Shear margins in glaciers and ice sheets
Abstract Analytical and numerical techniques are used to examine the flow response of a sloped slab of power-law fluid (power n ) subjected to basal boundary conditions that vary spatially across the flow direction, as for example near an ice-stream margin with planar basal topography. The primary a...
| Published in: | Journal of Glaciology |
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| Main Author: | |
| Format: | Article in Journal/Newspaper |
| Language: | English |
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Cambridge University Press (CUP)
1996
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| Subjects: | |
| Online Access: | http://dx.doi.org/10.1017/s0022143000030550 https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0022143000030550 |
| Summary: | Abstract Analytical and numerical techniques are used to examine the flow response of a sloped slab of power-law fluid (power n ) subjected to basal boundary conditions that vary spatially across the flow direction, as for example near an ice-stream margin with planar basal topography. The primary assumption is that basal shear stress is proportional to the basal speed times a spatially variable slip resistance. The ratio of mean basal speed to the speed originating from shearing through the thickness. denoted as r , gives a measure of how slippery the bed is. The principal conclusion is that a localized disturbance in slip resistance affects the basal stress and speed in a zone spread over a greater width of the flow. In units of ice thickness H , the spatial scale of spreading is proportional to a single dimensionless number R n ≡ ( r / n + 1) 1/ n +1 derived from n and r . The consequence for a shear zone above a sharp jump in slip resistance is that the shearing is spread out over a boundary layer with a width proportional to R n . For an ice stream caused by a band of low slip resistance with a half-width of w H , the margins influence velocity and stress in the central part of the band depending on R n in comparison to w . Three regimes can be identified, which for n = 3 are quantified as follows: low r defined as R 3 < 0.1 w , for which the central flow is essentially unaffected by the margins and the driving stress is supported entire by by basal drag; high r defined as R 3 > 1 w , for which the boundary layers from both sides bridge across the full flow width and the driving stress in the center is supported almost entirely by side drag; intermediate r , for which the driving stress in the center is supported by a combination of basal and side drag. Shear zones that are narrower than predicted on the basis of this theory (≈ R 3 ) would require localized softening of the ice to explain the concentration of deformation at a shorter scale. |
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