Ice Fabrics in a Vertical Flow Plane, Barnes Ice Cap, Canada

Abstract

AbstractAt a depth of about 75 m in the lower part of the accumulation area of the Barnes Ice Cap there is a change from fine-grained ice with a weakly-oriented c-axis fabric to coarser ice with a broad single-maximum fabric. At a depth of about 150 m the single maximum becomes elongate perpendicular to the direction of bubble elongation, and then splits into two distinct maxima making an angle of about 40–45° with respect to one another. At greater depths a third and finally a fourth maximum appear, forming the well known diamond pattern. Mean crystal size does not seem to increase in the transitions from one to two and thence to three maxima, but it may become more uniform. Crystal size does increase in the transition from three to four maxima, however. At the base of the glacier there is a 10–20 m thick layer of unusually-bubbly, fine-grained white ice with a strong single-maximum fabric.The depths to the transitions increase up-glacier and in place of the single-maximum fabric a small-circle pattern is found. Down-glacier the depths to the transitions decrease, systematically eliminating the higher zones. Thus in the lower part of the ablation area, ice with a four-maximum fabric appears at the surface.The independent variables governing these fabric transitions appear to be temperature T, stress τ, and cumulative strain ³oc. In a tentative stability diagram showing the fields in which given fabrics are stable in T–τ–³oc space, multiple-maximum fabrics occur at high temperatures (> —10°C) and at moderate to high stresses, weakly-oriented fabrics at low stresses or low cumulative strains, broad single-maximum fabrics at moderate stresses or moderate cumulative strains, and strong single-maximum fabrics at high stresses or large cumulative strains.