Geophysical anomalies along strike of the Southern Appalachian Piedmont

Bouguer gravity anomalies and aeromagnetic anomalies display a northward increase of values in the Charlotte and Carolina slate belts from Georgia to Virginia. Analysis of these data with constraints imposed by seismic reflection, seismic refraction, conductivity measurements, and surface geology imply that this increase is due to a corresponding increase in the percentage of mafic rocks in the upper crust. A regional trend in the Bouguer field is determined for an observed eastward decrease in crustal thickness based on seismic refraction measurements. A comparison of the calculated regional with surface boundaries shows that in places the crustal thinning occurs more than 50 kilometers west of the Inner Piedmont‐Charlotte belt transition. The comparison also shows that the crustal thinning may take place west of exposed Grenville basement. The evidence thus indicates that the southern Appalachian gravity gradient is not likely related to a crustal or lithospheric suture. The evidence further indicates that, because the upper crustal and deep crust/upper mantle variations are independent in terms of spatial position, the upper crust of the Charlotte/slate belts is decoupled from the lower crust, perhaps at subhorizontal boundaries seen on reflection data.