Sinistral Shear Research Articles

Sinistral strike-slip shearing along the Jiali shear zone around the Eastern Himalaya syntaxis region: Evidences for oligocene eastward limited translation of Tibet

In and around the Eastern Himalaya Syntaxis (EHS), the metamorphic complex and its structural boundaries are key tectonics to investigate the kinematics and thermal evolution of the continental collision. We provide new structural, kinematic and geochronological data along the northeastern boundary of the EHS, namely the Jiali strike-slip shear zone. Structures and various kinematic indicators record sinistral strike-slip shearing in the zone. Based on the microstructural features, crystallographic preferred orientation patterns of quartz and their slip systems, coupled with Raman Spectroscopy of Carbonaceous Materials (RSCM) geothermometry, the ductile sinistral shearing fabrics formed at upper greenschist and amphibolite facies (>500 °C). Two hornblende, one muscovite and five biotite separates from the mylonites in the shear zone were dated by the 40Ar/39Ar method. A hornblende plateau age of 23.7 Ma is interpreted as representing or being close to the time of sinistral movement. Muscovite and biotite 40Ar/39Ar ages from 22.9 to 16.9 Ma represent cooling ages following sinistral shearing. It is concluded on the basis of these data that the Jiali sinistral strike-slip shear zone was active in latest Oligocene. This sinistral movement on the Jiali shear zone is considered to link with the Ailao Shan-Red River shear zone, and form a unique, continuous structural boundary around the EHS. The Jiali-Ailao Shan-Red River sinistral shear zone and Gaoligong dextral shear zone formed a huge conjugate strike-slip shear pair around the EHS to accommodate the N–S shortening corresponding to northward indentation of the folded Indian continent. The conjugate strike-slip shear systems also contribute to transporting Tibet southeastward in a limited scale.

Control of pre-existing fabric in fracture formation, reactivation and vein emplacement under variable fluid pressure conditions: an example from Archean greenstone belt, India

Abstract. Most of the upper crustal fluid flows are strongly influenced by the pre-existing fractures/foliations in the rocks under a certain state of tectonic stress and fluid pressure condition. In the present study, we analyzed a wide range of crosscutting fractures that are filled with quartz veins of variable orientations and thicknesses, from the gold-bearing massive metabasalts (supracrustals) of the Chitradurga Schist Belt adjacent to the Chitradurga Shear Zone (CSZ), Western Dharwar Craton, southern India. The study involves the following steps: (1) analyzing the internal magnetic fabric, using anisotropy of magnetic susceptibility (AMS) studies, and determining strength of the host metabasalts, (2) quantifying the fluid pressure condition through lower hemisphere equal area projection of pole to veins by determining the driving pressure ratio (R′), stress ratio (ϕ), and susceptibility to fracturing, and (3) deciphering the paleostress condition using fault-slip analysis. We interpret the NNW–SSE to NW–SE (mean 337/69∘ NE) oriented magnetic fabric in the rocks of the region as having developed during regional D1/D2 deformation on account of NE–SW shortening. However, D3 deformation manifested by NW–SE to E–W shortening led to the sinistral movement along CSZ. As a consequence of this sinistral shearing, fractures with prominent orientations formed riedel shear components, with CSZ as the shear boundary. Subsequently, all the pre-existing fabrics along with the riedel shear components were reactivated and vein emplacement took place through episodic fluid pressure fluctuation from high to low Pf at shallow depth (∼ 2.4 km). However, NNW–SSE orientations were prone to reactivate under both high- and low-Pf conditions, thereby attaining maximum vein thickness along these orientations. The deduced paleostress from fault-slip analysis along with the kinematics of the fractures and veins are in good agreement with previously estimated regional tectonics. Thus, integrating multiple domains of studies helps in the logical interpretation of fluid flow conditions and vein emplacement mechanisms in the study area that has not been ventured before.

The Anatomy and Origin of a Synconvergent Grenvillian-Age Metamorphic Core Complex, Chottanagpur Gneiss Complex, Eastern India

Abstract Amphibolite facies supracrustal rocks interleaved with granite mylonites constitute a shallowly dipping carapace overlying granulite facies anatectic basement gneisses in the Giridih-Dumka-Deoghar-Chakai area that spans ~11,000 km2 in the Chottanagpur Gneiss Complex (CGC). Steep N-trending tectonic fabrics in the gneisses include recumbent folds adjacent to the overlying carapace. The basement and carapace are dissected by steep-dipping sinistral shear zones with shallow/moderately plunging stretching lineations. The shear zones trend NNE in the north (north-down kinematics) and ESE in the south (south-down kinematics). Chemical ages in metamorphic monazites in the lithodemic units are overwhelmingly Grenvillian in age (1.0–0.9 Ga), with rafts of older domains in the basement gneisses (1.7–1.45 Ga), granitoids (1.4–1.3 Ga), and the supracrustal rock (1.2–1.1 Ga). P-T pseudosection analysis indicates the supracrustal rocks within the carapace experienced postthrusting midcrustal heating (640–690°C); the Grenvillian-age P-T path is distinct from the existing Early Mesoproterozoic P-T path reconstructed for the basement gneisses. Quartz opening angle thermometry indicates that high temperature (~600°C) persisted during deformation in the southern shear zone. Kinematic vorticity values in carapace-hosted granitoid mylonites and in steep-dipping shear zones suggest transpressional deformation involved a considerable pure shear component. Crystallographic vorticity axis analysis also indicates heterogeneous deformation, with some samples recording a triclinic strain. The basement-carapace composite was extruded along an inclined channel bound by the steep left-lateral transpressional shear zones. Differential viscous extrusion during crustal shortening coupled with the collapse of the thickened crust caused midcrustal flow along flat-lying detachments in the carapace.

Lithological and structural controls on the emplacement of a Neoarchean plutonic complex in the Carajás province, southeastern Amazonian craton (Brazil)

New geological and geochronological (Pb-Pb evaporation) data from the Vila Jussara suite, a representative of the Neoarchean granitoids of the Carajás province, southeastern Amazonian craton (Brazil), are discussed. The Vila Jussara suite crops out as a series of coalescing plutons elongated in the E-W direction. These plutons generally have sigmoidal shapes, and steep dips (75–85°) that follow the regional trend. The centermost areas of plutons are generally slightly deformed, while the marginal portions display mylonitic aspect and are delimited by sinistral shear zones belonging to the transcurrent system of the Itacaiúnas shear belt. These granitoids cover a large compositional spectrum, with four main lithotypes: (i) Seriated biotite-hornblende monzogranite; (ii) biotite-hornblende tonalite; (iii) porphyritic biotite monzogranite-granodiorite; and (iv) porphyritic hornblende-biotite monzogranite-granodiorite. The microstructural analysis of the studied rocks reveals: (i) microfractures in feldspar phenocrysts, filled with a quartz-feldspar matrix; (ii) preferred orientation of euhedral feldspar crystals in an igneous matrix; and (iii) evidence of moderate-to high-temperature of solid-state deformation (>500 °C). These microstructures and field observations point to a continuum of deformation, from the (sub) magmatic state down to sub-solidus conditions. Dating of three types of granitoids out of four gives ages of 2754 ± 2.2 Ma, 2752 ± 5.7 Ma and 2745 ± 3 Ma. The different granitoid varieties of the Vila Jussara suite were thus emplaced simultaneously. It is proposed that emplacement of the granitoids was controlled by shear zones reactivated during oblique collision and that plutons where constructed by multiple injections of magmas, generating extensive hybridization.

Modification of pre-existing folds in a shear zone: A case study from Kumbhalgarh–Ranakpur area, South Delhi Fold Belt, Rajasthan, India

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

Deformation conditions and kinematics of fault rocks in the Hatagawa Fault Zone and the Shajigami Shear Zone, Northeast Japan: Insights from calcite and quartz microstructures

Calcite grains are plastically deformed at lower temperatures than are quartz grains, which means that investigating the microstructures of both minerals should provide useful information regarding the deformation history of faults. We investigated the conditions of deformation and kinematics of fault rocks within the Hatagawa Fault Zone and the Shajigami Shear Zone, Northeast Japan, using deformation microstructures and crystallographic preferred orientations of quartz and calcite. The Hatagawa Fault Zone trends NNW–SSE, and the associated foliated granite cataclasites show sinistral shear. Limestone mylonite samples showing sinistral shearing display girdles of calcite c-axes close to the YZ plane and Z-maximum, suggesting relatively high and low temperatures of deformation, respectively. The Shajigami Shear Zone trends NE–SW, and quartz microstructures in granodiorite mylonites of the shear zone indicate dislocation creep at temperatures of 400–500 °C. The limestone mylonites have Z-maximum patterns of calcite c-axes. These fault rocks were deformed during the Cretaceous (before the Cenozoic 40° counterclockwise rotation of Northeast Japan) in a kinematic framework characterized by a shortening direction that rotated from N–S to NW–SE. The kinematics and deformation ages of the fault rocks correspond to a change in the direction of motion of the Izanagi Plate at c. 100 Ma.

Tectonic evolution of the Chinese Tianshan Orogen from subduction to arc-continent collision: Insight from polyphase deformation along the Gangou section, Central Asia

AbstractThe Central Asian Orogenic Belt, as the largest accretionary orogen on Earth, is an ideal candidate to study the geodynamics of convergent plate boundaries through a prolonged period. The evolution of this orogen has been explained by different tectonic models, which incorporated one, or a combination, of the following mechanisms: lateral stacking of arc systems along major shear zones, arc amalgamation, oroclinal bending, and trench migration. Here we elucidate major mechanisms responsible for the tectonic evolution of the Central Asian Orogenic Belt, focusing on the Chinese Tianshan Orogen in the southern Central Asian Orogenic Belt. Structural observations from the ∼50-km-long Gangou section show evidence of polyphase deformation. The earliest episode of orogen-parallel sinistral shearing, constrained to the Early Devonian (ca. 399 Ma) by syn-deformational intrusions, was possibly controlled by oblique subduction. This was followed by an episode of ∼NE–SW contractional deformation, dated at ca. 356 Ma (40Ar/39Ar age of syn-deformational hornblende), and likely associated with an episode of trench advance. The third stages of deformation during the latest Carboniferous and Permian involved ∼NE-SW contraction, orogen-parallel extension, and dextral transpression. Our new geochronological data constrain the timing of orogen-parallel extension to ca. 303–293 Ma, and confirm that dextral activation along shear zones occurred during the Permian. The results highlight the role of trench migration, oblique tectonics, and syn-collisional orogen-parallel extension in the build-up of the Central Asian Orogenic Belt, and contribute to the pre-collisional reconstruction of this orogenic system.

CCAF-DB: the Caribbean and Central American active fault database

Abstract. A database of ∼250 active fault traces in the Caribbean and Central American regions has been assembled to characterize the seismic hazard and tectonics of the area, as part of the Global Earthquake Model (GEM) Foundation's Caribbean and Central American Risk Assessment (CCARA) project. The dataset is available in many vector GIS formats and contains fault trace locations as well as attributes describing fault geometry and kinematics, slip rates, data quality and uncertainty, and other metadata as available. The database is public and open source (available at: https://github.com/GEMScienceTools/central_am_carib_faults, last access: 23 March 2020), will be updated progressively as new data become available, and is open to community contribution. The active fault data show deformation in the region to be centered around the margins of the Caribbean plate. Northern Central America has sinistral and reverse faults north of the sinistral Motagua–Polochic fault zone, which accommodates sinistral Caribbean–North American relative motion. The Central Highlands in Central America extend east–west along a broad array of normal faults, bound by the Motagua–Polochic fault zone in the north and trench-parallel dextral faulting in the southwest between the Caribbean plate and the Central American forearc. Faulting in southern Central America is complicated, with trench-parallel reverse and sinistral faults. The northern Caribbean–North American plate boundary is sinistral off the shore of Central America, with transpressive stepovers through Jamaica, southern Cuba and Hispaniola. Farther east, deformation becomes more contractional closer to the Lesser Antilles subduction zone, with minor extension and sinistral shear throughout the upper plate, accommodating oblique convergence of the Caribbean and North American plates.

The Relationship Between Magmatism and Deformation Along the Intra‐arc Strike‐Slip Atacama Fault System, Northern Chile

AbstractMagmatic arcs may play a major role in the initiation, behavior, and abandonment of intra‐arc strike‐slip systems. Here we present zircon U‐Pb and (U‐Th)/He geochronology/thermochronology with new mapping to relate Coastal Cordillera arc magmatism to sinistral shear along the Atacama fault system (AFS) in northern Chile. New dates from 18 intrusions along the AFS between 24.6°S and 27°S compiled with published data record a minor Early Jurassic magmatic pulse (195–175 Ma), broad latest Jurassic to Early Cretaceous (150–120 Ma) pulse, and a minor younger (120–105 Ma) pulse. Mylonitization occurred only along the margins of Early Cretaceous plutons and surrounding Paleozoic metasedimentary rock, whereas Jurassic plutons and metasedimentary rocks away from Early Cretaceous plutons lack mylonitic fabrics. Early Cretaceous magmatism facilitated AFS deformation by thermally weakening the crust with elevated geothermal gradients that enabled mylonitization to take place at ~5‐ to 7‐km depths and low stresses. Spatial variability of pluton emplacement produced significant rheological heterogeneity, giving rise to a highly segmented fault system that did not originate as a regional‐scale shear zone. Synkinematic dikes (~120–117 Ma) cut mylonitic fabrics, and a postkinematic dike (~110 Ma) records the end stages of slip. The cessation of slip coincided with cooling below ~180 °C at ~116–99 Ma as arc magmatism migrated eastward and geothermal gradients relaxed, coeval with a major reorganization in plate motion and the onset of seafloor spreading in the south Atlantic.

Tectonic evolution of the Cretaceous Gyeongsang Back-arc Basin, SE Korea: Transition from sinistral transtension to strike-slip kinematics

The Gyeongsang Basin, the largest Cretaceous nonmarine sedimentary basin in Korea, formed as a continental back-arc basin related to NNW-directed subduction of the (proto-) Pacific Plate underneath the Eurasia Plate. The basin can be divided into an earlier-formed western back-arc depression and a later-formed eastern volcanic arc platform. We investigated the basin evolution and the associated tectonic settings, largely based on an analysis of structures individuated in the field in the back-arc region. From 127 Ma, the basin initiated as a NNE-trending narrow depression bordered by NNE-striking sinistral faults, and then progressively expanded under a transtensional kinematics induced by progressive trench roll-back. Sinistral shearing of inherited NNE-striking structures played an important role in basin subsidence, and secondary WNW- to NW-striking transverse faults acted as normal faults. The NNE-striking principal displacement zone in the west of the basin runs along the western marginal area of the Jinju and Daegu domains and passes through the Uiseong domain from south to north, but most of this zone is now preserved as deep structures. Volcanic activity starting at ca. 115 Ma was characterized mainly by episodic basaltic eruptions occurring contemporaneously with back-arc deposition of a sedimentary sequence. After ca. 90 Ma, a transtensional kinematics changed to a strike-slip one, and the basin expansion and sedimentation in the back-arc region terminated. During the strike-slip event, rhyolitic-dacitic volcanism increased in intensity as a large NE-trending volcanic arc developed close to subduction zone and its loading caused the stratal flexure in the back-arc region, and the orientations of the shortening and stretching axes remained NW–SE and NE–SW, respectively. Additionally, continuing sinistral shear generated local depressions along the faults located in the west of the back-arc region and within the volcanic arc.

Temporal variations in the dynamic evolution of an overriding plate: Evidence from the Wulong area in the eastern North China Craton, China

Abstract The evolution of overriding-plate deformation, and the mechanisms responsible for this deformation, are debated. One area where these processes can be investigated is the eastern North China Craton (NCC) in China, which was situated in an overriding-plate position relative to the subducting Paleo-Pacific Plate during the Jurassic and Cretaceous. Here we constrain the structural evolution of the Jurassic and Cretaceous using new structural, stress field, and geochronological data from the Wulong area. The results show that the first phase of deformation (D1) produced a series of SE-dipping reverse shear zones and parallel folds in response to NW-SE shortening at 157–146 Ma (Late Jurassic). Based on microscopic observations and quartz c-axis analysis, it is suggested that the temperature during D1 deformation was 500 ± 50 °C. A second phase of contractional deformation (D2) at 146–132 Ma (earliest Early Cretaceous) gave rise to numerous NE-SW–striking sinistral faults and shear zones. The majority of D2 structures display ductile fabrics in the southwest of the Wulong area and brittle deformational features in the northeast, thus indicating enhanced exhumation in the former area. Microstructures of D2 sinistral shear zones indicate deformation temperatures of 300–400 °C. Inversion of fault slip data from the sinistral faults demonstrate that N-S compression was responsible for the D2 structures. The third phase of deformation (D3) was related to WNW-ESE extension during the middle to late Early Cretaceous (132–100 Ma). This extensional phase produced a series of NE-SW–striking normal faults and reactivated pre-existing structures. Dikes and plutons were emplaced during the D3 deformation, synchronous with the peak destruction of the NCC. Our results indicate that the eastern NCC showed temporal variations in stress and strain during the Jurassic and Cretaceous. Consistent with the slab-driven model, we suggest that this behavior represents the response of the overriding-plate to changes in subduction kinematics.

Orogen‐Parallel Transition From a Decoupled Fore‐Arc Sliver to Andean‐Type Mountain Chain: Paleomagnetic and Geologic Evidence From Southern Chile (37–39°S)

AbstractThe Chiloé fore‐arc sliver is an approximately N‐S elongated crust block detached from South America along the dextral intra‐arc Liquiñe‐Ofqui fault zone (LOFZ). The sliver is internally dissected by active NW‐SE sinistral faults whose relations with the LOFZ are speculative, also due to widespread fluvioglacial and volcanic blanket hiding the substratum. We focus on the northern LOFZ end and on the Biobio fault, supposedly the northernmost of the sinistral fault set, reporting on the results from field investigation and paleomagnetism of 48 (mostly Oligo‐Miocene) volcanic sites. We find that the Biobio fault is an old inherited crust discontinuity that did not yield significant block rotation and deformation during the Cenozoic, thus testifying the end of sinistral shear at about 38°S. At the same latitudes, a northward transition from pure strike‐slip to transpressive LOFZ deformation occurs. Intense tectonic deformation and >90° clockwise rotations characterize the main LOFZ strand. Conversely, a supposedly western LOFZ strand displays counterclockwise rotations, similar to the pattern previously documented in the forearc; thus, it does not represent a LOFZ segment. LOFZ and sinistral fault kinematics must be related, and we suggest that crust pushed northward west of the LOFZ escapes laterally toward the trench along the sinistral faults. We also speculate that the northward increasing age of the subducting Nazca plate implies a concomitant decrease of heat transfer on the upper plate, thus an increasing crust rigidity that eventually inhibits strain partitioning and sliver decoupling from 38°S.

Timing of Paleozoic Exhumation and Deformation of the High-Pressure Vestgӧtabreen Complex at the Motalafjella Nunatak, Svalbard

The Vestgӧtabreen Complex exposed in the Southwestern Caledonian Basement Province of Svalbard comprises two Caledonian high-pressure units. In situ white mica 40Ar/39Ar and monazite Th-U-total Pb geochronology has resolved the timing of the tectonic evolution of the complex. Cooling of the Upper Unit during exhumation occurred at 476 ± 2 Ma, shortly after eclogite-facies metamorphism. The two units were juxtaposed at 454 ± 6 Ma. This was followed by subaerial exposure and deposition of Bullbreen Group sediments. A 430–400 Ma late Caledonian phase of thrusting associated with major sinistral shearing throughout Svalbard deformed both the complex and the overlying sediments. This phase of thrusting is prominently recorded in the Lower Unit, and is associated with a pervasive greenschist-facies metamorphic overprint of high-pressure lithologies. A c. 365–344 Ma geochronological record may represent an Ellesmerian tectonothermal overprint. Altogether, the geochronological evolution of the Vestgӧtabreen Complex, with previous petrological and structural studies, suggests that it may be a correlative to the high-pressure Tsäkkok Lens in the Scandinavian Caledonides. It is suggested that the Vestgӧtabreen Complex escaped to the periphery of the orogen along the sinistral strike-slip shear zones prior to, or during the initial stages of continental collision between Baltica and Laurentia.

Petrogenesis and tectonic significance of the Fenshuiling and Mengha granitic plutons, SW China: insights from bulk elements, zircon U-Pb ages and Sr-Nd-Hf isotopes

ABSTRACT The Ailaoshan shear zone developed plenty of Cenozoic granitic plutons, located between the Yangtze block in the east and the Simao block in the west. Although a mass of previous studies have enriched the understanding of petrogenesis of these plutons, there are few reports on the plutons within the Mojiang-Lvchun terrane, a suspect domain between the Yangtze and Simao blocks. Among two plutons investigated in this paper, the Fenshuiling pluton is characterized by strongly shoshonitic (K20 = 5.8–6.4 wt.%) nature, high Y (6.5–30.0 ppm) and Yb (0.88–2.97 ppm) contents, and has limited variations in La/Yb (8.6–20.5) and Dy/Yb (1.6–1.9) with increasing SiO2. In Mengha, the pluton is also strongly shoshonitic (K20 = 5.7–6.1 wt.%), high Y (18.7–32.3 ppm) and Yb (2.03–3.50 ppm) contents, and shares limited variations in La/Yb (17.0–17.6) and Dy/Yb (1.5–1.7) with increasing SiO2. These tendencies correspond with the differentiation from a shoshonitic mafic melt by olivine, clinopyroxene and plagioclase (gabbroic) dominated fractional crystallization. These plutons mainly composed of quartz syenites are characterized by island-arc basalt (IAB) patterns, which are consistent with those evolved by shoshonitic mafic melt derived from metasomatic lithospheric mantle within the Yangtze block. Zircon εHf(t) values of Fenshuiling and Mengha plutons range from −4.1 to −0.7 and −2.8 to −1.1, respectively, and their whole-rock εNd(t) values are −5.0 to −4.6 and −4.7 to −4.6, respectively. These Hf isotopes are similar to those from Neoproterozoic granitoids caused by a persistent Neoproterozoic subduction around the southwestern margin of the Yangtze block. This similarity supports a scenario that the enriched lithospheric mantle source area of these plutons caused by the Neoproterozoic subduction, also means that the suspect terrane belongs to the Yangtze block. The location of the Fenshuiling and Mengha plutons separated from those within the western margin of the Yangtze block is attributed to Cenozoic sinistral shearing events.

Plastic deformation and rheology of the sinistral strike‐slip ductile shear zone in the Bayan Nuru area, Alxa Zuoqi, NW China: Product of subduction of Paleo‐Pacific Plate

The Bayan Nuru ductile shear zone is located in the Yabulai–Bayan Nuru tectonic belt of the middle part of the southern Central Asian Orogenic Belt (CAOB) and east of the Langshan tectonic belt. This article is the first report on the Bayan Nuru ductile shear zone, and provides dynamic, kinematic, and geochronological data of the shear zone. Accurately determining the shear orientation and estimating the shearing age of the Bayan Nuruductile shear zone, has important reference value and significance for understanding the tectonic evolution in Alxa during “Yanshan movement” period. All shearing signatures, including asymmetrical folds, rotated porphyroclasts, “domino” structures, “mica fishes,” S–C fabrics, and asymmetric stress shadows, developed in the shear zone, indicate sinistral shear. The temperature environment at the time of formation of the shear zone was determined by the mineral deformation thermometer and EBSD quartz C‐axis fabric analysis. The temperature was 350–550°C. The differential stress (σ) was estimated to be 44.31–97.94 MPa and the strain rate(έ) to be 10−14 s−1(350°C) and 10−10s−1(550 °C). The average kinematic vorticity of the Bayan Nuru NNE‐trending ductile shear zone is 0.93, indicating that the shear zone was dominated by simple shear. The thickness of the ductile shear zone was reduced by 7%–22%. Combined with the age of granitic mylonite (289.53 ± 0.74 Ma) and the NNE‐trending spread feature, the Bayan Nuru ductile shear zone in the western part of the Langshan fault zone may have been formed in the context of continuous compressional intraplate deformation, caused by the subduction of the Paleo‐Pacific Plate at 170–150 Ma.

Transpressive Structures in the Ghadir Shear Belt, Eastern Desert, Egypt: Evidence for Partitioning of Oblique Convergence in the Arabian‐Nubian Shield during Gondwana Agglutination

Transpressional deformation has played an important role in the late Neoproterozoic evolution of the Arabian‐Nubian Shield including the Central Eastern Desert of Egypt. The Ghadir Shear Belt is a 35 km‐long, NW‐oriented brittle‐ductile shear zone that underwent overall sinistral transpression during the Late Neoproterozoic. Within this shear belt, strain is highly partitioned into shortening, oblique, extensional and strike‐slip structures at multiple scales. Moreover, strain partitioning is heterogeneous along‐strike giving rise to three distinct structural domains. In the East Ghadir and Ambaut shear belts, the strain is pure‐shear dominated whereas the narrow sectors parallel to the shear walls in the West Ghadir Shear Zone are simple‐shear dominated. These domains are comparable to splay‐dominated and thrust‐dominated strike‐slip shear zones. The kinematic transition along the Ghadir shear belt is consistent with separate strike‐slip and thrust‐sense shear zones. The earlier fabric (S1), is locally recognized in low strain areas and SW‐ward thrusts. S2 is associated with a shallowly plunging stretching lineation (L2), and defines ∼NW‐SE major upright macroscopic folds in the East Ghadir shear belt. F2 folds are superimposed by ∼NNW–SSE tight‐minor and major F3 folds that are kinematically compatible with sinistral transpressional deformation along the West Ghadir Shear Zone and may represent strain partitioning during deformation. F2 and F3 folds are superimposed by ENE–WSW gentle F4 folds in the Ambaut shear belt. The sub‐parallelism of F3 and F4 fold axes with the shear zones may have resulted from strain partitioning associated with simple shear deformation along narrow mylonite zones and pure shear‐dominant deformation in fold zones. Dextral ENE‐striking shear zones were subsequently active at ca. 595 Ma, coeval with sinistral shearing along NW‐ to NNW‐striking shear zones. The occurrence of upright folds and folds with vertical axes suggests that transpression plays a significant role in the tectonic evolution of the Ghadir shear belt. Oblique convergence may have been provoked by the buckling of the Hafafit gneiss‐cored domes and relative rotations between its segments. Upright folds, fold with vertical axes and sinistral strike‐slip shear zones developed in response to strain partitioning. The West Ghadir Shear Zone contains thrusts and strike‐slip shear zones that resulted from lateral escape tectonics associated with lateral imbrication and transpression in response to oblique squeezing of the Arabian‐Nubian Shield during agglutination of East and West Gondwana.

The structural geometry and metamorphic evolution of the Umm Gheig shear belt, Central Eastern Desert, Egypt: implications for exhumation of Sibai core complex during oblique transpression

Field and structural studies supported by remote sensing and thermobarometeric data are attempted to decipher the deformation and metamorphic histories of Umm Gheig shear belt which represents the eastern supracrustal rocks of the Sibai core complex in the Central Eastern Desert of Egypt. Umm Gheig shear belt is build up of three major structural domains (Abu Lassaf, Umm Luseifa, and Kab Ahmed) formed by four deformation events (D1–D4) during the late Cryogenian–Ediacaran (650–542 Ma). The border between these domains is controlled by major thrusts and sinistral-reverse shear zones. Abu Lassaf shear belt includes NW-oriented major folds, serpentinite klippes, and oppositely dipping bidirectional thrust forming pop-up structures. Umm Luseifa shear belt is marked by a major doubly plunging, curvilinear, asymmetrical anticline, series of fault bounded thrust slices, and bordered by two major NE- and SW-verging thrusts. Kab Ahmed shear belt includes the highly strained rocks and expresses as an imbricate NE-verging thrusts forming a simple piggyback imbricate thrust system with a unidirectional sense of thrust propagation to the NE. Presence of D3-related horsetail splays that grouped into a major of NW-trending sinistral shear zones in the Umm Gheig shear belt gives additional evidence for the formation and exhumation of Sibai core complex in oblique island arc collision zones and includes lateral extrusion in a transpressional collision zone along major strike-slip shear zones. The rocks of the Umm Gheig shear belt are subjected to prograde low-medium-grade regional metamorphism in the range of greenschist to lower amphibolite facies (350–620 °C at 2–6 kb). The structures in Umm Gheig shear belt are the products of an E–W shortening phase accompanied by compressional due to oblique shortening of the Arabian–Nubian Sheild between East and West Gondwana.

Chafalote Metamorphic Suite (Uruguay): Reflections on the evolution of the Punta del Este Terrane

The Chafalote Metamorphic Suite, located in the Uruguayan sector of the Punta del Este Terrane, southeasternmost Dom Feliciano Belt, comprises semipelitic, migmatitic metapelites, mafic granulites, and calc-silicate/amphibolitic gneisses. These supracrustal rocks occur as roof pendants in a granodiorite belonging to the Cerro Olivo Complex. The main structure of the Chafalote Metamorphic Suite is a composite S0/S1 banding, related to the main deformation event (D1) developed under upper-amphibolite/granulite facies conditions. This work presents an integrated study on the latter rocks, involving different geochronological methods (monazite U-Pb LA-ICP-MS and zircon U-Pb SHRIMP) and whole rock Sm-Nd isotope data in order to better understand the metamorphic evolution of the Chafalote Metamorphic Suite during the amalgamation of the SW Gondwana paleocontinent in the Neoproterozoic. The studied rocks record a peak metamorphic event (M2) occurred at around 660 Ma (monazite U-Pb ages), which was followed by decompression and partial melting (M3 event) at around 640 Ma (zircon U-Pb ages), probably during the late stages of the D1 event. The retrograde methamorphic event (M4) is related to the transition between the D1 and the development of NE-SW striking, low-grade sinistral shear zones (D2 event) and intrusion of syn-to post-orogenic Brasiliano granitoids of the Aiguá Batholith at around 600 Ma. The metamorphic conditions, together with geochronological and isotopic similarities, allow the correlation between the studied high-grade metasedimentary rocks and those from the Paso del Dragón Complex in Uruguay, and from the Várzea do Capivarita, Arroio Grande, and Telho Complex in Brazil. The latter rocks possibly share similar source areas and represent different portions of the Neoproterozoic fore-arc and/or back-arc basins developed in the Dom Feliciano Belt during the Brasiliano/Pan-African orogenic cycle.

Crustal movement and strain distribution in East Asia revealed by GPS observations

East Asia is bounded by the Indian plate to the southwest and the Pacific and Philippine plates to the east, and has undergone complex tectonic evolution since ~55 Ma. In this study, we collect and process three sources of GPS datasets, including GPS observations, GPS positioning time series, and published GPS velocities, to derive unified velocity and strain rate fields for East Asia. We observed southward movement and arc-parallel extension along the Ryukyu Arc and propose that the maximum principal stress axis (striking NEE) in North China could be mainly induced by westward subduction of the Pacific plate and the southward movement of the Ryukyu Arc. The large EW-trending sinistral shear zone that bounds North China has been created by eastward movement of South China to the south and westward subduction of the Pacific plate to the north. GPS velocity profiles and strain rates also demonstrate that crustal deformation in mainland China is controlled by northeastward collision of the Indian plate into Eurasia and westward subduction of the Pacific and Philippine Sea plates beneath Eurasia. In particular, the India-Eurasia continental collision has the most extensive impact, which can reach as far as the southern Lake Baikal. The viscous behavior of the subducting Pacific slab also drives interseismic deformation of North China. The crustal deformation caused by Philippine oceanic subduction is small and is limited to the region between the southeast coast of mainland China and Taiwan island. However, the principal compressional strain around eastern Taiwan is the largest in the region.

Structural Analysis of the Luowei Orefield in Xidamingshan, Guangxi, China

AbstractMany types of hydrothermal deposits (e.g. W, Bi, Pb, Zn, Ag) are confined by faults and hidden granodiorite in the Luowei Orefield in Xidamingshan, Guangxi, China. The orebodies in the Luowei W–Bi deposit are predominantly layered and distributed along bedding in sandstones of the Cambrian Xiaoneichong Formation. The orebodies in the Lujing Pb–Zn deposit are controlled mainly by west‐south‐west (WSW)‐trending faults, and those in the Fenghuangshan Ag deposit are controlled mainly by west‐north‐west (WNW)‐trending faults, which were reverse faults during mineralization and were later reactivated as sinistral strike‐slip faults. The Luowei fault was formed postmineralization and resulted in sinistral displacement of the subsurface granodiorite and the Cambrian strata. A tectonomagmatic mineralization model of the Luowei Orefield is proposed, and the following conclusions were made. (i) Under a regional N–S compressive stress regime, WSW‐ and WNW‐trending reverse faults and N–S‐trending tensional fractures were formed. (ii) Magma intruded along the tensional fractures. Under the force of magmatic thermodynamics, mineralizing fluid migrated along bedding planes in sandstones and formed W–Bi orebodies at favorable sites. Some fluid migrated along WSW‐ and WNW‐trending faults to sites farther from the magma source, forming vein‐type Pb–Zn and Ag orebodies. (iii) After mineralization, under ~E–W compression, a NW‐trending left‐lateral slip fault was formed, cutting the subsurface granodiorite and orebodies. Concurrently, sinistral shear slip occurred on WNW‐trending ore‐controlling faults. However, the small displacement on these faults did not change the overall distributions of the rock mass and orebodies.