Large volume of magma involved in South China Sea rifting: Implication for mantle breakup earlier than crust

Rheological differences may lead to discrepant thinning order of the crust and lithospheric mantle when the continent is passively stretched. The crust breaks up earlier than the lithospheric mantle under a low stretching rate when the lower crust is strong; otherwise, the lithospheric mantle may break up earlier than the crust. Here we report a new high-resolution wide-angle seismic velocity model from ocean bottom seismometers (OBS) across the northern continental margin of the South China Sea (SCS). Significant crustal structure and deformation style discrepancies are observed from the proximal to distal margin of the northern SCS. High-velocity lower crust (HVLC) up to 10 km in thickness was observed below the Yunli uplift and the oceanward margin, indicating magmatic underplating, while nearly no HVLC was found below the highly extended landward sags. Seismic data showed a low ratio of upper/lower crustal thickness below the underplating area, presumably caused by magmatic accretion and lower crustal flow. Largely seaward-dipping faults with small vertical offsets were observed on the seaward side of the HVLC. The OBS and multichannel seismic data favor a model with the lithospheric mantle breaking up prior to the crust at the SCS. Based on the observed stratigraphic relationship and volcanic rocks dating results, we conclude that erosion of the sedimentary strata is synchronous to the magmatic underplating, which indicates that lithospheric mantle breakup occurred at ∼43–38 Ma. The northern central SCS breakup occurred in two steps, in which the lithospheric mantle broke up ∼10 Myr earlier than the crust (at ∼31–29 Ma).