MAJOR TECTONIC ELEMENTS IN THE SOUTH CAROLINA WESTERN INNER PIEDMONT: KEY RELATIONSHIPS FROM CLEVELAND AND EASTERN TABLE ROCK 7.5-MINUTE QUADRANGLES

Recent re-mapping of Cleveland and eastern Table Rock quadrangles with refined map units has delineated a complex fold-fault chronology for this part of the South Carolina western Inner Piedmont. A key marker in the tectonic development sequence is the Seneca ductile fault at the base of the Six Mile thrust sheet (Poor Mountain and Tallulah Falls formations). Middle Ordovician quartzo-feldspathic gneisses of the Walhalla thrust sheet lie in the footwall. Brittle faulting and deep erosion have modified the original continuity of all structures.

The oldest macro-scale tectonic element (TE-1) is two systems of NW- and SW-vergent, overturned folds (F₁ and F₂) affecting Six Mile rocks. TE-2 involves inclined, N-NE striking folds (F₃) also affecting these rocks. The Seneca ductile fault (TE-3), emplaced northwestward, currently truncates the traces of earlier folds. NW- to SW-vergent, inclined-overturned F₄ folds (TE-4) deform the Seneca fault and all hanging wall-footwall rocks, producing structural windows in the Six Mile sheet. F₄ is contemporaneous with emplacement of the Eastatoee ductile fault (TE-5). Collectively, TE-6 is the development of a Type-1 interference pattern of broad folds of NE- and NW-strike (F₅ and F₆). Significant is Six Mile rocks preserved in synformal downwarps of both strikes, flanked by footwall-cored antiforms. South-directed compression has thrust Walhalla gneisses southward back over Six Mile rocks and F₄folds along E-W greenschist-grade shear zones (TE-7).

All earlier macro-scale fold patterns (F₁–F₆) were disrupted and rearranged by sets of NW-, N-, NE-, and E–W-striking oblique-slip to dip-slip faults and NE-striking cataclastic zones, collectively TE-8. Riedel relationships exist among these faults. Major dextral and sinistral, ~E–W-striking fault zones of TE-8 have 2-6 km of strike-parallel offsets, merging eastward with regional NE-striking faults. Widespread TE-8 Inner Piedmont oblique-slip to dip-slip normal faults resulted from changing Mesozoic stress episodes, involving NE-, E-, SE-, and S-directed extension. During significant NE-extension, F₅ Six Mile synforms were stretched into separate grabens, which internally preserve F₂ folds. NW diabase dikes (TE-9) formed by NE-extension. SE-extension was responsible for significant throw on regional NE faults.