Three-dimensional development of a natural thrust-related fold : an example from the Tuscarora Sandstone, Browns Mountain Anticlinorium, West Virginia

The purpose of this study is to test models for the origin of fault-related folds against a field example in order to determine which, if any, are viable. Although a variation of a model successfully fit the field example, a primary purpose of this work is not to propose a new model, but rather to show the need for multiple verifiable model tests in order to see which ones approximate reality. The Barclay anticline is a detached first-order parasitic fold to the Browns Mountain anticlinorium in the central Appalachians of West Virginia. It deforms the resistant Tuscarora Sandstone, which is exposed in two profiles, permitting comparison to models for fault-propagation folds, fault-bend folds, break thrusts, and folding that occurs after fault propagation. Geometry alone is an insufficient discriminator, because it eliminates only the fault-propagation fold model. Microscale deformation is also an insufficient discriminator, since it only records pervasive deformation resulting from diagenetic compaction and tectonic layer-parallel shortening, instead of the folding event. The distributions of layer-parallel slip surfaces, faults, and veins in the forelimb and hinge successfully differentiate between the fold models. The anticline exhibits preferential bedding-parallel slip in the forelimb with some hinge contraction, but no transport of material from forelimb to backlimb. Because folding occurred without material transport through the hinge, the anticline developed by break-thrusting with flexural slip and minor inner-arc contraction. Fold amplification occurred through hinge migration, perhaps because of kinking with only one rotating limb. Following thrust propagation, the anticline was translated through one footwall bend to its present position. The anticline did not develop self-similarly, which is consistent with a breakthrust origin.