Defining rates of landscape evolution in a south Tibetan graben with in situ-produced cosmogenic 10Be

Rades EF, Hetzel R, Strobl M, Xu Q, Ding L

Zusammenfassung

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 ~200m during the Holocene. In this study, cosmogenic 10Be 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 10Be concentrations. As a consequence, accurate exposure dating of sediment-covered terraces and beach ridges is difficult, because the pre-depositional 10Be concentration may exceed the post-depositional 10Be concentration from which exposure ages are calculated. This difficulty is illustrated by a rather inaccurate 10Be exposure age of 2.3±1.4ka (i.e. an error of 60%) for a terrace that is located 67m 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 10Be depth profiles is not feasible, because the pre-depositional 10Be component in these landforms varies with depth, which violates a basic assumption of this approach. 10Be 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.