Mechanics of Appalachian Foreland Folding in Pennsylvania and West Virginia

Abstract

The Sandhill well on the north-south Burning Springs anticline in West Virginia found little or no deformation below the Lower Devonian. Nevertheless the shortening produced in the Carboniferous and Devonian strata by the anticline--probably not less than ¼ mile--can not simply disappear but must be taken up somewhere, both laterally, beyond the abrupt ends of the anticline, and beneath, in the underlying strata. The pattern of fold trends south and east of the Burning Springs anticline suggests the presence of conjugate sets of strike-slip faults that have hitherto escaped recognition because, being in flat sediments, they show no stratigraphic displacement. Such faults may mark parts of the boundary of a block whose shifting produced the Burning Springs anticline; their orientation suggests that it moved roughly N.50°W. relative to the surrounding parts of the Appalachian Plateau. Complex structure reported in the Lower Devonian rocks under other foreland folds in northern West Virginia and southwestern Pennsylvania indicates that they are similar in mechanics and that the block extends northeast at least into west-central Pennsylvania. The location of the Burning Springs anticline over the western pinch-out of Upper Silurian salt suggests that the block has slipped forward on the salt layers, leaving the underlying strata relatively undisturbed. The hypothesis is therefore presented that the Appalachian foreland folds of Pennsylvania and West Virginia result from the slipping of the upper strata of a part of the Appalachian Plateau northwestward over their substratum, as a marginal effect of the deformation of the Valley and Ridge province on the southeast. The mechanics is exactly that demonstrated by Rich for the Pine Mountain thrust sheet in a similar position not far southwest; the block in question is larger but would have moved much less far.