3-D STRAIN ANALYSIS AND STRUCTURAL DEVELOPMENT OF THE CHILHOWEE GROUP IN THE WESTERN BLUE RIDGE PROVINCE NEAR FRONT ROYAL, VIRGINIA

We present new structural data from the Early Cambrian Chilhowee Group on the overturned western limb of the Blue Ridge anticlinorium near Front Royal, Virginia. Strain analysis on 6 oriented sandstone samples reveal two distinct strain geometries within the Chilhowee Group. The Harpers Formation records plane strain or slightly constrictional strain associated with regional NW-directed shortening and development of steeply SE-dipping cleavage. The overlying Antietam Formation records consistent flattening strain across a high strain gradient. The greatest amount of penetrative strain in the area can be seen in Antietam Formation outcrops in the hanging wall of the Alleghanian Happy Creek thrust fault, where NW-SE-trending Skolithos burrows have been stretched perpendicular to bedding, and detrital quartz grains have X:Z ratios of ~3:1 to ~4:1. Minor dynamic recrystallization of quartz to grains ~4 μm wide suggest ductile deformation took place under a high differential stress. The Antietam Formation in the footwall of the Happy Creek fault consists dominantly of dilational breccia cemented with Fe- and Mn-oxides. Within the breccia, clasts show much lower strain, as well as evidence of an earlier history of cataclastic deformation. We interpret penetrative strain within the Antietam Formation to be associated with top-NW shear along the Happy Creek fault. Quartz crystallographic preferred-orientation patterns of two Antietam Formation samples determined by electron backscatter diffraction are consistent with a flattening strain geometry and a component of top-NW shear. Gently-dipping foliation in the Antietam Formation is folded about a NE-trending axis, suggesting that regional shortening associated with the development of the Blue Ridge anticlinorium outlived shear along the Happy Creek fault.