Paleomagnetic data indicate that North America is now moving northward, and has had a measurable northward component of motion since late in Paleozoic time. The Gulf of Mexico is thought to have formed as a tensional feature in the wake of the continent. Twenty-two lines of supporting evidence include: earthquake-epicenter alignments, first-motion data (still very scanty), new offset information on a major fault, migration rates from paleomagnetic data, a new east-west graben, the apparently tensional nature of features such as the Mexico and Cayman Trenches, Appalachian?-type strata recently described from Mexico, tensional effects in the central Mississippi Valley, tensional indications along the St. Lawrence Valley, long-term subsidence of the Blake Plateau, the tapere shape of the Atlantic coastal plain, diapirism in the coastal plain, and several well-known items such as the general east-west orientation of grabens in, for example, Louisiana and southeastern Mexico.

The rate of north-south widening during Mesozoic and Cenozoic time has been approximately 2 cm/yr. This should have produced a Gulf 1,000-2,000 km wide in the north-south direction.

A time summary of 12 specific deformational events shows that the southeastern part of North America underwent an important change in tectonic style, during the Pennsylvanian-Permian-Triassic interval. The "Paleozoic style" can be described in terms of compression and left-lateral displacement along the southern Appalachian trend; the "Mesozoic-Cenozoic style" can be characterized in terms of north-south tension (and its corollary, right-lateral displacement along the same trend). Part of the difficulty inherent in understanding the tectonic history of the southern and southeastern part of North America arises from this major reversal. The northward migration of the continent in Mesozoic and Cenozoic time appears to have had the second-order result of thinning from east to west. This hinning may have caused a gentle widening (over a period of about 10⁸ years) of the North Atlantic basin. The proposed widening was not, however, nearly as great as that envisaged in the Wegener-du Toit hypothesis of continental drift.

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