Effects of metamorphism and structure on aeromagnetic anomalies over the Carolina slate belt near Roxboro, North Carolina

Interpretation of aeromagnetic anomalies to find the subsurface extent and attitudes of multiply deformed stratigraphic units near Roxboro, North Carolina is complicated by effects of regional metamorphism on rock magnetism. Anomaly amplitudes diminish markedly to the WNW near the onset of amphibolite facies. Thus, for similar lithologic mixes in this area, at higher metamorphic grades, magnetic susceptibility contrasts are markedly reduced. Modelling was done to determine the subsurface configuration of a magnetized greenschist facies metasedimentary unit in the faulted core of the Virgilina synclinorium. Magnetic anomalies appear to be caused by the entire fold structure in some locations and by only the upper limbs of the structure in others. Two alternative explanations are proposed: (1) the deeper part of the fold may have been displaced by thrusting, (2) metamorphic grade may increase with depth so that magnetic susceptibility contrasts diminish near the amphibolite facies boundary. The latter explanation is favored because it is consistent with surface data in the region. The study indicates that magnetic modelling has utility in complex metamorphic terrains to determine subsurface structure and subsurface configurations of metamorphic isograds.