Tectonics of the Aga Zone, Mongolia-Okhotsk belt

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The structure and tectonics of the Aga Zone are considered. It is shown that this zone is a system of tectonic nappes thrust over the Argun microcontinent. The zone is composed of two rock complexes related to the Variscan and Kimmerian structural stages. The Variscan stage (Silurian(?)-Early Carboniferous) comprises structural elements that correspond to the continental slope; the oceanic basin proper; the active continental margin, including an accretionary wedge; and an island arc and backarc basin. The Devonian age of the ophiolites of the Shilka Belt is specified. The formation of this set of tectonic units is related to the Middle Paleozoic pulse of the opening of the Mongolia-Okhotsk paleobasin. The Kimmerian stage (Middle Carboniferous-Early Jurassic) is characterized by a different style of structural evolution. A system of separate troughs filled with flyschoid sequences was formed on the Variscan basement. The unstable setting related to shortening and closure of the paleobasin brought about the spatial migration of sedimentation zones and the development of intraformational breaks in sedimentation, as well as unconformities. This stage was completed in the Lias by the general uplift of the territory and the formation of Jurassic and Cretaceous mollase along its periphery. The Aga allochthonous mass was ultimately formed in the Middle Jurassic. This event is recorded in emplacement of Middle-Late Jurassic granitic plutons that blocked the nappes. The granitic-metamorphic layer was formed in the Paleozoic and Early Mesozoic at the margin of the Aga Zone upon its conjugation with the adjacent continental masses; this layer is related to crustal anatexis. The bulk of the granitic rocks of the Aga Zone were generated in the Middle and Late Jurassic due to the collision of the North Asian continent with the Argun microcontinent.