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73-659: 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

146-606: A passive margin . 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. 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. Flooding of

219-400: A rising plume of molten material from the deep mantle. This would have built up a thick layer of depleted mantle underneath the cratons. A third model suggests that successive slabs of subducting oceanic lithosphere became lodged beneath a proto-craton, underplating the craton with chemically depleted rock. A fourth theory presented in a 2015 publication suggests that the origin of

292-629: 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. 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. In eastern and central Canada, much of

365-457: A solid residue very close in composition to Archean lithospheric mantle, but continental shields do not contain enough komatiite to match the expected depletion. Either much of the komatiite never reached the surface, or other processes aided craton root formation. There are many competing hypotheses of how cratons have been formed. Jordan's model suggests that further cratonization was a result of repeated continental collisions. The thickening of

438-1238: A term for mountain or orogenic belts . Later Hans Stille shortened the former term to Kraton , from which craton derives. Examples of cratons are the Dharwar Craton in India, North China Craton , the East European Craton , the Amazonian Craton in South America, the Kaapvaal Craton in South Africa, the North American Craton (also called the Laurentia Craton), and the Gawler Craton in South Australia. Cratons have thick lithospheric roots. Mantle tomography shows that cratons are underlain by anomalously cold mantle corresponding to lithosphere more than twice

511-648: Is a right-angled suture zone that extends eastward from Saskatchewan through collisional belts in the Churchill province , through northern Quebec , parts of Labrador and Baffin Island , and all the way to Greenland as the Rinkian belt and Nagssugtodidian Orogen. Westward it goes across Hudson Bay through Saskatchewan and then extends 90 degrees south through eastern Montana and the western Dakotas , downward through eastern Wyoming and western Nebraska , and

584-787: 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 . 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

657-573: 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. 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

730-452: Is strongly influenced by the inclusion of moisture. Craton peridotite moisture content is unusually low, which leads to much greater strength. It also contains high percentages of low-weight magnesium instead of higher-weight calcium and iron. Peridotites are important for understanding the deep composition and origin of cratons because peridotite nodules are pieces of mantle rock modified by partial melting. Harzburgite peridotites represent

803-974: Is then cut off by the Cheyenne belt - the northern edge of the Yavapai province (see Trans-Hudson Orogen map and the THOT Transect map. To the south, the orogen contributed to the subsurface Phanerozoic strata in Montana and the Dakotas that created the Great Plains . The Trans-Hudson orogeny was the culminating event of the Paleoproterozoic Laurentian assembly, which occurred after the Wopmay orogeny (West of Hudson Bay , ca. 2.1-1.9 Ga.). The Trans-Hudson orogeny resulted from

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876-553: Is used to distinguish the stable portion of the continental crust from regions that are more geologically active and unstable. Cratons are composed of two layers: a continental shield , in which the basement rock crops out at the surface, and a platform which overlays the shield in some areas with sedimentary rock . The word craton was first proposed by the Austrian geologist Leopold Kober in 1921 as Kratogen , referring to stable continental platforms, and orogen as

949-699: The Appalachian Plateau ). The Black Hills of South Dakota is one of the few remaining exposed portions of the Trans-Hudson orogenic belt. The peaks of the Black Hills are 3,000 to 4,000 feet above the surrounding plains, while Black Elk Peak - the highest point in South Dakota - has an altitude of 7,242 feet above sea level. These central spires and peaks all are carved from granite and other igneous and metamorphic rocks that form

1022-667: The Baltic Shield had been eroded into a subdued terrain already during the Late Mesoproterozoic when the rapakivi granites intruded. Trans-Hudson orogeny The Trans-Hudson orogeny or Trans-Hudsonian orogeny was the major mountain building event ( orogeny ) that formed the Precambrian Canadian Shield and the North American Craton (also called Laurentia ), forging the initial North American continent . It gave rise to

1095-627: The Basin and Range Province has been stretched up to 100% of its original width. The area experienced numerous large volcanic eruptions . Baja California rifted away from North America during the Miocene . This block of crust consists of Proterozoic to early Paleozoic shelf and Mesozoic arc volcano formations. The Holocene being an interglacial , a warm spell between episodes of extensive glaciation. Several climate events occurred in Laurentia during

1168-608: The Belt Supergroup , which is over 12 kilometers (7.5 mi) thick. 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. The breakup of Rodinia may have triggered an episode of severe ice ages (the Snowball Earth hypothesis.) There is some evidence that the fragments of Rodinia gathered into another short-lived supercontinent, Pannotia , at

1241-625: The Black Hills of South Dakota . The Trans-Hudson orogeny and the consequent upheaval of the continental crust in the middle Proterozoic eon caused the area around the Great Lakes to become a flattened plain, which in turn led to the creation of the intracontinental basin and the interior and central plains of the United States (the Great Plains are the westernmost portion of North America's Interior Plains , which extend east to

1314-637: The Caledonian orogeny . 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. Laurentia first assembled from six or seven large fragments of Archean crust at around 2.0 to 1.8 Gya. The assembly began when

1387-751: The Chattanooga Shale and the Antler Orogeny in the Western Cordillera. During the Carboniferous and Permian , Laurussia fused with Gondwana to form Pangaea . The resulting Alleghanian orogeny created the Central Pangean Mountains . 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

1460-461: The Grenville orogeny at 1.30 to 0.95 Gya, which accreted the 1.30 to 1.00 Gya 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

1533-458: The Hearne - Rae , Superior , and Wyoming cratons to form the cratonic core of North America in a network of Paleoproterozoic orogenic belts. These orogenic belts include the margins of at least nine independent microcontinents that were themselves sections of at least three former major supercontinents, including Laurasia , Pangaea and Kenorland (ca. 2.7 Ga ), and contain parts of some of

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1606-521: 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. Due to sea level fluctuation, these intervals led to mudstone deposits on Laurentia that act as a record of events. The late Ordovician brought a cooling period, although

1679-603: The Trans-Hudson orogen (THO), or Trans-Hudson Orogen Transect (THOT), (also referred to as the Trans-Hudsonian Suture Zone (THSZ) or Trans-Hudson suture ) which is the largest Paleoproterozoic orogenic belt in the world. It consists of a network of belts that were formed by Proterozoic crustal accretion and the collision of pre-existing Archean continents . The event occurred 2.0–1.8 billion years ago. The Trans-Hudson orogen sutured together

1752-674: 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. Chalk beds of the Niobrara Formation were deposited at this time, and accretion of crustal fragments continued along

1825-573: The Wopmay orogen of northwest Canada. During the assembly of the core of Laurentia, banded iron formation was deposited in Michigan, Minnesota, and Labrador. 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. The intense mountain building of

1898-406: The "cratonic regime". It involves processes of pediplanation and etchplanation that lead to the formation of flattish surfaces known as peneplains . While the process of etchplanation is associated to humid climate and pediplanation with arid and semi-arid climate, shifting climate over geological time leads to the formation of so-called polygenetic peneplains of mixed origin. Another result of

1971-684: The Appalachian coal beds in the U.S. 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 . 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 or subduction under

2044-639: 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 , and

2117-765: The Superior Craton, south of the Kisseynew Domain, and east of the Glennie Domain. The Superior Boundary zone is a narrow, southeastern, ensialic foreland zone bordering Superior Craton, comprising the Thompson Belt, Split Lake Block, and Fox River Belt. The Flin Flon greenstone belt is one of the largest Proterozoic volcanic-hosted massive sulfide (VMS) districts in the world, containing 27 Cu-Zn- (Au) deposits from which more than 183 million tonnes of ore have been mined. Most of mined VMS deposits in

2190-863: The THO mountain building (orogeny). The Northwestern hinterland zone is a complex tectonically deformed region that includes the Peter Lake, Wollaston, and Seal River domains, and other parts of the Cree Lake Zone, now included in Hearne Province. The Reindeer zone to the north is a 500 km wide collage of Paleoproterozoic (1.92-1.83 Ga) arc volcanic rocks, plutons, volcanogenic sediments, and younger molasse , divisible into several lithostructural domains. Most of these rocks evolved in an oceanic to transitional, subduction-related arc setting, with increasing influence of Archean crustal components to

2263-454: The Trans-Hudson orogeny formed thick, stable roots beneath the craton, possibly by a process of "kneading" that allowed low density material to move up and high density material to move down. 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

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2336-589: The Trans-Hudson orogeny, rifting at first separated the Superior craton from the rest of the continent. Then the Superior Craton reversed its direction and the ocean basin began to close. A subduction zone formed as the oceanic crust of the Superior Craton was subducted beneath the Hearne and Wyoming Craton with the Sask Craton in the middle. Volcanic arcs developed as the cratons collided, eventually resulting in

2409-595: The Western Cordillera. 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. The Atlantic and Gulf Coasts experienced eight transgressions in the Cenozoic. The Laramide orogeny continued to raise

2482-644: 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. Laurentia was formed in a tectonically active world. The subduction under the southeast margin of the continent is thought to have contributed to the formation of Rodinia . According to the Southwest U.S. and East Antarctica or SWEAT hypothesis , Laurentia became

2555-714: The collision of the Superior Craton of eastern Canada with the Hearne Craton in northern Saskatchewan and the Wyoming Craton of the western United States , with the Archean microcontinent Sask Craton trapped in the THO western interior. Similar to the Himalayas, the Trans-Hudson orogeny was also the result of continent-continent collision along a suture zone. Only the roots of this mountain chain remain, but these can be seen in northeastern Saskatchewan and in

2628-490: 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. These beds imply the presence of an equatorial climate belt that was hurricane free which lay inside 10° of

2701-557: The continental margin from the southwest. Two additional marine transgressions took place during the late Paleozoic: the Kaskaskia and Absaroka. The great continental mass of Pangaea strongly affected climate patterns. The Permian was relatively arid, and evaporites were deposited in the Permian Basin . Sedimentary beds deposited in the southwest in the early Triassic were fluvial in character, but gave way to eolian beds in

2774-403: The continental shelves, and oceanic crust formed on the margins of the continental cratons as the divergence continued. Eventually the divergence stopped, then reversed direction, and collision occurred between continental land masses. During the Wopmay orogeny, subduction occurred as oceanic crust of the Slave Craton was subducted beneath an eastward moving continental plate. Likewise, during

2847-477: The core of an independent continent with the opening of the North Atlantic in the Paleogene . 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 . The regression of the Sundance Sea in the late Jurassic was accompanied by deposition of the Morrison Formation , notable for its vertebrate fossils. During Cretaceous times,

2920-515: 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

2993-466: The core of the uplift. The nature and timing of this portion of the THO event in southern Laurentia is poorly understood, when compared to the exposed northern segments in Canada. The Black Hills offer the only surface exposure of the deformed and metamorphosed belt of Paleoproterozoic continental margin rocks in the collisional zone between the Archean Wyoming and Superior provinces. Based on geophysical evidence, this zone has been broadly interpreted to be

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3066-502: The craton from sinking into the deep mantle. Cratonic lithosphere is much older than oceanic lithosphere—up to 4 billion years versus 180 million years. Rock fragments ( xenoliths ) carried up from the mantle by magmas containing peridotite have been delivered to the surface as inclusions in subvolcanic pipes called kimberlites . These inclusions have densities consistent with craton composition and are composed of mantle material residual from high degrees of partial melt. Peridotite

3139-413: The craton. These included the North Slope of Alaska, which merged during the Early Devonian . Several small crust fragments accreted from the late Devonian through the Mesozoic to form the Western Cordillera. The Western Cordillera became a convergent plate margin during the Ordovician, and the Transcontinental Arch became submerged, only to reappear in the Devonian. The Devonian also saw the deposition of

3212-413: The cratons is similar to crustal plateaus observed on Venus, which may have been created by large asteroid impacts. In this model, large impacts on the Earth's early lithosphere penetrated deep into the mantle and created enormous lava ponds. The paper suggests these lava ponds cooled to form the craton's root. The chemistry of xenoliths and seismic tomography both favor the two accretional models over

3285-399: The crust associated with these collisions may have been balanced by craton root thickening according to the principle of isostacy . Jordan likens this model to "kneading" of the cratons, allowing low density material to move up and higher density to move down, creating stable cratonic roots as deep as 400 km (250 mi). A second model suggests that the surface crust was thickened by

3358-427: The crystalline residues after extraction of melts of compositions like basalt and komatiite . The process by which cratons were formed is called cratonization . There is much about this process that remains uncertain, with very little consensus in the scientific community. However, the first cratonic landmasses likely formed during the Archean eon. This is indicated by the age of diamonds , which originate in

3431-426: The depleted "lid" formed by the first layer. The impact origin model does not require plumes or accretion; this model is, however, not incompatible with either. All these proposed mechanisms rely on buoyant, viscous material separating from a denser residue due to mantle flow, and it is possible that more than one mechanism contributed to craton root formation. The long-term erosion of cratons has been labelled

3504-517: The equator. 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. The breakup of Rodinia began by 780 Ma, when numerous mafic dike swarms were emplaced in western Laurentia. Early stages of rifting produced

3577-500: The equator. This ecological conclusion matches the previous paleomagnetic findings which confirms this equatorial location. 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) in the Caledonian orogeny . This produced the continent of Laurussia. During this time, several small continental fragments merged with other margins of

3650-879: The extent of this cooling is still debated. More than 100 million years later, in the Permian , an overall warming trend occurred. 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. This opposition suggests that, during Permian global warm period, northern and northwestern Pangea (western Laurentia) remained relatively cool. [REDACTED] Africa [REDACTED] Antarctica [REDACTED] Asia [REDACTED] Australia [REDACTED] Europe [REDACTED] North America [REDACTED] South America [REDACTED] Afro-Eurasia [REDACTED] Americas [REDACTED] Eurasia [REDACTED] Oceania Craton The term craton

3723-471: The late Archean, accompanied by voluminous mafic magmatism. However, melt extraction alone cannot explain all the properties of craton roots. Jordan notes in his paper that this mechanism could be effective for constructing craton roots only down to a depth of 200 kilometers (120 mi). The great depths of craton roots required further explanation. The 30 to 40 percent partial melting of mantle rock at 4 to 10 GPa pressure produces komatiite magma and

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3796-412: The late Triassic. Pangaea reached its height about 250 Ma, at the start of the Triassic . 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. The central Atlantic ocean basin began opening at about 180 Ma. Florida, which had been a part of Gondwana before the assembly of Pangaea,

3869-405: The longevity of cratons is that they may alternate between periods of high and low relative sea levels . High relative sea level leads to increased oceanicity, while the opposite leads to increased inland conditions . Many cratons have had subdued topographies since Precambrian times. For example, the Yilgarn Craton of Western Australia was flattish already by Middle Proterozoic times and

3942-414: The middle Silurian . 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 . 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 . There

4015-403: The northwest. The zone overlies Archean basement exposed in structural windows that are now recognized as the Sask craton. The Wathaman-Chipewyan batholith is an Andean-type continental-margin, magmatic arc emplaced 1.86-1.85 Ga. The Flin Flon domain is in the center of the Trans-Hudson Suture Zone and extends over the border of the Manitoba-Saskatchewan segment east and west. It is west of

4088-403: The oldest cratonic continental crust on Earth . These old cratonic blocks, along with accreted island arc terranes and intraoceanic deposits from earlier Proterozoic and Mesozoic oceans and seaways, were sutured together in the Trans-Hudson Orogen (THO) and resulted in extensive folding and thrust faulting along with metamorphism and hundreds of huge granitic intrusions . The THO

4161-436: The overlying sedimentary layers composed mostly of limestones , sandstones , and shales . These sedimentary rocks were largely deposited 650–290 Ma. 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. Greenland

4234-420: The plume model. However, other geochemical evidence favors mantle plumes. Tomography shows two layers in the craton roots beneath North America. One is found at depths shallower than 150 km (93 mi) and may be Archean, while the second is found at depths from 180 to 240 km (110 to 150 mi) and may be younger. The second layer may be a less depleted thermal boundary layer that stagnated against

4307-554: The present Rocky Mountains into the Paleocene. 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. The southwestern portion of Laurentia consists of Precambrian basement rocks deformed by continental collisions. This area has been subjected to considerable rifting as

4380-436: The roots of cratons, and which are almost always over 2 billion years and often over 3 billion years in age. Rock of Archean age makes up only 7% of the world's current cratons; even allowing for erosion and destruction of past formations, this suggests that only 5 to 40 percent of the present continental crust formed during the Archean. Cratonization likely was completed during the Proterozoic . Subsequent growth of continents

4453-459: 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 Gya Yavapai province to Laurentia; the Mazatzal orogeny at 1.65 to 1.60 Gya, accreting the 1.71 to 1.65 Gya Mazatzal province; the Picuris orogeny at 1.49 to 1.45 Gya, which may have welded the 1.50 to 1.30 Gya Granite-Rhyolite province to Laurentia; and

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4526-442: The southern extension of the THO that was later truncated by the ~1.680 Ga. Central Plains orogen . Marine evidence indicates that the area initially opened to form an ocean called the Manikewan Ocean . Faulting, sedimentary and igneous rocks all indicate that divergence formed a rift valley that continued to spread until it resulted in a passive margin in which there was no tectonic activity. Shallow marine deposits formed on

4599-406: 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. The subduction under the southeast margin of the continent likely caused enrichment of the lithospheric mantle beneath the orogenic belts of the Grenville Province . Around 1.1 Gya,

4672-414: 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. When subsurface extensions are considered, the wider term Laurentian Shield

4745-515: The surrounding hotter, but more chemically dense, mantle. In addition to cooling the craton roots and lowering their chemical density, the extraction of magma also increased the viscosity and melting temperature of the craton roots and prevented mixing with the surrounding undepleted mantle. The resulting mantle roots have remained stable for billions of years. Jordan suggests that depletion occurred primarily in subduction zones and secondarily as flood basalts . This model of melt extraction from

4818-503: The typical 100 km (60 mi) thickness of mature oceanic or non-cratonic, continental lithosphere. At that depth, craton roots extend into the asthenosphere , and the low-velocity zone seen elsewhere at these depths is weak or absent beneath stable cratons. Craton lithosphere is distinctly different from oceanic lithosphere because cratons have a neutral or positive buoyancy and a low intrinsic density. This low density offsets density increases from geothermal contraction and prevents

4891-455: The upper mantle has held up well with subsequent observations. The properties of mantle xenoliths confirm that the geothermal gradient is much lower beneath continents than oceans. The olivine of craton root xenoliths is extremely dry, which would give the roots a very high viscosity. Rhenium–osmium dating of xenoliths indicates that the oldest melting events took place in the early to middle Archean. Significant cratonization continued into

4964-542: 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. The breakup of Pannotia produced six major continents: Laurentia, Baltica, Kazakhstania, Siberia, China, and Gondwana. Laurentia remained an independent continent until

5037-426: 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. Throughout the early Paleozoic, Laurentia was characterized by a tectonically stable interior flooded by the seas, with marginal orogenic belts . An important feature

5110-422: Was by accretion at continental margins. The origin of the roots of cratons is still debated. However, the present understanding of cratonization began with the publication in 1978 of a paper by Thomas H. Jordan in Nature . Jordan proposes that cratons formed from a high degree of partial melting of the upper mantle, with 30 to 40 percent of the source rock entering the melt. Such a high degree of melting

5183-454: Was left with Laurentia during the opening of the central Atlantic. This former Gondwana fragment includes the Carolina Slate belt and parts of Alabama. 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. Europe rifted away from North America between 140 and 120 Ma, and Laurentia once again became

5256-478: Was possible because of the high mantle temperatures of the Archean. The extraction of so much magma left behind a solid peridotite residue that was enriched in lightweight magnesium and thus lower in chemical density than undepleted mantle. This lower chemical density compensated for the effects of thermal contraction as the craton and its roots cooled, so that the physical density of the cratonic roots matched that of

5329-650: 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. 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

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