Sliding versus Till Deformation in the Motion of an Ice Stream Over Deformable Till

Throstur Thorsteinsson, and C. F. Raymond
Geophysics Program, University of Washington, Seattle, WA 98195, USA

Abstract.  The partitioning between till deformation and sliding in the fast flow of ice streams is examined assuming no adhesion of the till to the ice base and viscous fluid behavior for the till. We find for expected deformable till thickness of 10 m or less that the contribution to 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 sub-glacial till, unless there is a mechanism to continuously generate short scale roughness.

Method.  Our work addresses the partitioning of the basal motion between till deformation and slip at the ice base. The primary assumption is that till-ice boundary is a sharp, well-lubricated interface, which may be expected under conditions in which there is net heat flow to the base of the ice (Engelhardt, Humphrey and Kamb, 1990), melting of the ice and lack of adhesion of till components to it. In this circumstance, stress is transmitted between the ice and till by roughness elements of the ice-till interface. The resulting mean shear stress can cause deformation through the thickness of the till. Slip in the presence of the roughness elements can occur by local deformations in the till, which we analyze using the classic theory of ice sliding (Nye, 1969) with the primary modification that the ice is assumed to be rigid and the till to deform like an incompressible viscous fluid, thus reversing the role of ice and bed. In this regard, both the shearing through the till thickness and the slip at the ice till interface are mediated by deformation in the till. Both mean till deformation and slip increase with increasing softness of the till. The primary control variables affecting the relative amount of sliding and till deformation are the interface roughness and the vertical thickness of the till that can deform. We explore how the ratio of sliding to till deformation depends on these two variables and examine some implications concerning motions in the till imposed by the sliding.

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Figures from paper, 3 here, show the contribution of sliding and deformation depending on till layer thickness and ice-till interface roughness, and then 2 showing the instantaneous heat generation for λ = 0.3 m and λ = 30 m.

27 jan. 2003 © Þrõstur Þ.