Crustal structure beneath the Zhongsha Block and the adjacent abyssal basins, South China Sea: New insights into rifting and initiation of seafloor spreading

Abstract

Continental rifting, break-up, and onset of seafloor spreading are inherently controlled by the segmentation and structure of the continental domain suffering from extension. Today, the Zhongsha Atoll (ZS) is wedged between the Northwest Sub-basin (NWSB) and the Southwest Sub-basin (SWSB), two oceanic abyssal basins of the South China Sea (SCS). The nature of the crust and the structure of the transition from continental to oceanic domain are key to revealing the processes and dynamics during the rifting and break-up of the Zhongsha block. In this paper, we present a P-wave velocity model obtained from both forward modeling and tomographic inversion of wide-angle seismic line OBS2017-2. The results support the continental nature of the Zhongsha Block with a thickness of up to ~25 km. However, the transition from the thick continental domain of the ZS into both adjacent abyssal basins shows clear differences. To the north, a ~120 km wide domain of extended continental crust was observed. Farther north, the NWSB is characterized as a narrow basin with typical oceanic crust. The transitional domain between the continental and oceanic crust shows a ~30–40 km wide region with a high-velocity lower crust reflecting excessive magmatism. In contrast, the SWSB is characterized by a sharp transition from the thick continental crust of the ZS to thin oceanic crust which is probably underlain by serpentinized mantle. The strong rheological properties of the pre-rift crust in the western part of the SCS margin may be the reason that rifting concentrated on narrow rifts and thinning focused on necking domains, while the ZS avoided any intense extension. The configuration of rigid blocks thereafter affected the break-up position and the style of oceanic crust.