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 that trends NE-SW. Cloos (1947) was the first to recognize the structure as part of his seminal study of regional deformation. The anticline contains Precambrian to Paleozoic rocks metamorphosed to greenschist facies during the Alleghanian orogen. The core of the anticline is Precambrian Grenville basement with overlying Precambrian volcanic flows and Cambro-Ordivician sedimentary rocks. South Mountain anticline has well developed slaty cleavage in all rock types in the northern region of the anticline. Deformation is most intense in the Precambrian basement and decreases in younger-age rocks. We 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 field, tension gashes are common in the Weverton Quartzite and form a conjugate set that strikes NE-SW and dips moderately. Veins are also common, and are found in two conjugate sets; one parallels the tension gashes and the other set is orthogonal and moderately dipping. Cleavage is poorly developed in quartz-rich layers of the Weverton Quartzite, but within the Harpers Phyllite cleavage dips shallowly to the SE. We used the shape preferred orientation (SPO) of quartz grains as well as the anisotropy of magnetic susceptibility (AMS) of bulk samples to document cleavage in Weverton Quartzite. Both techniques indicate oblate fabrics with subvertical short-axes. Lattice preferred orientation (LPO) measurements from five samples show that there is no preferred orientation of the quartz c-axes.

These data provide information about the processes responsible for folding at Harpers Ferry. Tension gashes and veins suggest that extension occurs parallel to bedding and to the fold hinge. The subhorizontal AMS and SPO fabrics differ from cleavage in more phyllitic layers, indicating that cleavage refraction occurs based on lithologic differences. Last, the LPO data indicate that dislocation creep is not a dominant deformation mechanism.