The age and timing of folding in the central Appalachians from paleomagnetic results

Comparison of the paleomagnetic pole positions of late Paleozoic secondary magnetizations with a time-averaged reference apparent polar wander path for North America shows that sedimentary rocks within the central Appalachians were remagnetized in the later half of the Permian, between ca. 255 and 275 Ma. Ages of remagnetization do not vary for sites distributed along or across strike of the central Appalachian fold and thrust belt or across the Appalachian Plateau in New York, nor do they vary with stratigraphic position of the host rocks. However, the pattern of folding relative to remagnetization across the Valley and Ridge and Great Valley in Pennsylvania, Maryland, and West Virginia shows a distinct temporal variation. Individual fold tests yield postfolding magnetizations near the hinterland margin of the fold and thrust belt, synfolding magnetizations in the central part of the belt, and prefolding magnetizations near the foreland. Because of the similar age of the late Paleozoic magnetization, we conclude that rocks across the central Appalachians were remagnetized in a relatively short period of time as folding and thrusting propagated more slowly toward the foreland. We interpret this pattern to indicate that older folds near the hinterland grew prior to remagnetization, folds in the central region grew during remagnetization, and folds near the foreland grew after remagnetization. Rapid remagnetization relative to folding is also evident from the physical characteristics and magnetic properties of secondary hematite in remagnetized (synfolding) folds. In such folds, rocks were apparently remagnetized during an increment of fold growth, in which there was partial and possibly negligible fold-limb rotation. Collectively, these results may constrain the mechanism for basinal fluid mobilization during deformation. Current models for remagnetization involve the migration of chemically active brines into the foreland during tectonism. If realistic, our results bear on this model by suggesting that brine migration was not directly related to the successive emplacement of individual thrust sheets, as has been suggested for other fold and thrust belts.