Trench sediments of the Nikoro Group (Eastern Hokkaido, Japan): Implication for the origin of the Sea of Okhotsk.

Abstract:

The Nikoro Group in the Tokoro Belt of Eastern Hokkaido, Japan, has been regarded as a Late Cretaceous accretionary complex formed along the southern margin of the Sea of Okhotsk. Trench sediments overlying pelagic chert are composed mainly of turbidites and debrites. The turbidite sandstones consists essentially of volcanic rock fragments of basic to acidic compositions. The debrites contain sedimentary clasts and blocks of amphibolite, metagabbro, and tonalite in addition to volcano-lithic clasts. Majority of volcanic rock clasts, and amphibolites also, show chemical characteristics of volcanic arc. Basalt clasts with MORB-like composition are also contained in the debrites. U-Pb ages of detrital zircon range from Late Jurassic to Late Cretaceous, and of a tonalite block is Late Jurassic. No rocks or minerals of Precambrian and Paleozoic ages are so far known.

Major clastic source of turbidites was probably active volcanic arc on the Sea of Okhotsk. The debrites probably originated from slopes nearby, i.e. inner trench slopes, where tonalite, metagabbro, amphibolites, and oceanic basalts were exposed. These rocks, partly of Jurassic, probably comprised the basement of the Late Cretaceous arc. Absence of accretionary prism older than Late Cretaceous, in spite of occurrences of Jurassic tonalite, implies that the Nikoro Trench was juvenile crosscutting pre-existed arc-trench system of the Sea of Okhotsk.