ABSTRACT

Recent proposals for the origin of the Gulf of Mexico have sought to identify the geometry of the pre-drift blocks of continental crust and their positions relative to each other in a plausible reconstruction prior to the opening of the present Gulf of Mexico. Although a general consensus exists that the central Gulf of Mexico is underlain by more-or-less normal oceanic crust, there is considerable disagreement in the literature as to the exact location of the edges of oceanic crust in the Gulf. Yet, it is precisely the exact location of these oceanic crustal edges along both the northern and southern margins of the Gulf that provides the fundamental key to the opening of the Gulf of Mexico, the nature of subsequent rifted margin development and the original shape of the basin in which thickest Jurassic salt accumulated. A common theme in pre-drift reconstructions is the problem of too many crustal pieces for the existing space when the major continents (Africa, South America and North America) are rejoined across the Atlantic. Creative solutions to this room problem have included appeals to massive selective attenuation of continental crust, relegation of the Chortis Block (Honduras and Nicaragua) to the Pacific and largely speculative shear zones cutting across southern Florida and the Bahamas. Newly published regional gravity and magnetic maps of the Gulf of Mexico shed considerable light on these issues but have unfortunately been largely ignored. Ironically, the root cause for major difficulties with current kinematic reconstructions for the Gulf of Mexico can be shown to stem indirectly from a problem with the correct location of the oceanic crustal edge in the western Atlantic. That is, the room problem for crustal blocks in the Gulf of Mexico arises because conventional reconstructions of the North Atlantic are closed up too tightly, all the way back to the East Coast Magnetic Anomaly (ECMA), widely interpreted as a magnetic effect at the western edge of oceanic crust. We argue here that no less than four categories of evidence imply that this edge effect interpretation is inconsistent and erroneous: There are oceanic crustal edges without "edge effects" and "edge effects" without oceanic crustal edges in both the Gulf of Mexico and the western North Atlantic. Once this crucial (and dubious) assumption is abandoned, a simple and symmetrical explanation for the large-scale tectonic evolution of the two main U. S. passive margins can be developed naturally with two major beneficial results: 1) A simple pre-drift paleoreconstruction for the Gulf of Mexico with no associated room problem and 2) A consistent interpretation of the ECMA and the Gulf Coast Magnetic Anomalies as related to the long sought-after Alleghenian mega-suture. The kinematic development of the East and Gulf Coast continental margins is seen to possess a remarkable fundamental symmetry broken only by subsequent passive margin erosional and sedimentary effects.