The onset area of Ice Stream D, West Antarctica

Surface flow in a 10 000 km(2) expanse of the onset area of Ice Stream D, W est Antarctica, was measured by repeat, precise global positioning system s urveys over a 1 year interval. The pattern of velocity and strain rate show s the development of Ice Stream D, the major flow into which originates sou th of Byrd station and follows the course of a deep bed channel. Plotting o f the driving stress vs the ratio of velocity and ice thickness identifies the onset of streaming flow (roughly 140 km downstream of Byrd station) as a transition between deformation flow and sliding now Along the kinematic c enter line of the developing ice stream, the ice rheology is linear at stre sses below 0.6 bar, and appears temperate at the base well before the onset of streaming is reached. The onset corresponds to a maximum driving stress of 0.8 bar. It occurs downstream of a slight increase in longitudinal stra in rate where stronger along-flow lineations are apparent in Landsat imager y and after the ice has passed the center of an overdeepening in the bed ch annel. No current deviation from equilibrium is detected in this region, bu t a set of flow stripes misaligned with present flow indicates significant changes in flow have occurred in the past.