Experimental Recrystallization of Ice Under Stress

Polycrystalline ice in high-temperature creep (-5° to 0°C) under deforming stresses of a few bars undergoes rapid syntectonic recrystallization and large changes in texture and fabric that have prominent effects on flow. A simple original texture of small equant poly hedral grains recrystallizes at a shear strain 'Y of about 0.04 to a complex texture of coarse irregular interlocking grains with sutured boundaries and abundant strain shadows. Re crystallized grain size increases as stress decreases and temperature increases. Included air bubbles cause no detectable impediment to recrystallization or grain boundary migration. In simple shear (applied in torsion) the recrystallized c-axis fabric consists of two maximums, one at the pole of the shear plane and the other about 20° away from the shear direction. Fabric perfection mainly depends on total strain rather than time and develops pro gressively over the strain range 'Y ::::: 0.04-0.3. At a given total strain the fabric depends only weakly on stress: the maximums become somewhat weaker, and the angle between them increases slightly as the shear stress decreases from 4.1 to 1.9 bars. Under compressive stress superimposed across the shear plane the fabric maximums elongate and join to form n small-circle girdle, whose center reorients toward the compression axis. From the form of the fabric pattern it is possible to infer the stress character (as distinct from stress level and mean stress) over the full range of possible variation for a general state of stress. Also, it is notably possible to distinguish between simple and pure shear and to distinguish the shear plane from it! l conjugate plane in simple shear. The recrystallization fabric de veloped under simple shear has monoclinic symmetry and is incompatible in symmetry and orientation with the symmetries of stress, strain rate, and total strain but compatible with the symmetry of the velocity field measured in a coordinate system in which the stress re mains fixed. This conflicts with simple thermodynamic theories of crystal reorientation and also a basic assumption in the theoretical formulation of ice flow laws. A combined mechanism of thermodynamically controlled crystal reorientation modified by intracrystal line plastic flow is qualitatively compatible with the fabric. The favored crystal orientations indicated experimentally are those which would have nearly maximum elastic strain energy if the stre::s were homogeneous in the polycrystalline aggregate, but because of intracrystal line plastic anisotropy it is likely instead that the stress is not homogeneous and that crystals in the preferred orientations have minimum elastic strain energy. Statistical fluctua tions in perfection and orientation of fabric elements suggest that the recrystallization-re orientation mechanism is cooperative in nature. The crystal reorientations cause many-fold accelerations in creep rate as the recrystallization fabric develops. The seemingly steady creep observed between initial transient creep and the onset of recrystallization does not represent steady structural conditions, which require recrystallization. Fabrics observed in glacier ice deformed in pure shear to an equivalent strain of 'Y ::::: 0.6 are compatible with the experimentally produced fabrics, but the commonly observed multiple-maximum fabrics of glacier ice are entirely incompatible. An experimental test shows that the incompatibility is not due to a qualitative difference between crystal reorientation at and below the melting point. An orientation-controlling mechanism that becomes operative only at high strains (~1) and supersedes the mechanism operative in the experimental strain range appears to be indicated; this implies that truly steady structural conditions are not established at the lower strains. The complexity of the flow-induced and flow-affecting structural changes that occur in ice suggests caution in interpreting experimental creep results for other, more complicated geological materials. Nonsteady creep is probably the rule in nature, except in flow situations involving very large strains, comparable to glacier flow.