Shear Fabrics Research Articles

Effects of Chemical Alteration on Frictional Properties in a Deep, Granitic, Geothermal System in Cornwall: Direct Shear Experiments at Near In Situ Conditions

AbstractThe geochemical alteration of host rocks might affect the productivity and the potential for induced seismicity of geothermal systems. In addition to natural alteration, following production and heat extraction, re‐injected fluids at lower temperatures and different pressures may be in chemical disequilibrium with the rock, impacting mineral solubility and dissolution/precipitation processes. In this study, we investigate the effect of geochemical alteration on the frictional behavior of granites, and their seismogenic potential, by conducting direct shear experiments using samples with varying degrees of alteration. The samples originate from the Carnmenellis granite in Cornwall, SW England, and represent the formation used in the United Downs Deep Geothermal Power Project for heat extraction. Experiments were conducted on granite powders (referred to as gouges) at room temperature and 180°C, at simulated in situ confining and pore pressures of 130 and 50 MPa, respectively (∼5 km depth). With increasing degree of alteration, the frictional strength of the gouges decreases while frictional stability increases. At high temperature, frictional stability is reduced for all samples while maintaining the trend with alteration stage. Microstructural investigation of the sheared gouges shows alteration delocalizes shear by reducing grain size and increasing clay fraction, which promotes the formation of pervasive shear fabrics. Our work suggests that, within the range of tested pressures, more alteration of granite initially causes more stable shearing in a fault. This behavior with alteration is sustained at high temperatures, but the overall frictional stability is reduced which increases the potential for induced seismicity at higher temperatures.

Strain Localization in Sandstone‐Derived Fault Gouges Under Conditions Relevant to Earthquake Nucleation

AbstractConstraining strain localization and the growth of shear fabrics within brittle fault zones at sub‐seismic slip rates is important for understanding fault strength and frictional stability. We conducted direct shear experiments on simulated sandstone‐derived fault gouges at an effective normal stress of 40 MPa, a pore pressure of 15 MPa, and a temperature of 100°C. Using a passive strain marker and X‐ray Computed Tomography, we analyzed the spatial distribution of deformation in gouges deformed in the strain‐hardening, subsequent strain‐softening, and then steady‐state regimes at displacement rates of 1, 30, and 1,000 µm/s. We developed a machine‐learning‐based automatic boundary detection method to recognize the shear fabrics and quantify displacement partitioning between each fabric element. Our results show fabrics oriented along R1 and Y (including boundary) shears are the two major fabric elements. At rates of 1 and 30 µm/s, the relative amount of displacement on R1 shears is displacement dependent, increasing to ∼20% of the total displacement up to the strain‐softening stage, then decreasing to ∼10%–18% at the steady state. This trend is absent at the high rate where ∼18% of the displacement occurs on R1 shears throughout all investigated stages. At all rates, the relative amount of displacement on Y shears increases linearly with displacement to a total of larger than 50% at the steady state. Our study provides constraints on the development of the active slip zone, which is an important factor controlling heating and weakening associated with small‐magnitude earthquakes with limited displacement (mm‐dm), such as induced seismicity.

Finite strain, kinematic vorticity, rheological behavior and thermochronology of the Diancang Shan complex: Insights into channel flow of the southeastern Tibetan plateau

This study investigates the crustal flow within the southeastern orogenic belt of the Tibetan Plateau, with a focus on the Diancang Shan (DCS) complex. As an A-type gneiss dome, the DCS complex offers valuable insights into the deformation and evolution of intra-continental orogenic belts and the flow of deep crustal material. We have identified three main stages of deformation within the DCS complex since the Cenozoic. The initial stage (D1), spanning 41–30 Ma, was characterized by high-temperature pure shear deformation, which developed penetrative foliations, symmetric folds, and structural lenses. The subsequent major stage (D2), which shaped the overall architecture of the DCS complex from approximately 27–21 Ma, was primarily influenced by strike-slip stress regimes, resulting in shear fabrics with a top-to-the SE orientation in the releasing bend. The final stage (D3), initiated around 13 Ma, was marked by retrograde metamorphism at lower temperatures. Kinematic vorticity calculations indicate that the D1 stage was dominated by pure shear, while the D2 stage was primarily influenced by simple shear. The deformation temperature for the major deformation stage (D2) is estimated to range between 450 and 550 °C. During this stage, strain rates lie between 10−13–10−16 s−1 at 450 °C and 10−12–10−14 s−1 at 550 °C, indicating relatively low strain rates within the DCS complex. Our findings suggest that the formation of the DCS dome can be attributed to the ductile extrusion of channel flow, driven by the gravitational collapse of the Tibetan Plateau. Partial melting of the thickened middle and lower crust may have provided the channels and material sources. Furthermore, shear strain analysis (0.15–0.49) indicates that pure shear played a more significant role, contributing ∼ 65 % to the formation of the DCS dome.

Emplacement and associated sedimentary record of the Jurassic submarine salt allochthon of the Wurzeralm (Eastern Alps, Austria).

A fossil salt sheet emplaced in the Jurassic in submarine conditions is described in the Eastern Alps of Austria, providing unique insights into the emplacement of similar submarine structures and their potential control on depositional systems. The salt sheet is a plug-fed extrusion emplaced due to squeezing of a salt diapir under compression. The preserved mylonitic shear fabric in the evaporites indicates radial, south-directed emplacement of the salt sheet. Tectono-sedimentary relationships record the evolution of the salt structure, from initial diapiric growth, to salt sheet extrusion and posterior collapse. Syn-extrusion sediments record the variable bathymetry of the extruding salt sheet, with reefal carbonates building up on the crestal bulge while their deeper water equivalents accumulated on the extruding salt lobe. This is the first description of a salt allochthon still linked to its source diapir in the Eastern Alps.

Fabric controls fault stability in serpentinite gouges

SUMMARY Serpentinites are polymineralic rocks distributed almost ubiquitously across the globe in active tectonic regions. Magnetite-rich serpentinites are found in the low-strain domains of serpentinite shear zones, which act as potential sites of nucleation of unstable slip. To assess the potential of earthquake nucleation in these materials, we investigate the link between mechanical properties and fabric of these rocks through a suite of laboratory shear experiments. Our experiments were done at room temperature and cover a range of normal stress and slip velocity from 25 to 100 MPa and 0.3 to 300 µm s−1, respectively. We show that magnetite-rich serpentinites are ideal materials since they display strong sensitivity to the loading rate and are susceptible to nucleation of unstable slip, especially at low forcing slip velocities. We also aim at the integration of mechanical and microstructural results to describe the underlying mechanisms that produce the macroscopic behaviour. We show that mineralogical composition and mineral structure dictates the coexistence of two deformation mechanisms leading to stable and unstable slip. The weakness of phyllosilicates allows for creep during the interseismic phase of the laboratory seismic cycle while favouring the restoration of a load-bearing granular framework, responsible of the nucleation of unstable events. During dynamic slip, fault zone shear fabric determines the mode of slip, producing either asymmetric or Gaussian slip time functions for either fast or slow events. We report rate/state friction parameters and integrate our mechanical data with microstructural observations to shed light on the mechanisms dictating the complexity of laboratory earthquakes. We show that mineralogical and fabric heterogeneities control fault slip behaviour.

An Evolving Lithospheric‐Scale Wrench Fault System Along the Eastern End of the Altyn Tagh Fault: Kinematics and Quaternary Activity of the Heishan Fault System, Western China

AbstractThe Altyn Tagh fault (ATF) is a major intracontinental strike‐slip fault system that defines the northern margin of the Tibetan Plateau. The fault system loses its obvious surface expression north of the Qilian Shan fold‐and‐thrust belt, but may link eastward with multiple faults in the northern Hexi Corridor and southern Alxa block. To better understand the potential connectivity and displacement transfer between the ATF and the northern Hexi Corridor fault array, we carried out a multidisciplinary field and remote sensing‐based investigation of the actively deforming Heishan and Jinta'Nanshan region north of Jiayuguan City. We document Quaternary sinistral strike‐slip motion on the Heishan fault (HF) system and active sinistral transpression within a 70‐km‐long, E‐W deforming belt east of the Heishan, which is characterized by multiple left‐stepping, en‐échelon Quaternary folds and linking faults. Cosmogenic10Be dating of displaced terrace surfaces yields a vertical slip rate of ∼0.2 mm/a for the northern Heishan thrust and a sinistral strike‐slip rate of 0.6 ± 0.2 mm/a for the HF. Within the uplifted core of the Heishan massif, the HF contains ductilely deformed Paleozoic basement lithologies with dextral‐sense shear fabrics that indicate that the modern HF has reactivated and inverted an older shear zone. A previously published magnetotelluric profile across the HF system suggests that it roots into a steep, deeply penetrating fault that is unlinked to the Qilian Shan thrust wedge. Instead, we suggest that the active sinistral deformation belt of the Heishan‐Jinta'Nanshan represents the eastward‐evolving, upper crustal expression of the modern ATF system.

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.

Strain localisation and transcurrent reactivation in the granulite facies Kalinjala Shear Zone at Port Neill, South Australia

The central portion of the Kalinjala Shear Zone, in the Port Neill region, South Australia, is recognised as a northeast-trending anomalously magnetic corridor coinciding with a sub-vertical transpressional dextral shear zone. This shear zone is a primary lithospheric structure, separating the nucleus of the Archean Gawler Craton from a suite of Paleoproterozoic granites (ca 1850 Ma) and earlier paragneiss. At Port Neill, there is an apparent 17 km dextral offset of a pre-existing Bouguer gravity anomaly by the Kalinjala Shear Zone. This is attributed to Kimban-aged (ca 1740–1690 Ma) deformation that is accompanied by peak granulite facies metamorphic conditions reaching pressures of ∼1.0 GPa and temperatures of ∼800 °C. This is consistent with the presence of leucosomes, within the sheared fabric, in the axial surface of mylonitic folds and in mafic gneisses. Strain has been localised within and adjacent to mafic dykes, predominantly hosted in the granitic rocks, with the highest strained areas having the greatest evidence of leucosome extraction. The local orientation of stretching lineations in this shear zone do not necessarily correlate with the transport direction but may reflect along- and across-strike variations in finite shear strain into alternating zones dominated by either constrictional or a superimposed flattening strain. Overprinting the dominant dextral movement are sinistral shears, with horizontal strike-slip movements, which post-date the peak metamorphic assemblages and are generally confined to sites containing late K-feldspar-rich pegmatites. The major movement recorded in the Kalinjala Shear Zone was both horizontal and vertical, occurring in response to regional exhumation of the Cornian-aged (ca 1850 Ma) sequences in the east, during the Kimban Orogeny, and was basically controlled by the nature of lithospheric plate convergence during accretion of a continental backarc onto the Gawler Craton. KEY POINTS Dextral transpression with late-sinistral along-strike extension and east-up exhumation is associated with 17 km of horizontal offset along the central section of the Kalinjala Shear Zone. Deformation is dominated by three major Kimban-aged (ca 1730–1690 Ma) events, which overprint older ensialic backarc metasedimentary rocks, orthogneiss and granites of the Donington Suite. Granulite facies metamorphic conditions control the nature of deformation and strain distribution within the shear zone. The Kimban events record the collision of the Gawler Craton nucleus with the southern margin of the North Australian Craton.

Pore fabric anisotropy and elastic moduli of fault rocks from the Median Tectonic Line, Shikoku, southwest Japan

Pores and cracks have an important role in the evolution of fault rocks because they strongly influence the behavior of the fluids that promote rock alteration and trigger the mechanical instability of faults. We used rock physics model inversion of measured elastic wave velocity and porosity to estimate the grain elastic moduli and crack aspect ratios of a range of fault rocks (intact rocks, fractured rocks, transition rocks, and fault gouge) from the Median Tectonic Line in southwest Japan. Our results show distinct gaps in the evolutionary trends of crack aspect ratios and grain elastic moduli from intact rocks to fault rocks. Crack aspect ratios show a nonlinear trend from intact rock to fault gouge, and then these values in fault gouge were considerably higher than in fractured rock and transition rock. In contrast, grain elastic moduli decreased as fracture evolved with the development and subsequent extinction of shear planes and then increased markedly with the formation of fault gouge. Our results show that crack aspect ratios and grain elastic moduli are clearly related to the evolution of shear fabrics in faults. Therefore, they might be useful indicators of fault activity and maturity.

The transition from salt diapir to weld and thrust: Examples from the Northern Flinders Ranges in South Australia

AbstractThe interactions between salt diapirs, thrust welds and thrusts in contractional belts are poorly understood due to, first, the inability of seismic data to distinguish between thrusts and welds or resolve associated sub‐resolution deformation, and second, the paucity of good field examples. The Warraweena area in the Northern Flinders Ranges of South Australia contains examples of Neoproterozoic to Early Cambrian squeezed diapirs linked by steep reverse faults formed during the Delamerian Orogeny. Benefiting from good field exposures, we use geological mapping, cross‐section construction and conceptual structural models to assess the three‐dimensional geometry and evolution of the structures, the lateral transition from diapirs to linking faults and the variability of associated meso‐ and small‐scale deformation. Three discrete diapirs consist of narrow outcrops of Callanna Group megabreccia (Willouran in age) up to 5‐km long. Their diapiric origin is confirmed by local development of caprock, steepening of flanking strata in composite halokinetic sequences and reworked diapir and roof debris in adjacent strata. The surrounding rocks display only background levels of small‐scale deformation. In contrast, the linking faults show no evidence of precursor diapirism, have fault‐related anticlines up to 100s of m in wavelength in their hanging walls, and an associated increase in small‐scale deformation (i.e. millimetre to metre scale folds, fractures and shear fabrics). The transitions from diapirs to faults occur within less than 200 m as short thrust welds at the diapir terminations. The exposed structures are analogous to those found on the subsurface of other salt basins such as the Gulf of Mexico and the South Atlantic conjugate margins. The results of this work can aid geoscientists evaluating three‐way traps against squeezed diapirs, welds or faults, and can help them to predict the style and abundance of both halokinetic and small‐scale structures that are below seismic resolution.

Cenozoic deformation in the Tethyan Himalaya, SE Tibet: Insights from magnetic fabrics and structural analysis of Upper Triassic flysch

Anisotropy of magnetic susceptibility (AMS) analyses and structural observations of Triassic flysch in the Tsetang area, SE Tibet, were carried out to assess the Cenozoic tectonic evolution and kinematics of deformation of the eastern Tethyan Himalaya Thrust Belt. We analysed a total of 202 standard-sized specimens from four sample sites across the Renbu–Tsetang Thrust from north to south. Rock magnetism and the detailed mineralogy demonstrated that ferromagnetic and paramagnetic minerals both contribute to the AMS. However, the paramagnetic contribution is dominant in the study area. The AMS results, together with field-based structural investigations and analyses of microstructures, revealed that layer-parallel shortening (LPS) was accommodated mainly by isoclinal folds, cleavage, and thrust faults. Shear fabrics (K1 axes oriented parallel to the direction of local tectonic transport) were found mainly in the north thrust fault zone and were linked to fault movement. Microstructural analyses of quartz and metamorphic minerals such as muscovite (sericite) and chlorite in slates and sandstones imply that the peak temperature during the deformation did not exceed 300 °C. The shear AMS fabric in north Qonggyai, near the thrust zone, shows that the strain variation is related to the main activity (11–19 Ma) of the Renbu–Tsetang Thrust. The magnetic lineation that plunges towards the east in the southern Qonggyai area indicates extension that was linked to an E–W extensional event during the early Miocene. Based on the available geochronological data, two stages of tectonic deformation can be defined: (a) syncollisional layer-parallel shortening at ~40 Ma ago, and (b) E–W extension and continuing compaction resulting in shearing at 11–19 Ma.

Element mobility as a proxy for delineating shear zone boundaries in orogenic belts: A case study of Iwaraja shear zone, southwestern Nigeria

Shear zones are localised regions of intense deformation and are important archives of element mobility history during crustal deformation. The boundary is determined by examining shear deformation fabrics in affected rocks. However, the "mixture" of relicts of undeformed rocks coupled with newly formed shear fabrics affect the determination of the internal boundaries of the shear zone and its boundary with the unsheared protolith, which in turn limits the evaluation of the shear zone's width. A new approach involving element mobility across the shear zone is proposed as a proxy to address this problem and corroborate existing methods. The petrographic and geochemical analyses of granitic gneiss across the Iwaraja shear zone and its adjacent rock were carried out. The element mobility patterns and volume of chemical changes were evaluated using isocon and Cartesian element-distance plots to interpret the chemical changes in the granitic gneiss across the shear zone. The result indicates a very high enrichment of TiO2, Fe2O3, MnO, MgO, P2O5, and REEs with relatively similar mobility patterns and a high depletion of Na2O, Rb, and Th in the shear zone. Al2O3 is not significantly mobilized but the mobility pattern of K2O is erratic. Based on the chemical changes in the granitic gneiss, the ductile shear zone is a CaO–MgO–Fe2O3-REE-enriched and Na2O–Rb–Th-depleted system. Under greenschist facies condition, the chemical mobility of TiO2, MnO, P2O5, Th, Rb, Zn, Ba, Zr, Th, Hf, and REEs is sensitive to the changes in the deformation intensity between the strongly- and weakly-deformed zones whereas REE [Lu, Yb, Gd, Eu, Ho, Tb] is useful to determine the boundary between the weakly-deformed zone and the unsheared rock. These index minerals are useful in other similar ductile shear zones in any orogenic belt. Based on the element mobility patterns, the width of the affected granite gneiss in the shear zone is ~680 m.

Polygenetic mélange in the retrowedge foredeep of an active arc-continent collision, Coastal Range of eastern Taiwan

The Plio-Pleistocene Lichi Mélange in the Coastal Range of eastern Taiwan offers an excellent opportunity to study processes of mélange development at the continent-ocean interface of an active arc-continent collision. This paper presents new results of detailed geologic mapping, lithofacies analysis, magneto-biostratigraphy, paleocurrent, and paleoslope analyses in the southern Coastal Range to investigate the origins and significance of this mélange. The results show that the Lichi Mélange consists of mass-transport deposits including well-stratified block-in-matrix beds (olistostromes), extra-formational blocks (olistoliths), and broken formation with abundant soft-sediment deformation features that transition laterally into distal mega-slump beds and pebbly mudstones (subaqueous debrites). Abundant observations of depositional contacts and interbedding of mélange with contemporary (ca. 4–1 Ma) flysch units of the Fanshuliao and Paliwan formations confirm their sedimentary origin. Compacted sedimentological shear fabrics in olistostromal facies are broadly parallel to internal stratification and bedding, and are readily distinguishable from cross-cutting brittle fault zones related to post ~1 Ma west-vergent thrust faults. Paleoslope and paleocurrent analyses record down-slope gravity-driven transport toward the east and southeast.The data provide evidence for a polygenetic origin of the Lichi Mélange, in which sedimentary mass-wasting deposits are overprinted by younger tectonic shear zones. Slide blocks, conglomerate clasts, and detrital sand were all derived from an eroding source in the east-vergent eastern retrowedge of the Taiwan collisional orogen. The source area included tectonically accreted fragments of the two converging plates that represent shallow-crustal equivalents of the Miocene Yuli Belt and Eastern Slates exposed in the modern Central Range. Reconstructed stratigraphic panels record eastward progradation of olistostromal facies over distal basinal flysch deposits, which we infer resulted from eastward (oceanward) migration of a steep submarine slope at the leading edge of the retrowedge orogenic front. Thus, the Coastal Range basin evolved as a migrating retro-foredeep basin that formed on top of older, pre-collisional volcanic arc and forearc crust. These results demonstrate a unique type of sedimentary basin that is formed and then rapidly inverted at a convergent continent-ocean interface during the transition from intra-oceanic subduction to arc-continent collision. This revised history of the Lichi Mélange provides a new perspective on the dynamics of rapid crustal mixing and tectonic recycling at the convergent suture of an active arc-continent collision system.

Study of small-scale structures and their significance in unravelling the accretionary character of Singhbhum shear zone, Jharkhand, India

Localized strain within tabular ductile shear zones is developed from micro- to meso- to even large scales to form complex structures. They grow in width and length through linkage of segments with progressive accumulation of strain and displacement, and finally produce shear zone networks characterized by anastomosing patterns. Singhbhum shear zone (SSZ) represents a large composite zone characterized by a collage of different dismembered lithotectonic segments, with heterogeneous structural features, within a matrix typical of a shear zone. Structural features indicate that the material properties of protoliths have a great role in controlling the mechanics of deformation. Meso- and micro-scale structural studies of the east-central part of the SSZ reveal ‘tectonic complex like’ (? deeper level equivalent of melange type complex) assemblage of dismembered lithoteconic units. Shear-induced foliations, S, C and C′, were developed while the main mylonitic foliation is represented by C-plane. Apart from that, shear lenses are exceptionally well developed in both meso- and micro-scale in most of the units, particularly in schistose rocks. They were formed from different processes during progressive simple shear, which includes (1) anastomosing C-planes, (2) intersection between C- and C′-planes, (3) disruption of stretched out longer limbs of asymmetric folds, and (4) cleavage duplex. Fabrics recorded in rocks indicate that there was a progressive change in the development of predominantly flattening fabric (coaxial pure shear) in the northern part (outside the SSZ), to simple-shear non-coaxial type deformation producing shear fabric, dominating over the flattening fabric, in the southern part (within the SSZ) that is in close proximity with the Singhbhum Craton. Although an overall plane strain simple shear model is apparent, occasional presence of extensional features along two directions of the mylonitic foliation, demonstrative of three-dimensional deformation (simple shear and flattening: X > Y > 1 > Z), may indicate the stretching nature of the SSZ. From the orientation of oblique grain shape fabric [ISAmax (θ < 45°)], there is slight deviation from simple shear, i.e., a sub-simple nature of plane strain shear could be inferred. However, in conformity with simple shear model the ubiquitously developed stretching lineation shows consistency in orientation being parallel to the movement direction. There is no evidence of transpression. Shear sense indicators invariably indicate up-dip ductile thrust movement with vergence top-to-the south. Microstructural deformational characteristics indicate that peak temperature attained during the deformation in shear zone was ~600 °C. Prolonged period of metasomatism, induced by fluid influx, played an important role in strain softening during the development of SSZ.

Thrust Shear Sense and Shear Zone Fabrics in the Higher Himalaya, Siyom Valley, Eastern Arunachal Himalaya, India

Abstract The Higher Himalayan Crystallines (HHC) in the Siyom river section of eastern Arunachal Himalaya, display top-to- ESE compressional ductile shear from Pene to Menchuka. In this part of the Himalayan Metamorphic Belt (HMB), the base of the Main Central Thrust (MCT) is the southern boundary of the ductile shear fabrics in HHC. Across the MCT, a significant break in the grade of metamorphism is observed between the HHC and the Lesser Himalayan Sequences (LHS). The ductile shear fabrics documented in the HHC are primary S-C and S-C′ fabrics, asymmetric folds, porphyroclasts and porphyrobalsts, brittle- ductile structures and asymmetric boudins. Three phases of folding representing three deformation episodes (D1-D3) in HHC are recorded in the area. The rocks of the HHC in Siyom valley are completely transposed by the D2 deformation into NNW dipping C- plane. Ductile shear represented by grain scale structures include sigmoidal foliation, rotation of the inclusion trails in garnet porphyroblasts, asymmetric folds with consistent top-to-ESE sense of shear. The ductile to brittle-ductile shear fabrics in the area indicate that the thrust sense of shear in the HHC is consistent without any shear sense reversal from Pene to Menchuka. These compressional shear fabrics provide invaluable field evidences for constraining the evolution of this part of the Higher Himalaya.

The Role of Shear Fabric in Controlling Breakdown Processes During Laboratory Slow‐Slip Events

AbstractUnderstanding the physical mechanisms at the origin of slow‐slip events has been proven a very challenging task. In particular, little is known on the role of fault heterogeneity during slow slip. In this study, we provide evidences that fault fabric controls slip velocity time histories during slow‐slip events generated in the laboratory. We performed experiments using a double‐direct biaxial shear apparatus and two different fault gouges, homogeneous quartz powder, and heterogeneous anhydrite/dolomite mixture. We measure details of fault slip to resolve the slip velocity function and volumetric deformation that, coupled with an analysis of the resulting microstructure, allow us to infer the mechanical processes at play. Our results show that slow‐slip events can be generated for both fault gouges when k ~ kc with similar values of breakdown work. The shear fabric exerts a strong influence during the coseismic breakdown stage. In quartz, where most of the slip occurs on a very localized slipping surface, the peak slip velocity is attained near the final stage of friction breakdown and therefore a relevant amount of the mechanical work is absorbed during slip acceleration. In anhydrite/dolomite mixture, the peak slip velocity is suddenly reached after a relatively small drop in friction, accompanied by fault dilation, implying that most of the mechanical work is absorbed during slip deceleration. For anhydrite/dolomite mixture these results are likely related to heterogeneous slip distribution along the observed foliation. Taken together, these observations suggest that the mechanics of slow‐slip events depends on shear zone fabric.

Tectonic link between the Neoproterozoic dextral shear fabrics and Cenozoic extension structures of the Mekelle basin, Northern Ethiopia

The Mekelle basin (area similar to 5500 km(2)) is located at the immediate western margin of the Afar Depression, which has a history of collision, subsidence, uplift, magmatism and now rifting. The evolution of such small to medium size basins as the Mekelle in a complicated geodynamic set-up is a subject of debate, because several different mechanisms of basin formation could apply. To address the possible basin evolution scenarios, we combine structural and dynamic analyses of northern Ethiopia, with strong emphasis on the basin and basin bounding basement rocks. The northern basin bounding block is uplifted by about 1500 m compared with the western and southwestern basin-bounding blocks. In the basin, superposition of large-scale orthogonal brittle shear fabrics and the development of young, parallel to subparallel extensional fractures and domino faulting have produced a total of similar to 2200 m vertical displacement. These observations, coupled with the absence of gradual roll-down of the internal basin (i.e., absence of bending/folding of an elastic beam supported at both ends), imply that Mekelle basin is not an IntraCONtinental Sags (ICONS) or plate interior basin, as previously described, but is instead a multi-tectonic triggered basin. Integrated satellite images using ground-controlled measurements and deep borehole data were used to decipher the entire evolution of the basin starting from the late-collisional deformation through the regional up-doming to the sills/dikes injection and basin formation.

Evolution of Elastic and Mechanical Properties During Fault Shear: The Roles of Clay Content, Fabric Development, and Porosity

AbstractPhyllosilicates weaken faults due to the formation of shear fabrics. Although the impacts of clay abundance and fabric on frictional strength, sliding stability, and porosity of faults are well studied, their influence on elastic properties is less known, though they are key factors for fault stiffness. We document the role that fabric and consolidation play in elastic properties and show that smectite content is the most important factor determining whether fabric or porosity controls the elastic response of faults. We conducted a suite of shear experiments on synthetic smectite‐quartz fault gouges (10–100 wt% smectite) and sediment incoming to the Sumatra subduction zone. We monitored Vp, Vs, friction, porosity, shear and bulk moduli. We find that mechanical and elastic properties for gouges with abundant smectite are almost entirely controlled by fabric formation (decreasing mechanical and elastic properties with shear). Though fabrics control the elastic response of smectite‐poor gouges over intermediate shear strains, porosity is the primary control throughout the majority of shearing. Elastic properties vary systematically with smectite content: High smectite gouges have values of Vp ~ 1,300–1,800 m/s, Vs ~ 900–1,100 m/s, K ~ 1–4 GPa, and G ~ 1–2 GPa, and low smectite gouges have values of Vp ~ 2,300–2,500 m/s, Vs ~ 1,200–1,300 m/s, K ~ 5–8 GPa, and G ~ 2.5–3 GPa. We find that, even in smectite‐poor gouges, shear fabric also affects stiffness and elastic moduli, implying that while smectite abundance plays a clear role in controlling gouge properties, other fine‐grained and platy clay minerals may produce similar behavior through their control on the development of fabrics and thin shear surfaces.

Crustal seismic anisotropy of the Northeastern Tibetan Plateau and the adjacent areas from shear-wave splitting measurements

SUMMARYThe Northeastern Tibetan Plateau has thickened crust and is still undergoing strong active crustal shortening and deformation. Crustal anisotropy can provide clues to how the crust is currently deforming and evolving. We use an automatic method to analyse the upper-crustal anisotropy of the NE Tibetan Plateau and the adjacent region using local earthquakes recorded at 39 permanent seismic stations during the period 2009–2018. The majority of the dominant fast directions are consistent with the maximum horizontal stress orientation, suggesting that the upper-crustal anisotropy is mainly controlled by the regional or local stress field. Several fault-parallel measurements are observed for stations on or near to the main faults. These fault-parallel fast directions indicate that the main mechanism of upper-crustal anisotropy is associated with shear fabric caused by deformation. Fast directions neither fault-parallel nor stress-parallel are observed at stations lying several kilometres away from fault zones, likely reflecting the combined influence of stress-aligned microcracks and active faults. A comparison between our upper-crustal anisotropy parameters and those inferred from previous anisotropy studies that used receiver function and teleseismic shear wave splitting measurements suggests that the crust has the same deformation mechanisms as mantle anisotropy in the southern part of the Western Qinling Fault, whereas the upper-crustal anisotropic mechanism is different from those of lower crust and mantle anisotropy in the northern part of the Western Qinling Fault. These observations imply that the Western Qinling Fault may be an important boundary fault.

Orogenic Gold in Transpression and Transtension Zones: Field and Remote Sensing Studies of the Barramiya–Mueilha Sector, Egypt

Multi-sensor satellite imagery data promote fast, cost-efficient regional geological mapping that constantly forms a criterion for successful gold exploration programs in harsh and inaccessible regions. The Barramiya–Mueilha sector in the Central Eastern Desert of Egypt contains several occurrences of shear/fault-associated gold-bearing quartz veins with consistently simple mineralogy and narrow hydrothermal alteration haloes. Gold-quartz veins and zones of carbonate alteration and listvenitization are widespread along the ENE–WSW Barramiya–Um Salatit and Dungash–Mueilha shear belts. These belts are characterized by heterogeneous shear fabrics and asymmetrical or overturned folds. Sentinel-1, Phased Array type L-band Synthetic Aperture Radar (PALSAR), Advanced Space borne Thermal Emission and Reflection Radiometer (ASTER), and Sentinel-2 are used herein to explicate the regional structural control of gold mineralization in the Barramiya–Mueilha sector. Feature-oriented Principal Components Selection (FPCS) applied to polarized backscatter ratio images of Sentinel-1 and PALSAR datasets show appreciable capability in tracing along the strike of regional structures and identification of potential dilation loci. The principal component analysis (PCA), band combination and band ratioing techniques are applied to the multispectral ASTER and Sentinel-2 datasets for lithological and hydrothermal alteration mapping. Ophiolites, island arc rocks, and Fe-oxides/hydroxides (ferrugination) and carbonate alteration zones are discriminated by using the PCA technique. Results of the band ratioing technique showed gossan, carbonate, and hydroxyl mineral assemblages in ductile shear zones, whereas irregular ferrugination zones are locally identified in the brittle shear zones. Gold occurrences are confined to major zones of fold superimposition and transpression along flexural planes in the foliated ophiolite-island arc belts. In the granitoid-gabbroid terranes, gold-quartz veins are rather controlled by fault and brittle shear zones. The uneven distribution of gold occurrences coupled with the variable recrystallization of the auriferous quartz veins suggests multistage gold mineralization in the area. Analysis of the host structures assessed by the remote sensing results denotes vein formation spanning the time–space from early transpression to late orogen collapse during the protracted tectonic evolution of the belt.