Tectonic Influences on Surficial Processes and Deformation along the Nankai Accretionary Prism, Southwest Japan

Abstract

This dissertation presents interpretations of new, high-resolution multibeam bathymetric data, reprocessed 3D seismic data and drill cores from the southern Kumano Basin and Nankai accretionary prism off southwest Japan. These combined data sets show a widely variable surface morphology and provide insight into: 1) the distribution of landsliding along the prism, 2) a nested series of moderately-sized mass transport deposits (MTDs) along the seaward side of the forearc basin, 3) and record ~2.87 million years of structural and depositional history of a trench slope basin. We mapped and cataloged 718 individual landslide scars, 56% of which are part of complex (multi-slide) structures. One of the more prominent complexes in the forearc basin is completely contained within the 3D seismic volume and dates to ~0.3–0.9 Ma. A kinematic investigation revealed 10 individual landslides that originate from the same prominent scar as a likely result of earthquake cycle related faulting along a regional out-of-sequence thrust (megasplay fault). Fault related landsliding also occurs within a trench slope basin seaward of the outer ridge. The 3D seismic volume and drill core data permit a kinematic reconstruction of the basin since ~2.4 Ma. In the NE, deformation is accommodated by the main megasplay while deformation in the SW is along break-backward imbricate branches of the megasplay. We suggest that these differences are caused by subsurface geometry and seamount subduction and directly influence the depth and surface morphology of the trench slope basin via landsliding.