The Hebridean Terrane is one of the

terrane In geology, a terrane (; in full, a tectonostratigraphic terrane) is a crust fragment formed on a tectonic plate (or broken off from it) and accreted or " sutured" to crust lying on another plate. The crustal block or fragment preserves its ow ...

s that form part of the Caledonian orogenic belt in northwest

Scotland Scotland (, ) is a country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a border with England to the southeast and is otherwise surrounded by the Atlantic Ocean to ...

. Its boundary with the neighbouring Northern Highland Terrane is formed by the Moine Thrust Belt. The basement is formed by Archaean and

Paleoproterozoic The Paleoproterozoic Era (;, also spelled Palaeoproterozoic), spanning the time period from (2.5–1.6 Ga), is the first of the three sub-divisions ( eras) of the Proterozoic Eon. The Paleoproterozoic is also the longest era of the Earth's ...

gneiss Gneiss ( ) 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. Gneiss forms at higher temperatures a ...

es of the

Lewisian complex The Lewisian complex or Lewisian gneiss is a suite of Precambrian metamorphic rocks that outcrop in the northwestern part of Scotland, forming part of the Hebridean Terrane and the North Atlantic Craton. These rocks are of Archaean and Pale ...

, unconformably overlain by the

Neoproterozoic The Neoproterozoic Era is the unit of geologic time from 1 billion to 538.8 million years ago. It is the last era of the Precambrian Supereon and the Proterozoic Eon; it is subdivided into the Tonian, Cryogenian, and Ediacaran periods. It is prec ...

Torridonian In geology, the term Torridonian is the informal name for the Torridonian Group, a series of Mesoproterozoic to Neoproterozoic arenaceous and argillaceous sedimentary rocks, which occur extensively in the Northwest Highlands of Scotland. The st ...

sediments, which in turn are unconformably overlain by a sequence of Cambro–

Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. T ...

sediments. It formed part of the

Laurentia Laurentia or the North American Craton is a large continental craton that forms the ancient geological core of North America. Many times in its past, Laurentia has been a separate continent, as it is now in the form of North America, althoug ...

n foreland during the Caledonian

continental collision In geology, continental collision is a phenomenon of plate tectonics that occurs at convergent boundaries. Continental collision is a variation on the fundamental process of subduction, whereby the subduction zone is destroyed, mountains produ ...

Extent

The Hebridean Terrane forms the westernmost strip of mainland Scotland, most of the

Inner Hebrides The Inner Hebrides (; Scottish Gaelic: ''Na h-Eileanan a-staigh'', "the inner isles") is an archipelago off the west coast of mainland Scotland, to the south east of the Outer Hebrides. Together these two island chains form the Hebrides, whi ...

and all of the

Outer Hebrides The Outer Hebrides () or Western Isles ( gd, Na h-Eileanan Siar or or ("islands of the strangers"); sco, Waster Isles), sometimes known as the Long Isle/Long Island ( gd, An t-Eilean Fada, links=no), is an island chain off the west coas ...

. Similar rocks are also thought to be present in

Shetland Shetland, also called the Shetland Islands and formerly Zetland, is a subarctic archipelago in Scotland lying between Orkney, the Faroe Islands and Norway. It is the northernmost region of the United Kingdom. The islands lie about to the n ...

and have been proved west and north of the Outer Hebrides by BGS shallow boreholes and

hydrocarbon exploration Hydrocarbon exploration (or oil and gas exploration) is the search by petroleum geologists and geophysicists for deposits of hydrocarbons, particularly petroleum and natural gas, in the Earth using petroleum geology. Exploration methods Visi ...

wells. The full extent of this terrane to the west is obscured by the effects of

Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretace ...

rift In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics. Typical rift features are a central linear downfaulted depression, called a graben, or more commonly a half-grabe ...

ing.

Rock units

Lewisian complex

The Lewisian complex consists of mainly

granitic A granitoid is a generic term for a diverse category of coarse-grained igneous rocks that consist predominantly of quartz, plagioclase, and alkali feldspar. Granitoids range from plagioclase-rich tonalites to alkali-rich syenites and from quartz- ...

gneisses, subject to a series of metamorphic and tectonic events, interrupted by the intrusion of a major

dyke swarm A dike swarm (American spelling) or dyke swarm (British spelling) is a large geological structure consisting of a major group of parallel, linear, or radially oriented magmatic dikes intruded within continental crust or central volcanoes ...

Scourian complex

The

protolith A protolith () is the original, unmetamorphosed rock from which a given metamorphic rock is formed. For example, the protolith of a slate is a shale or mudstone. Metamorphic rocks can be derived from any other kind of non-metamorphic rock an ...

s for the Scourian complex were mainly

plutonic rocks intruded in the interval 3.0–2.7 Ga. They were metamorphosed to

granulite facies Granulites are a class of high-grade metamorphic rocks of the granulite facies that have experienced high-temperature and moderate-pressure metamorphism. They are medium to coarse–grained and mainly composed of feldspars sometimes associated ...

and deformed towards the end of the Archaean. At the beginning of the Proterozoic, the Scourian gneisses were locally affected by deformation and retrogression to

amphibolite facies Amphibolite () is a metamorphic rock that contains amphibole, especially hornblende and actinolite, as well as plagioclase feldspar, but with little or no quartz. It is typically dark-colored and dense, with a weakly foliated or schistose ...

in the Inverian event, which overlapped in time with the emplacement of the Scourie dykes.

Scourie dykes

This swarm of mainly

dolerite Diabase (), also called dolerite () or microgabbro, is a mafic, holocrystalline, subvolcanic rock equivalent to volcanic basalt or plutonic gabbro. Diabase dikes and sills are typically shallow intrusive bodies and often exhibit fine-grain ...

dykes, postdates the Scourian deformation and metamorphism and has therefore been used to identify the later Laxfordian event. Many of the dykes were intruded into hot

country rock Country rock is a genre of music which fuses rock and country. It was developed by rock musicians who began to record country-flavored records in the late 1960s and early 1970s. These musicians recorded rock records using country themes, vocal ...

Laxfordian event

This event is associated with the development of

shear zones Boudinaged quartz vein (with strain fringe) showing ''Fault (geology)">sinistral shear sense'', Starlight Pit, Fortnum Gold Mine, Western Australia In geology, shear is the response of a rock to deformation usually by compressive stress and f ...

, in which the Scourie dykes occur as concordant

amphibolite Amphibolite () is a metamorphic rock that contains amphibole, especially hornblende and actinolite, as well as plagioclase feldspar, but with little or no quartz. It is typically dark-colored and dense, with a weakly foliated or schistose (flaky ...

sheets. This deformation was accompanied by retrogression to amphibolite facies and subsequently locally to

greenschist facies Greenschists are metamorphic rocks that formed under the lowest temperatures and pressures usually produced by regional metamorphism, typically and 2–10 kilobars (). Greenschists commonly have an abundance of green minerals such as chlorite, ...

Torridonian sediments

The Torridonian is a sequence of Neoproterozoic sediments, mainly sandstones that rest

unconformably An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval ...

on an old land surface, with up to 300 m of relief locally. The Torridonian is divided into the older Stoer Group and the younger Sleat and Torridon Groups separated by an angular unconformity. Paleomagnetic data suggest that this unconformity represents a major time break. These sediments are interpreted to have been deposited during a period of

ing.

Stoer Group

The Stoer Group outcrops on the peninsula of Stoer, near

Assynt Assynt ( gd, Asainn or ) is a sparsely populated area in the south-west of Sutherland, lying north of Ullapool on the west coast of Scotland. Assynt is known for its landscape and its remarkable mountains, which have led to the area, along with ...

Sutherland Sutherland ( gd, Cataibh) is a historic county, registration county and lieutenancy area in the Highlands of Scotland. Its county town is Dornoch. Sutherland borders Caithness and Moray Firth to the east, Ross-shire and Cromartyshire (later c ...

. A basal

breccia Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix. The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of ...

is present in many areas with large clasts derived from the underlying Lewisian. The breccia passes up into muddy sandstones, often with well-preserved desiccation structures, and into deposits of a

braided river A braided river, or braided channel, consists of a network of river channels separated by small, often temporary, islands called braid bars or, in English usage, ''aits'' or ''eyots''. Braided streams tend to occur in rivers with high sediment ...

system, trough cross-bedded sandstones and conglomerates. A thin sequence of siltstones and fine sandstones alternate with muddy sandstones, suggesting deposition in a

lacustrine A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much larger ...

environment. The uppermost part of the sequence consists of trough cross-bedded sandstones thought to have been deposited by braided rivers.

Sleat Group

The Sleat Group, which outcrops on the Sleat peninsula on

Skye The Isle of Skye, or simply Skye (; gd, An t-Eilean Sgitheanach or ; sco, Isle o Skye), is the largest and northernmost of the major islands in the Inner Hebrides of Scotland. The island's peninsulas radiate from a mountainous hub dominated ...

, underlies the Torridon Group conformably, but the relationship with the Stoer Group is nowhere exposed. It is presumed to have been in deposited later that the Stoer Group, but possibly in a separate sub-basin. 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-grained feldspathic sandstones deposited in a fluvial environment with some less common grey

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especiall ...

s, probably deposited in a lacustrine environment.

Torridon Group

The Torridon Group infills an irregular land surface with up to 600 m of topography locally, cutting down through the previously deposited Stoer group sediments, resting in many areas directly on the Lewisian. The lowest part of this group consists of a basal breccia passing up into sandstones and subordinate shales deposited as

alluvial fan An alluvial fan is an accumulation of sediments that fans outwards from a concentrated source of sediments, such as a narrow canyon emerging from an escarpment. They are characteristic of mountainous terrain in arid to semiarid climates, but a ...

s building out into ephemeral lakes. The upper parts of the group consist of an overall fining-upward sequence of sandstones, interpreted to have been deposited in a bajada environment.

Cambro-Ordovician sediments

This thin strip of Lower Paleozoic strata lie unconformably on both the Lewisian complex and the Torridonian. The sequence is divided into the lower

Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ...

Ardvreck Group overlain by the Durness Group of Cambrian to lower

age.

Ardvreck Group

The ca. 200 m thick lower Cambrian Ardvreck Group consists of two formations, the basal

Eriboll Formation Eriboll (Scottish Gaelic: Earabol) is a village in Sutherland, Scotland. The village is situated on the south eastern shore of Loch Eriboll, in the northern part of the former county of Sutherland. The main A838 coast road connects the villages ...

and the overlying An-t-Sron Formation. The Eriboll Formation contains the Basal Quartzite member, with a basal

conglomerate Conglomerate or conglomeration may refer to: * Conglomerate (company) * Conglomerate (geology) * Conglomerate (mathematics) In popular culture: * The Conglomerate (American group), a production crew and musical group founded by Busta Rhymes ** ...

overlain by the heavily bioturbated Pipe Rock member with its distinctive ''

skolithos ''Skolithos'' (formerly spelled ''Scolithus'' or ''Skolithus'') is a common trace fossil ichnogenus that is, or was originally, an approximately vertical cylindrical burrow. It is produced by a variety of organisms in shallow marine environmen ...

trace fossil A trace fossil, also known as an ichnofossil (; from el, ἴχνος ''ikhnos'' "trace, track"), is a fossil record of biological activity but not the preserved remains of the plant or animal itself. Trace fossils contrast with body fossils, ...

s. The An-t-Sron Formation consists of calcareous siltstones of the Fucoid Beds and overlying sandstone of the Salterella Grit.

Durness Group

The Durness group lies conformably on the Ardvreck Group. It consists of several hundred metres of mainly

dolomite Dolomite may refer to: *Dolomite (mineral), a carbonate mineral *Dolomite (rock), also known as dolostone, a sedimentary carbonate rock *Dolomite, Alabama, United States, an unincorporated community *Dolomite, California, United States, an unincor ...

s with some

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms w ...

s and

chert Chert () is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a ...

s, although the effects of later faulting make precise estimates of thickness difficult. It is subdivided into seven formations.

History

The earliest event recorded in the terrane is the intrusion of the

s to the Scourian gneisses at about 3.0–2.7 Ga. Blocks within the Lewisian complex were then juxtaposed by Inverian and Laxfordian deformation from about 2.4–1.7 Ga. At about 1200 Ma, the terrane was affected by

extensional tectonics Extensional tectonics is concerned with the structures formed by, and the tectonic processes associated with, the stretching of a planetary body's crust or lithosphere. Deformation styles The types of structure and the geometries formed depend ...

causing rifting and the deposition of the thick coarse clastic sequence of the Torridonian. This was interrupted by a tilting event of unknown origin that caused a hiatus of about 200 Ma, followed by further deposition. At the start of the Cambrian, there was a marine transgression and deposition of shallow marine sandstone and carbonates continuing into the Ordovician. In the

Silurian The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleoz ...

the terrane became involved in the Caledonian continental collision, with the Highland Terrane being

thrust Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that ...

over this segment of the Laurentian foreland.

References

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