GEOPHYSICAL CONSTRAINTS ON THE STRUCTURE OF THE LAURENTIAN TRANFORM CONTINENTAL MARGIN BENEATH THE CENTRAL GULF OF MEXICO AND SOUTHEASTERN U.S.A. COASTAL PLAIN

A north-striking geophysical transect across the central Gulf of Mexico coastal plain shows that the early Paleozoic continental margin of southern Laurentia is preserved in a nearly pristine state beneath younger strata that were emplaced during the late Paleozoic Ouachita orogeny and formation of the modern Gulf of Mexico coastal plain. The thickness of the crystalline crust decreases abruptly across the margin over a distance of ~50 km, from 35 km beneath the Black Warrior foreland basin in northern Mississippi to 10 km beneath the Ouachita fold-and-thrust belt in central Mississippi. This abrupt decrease in crustal thickness is strikingly similar to modern transform margins, but very different from most rifted margins, which display much more gradual transitions in crustal thickness. Additionally, geophysical results do not support the presence of synrift magmatic rocks, underplated mafic rocks at the base of the crust, or abnormally thick oceanic crust adjacent to the margin, all of which are characteristics of modern transform margins. These new data combined with other transects across the margin farther west confirm previous suggestions that the central Gulf of Mexico coastal plain overlies an ~800-km-long transform segment of the Late Proterozoic–early Paleozoic southern Laurentian continental margin that extends continuously from western Arkansas at least as far eastward as eastern Mississippi.