Modelling of till microstructure development during subglacial deformation using ring shear experiments

Abstract

Micromorphology has become a principal tool in glacial sedimentology providing a wealth of data on the geometry and kinematics of deformation histories and aiding our understanding of progressive and polyphase deformation. This approach has been applied to the detailed microstructural analysis of fabric development in nine samples of experimentally deformed till subjected to simple shear in a ring shear apparatus in order to model the progressive microfabric development during simulated subglacial deformation. Thin section analysis revealed that the relative strength and complexity of the clast microfabrics increases with increasing cumulative shear distance (0–1152 cm) and cumulative shear strain (0–549), whereas microfabric strength increases rapidly during the initial stages of shearing (cumulative strain up to ∼20) and remains relatively stable at higher cumulative strain (>200). In addition microfabric development is heterogeneous reflecting the partitioning of deformation at a microscale. A conjugate pattern of clast long axis alignment identified in the un-sheared sample taken at the start of the experiment formed during the initial consolidation and draining of the till. This emphasises that water-rich unconsolidated tills have the potential to be highly responsive to any applied stress even at very low strains. This un-sheared fabric was overprinted at a very early stage of the experiment when cumulative shear strains were below 5. The complex microfabrics formed during simple shear record the progressive development of up-shear dipping P-shears, down-shear dipping R-shears, and subhorizontal Y-shears. Sigmoidal fabric geometries, comparable to S–C and ECC-type fabrics, associated with the Riedel shears record a consistent sinistral sense of shear, i.e. the direction of shear imposed by the rotating plate of the ring shear. Observed changes in the relationships between these microfabrics record the switching between the localised imposition of a compressional fabric and P-shear formation, to subhorizontal shear and the formation of Y-shears accompanied by extension and the growth of low-angle R-shears, and back again. This process may potentially lead to alternating phases of “stick” (compression) and “slip” (subhorizontal shear) recognised as a key feature of glacier motion occurring in response to soft-sediment bed deformation. Multiple cycles of microstructure overprinting and rejuvenation pose a major challenge on determining the cumulative strain of subglacially deformed tills of unknown history from micromorphological signatures alone.