Abstract
During a recent survey of the Nankai Trough region by JAMSTEC R/V KAIYO, ten piston cores were collected along a NW-SE transect through the Shikoku Basin, Kashinozaki Knoll, and Nankai Trough areas. The purpose was to demonstrate the influence of landslide processes on sediment distribution patterns on an accretionary prism. The Shikoku Basin is a flat abyssal plane covered by ca. 1 m thick hemipelagic mud, and underlain by a ca. 10 cm thick tuffaceous sand corresponding to the Aira-Tn tephra layer (25120 ± 270 yr. B.P.). The sedimentation rates in the Shikoku Basin are 3–4 cm/ky. At least three submarine landslide scars are observable on Kashinozaki Knoll. The abyssal plane surrounding the Kashinozaki Knoll is covered by a characteristic yellowish pumiceous mud intercalated with the hemipelagic mud. Immediately below thelandslide scars, these pumiceous mud layers thicken. Pumiceous mud was likely derived from the flanks of the volcanic Kashinozaki Knoll. These scars and deposits suggest that submarine landslides redistributed material from the knoll to the basin by mass-wasting. More than six trench turbidite beds were observed in a sequence that overlies a submarine landslide deposit at the foot of the accretionary prism within the Nankai Trough. The geomorphology of the axial channel suggests that the flow of turbidite deposits along the axial channel was blocked by this submarine landslide deposit, forcing the flow path and gradient of the channel to re-establish itself. In this event, sediment ponding would occur upstream of the blockage, while the downstream portion of the axial channel would be starved of sediment.