The Lackawanna synclinorium, Pennsylvania: A salt-collapse structure, partially modified by thin-skinned folding

The Lackawanna synclinorium is a 110-km-long structural trough in the Appalachian foreland of northeastern Pennsylvania. In map view, its hinge displays a concave-to-the-foreland (i.e., concave toward the west) curve. Historically, the entire synclinorium was thought to be an Alleghanian thin-skinned contractional structure that formed similarly to the fold trains of the central Appalachian Valley and Ridge province. Interpretation of seismic reflection data across the structure, however, suggests that the synclinorium formed primarily by the removal of salt. The trend of the central and northern synclinorium reflects the location of Upper Silurian Salina salt. In the southern synclinorium, the salt-collapse structure translated northwestward over a thrust ramp joining detachments in the Cambrian Waynesboro Formation and the Salina Group. Interpretation of seismic reflection data also suggests that the basement-cover contact deepens westward and that basement-penetrating reverse faults and a Neoproterozoic rift basin are present beneath the synclinorium. We speculate that (1) partial barriers due to sedimentation and/or reef formation, and (2) localized subsidence over a Neoproterozoic rift zone may have formed a Late Silurian restricted basin that allowed thick accumulations of Salina salt. Regional-scale salt/shale flow and/or foreland-directed (i.e., westward) fluid migration during the Alleghanian orogeny might have removed as much as 700 m of salt. The removal of this salt caused the overlying strata to collapse downward and form the synclinorium. Only in the southern synclinorium was the collapse structure modified significantly by décollement tectonics. Thus, the map-view curvature of the structure reflects greater tectonic shortening and westward translation (toward the foreland) in the southern synclinorium compared to the northern synclinorium.