FORMATION MECHANISM FOR BASINS ON THE WESTERN MARGIN OF SOUTH CHINA SEA

After examining the relationship among the Three Pagodas/Mae Ping/Red River fault systems and the sedimentary basins along the west margin of South China Sea, the authors pointed out that the extrusion model cannot explain the formation of the basins. The late Cretaceous - Eocene onset of extension in South China offshore basins coincides with a decrease in Pacific - Eurasia convergence rates, which vary from 120-140 m/a to 40 mm/a. The decrease in absolute convergence rate may have allowed slab rollback to initiate extension along the China offshore including the west margin of South China Sea. Since Oligocene, Australian plate has driven the Philippine Sea plate northward between the Eurasia and Pacific plates. The effect of the Philippine Sea plate would have removed the extensional force derived from rollback of Pacific plate. Uplift and erosion of China offshore basin and spread ridge as well as southward jumping of South China Sea reflect the decreasing influence on South China Sea of the Pacific plate subduction. It seems that the widespread Oligocene rifting across the west margin of South China Sea and the continued spreading of the South China Sea require a different extensional driving force. Slab pull force from southward subduction of a proto-South China Sea beneath Kalimantan may be a good choice. A great number of evidences support southward subduction beneath Kalimantan during the Oligocene to Mid-Miocene and the forces associated with this event would have the potential to generate the rifting of the basins and continued opening of South China Sea. Therefore, two forces were related to the subduction systems, i.e., the rollback of the Pacific plate and the traction force of the proto-South China Sea's southward subduction led to the extension of the basins along the west margin of South China Sea.