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Laurentia

From Wikipedia, the free encyclopedia
For the use of the surname "Laurentia", seeLaurentum.For thebioregionof the same name, seeLaurentia (bioregion).For the saint, seePalatias and Laurentia.For other uses, seeLaurentia (disambiguation).

Laurentia basement rocks

Laurentiaor theNorth American Cratonis a large continentalcratonthat forms theancient geological coreofNorth America.Many times in its past, Laurentia has been a separatecontinent,as it is now in the form of North America, although originally it also included the cratonic areas ofGreenlandand theHebridean Terranein northwestScotland.During other times in its past, Laurentia has been part of larger continents andsupercontinentsand consists of many smallerterranesassembled on a network of earlyProterozoicorogenic belts.Smallmicrocontinentsand oceanic islands collided with andsuturedonto the ever-growing Laurentia, and together formed the stablePrecambriancraton seen today.[1][2][3]

Etymology

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The craton is named after theLaurentian Shield,through theLaurentian Mountains,which received their name from theSt. Lawrence River,named afterSaint Lawrenceof Rome.[4]

Interior platform

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In eastern and central Canada, much of the stable craton is exposed at the surface as theCanadian Shield,an area ofPrecambrianrock covering over a million square miles. This includes some of the oldest rock on Earth, such as theArcheanrock of theAcasta Gneiss,which is 4.04 billion years (Ga) old, and theIstaq Gneiss Complexof 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 withsedimentary rockson the broad interior platform in theMidwestandGreat Plainsregions and is exposed only in northern Minnesota, Wisconsin, the New YorkAdirondacks,and theUpper 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 aremetamorphicorigneouswith the overlyingsedimentary layerscomposed mostly oflimestones,sandstones,andshales.[7]These sedimentary rocks were largely deposited 650–290 Ma.[8]

The oldest bedrock, assigned to the ArcheanSlave,Rae,Hearne,Wyoming,Superior,andNainProvinces, 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 theNares Strait,but this is aPleistoceneerosional 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 andDevoniantoPaleogeneformations on its western and eastern margins. The eastern and northern margins were heavily deformed during theCaledonian orogeny.[11][10]

TheIsua Greenstone Beltof western Greenland preserves oceanic crust containingsheeted dike complexes.These provide evidence to geologists thatmid-ocean ridgesexisted 3.8 Ga. TheAbitibi gold beltin the Superior Province is the largest greenstone belt in the Canadian Shield.[12]

Tectonic history

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Assembly

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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 theTrans-Hudson orogenic belt,which likely were similar to the modernHimalayas,[3]and theWopmay orogenof northwest Canada.[14]During the assembly of the core of Laurentia,banded iron formationwas deposited in Michigan, Minnesota, and Labrador.[15]

The resulting nucleus of Laurentia was mostly reworked Archean crust but with somejuvenile crustin the form ofvolcanic arcbelts. Juvenile crust is crust formed from magma freshly extracted from theEarth's mantlerather 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 theaccretionofisland arcsand 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-livedconvergent plate boundary.Major accretion episodes included theYavapai orogenyat 1.71 to 1.68 Gya, which welded the 1.8 to 1.7 Ga Yavapai province to Laurentia; theMazatzal orogenyat 1.65 to 1.60 Gya, accreting the 1.71 to 1.65 Ga Mazatzal province;[3]thePicuris orogenyat 1.49 to 1.45 Gya,[17]which may have welded the 1.50 to 1.30 Ga Granite-Rhyolite province to Laurentia; and theGrenville orogenyat 1.30 to 0.95 Gya, which accreted the 1.30 to 1.00 Ga Llano-Grenville province to Laurentia.

ThePicuris orogeny,in particular, was characterized by the intrusion of great volumes ofgranitoidmagma into the juvenile crust, which helped mature the crust and stitch it together.Slab rollbackat 1.70 and 1.65 Gya deposited characteristicquartzite-rhyolitebeds 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, andfertile(ripe for extraction of magma) mantle lithosphere.[3]Thesubductionunder the southeast margin of the continent likely caused enrichment of thelithosphericmantle beneath the orogenic belts of theGrenville Province.[18]Around 1.1 Gya, the center of the craton nearly rifted apart along theMidcontinent Rift System.This produced theKeweenawan Supergroup,whoseflood basaltsare rich in copper ore.[19]

Formation and breakup of Rodinia

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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 ofRodinia.[18][21][22]According to the Southwest U.S. and East Antarctica orSWEAT hypothesis,Laurentia became the core of thesupercontinent.It was rotated approximately 90 degrees clockwise compared with its modern orientation, withEast Antarcticaand Australia to the north (what is now the west),Siberiato the east (present north),BalticaandAmazoniato the south (present east), andCongoto the southwest (present southeast). TheGrenville orogenextended 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 Chinato the northwest, Baltica to the east, and Amazonia andRio de la Platato the south.[24]

The breakup of Rodinia began by 780 Ma, when numerousmaficdike swarmswere emplaced in western Laurentia.[25]Early stages of rifting produced theBelt Supergroup,which is over 12 kilometers (7.5 mi) thick.[26]By 750 Ma the breakup was mostly complete, andGondwana(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 (theSnowball Earthhypothesis.)[24]

Pannotia and after

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Pannotia 545 Ma, view centred on the South Pole.[27]

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 westernIapetus Ocean.Sometime in the earlyCambrian,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 middleSilurian.[10]During the early to middleOrdovician,several volcanic arcs collided with Laurentia along what is now the Atlantic coast of North America. This caused an episode of mountain-building called theTaconic orogeny.[30]As the mountains raised by the Taconic orogeny were subsequently eroded, they produced the immenseQueenston Delta,recorded in the rocks of theQueenston 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 marginalorogenic 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 majormarine transgressions(episodes of continental flooding) during the early Paleozoic, the Sauk and the Tippecanoe. During this time, theWestern Cordillerawas apassive 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 theshelfedge.[34]

The position of the equator during theLate Ordovicianepoch(c. 458– c.  444Ma) 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-beddedThalassinoidesfacies and loose shells or nonamalgamatedbrachiopodshell 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]

Laurussia

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Paleogeography of Earth in the middle Silurian, around 430 Ma. Avalonia and Baltica have fused with Laurentia to form Laurussia.

At the end of the Cambrian, about 490 Mya,Avaloniarifted 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 theCaledonian 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 theEarly 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 theChattanooga Shale[39]and theAntler Orogenyin the Western Cordillera.[40]

Formation of Pangaea

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Paleogeography of Earth in the late Carboniferous, around 310 Ma. Laurussia has fused with Gondwana to form Pangea.

During theCarboniferousandPermian,Laurussia fused with Gondwana to formPangaea.The resultingAlleghanian orogenycreated theCentral 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 extensivecoalbeds, 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 calledcyclothems.[44]

During thePennsylvanian,theAncestral Rocky Mountainswere 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, andevaporiteswere deposited in thePermian Basin.[47]Sedimentary beds deposited in the southwest in the early Triassic werefluvialin character, but gave way toeolianbeds in the late Triassic.[48]Pangaea reached its height about 250 Ma, at the start of theTriassic.[49]

Breakup of Pangaea

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The breakup of Pangaea began in the Triassic, with rifting along what is now the east coast of the U.S. that producedred beds,arkosic sandstone,and lakeshaledeposits.[48]The central Atlanticocean basinbegan 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]

TheGulf of Mexicoopened during the Late Triassic and Jurassic. This was accompanied by deposition of evaporite beds that later gave rise tosalt domesthat are importantpetroleum reservoirstoday.[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 thePaleogene.[10]

Fourorogeniesoccurred in the Mesozoic in the Western Cordillera: theSonoma,Nevadan,Sevier,andLaramide.The Nevadan orogeny emplaced the extensive batholiths of theSierra Nevada.[50]The regression of theSundance Seain the late Jurassic was accompanied by deposition of theMorrison Formation,notable for its vertebrate fossils.[48]

DuringCretaceoustimes, theWestern Interior Seawayran from the Gulf of Mexico to theArctic 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]Chalkbeds of theNiobrara Formationwere deposited at this time, and accretion of crustal fragments continued along the Western Cordillera.[48]

In the Cenozoic

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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 earlierbasement 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 theBasin and Range Provincein the middle Cenozoic and the uplift of theColorado Plateau.The Colorado Plateau was uplifted with remarkably little deformation. The flood basalts of theColumbia Plateaualso 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 theBasin and Range Provincehas been stretched up to 100% of its original width.[53]The area experienced numerouslarge volcanic eruptions.Baja California rifted away from North America during theMiocene.[49]This block of crust consists of Proterozoic to early Paleozoic shelf and Mesozoic arc volcano formations.[54][10]TheHolocenebeing aninterglacial,a warm spell between episodes of extensive glaciation.[52]

Paleoenvironmental change

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Several climate events occurred in Laurentia during thePhanerozoiceon. During the lateCambrianthrough theOrdovician,sea level fluctuated withice capmelt. Nine macro scale fluctuations of "global hyper warming", or high intensitygreenhouse gasconditions, occurred.[55]Due to sea level fluctuation, these intervals led to mudstone deposits on Laurentia that act as a record of events.[55]The lateOrdovicianbrought a cooling period, although the extent of this cooling is still debated.[56]More than 100 million years later, in thePermian,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, duringPermianglobal warm period, northern and northwestern Pangea (western Laurentia) remained relatively cool.[57]

Geological history

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  • Around 4.03 to 3.58Ga,the oldest intact rock formation on the planet, theAcasta Gneiss,was formed in what is nowNorthwest Territories(older individual mineral grains are known, but not whole rocks).[58]
  • Around 2.565 Ga,Arcticaformed as an independent continent.
  • Around 2.72 to 2.45 Ga, Arctica was part of the supercontinentKenorland.[clarification needed]
  • Around 2.1 to 1.84 Ga, when Kenorland broke apart, the Arctican craton was part of the landmassNenaalong withBalticaand EasternAntarctica.
  • Around 1.82 Ga, Laurentia was part of the supercontinentColumbia.
  • Around 1.35–1.3 Ga, Laurentia was an independent continent.
  • Around 1.3 Ga, Laurentia was part of the landmassProtorodinia.
  • Around 1.07 Ga, Laurentia was part of the supercontinentRodinia.
  • Around 750 Ma, Laurentia was part of the landmassProtolaurasia.Laurentia nearly rifted apart.
  • In theEdiacaran(635 to 541 ±0.3 Ma), Laurentia was part of the supercontinentPannotia.
  • In theCambrian(541 ±0.3 to 485.4 ±1.7 Ma), Laurentia was an independent continent.
  • In theOrdovician(485.4 ± 1.7 to 443.8 ±1.5 Ma), Laurentia was shrinking and Baltica was expanding.
  • In theDevonian(419.2 ± 2.8 to 358.9 ±2.5 Ma), Laurentia collided against Baltica, forming the landmassEuramerica.
  • In thePermian(298.9 ± 0.8 to 252.17 ±0.4 Ma), all major continents collided against each other, forming the supercontinentPangaea.
  • In theJurassic(201.3 ± 0.6 to 145 ±4 Ma), Pangaea rifted into two landmasses:LaurasiaandGondwana.Laurentia was part of the landmass Laurasia.
  • In theCretaceous(145 ± 4 to 66 Ma), Laurentia was an independent continent called North America.
  • In theNeogene(23.03 ± 0.05 Ma until today or ending 2.588 Ma), Laurentia, in the form of North America, collided withSouth America,forming the landmass America.


See also

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References

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Works cited

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