MULTIPLE DEFORMATION ASSOCIATED WITH TACONIC THRUST SHEETS IN THE GREAT VALLEY OF VIRGINIA

Previous geologic studies of Appalachian structure in the Great Valley of Virginia have consistently concluded that the region had a simple deformation history consisting of one phase of Alleghanian deformation. Our recent mapping in the southern end of the Mattanussen Syncline in Augusta County, Virginia, has revealed that the Martinsburg Formation can be divided into three thrust sheets, each with its own distinct poly-phase deformation history, structural geometry and package of lithologies. The structurally lowest domain includes several large map-scale folds that deform both the Martinsburg and the underlying Mid-Ordovician limestone sequence. These folds, previously thought to be parasitic to the Mattanussen Syncline, both fold the slatey cleavage in the Martinsburg and are older than the Syncline. This thrust sheet is structurally overlain by Martinsburg in which upright folds have a near vertical slatey cleavage in their axial planes. This thrust sheet is structurally overlain by a sheet in which folds are recumbent, have axial planes dipping less than 40 degrees to the east and consistently verge to the west. Each of these three thrust sheets is folded by open, broadly co-axial buckle folds that deform earlier structures and represent small scale examples of the fold that forms the main Mattanussen Syncline. Except for this last deformation, all the identified phases of deformation appear Taconic in age including the Staunton Thrust. The Martinsburg Formation has long been known to be one of the two most common decollement zones in the Applachian Orogen. The current study appears to be the first to unravel the kind of poly-phase deformation history one might expect in such a zone.