INFLUENCE OF REGIONAL MACROSCOPIC TYPE-1 SUPERPOSED FOLD PATTERN ON THE DEVELOPMENT OF BRITTLE FAULTS IN THE SOUTH CAROLINA INNER PIEDMONT

Folding, brittle faulting, and depth of erosion control the current distribution of the Six Mile thrust sheet and its klippen (with the Seneca fault at its base) in a 14-quadrangle area of the South Carolina Inner Piedmont. Backstripping the secondary brittle fault pattern reveals the principal cause for this distribution: a well-developed, late regional macroscopic interference pattern of Type-1 (egg-carton) superposed folds (NE-striking Set I and NW Set II) formed after Seneca thrust emplacement. Set I NE-plunging folds strike N45°E, and fold crest-crest wavelength is ca. 12-13 km. Three major Set I folds underlie the entire exposed width of the Walhalla thrust sheet; its gneisses core two flanking antiforms. An intervening Set I synformal downwarp of Six Mile paragneisses and schist above the Seneca has an erosional width of ca. 10 km. In Standingstone Mountain-Zirconia quadrangles, windows of footwall Walhalla gneiss exposed beneath Seneca-Six Mile sheet rocks are overturned, SW-vergent Set II culmination antiforms deformed by a younger open Set I antiform. The Type 1 interference fold pattern is seen locally on the mesoscale in Campobello quadrangle.

A developmental brittle fault sequence, involving NE, N, NW strikes, and cataclastic rocks, has been established for the Inner Piedmont in eastern Greenville and western Spartanburg counties, SC (Garihan, 2012, South Carolina Geology). A dense array of NE-striking oblique-slip faults follows the eastern Set I antiform-Walhalla belt or transects it at a shallow 20° angle. Farther west into Pickens County, a pattern of intersecting E-W, NE, and NW to N faults in Six Mile rocks of the Set I synform is markedly more complex, locally bounding triangular fault blocks. The N45°-75°E fault array dominant to the east is replaced to the west by five major E-W/ESE faults at its intersection with this Set I synform. Typical E-W/ESE fault offsets of Six Mile rocks preserved in the Set I synform vary between 2.5 and 3.5 km and result from both left and right normal-oblique movements in Cleveland and Table Rock quadrangles. NE, NW, and N graben-bounding normal-oblique faults terminate against or are displaced by the latest E-W/ESE brittle faults. The east to west change from dominant NE faults to E-W/ESE faults appears to have been influenced by the earlier interference fold pattern.