Xi Chen

The distribution and characteristics of the Moho reflections (M-reflections) in oceanic basins record the seafloor spreading process and provide crucial insights into the formation and evolution of the oceanic basins. Approximately 20,000 km of multichannel seismic reflection sections are used to characterize the M-reflections and the crustal structures in the South China Sea oceanic subbasins. The identified M-reflections are mainly distributed in the northwest subbasin, the east subbasin, and the northern and southern flanks of the southwest subbasin (SWSB). They exhibit a banded distribution pattern in two-way traveltime along the north–south direction, with shallower depths observed in the central region and gradually deepening toward the northern and southern sides. In addition, the M-reflection amplitudes generally weaken, and the reflection events become less continuous as the igneous oceanic crust thins along with the seafloor spreading prior to the second ridge jump event. Ultimately, the M-reflection becomes indistinguishable in proximity to fossil spreading centers. These observations suggest a decreasing magma supply toward the cessation of the spreading. Despite the significance of Moho interface imaging at the crust-mantle boundary, the distinct M-reflections are absent in the SWSB and the vicinity of the Zhenbei-Huangyan Seamounts Chain, which are attributed to several factors, especially the vague nature of Moho interfaces.