New insights into hydrothermal venting systems along a passive continental margin of the South China Sea

The South China Sea is associated with voluminous post-rift magmatic activities. The widespread emplacement of igneous intrusions drives magma degassing and thermogenic alteration of organic matter, resulting in broadly distributed hydrothermal venting systems situated above igneous bodies. Geophysical observations reveal >340 potential hydrothermal vent complexes (HTVCs), 430 subsurface fluid structures, and 76 forced folds within magma-sediment interaction zones of the northern South China Sea. After combining geophysical and geochemical data, this study postulates that the hydrothermal venting systems are characterized by the following key factors: (1) Intense hydrothermal activity typically occurs in regions of active tectonism or magmatism, primarily in the Northeast, Baiyun−Liwan, and Northwest zones; (2) fluid and gas emissions to the seafloor or into sedimentary strata occur chiefly above igneous bodies or along deep-seated faults, characterized by variable amounts of mantle-derived or thermogenic CO2 and thermogenic CH4; and (3) for the three investigated zones, the hydrothermal venting systems vary in their gas and/or fluid manifestations and compositions, controlled by spatially distinct structural, stratigraphic, and magmatic expressions. Detailed seismic interpretations indicate that most hydrothermal activity occurred during the Pliocene and Quaternary, with minor occurrences dated to the early−middle Miocene and late Miocene. By combining data from seismic images with geochemical analysis, our study provides compelling evidence of interactions between fluid seepage and igneous intrusions, indicating more complex fluid expulsion systems in the South China Sea than previously recognized. We therefore suggest an igneous control on the long-term fluid expulsion and seep dynamics, implying that additional undiscovered seeps likely occur near the hydrothermal vent complexes. The South China Sea, characterized by thick, organic-rich sedimentary packages and prolonged post-rift volcanism, represents an ideal setting for investigating how magmatism affects fluid and gas generation and seepage, similar to modern, young sedimented rifts.