The increasing catalogue of high-quality ice-penetrating radar data provides a unique insight in the internal layering architecture of the Greenland ice sheet. The stratigraphy, an indicator of past deformation, highlights irregularities in ice flow and reveals large perturbations without obvious links to bedrock shape. In this work, to establish a new conceptual model for the formation process, we analysed the radar data at the onset of the Petermann Glacier, North Greenland, and created a three-dimensional model of several distinct stratigraphic layers. We demonstrate that the dominant structures are cylindrical folds sub-parallel to the ice flow. By numerical modelling, we show that these folds can be formed by lateral compression of mechanically anisotropic ice, while a general viscosity contrast between layers would not lead to folding for the same boundary conditions. We conclude that the folds primarily form by converging flow as the mechanically anisotropic ice is channelled towards the glacier.

ABSTRACT. Results of numerical simulations of co-axial deformation of pure ice up to high-strain, combining full-field modelling with recrystallisation are presented. Grain size and lattice preferred orientation analysis and comparisons between simulations at different strain-rates show how recrystallisation has a major effect on the microstructure, developing larger and equi-dimensional grains, but a relatively minor effect on the development of a preferred orientation of c-axes. Although c-axis distributions do not vary much, recrystallisation appears to have a distinct effect on the relative activities of slip systems, activating the pyramidal slip system and affecting the distribution of a-axes. The simulations reveal that the survival probability of individual grains is strongly related to the initial grain size, but only weakly dependent on hard or soft orientations with respect to the flow field. Dynamic recrystallisation reduces initial hardening, which is followed by a steady state characteristic of pure-shear deformation.

In times of warming in polar regions, the prediction of ice sheet discharge is of utmost importance to society, because of its impact on sea level rise. In simulations the flow rate of ice is usually implemented as proportional to the differential stress to the power of the exponent n = 3. This exponent influences the softness of the modeled ice, as higher values would produce faster flow under equal stress. We show that the stress exponent, which best fits the observed state of the Greenland Ice Sheet, equals n = 4. Our results, which are not dependent on a possible basal sliding component of flow, indicate that most of the interior northern ice sheet is currently frozen to bedrock, except for the large ice streams and marginal ice.