P-RAMP DEVELOPMENT ON THE EASTERN PIEDMONT FAULT SYSTEM, LAKE MURRAY, SOUTH CAROLINA

The NE-striking Eastern Piedmont fault system is one of the largest faults along the southeastern coast of North America. This crustal fault system has a long-lived, complex history and is marked by ductile, brittle-ductile, and brittle dextral strike-slip deformation. Field studies in Slate Belt rocks along the south shore in the central part of Lake Murray show ϭ1 rotated clockwise during a period of dextral brittle-ductile movement. Dextral strike-slip initially was localized in foliation-parallel movement that reactivated a 055-065-striking axial planar cleavage produced by an older period of upright tight folding. P-ramps formed stepovers that cut 035-042 across foliation, disappear in adjacent zones of foliation-parallel movement, and are marked by dextral Z folding. Open antiforms are present on NW flanks of this set of P-shears, that plunge gently NE, lack cleavage development, and tighten with rotation into dextral foliation-parallel movement. Tight dextral folds also developed along 020-032 P-shears. In this set, antiforms on NW flanks are inclined, plunge NE, verge SE, and have a steep to overturned forelimb juxtaposed to the P-shears. An anti-clockwise, 020 transecting cleavage is present in many outcrops, and on different folds, is axial planar to this second fold set. Locally, a 020 crenulation cleavage also is present. The differences in fold form imply that the P-ramps formed under different styles of partitioned transpression during progressive deformation. Rotation of ϭ1 helps to explain the differences. Z-folding of the NE-striking fabric with 055 axes indicates shortening parallel to the foliation and strike slip-dominated dextral transpression. Once strike-slip was localized, components of convergent simple shear were partitioned onto the ramping P-shears. Movement was oblique, and the pure shear component produced the open folds. As deformation continued, ϭ1 rotated clockwise from strike-parallel (055-065) to obliquely convergent (~110). Fold vergence, anti-clockwise transecting cleavage, and local crenulation cleavage in the proximity of P-ramps indicate the rotation of ϭ1. The tighter-fold style resulted as shortening increased across the P-shear under pure shear-dominated dextral transpression.