The Old Red Sandstone (Middle Devonian) Orcadian basin was formed as a consequence of extensional collapse of the Caledonian orogen. Onshore study of these collapse-basins in Orkney and Shetland provides directions of extension during basin development. The origin of folding of Old Red Sandstone sediments, that has generally been related to a Carboniferous inversion phase, is discussed: syndepositional deformation supports a Devonian age and consequently some of the folds are related to basin formation. Large-scale folding of Devonian strata results from extensional and left-lateral transcurrent faulting of the underlying basement. Spatial variation of extension direction and distribution of extensional and transcurrent tectonics fit with a model of regional releasing overstep within a left-lateral megashear in NW Europe during late-Caledonian extensional collapse.
Later inversion (probably during the Upper Carboniferous) is characterized by E–W to NE–SW contraction. It induced reactivation of extensional faults as thrusts, development of small-scale folds and thrusts, and right lateral transcurrent movement of the major faults such as the Great Glen and Walls Boundary faults.