The distribution of layer parallel shortening fabrics in the Appalachian foreland of New York and Pennsylvania: Evidence for two non-coaxial phases of the A lleghanian orogeny

This paper presents a structural interpretation of a part of the central and northern Appalachian foreland using the correlation in orientation of such deformation features as mechanical twins, solution cleavage, crenulation cleavage, pencils, joints, and deformed fossils. Such a correlation suggests that, within the central Appalachians, the Alleghanian orogeny consists of two major phases: a deformation possibly as old as Pennsylvanian, herein called the Lackawanna phase, and a second deformation, termed the Main phase of Permian or younger age. Effects of the Lackawanna phase deformation are found mainly in the Hudson River Valley and Pocono plateau, while effects in the Main phase deformation are found throughout the Valley and Ridge and Alleghany Plateau. The Lackawanna phase is interpreted as the product of strike-slip motion, possibly between the Avalon microcontinent and North America. The Main phase may record the final convergence of Africa against North America and accreted terranes.