Variations in shear deformation rate with depth at Dome Summit South, Law Dome, East Antarctica

The variation of shear deformation rate with depth at the Dome Summit South (DSS) site, 4.7 km (~4 ice thicknesses) from the summit of Law Dome, East Antarctica, has been determined by repeated borehole inclination measurement. The results show that from the surface down to 1000 m (ice-equivalent depth) deformation rates increase as expected with the increase in stress, temperature and the development of stronger ice-crystal fabrics. There is a broad maximum in strain rate around 1000 m. Below this depth, strain rates decrease, with values in the basal ice ~1/3 of those at 1000 m. In DSS, Holocene ice with low, uniform impurity levels extends to a depth of 1110 m, so the decrease in shear rate below 1000 m is attributed not to any change in properties of the ice, but to shear stress reduction induced by the large-scale retarding effect of local bedrock hills. Below 1000 m, within the zone of retarded flow, there is a narrow spike, 14 m thick, in which the shear rate is ~5 times that in the ice immediately above and below. The shear-rate spike corresponds in depth to ice with high dust concentrations, small crystal size and strong vertical c-axis fabrics that was deposited at the Last Glacial Maximum. A surface velocity of 1.98 ± 0.03 m a^-1 obtained by integration of shear rate over the borehole depth is in agreement with the value of 2.04 ± 0.11 m a^-1 obtained by the global positioning system. The ratio of average column velocity to surface velocity determined by the borehole measurements is 0.74. A value of 0.76 is obtained from mass-balance considerations.<p></p>