Abstract: Studies on the crustal structures of the continental margin could contribute our understanding of the evolutionary history of the South China Sea (SCS). With gravity modeling works based on two newly acquired composite geophysical profile starts in the south of the South China Sea, we reconstruct the crustal structures in this area and discuss the stretching model. NH973-1profilestarts from the SW sub-basin of the SCS, and crosses the northern part of the central Nansha region. The NH973-2 profile lies in the more east and runs across east of the Reed Bank. Gravity modeling shows that the Moho depth is about 10~11km under the sea basin, and 15~21km under the southern margin. A sharp change from the sea basin to the continental margin could be observed. The thickness of the crust in the sea basin is between 6~7km, acting as typical oceanic nature. The crust in the Nansha region is extremely attenuated with thickness between 16 and 19km. Detailed analyses show that (1) a deeper Moho exists under the relic spreading ridge of the SW sub-basin; (2) a gravity high in the E sub-basin closed to the Reed Bank, which might be caused by the mixing of oceanic crust and deep magma; (3) the upper crust of Nansha region may have a high density zone, and on the lateral lithological features changes laterally. The crustal structures in the Nansha region and Reed Bank act similar features, and no high velocity layer has been found under either area. We suppose that the southern margin of the SCS should have experienced magma-poor rifting. Stretching factors were calculated along two profiles. The results indicate that fault-related stretching factors were different from the whole crust stretching factor. It suggested that the southern margin might have experienced non-uniform extension.

Key words: gravity modeling, crustal structure, magma-poor, stretching factor, South China Sea