Landscape Evolution of the Kyrgyz Range (Tien Shan): Deciphering Tectonic and Climate Inputs

The Kyrgyz Range, located on the northwestern edge of the Tien Shan, provides a unique opportunity to study the interplay between tectonic processes and climatic forces in shaping mountain landscapes. The apatite (U-Th-Sm)/He (AHe) thermochronometric system, sensitive to low-temperature (<100 °C) cooling histories, has the potential to detect million-year timescale changes in exhumation rates in glaciated regions. Previous studies in the Kyrgyz Range have identified increased exhumation rates over the last ~3 Ma (Bullen et al., 2003; Sobel et al., 2006).

In this study, we present seven new AHe ages from the Ala Archa valley, ranging from 3.3 ± 1.0 Ma to 7.5 ± 1.4 Ma. Samples were collected from granite outcrops along an elevation profile spanning 1,850 m (1792–3634 m), including the main trunk and a tributary valley with clear glacial imprints. 1D modeling of these ages reveal: (1) an onset of cooling at ~12–10 Ma, consistent with published work and interpreted as the start of exhumation in the Kyrgyz Range; and (2) a rapid increase in cooling rates between 0 and 3 Ma, recorded in the lower elevation samples (1792–2240 m).

Using 3D thermal-kinematic modeling with Pecube, we explored scenarios of topographic and tectonic evolution to explain these cooling ages. Our modeling shows that topographic evolution, specifically valley incision, can produce rapid and recent cooling ages even when rock-uplift rates are low (<0.5 km/Ma). Modeling further suggests that the onset of Pleistocene glaciations likely drove a phase of rapid valley incision in the Kyrgyz Range, emphasizing the impact of climatic forcing on exhumation.

Field constraints and reconstructed sediment volumes from the adjacent Chu Basin indicate a change in sedimentary dynamics between 4 and 2 Ma, supporting this scenario. These findings emphasize the critical role of glaciation in shaping the Kyrgyz Range, with glacial erosion contributing substantially to valley incision and exhumation rates.

This study underscores the importance of integrating thermochronology with topographic and thermal modeling to disentangle tectonic and climatic influences on mountain range evolution. Without accounting for changes in topography, variations in exhumation rates might be incorrectly attributed solely to tectonic uplift, potentially overlooking significant climatic impacts.