Progressive Deformation and Development of Fabric Across Zones of Shear in Glacial Ice
Discrete, ductile zones of differential shear are common in rocks, also occur in glacial ice, and are a response of crystalline materials to deformation at high temperatures. Such zones imply strain softening under shear deformation. They provide us with excellent opportunities for examining the nature of changes in fabric and texture with changes in state of strain.
Superimposed ice at the margin of the Barnes Ice Cap, Baffin Island, Canada, has been overridden and deformed by the main glacier. Deformation within the overridden ice is of variable intensity, and is characterized by zones of shear which are a few centimeters thick and which lie almost parallel to the bed rock and to the measured direction of maximum shear strain rate. The geometry of the shear zones, as delineated by air bubbles in the ice, indicates that deformation is very nearly one of simple shear. Changes from margin to center across a shear zone include an increase in grain size, the development of a weakly sutured texture, an increase in the aspect ratio of air bubbles, a decrease in the acute angle between the long axes of the bubbles and the shear zone boundaries, and an increase in the degree of preferred orientation of c axes. In a selected example, the fabric changes from a fairly uniform distribution of c axes outside the shear zone, through a double maximum fabric (not always well defined) of the kind observed in experiments, to an intense deformation and that it is related to a state of finite strain rather than to the stress or strain rate.
Quantitative measures of degree and direction of preferred orientations of c axes can be found using the Bingham probability distribution. It may thus be shown that a strong correlation exists between bubble orientation and the degree of preferred orientation of c axes, expressed as a concentration statistic. Bubble shape and inclination is related to the amount of shear strain undergone by the ice, and allows estimates of total shear strain across the shear zones to be made. There is considerable uncertainty involved in this procedure, but it appears that the maximum shear strain in the center of the selected shear zone (γ in the range 5 to 8) is about an order of magnitude greater than the maximum value of γ attained in previous experimental work. Considerable strains may be required to attain steady state conditions; such conditions may not obtain even in the centers of the shear zones. The results of this study provide some reconciliation between previous work on experimentally deformed ice at low strains and on glacial ice that has undergone extremely high strains.
KeywordsShear Strain Shear Zone Simple Shear Bubble Size Geodynamic Process
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