Variable Oceanic Crust Accretion at a Slow‐Spreading Ridge: A V p /V s Ratio Evidence Across the Southwest Sub‐Basin of the South China Sea

Abstract The opening of the Southwest Sub‐basin (SWSB) of the South China Sea was slow‐spreading and has been propagated toward the southwest. Yet, the structural variability and magmatic accretion process of the SWSB are still enigmatic. Here, utilizing a wide‐angle seismic profile traversing the central segment of the SWSB, we obtain detailed S‐wave velocity and V p /V s ratio models to investigate oceanic accretion process. In the first stage of seafloor spreading (ages >19.2 Ma), the V p /V s in the northern conjugate flank is more variable, while the V p /V s in the southern flank is lower and more convergence, with V p and V s decreased, indicating more vigorous magmatism on the northern flank but more thick cracks developed on the southern flank. In the second stage of spreading (ages ∼19.2–17 Ma), V p /V s ratios in different conjugated flanks indicate pronounced asymmetry in oceanic lithospheric structure. The V p /V s ratios in the southern flank reveal a partially serpentinized mantle overlain by possible continental fragments embedded in basaltic rocks, contrasting to the normal oceanic crust in the north. Our results suggest that magmatic budget in the SWSB during seafloor spreading was asymmetric and consistently stronger in the north flank of the spreading axis, which probably associated with a southward ridge jump event. The high V p /V s ratios at the extinct spreading center reveal the serpentinization beneath both flanks of the median valley. After the cessation of seafloor spreading, the tectonic extension of the frozen magma chamber formed a median valley composed of highly fractured mafic rocks overlain by remnant continental fragments.