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 northverging 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% NaCl eqv). Electron microprobe analyses (EMPA) of garnet amphibolites show metamorphic temperature of~510 C. The age of D1 event is determined by 40 Ar-39 Ar 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.

India (Tethyan Himalaya Series) in Central Myanmar: Implications for the Evolution of the Eastern Himalayan Syntaxis and the Sagaing Transform‐Fault System

“…These high‐P/low‐T conditions likely record local involvement of THS rocks into the subduction channel. In eastern S‐Tibet, the basal THS rocks experienced comparable‐T but lower‐P (∼600°C, 0.78 GPa; Dunkl et al., 2011; 510 ± 50°C, Fang et al., 2020; Figure 3a), and similar burial‐early exhumation histories like those inferred for Katha (∼49‒32 Ma; U‐Pb zircon, K(Ar)/Ar mica; for example, Aikman et al., 2008, 2012; Dunkl et al., 2011; Ratschbacher et al., 1994). In the same area of eastern S‐Tibet, post‐thrusting uppermost GHS granitoids have 48‒36 Ma U‐Pb zircon ages; the associated schists show higher‐T and lower‐P (∼630–660°C, 0.7–0.8 GPa; Ding, Zhang, Dong, et al., 2016; Ding, Zhang, Hu, et al., 2016) than the Katha rocks.…”

“…Figure 3a plots the Katha-rock P-T data together with THS data from central S-Tibet (Laskowski et al, 2016), eastern S-Tibet (Dunkl et al, 2011;Fang et al, 2020), and GHS and LHS data from Bhutan (Daniel et al, 2003). The P-T results are summarized in Supporting Information Table S4, and the petrology-derived from THERIAK/ DOMINO equilibrium-assemblage calculations and conventional thermobarometry-is detailed in Text S1 of Supporting Information S1 and Min et al (2022).…”

A timeline of the Cenozoic tectonic–magmatic–metamorphic evolution and development of ore resources in the Himalayas

“…The following are some important research results. The sericite Ar–Ar ages of the Middle Triassic slate in the Gyaca County area show that the southward thrust and the northward thrust are 47 Ma and 30–22 Ma, respectively (Fang et al, 2020) and the sericite Ar–Ar ages of the quartz vein in the Triassic are older at 56 Ma and 48 Ma (Zhao et al, 2019). The tectonic deformation of the THS in Qonggyai County shows that the compressional deformation times from the south and north are 50–45 Ma and 35–32 Ma, respectively (Montomoli et al, 2017).…”