The crustal structure of the southwestern South China Sea from seismic reflection and refraction data; implications to continental breakup, slow-spreading ridges, and subsequent mantle activity

The crustal structures of the continent-ocean transition (COT) zone and oceanic domain are key to revealing the tectono-magmatic evolution from rift to drift and the following seafloor spreading. We develop a comprehensive study of the deep seismic reflection imaging and tomographic inversion of a wide-angle seismic line that runs across the COT and extinct spreading center of the Southwest Subbasin (SWSB) in the South China Sea. We reveal a low-velocity (<3 km/s) region in the shallow upper crust of the Longmen Seamount, which may represent serpentinized mud or volcanoclastic. The mature oceanic crust is approximately 4-6 km thick with high-velocity bodies (7.2-7.5 km/s) overlapping the Moho, reflecting relatively rich magmatic additions during seafloor spreading. The northern and southern COT segments exhibit a prominent long-wavelength high magnetic anomaly and synbreakup volcano, indicating magmatic additions in the lower crust and lava flows with magma ascending along faults to the surfaces. However, the COT in the south is wider than that in the north. In addition, the southern COT is characterized by limited PmP reflections, well-developed rotated fault blocks and ocean-ward synrifting faults, and a low-velocity (7.5-7.8 km/s) upper mantle, suggesting that the southern COT is probably underlain by a local serpentinized mantle during continental lithosphere breakup. The differences between the southern and northern COTs on the origin of high-velocity lower crustal layers and faulting styles imply asymmetric continental breakup processes in the SWSB.