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Structure and evolution of the central Gulf of Mexico continental margin and coastal plain, southeast United States

D.L. Harry^,1 and J. Londono^,1

¹ Department of Geological Sciences, University of Alabama, Box 870338, Tuscaloosa, Alabama 35487-0338, USA

As in many continental margins, Mesozoic rifting in the Gulf of Mexico trended subparallel to older tectonic structures created by opening and closure of a prior ocean basin. Unlike the situation in most other margins, however, extension in the Gulf of Mexico did not involve widespread reactivation of these older tectonic features. This difference may be related to the unusual facts that (1) the antecedent passive continental margin was a transform rather than a rift margin and (2) the prior orogeny did not substantially thicken the crust. In order to characterize the interaction between Mesozoic rifting and prior tectonic events, two geophysical transects were constructed across the central Gulf of Mexico coastal plain and continental shelf. The transects show that the early Paleozoic passive continental margin buried beneath the late Paleozoic Ouachita orogen was little affected by compressional tectonism or Mesozoic extension during opening of the Gulf of Mexico. The thickness of the crust on this ancient margin decreases from 35 km to 10 km over a distance of only 50 km, consistent with previous interpretations that this was a transform margin. The Ouachita fold-and-thrust belt is shown to be a thin-skinned doubly vergent orogen that formed above a well-preserved southward-dipping subduction system. Relict Cambrian oceanic crust may exist beneath the southern part of the orogen, but its presence is not required by the data. The thin crust and shallow mantle in the Ouachita suture created a zone of relatively high strength that acted as a barrier to Mesozoic extension, which is restricted to regions south of the orogen. Extension was initially distributed over a broad area that included the Mississippi Interior Salt Basin and regions south of the Wiggins Arch. Extension became progressively more focused south of the Wiggins Arch with time, culminating in the onset of seafloor spreading beneath the modern continental rise. Mesozoic extension factors are ß = 1.4–1.9 beneath the Mississippi Interior Salt Basin, ß 2.9 immediately south of the Wiggins Arch, and ß 4 adjacent to the oldest oceanic crust in the central Gulf of Mexico.

Key Words: Gulf of Mexico • North America • rifting • Ouachita orogeny • gravity profiles

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