Clast-fabric development in a shearing granular material: Implications forsubglacial till and fault gouge

Elongate clasts in subglacial till and in fault gouge align during shearing , but the relation between clast-fabric strength and cumulative shear strai n for such materials is effectively unknown, This relation was explored in experiments with a large ring-shear de,ice in which a till and a viscous pu tty that contained isolated clasts were sheared to high strains. As expecte d, rotation of clasts in the putty is closely approximated by the theory of G.B. Jeffery, who derived the orbits of rigid ellipsoids in a slowly shear ing fluid. Clast rotation in the till, however, is strikingly different, Ra ther than orbiting through the shear plane as predicted by Jeffery, most cl asts rotate into the shear plane and remain there, resulting in strong fabr ics regardless of the aspect ratios and initial orientations of clasts, Thi s divergent behavior is likely due to slip of the till matrix along the sur faces of clasts, which is a natural expectation in a granular material but violates the no-slip condition of Jeffery's model, These results do not sup port the widespread belief that subglacial till deformation results in weak clast fabrics. Thus, many tills with weak fabrics thought to have been she ared sub-glacially to high strains, like many basal tills of the Laurentide Ice Sheet, may have been sheared only slightly with little effect on eithe r ice-sheet dynamics or sediment transport. In addition, these results indi cate that in simple sheer the rotation of clasts in till and in fault gouge is best analyzed with the model of A. March, who treated inclusions as pas sive markers.