VALLEY INCISION AND THE UPLIFT OF MOUNTAIN PEAKS

Simple end-member models describe the effect of isostatically compensated valley incision on the elevation of mountain peaks. These models allow calculation of the maximum portion of mountain peak elevations attributable to the development of observed relief and predict general downstream patterns of ridgetop uplift in response to erosion by fluvial processes. Analyses of topographic profiles across major drainages of the central Sierra Nevada and the Tibetan Plateau indicate that isostatically compensated valley incision could account for at most 5-10% of the present elevation of mountain peaks. In contrast, analyses of topographic profiles across the Himalaya show that as much as 20-30% of the present elevation of Himalayan peaks could be explained by isostatically compensated valley incision. The degree to which valley incision is compensated locally, however, depends on the flexural rigidity of the lithosphere, the size of the range, and its tectonic boundary conditions. These results show that while a tectonic control is required for late Cenozoic surface uplift of the Sierra Nevada, a significant portion of the elevation of Himalayan peaks may reflect late Cenozoic valley incision on the margin of the Tibetan Plateau.