Search

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 Fangshan area, SW of Beijing, lies at the junction of the NNE-trending Taihang mountain range and the southeastern portion of the Yanshan intraplate orogenic belt and has undergone at least five stages of deformation. Mid- to late Triassic extensional deformation ([D.sub.1]) is represented by the formation of the Fangshan tectonic dome during SE-directed extensional tectonics. This deformation was later modified by NNW-directed thrusting in the late Triassic ([D.sub.2]) and WNW-directed thrusting in the late Jurassic ([D.sub.3]). These [D.sub.1]-[D.sub.3] structures were deformed by the arcuate Beiling synclinc ([D.sub.4]), which formed during the intrusion of the 133-128 Ma Fangshan pluton. [D.sub.5] deformation is represented by Cretaceous to Quaternary NNE-striking high-angle normal faults. The ages of these events demonstrate that the principal ENE-trending tectonic framework of the Yanshan intraplate orogenic belt was established mainly in the mid- to late Triassic (the Indosinian event). The Jurassic to Cretaceous Yanshanian deformation with thick-skinned thrusting was associated with intrusion of numerous plutons, and strongly modified the older structures. The tectonic evolution of the Fangshan tectonic dome was linked genetically with the collision of the North China Block and the Siberian Craton, and the later collision of the North and South China blocks.

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]

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]