The impacts of base level and lithology on fluvial geomorphic evolution at the tectonically active Laohu and Hasi Mountains,northeastern Tibetan Plateau

摘要

Previous researches had emphasized tectonic impacts on the fluvial system at the tectonically active areas,while the effects of lithology and local base level change have received relatively rare attention.Here we investigated fluvial landforms at different spatial scales,focusing on knickpoints and channel network reorganization from an area affected by the Haiyuan Fault in the northeastern Tibetan Plateau.The geomorphic indices,i.e.,drainage pattern andχanomalies,were calculated and investigated.The results show that two regional radial drainages formed around the Laohu and Hasi Mountains.Within the interior of the radial drainage,tributaries from the southeast side of the Laohu Mountain experienced near 180°direction change.We interpret this as the gradual drainage capture originating from the height difference(~190 m)of the local base level between the two catchments.Some tributaries from the Hasi Mountain show alternating gorges and broad valleys controlled by lithology.Besides,tectonic uplift and the lowering of base level(from the incision of the Yellow River)triggered an autogenic positivefeedback transition from parallel to dendritic drainage patterns.These observations suggest that base level change and lithology play a crucial role in landscape evolution,even in a tectonically active region.