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Abstract

A combination of field investigation and micromorphological analysis has been applied to polydeformed Late Devensian rhythmites and glacigenic diamicton, exposed in Strathspey, Scotland. This provided information on the geometry, kinematics and relative ages of ductile and brittle structures, and records a complex subglacial deformation history. The deformation is interpreted as resulting from a single progressive event, associated with over-riding of proglacial lake sediments by wet-based ice. The earliest deformation (‘D1’) resulted from compaction/loading (pure shear) and imposed a bedding-parallel (S1) fabric throughout the rhythmites. S1 was subsequently deformed by kink bands and minor ductile shearing during ‘D2’. A later ‘D3’ event, characterized by soft-sediment deformation and fluidization of matrix-poor sands, was accompanied by an increase in pore water pressure. This lead to hydrofracturing of the rhythmites. The most intense deformation (‘D4’), which resulted from simple shear, was partitioned into the upper part of the sequence. It produced folding, thrusting and brittle microfaulting in response to NNW-directed ice-push. These findings indicate that, in general, subglacial deformation is not homogeneous and can extend to depths of >3 m below the presumed ice-sediment interface.

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