The geological evolution of the metamorphic Caledonides during the 100-million-year period from the phase of crustal thickening initiated by the Grampian orogeny to the emplacement of late granites at c. 400 Ma is reviewed. Shortening and westward transport associated with the Grampian orogeny were diachronous; deformation began in the weak Dalradian basin and extended north-westward into the area of crystalline Moine rocks. An important change in tectonic regime, coinciding roughly with the closing of the Iapetus Ocean, was recorded at c. 425 Ma by the onset of sinistral transcurrent motions on NE–SW lines parallel to the Caledonian belt. Regional uplift of the thickened crustal welt, accompanied by deep erosion, led to the removal of 20–30 km of overburden from the Highlands, mainly in Ordovician and Silurian times. Known Lower Palaeozoic deposits of Scotland cannot account for the volume of sediment so produced and it is suggested that other basins were located in a major transform zone beneath the northern North Sea.
The onset of late Caledonian magmatism coincided with the start of the post-collisional regime of strike-slip motion. The distribution of granites and appinites was influenced by transverse (NW–SE) dislocations in the lower crust and the emplacement of individual intrusions was often fault-controlled. The deep transverse lineaments acted as conduits for both sub-crustal and crustal melts over periods of up to 25 million years. It is suggested that mantle rocks metasomatized by fluids generated at an underlying subduction zone were the main source of the sub-crustal melts, and that magmatism was triggered from above by vertical and horizontal block movements.
This paper is a contribution to IGCP Project 27.