SEQUENCING POLYPHASE DEFORMATION WITHIN THE INNER PIEDMONT: FIELD EVIDENCE FROM NEAR MARIETTA, SOUTH CAROLINA PART 2

Four periods of deformation are recognizable in a borrow pit near Marietta, South Carolina; each is characterized by a progression of deformation styles. A third-period progression is found in a small bench of interlayered mica schist, metasomatic schist, and amphibolite. East-dipping, quartz-filled gash fractures (gashes) in metasomatic schist are kinematic indicators that show the rocks to be part of an unfolded forelimb of a second-period, northwest-verging, inclined fold. The second-period thrust (metasomatic schist—underlying mica schist contact) that unfolded the forelimb is folded by the third-period progression.

Asymmetric, third-period detachment folds indicate west-directed, layer-parallel shortening. Observations of the bench show that gashes localized deformation in metasomatic schist during that shortening. East-dipping thrust ramps nucleated at gashes, and thrusting dies out both up and down section away from them. Reverse movement is marked by upper-ramp, rollover thrust-tip folds. A footwall syncline is developed below rollover folds at the metasomatic schist—overlying amphibolite contact. A new deformation style developed when faulting connected with a basal detachment zone (Morley, 1994) in the underlying mica schist. Progressive deformation superimposed fault-propagation folding on gash-localized faults where the bench meets the borrow face. Slickenlines on interlayer surfaces indicate that flexural slip accommodated kink folding as up-to-the-west movement produced fault-propagation folding. Additional deformation was accommodated by a west-verging break thrust that cuts the anticlinal hinge zone; displacement decreases upward along the curved fault plane.

These relations are field evidence that properly oriented barriers will nucleate thrust ramps as upward deflection disrupts layer-parallel shortening within a layered sequence. Deformation styles develop progressively from initial thrust-tip folds to fault-propagation folds truncated by break thrusts.