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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.

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