THE INFLUENCE OF FRACTURES ON THE EVOLUTION OF GLACIER BED TOPOGRAPHY, TETON RANGE, USA

Current models reinforce the importance of subglacial bedrock topography at scales of centimeters to meters for processes of glacial sliding and erosion. Bed topography develops by erosion that exploits existing and newly-forming fractures in the subglacial bedrock. We tested the hypothesis that the spacing and orientation of preexisting fractures in the bedrock should dictate the dominant size and orientation of topographic features on glaciated bedrock floors of two valleys in the Teton Range. We measured fracture spacing and orientation and collected sub-cm topographic data by SfM photogrammetry at fifteen field sites in Alaska Basin (quartz monzonite and gneiss) and Darby Canyon (dolomite). Spectral analysis of the digital topography shows that within the quartz monzonite, significant roughness occurs at wavelengths similar to the spacing of open fractures in the bedrock, but is aligned either parallel or perpendicular to ice flow. Within the gneiss, roughness elements are instead oriented perpendicular to the strike of foliation and fractures. On the dolomite, the valley floor formed along downvalley-dipping bedding planes and the roughness is dominated by solution features. These results suggest that in massive crystalline rocks, fracture spacing influences the spatial scale of topographic roughness, but the direction of ice flow controls its orientation; meanwhile bed topography on rocks with a stronger fabric is dominated by that fabric. This could represent different modes of glacial erosion, or that the gneiss and dolomite are more susceptible to postglacial weathering and erosion, which has reasserted the influence of the fabric through the glacial overprint.