Seismic Constraint From Vp/Vs Ratios on the Structure and Composition Across the Continent-Ocean Transition Zone, South China Sea

At non-volcanic passive continental margins, seismic techniques often failed to uniquely define the nature of crustal domains. Here, we overcome this problem by studying the structure and composition of the continent-ocean transition (COT) in the Southwest Sub-basin of the South China Sea, using P and S wave seismic tomography and V-p/V-s ratios, providing unique constraints on lithology. Throughout the image domain, we can rule out large areas of exhumed mantle as V-p/V-s ratios are always <1.9 in the shallow basement layer. Instead, the COT is characterized by extended and fragmented continental crust, and possibly mafic aggregation at the bottom of the crust. In concert with observations from multichannel seismic reflection data, seismic velocities and V-p/V-s ratios suggest that the oldest oceanic crust was formed by starved magmatism, causing rugged basement, thin crust, nearly absent lower crust, and moderately serpentinized mantle below. Our results reveal that rifting occurred without un-roofing continental mantle.