Vorticity analysis in calcite tectonites: An example from the Attico-Cycladic massif (Attica,Greece)

Although calcite tectonites are widespread in nature their use to quantify flow vorticity is limited. We use new (micro-)structural, petrofabric and vorticity data to analyse the kinematics of flow in outcrop-scale calcite mylonite zones. These zones are genetically related to a crustal-scale NE-directed ductile thrust (Basal Thrust) that emplaced the Blueschist over the Basal unit during the exhumation of the Attico-Cycladic Massif. Calcite microstructures reveal that the last stage of deformation occurred at temperatures 200–300 °C achieved by mild heating, which is possibly related with the reburial of the Basal Thrust's footwall. Vorticity analyses were based on the degree of asymmetry of calcite c-axis fabrics as well as on the assumption that the orientation of the long axes of calcite neoblasts within an oblique foliation delineates the direction of instantaneous stretching axis. Both methodological approaches provide consistent estimates with a simple shear component between 55% and 82% (W_n = 0.76–0.96). The use of the stress axis (σ₁) orientation recorded by twin-c-axis-pairs to quantify vorticity generally gives significantly lower simple shear component. Comparison of our vorticity estimates with previous estimates inferred from quartz fabrics and rigid porphyroclasts reveals that exhumation-related deformation in the nappe pile was steady state.     

Deformation partitioning during folding and transposition of quartz layers

Banded iron formation (BIF) from the Quadrilátero Ferrı́fero (southeastern Brazil) shows a compositional layering with alternating iron-rich and quartz-rich layers. This layering was intensively folded and transposed at a centimeter/millimeter scale through a component of bedding-parallel shear related to flexural slip at middle to high greenschist facies conditions (400–450 °C). The microstructure and c-axis fabrics of normal limbs, inverted limb and hinge zones of a selected isoclinal fold were analyzed combining optical and scanning electron microscopy (SEM) and digital image analysis. In the normal limbs, recrystallized quartz grains show undulose extinction, relatively dry grain boundaries, c-axes at high angle to foliation and a pervasive grain shape fabric (GSF) indicating operation of crystal-plastic processes. In the inverted limb, quartz grains show more serrated and porous (“wet”) grain boundaries; the GSF is similar to that of the normal limb, but c-axes are oriented at 90° to those of the normal limb. We interpreted these characteristics as reflecting operation of solution-precipitation deformation in inverted limbs, as a consequence of grains having been rotated to an orientation that was hard to basal 〈a〉 glide, but easy to dissolution-precipitation creep. This deformation partitioning between crystal-plasticity and solution-transfer during folding/transposition of quartz may explain the common occurrence of layered quartz rocks, where individual layers show alternating c-axis fabrics with opposite asymmetries but a consistent GSF orientation. Such characteristics may reflect an earlier event of pervasive folding/transposition of a preexisting layering.     

鲁西青邑韧性剪切带运动学涡度及剪切作用类型

青邑韧性剪切带是晚太古代末期发育在鲁西前寒武纪基底花岗岩中一条规模较大的韧性剪切带。剪切带NW走向,面理直立,线理水平,剪切标志反映右行剪切。石英光轴法求得运动学涡度在0．96～0．99之间变化,极摩尔圆法求得糜棱岩化岩石运动学涡度为0．91,初糜棱岩运动学涡度为0．87,糜棱岩运动学涡度为0．81,超糜棱岩运动学涡度为0．60。运动学涡度表明,剪切带剪切作用类型为一般剪切,变形初期以单剪为主,随应变的增大,运动学涡度值逐渐减小,变形的纯剪分量不断增加,最后以纯剪为主。剪切作用类型及三维参照变形分析表明,青邑韧性剪切带属加长一变宽类型的一般剪切带并且在Y轴方向上有所增长。韧性剪切在太古代末期克拉通化过程中具有加厚陆壳的作用。     

西藏阿里札达韧性剪切带特征及其X光岩组分析

文中简述了西藏阿里札达盆地的地质背景、区域地层和札达韧性剪切带的基本特征。采用X射线衍射法对札达韧性剪切带中的石英、方解石和白云母等三种矿物,进行了X光岩组分析,确定了韧性变形岩石的组构特征、韧性剪切带的属性和变形岩石的应变类型,以及韧性剪切带形成时的温压条件。研究表明,韧性变形岩石均具不对称组构,反映韧性带属于南盘(下盘)俯冲型韧性剪切带,韧性变形是在高温、高压、低应变速率条件下发生的,处于>10km的地壳深度,岩石应变类型以压扁应变为主。     

东天山康古尔韧性变形带韧性挤压变形特征:以哈密黄山东地区为例

东天山康古尔断裂带和雅满苏断裂带之间石炭纪沉积-火山岩系中发育一条东西向韧性变形带，具两期性质不同的韧性变形，早期韧性挤压性质，晚期右行韧性剪切.对早期韧性挤压变形的宏观构造（糜棱面理、糜棱线理、对称石香肠构造、不对称褶皱等）和显微构造（压力影构造、布丁构造、动态重结晶、不同类型砾石变形特征等）特征做了详细的研究.糜棱岩中砾石有限应变Flinn图解判别岩石类型为L-S和SL型构造岩，三轴应变量的测量均揭示了一般压缩和少部分的平面应变类型.石英亚颗粒旋转重结晶及少量边界迁移重结晶，长石塑性变形、部分膨凸重结晶等变形特征；石英C轴组构揭示中温柱面 < a>和菱面 < a>滑移为主，后期叠加底面 < a>滑移.因此，推测早期韧性挤压变形温度范围在450~550℃.综合区域地质资料，康古尔变形带早期韧性挤压变形形成于300~290 Ma，塔里木板块和中天碰撞闭合后陆内南北挤压环境下.      

Fabric development during shear deformation in the Main Central Thrust Zone, NW-Himalaya, India

Quartz microfabrics and associated microstructures have been studied on a crustal shear zone—the Main Central Thrust (MCT) of the Himalaya. Sampling has been done along six traverses across the MCT zone in the Kumaun and Garhwal sectors of the Indian Himalaya. The MCT is a moderately north-dipping shear zone formed as a result of the southward emplacement of a part of the deeply rooted crust (that now constitutes the Central Crystalline Zone of the Higher Himalaya) over the less metamorphosed sedimentary belt of the Lesser Himalaya. On the basis of quartz c- and a-axis fabric patterns, supported by the relevant microstructures within the MCT zone, two major kinematic domains have been distinguished. A noncoaxial deformation domain is indicated by the intensely deformed rocks in the vicinity of the MCT plane. This domain includes ductilely deformed and fine-grained mylonitic rocks which contain a strong stretching lineation and are composed of low-grade mineral assemblages (muscovite, chlorite and quartz). These rocks are characterized by highly asymmetric structures/microstructures and quartz c- and a-axis fabrics that indicate a top-to-the-south sense that is compatible with south-directed thrusting for the MCT zone. An apparently coaxial deformation domain, on the other hand, is indicated by the rocks occurring in a rather narrow belt fringing, and structurally above, the noncoaxial deformation domain. The rocks are highly feldspathic and coarse-grained gneisses and do not possess any common lineation trend and the effects of simple shear deformation are weak. The quartz c-axis fabrics are symmetrical with respect to foliation and lineation. Moreover, these rocks contain conjugate and mutually interfering shear bands, feldspar/quartz porphyroclasts with long axes parallel to the macrosopic foliation and the related structures/microstructures, suggesting deformation under an approximate coaxial strain path.On moving towards the MCT, the quartz c- and a-axis fabrics become progressively stronger. The c-axis fabric gradually changes from random to orthorhombic and then to monoclinic. In addition, the coaxial strain path gradually changes to the noncoaxial strain path. All this progressive evolution of quartz fabrics suggests more activation of the basal, rhomb and a slip systems at all structural levels across the MCT.     

大别山北部糜棱岩中石英的高温应变

超高压变质岩的发现导致了人们对大别山构造格架的重新认识。最近的研究表明,超高压变质岩只产于大别山南部(Wang et al., 1992),而北部为古生代岛弧杂岩所占据,晓天-磨子潭断裂即为这些杂岩的北界(Wang et al., 1994)。晓天-磨子潭断裂作为左行走滑断裂已经得到公认,但笔者对该断裂南侧糜棱岩的研究表明,其并非左行走滑产物,而是早期变形的构造形迹。本文旨在分析这些糜棱岩中石英的显微构造,进而揭示其早期构造变形特征。     

秦岭商丹糜棱岩带石英组构特征变化及成因分析

商丹糜棱岩带岩湾、沙沟和商南等3个区段石英C-轴组构和显微构造特征表明,该带自西向东石英C-轴组构形式由单一环带型转为Y-轴方向的点极密型,然后变为Ⅰ型交叉环带型,构造变形环境从低绿片岩相过渡到中-高绿片岩相,然后变为高绿片岩相-低角闪岩相。构造变形环境差异是造成石英组构形式变化的主要因素。随着温压条件的升高,石英滑移系从以底面〈a〉滑移系和柱面〈a〉滑移系的共同作用为主转向以单一柱面〈a〉滑移系为主,进而底面〈a〉滑移系和柱面〈a〉滑移系又重新变得活跃,且菱面滑移系的作用也变得十分重要。     

Thermomechanics of an extensional shear zone,Raft River metamorphic core complex,NW Utah

A detailed structural and microstructural analysis of the Miocene Raft River detachment shear zone (NW Utah) provides insight into the thermomechanical evolution of the continental crust during extension associated with the exhumation of metamorphic core complexes. Combined microstructural, electron backscattered diffraction, strain, and vorticity analysis of the very well exposed quartzite mylonite show an increase in intensity of the rock fabrics from west to east, along the transport direction, compatible with observed finite strain markers and a model of ``necking'' of the shear zone. Microstructural evidence (quartz microstructures and deformation lamellae) suggests that the detachment shear zone evolved at its peak strength, close to the dislocation creep/exponential creep transition, where meteoric fluids played an important role on strain hardening, embrittlement, and eventually seismic failure.Empirically calibrated paleopiezometers based on quartz recrystallized grain size and deformation lamellae spacing show very similar results, indicate that the shear zone developed under stress ranging from 40 MPa to 60 MPa. Using a quartzite dislocation creep flow law we further estimate that the detachment shear zone quartzite mylonite developed at a strain rates between 10⁻¹² and 10⁻¹⁴ s⁻¹. We suggest that a compressed geothermal gradient across this detachment, which was produced by a combination of ductile shearing, heat advection, and cooling by meteoric fluids, may have triggered mechanical instabilities and strongly influenced the rheology of the detachment shear zone.     

Volume strain, strain type and flow path in a narrow shear zone

This study explores the state of finite strain and changes in the mean kinematic vorticity number, grain size, whole-rock chemistry and mineralogy across an upper amphibolite-facies shear zone in a metadiorite, northern Malawi, east-central Africa. P–T conditions during shear-zone formation and deformation were approximately 700–750^?°C and 5–7?kbar and are slightly less than P–T conditions for the regional peak of metamorphism. The major rock-forming minerals, plagioclase, hornblende, biotite, and quartz, were deformed by crystal-plastic processes accompanied by, except for hornblende, dynamic recrystallization. The modal abundance of all four major rock-forming minerals shows no systematic change from the country rock into and across the shear zone, indicating that shear-zone development was not associated with retrograde mineral reactions. The grain size of the major rock-forming minerals decreases within the shear zone. Plagioclase and hornblende, which occur as porphyroblasts outside the shear zone, exhibit a bimodal grain-size distribution within the shear zone. Quartz has a unimodal grain-size distribution in the shear zone. Major and trace element chemistry does not change systematically across the shear zone, implying no volume change in the mylonite. Matrix strain data for plagioclase and hornblende by the Fry method and fabric strain as deduced from R_f/φ analysis of plagioclase and quartz grains demonstrate a slightly constrictional strain type (K≈1.5) across the shear zone. The quantitative finite-strain data for the different residual minerals as obtained by unlike methods show no systematic variation, but recrystallized plagioclase grains record higher strain than the residual grains. The mean kinematic vorticity number changes from approximately 0.3 outside to approximately 0.8 within the shear zone, indicating that the bulk deformation path deviated from progressive simple shear. The estimates for finite strain and the degree of noncoaxiality account for approximately 50% of thinning normal to the shear zone.     

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.     

Application of General Shear Theory to the Study of Formation Mechanism of the Metamorphic Core Complex:A Case Study of Xiaoqinling in Central China

The kinematic vorticity number and strain of the mylonitic zone related to the detachment fault increase from ESE to WNW along the moving direction of the upper plate of the Xiaoqinling metamorphic core complex (XMCC) and the geometry of quartz c-axis fabrics changes progressively from crossed girdles to single girdles in the same direction. Therefore, pure shear is dominant in the ESE part of the XMCC while simple shear becomes increasingly important towards WNW. However, the shear type does not change with the strain across the shear zone, thus the variation of shear type is of significance in indicating the formation mechanism. The granitic plutons within the XMCC came from the deep source and their emplacement was an active and forceful upwelling prior to the detachment faulting. The PTt path demonstrates that magmatism is an important cause for the formation of the XMCC. The formation mechanism of the XMCC is supposed to be active plutonism and passive detachment. Crustal thickening and magmatic do     

Development of quartz c-axis crossed/single girdles under simple-pure shear deformation: Results of visco-plastic self-consistent modeling

Quartz c-axis fabrics are widely used to determine the shear plane in ductile shear zones, based upon an assumption that the shear plane is perpendicular to both the central segment of quartz c-axis crossed girdle and single girdle. In this paper the development of quartz c-axis fabric under simple-pure shear deformation is simulated using the visco-plastic self-consistent (VPSC) model so as to re-examine this assumption. In the case of no or weak dynamic recrystallization, the simulated crossed girdles have a central segment perpendicular or nearly perpendicular to the maximum principal finite strain direction (X) and the XY finite strain plane, and at a variable angle relative to the imposed kinematic framework that is dependent on the modeled flow vorticity and finite strain. These crossed girdles have a symmetrical skeleton with respect to the finite strain axes, regardless of the bulk strain and the kinematic vorticity, and rotate in a way similar to the shear sense with increasing bulk strain ratio. The larger the vorticity number the more asymmetrical their legs tend to be. In the case of strong dynamic recrystallization and large bulk strain, under simple shear the crossed girdle switches into single girdles, sub-perpendicular to the shear plane, by losing the weak legs. The numerical results in our models do not confirm the above-mentioned assumption.     

湖南郴州枞树板铅锌矿X光组构分析

作者通过对湖南郴州枞树板铅锌矿围岩及含矿构造带岩石的X光岩组分析,认为在构造变形过程中,原岩粒度越细,对变形的反映越敏感。在相同的变形条件下,较细的变质砂岩石的石英定向组构比较粗的变质砂岩内明显,板岩和粉砂质板岩的定向组构最明显。枞树板地区近EW向褶皱变形组构表现强烈,近SN向褶皱变形组构较弱。矿化热液的蚀变作用破坏了岩石中早期褶皱变形组构,但铅锌矿脉内的石英没有明显的生长组构,说明在矿化和含矿石英脉成生过程中没有明显的张性充填结晶表现,而是一种以交代为主的矿化形式。这与从其它地质地球化学研究取得的关于本     

舒兰韧性剪切带应变分析及石英动态重结晶颗粒分形特征与流变参数估算

舒兰北东向韧性剪切带位于佳木斯-伊通断裂带(佳-伊断裂带)中南段, 剪切带内糜棱岩具有明显左行走滑特征, 片麻理产状近NNE向.糜棱岩中长石有限应变Flinn图解判别岩石类型为L-S型构造岩, 属拉长型应变.石英C轴EBSD组构分析表明, 石英组构以中低温菱面为主, 滑移系为{0001} < 110>.剪切带内糜棱岩的剪应变为0.44, 不同方法计算所得运动学涡度值均大于0.95, 指示剪切变形以简单剪切为主.综合矿物变形温度计、石英C轴EBSD组构、石英的粒度-频数图及Kruhl温度计综合估计该韧性剪切带变形机制以位错蠕变机制为主, 变质相为低绿片岩相, 发生韧性变形和糜棱岩化温度范围在400~500 ℃之间.糜棱岩内石英动态重结晶新晶粒边界普遍具有锯齿状或港湾状结构, 利用分形方法对其重结晶新晶边界研究表明, 这些晶粒边界具有自相似性, 表现出分形特征, 分形维数值为1.195~1.220.根据石英重结晶粒径估算差应力值为24.35~27.59 MPa, 代表了舒兰韧性剪切带糜棱岩化作用过程的差异应力下限.使用不同实验方法估算、比较和分析了该剪切带古应变速率, 认为该速率应为10-12.00~10-13.18 s-1, 与区域性应变速率10-13.00~10-15.00 s-1对比, 说明舒兰韧性剪切带的应变速率与世界上大多数韧性剪切带中的糜棱岩应变速率一致, 是缓慢变形的结果, 其形成可能与早白垩世伊泽纳崎板块向欧亚大陆俯冲发生转向有关.      

Kinematic analyses of orogen-parallel L-tectonites from Pelling-Munsiari thrust of Sikkim Himalayan fold thrust belt: Insights from multiple,incremental strain markers

Fault rocks associated with the Pelling thrust (PT) in the Sikkim Himalayan fold thrust belt (FTB) change from SL tectonites to local, transport-parallel L-tectonites that are exposed in discontinuous klippen south of the PT zone. By estimating the incremental kinematic vorticity number (W_k) from quartz c-axes fabric, oblique fabric, and subgrains, we reconstruct a first-order, kinematic path of these L-tectonites. Quartz c-axes fabric suggests that the deformation initiated as pure-shear dominated (∼56–96%) that progressively became simple-shear dominated (∼29–54%), as recorded by the oblique fabric and subgrains in the L-tectonites. These rocks record a non-steady deformation where the kinematic vorticity varied spatially and temporally within the klippen.The L-tectonites record ∼30% greater pure-shear than the PT fault rocks outside the klippen, and the greatest pure-shear dominated flow among the published vorticity data from major fault rocks of the Himalayan FTB. The relative decrease in the transport-parallel simple-shear component within the klippen, and associated relative increase of transport-perpendicular, pure-shear component, support the presence of a sub-PT lateral ramp in the Sikkim Himalayan FTB. This study demonstrates the influence of structural architecture for fault systems for controlling spatial and temporal variations of deformation fabrics and kinematic path of deforming thrust wedges.     

Quartz fabrics and recumbent folds in the Sierra de Los Filabres (SE-Spain)

Abstract

An important generation of recumbent folds can be recognized in the Nevado-Filabride nappe complex in the Sierra de los Filabres in SE Spain.

Folding post-dates an initial phase of flattening and is prior to the main phase of non-coaxial deformation and thrusting in the upper part of the complex, involved in a large-scale movement zone with dominant sense of shear to the northwest.

Axial planes and axes of these folds are aligned sub-parallel to the plastic flow plane in quartz and the shear direction respectively. Relict quartz fabrics can be found however, which strongly suggest that some fold axes originated in this position and did not rotate towards parallelism with the shear direction. This apparently contradicts the generally accepted model of development of folds in mylonite zones.

The redistribution of linear structures of the older flattening fase over the folds, confirms that the original orientation of fold axes was close to the present and indicates that an important component of flexural slip was involved in fold formation.     

Structures,kinematic analysis,rheological parameters and temperature-pressure estimate of the Mesozoic Xingcheng-Taili ductile shear zone in the North China Craton

The NE to ENE trending Mesozoic Xingcheng-Taili ductile shear zone of the northeastern North China Craton was shaped by three phases of deformation. Deformation phase D₁ is characterized by a steep, generally E–W striking gneissosity. It was then overprinted by deformation phase D₂ with NE-sinistral shear with K-feldspar porphyroclasts forming a subhorizontal low-angle stretching lineation on a steep foliation. During deformation phase D₃, lateral motion accommodated by ENE sinistral strike-slip shear zones dominated. Associated fabrics developed at upper greenschist metamorphic facies conditions and show the deformation characteristics of middle- to shallow crustal levels. In some parts, the older structures have been in turn overprinted by late-stage sinistral D₃ shearing. Finite strain and kinematic vorticity in all deformed granitic rocks indicate a prolate ellipsoid (L-S tectonites) near plane strain. Simple shear-dominated general shear during D₃ deformation is probably of general significance. The quartz c-axis textures indicate prism-gliding with a dominant rhomb <a> slip and basal <a> slip system formed mainly at low-middle temperatures. Mineral deformation behavior, quartz c-axis textures, quartz grain size and the Kruhl thermometer demonstrate that the ductile shear zone developed under greenschist facies metamorphic conditions at deformation temperatures ranging from 400 to 500 °C. Dislocation creep is the main deformation mechanism at a shallow crustal level. Fractal analysis showed that the boundaries of recrystallized quartz grains had statistically self-similarities. Differential stresses deduced from dynamically recrystallized quartz grain size are at around 20–39 MPa, and strain rates in the order of 10⁻¹² to 10⁻¹⁴ s⁻¹. This indicates deformation of granitic rocks in the Xingcheng-Taili ductile shear zone at low strain rates, which is consistent with most other ductile shear zones. Hornblende-plagioclase thermometer and white mica barometer indicate metamorphic conditions of medium pressures at around ca. 3–5 kbar and temperatures of 400–500 °C within greenschist facies conditions. The main D₃ deformation of the ENE-trending sinistral strike-slip ductile shearing is related to the roll-back of the subducting Pacific plate beneath the North China Craton.     

Grain-size reduction of feldspars by fracturing and neocrystallization in a low-grade granitic mylonite and its rheological effect

Feldspar grain-size reduction occurred due to the fracturing of plagioclase and K-feldspar, myrmekite formation and neocrystallization of albitic plagioclase along shear fractures of K-feldspar porphyroclasts in the leucocratic granitic rocks from the Yecheon shear zone of South Korea that was deformed under a middle greenschist-facies condition. The neocrystallization of albitic plagioclase was induced by strain energy adjacent to the shear fractures and by chemical free energy due to the compositional disequilibrium between infiltrating Na-rich fluid and host K-feldspar. With increasing deformation from protomylonite to mylonite, alternating layers of feldspar, quartz and muscovite developed. The fine-grained feldspar-rich layers were deformed dominantly by granular flow, while quartz ribbons were deformed by dislocation creep. With layer development and a more distributed strain in the mylonite, lower stresses in the quartz-rich layers resulted in a larger size of dynamically recrystallized quartz grains than that of the protomylonite.     

Heterogeneous deformation and quartz crystallographic fabric transitions: natural examples from the moine thrust zone at the stack of glencoul,northern assynt

Quartz crystallographic fabric transitions in well-exposed mylonites immediately beneath the Moine Thrust at the Stack of Glencoul (NW Scotland) have been investigated by optical microscopy, X-ray texture goniometry and Orientation Distribution Function analysis. A progressive change is observed from asymmetrical kinked single girdle c-axis fabrics at 0.5 cm beneath the Moine Thrust, through asymmetrical Type I cross-girdle fabrics to symmetrical Type I cross-girdle fabrics at 30 cm beneath the thrust. This c-axis fabric transition is accompanied by a transition from asymmetrical single a-axis maximum fabrics (0.5 cm beneath the thrust) through asymmetrical two maxima fabrics to essentially symmetrical two maxima a-axis fabrics. ODF analysis of these S >L and L - S tectonites indicates that c-axis positions on the ‘leading edge’ of the fabric skeleton are related by a common (a) direction oriented within the XZ plane at a moderate angle to the lineation (X). In contrast, c-axis positions on the peripheral ‘trailing edge’ are related by a positive (r) rhomb pole oriented close to Z; (a) directions lying within this common rhomb plane progressively change through 180° in orientation traced around the c-axis fabric skeleton. Such contrasting ‘single crystal’ rhomb (a) preferred orientations on the ‘leading’ and ‘trailing’ edges of the fabric skeleton are interpreted as indicating localized (grain scale) plane strain and flattening deformation, respectively. They result in tectonites with essentially symmetrical c- and a-axis fabrics which display strongly asymmetrical positive (r) and negative (z) rhomb pole figures. The observed transition in quartz c- and a-axis fabrics is interpreted as indicating an increasing importance of non-coaxial plane-strain deformation as the Moine Thrust is approached. Even immediately (<1 cm) beneath the thrust, however, flow has still significantly departed from bulk simple shear and involved an important (heterogeneous) component of contemporaneous flattening deformation.