Crustal structure and rifting of the northern South China Sea margin: Evidence from shoreline-crossing seismic investigations

Seismic reflection and refraction data acquired from both onshore and offshore in the South China area and on the continental shelf of the northern South China Sea passive margin provide new constraints on the P-wave velocity structure beneath the continental shelf and contribute to constructing a continental rift evolution model. Based on the crustal thicknesses and P-wave velocities derived from the velocity structure of the seismic profile OBS2010-2 across the continental shelf, two distinct domains are identified: (a) the thinned/extended continental crust and (b) the transitional crust. The thinned continental crust has a thickness of 25-27km across the Hainan Rise, decreasing to 21-23km further south in the rift and faulted depression zone. The transitional crust is characterized by a thickness of 20-21km. High-velocity values (>7.0km/s) in the lower crust of the transitional domain may indicate possible magmatic underplating during rifting. Our results show that the Cathaysia Block extends from onshore South China to offshore into the South China Sea and is bounded by the transitional crust of the continental shelf. The initial rifting of the continental margin occurred in the rift and faulted depression zone and might be explained by the upwelling of the lithosphere based on a depth-dependent extension mechanism during the late Cretaceous or Paleocene rifting.