Microfabric and microshear evolution in deformed till

The bed-deformation model of glacier movement requires that ice masses shear their beds to high strains. For homogeneous basal tills that can result from such deformation, however, there are no sedimentological criteria for estimating strain magnitude. Thus, a large ring-shear device was used to shear two basal tills, and their particle fabrics and microshears were studied at various strains up to 108. Sand-particle fabrics in a longitudinal flow plane became steady at shear strains of 7-39; steady fabrics were moderately strong, with S₁ eigenvalues of 0.71-0.74. These values are smaller than those from experiments with pebble-sized particles, perhaps reflecting the greater probability of sand particles colliding with larger grains. Fabrics parallel to flow strengthened progressively, indicating that particles did not reorient with their long axes transverse to flow. R₁ and R₂ microshears developed and persisted to the highest strains attained. R₁ shears became more pervasive with strain and oriented closer to the shear plane. Movement along both sets of shears provides an explanation for the plunge of the steady-state sand fabric, approximately 10° "upglacier." These experiments provide a basis for estimating till-deformation magnitude, but results will require cautious application to the geologic record where deformation processes may be ambiguous.