Three-dimensional fabric analysis and deformation of the South Mountain anticline, Harpers Ferry, West Virginia

The South Mountain anticline is a large overturned fold in the central Appalachian tectonic zone. The anticline has slaty cleavage in all rock types in the northern region of the anticline that is well developed in phyllitic units and less visible in quartz-rich layers. I characterize deformation at the western limb of the fold near Harpers Ferry, West Virginia using field data and several laboratory techniques that have not previously been used in this region. Samples were collected from outcrops of the Cambrian Weverton Quartzite at South Mountain and Elk Ridge. In the laboratory, I measured lattice preferred orientation (LPO) measurements from five samples, which show that there is no preferred orientation of the quartz c-axes. I used the anisotropy of magnetic susceptibility (AMS) of bulk samples as well as the shape preferred orientation (SPO) of quartz grains to document cleavage in Weverton Quartzite. Both techniques indicate oblate fabrics with subvertical short-axes. In the field, cleavage is poorly developed in quartz-rich layers of the Weverton Quartzite, but within the Harpers Phyllite cleavage dips shallowly to the SE. Tension gashes are common in the Weverton Quartzite and are oriented in a conjugate set that strike NE-SW and dip moderately. Veins, also common, are oriented in two conjugate sets: one set oriented like the tension gashes and the other set striking 90° away, dipping moderately. These laboratory and field data provide information about mechanisms of folding at Harpers Ferry. The subhorizontal AMS and SPO fabrics differ from cleavage in more phyllitic layers, indicating that cleavage refraction occurs based on lithologic differences. LPO data indicate that dislocation creep is not a dominant deformation mechanism. Last, tension gashes and veins suggest that extension occurs parallel to bedding and to the fold hinge