TECTONIC IMPLICATIONS OF A COMPLEX KINEMATIC HISTORY IN A BASAL CHAUGA RIVER FORMATION PHYLLONITE, NORTHWEST SOUTH CAROLINA

Mesoscopic structural observations are described from Tommys Knob in Salem 7.5-minute quadrangle, South Carolina, southeast of the Brevard fault zone. At three exposures, overprinting relationships are preserved in the mylonitic fabric of a basal Chauga River Formation phyllonite. Overprinting of structures is clear evidence of a complex, polyphase kinematic (movement) history. Two episodes of movement are represented by C’-type shear bands. Shear band development is separated by an intermediate period of northwest-verging, asymmetric folding. Interpretations of C’-type shear-sense indicators demonstrate that left-lateral shear occurred prior to subsequent folding and right-lateral shear. Early left-lateral shear is identified by homogeneous shear-band fabrics, sporadic C’-shear bands, foliation fish, and deflection of both S-surfaces and intrafolial folds. The intermediate period of asymmetric folding deforms those early structures. Right-lateral C’-type shear bands occur sporadically and as a homogeneous fabric. These structures tend to obscure or destroy those developed by the two older periods of movement.

Mesoscopic evidence of left-lateral shear, along with older petrographic data, is used to suggest that the early, left-lateral movement fabric in phyllonite developed during the Middle Ordovician Blountian orogeny in the southern Appalachians. Conventional interpretations of the Brevard fault zone assign the later, right-lateral movement indicators to a part of the Alleghanian orogenic history. The intermediate period of mesoscopic folding also may have occurred during Alleghanian deformation. Alternately, no convincing evidence is available to rule out the possibility that right-lateral shearing during Acadian deformation caused the last two periods of movement.