Response of glacial landscapes to spatial variations in rock uplift rate

Abstract

The response of glaciated landscapes to rapid rock uplift, driven by tectonic convergence, is an important, often neglected, aspect of proposed interactions between plate tectonic processes and climate change. Rivers typically respond to more rapid rock uplift in part through increasing channel gradients. In contrast, the “glacial buzzsaw” hypothesis suggests that glaciers can erode as quickly as the fastest rock uplift rates (6–10 mm/yr) without any increase in mean elevations. However, it has not been established how this is achieved. We examined moving window maps, swath and longitudinal profiles, hillslope relief, and hypsometry for glacierized and formerly glacierized basins in areas of spatially variable rock uplift rate in the Southern Alps, New Zealand, and around Nanga Parbat, Pakistan, to determine whether glaciers have a specific response to rapid rock uplift. The response of these glaciated landscapes to rapid rock uplift (6–10 mm/yr) comprises (1) modest steepening of the longitudinal profiles in smaller glaciated basins, (2) maintenance of shallow downvalley slopes in larger glaciated basins (>∼30 km2, Southern Alps; >∼100 km2, Nanga Parbat), (3) development of tall headwalls, and (4) steepening of the basin as a whole, dominated by hillslope lengthening. Around Nanga Parbat, headwalls several kilometers high constitute >50% of the basin relief. At rapid rock uplift rates, although glaciers can incise the valley floor swiftly, they cannot prevent headwalls from reaching exceptional heights. The associated increase in mean distance between cirque heads (i.e., a decrease in drainage density) causes regional mean elevation to rise with increasing rock uplift rate. However, this is much less than the changes in elevation expected in unglaciated ranges.