THE SUGAR HOLLOW BASIN: A NEOPROTEROZOIC HALF GRABEN COMPLEX IN THE CENTRAL VIRGINIA BLUE RIDGE

The structural geometry of the central Appalachian Blue Ridge province is largely the result of Paleozoic contractional deformation; however, elements of the Neoproterozoic rift architecture are preserved in central Virginia. The stratigraphy of the western Blue Ridge includes the Mesoproterozoic granitic basement complex that is unconformably overlain by the Swift Run and Catoctin formations. The Swift Run Formation is a heterogeneous unit composed of meta-sedimentary conglomerate, meta-arkose, and phyllite along with interlayered greenstone.

20 km northeast of Charlottesville, the late Neoproterozoic Swift Run Formation crops out in the ~12 km² Sugar Hollow basin. On the western edge of this structure the Swift Run Formation is either absent or <20 m thick, to the east, over a distance of 3 km the unit thickens to over 300 m into the basin. Four NE-striking normal faults cut the basin with maximum vertical displacements of 100 m, framing an eastward thickening half graben complex. The eastern edge of the basin is cut by a mylonitic high-strain zone with a top-to-the NW sense of shear, placing basement on the Swift Run Formation.

Rocks in the Sugar Hollow area experienced penetrative deformation at the greenschist facies during the Paleozoic. This contractional deformation produced a pervasive SE-dipping foliation and a series of asymmetric open to overturned folds that are buttressed against the original normal faults that are partially inverted. Imbrication of these faults contributed to a tectonic thickening of the Swift Run Formation. A strong deformation gradient from west to east exists as foliation grades from moderate to mylonitic. Consistent top-to-the NW asymmetries developed under general shear conditions. Retrodeformation reveals ~30% NW-SE directed contraction across the Sugar Hollow basin.