Distribution of the gas vents at the seabed of the Mid-Okinawa Trough and their controlling factors

Gas venting at the seabed is recognized as a key geological process for carbon transfer from the lithosphere to hydrosphere. Understanding how methane is transported and the geological expressions in this process is necessary for quantatively evaluating its climatic impact on global carbon cycle. In this study, multi-beam echo-sounder (MBES) and multi-channel seismic (MCS) data acquired between 2013 and 2016 from the Mid-Okinawa Trough are used to show the geophysical features associated with gas vents. We identified the bundle-shaped gas flares in water column, the migration pathways consisting of cylindrical cluster of fractures in the shallow subsurface and the seabed domes underlain by carbonate-cemented sediments. They are regarded hereby as the geological indicators of gas vents, indicating how gases are vented in the Mid-Okinawa Trough. An example has been selected for description to represent the gas vents in the study area. We proposed that the distribution of gas vents in the study area is controlled by basin-scale tectonic activities after analyzing their spatial relationship with the faults. The oblique rifting in the Mid-Okinawa Trough resulted in the formation of the transtensional faults. The faults that formed in the tensile regime provide vertical conduits for gas-rich pore fluids to migrate, which caused that the gas vents were elongated along the normal faults. This study suggests that gas vents on the seabed can occur extensively in the extension-dominated tectonic regime.