Strain paths and folding of carbonate rocks near Blue Ridge, central Appalachians
Two-dimensional incremental and accumulating strain paths have been calculated from displacement paths preserved by syntectonic, fibrous growths of calcite in ten specimens from limbs of various folds of the Conococheague Formation. The folds are asymmetric, possess a slaty cleavage, and consist of multilayers of mechanically stiff dolomite interlayered with less stiff limestone in which displacement paths were measured. The strain paths are referred to a local, rotating coordinate frame which is approximately defined by the strata as they rotate during folding. The calculated strain paths give an estimate of the change in orientation of the principal directions of strain with respect to layering on fold limbs. As expected, the principal extensional strain begins at a high angle to layering and rotates toward layering as folding progresses. The sense of rotation reverses on opposite fold limbs, and one limb tends to have smaller magnitudes of rotation than the other. Similar strain paths occur on the limbs of finite-element computer models of asymmetric folds produced by buckling single layers inclined as much as 15° to the direction of boundary shortening. Although the natural folds are multilayers, comparison with the computer models indicates that strata near Blue Ridge were locally inclined as much as 15° to the direction of bulk shortening. If this direction was nearly horizontal, some strata may have had dips as high as 15°NW prior to late Paleozoic deformation.