Crustal Structure of the East Sub-basin, South China Sea Constrained by OBS Refraction/Reflection Data

The spreading history and mechanism of the South China Sea (SCS) has long been a debatable research topic. A recent long N-S trending OBS refraction/reflection survey (750 km) led by Sun Yat-Sen University was carried out across the East Sub-basin, SCS, covering the entire oceanic basin and part of continental crust to the south. Using this new dataset, we obtained a P-wave velocity model along the profile by forward/inversion of travel-time picks with Rayinvr program. Our result shows strong lateral variations in the upper and lower crust structure. The velocity model reveals the northern flank of the oceanic basin has wider and thicker oceanic crust compared to the southern flank, separated the extinct spreading center (i.e. the Huangyan-Zhenbei seamounts chain). The asymmetry feature of the oceanic crust may be caused by the southward ridge jump and faster spreading rate of the northern flank. Our result also shows a low velocity body (~50 km in length and 5 km in depth) sightly south of the Huangyan seamount, suggesting possible remnant partial melting near the spreading ridge even more than 10 Ma has passed since cessation of the spreading process. We recommend the northward subduction of the proto-South China Sea caused the opening and the asymmetry of the SCS.