The Bangong-Nujiang Suture Zone, Tibet Plateau: Its role in the tectonic evolution of the eastern Tethys Ocean

Abstract

The Tibet Plateau was generated from a long-term, multiple and complex convergence of many micro-blocks and the final Indian-Eurasian collision, which records the evolution of the eastern Neo-Tethys Ocean. In order to clarify the evolution of the eastern Neo-Tethys, we reviewed the Bangong-Nujiang Suture Zone (BNSZ) according to the geological and geophysical evidence. We analyzed the BNSZ deep structure by geophysical data, and subduction polarity and the closure process of the Bangong-Nujiang Ocean by combining geophysical and geochronological data. Bouguer gravity anomaly is used to identify tectonic units on both sides of the BNSZ. We used multi-scale wavelet decomposition to separate the gravity anomaly at different depths for comparative analysis. Subsequently, the Moho discontinuity was estimated by gravity inversion. The inversion results show that the Moho depth varies from south to north beneath the Tibet Plateau, indicating a compression environment in a NS direction during the collision of micro-blocks. The collision caused uplift of the Tibet Plateau and an increase in crustal thickness. There is a clear Moho gradient zone along the BNSZ, and Moho depth changes more than 5 km between the South Qiangtang Terrane (SQT) and North Lhasa Terrane (NLT). The relatively shallow Moho surface of SQT indicates that the SQT experienced crustal thinning, which probably can be attributed to the different delamination of the lithosphere beneath the NLT and SQT. The nearly E–W-trending gravity gradient zone correlates well with the BNSZ. There are several gravity lows south of the BNSZ, that correspond well with outcrops of volcanic rocks, which developed from the Late Jurassic to Early Cretaceous, indicating that the Bangong-Nujiang Ocean may have subducted southward at that time. The BNSZ is a gravity gradient zone in the central Tibet Plateau, and the gravity field is very different on both sides of the BNSZ, which may be an important tectonic boundary. A combined analysis of gravity anomaly and ophiolite characteristics reveals different evolution of the eastern, central and western BNSZ. The evolution of the eastern Neo-Tethys consists of Carboniferous-Permian breakup, Triassic-Early Jurassic extension, Middle-Late Jurassic subduction and Cretaceous extinction. In particular, the closure of the Bangong-Nujiang Ocean represents the extinction of the eastern Neo-Tethys, which closed in the Yanshanian Period (Jurassic to Cretaceous), with scissor-style movement from east to west. The eastern BNSZ closed in the Middle Jurassic, while the central and western BNSZ closed in the late Early Cretaceous, and the collision-related deformation occurred mainly in the Cretaceous.