Thermo-tectonic history of the Junggar Alatau within the Central Asian Orogenic Belt (SE Kazakhstan, NW China): Insights from integrated apatite U/Pb, fission track and (U-Th)/He thermochronology

The Junggar Alatau forms the northern extent of the Tian Shan within the Central Asian Orogenic Belt (CAOB) at the border of SE Kazakhstan and NW China. This study presents the Palaeozoic-Mesozoic post-collisional thermo-tectonic history of this frontier locality using an integrated approach based on three apatite geo-/thermochronometers:apatite U-Pb, fission track and (U-Th)/He. The apatite U-Pb dates record Carboniferous-Permian post-magmatic cooling ages for the sampled granitoids, reflecting the progressive closure of the Palaeo-Asian Ocean. The apatite fission track (AFT) data record (partial) preservation of the late Palaeozoic cooling ages, supplemented by limited evidence for Late Triassic (~230-210 Ma) cooling and a more prominent record of (late) Early Cretaceous (~150-110 Ma) cooling. The apatite (U-Th)/He age results are consistent with the (late) Early Cretaceous AFT data, revealing a period of fast cooling at that time in resulting thermal history models. This Cretaceous rapid cooling signal is only observed for samples taken along the major NW-SE orientated shear zone that dissects the study area (the Central Kazakhstan Fault Zone), while Permian and Triassic cooling signals are preserved in low-relief areas, distal to this structure. This distinct geographical trend with respect to the shear zone, suggests that fault reactivation triggered the Cretaceous rapid cooling, which can be linked to a phase of slab-rollback and associated extension in the distant Tethys Ocean. Similar conclusions were drawn for thermochronology studies along other major NW-SE orientated shear zones in the Central Asian Orogenic Belt, suggesting a regional phase of Cretaceous exhumation in response to fault reactivation at that time.