The latest Proterozoic and Paleozoic southern margin of the North American continent is interpreted to have been located in what is now northern Mexico. The interpretation is based on (1) the convergence across northern Mexico of the Cordilleran geosynclinal belton the west toward the Ouachita geosynclinal belt on the east, and (2) the lack of North American uppermost Proterozoic and Paleozoic shelf and cratonal platform rocks south of lat 28° in Mexico. Complex rifting in the latest Proterozoic to Cambrian is interpreted to have formed the southern margin and may have produced microcontinental blocks now represented by Precambrian crystalline basement rocks in eastern Mexico and possibly elsewhere in the Caribbean region. Precambrian crystalline rocks, associated Paleozoic sedimentary rocks, and perhaps far-traveled exotic terranes were accreted to North America in the late Paleozoic, and perhaps Triassic, and formed the nucleus of Mexico.

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