New low-temperature thermochronological data from 80 samples in eastern Kyrgyzstan are combined with previously published data from 61 samples to constrain exhumation in a number of mountain ranges in the Central Kyrgyz Tien Shan. All sampled ranges are found to have a broadly consistent Cenozoic exhumation history, characterized by initially low cooling rates (<1°C/Myr) followed by a series of increases in exhumation that occurred diachronously across the region in the late Cenozoic that are interpreted to record the onset of deformation in different mountain ranges. Combined with geological estimates for the onset of proximal deformation, our data suggest that the Central Kyrgyz Tien Shan started deforming in the late Oligocene-early Miocene, leading to the development of several, widely spaced mountain ranges separated by large intermontane basins. Subsequently, more ranges have been constructed in response to significant shortening increases across the Central Kyrgyz Tien Shan, notably in the late Miocene. The order of range construction is interpreted to reflect variations in the susceptibility of inherited structures to reactivation. Reactivated structures are also shown to have significance along strike variations in fault vergence and displacement, which have influenced the development and growth of individual mountain ranges. Moreover, the timing of deformation allows the former extent of many intermontane basins that have since been partitioned to be inferred; this can be linked to the highly time-transgressive onset of late Cenozoic coarse clastic sedimentation.

[1] The Kyrgyz Range, the northernmost portion of the Kyrgyzstan Tien Shan, displays topographic evidence for lateral propagation of surface uplift and exhumation. The highest, most deeply dissected segment lies in the center of the range. To the east, topography and relief decrease, and preserved remnants of a Cretaceous regional erosion surface imply minimal amounts of bedrock exhumation. The timing of exhumation of range segments defines the lateral propagation rate of the range-bounding reverse fault and quantifies the time and erosion depth needed to transform a mountain range from a juvenile to a mature morphology. New multicompositional apatite fission track (AFT) data from three transects from the eastern Kyrgyz Range, combined with published AFT data, demonstrate that the range has propagated over 110 km eastward over the last $7-11 Myr. On the basis of the thermal and topographic evolutionary history, we present a model for a time-varying exhumation rate driven by rock uplift and changes in erodability and the timescale of geomorphic adjustment to surface uplift. Easily eroded, Cenozoic sedimentary rocks overlying resistant basement control early, rapid exhumation and exhibit slow surface uplift rates. As increasing amounts of resistant basement are exposed, exhumation rates decrease while surface uplift rates are sustained or increase, thereby growing topography. As the range becomes high enough to cause ice accumulation and to develop steep river valleys, fluvial and glacial erosion becomes more powerful, and exhumation rates once again increase. Independently determined range-normal shortening rates also varied over time, suggesting a feedback between erosional efficiency and shortening rate.

[1] Basement-cored ranges formed by reverse faulting within intracontinental mountain belts are often composed of poly-deformed lithologies. Geological data capable of constraining the timing, magnitude, and distribution of the most recent deformational phase are usually missing in such ranges. In this paper, we present new low temperature thermochronological and geological data from a transect through the basement-cored Terskey Range, located in the Kyrgyz Tien Shan. Using these data, we are able to investigate the range's late Cenozoic deformation for the first time. Displacements on reactivated faults are constrained and deformation of thermochronologically derived structural markers is assessed. These structural markers postdate the earlier deformational phases, providing the only record of Cenozoic deformation and of the reactivation of structures within the Terskey Range. Overall, these structural markers have a southern inclination, interpreted to reflect the decreasing inclination of the reverse fault bounding the Terskey Range. Our thermochronological data are also used to investigate spatial and temporal variations in the exhumation of the Terskey Range, identifying a three-stage Cenozoic exhumation history: (1) virtually no exhumation in the Paleogene, (2) increase to slightly higher exhumation rates at~26-20 Ma, and (3) significant increase in exhumation starting at~10 Ma.

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