Preservation of a large-scale bedrock peneplain suggests long-term landscape stability in southern Tibet

Extensive areas with low relief constitute the dominant geomorphic element of the central and southern Tibetan Plateau and were previously interpreted to result from the progressive filling of internally drained basins with sediments derived from adjacent mountain ranges that have been largely eroded. Here we document a completely different low-relief landscape in the Lhasa block of southern Tibet (similar to 31 degrees N, similar to 90 degrees E), where a large-scale peneplain carved into Cretaceous granitoids is well preserved north of lake Nam. Around the town of Bangoin this bedrock peneplain occurs at a constant elevation of similar to 5,300 m in a region not affected by active faulting. The main planation surface has been incised by streams resulting in a local relief of up to 700 m. Progressive fluvial incision generated additional flat bedrock surfaces of limited extent that occur at lower elevations than the peneplain. Local hillslope gradients in the peneplain region increase with decreasing elevation, which is in marked contrast to the morphology of nearby mountain ranges that are uplifted in the footwall blocks of active normal faults. The subsiding hanging walls of these normal faults provide the necessary accomodation space for the sediment produced by erosion of the peneplain region. These relations demonstrate that the bedrock peneplain is the most stable geomorphic element present in southern Tibet.