Fig. 7: Modelling shear zone softening.

ELLE + VPFFT^34,57 results of simple shearing of ice 1 h, starting with a strong vertical point maximum of the c-axis orientations ( ±5°), a 16x weaker basal plane, and a stress exponent of 4⁶⁰. ELLE is a numerical simulation platform for modelling microstructure in geological processes. VPFFT indicates the viscoplastic deformation code. The simulation illustrates the evolution and reorientation of the crystal-preferred orientation (CPO) from the ice sheet into the shear margin. The modelled plane is horizontal (bedrock parallel) and is then subjected to simple shear with a vertical shear plane. The graph shows the normalised shear strain rate (at constant stress) vs. shear strain as well as pole figures (lower hemisphere; classical glaciologic projection looking down a drill core into the ice) until a shear strain of γ=30. The single-point maximum orientation of c-axes first divides into two-point maxima until a shear strain of γ=10. Between γ=10 and γ=20 a transition occurs, when the two-point maxima rotate towards a broad new single maximum perpendicular to the shear plane. The total softening is about a hundred at a shear strain of γ=30, and already 20 at a shear strain of γ=10.