The distribution of active rock uplift in the interior of the western Qilian Shan, NE Tibetan Plateau: Inference from bedrock channel profiles

摘要

The distribution of rock uplift rates in the interior of the western Qilian Shan is not yet fully resolved, which limits our knowledge about the uplift pattern and landscape evolution in the northeastern Tibetan Plateau. The stream power incision model suggests that the spatially differential rock uplift rate usually can exert a primary control on channel gradient, thus providing a powerful approach to map the rock uplift pattern of active orogens. Here we analysed the river longitudinal profiles in the western Qilian Shan and found a spatially differential pattern with higher steepness indices along the parallel ranges (e.g. the Danghe Nanshan, Daxue Shan, Tuolai Shan, and the Northern Qilian Shan) but lower values in the intramontane basins. Comparison of steepness indices to variations in lithology and annual precipitation revealed limited correlation of channel gradient with lithologic resistance and climate in this landscape. We also observed no systematic relationship between steepness indices with drainage area, sediment flux, or channel concavities. We argued that the systematic changes in channel steepness indices are tectonically controlled rather than the consequence of variable sediment flux, climate, or lithology. Thus, the normalized channel steepness pattern of the western Qilian Shan could reflect that the rock uplift is largely restricted to mountain ranges bounded by active thrust faults and imply distributed Cenozoic crustal shortening. By comparing the NNE-SSW-directed elevation swath profiles and the channel steepness pattern, we concluded that tectonic uplift rate should be higher in the Northern Qilian Shan than in the internal ranges. Thus, our study provides geomorphic evidence to favor both distributed crustal shortening and Asian mantle lithosphere underthrusting as the primary mechanisms to explain the landscape evolution in the NE Tibetan Plateau.