LABORATORY STUDIES OF TILL MECHANICAL BEHAVIOR AND FABRIC EVOLUTION DURING SHEAR

Shear deformation of subglacial till has been invoked widely to explain large sediment fluxes from some Pleistocene ice masses and genesis of subglacial landforms. Empirical studies of this process have usually involved either measurements beneath modern glaciers, in which only a small fraction of the bed can be studied, or studies of the geologic record, which is more accessible but difficult to interpret. In contrast, laboratory studies of till deformation have been relatively few, reflecting the weak experimental tradition in glacial geomorphology relative to most other disciplines of Earth science. In 1995, a ring-shear device that shears a large till specimen (0.6 m O.D., 80 mm thickness, 125 mm width) at either constant shear rates or stresses was constructed to help fill this void.

Experiments reveal unexpected mechanical behavior of till. Experiments at constant shear stresses illustrate that pore-water pressure is reduced during increases in shear rate, due to the effect of shear rate on dilation rate, which can result in slow episodic shear without external changes in forcing. Thus, pore-water pressure is not necessarily independent of shear rate, as is usually assumed. Similarly, during experiments designed to study plowing of clasts through the bed surface, pore-pressure diffusion does not keep pace with the rate of till compaction in front of clasts, resulting in pore-water pressure well in excess of hydrostatic that greatly reduces plowing resistance. Since such clasts couple a glacier to its soft bed, this process will weaken the ice-bed coupling, particularly at high sliding speeds, and cause slip of ice over the bed.

Ring-shear experiments also provide new techniques for interpreting the geologic record. Experiments on tills conducted to various shear strains allow till structural characteristics to be calibrated to shear strain. Macroscopic, elongate particles in shearing till align parallel to the shearing direction to form a strong fabric at shear strains of about 2. Alignment of clay-mineral particles in till, measured using high-resolution, X-ray texture goniometry, increases progressively up to shear strains as large as 140. These experiments, by linking till attributes to shear-strain magnitude, allow determination of the extent to which basal tills of the geologic record have been sheared.