Timing of India-Asia suturing: Evidence from a remnant peripheral foreland basin in Xigaze, South Tibet

Abstract

The absence of a fully preserved foreland basin system in the central-eastern Yarlung-Zangpo Suture Zone (YZSZ) of Tibet has hindered the understanding of the initial timing of the India-Asia collision along strike. To address this issue, remnant foredeep and wedge-top deposits of the foreland basin system are described from the Tethyan Himalaya sequence and the YZSZ in the Xigaze area. The foredeep is defined by the olistostrome of the Zongzhuo Formation, which consists of matrix and blocks, including chert, limestone, sandstone and volcanic rocks. Based on the matrix properties, the Zongzhuo Formation is divided into two units: a lower member (Zhilong unit) with a matrix composed of black shale and thin layers of siltstone and an upper member (Yongla unit) with a matrix composed of distinctly red shale or yellow-green siliceous mudstone with interbedded siltstone and limestone. The Zongzhuo Formation is dated to 65–57 Ma by detrital zircon U - Pb chronology and radiolarian assemblages. In the YZSZ, the Puxia and Jiabi units are interpreted as wedge-top deposits consisting of mudstone matrix and exotic blocks, such as sandstone, chert and limestone. The maximum depositional ages (MDAs) of the sandstone blocks from both units are ca. 65–59 Ma. Detrital zircon provenance analysis and trace element data exclude an intraoceanic arc origin and point to a source from the Gangdese arc and recycled accretionary wedge in the suture zone. Therefore, we propose that the initial collision between India and Asia occurred at 65 Ma in the central-eastern YZSZ.