Abstract: The Xisha block, covered by deep water, is a continental fragment formed during the process of lithospheric crustal thinning on the South China Sea, which recorded important information about the evolution of the South China block. In this study, integrated with the geological and geophysical data available, we studied the crust structure, tectono-stratigraphic of basins and faults in the Xisha block. We observed that the crust in the Xisha block is much thicker, which is significantly different from the crust structure of the surrounding basins. We observed small faults with high dip cut the basement underlying the Neogene reef buildup. These basement faults continued to cut upward and ceased their activity at the seismic horizon T60（~23Ma）. In contrast, to the north of the Xisha block the detachment fault system and extremely thinned the crust developed there. Integrated with the geological dynamic setting, we indicated that in the early Miocene, the activities of detachment faults in the Southwest Subbasin of the South China Sea facilitated the southward movement of the Nansha massif away from the Xisha block. The result from our study will not only improve our understanding of the break-up mechanism of the South China Sea, but also has significance implication for the petroleum exploration in this area.

Key words: South China Sea, Xisha block, wide rifting, detachment fault