Seismic evidence for magmatism-driven mud volcanism in the northern continental margin of the South China Sea

Abstract

The Chaoshan Depression in the northern South China Sea (SCS) margin is abundant with submarine mud volcanoes, and hosts two primary sets of hydrocarbon source rocks within its thick Mesozoic strata. However, the driving mechanism for submarine mud volcanism there remains enigmatic. Here multichannel seismic and ocean bottom seismometer data of line CS-L1 across several mud volcanoes in the Chaoshan Depression are processed and analyzed. Beneath the mud volcanoes, several vertical fuzzy reflection zones are imaged within the Mesozoic strata. They feature distinctly lower velocities than their host strata, and are interpreted as mud diapirs formed by vigorous fluid infilling. High-velocity bodies (HVBs) with Vp > 7.0 km/s are widely imaged below the lower crust and interpreted as post-drift magma underplating bodies. Beneath the mud volcanoes, a thicker HVB extending upward to the depth of ∼15 km is observed, implying locally stronger magma underplating. Moreover, the overlying crust of this thicker HVB also features higher velocities than the surrounding crust. This crustal high-velocity zone extends upward to the shallow depth of ∼9 km, probably representing local magma intrusions sourced from the underlying magma underplating body. This strong association between the submarine mud volcanism and the stronger deep magma underplating and intrusion indicates that the mud volcanism in the Chaoshan Depression of the northern SCS margin was probably magmatism-driven. The deep magma underplating and intrusion firstly caused the surface uplift and the formation of high-angle normal faults. Subsequently, the heat from these intruded magmas promoted hydrocarbon gas generation of the Mesozoic source rocks and increased the pore-fluid pressure, generating mud diapirs within the Mesozoic strata with abundant fluid charging. The submarine mud volcanoes were finally formed with abundant fluids migrating from these mud diapirs to the seafloor along the high-angle faults.