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The tectonic deformation reaction to multi‐plate convergence in the northern Guizhong Depression, as well as its geodynamic process, remain poorly understood. Two phases of intracontinental deformation are identified using field geological investigations, paleo‐stress inversion and seismic reflection sections: (1) early NE–SW‐directed compression during the Middle Triassic to Early Jurassic; (2) later NW–SE‐directed compression during the Middle Jurassic to Early Cretaceous. The reconstruction of paleo‐stress provides robust constraints on the spatiotemporal evolution of stress within the Guizhong Depression. The regional SW–NE‐directed compression during the Middle Triassic to Early Jurassic was driven by the collision and convergence between the Indochina and South China blocks, resulting in the formation of NW–SE‐directed compressional structures during the Indosinian Period. The later SE–NW‐directed paleo‐stress field was probably driven by the NW‐directed subduction of the Paleo–Pacific Plate during the Middle Jurassic to Early Cretaceous. [ABSTRACT FROM AUTHOR]

The genesis of the subduction mélange in the central Qiangtang terrane has been a long hot debate. However, little research has been conducted on the brittle failure within the accretionary wedge, which is very important to unveil the structural evolution of the mélange. In this study, based on the recognition of multiple deformational phases, we analyse the characteristics and formation history of the vein system in the Gangma Co mélange. Six groups of quartz veins are recognized. Foliation‐parallel extension veins (G1 veins), shear veins (G2 veins) and foliation‐perpendicular extension veins (G4 veins) are supposed to have formed during the subduction of oceanic crust, recording the repeated low‐angle thrust‐sense frictional sliding, tensile fracturing and stress changes generated by subduction‐related earthquakes. Subsequent vertical extension veins (G5 veins) are suggested to be related to the exhumation of the underplated mélange, while the horizontal extension veins (G6 veins) in the last phase represent the final horizontal thrusting. The temperature conditions for shear vein formation were examined by fluid inclusion analysis, ranging from 120 to 200°C, coinciding with the temperature conditions of the slow earthquake region where episodic tremors and slow slip occur. This contribution supports that the Gangma Co mélange represents an in situ subduction zone and that its internal vein system is a response to the tectonic evolution of the Longmu Co‐Shuanghu Tethys Ocean. [ABSTRACT FROM AUTHOR]

The spatio-temporal evolution of the Paleo-Tethys Ocean has been a hot and controversial issue in the world. Here we carry out petrographic, chronological, and geochemical study on garnet–phengite–quartz schist and mafic rocks in the Rongma area from the northern margin of the Southern Qiangtang block to determine the early Mesozoic tectonic evolution of the Shuanghu Paleo-Tethys Ocean in northern Tibet. The zircons from a phengite–quartz schist sample yielded concordant ages of 1936–393 Ma, indicating that its protolith deposited after ~ 393 Ma. Overgrowth zoning garnet with three stages of metamorphic evolution from garnet core to rim (i.e., Peak metamorphic, early retrograde metamorphic, and late retrograde metamorphic stages) in the schist was recognized, indicating that two subduction in a short time might be involved for its genesis. Two groups of phengite in the schist yielded 40Ar/39Ar plateau ages of 229 ± 1.4 Ma and 225 ± 1.3 Ma, respectively; thus, the late retrograde metamorphism of the schist might occur at ~ 229–225 Ma. Zircon U–Pb dating of a diabase in the area yielded crystallization age of 241 ± 1.1 Ma implying its formation in the early Triassic. The Hf-in-zircon and whole-rock Nd isotopes of the diabase show εHf(t) of − 0.6- + 19.1 and εNd(t) of − 0.8 to + 0.9, respectively. Combined with the whole-rock geochemical features of the early Triassic diabase (241 Ma) and gabbro (237 Ma), they indicate that these mafic rocks are formed in a back-arc extensional setting related to the subduction of the oceanic plate between the Northern and Southern Qiangtang blocks beneath the latter. Combined with regional data, our study favors that view bi-directional subduction of the Shuanghu Paleo-Tethys ocean in the Early Triassic and it was finally closed at ~ 237 to ~ 229 Ma. Our model will help us better understand the tectonic evolution of the Paleo-Tethys Ocean. [ABSTRACT FROM AUTHOR]

The pressure–temperature (P‐T) path of blueschist is usually applied to understanding the evolution of the subducted oceanic crust. Garnet plays a key role in calculating the metamorphic conditions, but sometimes it is absent in the blueschists. Nevertheless, the high‐P experimental simulation may serve as a useful tool to constrain the P‐T conditions. In the middle of the Longmu Co‐Shuanghu Suture, North Tibet, blueschist blocks embedded in the marble were discovered, where garnet is abundant in the core but absent in the marble‐contacting margin. These two kinds of blueschist contain similar mineral assemblage (glaucophane, epidote, and phengite) and show comparable basaltic character. Pseudosection modelling was applied to the garnet‐bearing blueschist that calculated the peak metamorphic condition of ca. 1.9 GPa and 530°C. As for the garnet‐absent blueschist, experiments (1.2–2.3 GPa and 500–800°C) were performed on the designed mixture of 93 wt% basalt and 7 wt% calcite. The results show that the total Fe content in the glaucophane and barroisite is positively correlated with the pressure but not with temperature, which could be regarded as a geobarometer. Accordingly, the peak condition was constrained at 2.3 GPa and 500–600°C. Therefore, the equivalent metamorphic conditions of the blueschists indicate that the Palaeo‐Tethys oceanic crust subducted into at least 70 km. Furthermore, both the experiments and the T‐X (CaCO3) pseudosection, constructed based on basaltic lithology, support that the absence of garnet in the blueschist is predominantly caused by the mixing of calcite. In summary, this study provides a valid attempt in exploring metamorphic conditions by experiment simulation, and evaluates the profound influence of the interaction between the basaltic crust and overlying carbonate‐bearing sediments in the subduction zone. [ABSTRACT FROM AUTHOR]

Located south of the Yarlung‐Tsangpo Suture Zone (YTSZ), the Upper Triassic rocks of the Tethyan Himalayan Sequence (THS) in the Shannan area, southeastern Tibet, serve as an ideal geologic entity to investigate the post‐collisional evolution of the eastern Tethyan Himalayan realm. Detailed field observations suggest that D2 north‐verging folds and thrusts dominate the deformational pattern of the newly named Luolin Group, while D1 south‐verging imbricated structures characterize the Langjiexue Group and the Nieru Unit. Illite crystallinity measurements in pelitic samples indicate that the metamorphic grade decreases from epizonal in the Luolin Group to anchizonal facies in the Langjiexue‐Nieru Unit. Fluid inclusions from two stages of syn‐deformational quartz veins are characterized by moderate homogenization temperature (~260°C) and low salinity (5% NaCleqv). Electron microprobe analyses (EMPA) of garnet amphibolites show metamorphic temperature of ~510°C. The age of D1 event is determined by 40Ar–39Ar dating of phyllite sample at ca. 47 Ma. Correlations with regional deformational events through the THS demonstrate that the D2 event occurred between ca. 30 and 22 Ma. These data imply that the Upper Triassic sequence is part of the fold‐thrust belt of the Himalayan Orogen, rather than the accretionary mélange within the YTSZ. [ABSTRACT FROM AUTHOR]

Models of in situ oceanic subduction and underthrusting is proposed to explain the formation and exhumation of the central Qiangtang mélange belt. Some studies have suggested that continental subduction occurred in central Qiangtang, but there is a lack of definitive evidence. Thus, the role of continental subduction in the formation and evolution of the central Qiangtang mélange belt has not been fully considered. We therefore studied the origin and structural evolution of the central Qiangtang mélange belt. Detrital zircon research indicates that the continental blocks in the Gangma Co mélange are part of the Carboniferous-Permian strata of the South Qiangtang terrane and have a Gondwanan affinity. Structural studies and geologic observations in the Gangma Co area show a change in the structural style and a pressure difference between the lower mélange and the upper Carboniferous-Permian strata. Structures in the Gangma Co mélange include D1 layer-parallel ductile structures, D2 horizontal contractional structures, D3 flattening structures, and D4 slightly horizontal contractional structures. According to our analysis, the newly proposed in situ continental subduction model best explain these findings. [ABSTRACT FROM AUTHOR]

The Upper Triassic Langjiexue Group, which lies immediately south of the Yarlung-Tsangpo Suture Zone in the Shannan area of southeastern Tibet, represents an important part of the Tethyan Himalayan Sequence (THS). Its provenance and palaeogeography have been the subject of debate. We present new data on petrographic composition, whole-rock geochemistry, and detrital zircon U-Pb geochronology to constrain the provenance of the Langjiexue Group. The dominance of quartz grains and felsic volcanic lithic fragments suggests that the sandstones are litho-quartzose. The trace element geochemical signatures (V-Ni-Th*10, Co/Th-La/Sc, Eu/Eu*-Th/Sc) suggest derivation from felsic igneous sources. The detrital zircon age spectra display three major peaks: a Meso-to-Neoproterozoic peak (1200-900 Ma, 7-18%), a Neoproterozoic-to-Late Cambrian peak (750-500 Ma, 32-65%), and a Late Carboniferous-to-Late Triassic peak (300-200 Ma, 11-33%). The maximum depositional age of early Carnian (236-235 Ma) is obtained by calculating weighted average ages of the youngest zircons (≤250 Ma). The youngest age cluster (300-200 Ma) is incompatible with sources from neighbouring terranes, including the South Qiangtang terrane, Lhasa terrane, THS, and Higher Himalayan Crystalline. Correlations of the Permian-Triassic zircons with those of time-equivalent strata in northwest Australia, west Burma, and the Banda Arc unveil a potential connection to the Tasmanides along the convergent margin of eastern Australia. The New England Orogen (300-230 Ma) could have supplied the Langjiexue Group with magmatic materials via continent-scale drainage systems or a submarine fan complex. This scenario provides a new perspective into the transport of detritus from distal orogens to sedimentary basins thousands of kilometres away. [ABSTRACT FROM AUTHOR]

The timing of the closure of the Bangong Ocean between the Lhasa and South Qiangtang Terranes in central Tibet and the resulting crustal thickening are still under debate. We integrate published apatite fission track and (U-Th)/He thermochronometer data with new zircon (U-Th)/He ages from eight samples and with structural profiles to document that the South Qiangtang Terrane experienced slow exhumation between 200 and 150 Ma, associated with the opening of the Bangong Ocean. Accelerated exhumation (around 0.2-0.3 mm/a) of the South Qiangtang Terrane was initiated at around 150 Ma. This exhumation event is interpreted to reflect collision between the Lhasa and South Qiangtang Terranes after closure of the Bangong Ocean, associated with crustal thickening via thick-skinned folding and thrusting within the South Qiangtang Terrane. The amalgamation of the Lhasa and South Qiangtang Terranes recorded here may represent the first stage of crustal thickening in the central Tibetan Plateau. [ABSTRACT FROM AUTHOR]

To explore the chemical weathering processes and the anthropogenic disturbance of weathering, 20 water samples were collected from the tributaries in the Nansihu Lake basin, a growing industrial area. The major ions in river waters were analyzed to identify and quantify the contributions of the different reservoirs. Based on stoichiometric analyses and end-member determination, the contributions of individual reservoirs were calculated for each tributary. In the study region, the averaged contributions of atmospheric inputs, anthropogenic inputs, evaporite weathering, carbonate weathering and silicate weathering were 2, 37, 28, 25 and 8 %, respectively. Combined with information regarding runoff and drainage area, the annual average contribution of TDS to waters was estimated to be 1.90 ± 0.95 ton/km from silicate weathering, 5.68 ± 2.84 ton/km from carbonate weathering. Furthermore, the associated consumption of CO was calculated to be approximately 7.50 × 10 mol/a. The industrial and mining activities were the main sources for anthropogenic inputs, and they produced non-CO acids (NCA). Of all protons involved in chemical weathering, 34 % was presumed to be originated from NCA, causing 2.74 × 10 mol/a of CO degassing. Moreover, industrial inputs could play a major role in the modification of the chemicals in the water system, and they could even change the carbonate weathering rate in such an intensively industrializing region. In North China, the chemical weathering associated with NCA was found to be significant for the first time. [ABSTRACT FROM AUTHOR]