Rift structures and its related unconformities on and adjacent the Dongsha Rise: insights into the nature of the high-velocity layer in the northern South China Sea

Abstract

It is curious that the typical feature of high-velocity layer (HVL) underneath the crust for the magma-rich margin was found in the magma-poor margin of the South China Sea. In order to better understand the nature and age of the HVL in the South China Sea, here we provide seismic imaging on the Cenozoic structure and sedimentation using a regional seismic reflection profile combined with existing drilling data and seismic refraction lines in the northeastern South China Sea. An important finding is that the extent of erosion since Neogene is only on the Dongsha Rise and is much smaller than that of the HVL observed across the northern margin of the South China Sea, which suggests the occurrence of the HVL has no influence on the erosion on Dongsha Rise through Neogene. On the regional seismic profiles, the different structures between the deepwater rifted margins and the continental shelf were also observed. The faults in the Zhuyi Sub-basins on the continental shelf are imaged to be high-angle and bound a series of grabens and horsts. In contrast, the deepwater region is characterized by tilted fault blocks and an array of listric normal faults, which extremely thinned the crust to be ~ 5–10 km. After middle Miocene, active magmatism occurred in the deepwater region at the time of post-spreading of oceanic crust. These listric faults on the hyper-extended crust is interpreted to be conduits for magma migration to the shallower levels. This typical structure in the deepwater region changed the geophysical nature of the HVL underneath the extreme thinned crust, which should be formed before the event of hyper-extension and is likely to be associated with the subduction of the paleo-Pacific toward the Asia in the late Cretaceous. Our study will answer the question why the South China Sea with the HVL underneath the crust is a magma-poor passive margin.