Borehole deformation experiments, Barnes Ice Cap, Canada

Abstract

Deformation measurements in four boreholes distributed along a flowplane on Barnes Ice Cap were evaluated using a Glen-type flow law with strain rate varying as stress cubed. The values obtained for the ice flow parameter, B, are broadly consistent with measurements on other natural ice masses. The ice through which one of the boreholes passes seems unusually stiff, however. This suggests that stresses in this location, near the equilibrium line, are lower than predicted theoretically or by finite-element modeling. Near the glacier surface the octahedral strain rate is dominated by the longitudinal and transverse strain rates. Despite this, the shear strain rate does not increase linearly with depth, as might be expected from Glen's flow law. This is a common problem in such analyses, and may reflect uncertainty either in the shear strain rates or in the estimate of the shear stress. Alternatively Glen's flow law may not be applicable in certain low-stress situations in which no single stress component is clearly dominant. Blue ice with a fabric having two maxima lying in a vertical plane normal to the flow direction appears to be slightly softer than blue ice with a broad single maximum fabric, and blue ice with a three or four maximum fabric is stiffer than that with a broad single maximum fabric, but the differences are not large. Bubbly white ice at the base of the glacier is appreciably softer than overlying blue ice; as discussed previously, this softness is attributed to the high bubble content.