Structural History of a Portion of the Blue Ridge, Northern Virginia

Abstract

The Front Royal area is on the west flank of the Blue Ridge uplift in northern Virginia. The rocks include Precambrian gneisses nonconformably overlain by Precambrian clastic and volcanic rocks and Cambrian to Ordovician siliceous clastic rocks, carbonate rocks, and graywacke-slate. Regional metamorphism of greenschist grade affected all of these rocks during the Paleozoic. Deformation accompanied metamorphism producing folds, slatey cleavage, and mineral lineation. Cleavage locally cuts across hinge surfaces of folds suggesting that cleavage formed late in the folding process after folds were well defined. The cleavage normal is interpreted as paralleling the short axis of a finite strain ellipsoid, and the lineation as paralleling the long axis. Both of these directions line in a northwest-southeast plane approximately perpendicular to the fold hinges. Pressure solution surfaces and quartz deformation lamellae also indicate that maximum compressive stress was at some time nearly perpendicular to cleavage. Superimposed folds distort the slatey cleavage and a fracture cleavage parallels the hinge surfaces of some of these folds. Striated fractures occur in all formations. The striations, which are interpreted as displacement directions, statistically lie in the northwest-southeast plane which is perpendicular to hinge lines of the bedding folds. Major reverse faults, high-angle longitudinal faults, high-angle transverse faults, and zones of en echelon tension fractures occur with displacements in the same northwest-southeast plane defined by the slatey cleavage, mineral lineation, and striations on fractures. Bedding folds, slatey cleavage and lineation, fracture cleavage, superimposed folds, striated fractures, reverse faults, high-angle longitudinal faults, transverse faults, and zones of en echelon tension fractures are a chronological sequence of structures related to each other by displacement directions statistically oriented in a northwest-southeast plane approximately perpendicular to fold hinges. The sequence indicates that the rocks were first ductile but became more brittle and fractured as deformation proceeded. Principal stress directions were constant enough throughout deformation to keep displacement directions of the various tectonic structures statistically oriented in the same plane. This kinematic uniformity implies, but does not prove, only one Paleozoic diastrophic event in the Blue Ridge of northern Virginia.