STRUCTURE OF THE NANKAI TROUGH ACCRETIONARY ZONE FROM MULTICHANNEL SEISMIC-REFLECTION DATA

New multichannel seismic reflection data collected over the Nankai Trough image the accretionary complex in two areas. The incoming Shikoku Basin sedimentary section consists of hemipelagic clays and thin terrigenous turbidites. The basin section is overlain by a trench wedge that is 12-16 km wide and 350-750 m thick at the thrust front. Accretionary deformation begins in a protothrust zone that is characterized by thickening and seaward tilting of the trench wedge. The zone in the western area is 4.5 km wide and is characterized by "kink' folds; the zone in the eastern area is only 2.5 km wide and does not exhibit such folds. The frontal thrusts in each area are imaged as fault plane reflections and ramp upward from within the basin hemipelagic section. The overthrusting sediments form fault-bend folds over these ramps. Thrust spacing at the toe of the slope is 1.5-2.5 km. The second thrust cuts up from an inferred decollement within the Shikoku Basin sedimentary section. -from Authors