Sedimentary successions and onset of the Mesozoic Qiangtang rift basin (northern Tibet), Southwest China: Insights on the Paleo- and Meso-Tethys evolution

The Mesozoic Qiangtang Basin is investigated to reconstruct the tectonic evolution of the Paleo- and Meso-Tethys Oceans. We present a detailed sedimentary facies analysis of the Nadi Kangri and Quemoco formations and the systematic zircon U Pb ages of the volcanoclastic rocks in the Nadi Kangri Formation to understand the onset and nature of the Mesozoic Qiangtang Basin. The Nadi Kangri Formation, consisting of volcanic-volcanoclastic facies associated with alluvial-diluvial facies association, which represents continental volcanic succession. The Quemoco Formation, consisting of braided fluvial, restricted platform and tidal flat facies associations, represents continental siliciclastic and marine carbonate successions. The deepening-upward transgressive sequences reconstructed from the two formations are indicative of a rift basin. Zircon U Pb ages of the volcanoclastic rocks in the bottom, middle, and upper sections of the Nadi Kangri Formation in the Fanghu area are 221.7 ± 1.3 Ma, 217.0 ± 1.5 Ma, and 207.1 ± 1.3 Ma, respectively. Combined with the previous zircon ages of the Nadi Kangri Formation, we interpreted that 221.7–220.4 Ma is the initial age for the onset of the Mesozoic rift basin. An evolution pattern for the Tethys Ocean and Mesozoic Qiangtang Basin has been proposed based on the magmatism, metamorphism, sedimentary sequence, and deformation data obtained from the Kunlun (KLT), Hoh Xil–Bayan Har (HBT), North Qiangtang (NQS), Central Uplift Belt (CUB), South Qiangtang (SQS), and North Lhasa subterranes/terranes (NLS) during the Early Triassic–Middle Jurassic. Based on the new data, we interpret a double-sided subduction and subsequently a collision of the Paleo-Tethys Ocean beneath the KLT to north and NQS to south, probably occurring at ca. 252-237 Ma and 237-227 Ma, respectively. This interpretation is consistent with the Late Triassic residual sea (ocean) model that attributed to the origin of the Hoh Xil–Bayan Har Basin. The QT and NLS experienced a strong extension between ca. 225 and 201 Ma owing to the rapid expansion of the Bangong Lake–Nujiang Ocean (Meso-Tethys Ocean), which is consistent with the massive volcanic eruption and deposition of volcanic sediments in the QT and NLS. In addition, the QT and NLS experienced a stabilized subsidence during 201–166 Ma, due to the existence of the mature Bangong Lake–Nujiang Ocean.