Mechanical anisotropy as a driver of shear margin and ice stream formation

Fast-flowing ice streams drain most of the inland ice from the Antarctic and Greenland ice sheets (GrIS). The Northeast Greenland Ice Stream (NEGIS), for example, extends for more than 500 km from the central GrIS ice divide to its outlets, with flow velocities up to ten times higher than in the surrounding ice. Despite extensive research, the mechanisms responsible for ice stream formation remain poorly understood. NEGIS, as a representative case of ice streams that are not topographically confined, has recently also been found to lack an area of elevated geothermal heat flux below. However, no model has so far been able to test whether ice stream initiation can solely result from the evolving internal properties of the ice itself, without relying on external forcing, given that slow-moving ice may be frozen to the bed before ice-stream formation.

Ice is strongly anisotropic because it deforms more easily parallel to its crystallographic basal plane than perpendicular to it along the crystal's c-axis. During deformation, this difference leads to a preferred alignment of the crystal lattice orientations. This anisotropy has significant implications for ice flow. We present a three-dimensional full-Stokes model of an analogue to NEGIS. In our modelling, ice first shows convergent flow towards the outlet gate. During flow, c-axis rotations calculated by our model cause the directional alignment of the easy-glide crystallographic basal planes parallel to the vertical shear plane, which make the ice effectively softer. Shear zones usually form in pairs due to the localized shearing, known as shear margins that bound the ice stream that can now flow much faster and extend further inland. Our results show that a fully developed, fast-flowing ice stream can form in only 1000–2000 years solely due to the evolving ice anisotropy. We perform several model runs up to 4000 years to explore the effect of varying boundary conditions, which result in different geometries of an ice-stream system. Ice streams in the system can potentially initiate and evolve by the formation and movement of shear margins in relation to the location of outlet gates within the drainage basin. This work stresses the importance of evolving ice anisotropy on ice-sheet mass balance and sea-level rise during global climate change.