Groups and Networks

The Lewisian, Torridonian and Moine of Scotland are basement (Lewisian) and cover rocks formed on the eastern margin of Laurentia between about 3000 Ma and 900 Ma (i.e. Archaean into the early Neoproterozoic). The Moine thrust separates relatively undeformed rocks to the west from those more severely deformed to the east. The Torridonian and Moine sequences represent at least two main phases of sedimentation that occurred from the late Mesoproterozoic to early Neoproterozoic times.

The Lewisian is exposed on the Outer Hebrides and also along the northwest coast of the Scottish mainland and is composed of a variety of gneisses, including the oldest rocks in the Britain, at c. 3000 Ma. The Lewisian Gneiss Complex contains a record of igneous, depositional and orogenic events that occurred over a period of 1400 million years. The Lewisian consists of several separate terranes which are characterized by protoliths of different ages, and by different Archaean and Palaeoproterozoic histories. During the latest Archaean and earliest Palaeoproterozoic, parts of the Lewisian Gneiss Complex were affected by granulite facies metamorphism (the ‘Scourian’ or ‘Badcallian’ event c. 2730-2490 Ma). A subsequent amphibolites facies deformation event, the Inverian event, had largely ended by the time of emplacement of the mafic dykes of the Scourie Dyke Suite. Following the Badcallian and Inverian tectono - thermal events, there was a period of crustal extension, possibly related to rifting. From about 2000 Ma, the extensional tectonic regime was replaced by the development of subduction zones and the onset of convergent tectonics, comprising successions of metamorphosed mafic volcanic rocks and a range of associated metasediments (i.e. the Loch Maree Group). Palaeoproterozoic, arc-related rocks also occur on the Outer Hebrides as the Ness gabbro–anorthosite complex, and the South Harris Igneous Complex (and associated metasediments in the Roineabhal Terrane). Subduction and formation of magmatic arcs was rapidly followed by a period of collisional tectonics (Laxfordian deformation) with associated magmatism and high pressure granulite facies metamorphism at around c. 1700 Ma. Following this Palaeoproterozoic (Laxfordian) orogenesis, there was a period of relative tectonic quiescence, during which the Lewisian gneisses were uplifted and exhumed.

Around 1300 Ma in the Mesoproterozoic, extension and rifting commenced once again marked by the deposition of the Stoer Group. This is a sedimentary sequence of c. 2 km thickness, comprising fluvial breccias, conglomerates, sandstones and siltstones, deposited in a fault-bounded rift basin. Towards the end of the Mesoproterozoic, extensional tectonics again gave way to a collisional regime, and a series of mountain belts (the Grenvillian orogeny) was formed around the globe from 1100 Ma to 950 Ma leading to amalgamation of the Rodinia supercontinent.

Towards the end of the Grenvillian Orogeny, the Torridon Group (deposited between 1000-970 Ma) was formed with up to 7 km of fluviatile, cross-bedded red sandstones deposited by braided river systems, flowing from the west, resting on the underlying Lewisian. The most recent interpretations view these sediments as accumulating in a foreland basin to the Grenville orogen, with sediment sources from the Labrador area of Canada.

The Moine Supergroup was also deposited during the early Neoproterozoic, and is now exposed to the east of the Moine Thrust Belt. The Moine Supergroup comprises a thick and extensive sequence of siliciclastic metasedimentary rocks, dominated by psammites with subordinate pelites. It is divided into three groups; the Morar, Glenfinnan and Loch Eil groups, deposited in fluvial and shallow marine environments. The Moine Supergroup was deposited unconformably upon a crystalline ‘Lewisianoid’ basement, with locally preserved basal conglomerates. The Moine was deposited between c. 950- 870 and recent interpretations view the Torridon and Morar groups as possible lateral equivalents. However, the Glenfinnan and Loch Eil groups may represent a distinctly younger succession. Although some sediment may have been derived from erosion of the Lewisian Gneiss Complex, much of it was probably derived from the Grenvillian mountain belt in NE Canada. The Moine succession contains evidence of at least four distinct tectonothermal events, with the Knoydartian and Morarian events in the Neoproterozoic.

Further Information

Bonsor, H.C., Strachan, R.A., Prave, A.R. & Krabbendam, M. (2012). Sedimentology of the early Neoproterozoic Morar Group in northern Scotland: implications for basin models and tectonic setting. Journal of the Geological Society, London, 169, 53-65.