The Seafloor Fabrics and Evolution of a Propagated Rift System in the South China Sea

The South China Sea (SCS) can be divided into several sub-basins. Among all, the eastern sub-basin of the SCS is the largest one. The eastern sub-basin holds the critical role of understanding the final evolution of the SCS seafloor spreading. Based on two marine geophysical surveys in 2014 and 2019, we can find clearly the seafloor fabrics and the change of the seafloor spreading from almost N-S direction to NW-SE direction. The SCS seafloor spreading ceased around 15 Ma. The analysis of the seafloor fabrics can help us understand the evolution of the seafloor spreading of the eastern SCS sub-basin. Our results show that the extinct mid-ocean-ridge of the SCS was segmented. A fossil overlapped rift system is also identified, which includes a propagated ridge and a failed ridge and several abandoned ridges. Several linear volcanic chains also exist in the eastern SCS sub-basin. The volcanic chains indicate that the SCS asthenosphere magma, probably linking to the Hainan plume, had involved in the SCS spreading. We suggest that the NW-SE spreading direction of the SCS started from ~23 Ma, when the mid-ocean ridge was segmented and one propagating rift started. Overall, the tectonic evolution of the eastern South China Sea sub-basin could be described in three stages. Before magnetic lineation C6b (~23Ma), the oceanic crust has spread in a N-S direction. After C6b, the seafloor spreading gradually changed from N-S direction to NW-SE direction. An area of fan-shaped seafloor fabrics accommodates the gradual change of the seafloor spreading direction. The SCS seafloor spreading ceased at the same time when the propagated rift finished.