STRUCTURAL GEOMETRY AND KINEMATICS OF THE SPOTSYLVANIA ZONE, CENTRAL VIRGINIA PIEDMONT

The Spotsylvania Zone forms the boundary between the early Paleozoic Chopawamsic terrane and the Mesoproterozoic Goochland terrane in the central Virginia Piedmont. This boundary was originally recognized as a sharp geophysical (magnetic and radiometric) lineament. The Spotsylvania Zone is ~10 km wide and includes the Lakeside mylonite zone and the Ca Ira mélange. Previous workers have interpreted the Spotsylvania Zone as 1) a significant thrust fault, but not a suture, along which rocks of the Goochland terrane were emplaced to the northwest in the late Paleozoic, 2) a major suture, and 3) a zone of predominantly brittle en-echelon faults. In southern Virginia the Spotsylvania Zone is on strike with the Hyco Zone, a segment of the Central Piedmont shear zone in the southern Appalachians. Biotite gneisses, amphibolites, granitic gneisses and granitic pegmatites are mylonitized within the Spotsylvania Zone. Foliation generally strikes to the northeast-southwest, dips moderately to gently southeast, and is folded into asymmetric northwest-verging folds. Elongation lineations plunge gently to the northeast and southwest. Asymmetric porphyroclasts and boudins, shear bands, and flanking folds consistently record dextral shear. Vorticity analysis (porphyroclast hyperbolic distribution method on ultramylonites) indicates general shear deformation (Wn=0.4-0.8). Folded and boudinaged pegmatite dikes record sectional strains of ~20:1. Three-dimensional strains include both apparent constrictional and flattening strains. Dynamically recrystallized quartz and feldspars are indicative of lower amphibolite facies deformation conditions. Collectively, these observations suggest that the Spotsylvania Zone is a transpressional high-strain zone that records both dextral strike-parallel movement and a component of contraction. Shear strain integration across the Spotsylvania Zone requires at least 50- 100 km of dextral offset between the Chopawamsic and Goochland terranes.