Simple Shear Zone Research Articles

Structural Evolution and Its Implication for the Emplacement of Gold Deposit in the Central Part of Burkina Faso, West Africa

In order to constrain the structural evolution in the central part of Burkina Faso and its implication for the emplacement of gold deposits, we undertook a structural mapping by coupling Landsat and aeromagnetic images interpretation to outcrop and core mapping followed by laboratory work. This approach reveals that the structural architecture in this locality mainly results from dextral transcurrent progressive deformation due to a NW-SE trending major stress. This architecture is similar to the Riedel-Tchalenko model. Initially, the dominant normal stress created an E-W constriction resulting in the development of N-S shear corridors. Subsequently, the tangential stress that took over this generated the progressive development of simple dextral shear zones with a NE-SW orientation that are locally taken up by ENE-WSW dextral shear bands associated with the Riedel's R structures. As a result of the cooling of the crust, we are witnessing the formation of sinistral NW-SE and dextral NNE-SSW strike-slip faults, respectively, which corresponds to Riedel's R' structures and Tchalenko's P structures. The development of NW-SE pre-Eburnean shear zones, particularly the N-S shear corridors, is synchronous with the circulation of gold-bearing fluids through the zone. However, the intersections of these directional corridors create zones suitable for gold concentration. Within these zones, ductile-brittle deformation following the emplacement of the shear bands has facilitated the remobilization and concentration of gold-bearing fluids within rocks with the appropriate rheological conditions. This is the case for Toega gold deposit.
 

Strain Localization: Analog Modeling and Anisotropy of Magnetic Susceptibility

AbstractThe majority of the strain in Earth crust and upper mantle is localized to the high strain zones developed at ductile‐to‐brittle condition at kilometer‐to‐micrometer scale. Therefore, they represent the key to understanding the deformation evolution of the lithosphere. The finite strain pattern recorded within these zones has been therefore a subject of research in geology. The methods studying rock magnetism such as the anisotropy of magnetic susceptibility (AMS) are frequently used techniques to characterize and quantify deformation and flow record in rocks. Numerous sedimentary, subsolidus and submagmatic deformation zones exhibit typical evolution of the AMS ellipsoid across the strain gradient suggesting indirect not straightforward correlation between AMS and strain ellipsoids. To document spatiotemporal and internal fabric evolution during strain localization, pure shear, simple shear, and shear zone (SZ) analog experiments were performed using shear‐thinning thixotropic material of plaster of Paris. The experimental results closely resemble the record from natural SZs in sedimentary rock systems but also in subsolidus SZs and submagmatic mushy systems. The magnetic fabric evolution across deformation zones is interpreted to be associated with the intersection and transposition of preexisting primary fabric with shear fabrics and evolution of synkinematic subfabrics. Their development is attributed to localization of deformation at microscale due to the self‐organized slip of anisometric particles forming microshear planes reflecting the symmetry of deformation. The experimental results when confronted with the natural examples implies that the localization and partitioning of deformation is one of the most important factors for the interpretation of AMS in deformation zones.

Monazite as a monitor of shear strain in orogenic crust

The Ramsay-Graham heterogeneous simple shear zone model revolutionized our understanding of finite strain variation in orogenic belts. Many deeply exhumed (>20 km depths) shear zones in orogenic continental crust are a consequence of sub-simple shear strain accumulation during contraction and displacement at high temperature. Here we explore the micro-scale record of this process using syn-kinematic monazite in two shear zones in a >20,000 km2 terrane of exhumed lower continental crust. Field observations combined with high-spatial resolution X-ray mapping, Th–U-Pb electron microprobe petrochronology, and electron backscatter diffraction analysis demonstrate that monazite grains can provide an important textural and temporal record of shear strain in mylonite and ultramylonite gneisses. Monazite grains exhibit aspect ratios of 1.7–4.6 parallel to the stretching lineation and parallel or oblique (<30°) to the penetrative foliation. Monazite EBSD data for grain orientation spread, i.e., average misorientation relative to the mean, are consistent with no significant dislocation-accommodated internal deformation. We conclude that dissolution precipitation creep of monazite produced compositionally-defined geometries compatible with the dextral sense of shear observed in outcrop and thin section. Syn-kinematic rims on monazite grains that develop in low strain domains during accumulation of shear strain can provide absolute ages for μm-to km-scale kinematics in continental orogens.

Shape of plutons in crustal shear zones: A tectono-magmatic guide based on analogue models

Plutons in crustal shear zones may exploit inherited structures, interfere with strain localizing or be deformed passively. To constrain the relative timing of such tectono-magmatic relationships in natural settings is not always straight-forward. We here present sandbox-type analogue model experiments simulating magma emplacement into simple and transtensional crustal shear zones to test the diagnostic potential of pluton shape with respect to timing and setting. Observations based on surface deformation and intrusion shape exemplify the interplay between evolving and inherited tectonic structures and magma uprising. We observe markedly asymmetric intrusions in association with dikes reflecting the regional stresses, fault pattern and finite strain field. At the same time, the presence of an intrusion modifies the tectonic evolution, but only transiently, resulting in short-lived faults, reactivation and inversion. Diagnostic attributes include the pluton's aspect ratio, its orientation and amplitude as well as dike association. Accordingly, syn-tectonic intrusions show the highest pluton amplitudes, but only intermediate elongation compared to other scenarios. They are oriented parallel to Riedel shears in simple shear, respectively to the compression direction in transtension. Post-tectonic intrusions are least elongated, have medium amplitudes and exploit Riedel shears. Pre-tectonic intrusions are characterized by lowest amplitudes but the highest aspect ratios and are parallel to the finite elongation direction. Intrusions in transtensional shear zones are generally of less elongate than those in simple shear zones. Experimental results are tested against observations from natural examples validating the diagnostic potential of pluton shape for the timing and the tectonic setting of the emplacement.

Submarine Active Faults and Morpho-Tectonics Around the Iberian Margins: Seismic and Tsunamis Hazards

The aim of this work is to make a synthesis at regional scale focused on the geophysical characterization of submarine faults around the Iberian margin to identify active structures and analyze their development in the framework of the present plate organization. Most of these submarine faults show seabed morphological expressions mapped with high-resolution swath bathymetry data, high-resolution parametric sub-bottom profiles and multichannel seismic profiles. Present active tectonics, deformation, seismicity, and tsunami-affected coastal areas is mainly focused on south Iberia at the Eurasian and Nubia plate boundary. Submarine active faults in these areas are represented by long strike-slip fault systems and arcuate fold-thrust systems. Their development takes place in response to present NW-SE convergence between the Eurasian and Nubia plates. We propose a strain partitioning model of the plate boundary into simple and pure shear zones to explain the distribution and mechanisms of active submarine faults along the Gulf of Cádiz, Gibraltar Arc and Alborán Sea in response to the present-day shear stress orientation. Nevertheless, deformation is also focused in the NW Iberian margin. Thus, along the Galician and Portuguese margin, several submarine faults mapped as thrust fault systems with high-seismic activity along the Iberian ocean-continent transition reflect the re-activation of former structures. We suggest that submarine active faults in the NW and W Iberia are also the response to the eastwards transfer of short-offset transform faults of the Mid Atlantic Ridge into the oceanic Iberian along a weakness as the former plate boundary between the oceanic Iberia and Eurasia domains. The distribution and activity of submarine faults mapped in this work from geophysical and bathymetric data are in good agreement with geodetic data and focal mechanisms.

Complex fold patterns developed by progressive deformation

Complex folding patterns include folds with curved hinges and/or curved axial surfaces. In simple shear zones, folds commonly nucleate with hinges at a high angle to the shear direction and progressively rotate towards parallelism with the shear/extension direction, giving rise to sheath folds. Axial surfaces also change in attitude with increasing strain, becoming parallel to the shear plane. However, when folds do nucleate with hinges along the extension direction but oblique to a shear component, they are not subjected to hinge rotation, but can experience a variable amount and sense of rotation of axial surfaces, developing into polyclinal folds. Foliated quartzite beds from the Cap de Creus massif in the eastern Pyrenees exhibit complex fold patterns produced during a single transpressional event. While folds in the embedding schists are harmonic and asymmetric, folds in the quartzite are strongly disharmonic and polyclinal, displaying large axial surface rotations. A plausible interpretation is that folds nucleate with hinges closely parallel to the extension direction, developing thickened short limbs that rotate synthetically with regard to the shear component. This may lead to strain partitioning, with localized antithetic rotations enhanced by progressive softening of the quartzite. Thus, fold interference patterns are not necessarily linked to polyphase tectonics but they may result from single-phase progressive deformation. In the here described cases, unfolding during progressive deformation is highly improbable.

Kinematic evolution of the Pieniny Klippen Belt in Cenozoic (Ukrainian Carpathians)

The data on surface, subsurface and deep structure of the Pieniny Klippen Belt (PKB) have been summarized and new results of its geological-structural and fault kinematic studies within the Ukrainian part of the Eastern Carpathians have been presented. In its contemporary appearance as a narrow strongly tectonized zone between the Outer Carpathians and Trans-Carpathian trough PKB was formed in Oligocene—Miocene in the place of ancient ocean. On the surface it is situated within the rootless Pieniny nappe and in the deep it is a steeply dipping deformation zone which developed under compression, simple dextral strike-slip, transtension and transpression. Mesostructures of horizontal shortening and vertical elongation, thrusts and thrust-related folds as well as structures of lateral extrusion — dextral strike-slip along sub-meridional slicken-sides correspond to SW—NE 220° trending compression and correlate with the stage of Alcapa microplate progression in NE direction in Oligocene—Early Miocene. In Early Miocene as a result of counter-clockwork rotation of Alcapa dextral strike-slip deformation is concentrated in boundary zone between Alcapa and accretionary prism of Ukrainian Flysch Carpathians. The stress fields of strike-slip type with orientation of compression axis SE 165—SW 215° and its “rotation” in a zone of simple shear correspond to this stage. Present deformation regime is characterized as transpressure with SW—NE orientation of compression axis.

Kinematics and shear heat pattern of ductile simple shear zones with ‘slip boundary condition’

Extrusion by Poiseuille flow and simple shear of hot lower crust has been deciphered from large hot orogens, and partial-slip boundary condition has been encountered in analogue models. Shear heat and velocity profiles are deduced from a simplified form of Navier–Stokes equation for simple shear together with extrusive Poiseuille flow and slip boundary condition for Newtonian viscous rheology. A higher velocity at the upper boundary of the shear zone promotes higher slip velocity at the lower boundary. The other parameters that affect the slip are viscosity and thickness of the shear zone and the resultant pressure gradient that drives extrusion. In the partial-slip case, depending on flow parameters (resultant pressure gradient, density and viscosity) and thickness of the shear zone, the velocity profiles can curve and indicate opposite shear senses. The corresponding shear heat profiles can indicate temperature maximum inside shear zones near either boundaries of the shear zone, or equidistant from them.

Shear Senses and Viscous Dissipation of Layered Ductile Simple Shear Zones

Velocity profiles and shear heat profiles for inclined, layered Newtonian simple shear zones are considered. Reverse fault-like simple shear of the boundaries and upward net pressure gradient act together in such shear zones. As the velocity of the boundary increases, the point of highest velocity shifts from the lower layer of less viscosity into the upper layer. The shear heat profile shows a temperature peak inside the lower layer. For a more viscous upper layer, the point of highest velocity is located inside the upper layer and shifts towards the upper boundary of the shear zone. The shear heat profile shows a maximum temperature within the upper layer. Depending on the flow parameters of the two layers, the slip rate of the boundary, and the dip and thickness of the shear zone, a shear sense in reverse to the relative movement of the shear zone boundaries may develop. These models can decipher thermo-kinematics of layered shear zones in plate-scale hot orogens.

Textural features as indicators of debris avalanche transport and emplacement, Taranaki volcano

The Pungarehu debris avalanche deposit was emplaced by the largest known collapse of the proto–Taranaki volcano, ca. 25,000 calibrated (cal.) years ago. This debris avalanche deposit displays a highly contrasting sedimentary character between its proximal and distal reaches. Examination of the deposit granulometry, sedimentary structures, and microscopic particle attributes provides new insights into debris avalanche transport and internal evolution processes. Initial collapse of the proto–Taranaki volcano during this event occurred near the Last Glacial Maximum, with snow and ice cover and substantial groundwater present. The collapsing, sliding large blocks of edifice material, “megaclasts,” were highly fractured by the landslide generation and the depressurization event, forming pervasive jigsaw textures. As the megaclasts moved, shear was focused in softer domains between the hardest, lava-dominated lithologies. These crush and shear zones developed a complex pattern of relative motion between horizontal and vertical parts of the landslide, rather than a simple basal shear zone that supported an upper pluglike mass. The sheared zones, concentrated in soft, pyroclastic lithologies, were areas of intense synflow fragmentation, producing a proto-interclast matrix between large blocks of coherent (albeit jigsaw fractured) lavas. Down flow, the interclast matrix component increased to become pervasive by ~23–25 km from the source, enveloping and preserving large megaclasts out to at least 30 km. The most distal exposures, limited by coastal erosion to ~25–27 km, show that the matrix was not completely water saturated, with only superficial penetration of the sand-dominated material into the margins of fractured lava domains, which still contained central void space. Evidence of multiple generations of particle fracturing is seen under scanning electron microscopy of sand-grade clasts, with initial decompression fractures crosscut by later cracks, pits, and scratches produced by collisional and frictional processes during transport. The findings from this study help to explain the formation of the highly irregular topography of debris avalanche deposits, with chaotically distributed (and probably temporary) zones of shear developing where softer lithologies occur in a collapsing mass, thus leading to differential velocity profiles of portions of the flowing mass in vertical and horizontal planes.

The development of the Deception Island volcano caldera under control of the Bransfield Basin sinistral strike-slip tectonic regime (NW Antarctica)

Abstract Deception Island is a small and volcanically active caldera volcano of Quaternary age, located in the marginal basin of Bransfield Strait, NW Antarctica. The distribution and orientation of fracture and fault systems that have affected the Deception volcanic edifice, and the elongated geometry of its volcanic caldera, are consistent with a model of Riedel deformation induced by a regional left-lateral simple shear zone. It is suggested that this caldera was formed above a magma chamber stretched under the influence of the regional transtensional regime with left-lateral simple shear. The collapse may have occurred in at least two phases: first, a small volume event occurred along the compressed flanks of the volcano edifice; and second, a large collapse event affected the stretched flanks of the volcano edifice.

Viscous dissipation pattern in incompressible Newtonian simple shear zones: an analytical model

An analytical model of shear heating in an inclined simple shear zone with Newtonian rheology under a reverse shear sense and an upward resultant pressure gradient is presented. Neglecting a number of secondary factors, the shear heat is expressed as a function of the total slip rates at the boundaries, pressure gradient, dip and thickness of the shear zone, and density, viscosity, and thermal conductivity of the rock. A quartic temperature profile develops with a point of maximum temperature near the bottom part of the shear zone in general. The profile is parabolic if pressure gradient vanishes leading to a Couette flow. The profile attains a bell shape if there is no slip at the boundaries, i.e., a true Pouseille flow. The present model of shear heating is more applicable in subduction channels and some extruding salt diapirs where the rheology is Newtonian viscous and pressure gradient drives extrusion.

Simple shear is not so simple! Kinematics and shear senses in Newtonian viscous simple shear zones

AbstractThis work develops an analytical model of shear senses within an inclined ductile simple shear zone with parallel rigid boundaries and incompressible Newtonian viscous rheology. Taking account of gravity that tends to drive the material downdip and a possible pressure gradient that drives it upward along the shear zone, it is shown that (i) contradictory shear senses develop within two sub-zones even as a result of a single simple shear deformation; (ii) the highest velocity and least shear strain develop along the contact between the two sub-zones of reverse shear; (iii) for a uniform shear sense of the boundaries, a zone of reverse shear may develop within the top of the shear zone if the pressure gradient dominates the gravity component; otherwise it forms near the bottom boundary; (iv-a) a ‘pivot’ defined by the intersection between the velocity profile and the initial marker position distinguishes two sub-zones of opposite movement directions (not shear sense); (iv-b) a pivot inside any non-horizontal shear zone indicates a part of the zone that extrudes while the other subducts simultaneously; (v) the same shear sense develops: (v-a) when under a uniform shear of the boundaries, the shear zone remains horizontal and the pressure gradient vanishes; or alternatively (v-b) if the shear zone is inclined but the gravity component counterbalances the pressure gradient. Zones with shear sense reversal need to be reinterpreted since a pro-sheared sub-zone can retro-shear if the flow parameters change their magnitudes even though the same shear sense along the boundaries is maintained.

A mid-crustal strain-transfer model for continental deformation: A new perspective from high-resolution deep seismic-reflection profiling across NE Tibet

Understanding why continental deformation departs from the theory of plate tectonics requires a detailed knowledge of three-dimensional structures at a lithospheric scale. In Tibet, the end-member models of continental deformation make distinctively different predictions on strain distribution and contrasting structural geometry as a function of depth. Specifically, the thin-viscous-sheet model predicts vertically coherent deformation while channel-flow and continental-subduction models predict the presence of sub-horizontal detachment zones within or at the base of the Tibetan crust during the Cenozoic deformation. To differentiate the above models, we conducted a high-resolution seismic-reflection survey across the active left-slip Kunlun fault and its nearby contractional structures. The results of this work show that the actively deforming middle Tibetan crust is dominated by discrete sub-horizontal simple-shear zones that terminate the sub-vertical, left-slip Kunlun fault above and mantle-cutting thrusts below. The flat shear zones appear to act as roof and floor thrusts of large duplex structures that transfer shortening strain from locally deformed and coupled lower crust and mantle lithosphere below to the high-strain domains of the upper crust above. The middle-crustal strain-transfer model proposed here implies that the weak Tibetan middle crust may not be active everywhere during the Indo-Asian collision. It also predicts that the kinematics of the activated portions of the middle crust, whether being deformed by simple shear or channel-flow deformation, may vary from place to place, depending strongly on the lateral variation of mechanical strength at different depths of the lithosphere. Our approach of establishing the kinematics of middle-crust deformation departs significantly from the early work that emphasizes exclusively the role of vertically varying rheology in controlling the mode of continental deformation.

Rift-related inheritance in orogens: a case study from the Austroalpine nappes in Central Alps (SE-Switzerland and N-Italy)

The Austroalpine nappe systems in SE-Switzerland and N-Italy preserve remnants of the Adriatic rifted margin. Based on new maps and cross-sections, we suggest that the complex structure of the Campo, Grosina/Languard, and Bernina nappes is inherited largely from Jurassic rifting. We propose a classification of the Austroalpine domain into Upper, Middle and Lower Austroalpine nappes that is new because it is based primarily on the rift-related Jurassic structure and paleogeography of these nappes. Based on the Alpine structures and pre-Alpine, rift-related geometry of the Lower (Bernina) and Middle (Campo, Grosina/Languard) Austroalpine nappes, we restore these nappes to their original positions along the former margin, as a means of understanding the formation and emplacement of the nappes during initial reactivation of the Alpine Tethyan margin. The Campo and Grosina/Languard nappes can be interpreted as remnants of a former necking zone that comprised pre-rift upper and middle crust. These nappes were juxtaposed with the Mesozoic cover of the Bernina nappe during Jurassic rifting. We find evidence for low-angle detachment faults and extensional allochthons in the Bernina nappe similar to those previously described in the Err nappe and explain their role during subsequent reactivation. Our observations reveal a strong control of rift-related structures during the subsequent Alpine reactivation on all scales of the former distal margin. Two zones of intense deformation, referred to as the Albula-Zebru and Lunghin-Mortirolo movement zones, have been reactivated during Alpine deformation and cannot be described as simple monophase faults or shear zones. We propose a tectonic model for the Austroalpine nappe systems that link inherited, rift-related structures with present-day Alpine structures. In conclusion, we believe that apart from the direct regional implications, the results of this paper are of general interest in understanding the control of rift structures during reactivation of distal-rifted margins.

Kinematic interpretation of shearband boudins: New parameters and ratios useful in HT simple shear zones

Shearband boudins (asymmetric boudins showing slip along the inter-boudin surface, which is synthetic with respect to the bulk shear sense) are ubiquitous and well exposed in HT simple shear zones. The present work aims to extend the methodology of analysis of shearband boudins developed by Goscombe and Passchier (2003). Such shearband boudins represent complex objects that require an adequate methodology for unambiguous kinematic interpretation. We propose new geometric parameters (Bbs, B-t, c′, D′, d, ψ′) in order to describe and identify, with confidence, the kinematics of boudinage in this particular geological framework. The key-observation for kinematic interpretation in simple shear regimes is the boudin axis (Lb) that is commonly present, excluding the necessity to identify the regional stretching lineation (Lx) in the metasedimentary matrix. In monoclinic HT simple shear zones, this approach involves the use of a local displacement plane (Sx), which is always normal to the boudin axis (Lb).

Cenozoic extensional tectonics of the Western Anatolia Extended Terrane, Turkey

The Western Anatolia Extended Terrane in Turkey is located on the eastern side of the Aegean Extended Terrane and contains one of the largest metamorphic core complexes in the world, the Menderes massif. It has experienced a series of continental collisions from the Late Cretaceous to the Eocene during the formation of the Izmir-Ankara-Erzincan suture zone. Based our field work and monazite ages, we suggest that the north-directed postcollisional Cenozoic extension in the region is the product of three consecutive stages, triggered by three different mechanisms. The first stage was initiated about 30 Ma ago, in the Oligocene by the Orogenic Collapse the thermally weakened continental crust along the north-dipping Southwest Anatolian shear zone. The shear zone was formed as an extensional simple-shear zone with listric geometry at depth and exhibits predominantly normal-slip along its southwestern end. But, it becomes a high-angle oblique-slip shear zone along its northeastern termination. Evidence for the presence of the shear zone includes (1) the dominant top to the north-northeast shear sense indicators throughout the Menderes massif, such as stretching lineations trending N10E to N30E; and (2) a series of Oligocene extensional basins located adjacent to the shear zone that contain only carbonate and ophiolitic rock fragments, but no high grade metamorphic rock fragments. During this stage, erosion and extensional unroofing brought high-grade metamorphic rocks of the Central Menderes massif to the surface by the early Miocene. The second stage of the extension was triggered by subduction roll-back and associated back-arc extension in the early Miocene and produced the north-dipping Alaşehir and the south-dipping Büyük Menderes detachments of the central Menderes massif and the north-dipping Simav detachment of the northern Menderes massif. The detachments control the Miocene sedimentation in the Alaşehir, Büyük Menderes, and Simav grabens, containing high-grade metamorphic rock fragments. The third stage of the extension was triggered by the lateral extrusion (tectonic escape) of the Anatolian plate when the North Anatolian fault was initiated at about 5 Ma. This extensional phase produced the high-angle faults in the Alaşehir, Büyük Menderes and Simav grabens and the high-angle faults controlling the Küçük Menderes graben.

Structural analysis of mylonitic rocks in the Cougar Creek Complex, Oregon–Idaho using the porphyroclast hyperbolic distribution method, and potential use of SC′-type extensional shear bands as quantitative vorticity indicators

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 structural evolution of sheath folds: A case study from Cap de Creus

It has long been recognised that within zones of intense non-coaxial deformation, fold hinges may rotate progressively towards the transport direction ultimately resulting in highly curvilinear sheath folds. However, there is a surprising lack of detailed and systematic field analysis of such “evolving” sheath folds. This case study therefore focuses on the sequential development of cm-scale curvilinear folds in the greenschist-facies El Llimac shear zone, Cap de Creus, Spain. This simple shear-dominated dextral shear zone displays superb three dimensional exposures of sheath folds defined by mylonitic quartz bands within phyllonite. Increasing amounts of fold hinge curvature ( δ) are marked by hinge segments rotating into sub-parallelism with the mineral lineation (Lm), whilst the acute angle between the axial-planar hinge girdle and foliation ( ω) also displays a sequential reduction. Although Lm bisects the noses of sheath folds, it is also clearly folded and wrapped-around the sheath hinges. Lm typically preserves a larger angle ( θ) with the fold hinge on the lower limb ( L) compared to the upper ( U) limb ( θL > θU), suggesting that Lm failed to achieve a steady orientation on the lower limb. Adjacent sheath fold hinges forming fold pairs may display the same sense of hinge arcing to define synthetic curvature, or alternatively opposing directions of antithetic curvature. Such patterns reflect original buckle fold geometries coupled with the direction of shearing. The ratio of long/short fold limbs decreases with increasing hinge curvilinearity, indicating sheath folds developed via stretching of the short limb, rather than migrating or rolling hinge models. This study unequivocally demonstrates that both hinges of fold pairs become curvilinear with sheaths closing in the transport direction recording greater hinge-line curvilinearity compared to adjacent return hinges. This may provide a useful guide to bulk shear sense.

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

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