AN UPDATE ON THE EASTERN PIEDMONT FAULT SYSTEM IN SOUTH CAROLINA

The eastern Piedmont fault system (EPFS) is an anastomosing network of dextral mylonite zones and brittle faults that extends from GA through southern VA and was first defined using aeromagnetic data. In central SC, the most prominent strand is the Modoc shear zone (MSZ), a 5 km-wide terrane bounding high strain zone. Here, the MSZ separates greenschist facies Carolina terrane rocks from amphibolite facies Dreher Shoals and Savannah River terrane rocks. Locally, the Carolina terrane crops out on both sides of the MSZ. Detailed mapping of the MSZ and greater EPFS has been ongoing since the 1980’s to uncover its significance in the state. A recent 2021-2023 earthquake swarm along the EPFS NE of Columbia, SC, highlights the importance of this effort.

Deformation in the MSZ and the EPFS in SC is separated into ductile and brittle phases. A regional mylonitic shear foliation, dextral shear-related folds, asymmetric porphyroclasts and mineral stretching-intersection lineations mark an early ductile phase. En echelon folds and pull-apart structures are also locally present. Faults and fractures characterize younger brittle deformation and are considered part of a network of riedel shears having dextral and sinistral components. NNE-striking dextral faults juxtapose rocks formed at different structural levels and occur along the short limb of asymmetric dextral shear folds. These dislocations are termed p-shears and accommodate shortening. NNW-striking sinistral faults are considered R’-shears. Historically, they have been the loci for seismicity, e.g. M4.1 2014 Edgefield earthquake.

A new high-resolution aeromagnetic survey funded through the USGS Earth Mapping Resource Initiative covers a portion of the EPFS in SC and is aiding ongoing mapping efforts, structural analyses, and earthquake studies. Reduced-to-pole aeromagnetic data shows that the EPFS consists of both left- and right-stepping ductile shear strands that in a dextral system accommodate oblique shortening and extension, respectively. A dextral p-shear splay recognized from aeromagnetic data appears to be the locus for the recent seismicity near Columbia, SC. Given the mapping, structural and geophysical studies, our current understanding of the terrane configuration is that of an extensive dextral duplex system in the EPFS across central SC.