Accretion-Collision Tectonics of Sakhalin and Circum-Pacific Orogeny

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The results obtained from this project are summarized as follows. 1) The Cretaceous accretionary complexes, called the Aniba complex, are composed of two tectonic units. The upper unit consisting of basalts, bedded cherts, hemipelagec siliceous shale, Melange, and terrigenous sediments in ascending order, thrust over thd lower the unit which is composed of dominant basalts and melanges characterized by abundant limestone blocks. The lower unit composing of five thrust sheets verging eastward, seems to be an underplating complex beneath the upper unit. 2) The Susunai complexes, high pressure metamorphic ones extending to the Kamuikotan metamorphic belt in Hokkaido, reveal a complete sequence of the tectonic pile composed of tens of thrust sheets which are stacked by low angle thrusts verging southward. The complexes consist mainly of greenschists, metachert, silicious shist, and metapelites associated with metagreywackes. The complex is divided into five domains based on lithologic assemblages. For example, Domain 4 is characterized by dominant basalts to dolerites overlain by basaltic sediments with pelagic limestones capped by a thin sequence of black shale. Three phases of deformation, D1 to D3 are discriminated. The rocks subjected the highest strain of D2, strongest deformation among them, show NE trending asymmetric folds, NW trending sheath folds and lineation of both the stretching and micro folding. Two metamorphic zones are defined in the surveyed areas, i. e. pumpellyite-actionlite zone in southern part and pumpellyite-epidote-actionlite zone in northern part. The P-T conditions at the peak of metamorphism are estimated to have been 200-300ﾟC and 4-5Kbar. The metamorphism coincides with D2 phase of deformation. 3) Major and minor element compositions of basaltic rocks exhibit that those from the Aniba complex represent oceanic island tholeiites features and alkaline affinity, whereas those from the Susunai complex reveal N-MORB to T-MORB features