Ice sliding over weak, fine-grained tills: Dependence of ice-till interactions on till granulometry

Abstract

Two fundamental aspects of ice-till interactions, the strength of the ice-till coupling and the vertical distribution of deformation in till, may be strongly dependent on till granulometry. In particular, results of theoretical analysis of several physical processes involved in such interactions suggest the following hypotheses: (1) fine-grained tills facilitate ice sliding with ploughing and little distributed deformation, and (2) coarse-grained tills facilitate strong ice-till coupling and relatively deep till deformation (~0.1 m). The theoretical analysis is limited to Coulomb-plastic tills under low subglacial effective stresses (0-100 kPa). Fine-grained tills are represented in the analysis by a clay-rich till from beneath Ice Stream B (ISB), West Antarctica, and a silty Pleistocene till from Ohio. For comparison, two coarse-grained, clast-rich tills are also considered (from beneath the Trapridge Glacier, Yukon, and the Breidamerkurjokull Glacier, Iceland). The mechanical condition for ice sliding over till is defined as the situation in which the strength of the ice-till interface is lower than the strength of the till itself. Model calculations predict that this condition is more likely to be met in fine-grained rather than coarse-grained tills because of (1) lower abundance of ploughing clasts (clast fraction ~0.01 vs. ~0.1), (2) widespread submergence of fine matrix particles even by a very thin basal water film (~10^(-6) m), and (3) greater susceptibility to interface smoothing due to ice-water surface tension. In addition, the theoretical analysis of ice-till interactions considers three potential mechanisms for distribution of deformation in tills of Coulomb-plastic rheology: (1) plastic deformation of till around a ploughing clast, which may affect till to depth of c. 2.7 to c. 4.5 times the clast diameter; (2) particle/clast bridging, which is typically observed to result in a shear-zone that is 10 times greater than the characteristic clast/particle diameter; and (3) vertical shear-zone migration due to water-pressure fluctuations. Combined, these three effects may result in distribution of a significant fraction of ice motion throughout ~0.1 m thickness of a coarse-grained, clast-rich till. However, lower clast abundance and smaller hydraulic diffusivity of a fine-grained till makes it a less favorable environment for significant strain distribution (predicted shear zone thickness ~0.01 m).

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