Intense magmatism in the form of widespread volcanoes and lava flows is identified in high-resolution 3-D seismic data over a post-rift sequence of the northern South China Sea (SCS). Such a magmatism post-dates the end of seafloor spreading in the SCS by at least 6.8 m.y. A detachment (boundary) fault propagating into a deep-seated magma chamber provided the main vertical pathway for magma migration. In turn, normal faults and dykes constituted a shallow plumbing system through which the magma migrated from the boundary fault and was extruded onto the paleo-seafloor. Volcanism occurred in the study area from ca. 8.2 Ma to ca. 1.1 Ma in the form of two distinct events, dated ca. 5.2 Ma and ca. 2.8 Ma, which are correlated with the Dongsha Event. Extrusive magma formed volcano edifices and extensive lava flows; the latter of which were confined to the troughs of sediment waves or, instead, flowed along submarine canyons. As a corollary, this study shows that in the SCS: (1) young magmatism is widespread on the northern continental margin, (2) seafloor morphology greatly influences the architecture of deep-water volcanoes, and (3) syn-rift faults (especially detachment faults) reactivated by regional tectonics closely control the magma plumbing systems.

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