A Late Triassic depositional age for the Xujiahe formation, Sichuan basin: Implications for the closure of the Paleo-Tethys Ocean

Abstract

The Late Triassic transition from marine carbonate rock to clastic in the Sichuan Basin, east of the Tibetan Plateau, not only records the evolutional process of peripheral foreland basin, but also preserves the evidence of closure of the Paleo-Tethys Ocean. The depositional age of the Xujiahe Formation, Late Triassic clastic, is debated. A new radiometric date based on sensitive high-resolution ion microprobe U–Pb ages for zircons grains from a tuff sample is presented here, which is used to reinterpret existing magnetostratigraphic data from the Xujiahe Formation. From these data, we infer a 224-210 Ma depositional age range for the formation. In addition, a comprehensive data set of seismic-reflection profiles and wire-line logs indicates there are four basinal scale unconformities (including U/C-1, U/C-2, U/C-3 and U/C-4) in the Late Triassic in the Sichuan basin. Together, the age constraints and hiatus relationships not only imply that the initiation of the Sichuan foreland basin is represented by the unconformity in the Late Carnian. This unconformity (U/C-2) is interpreted here to be the result of the closure of the Paleo-Tethys Ocean, and also indicate that the Sichuan foreland tectonics probably ceased by 211 Ma following the extensive lithosphere delamination from the Qiangtang to Songpan-Ganzi terranes. The new understanding of the tectonic and depositional evolution of the Sichuan foreland basin provides insight into the quasi-simultaneous collisional geodynamic processes of the Paleo-Tethys Ocean.