[SCG61-P06] Active tectonics of Muroto-off Ashizuri uplifted ridges, southwest Japan
Keywords:active fault, outer ridge, subbottom profiler
In order to detect sedimentation and deformation structures associated with uplift movements, deep-towed subbottom profiler (SBP) survey was conducted during R/V Hakuho-maru KH-15-2 and KH-16-5 cruises. SBP profiles were obtained from the Nishi-Muroto Knoll 45 km south of Cape Muroto and the unnamed knoll (called Minami-Ashizuri Knoll tentatively) 70 km south of Cape Ashizuri by deep-towing of a chirp system (EdgeTech DW-106) at approximately 15 m above the seafloor using ROV NSS (Navigable Sampling System). The Nishi-Muroto Knoll has NE-SW trending two major highs. The north summit shows few reflectors suggesting exposure of old sequence and the small basin at its hoot suggests relative uplift movement to the south summit. Steeply NW-dipping reflectors landward of the south summit and over a 100-meter-high scarp suggests large displacement at the hoot of this summit. The reflectors trenchward of the south summit shows increase of landward dip with burial depth in a piggyback basin. Such increase of dip angle is observed only at the surface sequence (max 15 m) and constant thickness layers are observed below it. This suggests recent beginning of landward tilting deformation. The boundary between the fan-shaped and parallel reflectors is estimated to be approximately 120 ka because it coincides with the horizon of disappearance of pink Globigerinoides ruber in a piston core sample.
SBP survey was also conducted across a NE-SW trending tomographic lineament at the unnamed knoll (called Minami-Ashizuri Knoll tentatively) 70 km south of Cape Ashizuri. The knoll is characterized by the horst structure bounded by a high angle fault and a flexure, which deformed the surface sequence suggesting recent activity. Our SBP survey successfully revealed recent activity along the topographic highs from Cape Muroto to off Cape Ashizuri.