Dynamics of the shear margin of ice stream B, West Antarctica

The ice streams in the West Antarctica Ice Sheet flow at several hundred metres per year. The lateral increase in speed from typical inland ice sheet speeds of a few metres per year to ice stream speeds of several hundred metres per year occurs over a short distance ( ∼ 2 km) in the outer part of the ice stream known as the marginal shear zone (MSZ). This thesis is an effort to understand the dynamics of the MSZ and to find out whether the velocity of the ice stream is controlled primarily by the stresses in its MSZs or by stresses at the base. This is done by determining the marginal shear stress in one of the marginal shear zones using the ice itself as a stress meter. The observed marginal shear strain rate of 0.14 a-1 is used to calculate the marginal shear stress from the flow law of ice determined by creep tests on ice cores from a MSZ. The resulting marginal shear stress is (2.2 +/- 0.3) x 10 5 Pa. This implies that 63 to 100% of the ice stream's support against gravitational loading come from the margins and only 37 to 0% from the base, so that the margins play an important role in controlling the ice stream motion.; An ice-stream flow model was developed. Using this model it is possible to match the observed surface velocity profile across the ice stream using a strain rate enhancement factor of 5. This is more than four times the value found in the experimental work but half the value from the modelling results of Echelmeyer et al (1994). The flow model suggests that the lateral shear stress integrated over the margins is larger than the basal shear stress integrated over the base, so that the ice stream is controlled at the sides rather than at the base.; c-axis measurements in ice from the middle of the MSZ reveal that there is an asymmetrical two-maxima fabric, as expected for ice under simple shear. 600 m away, between the middle of the MSZ margin and its outer edge, there is still a two-maxima fabric but the secondary maximum is much smaller and the primary maximum is much bigger. 500 m further, right at the boundary between the shear margin and the ice stream, there is only a single maximum. Outside the ice strewn the fabrics show a single, very diffuse maximum.