Abstract

Quaternary sedimentation around the Japanese Islands took place in a dynamic morphotectonic setting that included seismically active trenches, a large back-arc basin, and a tectonically complex slope environment characterized by en echelon folds and fault-bounded ridges with small intervening basins. These structural ridges created barriers to sediment transport by bottom currents and acted as dams on the slope behind which Quaternary sediments ponded. Sea levels higher than at present throughout most of the Pleistocene impeded escape of coarse sediments from the shelf, except through a few major submarine canyons that extended completely across the shelf and slope; thus, deeper-water sediments in both the Japan fore-arc and back-arc are dominantly hemipelagic muds and subordinate mud turbidites. A drop in sea level to about 130 m below the present level at approximately 18,000 yr B.P. resulted in deposition of widespread fluviomarine sands and gravels on the continental shelf and an increase in sand-silt turbidites in the latest Pleistocene record in both the fore-arc and the back-arc. Sandy turbidites interbedded with hemipelagic mud or mud turbidites occur particularly in the trench fill of Nankai Trough (Southwest Japan Trench), in some fore-arc basins on the South-west Japan slope, and in the western part of the Japan Sea back-arc basin. They are sparsely represented in the Japan Trench and the Northeast Japan slope, apparently owing to the poor development of throughgoing submarine canyons on the Northeast slope. Diatomaceous and calcareous muds and oozes occur in some parts of both the fore-arc and the back-arc, and volcanic ash and pumice are virtually ubiquitous throughout the Quaternary sediment record in nearly all parts of the Japan arc-trench system.

Quaternary sedimentation rates are less than about 30 m/m.y. in the open Pacific deep-sea basins seaward of the trenches, but they increase toward the fore-arc and reach values as high as 230 m/m.y. in the Japan Trench inner slope and 300–900 m/m.y. in the trench fill of Nankai Trough. Back-arc sedimentation rates for the total Quaternary range from 20–140 m/m.y., and latest Pleistocene-Holocene rates range from 65–230 mm/1,000 yr.

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