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A series of basement weakness zones trending northeast and northwest define a plate framework in the basement rocks of North America. The weakness zones began during the primordial solidification of the crust as a result of differential rotation between the equator and the poles, which created a left-lateral couple in the northern hemisphere.
The northeasterly weakness zones were formed by this hemispheric coupling as major tensional zones which define a series of northeast-trending horsts (Slave and Superior plates) and grabens (Arctic, Churchill, and Appalachian-Grenville plates). The northwest-trending weakness zones developed as faults flanking a major dragfold (Canadian shield) produced by the hemispheric coupling.
As horsts and grabens, the northeast-trending plates became the sites of Precambrian erosion and deposition. They later were deformed during Precambrian orogenies by severe coupling produced by right-lateral simple shear acting upon the northeast-trending weakness zones. The orogenic forces producing the simple shear were provided by periodic opening of the Pacific Ocean, which compressed North America during Precambrian and Paleozoic times.
The opening of the Atlantic Ocean, which began in the Triassic, forced North America west-southwestward over the eastern Pacific and eventually on to the mid-Pacific ridge. This caused the Laramide orogeny, which activated the northwest-trending weakness zones, producing coupling of the northwest-trending plates. The oblique encounter of North America and the East Pacific Rise also produced right-lateral, simple shear phenomena along the continent's western margin.
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