BRITTLE FEATURES MAPPED ALONG A SHEAR ZONE AT SALUDA DAM, CENTRAL SOUTH CAROLINA

Saluda Dam is located within the Eastern Piedmont Fault system in west central South Carolina. It lies within the Modoc Shear Zone, a 4-5 km wide dextral fault zone which experienced late Paleozoic high-grade metamorphism. A remedial design for the Saluda Dam called for the construction of a new berm at the toe of the existing earthen dam. To facilitate construction, an area of approximately 2400 ft. was excavated to competent rock, pressure- washed, and mapped before being backfilled with concrete.

Four Paleozoic ductile deformational events are recognized in this area. Key deformational events visible in the Dam excavation include the Lake Murray event, D2, and the Irmo event, D4. D2 took place during the late Carboniferous and is characterized by high-grade amphibolite facies metamorphism with regional folding. The final late Paleozoic deformation, D4, produced early ductile structures (asymmetric dextral folds) and transitions to late brittle fold-fault features. From approximately 268 to 100 ma, 20 to 30 km of material was eroded from the region, during which time most seismic activity was produced by basement readjustment from differential loading and unloading. Brittle features observed in the Saluda Dam foundation are interpreted to be the result of this readjustment as well as tectonic movement associated with latest Alleghanian deformation and initial Triassic rifting.

Brittle geologic features were mapped at 1:120 scale and described by orientation, spacing, continuity, openness, mineralization, filling, weathering, surface, and offset. Discontinuous fracture sets, Riedel shears, offsets, exfoliation joints, brecciated zones, en echelon vein arrays, and conjugate fault sets were observed. A total of 269 faults were mapped, varying in orientation, but typically displaying a dextral component often striking NNW to NW. Two major thrust faults were mapped trending approximately EW. Left lateral faults were few, however one fault striking NE offset an extensive felsic dike by 50ft. Zeolite mineralization, indicative of relatively younger faults, and epidote, indicative of relatively older faults, were observed on some exposed fracture faces and faults. In some cases, brittle features unearthed in the foundation played a role in determining final depth of excavation and joint construction position.