DUCTILE STRUCTURAL FEATURES WITHIN THE MODOC ZONE, SALUDA DAM, COLUMBIA, SC

Excavations at a hydroelectric dam remediation project in central South Carolina exposed high-grade, polyphase deformed rocks within the Modoc zone. The Modoc zone is part of the Eastern Piedmont Fault System and separates the Dreher Shoals terrane from the Carolina terrane. Excavations for the foundation of a new backup dam provided a unique opportunity to examine a 2400 ft section across strike in the core of the Modoc zone.

Detailed mapping at 1:120-scale recorded multiple deformational events consistent with those exposed in the Lake Murray spillway approximately 1000 feet to the south, but revealed much more information about the complexity of individual deformations and the relationships between deformations. On the basis of regional correlations, one episode of pre-Alleghanian (D-1) and two episodes of Alleghanian deformation (D-2 and D-4) were observed. Evidence for D-3 deformation is lacking in the Modoc zone but is common in the adjacent Carolina terrane. D-4 ductile deformation is characterized by dextral shear. Typical D-4 ductile structures include tight to closed, moderately to steeply northeast plunging asymmetric folds, en echelon boudins, 1-5 inch wide shear bands, and mesoscopic kinematic indicators in the form of rotated porphyroblasts and sigmoidal fabrics. Foliation and lineation mapped provide evidence of a macroscopic D-4 fold system. D-2 deformation was a significant shearing event and produced the major foliation. D-2 structures include mylonitized fabrics and migmatization features. Mylonitized gneiss, schist, and amphibolite have similar fabric elements, which indicate that these rocks were subjected to the same deformation. Migmatized zones in psammitic biotite gneiss and garnet pelitic schist suggest that sheeted “plagio-granite” layers characteristic of the Modoc zone were derived in situ. D-1 structures are sparse isoclinal folds. They are transposed and rootless folds tucked between the main foliation of D2; hinge lines are typically near horizontal and approximately parallel to the main foliation.