KINEMATIC AND TEMPORAL SIGNIFICANCE OF FRACTURES IN THE WESTERN BLUE RIDGE PROVINCE, NORTH-CENTRAL VIRGINIA

In an attempt to better understand the kinematic and temporal significance of brittle deformation in the Blue Ridge province of north-central Virginia we systematically measured fractures during bedrock mapping in the Swift Run Gap 7.5’ quadrangle. The bedrock includes Mesoproterozoic basement units, Neoproterozoic metasedimentary and metavolcanic rocks of the Swift Run and Catoctin Formations, and siliciclastics of the early Cambrian Chilhowee Group (Weverton, Harpers, and Antietam Formations). The basement complex is cut by a number of NE-striking high-strain zones. Cover units are folded into a series of NW-verging, gently plunging structures above the Blue Ridge thrust. Many rock types display a weakly- to strongly-developed penetrative, NE-striking, SE-dipping foliation. Systematic fractures were measured using the traverse method and data was typically collected along perpendicular traverse lines from 5 to 50 m in length. Fracture densities ranged from <0.1 to 4.0 m^-1. Although fracture density was highly variable within the same rocktype, basement units generally had the lowest fracture density and well-cemented quartz sandstones in the Antietam Formation the highest. The basement complex is characterized by numerous variably oriented fracture sets. Although dominant sets occur at individual outcrops, there is no systematic pattern at the quadrangle scale. The Swift Run and Catoctin Formations are dominated by a set of E-striking (080°-100°) subvertical joints. The Weverton and Harpers Formations are characterized by 2 dominant sets: an older, NE-striking (020°-040°), moderately SE-dipping set and a younger, E-striking (090°-110°) subvertical set. The earlier joint set was tilted from a subvertical orientation during folding. The Antietam Formation is dominated by NW- to NNW-striking (320° -340°) subvertical fractures. Many of the fractures in the basement complex appear to be shear fractures with complex kinematics. The E-striking, subvertical extension fractures in the cover rocks may be related to dextral contraction during the early stages of the Alleghanian orogeny. The NW- to NNW-striking fracture set is parallel to topographic lineaments, brittle faults, and diabase dike orientations and may have developed during Mesozoic extension.