Evolution, as affecting the earth's crust

Abstract

Well-documented tectonic events in the central Appalachians of Pennsylvania are: (1) the early Paleozoic Taconian orogeny that occurred during convergence of Laurentian and the Chopawamsic-Wilmington complex magmatic arc over an east-dipping subduction zone, and resulted in intense metamorphism and deformation in the Piedmont, and (2) the late Paleozoic Alleghanian orogeny that resulted in the thrust and fold belt in the Valley and Ridge province and dextral shear zones in the Piedmont. Unlike the Paleozoic tectonic history for the northern and southern Appalachians, the north-central part of the orogen in Pennsylvania lacks evidence for Acadian deformation and metamorphism. The relative chronological order of deformation and metamorphic events in the eastern Piedmont of Pennsylvania, combined with published geochronology suggests the previously undocumented Acadian deformation possibly exists as a transcurrent conjugate shear zone pair.The Rosemont shear zone is a dextral transcurrent shear zone that is the boundary between the type-section Wissahickon Formation of the Philadelphia structural block (to the southeast) and the West Chester and Avondale Grenvillian basement massifs (to the northwest). The Crum Creek shear zone is the sinistral antithetic structure to the Rosemont zone, and developed internal to the Philadelphia block. Geometric and metamorphic history similarities, opposing offsets, angular relationships, and relative timing of local deformation events supports a conjugate model for these shear zones. East-west oriented bulk shortening and north-south oriented bulk elongation directions are inferred from the conjugate geometry. The Rosemont-Crum Creek system crosscuts and deforms regional Taconian structures and metamorphic zones. In turn, the Rosemont zone is truncated by the Alleghanian Pleasant Grove-Huntingdon Valley shear zone. The available geochronology brackets the movement on the Rosemont-Crum Creek system from Devonian to early Mississippian time. This timing correlates with the Acadian metamorphism in the central Appalachians, suggesting that the Acadian orogeny was manifest as a strike-slip shear system in the Piedmont of eastern Pennsylvania. The lack of regional thrusting and subsequent crustal thickening associated with transcurrent deformation could explain the lack of regionally extensive Acadian metamorphism in the Pennsylvania Piedmont.