Subglacial hydrology as a control on ice stream shear margin locations

Abstract:

Ice streams are fast-flowing bands of ice separated from the nearly stagnant ice in the adjacent ridge by zones of highly localized deformation known as shear margins. However, it is presently unclear what mechanisms can control the location of shear margins, and possibly allow them to migrate. Within the shear margin, the transition from a freely slipping bed beneath the ice stream to a locked bed beneath the ridge concentrates stresses on the locked bed. We show that subglacial hydrological processes can select the shear margin location by strengthening the subglacial till within the margin, reducing the stress concentration associated with the transition from a slipping to a locked bed.

Our study uses a two-dimensional thermo-mechanical model in a cross-section perpendicular to the direction of flow. We show that the intense straining at the shear margins can generate large temperate regions within the deforming ice. Assuming that the melt generated in the temperate ice collects in a drainage channel at the base of the margin, we show that the channel locally decreases the pore pressure in the subglacial till. For a Coulomb-plastic rheology, this depressed pore pressure leads to a basal shear strength substantially higher than that inferred under the majority of the stream. Our results show that the additional basal resistance produced by the channel can offset the large stresses concentrated on the locked bed, allowing the drainage channel to select the margin location. Matching the model to surface velocity data near Dragon Margin within Whillans ice stream B2, we show that a stable shear margin occurs when the slipping-to-locked bed transition is less than 500 m away from a channel operating at an effective pressure of 200 kPa if the basal hydraulic transmissivity is equivalent to that of a water-film ~0.2 mm thick. Extending these results we explore how the shear margin location varies with the effective pressure of the channel and hydraulic properties of the bed.