Variations of strain and microstructure in folded Pipe Rock in the Moine Thrust Zone at Loch Eriboll and their bearing on the deformational history

Abstract

Synopsis Using deformed Skolithos pipes as strain markers, a folded sequence of Pipe Rock quartzites in the Heilam Nappe at Loch Eriboll is shown to have deformed dominantly by flexural flow. However, elliptical pipe sections on bedding evince a pre-folding strain. Microstructures observed in the hinge zone imply deformation dominated by the intercrystalline diffusion mechanism, pressure solution. On the limbs microstructural evidence for dislocation creep has almost obliterated features associated with pressure solution. It is concluded that the quartzite sequence deformed by pressure solution during an initial phase of layer parallel shortening and subsequently, when buckling was initiated and fold amplification occurred, the mechanism switched to dislocation creep. It is argued that this change in mechanism is related to changes in strain rate.