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Transpressional shear zones commonly occur in ancient and modern convergent plate boundaries to accommodate oblique plate convergence. The early Paleozoic Qilian orogen in northeastern Tibet records the subduction of Proto-Tethyan Ocean lithosphere and the accretion-collision of various magmatic arcs and continental terranes. This study focused on the Datong ductile shear zone, which represents the central part of the WNW-ESE-striking ductile shear zone along the northern margin of the Qilian block in the Qilian orogen. This structure bears key information about the evolution of oblique convergence during the early Paleozoic orogeny. The kinematics and timing of the Datong ductile shear zone were investigated via fieldbased, microstructural, and mica 40Ar/39Ar dating analyses. Mesostructural and microstructural data showed predominantly dextral strike-slip shearing within the Datong ductile shear zone. Microstructural features and quartz c-axis crystallographic preferred orientation patterns indicated that dextral ductile shearing occurred under lower-amphibolite-facies conditions (~500-550 °C and ~5.6 kbar) within the shear zone. Microstructures of quartz showed subgrain rotation (SGR) and grain boundary migration (GBM), suggesting dislocation creep-dominated deformation. A strain rate of 10-12 s-1 and a differential stress of 25-39 MPa were estimated by the rheological flow law and quartz paleopiezometry. Finite strain measurements indicated that all deformed rocks of the Datong ductile shear zone exhibit a weakly oblate ellipsoid near the plane strain. Kinematic vorticity (ranging 0.47-0.83) analysis suggested the coexistence of simple shear and pure shear strains within the Datong ductile shear zone, indicating a transpressional setting. Biotite and muscovite 40Ar/39Ar data showed that transpressional shearing deformation started in the Ordovician (before 453 Ma) and lasted to the Silurian (ca. 430 Ma). Our new data combined with regional geological data show that the deformation type, kinematics, and dynamics of the Datong ductile shear zone were controlled by the southward oblique subduction of the Paleo-Qilian Ocean (Proto-Tethyan Ocean) and the following oblique collision between the Qilian block and the Alxa block. The intensive transpressional deformation along the northern Qilian block may reflect strong coupling between the subducting Paleo-Qilian oceanic slab and the overriding Qilian block as well as a high degree of convergence obliquity during the ongoing early Paleozoic convergence. [ABSTRACT FROM AUTHOR]

Various factors are inferred to influence the structural evolution of fold-and-thrust belts or thrust wedges, but the effects of lateral friction (such as that exerted by the opposite block of a strike-slip fault in nature or by sidewalls in analogue models) have rarely been given sufficient attention and hence are still far from being fully understood. Up to now, seaward-vergent and dually vergent thrust wedges have been satisfactorily explained by the classic Coulomb wedge theory and can be well reproduced in analogue and numerical models. In contrast, landward-vergent thrust wedges, which are uncommon in nature (e.g., those occurring in the Cascadia subduction zone), have not yet been adequately explained by the classic Coulomb wedge theory and can only be reproduced in a few analogue models under particular conditions. In order to look closer at the effects of lateral friction on the structural evolution of fold-and-thrust belts and to explore the origin of landward-vergent thrust wedges, we conducted a series of sandbox models under different conditions of lateral friction and various ratios of initial average lateral frictional stress to basal shear stress (τs/τd). Results of our models revealed that in the presence of a weak or ductile basal décollement, the direction of lateral friction can dictate not only the deformation propagation direction and the surface slope direction, but also the structural vergence of fold-and-thrust belts. Fold-and-thrust belts normally show a structural vergence in the same direction as the direction of lateral friction but a deformation propagation direction and surface slope direction opposite to the direction of lateral friction. According to the results of models and the relationships between structural vergence and lateral friction direction occurring both in Cascadia and the analogue models, we propose that an extremely high τs/τd ratio due to an extremely weak basal décollement and landward-directed lateral friction exerted by the opposite blocks of basement-involved strike-slip faults are probably the two key factors responsible for the origin of landward-vergent thrust wedges in Cascadia. This work illustrates that effects of lateral friction should be properly taken into account both in the kinematic analysis and designs of models to simulate fold-and-thrust belts in the presence of weak or ductile basal décollements. [ABSTRACT FROM AUTHOR]

The Lincang-Inthanon tectonic belt is a major tectonic boundary within the south-eastern Tibetan Plateau and Indo-China Peninsula, which are typical examples of tectonic extrusion in SE Asia. The Lincang strain zone, Lancang-Gengma fault, and Inthanon metamorphic complex make up this nearly N--S-striking tectonic belt, which separates the Baoshan--Shan Thai and Simao-Indochina terranes. New petrographic, structural, and mica 40Ar/39Ar geochronological studies were conducted to reveal their deformation styles and constrain the timing of their tectonic evolution. W--E-directed compression related to the subduction of the Paleotethyan Ocean with subsequent continental collision and sinistral ductile shearing in the early Oligocene are recorded along the Lincang strain zone. The Lancang-Gengma fault zone switched from sinistral shearing to dextral motion in the late Cenozoic and shows a deformation history similar to that of the parallel Red River fault. The Inthanon metamorphic complex may have experienced crustal shortening in the early Cenozoic, followed by sinistral transtension in the early Miocene. The Lincang-Inthanon tectonic belt shows many lithological, tectonic evolutionary, and metamorphic similarities with the Gaoligong, Chongshan, and Ailaoshan--Red River shear zones. Therefore, the sinistral shearing along the Lincang-Inthanon tectonic belt and the Chongshan shear zone in the north, which may have initiated since the early Oligocene, played an important role in adjusting differential extrusion and rotation of the Baoshan--Shan Thai and SimaoIndochina terranes. Our results delineate the regional tectonic framework and provide insights into the characteristics and geodynamics of intracontinental deformation in the eastern India-Eurasia oblique convergence zone. [ABSTRACT FROM AUTHOR]

The Himalayan Orogen is a typical continental collision orogenic belt that formed during India-Asia collision. The orogeny involved a transition in tectonic regime, which led to E-W-trending extension during the middle Miocene. At the same time, widespread post-collisional adakitic magmatism occurred in southern Tibet, particularly in the Lhasa Terrane, and these rocks provide a valuable record of the collisional geodynamic processes within the lithosphere. Few studies have focused on the middle Miocene adakitic rocks of the Himalayan Orogen, so further research is required to constrain their origin and geodynamics. This study presents new geochronological and geochemical data, including the whole rock Sr-Nd and zircon Hf isotopic compositions from the Mayum pluton in the Mayum-Gong Tso area, Northern Himalaya. Zircon U-Pb ages show that the Mayum granodiorite porphyries crystallized at 16.67 ± 0.14 Ma (mean square weighted deviation [MSWD] = 0.82, n = 29) and 16.68 ± 0.16 Ma (MSWD = 1.13, n = 28), indicating that they formed during the middle Miocene. The major and trace element characteristics are as follows: SiO2 = 65.79-67.31 wt%, Al2O3 = 15.28-16.00 wt%, MgO = 1.77-1.89 wt%, Y = 12.0-13.5 ppm, Yb = 0.11-0.99 ppm, Sr = 719-822 ppm, (La/Yb)N = 21.89-27.02, Sr/Y = 56.1-65.5, and the rocks have weak negative Eu anomalies (Eu/Eu* = 0.76-0.85), indicating that they are adakitic rocks and show high Sr-Ba granite affinity. The Mayum granodiorite porphyries have high K2O contents (3.42-3.65 wt%), Na2O (3.76-4.04 wt%), and K2O/Na2O ratios of 0.91-0.95, indicating a high-K calc-alkaline affinity. All samples are enriched in large-ion lithophile elements and depleted in high field strength elements. The initial 87Sr/86Sr ratios are 0.709262-0.709327 and εNd(t) values are -6.36 to -7.07, which correspond to two-stage Nd model ages [TDM2(Nd)] of 1405-1348 Ma. In situ zircon Lu-Hf isotopic compositions are variable, with 176Hf/177Hf ratios of 0.2823845-0.282824, εHf(t) values of -13.37 to +2.17, and two-stage Hf model ages [TDM2(Hf)] of 1704-841 Ma. These geochemical and Sr-Nd-Hf isotopic characteristics indicate that the Mayum granodiorite porphyries may be derived from partial melting of the subducted Indian thickened ancient mafic lower crust (~92%) mixed with depleted mid-ocean ridge basalt mantle material input (~8%) slightly. Crust-mantle interaction was induced by asthenospheric upwelling followed by the underplating of the Himalayan lower crust beneath Southern Tibet during the middle Miocene in response to significant changes in the geodynamics of the India-Asia collisional orogen. These deep geodynamic processes reflect the break-off or rollback of the subducted Indian continental slab, which caused asthenospheric upwelling, the input of juvenile heat, and the addition of depleted mantle material. This study indicates that the middle Miocene Himalayan adakitic rocks, which include the Miocene Yardoi two-mica granite, Gyangzê granite porphyry dike, Bendui two-mica granite, Langkazi enclave, Kuday dacitic dike, Lasa pluton, and Mayum pluton, form a belt of adakitic rocks in the Northern Himalaya to the south of the Indus-Tsangpo Suture Zone. These adakitic rocks have similar temporal and spatial distributions, geochemical features, and Sr-Nd isotopic compositions, indicating that their petrogenesis and geodynamic settings were similar. At that time there was widespread initiation of N-S-trending rifting, exhumation of central Himalayan eclogites, and large changes in the compositions of Himalayan leucogranites in the Himalayan Orogen. These can be attributed to significant changes in the characteristics of the subducted Indian continental lithosphere during the middle Miocene. The middle Miocene Himalayan adakitic rocks provide valuable insights into this transition in the tectonic regime and deep geodynamic processes. [ABSTRACT FROM AUTHOR]

Recycled crustal components have been widely identified in the source of continental basalts with geochemical features similar to oceanic island basalts (OIBs). However, the mechanism of how these recycled materials are involved remains highly debatable. Here we conduct comprehensive geochemical analyses (including whole-rock, olivine, and melt inclusion) and numerical modeling on Late Cretaceous Ji'an basalts from South China interior, aiming to investigate the possible role of recycled crustal components in basalt petrogenesis driven by the subducted paleo- Pacific oceanic plate. The Ji'an basalts show geochemical characteristics akin to OIBs and have depleted asthenospheric mantle-like Sr- Nd-Pb-Hf isotopic compositions with moderately radiogenic Os. Their olivine-hosted melt inclusions have low H2O and highly negative dD values and olivine phenocrysts are mainly characterized by depletion of 18O with d18O values lowering to 3.9?. These features are consistent with positive Sr and Eu anomalies in some whole-rock samples. The combined geochemical data suggest that the primary magmas were derived from an asthenospheric mantle enriched by melts from an altered gabbroic oceanic crust, which had experienced intensive dehydration. Further numerical modeling shows that melting of the dehydrated oceanic crust can occur along the torn flank of the subducting lithosphere, in the case that the slab is strongly thinned and fractured. The low d18O preserved in olivine and the estimated slab age (<300 Ma) from the radiogenic whole-rock Os and Pb compositions also require the involvement of a recently recycled slab, probably represented by the subducted paleo-Pacific oceanic plate. Rollback of the subducting paleo-Pacific slab might create a slab window, in which melt from the torn/fractured slab reacted with the upwelling asthenosphere to form an enriched mantle source for the Ji'an basalts and similar counterparts. [ABSTRACT FROM AUTHOR]

The most striking structural features in the interior of the Shan Plateau, southeast of the eastern Himalayan syntaxis, are a series of NE-trending faults that exhibit sinistral movement and an arcuate geometry. Their origin and tectonic evolution remain poorly understood. Furthermore, a switch in slip sense is recorded along many of these faults, but the timing of kinematic reversal is still unclear, hindering an understanding of the causal geodynamic mechanisms. We conducted an integrative study of apatite and zircon (U-Th)/He thermochronology, 40Ar/39Ar geochronology, and structural and geomorphic analysis to decipher the evolution of two major NE-trending faults: the Nantinghe and Dayingjiang faults. At least three deformation stages are identified within the Nantinghe fault zone, including top-to-the-SE/ESE thrusting, dextral ductile strike-slip shearing, and sinistral movement. Zircon and apatite (U-Th)/He data, collected from the northeastern terminus of the Nantinghe fault, reveal rapid cooling in the early Miocene. Combined with the 40Ar/39Ar data from sinistrally sheared mylonite, left-lateral movement on the Nantinghe fault is inferred to have initiated as early as ca. 20 Ma. The Dayingjiang fault reactivated as a sinistral brittle fault along the dextral Yingjiang shear zone. A two-stage thermal history is identified along the shear zone, with prominent cooling during dextral ductile shearing in the early- to mid-Miocene followed by a lower-magnitude cooling episode at ca. 11 Ma caused by sinistral transtension along the Dayingjiang fault. The evolution of the Nantinghe and Dayingjiang faults suggests that the NE-trending fault system in the Shan Plateau may have developed along preexisting structures and underwent diachronous slip-sense inversion in the late Cenozoic. The northward advance of the eastern Himalayan syntaxis caused a major change in both the regional stress field and fault geometries in the eastern India-Eurasia oblique convergence zone, contributing to the inversion of fault kinematics. [ABSTRACT FROM AUTHOR]