The structure of Tosa Terrace and Nankai Trough off southeastern Japan

Abstract

Continuous seismic profiles recorded south of Shikoku, Japan, reveal the shallow structure of the Tosa Terrace, continental slope, Nankai Trough, and adjacent Shikoku Basin. Tosa Terrace contains at least 1.5 km of terrestrially derived horizontally bedded sediments trapped by an outer structural dam. Deposition has probably continued from early Neogene time. These sediments unconformably overlie a deformed sedimentary unit of probable Paleogene age that appears truncated at its upper exposed surface. The deformed sediments and igneous basement of the continental slope have been faulted to form benches on which more recent sediment has accumulated, and scarps which expose basement. An average of approximately 600 m of interbedded turbidite and pelagic sediments are present in the Shikoku Basin. Sediment thickness in Nankai Trough averages 1 km. Moated knolls suggest turbidity currents transporting sediment off the Shichito-Iwo Jima Ridge into the Shikoku Basin. Recent turbidites have been deposited in the Nankai Trough and are folded where they abut the continental slope. This and a plunging basement at the foot of the continental slope suggest seafloor spreading.