Determining conditions that allow a shear margin to coincide with a Röthlisberger channel

Abstract

The mass loss from the West Antarctic Ice Sheet is dominated by numerous rapidly flowing ice streams, which are separated from stagnant ice in the adjacent ridges by zones of concentrated deformation known as shear margins. Because the discharge from a single ice stream depends sensitively on the ice stream width, determining the physical processes that control shear margin location is crucial to a full understanding of ice stream dynamics. Previous work has shown that the transition from a deforming to an undeforming bed within a shear margin concentrates large stresses on the undeforming bed beneath the ridge. In this paper we investigate how the presence of a drainage channel collocated with the transition from a deforming to an undeforming bed perturbs the stress field within the shear margin. We show that the channel limits the maximum shear stress on the undeforming bed and alters the yield strength of the till by changing the normal stress on the ice‐till interface. By comparing the maximum stress with the till strength, we show that the transition from a deforming to an undeforming bed can occur across a channel whenever the water flux in the channel exceeds a critical value. This critical flux is sensitive to the rheology and loading of the shear margin, but we conclude that there are some scenarios where the transition from a deforming to an undeforming bed can be collocated with a drainage channel, though this configuration is probably not typical.

Contrasting Hydrological Controls on Bed Properties During the Acceleration of Pine Island Glacier, West Antarctica

“…Moreover, with several margins (e.g., M5B, M5C, and M7B) showing high strain rates but no significant change in basal traction over time, it is unlikely that changes along margins M1A, M3A, M5A, and M7A are merely an artifact of simplified physics in our model. Given that the reported increase in basal drag often occurs along subglacial drainage pathways, we propose that the modeled basal strengthening might be a direct result of hydrological processes associated with efficient hydrology and high basal water fluxes, an interpretation which is consistent with previous work (Platt et al, ; Schroeder et al, ).…”

“…The shear margins that coalesce with basal melt water pathways are almost exclusively situated in the southern sector of PIG, where modeled basal melt rates are high, and the basal hydrological system is inferred to be composed of efficient channels (section ). It is therefore possible that these shear margins formed in unison with a regional hydrological system that now locks them in place, as proposed in recent theoretical work (Elsworth & Suckale, ; Meyer et al, ; Perol et al, ; Platt et al, ). It has been suggested that high deformation rates along shear margins could result in relatively thick layers of temperate ice, which would impede conduction of heat away from the bed and generate high basal melt rates (Beem et al, ; Jacobson & Raymond, ; Perol et al, ; Raymond et al, ; Schoof, ; Suckale et al, ).…”

“…With large volumes of water produced, efficient channels with low‐water pressure may develop and draw additional water from their surroundings (Rothlisberger, ; Weertman, ), thereby strengthening the bed in a manner that is consistent with this study. Theoretically, these channels are capable of reducing the pore pressure along the bed over a distance of several kilometers, thus providing a mechanism to increase the local basal drag and stabilize the position of the margin when ice‐flow acceleration increases the amount of shearing along the margin (Elsworth & Suckale, ; Meyer et al, ; Perol et al, ; Platt et al, ).…”

“…Analysis of airborne radio‐echo sounding data points to the possible past migration of some of these margins, including margins of T3 and the western margin of T7 (Karlsson et al, ). Without sufficient topographical confinement or thermal boundary conditions, the overall mechanisms responsible for such migration are still uncertain (Platt et al, ; Raymond et al, ; Schoof, ), although one recent study (Kyrke‐Smith et al, ) suggested that the width and spacing of ice streams may be defined with a natural length‐scale resulting from the distribution of meltwater and effective pressure at the bed.…”

The role of subtemperate slip in thermally driven ice stream margin migration

“…Both of these processes would lead to subtemperate sliding, that is, sliding at temperatures below the melting point. Additionally, the high stress concentrations may be alleviated by mechanical failure or damage production in the ice itself (Pralong and Funk, 2005).…”