THE DEEP STRUCTURE OF THE JUNCTION AREA BETWEEN THE KURIL-KAMCHATKA AND ALEUTIAN-COMMANDER ISLAND ARCS FROM GEOLOGICAL AND GEOPHYSICAL STUDIES

The article presents information on the deep structure of dynamically active regions of Central and Eastern Kamchatka in the area of island arc junctions. It also discusses the supposed genetic relation of the ultrabasic rocks and copper-nickel mineralization to the structural features of lithosphere in the continent-ocean transition zone. The paper presents the results of integrated interpretation of geophysical data derived from various methods such as earthquake converted-wave method (ECWM), magnetotelluric sounding (MTS), gravity and magnetic surveys, as well as geological surveys carried out in the area of the Klyuchevskaya group of volcanoes and Cape Kamchatka (the east coast of Kamchatka). For the first time, a geological and geophysical model has been developed for the Krapivnoe Village – Cape Africa geophysical profile. The model shows the main boundaries of the lithosphere and identifies previously unknown elements of the Earth's crust and upper mantle structure. In the western part of the model, a NE dipping upper mantle layer is identified, the present-day movement of which can be explained by the involvement of the lower layer of Eastern Kamchatka lithosphere in subduction of the oceanic plate under the Aleutian-Commander arc. The dynamics of interaction between the oceanic and continental lithospheric plates is shown. Two subduction zones that existed successively in the past are represented schematically. The upper part of the second subducting slab contains a 12–16 km thick layer consisting of ophiolite sheets and blocks of ultrabasic and basic composition. One of the identified blocks in the layer is presumably part of an ultrabasic massif with copper-nickel mineralization. There is a tension zone lower in the section, which is confined to the area of maximum bending of the slab. Natural thermal water pouring out of the ground in the area of Cape Africa suggests the circulation of high-temperature solutions in the tension zone. The upper part of the model includes a block identified as a fragment of the Achaivayam-Valaginsky terrane. The block is represented by discrete east-tilted sheet-like fragments of varying density, which consist of two layers exhibiting a combined thickness of 6 km. The upper layer is represented by an allochthonous sheet, with its frontal part made up of Palaeocene-Eocene deposits thrust over the western slope of the Tyushevsky trough filled with Oligocene-Miocene deposits. The thrust was caused by the Central Kamchatka rift extension. The Tyushevsky trough is derived from the accretion zone. Two areas where the maximum depth of the trough base is estimated at 6 and 7 km are inherited from the deep-sea paleo-trenches formed during successive subductions.