ONGOING ACCRETIONARY PRISM FORMATION: SUBMERSIBLE DIRECT OBSERVATIONS OF THE TENRYU SUBMARINE CANYON IN THE NANKAI ACCRETIONARY PRISM

The geologic architecture of the eastern Nankai accretionary prism was directly observed by two submersibles to the Tenryu canyon. It was clarified that the architecture consists of repetition of thrust-anticlines of turbidite sequences which are considerably deformed. Previously the geologic structures of the Nankai accretionary prism were analyzed mainly by 2D-3D seismic data analysis and observation of the drill cores. In this study, we used the manned submersible Shinkai 6500 and the unmanned submersible Kaiko operated by Agency for Japan Marine Science and Technology. The beds of the accretionary prism are exposed at the walls of NNE running Tenryu canyon, which show extremely ragged morphology with maximum 1000 meters height. We collected data and rock samples necessary for structural analyses, from the shallower fore-arc basin deposits the prism proper into the canyon bottom with 1000 m in burial depth at the maximum. A total of eleven dives were performed during four consecutive cruises; KR97-05 and KR97-06 cruises by R/V Kairei-Kaiko and YK03-03, YK05-08 and YK06-02 cruises by R/V Yokosuka-Shinkai 6500 cruises. The unmanned Kaiko dived three times at the mouth of the Tenryu canyon, and the manned Shinkai dived eight times at the sidewalls of the canyon. The strata in the Nankai prism are composed mainly of gently folded turbiditic beds, approximately several to several tens of cm in thickness. Muddy layers in these turbidites commonly show vein structures and fracture cleavages, oriented approximately N-S and E-W near the large thrust faults. Porosity and uniaxial compression strength of these rocks are ca. 40-50% and 1-10 MPa, respectively. We discuss the process and mechanism of ongoing off-scraping accretion based on the above data.