The Moine in NW Scotland
(The following text is adapted from an essay on Assynt Geology hosted at earth.leeds.ac.uk and written by Rob Butler, University of Aberdeen.)
Overlying the Durness carbonates in southern Assynt is a sequence of rocks that are now called the Moine. Some nineteenth century researchers, notably Murchison, considered the Moine to represent rocks younger than the Durness carbonates and that the boundary between them was simply depositional. But the Moine, although locally showing sedimentary features, has been metamorphosed and does not lie with depositional continuity upon the carbonates. Rather it has been carried on by a fault – the Moine Thrust – which represents the front of the Caledonian mountain belt.
The Moine sediments were deposited unconformably on continental crust very much like the Lewisian, rather like the Torridonian of the foreland. Some people consider the Moine and Torridonian to be equivalent. Detrital grains of zircon shows the Moine sediments to be derived from rocks older than about 1000 million years – so the Moine is younger than this. The oldest igneous rocks that intrude the Moine are dated at about 875 million years, so the Moine must be older than that. Unlike the Torridonian, the Moine sediments are widely believed to be marine deposits that probably accumulated on an ancient continental shelf. The Moine rocks of NW Scotland have all been strongly deformed and metamorphosed. Some geologists have suggested that the history of magmatism, metamorphism and deformation records a period of mountain building called the “Knoydartian”. Other geologists suggest that their history records rifting. Note - the Moine rocks are extremely complex and still not fully understood!
The main mountain building events in Scotland.
Geologists generally agree that the main event to effect the Moine after its deposition was a period of mountain building that happened about 470 to 450 million years ago that happened in response to thickening the old crust by two different mechanisms. One involved stacking of sheet upon sheet of crust, a process known as thrusting. The other involved large-scale folding, or nappe stacking. The thickened crust warmed up metamorphosing the Moine rocks (again). The deformation and metamorphism at this time formed part of a very much larger mountain belt – the Caledonian orogen. This mountain building period was complex and drawn out and finished in the collision between a southern continent (called Avalonia) and a northern one (called Laurentia), broadly along the modern England-Scotland border. The collision came about through the closure of an intervening ocean called Iapetus, in much the same way as the modern Himalayas and Tibet have formed in response to the collision between Eurasia and the Indian subcontinent with the closure of the Tethys ocean. Final collision was preceded by periodsof terrane/island arc accretion. There remains much controversy within the geological community as to how much of the framework of the British Isles is made up of a collage of distinct terranes, stitched together by the long-lasting Caledonian orogeny. The main deformation in the Scottish Highlands SE of the Great Glen Fault (called the Grampian) is about 470 million years old but may be younger (about 430 million years old) in the Moine, a phase of the Caledonian known as the Scandian.
The Moine Thrust Belt
Although the timing of deformation in the Moine is yet to be fully resolved, the whole lot was pushed westwards. The Moine has been carried onto rocks of the foreland during the late stages of Caledonian mountain building along a major fault, the Moine Thrust. Estimates suggest that this movement exceeded 100 km. Small scale structures along the thrust show that the Moine moved towards the WNW, although the region is very complex - early geologists termed the area a “zone of complication”! In some places thin strips of Cambrian strata have been stacked up many times. An original stack of Pipe Rock, about 75m thick, has been piled up to a thickness of over 800m. Thin sheets of Lewisian gneiss caught up in the Moine Thrust Belt have been carried for many tens of kilometres. The Moine Thrust Belt is generally taken to be the edge (or “front”) of the Caledonian mountain belt. Many other, younger, mountain belts (e.g. Alps, Himalayas) have similar features at their margins.