SUPERIMPOSED OBLIQUE EXTENSION ON THE SOUTH GEORGIA RIFT

The South Georgia rift (SGR) is a buried Triassic rift that lies oblique to the southeast coast of the United States. Related, NE-trending basins with differing polarities underlie parts of South Carolina, Georgia, Florida, and Alabama. Rifting in South Carolina, however, has not been clearly addressed. Examination of aeromagnetic images shows a complex arrangement of faults, dikes, and Mesozoic basins in South Carolina. The most recognizable SGR features on aeromagnetic images are a NE-trending lineament (Ehrhardt fault) that sinisterly offsets an E-trending lineament (Magruder fault) which can be traced toward the coast. Both faults are interpreted to be SGR margins (Chowns, 2009), but recent drilling shows red beds subcrop to the northwest and north of each fault respectively. South of the Magruder fault lie three, en echelon, NW-trending, lenticular-shaped geophysical lows; and from west to east, those geophysical lows are referred to as GL1, GL2, and GL 3 (GL-series). Lack of previous acknowledgment may be because the GL-series have a geophysically subdued footprint, are NW-trending features, and are spatially restricted to the SGR. Specifically, GL1 merges with the Magruder fault to the west and is separated from GL2 by a NE-striking lineament. GL1 is bounded by a lineament on the northeast, whereas GL2 is bounded by a lineament on the southwest. That bounding lineament bends to the northeast, separates GL2 and GL3, and becomes the northeast margin of GL3.

Using models as a guide, the GL-series are interpreted to be small asymmetric half-grabens adjacent to the Magruder fault. The polarity of the GL half-grabens is inferred to switch west-to-east from down-to-the-south to down-to-the-north back to down-to-the south on the basis of the location of bounding lineaments. The NW-trend of the half-grabens indicates that related extension would have to have been in a NE-SW-orientation. A rotating extension vector could explain that orientation, which resulted from initial NW-SE extension followed in a relatively short-time frame by oblique NE-SW extension. This subsequent oblique extension overprinted initial extension and produced the graben-in-graben structures. If that is true, superimposed oblique extension affected the SGR in the Late Triassic contemporaneous with the initial opening of the Atlantic.