C33A-0359:
Stabilizing Mechanism of Siple Coast Ice Streams Margins from a Thermomechanically Derived Triple-Valued Lateral Shear Stress Law
Wednesday, 17 December 2014
Thibaut Perol, Harvard University, Cambridge, MA, United States and James R Rice, Harvard Univ, Cambridge, MA, United States
Abstract:
Modeling efforts supported by the few field observations available show that intense straining at the margins of Siple Coast ice streams (SCIS) can heat, and even partially melt, the ice adjacent to the bed. This enhances deformation in the margins and lowers the lateral resistance to ice stream flow. Combining a steady state 1D thermal model for a uniformly sheared column of ice with a temperature dependent ice rheology we solve for the average lateral shear stress in the column, showing that this stress has a triple-valued dependence on lateral strain rate. Using measured strain rates from Joughin et al. [2002] we show that for nearly all active SCIS margins the inferred shear stress sits in the low-strength valley of the triple-valued curve. This means that the margins support a lower lateral stress than the adjacent ice of the more rapidly moving stream, and thus the basal resistance must be enhanced near the margin. Such an enhancement could come from a channelized drainage system that develops to accommodate the melt generated within the temperate margin. To study this we develop a simple model for the subglacial hydrology in which water flows laterally in a thin film between the ice and till towards a channel that operates at a lower water pressure [Rothlisberger, 1972]. The resulting pore pressure profile is then used to calculate the basal resistance near the margin assuming a Coulomb plastic rheology for the till. We predict strengthening within 2 km of the margin. Finally we use a path-independent integral (Rice [1968]) to quantify how this additional basal resistance lowers the stress concentrated on the locked portion of the bed adjacent to the stream. We find that for realistic values of the film thickness the additional basal resistance can strongly reduce the stress concentration, allowing for a smooth transition from a deforming to an undeforming bed at the margin.
