Abstract:
The principal new feature in the Cenozoic evolution of the Northwest Pacific is based on the fact that two arc terranes of Upper Cretaceous–Paleogene age were incorporated in the Kamchatka orogen during the early Tertiary and at the end of the Miocene, respectively. These two arc terranes, so-called Achaivayam–Valaginskaya (Campanian–Lower Paleocene) and Kronotskaya (Coniacian–Eocene) arcs, are separated by a suture zone that contains deformed slices of young (Paleocene–Lower Eocene) oceanic crust. Paleomagnetic data on the original paleolatitudes of the arc positions as well as geochemical data characterized the arcs as intraoceanic, originated in the Northwest Pacific. The arcs were separated by the Vetlovka oceanic plate, suggested to be a trapped fragment of the Kula plate, in which a new spreading zone developed in the early Tertiary. The principal event in the reconstructed evolution is the Eocene collision of the extended Achaivayam–Valaginskaya arc terrane with the continental margin of Asia. The margin is interpreted as the southeastern (facing the Pacific) Cretaceous accretionary boundary of the Okhotsk microplate docked against the Asian continent at ∼55–65Ma. The arc–continent collision developed progressively northward from the end of the Paleocene–Early Eocene in Southern Kamchatka to the Middle Eocene in the Olutorka region. The different stages of the collision occurred simultaneously along strike of the Achaivayam–Valaginskaya arc. According to the geodynamic model based on geological data for Southern Kamchatka, collision included: (1) the entrance of the continental margin in the subduction zone, deformation of the overriding plate and subduction of fore-arc block; (2) arc obduction on the continental margin, blocking of continental margin subduction; and (3) subduction reversal with onset of oceanic subduction at the eastern side of the deformed arc. This subduction reversal produced deformation and reverse faulting along the eastern side of the arc, thus allowing accretion of tectonic slices of young oceanic crust of the Vetlovka plate, which began to subduct. During the subduction reversal, the subducted plate dipping to the southeast was broken off and detached at depth under the arc area. The subduction polarity reversal, the most characteristic feature of the collision model, is strongly supported by structural and geologic data of Southern Kamchatka and explains the main tectonic, metamorphic and magmatic events related to and postdating the collision.
