Avalonia

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Current extent of Avalonia highlighted in yellow Avalonia today.jpg
Current extent of Avalonia highlighted in yellow

Avalonia was a microcontinent in the Paleozoic era. Crustal fragments of this former microcontinent underlie south-west Great Britain, southern Ireland, and the eastern coast of North America. It is the source of many of the older rocks of Western Europe, Atlantic Canada, and parts of the coastal United States. Avalonia is named for the Avalon Peninsula in Newfoundland.

Contents

Avalonia developed as a volcanic arc on the northern margin of Gondwana. It eventually rifted off, becoming a drifting microcontinent. The Rheic Ocean formed behind it, and the Iapetus Ocean shrank in front. It collided with the continents Baltica, then Laurentia, and finally with Gondwana, ending up in the interior of Pangea. When Pangea broke up, Avalonia's remains were divided by the rift which became the Atlantic Ocean.

Extent

The terranes of Avalonia with modern borders for orientation: 1 Laurentia; 2 Baltica; 3 Proto-Tethys Ocean; 4 Western Avalonia; 5 Eastern Avalonia.
US: United States; CT: Connecticut; MA: Massachusetts; NH: New Hampshire; ME: Maine; RI: Rhode-Island
CA: Canada; NB: New Brunswick; NFL: Newfoundland; NS: Nova-Scotia; PE: Prince Edward Island
Europe: IE: Ireland; UK: United Kingdom; FR: France; BE: Belgium; NL: Netherlands; DE: Germany; PL: Poland AVALONIA.svg
The terranes of Avalonia with modern borders for orientation: 1 Laurentia; 2 Baltica; 3 Proto-Tethys Ocean; 4 Western Avalonia; 5 Eastern Avalonia.
US: United States; CT: Connecticut; MA: Massachusetts; NH: New Hampshire; ME: Maine; RI: Rhode-Island
CA: Canada; NB: New Brunswick; NFL: Newfoundland; NS: Nova-Scotia; PE: Prince Edward Island
Europe: IE: Ireland; UK: United Kingdom; FR: France; BE: Belgium; NL: Netherlands; DE: Germany; PL: Poland

When the term "Avalon" was first coined by Canadian geologist Harold Williams in 1964, he included only Precambrian rocks in eastern Newfoundland. More than a decade later he extended the term to include all exotic rocks from Newfoundland down to Rhode Island. Since the introduction of the term terrane in the 1980s, Avalonia has been referred to as "platform", "composite terrane", "superterrane", "East" and "West Avalonia", and "Avalon sensu lato". "Avalonia" can thus refer exclusively to rocks in Newfoundland (Avalonia sensu stricto), an assembly of terranes, or a single tectonic unit. [1]

Avalonia is the largest of the peri-Gondwanan terranes, a series of continental blocks that more or less simultaneously broke off the margins of the southern supercontinent Gondwana and therefore share an early Paleozoic marine fauna. They were pulled northward and eventually collided to form the Appalachian, Caledonide, and Variscan orogens. West Avalonia, in North America, stretches from New England to Atlantic Canada, and East Avalonia, in Europe, from southern Britain to Brabant. [2]

Other Peri-Gondwanan terranes, sometimes called "Greater Avalonia", include Carolina in the Appalachian and the deep bedrock of Florida in North America, Oaxaquia and Yucatán in Mexico, and the Chortis Block in Central America. [2] The North American terranes of Ganderia and Carolinia are sometimes grouped with Avalonia because they were transferred together across the Iapetus Ocean. [3] Sometimes the Meguma terrane in Nova Scotia is also included. [4]

The Avalonian part of Great Britain almost exactly coincides with England and Wales. Elsewhere in Europe, parts of Avalonia are found in the Ardennes of Belgium and north-eastern France, north Germany, north-western Poland, south-eastern Ireland, and the south-western edge of the Iberian Peninsula.

Part of the British-Belgian section formed an island in the Carboniferous, affecting the disposition of coalfields; this is known by names such as the 'London-Brabant Island'. Its bulk had an effect on the geological structure between the Ardennes and the English Midlands by influencing the subsequent crustal folding resulting from the Variscan collision.

In Canada, Avalonia comprises the Avalon Peninsula of southeast Newfoundland, southern New Brunswick, part of Nova Scotia, and Prince Edward Island. In the United States, Avalonia consists of northern coastal Maine, all of Rhode Island, and other sections of coastal New England.

Development

Avalonia on its journey to Baltica at 480 Ma (Early Ordovician) Avalonia-480Ma.jpg
Avalonia on its journey to Baltica at 480 Ma (Early Ordovician)
Location of the Caledonian/Acadian mountain chains in the Early Devonian Epoch. Present day coastlines are shown for reference. Red lines are sutures, capitalized names are the different continents/super-terranes that joined during the Caledonian orogeny. Caledonides EN.svg
Location of the Caledonian/Acadian mountain chains in the Early Devonian Epoch. Present day coastlines are shown for reference. Red lines are sutures, capitalized names are the different continents/super-terranes that joined during the Caledonian orogeny.

The basement of Avalonia is poorly known, but, based on isotopic analyses, proto-Avalonia most likely evolved together with Carolina about 800 Ma from volcanic arcs far offshore from the supercontinent Rodinia, most likely outboard continental terranes of more obvious West African affinities, such as Cadomia and Iberia. About 650 Ma the Avalonian belt collided with Gondwana. [5] [6]

Avalonia originally developed along the shores of Rodinia together with island arcs now found in the Arabian-Nubian Shield (900–700 Ma) and Tocantins in central Brazil (950–900 Ma) and the basement of Avalonia is most likely of the same age. [7]

During the Precambrian-Cambrian transition, Avalonia was located in a cool-water environment and even underwent a glaciation. The Moroccan Anti-Atlas range in West Gondwana was, in contrast, characterised by evaporites, evidence of desiccation, and had thrombolites. Avalonia and Gondwana must therefore have been located far from each other during this period. The two did, however, share a geological history from later in the early Cambrian until the mid-Orodovician. [8]

Subduction evolved along the shores of Gondwana, which caused Avalonia to rift away and begin its northbound journey towards Baltica during late Cambrian and Early Ordovician. The Rheic Ocean opened behind the Avalonian microcontinents. [9] This independent movement of Avalonia started from a latitude of about 60° South. [10]

The eastern end of Avalonia collided with Baltica at 30°S, an event which closed the Tornquist Sea, during the Late Ordovician. In Avalonia, folding, faulting, and volcanism followed – as evidenced in the Welsh Borderland and the Taconic deformation in Laurentia – some or all of which are related to the collision. Plinian eruptions resulted from the subduction beneath Avalonia and produced thick layers of K-bentonite in southwestern Baltica, while the Millbrig eruptions occurred in Laurentia. This Late Ordovician magmatism peaked between 457 and 449 Ma. [11]

In the late Silurian and lower Devonian, the combined Baltica and Avalonia collided progressively, with Laurentia, beginning with the long extremity of Avalonia which is now attached to North America. The result of this was the formation of Euramerica. At the completion of this stage, the site of Britain was at 30°S and Nova Scotia at about 45°S. This collision is represented by the Caledonian folding or in North America as an early phase in the Acadian orogeny. [12]

In the Carboniferous, the new continent and another terrane, Armorica which included Iberia, drifted in from Gondwana, trapping Avalonia between it and the continent so adding Iberia/Armorica to Euramerica. [13] This was followed up by the arrival of Gondwana. The effects of these collisions are seen in Europe as the Variscan folding. In North America it shows as later phases of the Acadian orogeny. This was happening at around the Equator during the later Carboniferous, forming Pangaea with Avalonia near its centre but partially flooded by shallow sea. [14]

In the Jurassic, Pangaea split into Laurasia and Gondwana, with Avalonia as part of Laurasia. [15] In the Cretaceous, Laurasia broke up into North America and Eurasia with Avalonia split between them. [16]

Iberia was later rotated away again as the African part of Gondwana strike-slipped past it. This last movement caused the Alpine orogeny including the raising of the Pyrenees during the Miocene and Pliocene. As a result of this, part of Avalonia is now to be found on each side of the Straits of Gibraltar. [17]

See also

Related Research Articles

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Laurasia was the more northern of two large landmasses that formed part of the Pangaea supercontinent from around 335 to 175 million years ago (Mya), the other being Gondwana. It separated from Gondwana 215 to 175 Mya during the breakup of Pangaea, drifting farther north after the split and finally broke apart with the opening of the North Atlantic Ocean c. 56 Mya. The name is a portmanteau of Laurentia and Asia.

<span class="mw-page-title-main">Iapetus Ocean</span> Ocean that existed in the late Neoproterozoic and early Paleozoic eras

The Iapetus Ocean was an ocean that existed in the late Neoproterozoic and early Paleozoic eras of the geologic timescale. The Iapetus Ocean was situated in the southern hemisphere, between the paleocontinents of Laurentia, Baltica and Avalonia. The ocean disappeared with the Acadian, Caledonian and Taconic orogenies, when these three continents joined to form one big landmass called Euramerica. The "southern" Iapetus Ocean has been proposed to have closed with the Famatinian and Taconic orogenies, meaning a collision between Western Gondwana and Laurentia.

<span class="mw-page-title-main">Pannotia</span> Hypothesized Neoproterozoic supercontinent from the end of the Precambrian

Pannotia, also known as the Vendian supercontinent, Greater Gondwana, and the Pan-African supercontinent, was a relatively short-lived Neoproterozoic supercontinent that formed at the end of the Precambrian during the Pan-African orogeny, during the Cryogenian period and broke apart 560 Ma with the opening of the Iapetus Ocean, in the late Ediacaran and early Cambrian. Pannotia formed when Laurentia was located adjacent to the two major South American cratons, Amazonia and Río de la Plata. The opening of the Iapetus Ocean separated Laurentia from Baltica, Amazonia, and Río de la Plata. In 2022 the whole concept of Pannotia was put into question by scientists who argue its existence is not supported by geochronology; "the supposed landmass had begun to break up well before it was fully assembled".

<span class="mw-page-title-main">Baltica</span> Late-Proterozoic to early-Palaeozoic continent

Baltica is a paleocontinent that formed in the Paleoproterozoic and now constitutes northwestern Eurasia, or Europe north of the Trans-European Suture Zone and west of the Ural Mountains. The thick core of Baltica, the East European Craton, is more than three billion years old and formed part of the Rodinia supercontinent at c.Ga.

<span class="mw-page-title-main">Acadian orogeny</span> North American orogeny

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<span class="mw-page-title-main">Caledonian orogeny</span> Mountain building event caused by the collision of Laurentia, Baltica and Avalonia

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.

<span class="mw-page-title-main">Variscan orogeny</span> Collision of tectonic plates resulting in the creation of mountains

The Variscan or Hercynianorogeny was a geologic mountain-building event caused by Late Paleozoic continental collision between Euramerica (Laurussia) and Gondwana to form the supercontinent of Pangaea.

The Rheic Ocean was an ocean which separated two major palaeocontinents, Gondwana and Laurussia (Laurentia-Baltica-Avalonia). One of the principal oceans of the Palaeozoic, its sutures today stretch 10,000 km (6,200 mi) from Mexico to Turkey and its closure resulted in the assembly of the supercontinent Pangaea and the formation of the Variscan–Alleghenian–Ouachita orogenies.

<span class="mw-page-title-main">Paleo-Tethys Ocean</span> Ocean on the margin of Gondwana between the Middle Cambrian and Late Triassic

The Paleo-Tethys or Palaeo-Tethys Ocean was an ocean located along the northern margin of the paleocontinent Gondwana that started to open during the Middle Cambrian, grew throughout the Paleozoic, and finally closed during the Late Triassic; existing for about 400 million years.

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<span class="mw-page-title-main">Gondwana</span> Neoproterozoic to Cretaceous landmass

Gondwana was a large landmass, sometimes referred to as a supercontinent. It was formed by the accretion of several cratons, beginning c. 800 to 650Ma with the East African Orogeny, the collision of India and Madagascar with East Africa, and was completed c.600 to 530 Ma with the overlapping Brasiliano and Kuunga orogenies, the collision of South America with Africa, and the addition of Australia and Antarctica, respectively. Eventually, Gondwana became the largest piece of continental crust of the Palaeozoic Era, covering an area of about 100,000,000 km2 (39,000,000 sq mi), about one-fifth of the Earth's surface. It fused with Euramerica during the Carboniferous to form Pangea. It began to separate from northern Pangea (Laurasia) during the Triassic, and started to fragment during the Early Jurassic. The final stages of break-up, involving the separation of Antarctica from South America and Australia, occurred during the Paleogene (from around 66 to 23 million years ago. Gondwana was not considered a supercontinent by the earliest definition, since the landmasses of Baltica, Laurentia, and Siberia were separated from it. To differentiate it from the Indian region of the same name, it is also commonly called Gondwanaland.

<span class="mw-page-title-main">Laurentia</span> A large continental craton that forms the ancient geological core of the North American continent

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, although originally it also included the cratonic areas of Greenland and also the northwestern part of Scotland, known as the Hebridean Terrane. During other times in its past, Laurentia has been part of larger continents and supercontinents and itself consists of many smaller terranes assembled on a network of Early Proterozoic orogenic belts. Small microcontinents and oceanic islands collided with and sutured onto the ever-growing Laurentia, and together formed the stable Precambrian craton seen today.

<span class="mw-page-title-main">Pangaea</span> Supercontinent from the late Paleozoic to early Mesozoic eras

Pangaea or Pangea was a supercontinent that existed during the late Paleozoic and early Mesozoic eras. It assembled from the earlier continental units of Gondwana, Euramerica and Siberia during the Carboniferous approximately 335 million years ago, and began to break apart about 200 million years ago, at the end of the Triassic and beginning of the Jurassic. In contrast to the present Earth and its distribution of continental mass, Pangaea was centred on the equator and surrounded by the superocean Panthalassa and the Paleo-Tethys and subsequent Tethys Oceans. Pangaea is the most recent supercontinent to have existed and the first to be reconstructed by geologists.

<span class="mw-page-title-main">Carolina terrane</span> Exotic terrane from central Georgia to central Virginia in the United States

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<span class="mw-page-title-main">Armorican terrane</span> Microcontinent or group of continental fragments rifted away from Gondwana

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Ganderia or Gander Terrane is a terrane in the northern Appalachians which broke off the supercontinent Gondwana c.570 million years ago (Ma) together with Avalonia, Megumia, and Carolinia.

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References

Notes

  1. Pollock, Hibbard & van Staal 2012 , Avalonia: Definition and extent, p. 267
  2. 1 2 Murphy et al. 2001 , Geological Setting, p. 46
  3. Keppie & Keppie 2014 , Introduction, p. 6
  4. Landing et al. 2022 , Avalonia as zone, terrane, and microcontinent, pp. 4–6
  5. Murphy et al. 2001 , 800-625 Ma, p. 50
  6. McNamara et al. 2001 , Abstract
  7. Nance, Murphy & Keppie 2002 , Development of proto-Avalonia, pp. 13–16
  8. Landing, Westrop & Bowring 2013 , Introduction
  9. Golonka et al. 2009 , The Rifting of Avalonian terranes
  10. Golonka 2002 , Fig. 6
  11. Torsvik & Rehnström 2003 , Avalonia and Baltica and the tale of convergence K-bentonites, pp. 77-80
  12. Golonka 2002 , Fig. 8
  13. Golonka 2002 , Fig. 16
  14. Golonka 2002 , Fig. 17
  15. Golonka 2002 , Fig. 26
  16. Golonka 2002 , Fig. 31
  17. Golonka 2002 , Fig. 38

Sources