MAGNETIC SIGNATURE OF THE PENNSYLVANNIA PIEDMONT: ORIGIN AND RELATIONSHIP TO DEFORMATION AND METAMORPHIC EVENTS

The Appalachian Piedmont in southeastern Pennsylvannia exhibits a‘ curly maple’ contour pattern on aeromagnetic maps. South of the Holtwood Dam on the Susquehanna River, strong positive anomalies occur over some areas of multiply deformed mica-schists. Anomalies vary over 1300nT in aeromagnetic surveys and four times that value on the ground. In the 1x3 km field area, a dramatic range (10^-4-10^-2) of magnetic susceptibilities was recorded. Irregular patterns of magnetic susceptibility cut across lithologic and structural fabrics, locally with boundaries at decimeter-scale. In hand sample, magnetite grains are as large as 1cm in diameter. However, rock magnetic studies show very low natural remanent magnetism intensities, 90% of cores measured are under 0.2 A/m. This suggests that the anomalies are produced almost entirely by induced magnetization. Thin section analysis reveals that susceptibility increases with the amount of magnetite in the sample. This occurs either as platy or somewhat euhedral grains of several ages within a 3-stage deformation cycle. Euhedral grains are generally homogeneous, whereas platy grains commonly include hematite exsolution lamellae with one or more sets of perpendicular secondary lamellae within. The relative ages of the different phases of magnetite growth in relation to metamorphic events and the possible effects of migrating chemical fronts are under continuing investigation.