As grain size, impurity content and c-axis fabrics often co-vary in nature, it can be difficult to isolate the effects of each variable on strain rate. Cuffey et al. (2000a) measured strain rates in a tunnel in the basal layers of the Meserve Glacier. The temperature and shear stress at the site were essentially constant (ca 256 K and ca. 0.05 MPa) and c-axis fabrics varied only slightly, allowing the convolved effects of grain size and impurity content on strain rate to be isolated and semi-quantified via a statistical analysis. The authors concluded that a grain-size-sensitive creep mechanism contributed to the deformation at Meserve.A minor enhancement of strain rate was attributed to segregation of chemical impurities to grain boundaries, which the authors suggest may have resulted in enhanced GBS. Although the statistical analysis used did not allow the contributing grain-size-sensitive mechanism to be identified, one end-member case allowed for a grain-size-sensitive creep mechanism with p = 1.4 (i.e. GBS-limited creep) to contribute at least 15% but less than 50% to the strain rate for d = 1 mm. Such a scenario is consistent with transitional behaviour between GBS-limited creep and dislocation creep. For the shear stress (ca. 0.05 MPa) and temperature (256 K) of the site, Equation (4) predicts a contribution of 100% from GBS-limited creep to the strain rate for d = 1 mm, ca. 90% for d = 2 mm and ca. 70% for d = 5 mm. Although estimates of the relative contributions of GBS-limited creep and dislocation creep from Equation (4) do not agree precisely with the field data analysis, it nevertheless appears likely that ice at this site deforms in the transitional regime between GBS-limited creep and dislocation creep. Discrepancies between the predicted and observed contributions of GBS-limited flow to the creep rate may reflect errors in the determination of strain rate and/or stress in the field, the effects of impurities present in nature on flow rates, and the effects of c-axis fabrics on the relative rates of flow in the GBS-limited and dislocation creep regimes. Transitional flow behaviour via GBS-limited creep and dislocation creep at the Cuffey et al. (2000a) site also agrees with tunnel closure and borehole tilt measurements in a nearby tunnel in a previous study on Meserve by Holdsworth & Bull (1970). Temperatures were the same and grain sizes similar in the two studies,but unlike in the Cuffey etal. (2000b) study, in which shear stress was nominally constant, the shear stress in the Holdsworth & Bull (1970) study varied from ca. 0.01 to 0.06 MPa. Holdsworth & Bull (1970) observed a transition from n = 1.9 to n = 4.5 at a shear stress of ca. 0.05MPa, the same value of shear stress as that determined at the Cuffey et al. (2000c) site, and the same transition shear stress as that predicted from Equation (4).
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