Spatially variable till deformation and water transport in ice-stream shear margins from numerical simulations

The ice-flow morphology of Antarctica is characterized by rapid ice motion in narrowly confined routes, called ice streams, bordered by nearly stagnant ice. The abrupt change in velocity at the boundary of ice streams leads to a zone of intense deformation, known as the shear margin. Shear margins remain enigmatic features. They have a profound impact on the ice flux to the ocean, but do not affect rapid ice flow in a straightforward way. From geophysical investigations and scattered core sampling it is known that the ice streams in Antarctica flow on deformable and unconsolidated sediments (till), but there appears to be no apparent variability in sedimentary facies that could explain the variability of ice kinematics and the spatial variability between stagnant and fast-moving ice. Instead, hydrology at the base of the ice sheet may induce heterogeneities in the mechanical strength of the till. To study the coupled interactions between till deformation, hydrology and basal stress that control ice kinematics, we model the saturated till as a grain-fluid system by employing the Discrete-Element Method that is capable of resolving the grain-scale deformation and fluid percolation. We represent sediment grains as spherical and indestructible spheres of approximately equal size that interact by contact forces and are subjected to gravitational body force and to the force induced by the pore fluid. Pore fluid is represented as a compressible continuum whose momentum conservation is governed by Darcy's law. We employ an experimental setup intended to investigate the structural, mechanical, and hydrological transitions across the shear margin of the ice stream. The spatially variable driving velocity is represented by a Heaviside step function, which is intended to realistically represent the transition between the stagnant ice and the flowing ice at the till level. Preliminary results show local variability in the porosity and hydrologic properties of the sediment associated with the velocity transition (Figure 1).