Tracking the multi-stage exhumation history of the western Chinese Tianshan by apatite fission track (AFT) dating: Implication for the preservation of epithermal deposits in the ancient orogenic belt

摘要

The western Chinese Tianshan, located in the southern domain of the Central Asian Orogenic Belt (CAOB), was originally established by multiple accretion-collision processes in the Paleozoic, and was modified by complicated intracontinental tectonic evolution in the Mesozoic to Cenozoic. Understanding the timing and mechanism of the latter geological processes is critical to unravel the preservation conditions of the epithermal deposits in the western Chinese Tianshan. This work presents new apatite fission track (AFT) data for three mountain ranges (Keguqinshan, Wusunshan and Nalatishan) of the western Chinese Tianshan to track their exhumation history. Our AFT data gave a wide range of ages from 76.8 +/- 5.5 Ma to 182.3 +/- 9.9 Ma, and the mean confined fission track lengths are between 9.8 +/- 0.5 mu m and 12.3 +/- 0.2 mu m. The new data, in combination with the thermal history modeling results, allow us to summary three main episodes of exhumation in the western Chinese Tianshan, including the Early Permian (300-280 Ma), Late Triassic-Late Jurassic (230-140 Ma), and Late Oligocene-Early Miocene (30-20 Ma). The first episode may have been caused by terrane accretion-collision in the late Paleozoic, leading to uplift of the Keguqinshan that supplied abundant sediments to form a critical cover of ca. 4 km thick on these epithermal deposits. The second episode was likely related to closure of the Mongol-Okhotsk Ocean in the Mesozoic, and an extremely slow exhumation rate (ca. 0.0083 mm/y) as well as a moderate erosion of ca. 0.8 km thick are estimated. The last episode is regarded to be the result of collision between the Indian Plate and the Eurasia Plate in the Cenozoic, and this significant tectonic event triggered rapid exhumation (up to ca. 0.1 mm/y) of the whole region in the western Chinese Tianshan, which removed more than ca. 3.2 km pre-Cenozoic sedimentary cover and sent the epithermal deposits to the subsurface level. (C) 2017 Elsevier B.V. All rights reserved.