Microstructures and microshears as proxy for strain in subglacial diamicts: Implications for basal till formation

Abstract

Ring-shear experiments were used to study subglacial sediment deformation and the development of S-matrix microstructures and microshears to constrain glacially induced deformation in diamicts. The experiments approximated subglacial conditions, and the diamict was sheared and sampled incrementally to strains of 0, 7, 18, 33, 57, and 107. At strains between 7 and 18, a steady association of S-matrix microstructures developed, which did not evolve further despite continued shearing. The I_L-index, a measure of microshear-orientation strength parallel to the shearing direction correlates log-linearly with strain, which indicates its potential use as strain proxy. The ring-shear data were compared with the characteristics of Quaternary basal tills from Denmark, Poland, and Svalbard. Based on I_L-index values, we conclude that these tills only experienced strain of ∼10¹, which is orders of magnitude lower than expected for deformation tills. This suggests that pervasive subglacial deformation and sediment advection in the mobile layer at the ice-bed interface may be less significant than previously assumed.