Abstract:

Several seismic profiles and one oceanbottomseismic profile crossing the Northwest Subbasin, South China Sea, are introduced in this article. Based on the interpretations of these profiles and combined with structural geology, we have discussed the Cenozoic tectonosedimentary characteristics and extension model of this subbasin. The results show that the Northwest Subbasin began its seafloor spreading since 30 Ma. The extension activities mostly happened during the Oligocene. The rifts distributed symmetrically around the spreading center, which is occupied by igneous body now. Sediments in this period are characterized by chaotic and discontinuous reflectors, indicating riftfilling clastic sediments. Further to the west, the subbasin has more rifted depression characteristics. After 25 Ma the spreading center migrated to the south, the Southwest Subbasin began its spreading. The NW direction stress caused by it may have ceased the opening of the Northwest Subbasin. The Northwest Subbasin experienced thermal cooling and exhibited a broad subsidence. The ZhongXisha block may have influenced the structural distribution of the basin. The deep crustal structure showed by the velocity model of OBS profile are also symmetrical around the spreading center, which indicates that the Northwest Subbasin experienced its extension in pure shear mode.

Key words:

Key words: the Northwest Subbasin, South China Sea; Cenozoic; tectonosedimentary characteristics; extension model