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The Japan Trench off northern Honshu Island is associated with a backarc basin, a magmatic arc, a forearc basin, and a continental shelf that is submerged deeper than the usual shelf above 200 m water depth. The latest period of arc magmatism began to build the volcanic backbone of Honshu Island in late Oligocene to early Miocene time, and an intense Miocene period of volcanism produced green tuff, a thick complex of altered felsic to mafic volcanic rock interbedded with marine sediment. Other times of accelerated volcanism occurred in the Pliocene and Pleistocene (Cadet and Fujioka, 1980).

The Pacific side of Honshu Island is built on an older continental framework encompassing the Kitakami massif, a large body of Mesozoic and Paleozoic rock overlain by a transgressive Cretaceous sediment sequence. In the adjacent submerged area is a deep basin filled with Cretaceous to Recent sediment (see diagrammatic section). The seaward flank of the basin is truncated by subaerial erosion as is the adjacent 35-km-wide section of Cretaceous rock that comprises the seaward part of the margin. The unconformity marking the top of the Cretaceous complex is prominent in many multichannel records across this margin, and it marks an abrupt change from rocks with a low to rocks with a high acoustic velocity.

Paleogene sediment also fills part of the basin, but its seaward extent is limited. Neogene sediment with a basal Oligocene unit underlain in turn by the Cretaceous complex was penetrated at DSDP site 439 (see diagrammatic section), and geophysical

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