The continent-ocean transition at the northern margin of the South China Sea

The northern margin of the South China Sea (SCS) has particular characteristics compared with typical passive margins. Several tectonic movements and volcanic activities have happened in the SCS and resulted in diverse structures along its northern margin. Based on several crustal-scale, high-resolution, multi-channel seismic reflection profiles and satellite gravity data across the northern SCS margin, this paper analyses the structures, volcanoes and deep crust of the continent-ocean transition zone (COT) at the mid-northern margin of the SCS to study the pattern and extension model of the SCS. Gravity modelling revealed that a stepped crustal thinning occurs across the COT and a High Velocity Layer (HVL) with 0.8-6 km thick is located in the slope below the lower crust. Results show that the COT close to the Dongsha uplift is characterized by a syn-rift depression and structural highs with igneous rocks bounded by seamounts seaward. Farther west, the COT shows a zone including syn-rift depressions, structural high with igneous sills, volcanic zone, and tilted fault blocks bounded by normal fault seaward. Reflector T60 may represent an important unconformity in Pearl River Mouth Basin indicating a delayed rift in the northern SCS margin related to a ridge jump in 23.8 Ma. Our study delineates an extent of HVL just located in the eastern portion of the northern SCS margin based on the geophysical data. The non-coupling of the spatial extent between HVL and COT shows that the HVL may not be a necessary sign of the COT at the northern SCS margin. The magmatic intrusions and HVL may result from post-rifting underplating due to the partial melting from lithospheric extension and the passive mantle/asthenospheric upwelling. This study proposed an intermediate rifting mode to explain the mid-northern margin of the SCS which is closer to the magma-poor margins than volcanic margins.