The deformation structure of submarine landslide recorded in the incoming sediments on the Kashinosaki Knoll, in the Nankai Trough

The Nankai accretionary prism of southeast Japan is one of the most extensively investigated convergent plate margins in the world. As a part of multi-stage project of the IODP Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) Expedition 333, we drilled Site C0012, a presubduction input site positioned on the crest of bathymetric high (the Kashinozaki Knoll) in the Shikoku Basin, approaching the Nankai Trough. In this study, we seek to determine the deformation features of the incoming sediments in the vicinity of the structural high, correlated with regional topography and tectonic setting. Based on the characteristics of split core sections and X-ray computed tomography scan image observation, we identify 4 major structural units from top to bottom of the incoming sediments at Site C0012; (1) faulted and sheared subhorizontal beds (0-15 m CSF), (2) inclined beds with high dip angle (up to 70°) and chaotic sediments (15-85 m CSF), (3) subhorizontal beds with faults and shear zones (85-145 m CSF), and (4) highly disturbed sediments with chaotic structures (145-180 m CSF). Boundaries at 15 m CSF and 85 m CSF correspond to hiatuses in the paleomagnetic and biostratigraphic records. In order to resolve more detailed deformation structures, we conducted microstructural and fabric analyses on discrete samples using optical microscopy and Scanning Electron Microscope (SEM) analyses. From these results of shipboard and post-cruise analyses, we suggest that the hiatus and angular unconformity at ~15 m CSF represent the top of a package of slumped sediments. Internal deformation structures of the deformed mass include a sheath fold, high angle bedding dips, and chaotic structures below the unconformity within the interval between 15 to 85 m CSF. We primarily focus on meso- and micro-structural analyses of internal deformation structures within the incoming hemipelagic sediments to demonstrate the origin and evolution of gravity-driven deformation and to typify the post-failure deformation structures.