WILSON CYCLES, TECTONIC INHERITANCE, AND RIFTING ON THE NORTH AMERICAN GULF OF MEXICO CONTINENTAL MARGIN

The tectonic evolution of the North American Gulf of Mexico continental margin is characterized by two Wilson Cycles – repeated episodes of opening and closing of ocean basins along the same structural trend. This evolution includes i) the Precambrian Grenville orogeny; ii) formation of a rift/transform margin during Late Precambrian opening of the Iapetus Ocean; iii) the Late Paleozoic Ouachita orogeny during assembly of Pangea; and iv) Mesozoic rifting during opening of the Gulf of Mexico. Unlike the Appalachian/Caledonian orogen, where Wilson Cycles were first recognized, the Ouachita orogen does not involve large amount of crustal shortening or thickening. Instead, the Ouachita orogen is underlain by the lower plate of a relatively pristine Paleozoic subduction system that is characterized by a mantle shoal rather than a crustal isostatic root. A finite element model simulating extension on the margin demonstrates that this preexisting structure exerted fundamental controls on the style of Mesozoic rifting. The shallow mantle created a strong lithosphere beneath the orogen, causing extension to initiate adjacent to, rather than within, the orogen. This is in contrast to rifting on the Atlantic Pangean margins, where the Appalachian/Caledonian orogeny created a thick crustal root, resulting in a weak lithosphere and focusing of extension within the interior of the orogen. Major features of the modern Gulf margin, including the Interior Salt Basin, outboard unextended Wiggins Arch, and an unusually broad region of extension beneath the coastal plain and continental shelf, are direct consequences of the pre-rift structure of the margin.