Tectonic significance of the cold seepage zones in the eastern Nankai accretionary wedge – an outcome of the 15 years’ KAIKO projects

The tectonic significance of the cold fluid venting in the eastern Nankai accretionary wedge has been elucidated based upon results of precise mapping and submersible observation executed since 1984 under the French–Japanese cooperative projects (KAIKO I, II, KAIKO-Nankai and KAIKO-Tokai) in conjunction with studies of the Japan Trench, Sagami Bay and other regions. The very sharp boundary of benthic communities associated with temperature anomalies indicates the occurrence of focused upflow of nutrient-rich fluids through cracks in the carbonate crust existing on a near-surface horizon of permeable sediment. The linear alignment of venting zones is a surface expression of the shallow detachment faults. Methane-rich fluids expelled from the conduit along a fault migrate in the overlying permeable sediment and react with circulating sea water containing sulfate to produce hydrogen sulfide and carbonate precipitate. En echelon or zigzag patterns of the cracks are developed in the carbonate crust under the influence of tectonic shear stress. It has been concluded that bivalve communities grow on a thin veneer of sediment above the cracks of carbonate through which fluid containing hydrogen sulfide diluted with sea water flows out. On the other hand, bacterial mats seem to flourish either on thin sediment overlying the fault conduit or over a thick cover of sediment without carbonate crust. In the former case, bivalves cannot grow because venting fluids are too dense with methane and hydrogen sulfide to maintain the oxygen-rich condition of the bottom water. In the western Nankai Trough fine-grained less permeable sediment cover seems to prevent circulation of fluids, whereas methane appears to be widespread in the deeper sedimentary layer there. Major distinctions in modes of seepage among sites are presumed to be caused by: (1) the tectonic regime causing channels of upward fluid venting; (2) rates and composition of venting fluids affected by mixing rates of sea water; and (3) the permeability and thickness of the near-bottom sediment.