Current Status and Challenges for Three-Dimensional Deep Seismic Survey in the South China Sea

The deep velocity structures are the basis for building the formation and revolution of the South China Sea (SCS). Two-dimensional (2D) and three-dimensional (3D) OBS surveys are successful technique to acquire the deep seismic structures. The results from 3D OBS surveys in the SCS have enhanced the knowledge for theoretical concepts. In the Southwest sub-basin, the asymmetrical velocity structures have been presented across the extinct spreading ridge (ESR) due to the development of detachment faults. In the central part of the east sub-basin, four types of crust have been defined:thin oceanic crust (< 5 km); typical oceanic crust (5-6 km); thick oceanic crust hosting post-spreading volcanoes ( > 6 km) with significant intrusive roots; and thick oceanic crust with enhanced spreading features ( > 6 km) but without significant roots. The post-spreading volcanoes (Zhenbei-Huangyan seamounts chain) were formed 6-10 Ma ago during a N-S tensional episode, several millions of years after seafloor spreading ceased in the SCS, and emplaced where the crust was the weakest, might have been influenced by the Hainan plume activity through a buoyancy-driven partial melting mechanism. The study on Dongsha uplift highlights magmatic activities through upwelling channels after rifting ceasing. The velocity structure in the Manila subduction zone (northeastern SCS) shows the nature of the crust and its relationship with the Manila subduction slab. A 3D OBS survey will be carried out in 2018 in the area of the IODP Legs 367-368, aiming at building a model of lithospheric rifting and breakup mechanism in the continent-ocean-transition (COT) zone of the SCS. The joint 2D and 3D survey method will play a major role in the understanding of the SCS geodynamics by integrating IODP drilling results with information on the deep structures.