USING RATE-AND-STATE FRICTION TO ESTIMATE SLIDING STABILITY OF GLACIERS

The discharge of ice into the ocean by rapid slip is the primary mechanism by which glaciers currently contribute to sea-level rise, but it is also one of the most poorly constrained aspects of glacial behavior. Paleo glaciers such as the Laurentide also likely accommodated a majority of their motion through basal slip, which resulted in a wide range of basal landform formation. In order to more accurately constrain modern and paleo glacier movement, an improved understanding of glacial slip is needed. Unfortunately, modern-day glacier beds are difficult to access. Due to this difficulty, we have conducted a series of laboratory shear experiments on basal till units of the Laurentide Ice Sheet to understand how till composition could control glacier slip.

We estimate the sliding resistance of till units as a function of sliding speed for comparison with varying till properties such as grain size distribution. Using the response in sliding resistance to an instantaneous increase in sliding velocities provides a dataset capable of estimating the sliding stability of glaciers that moved primarily by basal slip. This system of slip response (rate-and-state friction) is commonly used in fault mechanics but only recently has it been implemented to study glacier motion. We examine till deformation characteristics related to rate-and-state dependent friction in a modified bi-axial shearing apparatus. Tills with a range of grain size distributions, once deposited by southern lobes of the Laurentide Ice Sheet, were collected and placed in a 10x10x2 cm sample chamber under 100 kPa of effective stress and sheared in velocity-stepping tests to gather rate-and-state friction values. Results reveal both velocity weakening or strengthening properties, describing the sub-glacial till’s ability to sustain stable slip or unstable slip that could nucleate subglacial icequakes.