Deformation in the folded Cambrian to Devonian sedimentary rocks of the central Appalachians has been investigated, mainly in the Potomac River Valley. Interpretations of deforming mechanisms are based on the study of folds, rock cleavages, faults, and lineations. An attempt is made to present a kinematic analysis of deformation. Effects of deforming movements, such as preferred orientations, cleavages, small-scale thrusts, and alterations of primary stratigraphic thicknesses are analyzed and correlated.
Changes in deformation are related to (1) differences in location within the folded Appalachian belt; (2) different stratigraphic horizons; (3) different positions in given folds; (4) lithological differences.
It has been found that:
1. Deformation decreases westward from the South Mountain-Blue Ridge belt. This corroborates the previous findings of Cloos and Fellows;
2. Less competent limestones are deformed farther toward the west than the more competent sandstones. Each formation of differing relative competency has its own tectonite front, and thus, for the succession as a whole, there is a series of tectonite fronts approximately parallel to the western edge of the Appalachian Valley;
3. Distortion of primary thicknesses are correlated with cleavage-bedding angles in separate beds of a given fold in which transverse cleavage is prominent;
4. Changes in thickness corresponding to changes in cleavage-bedding angles occur in the less competent rocks west of the Appalachian Valley; they are inferred for the rocks in the Valley;
5. Radical changes in thickness often have been caused by bedding-plane slip, interleafing, and piling-up of shales between more competent layers;
6. Bedding-plane thrusts cause repetition of individual layers;
7. Bedding-plane slip and thrusting parallel to bedding are the predominant modes of distortion west of the Appalachian Valley.