37. Large Mass Transport Deposits in Kumano Basin, Nankai Trough, Japan

Large-scale landsliding is a common process in the Kumano Forearc Basin of the Nankai Trough accretionary prism. We use a 3D seismic data volume to map the seafloor reflection, which shows that there are two surficial landslides, one rotational slump ~3.4 km wide, 1.8 km long and 150 m thick and one disintegrative slide that has left a seafloor scar ~ >3.65 km wide, 2.6 km long and ~200 m deep. We see no evidence for any deposits related to the latter in our data, so the entire mass must have been transported as debris flows/turbidites outside the area covered by 3D data. The slump failures occurred along a bedding plane that dips ~5–7° landward, but the disintegrative landslide has a gently-dipping base and is associated with steep normal fault scarps. Several large subsurface mass-transport deposits (MTD)s are mapped in the 3D seismic data – all have slid along single landward-dipping bedding planes. Their bases range in depth from 140 to 700 m below sea floor (mbsf). The thickest MTD is ~6.5 km² × 155 m thick, encompassing a volume of ~1.0 km³. The three other large MTDs range from 0.3 to 0.6 km³ in volume. The toes of the MTDs are imbricated, and the imbricate structure, as imaged in continuity displays, is aligned parallel to the slope. Many less extensive, thinner (<20 m thick) MTDs are also present in the Kumano Basin. Regional seismic-stratigraphy and age-constraints on MTD-correlative seismic reflections drilled at IODP drill Sites C0009 and C0002 reveal that four of the investigated MTDs are younger than 0.3–0.44 Ma, three are 0.44–0.9 Ma, and three others are between ~0.9 and 1.24 Ma.