The role of grain behaviour in subglacial deformation

Khatwa, Anjana (1999) The role of grain behaviour in subglacial deformation. University of Southampton, Doctoral Thesis.

Abstract

The aim of this thesis is to examine subglacial deformation through the interaction of grains at the micro-scale and characterise such behaviour to rheological deformation.

Research on grain behaviour continued in the form of micromorphological analysis. This study has developed a new semi-automated technique for microfabric analysis using digitising and an orientation program. For thin section analysis, a modified micromorphological classification scheme is proposed, which utilises geological terminology rather than soil science based nomenclature. The scheme also incorporates a new suite of structures identified during the course of this study. These new structures are prominently rotational in behaviour and are present across a wide range of Quaternary diamictons and contemporary sediment facies. Among many findings, subglacial deformation microstructures have been identified in sediments interpreted to be lodgement tills. The most intriguing microstructure found during this study is a grain plaster feature. This is a matrix layer that surrounds the grain and comes in many forms. Microfabric analysis on the plaster layer revealed an oriented fabric parallel to the grain edge, suggesting a rotational forming mechanism. Studies of grain behaviour across sequences exhibiting increasing homogenisation were also conducted. Micro-scale features are found to survive homogenisation, unlike macro-scale structures, by dispersion and diffusion into the surrounding matrix where strain is distributed.

Grain behaviour can be used to characterise rheological behaviour. During plastic deformation grain bridging is a dominant mechanism where threshold failure points are reached quickly by grain fracture. As grain size breaks down the matrix cushions interparticle stresses, so that components in the bridge slide out and disperse into the deforming matrix. As deformation becomes distributed across the finer matrix a transition to steady state viscous (or pseudo-viscous) rheology occurs. In this environment grains rotate through the fabric as individual components, dispersing and diffusing entrained bedrock material.

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