Crustal structure of the basin in the Southwest Subbasin, South China Sea

Using two-dimensional seismic tomography, we reported a detailed P-wave velocity model of the basin area and the northern margin in the southwest SWSB. We used two OBS profiles (OBS973-1 and OBS973-3), and 12 OBSs were involved into forward modeling and inversion. The whole profile is approximately 311-km-long. The average thickness of the crust beneath the basin is 5.33 km, and the Moho interface is about 10-12 km. No High Velocity Bodies (HVBs) are observed, and only two thin high-velocity structures ( 7.3 km/s) in the layer 3 are identified beneath the northern continent-ocean transition (COT) and the extinct spreading center. It is suggested that the basin area is a typical oceanic crust. Combined with other refraction profiles in the SWSB, the thickness of crust became thinner from the east to the west, indicating a decreasing magma supply. Besides, the continental block shows asymmetric crustal thickness: the southern margin represents thicker crust than the northern margin, which may be related to the large scale of detachment fault systems developed in the southern margin. Revealed from the multi-channel seismic (MCS) profile, the profile here shows asymmetric structural characteristics between the north and south section of the spreading center, which may be controlled by detachment faults. The initial rifting is likely to occur in the south of our study area.KEY WORDS crustal structure; South China Sea; Southwest Sub-basin Extinct spreading center, Asymmetric extension; Thinned crust