The Moine Thrust Belt or Moine Thrust Zone is a linear tectonic feature in the Scottish Highlands which runs from Loch Eriboll on the north coast 190 kilometres (120 mi) southwest to the Sleat peninsula on the Isle of Skye. The thrust belt consists of a series of thrust faults that branch off the Moine Thrust itself. Topographically, the belt marks a change from rugged, terraced mountains with steep sides sculptured from weathered igneous, sedimentary and metamorphic rocks in the west to an extensive landscape of rolling hills over a metamorphic rock base to the east. Mountains within the belt display complexly folded and faulted layers and the width of the main part of the zone varies up to ten kilometres (six miles), although it is significantly wider on Skye.
Discovery The presence of metamorphic gneisses and schists lying apparently stratigraphically above sedimentary rocks of lower Paleozoic age in the Northwest Highlands had been known since the early 19th century, convincing Roderick Murchison that the change was a purely metamorphic effect and that the upper gneiss was younger than the sediments beneath. Initially he was supported in this interpretation by Archibald Geikie and James Nicol. After further fieldwork, Nicol changed his mind and advocated instead that the contact at the base of the upper gneisses was tectonic, starting what was known as the Highlands Controversy. A tectonic interpretation was supported by, amongst others, Charles Lapworth who had corresponded with Albert Heim on similar structures in the Alps. In 1883 and 1884 the survey geologists Ben Peach and John Horne were sent into the area by the survey's director Archibald Geikie to carry out detailed mapping. The results of the mapping proved conclusively to Peach and Horne that the contact was tectonic and they were eventually able to persuade Geikie when he visited them briefly in the field in October 1884. In November that year Peach and Horne's preliminary results were published and Geikie published a paper in the same issue of Nature in which he coined the term "thrust-plane" for these low-angle faults, although the term was probably already in use before then. By 1888 the term "Moine Thrust" was being used for the tectonic break at the base of Moine schists (what is now called the Morar Group of the Wester Ross Supergroup). The Moine schists are named for the Moine peninsula in Sutherland. The recognition of the Moine Thrust Belt in the early 1880s was a milestone in the history of geology as it was one of the first thrust belts discovered and where the importance of large scale horizontal rather than vertical movements became apparent. Detailed mapping of the Moine Thrust Belt by the survey continued for another two decades, culminating in the classic survey memoir The Geological Structure of the North-West Highlands of Scotland, published in 1907.
Caledonian structure
The Moine Thrust Belt was formed during the Scandian orogenic phase Caledonian Orogeny cycle as part of the collision between Laurentia and Baltica. It is the most westerly Scandian structure in Scotland apart from the Outer Isles Fault in the Outer Hebrides, which is developed within the Hebridean terrane. The Moine Thrust Belt defines the boundary between the Hebridean terrane to its northwest and the Northern Highlands terrane to its southeast. The thrust carried metamorphic material over 200 km across Scotland entirely masking the geology of the previous terrane. However, small windows, such as the Assynt window and the Glen Achall imbricated thrust system, allow geologists to estimate what the geology of Scotland was like before the Caledonian Orogeny. The relationship between the Moine Thrust Belt and other Scandian age structures in Scandinavia and East Greenland remains unclear, due to uncertainties associated with the Great Glen Fault zone. This major sinistral (left-lateral) strike-slip fault was also active during the late stages of the orogeny, but continued to move during the early Devonian and appears to truncate the southern end of the thrust belt. The total late Caledonian displacement on the Great Glen Fault is poorly constrained, making reconstruction of the southern part of the orogenic belt difficult.
Involved rock units The stratigraphic sequence of the footwall of the Moine Thrust is the full sequence characteristic of the Hebridean terrane.
Lewisian complex The Lewisian complex consists of mainly granitic gneisses that are of Archaean and Paleoproterozoic age. They form the basement to both the Stoer Group, the Wester Ross Supergroup and the Loch Ness Supergroup of the Northern Highlands terrane, in both the footwall and hanging wall of the Moine Thrust.
Torridonian The Torridon and Sleat groups are of Neoproterozoic age and consists mainly of sandstone with a maximum preserved thickness of over 8 km. The unconformity at the base of these groups is highly irregular, showing that it was deposited on an eroded land surface.
Lower Palaeozoic The Cambrian to lower Ordovician rocks consist of two groups, the Ardvreck Group and the Durness Group. The Ardvreck Group lies above an angular unconformity over various parts of the Torridon Group and locally over the Lewisian. It is a sequence of mainly quartz arenites. The lowermost part of the Eriboll Formation, the Basal Quartzite Member, is often pebbly at its base. The overlying Pipe Rock Member is a distinctive quartz arenite with many white weathering skolithos trace fossils that act as strain markers in areas of more ductile deformation. The uppermost two parts of the Ardvreck Group form the An t-Sron Formation, with the dolomitic Fucoid Beds Member being overlain by the quartz arenites of the Salterella Grit Member. The succeeding Durness Group consists mainly of dolomites, with some limestone and chert. The distinctive character of this sequence enabled detailed mapping, even in areas of relatively poor exposure and allowed sections repeated by thrusting to be recognised.
Morar Group The Morar Group, like the Torridon Group, is of Neoproterozoic age and interpreted to be a lateral equivalent of that unit within the overall Wester Ross Supergroup. The Morar Group forms the lowest tectonostratigraphic unit of the Neoproterozoic metasediments, lying tectonically beneath the younger Loch Ness Supergroup.
Individual thrusts
… excerpt ends here. Continue reading the full article.
