Thin-Skinned Tectonics in the Plateau and Northwestern Valley and Ridge Provinces of the Central Appalachians

Abstract

Folds in the Appalachian Plateau and northwestern Valley and Ridge provinces of the Central Appalachians involve allochthonous Paleozoic rocks which have been transported westward along nonoutcropping low-angle detachment thrust faults. Deep wells in the Plateau and along the Appalachian Structural Front southeast of the Plateau penetrate major overthrusts which form part of the sole fault system. The great folds along the arcuate Structural Front were formed in passive response to steplike upward shearing of the detachment thrusts from a mid-Cambrian shale glide zone in the Valley and Ridge province up through competent carbonate rocks into glide zones in the Upper Ordovician or Upper Silurian beneath the Plateau. The folds evidence shortening of the stratified sequence superincumbent on the sole thrusts. Anticlines in the Plateau and in local parts of the Valley and Ridge provinces terminate or change trend abruptly in groups along west-to-northwest-trending lineaments. Some of the lineaments appear to reflect tear-faulting along the margins of semi-independently advancing thrust blocks, whereas others, along which no faulting is noted, seem to be zones at which the sole thrusts change stratigraphic position along the structural strike. The termination of sets of anticlines along the lineaments and the absence of uniformly offset counterparts across the lineaments in adjacent blocks indicate that a large part of the net subhorizontal translation of the thrust sheets occurred prior to most of the folding, very early in the deformation. Both limbs of most of the anticlines in the north-western two thirds of the folded Plateau region are broken by synclineward-dipping thrust faults at the Lower Devonian level. These thrust faults, which uplift both anticlinal flanks above a relatively depressed former axial zone, appear to flatten into bedding in evaporites of the Upper Silurian Salina group. The faults facilitate movement of material up and out of the tightening synclines onto the anticlines and are thus simply an extension of the flexural-slip folding process. Structural relief of the anticlines fault-folded in this manner is significantly reduced beneath the Salina decollement zone. The tectonic style and mode of deformation of the “folded” Central Appalachians is, thus, practically identical with that observed in the “thrust-faulted” Southern Appalachians. The only important difference between the two regions is that the upward-shearing segments, or “toes,” of thrust sheets in the Southern Appalachians have been exposed by erosion, whereas the toes of the major Central Appalachian thrust sheets are stil covered by a mile or more of stratified rocks.