Laurasia


Laurasia was the more northern of two large landmasses that formed part of the Pangaea supercontinent from around , the other being Gondwana. It separated from Gondwana during the breakup of Pangaea, drifting further north after the split and finally broke apart with the opening of the North Atlantic Ocean 56 Mya. The name is a portmanteau of Laurentia and Eurasia.
Laurentia, Avalonia, Baltica, and a series of smaller terranes, collided in the Caledonian orogeny c. 400 Mya to form Laurussia. Laurussia then collided with Gondwana to form Pangaea. Kazakhstania and Siberia were then added to Pangaea 290–300 Mya to form Laurasia. Laurasia finally became an independent continental mass when Pangaea broke up into Gondwana and Laurasia.

Terminology and origin of the concept

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 from c. 430–420 Mya to form Laurussia. In the Late Carboniferous, Laurussia and Gondwana collided and formed Pangaea. Siberia and Kazakhstania finally collided with Baltica in the Late Permian to form Laurasia. A series of continental blocks that now form East Asia and Southeast Asia were later added to Laurasia.
In 1904–1909, Austrian geologist Eduard Suess proposed that the continents in the Southern Hemisphere were once merged into a larger continent called Gondwana. In 1915, German meteorologist Alfred Wegener proposed the existence of a supercontinent that he called Pangaea. In 1937, South African geologist Alexander du Toit proposed that Pangaea was divided into two larger landmasses, Laurasia in the Northern Hemisphere and Gondwana in the Southern Hemisphere, separated by the Tethys Ocean.
"Laurussia" was defined by Swiss geologist Peter Ziegler in 1988 as the merger between Laurentia and Baltica along the northern Caledonian suture. The "Old Red Continent" is an informal name often used for the Silurian-Carboniferous deposits in the central landmass of Laurussia.
Several earlier supercontinents proposed and debated in the 1990s and later included earlier connections between Laurentia, Baltica, Siberia. These original connections apparently survived through one and possibly even two Wilson Cycles, though their intermittent duration and recurrent fit is debated.

Proto-Laurasia

Pre-Rodinia

Laurentia and Baltica first formed a continental mass known as Proto-Laurasia as part of the supercontinent Columbia which was assembled 2,100–1,800 Mya to encompass virtually all known Archaean continental blocks. Surviving sutures from this assembly are the Trans-Hudson orogen in Laurentia; Nagssugtoqidian orogen in Greenland; the Kola-Karelian and the Volhyn–Central Russia orogen and Pachelma orogen in Baltica, the Akitkan Orogen in Siberia.
Additional Proterozoic crust was accreted 1,800–1,300 Mya, especially along the Laurentia-Greenland-Baltica margin. Laurentia and Baltica formed a coherent continental mass with southern Greenland and Labrador adjacent to the Arctic margin of Baltica. A magmatic arc extended from Laurentia through southern Greenland to northern Baltica. The breakup of Columbia began 1,600 Mya, including along the western margin of Laurentia and northern margin of Baltica, and was completed c. 1,300–1,200 Mya, a period during which mafic dike swarms were emplaced, including MacKenzie and Sudbury in Laurentia.
Traces left by large igneous provinces provide evidences for continental mergers during this period. Those related to Proto-Laurasia includes:
  • 1,750 Mya extensive magmatism in Baltica, Sarmatia, southern Siberia, northern Laurentia, and West Africa indicate these cratons were linked to each other;
  • a 1,630–1,640 Mya-old continent composed of Siberia, Laurentia, and Baltica is suggested by sills in southern Siberia that can be connected to the Melville Bugt dyke swarm in western Greenland;
  • a major large igneous province 1,380 Mya during the breakup of the Nuna supercontinent connects Laurentia, Baltica, Siberia, Congo, and West Africa.

    Rodinia

In the vast majority of plate tectonic reconstructions, Laurentia formed the core of the supercontinent Rodinia, which formed 1,260-900 Mya. However, the exact fit of various continents within Rodinia is debated. In some reconstructions, Baltica was attached to Greenland along its Scandinavian margin while Amazonia was docked along Baltica's Tornquist margin. Australia and East Antarctica were located on Laurentia's western margin.
Siberia was located near but at some distance from Laurentia's northern margin in most reconstructions. In the reconstruction of some Russian geologists, however, the southern margin of Siberia merged with the northern margin of Laurentia, and these two continents broke up along what is now the -long Central Asian Foldbelt no later than 570 Mya and traces of this breakup can still be found in the Franklin dike swarm in northern Canada and the Aldan Shield in Siberia.
The Proto-Pacific opened and Rodinia began to breakup during the Neoproterozoic as Australia-Antarctica rifted from the western margin of Laurentia, while the rest of Rodinia rotated clockwise and drifted south. Earth subsequently underwent a series of glaciations - the Varanger and the Rapitan and Ice Brook glaciations - both Laurentia and Baltica were located south of 30°S, with the South Pole located in eastern Baltica, and glacial deposits from this period have been found in Laurentia and Baltica but not in Siberia.
A mantle plume forced Laurentia and Baltica to separate ca. 650–600 Mya and the Iapetus Ocean opened between them. Laurentia then began to move quickly 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.

Pannotia

Laurentia, Baltica, and Siberia remained connected to each other within the short-lived, Precambrian-Cambrian supercontinent Pannotia or Greater Gondwana. At this time a series of continental blocks called as Peri-Gondwana, that now form parts of Asia, the Cathaysian terranes, namely Indochina, North China, South China , Cimmerian terranes, Sibumasu, Qiangtang, Lhasa, Afghanistan, Iran, 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. This northward drift of terranes across the Tethys Ocean also included the Hunic terranes, now spread from Europe to China.
Pannotia broke apart in the late Precambrian into Laurentia, Baltica, Siberia, Gondwana. A series of continental blocks, the Cadomian, Avalonian, Cathaysian, Cimmerian terranes, broke away from Gondwana and began to drift north.

Euramerica/Laurussia

Laurentia remained almost static near the Equator throughout the early Palaeozoic, separated from Baltica by the up to -wide Iapetus Ocean. In the Late Cambrian, the mid-ocean ridge in the Iapetus Ocean subducted beneath Gondwana which resulted 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.
Avalonia rifted from Gondwana in the Early Ordovician and collided with Baltica near the Ordovician–Silurian boundary. Baltica-Avalonia was then rotated and pushed north towards Laurentia. The collision between these continents closed the Iapetus Ocean and formed Laurussia, also known as Euramerica. Another historical term for this continent is the Old Red Continent or Old Red Sandstone Continent, in reference to abundant red beds of the Old Red Sandstone during the Devonian. The continent covered including several large Arctic continental blocks.
With the Caledonian orogeny completed Laurussia was delimited thus:
  • The eastern margin were the Barents Shelf and Moscow Platform;
  • the western margin were the western shelves of Laurentia, later affected by the Antler orogeny;
  • the northern margin was the Innuitian-Lomonosov orogeny which marked the collision between Laurussia and the Arctic Craton;
  • the southern margin was a Pacific-style active margin where the northward directed subduction of the ocean floor between Gondwana and Laurussia pushed continental fragments towards the latter.
During the Devonian the combined landmass of Baltica and Avalonia rotated around Laurentia, which remained static near the Equator. The Laurentian warm, shallow seas and on shelves a diverse assemblage of benthos evolved, including the largest trilobites exceeding. The Old Red Sandstone Continent stretched across northern Laurentia and into Avalonia and Baltica but for most of the Devonian a narrow seaway formed a barrier where the North Atlantic would later open. Tetrapods evolved from fish in the Late Devonian, with the oldest known fossils from Greenland. Low sea-levels during the Early Devonian produced natural barriers in Laurussia which resulted in provincialism within the benthic fauna. In Laurentia the Transcontinental Arch divided brachiopods into two provinces, with one of them confined to a large embayment west of the Appalachians. By the Middle Devonian, these two provinces had been united into one and the closure of the Rheic Ocean finally united faunas across Laurussia. High plankton productivity from the Devonian-Carboniferous boundary resulted in anoxic events that left black shales in the basins of Laurentia.

Pangaea

The subduction of the Iapetus Ocean resulted in the first contact between Laurussia and Gondwana in the Late Devonian and terminated in full collision or the Variscan orogeny in the early Carboniferous. The Variscan orogeny closed the Rheic Ocean and the Proto-Tethys Ocean to form the supercontinent Pangaea. The Variscan orogeny is complex and the exact timing and the order of the collisions between involved microcontinents has been debated for decades.
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. Heavy rainfall resulted in high groundwater tables, in turn resulting in peat formation and extensive coal deposits.
During the Cambrian and Early Ordovician, when wide oceans separated all major continents, only pelagic marine organisms, such as plankton, could move freely across the open ocean and therefore the oceanic gaps between continents are easily detected in the fossil records of marine bottom dwellers and non-marine species. By the Late Ordovician, when continents were pushed closer together closing the oceanic gaps, benthos could spread between continents while ostracods and fishes remained isolated. As Laurussia formed during the Devonian and Pangaea formed, fish species in both Laurussia and Gondwana began to migrate between continents and before the end of the Devonian similar species were found on both sides of what remained of the Variscan barrier.
The oldest tree fossils are from the Middle Devonian pteridophyte Gilboa Fossil Forest in central Laurussia. In the late Carboniferous, Laurussia was centred on the Equator and covered by tropical rainforests, commonly referred to as the coal forests. By the Permian, the climate had become arid and these Carboniferous rainforests collapsed, lycopsids were replaced by treeferns. In the dry climate a detritivorous fauna - including ringed worms, molluscs, and some arthropods - evolved and diversified, alongside other arthropods who were herbivorous and carnivorous, and tetrapods - insectivores and piscivores such as amphibians and early amniotes.