An experimental baseline for ice-till strain indicators

Abstract

Subglacial till can deform when overriding ice exerts shear traction at the ice-till interface. This deformation leaves a strain signature in the till, aligning grains in the direction of ice flow and producing a range of diagnostic microstructures. Constraining the conditions that produce these kinematic indicators is key to interpreting the myriad of features found in basal till deposits. Here we use a cryogenic ring shear device with transparent sample chamber walls to slip a ring of temperate ice over a till bed from which we examine the strain signature in the till. We use cameras mounted to the side of the ring shear and bead strings inserted in the till to estimate the strain distribution within the till layer. Following completion of the experiment, we extract and analyze AMS samples and create thin sections of the till bed for microstructure analysis. We then compare the AMS and microstructures with the observed strain history to examine the relationship between kinematic indicators and strain in a setting where shear traction is supplied by ice. We find that AMS fabrics show a high degree of clustering in regions of high strain near the ice-till interface. In the upper most zone of till, k1 eigenvector azimuths are generally aligned with ice flow, and S1 eigenvalues are high. However, S1 eigenvalues and the alignment of the k1 eigenvector with ice flow decrease nonlinearly with distance from the ice-till interface. There is a high occurrence of microshears in the zone of increased deformation.