The basement versus the no-basement hypotheses for folding within the appalachian plateau detachment sheet
Within the Appalachian plateau detachment sheet of southwestern Pennsylvania tectonic thickening is a central element to the growth of anticlines in a Silurian-Devonian section containing a three-tiered mechanical stratigraphy (that is, a basal detachment zone, a lower imbrication zone, and an upper wedge zone). The detachment zone is predominantly within disturbed shale of the Silurian Vernon Formation which sits above a disturbed surface on the Lockport dolomite. Large-scale anticline growth is, in part, a consequence of small-scale blind imbrication of the more competent mechanical layers in the lower portion of the detachment sheet (the imbrication zone). Here, salt within the Silurian Syracuse Formation hosts secondary detachment responsible for imbrication and the development of triangle zones in the core of the anticlines. Some fold amplification is also accomplished by extensive, smaller-scale thrust wedging and concomitant tectonic thickening of the less competent Devonian section in the upper portion of the detachment sheet (the wedge zone). These spatially periodic anticlines are separated by horizontally bedded synclines characterized by the absence of thrust wedging and fault imbrication across all three zones. Tectonic thickening continued behind the foreland propagation of the detachment-tip line within the Vernon shale as indicated by a systematic hinterland increase in the cross-sectional width and amplitude of the anticlines. On the basis of seismic images, each detachment sheet anticline is situated above prominent, periodically-spaced, pre-Alleghanian structures in the footwall of the detachment sheet. These footwall structures arise from a combination of thrust imbrication at the depth of the Ordovician Trenton Group and high-angle, basement-involved faulting at the depth of the Cambrian Gatesburg Formation. Some of the high-angle fault displacement is a consequence of Alleghanian inversion on faults associated with the extensional Rome Trough and other basement structures developed during the Late Proterozoic rifting of the eastern margin of Laurentia. Evidence for tectonic inversion is present in seismic reflection images that show buttress anticlines in the Cambro-Ordovician carbonate section. The superposition of detachment-sheet anticlines above footwall structures strongly suggests that the foreland transport was disrupted by topographic bumps on the base of the detachment zone. Bending of the detachment sheet over these irregularities may have promoted the spatially periodic collapse and concomitant tectonic thickening in the over-riding sheet as it was pushed laterally toward the foreland. A strain hardening of the detachment zone part of the detachment sheet at the topographic irregularities permitted the growth of double-sided tapered wedges (that is, each anticline). The overall structural profile of the detachment sheet is consistent with the growth of a Coulomb wedge whose low basal friction is interrupted by patches of strain hardening.