the Fucha Shan ductile shear zone. Two phases of sinistral ductile shear deformation in the shear zone are identified. The main phase occurred under amphibolite‐facies metamorphic conditions (500°C–650°C, 2.8–5.2 Kbar). U–Pb dating of synkinematic titanite indicate deformation occurred ca. 142–140 Ma. This age information is consistent with U–Pb dating of zircon from crosscutting granitic dikes that provide an upper limit of the shearing of ca. 134 Ma. A later, but less well‐developed phase of sinistral ductile shearing variably overprints older structures within the Fucha Shan shear zone at greenschist‐facies metamorphic conditions. U–Pb dating of apatite grains in rocks that record this later event are interpreted to indicate that shearing occurred ca. 118–108 Ma. These results provide new insight into Early Cretaceous shear deformation history within the southern TLFZ in the context of the late Mesozoic intracontinental deformation in East Asia.

Intracrystalline vorticity record of flow kinematics during shear zone reactivation

The vorticity axis of deformation of a shear zone is a rotational axis controlled by the relative movement of its boundaries. When characterized, it is a reliable tool to determine the ductile flow geometry (monoclinic general, monoclinic transpressive, or triclinic) and an asset to identify the kinematics of complex oblique collisions. Advancements in analysis of electron backscattered diffraction (EBSD) maps now permit inference of vorticity axes – known as crystallographic vorticity axes (CVAs) – from the intragranular lattice orientation dispersion of strain-bearing minerals. However, natural shear zones may preserve a record of multiple shear events with contrasting kinematics and it is not understood to which extent the quartz lattice may preserve a kinematic record through multiple deformation events. Herein, we characterize the finite flow kinematic record produced during reactivation of the Eastern Highlands shear zone (EHSZ), in an oblique collision within the Canadian Appalachians. We integrate field and thin section observations, quartz lattice preferred orientations, EBSD maps, and grain classification, leading to a new method to determine and compare CVA within relict and recrystallized quartz grains. Using this method, we were able to conclude that the EHSZ was reactivated in an heterogenous monoclinic flow environment.

Fabrics and geochronology of the Taibai ductile shear zone: Implications for tectonic evolution of the Qinling Orogenic Belt, central China

The Taibai ductile shear zone affecting the northern margin of the Taibai granitic pluton which emplaced in the North Qinling Orogenic Belt, is crucial to understand the amalgamation and post-collision history between the North China Block and the South China Block. The shear sense indicators and the kinematic vorticity numbers (0.42–0.59) suggest that the Taibai shear zone is an elongating general shear zone involving both sinistral shear and pure shear components. The quartz lattice preferred orientations mainly indicate the activation of prism 〈a〉 slip system combined with rhomb 〈a〉 slip to prism 〈a〉 slip system. The dynamic recrystallization of quartz is accommodated by sub-grain rotation and bulging recrystallization while plagioclase mainly displays bulging recrystallization. These characteristics suggest that the mylonites have experienced ductile shear deformation under greenschist facies conditions at temperatures of ∼400–550 °C. Zircons from granitic mylonite yield upper and lower intercept ages of 425 ± 4 Ma and 144 ± 1 Ma, respectively, suggesting the protolith of the mylonite crystallized at ca. 425 Ma and was involved into a hydrothermal activity at ca. 144 Ma. Muscovite 40Ar/39Ar dating of the mylonite sample yield a plateau age of 346 ± 5 Ma and an inverse isochron age of 344 ± 4 Ma, constraining the ductile shear deformation occurring during the Early Carboniferous time. We suggest that the Taibai shear zone was formed by NE-SW oblique compression with top-to-the-NW shear sense during the Early Carboniferous time, following the N-S shortening caused by collision between the North China Block and South China Block along the Shangdan suture at ca. 400 Ma.

The architecture of long-lived fault zones: insights from microstructure and quartz lattice-preferred orientations in mylonites of the Median Tectonic Line, SW Japan

We combine field mapping with quartz microstructure and lattice preferred orientations (LPO) to constrain the mechanisms and spatio-temporal distribution of deformation surrounding the Median Tectonic Line (MTL), SW Japan. In the study area, the MTL occurs either as a narrow gouge zone or as a sharp contact between hanging-wall quartzofeldspathic mylonites to the north and footwall pelitic schists to the south. Along the northern margin of the MTL, there exists a broad zone of mylonitic rocks, overprinted by cataclastic deformation and a damage zone associated with brittle deformation. The mylonitic shear zone is dominated by coarse-grained protomylonite up to ~ 100 m from the MTL, where fine-grained ultramylonite becomes dominant. We observe a systematic variation in quartz LPO with distance from the MTL. In protomylonites, quartz LPOs are dominantly Y-maxima patterns, recording dislocation creep by prism<a> slip at ~ 500 °C. Closer to the MTL, we observe R- and Z-maxima, and single and crossed girdles, reflecting dislocation creep accommodated by mixed rhomb<a> and basal<a> slip, likely under cooler conditions (~ 300 °C–400 °C). Some ultramylonite samples yield weak to random LPOs, interpreted to result from the influx of fluid into the shear zone, which promoted deformation by grainsize-sensitive creep. Following cooling and uplift, deformation became brittle, resulting in the development of a narrow cataclasite zone. The cataclasite was weakened through the development of a phyllosilicate foliation. However, healing of fractures strengthened the cataclasites, resulting in the development of anastomosing cataclasite bands within the protomylonite.

Large-scale strain localization induced by phase nucleation in mid-crustal granitoids of the south Armorican massif

The reduction of grain size is believed to play a critical role in strain localization to form shear zones. Although many mechanisms have been proposed, the source of grain size reduction remains debated. The South Armorican Shear Zone (SASZ) is a crustal-scale strike-slip fault that deforms granitoids at mid-crustal conditions. The SASZ records the transition from protolith to ultramylonite, representative of increasing ductile shear strain. To investigate the evolution of strain localization, the different states of deformation were studied using a combination of detailed microstructural, chemical and electron backscatter diffraction analyzes. Increasing strain from protolith to ultramylonite resulted in (1) grain size reduction, (2) the development of interconnected monophase layers of mica and incipient mixed-phase zones composed of phengite-quartz ± K-feldspar in protomylonite and low-strain mylonite, and (3) the formation of fine-grained mixed-phase zones composed of K-feldspar-quartz ± phengite in high-strain mylonite and ultramylonite. We propose that the causes for interconnection of mica are the formation of cracks in the protolith combined with fluid-assisted nucleation. The latter process also plays a major role in phase mixing, as attested by the precipitation of K-feldspar at triple junctions in quartz-rich layers and in fine-grained tails of inherited K-feldspar porphyroclasts in ultramylonite. The transition from quartz-rich layers and mixed-phase zones is accompanied by a strong dispersion of the quartz lattice preferred orientation. These microstructural and textural evidences suggest that phase nucleation is the major process behind phase mixing, possibly accompanied by the action of grain-boundary sliding to open cavities during deformation. Instead of a single process, we therefore highlight a succession of weakening processes in the evolution of the SASZ, starting with the crystallization of mica as a first weakening material, and then evolving with the formation of very fine-grained mixed-phase zones made principally of feldspar, quartz and phengite.

Characterizing a middle to upper crustal shear zone: Microstructures, quartz c-axis fabrics, deformation temperatures and flow vorticity analysis of the northern Ailao Shan-Red River shear zone, China

Structural and microstructural characteristics, deformation temperatures and flow vorticities of the northern Ailao Shan (ALS) high-grade metamorphic belt provide significant information regarding the nature and tectonic evolution of the Ailao Shan-Red River (ASRR) shear zone. Mineral deformation mechanisms, quartz lattice-preferred orientation (LPO) patterns and the opening angles of quartz c-axis fabrics of samples from the Gasa section indicate that the northern ALS high-grade metamorphic belt has experienced progressive shear deformation. The early stage shearing is characterized by a gradual decrease of deformation temperatures from >650°C at the northeastern unit to ca. 300°C at the southwestern unit, that results in the formation of migmatites, mylonitic gneisses, thin bedded mylonites, mylonitic schists and phyllonites from the NE to SW across the strike of the shear zone. The late stage low-temperature (300–400°C) shearing is superimposed on the early deformation throughout the belt with the formation of discrete, small-scale shear zones, especially in the thin-banded mylonitic rocks along both margins. The kinematic vorticity values estimated by rotated rigid porphyroclast method and oblique grain-shaped/quartz c-axis-fabric method imply that the general shear-dominated flow (0.49–0.77) progressively changed to a simple shear-dominated flow (0.77–1) toward the late stage of ductile deformation. The two stages of shearing are consistent with early shortening-dominated and late extrusion-controlled regional tectonic processes. The transition between them occurred at ca. 27Ma in the ALS high-grade metamorphic belt along the ASRR shear zone. The large amount of strike-slip displacement along the ASRR shear zone is predominantly attributed to accelerated flow along the shear zone during the late extrusion-controlled tectonic process.

Domainal petrofabric analysis of micaceous quartzite using EBSD data: Role of muscovite in LPO evolution of quartz

Lattice Preferred Orientation (LPO) of quartz and muscovite are measured in a micaceous quartzite using SEM based Electron Backscatter Diffraction (EBSD) analysis. The measurements were done in a thin section prepared parallel to the K1K3 plane of the anisotropy of magnetic susceptibility (AMS) ellipsoid, which is equivalent to the XZ plane of the strain ellipsoid. Bulk data from the sample reveal that quartz c-axis develop an oblique single girdle LPO pattern indicating a dextral sense of shear. A similar pattern is produced by poles to the basal planes (001) of muscovite, which implies that muscovite controls the LPO of quartz in the rock. Petrofabric analysis of quartz is performed in domains viz. (I) not containing muscovite, (II) between two muscovite grains, and (III) rim of quartz grains around a muscovite grain. Quartz LPO pattern in Domain-III is noted to be similar to the bulk LPO recorded from the whole sample, thus confirming that muscovite was critical in textural evolution of the rock. The study thus establishes that a minor mineral phase in a rock can control the LPO of the major mineral phase, thus playing a significant role in development of bulk texture of the rock. The advantages of domainal petrofabric analysis in evaluation of slip systems in minerals in different parts of a rock, and vorticity quantification are discussed.

Midcrustal discontinuities and the assembly of the Himalayan midcrust

AbstractDetailed quartz lattice preferred orientation (LPO) data define two structural discontinuities in the exhumed high‐grade metamorphic core of the Himalaya exposed in the upper Tama Kosi region of east central Nepal. The structures are marked by abrupt breaks in a general trend of up structural section increasing quartz LPO‐defined deformation temperatures. Deformation associated with the upper structural discontinuity, which occurs within sillimanite grade rocks, is postpeak metamorphism in both the hanging wall and the footwall. New geochronologic data constrain the timing of metamorphism in the hanging wall of the upper discontinuity to between 24 and 16 Ma, indistinguishable from previously published ages for the footwall. Movement across this structure represents Early Miocene strain localization and thickening in the Himalayan midcrust. Movement across the lower discontinuity, which occurs between staurolite and kyanite grade rocks, appears to be synmetamorphic with material in its footwall at approximately 10 Ma, but postpeak metamorphism for material in its hanging wall. This movement is interpreted to reflect the underplating and incorporation of material into the metamorphic core. The recognition of two thrust‐sense discontinuities in the exhumed Himalayan core in the Tama Kosi region is consistent with other similar structures recognized along the Himalaya. The widespread nature of these structures reinforces that they are important to our understanding of the evolution of the kinematics of large, hot orogens.

Microstructures and quartz lattice‐preferred orientations in the eclogite‐bearing migmatitic gneisses of the D'Entrecasteaux Islands, Papua New Guinea

Lattice preferred orientations (LPO) of quartz in gneiss domes of the D'Entrecasteaux Islands, Woodlark Rift shed insight into exhumation of the world's youngest (~5–7 Ma) eclogite‐bearing terrane at cm/yr rates. We focus on deformation that affected the terrane as it transited between lower crustal depths and the surface, including: (1) grain‐scale deformation mechanisms; and (2) style of flow and mode of emplacement of the domes. Electron‐backscatter diffraction was used to analyze microstructure and LPOs of 37 quartzofeldspathic gneiss samples that enclose meter‐scale mafic blocks preserving original eclogite‐facies assemblages. During exhumation of the ultrahigh‐pressure (UHP) terrane, gneisses were retrogressed in the amphibolite facies at lower crustal depths. The LPOs change from dome cores to carapaces, consistent with decreasing deformational temperatures. In the relatively chilled outer carapaces of the domes, the quartz LPOs consist of mostly crossed‐girdle [c]‐axis patterns, with some cleft‐girdle and small‐circle LPOs, and record dislocation creep accommodated by mixed‐ &lt; a &gt; slip. In the cores of the migmatitic domes, a chessboard pattern of subgrains is common, and quartz LPOs primarily record prism‐[c] slip, probably at &gt;630 °C. Other microstructures indicate recovery by high‐temperature grain‐boundary migration. Grain‐boundary mobility was anisotropic, leading to strong grain‐shape fabrics oblique to foliation, but not obviously relatable to shear sense. Evidence for melt‐present deformation is abundant, and microstructures (including partially dissolved feldspar grains) indicate some deformation by fluid‐assisted grain‐boundary diffusion creep. LPOs in carapace rocks are symmetrical, recording flow that was dominantly coaxial. We interpret the gneiss domes to have been emplaced into the rift as partially molten diapirs.

Mica-controlled anisotropy within mid-to-upper crustal mylonites: an EBSD study of mica fabrics in the Alpine Fault Zone, New Zealand

Abstract The lattice preferred orientation (LPO) of both muscovite and biotite were measured by electron backscatter diffraction (EBSD) and these data, together with the LPOs of the other main constituent minerals, were used to produce models of the seismic velocity anisotropy of the Alpine Fault Zone. Numerical experiments examine the effects of varying modal percentages of mica within the fault rocks. These models suggest that when the mica modal proportions approach 20% in quartzofeldspathic mylonites the intrinsic seismic anisotropy of the studied fault zone is dominated by mica, with the direction of the fastest P and S wave velocities strongly dependent on the mica LPOs. The LPOs show that micas produce three distinct patterns within mylonitic fault zones: C-fabric, S-fabric and a composite S–C fabric. The asymmetry of the LPOs can be used as kinematic indicators for the deformation within mylonites. Kinematic data from the micas matches the kinematic interpretation of quartz LPOs and field data. The modelling of velocities and velocity anisotropies from sample LPOs is consistent with geophysical data from the crust under the Southern Alps. The Alpine Fault mylonites and parallel Alpine schists have intrinsic P-wave velocity anisotropies of 12% and S-wave anisotropies of 10%.

Origin of felsic granulite microstructure by heterogeneous decomposition of alkali feldspar and extreme weakening of orogenic lower crust during the Variscan orogeny

AbstractThis study answers the question of origin and evolution of a granulitic microstructure typically developed in felsic granulites of the European Variscan belt. It shows that the precursor of the Variscan felsic granulites was a high‐pressure alkali feldspar‐rich coarse‐grained layered orthogneiss. Its S1 subhorizontal layering is defined by the alignment of alkali feldspar porphyroclasts alternating with monomineralic bands of quartz and bands rich in plagioclase and garnet. The alkali feldspar porphyroclasts contain inclusions of quartz, garnet, kyanite, biotite and rutile, reflecting peak P–T conditions of 1.6–1.8 GPa and 850 °C during S1 formation. Superimposed steep folds and steep cleavage, S2, are associated with recrystallization of alkali feldspar, plagioclase and quartz, and garnet chemistry modifications that correspond to 0.9–1.0 GPa and 800 °C. During exhumation, involving 0.8 GPa decompression and cooling, the probably perthitic alkali feldspar underwent an unusual process of heterogeneous decomposition along irregular reaction fronts forming a fine‐grained matrix composed of plagioclase and K‐feldspar grains. Regular grain distributions in the matrix, nucleation‐dominated crystal size distribution and preservation of lattice orientation of the parental perthite crystals are all explained by a discontinuous precipitation process. This heterogeneous decomposition of alkali feldspar solid solution is controlled by chemically and strain induced grain‐boundary migration. During exhumation and decompression, the fine‐grained matrix underwent viscous deformation, forming the typical microstructure of the Variscan granulites. Random phase distributions, minor coarsening and feldspar textures are interpreted as a result of strain softening due to diffusion creep‐accommodated grain‐boundary sliding. Subordinate large quartz ribbons were rheologically stronger than the feldspar‐dominated matrix due to the activity of different deformational mechanisms. Finally, in mid‐crustal levels, the subvertical structure was overprinted by a perpendicular steep fabric associated with the growth of sillimanite, heterogeneous hydration and local partial melting, development of aggregate phase distributions and significant coarsening. This evolution is accompanied with the development of a strong lattice preferred orientation of quartz, K‐feldspar and plagioclase, reflecting a switch to dislocation creep mechanism and a general hardening of the granulites under amphibolite facies conditions.

Lattice-preferred orientations of late-Variscan granitoids derived from neutron diffraction data: implications for magma emplacement mechanisms

The lattice-preferred orientation (LPOs) of two late-Variscan granitoids, the Meissen monzonite and the Podlesí dyke granite, were determined from high-resolution time-of-flight neutron diffraction patterns gained at the diffractometer SKAT in Dubna, Russia. The results demonstrate that the method is suitable for the LPO analysis of polyphase, relatively coarse-grained (0.1–6 mm) rocks. The Meissen monzonite has a prominent shape-preferred orientation (SPO) of the non-equidimensional minerals feldspar, mica and amphibole, whereas SPO of the Podlesí granite is unapparent at the hand-specimen scale. The neutron diffraction data revealed distinct LPOs in both granitoids. The LPO of the non-equidimensional minerals feldspar, mica and amphibole developed mainly during magmatic flow. In the case of the Meissen monzonite, the magmatic flow was superimposed by regional shear tectonics, which, however, had no significant effect on the LPOs. In both samples, quartz shows a weak but distinct LPO, which is atypical for plastic deformation and different in the syn-kinematic Meissen monzonite and the post-kinematic Podlesí granite. We suggest that, first of all, the quartz LPO of the Meissen monzonite is the result of oriented growth in an anisotropic stress field. The quartz LPO of the Podlesí granite, which more or less resembles a deformational LPO in the flattening field of the local strain field, developed during magmatic flow, whereby the rhombohedral faces of the quartz crystals adhered to the (010) faces of aligned albite and to the (001) faces of zinnwaldite. Due to shape anisotropy of their attachments, the quartz crystals were passively aligned by magmatic flow. Thus, magmatic flow and oriented crystal growth are the major LPO-forming processes in both granitoids. For the Meissen monzonite, the solid-state flow was too weak to cause significant crystallographic re-orientation of the minerals aligned by magmatic flow. Finally, the significance of our results for the evaluation of the regional tectonic environment during magma emplacement is discussed. The discussion on the regional implications of the more methodologically oriented results provides the basis for future, more regionally aimed studies in view of the fabric characteristics of such plutons and their developing mechanisms.

Microfabric characteristics and rheological significance of ultra‐high‐pressure metamorphosed jadeite‐quartzite and eclogite from Shuanghe, Dabie Mountains, China

AbstractQuantitative analysis of the structural evolution of jadeite‐quartzite, a rare ultra‐high pressure (UHP) rock type from the Dabie Mountains of eastern China, sheds light on the formation and evolution of UHP orogenic belts worldwide. Geological mapping of the Shuanghe area, where jadeite‐quartzites crop out, was carried out to determine the spatial relationships between different UHP rocks within this orogen. The deformation mechanisms of jadeite‐quartzite, geodynamical parameters (stress, strain, strain rate), and microstructure including lattice preferred orientation (LPO) were determined from six jadeite‐quartzite samples from the Shuanghe area. LPOs of clinopyroxene (jadeite and omphacite), garnet, rutile and quartz from these jadeite‐quartzite samples are compared with those of three eclogites preserving different degrees of deformation from the Shuanghe area. Microstructural LPOs of jadeite, omphacite, garnet, rutile and quartz were determined using electron backscattered diffraction (EBSD) analysis. Quartz fabrics were largely recrystallized during late, low‐grade stages of deformation, whereas garnet shows no strong LPO patterns. Rutile fabrics show a weak LS fabric along [001]. Jadeite and omphacite show the strongest eclogite facies LPO patterns, suggesting that they may provide important information about mantle deformation patterns and control the rheology of deeply subducted continental crust. Microstructural data show that the jadeite LPO patterns are similar to those of omphacite and vary between L‐ and S‐types, which correlate with prolate and oblate grain shape fabrics (SPO); quartz LPOs are monoclinic. Microstructural analysis using TEM shows that the dominant slip systems of jadeite in one sample are (100)[001], (110)[001] and (1 1 0)1/2[110], while in another sample, no dislocations are observed. Abundant dislocations in quartz were accommodated by the dominant slip system (0001)[110], indicating basal glide and represents regional shearing during the exhumation process. This suggests that dislocation creep is the dominant fundamental deformation mechanism in jadeite under UHP conditions. The protoliths of jadeite‐quartzite, metasedimentary rocks from the northern passive continental margin of the Yangtze craton, experienced the same deep subduction and were deformed under similar rheological conditions as other UHP eclogite, marble and paragneiss. Experimental UHP deformation of quartzo‐feldspathic gneiss with a chemical composition similar to the bulk continental crust has shown that the formation of a jadeite–stishovite rock is associated with a density increase of the host rock similar to the eclogite conversion from basaltic protoliths. The resulting rock can be denser than the surrounding mantle pyrolite up to depths of 660 km (24 GPa). Thus, processes of deep continental subduction may be better‐understood through understanding the rheology and mechanical behaviour of jadeite. Jadeite‐quartzites such as those from the Shuanghe may be exhumed remnants of deeply‐subducted slabs of continental crust, other parts of which subducted past the ‘depth of no return’, and remain in the deep mantle.

Amphibole and lower crustal seismic properties

Measurements of seismic anisotropy frequently offer insights of the deformation kinematics in-situ within the Earth. In the continental crust, seismic anisotropy is commonly ascribed to regionally aligned lattice preferred orientation (LPO) of mica crystals. However, determinations of composition suggest that the deep continental crust contains rather little mica. In this contribution, the role of amphibole, a far more plausible constituent, especially for deep continental crust of basic composition, is assessed. A field analogue approach is adopted, where the LPO are measured, correlated with the strain state of the rocks, and used to calculate the seismic properties. The seismic properties may be up-scaled through the specimen using the modal proportions of the constituent mineral phases. To examine the role of composition in controlling the intensity of seismic anisotropy, seismic properties are computed using an array of modal compositions (so-called ‘rock recipes’). The case history comes from the Central Block of the Lewisian of NW Scotland, where a regional basic dyke swarm (the Scourie dykes) is locally deformed within amphibolite facies shear zones, conditions that are representative of the deep crust. Shear strain (γ) profiles across a 100 m wide shear zone, using deflected banding in the host gneisses, yield values of 0<γ<15 or greater. A Scourie dyke deflected into the shear zone exhibits hornblende with distinct preferred dimensional orientations representative of such strains. In contrast, plagioclase (and quartz) clots are almost totally insensitive to deformation (due to grain-size sensitive creep deformation) and yield approximately constant apparent strains of γ=1–2. Hornblende–plagioclase–quartz LPOs, combined in their modal proportions and modulated by their individual single-crystal elastic properties, define the seismic properties of the shear zone. Rock recipe modelling demonstrates that it is the hornblende that dominates the seismic response and that plagioclase and quartz serve simply to dilute the intensity of anisotropy. The values calculated indicate significant (up to ~13%) seismic anisotropy for strongly sheared amphibolites. Thus amphibole, aligned through deformation, is likely be a major contributor to the seismic anisotropy of the deep continental crust.

Microstructural-deformation record of an orogen-parallel extension in the Vepor Unit, West Carpathians

Deformation microstructures of quartzo-feldspathic rocks have been used to analyse the orogen-parallel stretching of the Vepor Unit in the West Carpathians during Cretaceous convergence. This process occurs contemporaneously with the burial of the Vepor basement related to the overthrusting Palaeozoic volcano-sedimentary sequences of the Gemer Unit. The microstructures develop along a metamorphic gradient indicating an increase of pressure and temperature towards the structural footwall. The recrystallization of quartz occurs in the field of subgrain rotation recrystallization (SGR) while the plagioclase and K-feldspar disintegrate into a neocrystallized muscovite-albite matrix. Palaeopiezometry of the recrystallized quartz in conjunction with P–T estimates yield strain rates of 9 × 10 −12 to 6 × 10 −14 s −1, depending on temperature (380–480 °C) and choice of piezometer and flow law calibration. Strain analysis of the recrystallized quartz aggregates and quartz texture analysis indicate an overprint of the Variscan magmatic and high grade fabrics by a Cretaceous plane strain deformation resulting in vertical shortening and horizontal orogen parallel East–West stretching. Spatial distribution and conflicting sense-of-shear of the asymmetrical quartz lattice preferred orientation patterns indicate that the large-scale pure shear deformation of the Vepor Unit is locally accommodated by simple shear zones. Observed metamorphic gradients across the Vepor Unit are interpreted to result from later large-scale folding and heterogeneous exhumation of the orogen-parallel deformation fabric.

Lattice preferred orientation and seismic anisotropy in sedimentary rocks

SUMMARY Although it is well known that sedimentary rocks can be seismically anisotropic, there have been few detailed investigations of the underlying cause of such anisotropy. Here, we investigate anisotropy due to the preferred orientation of minerals, or lattice preferred orientation (LPO), in a suite of sedimentary rocks. Seismic properties are predicted by averaging single-crystal elastic constants of minerals according to their crystal orientation and modal volume fraction in the rock aggregate. Both Electron Backscattered Diffraction (EBSD) and X-ray Texture Goniometry (XTG) are tested as quantitative techniques for measuring the LPO of sedimentary rocks. Although EBSD has promise for future LPO measurements in polymineralic sedimentary rocks, problems currently remain in measuring low-symmetry phases (e.g. feldspars) and very small clay or mica particles. However, the LPO of very fine-grained phyllosilicates can be measured using XTG and the LPO of low-symmetry minerals can be measured using manual EBSD pattern analysis. Here, we use such a hybrid approach to estimate LPO in a suite of sedimentary samples. The seismic properties calculated from LPO data show anisotropy values for P-waves ranging from 1.5 per cent in sandstones, to over 3.5 per cent in a siltstone, to 12 per cent in a shale. The effect of thin multilayering on long-wavelength propagation in a siltstone is predicted by applying Backus-type averaging. The layering does not enhance the anisotropy because of the small differences in density and in elasticity between the two layer types. The LPO of phyllosilicates and to a lesser extent dolomite and siderite seem to contribute significantly to the seismic anisotropy of phyllosilicate-rich rocks (siltstones and shales). The weak LPO of quartz in sandstones causes a few per cent anisotropy. Cumulatively, our results suggest that field observations of seismic anisotropy have the potential to be used as an indicator of rock lithology.

Petrofabrics and seismic properties of garnet peridotite from the UHP Sulu terrane (China): Implications for olivine deformation mechanism in a cold and dry subducting continental slab

Lattice-preferred orientations (LPO) of olivine, diopside, enstatite and garnet from the Zhimafang garnet peridotite body in the Sulu ultrahigh-pressure (UHP) metamorphic terrane (China) were measured using the electron backscatter diffraction (EBSD) technique. The peridotite was captured from a mantle wedge immediately adjacent the subducted Yangtze slab and then experienced the UHP metamorphism at 750–950 °C and 4–7 GPa. The olivine LPO is characterized by the [001] axis close to the stretching lineation and the (100) plane subparallel to the foliation, indicating the prevailing of (100) [001] slip. Enstatite LPO displays the dominance of (100) [001] slip. Diopside developed complex LPO patterns that are difficult to explain using a single slip system of (100) [001]. Garnet is almost randomly oriented due to its low volume fractions, cubic symmetry and the presence of numerous slip systems. Calculated seismic properties of the peridotite yield a maximum P-wave velocity normal to the foliation and a minimum along the foliation, with anisotropy up to 8% in strongly sheared samples. The S-wave velocity pattern is complex but the fast polarization plane generally normal to the foliation. The inferred shear sense from the olivine LPO is top-to-SE, in contrary to exhumation-induced top-to-NW thrusting recorded in the quartz LPO, implying that the olivine LPO formed at early UHP metamorphic conditions. The olivine crystals have relatively low water contents (141–475 H/10 6 Si), indicating a fluid-deficient environment for the LPO formation. The present study suggests that a combination of low temperature and UHP plays a much more important role than the water content to promote the activation of (100) [001] slip in olivine.

Quartz lattice preferred orientation patterns and static recrystallization: Natural examples from the Ryoke belt, Japan

Research Article| July 01, 2006 Quartz lattice preferred orientation patterns and static recrystallization: Natural examples from the Ryoke belt, Japan Miharu Otani; Miharu Otani 1Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8602, Japan Search for other works by this author on: GSW Google Scholar Simon Wallis Simon Wallis 1Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8602, Japan Search for other works by this author on: GSW Google Scholar Author and Article Information Miharu Otani 1Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8602, Japan Simon Wallis 1Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8602, Japan Publisher: Geological Society of America Received: 14 Nov 2005 Revision Received: 13 Feb 2006 Accepted: 03 Apr 2006 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 The Geological Society of America, Inc. Geology (2006) 34 (7): 561–564. https://doi.org/10.1130/G22430.1 Article history Received: 14 Nov 2005 Revision Received: 13 Feb 2006 Accepted: 03 Apr 2006 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Miharu Otani, Simon Wallis; Quartz lattice preferred orientation patterns and static recrystallization: Natural examples from the Ryoke belt, Japan. Geology 2006;; 34 (7): 561–564. doi: https://doi.org/10.1130/G22430.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Quartz c-axis lattice preferred orientation (LPO) patterns have been widely used to analyze different aspects of mid-crustal ductile deformation. However, despite the common presence of annealed microstructures in high-grade metamorphic belts, the effects of static recrystallization on such quartz LPOs are unclear. A suite of samples of quartz schist from the Nukata area of the Ryoke metamorphic belt was collected along profiles of several kilometers that reach the postorogenic Shinshiro tonalite. The samples show an increase in grain size and changes in microstructure, such as straightening of grain boundaries and formation of quartz-in-quartz inclusions that represent annealing related to contact metamorphism. In contrast to the clear microstructural changes, quartz c-axis LPO patterns show only very minor modifications even after strong annealing. This result implies that quartz LPO patterns should not be obliterated by static recrystallization and quartz-rich tectonites that lack such LPO patterns deformed by processes other than dislocation creep. Likely processes in high-grade rocks are grain boundary sliding assisted by melt migration and diffusion creep, both of which are characterized by Newtonian rheologies. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Quartz lattice preferred orientation patterns and static recrystallization: Natural examples from the Ryoke belt, Japan

Quartz c-axis lattice preferred orientation (LPO) patterns have been widely used to analyze different aspects of mid-crustal ductile deformation. However, despite the common presence of annealed microstructures in high-grade metamorphic belts, the effects of static recrystallization on such quartz LPOs are unclear. A suite of samples of quartz schist from the Nukata area of the Ryoke metamorphic belt was collected along profiles of several kilometers that reach the postorogenic Shinshiro tonalite. The samples show an increase in grain size and changes in microstructure, such as straightening of grain boundaries and formation of quartz-in-quartz inclusions that represent annealing related to contact metamorphism. In contrast to the clear microstructural changes, quartz c-axis LPO patterns show only very minor modifications even after strong annealing. This result implies that quartz LPO patterns should not be obliterated by static recrystallization and quartz-rich tectonites that lack such LPO patterns deformed by processes other than dislocation creep. Likely processes in high-grade rocks are grain boundary sliding assisted by melt migration and diffusion creep, both of which are characterized by Newtonian rheologies.