Seismic imaging of oceanic crust across the northwestern subbasin, South China Sea

Located at the northwestern part of the South China Sea(SCS) among the east subbasin, the Zhongsha Islands(Maccelesfield Bank), and the Xisha Islands(Paracel), the NW subbasin is the smallest ocean basin in all the three subbasins of SCS. It presents all major stages from rifting in the late Oligocene to initial seafloor spreading in the mid-Miocene, thus provides an ideal place to study the rifting and initial seafloor spreading process in great detail. In autumn 2006, a wide angle seismic profile has been acquired by the SIO, SOA of China, which starts from Pearl River Mouth Basin, goes across the NW subbasin and the Zhongsha Islands, ends at east subbasin of South China Sea, 484km in length. The refraction/reflection data were recorded by 13 three components Ocean Bottom Seismometers (OBS) and seismic energy was provided by 22 air-gun array with different volumes with whole volume of 5160inch³. A detailed velocity model was obtained by using an interactive trial-and-error 2D ray-tracing method. Data inversion and analysis show that, the crust thickness under the continental slope decreases from 21km to 11km, the crust thickness of the NW subbasin is about 7.7km, and the moho depth ascends from 21km to 11km. The crust of the NW subbasin similar to the east subbasin in structure is confirmed to be oceanic crust, with thicker layer 1(sedimentary layer) and thinner layer 2 especially obvious in east subbasin, which is some different from typical oceanic crust structure. The tectonic geometry and velocity structure of the NW subbasin and its margins on both sides shows symmetrical and conjugate and indicates pure shear mode in continental margin rifting mechanism. We don´t find clear seismic signals from high velocity layer under the lower crust of the continental margin in the northern of the NW subbasin, which supplies new evidence for the viewpoint that western part of the northern continental margin of South China Sea is non-volcano crust.