The thrust and shear zones of the Moine thrust zone and the NW Scottish Caledonides

Abstract

The Caledonian thrust belts of NW Scotland exhibit many features typical of foreland fold-and-thrust belts. The major structures propagated westwards towards the foreland, the thin carapace of Cambro-Ordovician sediments being accreted in imbricate and duplex zones beneath the major thrust sheets. On a broad scale, the rules of thrust geometry are obeyed but many local anomalies are found, particularly in the Assynt region. In Assynt, several large fault-bounded gravity-spreading structures, surge zones, have been identified. Extensional faults have been traced into strike-slip and then contractional faults. There are zones of extensional and contractional imbricate faults and extensional and contractional ductile flow. Similar structures have been found along the length of the Moine thrust belt, suggesting that gravity spreading was an important mechanism during the later stages of movement. Within the main Caledonian fold belt there are several major tectonic slides, which are recognized by a local increase in intensity of deformation. The majority are NW-verging and were originally gently dipping. In the Highlands SE of the Great Glen there are early major SE-verging recumbent folds associated with considerable crustal thickening reflected in high pressure (~12kbar) metamorphism. Many of the major slides may have formed by spreading away from this thickened crust. The Caledonian deformation and metamorphism occurred later in the Moines and Moine thrust zone than in the Dalradian and Grampian Moines to the SE of the Great Glen. This may reflect different levels of deformation; alternatively, it may argue for a large strike-slip displacement on the Great Glen fault, juxtaposing crustal blocks which had experienced different ages of Caledonian deformation and metamorphism. The late movement on the Moine thrust involved spreading from an area of thickened Caledonian crust formed either by the stair-step trajectory of the basal thrust to the Moines and, or, by thickening due to intrusion of large masses of late Caledonian granites