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.