GEOLOGIC AND TOPOGRAPHIC CONTROLS ON FAST-FLOW IN THE LAURENTIDE AND CORDILLERAN ICE SHEETS

Ice streams are fast flowing arteries which play a vital role in the d ynamics and mass balance of present-day ice sheets. Although not fully understood, fast flow dynamics are intimately coupled with geologic, topographic, thermal, and hydrologic conditions of the underlying bed. These are difficult observables beneath contemporary ice sheets, hind ering elucidation of the processes which govern ice stream behavior. F or past ice sheets the problem is antithetic. Geologic evidence of for mer ice streams exists, but spatial and temporal histories are uncerta in; however, detailed knowledge of bed geology and topography is avail able in many places. We take advantage of this information to compile terrain characteristics relevant to fast flow dynamics in the Laurenti de and Cordilleran Ice Sheets. Using seed points where fast flowing Wi sconsinan ice has been geologically inferred, discriminant analysis of a suite of North American geologic and topographic properties yields a concise measure of ice-bed coupling strength. Our analysis suggests that the interior plains and continental shelf regions of North Americ a have low basal coupling relative to areas of variable relief or expo sed bedrock in the Cordillera and on the Canadian Shield. We conclude that the interior plains and continental shelves are both topographica lly and geologically predisposed to large-scale basal flows (i.e., ice streams or surge lobes). This result holds independent of whether the mechanism of fast flow is sediment deformation or decoupled sliding o ver the bed.