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: Landscape evolution of a bedrock peneplain on the southern Tibetan Plateau revealed by in situ-produced cosmogenic 10Be and 21Ne.-Geomorphology, [153][154][192][193][194][195][196][197][198][199][200][201][202][203][204] AbstractLow-relief bedrock surfaces that occur at high altitude are a common feature of Cenozoic mountain belts and have often been used to infer a significant amount of rock uplift after their generation at low elevation. The timescale over which such surfaces can be preserved at high elevation and the rate at which they are modified by weathering and erosion are poorly known. Here we use cosmogenic 10 Be and 21 Ne to quantify the landscape evolution of a bedrock peneplain in southern Tibet that occurs at an altitude of ~5300 m. The peneplain is developed in Cretaceous granitoids and Jurassic metasediments of the northern Lhasa block (90°E, 31°N) and originally had a minimum extent of ~150 km east-west and ~75 km northsouth. It has been dissected by small rivers that generated a few hundred meters of relief and formed additional bedrock surfaces of limited extent at lower elevation. Local denudation rates for the peneplain and the lower bedrock surfaces -based on 10 Be concentrations in grus samples and amalgamated quartz clasts -cluster between 5 and 11 m Ma -1 (mean = 8.1 m Ma -1 ) and are thought to be representative for the long-term rate of downwearing of these surfaces. Samples from bedrock outcrops and bedrock blocks yield more variable rates (3-20 m Ma -1 ), which partly overestimate the long-term lowering rate, presumably due to block tilting and bedrock inhomogeneity. Spatially averaged 10 Be denudation rates for small river catchments range from 9 to 16 m Ma -1 (mean = 11.8 m Ma -1 ) and are only slightly higher than the local denudation rates. Hence, the incision and widening of valleys proceeds at low rates, which demonstrates that the landscape of the peneplain region is remarkably stable. The combined 21 Ne and 10 Be data in a subset of the samples suggest that the bedrock surfaces have experienced a simple exposure history without periods of burial. Thus, our data do not provide any evidence for extended periods of shielding by ice during past glaciations, which is consistent with the absence of erratic boulders and moraines in the peneplain region.

The Tangra Yum Co graben is one of the active structures that accommodate the east-west extension of the southern Tibetan Plateau and hosts one of the largest Tibetan lakes, which experienced lake-level changes of~200 m during the Holocene. In this study, cosmogenic 10 Be is employed to: (1) quantify catchment-wide denudation rates in fault-bounded mountain ranges adjacent to the Tangra Yum Co graben; (2) date palaeo-shorelines related to the Holocene lake-level decline; and (3) determine the age of glacial advances in this region. The fault-bounded, non-glaciated mountain range north of Tangra Yum Co -and presumably most other areas around the lake -erode at low rates of 10-70 mm/ka. Owing to the slow erosion of the landscape, the sediments delivered to Tangra Yum Co have high 10 Be concentrations. As a consequence, accurate exposure dating of sediment-covered terraces and beach ridges is difficult, because the pre-depositional 10 Be concentration may exceed the post-depositional 10 Be concentration from which exposure ages are calculated. This difficulty is illustrated by a rather inaccurate 10 Be exposure age of 2.3 ± 1.4 ka (i.e. an error of 60%) for a terrace that is located 67 m above the lake. Nevertheless, the age is consistent with luminescence ages for a series of beach ridges and provides further evidence for the decline of the lake level in the late Holocene. At Tangra Yum Co exposure dating of beach ridges via 10 Be depth profiles is not feasible, because the pre-depositional 10 Be component in these landforms varies with depth, which violates a basic assumption of this approach.10 Be ages for boulders from two moraines are much older than the early Holocene lake-level highstand, indicating that melting of glaciers in the mountain ranges adjacent to Tangra Yum Co has not contributed significantly to the lake-level highstand in the early Holocene.

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