IMPACTS OF EFFECTIVE PRESSURE OSCILLATIONS ON TILL DEFORMATION

Cyclic effective pressure oscillations are common in glaciers and ice streams, often through tidally modulated loading of the grounding zone or diurnal meltwater flux towards the bed. When exposed to effective pressure oscillations, soft bedded glaciers and ice streams are hypothesized to produce deeper till deformation than under statically loaded conditions. The changes in deformation have impacts on glacial slip by altering factors such as basal drag, till consolidation and sediment transport. Examining these effects directly in the field is challenging as access to the ice-till interface is limited, thus our understanding of these processes is also limited. To alleviate these difficulties, we use a large diameter cryogenic ring shear device to experimentally simulate the basal environment by rotating a ring of temperate ice over a till bed while continually modulating the effective pressure. The ice ring was spun at a constant velocity of 100 m/yr and effective pressure was modulated on a 24 hour period with amplitudes of ~10, 25, and 40 kPa around a mean value of 45 kPa. The kinematic history of till deformation is recorded through strain markers such as magnetite grains that become oriented from strain. Within the experiment we analyzed strain kinematics in the till using anisotropy of magnetic susceptibility (AMS) alongside buried strain beads. AMS allows us to determine the magnitude, direction, and clustering of the shear markers within the till, and determine how shear evolves with depth. These methods allow us to quantitatively analyze strain kinematics under well controlled conditions. The oriented magnetite grains are sampled through the collection of sediment at multiple depths, which are then analyzed using AMS to ascertain any fabric within the till. Results indicate that maximum shear strain is localized near the ice-till interface, and decreases exponentially with depth, however the depth of shear is greater under cyclically loaded conditions than under comparable statically loaded conditions.