Core Of North America

Most of the core of North America was assembled by the collision of Archean cratons in the Paleoprotero-zoic, between 2.5 and 1.6 billion years ago. Paleo-magnetic data from the slave, superior, Churchill, and Wyoming cratons all show relative convergence of thousands of miles (up to 4,000 km) in the Late Paleoproterozoic, and wide collisional orogens developed between these cratons. At this time the Churchill Province consisted of the previously joined Rae and Hearne Provinces, and probably also the Wyoming craton in the south. These were joined with Baltica to form a Late Paleoproterozoic supercontinent called Nuna, after an Inuit name for the lands bordering the northern oceans.

During the formation, Nuna oceanic crust was subducted beneath the Churchill Province, forming extensive belts of Paleoproterozoic plutonic rocks that form large convergent margin arc sequences, now deeply eroded. The margins of the slave and superior cratons that collided with the Churchill show more passive margin-type sequences, and as the magmati-cally thickened and heated crust of the Churchill Province was caught between the colliding slave and superior cratons, it became strongly deformed and was extruded laterally in escape tectonics as the convergence continued. The extrusion was accompanied by thickening of the province, which formed a large collisionally thickened plateau, similar to deep levels of the Tibetan plateau forming in the present plate mosaic as a result of the India-Asia collision. The Churchill Province therefore preserves many large continental scale ductile shear zones formed during the Paleoproterozoic amalgamation of Nuna. After the collision was complete, the Churchill Province experienced extensional collapse and then intrusion of post-orogenic granitoid rocks, including rapakivi granites and other intraplate magmas.

The margins of the slave and superior cratons that collided with the Churchill Province in the Late Paleoproterozoic show large-scale fold-thrust belts and nappes that were directed away from the core of the orogen in the Churchill Province. The rocks in these fold-thrust belts include rifted margins formed between 2.1 and 2.0 billion years ago overlain by passive margin sediments that were caught in the thrusting at 1.6-1.9 billion years ago.

some of the Paleoproterozoic suture zones surrounding the superior craton contain remnants of closed oceans. On the Ungava Peninsula a 2.0 billion-year-old ophiolite, 1.92 billion-year-old oceanic plateau, and 1.87-1.83-billion-year-old oceanic island arc were thrust over a rifted margin of the superior craton to the south, and similar sequences are known from the western side of Hudson's Bay. Overall, the structural style and types of rocks in these Paleopro-terozoic orogens are very similar to orogenic belts formed in the Paleozoic and younger times.

During the formation of the Nuna supercontinent, the slave, superior, and other smaller blocks including the Nain craton collided with the Churchill Province, and these cratons all had passive margins on their trailing, or back sides. Soon after they collided with the Churchill Province, most of these passive margins experienced collision of arc terranes; then the direction of subduction along those margins reversed, so that after the collision, the margins were converted to Andean-style convergent margins. On the western side of the Slave Province the Wopmay orogen formed in this way, where a rift to passive margin formed on the Slave crust and was followed by the accretion of an arc and collision with the Great Bear magmatic arc at 1.88 billion years ago. Other belts experienced similar collisions at 1.85 and 1.84 billion years ago, showing that material continued to be accreted to Nuna, or ancestral North America, after the heartland formed.

The growth of North America continued at 1.8 to 1.6 billion years ago with the accretion of juvenile crust that underlies much of the Central Plains and the southwestern United States in the Central Plains, Yavapai, and Mazatzal orogens.

In the Mesoproterozoic from 1.6 to 1.0 billion years ago, the tectonics of North America were dominated by the formation of the Grenville orogen. The Grenville orogen forms a belt of high-grade meta-morphic rocks that stretches from Newfoundland and Labrador through eastern Canada to the Great Lakes and Adirondack region of the United States, then through to Texas and the Sierra Madre of Mexico. The Grenville orogen is part of a worldwide belt of similar orogens with a total length of more than 6,000 miles (10,000 km) that formed as many continental nuclei and cratons similar in aspect to North America collided to form the supercontinent of Rodinia. These continental fragments included North America, Baltica, Amazonia, West Africa, the Kalahari craton, Congo craton, India, Australia, and parts of Siberia. Rodinia existed as a supercontinent from about 1.0 billion years ago until about 750 million years ago, when widespread rifting caused many of these fragments to be dispersed in a new phase of breakup in the supercontinent cycle. Some of the continents rifted off somewhat earlier, some even as early as 900 million years ago, not very long after Rodinia had just finished forming.

After the breakup of Rodinia in the Mesoprotero-zoic, many of the continental fragments reassembled on the opposite side of the globe in an "inside-out" configuration, where fragments formerly on the exterior parts of Rodinia found themselves located on the inside of the next supercontinent, Gondwana. This progressive breakup and reamalgamation also left formerly interior parts of the continents, including North America, outside the Gondwana supercontinent altogether or located on its outer fringes. Gond-wana formed by the joining of the main blocks of the Congo and Kalahari cratons, West Africa, Amazon and Rio del Plata cratons, India, East Antarctica, and Australia at about 545 million years ago.

During the evolution of Gondwana, North America formed a northern supercontinent known as Lau-rentia. During the Paleozoic and Mesozoic, North America saw its most significant evolution along the western and eastern margins of the continent, in the Cordilleran or Rocky Mountain fold belts and the Applachian-Ouachita orogen. Other belts on the fringes of North America include the Caledonian belt and the Innuitian belt in the Canadian Arctic.

Continue reading here: Appalachians

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