Continental passive margins are commonly classified into volcanic (or magma-rich) and non-volcanic (or magma-poor) types based on the magmatic budget. It is difficult, however, to use either of these models to define the northern margin of the South China Sea (SCS). Based on 2-D/3-D time/depth-migrated seismic data and borehole data from the Pearl River Mouth Basin in the northern SCS, we examine the detachment fault systems and syn-rift magmatism in the Baiyun and Liwan Sags. The detachment fault systems and related tilted blocks of the northern SCS established in the Baiyun and Liwan Sags in previous studies may be mainly constrained to the Baiyun Sag. The Liwan Sag, however, acts as a “sag-type” basin accompanied by magmatic domes. Radiometric age of igneous samples from wells, coupled with multi-horizon onlapping terminations within syn-rift layers, reveal four-phase rift-related magmatism in the northern SCS. The crustal thinning in the Baiyun Sag is controlled by a detachment fault, but in the Liwan Sag, it is mainly controlled by a preferential thinning of the lower crust. High thermal conditions and intense syn-rift magmatism weakened the lower crust, which contributes to the depth-dependent thinning in the Liwan Sag.

The formation of a new plate boundary and creation of the first oceanic crust, two of the most important processes of plate tectonics, still remains little understood. While older studies used to assumed a sharp ocean-continent boundary between continent and ocean, recent studies suggest a progressive ocean-continent transition (OCT) between unequivocal continental and oceanic crusts. In the latter view, breakup is not instantaneous but a lasting phase, which raises questions about the nature of the OCT basement and the processes operating between continental and lithospheric breakup. Based on detailed observations of high-quality and yet unpublished reflection seismic data, we describe and interpret the characteristic structures of the NW-South China Sea OCT and their relationship with overlying syn-breakup phase sediments. We show that the OCT displays a transition from fault-dominated rifting to magma-dominated seafloor spreading. On its continent-ward side, the OCT is made of hybrid crust where tectonic thinning of continental crust is compensated by syn-extensional magmatic thickening. Oceanward, the hybrid crust evolves into a fully magmatic but fault-dominated proto-oceanic crust, and finally turns into a mature Penrose-type oceanic crust. Relying on the growth structures observed in the syn-breakup sedimentary sequences and magmatic additions, we propose a kinematic restoration of the breakup phase. We suggest out-of-sequence flip-flop faulting to explain the switch from asymmetrical, fault-dominated-extension, to fully magmatic and largely symmetrical syn-extension accretion recorded in the syn-breakup sedimentary sequences overlying the OCT.