Measurements of Fast ice Flow of the Malaspina Glacier to Explore Connections Between Glacial Erosion and Crustal Deformation in the St. Elias Mountains, Alaska

In the St. Elias range of southern Alaska, dynamic tectonics and massive, fast-moving temperate glaciers provide an ideal opportunity to explore linkages between concentrated glacial erosion and localized deformation. With the potential for rapid erosion, the Seward Throat funnels a large volume of ice through the St. Elias Mountains, slicing through inactive and active structures alike and spreading out to form the Malaspina piedmont glacier. While glacial erosion processes are complex, erosion rates tend to scale with the sliding velocity, making it a useful indicator of erosive potential. In simple glacial valleys, conservation of mass dictates that reaches with high ice speeds tend to be relatively shallow; interestingly, these areas where the glacier bed is relatively high are precisely where the erosion is expected to be fastest. Hence, bedrock uplift must be concentrated for the elevated parts of the glacier bed to maintain their positions. Systematic studies of glacier speed and ice thickness hold considerable promise for elucidating patterns of active tectonics. We investigate the variation in velocity, and thus the probable variation in erosion rate, along a portion of the glacier's length. In late July 2007, 10 targets were placed on the lower Seward Throat and surveyed for 8 days. 7 targets were placed along 6 km of a flow line to define the velocity variation through steep and gentle sections of the glacier. Preliminary determination of surface velocities reveal large longitudinal strain rates with velocities varying from ~1400 m/year to ~1800 m/year within 1500 m with surface slopes ranging from ~1° to 3°. Previously calculated balance velocities, which suggest that most of the motion results from sliding, compare encouragingly. Assuming a 1 bar basal shear stress to estimate the ice thickness, basal sliding comprises between 83% and 95% of the surface velocity. To extend the spatial coverage of our measurements, our survey results will be examined in the context of detailed, unpublished velocity measurements collected by the USGS (Robert Krimmel and Austin Post) in the 1970s, as well as surface velocities derived from Radarsat-1 interferometric synthetic aperture radar data collected from 2000 to the present. The InSAR velocities were derived using a speckle tracking technique over 24- day periods, describing the interannual as well as seasonal variability in speed. The velocities from different time periods are influenced by climate and possible surge behavior; hence we expect temporal variations in the absolute values of the velocities but not in the spatial pattern. Analysis of the surface strain rates will enable us to improve calculations of basal velocities by relaxing assumptions such as uniform basal shear stress and negligible sidewall drag. An enhanced understanding of the glacier's dynamics, combined with information about ice depth from upcoming air-borne ice penetrating radar measurements, will enable us to better investigate areas with high erosive potential in the context of the active deformation and seismic activity in the region studied by the St. Elias Erosion/Tectonics Project.