Ice fabrics from a borehole at the top of the south dome, Barnes Ice Cap, Baffin Island

摘要

Crystallographic fabrics from a borehole at the top of the south dome of Barnes Ice Cap, Baffin Island, Canada, were studied. Fabric intensity in coarse-grained ice increases with increasing depth, and the pattern changes from an initial weakly oriented one to one involving two c-axis maxima within a small circle centered on the vertical. The two maxima are aligned in a plane in the direction of maximum horizontal extension; they become distinct by about 180 m and then dominate the fabric until about 215 m; below this depth, the small-circle pattern dominates. Fine-grained ice displays similar but more variable fabric patterns, complicated by the inheritance of a vertical single maximum from the initial state. Measured surface strain rates suggest a coaxial deformation history intermediate between pure shear and uniaxial compression. Such stress configurations are known to lead to two-maximum and small-circle fabric patterns, respectively, on the basis of experiments and models involving plastic basal glide alone. With increasing strain, the angle between the two maxima, or the opening angle of the small circle, decreases in a similar manner both in the fabric diagrams for natural ice and in the fabric patterns predicted by models of plastic glide. Both the two-maximum and the small-circle patterns are considered to be equivalent to the single maximum pattern found in natural ice subjected to simple shear. In all cases, the fabric becomes well-defined at natural octahedral unit shears of about 1.0, and in all cases the fabric development is inferred to be largely controlled by basal glide.