TERTIARY-QUATERNARY REACTIVATION OF EARLY MESOZOIC FAULTS IN THE SOUTH GEORGIA RIFT

Recent reprocessing of seismic reflection data collected in the period 1975-1985 and modeling of the potential field has resulted in new information on Cenozoic faulting and its relationship to Mesozoic structure in the epicentral area of the 1886 Charleston, South Carolina, earthquake. The area lies within the South Georgia Rift, a large Mesozoic extensional terrane buried beneath the Atlantic coastal plain in parts of Alabama, Georgia and South Carolina. The reflection data image a fault-bounded extensional basin approximately 20 km in width between Summerville and Charleston, South Carolina. This feature is delineated by reflections imaging early Mesozoic volcanic and sedimentary units and by potential field anomalies. Cenozoic compressional reactivation of Mesozoic extensional faults is imaged in the interior of the basin. The northwestern boundary of the feature is marked by a sharp gradient in total magnetic intensity. The Cretaceous and Tertiary coastal plain sediments show monocline folds in association with diffractions and truncated reflections from the underlying Mesozoic units at four locations along this magnetic gradient, indicating that the northwestern boundary of the basin is bounded by faults that have been reactivated in the Cenozoic. The imaged Cenozoic deformation is in close association with the epicenters of instrumentally located earthquakes. The modeling of the potential field suggests that the Mesozoic section is composed largely of mafic rock to a depth of at least 4 km. It appears that the 1886 shock and current seismicity is due to compressional reactivation of extensional faults in a zone of intense early Mesozoic faulting and volcanism, within the larger South Georgia Rift.