The strain and textural history of thin-skinned tectonic zones: examples from the Assynt region of the Moine thrust zone, NW Scotland

Abstract

The Moine thrust zone at Assynt is a classical example of a foreland propagating, thin-skinned thrust zone, ideal for an examination of thrust-related folds, deformation textures and strains and complex incremental strain history. Divergent transport directions, oblique trending folds, duplex zones and extensional strains normal to the main transport direction may all be explained in terms of thrust propagation, leading to the development of oblique to lateral ramps. The majority of thrusts cut up section from basement to cover in the transport direction but there is also localised extensional flow and thinning of the thrust sheets. In northern Assynt, the thrust zone involves a wide vertical zone of sinistral shear, within which forethrusts, backthrusts and associated folds and cleavages are oblique to the general transport direction. It is suggested that north of this shear, the thrusts moved further, probably under a thicker cover, while to the south, movement was more intermittent, probably under a thinner cover. This variation is probably due to a change in thrust geometry in the Moines, east of the Moine thrust zone, causing a variation in gravitational potential along the length of the Moine thrust.