Seismic velocity structure and tectonic evolution of the Continent-ocean transition in the mid-northern South China Sea

Abstract

Continent-ocean transition (COT) is a key area for studying the lithospheric replacement from the thinned continental to oceanic crust. The extent and nature of the COT in the mid-northern margin of the South China Sea (SCS) have been reported recently after multiple geological and geophysical surveys, including IODP Expeditions 367/368/368X. However, the debate still remains due to limited constraints on the deep crustal structure and lateral variations. A 172 km long wide-angle reflection/refraction profile was conducted. The detailed crustal structure of the COT was mapped based on the forward and inverse modeling. The crust thickness decreased from ∼12 km in the thinned continental domain to ∼7 km in the COT, and then transitioned to ∼5 km in the oceanic domain. A ∼ 3 km thick layer with high velocity of 7.0–7.5 km/s at the bottom of the crust occurred continuously from the thinned continental to COT domains, which is interpreted as syn-rift underplating. Based on the features of crustal structure, sedimentary sequence and gravity anomaly, the landward and oceanward boundaries of the COT were determined which delimit a ∼ 20 km wide COT. The COT documents prominent tectono-magmatic interaction and rapid transition from final rifting to seafloor spreading. Eventually a model was proposed to explain the breakup processes characterized by brittle faulting in the upper crust coevally with ductile flow in the lower crust.