Caledonian orogeny
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.
"Caledonian Mountains" redirects here. Not to be confused with Caledonia Mountain or Mountains of Scotland.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.[2] Current understanding has it that the Caledonian orogeny encompasses a number of tectonic phases that can laterally be diachronous. 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[edit]
In the Neoproterozoic most of the Earth's landmasses were united in the Rodinia supercontinent. Gondwana formed its bulk.[Note 1] Near the end of the Neoproterozoic, during the breakup of this supercontinent, Laurentia[Note 2] and Baltica[Note 3] 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 (W and E 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 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.[4]
Either in the Late Precambrian or Early Ordovician[Note 5] the Avalonia microcontinent[Note 6] started to drift northwestward form the Gondwana northern margin (Amazonia and NW 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.[5] Its drift was towards the Baltica and Laurentia Ordovician positions which by then were 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).[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.
Early orogenic phases[edit]
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.[9][10]
van Roermund and Brueckner (2004) proposed 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.[11] 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.[7][12]
Controversies[edit]
According to some authors, the Caledonian continental collisions involved another microcontinent, Armorica (southern Portugal, most of the north of France and parts of southern Germany and the Czech 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 current Armorican and Bohemian Massifs are the most important. The ocean between the combined continental mass of Laurentia, Baltica and Avalonia (called Euramerica, Laurussia or Old Red Continent) and Armorica is called the Rheic Ocean.
The paleogeographic position of the Armorica crustal fragments between the Ordovician and Carboniferous 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 Carboniferous Variscan orogeny (about 340 million years ago). The Rhenohercynian basin, a back-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]