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Faults in the crust at seismogenic depths are embedded in a fluid‐saturated, elastic, porous material. Slip on such faults may induce transient pore pressure changes through dilatancy or compaction of the gouge or host rock. However, the poroelastic nature of the crust and the full coupling of inelastic gouge processes and the host rock have been largely neglected in previous analyses. Here, we present a linearized stability analysis of a rate‐and‐state fault at steady‐state sliding in a fully‐coupled poroelastic solid under in‐plane and anti‐plane sliding. We further account for dilatancy of the shear zone and the associated pore pressure changes in an averaged sense. We derive the continuum equivalent of the analysis by Segall and Rice (1995, https://doi.org/10.1029/95jb02403), and highlight a new parameter regime where dilatancy stabilization can act in a highly diffusive solid. Such stabilization is permitted since the time scale of flux through the shear zone and diffusion into the bulk can be very different. A novel aspect of this study involves analyzing the mechanical expansion of the shear layer causing fault‐normal displacements, which we describe by a mass balance of the solid constituent of the gouge. This effect gives rise to a universal stabilization mechanism in both drained and undrained limits. The importance of the mechanism scales with shear‐zone thickness and it is significant for wider shear zones exceeding approximately 1 cm. We hypothesize that this stabilization mechanism may alter and delay an ongoing shear localization process. Key Points: We analyze stability of a rate‐and‐state fault in a poroelastic solid with fully coupled dilatancyWe show that dilatancy stabilization can also occur in a highly diffusive bulk if shear zone permeability is lowWe identify a new stabilizing mechanism associated with the mechanical expansion of the shear zone [ABSTRACT FROM AUTHOR]

We analyze the 3D geomorphology of a boudinaged amphibolite layer encased in marble using meter-scale tomography by serial sectioning, high resolution imaging and 3D reconstruction of a 2 m³ block from a quarry in the high-grade core of the Naxos core complex. Local uncertainties in the model are resolved by dissolution of marble in selected slabs, yielding a large, highly accurate 3D model. Results show five generations of brittle extension of the amphibolite in ductile marble, although examination of the boudinaged layer on the outside of the block only suggested one generation. Most prominent are the rotated domino boudins, where part of the dilatancy between domino boudins can be shown to be due to later reactivation. Shear fractures between domino boudins have relatively large throw gradients, indicating ductile rupture. Layer-parallel stretch as measured by reconstruction of domino boudins decreases with amphibolite layer thickness although the layer has undergone the same bulk stretch everywhere. This suggests that with decreasing layer thickness deformation becomes delocalized, offering tantalizing glimpses of rheology under metamorphic conditions. [ABSTRACT FROM AUTHOR]

A classification of boudinage and boudins is made presenting a simplified parametric description of boudin block geometry end-members. The most important shearband boudins parameters, ratios, microstructural domains and internal evolutions are described and interpreted. Shearband boudins are related to a large number of geological meso- and microstructures (e.g. pinch-and-swell, drawn or torn boudins, mica-fish, porphyroclasts, foliation boudinage, tectonic lozenges and C-S-C′). Several factors can control the asymmetric boudinage process, such as the obliquity of a competent layer, the orientation of an initial fracture, boudin axial ratio, the viscosity ratio between matrix and layer, flow type and shear strain. This review highlights the correlation of shearband boudins with other asymmetrically deformed structures and shows their geological relevance. Finally, this study presents a phenomenological framework for shearband boudins. [ABSTRACT FROM AUTHOR]

Mafic granulites in the Fuping Complex occur as lenses or boudins within high-grade TTG (Trondhjemite–Tonalite–Granodiorite) gneisses. Petrographic observations reveal four generations of mineral assemblage in the granulites: an inclusion assemblage of hornblende + plagioclase + ilmenite + quartz within garnet core; an inferred peak assemblage composed of garnet ± hornblende + plagioclase + clinopyroxene + rutile/ilmenite + quartz; a decompression assemblage characterized by symplectites of clinopyroxene ± orthopyroxene + plagioclase, coronae of plagioclase ± clinopyroxene ± hornblende around embayed garnet porphyroblasts or a two-pyroxene association; and a late amphibolite-facies retrogressive assemblage. Two representative samples were used for pseudosection modeling in NCFMASHTO model system to determine their metamorphic evolution. The results show that these granulites experienced a high-pressure stage of metamorphism with peak P–T conditions of 12–13 kbar and 760–800 °C (P max ) and a post-peak history under P–T conditions of ∼9.0 kbar and 805–835 °C (T max ), indicating a nearly isothermal decompression process (ITD) with a slight heating. Metamorphic evolution from the P max to the T max is predicted to be dominated by garnet breakdown through continuous metamorphic reactions of garnet + quartz ± diopside = hornblende + plagioclase + liquid and garnet + quartz + hornblende = plagioclase + diopside + liquid + orthopyroxene. Further metamorphic evolution after the T max is dominated by cooling, suggesting that high-pressure (HP) granulites may also exist in the Fuping Complex. Metamorphic zircons in the Fuping HP mafic granulites have left inclined REE patterns, Ti contents of 1.68–6.88 ppm and crystallization temperatures of 602–712 °C. SIMS zircon U–Pb dating on these zircons yields 207 Pb/ 206 Pb ages of 1891 ± 14 Ma and 1849 ± 6 Ma, interpreted to represent the cooling stage of metamorphism. The P–T–t evolution of the Fuping HP mafic granulites records well the protracted Paleoproterozoic orogenic event occurred in the central North China Craton. [ABSTRACT FROM AUTHOR]

The evaporite deposits examined in this study are located in the central part of middle Dalmatia, Croatia. In this region, Upper Permian evaporite sediments were deposited under favourable conditions onto the Variscan basement around the northern margins of Gondwana. These sediments can be subdivided into three members, a lower evaporite unit (an anhydrite member), a middle evaporite unit (a gypsum member), and an upper unit (a clastic member), and are mainly comprised of secondary gypsum that formed via the hydration of precursor anhydrite rocks. The middle evaporite unit comprises beds of gypsum as well as early diagenetic dolomites that contain gypsum sequences, extending up to 60 m maximum thickness, and overlying clastic sequences that themselves are up to 20 m thick. These Upper Permian evaporite sediments contain horizontal, irregular, gypsum lithofacies that exhibit pronounced enterolithic and boudinage structures. The characteristics of these sediments are indicative of deposition in supratidal and sabkha settings (i.e., early diagenetic dolomites and evaporites) within a shallow epicontinental marine environment with highly varied coastlines, bays, and lagoons. The secondary gypsum seen within this Upper Permian middle evaporite unit displays alabastrine and porphyroblastic secondary textures and includes corroded anhydrite relics; associated minerals include muscovite, chlorite, potassium (K)-feldspar, quartz, and amphibole. The Upper Permian evaporite sediments discussed in this study are composed of irregular, locally brecciated secondary gypsum that probably formed as a result of multiple synsedimentary collapse of pre-existing soluble mineralsand/or synsedimentary and post-sedimentary tectonics. [ABSTRACT FROM AUTHOR]

In multiply deformed terrains multiphase boudinage is common, but identification and analysis of these is difficult. Here we present an analysis of multiphase boudinage and fold structures in deformed amphibolite layers in marble from the migmatitic centre of the Naxos metamorphic core complex. Overprinting between multiple boudinage generations is shown in exceptional 3-D outcrop. We identify five generations of boudinage, reflecting the transition from high-strain high-temperature ductile deformation to medium- to low-strain brittle boudins formed during cooling and exhumation. All boudin generations indicate E-W horizontal shortening and variable direction of bedding parallel extension, evolving from subvertical extension in the earliest boudins to subhorizontal N-S extension during exhumation. Two phases of E-W shortening can be inferred, the first associated with lower crustal synmigmatic convergent flow and the second associated with exhumation and N-S extension, possibly related to movement of the North Anatolian Fault. [ABSTRACT FROM AUTHOR]

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The "island arc junctions" are the regions with plural island arcs, uplift/geanticline belts. In the Japanese Islands, the island arc junctions are situated in the three districts; the north, the central, and the south of Japan, where the Chishima (Kuril), Izu-Mariana, and Ryukyu arcs intersect the Honshu arc. There, during the Plio-Pleistocene, vast volcanic activities had repeatedly occurred and formed calderas/cauldrons with gold vein-fracture systems. Such zones are still the active volcano-geothermal areas, and earthquake activities are continuing. All of these geologic phenomena are not independent, they are mutually related. In the images of the seismic tomography, the vertical structures composed of the high and low velocity zone cut through the seismic plane, the 'subducting' lithosphere plate, which is rooted at least 100-200 km in depth. Around the depth of 20-50 km±, the low velocity lenses like a wiener or boudinage spread over 200 km or more. Shallowest low velocity spots below the surface are located directly beneath the active volcanoes. Therefore, this is considered a magma, melted layer. These geological features suggest that the island arc junctions are the intersection sites of different tectonic directions/geanticlines, that is, the deep-seated fracture zones rooted in the mantle or the outer core. The area provides a conduit for mantle energy to rise to the Earth surface - like a chimney. This mechanism isn't compatible with Plate Tectonics since Island Arcs and their junctions have to be compressional regimes associated with subduction. The locations of the island arc junctions as high potential areas of the Quaternary gold deposits in the Circum- Pacific region are pointed out on both sides of the Western and Eastern Pacific. Those sites are not only the island arc junctions in the Western Pacific but also the intersection of the submarine fracture zones/ridge and the land arcs or the areas of the bending land arcs in the Eastern Pacific. In recent years, there has been interesting discussions that gold veins are created by earthquakes and furthermore, the occurrence of earthquakes is related to the injection of "supercritical fluids". "Supercritical fluids" will be the key to this problem. [ABSTRACT FROM AUTHOR]

Many normal fault systems and, on a smaller scale, fracture boudinage often exhibit asymmetry with one fault dip direction dominating. It is a common belief that the formation of domino and shear band boudinage with a monoclinic symmetry requires a component of layer parallel shearing. Moreover, domains of parallel faults are frequently used to infer the presence of a décollement. Using Distinct Element Method (DEM) modelling we show, that asymmetric fault systems can emerge under symmetric boundary conditions. A statistical analysis of DEM models suggests that the fault dip directions and system polarities can be explained using a random process if the strength contrast between the brittle layer and the surrounding material is high. The models indicate that domino and shear band boudinage are unreliable shear-sense indicators. Moreover, the presence of a décollement should not be inferred on the basis of a domain of parallel faults alone. [ABSTRACT FROM AUTHOR]

The present study determines the role of viscosity on the development of rectangular torn boudin and its various types, defined by the curvature of their exterior and face margins. Numerical modeling was performed with the help of Finite Element Method considering Maxwell visco-elastic materials in commercial code ANSYS. Seven different viscosities were used and interchanged among the boudin, inter-boudin and matrix materials to understand the effect of viscosity ratios, specifically of relative viscosity of inter-boudin material. Results show that the viscosity of inter-boudin material has significant control on the shape of torn boudins apart from the viscosity ratio of boudin to matrix material. Bone-shaped boudin develops only when the inter-boudin is more competent than boudin and it becomes more prominent when matrix is also competent than boudin, but incompetent than inter-boudin. When boudins are stiffer than inter-boudin, barrel-shaped and fish-head boudins with concave faces develop. Exterior or face margins remain almost straight when boudin is relatively rigid compared to its surrounding matrix materials, or when there is no or very little viscosity contrast between boudin and inter-boudin material even in case of large boudin-matrix viscosity contrast. Therefore, the relative viscosity among the boudin, inter-boudin and matrix materials can be estimated qualitatively by studying the shape of boudin in the field. [ABSTRACT FROM AUTHOR]

Observations of physical and chemical changes across strain gradients can provide information about the processes that lead to localization and therefore provide better tools for prediction of spatial and temporal strain patterns. In contrast to the many chemical and microstructural studies of natural shear zones in metapelites and mafic lithologies, few have described kilometer-scale granitic strain gradients despite the fact that granitoid bodies make up much of the orogenic crust. This study reports microstructural and compositional data across two amphibolite-facies strain gradients from middle crust of the Grenville orogen. The kilometer-scale gradients, defined by stretched enclaves and foliation intensity, form parts of the Boundary and Bad River shear zones, located along the east and west borders of the Bad River granite in the southwestern Grenville Front tectonic zone in Ontario, Canada. The granite is bounded on both sides by older orthogneiss (ca. 1720 Ma igneous crystallization age). Zircon U-Pb ages indicate that the granite crystallized from a magma at ca. 1465 Ma, with metamorphic growth at ca. 1020 Ma. Zircons in the orthogneiss record metamorphic growth during these two younger episodes. These age determinations are consistent with other regional dates and indicate that the strain gradients in the Bad River granite formed during the Grenville orogeny. Whole-rock analyses reveal minor heterogeneity in major-element distribution in the granite that can be attributed to igneous processes, and homogeneity in the trace elements, indicating that strain did not affect the bulk-rock composition. In contrast, microstructural and chemical analyses across the two strain gradients indicate some correlation with strain: slight changes in mineral compositions, development of crystallographic preferred orientations in quartz, an increase in recrystallized fraction, a reduction in recrystallized grain size, and the development of a mixed-phase matrix. However, most of these variations are subtle and occur at different positions across the gradients. The spatial distribution of the microscale changes suggests a change in deformation mechanisms toward the margin, accompanying increased localization. The rheological weakening at the Bad River granite margins was the product of microstructural, rather than mineralogical, change, in contrast to nearby shear zones in more mafic units. [ABSTRACT FROM AUTHOR]

The Comechingones pegmatitic field comprises granitic pegmatites and leucogranites hosted by mylonitic rocks from the Guacha Corral shear zone. Pegmatites consist of variably sized, internally zoned bodies, with late replacement units. Leucogranites are fine-grained rocks that form small bodies with tabular-to lens-like shapes. Field relationships and petrographic observations suggest a synkinematic emplacement of both rock types with respect to the main deformational event of this shear zone. U-Pb LA-ICP-MS analyses on zircon crystals indicate crystallization ages of 473.8 ± 3.9 and 474.8 ± 12.2 Ma for the granitic pegmatites and leucogranites, respectively, suggesting that they originated coevally at around 474 Ma. This age provides a first-order time constraint on the deformation of the Guacha Corral shear zone. Using ages and pressure estimations, along with an average pressure gradient of 1GPa/35 km for the continental crust, an exhumation rate of approximately 0.1 mm/a and 10.5 km of total amount of exhumation were calculated for the time span between the early Ordovician and the late Devonian in this part of the Sierras Pampeanas. The inherited ages obtained in zircon crystals from the leucogranites display a distribution pattern that strongly resembles those of the metasedimentary rocks of the Pampean metamorphic belt, suggesting that equivalents of these rocks should have been involved in the melting processes that originated the leucogranites. [ABSTRACT FROM AUTHOR]

The Ngaoundéré granite pluton, in Central-North Cameroon, located near the Central Cameroon Shear zone (CCSZ), and previously studied for its petrography and geochemistry, is characterized by the absence of macroscopic markers of deformation. In this study, we report microstructures and magnetic fabrics (AMS) of this pluton and discuss the relationship with the Pan-African evolution of the CCSZ. The pluton consists of a porphyritic Hbl-Bt-monzogranite at its rim and a porphyritic biotite-granite at its core, a petrographic distribution denoting a normal zoning pattern, i.e. more silicic toward the centre. As expected, magnetic susceptibilities values also exhibit a zoning pattern in agreement with petrographic zonation. Thermomagnetic data indicate that this pluton is dominantly ferromagnetic in behaviour. As indicated by its microstructures, the pluton has suffered a continuum of deformation from the magmatic state to the high temperature solid-state during magma crystallization and solidification. The magnetic foliations dominantly strike NE-SW and dip moderately to steeply and the lineations mostly plunge shallowly to the NE or SW, roughly parallel to NE-to ENE-trending Central Cameroun Shear Zone (CCSZ). The foliation poles define a girdle pattern with a zone axis (52°/11°) rather close to the best line of the lineations (44°/21°). These fabrics correlate with the structures of the country rocks ascribed by several workers to a regional transpression. Toward the margins of the pluton, particularly the northern one, the lineations tend to rotate from NE to N in azimuth. This change is interpreted as due to strain partitioning, simple shearing with NE-SW extension being relayed by compression toward the northern pluton border. This new magnetic fabric study suggests that the Ngaoundéré pluton (poorly dated at c. 575 Ma) was emplaced during the late stages of the CCSZ dextral transpressive movement. It also provides some more constraints on the correlation between the CCSZ system and the shear zone system of NE-Brazil. [ABSTRACT FROM AUTHOR]

The shortening and extension of mechanically layered ductile rock generates folds and pinch-and-swell structures (also referred to as necks or continuous boudins), which result from mechanical instabilities termed folding and necking, respectively. Folding and necking are the preferred deformation modes in layered rock because the corresponding mechanical work involved is less than that associated with a homogeneous deformation. The effective viscosity of a layered rock decreases during folding and necking, even when all material parameters remain constant. This mechanical softening due to viscosity decrease is solely the result of fold and pinch-and-swell structure development and is hence termed structural softening (or geometric weakening). Folding and necking occur over the whole range of geological scales, from microscopic up to the size of lithospheric plates. Lithospheric folding and necking are evidence for significant deformation of continental plates, which contradicts the rigid-plate paradigm of plate tectonics. We review here some theoretical and experimental results on folding and necking, including the lithospheric scale, together with a short historical overview of research on folding and necking. We focus on theoretical studies and analytical solutions that provide the best insight into the fundamental parameters controlling folding and necking, although they invariably involve simplifications. To first order, the two essential parameters to quantify folding and necking are the dominant wavelength and the corresponding maximal amplification rate. This review also includes a short overview of experimental studies, a discussion of recent developments involving mainly numerical models, a presentation of some practical applications of theoretical results, and a summary of similarities and differences between folding and necking. [ABSTRACT FROM AUTHOR]

The role of mass transport deposits (MTDs) in redistributing sediment from the shelf-break to deep water is becoming increasingly apparent and important in the study of basins. While seismic analysis may reveal the general morphology of such deposits, it is unable to provide information on the detailed geometry and kinematics of gravity-driven transport owing to the limits of seismic resolution. Outcrop analysis of ancient MTDs may therefore provide critical observations and data regarding the internal deformation and behavior during slope failure. One such field area where geometry and kinematics are clearly exposed is Cerro Bola in the Paganzo Basin of northwestern Argentina. This 8 km strike section exposes a mid to late Carboniferous succession, comprising fluvio-deltaic sediments, turbidites and MTDs. Our work focuses on the main MTD that is up to 180 m thick and is characterized by a silty matrix, containing sandstone blocks and siltstone rafts. Although we consider a single slope failure as the most likely scenario, a possible double failure might also explain the occurrence of a folded turbidite marker in the upper zone of the MTD. The MTD is host to a variety of deformational features such as folding, boudinage, shear zones, allochthonous strata, and secondary fabrics among others. These deformational features vary in intensity, scale and style, both vertically and laterally across the deposit. The vertical variation is the most notable, and the entire deposit can be subdivided into lower, middle and upper zones according to variations in texture and structures, including sandstone blocks, sand streaks and blebs in the matrix, folding on a variety of scales, and shear zones. The middle part of the MTD is characterized by the abundance of siltstone rafts. Various models are proposed for the origin of blocks and rafts within the MTD: erosion of underlying strata; fragmentation of the original protolith; or a mixture of both. Significantly, specific strain cells occur around the blocks, and so the kinematics of deformation structures in the matrix of the MTD are very largely governed by their proximity and position relative to blocks, and may not relate to the overall kinematics of the MTD. This casts serious doubt on the ability to interpret overall movement directions from core or dip-meter data in the subsurface. [ABSTRACT FROM AUTHOR]

Macroscopic structures are investigated in a zone of highly contorted migmatites from the southern Prince Charles Mountains, Antarctica. Here, L-tectonite fabrics, rods, mullions, boudin pods, elongate enclaves, and fold hinges, are persistent linear features all plunging gently to the northeast. In contrast, amoeboid folds, ptygmatic folds and folded boudins with different orientations are the characteristic structures in transverse sections (perpendicular to the lineation). No consistent shear sense is recognised in any dimension. Together with strain and shape analysis, these observations strongly suggest that the deformation pattern is one of folding and stretching by constriction. Previous timing constraints indicate that this deformation overlapped with the waning stages of anatexis during decompression at approximately 510 Ma, up to 30 million years after initial orogeny at 540 Ma. The zone affected by constriction is several kilometres wide and has a contorted flower-like shape confined between two broad domal antiforms. In this context, the constricted zone is interpreted as a relatively late tectonic feature that could have formed via deep-seated viscous collision in response to orogenic collapse and doming. [ABSTRACT FROM AUTHOR]

This work focuses on the development of a regional P-T-path from the Malpica-Lamego Ductile Shear Zone, NW Portugal, based on the microstructures of shearband boudins evolved during progressive simple shear. The combination of microstructural analysis, fluid inclusion studies, crystallographic preffered orientation and fractal geometry analyses, allows to link several stages in the internal evolution of the boudin to regional P-T conditions. The boudinage process is initiated under differential stress after the original layer achieved sufficient viscosity contrast relative to the surrounding matrix. Two main transformations occur simultaneously: i) change in the external shape with continuous evolution from tabular rigid body to sigmoidal asymmetric morphology (shearband boudin) and ii) localized dynamic recrystallization in the sharp-tips of the structure (acute edge of shearband boudin), and along the boudin's margin and grain boundaries. Smaller recrystallized grains, particularly in the sharp-tip domains, accommodate most of the external strain, and larger relict grains are preserved in the centre. Dynamic recrystallization under constant strain rates and strain partitioning inside the boudins is indicated by fractal geometry based on grain boundary and grain area analysis. Progressive deformation leads to the generation of structural and textural heterogeneous domains inside the boudins, and is recorded by quartz c -axis orientation analysis and fluid inclusion studies. The last deformation episode shows the final formation of the blunt-tip domain and internal secondary shear planes. The regional P-T path begins with the crystallization of andalusite after an internal shearband boudin dilation event and ends with quartz dynamic recrystallization on boudin tips. The main deformation stage (310/315 Ma) led to reactivation of internal secondary shear zones with sillimanite crystallization. [ABSTRACT FROM AUTHOR]

The Northern Snake Range is a classic example of a metamorphic core complex, Basin-and-Range province, United States. It is composed of a plastically deformed footwall and a brittlely deformed hanging wall, separated by the Northern Snake Range low-angle detachment (NSRD). Brittle deformation, however, is not confined to the hanging wall. This paper focuses on exposures in Cove Canyon, located on the SE flank of the Northern Snake Range, where penetrative, homogeneous faults are well exposed throughout the hanging wall, footwall and NSRD, and overprint early plastic deformation. These late-stage fault sets assisted Eocene-Miocene extension. Detailed analysis of the faults reveals the following: (1) The shortening direction defined by faults is similar to the shortening direction defined by the stretching lineation in the footwall mylonites, indicating that the extensional kinematic history remained unchanged as the rocks were uplifted into the elastico-frictional regime. (2) After ∼17 Ma, extension may have continued entirely within elastic-frictional regime via cataclastic flow. (3) This latest deformation phase may have been accommodated by a single, continuous event. (3) Faults within NSRD boudins indicate that deformation within the detachment zone was non-coaxial during the latest phase of extension. [ABSTRACT FROM AUTHOR]

Our analyses of microboudinage structures of piemontite grains embedded within six samples of metachert, one collected from an ultrahigh-pressure (UHP) metamorphic unit at Lago di Cignana in Italy of the Western Alps, and the other five from surrounding high-pressure (HP) metamorphic units in Italy and France, have revealed that the structures are all symmetrical in type, and were presumably produced in coaxial strain fields. Stress–strain analyses of the microboudinaged grains revealed significant contrasts in the stress and strain histories of the UHP and HP metamorphic units, with the differential stress recorded by the UHP sample being unequivocally lower than that recorded by the five HP samples. In addition, our analyses showed that the UHP sample underwent stress-relaxation during microboudinage, whereas the five HP samples did not. On the basis of these observations and analyses we discuss the mechanical decoupling of the UHP and HP units that led to different histories in differential stress between the units during exhumation of the Western Alps. [ABSTRACT FROM AUTHOR]

In the (ultra-)high- P-low- T metamorphic terrane of the Chinese South Tianshan, discontinuous mafic blocks and boudins (former upper oceanic crust) are now embedded in voluminous (mainly metasedimentary) host rocks. Two different models were proposed and relate the occurrence of both high- P and ultra-high- P mafic and metasedimentary rocks to either (i) a tectonic mélange style exhumation, with no exhumation of coherent units, but different lithologies derived from different depths juxtaposed and intermingled during exhumation in the subduction channel, or (ii) the evolution of two coherent metamorphic belts: one with high- P and the other with ultra-high- P conditions. In contrast to most previous studies in the Chinese South Tianshan which focused either on single eclogites or metasedimentary rocks (assumed as representative), this study concentrates on the systematic investigation of both mafic boudins and their immediate sedimentary host rocks, because the investigation of both lithologies and the comparison of their metamorphic evolution is crucial to reconstruct the geodynamical context of the whole (ultra-)high- P-low- T metamorphic complex. Several sample pairs consisting of both lithologies were geochemically investigated and their respective metamorphic evolution was reconstructed using geothermobarometry and thermodynamic modelling. The latter approach considers changes in the mineral assemblage during the metamorphic evolution, as well as changes in mineral composition, which may help to determine the metamorphic history of a rock despite the preservation of critical mineral assemblages. All samples experienced a clockwise P-T path with overall maximum P-T conditions of 540-550 °C and 1.9-2.25 GPa for the host rocks, and 555-575 °C and 2.2-2.5 GPa for the eclogites. Peak-metamorphic temperatures of ~525-540 °C of the metasedimentary host rocks were also confirmed by Raman spectroscopy of carbonaceous material. Results from thermobarometry and thermodynamic modelling are consistent with the observation that none of the samples contains mineral relicts indicating UHP conditions (like coesite in garnet) and neither conventional thermobarometry, nor thermodynamical modelling resulted in P-T conditions in the stability field of coesite. Thus, no evidence of ultra-high- P conditions was found. Given that the whole sampled river valley lies within the proposed 'ultra-high- P sub-belt' and considering former studies, which showed that at other places within this 'unit' both ultra-high- P and high- P rocks are now juxtaposed on a small scale, the formation of the whole (ultra-)high- P-low- T metamorphic belt in the Chinese South Tianshan as a tectonic mélange style exhumation is more convincing than the formation and juxtaposition of two coherent metamorphic units with high- P and ultra-high- P conditions respectively. [ABSTRACT FROM AUTHOR]

Pinch-and-swell structures are commonly interpreted to evolve out of viscosity contrasts, which are induced by geometric interactions and material imperfections. From materials science an additional localization phenomenon is well established, where localization emerges out of steady state for critical material parameters and/or loading rates. Here we investigate the conditions under which this second type of instabilities prevails and whether geological materials necessarily require a trigger by imperfections in order to generate instability. We focus on imperfections in terms of grain size variations embedded in a deformation environment controlled by thermo-mechanical feedbacks. We introduce a random distribution of grain sizes over two orders of magnitude in a central layer embedded in a matrix with a diffusion creep rheology. The rheology of the layer evolves with dislocation and diffusion creep as end-member deformation mechanisms. Applying pure shear extension, the 3-layer model is subjected to natural deformation conditions. The central layer quickly establishes a viscous steady state as a natural response of the system due to relaxation and energy optimization. Upon continued loading, localization then intriguingly arises out of a homogeneous state. We present an analysis which confirms that this type of instability is indeed physically admissible. Using vibration analysis, we verify the robustness of the numerical solution by first identifying the natural mode shapes and frequencies of the simulated structure and material parameters, including geometric imperfections. In a second step, the eigenmodes are perturbed and superposed to the initial conditions. We conclude that this pattern of perturbations guides the onset of strain localization. Boudinage can therefore be seen both as a geometric problem and/or a material bifurcation, which evolves out of homogeneous state. The latter class offers the great possibility of extracting fundamental material parameters out of localized structures directly from field observations. [ABSTRACT FROM AUTHOR]

Asymmetric boudins that are widespread in the Seh-Ghalatoun area in the Neyriz, southwestern Iran are good indicators to estimate the kinematic vorticity number (W) and sense of shear. This area is part of the Sanandaj-Sirjan HP-LT metamorphic belt located within the Zagros orogenic belt. Boudins have been analyzed on the meso- and micro-scales. Domino and shearband boudins are developed in deformed quartzitic layers associated with quartzo-feldspathic gneiss, amphibolite, mica-schist and marble. Shearband boudins with backward rotation and domino boudins with forward rotation, mantled porphyroclasts, drag folds, and asymmetric folds which all display a dextral sense of shear. The mean kinematic vorticity number was estimated by using the asymmetric boudin hyperbolic distribution method and quartz caxis patterns. The estimated mean kinematic vorticity number is W = W = 0.85 ± 0.04, which indicates 34% pure shear and 66% simple shear components of deformation. The estimated mean deformation temperature based on the opening angle of the quartz c-axis fabrics is 530 ± 50 °C. These quantitative analyses suggest a sub-simple shear deformation developed at the epidote-amphibolite to amphibolite facies conditions at depth along the Sanandaj-Sirjan HP-LT metamorphic belt, which is associated with a transpressional flow regime with shortening perpendicular to the shear zone boundary. [ABSTRACT FROM AUTHOR]

The Himalaya is a natural laboratory for studying mountain-building processes. Concepts of extrusion and duplexing have been proposed to dominate most phases of Himalayan evolution. Here, we examine the importance of these mechanisms for the evolution of the Himalayan crystalline core via an integrated investigation across the northern Kathmandu Nappe. Results reveal that a primarily top-to-the-north shear zone, the Galchi shear zone, occurs structurally above and intersects at depth with the Main Central thrust (MCT) along the northern flank of the synformal Kathmandu Nappe. Quartzc-axis fabrics confirm top-to-the-north shearing in the Galchi shear zone and yield a right-way-up deformation temperature field gradient. U-Pb zircon dating of pre-to-syn- and post-kinematic leucogranites demonstrates that the Galchi shear zone was active between 23.1 and 18.8 Ma and ceased activity before 18.8–13.8 Ma. The Galchi shear zone is correlated to the South Tibet detachment (STD) via consistent structural fabrics, lithologies, metamorphism, and timing for four transects across the northern margin of the Kathmandu Nappe. These findings are synthesized with literature results to demonstrate (1) the broad horizontality of the STD during motion and (2) the presence of the MCT-STD branch line along the Himalayan arc. The branch line indicates that the crystalline core was emplaced at depth via tectonic wedging and/or channel tunnelling-type deformation. We proceed to consider implications for the internal development of the crystalline core, particularly in the light of discovered tectonic discontinuities therein. We demonstrate the possibility that the entire crystalline core may have been developed via duplexing without significant channel tunnelling, thereby providing a new end-member model. This concept is represented in a reconstruction showing Himalayan mountain-building via duplexing from the Oligocene to Present. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM AUTHOR]

Purpose. To analyze the influence of structural and morphological, and elastic characteristics of defects on the features of the structure and deformation of some sandstones based on the modeling approach and interpretation of experimental data of velocity of longitudinal and transverse acoustic waves. Methodology. We have involved a series of experimental studies of density, porosity, and determination of the velocity of propagation of longitudinal and transverse acoustic waves in samples of sandstone with different structural organization. Findings. The elastic parameters of defects of solid structure were calculated based on the velocities of propagation of acoustic waves for sandstones with different effective Poisson's ratio. The normal and tangential compliances with the effective Poisson's ratio of sandstone were comparatively analyzed. Poisson's ratio of the medium matrix was calculated based on the variations of the velocities of propagation of acoustic waves under the influence of external pressure. Originality. The deformation behavior of sandstones was considered on the basis of the model of microinhomogeneous medium as an elastic matrix and planar defects inclusions. Theoretical estimates of the parameters of defects, based on the elastic constants of linear elasticity theory, indicate the adequacy of the model to describe the strain state of imperfectly elastic rock. The possibility of the existence of negative Poisson's ratio in sandstones with high normal compressibility and the contribution of the contact interactions were theoretically justified. Practical value. Theoretical estimates of the parameters of defects, based on the elastic constants of linear elasticity theory, indicate the adequacy of the model to describe the strain state of imperfectly elastic rock. There were obtained the relations for evaluation of normal and tangential compliance on the basis of effective elastic moduli microinhomogeneous medium, elastic moduli and the Poisson's ratio of the matrix medium. It is shown that the concentrations of defects in their compliance and effective Poisson's ratio have an inverse relationship, which determines the strain-elastic properties of the medium of propagation of acoustic waves. [ABSTRACT FROM AUTHOR]

The boudin segments of a torn competent layer experience synthetic rotation in layer-parallel simple shear. As long as the individual segments in a boudin train are constrained by their neighbors, even a highly viscous boudin deforms internally to create the necessary space for rotation. The rotation rate is then much smaller compared to the case of an isolated segment. Hence, a small tilt of boudin segments is not indicative of low strain. The rotation rate at this stage largely depends on the aspect ratio of the boudin segments and the scaled gap width. Once the tilted boudins are no longer constrained by their neighbors, the rotation rate greatly accelerates. In the case of a low viscosity ratio between the boudins and the host, the boudin segments develop complex shapes, which may give an impression of shear-band boudins forming under the opposite shear sense. We furthermore investigate the behavior of boudin trains of finite length. The terminal segments are displaced out of the shear plane, deforming into isoclinal folds, and separate into groups of boudin segments that rotate into the shear direction and eventually lead to an overall chaotic appearance of the structure. Natural examples of domino boudinage from a high shear –strain detachment zone in the Western Cyclades (Greece) show many similarities with the modeled structures suggesting that, under simple shear deformation, the rotation and separation of boudin segments is an indicator for high shear strain. [ABSTRACT FROM AUTHOR]

We used power-law viscous plasticine as a rock analogue to simulate chocolate tablet boudinage of rocks undergoing dislocation creep. A competent plasticine layer, oriented perpendicular to the main shortening direction, Z, underwent two phases of plane strain in a weaker plasticine matrix, with the principal stretching axis, X , and the axis of no-change, Y , replacing each other from the first to the second phase. In each phase of plane strain, boudinage was controlled by an initial phase of viscous necking followed by extension fracture along the neck domain. Increase in the magnitude of finite strain ( e ) and decrease in layer thickness ( H i ) result in a decrease in the boudin width ( W a ) and an increase in the number of boudins ( N ). Given the viscosity ratio between layer and matrix ( m ) is higher than ca. 5, the number of boudins decreases and the boudin width increases with increasing values of m . An unexpected result of the present study is that in each experiment, the number of boudins was significantly higher during the second phase of plane strain. This difference should be related to additional drag of the matrix plasticine on the stiff layer in the neck domains formed during the first phase of boudinage. The aspect ratio of the second generation of boudins ( W d = W a / H i ) is compatible with aspect ratios of natural boudins and with aspect ratios calculated using analytical solutions. [ABSTRACT FROM AUTHOR]

Abstract: With the help of 2D-finite element modeling the present study analyses the role of syntectonic migmatisation on the development of concave-face boudins within amphibolite dykes in Chhotanagpur Granite Gneiss Complex of Jasidih-Deoghar area, eastern India. Amphibolitic bands embedded in quartzofeldspathic gneiss show concave-face boudins with varied face geometries, resulted due to rheological changes during syntectonic migmatisation. Detailed study reveals that due to couple effect of H2O infiltration and potassium (K+)-metasomatism associated with the invasion of pegmatitic fluid, pyroxene converted to amphibole and later to biotite at the marginal part of the amphibolitic bands and especially, near the separation zone of boudin. In this study, three types of models are prepared to simulate three different patterns of syntectonic rheological changes that can best explain the features observed in the field. Type I is a symmetric rim model representing equal amount of rheological changes in all directions of a rectangular boudin object. Other two are asymmetric rim models (Type II and Type III) with different amount of rheological changes along length and width of the boudin block. The analysis also takes into account the effects of rate of syntectonic rheological changes (D). The study reveals that the pattern and rate of rheological changes have strong influences on the development of concave-face boudin. Type I model produces barrel-shaped fish-mouth boudin with extremely sharp corners, whereas Type III model produces more lensoid shape with relatively tighter fish mouth. For all types of model, U-shaped concave-face boudin develops at lower rate of rheological changes and the face geometry gradually transforms to V-shape with increasing the rate. The progressive change of face curvature (FC), exterior curvature (EC) and aspect ratio (AR) depends on the timing of rheological inversion during progressive deformation. [Copyright &y& Elsevier]

Abstract: We study the evolution of the Eocene–Recent Phu Khanh Basin opened during the rifting of the South China Sea (SCS). This sub-basin formed when continental crust ruptured along the East-Vietnam Boundary Fault (EVBF) at the western edge of the SCS. Using high quality long-streamer seismic lines we interpret structures that highlight the different phases of the SCS rifting and processes related to crustal boudinage. Extreme crustal thinning and mantle uplift that sometimes places sediments in contact with the Moho discontinuity mark the central part of the basin. The mantle is shallowest there and marks the final rupture of the continental crust during an intense phase of mantle upwelling. There, a low-angle detachment fault separates several crustal blocks from the Moho. The cylindrical axis of the Moho rise is roughly parallel to the trend of the South China Sea propagator. Above the mantle, the upper and lower crusts form large crustal boudins. The network of normal faults is dense in the upper crust and occasionally propagates into the lower crust. However, the lower crust is missing at some places. The seismic facies above the Moho rise is poorly stratified and might have been affected by a certain degree of metamorphism. At the apex of mantle uplift, there are frequent indications of fluid circulations, including volcanic edifices and gas escapes features. Three stages of extension are clearly identifiable, with ages of the two youngest constrained by well calibration: the first and oldest rift sequence is situated between the tilted pre-rift basement and the Oligocene horizons (32Ma); the second is delimited by the Oligocene to the Mid Miocene (15.5Ma) horizons, and the third is bound by the Mid Miocene and the Upper Miocene (before 10.5Ma) horizons. These three rift episodes formed in at least two extension directions, the first N–S and the second NW–SE. The distinct Mid Miocene (15.5Ma) horizon is tilted and the above layers show a diverging reflection. These are in turn sealed by an erosional unconformity before 10.5Ma. Although tectonic activity appears diachronous from north to south, we suggest that cessation of rifting did not occur before 12–10.5Ma. This differs from models derived from magnetic anomalies observed in the South China Sea (15.5 to 20Ma). [Copyright &y& Elsevier]

The article offers information on "International Conference on Deformation Mechanisms, Rheology and Tectonics" that was held in Leuven, Belgium from September 16-18, 2013. It further states that discussions were held after every presentation which include discrete element modeling of boudinage, and experimental deformation of harzburgite.