Northeastern Section–41st Annual Meeting (20–22 March 2006)

Paper No. 9
Presentation Time: 11:00 AM


BOSBYSHELL, Howell, Department of Geology and Astronomy, West Chester Univ, 750 South Church Street, West Chester, PA 19383, WISWALL, C. Gil, Geology & Astronomy, West Chester Univ, 750 S. Church St, West Chester, PA 19383 and BLACKMER, Gale C., Bureau of Topographic and Geologic Survey, 3240 Schoolhouse Rd, Middletown, PA 17057,

Numerous models have been proposed to explain the evolution of the southeast PA Piedmont. Kinematically, these models cover the spectrum from pure dip-slip to pure strike-slip motions. Our work indicates that the early assembly of this terrane was predominantly accomplished via orthogonal convergence. The Embreeville Thrust and Street Road Fault are two key structures where this shortening was concentrated.

The Embreeville thrust (ET) trends ENE and is well constrained in the Unionville, Coatesville, and a portion of the West Chester quadrangles. To the east, the thrust is apparently truncated by a fault. The westward extent of the thrust has yet to be determined. In the Unionville quadrangle, the trace of the thrust is parallel to but not coincident with the Cream Valley fault, a later, high-angle fault with transpressional kinematics. The ET emplaces amphibolite facies Doe Run schist member of the Glenarm Wissahickon over greenschist facies Peters Creek Schist. A band of muscovite-chlorite-garnet phyllite occurs between the two map units and is interpreted as the fault rock contained in the ET shear zone.

The Street Road Fault (SRF) trends NE across the West Grove, Kennett Square, West Chester, and Media quadrangles. Where best exposed in the West Chester quadrangle, it places the Precambrian Avondale Massif and its cover sequence, Setters Formation and Cockeysville Marble, above Doe Run schist. To the east, where it separates the West Chester and Avondale Massifs, the SRF is not well defined but its trace is marked by a linear array of ultramafic bodies.

Both shear zones are characterized by shallow southeast dipping mylonitic foliation which wraps porphyroclasts formed during peak metamorphism. Kinematic indicators show a consistent top to the NW sense of motion. Lack of cross-cutting relationships precludes establishment of the relative age of deformation in the two shear zones. However, preliminary monazite geochronology indicates that peak metamorphism in the Doe Run schist is Silurian and that fabric-forming deformation is early Devonian, providing some constraint on the absolute age of shear zone deformation.