Using Bed Conditions of the Seward/Malaspina Glacier System to Investigate Erosional and Tectonic Interplays in the St. Elias Mountains, Alaska

The St. Elias range in southeastern Alaska is a unique area with ideal characteristics for examining interactions between glacial and tectonic processes. Extensive observational data sets, both glaciological and tectonic, exist for the Seward/Malaspina Glacier system. Geodetic, structural, and thermochronological data suggest that considerable crustal strain is focused in this region, especially around the Seward throat directly upglacier from the Malaspina piedmont lobe. The massive discharge of ice funneling through this narrow valley and its high velocities suggest that this area is eroding exceptionally rapidly. Surface velocities of Seward Glacier through this constricted channel are available from high-resolution InSAR data, supplemented by field measurements. In contrast, the position of the glacier bed, its properties, including effective basal roughness, and the sliding velocity, remain poorly defined, yet they are central to understanding erosion patterns. To characterize the glacier bed and to constrain the sliding velocity, we use a numerical model and available data on the glacier, including surface velocities, radar-based depth measurements, and SRTM DEM elevation data. We first use the surface-velocity azimuth data to define a flowband, within which we calculate the flow field using a full-stress finite-volume ice-deformation model with prescribed basal conditions that represent sliding. From an initial profile determined from the sparse thickness data and conservation of mass principles, the bed profile is varied to minimize the deviations of the modeled vertical velocities, to ensure the stability of the known ice surface. Once near steady-state velocities are obtained, both the bed and the sliding properties are varied to define basal conditions as precisely as possible to provide a basis for calculating erosion rates, in Seward Glacier. The intersections of the glacier and active thrust features, which are difficult to determine in the field due to the extensive snow and ice coverage, are of particular interest. Rapid uplift along active thrusts may be required to maintain transverse ridges underneath the glacier; these zones where the ice has to funnel through a smaller cross-sectional area should be loci of high sliding velocities and thus exceptionally rapid erosion. The basal conditions, when used in conjunction with the known and inferred tectonic patterns of the region, promise to contribute to a better understanding of both glacial erosion and regional tectonics.