THE ROLE OF THE MARGINS IN THE DYNAMICS OF AN ACTIVE ICE STREAM

A transverse profile of velocity was measured across Ice Stream B, West Antarctica, in order to determine the role of the margins in the force balance of an active ice stream. The profile extended from near the ice-stream center line, through a marginal shear zone and on to the slow-moving ice sheet. The velocity profile exhibits a high degree of shear deformation within a marginal zone, where intense, chaotic crevassing occurs. Detailed analysis of the profile, using analytical and numerical models of ice flow, leads to the following conclusions regarding the roles of the bed and the margins in ice-stream dynamics: (i) The overall resistive drag on the ice stream is partitioned nearly equally between the margins and the bed and, thus, both are important in the force balance of the ice stream. (ii) The ice within the chaotic zone must be about 10 times softer than the ice in the central part of the ice stream. (iii) The average basal shear stress is 0.06 x 10(5) Pa. This implies that the entire bed cannot be blanketed by the weak, deformable till observed by Engelhardt and others (1990) near the center of the ice stream-there must be regions of increased basal drag. (iv) High strain rates and shear stresses in the marginal zones indicate that strain heating in the margins may be significant. While the exact quantitative values leading to these conclusions are somewhat model and location-dependent, the overall conclusions are robust. As such, they are likely to have importance for ice-stream dynamics in general.