The effects of grain‐scale deformation on the Bloomsburg Formation Pole

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Abstract
Previous paleomagnetic results from the Silurian Bloomsburg Formation in the central Appalachian Valley and Ridge Province suggest that the Bloomsburg's characteristic magnetization is a synfolding magnetization acquired during the Acadian Orogeny. However, there is little geological evidence to support extensive Acadian folding in the central Appalachians. A more favorable explanation is that the characteristic magnetization originated as a prefolding Silurian magnetization and has been altered by grain‐scale deformation to mimic a synfolding Devonian remagnetization. In order to investigate whether penetrative deformation has altered the Bloomsburg Formation's characteristic signal, the relationship between strain and remanence was examined around three central Appalachian Valley and Ridge folds. Structural analysis of mesoscopic and microscopic features along with center‐to‐center finite strain analysis at these three folds indicates a deformation sequence that includes prefolding layer‐parallel shortening overprinted by top‐to‐the‐foreland, bedding‐parallel shear and flexural slip/flow folding. Inclination of the characteristic magnetization varies systematically with the magnitude of finite strains. This relationship between strain and remanence suggests that the characteristic magnetization has been progressively rotated toward shallower inclinations by penetrative deformation analogous to rigid particle rotation in a viscously deforming matrix. Using directions from sites with relatively low finite strain values, a new Bloomsburg Formation pole is calculated at 18.7°N, 107.4°E (K = 58.4, A95 = 8.2°). This new pole falls on the Silurian track of the North American apparent polar wander path between the Silurian Wabash Reef pole and the Silurian‐Devonian Andreas pole. In addition, these results suggest that previously reported 20°–30° difference in declination of site mean directions north arid south of the Pennsylvania salient may also be the result of grain‐scale reorientation of the remanence‐carrying grains and not oroclinal bending.