TESTING THE "GLACIAL BUZZSAW" HYPOTHESIS IN THE MOUNTAINS OF THE WESTERN UNITED STATES

It has recently been suggested that glaciers can exert strong control on the topographic development of tectonically active mountain ranges. In northern Pakistan, Brozovic et al. (1997) found that across climatically diverse areas of very different uplift rates and topographic characteristics, mean and modal surface altitudes corresponded closely to the altitude of late Pleistocene glacier equilibrium lines (ELAs). On that basis they postulated the existence of a “glacial buzzsaw” which limits surface uplift and relief in mountainous areas experiencing rock uplift. In this study we attempted to test this “buzzsaw” hypothesis by applying an approach similar to that of Brozovic et al. to the mountains of the western United States. Digital Elevation Models (DEMs) of the western United States were subsampled along three representative, 50 km wide, west-to-east swaths from the Pacific Ocean to the western Great Plains. From the DEMs, mean and maximum elevations across the swaths were determined and then compared to published and unpublished late Pleistocene ELA data.

Our results indicate that late Pleistocene ELAs are strongly correlated with surface altitudes across all ranges within the swaths, but more closely approximate maximum, rather than mean, altitudes. The central swath (across OR, ID, and WY) produced the most consistent correlation between maximum elevation and late Pleistocene ELA which is manifest in smooth eastward increases in both maximum altitudes in glaciated ranges and paleoELAs. The strong correlations found between altitudes of glaciated ranges in the Western United States and late Pleistocene ELAs in those ranges lends support to the “glacial buzzsaw” hypothesis that glaciation is an important limiting factor in the topographic development of mountain ranges. However, the fact that paleoELAs are most closely associated with maximum altitudes rather than mean altitudes suggests that the processes by which glaciers have influenced topographic development in the mountains of the western United States may be different from those operating in Pakistan.