DEFORMED DEVONIAN-AGED BEDROCK EXPOSED IN STRUCTURALLY-CONTROLLED BEDROCK STREAM CHANNELS IN NORTHWESTERN PENNSYLVANIA: POSSIBLE EVIDENCE FOR GLACIALLY-INDUCED TECTONISM DURING THE QUATERNARY

Meandering stream channels incised into flat-lying Devonian-aged bedrock in Erie, PA expose structural deformation in the form of joints, thrust faults, and anticlines. Joint orientations broadly fall into one of two regional trends (broadly N-S or E-W; Engelder, J. Struct. Geol, 1985) and are pervasive in bedrock exposures where they appear to be controlling the orientations of individual arms of each stream meander. Contractional structures (as mapped in Elk Creek, Four-mile Creek, Mill Creek, Walnut Creek, and tributaries) are localized in their occurrence and die out within meters of either side of the creeks; they are exposed in the walls of cut-in-bedrock terraces covered by relatively thin (up to 2 m thick) fluvial deposits. Surfaces of fluvial deposits on the highest standing (oldest?) terraces appear to be disrupted above each structure whereas fluvial deposits on the lower standing (younger?) terraces expose these structures, but have no topographic disruption of their surface. This suggests structural deformation occurred earlier during stream incision. The orientations of mapped contractional structures in relation to stream directions suggests both right and left lateral slip likely occurred. Specifically, streams with orientations closer to true north appear to exhibit left lateral motion whereas streams that trend more WNW appear to exhibit right lateral motion. The general orientation of the contractional structures is consistent with the E-W regional joint trend suggesting that the anticlines and thrust faults represent contractional re-activation of this particular joint set. Specifically, the N-S joint set experienced lateral displacement causing some E-W-trending joints to contract. Fault displacement appears to be on the order of cm, but has not been fully constrained and could reflect only minor lateral slip on the N-S joint set. The timing and stress orientations represented by the contractional structures are consistent with Quaternary ice movement in this region and may have been induced by ice retreat – an interpretation consistent with the proposed origin of similar structures in eastern Canada. Future work should explore additional segments of bedrock-incised streams in NW PA, quantify fault offset and stress magnitudes, and further constrain the timing of deformation.