THE ROLE OF TRANSCURRENT SHEAR ZONES IN THE HISTORY OF THE WISSAHICKON FORMATION

Structural studies from the mid 1980s through 1990s led to the recognition and characterization of a system of steeply dipping shear zones in the central Appalachian Piedmont of southeastern Pennsylvania. Dextral displacement on these ductile shear zones allowed significant horizontal displacement of adjacent terranes, making interpretations of a widespread Wissahickon Formation suspect.

The type locality for Wissahickon Formation underlies Philadelphia and is well exposed along Wissahickon Creek. It is part of a terrane bounded to the west by the Rosemont shear zone and to the north by the Cream Valley-Huntingdon Valley shear zone. Amphibolite facies metamorphism of this Wissahickon schist is overprinted by later syntectonic greenschist facies metamorphism along the Cream Valley-Huntingdon Valley shear zone. Structural interpretation indicates that this terrane has been translated southwestward with respect to autochtonous Laurentia.

Other metasedimentary lithologies mapped as Wissahickon Formation are not likely to be correlative as they lie on the other side of proposed terrane boundaries within regions that do not share the same history of metamorphism and deformation. Early tectonic interpretations relied on this lithologic correlation in suggesting a simple collisional history for this part of the central Appalachians. The recognition of significant transcurrent displacements along ductile shear zones indicates a tectonic history of oblique convergence and orogen-parallel displacement of discrete terranes along this part of the Laurentian margin.

These investigations, conducted by the structural geology group at Temple University, were built on a foundation of earlier detailed metamorphic studies and laid the groundwork for more recent studies that divide the Wissahickon Formation on the basis of timing of metamorphism.