ABSTRACT

Regional structures beneath the northern Gulf of Mexico coastal plain clearly reveal the fit of the American continents following Late Paleozoic Appalachian-Ouachita orogeny. Most diagnostic is a wrench fault zone within the former Gondwana megacontinent, recognizable from the Florida panhandle to western Mississippi, along which future South America moved northwest against the southern edge of eastern North America to form the Ouachita foldbelt, while future Africa was already subducting that continent along the Appalachian belt. Extending westward from this in southwestern Alabama is the Wiggins arch, underlain by "granite" and phyllite of Late Pennsylvanian into Early Permian age, apparently part of a volcanic arc. Along its north side and that of its counterparts farther west are shallow-marine strata of similar age. These appear to occupy a remnant ocean on North American oceanic crust, which was uncoupled by the close approach of future South America and subducted very briefly beneath North America, while the sediments on its leading edge were peeled off and thrust onto the continent. The inferred volcanic arc and remnant ocean, and the Late Triassic Interior rift system that separates them from the Ouachita foldbelt, terminate abruptly in East Texas against a wrench fault that transferred this rifting south-southwest to the Rio Grande embayment area.

Taking the Pickens-Quitman-Gilbertown-Pollard and the Mexia-Talco fault zones, which mark the approximate updip limit of appreciable Louann Salt, to represent these northward-converging wrench faults, northern South America's Guyana shield fits snugly against them, with slight overlap onto the intervening Wiggins and Sabine uplifts. With Africa conventionally joined to South America, this continental fit also places West Africa's two prominent capes opposite pronounced salients of the Appalachian foldbelt, while capes Cod and Hatteras face the principal West African basins, which occupy salients in the equivalent African foldbelt. Also, pre-Mesozoic Florida lies in the reentrant between the southern continents, seemingly separated from each by an arm of a triple rift system whose third arm passes between them.

From this tightly-closed condition the Gulf of Mexico opened by relative right-lateral translation of eastern North America (Florida to Nova Scotia) against the bulge of northwestern Africa, actually a rotation about a pole near the central southern edge of the Sahara. The Interior rift system formed while the sialic block that includes the Wiggins and Sabine uplifts remained briefly with South America as North America moved north-northeast along the East Texas transform and a parallel one at the east end of the Wiggins arch. The rifting jumped south of these uplifts after extension of perhaps 50 mi (80 km) to open the present Gulf basin. Almost 200 mi (320 km) of such translation brought Cape Hatteras solidly against Africa's Cap Blanc, ending this movement in earliest Jurassic time.

Magnetic anomaly lineaments off the southeastern states, used as crude isochrons, show that the central North Atlantic Ocean (CNAO) began to open by southern North America swinging westward to disengage the obstructing capes. The Blake Plateau basin and Carolina trough opened first, then the spreading jumped east of the latter and propagated northward along the entire CNAO, followed by a second jump marked by development of the Blake Spur magnetic anomaly lineament. Changes in North America's movement direction throughout this period were recorded by the Great Abaco fracture zone, as reinterpreted, along which the early CNAO terminated southward against the Florida-Bahama platform. Right-lateral offsets of this fracture zone totalling about 65 mi (100 km) are associated with the two spreading-center jumps.

The Late Triassic intercontinental transform continued across northwestern Morocco into the Tethyan sea, which then extended west into what's now the northern CNAO. As the CNAO opened to its full length it appropriated this tip of Tethys and left the intervening Moroccan Meseta with Africa. Tethys' venture into the future CNAO followed extensive earlier Triassic rifting in eastern North America, which formed an inferred graben system responsible for the Brunswick magnetic anomaly lineament and prepared the way for subsequent intercontinental translation.