Sliding versus till deformation in the fast motion of an ice stream over a viscous till

Abstract

The partitioning between till deformation and sliding in the fast flow of ice streams with active basal melting is examined assuming no adhesion of the till to the ice base and incompressible viscous fluid behavior for the till. For deforming-till thickness of 10 m or less the predicted contribution to basal motion by sliding is larger than shearing in the till unless there is short-scale roughness with wavelengths less than order 0.1 m on the ice sole. At such short scales strain heating within the till and focused melting on the ice sole would quickly eliminate the roughness. Thus, fast flow over a till bed would be expected to be mostly by sliding over the subglacial till. More realistic continuum behavior of the till including non-linear and compressible deformation strengthens the conclusion. If sliding is not dominant, then there must be adhesion of the till to the ice base, some mechanism that continuously generates short-scale roughness on the ice-till interface, or very weak internal slip boundaries within the till.