Discrepancy between fluvial incision and denudation rates at the western Pamir margin

Abstract:

Surface process rates provide keys to unravel controls of tectonics and climate on mountain evolution. In the Pamir, significant discrepancies exist between fluvial incision and denudation rates. Optically stimulated luminescence (OSL)-based fluvial incision outpaces cosmogenic nuclide (Be-10)-based denudation by up to 10 times at the western Pamir margin.

Differences in averaging times of determined rates cannot explain the contrast. The millennial denudation rates are highest (1.0 – 1.5 mm/yr) where long-term (10⁴years) fluvial incision is moderate (2 – 5 mm/yr). In contrast, denudation is lower (~0.8 mm/yr) where incision is highest (7 – 10 mm/yr).

Process rates across the Pamir reveal differing control factors. The longitudinal profile and valley profiles of the Panj highlight links between fluvial incision and tectonic structures. Convex river reaches across the Pamir domes correspond to intense incision, whereas graded river segments coincide with southern dome boundaries. The indicated river captures induce intense incision and base level lowering for basins at the Pamir margins. Both, rapid incision and base level changes, maintain the steep slopes (0.75 quartiles) that are the predominant driver (R² of ~0.8) for high denudation rates at the Pamir margins (0.5 – 1.5 mm/yr). However, highest denudation and lowest discrepancy to fluvial incision (~2 - 3 times) coincides with the areas receiving the highest annual precipitation. Multiple linear regression of denudation rates with the 0.75 quartiles of basin slopes and precipitation shows an R² of 0.93. We propose that river captures are responsible for driving the intense incision along the Panj and consequently, provide the conditions for hillslope adjustment that contributes to the high denudation at the Pamir margins. Winter precipitation, and related concentrated discharge during the melting season, may act as limiting factor to hillslope adjustment and consequently, to denudation processes.