An antiplane strain model for evaluating shear-margin stability (Ortholine v1.0)

Abstract. Shear margins—the lateral boundaries of ice streams and glaciers—play a key role in the mass balance of our ice sheets. In glaciers, shear margins are often controlled by topography, whereas in ice streams they are inherently dynamic and can migrate. Margin migration alters the width of fast-moving ice and thus the force balance that controls ice speed and mass loss. Quantifying whether a shear margin is stable or prone to migration is hence important for estimating the future mass balance, but requires understanding the processes governing its current position. Numerous englacial and subglacial processes may contribute, including shear heating, fabric development, basal topography, lithology, and hydrology. Although the potential importance of these processes has been established, we currently lack models aimed at evaluating their relative importance at a specific field site and we aim to fill this gap. Here, we introduce "Ortholine'', an antiplane-strain free-boundary model developed to evaluate the relative importance of different englacial and subglacial processes in controlling a specific shear-margin location. By focusing on the cross-sectional force balance of ice streams, Ortholine complements commonly used flow-line models. After deriving and verifying Ortholine, we outline the four key steps for applying it to a field site, using Institute Ice Stream in the Weddel Sea Sector, Antarctica, as a proof-of-concept. We have selected Institute Ice Stream as an example, because of its intermediate complexity and the sufficient availability of field data to guide the model setup.