Spatial variations in 10Be-derived catchment wide denudation rates and the timing of glaciation in the NW Pamir SW Tien Shan Mountains, Tajikistan

Abstract:

Mountain topography evolves in response to interactions between climate and tectonic processes. This study investigates the role of glaciation and catchment wide denudation rates in forming the 40.000 km² large Vakhsh catchment in Tajikistan. The Vakhsh river is located in the Pamir – Tian Shan transition zone. The river drains the North Pamir as well as parts of the Alay Range, which is the western extension of the Tian Shan Mountains. The southward Vakhsh -Trans -Alay thrusting is the expression of ongoing tectonic activity in this area. Today the rate of convergence between the two mountain ranges adds up to at least 15 mm/yr.

We analyzed the upstream part of the Vakhsh catchment with in situ-produced cosmogenic ¹⁰Be and ²⁶Al in quartz. Modern river samples, terrace depth profiles, and moraine boulder samples were used for quantifying modern and paleo-denudation rates, and timing of the most recent glaciation. A recessional moraine was dated with a total number of 18 boulders and a sequence of five lateral moraines with 41 samples. Results from well-preserved moraines indicate in the North Pamir a maximal glacial extent at around 17 ka (+/- 1.9 ka). The analysis of modern river samples derived from several locations along the main channel of the Vakhsh river reveal denudation rates between 1.8 – 2.6 mm/yr. Spatial variations in denudation were also evaluated using cosmogenic nuclide-derived denudation rates from rivers ranging between Strahler order 4 and 7. Preliminary results for these rates vary between 0.7 and 2.9 mm/yr within the catchments. A correlation between Strahler order and the denudation rate has not been observed. Exposure ages for the lowest, middle, and highest of the 10 terraces are all ~3.2 ka (+/- 0.29 ka) indicating synchronous and rapid terrace formation in the late Holocene. Taken together, these results indicate: a) the most recent maximum extent of glaciation is in this valley is ~17 ka, b) terrace formation significantly post-dates glaciation and is most likely not controlled by glacial-interglacial climate variations, and c) catchment wide denudation rates in this semi arid, tectonically active, and glaciated setting are high, and of a similar magnitude as denudation rates of the monsoon influenced Himalaya.