Anatomy of trench-slope basins: Examples from the Nankai Trough

Abstract

Along convergent margins, forearc basins and trenches are most important as major sedimentary basins of terrigenous materials. In addition to them, small basins are generally developed on the slope between the trench-slope break and the toe of the slope. In this study some characteristic features of the inner trench-slope basins of the Nankai Trough are shown on the basis of the sea-beam maps and single-channel seismic profiles obtained on board RV Jean Charcot on Leg 1 of the Kaiko Project Phase I (1984). The study areas cover the inner trench slope off Shikoku and off the Enshu-nada Sea, central Honshu. In both areas the inner trench-slope basins vary in size from a few kilometers to 10 km wide and 10–25 km long, being filled with turbidites 200–1000 m thick. These basins are controlled tectonically, indicating some contrasting features between those off Shikoku and those off the Enshu-nada Sea. The basins off Shikoku are developed in front of major thrusts (thrust-controlled), whereas the basins off the Enshu-nada Sea represent the synclinal axes of the intensely folded accretionary wedges (fold-controlled). In-filling sediments are characterized by turbidites transported mostly longitudinally and sometimes by slide deposits supplied laterally. Some of the sediments seem to have been subject to recent tectonic activities. These features are useful for a better understanding of ancient forearc basin development.