Dynamics of the shear margin of Ice Steam B, West Antarctica

• Main Author: Jackson, Miriam
• Format: Thesis
• Language: English
• Published: California Institute of Technology 1999
• Subjects: ice; Antarctica; glaciology; ice streams; Geophysics; FOS Earth and related environmental sciences; West Antarctica; Antarc*; Ice Sheet
• Online Access: https://dx.doi.org/10.7907/8254-rc11; https://resolver.caltech.edu/CaltechETD:etd-08032006-130753

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. 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). The ice in this zone is highly crevassed and chaotically jumbled. 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 test specimen orientation relative to the stress axes in the tests is chosen on the basis of c-axis fabrics so that horizontal shear across vertical planes parallel to the margin is applied to the ice specimens in the test. 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 comes 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. The marginal shear stress value is twice that given by the ice-stream model of Echelmeyer et al. (1994), and the corresponding strain-rate enhancement factors differ greatly (E [...] 1 - 2 from the creep tests vs. E [...] 10 - 12.5 from the model of Echelmeyer et al. (1994)). This large discrepancy may possibly be explained by recrystallization of the ice during or shortly after coring. Estimates of the expected recrystallization time scale range widely but include the ~ 1-hour time scale of coring and leave the likelihood of recrystallization uncertain. However, the observed two-maximum fabric type is not what is expected for annealing recrystallization from the sharp single-maximum fabric that would be expected in situ at the high shear strains involved ([gamma] ~ 20). Experimental data from Wilson (1982) suggest that if the core did recrystallize, the prior fabric was a two-maximum fabric not substantially different from the observed one, which implies that the measured flow law and derived marginal shear stress are applicable to the in-situ situation. An ice-stream flow model was developed to explore the discrepancy in enhancement factors. Using this model, which is similar to the model of Echelmeyer et al. (1994), 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. 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. It was thus not possible to reconcile fully the results from the experimental work and from modelling, since the modelling still suggests that there is substantial flow enhancement in the MSZ. There may be variation of the enhancement factor with depth, so that at 300 m depth the enhancement factor is close to 1, but increases at greater depths. 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 stream the fabrics show a single, very diffuse maximum.