The nature of the high-velocity layer, northeastern margin of South China Sea

Abstract:

We present a new 2-D forward P-velocity model of the crustal structure along the profile OBS2012 that delineate the Moho morphology and magmatic features of the margin in the northeastern-most South China Sea. These data reveal a normal continental crust ~27-29km thick in the Pearl River Mouth Basin (PRMB) and the thickness decreased moderately seaward. However, the crust thickened rapidly under the Dongsha rise accompanying the increasing of Moho depth and the thinning of sediment layer. In the Tainan Basin, which was highly stretched, the crust sharply thinned from 15 km to 8 km and the Moho depth shallows dramatically forming a convex in the distal model. A high velocity layer (HVL) (~7.0-7.6 km/s), with ~5-7 km thick in the low crust, was imaged under Dongsha uplift. What’s significant about the spatial position of the high velocity body is that the northeast of SCS high magnetic belt coincides with it, which perhaps is associated with a Mesozoic volcanic arc caused by the subduction of paleo-Pacific plate. Moreover, another HVL, with ~3-5 km thick, was found in the Tainan basin. We take this to be magmatic underplating, which caused by the lithosphere extension during postrift seafloor spreading.

Key words: high velocity layer (HVL), northeastern South China Sea, subduction of paleo-Pacific plate, Mesozoic volcanic arc, magmatic underplating