Description: Deformation localisation can lead to a variety of structures, such as shear zones and bands that range from grain to crustal scale, from discrete zones to anastomosing networks, and shear zone related folds. We present numerical simulations of the deformation of an intrinsically anisotropic material with a single maximum crystal preferred orientation (CPO) in simple shear. We use the Viscoplastic Full-Field Transform (VPFFT) crystal plasticity code coupled with the modelling platform ELLE to achieve very high strains. The VPFFT-approach simulates deformation by dislocation glide, taking into account the different available slip systems and their critical resolved shear stresses. We vary the anisotropy of the material from isotropic to highly anisotropic, as well as the orientation of the initial CPO. To visualize deformation structures, we use passive markers, for which we also systematically vary the initial orientation. At low strains the amount of strain rate localisation and resulting deformation structures depend on the initial CPO in all anisotropic models. Three regimes can be recognised: distributed shear localisation, synthetic shear bands and antithetic shear bands. However, at very high strains localisation behaviour always tends to converge to a similar state, independent of the initial CPO. Shear localisation is often detected by folded layers, which may be parallel to the anisotropy (e.g. cleavage formed by aligned mica), or the deformation of passive layering, such as original sedimentary layers. The resulting fold patterns vary strongly, depending on the original layer orientation. This can result in misleading structures that seem to indicate the opposite sense of shear.

Description: An overview of the deformation and recrystallization mechanisms that are active in the North Greenland Eemian Ice Drilling (NEEM) ice core is given, based on microscale models, light microscopy and cryogenic electron backscatter diffraction (cryo-EBSD). The Holocene ice (0-1419 m depth) deforms by dislocation creep with basal slip accommodated by non-basal slip. The amount of non-basal slip is controlled by the extent of strain induced boundary migration (SIBM). The most important recrystallization mechanisms and processes in the Holocene ice are grain dissection, strain induced boundary migration (SIBM), and bulging nucleation. In the glacial ice (1419-2207 m of depth) basal slip is accommodated by both non-basal slip and grain boundary sliding (GBS). Rotation recrystallization is more important, while SIBM is less important in the glacial ice compared to the Holocene ice. In the Eemian ice (2207-2540 m depth), which is at high temperature, different microstructures occur depending on the impurity content of the ice. The difference in microstructure and deformation mechanisms, between interglacial and glacial ice can have important consequences for ice rheology and ice sheet dynamics.

Description: The flow of glaciers and polar ice sheets is controlled by the highly anisotropic rheology of ice crystals that have hexagonal symmetry (ice lh). To improve our knowledge of ice sheet dynamics, it is necessary to understand how dynamic recrystallization (DRX) controls ice microstructures and rheology at different boundary conditions that range from pure shear flattening at the top to simple shear near the base of the sheets. We present a series of two-dimensional numerical simulations that couple ice deformation with DRX of various intensities, paying special attention to the effect of boundary conditions. The simulations show how similar orientations of c-axis maxima with respect to the finite deformation direction develop regardless of the amount of DRX and applied boundary conditions. In pure shear this direction is parallel to the maximum compressional stress, while it rotates towards the shear direction in simple shear. This leads to strain hardening and increased activity of non-basal slip systems in pure shear and to strain softening in simple shear. Therefore, it is expected that ice is effectively weaker in the lower parts of the ice sheets than in the upper parts. Strain-rate localization occurs in all simulations, especially in simple shear cases. Recrystallization suppresses localization, which necessitates the activation of hard, non-basal slip systems.This article is part of the themed issue {\textquoteleft}Microdynamics of ice{\textquoteright}.

Description: Microstructures from deep ice cores reflect the dynamic conditions of the drill location as well as the thermodynamic history of the drill site and catchment area in great detail. Ice core parameters (crystal lattice-preferred orientation (LPO), grain size, grain shape), mesostructures (visual stratigraphy) as well as borehole deformation were measured in a deep ice core drilled at Kohnen Station, Dronning Maud Land (DML), Antarctica. These observations are used to characterize the local dynamic setting and its rheological as well as microstructural effects at the EDML ice core drilling site (European Project for Ice Coring in Antarctica in DML). The results suggest a division of the core into five distinct sections, interpreted as the effects of changing deformation boundary conditions from triaxial deformation with horizontal extension to bedrock-parallel shear. Region 1 (uppermost approx. 450 m depth) with still small macroscopic strain is dominated by compression of bubbles and strong strain and recrystallization localization. Region 2 (approx. 450{\textendash}1700 m depth) shows a girdle-type LPO with the girdle plane being perpendicular to grain elongations, which indicates triaxial deformation with dominating horizontal extension. In this region (approx. 1000 m depth), the first subtle traces of shear deformation are observed in the shape-preferred orientation (SPO) by inclination of the grain elongation. Region 3 (approx. 1700{\textendash}2030 m depth) represents a transitional regime between triaxial deformation and dominance of shear, which becomes apparent in the progression of the girdle to a single maximum LPO and increasing obliqueness of grain elongations. The fully developed single maximum LPO in region 4 (approx. 2030{\textendash}2385 m depth) is an indicator of shear dominance. Region 5 (below approx. 2385 m depth) is marked by signs of strong shear, such as strong SPO values of grain elongation and strong kink folding of visual layers. The details of structural observations are compared with results from a numerical ice sheet model (PISM, isotropic) for comparison of strain rate trends predicted from the large-scale geometry of the ice sheet and borehole logging data. This comparison confirms the segmentation into these depth regions and in turn provides a wider view of the ice sheet.This article is part of the themed issue {\textquoteleft}Microdynamics of ice{\textquoteright}.