Vp/Vs ratios beneath a hyper-extended failed rift support a magma-poor continental margin in the northeastern South China Sea

Abstract

A wide-angle refraction and coincidental single-channel reflection seismic survey (line OBS2019–1) was acquired across the northeastern continental margin of the South China Sea (SCS). S-wave velocity and Vp/Vs ratios are obtained based on a previously published P-wave velocity model. The upper and lower crusts in the continental slope area show an average Vp/Vs ratio of 1.87 and 1.77, respectively. The high average Vp/Vs ratio (> 1.80) in the upper crust reflects large-scale magmatic intrusions. The magmatic conduits effectively connected the high velocity lower crustal anomaly and volcanic bodies in the continental slope area, indicating that post-spreading magmatism caused the high velocity lower crustal anomaly. The Vp/Vs ratios of high velocity lower crustal anomaly with Vp of 7.0–7.4 km/s below the continental slope area are 1.74–1.88, indicating compositions of gabbroic facies associated with post-spreading magmatic underplating. The average crustal Vp/Vs ratio of the Southern Depression (a failed rift) of the Tainan Basin is 1.74, indicating the affiliation of the continental crust. The high velocity lower crustal anomaly with Vp of 6.8–8.0 km/s below this failed rift shows Vp/Vs ratios of 1.78–2.06, falling within the mantle serpentinization domain. The maximum degree of serpentinization is estimated at about 29%. Mantle serpentinization and lack of syn-rifting magmatism below the failed rift provide new evidences supporting a magma-poor continental margin in the northeastern SCS, which was later underplated and volcanically intruded after continental breakup.