Abstract

Along the International Program of Ocean Drilling (IPOD) Japan Trench transect, a mid-Cenozoic period of tectonism without arc volcanism separated an earlier Cretaceous to Paleogene and a later Neogene period of plate convergence and subduction. The Cretaceous to early Paleogene period of convergence is evidenced by andesitic volcanism associated with a large syncline, probably a forearc basin. The seaward flank of the basin is a tectonically thickened sediment sequence inferred to be an accretionary complex. The arc, the syncline, and the thickened sediment sequence resemble an arc-trench system of the size and structure of the Great Valley forearc basin and Franciscan accretionary complex of California, and they were probably continuations of the Yezo geosyncline and associated sequences exposed to the north on Hokkaido. The arc-trench system is contemporaneous with the adjacent Shimanto arc-trench system southwest of Tokyo Bay. In the early Paleogene, volcanism ceased, and part of the thickened sediment sequence of the Cretaceous-Paleogene margin was emergent as a 160-km-wide landmass which is recognized in seismic records as a later Paleogene sediment source. At the end of the Paleogene, this landmass began to subside and volcanism began again, but it was located first briefly on the east, and then on the west of the Cretaceous arc along the present arc of northern Honshu. The land-mass erosion surface is clearly visible as an angular unconformity in multichannel seismic-reflection records; the sequence of Neogene sediment, explosive volcanism, and benthic foraminiferal assemblages recording subsidence from subareal conditions to the present bathyal depths have been studied in Deep Sea Drilling Project (DSDP) cores. Cretaceous subduction appears to have resulted in a very extensive accretionary complex, whereas the Neogene subduction period appears to have been marked by little net accretion in the forearc area but massive subsidence and some erosion of the front of the convergent margin.

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