The transition zone between the modern northern and southern Appalachian Mountains is located in Pennsylvania, where the structural orientation of the fold-and-thrust belt changes from north–south to east–west, and the orogeny narrows significantly. Vintage studies of wide-angle reflection and temporary broadband seismic data suggest that the crust beneath the ∼8 km
of foreland basin sequences was thickened and heavily intruded around the margins of and beneath a failed Neoproterozoic rift. We use receiver function analysis of broadband seismic data recorded by additional and permanent stations, along with forward and inverse modeling of Bouguer gravity data to constrain the geometry and depth extent of mafic intrusion and underplating in the rift, as well as the role of this Proterozoic heterogeneity on the location and geometry of the curvature of the Appalachian orogen. The receiver function analyses suggest that the crust is ∼47–49 km thick beneath the ancient rift, about 5–7 km thicker than the surrounding area. Inverse models of gravity data indicate that the ∼300 km-long zone of thickened, high density crust is bounded on both the NW and SE sides by steep contacts; its shorter NE and SW margins are also steep contacts interpreted as crustal-scale faults. Forward modeling of the gravity data, constrained by the receiver function crustal thickness estimates, sparse seismic reflection data and Euler deconvolution solutions, implies that the Proterozoic rift has been heavily intruded as well as thickened by a 7–10 km mafic underplate. Its margins appear to have been sheared along NE-striking fault zones that parallel Appalachian thrust sheet transport directions. These combined results suggest that the mid- and lower-crust of the Proterozoic rift was enriched with pyroxene, which strengthened the crust locally and localized compressional strain along its margins during the North American–African collision. Compressional strain in the pre-Appalachian crystalline crust to the southwest and northeast of the Proterozoic rift may have been more distributed, leading to the formation of oblique-slip faults orthogonal to the axes of folds, and leading to the curvature in the Pennsylvania salient. Additionally, the thrust sheets are stacked most thickly in front of the rift, suggesting that the rift served as a backstop during collision.