POLYPHASE DEFORMATION IN THE INNER PIEDMONT: GEOLOGY OF THE SOUTHERN HALF OF FINGERVILLE WEST QUADRANGLE, SPARTANBURG COUNTY, SOUTH CAROLINA

A prominent feature in the South Carolina portion of Fingerville West quadrangle is a discontinuous NE-striking outcrop of silicified breccia (SB). The SB is referred to here as the Pax Mountain fault zone (PMf) which can be traced SW across a number of quadrangles. Along strike small outcrops and float bands of SB mark the location of the PMf. As elsewhere in the Inner Piedmont, the presence of SB indicates the proximity of subsequent faulting; and where seen in cut banks, faulting is defined by narrow, subvertical-damage zones that imply strike-slip. Changes in structural position across faults also indicates a component of oblique-slip. Faulting occurs both to the N and S of the PMf along NE projections of springs and, commonly, is characterized by right-lateral offset. Map patterns, however, show left-lateral offset occurs immediately adjacent to the PMf. Both the PMf and associated faulting cut diagonally NE across two, older, W-verging thrust sheets. Throughgoing faults are mapped by using the tip lines of the two thrust sheets as markers. The two thrust sheets are characterized by distinct lithologies. The upper-thrust sheet is characterized by sillimanite schist and is traced easily along strike by float blocks of sillimanite schist. The lower-thrust sheet is dominated by a thick package of amphibolite, and the contact with footwall lithologies is marked by a thin layer of sheared, quartz-sillimanite mylonite. To the W of the tip line of the lower-thrust sheet, a series of inclined, W-verging, tightly folded layers of schist, amphibolite, and quartzite crop out. Layers of calcsilicate also are found randomly, and pods of ultramafic occur in the northwest corner of the mapped area. These interlayered rocks are referred to as the “mixed unit” because of poor outcrop and subsequent fault offset. The outcrop pattern of the “mixed unit” and the lower-thrust sheet are complicated further by the intrusion of fine-crystalline leucocratic gneiss and by macro-scale, W-verging asymmetric folds; both of which have been subsequently faulted. W-verging folding is characterized by a steep near-vertical, western forelimb and a gentler, eastern backlimb and is marked by belts of leucocratic gneiss and of schist, that strike approximately N across the lower thrust sheet.