Investigation of Glacial Retreat, Terrace Abandonment, and Catchment-wide Denudation Rates in the Vakhsh Catchment, Tajikistan

Abstract

The development of the Vakhsh catchment is linked to the tectonic development of the Pamir during the India-Asia collision. The northward motion of the Pamir block relative to Eurasia, as well as its uplift and shortening, confined the Vakhsh River system between the western Tian Shan and the Pamir along the Pamir Thrust System deformation zone (PTS). The northern Pamir overthrusts the western Tian Shan and the Alai Valley experiences shortening along the PTS. At the present, the Vakhsh catchment drains two orogens along a thrust system and provides an opportunity for understanding denudation associated with plate convergence. During the Quaternary, the Vakhsh River catchment experienced changes in climatic conditions such as Quaternary glaciation and the Holocene thermal optimum, which caused fluctuations in sediment supply and water discharge. This study investigates the timing of glaciation, terrace formation as well as modern and paleo-denudation rates in the context of the tectonic and paleo-climatic conditions of the region. Glacial deposits, such as moraines, are archives that record advance and retreat cycles driven by climate change. Collecting spatial and temporal data about glaciations is essential for the reconstruction of both, the regional and global timing of ice ages and climate change. The investigation of glacial extent and timing in the Vakhsh River catchment was addressed by studying the glacial deposits near the former terminus of the Fedchenko Glacier. The Fedchenko Glacier is particularly important for understanding the glacial chronology in Central Asia, because it is the longest (~72 km) glacier outside the Polar Regions and the timing of its advance and retreat may be representative for the climate evolution of the Westerly-dominated Central Asia region. The present-day climate is dominated by the Mid-Latitude Westerlies and supplies the region with 400 – 1,000 mm annual precipitation. The present-day mean annual temperatures range between -6.7 and 12 ˚C. Inter alia, the interaction between precipitation and temperature induces flooding and increases mass movement events during snow melt. To understand and quantify the temporal and spatial influence of climate and tectonics on the evolution of mountain belts and the fluvial systems within is of a major importance for human existence in this challenging environment. To target the timing and possible causes for fluctuations in sediment supply, I investigated a flight of fluvial terraces along the main trunk. The incision into the investigated terraces started at Holocene times ~ 3 ka ago and was accompanied by high incision rates of 4 – 29 mm/yr. The paleodenudation rates associated with the terraces range between 1.75 ± 0.41 mm/yr and 2.01 ± 0.36 mm/yr. The modern denudation rates cover the same magnitude (1.28 ± 0.16 – 1.94 ± 0.26 mm/yr) along the main trunk. The investigation of the tributaries yielded the greatest variation in modern denudation rates in the western Tian Shan tributaries (0.18 ± 0.02 mm/yr to 2.05 ± 0.27 mm/yr). The Alai Valley displays an intermediate signal of 1.14 ± 0.14 mm/yr and the northern Pamir tributaries display rates (1.78 ± 0.23 mm/yr and 1.63 ± 0.21 mm/yr) at the higher end of the spectrum. Additionally 26Al/10Be ratios were used to test, whenever the tributaries are prone to store or mix fresh sediment with shielded material. The results indicate no immediate storage or mixing (26Al/10Be ratios: 6.0 ± 1.0 to 7.6 ± 1.2), except for one western Tian Shan catchment (26Al/10Be ratio: 5.2 ± 0.5).