The Southern Tian-Shan: Upgrading the geologic synthesis

Abstract

New geological results concerning the 2500 km length Southern Tian-Shan (STS) Late Paleozoic collisional belt are revised. The STS basement consists mainly of Proterozoic continental crust fragments. Precambrian terrains of the STS immediate environment (Ishim-Naryn microcontinent with its Middle Tian-Shan outcrop, Central Tian-Shan, Karakum-Tajik, Tarim) as well as presumed blocks inside the STS belt (Kyzylkum-Alay etc.) embrace the ancient Precambrian crust of ca. 2700–1800 Ma and its parts that were reworked and converged in the Late Proterozoic (900–600 Ma) active margin. The STS orogenic belt in the Turkestan ocean had emerged not later than 750 Ma in the Kyzylkum segment; before the Middle Ordovician in the central segment between Tarim and Central TS terrains; not later than Late Ordovician in the eastern (Chinese) segment. Active margins or Paleozoic island arc magmatism started in the Late Ordovician and developed in the Silurian — Early Devonian. Passive margin sedimentation in Devonian to Early Carboniferous expands thanks to intraplate basalt or bimodal volcanism. The general convergence with some back-arc rifting and spreading resumed in Visean and completed with continental collision in the Late Carboniferous. The great Bukantau — Kokshaal — Halyktau top-to-the-south thrust belt is the most expressive result of the collision, despite the fact that some top-to-the-north thrusts, both earlier and later, are also known. The Mississippian and Early Pennsylvanian magmatism of the active Kazakhstan margin as well as HP-UHP metamorphism emerged in the northern flank of the STS suture in the rear position to the main thrust belt direction. This active margin is better-presented in the western STS segment and implies greater volume of the ocean space consumed here. The Early Permian post-collisional magmatism of the STS and its frame, 290–270 Ma at its peak, presents a variety of granitoids, alkali rocks and also basalts with some ultrabasites. Among them are Rapakivi-type A-granite magmas that intruded the marginal (lower) part of the Kokshaal-Halyktau thrust continuity and expanded the Tarim margin in the unusual frontal position. This could be the result of Tarim mantle plum impact. In the Central Asian Orogenic Belt, well manifested alpine-type structure of STS, as much as poor persistence of ophiolites, absence of primitive arcs, keep the STS aside of accretionary “Altaids” such as the Eastern Kazakhstan or Junggar margin of the same Tian-Shan mountains.