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

A transverse profile of velocity was measured across Ice Stream B, Wes t Antarctica, in order to determine the role of the margins in the for ce 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 s low-moving ice sheet. The velocity profile exhibits a high degree of s hear deformation within a marginal zone, where intense, chaotic crevas sing occurs. Detailed analysis of the profile, using analytical and nu merical models of ice flow, leads to the following conclusions regardi ng the roles of the bed and the margins in ice-stream dynamics: (i) Th e overall resistive drag on the ice stream is partitioned nearly equal ly between the margins and the bed and, thus, both are important in th e force balance of the ice stream. (ii) The ice within the chaotic zon e must be about 10 times softer than the ice in the central part of th e 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, def ormable 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 th at strain heating in the margins may be significant. While the exact q uantitative 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.