STRAIN ANALYSIS OF THE EASTERNMOST APPALACHIAN OROCLINE

The northernmost central Appalachian fold-thrust belt is a classic example of a blind thrust system that abruptly changes orientation by nearly 90°, and defines the Pennsylvania orocline. Previous research has quantified shortening distribution across the central Valley and Ridge Province, yet no such data has been collected at the easternmost extent of the arc in Pennsylvania. In the easternmost portion of the Pennsylvania orocline, folding in the Appalachian plateau is less pronounced than in the central and western extents, suggesting complications in the transition from the valley and ridge to Appalachian plateau that limits stress migration northward. Underlying this region in northeastern Pennsylvania (NEPA) are intrusions of high density rock, a product of a failed neoproterozoic rift, that contribute to a local gravity anomaly, or Scranton Gravity High (SGH). The Lackawanna Synclinorium, a structure formed by the removal of salt, geographically coincides with the SGH. These two features could have limited the development of strain northward by (1) constraining stress accumulation with denser material, and/or (2) accommodating stress through the migration of weak materials (salt). In the case of the first hypothesis, grain-scale strain would be more pronounced south of the SGH than in the central parts of the orocline as stress builds up in the valley and fold system. For the second hypothesis, stress may have been preferentially accommodated by the migration of salt, resulting in no significant increase in strain within the orocline. To explore these hypotheses, we constructed a balanced cross-section and conducted microscopic strain analyses. Microscopic strain analysis was performed on three orthogonal thin-sections from oriented samples of quartz-rich competent rocks using normalized fry and Rf-Φ methods. 2D ellipses from the normalized fry method were then processed by Geological Programs for Mathematica to extract an oriented 3D ellipse for each sample, and ellipticity, axes orientations and Flinn diagrams for all samples were analyzed to more easily identify distinct microscopic deformation patterns in eastern PA. We present this strain data with a balanced cross-section to gain a better understanding of how pre-existing structures affect the development of strain in the Appalachians.