The Crum Creek Shear Zone, Delaware County, Pennsylvania And The Application Of A Ductile Conjugate Pair Model

Abstract

The sinistral Crum Creek shear zone is a zone of ductile strain located in Delaware County, Pennsylvania, and is internal to the Philadelphia structural block. The sinistral Crum Creek shear zone is an antithetic conjugate structure to the dextral Rosemont shear zone. Geometry, opposing shear sense, angular relationships and post-Taconian timing support a conjugate model for these structures. The Crum Creek shear zone is 1.5 to 3.0 kilometers wide, strikes approximately north-south, dips steeply to the east, and affects the schists and amphibolites of the Wissahickon Formation and the western margin of the Springfield granodioritic gneiss. The trace of earlier structures into the Crum Creek zone as well as the shape of the western margin of the Springfield gneiss indicate sinistral offset across the shear zone. This sinistral displacement is supported by the presence of Type I S-C mylonite developed along the western margin of the Springfield gneiss. Taconian isograds in the Wissahickon Formation were displaced approximately 4 kilometers across the Crum Creek shear zone in a manner consistent with sinistral offset Transposition of the Taconian isograds along the margins of the shear zone indicates that deformation along the Oum Creek shear zone occured atl.east after the peak of Taconian metamorphism. Opposing shear sense, identical geometries and an angular difference of 35° to 400 between the Crum Creek and Rosemont shear zones support a model of conjugate ductile structures. The implications are that the principal compressive stress array was oriented relative to present day coordinates such that a1 plunged shallowly west-northwest, a2 plunged steeply to the northeast and <J3 plunged shallowly south-southwest. This stress array indicates that the maximum compression direction was oriented at a high angle to the Crum Creek shear zone (>65°), and that structural escape occurred subhorizontal. These conclusions are consistent with the geometry of a map-scale conjugate box fold that developed between the conjugate shear zones.