Laurentia
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]
For the use of the surname "Laurentia", see Laurentum. For the bioregion of the same name, see Laurentia (bioregion). For the saint, see Palatias and Laurentia. For other uses, see Laurentia (disambiguation).Etymology[edit]
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]
Interior platform[edit]
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]
Tectonic history[edit]
Assembly[edit]
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]
Formation and breakup of Rodinia[edit]
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]
Paleoenvironmental change[edit]
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]
