Paleozoic Tian-Shan as a transitional region between the Rheic and Urals-Turkestan oceans

Abstract The Upper Paleozoic orogenic belt of South Tian-Shan (STS) in Kyrgyzstan, Uzbekistan and Tajikistan consists of two structural domains: the south-vergent Bukantau–Kokshaal (BK) in the north and continuing into Xinjiang (China), and the north-vergent Zeravshan–Hissar (ZH) in the south, in Tajikistan. The Bukantau–Kokshaal fold belt was thrust south onto the Kyzylkum–Alai and Tarim continents in the Late Carboniferous. The BK belt is the most prominent collision-related, alpine-type part of the Paleozoic Tian-Shan and, as a prolongation of the Tian-Shan structure, shows close resemblance to the western (outer, west-vergent) part of the Urals. The Kazakhstan continent acts as a hinterland to the BK collision belt. Kazakhstan was constructed by accretion processes in which ancient (presumably Gondwanan) continental terranes and ocean-derived crustal elements of the Early Paleozoic to Early Carboniferous age played a role. The main episode of terrane amalgamation took place during the Middle and Late Ordovician. This appears to reflect active margin development in the Paleoasiatic Ocean, and resembles processes occurring in the recent Western Pacific. Geological differences in construction and protolith age of continental crust in the region are in general agreement with Pb– and Sm–Nd isotopic data. Relatively early (Visean) north-vergent thrust structures in Zeravshan–Hissar and eastern Alai (southwestern STS) bear some resemblance to the Central European Hercynides of Rheic origin, although this region became the location of active margin tectonic processes associated with the closure of the Paleotethys Ocean during the Carboniferous. Post-collisional magmatism occurred from ca. 300 to 270 Ma and is represented by a variety of magma types from A-type granites to nepheline syenites. The spatial distribution of plutons appears to be controlled by transtensional structures associated with east–west, left-lateral wrench faulting. The presence of coeval alkali intrusions and plateau basalts in adjacent areas suggests that this magmatism may have been associated with a mantle plume.

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