Rotated paleomagnetic poles from the Sassamansville syncline, Newark basin, southeastern Pennsylvania

Abstract

A paleomagnetic fold test was conducted at eight sites collected from the Sassamansville syncline which folds the Norian age Passaic Formation along the border fault of the Newark basin in southeastern Pennsylvania. Thermal demagnetization isolated two components of magnetization; a synfolding age, intermediate unblocking temperature, normal polarit B component with intermediate inclinations and northwesterly declinations and a prefolding age, high unblocking temperature, dual polarity C component with shallow inclinations and northwesterly‐southeasterly declinations. The paleomagnetic poles calculated from these magnetizations are both rotated ≈20°±6° counterclockwise from contemporary paleopoles isolated from other parts of the Newark basin. A paleomagnetic study of the Sassamansville diabase (Kodama and Mowery, 1993) indicates a 15°±5° counterclockwise rotation, so the 20° rotation of the Passaic Formation remanence at the Sassamansville syncline is interpreted to be due to a 15° block rotation of the Sassamansville area. The remainder although not statistically distinguishable may be due to grain‐scale strain based on rock and magnetic fabric measurements. Anisotropy of magnetic susceptibility, low temperature:high temperature susceptibility ratios and X ray pole figure goniometry results suggest that a tectonic fabric developed in the phyllosilicates, which has vertical minimum axes and maximum extensional axes parallel to the fold axis, could have caused the small (≤5°) grain‐scale rotation of the B and C component magnetizations. Fry analysis of quartz grains, however, does not record a significant finite strain in the rocks and may remain the best method to identify if a large grain‐scale rotation of remanence has occurred. If local block rotations also affected some of the sites collected from the nearby Jacksonwald syncline, the effect on the B and C component paleopoles reported by Witte and Kent (1991) and Witte et al.(1991) from the Newark basin is small enough that the discrepancy between Newark basin paleopoles and coeval poles from the southwestern United States is not appreciably reduced. When the 20° rotations are removed from the Sassamansville paleomagnetic data the B component magnetization yields a high latitude paleomagnetic pole which supports the Newark B remagnetization pole of Witte and Kent (1991) and the Moat Volcanics paleopole of Van Fossen and Kent (1990). The C component magnetization yields a pole consistent with the Norian age pole from the Passaic Formation reported by Witte et al. (1989).