Marco Antonio Lopez-Sanchez , Thomas Chauve, Maurine Montagnat, Andrea Tommasi 

We explore the links between strain localisation and microstructural evolution in ice Ih deformed by dislocation creep. Using digital image correlation (DIC), we monitored the evolution of the strain field in two coarse-grained columnar ice samples deformed by creep (uniaxial compression at constant load) at -7 °C and 0.5 MPa up to 9.5% bulk shortening. After a brief transient (<0.2% bulk strain), in which strain localises nearby grain boundaries, viscoplastic strain concentrates in a few narrow intracrystalline shear bands that eventually extend over multiple grains. A comparison of pre- and post-deformation crystal orientation maps shows that strain localisation in shear bands is mainly accommodated by basal slip without producing significant dislocation-related substructures. Severe dynamic recrystallization develops locally at grain boundaries that act as barriers to dislocation motion, particularly where basal shear transfer is ineffective. These observations are compared to full-field simulations reproducing the initial microstructure and experimental setup, which predict the stress and strain rate fields for deformation entirely accommodated by dislocation slip on the known slip systems in ice. The present data indicate that during the deformation of coarse-grained ice Ih at high homologous temperatures: (1) recrystallization does not drive strain localisation but accommodates strain incompatibility, and (2) large strains can be accommodated by unimpeded basal slip with no formation of dislocation substructures. Observation (2) implies that intragranular orientation gradients are unreliable gauges of viscoplastic strain intensity at the grain scale and that the proportion of dislocation types in subgrains does not measure the relative contribution of different slip systems to deformation. Finally, we discuss the implications of this study for the interpretation and modelling of deformation by dislocation creep in rocks.