Distribution of methane hydrate BSRs and its implication for the prism growth in the Nankai Trough

Detailed mapping of a bottom simulating reflector (BSR), which marks the phase transition from the methane hydrate layer above the reflector to free gas below, was conducted in the Nankai accretionary prism off Shikoku and Tokai. BSRs are widely distributed in the prism slope from the toe region to the forearc basin. BSR positions provide us with the regional heat flow variations using pressure-temperature conditions for methane hydrate stability. Estimated heat flows generally show constant values about 50 mW/m(2) shallower than the middle slope of the prism, and gradually increase seaward in the lower prism slope. Occurrences of BSRs are regarded as accumulation of free gas beneath the base of a gas hydrate stability field (BGHS) and/or concentration of methane hydrate above the BGHS. These conditions can be accomplished by updip migration of methane gas because it is unlikely that such methane concentration is completed by in situ biogenic methanogenesis within sediments including low total organic carbon. Moreover, sedimentation, uplifting, and sediment stacking by thrust faulting cause upward migration of the BGHS and migration of methane from the dissociated hydrate to new BGHS. Such recycling of methane gas may have actively occurred in accretionary prisms. In contrast, there are five regions of no BSRs: the Nankai Trough floor, prism toe, slope basin, steep slope, and deep-sea canyon. The trough floor, the prism toe and the slope basin are characterized by young sediments with low production of methane gas and sub-horizontal strata unsuitable for migration of gases and fluids. Erosion at the steep slope and the canyon causes removal of hydrated sediments and downward movement of the BGHS. BSR distribution and thermal structure estimated from BSR positions offer information about active processes occurring in accretionary prisms. (C) 2002 Elsevier Science B.V. All rights reserved.