Anatomy of Eastern Nankai accretionary prism toe at Tenryu deep-sea canyon by submersible surveys and various physical tests

Detailed geologic structures, physical properties and soil mechanical tests of the superficial part of the Nankai accretionary prism toe along the Tenryu deep-sea canyon on the eastern Nankai Trough were disclosed as follows. The proto-thrust was observed at seabed on the Nankai Trough floor as a fresh fault cliff of 3-4 m height, which is likely to be deformed in brittle fashion as the very recent expression. The Tenryu frontal thrust zone of approximately more than 200 m wide as a fault breccia zone is composed of small mounds and ridges of 2-3 m relief, where highly deformed sedimentary rocks are exposed. The coherent unit zone is composed of several 10 cm thick turbidites with small displacement by sharp microfault. These strata form an open fold and thrust system. Uniaxial compression brittle strengths of rock samples in coherent unit zone attain approximately 1.1-1.2 MPa, whereas the rock samples of the Tenryu frontal thrust are deformed ductilely at a yield stress of 0.6 MPa. Maximum burial depths of the rock samples of the Tenryu frontal thrust zone and coherent unit zone were estimated by consolidation test as approximately 350 mbsf and 200 mbsf, respectively, corresponding to the burial depth estimated by their physical properties, e.g. porosity being 40-50%, P-wave velocity being 1.7-1.8 m/s, etc. This means that the prism toe in this area was up-lifted from 200-350 mbsf due to the upward push by the Paleo-Zenisu ridge collision/subduction during accretionary stage.