Seismic Structure of the Lower Ocean Crust in the South China Sea

Over 60% of the global oceanic crust forms at intermediate to fast spreading mid-ocean ridges, making the study of the lower ocean crust crucial for understanding magma processes, and the interactions between hydrothermal circulation and serpentinization during seafloor spreading. However, direct observations of gabbroic cumulates in the lower crust remain scarce. Previous studies have identified lower crustal reflections (LCRs) beneath mature oceanic crust in the big oceans, with these reflections formed due to faulting, lithological layering, or ductile shear zones. Here, we employed extensive multi-channel seismic data to investigate the crustal structure in the East Sub-basin of the South China Sea. Our analysis revealed a network of LCRs located between the COT and the early-formed oceanic crust (before the ridge jump) on both sides of the basin, where the basement exhibits a relatively smooth with minor faults, and the Moho layer is distinctly identifiable. These LCRs are concentrated in the central segment of the basin, particularly in the right side of the Zhongnan Fault Zone, while they become sparse or completely absent in other regions. Additionally, our examination of LCR length and dip angles along ridge-normal and ridge-parallel profiles revealed remarkable uniformity, challenging the shear zone model that anticipated only ridge-ward LCRs in the southern part of the basin. We propose that the observed LCRs are magma intrusions, potentially formed during periods of unstable seafloor spreading following continental breakup or subsequent magma intrusions occurred nearby off-axis ridges. This study enhances our understanding of the magma processes during crustal accretion in the marginal basin.