Spontaneous Formation of an Internal Shear Band in Ice Flowing Over Topographically Variable Bedrock

Abstract

AbstractIce surface speed increases dramatically from upstream to downstream in many ice streams and glaciers. This speed‐up is thought to be associated with a transition from internal distributed deformation to highly localized deformation or sliding at the ice‐bedrock interface. The physical processes governing this transition remain unclear. Here, we argue that highly localized deformation does not necessarily initiate at the ice‐bedrock interface, but could also take the form of an internal shear band inside the ice flow that connects topographic highs. The power‐law exponent n in the ice rheology amplifies the feedback between shear heating and shear localization, leading to the spontaneous formation of an internal shear band that can create flow separation within the ice. We model the thermomechanical ice flow over a sinusoidal basal topography by building on the high‐resolution Stokes solver FastICE v1.0. We compile a regime diagram summarizing cases in which a sinusoidal topography with a given amplitude and wavelength leads to shear band formation for a given rheology. We compare our model results to borehole measurements from Greenland and find evidence to support the existence of an internal shear band. Our study highlights the importance of re‐evaluating the degree to which internal deformation contributes to total deformation in the ice column and to the flow‐to‐sliding transition.