TECTONOTHERMAL EVOLUTION OF THE NEOPROTEROZOIC GRAMPIAN AND APPIN GROUPS, SOUTHWESTERN MONADHLIATH MOUNTAINS, SCOTLAND

Metasedimentary rocks of the Neoproterozoic Grampian and Appin groups exposed in the Glen Roy district (Monadhliath Mountains, Scotland) wer e strongly folded during an initial progressive tectonothermal event o f the Caledonian orogeny. The attitude and style of the first folds va ries from open, upright NE-trending structures in the west (e.g. the A ppin Synform) to recumbent NW-facing nappes in the east (e.g. the Trei g Synform). These early nappes occur only south of the Corrieyairack C omplex and their amplitude decreases towards the NW. Ductility contras ts induced sliding such as that locally recorded along the Grampian-Ap pin group lithostratigraphical boundary. Subsequently the early struct ures were deformed by tight, upright 'D2' folds. NW-thrusting along th e Eilrig Shear Zone, during 'D2', is interpreted as occurring in respo nse to a transfer of bulk transport to deeper structural levels relati ve to 'D1'. Prograde metamorphism accompanied deformation, with the th ermal peak in the Glen Roy district (T = 500-600 degrees C; P = 7.0-8. 0 kbar) coinciding with 'D2' folding. It is suggested that the regiona l variation in the style of deformation was controlled, at least in pa rt, by pre-existing major structures, most notably fundamental, transv erse basement lineaments. Temporal changes in the style of deformation occurred in response to changing crustal architecture during progress ive deformation. The study demonstrates the diachronous nature of defo rmation and also illustrates how its is possible to preserve areas wit h simple deformation histories in the centre of a major orogenic belt.