Sedimentation across the Japan Trench off northern Honshu Island

Abstract

Summary The convergent margin along the International Program of Ocean Drilling (IPOD) Japan Trench transect was sampled by dredging, piston coring, and drilling by the Deep Sea Drilling Project (DSDP). This sampling was within an extensive network of single-and multichannel seismic reflection records. The Quaternary uplift of northern Honshu Island, climatic change, and lowered sea-levels, provided coarser grained sediment during the Quaternary than during the late Miocene and Pliocene. Within the Quaternary sediments, those in the forearc basin, in a slope basin, and on the trench floor are coarser grained and accumulated more rapidly than the clayey sediment from the trench slope. But overall, the sediment sampled from all environments is mainly clay and silt. Rates of accumulation varied greatly and prior to 3 Ma ago they were higher in trench slope basins than in the forearc basin. Tectonism along the Japan Trench transect must have changed the Miocene-Pliocene morphology by tilting some of the former trench slope basins; this may have actually triggered slumping and erosion rather than allowing sediment deposition. The lower slope has a relatively large number of channels and a greater amount of slumped material then is found elsewhere; thus it may contain vertical and lateral changes of facies in sediments deposited over relatively short periods of time. Morphological features and sedimentation patterns on the lower slope are greatly affected by local tectonic structure which has changed significantly in the past 2–3 Ma. Slope basin and trench-floor deposits are distinguished from those of the main forearc basin by the relative abundance of material deposited by mass movement. Channels commonly cross the forearc basin and trench slope and provide the transport paths for coarse sediment to any part of the margin. No specific characteristics appear exclusively in a given environment, but the relative abundance of features can help identify broad environments of deposition.