Early Eocene Arc–Continent Collision in Kamchatka, Russia: Structural Evolution and Geodynamic Model

Abstract

The major event in the Cenozoic evolution of the Kamchatka orogen was the Early Eocene collision of the Ozernoy-Valagina (Olutorsky) arc terrane with the continental margin of Asia. The arc–continent collision developed progressively from southwest to northeast from the Early Eocene in South Kamchatka to the early Mid Eocene in the Olutorsky region. The nature of the continental margin of western Kamchatka and alternative models for the Early Eocene arc–continent collision in South Kamchatka are discussed based on a review of geological and geochronological data and physical modelling results. In South Kamchatka, rapid heating, metamorphism and granite formation occurred in the continental margin (Sredinny massif) during its burial in the subduction zone in the Early Eocene, synchronously with the arc–continent collision. The physical modelling results suggest delamination of crustal slices and tectonic thickening of the crust could take place within the continental margin during the collision resulting in the peak metamorphism in the margin. The syncollisional exhumation of metamorphic crust was produced by a compressional and buoyancy-driven squeezing up of subducted slices of continental crust and erosional unloading. The arc was subsiding during the collision, and the exhumed material of the margin was accumulated in the forearc and foreland basins. The physical models support that subduction reversal and slab detachment likely occurred at latest stages of the collision that resulted in the onset of oceanic subduction at the back side of the accreted arc. The geochemical features of the Kamchatkan volcanic arcs before and after the Early Eocene arc–continent collision provide evidence for subduction flip during the collision.