Thermal history of the Junggar Alatau (SE Kazakhstan, NW China): insights from apatite fission track thermochronology.

Abstract

The Junggar Alatau is located along the border of SE Kazakhstan and NW China, representing the western extent of the northern Chinese Tian Shan within the Central Asian Orogenic Belt. This study applies apatite U–Pb and low temperature thermochronology to constrain the exhumation history of the Junggar Alatau through the Meso-Cenozoic period. Apatite U–Pb results record Ordovician-Permian ages, reflecting the post-magmatic cooling of granitoids that intruded during the progressive closure of the Palaeo-Asian Ocean. Apatite fission track data obtained from Palaeozoic basement and (meta)sedimentary samples record (partial) preservation of post-magmatic cooling ages and suggests at least two subsequent cooling periods during the Late Triassic (~230–210 Ma) and Early Cretaceous (~150–120 Ma). Permian-Triassic cooling signals are preserved in low-relief areas distal to major NW-SE orientated shear zones, reflecting post-magmatic cooling during the Palaeo-Asian Ocean closure and regional exhumation in response to the Qiangtang collision. The Early Cretaceous rapid cooling signal is localised for samples taken along the Central Kazakhstan Fault Zone (CKFZ). The record of younger signals localised in the CKFZ suggests the reactivation of faulting during the Early Cretaceous, which can be linked to a phase of slab-rollback and associated extension in the distant Tethys Ocean. Cretaceous exhumation is thought to have induced rapid cooling in the exhuming footwalls with respect to subsiding hanging walls of the CKFZ. Additionally, few samples from higher relief areas record Late Cretaceous cooling, although there is no consensus on the cause. The results obtained in this study indicate that the extent of Cenozoic exhumation within Central Asia has not propagated to the Junggar Alatau. Results obtained in this work fit with those obtained from samples along other major NW-SE orientated shear zones in the vicinity of the study area, improving the overall understanding of strain propagation through Central Asia.