The Geometry And Conditions Of Deformation Of The Rosemont Shear Zone, Southeastern Pennsylvania Piedmont

Abstract

Re-examination of the Rosemont Fault (southeast Pennsylvania Piedmont) using the ductile shear zone approach reveals a large (1.5-3.0 km wise) ductile shear zone. The Rosemont shear zone strikes parallel to the Rosemont fault line (N30-40°E) and is moderate to steeply dipping. Numerous perpendicular transects across the Rosemont zone reveal a complex geometry. Unlike a classic tabular-shaped shear zone the boundaries of the Rosemont zone are not parallel. The eastern boundary strikes approximately N35°E and is very steeply dipping (70-90°E) while the western boundary has a similar strike but a variable dip between 40-60°E. In cross-section the Rosemont zone has a semi-wedge shape that widens in the upward direction. The variability i the horizontal width of the zone as well as the orientations of the boundaries are most likely the result of large competency difference between different lithologies associated with the Rosemont zone. Consistent dextral shear sense was determined using 1) asymmetric tails around porphyroclasts in thin-section, 2) numerous small (meter scale) parasitic shear zones with obvious right lateral displacement, and 3) dextral transposition of earlier structures into the zone as seen on the regional scale. Correlation of a sliver of mafic gneiss with the Wilmington Complex supports a model of dextral displacement with a minimum offset of 20 kilometers. Six phases of deformation were recognized. The first phase was a postulated overthrusting of the Wilmington Complex onto the Wissahickon Group. This thrusting event was followed by four phases of transcurrent ductile shearing and finally a weak phase of reverse ductile displacement possibly as a result of uplift. Retrodeformation of the largest ductile structures (phase 5: Martic zone; phase 3: Rosemont zone) suggests that the Baltimore Gneiss was once adjacent to the Wilmington Complex.