The slow motion and changes of glaciers and large ice masses are governed by the deformation of polycrystalline ice. The constitutive law of polycrystalline ice for ice-sheet modelling is that of an incompressible, non-linear viscous fluid. Deviatoric stresses in ice sheets are generally lower than 0.1 MPa and strain rates are typically between 10-10 and 10-13s-1, but can reach 10-7s-1 in temperate glaciers. It is difficult to obtain valuable information on the ice-flow law under low stress conditions. Laboratory tests take too long to obtain a significant amount of deformation under these conditions and extrapolation from tests performed at higher stresses introduces significant uncertainty. Much progress has been made, however, with the study of the ice structure in deep ice-cores recently retrieved from Antarctica and Greenland. The sensitivity of strain rate to stress in ice sheets is characterized by a stress exponent lower than 2. The rate-controlling processes are not totally clear, but basal slip is the dominant deformation mode (Montagnat & Duval, 2000; Montagnat et al., 2003).

The deformation of polar ice induces the development of lattice-preferred orientations (fabrics) producing a non-random distribution of c-axis orientation. Owing to the very large anisotropy of ice crystals (Duval et al., 1983) and the preponderance of intracrystalline dislocation glide in polar ice (Alley, 1992), initially isotropic ice formed after transformation of snow and firn near the surface becomes anisotropic as fabrics develop. Very large variations of strain rates with the applied stress direction

Thorsteinsson, Will Harrison, Dan Elsberg, John Morack, Mark Zumberge and Eric Husmann contributed ideas and data. Greg Lamorey, Richard Alley, Larry Wilen and others from the Siple Dome Deep Drilling Project contributed their data for this analysis.

have been found using both laboratory and in situ measurements (Russell-Head & Budd, 1979; Pimienta et al, 1987; Budd & Jacka, 1989). Up to now, models describing the evolution of ice sheets have not accounted for the changing rheological properties of ice with time. A first attempt, however, has been made by Mangeney et al. (1997) and Salamatin & Malikova (2000).

This work focuses on the deformation modes of polycrystalline ice for conditions prevailing in glaciers and ice sheets. We will show that intracrystalline dislocation glide is compatible with in situ deformation measurements and the development of fabrics. This paper is also intended to clarify the relationship between recrystallization and fabrics. Emphasis is placed on the influence of impurities and crystal size on strain rate in the low stress conditions of ice sheets. The role of a liquid phase in the deformation of ice is also discussed.

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