Structural controls on widespread methane seeps in the back-arc basin of the Mid-Okinawa Trough

Abstract

Methane trapped in marine sediments surrounding continental margins can be driven by tectonic stress to escape into the oceans. What geological processes control their spatial occurrences in an extension-dominated tectonic setting is not fully understood. Here we use the multibeam echo-sounder (MBES) and the multichannel seismic (MCS) data to reveal the distribution of the gas seeps in the back-arc basin of the Mid-Okinawa Trough in the years 2013–2016. They are manifested by (a) the gas plumes in the water column, occurring either in isolation or as clusters; (b) the irregularly shaped areas of the medium- to the high-backscatter intensity at the level of the seabed and (c) some seismic gas chimneys below these areas. The spatial combination of these signature points toward the widespread upward migration of the gas-rich pore fluids. Two types of gas seeps (A and B) sustained by methane supply have been proposed. The results of the isotopic analysis (δ13CCH4 and δDCH4) show that methane having a mixture of thermogenic and microbial sources migrated upwards along the faults to feed type A gas seeps, while in type B ones the thermogenic methane preferentially migrated through the fracture clusters embodied as the seismic gas chimneys. The tensile faults formed during the ongoing rifting episode of the incipient back-arc basin and provided the permeable migration conduits for the gases trapped in the shallow subsurface. We propose that this faulting driven by back-arc extension determines, to a large extent, the potential locations of the temporal gas seeps in the Mid-Okinawa Trough. Some of the other free gases accumulated in the less faulted sediments and the spatial distribution of their seepage is controlled by the topographic high. Tensile faulting can disturb the overpressure build-up within the shallow sediments by releasing it prior to the seal failure in the extension-dominated regime.