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The Mae Ping shear zone (MPSZ) is one of the major ductile strike-slip systems associated with the Cenozoic extrusion tectonics in Southeast Asia. However, its sinistral shear lacks a robust temporal constraint. This study attempts to acquire the deformation timing by applying 40Ar/39Ar thermochronology on a sheared pegmatitic leucogranite showing sinistral S–C fabrics with a thrust component. The contact of the leucogranite sub-paralleling to the major foliation in host gneiss indicates it could be a pre- to syn-shearing intrusion. Most minerals, including garnet, muscovite, K-feldspar, albite, and quartz, exhibit ductile to brittle deformation. Mineral microstructural analysis suggests a retrograde sinistral shear from > 600 to 250 °C. In situ 40Ar/39Ar dating on muscovite yield ages mainly within 42–38 Ma with calculated closure temperatures of 435–330 °C. Fine-grained muscovite aggregates are slightly older than fish, implying that grain size reduction may not always reset 40Ar/39Ar ages. The K-feldspar 40Ar/39Ar step heating age spectrum with two segments of contiguous steps at 24.5 and 35.4 Ma may reflect the coexistence of high-T porphyroclast and low-T K-rich fine-grain recrystallizing at pressure shadows. The reconstructed cooling path and inferred deformation temperatures constrain a shear duration of 42–30 Ma for the MPSZ. The activation of the MPSZ before 42 Ma could be linked to the Eocene metamorphism within a transpressional regime triggering crustal thickening that may further induce the leucogranitic melt. This study also shows leucogranite can be a nice thermal history recorder for a shear zone regarding its petrogenesis and suitable mineral assemblage for thermochronology.

Petrofabrics and microstructural characteristics of the quartz-rich mylonites were investigated in the NW–SE-trending Gol-e-Gohar shear zone with length of ca. 25 km within the Zagros Transpression Zone, SW Iran. The semi-quantitative vorticity analysis using quartz c-axis-fabric (Rxz/β method) and rigid porphyroclast geometry (Rigid Grain Net method) reveals that mean kinematic vorticity number (Wm) increases towards the Baghat Thrust Fault displaying the occurrence of spatial and temporal variations of flow during progressive deformation. The Wm values show pure shear-dominated flow progressively was changed to a simple shear-dominated flow toward to the late stages of ductile deformation, implying that the accelerating flow path was accompanied with progressive cooling and strain localization at the lower parts of the zone. The measured vertical thinning/dip-parallel elongation suggests that thrust emplacement and metamorphic rock extrusion likely occurred by a combination of simple shear with a pure shear component of deformation (i.e., general shear). The estimated deformation temperatures based on the quartz c-axis fabric patterns and their opening angles highlight greenschist-to-amphibolite-facies conditions.

The forty-year history of the Journal of Structural Geology has recorded an enormous increase in the description, interpretation and modelling of deformation structures. Amongst factors that control deformation and the resulting structures, mechanical anisotropy has proven difficult to tackle. Using a Fast Fourier Transform-based numerical solver for viscoplastic deformation of crystalline materials, we illustrate how mechanical anisotropy has a profound effect on developing structures, such as crenulation cleavages, porphyroclast geometry and the initiation of shear bands and shear zones.

Deformed amphibole in the plagioclase amphibolite mylonite of the Guandi Complex, Xishan, Beijing, is the research object in this study. The amphibole nanodeformation under the middle crust was analyzed using microstructural analysis and high-resolution transmission electron microscopy (TEM). Microscope observations show that the amphibolite deformations in the plagioclase amphibolite mylonite are δ and σ type porphyroclasts, and the porphyroclast tail is composed of new long-columnar crystals. Using transmission electron microscopy (TEM, and this acronyms would be defined only once), the authors observed the nanodeformation characteristics of the amphibole porphyroclast core and mantle. Dislocation tangles are dominant in the porphyroclast core, and inside the new crystal, there is little or no dislocation. Swelled new crystals surrounded by dislocation can be observed in the transition zone between the porphyroclasts and new crystals. The deformed amphibole microstructure and submicrostructure represent typical brittle–ductile transitional deformation. The deformation process can be divided into two stages: the disordered dislocation increment stage and the dislocation reduction and ordering stage. Crystalline plastic deformation occurs in the amphibole in the plagioclase amphibolite mylonite of the Xishan area, Beijing. The crystalline plastic deformation temperature in amphiboles is higher than that in plagioclase.

Pre-existing structures (e.g., folds, foliations, lineations) are usually rotated and modified within a shear zone, depending on their attitude with respect to the kinematic framework (i.e., orientation of instantaneous stretching axes (ISA), kinematical vorticity number of the flow (Wk)) of the shear zone. In addition, new folds may also form within a shear zone, and it is not always easy to distinguish between the pre- and syn-shearing structures in the field, especially if they form on the same rock type. The present contribution describes the reorientation and shape modification (tightening) of pre-existing and syn-shearing folds in metamorphosed calcareous rocks of the Kumbhalgarh Group (part of Delhi Supergroup) due to shearing along the Ranakpur Shear Zone (RSZ), in Kumbhalgarh–Sayra–Ranakpur area of South Delhi Fold Belt (SDFB), also called South Delhi Terrane (SDT). From field-based study and measurements, it is shown that the shallow-plunging, upright second generation (DF2) folds of SDFB/SDT have been rotated to subvertical, tight folds within the RSZ. Fold shape analysis using layer thickness and limb dip of folds (i.e., Ramsay’s classification) and by Fourier transform shape analyses of fold profile sections corroborates and roughly quantifies the tightening and shape modification of pre-existing folds within the RSZ. In contrast, syn-shearing folds have formed on the foliations in calc-silicate rocks which show strongly non-cylindrical geometry with apical deflection in an oblique direction. From the available shear sense indicators like rotated porphyroclast tails and vergence of asymmetric folds, the shear sense of the RSZ is interpreted as oblique reverse (east-side-up) with subordinate sinistral (east-towards-north) shear component, which is similar to the shear sense interpreted by some earlier workers.

A deformed layered gabbro and a mylonitic gabbro sample from the marginal shear zone of the Neyriz mantle diapir in Iran were analyzed using electron backscatter diffraction (EBSD). Both samples have the common amphibole crystallographic preferred orientation (CPO) in which (100) lies perpendicular to foliation and parallel to lineation. Amphibole grains in the layered gabbro sample have little internal deformation, whereas in the mylonitic gabbro sample the amphibole grains are strongly distorted and contain low angle grain boundaries. There is a subtle change in CPO as a function of grain size in the mylonitic gabbro. Coarse grains (porphyroclasts) have a (100) CPO oriented with the main foliation reference frame whilst fine grains have a (100) CPO oriented with the C′ shear bands. Detailed analysis of porphyroclast distortions and subgrain boundary trace analysis suggests that hard slip systems, most particularly (110) control intracrystalline deformation. Schmid factor analysis suggest that these slip systems are not involved in foliation formation but are linked kinematically to C′ shear bands. It is unlikely that the slip systems that control intracrystalline deformation are important in CPO formation. We interpret that subgrain rotation recrystallization lead to grain size reduction and the elongate recrystallized grains were rotated towards the C′ shear bands by grain boundary sliding. This rigid body rotation, possibly in combination with easy slip on (100) are considered the main cause of CPO formation. Amphibole zonation patterns in the layered gabbro sample suggest that oriented growth of amphibole may have contributed to CPO.

This study focuses on a mylonitic skarn at the margins of a harzburgite of the Neyriz mantle diapir, Iran. The studied sample contained garnet porphyroclasts in a wollastonite and pectolite matrix. The microstructures of porphyroclasts and matrix are analyzed using electron backscatter diffraction (EBSD). Detailed analysis of garnet porphyroclast distortions and subgrain boundary trace analysis suggests that (110) 〈110〉 dislocation control intracrystalline deformation of garnet grains. Wollastonite and pectolite crystallographic preferred orientations (CPOs) are dominated by (100) parallel to foliation plane and 〈010〉 parallel to lineation. However, a dominance of low angle misorientation axes parallel to 〈010〉 precludes 〈010〉 dislocations from significant involvement in intracrystalline deformation. Schmid factor analysis also shows this slip system does not have high integrated Schmid factor. These observations suggest that oriented grain growth may be responsible for wollastonite and pectolite CPO development. Schmid factor analysis suggests (001) 〈100〉 and (100) 〈001〉 slip systems in the wollastonite and pectolite were involved in intracrystalline deformation and are linked kinematically to S 2 and reactivated S 1 planes.

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. 27 Ma 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.

Asymmetric porphyroclast systems have long been used as independent shear-sense indicators based on their clast-tail asymmetry and/or stair-stepping tail geometry. Over the last two decades, advances in exploring the mechanical behavior of rigid clasts in a viscous flow has shed new light on understanding the formation of porphyroclast systems embedded in noncoaxial viscous flow and subsequently quantifying many aspects of flow kinematics. Using examples from naturally deformed rocks combined with results from recent advances, our study indicates that: (1) in general, only porphyroclast systems composed of equant-shaped clasts can be treated as independent shear-sense indicators; (2) special attention should be given to β-type porphyroclast systems composed of high-angle intermediate aspect-ratio clasts, which may contain meaningful information on bulk flow kinematics; (3) bookshelf structures cannot serve as independent shear-sense indicators. Multi-criteria and thorough three-dimensional shape analyses are suggested for a proper interpretation of flow kinematics in ductile shear zones. publishedVersion

In the Garhwal Himalaya, the Main Central Thrust Zone (MCTZ) is bounded by the Main Central Thrust (MCT) in the south and Vaikrita Thrust (VT) in the north. The MCTZ along the Alakhnanda, Bhagirathi and Yamuna valleys is marked by different small-scale structures and are classified as early structures i.e. pre-thrusting, structures due to progressive ductile shearing (syn-thrusting) and late structures (post-thrusting). The detailed study of small-scale structures/kinematic indicators such as sigmoidal foliations, shear band (S-C) structures, asymmetric porphyroclast tails, intergranular faults, folded layering and sheath folds suggest top to SSW sense of movement in the rocks of MCTZ of Garhwal Himalaya. The strain studies and crystallographic preferred orientation of quartz reveal that mesoscopic ductile shear zones grow in a narrow zone in response to very high strain, which deformed the internal crystallographic fabric of rock. The study of compressive stress from several mesoscopic sigmoidal foliations suggest that, in MCTZ the maximum compressive stress direction acted in the NNE-SSW direction horizontally synchronous to the northward movement of Indian Plate.

Porphyroclasts have been widely used to determine the shear sense, vorticity and other kinematic parameters of shear zones. Previous studies have revealed that several factors affect the rotation angle of porphyroclasts. These studies mostly regard porphyroclasts as rigid objects or viscous deformable objects to simulate porphyroclast rotation in the upper or lower crust. However, research on how these porphyroclasts behave in the brittle-viscous transition zone is still limited. In this study, a two-dimensional viscoelastic finite element model is presented to simulate the rotation and deformation of an elastic porphyroclast embedded in a viscoelastic matrix with various mechanical parameters (Young's modulus, Poisson's ratio and viscosity coefficient) and different boundary conditions (load magnitude and load time). The modelling results demonstrate that the rotation angle of the porphyroclast is strongly influenced by the Young's modulus, Poisson's ratio and viscosity coefficient of the matrix and boundary conditions. The modelling results have a level of consistency with the natural examples of porphyroclast-matrix systems of the Taili and Tianhuang ductile shear zones of the North China Craton. These results are helpful to further explain the rotation and deformation mechanism of porphyroclasts in the brittle-viscous transition zone.

Feldspar, which is the most abundant crustal mineral, exhibits brittle behavior while quartz and mica deform plastically by dislocation creep, under the greenschist-facies conditions (300–450 °C). Pull-apart microboudinage, which formed by repeated cycles of tensile fractures, is a common microstructure observed in feldspar porphyroclasts embedded in the ductile matrix of quartz in granitic mylonites from the Red River, Chongshan and Gaoligong shear zones (Yunnan, China). These crustal-scale shear zones, which lie in the southeastern margin of the Tibet Plateau, have played a critical role in the southeastward lateral extrusion of the Indochina composite block in response to the Indo-Asian continental collision. Here we developed a feldspar microboudinage paleopiezometer. In each feldspar porphyroclast, the differential stresses estimated from microboudins progressively increases with sequential fracturing, depicting an evolution during exhumation of the mylonite upwards the brittle-ductile transition. The results from the feldspar microboudinage piezometer and the quartz recrystallized grain-size piezometer are largely consistent within the range of uncertainties. The microboudinage piezometer, combined with results of finite strain measurements, deformation temperature estimates, and thermochronological data, can yield the valuable information about rheological properties of naturally deformed rocks. It can also be directly used or adapted to a wide range of geological problems, for example, fractured pebbles in an unlithified matrix, chromite pods in serpentinized mantle rocks, and strong boudins in a soft material at any scale.

How middle to lower crust in Southeastern Tibetan Plateau flowed in response to the Indian-Eurasian continental collision remains hotly debated in the last decades. Metamorphic complexes along crustal mobile zones, e.g., the Diancang Shan (DCS) complex along the Ailao Shan-Red River (ASRR) metamorphic belt may provide key knowledge on our understanding of the middle-lower crustal flow during the collision. In this contribution, integrated structural, microstructural, kinematic, and thermo-chronological studies of the DCS complex reveal that the complex constitutes a destructed A-type dome. High-grade metamorphic rocks of Proterozoic protoliths form the core (Unit I) that is covered by low-grade metamorphic rocks of Mesozoic age in the limbs (Unit II). The eastern limb of the DCS dome is cut by a normal shear zone (Unit III). Three phases (D1, D2 and D3) of Cenozoic deformation structures are recognized in the complex. The D1 and D2 structures constitute the dominant structural style of the Units I and II. The D1 structures consist of penetrative S1 foliations, mineral lineations (L1), symmetrical intrafolial folds (F1) and tectonic lenses of granitic and quartz veins in rocks from the Units I and II. Mylonitic foliations (S2//S1), NNW-SSE plunging subhorizontal stretching lineations (L2), widespread A-type folds (F2), asymmetric porphyroclasts and tectonic lenses form the dominant structural styles in the D2 deformation. D3 structures are new foliations S3 and hot striae L3 in the normal-slipping shear zone (Unit III). The S2//S1 foliations define the domal framework of the DCS complex, which is truncated by the D3 shear zone. The dome is NW-SE elongated and has a hinge line paralleling to the L2//L1 stretching lineations. Synthetic analysis of structural and microstructural styles, and kinematic vorticity data estimated by rotated rigid porphyroclast method (0.38–0.69) and oblique grain-shape/quartz c-axis-fabric method (0.72–0.98) implies that the D1 and D2 deformations formed a progressive process from early pure shear-dominated (D1) to simple shear-dominated (D2) shearing. Shear sense indicators from different parts (Units I and II) of the dome are compatible with top-to-the southeast shearing during D2 deformation. It is shown that the D1-D2 deformation was characterized by a successive process from penetrative flow (D1) and ductile shearing at middle to lower crustal levels to doming (D2) that was coeval with exhumation of the DCS complex since 27 Ma or earlier. We conclude that these processes, i.e., the penetrative flow, ductile shearing, doming and exhumation of the DCS complex, were attributed to lateral subhorizontal middle to lower crustal flow along the ASRR metamorphic belt in the Southeastern Tibetan Plateau during the Indian and Eurasian plate collision.

The rheology of rocks at depth remains a key point in earth sciences. Deformation of high-temperature–high-pressure rocks from the subduction zone has not been fully studied; in particular, the deformation behavior of eclogites remains poorly understood. This research is focused on the microstructure and fabric of retrograded eclogites from the Hongqiyingzi Complex in Chicheng, north Hebei, China, based on photomicrography, scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD) analysis. The analytical results show that plastic deformation occurred in the garnets during exhumation, but they do not show an obvious lattice preferred orientation (LPO). This can be interpreted as being caused by the simultaneous activation of multiple slip systems in the garnets during deformation by dislocation creep. The plagioclases have a special fabric; the (001) LPO presents a maximum in Z direction which can be correlated with a new [100](001) slip system. The misorientation angle distribution (MAD) of the plagioclases in the deformed retrograded garnet augens shows a special bimodal distribution with peaks in both a low-angle range ( 140°). In the retrograded garnet augens (i.e. “white eye socket” garnets) the maximum peak moves from a high-angle range to a low-angle range when we analyze only those plagioclases surrounding the residual garnet porphyroclast. Deformation behavior is controlled by the crystallographic orientation of the host grain and the grain boundary sliding process. Hornblendes in different layers and the retrograded garnet augens show almost the same strong LPO patterns, which are correlated with the [001](010) slip system; and MAD diagrams show a peak in a low-angle range (

We present microstructural and crystallographic preferred orientation (CPO) data on quartz deformed in the middle crust to explore the interaction and feedback between dynamic recrystallization, deformation processes, and CPO evolution. The sample investigated here is a moderately deformed quartz-rich mylonite from the Blue Ridge in Virginia. We have created high-resolution crystallographic orientation maps using electron backscatter diffraction (EBSD) of 51 isolated quartz porphyroclasts with recrystallized grain fractions ranging from 10 to 100%. Recrystallized grains are internally undeformed and display crystallographic orientations dispersed around the orientation of the associated parent porphyroclast. We document a systematic decrease in fabric intensity with recrystallization, suggesting that progressive deformation of the recrystallized domains involves processes that can weaken a pre-existing CPO. Relationships between recrystallization fraction and shear strain suggest that complete microstructural re-equilibration requires strains in excess of γ = 5. Variation in the degree of recrystallization implies that strain was accumulated heterogeneously, and that a steady-state microstructure and rheology were not achieved.

The Karakoram Shear Zone (KSZ) is a northwest-southeast trending dextral ductile shear zone that has mylonitized the Tangste and Darbuk granitoids of the southern margin of the Asian plate. Kinematic vorticity (Wk) has been estimated in 6 mylonitized Tangste granite samples, using Porphyroclast Hyperbolic Distribution (PHD) and Shear Band (SB) Analyses methods on well-developed quartz and feldspar porphyroclasts, and synthetic and antithetic shear bands respectively to visualize the overall deformation of the KSZ. The PHD and SB analyses yield Wk values ranging from Wk=0.29 to 0.43 and 0.45 to 0.93, respectively, thus indicating distinct pure and simple shear dominant regimes during different stages of the evolution of the KSZ. Strain has essentially been pure shear when southern edge of the Asian plate was initially juxtaposed against the Indian plate around 70 Ma, and changed to simple shear, possibly during the reactivation of this shear zone during 21-13 Ma to produce the shear bands.

We present microfabrics in high-pressure, metamorphic, partly serpentinized peridotite mylonites from the Voltri Massif, in which porphyroclasts and matrix record independent deformation events. The microfabrics are analysed using polarization microscopy and electron microscopy (SEM/EBSD, EMP). The mylonites contain diopside and olivine porphyroclasts originating from the mantle protolith embedded in a fine-grained matrix consisting mainly of antigorite and minor olivine and pyroxene. The porphyroclasts record brittle and crystal-plastic deformation of the peridotite at upper-mantle conditions and differential stresses of a few hundred MPa. After the peridotites became serpentinized, deformation occurred mainly by dissolution–precipitation creep resulting in a pronounced foliation of the antigorite matrix, crenulation cleavages and newly precipitated olivine and pyroxene from the pore fluid at sites of dilation, i.e. in strain shadows next to porphyroclasts and folded fine-grained antigorite layers. Antigorite reveals a pronounced associated shape preferred orientation (SPO) and crystallographic preferred orientation (CPO) with the basal (001) cleavage plane oriented in the foliation plane. In monomineralic antigorite aggregates at sites of stress concentration around porphyroclasts, a characteristically reduced grain size and deflecting CPO as well as sutured grain boundaries indicate also some contribution of crystal-plastic deformation and grain-boundary migration of antigorite. In contrast, the absence of any intragranular deformation features in newly precipitated olivine in strain shadows reveals that stresses were not sufficiently high to allow for significant dislocation creep of olivine at conditions at which antigorite is stable. The porphyroclast microstructures are not associated with the microstructures of the mylonitic matrix, but are inherited from an independent earlier deformation. The porphyroclasts record a high-stress deformation of the peridotite with dislocation creep of olivine in the upper mantle probably related to rifting processes, whereas the serpentinite matrix records dominantly dissolution–precipitation creep and low stresses during subduction and exhumation.

Serpentinites (frequently cross cut by gabbroic dikelet), collected from Northern Central Indian Ridge (NCIR), Indian Ocean, contain both Cr-rich (Group I) and Cr-poor (Group II) variety of spinel. Based on mineralogy they can be classified as non-residual and residual spinel, respectively. While, non residual spinel (TiO2 up to 0.5 wt %; Group I) display the evidences of peridotite-gabbro interaction, residual spinel (TiO2

This paper documents the 40–60 km wide ENE–WSW trending Mubarak–Barramiya shear belt (MBSB) in the Central Eastern Desert of Egypt by examining its structural styles, kinematics and geometry. Our study revealed the existence of prevalent dextral and minor sinistral conjugate shear zones. The MBSB is metamorphic belt (greenschist–amphibolite) characterized by at least three post-collisional (740–540 Ma) ductile Neoproterozoic deformation events (D 1 , D 2 and D 3 ) followed by a brittle neotectonic deformation (D 4 ). D 1 event produced early top-to-the-northwest thrust displacements due to NW–SE shortening. D 2 produced discrete zones of NNW-trending upright folds and culminated in initiation of major NW-trending sinistral shear zones of the Najd Fault System (NFS, at c. 640–540 Ma ago) as well as steeply dipping S 2 foliation, and shallowly plunging L 2 lineation. NW-to NNW-trending F 2 folds are open to steep and vary in plunge from horizontal to vertical. D 2 deformational fabrics are strongly overprinted by D 3 penetrative structures. D 3 is characterized by a penetrative S 3 foliation, steeply SE- to NW-plunging and shallowly NE-plunging stretching lineations (L 3 ), asymmetric and sheath folds (F 3 ) consistent with dextral sense of movement exhibited by delta- and sigma-type porphyroclast systems and asymmetric boudinage fabrics. D 2 –D 3 represent a non-coaxial progressive event formed in a dextral NE- over NW-sinistral shear zone during a partitioned transpression in response to E–W-directed compression during oblique convergence between East and West Gondwana developed due to closure of the Mozambique Ocean and amalgamation of the Arabian–Nubian Shield in Cryogenian-early Ediacaran time.

The Ronda peridotite massif of southern Spain exposes subcontinental lithospheric mantle that records pressure–temperature data and microstructures formed during exhumation beneath the rapidly extending Alboran domain. The peridotite is zoned from garnet- and spinel-bearing mylonites at the structural top, to spinel-bearing tectonites, to melt-percolated spinel-bearing granular peridotites, to plagioclase-bearing tectonites at the structural base. We find microstructural evidence of melt present in the spinel zones prior to the deformation event which exhumed the peridotites, and we therefore reinterpret the spinel tectonites as being a result of deformational overprinting of part of the granular domain. We also reinterpret garnet intergrown with spinel in the mylonite zone as part of the pre-mylonitic porphyroclast assemblage, rather than as a syn-mylonite assemblage. This places mylonite formation within the spinel field, rather than right on the garnet-spinel transition (18 kb). Two-dimensional thermal modeling indicates that these conditions require removal of lithospheric mantle below 100 km followed by exhumation along a low angle shear zone. Excision of material during exhumation is required to explain the steep thermal gradients observed. These results shed light on the mechanisms of back-arc extension, as well as the emplacement of orogenic lherzolites.

Multiple methods were applied to study the deformation characteristics of hornblende in Archean plagioamphibolite mylonite from the Western Hills (Beijing), including optical microscopy (OM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and electron probe microanalysis (EPMA). The hornblendes are σ and δ type porphyroclasts with the new-born needle shaped grains as their tails. The analysis of lattice preferred orientation (LPO) of both the porphyroclasts and the new-born grains shows that the main slip system of the deformed hornblende is (100)〈001〉, suggesting that the fabric characteristics of new-born grains inherit that of porphyroclasts. Sub-microstructures show the porphyroclast core is dominated by dislocation tangle, little or no dislocations in the new-born grains, and the subgrains confined by dislocations in the transition zone between porphyroclasts and new-born grains. By using plagioclase-hornblende geothermometry and hornblende geobarometry, the estimated temperature and pressure of porphyroclasts are 675.3–702.9°C and 0.29–0.41 GPa and those of new-born grains are 614.1–679.0°C and 0.11–0.31 GPa. The bulging recrystallization is summarized as deformation mechanisms of hornblende by the discussions of the microstructures, EBSD fabric, sub-microstructures, and the deformed temperature and pressure.

We report here on a study of three deformed granitoids: two mylonites and an ultramylonite from the inner ductile shear zone of the Ryoke metamorphic belt, SW Japan. Monophase layers composed of quartz, plagioclase or K-feldspar are present in all samples. The plagioclase-rich layers consist of grains 6–10 μm in size, and sometimes include patchy K-feldspar and quartz, indicating solution-precipitation. In the mylonite, the fine-grained plagioclase is mainly An23–25 and, the composition of plagioclase porphyroclast is An21–39 without any significant maximum. The An compositions together with textural observations indicate that fine-grained plagioclase nucleated from solution with mass transfer during deformation. In the ultramylonite, fine-grained plagioclase is widely changed to be An15–37, indicating that the grain-size-reduction process includes fracturing of original plagioclase porphyroclasts in addition to the solution–precipitation process, which results in the composition concentrated around An30. In all samples, the crystallographic orientations of fine-grained plagioclases are almost random and do not correlate with neighbouring porphyroclasts. Grain-size-sensitive creep occurred during rock deformation subsequent to the process of solution–precipitation that involved mass transfer via fluids.

This paper documents the 20 to 60 km wide N–S trending Angavo Shear Zone (ASZ) in central Madagascar and its tectonic implications by examining its structural styles, kinematics and geometry. Our study indicates that the ASZ is characterized by at least two ductile Late Proterozoic deformation events (D1 and D2) followed by a brittle neotectonic deformation (D3). The early D1 event produced a regionally extensive S1 foliation, stretching/flattening mineral lineation L1 and symmetrical structural fabrics such as recumbent and isoclinal intra-folial folds (F1), implying a flattening deformation. D1 deformational fabrics are locally overprinted by D2 structures. D2 is characterized by a penetrative S2 foliation, shallow south plunging L2 lineation, asymmetric and sheath folds (F2) consistent with a right lateral sense of movement exhibited by delta- and sigma-type porphyroclast systems and asymmetric boudinage fabrics. D2 represents a non-coaxial flow regime formed in a dextral west over east shear zone during a partitioned transpression in response to east–west-directed compression during the assembly of Gondwana. A close resemblance with the Achankovil shear zone in India is noticed; however the continuation of the ASZ in Africa is uncertain.

Plagioclase porphyroclasts with well-preserved idiomorphic shapes and zoning, and showing limited clast interaction, are ubiquitous in the flat-lying Zhangbaling schist that is exposed east of the Tan-Lu fault in southeast China. Plagioclase porphyroclasts define rigid particles whose distribution be related to the kinematic vorticity of the schist using the methods of porphyroclast hyperbolic distribution (PHD) and modified Rf/ϕ (Passchier/Wallis plot). The kinematic vorticity numbers calculated from this approach range from Wk = 0.72 to 0.82, increasing progressively from south to north along the Zhangbaling belt. Such a Wk distribution indicates that the Zhangbaling schist was deformed uniformly under simple-shear dominated general shear, and that the Zhangbaling ductile crust experienced relatively even crustal thinning deformation. The subhorizontal Zhangbaling schist is considered a mid-crustal attachment zone that coupled the rigid upper crust to a subvertical, wrench shear zone in the lower crust.

Optical microscopy, electron probe microanalysis, and electron backscatter diffraction methods have been used to examine a broad range of garnet microstructures within a high strain zone that marks the western margin of a major transpression zone in the southern New England Appalachians. Garnet accommodated variable states of finite strain, expressed as low strain porphyroclasts ( Type 1 ), high strain polycrystalline aggregates ( Type 2 ), and transitional morphologies ( Type 3 ) that range between these end members. Type 1 behaved as rigid porphyroclasts and is characterized by four concentric Ca growth zones. Type 2 help define foliation and lineation, are characterized by three Ca zones, and possess a consistent bulk crystallographic preferred orientation of (100) symmetrical to the tectonic fabric. Type 3 show variable degrees of porphyroclast associated with aggregate, where porphyroclasts display complex compositional zoning that corresponds to lattice distortion, low-angle boundaries, and subgrains, and aggregate CPO mimics porphyroclast orientation. All aggregates accommodated a significant proportion of greenschist facies deformation through grain boundary sliding, grain rotation and impingement, and pressure solution, which lead to a cohesive behavior and overall strain hardening of the aggregates. The characteristic CPO could not have been developed in this manner, and was the result of an older phase of partitioned amphibolite facies dislocation creep, recovery including chemical segregation, and recrystallization of porphyroclasts. This study demonstrates the significance of strain accommodation within garnet and its affect on composition under a range of PT conditions, and emphasizes the importance of utilizing EBSD methods with studies that rely upon a sound understanding of garnet.

It has been debated for decades whether rigid inclusions, such as porphyroclasts and porphyroblasts, do or do not rotate in a softer matrix during deformation. Experiments and numerical simulations with viscous matrix rheologies show ongoing rotation of circular inclusions, whereas using Mohr-Coulomb plasticity results in nonrotation. Because the rocks in which inclusions are found normally undergo deformation by dislocation creep, we applied a full-field crystal plasticity approach to investigate the rotation behavior of rigid circular inclusions. We show that the inclusion9s rotation strongly depends on the anisotropy of the matrix minerals. Strongly anisotropic minerals will develop shear bands that reduce the rotation of inclusions. Inhibition of rotation can only occur after a significant amount of strain. Our results may help to explain why geologic rigid objects often show evidence for rotation, but not necessarily in accordance with the viscous theory that is usually applied to these systems.

A description of the systematic errors associated with the measurement of the vorticity number from poryhroclasts in natural systems is presented and discussed. We show that strong biases and systematic errors could derive both from some erroneous physical (i.e. no slip across clast/matrix boundary, homogeneity within the matrix) as well as geometrical assumptions (i.e. the radius ratio and angular measurements carried out in two dimensions on outcrop surfaces and thin section). By comparing natural datasets of porphyroclast shape preferred orientation (SPO) with different theoretical curves plots, we suggest that at least one of the Jeffery physical assumptions can be tested when applying vorticity techniques. The comparison of different possible sources of systematic errors indicates that, for medium-to-low vorticity numbers (Wm , 0.8), vorticity data are strongly biased and that a minimum systematic error of 0.2 should be taken into account. Finally, we use data from natural shear zones from the Southern Variscan Belt in Sardinia to test and discuss the starting assumptions of the Jeffery model. One of the main challenges of modern structural geology is to try to obtain information about the rheology and kinematics of flow occurring in the deepest crust and upper mantle. Several microstructure fabrics (e.g. fabric asymmetry) associated with mylonites represent potentially important sources of information to obtain kinematic data as well as for estimation of deformation parameters (see Passchier & Trouw 2005 for a review). In particular, porphyroclast systems have been shown to be a sensitive indicator of flow parameters and can be used to constrain the relative ratio of combined pure and simple shear (i.e. the vorticity of the flow). Several theoretical and practical studies indicate that syn-kinematic recrystallization of mantles (sor d-shaped tails) around porphyroclasts and shape preferred orientation (SPO) of porphyroclasts may store information on the amount of shear strain. With the aim of developing a vorticity gauge, theoretical and experimental models for study of the rotation and behaviour of elliptical porphyroclasts in a fluid have been proposed and developed. The first attempt began with the pioneering works of Jeffery (1922) and Eshelby (1957) that found solutions for rigid objects immersed in a viscous and linear-elastics fluid, respectively. Bretherton (1962), Ghosh & Ramberg (1976) and Bilby & Kolbuszewski (1977) further adapted the basic Jeffery analytical solution to material science and structural geology. Subsequent investigations used the previous theoretical calculation to develop efficient strain and vorticity gauge techniques capable of quantifying the flow parameters within shear zones (Passchier 1987; Wallis 1992; Simpson & De Paor 1993; Holcombe & Little 2001). Three main analytical techniques (Passchier 1987; Wallis 1995; Holcombe & Little 2001) are commonly employed to characterize flow within shear zones using rigid porphyroclats (Jessup et al. 2007). All these techniques use the measurement of the axial ratio of porphyroclast R (the shape factor; Fig. 1) and the angle of long axis porphyroclast with respect to foliation or stretching direction h (Fig. 1) and the distribution of the cross-plotted curves to define the mean vorticity number Wm. These curves define a threshold number Rc, below which the porphyroclasts continuously rotate and above which they record stable sink position. However, as many authors (Passchier 1987; Marques & Coelho 2001) have already noted, the Jeffery model assumes: (1) rigid particles immersed in a non-confined viscous flow, (2) a perfect coupling on matrix/particle interface and (3) no interference between particles during deformation. Use of the Jeffery model implies that these physical assumptions are met, and/or need to be, at least preliminarily, tested. Although techniques to estimate the vorticity of flow have become routinely used on sheared rocks in a variety of tectonic settings (e.g. Klepeis et al. 1999; Xypolias & Doutsos 2000; Xypolias & Koukouvelas 2001; Bailey & Eyster 2003; Law et al. 2004; Carosi et al. 2006, 2007; Iacopini et al. 2008; Sarkarinejad et al. 2008, 2010; Frassi et al. 2009; Larson & Godin 2009), relatively few detailed analysis of error sources have been proposed and a critical discussion From: Prior, D. J., Rutter, E. H. & Tatham, D. J. (eds) Deformation Mechanisms, Rheology and Tectonics: Microstructures, Mechanics and Anisotropy. Geological Society, London, Special Publications, 360, 301–318. DOI: 10.1144/SP360.17 # The Geological Society of London 2011. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics at University of Aberdeen on November 19, 2012 http://sp.lyellcollection.org/ Downloaded from

In order to shed light on the cause for onset of strain localisation in plagioclase-rich rocks we have performed detailed microstructural analyses on a sheared anorthosite–leucogabbro using optical microscopy, electron backscatter diffraction (EBSD) and chemical analyses. The analysed sample is from an Archaean unit, SW Greenland, deformed at lower to mid crustal conditions ( T = 675–700 °C and moderate pressure). The initial deformation occurred dominantly by dislocation creep and the grain size was reduced primarily by subgrain rotation recrystallisation. Recrystallised plagioclase grains (average size 80 μm) are dominantly found in (i) clusters, (ii) lenses and (iii) continuous bands subparallel to shear zone boundaries. Recrystallised grains in clusters and lenses display inherited crystallographic orientations. Their bulk crystallographic preferred orientation (CPO) is random; however, crystallographic characteristics show that parent and daughter grains have the same misorientation axes and possibly the same active slip systems. Recrystallised grains in continuous bands show a CPO with a single dominant active slip system, (001) , aligned with the structural (XYZ) framework. For these parent and daughter grains, misorientation axes are random and the dominant slip system is different. Grain rotations of recrystallised grains are traceable back to the orientation of the adjacent porphyroclast. We infer that the cause for strain localisation is recrystallisation and development of a CPO in continuous recrystallised bands. Microstructures in combination with misorientation and slip system analyses indicate a possible change from dislocation creep in clusters and lenses to dislocation-accommodated grain boundary sliding (DisGBS) in continuous bands. This inferred shift in dominant deformation mechanism would lower the strength of the shear zone.

The Turon de Tecouere peridotite, North Pyrenean fault zone, France, contains protomylonites grading to ultramylonites. Grain size reduction to 0.005–0.025 mm took place by reaction during deformation. Within the protomylonite domain, the size of the porphyroclasts suggests an initial grain size in this domain of 2–10 mm. In the ultramylonite domain, porphyroclasts are polycrystalline with an internal grain size of 0.1–0.2 mm, indicating that the ultramylonite domain had a finer initial grain size than the protomylonite domain. Lattice-preferred orientations and misorientations within a polycrystalline porphyroclast in the ultramylonite domain suggest an early episode of dislocation creep accommodated dynamic recrystallization, restricted predominantly to the present-day ultramylonite domain. Later grain size reduction by reaction, resulting in very fine-grained ultramylonites, was concentrated in the same domain. For reaction-involved deformation, initial grain size may be crucially important as it controls the surface area available for nucleation of new grains. As a result of the finer initial grain size within the present-day ultramylonites, a critical percentage of fine-grained matrix developed in the present-day ultramylonite domain earlier than in the present-day protomylonite domain, resulting in localization of further deformation in the ultramylonites. This study suggests that zones of locally smaller grain size can act as heterogeneities that control the location and width of shear zones formed by reaction-softening.

A Late Cretaceous lamprophyre dyke in the Villany Mts (S Hungary), situated in the Tisza unit, contains abundant spinel lherzolite xenoliths with porphyroclastic textures. Mineral chemistry suggests a relatively fertile mantle, which experienced only 5–7% melt extraction. Differences in porphyroclast and neoblast chemistry and thermobarometric calculations suggest that the mantle section represented by the xenoliths experienced recrystallization at lower PT as it was transported to shallow mantle depths close to the plagioclase stability field, followed by later relaxation. Based on volcanological and sedimentological constraints from the Villany Mts and the neighboring Mecsek Mts, we suggest that the uprise of the subcontinental mantle material was related to a Cretaceous rifting event and lithospheric deformation of the southwestern part of the Tisza unit. Mantle upwelling and formation of lamprophyre melts can be related to generation or reactivation of deep fractures of the lithosphere, during a period of lithospheric extension between the major nappe emplacements (Albian–Cenomanian and Paleocene) of the region.

Polymer jacketed porphyritic samples of 70% halite + 30% coarse calcite were subjected to torsion deformation to investigate the effects of a mixture of coarse calcite on the microstructure and mechanical properties of a two-phase aggregate. The experiments were run at 100 and 200 °C, a confining pressure of 250 MPa, and a constant shear strain rate of 3E-4 s −1 . Ultimate strengths of single-phase halite synthetic aggregates at 100 and 200 °C were ca. 32 and 8 Nm, respectively, and of the two-phase aggregate 39 and 18 Nm, respectively; this shows that the two-phase aggregate was much stronger, especially at 200 °C. Stepping strain rate tests show that the two-phase aggregate behaved as power-law viscous, with stress exponents of ca. 19 and 13 at 100 °C at 200 °C, respectively. From these high exponents, we infer that the active deformation mechanisms were not efficient enough to rapidly relax the applied stress. Halite stress exponents at 100 and 200 °C are typically much lower, in the order of 4–6, which means that the calcite porphyroclasts were obstacles to halite plastic flow and hampered stress relaxation. The drop of the stress exponent with temperature indicates that the main deformation mechanism(s) was temperature sensitive. Matrix halite deformed plastically, while calcite rotated rigidly or deformed in a brittle fashion, with grain size reduction by fracturing (e.g. bookshelf and boudinage). We conclude that halite was softer than calcite in the investigated temperature range. Strain was homogeneous at the sample scale but not at the grain scale where the foliation delineated by plastically flattened halite contoured the rigid calcite clasts. The microstructures experimentally produced at 100 and 200 °C are very similar and find their counterparts in natural mylonites: rolling structures, σ and δ porphyroclast systems, bookshelf and boudinage in brittle calcite porphyroclasts, and ductile y and c ′ micro shear bands in the halite matrix.

Grain-scale numerical experiments involving simple shear of a two-phase non-linear viscous material are described and compared with mineral fish or lozenge-shaped porphyroclasts, such as muscovite. Two types of 2D models are considered; either a single elongate grain or two parallel elongate grains, in both cases supported by a weaker polygonal grain matrix. The relative viscosities of the contrasting grain structures were systematically varied, allowing us to observe the effects of non-linear viscous rheology on the resulting microstructure and flow patterns. The results show that the finite rotation of the hard elongate grain was similar within any one experiment, but was largely influenced by viscosity contrast, the geometry of the model and the imposed shear strain. Models involving single elongate hard grains show increasing instability at their ends and less strain compatibility with the deforming matrix grains, as the viscosity contrast is increased. In the paired grain models the greatest variation in the matrix grain microstructure is seen in the region where the two hard grains are oriented at a high-angle to the direction of shear. Finally, we consider the changes in intragranular stress by comparing microstructural observations using different viscosities with the distribution of stress in space and during progressive shear. In the plane approximately parallel to the maximum principal stress direction ( σ 1 ), a localised change of stress occurs across and along the interface between the hard and soft grains. Variations in the mean stress at these boundaries are directly attributable to changes in the minimum principal stress. We propose that with shear strains greater than γ = 2 it is the minimum principal stress that can control diffusion processes at the grain boundary rather than mean stress. In conclusion we suggest that our models have the potential for providing useful insights into why metamorphic reactions can occur at the interface between a porphyroclast and matrix at high shear strains and how stress distribution can control the initiations of such reactions.

Two vorticity methods have been proposed to estimate kinematic vorticity number in naturally deformed quartzites. These combine the degree of quartz c -axis-fabric asymmetry either with the strain-ratio in the XZ- plane of finite strain ( R XZ ) or with the angle, δ , between the oblique-grain-shape fabric and the main foliation. Here, it is suggested that the incorporation of R XZ data with angle δ values can also serve as an additional method of vorticity analysis. The use of a new nomogram that provides a rapid means of evaluating the consistency of vorticity estimates obtained by the three methods is also discussed. A comparison of vorticity estimates from low-grade test samples using all methods yields that three methods may record different parts of deformation history. This hypothesis was then used to discuss the spatial and temporal variation of kinematic vorticity in a crustal-scale shear zone. This natural example showed that the new method is able to record late-stage changes in the flow regime, which are slightly or not detectable by previous methods. Also, it is noteworthy that vertical vorticity profiles based on results of rigid porphyroclast analysis tend to ‘average’ profiles obtained by the estimates of quartz based methods.

The mylonitization of the Pankenushi gabbro in the Hidaka metamorphic belt of central Hokkaido, Japan, occurred along its western margin at ≈600 MPa and 660–700 °C through dynamic recrystallization of plagioclase and a retrograde reaction from granulite facies to amphibolite facies (orthopyroxene + clinopyroxene + plagioclase + H2O = hornblende + quartz). The reaction produced a fine-grained (≤100 μm) polymineralic aggregate composed of orthopyroxene, clinopyroxene, quartz, hornblende, biotite and ilmenite, into which strain is localized. The dynamic recrystallization of plagioclase occurred by grain boundary migration, and produced a monomineralic aggregate of grains whose crystallographic orientations are mostly unrelated to those of porphyroclasts. The monomineralic plagioclase aggregates and the fine-grained polymineralic aggregates are interlayered and define the mylonitic foliation, while the latter is also mixed into the former by grain boundary sliding to form a rather homogeneous polymineralic matrix in ultramylonites. However in both mylonite and ultramylonite, plagioclase aggregates form a stress-supporting framework, and therefore controlled the rock rheology. Crystal plastic deformation of pyroxenes and plagioclase with dominant (100)[001] and (001)1/2 1 1 ¯ 0 > slip systems, respectively, produced distinct shape- and crystallographic-preferred orientations of pyroxene porphyroclasts and dynamically recrystallized plagioclase grains in both mylonite and ultramylonite. Euhedral to subhedral growth of hornblende in pyroxene porphyroclast tails during the reaction and its subsequent rigid rotation in the fine-grained polymineralic aggregate or matrix produced clear shape- and crystallographic-preferred orientations of hornblende grains in both mylonite and ultramylonite. In contrast, the dominant grain boundary sliding of pyroxene and quartz grains in the fine-grained polymineralic aggregate of the mylonite resulted in their very weak shape- and crystallographic-preferred orientations. In the fine-grained polymineralic matrix of the ultramylonite, however, pyroxene and quartz grains became scattered and isolated in the plagioclase aggregate so that they were crystal-plastically deformed leading to stronger shape- and crystallographic-preferred orientations than those seen in the mylonite.

Mylonitic rocks of the Cougar Creek Complex of northeastern Oregon and west-central Idaho provide an opportunity to document the deformational structures produced during general non-coaxial shear within quartz-feldspar mylonites and to explore the potential role of SC′-type extensional shear bands in vorticity analysis. Well-developed feldspar porphyroclasts within six mylonite zones were utilized to estimate bulk kinematic vorticity ( W k ) using the porphyroclast hyperbolic distribution (PHD) method. W k values for the Cougar Creek mylonites range from W k = 0.26 to W k = 0.37. Synthetic and antithetic shear band inclinations were measured relative to observed shear zone boundaries within five mylonite zones with estimated W k values and compared to the non-coaxial flow field geometries and eigenvector orientations. In each mylonite zone, synthetic SC′-type shear band populations exhibit a range of inclination with maximum inclination lying approximately parallel to the acute bisector (AB) of the eigenvectors. Similarly, antithetic shear band populations show a range of inclination near the obtuse bisector (OB) of the eigenvectors. We infer that SC′-type extensional shear bands form initially parallel to AB and OB and rotate towards the flow plane with progressive deformation, decreasing their inclination relative to the shear zone boundary. AB and OB have significance in the strain field in that they represent orientations of maximum angular strain rate. Thus, planes perpendicular to AB and OB are mechanically favorable for small zones of localized simple shear (shear bands) within the heterogeneous bulk strain of the mylonite. Orientation analysis of populations of SC′-type shear bands may provide a direct, quantitative means of estimating W k .

The role of fluids in the deformation of continental serpentinites is investigated from structural, microstructural and petrographic analyses applied to a km-scale porphyroclast mantled in a viscous matrix of amphibolites. The clast is sited within a shear zone of the Palaeozoic Maures massif, France. Syntectonic fluid–rock interactions occurred from km to mm scales, at first on the clast borders (along the main rheological boundaries) then within the clast. They are accommodated macroscopically by slickenfibers faults and microscopically by shear microcracks within crack-seal veins, typifying an intermediate, brittle–ductile behaviour of serpentinites. Three main stages of deformation–serpentinisation processes occurred in relation with the left-lateral movement of the hosted shear zone. They developed under metamorphic conditions evolving from amphibolites to green-schists facies conditions (∼ 400 MPa/550 °C to ∼ 200 MPa/

The island of Corsica (France) plays a central role in any reconstruction of Western Mediterranean geodynamics and paleogeography but several key aspects of its geological evolution are still uncertain. The most debated topics include the interpretation of the Corsican orogen as the result of an east- or west-directed subduction, and the actual involvement of the Variscan basement of Corsica in the Alpine orogenic cycle. This study integrates 40 Ar– 39 Ar laserprobe, mesostructural, microtextural, and microchemical analyses and places relevant constraints on the style, P – T conditions, and timing of Alpine-age, pervasive ductile shear zones which affected the Variscan basement complex of central Corsica, a few kilometers to the west of the present-day front of the Alpine nappes. Shear zones strike ~ NNE–SSW, dip at a high angle, and are characterized by a dominant sinistral strike-slip component. Two of the three investigated shear zones contain two texturally and chemically resolvable generations of white mica, recording a prograde (burial) evolution: (1) deformed celadonite-poor relicts are finely overgrown by (2) a celadonite-rich white mica aligned along the main foliation. White mica from a third sample of another shear zone, characterized by a significantly lower porphyroclast/matrix ratio, exhibits a nearly uniform high-celadonite content, compositionally matching the texturally younger phengite from the nearby shear zones. Mineral-textural analysis, electron microprobe data, and pseudosection modeling constrain P – T conditions attained during shearing at ~ 300 °C and minimum pressures of ~ 0.6 GPa. In-situ 40 Ar– 39 Ar analyses of coexisting low- and high-celadonite white micas from both shear zones yielded a relatively wide range of ages, ~ 45–36 Ma. Laser step-heating experiments gave sigmoidal-shaped age profiles, with step ages in line with in-situ spot dates. By contrast, the apparently chemically homogenous high-celadonite white mica yielded concordant in-situ ages at ~ 34 Ma, but a hump-shaped age spectrum, with maximum ages of ~ 35 Ma and intermediate- to high-temperature steps as young as ~ 33–32 Ma. Results indicate that the studied samples consist of an earlier celadonite-poor white mica with a minimum age of ~ 46 Ma, overgrown by a synshear high-celadonite white mica, developed at greater depth between ~ 37 and 35 Ma; faint late increments in shearing occurred at ≤ 33–32 Ma, when white mica incipiently re-equilibrated during exhumation. Results suggest that ductile shearing with a dominant strike-slip component pervasively deformed the Corsican basement complex during the emplacement and progressive thickening of the Alpine orogenic wedge and broaden the extent of the domain affected by the Alpine tectonometamorphic events. Integration of petrological modeling and geochronological data shows that the Variscan basement of central Corsica, close to the Alpine nappes, was buried during the late Eocene by ≥ 18 km of Alpine orogenic wedge and foreland deposits. Our results, combined with previously published apatite fission-track data, imply an overburden removal ≥ 15 km from the late Eocene (Priabonian) to the early Miocene (Aquitanian), pointing to a minimum average exhumation rate of 1.3–1.5 mm/a.

Kinematic vorticity (Wk) is a dimensionless measure of rotation relative to finite stretching and is essential for complete understanding of flow in ductile shear zones. The porphyroclast hyperbolic distribution (PHD) method is a widely used technique for estimating Wk based on the acute angle between the flow eigenvectors determined from the orientations of back-rotated s porphyroclasts. The utility of the PHD was tested with Tertiary ultramylonites from the Santa Catalina Mountains, Arizona, Proterozoic ultramylonites from the Virgin Mountains, Nevada, and a Paleozoic ultramylonite from the Hylas zone in the Virginia Piedmont. Bootstrapping statistics and a computational sieving process were used to analyze the PHD data set. Average standard deviations of the bootstrapped data sets yield a 1s standard error of � 9% for a Wk-value measured as a % simple shear. Sieving results imply that back-rotated porphyroclasts may not orient parallel to the extensional eigenvector. The PHD method is useful for discerning between deformations that are pure shear dominated, general shear, or simple shear dominated, but is not accurate enough to report precise Wk-values. When performed on multiple sections the PHD method can identify zones of monoclinic versus triclinic shear. 2007 Elsevier Ltd. All rights reserved.

This study investigates the microstructure and texture variations across a mm-scale shear zone in Carrara marble of the Alpi Apuane (Italy). The microstructures have been investigated for grain size, texture, and shape fabrics. Textures have been measured with Computer-Integrated Polarization Microscopy (CIP) and Electron Back Scattered Diffraction (EBSD) separating porphyroclast and recrystallized grains. The deformation, which post-dates an earlier deformation phase and subsequent annealing, is strongly localized. The microstructures and textures change across the shear strain gradient and are interpreted to preserve a time sequence of progressive stages of deformation. The bulk shear strain rate is estimated to be about 10 � 11 sec � 1 at deformation temperatures of approximately 325 � C � 30 � C. The protomylonite is characterized by a core mantle structure with a bimodal grain size distribution which changes gradually to a completely dynamically recrystallized microstructure with a unimodal grain size distribution in the mylonitic center of the shear zone. Core-mantlestructures are produced by dominant rotation recrystallization accompanied by some grain boundary migration. The microstructural transition from protomylonite to mylonite coincides with a change in texture. With increasing strain the single c-axis maximum of an earlier inherited texture in the protomylonite is replaced by a similar texture in a different orientation (maximum normal to the shear plane) which is consistent with dominant basal hai and r h� 2201i slip. The microstructural and textural variations depend on the proportion of recrystallized grains. As dynamic recrystallization progresses with finite strain the texture development is finite strain-dependent. The comparison of the microstructures and textures to other natural and to experimental examples explains the progressive change of the texture and demonstrates the texture evolution produced by dynamic recrystallization. 2006 Elsevier Ltd. All rights reserved.

Two spinel lherzolite xenoliths from Hungary that contain pyroxene–spinel symplectites have been studied using EPMA, Laser ablation ICP-MS and universal stage. Based on their geochemical and structural characteristics, the xenoliths represent two different domains of the shallow subcontinental lithospheric mantle beneath the Pannonian Basin. The occurrence of symplectites is attributed to the former presence and subsequent breakdown of garnets due to significant pressure decrease related to lithospheric thinning. This implies that both mantle domains were once part of the garnet lherzolitic upper mantle and had a similar history during the major extension that formed the Pannonian Basin. Garnet breakdown resulted in distinct geochemical characteristics in the adjacent clinopyroxene crystals in both xenoliths. This is manifested by enrichment in HREE, Y, Zr and Hf towards the clinopyroxene porphyroclast rims and also in the neoblasts with respect to porphyroclast core compositions. This geochemical feature, together with the development and preservation of the texturally very sensitive symplectites, enables us to determine the relative timing of mantle processes. Our results indicate that garnets had been metastable in the spinel lherzolite environment and their breakdown to pyroxene and spinel is one of the latest processes that took place within the upper mantle before the xenoliths were brought to the surface. © 2006 Published by Elsevier B.V.

The Tres Arboles ductile fault zone in the Eastern Sierras Pampeanas, central Argentina, experienced multiple ductile deformation and faulting events that involved a variety of textural and reaction hardening and softening processes. Much of the fault zone is characterized by a (D2) ultramylonite, composed of fine-grained biotite + plagioclase, that lacks a well-defined preferred orientation. The D2 fabric consists of a strong network of intergrown and interlocking grains that show little textural evidence for dislocation or dissolution creep. These ultramylonites contain gneissic rock fragments and porphyroclasts of plagioclase, sillimanite and garnet inherited from the gneissic and migmatitic protolith (D1) of the hangingwall. The assemblage of garnet + sillimanite + biotite suggests that D1-related fabrics developed under upper amphibolite facies conditions, and the persistence of biotite + garnet + sillimanite + plagioclase suggests that the ultramylonite of D2 developed under middle amphibolite facies conditions. Greenschist facies, mylonitic shear bands (D3) locally overprint D2 ultramylonites. Fine-grained folia of muscovite + chlorite ± biotite truncate earlier biotite + plagioclase textures, and coarser-grained muscovite partially replaces relic sillimanite grains. Anorthite content of shear band (D3) plagioclase is c. An30, distinct from D1 and D2 plagioclase (c. An35). The anorthite content of D3 plagioclase is consistent with a pervasive grain boundary fluid that facilitated partial replacement of plagioclase by muscovite. Biotite is partially replaced by muscovite and/or chlorite, particularly in areas of inferred high strain. Quartz precipitated in porphyroclast pressure shadows and ribbons that help define the mylonitic fabric. All D3 reactions require the introduction of H+ and/or H2O, indicating an open system, and typically result in a volume decrease. Syntectonic D3 muscovite + quartz + chlorite preferentially grew in an orientation favourable for strain localization, which produced a strong textural softening. Strain localization occurred only where reactions progressed with the infiltration of aqueous fluids, on a scale of hundreds of micrometre. Local fracturing and microseismicity may have induced reactivation of the fault zone and the initial introduction of fluids. However, the predominant greenschist facies deformation (D3) along discrete shear bands was primarily a consequence of the localization of replacement reactions in a partially open system.

We investigate the behavior of the isolated mantled porphyroclast in a shear zone. The method employed is a finite element model. Three distinct phases, clast, mantle and matrix are present, the rheologies are power-law with exponents ranging from 1 to 5 and the far-field boundary condition is simple shear. The effective viscosity of the mantle is assumed to be less than those of the clast and the matrix. We show for which parameter sets mantled porphyroclasts reach super-horizontal stabilization with respect to the shear plane and sense. Clasts in natural mylonites frequently exhibit similar orientations, which are interpreted as stable inclinations. The systematic examination of the matrix‐mantle‐clast system allows for the construction of attractor maps that can be directly used as gauges for (i) the effective viscosity contrast between matrix and mantle, (ii) the production rate of mantle material around the clast as a function of the bulk shear strain, and (iii) for the total shear strain. The necessary data required to use the attractor maps are simple geometrical parameters that can be measured in the field, i.e. clast aspect ratio, clast inclination versus the shear plane, mantle thickness, and mantle and clast area. This new method successfully reproduces the characteristics of natural porphyroclasts and is in good agreement with data from natural shear zones. q 2005 Elsevier Ltd. All rights reserved.

We used analogue modelling to investigate the factors controlling tail geometry in porphyroclast systems. Results show that: (1) σ inclusions can develop in both slipping and non-slipping modes, but δ-inclusions only form in the latter. (2) σ inclusions develop when the mantle production rate is constant and the mantle is transected by the separatrix. δ inclusions form when the mantle is initially outside the separatrix and later comes inside this line. (3) In the slipping mode, the wedge-shaped tail of σ-inclusions always has a straight external surface parallel to the shear plane, whereas in the non-slipping mode the external surface is curved inwards (external embayments). (4) Together with earlier theoretical results, σ- and δ inclusions always show stair-stepping of tails when embedded in a viscous matrix under homogeneous simple shear deformation. (5) Maximum stair-stepping occurs in the slipping mode and is at least equal to the inclusion diameter. If our models bear significant similarity to nature, then (i) the straight or curved character of σ-inclusions could mean that they had, respectively, a slipping or non-slipping interface with the surrounding recrystallized matrix, and (ii) δ-inclusions may result from shear deformation under retrogressive metamorphic conditions in thrust systems.