The Caledonian orogeny was a mountain-building cycle recorded in the northern parts of the British Isles, the Scandinavian Caledonides, Svalbard, eastern Greenland and parts of north-central Europe. The Caledonian orogeny encompasses events that occurred from the Ordovician to Early Devonian, roughly 490–390 million years ago (Ma). It was caused by the closure of the Iapetus Ocean when the Laurentia and Baltica continents and the Avalonia microcontinent collided.
The orogeny is named for Caledonia, the Latin name for Scotland. The term was first used in 1885 by Austrian geologist Eduard Suess for an episode of mountain building in northern Europe that predated the Devonian period. Geologists like Émile Haug and Hans Stille saw the Caledonian event as one of several episodic phases of mountain building that had occurred during Earth's history.McKerrow et al. (2002) Current understanding has it that the Caledonian orogeny encompasses a number of tectonic phases that can laterally be diachronous, 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.
Palaeogeographic evolution prior to the orogeny In the Neoproterozoic most of the Earth's landmasses were united in the Rodinia supercontinent. The majority of its bulk consisted of the landmass of Gondwana. Near the end of the Neoproterozoic, during the breakup of this supercontinent, Laurentia and Baltica rifted from the western (Amazonian craton) and northern (African) margins of Gondwana respectively.
Laurentia first drifted westward away from Gondwana and then migrated northward. This led to the opening of the Iapetus 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 MaWen et al. (2020) or 590 Ma.Robert et al. (2021) Then the part between Laurentia and Gondwana (to the east), opened c. 550 Ma. 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 the Tornquist 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 insufficient palaeomagnetic data but must have occurred in similar times as those of Laurentia and the Iapetus Ocean.
Either in the Late Precambrian or Early Ordovician, the Avalonia microcontinent 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 and spreading of the Rheic Ocean to its south, which separated it from Gondwana. This rifting and opening were coeval with and may be related to subduction onset in the Iapetus Ocean.Pollock et al. (2009) 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 to slab pull created by the subduction of the Iapetus Ocean beneath the margin of Laurentia to its northwest and possibly also by ridge push created by the spreading of the Rheic Ocean. It migrated across the Iapetus Ocean orthogonally (at a right angle).Keppie and Keppie (2014) 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.Pharaoh et al. (1993) 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.Torsvik & Rehnström (2003)
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.
Early orogenic phases McKerrow et al. (2000) give a definition of the Caledonian orogeny which includes "all the Cambrian, Ordovician, Silurian and Devonian 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, the Taconic and Acadian orogenies in what today is North America are included in the phases of the Caledonian orogeny.
Some early phases of deformation and metamorphism are recognised in the Scandinavian Caledonides. The first phase that is often included in the Caledonian orogeny is the Finnmarkian 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 (Late Cambrian). It continued to c. 460 Ma and was reactivated in the Scandian phase at ~425–415 Ma.Kirkland et al. (2008)Rice et al. (2003)
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 the Jämtlandian Orogeny. It involved the Seve Nappe Complex of the Swedish Caledonides in central Sweden, which is interpreted as the stretched outermost edge of Baltica. Contrary to the previous opinion that it had been subducted beneath an oceanic island arc, they propose that it involved a collision with a continental fragment.
The Shelveian Orogeny occurred particularly in the Shelve area in Shropshire, in eastern Wales and in the English Midlands in the Late Ordovician and was related to the Taconic orogeny. It formed the Shelve Anticline and Rytton Castle Syncline and was the most important tectonic event in the area between the Cambrian and Devonian. Folding was accompanied by late stage igneous intrusions. The event caused a major unconformity in Shropshire with considerable erosion before the deposition of sediments in the Llandovery Epoch of the Silurian (444–443 Ma). There was no break in sediments in the area until the end of the Early Devonian, which was caused by the Acadian Orogeny in the British Isles.Toghill (1990) It was associated with dextral (right-lateral) strike-slip movement in the Pontesford-Linley fault system and folding in pre-Ashgill strata, uplift of the adjacent Towi Anticline and igneous activity.McKerrow et al. (2000)
Main orogenic phases
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 (see Acadian orogeny article for this orogeny).
During the final part of its northwestward migration, Avalonia converged 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 created continental collisions between them, the mentioned orogenic events and the closure of the Iapetus and Tornquist oceans.
Continental collisions started in the Mid Silurian and mountain building and ended in the Early Devonian (420–405 Ma).Fossen & Dunlap. 1998Torsvik et al. (1996)
Grampian orogeny
The Grampian orogeny involved collisions between two landmasses of Laurentia and an oceanic island 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 western Ireland. The other parts of the British Isles (England and Wales and the rest of Ireland) were part of the Avalonia microcontinent.
Eastern and Western Avalonia 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 the Laurentia tectonic plate (what is now North America) to the west in the area of the northern Appalachians and the Maritimes. Eastern Avalonia refers to a) the part which amalgamated with Baltica, 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).Schofield et al. (2016)
Docking Eastern Avalonia with Baltica The easternmost part of Eastern Avalonia amalgamated with Baltica through an oblique soft docking governed by dextral strike-slip convergence and shear, rather than through an orogen-causing hard continental collision. This is indicated by the absence of orogenic structures or high-pressure metamorphic rocks, which are either not present or buried. This event occurred close to the end of the Ordovician, 440 Ma.MONALISA Working Group (1997) It docked with the Baltica margins in southern Denmark, the south-western corner of the Baltic Sea and Poland. It came to comprise Silesia in Poland, northern Germany, the Netherlands, Belgium and part of north-eastern France (the Ardennes Mountains).
The Anglo-Brabant massif or London-Brabant Massif in central and southern England and in Belgium is a large basement massif. It is part of a magmatic belt which, starting from the Lake 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 the Rheic Ocean, which took place soon after, occurred through subduction along the southern margin of this massif.Pharaoh (2018)
Trans-European Suture Zone The Trans-European Suture Zone or Tornquist Zone is the area of the suture of Baltica and Eastern Avalonia. It runs from a portion of the North Sea close to Denmark, through southern Denmark, a portion of the Baltic Sea between Denmark and Poland (by Germany's Rügen Island), and through Poland. It then follows the eastern margin of the Eastern Carpathian Mountains in western Ukraine. Finally, it runs to the Black Sea. However, in the Sudetes Mountains and the Eastern Carpathians, it evolved through the Variscan and the Alpine orogenies, rather than the Caledonian one.Averill et al. (2006)
Scandian Phase
The Scandian phase involved a collision between eastern Greenland on the eastern margin of Laurentia and the margin of the Baltoscandian platform of the Fennoscandian peninsula of Baltica. It involved the Scandinavian Caledonides in what is now Norway and the Swedish areas by its border. It occurred from the Wenlock Epoch of the Silurian to the Mid Devonian (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.Corfu et al. (2014).