Cross-divide topographic contrasts created by asymmetrical glaciation: A case study from the northeastern Qilian Shan

Drainage divide migration has drawn growing attention in recent years because it can induce changes in drainage areas and confound inference of spatial or temporal changes in tectonic or climatic forcing from river profiles. Recent studies have used different metrics of divide stability, such as cross-divide contrasts in topography, to quantify a divide’s susceptibility to migration. These metrics are based on expectations of cross-divide differences in fluvial or hillslope erosion rates, yet glacial erosion may be the primary driver of topographic evolution and drainage reorganization in many mid-latitude mountain ranges. Here we report a case study in the northeastern Qilian Shan, a northwest-southeast-trending mountain belt on the northeast margin of the Tibetan Plateau. The northeast-facing range front in Qilian Shan today receives less solar insolation but more summer monsoonal precipitation than the southwest-facing front and thus hosts more small, high elevation valley glaciers. We quantify cross-divide contrasts in topography using different metrics and find stronger glacial modification of topography on northeast-facing slopes than on southwest-facing slopes. The northeast-facing range front displays oversteepened U-shaped valleys and evidence of extensive Quaternary glaciation, whereas the southwest-facing front is incised by V-shaped valleys that hosted only small Quaternary glaciers. Near the drainage divide, valleys on the northeast-facing front have steeper headwalls and higher headwall relief than valleys on the southwest-facing front. Based on these observations, we proposed a conceptual model of divide migration in the northeastern Qilian Shan: during the last glacial period, strong glacial modification on the northeast-facing range front caused headward expansion of valleys and drove southwestward divide migration. Since the onset of the present interglacial period, faster postglacial hillslope processes in northeast-facing valleys have sustained this southwestward divide migration. We develop a numerical model to test this conceptual model and discuss the impact of Quaternary glaciation on drainage reorganization in the Qilian Shan. We suggest that Quaternary glaciation and following postglacial adjustment have important impacts on divide migration and drainage reorganization in mid-latitude mountain ranges.