GSA Connects 2021 in Portland, Oregon

Paper No. 70-9
Presentation Time: 10:15 AM


SHAH, Anjana, U.S. Geological Survey, Denver Federal Center, Bldg 20, MS 973, Denver, CO 80225, PRATT, Thomas L., U.S. Geological Survey, Geologic Hazards Science Center, 12201 Sunrise Valley Drive, Reston, VA 20192 and HORTON Jr., James, U.S. Geological Survey, Florence Bascom Geoscience Center, 926A National Center, Reston, VA 20192

The South Georgia rift basin, extending from South Carolina to Alabama, is the southernmost and largest of the early Mesozoic rift basins in the eastern U.S. It contains the epicenter of the 1886 ~M7 earthquake that caused extensive damage to Charleston, SC. New high-resolution aeromagnetic data, collected over a ~212 x 134 km area covering the northeastern part of the basin and the Charleston seismic zone, image structural and igneous features that are now buried beneath hundreds of meters of sediments.

Round magnetic anomalies that coincide with gravity highs likely represent mafic or possibly ultramafic intrusions; drill data suggest some are rift-related. Linear, N-NNW-trending, 1-3 km-wide and up to 90 km-long magnetic highs are typical of Jurassic dikes observed over much of the eastern seaboard. Some interpreted dikes are present within the rift basin and cross its northern boundary; these and others appear to radiate from larger intrusions.

Outside of the rift basin, magnetic lineaments are primarily NE-trending, consistent with Paleozoic structures of the Appalachian orogen. Some magnetic anomalies show intricate patterns that suggest a complex faulting history. The northern boundary of the basin, interpreted from previous geophysical and drill-hole data, trends east-west in this region, but the new data show it comprises smaller NE- and SE-trending magnetic lineaments, suggesting conjugate faults along the basin boundary. This could occur if segments of the Mesozoic boundary formed along NE-trending Paleozoic structures that acted as inherited zones of weakness, with conjugate SE structures forming during rifting.

Within the rift basin are a series of 6-15 km long E-ESE-trending lineaments that are parallel or subparallel to the basin boundary and terminate along NE-trending anomalies up to 50-km long. These NE-trending anomalies also suggest inherited Paleozoic structures that influenced locations of Mesozoic rift structures. Near the locus of modern seismic activity within the Charleston seismic zone near Summerville, legacy seismic reflection data show post-Eocene faulting or folding in places coinciding with both the E-ESE- and NE-trending magnetic lineaments. These lineaments, with sources at 1-6 km depth, are candidates for faults that could slip in the modern Charleston seismic zone.