GEOLOGIC MAP OF THE CLARKS HILL AND COLLIERS 7.5-MINUTE QUADRANGLES, EDGEFIELD AND MCCORMICK COUNTIES, SOUTH CAROLINA

The South Carolina Geological Survey produced 1:24,000-scale maps of the Clarks Hill and Colliers quadrangles as part of the USGS STATEMAP Program. This mapping assists studies of earthquake hazard in the Eastern Piedmont Fault System (EPFS), investigates mineral resources, and will complete the Clarks Hill Lake 1:100k sheet.

Three lithotectonic elements are identified in the map area: 1) Neoproterozoic-Early Cambrian Carolina terrane (Ct), 2) Neoproterozoic Savannah River Terrane (SRt), and 3) the Paleozoic Ct consists of polydeformed, regionally metamorphosed, greenschist-facies intermediate-felsic volcanic and siliciclastic sedimentary rocks assigned to the Persimmon Fork, Richtex, and Emory Formations. SRt consists of migmatitic amphibolite-facies ortho- and paragneisses. Map relations in the SRt show an increase in migmatization from paleosome-dominated stromatitic metatexitic migmatites in the north to neosome-dominated diatexitic migmatites in the south. A schollen migmatite member containing blocks of amphibolite and paragneiss paleosome in a fine-to-coarse-grained neosome matrix separates the two endmembers. MSZ separates the two terranes and contains mylonitic granitic orthogneiss, biotite-amphibole paragneiss, feldspathic quartzite, and biotite-white mica-garnet schist.

Two early generations of folding overprint the Ct and SRt prior to development of MSZ. MSZ is a dextral ductile strand of the EPFS. Ductile structures formed in MSZ during an early phase of EPFS deformation when rocks were at deeper structural levels and include composite S-C-C’ foliations, mineral lineations, delta- and sigma-porphyroclasts, and dextral Z folds. As deformation continued, MSZ rocks were exhumed above the ductile-brittle transition zone, and brittle faulting accommodated strain. A network of brittle Riedel shears overprints MSZ rocks and mesoscale dextral X-, R-, and P-shears are observed in all map units. Map-scale P-shears, such as the NE-striking Ray Creek fault, dextrally offset map units. In the present-day stress field, NE- and NW-striking EPFS brittle faults are locally reactivated and are loci for low magnitude seismicity. The Ray Creek fault may be a structural model for a recent earthquake swarm in the EPFS NE of Columbia, SC.