Postseafloor Spreading Volcanism in the Central East South China Sea and Its Formation Through an Extremely Thin Oceanic Crust

P wave velocity models were obtained by forward and inverse modeling from 38 ocean bottom seismometers deployed in the central East subbasin of the South China Sea (SCS). Four types of crust have been defined: (a) thin oceanic crust (<5 km), (b) typical oceanic crust (5-6 km), (c) thick oceanic crust hosting postspreading volcanoes (>6 km) with significant intrusive roots, and (d) thick oceanic crust with enhanced spreading features (>6 km) but without significant roots. Within the central East subbasin, the thin oceanic crust, only identified inside an 80 km wide zone, is located within an overall 150 km wide domain characterized by N055° seafloor spreading trends. The postspreading volcanoes were formed during a N-S tensional episode around 6-10 Ma, several millions of years after seafloor spreading ceased in the SCS. Seafloor spreading (N055° and N145°) and postspreading (N000° and N090°) features are observed in the morphology of some of these volcanoes. The rupture of the brittle thin oceanic crust was focused where the crust was the weakest, i.e., at the intersection of the extinct spreading ridge with former fracture zones. From geological and geophysical arguments, we suggest that the postspreading volcanism might have been influenced by the Hainan plume activity through a buoyancy-driven partial melting mechanism.