Rift evolution at the V-shaped tip of the Southwest Sub-sea basin, South China Sea: Insight into the formation of two phases of the proto-oceanic crust.

This study presents evidence of the evolution of proto-oceanic crust at the South China Sea through magmatic, tectonic, and stratigraphic analysis. The evidence was obtained by interpreting seismic profiles across the V-shaped tip of the Southwest Sub-basin(SWSB) of the South China Sea (SCS). SCS is the largest marginal sea of Southeast Asia located at the intersection of the Tethys and Pacific subduction zones. The V-shaped tip of the SWSB is located at the end of the propagation of the SCS Ridge, where the lithosphere is in a critical state of breakup. This region formed an Oceanic-Continental Transition(OCT) that differs from the Atlantic rift margin and the Penrose crust. It is considered a special type of OCT with complex composition and structure and is called the proto-oceanic crust. However, the insufficient research on the OCT at the SCS hindered our comprehension of its rift margin evolution and dynamic mechanisms. Our study establishes the tectonic-stratigraphic framework and reconstructs the rift margin evolution of the SWSB using high-quality multi-channel seismic profiles across its V-shaped tip. Based on our interpretation of magma, faults, and sequences, we present the following perspectives. The transition from continental crust to proto-oceanic crust occurred through the 'crocodile's mouth' structure, where erupted magma and bottom-invaded magma clamped the tip of the continental crust. The direction change of the seafloor spreading during the propagation process of the SCS ridge results in stratigraphic deformation and denudation, leading to the formation of the outer high and the two phases of the proto-oceanic crust. Combined with the analysis of gravimagnetic and topographic data, it is found that the two phases of proto-oceanic crust were formed before and after 20Ma when the direction of the seafloor spreading direction changed. These findings suggest that the establishment of mantle convection during the lithospheric extension and break-up of the V-shaped tip of the SWSB was jointly controlled by the evolution of the tectonic stress field and the propagation of the SCS ridge. This interaction led to the formation of the two phases of the proto-oceanic crust. This study is significant for understanding the formation and evolution of the OCT and provides insights into the interaction between magma and tectonics, as well as the dynamic mechanisms of the rift in the context of a marginal sea, which differs from the Atlantic Ocean rift.