Prior to Leg 190 drilling, we conducted a three-dimensional seismic reflection survey across the central Nankai Trough south of Shikoku Island. These new data helped establish the regional structural setting of the Leg 190 drill sites. Seaward of the trench axis, buried basement topography of the Kinan Seamount chain is imaged as a broad ridge overlain by 0.7 s (750 m) of sediment. Seaward of the ridge, the sediment thickness increases to ~1.75 s (2 km); the deepest ~1.0 s of sediment onlaps the ridge, indicating that the ridge was formed before sedimentation began. The youngest 0.6 s (700 m) of hemipelagic sediment is draped over the ridge and continues into the trench axis, where it is buried by 0.3 s (350 m) of trench turbidites.
The trench strata and the upper hemipelagic deposits are stripped off the subducting plate and accreted. The accretionary complex is divided into several segments based on structural styles. Between the deformation front and the frontal thrust is a zone of diffuse structural thickening, the protothrust zone. The frontal section of the accretionary prism is characterized by 700- to 800-m-thick thrust-bounded slices. There are 12 major thrust packets with a thrust spacing of ~1.5-2.0 km. The thrusts sole into a well-defined décollement ~0.25-0.30 s above the oceanic crust. Approximately 20 km landward of the frontal thrust is a major out-of-sequence thrust (OOST) that thickens the prism from 1.6 to 1.9 s. A 6.25-km-wide slope basin with ~0.6 s of sediment is ponded behind this OOST. Strata within the basin are tilted landward with dips that increase with depth, indicating continued relative uplift along the OOST during sedimentation.
Approximately 30 km landward of the frontal thrust, the thrust packets abruptly increase to at least 1 s in thickness, with thrust spacing increasing to 2-3 km. These thrust slices are formed of strata equivalent to the deeper section seaward of the Kinan Seamounts. The décollement cuts down closer to the oceanic crust beneath these thrust packets. Another major slope basin has formed above these thrust slices. Landward of this zone, the prism is characterized by less distinct, landward-dipping reflections; the décollement is obscured by the seafloor multiple.
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