TheCaledonian orogeny was amountain-building cycle recorded in the northern parts of theBritish Isles, theScandinavian Caledonides,Svalbard,eastern Greenland and parts of north-central Europe. The Caledonianorogeny encompasses events that occurred from theOrdovician to EarlyDevonian, roughly 490–390 million years ago (Ma). It was caused by the closure of theIapetus Ocean when theLaurentia andBaltica continents and theAvaloniamicrocontinent collided.
The orogeny is named forCaledonia, theLatin name forScotland. The term was first used in 1885 byAustrian geologistEduard Suess for an episode of mountain building in northern Europe that predated theDevonianperiod. Geologists likeÉmile Haug andHans Stille saw the Caledonian event as one of several episodic phases of mountain building that had occurred duringEarth's history.[2] Current understanding has it that the Caledonian orogeny encompasses a number oftectonic phases that can laterally bediachronous, meaning that different parts of the mountain range formed at different times. The name "Caledonian" can therefore not be used for an absolute period of geological time, it applies only to a series of tectonically related events.
In theNeoproterozoic most of the Earth's landmasses were united in theRodiniasupercontinent. The majority of its bulk consisted of the landmass ofGondwana.[Note 1] Near the end of the Neoproterozoic, during the breakup of this supercontinent,Laurentia[Note 2] andBaltica[Note 3]rifted from the western (Amazonian craton) and northern (African) margins of Gondwana respectively.
Laurentia firstdrifted westward away from Gondwana and then migrated northward. This led to theopening of theIapetus Ocean between Laurentia, Baltica and Gondwana. Its initial opening phase was between the adjacent Laurentia and Baltica (to the West and East respectively) and caused the two to breakup c. 615 Ma[3] or 590 Ma.[4] Then the part between Laurentia and Gondwana (to the east), opened c. 550 Ma.[Note 4] Further spreading of the Iapetus Ocean also caused Laurentia and Baltica to move away from each other.
Baltica drifted northward, too. This involved the opening of theTornquist Ocean which separated it from the northern margin of Gondwana to the south. The onset of Baltica rifting and the Tornquist Ocean opening are difficult to date due to insufficientpalaeomagnetic data but must have occurred in similar times as those of Laurentia and the Iapetus Ocean.[4]
Either in the LatePrecambrian or EarlyOrdovician,[Note 5] theAvaloniamicrocontinent[Note 6] started to drift northwestward from the northern margin of Gondwana (Amazonia and northwest Africa) close to the original position of Baltica which had been to its north. Its rifting involved the opening andspreading of theRheic Ocean to its south, which separated it from Gondwana. This rifting and opening were coeval with and may be related tosubduction onset in the Iapetus Ocean.[5] The drift of Avalonia was towards the positions where Baltica and Laurentia had been in the Ordovician; these continents were by then further north. It also involved the consumption of both the Iapetus Ocean and the Tornquist Ocean along its northern margin.
Avalonia's motion was related toslab pull created by thesubduction of the Iapetus Ocean beneath the margin of Laurentia to its northwest and possibly also byridge push created by the spreading of the Rheic Ocean. It migrated across the Iapetus Oceanorthogonally (at aright angle).[6] Its drift included an up to 55°counterclockwise rotation with respect to the subduction zone to its north, mainly in the 470–450 Ma timeframe.[7] It moved significantly faster than Baltica but slowed down to a rate comparable to that of the latter in the Late Ordovician when it got close to it.[8]
The main phases of the Caledonian orogeny resulted from the convergence of Baltica, Laurentia and Avalonia which led to the closure of the Iapetus Ocean.
McKerrow et al. (2000) give a definition of the Caledonian orogeny which includes "all theCambrian,Ordovician,Silurian andDevonian tectonic events associated with the development and closure of those parts of the Iapetus Ocean which were situated between Laurentia (to the NW) and Baltica and Avalonia (to the SE and east) ... and each tectonic event throughout this 200 million years can be considered as an orogenic phase." This includes tectonic events which were smaller, localised and predated the more well-known main phases of this orogeny.
In this definition, theTaconic andAcadian orogenies in what today is North America are included in the phases of the Caledonian orogeny.
Some early phases ofdeformation andmetamorphism are recognised in theScandinavian Caledonides. The first phase that is often included in the Caledonian orogeny is theFinnmarkian Orogeny, which was an early deformation event in Arctic (northern)Norway which preceded the Scandian phase (see below) in this area. Its onset has been dated at c. 500 Ma (LateCambrian). It continued to c. 460 Ma and was reactivated in the Scandian phase at ~425–415 Ma.[9][10]
According to van Roermund and Brueckner (2004), there was a distinct orogenic event which was separate and slightly younger than that of the Finnmarkian one, which they dated at 455 Ma. They named it theJämtlandian Orogeny. It involved the Seve Nappe Complex of the Swedish Caledonides in centralSweden, which is interpreted as the stretched outermost edge of Baltica. Contrary to the previous opinion that it had been subducted beneath an oceanicisland arc, they propose that it involved a collision with a continental fragment.
TheShelveian Orogeny occurred particularly in the Shelve area inShropshire, in easternWales and in theEnglish Midlands in the LateOrdovician and was related to theTaconic orogeny. It formed the ShelveAnticline and Rytton CastleSyncline and was the most important tectonic event in the area between theCambrian andDevonian.Folding was accompanied by late stageigneous intrusions. The event caused a majorunconformity in Shropshire with considerable erosion before the deposition of sediments in theLlandovery Epoch of theSilurian (444–443 Ma). There was no break in sediments in the area until the end of the EarlyDevonian, which was caused by the Acadian Orogeny in theBritish Isles.[11] It was associated withdextral (right-lateral)strike-slip movement in the Pontesford-Linleyfault system and folding in pre-Ashgill strata,uplift of the adjacent Towi Anticline and igneous activity.[7][12]
The main orogenic events or phases of the Caledonian orogenic cycle were related to the final closure of the Iapetus Ocean. They were, in sequential order, the Grampian phase, the docking of Eastern Avalonia with Baltica, the Scandian phase and the Acadian phase. The latter involved: A) the docking of England and Wales (which were part of eastern Avalonia) with eastern and southern Ireland with Scotland and the rest of Ireland (which were part of Laurentia). B) the amalgamation of terranes of Western Avalonia with the eastern margin of the main landmass of Laurentia (seeAcadian orogeny article for this orogeny).
During the final part of its northwestward migration, Avaloniaconverged with Baltica and Laurentia to its northeast and northwest respectively. After its amalgamation with Eastern Avalonia, Baltica converged with Laurentia in a westward direction. The combined convergence of this microcontinent and the two continents createdcontinental collisions between them, the mentioned orogenic events and the closure of the Iapetus and Tornquist oceans.
Continental collisions started in the MidSilurian and mountain building and ended in the EarlyDevonian (420–405 Ma).[13][14]
TheGrampian orogeny involved collisions between two landmasses of Laurentia and an oceanicisland arc in the Iapetus Ocean outboard the main margin of the Laurentia tectonic plate (the future North America). There two Laurentian landmasses were Scotland and northern and westernIreland. The other parts of the British Isles (England andWales and the rest of Ireland) were part of the Avalonia microcontinent.
Two parts of Avalonia have been distinguished, a western and an eastern one. The term Western Avalonia refers to the westernmost part of the microcontinent which amalgamated the east coast of the main part of theLaurentiatectonic plate (what is now North America) to the west in the area of the northernAppalachians and theMaritimes. Eastern Avalonia refers to a) the part which amalgamated withBaltica, b) England a Wales and eastern and south-eastern Ireland which amalgamated with Scotland and the north and west of Ireland (which were part of Laurentia).[15]
The easternmost part of Eastern Avalonia amalgamated with Baltica through an oblique soft docking governed bydextralstrike-slipconvergence andshear, rather than through an orogen-causing hardcontinental collision. This is indicated by the absence of orogenic structures or high-pressuremetamorphic rocks, which are either not present or buried. This event occurred close to the end of theOrdovician, 440 Ma.[16] It docked with the Baltica margins in southernDenmark, the south-western corner of theBaltic Sea andPoland. It came to compriseSilesia inPoland, northernGermany, theNetherlands,Belgium and part of north-easternFrance (theArdennes Mountains).
TheAnglo-Brabant massif orLondon-Brabant Massif in central and southernEngland and inBelgium is a largebasement massif.[Note 7] It is part of amagmatic belt which, starting from theLake District, to the north of this massif, bears record of the subduction of part of the Tornquist Sea beneath Avalonia and its closure. The closure of theRheic Ocean, which took place soon after, occurred through subduction along the southern margin of this massif.[17]
TheTrans-European Suture Zone or Tornquist Zone is the area of thesuture of Baltica and Eastern Avalonia. It runs from a portion of theNorth Sea close toDenmark, through southern Denmark, a portion of theBaltic Sea between Denmark andPoland (by Germany'sRügen Island), and through Poland. It then follows the eastern margin of theEastern Carpathian Mountains in westernUkraine. Finally, it runs to theBlack Sea. However, in theSudetes Mountains and the Eastern Carpathians, it evolved through theVariscan and theAlpine orogenies, rather than the Caledonian one.[18]
The Scandian phase involved a collision between easternGreenland on the eastern margin of Laurentia and the margin of the Baltoscandian platform of theFennoscandian peninsula of Baltica. It involved theScandinavian Caledonides in what is now Norway and the Swedish areas by its border. It occurred from theWenlock Epoch of theSilurian to the MidDevonian (430–380 Ma). Gee et al. (2013) and Ladenberger et al. (2012) propose a revised onset dating set at 440 Ma, however, there is no consensus about this.[19]
The Scandian orogenic event also led to the formation of mountains ofQueen Louise Land (or Dronning Louise Land) in north-easternGreenland. It is an exposed N–S trendingthrust zone which marks the western limit of intense Caledonian deformation. The dominant structures are interpreted as having resulted fromsinistraltranspression, which involvedstrain partitioning of regional deformation between sinistralstrike-slip movements in the east and NW-directed obliquethrusting andfolding further to the west.[20]
This orogenic event also affectedScotland and the outerHebrides, causing thrusting in the NorthernHighlands which culminated in the development of theMoine Thrust Belt,Ben Hope Thrust and Naver-Sgurr Beag Thrust (435–420 Ma)[21] and led toigneous intrusion inGalloway and theSouthern Uplands (c. 400 Ma) in Scotland and the enlargement of theLake Districtbatholith in northernEngland. All this spanned theIapetus Suture zone (see below). It also caused northeast trending strike-slip faults, such as theGreat Glen Fault which affected theMoine Supergroup and theDalradian rocks in Scotland and theShetland Islands through theWalls Boundary Fault, which is the northeast-ward extension of the Great Glen Fault.[22]
As mentioned above, theBritish Isles were separated and belonged to two different tectonic plates: Laurentia (Scotland and northern and westernIreland) and Avalonia (England andWales and the rest of Ireland). The EarlyDevonian Acadian event in this area saw the amalgamation of these landmasses to form theBritish Isles as they are now. This occurred through NW-dippingsubduction of Avalonianoceanic crust beneath the southern margins of the Laurentian landmasses.
Since the 1980s the termAcadian, which referred to the LateSilurian to EarlyDevonian orogeny in the NorthernAppalachians, and theMaritime Provinces ofCanada has been applied to the early Devonian deformation phase in the British Caledonides by analogy with the one that occurred in what is nowNorth America.[17] Late Caledonian orogeny is another term used in reference to this phase.
This phase involved a soft docking or soft collision rather anorogen-causing hardcontinental collision like the Eastern Avalonia docking with Baltica.
This orogenic event has been interpreted as a late Caledonian phase and as having been driven by the closure of theIapetus Ocean. However, there is also an argument that it would more appropriate to regard it as a proto-Variscan orogeny. This is because this Devonian event postdated the collision of Avalonia with Laurentia by 15–20 million years and was coeval with the early phase of the Variscan orogeny (Eo-Variscan or Ligerian) and because it was not related to the Iapetus Ocean.[17]
It also has been argued that, although the Acadian orogeny in the British Isles involved the Iapetus Ocean closure, its driving force was actually a push from the south caused by the northward subduction of theRheic Ocean which lay to the south of Avalonia and separated it fromGondwana. The closure of this ocean involved the (early) Eo-Variscan collision of Gondwana-related terranes in which Eastern Avalonia was peripherally involved.[17][23]
Subduction of theIapetus Ocean occurred beneath theMidland Valleyterrane of Scotland. There is a Trans-Suture Suite ofintrusive plutons which straddle both sides of the trace of theIapetus Suture in theSouthern Uplandsterrane ofScotland (to the north of the suture) and the Lakesman-Leinster terrane of northernEngland and easternIreland (to the south of the suture) which were at the Laurentia and Avalonia margins respectively. The emplacement of the plutons occurred after the subduction of the Iapetus Ocean ended.[24][25]
The Southern Uplands terrane is thought to be anaccretionary wedge. Deep marine sedimentation here in response to subduction begun 455 Ma and marked the switch from an initial SE-dipping Iapetussubduction under Avalonia to a NW-dipping one beneath Laurentia. About 430 Ma accretion in the Southern Uplands and Ireland switched from beingorthogonal (at a right angle) to a sinistrally (left-lateral)transpressive one as indicated bycleavage transectingfolds counterclockwise.Turbidite deposition in theoceanic trench overlapped onto the Lakesman-Leinster terrane. Laurentia-Avalonia convergence and Iapetus Ocean subduction ceased by C. 420 Ma as indicated by a mid-Silurian weakening of deformation in the accretionary wedge.[26]
Magma production should be larger inconvergent tectonic regimes duringsubduction and markedly reduced with the change to post-subductioncollisional regimes. However, during Iapetussubduction (455–425 Ma) this was low andintrusive rocks were largely absent across all terranes in the concerned area in this period. Most Acadianmagmatism occurred post-subduction (425-390 Ma) in a regional tectonic setting with alternatingtranspression andtranstension phases. High rates of magma generation coincided with a c. 418–404 Ma Early Devoniansinistral transtension phase. This decreased during the 404–394 Ma Acadian transpression.[26]
In addition, the Southern Uplands accretionary wedge lacks evidence of the presence of avolcanic arc as usually found near subduction zones.[27] This has led to the hypotheses that arc rocks were eroded and thus have not been preserved, that the arc was displaced by lateral movement alongstrike-slip faults or that this is due toflat–slab subduction, which reducesmagmatism rates.[26]
Nelison et al. (2009) propose an Iapetus Ocean subductingslab breakoff model to account for the intrusive rocks in the Grampian terrane being emplaced post-subduction. However, Miles at al. (2016) note that the intrusive rocks in the Trans-Suture Suite and in all the terranes in the region are similar in age and geochemistry. Thus, they argue that the common mechanism for the whole region involved an Iapetus Oceanslab which did not just break off. It also peeled back below theIapetus Suture for c. 100 km to the SE below Avalonia. Thus they invoke a model of slab drop-off caused bylithospheric mantledelamination.
The Lakesmanterrane covers the north ofEngland down to theWensleydale inNorth Yorkshire and crosses theIrish Sea passing by the Island ofAnglesey offWales. Its continuation in easternIreland is the Leinster terrane. The combined terrane is termed Leinster-Lakesman terrane. It lies on the southern margin of theIapetus Suture. It includes theLake District and theIsle of Man.
The Acadian Orogeny affected the Lakesman terrane and northWales.Transpression resulted in regionally clockwise transectingsinistral transpressivecleavages which were superimposed on pre-existing structures.Folding northwest of the Iapetus Suture is weak and this northward weakening of deformation may indicate that it is linked withRheic Oceansubduction rather than Iapetus Ocean closure.[26]
The Lake District in north-westernEngland was at the north-western margin of the English part of Eastern Avalonia which converged and collided with Scotland and was thus involved in the Acadian phase. Generally, Acadian deformationmetamorphosedmudrocks throughout various geologic formations of the district intoslates by creating slatycleavages.[28]
The EarlyPalaeozoic rocks in theIsle of Man in theIrish Sea crop out close to or probably onIapetus suture. The island lies immediately to its SE.
The island is composed mainly of theManx Group and theDalby Group which were deformed in asinistraltranspression zone during the sinistral,oblique closure of the Iapetus Ocean.Folds are transected clockwise by theircleavage, major strike-parallel sinistral faults andductileshear zones thought to be related to this transpression.[35] All primary folds have the same style and are associated with the same regional cleavage suggesting that they are roughly coeval. There is ductile deformation in some localities and a broadshear zone in theLangness Peninsula which deform the primary cleavage and are thought to have formed during or soon after the main deformation phase.[36]
The Dalby Group wasoverthrust onto the Manx Group, probably in the early Devonian. During the final stage of the Iapetus Ocean closure itsturbidites were deposited from the NE into a marine basin which bridged theAvalonia andLaurentia margins. The tectonic contact between the two groups has been correlated either with theWindermere Supergroup (Lake District) turbidites or the Riccarton Group, (Southern Uplandsterrane).The former hypothesis implies that the Dalby Group was originally deposited on the Manx Group and was subsequently faulted into its present day relationship. The latter one implies that it is the toe end of theSouthern Uplands turbiditeaccretionary wedge onlapping or thrust onto the Avalonia continental margin.[37]
The broad deformation style and age of the Manx Group are very similar to the equivalent features of theSkiddaw Group in the Lake District and theRibband Group in SE Ireland. This group is thought to be their regional equivalent. It underwent two main deformation phases which also affected the Dalby Group: a) a pervasive slaty cleavage associated with gently to moderately plunging folds which also affected many of the minorigneous intrusions, b) a gently dippingcrenulationcleavage associated with small foldsverging towards thebedding dip direction.[31]
There are several ductile shear zones which run subparallel to the Manx Group northeast-oriented boundary faults which indicate predominantly sinistral shear and possibly a transition fromorthogonal compression totranspression during the later stages of Acadian deformation. This makes the island more similar to the Southern Uplands terrane of Scotland than the Lake Districtinlier in this respect.[31]
In Ireland the Acadian Orogeny affected the four mainterranes of the island: Grampian, Midland Valley, Longford-Down and Leinster. Tectonic deformation was mild as the collision was stronglyoblique withsinistraltranspression and without substantialcrustal thickening.Devonian toCarboniferous rocks restunconformably onCambrian toSilurianfolded andcleaved rocks. There wereigneous intrusions with plutons andbatholiths.
The terrane has three relief belts. The northern belt and the northernmost part of the Central Belt underwentpure shear deformation with an axial planarcleavage and a stretchinglineation perpendicular to the fold hinges. The Southern Belt and the rest of the Central belt underwent sinistraltranspression. This reflects a LateOrdovician–Silurian change from anorthogonal to anoblique tectonic plate collision. In the Central Belt the cleavage transects folds in a clockwise sense and is accompanied by a sub-horizontal stretching lineation. In the Southern belt the TinureFault is the surface expression of the Iapetus Suture zone.[38]
TheIapetus Suture is thelineament where the Caledonian collision closed the Iapetus Ocean. In Ireland it runs from the estuary of theRiver Shannon on the Atlantic coast toClogherhead on theIrish Sea. It crosses this sea and is exposed in theNiarbyl Fault in the southern part of the northern coast of theIsle of Man. In Britain it runs roughly parallel to theAnglo-Scottish border. It consists of a series of faults with no traces ofsubduction, such asophiolite remnants oroceanic trench-derived rocks.[42]
The Iapetus Suture also extends along the margin of the Baltoscandian platform of theFennoscandian Peninsula which collided with the eastern margin ofGreenland along the eastern margin of Laurentia in the Scandian orogeny.
According to some authors, the Caledonian continental collisions involved another microcontinent,Armorica (southernPortugal, most of the north ofFrance and parts of southern Germany and theCzech Republic), even smaller than Avalonia.[43] This microcontinent probably did not form one consistent unit, but was instead a series of fragments, of which the currentArmorican andBohemian Massifs are the most important. The ocean between the combined continental mass of Laurentia, Baltica and Avalonia (calledEuramerica, Laurussia or Old Red Continent) and Armorica is called theRheic Ocean.
The paleogeographic position of the Armorica crustal fragments between the Ordovician andCarboniferous is highly disputed though. There are indications that the Bohemian Massif started moving northward from the Ordovician onward,[44] but many authors place the accretion of the Armorican terranes with the southern margin of Laurussia in the CarboniferousVariscan orogeny (about 340 million years ago). TheRhenohercynian basin, aback-arc basin, formed at the southern margin of Euramerica just after the Caledonian orogeny. According to these authors, a small rim from Euramerica rifted off when this basin formed. The basin closed when these Caledonian deformed terranes were accreted again to Laurussia during the Hercynian orogeny.[45]
