Abstract
The southeastern United States has experienced two complete successions of Wilson cycles: (1) the assembly and break up of Rodinia and the opening of the Iapetus Ocean; and (2) the closing of Iapetus ocean, the assembly of the supercontinent Pangaea and its subsequent break up, and the opening of modern Atlantic Ocean. Evidence of these supercontinent cycles is recorded in the crust of Alabama and adjacent areas, but is covered by as much as 7 km of Coastal Plain sediments in the southern portion of the state. In this study, we use airborne gravity and magnetic data to develop crustal models along transects that cross major tectonic structures and the ancient North American (Laurentian) margin. Models derived from gravity and magnetic data are constrained by well-log information, geologic mapping, and nearby previous geophysical studies. Results show that a pronounced east-west trending gravity low observed in southern Alabama can be interpreted as the suture between relict Gondwanan crust and Peri-Gondwanan or Laurentian crust. The best-fit models suggest that the crust thickens from south to north, with a change in crustal thickness near the suture zone. Laurentian crust is characterized by northeast-southwest trending lineations in magnetic data that can be traced beneath the Coastal Plain until truncated by the tectonic suture with Gondwanan-affiliated crust. This truncation is marked by the Brunswick Magnetic Anomaly. The denser crystalline rocks of Piedmont and Valley and Ridge provinces in northern and central Alabama correspond to minor gravity highs. In southwestern Alabama, the crust of the Wiggins terrane appears to be a unique tectonic entity relative to other areas and shares similarities to the crust beneath the Mississippi Gulf coast. Sharp magnetic gradients and long-wavelength gravity gradients along faults such as the Towaliga fault, Alexander City fault, and Bartletts Ferry fault suggest these structures are major, crust-penetrating features. In the southeast, the gravity and magnetic data are consistent with the presence of Mesozoic rift basins and suggest the along-strike continuation of the South Georgia basin.