The Fabric of Polycrystalline Ice Deformed in Simple Shear: Experiments in Torsion, Natural Deformation and Geometrical Interpretation

Abstract

Three cylinders of artificial ice have been deformed in torsion at about –10℃ up to finite shear strains γ of 0.6, 0.95 and 2. The initial random lattice orientation rapidly evolves into a bimodal distribution of the basal slip planes as already observed by Kamb (1972) and Duval (1981) for low-strains experiments near the melting point. For the γ = 0.6 and 0.95 experiments, one family of grains (> 50%) corresponds to basal planes tending to parallel the imposed shear plane; the basal planes of the other family make a broader maximum at about 60° from the shear plane. The direction of minimum concentration between the two populations approximately corresponds to the flattening plane or to the elongation direction of the strain ellipsoid. With increasing strain (γ = 2) the second submaximum vanishes and only the principal maximum parallel to the shear plane remains. This evolution is conformable with the data of Hudleston (1977) in a natural shear zone in glacial ice; it also compares remarkably well with Etchecopar's (1977) geometrical computer model of simple shear in the same range of γ values. Single slip on the basal plane with no preferential slip direction in that plane can explain the analogy between fabrics in ice deformed in plane strain and fabrics obtained from the two-dimensional computer model.The bimodal distribution reflects predominant slip on the basal plane; the progressively increasing heterogeneous strain enhances internal distorsion, rigid body rotation and recrystallization of grains unfavorably oriented for further slip, leading to the unimodal distribution. The adequacy of fabric analyses to infer the strain regime and the sense of shear in plastically deformed rocks is strengthened.