DIFFERENTIATION OF STRUCTURAL SIGNATURES IN PROTEROZOIC ROCKS OF THE READING PRONG AT ANTIETAM RESERVOIR, PENNSYLVANIA

Billion-year-old rocks in the Reading Prong in eastern Pennsylvania record four collisional orogenies and two rifting events. In a 0.5 km long roadcut near Antietam Reservoir east of Reading, Pennsylvania, exposed meta-granodiorite rocks preserve brittle and ductile structural signatures characteristic of these major events. Slickenlined surfaces and foliation measurements were collected at three locations (north, middle and south) across the outcrop. The foliation is generally NE-striking but shows tight folding at each location, with differences in foliation dip and fold axes across all three sites. Slickenlined surfaces at each location were grouped according to similar orientations, producing 5 distinct groups at the northern site, 2 in the middle, and 4 in the south. The two most distinctive fracture groups, one striking N, the other striking NW were correlated across all three locations based on similar orientations and fracture characteristics. Subtle differences in these fracture orientations across the three sites indicate the N-striking group of fractures formed and was later rotated before the second, NW-striking group formed. Preliminary analysis shows foliation (S1 fabric), tight folding of foliation at each data location (F2 folding), distortion of folded foliation from north to middle to south (F3 folding), and at least two distinct fault sets (S4 and S6). An additional outcrop-scale distortion, F5, is hypothesized to account for incongruous variations in orientations of the two fault groups along the outcrop. Further analysis will explore whether these structural signatures can be linked to the six major tectonic events that have affected the region. The overprinting of brittle and ductile features at this location and other outcrops in the Reading Prong can expose details of Proterozoic and Phanerozoic deformation events that contributed to the growth of the Appalachian Mountains.