Late Paleozoic Exhumation of the West Junggar Mountains, NW China

Abstract

The Tianshan and Altai mountain belts dominate the present-day topography of central Asia and are major constituents of the Central Asian Orogenic Belt. These mountain belts have been extensively studied using low-temperature thermochronology to understand their uplift and exhumation history. In comparison, few studies have focused on the spatial extent and timing of intracontinental deformation in the intervening ranges, such as the West Junggar Mountains. Apatite fission track (AFT) and apatite U-Pb dating of igneous samples from the West Junggar Mountains reveals AFT central ages ranging from latest Carboniferous to Middle Jurassic and mean confined track lengths between 14.1 and 12.0 mu m, which are negatively correlating to AFT central ages. Apatite U-Pb dating produced mid-Carboniferous-early Permian ages within uncertainty of zircon U-Pb crystallization ages, indicating cooling to below (similar to)450-550 degrees C immediately following emplacement. Thermal history modeling of our AFT data combined with published (U-Th)/He data predicts rapid cooling to <60 degrees C during the early-middle Permian (285-260 Ma), followed by slower cooling rates during the Mesozoic-Cenozoic. The rapid cooling event coincides with (1) the timing of major basinward thrusting in the Junggar Basin adjacent to the study area and (2) exhumation in the Chinese Altai to the north. In contrast to previous studies, our results imply that the West Junggar Mountains do not record significant exhumation during the Mesozoic-Cenozoic. This suggests that Mesozoic deformation of the Central Asian Orogenic Belt was less pervasive and widespread than previously assumed.