Field measurements of surface deformation across the northern shear margin of Whillans Ice Stream, West Antarctica, were analyzed to better understand the processes controlling the position and migration of this margin. Four lines of poles extending ∼6 km from the interstream ridge into the shear margin were installed in November 1997 and resurveyed in January 2000, using GPS methods. A band of arcuate crevasses form where the surficial shear stress reaches ∼130 kPa, and a chaotic zone of crevassing occurs where this stress reaches a maximum of ∼270 kPa. Inspection of the RAMP mosaic indicates that the band of arcuate crevasses is separated from the chaotic zone by a narrow zone (∼250 m) with a relatively undisturbed surface morphology. A force-budget analysis suggests the transition from no basal sliding to full basal sliding must be restricted to a relatively narrow band, ∼1.5 km wide beneath the chaotic zone. On the ice stream, resistance from the bed is near zero and the driving stress is balanced by gradients in lateral shearing. Basal drag reaches a maximum close to the shear margin where basal conditions transition from well lubricated (beneath the ice stream) to no basal slip (beneath the ridge). The zone of elevated basal drag is ∼4 km wide. Within this zone, lateral drag is reduced and becomes increasingly restricted to upper layers. Localized meltwater production under the region of elevated basal drag is similar in magnitude to melting under the ice stream. The uniform bed morphology, inferred from radar sounding, excludes the presence of abundant basal water either in a water film at the glacier bed or in a thick saturated till layer. This suggests that most of the meltwater produced under the shear margin is diverted towards the ice stream, possibly playing a crucial role in maintaining lubricated conditions allowing streaming flow.
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