Late Palaeozoic and Meso-Cenozoic tectonic evolution of the southern Kyrgyz Tien Shan: Constraints from multi-method thermochronology in the Trans-Alai, Turkestan-Alai segment and the southeastern Ferghana Basin

In this paper we present new geochronological and thermochronological data from the Turkestan-Alai segment of the southern Kyrgyz Tien Shan, the Ferghana Basin transition and the Trans-Alai Pamir foreland. In the southern and central parts of the Alai Range, towards the Alai Valley, mainly post-collisional granitoids were targeted for zircon U/Pb dating by Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS), apatite fission-track (AFT) and apatite (U–Th–Sm)/He (AHe) thermochronology. These granitoids, and their thermally metamorphosed contact aureole, yielded Middle Permian zircon crystallization ages of 284 ± 4 to 264 ± 5 Ma. AFT ages (both using the traditional external detector approach as well as using LA-ICP-MS) for these samples are constrained to the Miocene and range between 22.3 ± 1.1 and 8.4 ± 0.3 Ma, while the AHe ages are somewhat younger, between 12.5 ± 0.8 and 5.0 ± 0.3 Ma. This low-temperature thermochronological data suggests an Early Miocene onset of denudation in this area in the Tien Shan orogen, continuing through the Miocene and Pliocene. This denudation is interpreted as a response to reactivation of the mountain belt in the framework of India–Eurasia convergence, and specific to this region, of the Pamir–Tien Shan interaction. On the northern slopes of the Alai Range, at the edge of the Ferghana Basin, a Late Palaeozoic conglomerate yielded a reset Early Cretaceous AFT age of 118 ± 7 Ma, pointing to an older denudation event with respect to those identified in the southern and central parts of the Alai Range. A Mesozoic sediment sample from the same region only contains a Late Triassic age component of ∼223 Ma. Mesozoic and Cenozoic age components on the other hand are all detected in the Cenozoic sediments of the Trans-Alai Range, at the transition between the southern Alai Valley and the Pamirs, revealing several Meso-Cenozoic denudation events in the adjoining basement source rocks. The age components in these detrital apatites are ∼207 Ma, ∼75 Ma, ∼18 Ma and ∼3 Ma.

► Early-Middle Permian post-collisional granites in the Southern Tien Shan are dated at 284 – 264 Ma by zircon U/Pb LA-ICP-MS.
► Early Cretaceous and Early Miocene to Late Miocene apatite fission-track basement ages are reported for the Alai Range.
► Late Triassic, Late Cretaceous, Miocene and Pliocene AFT age components are found in the detrital record of the Alai Basin.
► Multichronologic Late Palaeozoic and Meso-Cenozoic ages constrain the tectonic evolution of the Kyrgyz Southern Tien Shan.