SEQUENCING POLYPHASE DEFORMATION WITHIN THE INNER PIEDMONT: FIELD EVIDENCE FROM NEAR MARIETTA, SOUTH CAROLINA

During investigation of the Seneca fault, Garihan (2000) identified polyphase deformation in a borrow pit near Marietta, South Carolina. We revisited this pit within a klippe of the Six Mile thrust sheet. Mesoscopic observations define four distinct periods of deformation. They modify the interlayered sequence of psammatic gneiss, metasomatic schist, and amphibolite exposed by mining. Each period is characterized by a set of structures that are a progression of deformation styles which deforms older features.

The first deformation period is characterized by small, rootless isoclines and near-recumbent isoclines with attenuated limbs. Parasitic Z-folds are present on overturned limbs. Isoclines are initially re-folded by a set of northwest-verging, inclined, closed folds (second period). A single-bed duplex is located on a backlimb of one such fold and probably reflects delamination of the interlayered sequence (Woodward, 1997) during initial second period buckling. Northwest-verging imbricate thrusts cut through overturned limbs of isoclines and forelimbs of inclined folds. Limbs of both fold types are unfolded by rotation along the thrusts, and ramping is located at parasitic Z-folds. All first and second period structures are re-folded by west-verging, inclined detachment folds and fault-propagation folds (third period). West-verging thrusts cut through anticlinal hinge zones of closed, fault-propagation folds and through their shallow-dipping backlimbs. A zone of high-angle, left-oblique faults (fourth period) subsequently offsets older structures.

First period isoclines are related to the Paleozoic emplacement of the Six Mile thrust sheet. Fourth period, left-oblique faulting is Mesozoic in age. If these relations are true, two other separate periods of Paleozoic deformation, characterized by distinctive structural styles, affected the interlayered sequence at the borrow pit.