Flow of weak rocks in Appalachian Plateau folds

A thin-skinned fold model of Appalachian Plateau folds in which layers do not thin in synclines is not possible geometrically. However, a two-layer model consisting of a gently folded competent layer overlying a deformed zone in which material flows from the synclines to the anticlines is both reasonable geometrically and supported by geologic evidence from the Plateau province of Pennsylvania and south-central New York. The deformed zone is composed of weak rocks-evaporites and shale-which have been deformed much more than the competent rocks above; the decollement marks the lower boundary of this deformed zone. Care should be taken in reconstructing the depocenters of rocks composing the deformed zone, because the original thicknesses have been altered by later deformation. The Appalachian Plateau of Pennsylvania and New York may be divided into two regions on the basis of fold mechanics. In the northwestern and far western Plateau of Pennsylvania and south-central New York, Region I, the salt-bearing part of the Silurian Salina Group is the deformed zone; no other part of the stratigraphic section has flowed significantly. Synclines are broader than anticlines in the higher relief folds of Region I because much material has been removed from beneath the synclines and flanks to fill anticlines. In southwestern and central Pennsylvania, Region II, the anhydrite-rich Tonoloway formation and/or the Upper Ordovician Reedsville Shale flowed as well as salt. Complex thrust faulting and disharmonic folding at the level of the Oriskany Sandstone contribute significantly to the structural relief, but it cannot provide the major part of the relief. Balancing the shortening present at the Oriskany level requires either stretching at this level in the synclines or significant layer-parallel shortening at shallower stratigraphic levels.