Dynamic processes from plate subduction to intracontinental deformation: Insights from the tectono-sedimentary evolution of the Zhaosu–Tekesi Depression in the southwestern Chinese Tianshan

Abstract

The Zhaosu–Tekesi Depression, located to the northern margin of the South Tianshan orogenic belt (STB), is an ideal region for investigating Paleozoic orogenic processes and later modifications of the southwestern Chinese Tianshan. In this study, we employ 2D seismic and borehole data revealing the geological structure and tectonic evolution of the Zhaosu–Tekesi basin, and we report new zircon U–Pb geochronological and Hf isotopic data and whole-rock elemental and Sr–Nd isotopic analytical results of the Late Silurian magmatic rocks, as well as the whole-rock trace element data of the Early Carboniferous carbonate rocks. Borehole data (wells ZC1 and ZC2) and seismic imaging suggest that the Zhaosu–Tekesi Depression is composed of Silurian, Lower Carboniferous, Permian, Jurassic, Upper Cretaceous and Paleogene strata, with six unconformity surfaces (C1a/S3, P1/C1a, P2t/P1, J/P2t, K2d/J and E/K2d). The Silurian strata exhibit southward thrust structures and consist of basalt, andesite, dacite, tuff, minor muddy limestone and diorite intrusions. Zircon SHRIMP U–Pb dating for the well ZC1 diorites yield a crystallization age of ∼422Ma, with positive εHf(t) values (3.3–10.6). The diorites and andesites are calc-alkaline and show enrichment in LILEs (such as Th and U) and depletion in Nb, Ta and Ti, relatively high initial 87Sr/86Sr ratios (0.703376–0.706737) and low εNd(t) values (−0.5 to 1.4), and high Th/Yb (0.29–0.35) and Ta/Yb (1.31–1.58) ratios, indicating their formation in an active continental margin setting. The Lower Carboniferous strata mainly comprise oolitic limestone, bioclastic micritic limestone and interbedded calcareous siltstone. High Sr/Ba (1.31–2.33), V/Sc (10.4–18.6) and low Th/U (0.01–0.23) ratios suggest that they were potentially deposited in the mixing near-shore marine environment. These results, in combination with previous data, indicate that the southwestern Chinese Tianshan is controlled by northward subduction of the South Tianshan Ocean during the Late Silurian to Early Carboniferous. The presence of the Lower Permian terrigenous strata and rift basin structure constrains the timing of collision between the CTB–KYB and the Tarim Craton as the late Carboniferous. Thrusting reactivation and syn-tectonic growth strata in the basin suggest that the southwestern Chinese Tianshan experienced Late Cretaceous compression and Neogene modifications after the Late Permian post-faulting thermal subsidence, Triassic peneplanation and Jurassic regional subsidence. Intracontinental subduction due to distant effect is the main tectonic control on the Miocene rapid uplift of the Tianshan. The Zhaosu–Tekesi basin evolution sheds light on dynamic processes of the southwestern Chinese Tianshan from plate subduction to intracontinental deformation.