FOLD-THRUST BELT STRUCTURES OF THE LACKAWANNA SYNCLINORIUM: IMPLICATIONS FOR THE TECTONIC EVOLUTION OF THE CENTRAL APPALACHIANS IN PENNSYLVANIA

The Lackawanna synclinorium is a 110 km-long salt-collapse structure in the Appalachian foreland of northeastern Pennsylvania that formed during the Alleghanian orogeny. In map view, the synclinorium fold hinge displays concave-to-the-foreland (i.e., concave toward the northwest) curvature. The southern part of the synclinorium trends 075°, parallel to the fold trains of the Valley and Ridge province to the south, whereas the northern part of the structure trends 010° at a high angle to the gentle folds of the Appalachian plateau to the west. Mesoscopic layer-parallel shortening (LPS) structures (e.g., folds, faults, cleavage) within the synclinorium developed in the coal-bearing Pennsylvanian strata above a regional detachment. Field data indicate that the dominant trend of these structures throughout the synclinorium is ~075°, sub-parallel to the fold hinge of the southern synclinorium but oriented at a high angle to the northern synclinorium. Shortening fabrics determined by finite-strain analysis and the anisotropy of magnetic susceptibility (AMS) trend sub-parallel to the mesoscopic LPS structures. In general, the southern region of the synclinorium is characterized by a greater abundance of LPS structures and larger finite-strain ratios compared to the northern region, suggesting the map-view curvature of the synclinorium resulted from greater shortening of strata to the south. During Alleghanian deformation, the horizontal component of the shortening direction in northeastern Pennsylvania rotated 46° clockwise, from 310° to 356°, as indicated by crosscutting relationships. The dominant ~075° LPS fabric within the synclinorium represents a later stage of Alleghanian progressive deformation. These results suggest that the traditional phases of Alleghanian deformation (the Lackawanna phase and Main phase) are simply pulses of shortening during an orogenesis where the shortening direction changed azimuth through time. Furthermore, the Lackawanna synclinorium is not a Lackawanna-phase contractional structure, as previously thought, and its formation by salt collapse overprinted the mesoscopic structures that formed earlier in the Pennsylvanian strata.