Cumulative deformation in the barnes ice cap and implications for the development of foliation

Abstract

In recent years, it has become apparent that the development of foliation and crystallographic fabric in glacier ice is strongly dependent upon cumulative deformation. However, because glaciers lack suitable strain markers and past velocity fields are unknown, the nature of the cumulative deformation is not well understood. Part of the Barnes Ice Cap, Baffin Island N.W.T., Canada has undergone little overall change in position in the past 2000 years, so a long-term steady-state velocity field can be estimated from present-day measurements in a section in which flow is essentially two-dimensional. By means of a numerical model, particle paths, isochrons, strain rates, and cumulative strains are computed for this section. There is no correspondence between the principal directions of strain rate and cumulative strain in most of the ice, the greatest divergence of about 45° being near the base of the glacier where the maximum cumulative extension direction, X, is sub-horizontal. All ice particles initially undergo a restricted amount of pure shear, and then pass gradually into a long domain of nearly simple shear. They return to a second short stage of nearly pure shear near the margin. The effects of the early stage of pure shear are never eliminated, but the cumulative strain is increasingly dominated by simple shear down-glacier. Cumulative strain magnitude, magnitude of strain rate, and age of the ice all increase rapidly with depth, and change much more slowly with distance down-glacier. We argue that foliation is formed during flow by modification of primary inhomogeneities in ice. The three most important inhomogeneities are probably sedimentary layering, crevasse infillings, and irregularities such as ice glands or lenses. During deformation, the first will behave as isochrons, the second will rotate as material planes, initially perpendicular to the isochrons, and the third will be stretched parallel to the X-direction. The numerical model indicates that the isochrons, planes representing crevasse infillings, X-direction, and particle paths are within 1° of mutual parallelism in the most distal ice. Thus a single subhorizontal foliation, dipping slightly more steeply up-glacier than the flow lines, and curving upwards near the margin would be produced in this case. This corresponds with observations.