Recrystallization 61
of rotating basal planes clockwise on the left and counterclockwise on the
right in Figure 4.14d. Thus, as the crystals grow, the c-axes rotate toward
the compression axis (Alley, 1992), with the result that the mean angle
between the compression axis and the c-axes is typically only ∼30
◦
–35
◦
,
not 45
◦
(Kamb, 1972; Hooke and Hudleston, 1980). Crystals that have
been rotated too far, and thus become highly stressed, are resorbed, while
nucleation develops new crystals in more favorable orientations.
If the ice at this depth is close enough to the bed, drag exerted
by the bed results in a stress configuration approximating simple shear
parallel to the bed (Figure 4.14c). Crystals with vertical c-axes are
then preferred. The resulting fabrics, which are common in ice sheets
(Gow and Williamson, 1976; Hooke and Hudleston, 1980), have single
maxima that range from relatively broad (Figure 4.13d)toquite tight
(Figure 4.13e).
The fabrics in Figures 4.13b,c,d all seem to form under roughly
equivalent cumulative strain. The differences among them are primarily
caused by stress configuration. As a class, we will refer to them as broad
single-maximum fabrics.
Although the increase in creep rate associated with recrystallization
usually begins at effective strains, ε
e
(see Equation (2.11)) of ∼0.01 in
the laboratory (Figure 4.8b), broad single-maximum (and equivalent)
fabrics are not particularly evident until ε
e
∼
=
0.04 and only become well
developed at ε
e
=0.4 (Kamb, 1972; Jacka and Maccagnan, 1984). In the
field, Hooke and Hudleston (1980) found that such fabrics first appeared
at ε
e
∼
=
0.7−0.8. For reference, circles that have been deformed into
ellipses by strains of these magnitudes have axial ratios of 1.02, 1.08,
2.22, and ∼4.5, respectively. Thus, creep rates increase long before a
detectable preferred c-axis orientation develops.
In simple shear at cold temperatures or high strain rates (or high
cumulative strains), the single-maximum fabric strengthens (Figure
4.13e). However, at lower strain rates and temperatures above −10
◦
C,
an unexpected fabric appears. First, the single maximum splits in two,
with a maximum on either side of the shear direction (Figure 4.13f). The
basal planes corresponding to these c-axis orientations are still parallel to
the shear direction, but do not have optimal orientations (Figure 4.14e).
Then, with increased cumulative strain, strain rate, or temperature, first
one and then a second maximum appears inclined to the direction of
shear (Figures 4.13g and 4.14f). These planes are definitely not well
oriented for glide, and thus must stiffen the ice, at least slightly. These
multiple-maximum fabrics appear at ε
e
∼
=
1.3 (Hooke and Hudleston,
1980). The corresponding axial ratio of the strain ellipse is ∼15.
The origin of these multiple-maximum fabrics is not understood.
They have been attributed to annealing under conditions of near stagna-
tion (Budd and Jacka, 1989) and have been reproduced in the laboratory