Strain weakening plays an important role in the continent-scale flow of rocks and minerals, including ice. In laboratory experiments, strain weakening coincides with microstructural changes including grain size reduction and the development of crystallographic preferred orientation (CPO). To interrogate the relative contributions of CPO development and grain size towards the strain weakening of viscously anisotropic minerals at very high homologous temperatures (Th ≥ 0.9), we deformed initially isotropic polycrystalline ice samples to progressively higher strains. We subsequently combined microstructural measurements from these samples with ice flow laws to separately model the mechanical response arising from CPO development and grain size reduction. We then compare the magnitudes of strain weakening measured in laboratory experiments with the magnitudes of strain weakening predicted by the constitutive flow laws. Strain weakening manifests as strain rate enhancement after minimum strain rate under constant load conditions, and as a stress drop after peak stress under constant displacement rate conditions. However, flow laws that only consider grain size evolution predict a nominally constant sample strength with increasing strain. On the other hand, flow law modelling that solely considers CPO effects can accurately reproduce the experimental strain weakening measurements. These observations suggest that at high homologous temperatures (Th ≥ 0.9), CPO development governs the strain weakening behaviour of viscously anisotropic materials like ice. Grain size, on the other hand, plays a negligible role in strain weakening under such conditions. Overall, we suggest that geodynamic and glaciological models should incorporate evolving CPOs to account for strain weakening, especially at high homologous temperatures.

Strain weakening during plastic deformation can be achieved via strain energy reduction due to intragranular boundary development and grain boundary formation. To examine intragranular boundary formation at high temperatures (Th≈0.9), we analysed electron backscatter diffraction (EBSD) data of coarse-grained ice deformed at -30°C. Misorientation and weighted Burgers vector (WBV) statistics were calculated along planar intragranular boundaries. Neighbour-pair and random-pair misorientation distributions intersect at misorientation angles of 10–30°, indicating an upper limit to the misorientation threshold angle at which neighbouring grains begin to interact, e.g., rotate relative to each other. Misorientation angles change markedly along each analysed intragranular boundary, linking low- (<10°) and high-angle (10–38°) segments, with each segment exhibiting distinct misorientation axes and WBV directions. We suggest that these boundaries might be produced by the growth and intersection of individual boundary segments comprised of dislocations with distinct slip systems. This new kinematic model does not require a change in the boundary geometry after its formation, as required by the other models, to modify the crystallographic geometry of a planar boundary. Misorientation axis distributions are fundamentally different between intragranular boundaries (mostly confined to the ice basal plane) and grain boundaries (largely dispersed). This observation suggests a strong crystallographic control of intragranular boundary development via subgrain rotation. The apparent lack of crystallographic control for grain boundaries, on the other hand, suggests that misorientation axes become randomized upon grain boundary formation, likely due to the operation of other mechanisms/processes that can modify misorientation axes.