In 1981 the U.S. Geological Survey conducted a high-resolution, 12-channel seismic-reflection survey offshore of Charleston, South Carolina, to study young faults that may be earthquake sources. We collected data along 24 profiles, totaling about 600 km, with shotpoints at 10-m intervals. The Helena Banks fault zone (HBFZ) was crossed by 13 high-resolution profiles which, combined with 5 conventional 64-channel profiles acquired in 1979 across the fault zone, provide greater definition of this structure than any similar feature along the Atlantic seaboard or continental margin. The HBFZ trends N66°E and is about 110 km long (the same trend as the underlying Kiawah Triassic(?) basin); it comprises several segments 10–40 km long that trend between N68°E and N77°E, with a mean of N72°E. The left-stepping echelon pattern of the segments (connected by normal faults observed on two profiles) in the HBFZ suggests slip could possibly be left lateral despite high-angle reverse separation (mean fault dip 70° ± 5°) of Coastal Plain strata seen on the individual reflection profiles. Aeromagnetic data constrain the maximum ratio of horizontal to vertical slip from about 10:1 to 100:1. Our interpretation is consistent with the present-day, northeast-trending, maximum horizontal compressional stress field; however, the reflection data cannot independently prove that the main sense of motion on the HBFZ is strike slip. Examination of vertical separations on six segments across the HBF indicates a linear increase with depth of 51 ± 12 m/km which is similar to 36 m/km and 48 m/km reported previously as measured on multichannel seismic-reflection profiles crossing the Gants and Cooke faults, respectively, on land in the meizoseismal area of the 1886 earthquake. We interpret these results to indicate that, as a first approximation, the stress field did not vary significantly in direction or magnitude from Late Cretaceous time to Pleistocene time (and likely to the present) in the Charleston region. The trend of the HBFZ as mapped by the reflection profiles follows the narrow Kiawah Triassic(?) basin, which was delineated by the second vertical derivative of total magnetic intensity. Several reflection profiles indicate that the HBFZ is a compressional reactivation of an extensional Triassic(?) fault zone bounding the basin.
- Geological Society of America