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

<span class="mw-page-title-main">Cambrian</span> First period of the Paleozoic Era, 539–485 million years ago

The Cambrian is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran period 538.8 Ma to the beginning of the Ordovician Period 485.4 Ma.

<span class="mw-page-title-main">Laurasia</span> Northern landmass that formed part of the Pangaea supercontinent

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 existed in the late Neoproterozoic and early Paleozoic eras of the geologic timescale. It lay 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

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. A 2022 paper argues that Pannotia never fully existed, reinterpreting the geochronological evidence: "the supposed landmass had begun to break up well before it was fully assembled". However, the assembly of the next supercontinent Pangaea is well established.

<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

The Acadian orogeny is a long-lasting mountain building event which began in the Middle Devonian, reaching a climax in the Late Devonian. It was active for approximately 50 million years, beginning roughly around 375 million years ago (Ma), with deformational, plutonic, and metamorphic events extending into the early Mississippian. The Acadian orogeny is the third of the four orogenies that formed the Appalachian Mountains and subsequent basin. The preceding orogenies consisted of the Grenville and Taconic orogenies, which followed a rift/drift stage in the Neoproterozoic. The Acadian orogeny involved the collision of a series of Avalonian continental fragments with the Laurasian continent. Geographically, the Acadian orogeny extended from the Canadian Maritime provinces migrating in a southwesterly direction toward Alabama. However, the northern Appalachian region, from New England northeastward into Gaspé region of Canada, was the most greatly affected region by the collision.

<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 paleocontinents, Gondwana and Laurussia (Laurentia-Baltica-Avalonia). One of the principal oceans of the Paleozoic, 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.

The Proto-Tethys or Theic Ocean was an ancient ocean that existed from the latest Ediacaran to the Carboniferous.

<span class="mw-page-title-main">Geology of England</span> Overview of the geology of England

The geology of England is mainly sedimentary. The youngest rocks are in the south east around London, progressing in age in a north westerly direction. The Tees–Exe line marks the division between younger, softer and low-lying rocks in the south east and the generally older and harder rocks of the north and west which give rise to higher relief in those regions. The geology of England is recognisable in the landscape of its counties, the building materials of its towns and its regional extractive industries.

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

Gondwana was a large landmass, sometimes referred to as a supercontinent. The remnants of Gondwana make up around two-thirds of today's continental area, including South America, Africa, Antarctica, Australia, Zealandia, Arabia, and the Indian Subcontinent. The other contemporary supercontinent at this time was Laurasia, both having broken off Pangaea.

<span class="mw-page-title-main">Laurentia</span> Craton forming the geological core of North America

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 the Hebridean Terrane in northwest Scotland. During other times in its past, Laurentia has been part of larger continents and supercontinents and 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">Carolina terrane</span> Exotic terrane from central Georgia to central Virginia in the United States

The Carolina Terrane, also called the Carolina Superterrane or Carolinia, is an exotic terrane running ~370 miles (600 km) approximately North-South from central Georgia to central Virginia in the United States. It constitutes a major part of the eastern Piedmont Province.

The Tornquist Sea or Tornquist Ocean was a sea located between the palaeocontinents Avalonia and Baltica about 600 to 450 million years ago. The remains of the sea today form a suture stretching across northern Europe.

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

The Armorican terrane, Armorican terrane assemblage, or simply Armorica, was a microcontinent or group of continental fragments that rifted away from Gondwana towards the end of the Silurian and collided with Laurussia towards the end of the Carboniferous during the Variscan orogeny. The name is taken from Armorica, the Gaulish name for a large part of northwestern France that includes Brittany, as this matches closely to the present location of the rock units that form the main part of this terrane.

<span class="mw-page-title-main">Geology of Massachusetts</span>

The geology of Massachusetts includes numerous units of volcanic, intrusive igneous, metamorphic and sedimentary rocks formed within the last 1.2 billion years. The oldest formations are gneiss rocks in the Berkshires, which were metamorphosed from older rocks during the Proterozoic Grenville orogeny as the proto-North American continent Laurentia collided against proto-South America. Throughout the Paleozoic, overlapping the rapid diversification of multi-cellular life, a series of six island arcs collided with the Laurentian continental margin. Also termed continental terranes, these sections of continental rock typically formed offshore or onshore of the proto-African continent Gondwana and in many cases had experienced volcanic events and faulting before joining the Laurentian continent. These sequential collisions metamorphosed new rocks from sediments, created uplands and faults and resulted in widespread volcanic activity. Simultaneously, the collisions raised the Appalachian Mountains to the height of the current day Himalayas.

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.

The geology of Newfoundland and Labrador includes basement rocks formed as part of the Grenville Province in the west and Labrador and the Avalonian microcontinent in the east. Extensive tectonic changes, metamorphism and volcanic activity have formed the region throughout Earth history.

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