Deformation mechanisms and strain history of a minor fold from the Appalachian Valley and Ridge Province

Abstract

We have re-examined a minor fold in the Silurian McKenzie limestone, collected from the Cacapon Mountain anticline where the anticline crosses the Potomoc River. The fold was originally studied by James Conel (1962). We have determined the strain and deformation mechanisms in both the hinge and the limbs of one layer. The layer is towards the inner arc of a multilayer containing one other bed of comparable thickness and numerous thinner beds, all separated by thin shale beds and enclosed in shale. Intragranular deformation mechanisms related to folding include faults and replacement veins. The faults represent a complex interrelationship between shear displacement, pressure solution, and extension veins containing fibrous calcite. The faults are curved and have the effect of moving material into the inner arc of the hinge zone. The replacement veins occur normal to bedding on the outer arc of the hinge. Pressure solution zones normal to bedding are absent and so is cleavage. Intragranular strain is measured on twinned calcite using the least-squares strain gage technique. Based on all the data, the maximum compressive strain, ϵ 1, is everywhere subparallel to layering and approximately perpendicular to the fold axis. The maximum extension strain is everywhere subparallel to the fold axis. The largest ϵ 1 values (−12.7 and −11.0%) occur in the inner arc of the hinge; the smallest ϵ 1 (−2.1%) is in the outer arc of the hinge. The limbs have intermediate values of ϵ 1. Intragranular layer-parallel shear strain on the limbs is small and indicates a relative motion of material away from the hinge in the inner arc with respect to the outer arc.