Shear heating and weakening of the margins of West Antarctic ice streams

Ice streams are fast flowing bands of ice separated from stagnant ridges by shear margins. The mechanisms controlling the location of the margins remain unclear. We use published ice deformation data and a simple one-dimensional thermal model to show that West Antarctic ice stream margins have temperate ice over a substantial fraction of their thickness, a condition that may control their width. The model predicts a triple-valued relation between the thickness-averaged lateral shear stress and the lateral shear strain rate. Observed strain rates at the margins imply that they support slightly less lateral shear stress than adjacent ice within the stream. This requires enhanced basal resistance near the margin. We suggest, in agreement with the limited observations, the presence of a channelized drainage system at the margin that reduces the pore fluid pressure at the ice-till interface, thus increasing the shear stress acting on the yielding Coulomb plastic bed.