Large-scale igneous intrusion emplacement as a trigger for fluid seepage on the northern South China Sea margin

Abstract

Widespread mud eruptions have been identified along the northern margin of the South China Sea. However, the ultimate driving force for initiation of the eruptions remains to be discussed. Here, we investigated the discovered hydrothermal vents and the presence of gas/fluid occurring above deep-seated igneous intrusions and volcanic edifices and consider them as a likely trigger. Using a one-dimensional (1D) numerical model, SILLi 1.0, we studied the thermal effect of a 100-m-thick dolerite sill complex emplaced at 1100 °C and 3.8 km depth within the organic-rich Triassic series (∼3.5 wt% total organic carbon). The modeled scenarios also account for variations in sill count and thickness and the effects of erosion. The calculated CH4 produced from contact metamorphism reaches a peak of ∼700 kg/m2/yr shortly after emplacement of the first sill in the complex. When scaled to a sill size of 100 m by 25 km2, i.e., a sill volume of 2500 km3, ∼42−550 Mt of CH4, depending on individual sill thickness, have been produced within 10,000 yr of sill emplacement from sedimentary organic matter in the aureole, with peak generation of ∼17.5 Mt CH4/yr just after emplacement. The newly discovered vents are suggested to have formed as a direct result of sill emplacement and by overpressure generated in the sediments around the sill by hydrothermal fluids and gases liberated during organic matter degradation. Combining data from seismic images with the likely production of carbon gas released from contact metamorphism with igneous intrusions, we suggest that volcanically induced gas release is common along the South China Sea margin. This study supports the important role of magmatism in driving regional-scale gas/fluid seepage. Our findings also suggest that the hydrothermal vents and intrusive magmatic bodies provide postemplacement fluid focusing pathways to shallower strata or the seafloor over long time scales.