CHARACTER OF TWO UNUSUAL BRITTLE-DUCTILE SHEAR ZONES IN THE WESTERN INNER PIEDMONT, SOUTH CAROLINA

Greenschist grade ductile shear zones occur locally within Table Rock biotite quartzo-feldspathic gneiss of the Walhalla fold-nappe in Table Rock quadrangle (western Inner Piedmont, SC). In outcrop the zones (<1m to tens of meters wide) typically are muscovite-rich, schistose, and oriented parallel to regional foliation-compositional layering in enclosing gneiss. Regional shear sense indicators developed in these rocks indicates SW-directed shearing along the N- and NE-striking ductile zones.

Two unusual shear zones are exposed below the Table Rock Reservoir dam; one is 10-11m wide. The two zones contain abundant dismembered, lenticular fragments and irregular blocks of Table Rock gneiss, lesser amphibolite, and biotite gneiss floating in a medium- to coarse-crystalline pyrite (2-3%)-garnet-muscovite schist matrix. Fragments range from 2-50 cm in length. Their long axes generally are parallel or at a slight angle to schistosity in enveloping schist. Locally, internal compositional layering at the margins of individual blocks is truncated abruptly by enclosing schistosity, indicating blocks are rotated relative to each other. Pre-shearing, variably oriented isoclinal folds of compositional layering in gneiss are preserved within individual fragments. Elsewhere crochet hook-shaped, tight folds with thickened noses and thinned-out limbs within schist resemble intrafolial folds. The distribution of separate, isolated gneiss fragments in a few places suggests transposition, flattening and significant disruption of earlier, originally continuous gneiss layers. Some warped lenses of gneiss with sigmoidal shapes resemble mica fish at a larger scale (<0.5 m). One elongated piece appears rolled >180° into a crude circular structure by shearing. Dismemberment of brittle gneiss blocks in a ductile greenschist grade matrix indicates conditions in the crust near the brittle-ductile transition.

Post-isoclinal, post-shearing, polyphase folding has affected the secondary schistosity of the two shear zones and the compositional layering of adjacent non-schistose gneisses. Along their hingelines, a re-folded system of inclined, open folds plunges 3°/N58°W at their exposed northwest end and 55°/S64°E at the southeast end. The youngest set of upright, gentle folds plunges 20°/east.