Geological anatomy of the Upper Triassic sequence in southeastern Tibet: Implication for tectonic evolution of the eastern Himalayan Orogen

Abstract

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.