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

摘要

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 Be-10 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 10-70mm/ka. Owing to the slow erosion of the landscape, the sediments delivered to Tangra Yum Co have high Be-10 concentrations. As a consequence, accurate exposure dating of sediment-covered terraces and beach ridges is difficult, because the pre-depositional Be-10 concentration may exceed the post-depositional Be-10 concentration from which exposure ages are calculated. This difficulty is illustrated by a rather inaccurate Be-10 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 Be-10 depth profiles is not feasible, because the pre-depositional Be-10 component in these landforms varies with depth, which violates a basic assumption of this approach. Be-10 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. Copyright (c) 2015 John Wiley & Sons, Ltd.