Abstract. Torsion experiments were performed in polycrystalline ice at high temperature
(0.97 Tm) to reproduce the simple shear kinematics that are believed to
dominate in ice streams and at the base of fast-flowing glaciers. As clearly
documented more than 30 years ago, under simple shear ice develops a
two-maxima c axis crystallographic preferred orientation (CPO), which
evolves rapidly into a single cluster CPO with a c axis perpendicular to the
shear plane. Dynamic recrystallization mechanisms that occur in both
laboratory conditions and naturally deformed ice are likely candidates to
explain the observed CPO evolution. In this study, we use electron
backscatter diffraction (EBSD) and automatic ice texture analyzer (AITA) to
characterize the mechanisms accommodating deformation, the stress and strain
heterogeneities that form under torsion of an initially isotropic
polycrystalline ice sample at high temperature, and the role of dynamic
recrystallization in accommodating these heterogeneities. These analyses
highlight an interlocking microstructure, which results from
heterogeneity-driven serrated grain boundary migration, and sub-grain
boundaries composed of dislocations with a [c]-component Burgers vector,
indicating that strong local stress heterogeneity develops, in particular,
close to grain boundaries, even at high temperature and high finite shear
strain. Based on these observations, we propose that nucleation by bulging,
assisted by sub-grain boundary formation and followed by grain growth, is a
very likely candidate to explain the progressive disappearance of the
c axis CPO cluster at low angle to the shear plane and the stability of the
one normal to it. We therefore strongly support the development of new
polycrystal plasticity models limiting dislocation slip on non-basal slip
systems and allowing for efficient accommodation of strain incompatibilities
by an association of bulging and formation of sub-grain boundaries with a
significant [c] component.
Seismic anisotropy in subduction zones and lattice-preferred orientation of olivine
