KINEMATIC AND TEMPORAL HISTORY OF BRITTLE DEFORMATION IN SHENANDOAH NATIONAL PARK, BLUE RIDGE PROVINCE, VIRGINIA

Shenandoah National Park lies astride the Blue Ridge Mountains in central and northern Virginia and is underlain by Mesoproterozoic granitoid basement, a late Neoproterozoic sedimentary and volcanic rift-sequence, and early Cambrian passive margin siliciclastics. These rocks experienced greenschist facies deformation during the mid-Paleozoic and were thrust northwestward over Cambrian-Ordovician sedimentary rocks of the Valley & Ridge. In contrast to the ductile deformation history of the region, the kinematic history of brittle deformation in the Shenandoah National Park region is poorly understood.

All bedrock units in the Blue Ridge are variably fractured. Basement granitoids are complexly fractured, whereas the cover sequence contains three regionally prominent fracture sets that include: 1) an older NE-striking, moderately SE-dipping set, 2) a subvertical WNW- to NW-striking set, and 3) a younger NNW- to N-striking set. A suite of NW- to NNW-striking transverse faults cut units in both the Blue Ridge and Valley & Ridge structures. Many transverse faults correspond to well-developed lineaments and most topographic gaps are located on transverse faults. These steeply dipping faults experienced normal dip-slip movements with maximum displacements of ~150 m. Linear breccia zones in well-cemented Cambrian quartz sandstones parallel the NNW-striking faults and fracture sets. These breccia zones have been interpreted as both collapse breccias and tectonic breccias associated with late Paleozoic thrusting, but they may be dilational breccias developed during extension associated with transverse normal faults.

The WNW-striking joints developed during the later stages of the Alleghanian Orogeny and are similar to joints observed in the Valley & Ridge. The NNW-striking joints and faults are subparallel to a suite of Jurassic diabase dikes. The NNW-striking joints, transverse faults, and breccia zones record modest amounts of ENE- to E-directed extension and developed at the western edge of a region that experienced transtensional deformation in the Jurassic.