Regional free-air gravity data from the Gulf of Mexico define deep-seated linear features which we interpret as outer marginal basement highs. Highs are arranged symmetrically around the deep water Gulf. Abrupt changes in trend occur along three well-defined zones on both sides of the central Gulf. The marginal high off Galveston parallels the Cretaceous Edwards Reef trend 230 km to the northwest. We interpret the seaward limits of these outer marginal highs as close to the landward edges of oceanic crust. The crust in the area from the continental hinge zone (near which the Edwards Reef developed) to the outer high is thinned, faulted, and intruded by mafic dikes, but probably has a nearly continental overall composition.

We infer that the Gulf opening followed a pattern of early rifting and subsequent sea-floor spreading. Our model implies that the thick salt deposits underlying the modern Texas slope were deposited on sediments overlying oceanic crust. Thinner salt deposits overlie pre-salt sediments on rift-stage crust both northwest of the Texas outer marginal high and in the Sigsbee knolls. A change in the location of the sea-floor spreading center led to separation of the two main salt depo-centers beneath the Sigsbee Knolls and along the Texas-Louisiana shelf and slope. Recent paleomagnetic evidence (L. Sanchez Barreda, personal communication, 1981) indicates a post-Permian, 24° clockwise rotation of Chiapas relative to Oaxaca (this is consistent with our rotational model). The Salina Cruz f ult, crossing the Isthmus of Tehuantepec between the Permian outcrops in Chiapas and Oaxaca, was probably a major transform fault active during the Gulf opening.