A MAGNETIC INVESTIGATION OF DIABASE DIKES IN THE CENTRAL PIEDMONT, VIRGINIA

Based exclusively on outcrop data Mesozoic diabase dikes appear to be dextrally offset along the Lakeside fault zone in the central Virginia Piedmont. The Lakeside fault zone strikes NNE, forms the western boundary fault of the Farmville Mesozoic Basin, and may link up with the central Piedmont/Spotsylvania high-strain zone. Paleozoic mylonitic rocks are common in the Lakeside fault zone, but post-Paleozoic brittle deformation in this zone produced cataclasites and may have dextrally offset diabase dikes. Given the paucity of bedrock exposures in the Piedmont, a magnetic survey was completed to better define the map pattern of the diabase dikes. 1000 magnetic readings were taken using a proton-precession magnetometer along a series of traverses, with a 15 m spacing between readings. The magnetic survey covered an area of 4 km² (2.8 km x 1.5 km).

The diabase is highly magnetic compared to the surrounding amphibolite, granitic gneiss, and mylonitic gneiss country rock and showed magnetic relief of up to 1500 gammas. Based on contoured magnetic data a single dike (10–30 m thick) was recognized. The dike is characterized by two N-striking segments that are linked by a NNE-striking segment. The N-striking segments may have formed different en-echelon dikes and were linked by a NNE-striking transverse fracture. Based on the total magnetic field profile we estimate that dike segments dip steeply to the east-southeast. The magnetic data are inconsistent with dextral offset of the diabase dikes; rather we interpret there to be a single dike with distinct segments of different orientations.