Martic Shear Zone Deformation In The Honeybrook Upland Northern Chester County, Pennsylvania

Abstract

This study is an investigation of the effects of Martic shear zone deformation in the Honey Brook Upland, a region north of the Chester Valley, southeastern Pennsylvania. The Chester Valley and rocks adjacent to its southern boundary lie within the Martic shear zone, a zone of high ductile strain in southeastern Pennsylvania. This study found planar fabric in rocks in the southern and central regions of the Honey Brook Upland which resemble that of the Martic shear zone. Along the southern boundary of the Upland, a mylonitic fabric dips steeply to vertically, and its strike while varying, averages approximately N75E, similar in attitude to the Martic zone. Slightly north of the southern shear band, in the central Upland section, grain size is slightly coarser, although strike and dip resemble the southern shear zone and structures vary. Metamorphic facies assemblages in the southern and central regions consist of an upper amphibolite facies mineral assemblage overprinted by greenschist facies. The strong planar fabric composed of relict high temperature mineral assemblages on which a greenschist facies assemblage has been superimposed, is common in the Martic shear zone. This investigation found that the strong planar fabric, while enhanced by greenschist facies metamorphism, is primarily due to the amphibolite facies metamorphism and accompanying deformation.