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

• Main Author: Jackson, Miriam
• Format: Thesis
• Language: English
• Published: 1999
• Subjects: West Antarctica; Antarc*; Antarctica; Ice Sheet
• Online Access: https://thesis.library.caltech.edu/3002/; https://thesis.library.caltech.edu/3002/1/Jackson_m_1999.pdf; 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 ...