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.