Mechanical and hydrologic properties of Whillans Ice Stream till: Implications for basal strength and stick-slip failure

Ice streams transport large volumes of inland ice to the ocean and play a key role in the mass balance of the Antarctic ice sheet. The rate and style of ice stream basal slip are governed, in part, by the underlying till whose physical properties are poorly constrained. To address this problem, we conducted a suite of laboratory measurements to document the permeability, stiffness, consolidation behavior, and compressional wave speeds of Whillans Ice Stream till samples. We investigated the effects of stepped and cyclic loading on the evolution of the till. Initial permeabilities were 3.8–4.9 × 10⁻¹⁷ m² (porosities 28.1–31.8%), which decreased to 2.0 × 10⁻¹⁹ m² (20.4%) at 10 MPa effective stress. P wave velocities span from 2.26 to 3 km/s over this effective stress range and are well described by an effective medium model. The laboratory measurements were used to parameterize a 1-D numerical model to predict the till's response to stress perturbations. Perturbations corresponding to tidal periods produce a drained and strengthened layer tens of centimeter thick. For perturbations over time scales of weeks to months, as expected for till motion over basement features, the drained zone is a few meters thick. This strong layer can become brittle upon unloading and may facilitate observed stick-slip motion. Extrapolation of our effective medium model suggests that low basal effective stresses, on the order of a few tens of kPa, are needed to produce seismic velocities observed in the field (V_p~1750 m/s; V_s~160 m/s) and provides an approach to quantify and monitor in situ conditions.