Interpretation of regional gravity anomalies on the margin of the northwest Gulf of Mexico

Free-air gravity anomalies along portions of 27 shiptracks observed in 1970 during five cruises of the USNS "GEORGE B. KEITHLEY" in the area of the Texas continental shelf and part of the continental slope reveal a continuous linear maximum, bifurcating at the latitude of Baffin Bay, of as much as +32 mgals which may extend along the entire Texas continental shelf at distances of 40-75 km offshore. An oval positive anomaly of +57 mgals exists offshore Port Isabel, Texas. A regional gravity map, prepared from the "KEITHLEY" data and land Bouguer gravity data available in the literature reveals that a positive Bouguer anomaly similar to the linear free-air maximum offshore is present southwest of Freeport, Texas, while the oval anomaly offshore Port Isabel is shown to extend onshore and is associated with sediment loading on the continental margin by the Rio Grande Delta. A crustal section perpendicular to the coastline in the vicinity of Port O'Conner, Texas, constructed using a two-dimensional gravity model, indicates that the Freeport maximum and the linear shelf maximum may be caused by high density, probably ultramafic intrusions up to 14 km wide within the basement of oceanic crust offshore and the Paleozoic meta-sediments onshore. Magnetic models of the intrusion using an aeromagnetic profile extending southeast from the vicinity of Galveston yield depths and widths for the intrusive similar to those indicated by the gravity model near Port O'Conner, approximately 200 km to the south. Plotting the locations of the gravity maximum along 18 of the shiptracks indicated apparent strike-slip displacement of the intrusive, perpendicular to the coast, of as much as 14 km. Integration of the gravity and magnetic data presented in this thesis with modern examples of extensive ultramafic intrusions and a recently published model for a late Triassic opening of the Gulf of Mexico by sea-floor spreading suggests that the ultramafic intrusions along the Texas coastline may represent a "relict" site of the original rifting of that time. Several curvilinear free-air minima are present on the south Texas continental shelf. These are thought to be caused by low density, highly pressured marine shale diapirs. Isolated Bouguer minima over the upper continental slope substantiate that the "hummocky" topography of that area is due to salt diapirism.