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Tectonothermal evolution of the Neoproterozoic Grampian and Appin groups, southwestern Monadhliath Mountains, Scotland

¹ British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, UK
² British Geological Survey, Sir Kingsley Dunham Centre, Keyworth, Nottingham NG12 5GG, UK

Metasedimentary rocks of the Neoproterozoic Grampian and Appin groups exposed in the Glen Roy district (Monadhliath Mountains, Scotland) were strongly folded during an initial progressive tectonothermal event of the Caledonian orogeny. The attitude and style of the first folds varies from open, upright NE-trending structures in the west (e.g. the Appin Synform) to recumbent NW-facing nappes in the east (e.g. the Treig Synform). These early nappes occur only south of the Corrieyairack Complex and their amplitude decreases towards the NW. Ductility contrasts induced sliding such as that locally recorded along the Grampian-Appin group lithostratigraphical boundary. Subsequently the early structures were deformed by tight, upright ‘D2’ folds. NW-thrusting along the Eilrig Shear Zone, during ‘D2’, is interpreted as occurring in response to a transfer of bulk transport to deeper structural levels relative to ‘D1’. Prograde metamorphism accompanied deformation, with the thermal peak in the Glen Roy district (T = 500–600 °C; P = 7.0–8.0 kbar) coinciding with ‘D2’ folding. It is suggested that the regional variation in the style of deformation was controlled, at least in part, by pre-existing major structures, most notably fundamental, transverse basement lineaments. Temporal changes in the style of deformation occurred in response to changing crustal architecture during progressive deformation. The study demonstrates the diachronous nature of deformation and also illustrates how its is possible to preserve areas with simple deformation histories in the centre of a major orogenic belt.

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