Laurasia

Laurasia (/lɔːˈrʒə, -ʃiə/),[1] a portmanteau for Laurentia and Eurasia, was the more northern of two supercontinents (the other being Gondwana) that formed part of the Pangaea supercontinent around 335 to 175 million years ago (Mya). It separated from Gondwana 215 to 175 Mya (beginning in the late Triassic period) during the breakup of Pangaea, drifting farther north after the split.

Laurasia
Laurasia and Gondwana as part of Pangaea 200 Mya
Historical continent
Formed1,071 Mya (Proto-Laurasia)
253 Mya
TypeSupercontinent
Today part of
Smaller continents
Tectonic plate

Laurentia, the Palaeozoic core of North America and continental fragments that now make up part of Europe, collided with Baltica and Avalonia in the Caledonian orogeny c. 430–420 Mya to form Laurussia. In the Late carboniferous Laurussia and Gondwana formed Pangaea. Siberia and Kazakhstania finally collided with Baltica in the Late Permian which formed Laurasia.[2] The North China and South China cratons later collided with Siberia.

Proto-Laurasia

Laurentia and Baltica first formed a continental mass known as Proto-Laurasia by the convergence and suturing of a series of Archaean continental blocks 1,950-1,800 Mya. Additional Proterozoic crust was accreted 1,800-1,600 Mya of which the only well-preserved sutures are the interior Trans-Hudson orogen and periferal Svecokarelian/Svecofennian orogen.[3] The southern margin (modern coordinates) of Siberia merged with the northern margin of Laurentia, and the two continents broke up along what is now the 3,000 km (1,900 mi)-long Central Asian Foldbelt. Traces of this breakup can still be found in the Franklin dike swarm in northern Canada and the Aldan Shield in Siberia.[4]

Laurentia and Baltica formed part of Rodinia, a supercontinent that began to breakup during the Neoproterozoic. As the Proto-Pacific opened 750–600 Mya both continents drifted southward away from the Equator together and rotated clockwise. As they approached the South Pole during the Varanger Glaciation, a mantle plume forced them to separate ca. 650–600 Mya and the Iapetus Ocean opened between them. Laurentia then began to move quickly (20 cm/year (7.9 in/year)) north towards the Equator where it got stuck over a cold spot in the Proto-pacific. Baltica remained near Gondwana in southern latitudes into the Ordovician.[5]

Left: Laurasia as part of Pannotia at 600 Mya.
Right: Laurasia during the breakup of Pannotia at 550 Mya.
View centred on the South Pole.

Laurasia — Laurentia, Baltica, and Siberia — remained connected to each other within the short-lived, Precambrian-Cambrian supercontinent Pannotia. At this time a series of continental blocks that now form part of Asia, the Cathaysian terranes — Indochina, North China, and South China — and Cimmerian terranes Sibumasu, Qiangtang, Lhasa, Afghanistan, Iran, and Turkey — were still attached to the Indian–Australian margin of Gondwana. Other blocks that now form part of southwestern Europe and North America from New England to Florida were still attached to the African-South American margin of Gondwana.[6] This northward drift of terranes across the Tethys also included the Hunic terranes, now spread from Europe to China.[7]

Pannotia broke apart in the late Precambrian into Laurasia, Baltica, Siberia, and Gondwana. Meanwhile, the Cadomian–Avalonian, Cathaysian, and Cimmerian terranes broke away from Gondwana north across the Tethys Ocean.[8]

Reformation

Laurasia during (left) the closure of the Iapetus Ocean at 430 Mya (view centred on 0°,-60°) and (right) Rheic Ocean at 330 Mya (right, view centred on -30°,-30°).

In the Late Cambrian, the mid-ocean ridge of the Iapetus Ocean was subducted beneath Gondwana, resulting in the opening of a series of large back-arc basins. During the Ordovician, these basins evolved into a new ocean, the Rheic Ocean, which separated a series of terranes  Avalonia, Carolinia, and Armorica  from Gondwana. The Iapetus Ocean finally closed in the Ordovician-Silurian (480-420 Mya) when these terranes collided with Laurentia and Baltica to form Laurussia (i.e. Laurasia except Siberia).[9]

During the Silurian–Devonian (425–400 Mya) Laurussia subsequently collided with Gondwana in the Variscan orogeny closing the Rheic Ocean (between Avalonia and Armorica) and the Proto-Tethys Ocean (between Armorica and Gondwana) and then with Siberia in the Uralide orogeny to form the supercontinent Pangaea.[10]

Pangaea was completely assembled by the Permian except for the Asian blocks. The supercontinent was centred on the Equator during the Triassic and Jurassic, a period that saw the emergence of the Pangaean megamonsoon.[11] Heavy rainfall resulted in high groundwater tables, in turn resulting in peat formation and extensive coal deposits.[12]

Asian blocks

During the Neoproterozoic-Early Paleozoic break-up of Rodinia the opening of the Proto-Tethys Ocean split the Asian blocks — Tarim, Qaidam, Alex, North China, and South China — from the northern shores of Gondwana (north of Australia in modern coordinates) and the closure of the same ocean reassembled them along the same shores 500-460 Mya resulting in Gondwana at its largest extent.[9]

The break-up of Rodinia also resulted in the opening of the long-lived Paleo-Asian Ocean between Baltica and Siberia in the north and Tarim and North China in the south. The closure of this ocean is preserved in the Central Asian Orogenic Belt, the largest orogen on Earth. In the Early Devonian, the Palaeo-Tethys opened, separating the Asian blocks from the northern shores of Gondwana and by 245 Mya these had collided with Kazakhsatania and Siberia and were now part of Laurasia.[13]

During the Cambrian, North China and Siberia drifted into latitudes further north than those occupied by continents during the previous 500 million years. By the Devonian, North China was located near the Arctic Circle and it remained the northernmost land in the world during the Carboniferous Ice Age between 300 and 280 million years ago. No evidence, though, exists for any large-scale Carboniferous glaciation of the northern continents.

Siberia moved southwards and joined with Kazakhstania, a small continental region believed today to have been created during the Silurian by extensive volcanism. When these two continents joined together, Laurasia was nearly reformed, and by the beginning of the Triassic, the Tarim craton had rejoined the redeveloping Laurasia as it collided with Gondwana to form Pangaea. North China became, as it drifted southwards from near-Arctic latitudes, the last continent to join with Pangaea.

Final split

In the Triassic-early Jurassic (c. 200 Mya), the opening of the Central Atlantic Ocean was preceded by the formation of a series of large rift basins, such as the Newark Basin, between eastern North America, from what is today the Gulf of Mexico to Nova Scotia, and in Africa and Europe, from Morocco to Greenland.[14]

By c. 83 Mya spreading had begun in the North Atlantic between the Rockall Plateau, a continental fragment sitting on of the Eurasian Plate, and North America. By 56 Mya Greenland had become an independent plate, separated from North America by a spreading zone in the Labrador Sea. By 33 Mya spreading had ceased in the Labrador Sea and relocated to the Mid-Atlantic Ridge.[15] The opening of the North Atlantic Ocean had effectively broken Laurasia in two.

See also

References

Notes

  1. OED
  2. Torsvik et al. 2012, From Laurentia to Laurussia and Laurasia: Overview, p. 6
  3. Olesen 1987, General geology, pp. 2-3
  4. Yarmolyuk et al. 2006, p. 1031; Fig. 1, p. 1032
  5. Torsvik et al. 1996, Abstract
  6. Scotese 2009, p. 71
  7. Stampfli 2000, Palaeotethys, p. 3
  8. Scotese 2009, The break-up of Pannotia, p. 78
  9. Zhao et al. 2018, Closure of Proto-Tethys Ocean and the first assembly of East Asian blocks at the northern margin of Gondwana, pp. 7-10
  10. Rey, Burg & Casey 1997, Introduction, pp. 1–2
  11. Parrish 1993, Paleogeographic Evolution of Pangea, p. 216
  12. Parrish 1993, Geological Evidence of the Pangean Megamonsoon, p. 223
  13. Zhao et al. 2018, Closure of Paleo-Asian Ocean: collision of Tarim, Alex and North China with East Europe and Siberia, pp. 11-14
  14. Olsen 1997, Introduction, p. 338
  15. Seton et al. 2012, Rockall–North America/Greenland, p. 222

Sources

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