OUTCROP-SCALE STRUCTURAL ANALYSIS OF THE BLUE MOUNTAIN RIDGE NEAR PORT CLINTON, PA

Outcrop-scale fold and fault relationships in quartzite of the Silurian Tuscarora Formation along the Blue Mountain ridge near Port Clinton, PA record previously undocumented structural complexity. Published geologic maps of the study area show north-dipping strata striking 70°, parallel to Blue Mountain, and a ridge-parallel thrust fault dipping north at the base of the Tuscarora Fm. No detailed maps or publications have analyzed outcrop-scale secondary structures, which are instrumental in deciphering kinematics and tectonic history. In outcrops along the ridge, the Tuscarora Fm. shows intense deformation, with juxtaposed upright and overturned bedding associated with thrust-related, recumbent, isoclinal anticlines. Fold hinges trend 60°-75° and thrust fault surfaces strike 55°-70° dipping 25°-47° SE. Slickenlines on the thrust fault surfaces rake ~90°, indicating dip-slip thrust faulting perpendicular to fold hinges. Folds and thrust faults are compatible with a single deformation event with a horizontal maximum compressive stress trending 145°-160°. The southward dip of thrust faults and predominantly south-dipping bedding along the Tuscarora Fm. ridge are opposite those shown on published geologic maps. Outcrops along the ridge expose near-vertical, fold-perpendicular fault surfaces striking ~160°. These surfaces are perpendicular to the fold hinges and thrust faults. Slickenlines on fold-perpendicular fault surfaces change orientation in tandem with changing bedding dips around anticlinal fold hinges. Unfolding the slickenlines along with bedding returns the slickenlines on fold-perpendicular, vertical fault surfaces to approximately horizontal, suggesting that strike-slip faulting occurred before folding. It is unclear whether strike-slip faulting represents a separate deformation event, tear faulting in the hanging wall of the developing thrust fault, or some other mechanism; this is the subject of ongoing investigation. The discrepancies between structures observed in this detailed, outcrop-scale study, and those shown on existing geologic maps, highlight the need for continued emphasis on detailed geologic mapping, structural analysis, and re-interpretation of structure and tectonics in the Appalachians of central Pennsylvania.