Evidence for pre-Caledonian discontinuities in the Achnashellach Culmination, Moine Thrust Zone: the importance of a pre-thrust template in influencing fold-and-thrust belt development

Abstract

South of the Loch Maree Fault in the Northwest Highlands of Scotland, an unexplained step-wise thickening of Torridon Group strata, from c. 200–1000 m, occurs towards the Loch Maree Fault, within the trailing edges of the stacked thrust sheets of the Achnashellach Culmination in the Caledonian Moine Thrust Zone. This thickening cannot be explained readily by Caledonian thrust tectonics alone, and suggests that thrusting was superimposed upon a pre-Caledonian non-layer-cake template giving rise to a thrust-parallel thickness change of Torridon Group strata in the thrust belt. Cross-section constructions within the culmination constrain discrete abrupt thickness changes of the Torridon Group succession preserved within the Coire nan Clach and Toll Ban thrust sheets. We infer the existence of a pre-existing discontinuity in the form of either a set of pre-Caledonian faults striking parallel or sub-parallel to the long-lived Loch Maree Fault in its southwestern wall, or palaeovalleys creating locally greater thicknesses of Torridon Group sediments in the pre-thrust template. Such palaeovalleys may have been eroded along pre-existing discontinuities. In either case, these discontinuities will have contributed to generating step-wise thickness changes in preserved Torridon Group strata prior to Cambro-Ordovician overstep and then contributed to controls on the observed (lateral) variations in thrust architecture and the northwards step-wise thinning of the Achnashellach Culmination towards the Loch Maree Fault. This northern termination of the Achnashellach Culmination demonstrates the importance of the pre-thrust template in constraining the three-dimensional architecture of lateral changes within fold-and-thrust belts.