Creep in Columnar Ice: Insights from Laboratory Experiments and Simple Numerical Modelling

Tooley, Lauren

Ice deformation experiments and related numerical modelling are necessary to increase our understanding of ice sheet flow, which is of extreme importance in predicting sea level rise in relation to global warming and climate change. We present the results of high temperature (?5?) uniaxial shortening experiments and finite element modelling of ice forced through a constriction. In our experiments we applied constant loads to columnar ice polycrystals oriented with their column axis normal to the shortening direction. We deformed samples to several increments of strain, between 0% and 14%. At these conditions the crystallographic preferred orientation (CPO) of columnar ice evolves from a large circle CPO to a CPO where two clusters exist at ~30° from the shortening direction. The rotation of grains via intracrystalline glide on basal planes is responsible for the CPO evolution at low strain (< 7.2% strain). At high strain, grain boundary migration recrystallisation is dominant, encouraging the growth of optimally aligned grains and the consumption of grains poorly aligned for basal slip.
Our numerical model shows heterogeneous stress and strain rate fields developed in ice pushed through a constricting narrowing channel. A significant high stress (1MPa), high strain rate (1.0 x 10?7 s?1) and low velocity (2 x 10?9 s?1) zone develops along the ice-constriction interface. An optimised CPO would rapidly develop within this zone. A less significant high stress and high strain rate zone develops further into the ice interior where the velocity is faster and the ice is in extension.
Ice is an ideal analogue for quartz because of its comparable basal-slip anisotropy. We present a novel columnar ice uniaxial shortening experiment that can be used to test the results of an existing numerical model for quartz dynamic recrystallisation. By deforming ice around a wooden dowel we simulated quartz deformation around a rigid porphyroclast. The CPO at 8% strain indicates that the deformation remains approximately two-dimensional, allowing for direct comparison with the 2D model outputs. This experiment should inspire more columnar experiments of this type and provide a means to further constrain quartz deformation conditions and mechanisms.

123 pages A4, digital appendix with 3D models

2016Tooley