PALEOMAGNETIC CONSIDERATIONS OF THE DEVELOPMENT OF THE PENNSYLVANIA SALIENT IN THE CENTRAL APPALACHIANS

Late Paleozoic remagnetizations of central Appalachian clastic and carbonate rocks of Ordovician to Devonian age constrain the structural evolution of the Pennsylvania salient. New paleomagnetic results from 26 sites located on nine folds combined with previously published results from 60 sites yield a mean direction for the northeastern limb of the salient at 172-degrees/1-degrees (k = 49) and a mean direction for sites southwest of the salient at 173-degrees/1-degrees (k = 70). These two directions are not statistically different from each other and are not significantly rotated from the expected Permian paleomagnetic direction for North America. Individual fold test results show that this secondary magnetization is synchronous with or even older than folding across the region. These paleomagnetic results suggest that curvature of fold axes around the salient is an original feature of fold growth and not the result of post-orogenic oroclinal bending. In contrast, analysis of the primary magnetization in the Silurian Bloomsburg Formation shows that directions from the two limbs of the salient are significantly different. New results from twenty sites yield a mean direction for the northeastern limb at 4-degrees/-40-degrees (k = 31), and for the southwestern limb at 338-degrees/-38-degrees (k = 23). Combining these results with previously published directions from the Bloomsburg Formation yields a mean direction for the northeastern limb at 1-degrees/-34-degrees (k = 32) and for the southeastern limb at 343-degrees/34-degrees (k = 30). The difference in declination does not appear to be the result of internal deformation of the rocks or acquisition of magnetization during a period of rapid apparent polar wander. Rather, these directions indicate that the northeastern limb has been rotated clockwise 18 +/- 9-degrees relative to the southwestern limb. The sense and amount of this rotation is consistent with the curvature of the orogen around the salient. However, within each limb of the salient the primary component declinations do not directly correlate with the orientation of the first-order fold axes. Collectively therefore, these paleomagnetic results suggest that the structural evolution of the salient included pre-orogenic rotations or rotations that occurred early in the Alleghanian orogeny followed by later remagnetization and folding of the central Appalachian foreland.