Major late Caledonian and Hercynian shear movements on the Great Glen Fault

Synthesis of palaeomagnetic data from Ordovician-Devonian rock formations of North Scotland, involving “original” as well as overprinted magnetizations, has uncovered two major phases of transcurrent motion on the Great Glen Fault. It is inferred that the first of these displacements occurred in the late Middle Devonian and was of left-lateral character: it led to a minimum of 35° clockwise rotation of the Northern Highlands, corresponding to an offset of the order of 600 km along the Great Glen Fault. The second major movement took place during Hercynian time, amounting to c. 300 km in the dextral sense. The new reconfiguration of Scotland prior to the late Middle Devonian gives supporting evidence for late Proterozoic and Caledonian zonal correlations between the Central and Northern Highlands, as well as being compatible with various features of the Old Red Sandstone rocks of the Orcadian Basin. A recent proposal of a c. 2000 km sinistral offset along the Great Glen Fault in the Carboniferous is shown to be wrong, but the new evidence for superimposed offsets in opposite directions opens new perspectives for reconsidering megascale mid-late Palaeozoic displacements along the North Atlantic orogenic belt.