Along-strike variations in the structure of the Nankai Trough accretionary prism: correlation with variations in subducting basement topography and overlying sediments

Variations in the basement topography and overlying sediments of the subducting Philippine Sea plate affect the structure of the frontal portion of the Nankai Trough accretionary prism. We analyzed seismic reflection and multibeam bathymetry data sets across the northern Shikoku Basin to evaluate differences in along-strike structures of the Nankai prism. The proto-thrust zone (PTZ), frontal thrust (FT), and imbricate thrust zone (ITZ), all show variations in geometry that can be associated with regional variations in subducting topography and sediment cover. Based on structural and geometric characteristics of the prism identified on our seismic lines, we define four distinct segments along the Nankai Trough: Ashizuri, Muroto, western Kii Peninsula, and Kumano Basin. The critical taper angle of the prism toe along the Ashizuri area is larger (8-10°) than that of the Muroto area (4 - 5°). The tape angle increases to the east from 6 - 7° off western Kii Peninsula to 7 - 10° off Kumano Basin. Our seismic profiles show that Miocene turbidites are subducted beneath the décollement in the Ashizuri and Kumano areas, but not beneath the Muroto and western Kii areas. These Miocene turbidites are strongly controlled by the basement relief and smaller scale topographic irregularities. The different taper angles may arise from differences in décollement strength or internal sediment strength, or to overpressures related to the Miocene turbidites in the subducting section. These variations in basement morphology and overlying sediment not only affect the accretionary prism, they must also influence regional fluid flow patterns and the shallow portion of the seismogenic zone.