Thermochronology of the highest Central Asian massifs (Khan Tengri - Pobedi, SE Kyrgyztan): evidence for Late Miocene (ca. 8 Ma) reactivation of Permian faults and insights into building the Tian Shan

Abstract Geological study of the Khan Tengri and Pobedi massifs highlights two phases of compressional fault displacements. A Late Permian/Triassic displacement phase is highlighted (1) by biotite 40Ar/39Ar ages of 265–256 Ma, suggestive of cooling of Pobedi mid-crustal granulites during >8 km of top-to-the-north motion of the Pobedi Thrust; (2) by 40Ar/39Ar dating of syn-kinematic phengite at 249–248 Ma, which suggests a crystallization age during top-to-the-south motion of the Khan Tengri Thrust shear zone; (3) by Apatite Helium (AHe) ages of 280-240 Ma on the crystalline basement below the Mesozoic peneplain, which gives insight into the final exhumation stage. Cenozoic reactivation of the Pobedi Thrust is indicated by AHe thermochronology with a mean age of 8.3 ± 2.5 Ma, in agreement with ∼3 km exhumation and ∼4.2 km of top-to-the-north motion in the Late Miocene. Along a north-south transect of the west Tian Shan range from Issyk-Kul Lake to Tarim Basin, compiled thermochronological data outline out-of-sequence deformation beginning with the activation of north-directed crustal-scale faults at 22-15 Ma along the northern margin of the north Tian Shan, followed by 20-10 Ma motion at the boundary between the Middle and the South Tian Shan, and ending with the