Ice-Sheet Flow Properties Derived from Bore-Hole Shear Measurements Combined With Ice-Core Studies

Abstract

A 345 m deep bore hole in ice about 385 m thick, near the edge of Law Dome, Antarctica, was drilled in 1974 about 3 km up-stream from the site of a previous bore hole, nearly reaching the bed, obtained in 1969. The core from this new bore hole has been studied comprehensively, particularly with regard to the ice-crystal orientation fabrics. Samples of the ice core were subjected to simple shear at temperatures and deviatoric stresses which match thein situconditions of the ice sheet.Similar studies of randomly-oriented laboratory-made polycrystalline ice were undertaken. Long-term tests, lasting for up to two years, were required to determine minimum strain-rates. The flow law for the anisotropic ice was thus determined as a function of that for the isotropic ice together with a measure of c-axis fabric strength perpendicular to the shear plane.Core studies indicate that the upper part of the ice sheet has a polycrystalline structure appropriate to the surface longitudinal stress. Deeper in the core a strong concentration of near-verticalc-axes develops. Ice having very large crystals with multiple maxima fabrics was found in the lower quarter of the ice thickness.Shear measurements in the bore hole indicate the existence of high strain-rates in the zone of verticalc-axes, and of lower shear-rates below that level. The low values of shear-rates in the basal region cannot be explained in terms of crystallographic changes alone, and therefore it is inferred that the shear stress decreases in this layer–a result which also provides a possible explanation for the development of the observed basal crystal structure.