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The marginal shear stress of Ice Stream B, West Antarctica

Jackson, Miriam and Kamb, Barclay (1997) The marginal shear stress of Ice Stream B, West Antarctica. Journal of Glaciology, 43 (145). pp. 415-426. ISSN 0022-1430.

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To ascertain whether the velocity of Ice Stream B, West Antarctica, may be controlled by the stress in its marginal shear zones (the "Snake" and the "Dragon"), we undertook a determination of the marginal shear stress in the Dragon near Camp Up B by using 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 depth of 300 m in the Dragon, obtained by using a hot-water ice-coring drill. The test-specimen orientation relative to the stress axes in the tests is chosen on the basis of c-axis fabrics so that the test applies horizontal shear across vertical planes parallel to the margin. The resulting marginal shear stress is (2.2 ± 0.3) × 10^5 Pa. This implies that 63-100% of the ice stream's support against gravitational loading comes from the margins and only 37-0% 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 and others (1994} and the corresponding strain-rate enhancement factors differ greatly (E ≈ 1-2 vs 10-12.5). This large discrepancy could be explained by recrystallization of the ice during or shortly after coring. Estimates of the expected recrystallization time-scale bracket the ~1 h 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 (γ~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. These issues need to be resolved by further work to obtain a more definitive observational assessment of the marginal shear stress.

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Additional Information:© 1997 International Glaciological Society. MS received 1 October 1996 and accepted in revised form 12 March 1997. We thank H. Engelhardt for carrying out the core drilling that furnished the core studied here, and also for providing the temperature profile in Figure 8b and for help with preparation of the test apparatus and instrumentation. He was ably assisted in the difficult job of drilling in the highly crevassed Dragon by K. Echelmeyer, W. Harrison and several field assistants (C. Larsen, K. Petersen, K.-N. Swenson, S. Schmidt, E. Miya and others). We thank A. Gow, K. Echelmeyer, W. Harrison and an anonymous reviewer for helpful comments. The work was made possible by grants OPP-9018703 and OPP-9319018 from the U.S. National Science Foundation.
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Issue or Number:145
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ID Code:47277
Deposited On:17 Jul 2014 18:37
Last Modified:03 Oct 2019 06:52

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