Mode of mixture of oceanic fragments and terrigenous trench fill in an accretionary complex: Example from southern Sakhalin

Abstract

Four modes of mixture of oceanic materials such as cherts, limestones and basalts, and terrigenous trench fill sediments are recognized in the Cretaceous accretionary complexes in southern Sakhalin, Far eastern Pacific margin of the U.S.S.R. The first mode is a thin-skinned stacking of pelagic bedded cherts with black shale, due to duplex formation in association with shallow underplating or off-scraping. The second mode is a mixture of the uppermost part of the oceanic crust with terrigenous trench fill sediments due to olistostrome formation. Reconstruction of the original sequence in ascending order suggests that the basement is pillow basalt, with overlying pelagic sediments of chert and limestone, olistostrome caused by collapse of oceanic crust due to its bending before encountering a trench, and finally trenchfill sediments of turbidite. This sequence is imbricated by thrusting, which appears to have occurred at the time of underplating. The third mode is a tectonic mixing of oceanic fragments and terrigenous sediments due to shear which is related to underthrusting of the oceanic plate. A reduction in the supply of trench fill sediments causes copious accretion of the upper part of oceanic crust. The fourth mode is in-situ basalt volcanism at the trench, where thick baked margins of terrigenous shale are formed around the injected basalts. This mode appears to represent a ridge-trench encounter. These four different modes of mixing seem to be usual in ancient accretionary complexes in orogenic belts.