GEOPHYSICAL TRANSECTS ACROSS THE MARGINS OF THE CARROLL KNOB MAFIC/ULTRAMAFIC COMPLEX, MACON COUNTY, NORTH CAROLINA

Geophysical surveys were conducted along three transects crossing margins of the Carroll Knob mafic/ultramafic complex in western North Carolina. The transects include (1) magnetic, refraction seismic, and resistivity profiles on Jones Creek Road in the NW part of the complex; (2) magnetic, resistivity, electromagnetic, and ground penetrating radar profiles near Shope Fork Road in the western part of the complex; and (3) magnetic, refraction seismic, resistivity, and ground penetrating radar near Carroll Knob, close to the Coweeta hydrologic station. At the margins of the complex, altered mafic and ultramafic units (metadunites, metapyroxenites, metagabbros, amphibolites) grade into metasedimentary host rock. Lithologies within and on the margins of the complex are interlayered on scales from meters to tens of meters. In the Jones Creek Road area, magnetometer data suggest that magnetic anomalies strike northeast, roughly parallel to the regional foliation. Strong discontinuities in the magnetic signal coincide with metadunite/amphibolite contacts. Multi-electrode Wenner resistivity traverses along this profile show high resistivities coincident with metadunite outcrops and lower resistivities in amphibolite zones. The data suggest that the resistive bodies extend to at least 20 meters depth and are overlain by a more conductive, presumably weathered zone, 2-10 meters thick. Refraction seismic data image irregular contacts that are inferred to represent the base of a saprolitic zone of weathering. The seismic profiles show no consistent variations across lithologic contacts. On the Shope Fork Road profile both resistivity and ground penetrating radar surveys indicate subhorizontal layering that contrasts with the steeper dip of the regional foliation. Electromagnetic surveys with the EM-34 instrument proved insensitive to variations in this highly resistive setting. A conductive body of 40 meters width with a strong magnetic signal could not be clearly correlated with surface lithologies. Resistivity profiles at the Carroll Knob site show variations at amphibolite/pyroxenite and amphibolite/schist contacts. Variable magnetic susceptibilities within lithologic units and significant elevation differences within the study areas make quantitative interpretation of magnetic data difficult.