Breakup mechanism of the northern South China Sea: Evidence from the deep crustal structure across the continent-ocean transition

To improve constraints on breakup processes that eventually lead to seafloor spreading and the subsequent formation of new oceanic crust in the northern margin of the South China Sea (SCS), we collect a 172-km-long deep-penetration wide-angle seismic data OBS2018-L5. In the continental domain, the results present a gradually thinning crust with thickness ranging from ∼17 km to ∼8 km, with a slight thickening crust at the Outer Margin High. The attenuated continental crust is fairly rough and highly offset by distinctly different scale faults system. In the oceanic domain, the top of the basement is highly reflective with weak faulting and relatively constant crust thickness ranging from 5 km to 8 km (average 6 km). The continent-ocean boundary is characterized by an abrupt change in crustal thickness accompanied by a specific boundary between mostly positive gravity anomalies in the oceanic basin and grossly negative anomalies in the thinned continental crust, and between low amplitude, out-of-shape magnetic anomalies in the thinned continental domain and high amplitude, linear anomalies in the oceanic domain. A 0–5 km thick high-velocity layer (7.0–7.5 km/s) is present in the distal margin, which might represent mafic magma underplating. We also identify other magmatic features such as distinct buried volcanic edifices overprinting the original velocity structure. We propose that three mechanisms control the extension and breakup of the SCS: (1) the rapid transition from continental to oceanic crust, (2) elevated potential mantle temperature, and (3) the ductile lower crustal necking. Their interactions lead to the intense thinning of the continental lithosphere, decompression melting of the enriched asthenosphere followed by fast upwelling of mafic melt, relatively strong volcanic activities during the final stage of syn-rift and breakup, and finally generating a margin with intermediately rich magma, referred to as in intermediate-type margin.