Spatial distribution of cosmogenic 10Be derived denudation rates between the Western Tian Shan and Northern Pamir, Tajikistan

The northward motion of the Pamir mountains relative to Eurasia results in uplift and shortening of the Pamir and Tian Shan orogens. The Vakhsh River catchment (Tajikistan) is situated along a thrust system in the collision zone between the Northern Pamir and the Western Tian Shan. In this study, spatial variations in catchment-wide denudation rates are reported from the Vakhsh River catchment with in situ-produced cosmogenic 10Be and 26Al of 22 samples from active river channels. Samples distributed along 140 km of the main Vakhsh River yield denudation rates between 1.28 ± 0.16 to 1.94 ± 0.26 mm/yr. In detail, samples from rivers draining the Northern Pamir indicate denudation rates of ~1.7 mm/yr, and a river draining the Alai Valley indicates denudation rates of 1.14 ± 0.14 mm/yr. In contrast, rivers draining the Western Tian Shan show increasing denudation rates from 0.18 ± 0.02 mm/yr in the west to 2.70 ± 0.36 mm/yr in the east, coincident with a small increase in the fluvial normalized steepness index (ksn). Measured 26Al/10Be ratios range from 5.2 to 7.6, indicating a low influence of sediment storage and reworking of partially shielded material. In addition, locations resampled one year later confirm previously calculated denudation rates along the main trunk. Analysis of in situ-produced cosmogenic 10Be from depth profiles collected from a sequence of terraces preserved along the main trunk of the Vakhsh River indicate terrace incision starting at ~3 kyr as well as paleo-denudation rates ranging between 1.75 ± 0.54 to 2.01 ± 0.39 mm/yr (2 s.d. errors). All these results suggest consistent high denudation rates in the Northern Pamir block, and a spatial (along strike) gradient in Western Tian Shan denudation rates. The most likely explanation for the spatial gradient in Western Tian Shan and Vakhsh River denudation rates is a regional gradient in tectonic rock uplift.