Normal faulting and in situ stress in the South Carolina coastal plain near Charleston

Abstract

In situ stress measurements were made to depths of 344 m in Atlantic Coastal Plain sediments near Charleston, South Carolina. The magnitudes of the least principal compressive stress were found to be considerably sublithostatic: 30.6 b at 193 m, 33.7 b at 209 m, 35.0 b at 297 m, and 41.6 b at 344 m. These data, combined with simple faulting theory, indicate that the least principal horizontal stress at depths of 297 and 344 m is sufficiently less than the lithostatic load to result in normal-type fault motion on favorably oriented faults. Stratigraphic evidence from three test wells and 20 auger holes in the area supports existence of at least one normal fault in the vicinity of the wells in which the stress measurements were made. We interpret these results to suggest that normal faults in coastal plain sediments near Charleston are currently active. Because the direction of relative horizontal extension appears to be northeast-southwest, or parallel to the trend of the continental margin, we infer that this stress field is of tectonic origin.