SEISMIC, WELL, AND GRAVITY CONSTRAINTS ON THE STRUCTURE AND EVOLUTION OF THE MISSISSIPPI COASTAL PLAIN AND GULF OF MEXICO

Crustal scale geological transects crossing the Gulf of Mexico (GOM) continental margin in Mississippi are constructed on the basis of seismic reflection and refraction data, well data, and bouguer gravity data. The transects reveal an early Cambrian passive continental margin preserved beneath the Late Paleozoic Ouachita fold and thrust belt (OFTB) that was little affected by later tectonic events. The crystalline crust on this ancient margin decreases from 35 km-thick to 10 km over a distance of only 50 km, which is similar to modern transform margins. The OFTB is a thin-skinned doubly vergent orogen bounded to the north by the late paleozoic Black Warrior foreland basin (BWB) and to the south by the Mississippi Interior Salt basin (MISB). Middle and late Paleozoic sedimentary rocks in the BWB reach a maximum thickness of about 17 km beneath the frontal Ouachita thrusts. Mesozoic extension appears to be restricted to the allocthonous terrane south of the OFTB. This is attributed to the presence of a shallow mantle in the late Paleozoic suture zone, which resulted in a zone of relatively high strength beneath the OFTB. This zone of strength acted as a barrier to Mesozoic extension, restricting deformation to regions south of the suture zone. The data are consistent with interpretations elsewhere in the Ouachita system that depict preservation of Cambrian oceanic crust emplaced beneath the allocthonous terrane in a southward dipping subduction system. Mesozoic extension appears to have been distributed over a broad area that included both the MISB and regions south of the Wiggins arch (WA). Extension became more focused south of the WA with time, ultimately leading to continental breakup and the onset of seafloor spreading in the central GOM beneath the modern continental rise. Maximum Mesozoic extension factors are 1.4-1.9 beneath the MISB, 2.9 immediately south of the WA, and about 4 adjacent to the oldest oceanic crust in the central GOM.