Appalachian thrusting, lithospheric flexure, and the Paleozoic stratigraphy of the Eastern Interior of North America

Abstract

The Appalachian Basin is interpreted to be a multistage foreland basin developed by lithospheric downwarp under the loads of successive Taconic, Acadian, and Alleghanian overthrusts in the adjacent Appalachian Mountains. By quantifying this model we show how the stratigraphic record of the foreland basin can be used to constrain the timing, areal distribution, and thickness of the orogenic overthrusts. The cumulative present-day thickness of these allochthonous units is suggested to range from 4 to 18 km with the greatest thickness in the vicinity of Pennsylvania and Virginia. These model load thicknesses compare favourably with those implied by the COCORP seismic profile across the southern Appalachians and Atlantic Coastal Plain. We further suggest that, for reasons of isostatic balance, these thick overthrusts are likely to rest on the old Cambro-Ordovician continental margin, a conclusion also in keeping with, but independent of, COCORP results. The distribution of model loads is qualitatively correlative with observed patterns of regional Bouguer gravity anomalies but we have not as yet attempted quantitative gravity calculations.Flexural interactions between the Appalachian Basin and the contemporaneous intracratonic Michigan and Illinois basins produced the interbasinal Kankakee, Findlay–Algonquin, and Cincinnati arches as well as the Jessamine and Nashville domes. These arches and domes existed in fluctuating submergent and emergent conditions, alternately yoking together and decoupling the foreland basin and one or both of the intracratonic basins. The location and magnitude of Appalachian overthrusting and the lithosphere's rheological behaviour are the primary controls on arch development. The most satisfactory stratigraphic results are achieved using a lithospheric model with a temperature-dependent Maxwell viscoelastic rheology. In such a lithosphere the lower regions relax load-induced stress on time scales of 1–200 Ma but the upper regions are too viscous to flow on time scales less than the age of the Earth.We propose no explanation for the initiation of subsidence in the intracratonic Michigan and Illinois basins. Nevertheless, we show that the sediment record of these basins is likely to have been substantially modified by the influence of Appalachian overthrusts. This influence should be removed before attempting to interpret the sedimentary record of the intracratonic basins in terms of a basin-initiating mechanism.Viscoelastic relaxation is shown to provide a natural explanation for the unconformities that bound Sloss' sedimentary sequences in those parts of the basins well removed from the overthrusts. The onset and termination of erosion that created the unconformities correlate with the termination and initiation, respectively, of overthrust episodes. Sloss' sequences are seen as marking intervals of major orogenic overthrusting in the Appalachians. Insofar as the periods of thrusting are a consequence of worldwide plate tectonic reorganization, the stratigraphic sequences may have worldwide synchroneity but they are shown to have a more immediate local tectonic origin.