METAMORPHIC FOLIATIONS, FLUID SEEPAGES AND EXHUMATION PROBLEMS REVISITED: MAJOR TO MICRO STRUCTURES OF THE PRESENT NANKAI ACCRETIONARY PRISM BY SUBMERSIBLE STUDY

Exhumation of metamorphic or deformed rocks is one of the eternal problems in the convergent plate boundaries. We elucidated an interesting type of such exhumation in Nankai accretionary prism, Japan, through direct observation and sampling by using JAMSTEC submersibles. Two deep eroded transects along canyons off Kii and Tenryu areas were targeted for observation and sampling. As a result the following interpretations were obtained. Most of the rocks are of turbidite in origin, and considerably deformed in spite of their young age, less than 3 Ma. In particular rocks along the out-of-sequence faults, one in the Kumano thrust, another in the Tokai thrust, have texture of so-to-speak slate or even schist, including strong illite preferred orientation in S-C structure of high crystallinity showing 220 degrees Celcius of maximum experienced temperature at most. At least five deformation stages are elucidated, one parallel to subparallel to bedding plane, others oblique, including deformation bands and kink bands. Also fluid isotope chemistry of oxygen and carbon of calcite veins and cements from the seepage zones indicated high temperature character, suggesting deep thermal origin, similar to those from active faults or thermally induced hot spring fluid, different from those from the surface of the prism that is by surface circulation of biogenic fermentation origin. Thus, the rocks along the two canyons indicated exhumed prism rocks and deep origin fluid as well as the thrust and folded internal structures. The final stage of exhumation might be attributed to almost horizontal thrusting, which is caused by large scale land sliding by gravitational collapse due to the effect of the Zenisu ridge collision-subduction to Nankai prism.