Abstract

The dominant structures of the Valley and Ridge province in central Pennsylvania are traditionally thought to be open, concentric folds of great length and parallel to one another. However, the majority of the folds possess planar limbs and narrow hinges and therefore do not conform to the concentric fold geometry. Furthermore, many folds change radically in profile along trend, and even vanish.

Parallel-sided zones of rotated bedding, as much as 50 feet in width, have been observed transecting bedding at a large angle. The zone boundaries are defined by the abrupt flexure of the bedding, which is continuous across the structures and of constant thickness. Slickensides on bedding surfaces indicate that the flexural slip mechanism was operative in the development of these structures. Because of this geometry and because of the inferred deformational mechanism, these structures are interpreted as large kink bands.

Structural models which utilize the kink band concept accurately represent the fold geometry both in the cross sections and the changes along trend. Because folds ranging in size from a few feet to the largest in the province can be represented by such models, it is suggested that kink banding was an important deformational process in the development of the fold geometry.

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