Tectonics of the Verkhoyansk-Kolyma Mesozoides in the context of plate tectonics

Abstract

The Verkhoyansk-Kolyma Mesozoides, located to the northeast of the ancient Siberian platform, form a part of a Mesozoic orogenic belt which lies between the Siberian and North American platforms, the Pacific Ocean and Arctic Ocean. The outer part of the Verkhoyansk-Kolyma Mesozoides is adjacent to the Siberian platform and comprises the Verkhoyansk foldbelt, composed primarily of Carboniferous-Middle Jurassic clastic sequences and lower-middle Paleozoic and Riphean terrigenous-carbonate series deposited on the Siberian passive continental margin. Northeastward, the passivemargin shelf deposits grade into Upper Permian to Lower Jurassic black-shale sequences of the Kular Nera slate belt which formed at a continental rise and in a marginal sea. Thick (up to 8–10 km), largely carbonate Ordovician-Devonian deposits of the Cherskiy, Selennyakh and Ulakhan-Tas ranges lie allochthonously over the Mesozoic and represent formations of the Omulevka carbonate platform. Ophiolites of uncertain age and a serpentinite melange have been tectonically juxtaposed against the latter. The time of emplacement for the Ophiolites is constrained by their occurrence in a Bathonian-Callovian olistostrome. The northeast-trending Alazeya volcanic arc, of Carboniferous to early Jurassic age, is located in the central part of what was once considered as the Kolyma massif. An accretionary wedge associated with the arc is buried beneath Late-Cenozoic deposits and can be traced by a large magnetic anomaly which extends to the south and east. The Omolon and Okhotsk massifs have an Archean granulite basement and are independent microcontinents. The available paleomagnetic data do not allow of the quantitative assessment of the amount ofseparation of the various allochthonous terranes, either from each other or from the Siberian continent. An affinity between Paleozoic and Mesozoic faunas of the Mesozoides and those ofthe Siberian platform indicates that their relative separation did not exceed a few thousand kilometers. Remnants of basins with oceanic crust found in the inner areas of the Verkhoyansk-Kolyma Mesozoides indicate that they are more comparable to present-day marginal seas of eastern Asia than to the open Pacific Ocean. Early folding occurred as a result of the pre-mid-Jurassic attachment of the Prikolyma and Omolon terranes to the Alazeya arc, which led to the formation of the large, heterogeneous, Kolyma-Omolon microcontinent. Later folding was due to a collision of the Kolyma-Omolon microcontinent with the Omulevka platform and the Verkhoyansk passive margin. This deformation propagated from the inner (Cherskiy) areas, in Bathonian time, towards the Siberian platform, where it culminated in frontal thrusts in the latest Cretaceous. Transverse and longitudinal belts of Upper-Mesozoic granitoids are closely related to collision processes. Their long history (about 90 m.y.) reflects the duration of the collision.