DIGITALLY ENHANCED REGIONAL AEROMAGNETIC AND GRAVITY DATA PERMIT TRACING SURFACE GEOLOGY IN ALABAMA SOUTHWARD BENEATH THE GULF COASTAL PLAIN

The basement beneath the Gulf Coastal Plain in Alabama offers a unique setting where NE-trending Appalachian rocks and structures exposed at the surface are near-orthogonally onlapped and progressively buried southward beneath sedimentary units of the Gulf Coastal Plain. Surface geology can be progressively traced southwestward using aeromagnetic and gravity signatures to depths of ~6 km. In the subsurface, Appalachian trends are truncated by the EW-trending Brunswick-Charleston-Altamaha (BCA) magnetic low, previously interpreted as the suture between the Gondwanan Suwannee terrane and Laurentia. Digital aeromagnetic and gravity maps provide greater resolution of this progression than ever seen before, permitting comparison of how their geophysical signatures change with increasing depth. Southwardly buried Grenvillian and/or Appalachian structures include (west to east) the New York-Alabama lineament, the Amish anomaly, the Talladega-Cartersville-Great Smoky thrust, the Brevard fault zone, faults flanking the Pine Mountain window, and the central Piedmont suture bounding peri-Gondwanan Carolinian arc terranes. Surprisingly, the Alexander City fault zone, west of the Brevard, is one of the most prominent anomalies, traceable southwestward for more than 100 km to its termination along the BCA anomaly. Magnetic and gravity signatures for Laurentian margin and Appalachian crust are sharply truncated by the E–W-trending BCA low. High-frequency magnetic lineaments of the peri-Gondwanan rocks appear to be dragged into parallelism with the BCA anomaly. The Suwannee terrane contains broad, subparallel, arcuate, ~N45^oE-trending magnetic anomalies intruded by elliptical anomalies interpreted as plutons that likely reflect internal lithologic structure and superposed E-W Mesozoic failed-rift trends.