The down-stress transition from cluster to cone fabrics in experimentally deformed ice

Abstract

During plastic deformation of polycrystalline ice 1h, ice crystals become crystallographically aligned due to dislocation glide, primarily on the basal slip system. Such crystallographic preferred orientation (CPO) introduces a viscous anisotropy in ice, and thus strongly influences the kinematics of the flow of glaciers and ice sheets. Two key mechanisms exert different controls on CPO. In axial compression, recrystallization dominated by lattice rotation yields a cluster of c-axes parallel to compression, and recrystallization dominated by grain boundary migration (GBM) yields a cone-shaped distribution of c-axes with the cone axis parallel to compression. The transition between these dominant mechanisms of CPO formation has not been well quantified. In this study, we explore how this transition varies with stress. Ice deformation experiments were conducted using a high-pressure, gas-medium apparatus to prevent fracturing of samples at relatively high stresses. Samples were deformed in uniaxial compression at a temperature of ∼−10 °C and a confining pressure of 10 MPa. Fabricated ice samples with starting average grain sizes of either ∼0.23 mm or ∼0.63 mm were each deformed to an axial strain of ∼0.2 at a nominally constant strain rate in the range 1.2×10−6 to 2.4×10−4 s−1, yielding flow stresses of 1.17 to 4.31 MPa. High-quality electron backscatter diffraction reveal the grain size, shape, subgrain structure, and CPOs formed at different stresses. All deformed samples have strong, non-random CPOs with c-axes concentrated in cones. The cone angle and CPO strength are observed to decrease with increasing stress. As stress increases, the fraction of grains with highly curved or lobate grain boundaries decreases and the fraction of polygonal grains with straight grain boundaries increases. Based on these observations, we propose that a transition in the dominant mechanism of CPO formation occurs with increasing stress, from GBM, which consumes grains with low Schmid factors at low stress, to lattice rotation caused by slip primarily on the basal slip system, which causes c-axes to rotate to become parallel to the shortening direction at high stress. Mapping out the transition from cluster (rotation-dominated) to cone (GBM-dominated) CPOs as a function of stress (this study) and temperature (future studies) allows for a robust extrapolation to, and a fundamental understanding of the CPOs formed at, glaciological stresses and temperatures.