BACKFOLDING, PARTITIONED TRANSPRESSION, AND FAULT REACTIVATION IN NORTHWESTERN SOUTH CAROLINA

Systematic mapping in the Jocassee Gorges area of South Carolina shows that the dextral Salem transpressional system (STS) influenced deformation styles in three, subparallel structural domains. From northwest to southeast, the STS domains are the Brevard fault zone (BFZ), the Devils Fork block (DFB), and the Keowee fault zone (KFZ). Folds are recognizable in all domains. Faulting, however, appears to be confined to the boundary domains and, in part, resulted from fault reactivation. The NW BFZ is delineated by strike-slip faults and thrust imbrication, whereas the SE KFZ is delineated by normal faulting and thrust imbrication. Macroscopic, northwest-verging, asymmetric folds deform the intermediate, 8-km wide DFB. The width of the STS is marked by a pronounced dextral S-C' fabric in Chauga River Formation phyllites.

Our mapping shows that a southeast-verging backfold along the NW boundary localized the STS. Backfolding may have occurred when Blountain-age (?) thrusting became locked in the foreland. At many structural levels, the backfold is now overprinted by reactivation along the Rosman fault. During the Acadian orogeny, partitioned transpression resulted from difficulty of superimposing oblique compressive strain on the backfold's steeply dipping SE forelimb. Progressive deformation during this event localized dextral strike-slip faulting in the BFZ, uplifted and laterally extruded the DFB to the SW, and initiated normal faulting in the KFZ. The STS was overprinted by fault reactivation when oblique compressive stress was applied again during the Alleghanian orogeny. Footwall shortcut thrusts, ‘pop-up' structures, and back thrusts developed in the BFZ. Footwall shortcut thrusts and hanging wall ‘break-back bypass'(?) thrusts were produced in the KFZ. Asymmetric folds developed in the DFB. Fold asymmetry indicates simple shear and implies that transpression also may have occurred during the Alleghanian orogeny.