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

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.

Automated summary

This article focuses on the crustal structure of the continent-ocean transition zone in the mid-northern margin of the South China Sea. Through geological and geophysical surveys, as well as modeling analysis, the landward and oceanward boundaries of the transition zone were determined. The variations in crustal thickness and tectonic activities during the transition were revealed.