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

Ice 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. Plain Language Summary On its way toward the ocean, ice speeds up dramatically from less than 1 m/year upstream to more than a kilometer per year downstream. In this paper, we investigate the physical processes controlling this speed-up. Specifically, we focus on the role of the bedrock topography and rheology in facilitating the transition from this slow to rapid motion. We use a two-dimensional numerical model to simulate the flow field within a slab of ice flowing down a ramp over a simplified topography. We find that including the bedrock topography can lead to a zone of highly localized deformation within the ice above topographic highs. We also find that a non-linear rheology amplifies this localization. We compare our model results to borehole measurements from Greenland and find evidence that supports the existence of this highly localized deformation zone. This study indicates that the localized deformation induced by bedrock topography and amplified by non-linear rheology could be one physical mechanism that governs the speed-up of the ice flow. [ABSTRACT FROM AUTHOR]