Along-strike Variations in Cosmogenic Derived Denudation Rates in the Western Tian Shan, Tajikistan

Abstract:

The Pamir-Tian Shan constitutes the northwestern corner of the India-Asia collision where the indentation of the Pamir has resulted in crustal shortening and rock uplift. To understand the topographic response to tectonic, lithologic and climatic variations over millennial timescales, previous studies determined catchment-wide denudation rates with in situ-produced cosmogenic ¹⁰Be of samples from active river channels in the Pamir. These studies provided denudation rates that range from 0.05 (Pamir Plateau) to ~2 mm/yr (northern Pamir). In this study, we investigate catchments in the Tian Shan, northwest of the Pamir. We quantified along-strike variations in denudation rates along the largest river system in the area, the Zeravshan river and its major tributary the Yagnob river (total drainage area of >12,000 km²). These rivers drain the crest of the Tian Shan facing the Westerlies. We compared these rates with the locations of Quaternary faults, lithologic units, and topographic metrics such as the normalized steepness index upstream of the sample locations. Our results are also compared with rates obtained from 22 published cosmogenic nuclide samples from the Surkhob-Vakhsh river system which drains the frontal ranges of the Pamir.

We present 9 new cosmogenic ¹⁰Be analyses of river sediments. From east (69.5 ^o E) to west (67.5 ^o W), samples distributed along ~200 km of the Zeravshan river yielded denudation rates decreasing from 1.37 ± 0.27 to 0.77 ± 0.17 mm/yr downstream. The rates measured along the Yagnob river yielded lower values between 0.57 ± 0.11 and 0.69 ± 0.13 mm/yr. These rates correlate positively with the fluvial normalized steepness index. Quaternary faults mostly parallel the studied rivers and lack significant historical and recent seismicity. The denudation rates of the Surkhob-Vakhsh river system are similar but generally higher than those derived from Zeravshan-Yagnob system. This is interpreted by incision of the Surkhob-Vakhsh into easily erodible sediments (clastic rocks with evaporites of the Tajik basin), high discharge due to more voluminous hinterland glaciations, and high tectonic strain rates along the Pamir front.