During Leg 190, we successfully cored six sites, meeting most of the leg objectives and also revealing some surprising new findings. Lithostratigraphy and sediment diagenesis vary markedly in three dimensions throughout the Nankai-Shikoku Basin system. These variations in turn exert considerable influence on physical properties, hydrology, fluid-sediment geochemistry, and microbial activity. Discontinuities in porosity, P-wave velocity, electrical conductivity, and grain density generally correspond to lithologic and diagenetic boundaries.
We demonstrated that there is a large contrast among many key properties, including the diagenetic, lithologic, and geochemical (including microbial activity) character of the incoming sequences, between the Muroto and Ashizuri Transects. Such contrasts may be tied to variations in mechanical, structural, and hydrologic behavior along the strike of this margin. In spite of these contrasts, we found that the décollement at both transects occurs at the level of an almost coeval horizon (6-7 Ma). As at Site 808, the décollement marks an abrupt increase in porosity (and a corresponding decrease in P-wave velocity), suggesting some combination of undercompaction below and overcompaction above.
A broad low-chloride pore-water anomaly was found in the lower section of the incoming, as well as accreted, sediments along the Muroto Transect, as first identified at Site 808. The progressive decrease in the magnitude of the low-chloride anomaly from prism to basin indicates an important role of in situ diagenetic dehydration reaction and possible fluid migration process.
We also documented the accretionary history of the Nankai Trough prism for the first time and revealed a phenomenally rapid rate of growth of the prism during the Pleistocene. Our results will provide a basic framework for further mechanical, hydrogeological, and geochemical studies of this accretionary prism. Leg 190 results will also be useful for understanding the tectonics of accretionary prism evolution, thus providing a link between prism-toe processes and highly deformed accretionary complexes, the dominant components of orogenic belts.