Seismic Imaging of an Intracrustal Deformation in the Northwestern Margin of the South China Sea: The Role of a Ductile Layer in the Crust

The continental margins of the South China Sea (SCS) have undergone episodic rifting since the Cenozoic, and there are ongoing debates surrounding the processes of crustal deformation and seafloor opening. In this work, we present a P-wave velocity model extending from the north of Xisha Trough to the Zhongshanan Basin in the northwestern SCS margin, using ocean bottom seismometer data of the wide-angle seismic profile OBS2013-1. The results show that the crust thins symmetrically across the western Xisha Trough, from more than similar to 20 km at the flanks to similar to 10 km in the central valley where the sedimentary layers thicken to over 6 km. In the Zhongsha Trough, closer to the deep basin, the upper crust is detached in a similar to 20 km wide region and the lower crust has seismic velocities increased by more than similar to 0.3 km/s. The top boundary of the lower crust is located at a depth of similar to 13 km across the Zhongsha Trough, and a similar to 5 km thick midcrustal ductile layer is imaged. A similar to 50 km wide ocean-continent transition region beneath the Zhongshanan Basin characterizes a similar to 6 km thick continental crust underlain by serpentinized and magnetized upper mantle. These observations, together with plate reconstructions based on gravity and magnetic analysis, suggest that deformation of the continental margin was controlled by a ductile crustal layer. Magmatism, associated with the early stage oceanic accretion, has mixed with the highly extended continental crust. Developments of the failed rifted basins were controlled by the westward propagation of the continental breakup.

Automated summary

The study used ocean bottom seismometer data to investigate the depth variations and geological structure of the continental crust in the South China Sea. The research suggests that deformation of the continental margin was controlled by a ductile crustal layer, with magmatism associated with early stage oceanic activity.