Tectonically Controlled Terminal Crustal Extension in the Southwestern South China Sea: New Wide‐Angle Seismic Evidence

Abstract We present a 780‐km‐long P‐wave velocity model derived from active‐source wide‐angle seismic data spanning the conjugate Zhongjiannan Basin and Nansha Block margins and the oceanic propagating tip between them. The model reveals different crustal structural domains and velocity variations between the margins. The Zhongjiannan Basin margin exhibits a hyper‐thinned crust (∼7–12.6 km thick) characterized by intense upper crustal thinning. In contrast, the Nansha Block margin, shows a wider crustal thickness range (6–23 km thick), and features a more uniform thinning, preserving a ∼5 km‐thick middle‐crust layer with Vp values of 6.0–6.4 km/s. The different styles of crustal thinning observed between the two conjugate margins reveal asymmetric extension, likely resulting from southward rift migration. This asymmetry extends into the oceanic domain, where the velocity structure reveals clear differences in crustal thickness and velocity gradients on either side of the extinct spreading center. The northern flank exhibits a high velocity gradient and the absence of typical oceanic layer 3, suggesting a partially serpentinized mantle underlying a thin magmatic crust. In contrast, the southern flank retains a two‐layer velocity structure comparable to that of typical oceanic crust. Our study demonstrates that the final stage of basin opening in the South China Sea was tectonically dominated and characterized by asymmetric rifting, differential magmatic accretion, and a limited magmatic budget.