The evolution of Neogene and Quaternary sedimentation in the fore-arc region off northern Honshu is evaluated using multichannel and single-channel seismic records in conjunction with the drill holes of the Japan Trench Transect (DSDP/IPOD Legs 56-57). The outer forearc region, which consisted of older sedimentary rocks and some calc-alkaline volcanic rocks, was subaerially exposed and eroded during the Paleogene and part of the Neogene. The deep sea terrace (fore-arc basin) region subsided below sea level in the early Miocene; most rapid subsidence occurred during the early to middle Miocene. Submergence progressed seaward so that the last vestige of the Oyashio landmass, which is now under the upper trench slope, was below sea level in the latest Miocene. Sediment sources to the outer fore-arc basin changed progressively from lithic, predominantly nonvolcanic material derived from the uplifted landmass during the late Paleogene-early Neogene to volcanic, arc-derived sediment rich in volcanic glass, Plagioclase, and volcanic lithic fragments. The volcaniclastic sediment was probably derived both from Honshu to the west and Hokkaido to the northwest.In response to subsidence the sedimentary depocenters in the fore-arc basin migrated generally seaward through time; the greatest relative seaward migration occurred between the late Miocene and Pliocene. Thick sediment sequences accumulated in slope basins on the trench inner slope. Sediment from the arc moved seaward to spill over the slope via large channels.An abrupt change in morphology and patterns of sedimentation apparently took place in the late Pliocene, coincident with a peak in explosive volcanism recorded in the form of ash layers and increased glass contents in sediment. The deep sea terrace was uplifted several hundreds of meters and a major channel crossing the fore-arc region was tilted landward and filled. At about the same time the midslope terrace basin was created and began rapidly accumulating sediment. The older basins, lower on the trench inner slope, were destroyed, possibly by steep seaward tilting, or filled. Large slump masses were sloughed-off downslope to the trench. Little sediment now accumulates on the trench inner slope in the vicinity of the sites, and older strata crop out on the slope. The locus of deposition has shifted northward off Hokkaido where a large channel feeds sediment to the slope. Large slump masses now fill the trench and are being accreted, creating a "toe" to the slope in this region.

No abstract