UBC Theses and Dissertations
Relation between secondary structures in Athabasca Glacier and laboratory deformed ice Stanley, Alan David
Glacier movement produces numerous secondary structures including layers formed by different types of ice and the preferred crystallographic orientation of constituent grains. This thesis describes structures on Athabasca Glacier and shows how they are related to systems of stress that produce glacier flow. The surface of Athabasca Glacier can be divided into an area of prominent layers of coarse ice near the glacier margin and another formed by less distinct thick layers of fine ice in the central quarter of the ice tongue to within 800 m of the terminus. The coarse layers trend subparallel with the glacier walls and dip steeply towards the centre. In contrast, layers of fine ice near the glacier centre are near vertical and trend parallel with the direction of flow. The layers are deformed about a transverse vertical plane into a series of "similar" folds with limbs commonly separated by narrow cracks subparallel with the axial plane. Because the coarse layers near the margins, and the fine layers near the centre do not change in shape, size or attitude down the length of the glacier they must be formed at or near their present position. Cv measurements of ice grains at 25 locations on the ablation surface give fabric diagrams that represent real stress fabrics that have two or more areas of concentration containing up to 7% of the data. The diagrams may be separated into two distinct groups according to their location on the ice surface. Fabric diagrams from coarse layers near the margins have two or more maxima clustered near the pole to the layering. Diagrams from contorted fine layers near the middle of the glacier have most data concentrated in the north east quadrant, but maxima are independent of the attitude of any ice layers. In most diagrams, maxima fall on the locus of a small circle of constant radius. The observed radius lies between 30° and 50°, and the centre, located in approximately the same position in all diagrams, represents a line subparallel with the direction of glacier flow. The two types of ice and their distinct fabric indicate that two different stress systems exist in a glacier. Ice near the margin is under shear while that near the centre is under compression. In laboratory experiments, increase in the rate of creep may be attributed to some process of recrystallization. Test specimens that have recrystallized under compression are composed of small grains with Cv axes that tend to be oriented in a small circle about the unique stress axis. Fabrics of compressed ice are identical to those obtained from ice near the centre of many glaciers and show that if ice deforms most readily by glide within the basal plane, the final orientation fabric depends upon the local plane of movement and not the plane of maximum resolved shear stress.
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