Verkhoiano-Chukotka Folded Region

a region of Mesozoic folding in northeastern USSR. The Verkhoiano-Chukotka folded region borders the Siberian platform on the west and is separated from it by the Priver-khoiansk marginal depression. On the east it is separated from the Cenozoic folded structures of the Kamchatka-Koriak system by the Okhotsk-Chukotka regional volcanogenic belt. On the north the structures of the Verkhoiano-Chukotka folded region are under the seas of the Arctic Ocean, and on the south, the Sea of Okhotsk.

Orographically the region includes the mountain systems of the Verkhoiansk and Cherskii ranges, the Anadyr’, Kolyma, and Iukagir plateaus, and the Anadyr’, Kolyma, and Iano-Indigirka lowlands. The general layout of the large orographic elements follows the Mesozoic structural scheme: ridges and highlands correspond to folded zones, plateaus to rigid median massifs. Among these are the Kolyma, Omolon, Okhotsk, Taigonos, and Chukotka massifs. The central portion of the Kolyma massif is composed of Precambrian crystalline rocks that are covered with a sheath of Upper Paleozoic, Mesozoic, and Cenozoic deposits. Along the periphery the folded foundation, which is composed of carbonaceous Ordovician, Silurian, Devonian, and Carboniferous series crumpled into folds, emerges on the surface, forming marginal elevations—the Polousnensk, Tas-Khaiakhtakh, Cherskii, and Prikolyma. The Omolon, Taigonos, and Okhotsk massifs are made up of Proterozoic crystalline schists and gneisses covered with a sheath of Ripheus, Paleozoic, and Mesozoic deposits. The massifs are broken by many ancient, in some places rejuvenated, faults that are revealed in a topography of upthrust ridges (Polousnyi crest and others) and intermontane depressions— grabens (Chaun and other valleys).

Between the Kolyma massif and the Siberian platform lies the Verkhoiano-Kolyma folded system, which is composed of a series of terrigenous Permian deposits many kilometers thick—the Middle Jurassic series (Verkhoiansk complex). In its western part this system is represented by the Verkhoiansk anticlinal zone, and in the east by the Iano-Kolyma synclinal zone. The former is made up primarily of Permian rocks crumpled into symmetrical linear folds; the latter is filled chiefly with Triassic and Jurassic deposits, which attain maximum multikilometer thickness at their junction with the Kolyma massif (In’iali-Debin synclinorium). Here the large granite intrusions of the Upper Jurassic and Lower Cretaceous ages are developed.

The extreme northeast of the Verkhoiano-Chukotka folded region is occupied by the Aniui-Chaun folded system, which includes the Berezovka, Aniui, and Chaun-Chukotka folded zones; the first two zones are divided by the Upper Jurassic-Cretaceous Oloi depression. Terrigenous and vul-canogenic sedimentary series of the Triassic and Lower Jurassic periods that were complexly dislocated and broken by faults were involved in the building of folded zones.

Two stages of tectonic movement played a fundamental role in the development of the Verkhoiano-Chukotka folded region. During the first stage (Upper Carboniferous) the cleavage of the eastern margin of the Siberian platform and the laying of géosynclinal depressions in that area occurred, and the rock strata in the Paleozoic miogeosynclines, which framed the old nucleus of the Kolyma massif, underwent folding. During the second stage (Upper Jurassic-Lower Cretaceous) the formation of the folded structures, pierced by granite intrusions and broken by clefts, was completed. General upheavals during this period were accompanied by the formation of postgeosynclinal structures: the Priver-khoiansk marginal depression and the Zyrianka depression, composed of molasse and carboniferous deposits of the Upper Jurassic and Lower and Upper Cretaceous periods, crumpled into slanting folds. In the middle of the Cretaceous period the Verkhoiano-Chukotka folded region was transformed into a mountainous region.

The Verkhoiano-Chukotka folded region is one of the most important mining provinces in the world. Large deposits of gold (in the areas of the Berelekh, Nera, Aian-Iuriakh, and other rivers), tin (Deputatskii and Chukotka), polymetallic ores and ores of rare earth elements (Im-tandzha, Tokichanskoe, Bulunginskoe, and other villages), mercury, and coal (Zyrianka basin) are located in the region.