Interdisciplinary and international cooperative studies on fluid circulations in the Nankai Trough accretionary prism

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The purpose of this project is to promote interdisciplinary studies on fluid circulation, crustal deformation and biological activities in the Nankai Trough accretionary prism, as part of the Japan-France cooperative KAIKO-TOKAI Project. Since the beginning of the KAIKO project in 1984, it has been known that the northwestward subduction of the Philippine Sea plate under the Eurasian plate forms a huge accretionary prism along the Nankai Trough, where circulating fluid in sedimentary layrs, accompanied with cold seepage on the sea bottom, likely plays an important role in causing large-scale crustal deformation such as big earthquakes. The KAIKO-TOKAI Project aims at elucidating geophysical, geological, geochemical and ecological characteristics of the accretionary prism, by using research vessels and submersibles. During 1994-1996,3 cruises of R/V Hakuho Maru (Ocean Research Institute, University of Tokyo), 2 cruises of R/V Tansei Maru (Ocean Research Institute, University of Tokyo), 1 cruise of R/V L'Atalante (IFREMER,France), 12 dives of DSRV Shinkai 2000 (JAMSTEC) and 8 dives of DSRV Shinkai 6500 (JAMSTEC) were devoted for this project. Results obtained during these cruises and dives have been presented and discussed at the KAIKO-TOKAI Steering Committee meetings, which were held in Japan and France once in each year. Remarkable results so far obtained are as follows : (i) another subduction was identified along the southern rim of the Zenisu Ridge by OBS observations and differential GPS measurements, (ii) Detailed crustal structure has been revealed by 120 channel seismic surveys, (iii) H_2S production by sulfate reduction (methane oxidation) was first confirmed through isotope measurements of methane dissolved in pore fluids from accretionary prism sediment, etc. The KAIKO-TOKAI project will last till the end of the 1997 fiscal year, when an international symposium will be held in Japan in order to review this project totally.