COLLISION OF THE OLYUTORSKY TERRANE, KAMCHATKA

Throughout much of Kamchatka, the footwall of the collision zone is marked by the Ukelayat flysch and its stratigraphic equivilants, which are overthrust by the Olyutorsky arc. The Ukelayat flysch is a 10-to-15-km-thick zone of deformed turbidites that accumulated in forearc region of the Okhotsk-Chukotka continental Arc that was active in the mid to Late Cretaceous and extends across NE Asia eastward to the western Alaskan margin. This Andean-style continental arc was build on a collage of terranes accreted in the mid-Cretaceous. The collision of the Olyutorsky Arc occurred in the Eocene, but was diachronous. Detailed geochronology along the suture zone in a number of study areas in Kamchatka suggest that the collision proceeded from south to north. In southern Kamchatka (Sredinnyi Range), collision is inferred to be to be synchronous with peak metamorphic conditions attained in the Early Eocene (~52 Ma) while post orogenic exhumation occurred in the middle to late Eocene (~40-42 Ma). Further, the Sredinnyi Range is deeply exhumed to depths of 22- 33 km based on peak pressure estimates from rocks in the core of the range. In central and northern Kamchatka, collision occurred slightly later (~45 Ma), and the suture is only shallowly exhumed (~3-4 km). In these northern areas, post-orogenic exhumation was slow (50 m/Myr), and much of the orogenic belt remained underwater and has accumulated marine strata since collision. Many workers contend that the Shirshov and Bowers ridges in the southern Bering Sea represent the submarine continuation of the Olyutorsky arc. If this is the case, collision must have predated the establishment of the Aleutian Ridge; in fact it is likely that collision of the Olyutorsky terrane changed the plate geometry and forced the initiation of outboard subduction at ~45 Ma.