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

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Abstract
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 1000m (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 rale 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, with in 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 Glaciol 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 ± 0.11m 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.

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