STRUCTURAL GEOLOGY AND MAGNETIC SUSCEPTIBILITY ANISOTROPY OF THE LACKAWANNA SYNCLINORIUM, PENNSYLVANIA: INSIGHT INTO THE FORMATION OF AN ALLEGHANIAN OROCLINE IN THE CENTRAL APPALACHIANS

The Lackawanna synclinorium is a 110 km-long structural trough in eastern Pennsylvania that involves Paleozoic strata and straddles the boundary between the central and northern Appalachians. Its axis curves in map view, so the fold has a concave-toward-the-foreland crescent shape. The 075°-trending southern third of the synclinorium lies in the Valley and Ridge fold-thrust belt, parallel to other folds of the belt, while the 030°-trending central and northern thirds cut into the Plateau province of the Appalachian foreland at a high angle to the 080°-trending gentle folds of the Plateau. To understand the origin of the synclinorium and its curvature, we conducted field mapping, seismic reflection analysis, Fry strain analysis, and magnetic susceptibility anisotropy (AMS) analysis. Our results suggest that the northern two-thirds of the synclinorium are a salt-collapse structure whose position and trend reflect the location of Silurian salt deposits, not regional shortening. Basement-penetrating faults formed during Neoproterozoic rifting may have controlled the distribution of this salt. In the southern third of the synclinorium, Paleozoic strata translated westward over a thin-skinned thrust ramp. Thus, the map-view curvature of the synclinorium developed when its southern third was incorporated into the Valley and Ridge fold-thrust belt and underwent greater shortening and westward translation relative to the northern synclinorium. This led to a clockwise rotation of the southern part of the trough from which the synclinorium formed, a history that makes the present crescent shape of the synclinorium an intersection orocline. Significantly, rotation of rocks in the trough does not require that thin-skinned faults and folds of the southern third were themselves rotated comparably in map view. In fact, second-order folds and faults, dominant cleavage, finite strain long axes, and K_max AMS axes in the region trend sub-parallel to the southern synclinorium, and thus trend at a high angle to the northern synclinorium. This relation, as well as crosscutting relations, suggests that 075°-trending structures formed with their present orientation. The distinction between rock rotation and structure rotation in the synclinorium's evolution may complicate interpretation of paleomagnetic data.