To illustrate this generalized description of ice-sheet flow, a comparison is made between data from studies on the Byrd Station borehole in Antarctica with the deformation mechanism map for ice. In Fig. 60.5, the measured grain size versus depth profile (Gow etal., 1968) and estimated shear stress versus depth profiles (Frost & Ashby, 1982) for the Byrd Station drill site are plotted. Following Frost & Ashby (1982), shear stresses T were estimated from T = pgh sin (a), where p is ice density, g is the acceleration due to gravity, h is the depth in the ice and a is the surface slope. Temperatures increase with increasing depth and are shown at discrete points along the borehole, ranging from ca. 245 K at the surface to ca. 273 K at the base. Superimposed on this plot is the T versus d deformation mechanism map for ice constructed for T = 273 K.
1000 43 2164 ju
1000 43 2164 ju
10 100 grain size (mm)
Figure 60.5 Plot of shear stress and depth versus grain size for the deep ice-core from Byrd Station, Antarctica. Shear stresses t were estimated from the depth h using t = pgh sin(a), where p is density, g is gravitational acceleration and a is the surface slope (ca. 2.5 X 10-3, after Frost & Ashby, 1982). Grain-size versus depth data (solid symbols connected schematically with the solid curve) are from Gow etal. (1968). Superimposed on the figure is a shear stress versus grain size deformation mechanism map constructed for a temperature of ca. 273 K. The heavy solid line is the boundary between GBS-limited creep and dislocation creep; the dotted-dashed lines are strain-rate contours. Temperatures measured at discrete depths along the borehole (from Gow et al., 1968) are indicated in the figure.
It must be emphasized that the deformation map represents a limiting case, with the boundary between mechanisms and the shear-strain-rate contours within each creep regime calculated only for the highest temperature at Byrd, ca. 273 K. However, the boundary between the GBS-limited creep and dislocation creep regimes shifts downward in stress in Fig. 60.5 only slightly, by a factor of only 1.3, when calculated for a temperature of 245 K.
As suggested in Fig. 60.5, the ice at Byrd Station probably deforms within the GBS-limited creep regime over nearly the entire depth of the ice sheet. At the base of the ice sheet, the ice is expected to deform in a transitional regime between GBS-limited creep and dislocation creep. The observed sharp increase in the rate of grain growth with increasing depth occurs at a temperature of ca. 256 K, in excellent agreement with the temperature of the onset of pre-melting inferred from creep data (ca. 255 K, Goldsby & Kohlstedt, 2001). A similarly sharp increase in the rate of grain growth with increasing depth is observed at the GRIP site in Greenland at a temperature of ca. 258K (Thorsteinsson et al.,
1997). As the ice at 256K at Byrd deforms well within the GBS-limited creep regime (in which non-basal slip is insignificant), the increased rate of grain growth with depth at T > 255 K probably results from enhanced grain-boundary mobility, rather than an increase in driving force due to variations in strain energy (dislocation density) caused by the activation of non-basal slip systems.
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