Fabric development

Tertiary creep is associated with migration recrystallization (Steinemann, 1958b). Fabrics are considered to be 'recrystallization fabrics' because the orientation of grains is related to the nucleation and migration of grains (Duval, 1981; Alley, 1992). Plastic strain associated with the formation of recrystallization fabrics is too low to produce important rotation of the lattice. Figure 59.2a shows a typical creep curve obtained at -3°C with the c-axis orientation measured after a strain of about 30%. Recrystallization fabrics are stress-controlled and recrystallized grains are well oriented for basal slip (Duval, 1981). Most of the fabrics that form in temperate glaciers result from migration recrystallization.

Figure 59.2 Creep curves and fabric data for initially isotropic ice samples deformed in uniaxial compression at 0.2 MPa. (a) Temperature 9 = -3°C (from Jacka & Maccagnan, 1984). (b) Temperature 9 = -15°C (from Jacka & Li, 2000).

Stress Strain Curve Ice

Figure 59.2 Creep curves and fabric data for initially isotropic ice samples deformed in uniaxial compression at 0.2 MPa. (a) Temperature 9 = -3°C (from Jacka & Maccagnan, 1984). (b) Temperature 9 = -15°C (from Jacka & Li, 2000).

In ice sheets, fabrics develop under the effect of intracrystalline slip (Azuma & Higashi, 1985; Alley, 1988) as long as migration recrystallization does not occur. High finite strain is therefore required to obtain significant preferential c-axis orientation. Figure 59.2b shows a creep curve obtained at — 15°C under compression at 0.2 MPa by Jacka & Li (2000). Tertiary creep and, as a consequence, migration recrystallization were not initiated in this laboratory test. The weak fabric pattern is in accordance with this assertion (Fig. 59.2b). Figure 59.3 clearly illustrates the marked difference between fabrics induced by migration recrystallization and those induced by the rotation of the lattice by slip. Small-grained ice is associated with rotation recrystallization, which is stabilized by the presence of fine particles in glacial ice.

In conclusion, migration recrystallization readily occurs in temperate glaciers, but it is involved in ice sheets only at relatively high temperature and if the stored energy is sufficiently high. Microparticles can prevent the transition between rotation and migration recrystallization.

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