Abstract

It has become generally accepted that the Alleghanian deformational structures in the Valley and Ridge province of the central Appalachians were produced by northwestward movement of the Paleozoic rocks on décollements at depth. The folds in central Pennsylvania represent conversion of transport on the décollements into deformation of the overlying moving block. Abundant slicken-sides on bedding surfaces and constant bed-normal thicknesses indicate that they are flexural slip folds. But they are not concentric in profile. Rather, the folds possess planar limbs and narrow hinges. The presence of large and small kink bands, and the similarity of geometry (large zones of constant bed orientation, abrupt changes in bed attitude) and deformational mechanism of kink bands and these folds indicate a genetic relation. That is, kink bands can be combined to produce all the observed varieties of fold profiles. Thus, the folds in this province appear to be a widespread, large-scale kink-band deformation.

In cylindrical kink-band folds, bedding in both limbs has been rotated about the same axis. The presence of two differently oriented slickenside sets on single bedding surfaces indicates that in many of the folds, each limb has been rotated about a different axis. The resulting fold geometry is noncylindrical, giving rise to the doubly plunging, en echelon, and other complex folds that are prevalent in the province. These noncylindrical structures, reflecting a pervasive interfingering of kinematic domains (kink bands) of different orientations, constitute the regional arcuation of the Valley and Ridge folds in Pennsylvania.

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