STRAIN VARIATION AND METAMORPHISM IN OUTCROP SCALE DUCTILE SHEAR ZONES IN THE WILMINGTON COMPLEX, SE PENNSYLVANIA PIEDMONT

Outcrops of amphibolite facies tonalitic orthogneiss of the Wilmington Complex near Glen Riddle in the Southeastern Pennsylvania Piedmont are cross-cut by one to two meter wide, sub-vertical, northeast striking ductile shear zones. Macroscopic and microscopic observations suggest that shear zones formed in a transpressive regime. Deflection of older foliation at shear zone margins suggests that strike parallel dextral motion and east side down vertical motion were important components of shear zone deformation. Sense of shear determined by using asymmetric microstructures, including tails on feldspar porphyroclasts and C'- shear bands, is consistent with outcrop-scale observations.

Foliation and lineation orientations vary across the width of the shear zones. At shear zone boundaries, foliation strikes northwest and dips moderately northeast; lineation has a moderate plunge in the plane of foliation. In the center of shear zones, foliation is subparallel to shear zone boundaries and lineation plunges steeply. The relationship of dip to plunge in the zone is similar to those predicted by published kinematic models of transpressive shear zones.

Petrographic observations and elemental X-ray maps constrain metamorphic conditions during deformation. Microstructures in plagioclase and quartz suggest that subgrain rotation and grain boundary migration were important deformation mechanisms, consistent with deformation under amphibolite facies conditions as inferred from mineral assemblages (Hbl + Pl + Bt ± Ep ± Cum ± Grt). X-ray maps show garnet with a low-Ca core and an overgrowth that exhibits distinctly higher, then gradually decreasing Ca content. Hornblende exhibits Al enrichment and Mg depletion in rims, and plagioclase is zoned to lower Ca and Al concentrations. This zoning suggests mineral growth by reactions such as An + Tr=Grs + Prp + Ts + Qtz or An + Act=Grs + Alm +Ts + Qtz in response to increased pressure. Structural and metamorphic evidence suggests that transpressive shear zones in the Wilmington Complex formed in a convergent tectonic setting associated with crustal thickening.