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. [1] [2] [3]
The craton is named after the Laurentian Shield, through the Laurentian Mountains, which received their name from the St. Lawrence River, named after Saint Lawrence of Rome. [4]
In eastern and central Canada, much of the stable craton is exposed at the surface as the Canadian Shield, an area of Precambrian rock covering over a million square miles. This includes some of the oldest rock on Earth, such as the Archean rock of the Acasta Gneiss, which is 4.04 billion years (Ga) old, and the Istaq Gneiss Complex of Greenland, which is 3.8 Ga. [5] When subsurface extensions are considered, the wider term Laurentian Shield is more common, not least because large parts of the structure extend outside Canada. In the United States, the craton bedrock is covered with sedimentary rocks on the broad interior platform in the Midwest and Great Plains regions and is exposed only in northern Minnesota, Wisconsin, the New York Adirondacks, and the Upper Peninsula of Michigan. [6] The sequence of sedimentary rocks varies from about 1,000 m to in excess of 6,100 m (3,500–20,000 ft) in thickness. The cratonic rocks are metamorphic or igneous with the overlying sedimentary layers composed mostly of limestones, sandstones, and shales. [7] These sedimentary rocks were largely deposited 650–290 Ma. [8]
The oldest bedrock, assigned to the Archean Slave, Rae, Hearne, Wyoming, Superior, and Nain Provinces, is located in the northern two thirds of Laurentia. During the Early Proterozoic they were covered by sediments, most of which has now been eroded away. [1]
Greenland is part of Laurentia. The island is separated from North America by the Nares Strait, but this is a Pleistocene erosional feature. The strait is floored with continental crust and shows no indications of a thermal event or seaway tectonism. [9] [10] Greenland is composed mostly of crust of Archean to Proterozoic age, with lower Paleocene shelf formations on its northern margin and Devonian to Paleogene formations on its western and eastern margins. The eastern and northern margins were heavily deformed during the Caledonian orogeny. [11] [10]
The Isua Greenstone Belt of western Greenland preserves oceanic crust containing sheeted dike complexes. These provide evidence to geologists that mid-ocean ridges existed 3.8 Ga. The Abitibi gold belt in the Superior Province is the largest greenstone belt in the Canadian Shield. [12]
Laurentia first assembled from six or seven large fragments of Archean crust at around 2.0 to 1.8 Ga. [3] [13] The assembly began when the Slave craton collided with the Rae-Hearne craton, and the Rae-Hearne craton collided shortly after with the Superior Craton. These then merged with several smaller fragments of Archean crust, including the Wyoming, Medicine Hat, Sask, Marshfield, and Nain blocks. This series of collisions raised the mountains of the Trans-Hudson orogenic belt, which likely were similar to the modern Himalayas, [3] and the Wopmay orogen of northwest Canada. [14] During the assembly of the core of Laurentia, banded iron formation was deposited in Michigan, Minnesota, and Labrador. [15]
The resulting nucleus of Laurentia was mostly reworked Archean crust but with some juvenile crust in the form of volcanic arc belts. Juvenile crust is crust formed from magma freshly extracted from the Earth's mantle rather than recycled from older crustal rock. [3] The intense mountain building of the Trans-Hudson orogeny formed thick, stable roots beneath the craton, [3] possibly by a process of "kneading" that allowed low density material to move up and high density material to move down. [16]
Over the next 900 million years, Laurentia grew by the accretion of island arcs and other juvenile crust and occasional fragments of older crust (such as the Mojave block). This accretion occurred along the southeastern margin of Laurentia, where there was a long-lived convergent plate boundary. Major accretion episodes included the Yavapai orogeny at 1.71 to 1.68 Gya, which welded the 1.8 to 1.7 Ga Yavapai province to Laurentia; the Mazatzal orogeny at 1.65 to 1.60 Gya, accreting the 1.71 to 1.65 Ga Mazatzal province; [3] the Picuris orogeny at 1.49 to 1.45 Gya, [17] which may have welded the 1.50 to 1.30 Ga Granite-Rhyolite province to Laurentia; and the Grenville orogeny at 1.30 to 0.95 Gya, which accreted the 1.30 to 1.00 Ga Llano-Grenville province to Laurentia.
The Picuris orogeny, in particular, was characterized by the intrusion of great volumes of granitoid magma into the juvenile crust, which helped mature the crust and stitch it together. Slab rollback at 1.70 and 1.65 Gya deposited characteristic quartzite-rhyolite beds on the southern margin of the craton. This long episode of accretion doubled the size of Laurentia but produced craton underlain by relatively weak, hydrous, and fertile (ripe for extraction of magma) mantle lithosphere. [3] The subduction under the southeast margin of the continent likely caused enrichment of the lithospheric mantle beneath the orogenic belts of the Grenville Province. [18] Around 1.1 Gya, the center of the craton nearly rifted apart along the Midcontinent Rift System. This produced the Keweenawan Supergroup, whose flood basalts are rich in copper ore. [19]
Laurentia was formed in a tectonically active world. [20] [3] The subduction under the southeast margin of the continent is thought to have contributed to the formation of Rodinia. [18] [21] [22] According to the Southwest U.S. and East Antarctica or SWEAT hypothesis, Laurentia became the core of the supercontinent. It was rotated approximately 90 degrees clockwise compared with its modern orientation, with East Antarctica and Australia to the north (what is now the west), Siberia to the east (present north), Baltica and Amazonia to the south (present east), and Congo to the southwest (present southeast). The Grenville orogen extended along the entire southwest (present southeast) margin of Laurentia, where it had collided with Congo, Amazonia, and Baltica. Laurentia lay along the equator. [23]
Recent evidence suggests that South America and Africa never quite joined to Rodinia, though they were located very close to it. Newer reconstructions place Laurentia closer to its present-day orientation, with East Antarctica and Australia to the west, South China to the northwest, Baltica to the east, and Amazonia and Rio de la Plata to the south. [24]
The breakup of Rodinia began by 780 Ma, when numerous mafic dike swarms were emplaced in western Laurentia. [25] Early stages of rifting produced the Belt Supergroup, which is over 12 kilometers (7.5 mi) thick. [26] By 750 Ma the breakup was mostly complete, and Gondwana (composed of most of today's southern continents) had rotated away from Laurentia, which was left isolated near the equator. [25] The breakup of Rodinia may have triggered an episode of severe ice ages (the Snowball Earth hypothesis.) [24]
There is some evidence that the fragments of Rodinia gathered into another short-lived supercontinent, Pannotia, at the very end of the Proterozoic. This continent broke up again almost at once, and Laurentia rifted away from South America at around 565 Ma to once again become an isolated continent near the equator, separated from Gondwana by the western Iapetus Ocean. Sometime in the early Cambrian, around 530 Ma, Argentina rifted away from Laurentia and accreted onto Gondwana. [28]
The breakup of Pannotia produced six major continents: Laurentia, Baltica, Kazakhstania, Siberia, China, and Gondwana. [29] Laurentia remained an independent continent until the middle Silurian. [10] During the early to middle Ordovician, several volcanic arcs collided with Laurentia along what is now the Atlantic coast of North America. This caused an episode of mountain-building called the Taconic orogeny. [30] As the mountains raised by the Taconic orogeny were subsequently eroded, they produced the immense Queenston Delta, recorded in the rocks of the Queenston Formation. [29] There was also violent volcanic activity, including the eruption that produced the Millburg/Big Bentonite ash bed. About 1,140 cubic kilometers (270 cu mi) of ash erupted in this event. However, this does not seem to have triggered any mass extinction. [31] [32]
Throughout the early Paleozoic, Laurentia was characterized by a tectonically stable interior flooded by the seas, with marginal orogenic belts. [29] An important feature was the Transcontinental Arch, which ran southwest from the lowlands of the Canadian Shield. The shield and the arch were the only portions of the continent that were above water through much of the early Paleozoic. [33] There were two major marine transgressions (episodes of continental flooding) during the early Paleozoic, the Sauk and the Tippecanoe. During this time, the Western Cordillera was a passive margin. [29] Sedimentary rocks that were deposited on top of the basement complex were formed in a setting of quiet marine and river waters. The craton was covered by shallow, warm, tropical epicontinental or epicratonic sea (meaning literally "on the craton") that had maximum depths of only about 60 m (200 ft) at the shelf edge. [34]
The position of the equator during the Late Ordovician epoch (c. 458 – c. 444 Ma) on Laurentia has been determined via extensive shell bed records. [35] Flooding of the continent that occurred during the Ordovician provided the shallow warm waters for the success of sea life and therefore a spike in the carbonate shells of shellfish. Today the beds are composed of fossilized shells or massive-bedded Thalassinoides facies and loose shells or nonamalgamated brachiopod shell beds. [35] These beds imply the presence of an equatorial climate belt that was hurricane free which lay inside 10° of the equator. [35] This ecological conclusion matches the previous paleomagnetic findings which confirms this equatorial location. [35]
At the end of the Cambrian, about 490 Mya, Avalonia rifted away from Gondwana. By the end of the Ordovician, Avalonia had merged with Baltica, and the two fused to Laurentia at the end of the Silurian (about 420 Ma) [30] in the Caledonian orogeny. This produced the continent of Laurussia. [30] [10]
During this time, several small continental fragments merged with other margins of the craton. These included the North Slope of Alaska, which merged during the Early Devonian. [36] Several small crust fragments accreted from the late Devonian through the Mesozoic to form the Western Cordillera. [37]
The Western Cordillera became a convergent plate margin during the Ordovician, and the Transcontinental Arch became submerged, only to reappear in the Devonian. [38] The Devonian also saw the deposition of the Chattanooga Shale [39] and the Antler Orogeny in the Western Cordillera. [40]
During the Carboniferous and Permian, Laurussia fused with Gondwana to form Pangaea. The resulting Alleghanian orogeny created the Central Pangean Mountains. [41] [42] [10] The mountains were located close to the equator and produced a year-round zone of heavy precipitation that promoted the deposition of extensive coal beds, including the Appalachian coal beds in the U.S. [43] Meanwhile, Gondwana had drifted onto the South Pole, and cycles of extensive glaciation produced a characteristic pattern of alternating marine and coal swamp beds called cyclothems. [44]
During the Pennsylvanian, the Ancestral Rocky Mountains were raised in the southwestern part of Laurentia. This has been attributed either to either the collision with Gondwana [45] or subduction under the continental margin from the southwest. [46] Two additional marine transgressions took place during the late Paleozoic: the Kaskaskia and Absaroka. [29]
The great continental mass of Pangaea strongly affected climate patterns. [43] The Permian was relatively arid, and evaporites were deposited in the Permian Basin. [47] Sedimentary beds deposited in the southwest in the early Triassic were fluvial in character, but gave way to eolian beds in the late Triassic. [48] Pangaea reached its height about 250 Ma, at the start of the Triassic. [49]
The breakup of Pangaea began in the Triassic, with rifting along what is now the east coast of the U.S. that produced red beds, arkosic sandstone, and lake shale deposits. [48] The central Atlantic ocean basin began opening at about 180 Ma. [49] Florida, which had been a part of Gondwana before the assembly of Pangaea, was left with Laurentia during the opening of the central Atlantic. This former Gondwana fragment includes the Carolina Slate belt and parts of Alabama. [10]
The Gulf of Mexico opened during the Late Triassic and Jurassic. This was accompanied by deposition of evaporite beds that later gave rise to salt domes that are important petroleum reservoirs today. [48] Europe rifted away from North America between 140 and 120 Ma, [49] and Laurentia once again became the core of an independent continent with the opening of the North Atlantic in the Paleogene. [10]
Four orogenies occurred in the Mesozoic in the Western Cordillera: the Sonoma, Nevadan, Sevier, and Laramide. The Nevadan orogeny emplaced the extensive batholiths of the Sierra Nevada. [50] The regression of the Sundance Sea in the late Jurassic was accompanied by deposition of the Morrison Formation, notable for its vertebrate fossils. [48]
During Cretaceous times, the Western Interior Seaway ran from the Gulf of Mexico to the Arctic Ocean, dividing North America into eastern and western land masses. From time to time, land masses or mountain chains rose up on the distant edges of the craton and then eroded down, shedding their sand across the landscape. [51] Chalk beds of the Niobrara Formation were deposited at this time, and accretion of crustal fragments continued along the Western Cordillera. [48]
Northeast Mexico was added to the North American craton relatively recently in geological time. This block was formed from the Mesozoic to nearly the present day, with only small fragments of earlier basement rock. It moved as a coherent unit after the breakup of Pangaea. [10] The Atlantic and Gulf Coasts experienced eight transgressions in the Cenozoic. [52] The Laramide orogeny continued to raise the present Rocky Mountains into the Paleocene. [52] The Western Cordillera continued to suffer tectonic deformation, including the formation of the Basin and Range Province in the middle Cenozoic and the uplift of the Colorado Plateau. The Colorado Plateau was uplifted with remarkably little deformation. The flood basalts of the Columbia Plateau also erupted during the Cenozoic. [52]
The southwestern portion of Laurentia consists of Precambrian basement rocks deformed by continental collisions. This area has been subjected to considerable rifting as the Basin and Range Province has been stretched up to 100% of its original width. [53] The area experienced numerous large volcanic eruptions. Baja California rifted away from North America during the Miocene. [49] This block of crust consists of Proterozoic to early Paleozoic shelf and Mesozoic arc volcano formations. [54] [10] The Holocene being an interglacial, a warm spell between episodes of extensive glaciation. [52]
Several climate events occurred in Laurentia during the Phanerozoic eon. During the late Cambrian through the Ordovician, sea level fluctuated with ice cap melt. Nine macro scale fluctuations of "global hyper warming", or high intensity greenhouse gas conditions, occurred. [55] Due to sea level fluctuation, these intervals led to mudstone deposits on Laurentia that act as a record of events. [55] The late Ordovician brought a cooling period, although the extent of this cooling is still debated. [56] More than 100 million years later, in the Permian, an overall warming trend occurred. [57] As indicated by fossilized invertebrates, the western margin of Laurentia was affected by a lasting southward bound cool current. This current contrasted with waters warming in the Texas region. [57] This opposition suggests that, during Permian global warm period, northern and northwestern Pangea (western Laurentia) remained relatively cool. [57]
In geology, a supercontinent is the assembly of most or all of Earth's continental blocks or cratons to form a single large landmass. However, some geologists use a different definition, "a grouping of formerly dispersed continents", which leaves room for interpretation and is easier to apply to Precambrian times. To separate supercontinents from other groupings, a limit has been proposed in which a continent must include at least about 75% of the continental crust then in existence in order to qualify as a supercontinent.
Rodinia was a Mesoproterozoic and Neoproterozoic supercontinent that assembled 1.26–0.90 billion years ago (Ga) and broke up 750–633 million years ago (Ma). Valentine & Moores 1970 were probably the first to recognise a Precambrian supercontinent, which they named "Pangaea I." It was renamed "Rodinia" by McMenamin & McMenamin 1990 who also were the first to produce a reconstruction and propose a temporal framework for the supercontinent.
The Proterozoic is the third of the four geologic eons of Earth's history, spanning the time interval from 2500 to 538.8 Mya, the longest eon of the Earth's geologic time scale. It is preceded by the Archean and followed by the Phanerozoic, and is the most recent part of the Precambrian "supereon".
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.
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.
Kenorland is a hypothetical Neoarchean supercontinent. If it existed, it would have been one of the earliest known supercontinents on Earth. It is thought to have formed during the Neoarchaean Era c. 2.72 billion years ago by the accretion of Neoarchaean cratons and the formation of new continental crust. It comprised what later became Laurentia, Baltica, Western Australia and Kalaharia.
Columbia, also known as Nuna or Hudsonland, is a hypothetical ancient supercontinent. It was first proposed by John J.W. Rogers and M. Santosh in 2002 and is thought to have existed approximately 2,500 to 1,500 million years ago (Ma), in the Paleoproterozoic era. The assembly of the supercontinent was likely completed during global-scale collisional events from 2,100 to 1,800 Ma.
Arctica, or Arctida is a hypothetical ancient continent which formed approximately 2.565 billion years ago in the Neoarchean era. It was made of Archaean cratons, including the Siberian Craton, with its Anabar/Aldan shields in Siberia, and the Slave, Wyoming, Superior, and North Atlantic cratons in North America. Arctica was named by Rogers 1996 because the Arctic Ocean formed by the separation of the North American and Siberian cratons. Russian geologists writing in English call the continent "Arctida" since it was given that name in 1987, alternatively the Hyperborean craton, in reference to the hyperboreans in Greek mythology.
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.
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. 1 Ga.
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.
The geological history of the Earth follows the major geological events in Earth's past based on the geological time scale, a system of chronological measurement based on the study of the planet's rock layers (stratigraphy). Earth formed about 4.54 billion years ago by accretion from the solar nebula, a disk-shaped mass of dust and gas left over from the formation of the Sun, which also created the rest of the Solar System.
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.
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 C-shaped, with the bulk of its mass stretching between Earth's northern and southern polar regions 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.
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.
This is a list of articles related to plate tectonics and tectonic plates.
A paleocontinent or palaeocontinent is a distinct area of continental crust that existed as a major landmass in the geological past. There have been many different landmasses throughout Earth's time. They range in sizes, some are just a collection of small microcontinents while others are large conglomerates of crust. As time progresses and sea levels rise and fall more crust can be exposed making way for larger landmasses. The continents of the past shaped the evolution of organisms on Earth and contributed to the climate of the globe as well. As landmasses break apart, species are separated and those that were once the same now have evolved to their new climate. The constant movement of these landmasses greatly determines the distribution of organisms on Earth's surface. This is evident with how similar fossils are found on completely separate continents. Also, as continents move, mountain building events (orogenies) occur, causing a shift in the global climate as new rock is exposed and then there is more exposed rock at higher elevations. This causes glacial ice expansion and an overall cooler global climate. The movement of the continents greatly affects the overall dispersal of organisms throughout the world and the trend in climate throughout Earth's history. Examples include Laurentia, Baltica and Avalonia, which collided together during the Caledonian orogeny to form the Old Red Sandstone paleocontinent of Laurussia. Another example includes a collision that occurred during the late Pennsylvanian and early Permian time when there was a collision between the two continents of Tarimsky and Kirghiz-Kazakh. This collision was caused because of their askew convergence when the paleoceanic basin closed.
The East Antarctic Shield or Craton is a cratonic rock body that covers 10.2 million square kilometers or roughly 73% of the continent of Antarctica. The shield is almost entirely buried by the East Antarctic Ice Sheet that has an average thickness of 2200 meters but reaches up to 4700 meters in some locations. East Antarctica is separated from West Antarctica by the 100–300 kilometer wide Transantarctic Mountains, which span nearly 3,500 kilometers from the Weddell Sea to the Ross Sea. The East Antarctic Shield is then divided into an extensive central craton that occupies most of the continental interior and various other marginal cratons that are exposed along the coast.
The Yavapai orogeny was an orogenic (mountain-building) event in what is now the Southwestern United States that occurred between 1710 and 1680 million years ago (Mya), in the Statherian Period of the Paleoproterozoic. Recorded in the rocks of New Mexico and Arizona, it is interpreted as the collision of the 1800-1700 Mya age Yavapai island arc terrane with the proto-North American continent. This was the first in a series of orogenies within a long-lived convergent boundary along southern Laurentia that ended with the ca. 1200–1000 Mya Grenville orogeny during the final assembly of the supercontinent Rodinia, which ended an 800-million-year episode of convergent boundary tectonism.