Reorientation of remanent magnetism during tectonic fabric development: an example from the Waynesboro Formation, Pennsylvania, U.S.A.

Abstract

A paleomagnetic study of the late Early-early Middle Cambrian Waynesboro Formation in south, central Pennsylvania reveals a pre-folding magnetization which is close to the expected Cambrian field at three sites, while other site mean directions are rotated by as much as 82° from the expected Cambrian field. These results are based on detailed thermal demagnetization analysis of 84 red siltstone and sandstone samples from nine sites collected over a 163 km 2 area. Local faulting, fault block rotation, and remanence acquisition during rapid apparent polar wander do not appear to be responsible for the observed remanence deviations. Total strain at the sites, as indicated semiquantitatively by the anisotropy of magnetic susceptibility (AMS) of 28 samples, has been studied to test the hypothesis that deformation has rotated the remanence directions. AMS data vary considerably both between sites and within sites and show a path of progressive deformation on a Flinn diagram. There is a strong, positive correlation between the deviation of a sample's remanence from the expected Cambrian field and its position along the deformation path, suggesting that finite strain/fabric development has rotated the magnetizations. The study area is structurally contained in the South Mountain Anticlinorium whose southern margin forms the displacement transfer zone to the southern Appalachian Alleghenian thrust sheets (Elliott, 1976). If thrusting had continued and incorporated the Waynesboro rocks into these thrust sheets, the rotated magnetizations could have been incorrectly interpreted to result from rigid thrust sheet rotation.