Shear Variation at the Ice-Till Interface Changes the Spatial Distribution of Till Porosity and Meltwater Drainage

Many subglacial environments consist of a fine-grained, deformable sediment bed, known as till, hosting an active hydrological system that routes meltwater. Observations show that the till undergoes substantial shear deformation as a result of the motion of the overlying ice. The deformation of the till, coupled with the dynamics of the hydrological system, is further affected by the substantial strain rate variability in subglacial conditions resulting from spatial heterogeneity at the bed. However, it is not clear if the relatively low magnitudes of strain rates affect the bed structure or its hydrology. We study how laterally varying shear along the ice-bed interface alters sediment porosity and affects the flux of meltwater through the pore spaces. We use a discrete element model consisting of a collection of spherical, elasto-frictional grains with water-saturated pore spaces to simulate the deformation of the granular bed. Our results show that a deforming granular layer exhibits substantial spatial variability in porosity in the pseudo-static shear regime, where shear strain rates are relatively low. In particular, laterally varying shear at the shearing interface creates a narrow zone of elevated porosity which has increased susceptibility to plastic failure. Despite the changes in porosity, our analysis suggests that the pore pressure equilibrates near-instantaneously relative to the deformation at critical state, inhibiting potential strain rate dependence of the deformation caused by bed hardening or weakening resulting from pore pressure changes. We relate shear variation to porosity evolution and drainage element formation in actively deforming subglacial tills.