Impact of glaciations on the exhumation history of the Kyrgyz Range – Western Tien Shan (Kyrgyzstan).

Constraining the effect of global climatic changes on Earth-surface processes is crucial to our understanding of landscape evolution. One debated question is the impact of Cenozoic cooling and subsequent glaciations on the spatial and temporal distribution of erosion in mountain ranges. The apatite (U-Th-Sm)/He thermochronometric system can record low temperature (<100 ◦C) cooling histories and thus potentially has the sensitivity to detect million-year timescale changes in exhumation rates in glaciated regions. Previous thermochronology studies in the Kyrgyz Range (Western Tien Shan, Kyrgyzstan) have identified an increase of exhumation rates over the last 3 Ma, which have been hypothesized to result from enhanced glacial erosion (Bullen et al., 2003; Sobel et al., 2006). Furthermore, an analysis of published global thermochronology data identified the Kyrgyz Range as one of the few locations globally with the potential to record the effect of Pleistocene glaciations (Schildgen et al., 2018). In this study, we present new AHe ages for 3 samples collected along the main trunk of the Ala Archa valley and 6 samples collected in a tributary valley exhibiting clear glacial imprint. The samples were collected from granite outcrops over an elevation range of 1850 m (lowest sample: 1792 m – highest sample: 3634 m). These new samples exhibit: (1) an onset of cooling at 12 - 10 Ma, in agreement with published work and interpreted as the start of exhumation in the Kyrgyz Range; (2) a rapid increase in cooling rates between 2 and 3 Ma recorded in the lower elevation samples (1792 – 2240 m), which could have been caused by glacial incision and valley widening during the onset of Pleistocene glaciations (2.6 Ma) and; 3) a negative age-elevation relationship above 3600 m (5.6 ± 0.7 Ma) potentially demonstrating valley widening due to lateral glacial erosion. These results suggest that the onset of the Pleistocene glaciations had a strong impact on the Western Tien Shan, both at higher and lower elevations. --- Bullen, M. E., Burbank, D. W., and Garver, J. I.: Building the Northern Tien Shan: Integrated thermal, structural, and topographic constraints, Journal of Geology, 111, 149–165, https://doi.org/10.1086/345840, 2003. Schildgen, T. F., Van Der Beek, P. A., Sinclair, H. D., and Thiede, R. C.: Spatial correlation bias in late-Cenozoic erosion histories derived from thermochronology, Nature, 559, 89–93, https://doi.org/10.1038/s41586-018-0260-6, 2018. Sobel, E. R., Oskin, M., Burbank, D. W., and Mikolaichuk, A.: Exhumation of basement-cored uplifts: Example of the Kyrgyz Range quantified with apatite fission track thermochronolgy, Tectonics, 25, https://doi.org/10.1029/2005TC001809, 2006.