Asymmetric continental crust stretching and seafloor spreading of the southwestern South China Sea: New insights from wide-angle seismic data

Along-strike heterogeneity of pre-existing Mesozoic structures have caused different rift-to-drift features from east to west of the South China Sea (SCS). The southwestern margins have undergone prolonged extension and propagating seafloor spreading, leading to crustal structures significantly different from the northeastern SCS margin. The geodynamic mechanism of the termination of seafloor spreading in the SCS is still enigmatic.

In this study, we present a ~780 km long P-wave velocity model across the two conjugate continental margins and the oceanic basin at the southwestern propagating tip in the SCS from a wide-angle seismic refraction profiling. We use joint refraction and reflection seismic tomographic inversion and a layer-stripping approach to obtain the velocity structure. We show that the crustal thickness varies from ~6–23 km in the extended continental domain to <5 km in the oceanic basin. Significant asymmetry in velocity structures is observed in both the continental and oceanic domains, across the mid-ocean ridge. The continental crust of the Nansha Block in the southwestern SCS margin has a velocity structure and a mid-crustal layer (Vp=6.0-6.5 km/s) comparable to those of the Xisha Block. In contrast, the conjugate continental crust in the northern margin features a thinner upper crust with a high velocity gradient and lacks a mid-crustal layer. Stretching factors show that the upper and lower crusts of the southern continental margin are uniformly extended. However, the upper crust of the northern continental margin is much more stretched than its lower crust. The fault-driven stretching factor calculated on the coincident multi-channel seismic profile is much smaller than the stretching factors of the upper crust calculated from crustal thinning, suggesting that the initial crustal thickness may be much less than 32 km. In the ocean basin, we observe high P-wave velocities with high gradient at 2-3 km below the top of the sedimentary basement, and they increase to 7.5 km/s at a depth of 6 km below the basement. Our results suggest that a highly serpentinized mantle underlies a thin oceanic crust at the southwestern tip of the SCS basin, and the magmatic budget was inadequate near the end of the southwestward spreading propagation of the SCS.