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54-591: 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

108-572: A broad textural category for medium- to coarse-grained metamorphic rock that shows poorly developed schistosity, with compositional layering over 5 millimeters (0.20 in) thick and tending to split into plates over 1 centimeter (0.39 in) thick. Neither definition depends on composition or origin, though rocks poor in platy minerals are more likely to produce gneissose texture. Gneissose rocks thus are largely recrystallized but do not carry large quantities of micas, chlorite or other platy minerals. Metamorphic rock showing stronger schistosity

162-461: A distinct cleavage . Gneisses are common in the ancient crust of continental shields . Some of the oldest rocks on Earth are gneisses, such as the Acasta Gneiss . In traditional English and North American usage, a gneiss is a coarse-grained metamorphic rock showing compositional banding ( gneissic banding ) but poorly developed schistosity and indistinct cleavage . In other words, it

216-655: 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. 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

270-504: A series of igneous intrusions in the Assynt Window and partly from the results of radiometric dating of deformed rocks from the footwall or hanging-wall of the thrusts. The intrusions near Loch Borralan and Loch Ailsh have similar chemistry, both being forms of syenite , and have been dated at 430±4 Ma and 430.6±0.3 Ma respectively suggesting that they were intruded at about the same time. The Loch Ailsh intrusion outcrops within

324-454: Is a common and widely distributed type of metamorphic rock . It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks . This rock is formed under pressures anywhere from 2 to 15 kbar, sometimes even more, and temperatures over 300 °C (572 °F). Gneiss nearly always shows a banded texture characterized by alternating darker and lighter colored bands and without

378-475: Is a gneiss resulting from metamorphism of granite, which contains characteristic elliptic or lenticular shear-bound grains ( porphyroclasts ), normally feldspar , surrounded by finer grained material. The finer grained material deforms around the more resistant feldspar grains to produce this texture. Migmatite is a gneiss consisting of two or more distinct rock types, one of which has the appearance of an ordinary gneiss (the mesosome ), and another of which has

432-488: Is a metamorphic rock composed of mineral grains easily seen with the unaided eye, which form obvious compositional layers, but which has only a weak tendency to fracture along these layers. In Europe, the term has been more widely applied to any coarse, mica -poor, high-grade metamorphic rock. The British Geological Survey (BGS) and the International Union of Geological Sciences (IUGS) both use gneiss as

486-663: 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

540-518: Is called gneissic banding. The darker bands have relatively more mafic minerals (those containing more magnesium and iron ). The lighter bands contain relatively more felsic minerals (minerals such as feldspar or quartz , which contain more of the lighter elements, such as aluminium , sodium , and potassium ). The banding is developed at high temperature when the rock is more strongly compressed in one direction than in other directions ( nonhydrostatic stress ). The bands develop perpendicular to

594-449: Is classified as schist, while metamorphic rock devoid of schistosity is called a granofels . Gneisses that are metamorphosed igneous rocks or their equivalent are termed granite gneisses, diorite gneisses, and so forth. Gneiss rocks may also be named after a characteristic component such as garnet gneiss, biotite gneiss, albite gneiss, and so forth. Orthogneiss designates a gneiss derived from an igneous rock , and paragneiss

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648-406: Is evidence of deformation of the syenite while it was still hot, suggesting that thrusting started soon after it was intruded. In respect of it being 'the classic orogenic front of significant importance in both modern and historical tectonics research', the International Union of Geological Sciences (IUGS) included the 'Moine Thrust Zone' in its assemblage of 100 'geological heritage sites' around

702-592: 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 . In addition to the Sole Thrust at the base of the northern section of the structure and the Moine Thrust at the top of

756-517: Is one from a sedimentary rock . Both the BGS and the IUGS use gneissose to describe rocks with the texture of gneiss, though gneissic also remains in common use. For example, a gneissose metagranite or a gneissic metagranite both mean a granite that has been metamorphosed and thereby acquired gneissose texture. The minerals in gneiss are arranged into layers that appear as bands in cross section. This

810-572: Is probably derived from the Middle High German noun gneist "spark" (so called because the rock glitters). Gneiss is used as a building material, such as the Facoidal gneiss. It's used extensively in Rio de Janeiro . Gneiss has also been used as construction aggregate for asphalt pavement . Wester Ross Supergroup The Wester Ross Supergroup is one of the subdivisions of

864-510: Is subjected to extreme temperature and pressure and is composed of alternating layers of sandstone (lighter) and shale (darker), which is metamorphosed into bands of quartzite and mica. Another cause of banding is "metamorphic differentiation", which separates different materials into different layers through chemical reactions, a process not fully understood. Augen gneiss , from the German : Augen [ˈaʊɡən] , meaning "eyes",

918-710: Is truncated by the SW–NE trending Strathcarron Fault. It may originally have been continuous with the Kishorn Thrust. The Kishorn Thrust extends from the Achnashellach Culmination, just north of Loch Carron , where it is truncated by the Strathcarron Fault. It continues southwestwards to Loch Kishorn , becoming the basal thrust. On the Isle of Skye , the Kishorn Thrust also marks the base of

972-795: 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

1026-794: The Neoproterozoic sequence of sedimentary rocks (or their metamorphic equivalents) in the Scottish Highlands . It lies unconformably on medium to high-grade metamorphic rocks and associated igneous rocks of the Archaean and Paleoproterozoic age Lewisian complex or locally over the Mesoproterozoic sedimentary rocks of the Stoer Group . The contact between the Wester Ross Supergroup and

1080-737: 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

1134-469: The Assynt Window, it becomes the sole thrust structure, before the Kinlochewe Thrust branches off to the south. This thrust forms the base of the belt from Eriboll to the southern end of the Assynt Window where it merges with the Moine Thrust itself. It is thought to be the youngest formed thrust structure in the belt, which is consistent with a foreland propagating thrust sequence. Along strike

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1188-576: The Ben More thrust sheet and predates movement on that structure. The Loch Borralan intrusion lies beneath the Ben More Thrust. Locally the intrusion has been said to cut across the Ben More Thrust, but a lack of the metamorphic effects found elsewhere along the margin of the intrusion suggest that this contact is tectonic rather than intrusive. In the upper part of the Borralan intrusion, there

1242-657: The Caledonian thrust structures. Further deep profiles acquired in the same area, the DRUM and GRID lines confirmed these features. Onshore in Shetland the Wester Keolka Shear was proposed to represent the Moine Thrust, although more recent radiometric dating results suggest that this is unlikely to be the case. The link between the structures imaged offshore and those onshore remains unclear. Southwest of Skye

1296-661: The Hebridean Terrane. 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. The Torridon and Sleat groups are of Neoproterozoic age and consists mainly of sandstone with

1350-511: The Hebridean Terrane. Unlike the other thrusts, there is a broad zone (up to 600 m in thickness) of the Morar Group in the hanging-wall that is intensely deformed into mylonite, indicating that it originated at a significantly deeper crustal level. Total displacement on this one structure has been estimated as several tens of kilometres based on the change in crustal level and the thickness of mylonites developed. In its central section, south of

1404-576: The Iona and Tarskavaig groups and those on Shetland are likely but not proven. The Torridon Group is the main part of what used to be termed the "Torridonian", although that also included the Stoer Group, which is now known to be a completely separate and older sequence. The Sleat Group, which outcrops on the Sleat peninsula on Skye , underlies the Torridon Group conformably, but the relationship with

1458-584: The MOIST (Moine and Outer Isles Seismic Traverse) deep seismic profile was acquired north of the Scottish mainland by the BIRPS group in 1981 a series of east-dipping fault zones were imaged, two of which were interpreted as potential continuations of the Moine Thrust. The presence of half graben basins in the hanging walls of these faults was interpreted to represent Devonian and Mesozoic extensional reactivation of

1512-723: The Moine Thrust Belt bringing mainly Torridonian sandstones over foreland Cambrian-Ordovician limestones and quartzites. Locally on the Sleat peninsula the Kishorn thrust sheet is eroded to form the Ord window in which the foreland rocks once more appear. At the southwestern end of the Sleat peninsula, the Tarskavaig Thrust carries Tarskavaig Group metasediments over the Sleat Group of the underlying Kishorn thrust sheet. When

1566-608: 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. 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

1620-611: The Stoer Group is nowhere exposed. It is metamorphosed to greenschist facies and sits within the Kishorn Nappe, part of the Caledonian thrust belt , making its exact relationship to the other outcrops difficult to assess. The sequence consists of mainly coarse feldspathic sandstones deposited in a fluvial environment with some less common grey shales , probably deposited in a lacustrine environment. This sequence of variably metamorphosed and deformed sandstones (psammites)

1674-485: The Tarskavaig Thrust on the southwestern end of the Sleat peninsula. The group consists of a mixture of psammites, semi-pelites and pelites , which are deformed and no sedimentological analysis has been carried out, nor have the group been sampled for detrital zircons. From its structural position it has been tentatively correlated with the lower part of the supergroup. The Westing, Sand Voe and Yell Sound groups on Shetland are tentatively correlated with other groups within

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1728-455: The Wester Ross Supergroup based on lithological similarities, the ages of the youngest detrital zircons and the timing of later metamorphism. The age of this sequence is constrained by a combination of detrital zircon geochronology and by the dating of later metamorphic events. The youngest detrital zircon and rutile ages are in the range 1070–1000 Ma, providing an upper bound. Metamorphic ages of 950–940 Ma have been calculated for garnets from

1782-458: The amphibolite or granulite facies. These form most of the exposed rock in Archean cratons . Gneiss domes are common in orogenic belts (regions of mountain formation). They consist of a dome of gneiss intruded by younger granite and migmatite and mantled with sedimentary rock. These have been interpreted as a geologic record of two distinct mountain-forming events, with the first producing

1836-538: The appearance of an intrusive rock such pegmatite , aplite , or granite the ( leucosome ). The rock may also contain a melanosome of mafic rock complementary to the leucosome. Migmatites are often interpreted as rock that has been partially melted, with the leucosome representing the silica-rich melt, the melanosome the residual solid rock left after partial melting, and the mesosome the original rock that has not yet experienced partial melting. Gneisses are characteristic of areas of regional metamorphism that reaches

1890-426: The belt, many other thrust faults are developed, some of which are large enough to be named and mapped more regionally. The thrust sheet carried by each thrust is named after the thrust beneath it, although the term " nappe " is also used. The Moine Thrust in the strict sense is the uppermost thrust in the belt and the one that carries the most displacement. In all cases it carries rocks of the Morar Group over rocks of

1944-416: The bottom of the deck in the other direction. These forces stretch out the rock like a plastic , and the original material is spread out into sheets. Per the polar decomposition theorem , the deformation produced by such shearing force is equivalent to rotation of the rock combined with shortening in one direction and extension in another. Some banding is formed from original rock material (protolith) that

1998-577: 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

2052-559: The course of the Moine Thrust Belt becomes uncertain. It is assumed that it must pass to the southeast of the mainly Lewisian islands of Coll and Tiree . It is often shown passing through the Sound of Iona west of Mull , although no thrusts are actually exposed. It is presumed to be truncated by the Great Glen Fault zone somewhere southwest of Mull. Movement on the Moine Thrust Belt has been dated partly on its relationship with

2106-402: The direction of greatest compression, also called the shortening direction, as platy minerals are rotated or recrystallized into parallel layers. A common cause of nonhydrodynamic stress is the subjection of the protolith (the original rock material that undergoes metamorphism) to extreme shearing force, a sliding force similar to the pushing of the top of a deck of cards in one direction, and

2160-486: 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. The Morar Group, like the Torridon Group,

2214-645: The footwall of the Sole Thrust changes from Lewisian in the south to Cambrian in the north. In the Eriboll section this thrust carries Lewisian gneiss over the Pipe Rock. Its outcrop is complicated by the effects of later thrusting that both folds and offsets the Arnaboll Thrust. The exposures on the flank of Ben Arnaboll have particular importance as it was here that Lapworth first described the highly deformed rock type mylonite and also where Geikie coined

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2268-595: The granite basement and the second deforming and melting this basement to produce the domes. However, some gneiss domes may actually be the cores of metamorphic core complexes , regions of the deep crust brought to the surface and exposed during extension of the Earth's crust . The word gneiss has been used in English since at least 1757. It is borrowed from the German word Gneis , formerly also spelled Gneiss , which

2322-483: The lower part of the Morar Group, confirming that these rocks were affected by the Renlandian Orogeny (960–920 Ma) and providing a lower bound. Together these data give a depositional age range of 1000–960 Ma. The Sleat and Torridon groups have been interpreted as deposited in a rift setting. However the scale of the interpreted river systems that deposited the Torridon Group rocks and the consistency in

2376-457: 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

2430-521: The middle amphibolite to granulite metamorphic facies . In other words, the rock was metamorphosed at a temperature in excess of 600 °C (1,112 °F) at pressures between about 2 to 24 kbar . Many different varieties of rock can be metamorphosed to gneiss, so geologists are careful to add descriptions of the color and mineral composition to the name of any gneiss, such as garnet-biotite paragneiss or grayish-pink orthogneiss . Continental shields are regions of exposed ancient rock that make up

2484-621: The mountain belt formed by the Grenville Orogeny. The Iona Group is exposed on the island of Iona and consists of two units separated by a zone of mylonite . The group has been assigned to the Wester Ross Supergroup based on its structural position beneath the Moine Thrust Zone and the ages of detrital zircons , which are similar to those found in the Sleat Group. Rocks of the Tarskavaig Group lie above

2538-564: The next youngest of the Neoproterozoic sequences in the Scottish Highlands, the Loch Ness Supergroup , is everywhere a tectonic one. The Wester Ross Supergroup consists of several groups that are geographically or structurally isolated from each other. The certainty of the correlation between the groups is variable, with the Torridon, Sleat and Morar groups considered as very likely to be lateral equivalents of each other, while

2592-427: The paleocurrent directions suggest that most of the sequence was deposited in a single basin. This is also true for the Morar Group, which has very similar thickness, sedimentary facies and interpreted paleocurrents to the Torridon Group. Combined with the detrital zircons that indicate a dominantly Grenvillian source, this has led to a reinterpretation that the supergroup was deposited within a foreland basin, in front of

2646-596: The stable cores of continents. The rock exposed in the oldest regions of shields, which is of Archean age (over 2500 million years old), mostly belong to granite-greenstone belts. The greenstone belts contain metavolcanic and metasedimentary rock that has undergone a relatively mild grade of metamorphism, at temperatures of 350–500 °C (662–932 °F) and pressures of 200–500 MPa (2,000–5,000 bar). The greenstone belts are surrounded by high-grade gneiss terrains showing highly deformed low-pressure, high-temperature (over 500 °C (932 °F)) metamorphism to

2700-461: The term "thrust plane". The Ben More Thrust is the largest and most continuous of the thrust faults developed between the Moine and Sole Thrusts within the Assynt Window. This thrust is developed within the lower part of the belt in the Assynt Window. The Kinlochewe Thrust branches off the Moine Thrust's central section and runs southwards past Kinlochewe to the Achnashellach Culmination, where it

2754-492: 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 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

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2808-480: The world in a listing published in October 2022. The organisation defines an IUGS Geological Heritage Site as 'a key place with geological elements and/or processes of international scientific relevance, used as a reference, and/or with a substantial contribution to the development of geological sciences through history.' 58°8′N 4°51′W  /  58.133°N 4.850°W  / 58.133; -4.850 Gneiss Gneiss ( / n aɪ s / nice )

2862-428: 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. The stratigraphic sequence of the footwall of the Moine Thrust is the full sequence characteristic of

2916-494: Was originally considered as separate from the "Torridonian" and formed the lower section of the Moine Supergroup . Comparisons between the Torridon Group with areas of lower strain in the Morar Group have found many similarities in lithology, thickness and interpreted depositional environment . This group is now thought to have been deposited as part of the same depositional basin, a foreland basin developed in front of

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