Continental Margin of Kamchatka Peninsula, Russia: the Mode and Nature of Crustal Growth in the Accretionary Orogen

Tectonic accretion of island arc terranes is the process widely developed in Pacific Rim in the present and in the past. The mode and nature of crustal growth of continental margins during arc accretion are various and essentially determined by deformation of the margin. The Cenozoic Kamchatka orogen formed by the accretion of two island arc terranes: Achaivayam-Valaginskaya arc (A-V, Eocene) (2) and Kronotskaya arc (terminal Miocene) to the continental margin of Asia. During the Early Eocene, the southern segment of the A-V arc collided with the Sredinny metamorphic massif, which was the frontal part of the Asian continental margin (3). New results from SHRIMP dating of zircons (1) from metamorphic rocks of Sredinny massif (Kolpakovskaya series) show that the massif contains an abundance of Archean, Proterozoic and Phanerozoic detrital zircon cores, and ubiquitous 77 Ma rims. The youngest ages are from four 47-53 Ma unzoned zircon cores, with dull cathodoluminescence, and irregular morphology. We regard the 47-53 Ma episode of zircon growth in the Sredinny massif as evidence for superimposed metamorphism induced by continental margin subduction at the beginning of its collision with the A-V arc in the early Eocene. Physical modeling experiments of arc-continent collision suggest that deformation at continental margin is controlled by strength of the subducting crust. Failure, accretion and erosion-activated extrusion/exhumation of the subducted crust occur in the continental margin in the case when the margin is weakened by pre-existing faulting, extension, or heating. At the beginning of the continental margin subduction, crust of the margin fails along the continent-vergent thrust. The subducted crustal slice is, then, completely scraped from the mantle base and accreted to the fore-arc block. Subsequent thrusting and thickening of the subducting crust within the continental margin lead to formation of the accretionary orogen composed of crustal slices in front of the collided arc. Erosional unloading causes the previously subducted crustal slice of the continental margin to slide with a strong horizontal compressional squeezing, and buoyant force. After some subduction of the continental margin, the overriding plate fails in the arc area along the continent-vergent fault, which results in subduction of the fore-arc block. In Kamchatka, the Sredinny massif is considered to represent the crustal slice detached from the mantle base of the leading edge of the Asian continental margin, accreted and exhumed in front of the collided A-V arc. This work is supported by Russian Foundation of Basic Research, project no 316, Geosphaera Research Center, and DOE. (1) Bindeman I.N., Vinogradov V.I., Valley J.W., Wooden J.L. (2001) Archean protolith, and accretion of crust in Kamchatka: SHRIMP dating of zircons from Sredinny and Ganal Massifs, subm to J Geol (2) Garver J.I., Soloviev A.V., Bullen M.E., Brandon M.T. (2000) Toward a more complete record of magmatism and exhumation in continental arcs, using detrital fission-track thermochronometry. Phys Chem Earth (A) 25, 565. (3) Konstantinovskaia E.A. (2001) Arc-continent collision and subduction reversal in the Cenozoic evolution of the Northwest Pacific: an example from Kamchatka (NE Russia), Tectonophysics, 333, 75-94.