Based on geophysical data, the northern margin of the South China Sea since the Cretaceous includes three types of crust; continental, transitional and oceanic. The crustal thickness changes from about 34 km to 8 km from land to sea. Seismically, the crust can be subdivided into three layers and the lower crust shows stronger reflectivity, which indicates the lower crust is on accretion. Its reflective interfaces can be a detachment surfaces. The formation of the northern margin and the basin of the South China Sea experienced two periods of spreading in the early Cretaceous (126-120 Ma) and the early Tertiary (32-17 Ma). Its dynamics is the plume tectonics caused by the subduction of the Australian-Indian oceanic plate down to the South China Sea about 670 km. The northern margin of the South China Sea has similar features to the Atlantic margin. The oceanic crust of the South China Sea originated from the extension of the passive margin. The continental remnants in the South China Sea came from the breaking and shifting of the South China - Indochina convergent plate.