Tectonothermal evolution of the Triassic flysch in the Songpan–Garzê orogen, eastern Tibetan plateau

Abstract

Abstract Low temperature metamorphic indicators were used to investigate the tectonothermal evolution of the Triassic flysch in the Songpan–Garze orogen, eastern margin of the Tibetan plateau. Iso-thermal zones mapped with illite crystallinity (Kubler index, KI) describe a complex pattern of the diagenetic, anchimetamorphic and epimetamorphic zones. From this pattern, a general increase in grade toward the Longmenshan fault belt is observed, and across the Longmenshan fault belt, greenschist facies and anchizonal rocks on its NW side are juxtaposed to diagenetic rocks in the Sichuan basin on its SE side. This juxtaposition is marked by KI jumps of 0.23°Δ2θ in the SW portion of the Longmenshan fault through 0.39°Δ2θ in the Middle, and reaching 0.71°Δ2θ in the NE of the Longmenshan fault thus describing a clockwise rotation of the upper part of the Longmenshan fault after metamorphism. Across the Xianshuihe fault, the truncated KI zones within the Triassic flysch suggest a total offset of roughly 50–60 km due to post-metamorphic sinistral strike-slip. Peak metamorphic conditions are estimated at 365–431 °C with a pressure facies of low to intermediate type (3 kbar). Compression at the end of the Triassic was induced by the interaction of the South China, North China and North Tibetan blocks, which caused the closure of the Paleo-Tethys Ocean and led to the folding of the Triassic flysch within the Songpan–Garze basin. Very low to low grade metamorphism was caused by crustal thickening due to an accretionary wedge setting, which led to an increase in the thickness of the Triassic flysch to > 10 km, and resulted in a temperature increase in those rocks due to burial. Finally, the Indo-Asia collision caused the formation of the Longmenshan and the Xianshuihe strike-slip faults in the early Tertiary and disturbed the distribution of the metamorphic zones.