HISTORIC SEISMICITY IN NW SOUTH CAROLINA: REACTIVATION OF THE TRIASSIC MARIETTA-TRYON GRABEN SYSTEM?

Although sparse in occurrence, historic seismic events in northwestern South Carolina appear to be spatially coincident with mapped cataclastic zones of the Marietta-Tryon graben (MTG) system. The MTG consists of an ENE-WSW trending zone (~25 x 75 km) of silicified cataclastic breccias within crystalline basement rocks of the inner piedmont geologic province, and sits astride the topographic boundary (Blue Ridge escarpment) between the Blue Ridge and Piedmont geomorphic provinces. Previous workers have suggested the MTG represents the eroded remnants of a Triassic extensional basin, preserved now only in the down-dip continuation of basin-forming faults within the crystalline basement. The surrounding region is marked today by strong structural control (cataclastic zones) of stream drainage (e.g. South Fork of Saluda), clear evidence for stream capture (e.g. Tugaloo), stranded gravel deposits along river drainages (e.g. Table Rock Reservoir area), and an abundance of natural waterfalls (e.g. Rainbow, Raven Cliff Falls). Such geomorphic features suggest a landscape that is distinctly at topographic disequilibrium. During the period 1911-1925, several earthquakes occurred in the region surrounding the North Carolina - South Carolina border. Although epicenters for these events are not well known due to lack of instrumentation, damage reports assembled at the time provide at least provisional locations. In particular, seismic events in Greenville Co. (1911) and Pickens Co. (1924) appear to coincide with subsequently mapped traces of fault zones within the MTG. Such seismicity pre-dates well-documented reservoir-induced seismicity in the region, and may be reflected as well in cavernous porosity within cataclastic zones identified from groundwater studies. Historic seismic activity on these ancient faults would appear to argue for Cenozoic tectonism in the southeastern U.S., and suggests that these features might be the focus of future seismic hazards.