Crystallographic preferred orientation (CPO) development governs the
strain weakening in minerals with strong viscous anisotropy at high
homologous temperatures (≥ 0.9): insights from up-strain ice deformation
experiments
Abstract
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.