ORIGIN OF APPALACHIAN FORELAND PLATEAUS: AN ISOSTATIC RESPONSE TO THE EROSION OF FOLD-THRUST BELTS

A foreland plateau is a broad elevated region on the craton side of an inactive and deeply eroded orogenic belt. Unlike the higher-elevation and better-known plateaus that occur in association with Cenozoic orogens (e.g., the Tibet Plateau, the Altiplano, and the Colorado Plateau), foreland plateaus are not bordered by high mountains, they have been incised by fluvial erosion, and they slope gradually toward the continental interior. Foreland plateaus are geographically identified as plateaus because their average elevation is higher than that of both the interior lowlands on the craton side and the fold-thrust belt on the orogen side. On the craton side of the northern and central Appalachians, four foreland plateaus have developed: the Deep Valleys Plateau, Pocono Plateau, and Allegheny Plateau of Pennsylvania, and the Catskill Mountains of New York. All are underlain by cratonic crust covered by platform strata. On the hinterland side of each plateau, foreland-basin deposits buried the platform strata. Since these deposits were sourced from the adjacent fold-thrust belt, the region now within the foreland plateau was topographically lower than the adjacent fold-thrust belt at the time of deposition. Present-day topography, therefore, represents a topographic inversion, in that the foreland-basin deposits are now higher than the roots of their source. What caused this inversion? Anders et al. (2021) addressed this question in relation to a midwestern example, the Ozark Plateau. In their model, produced using the Landlab platform, they assumed that the deformed, more argillaceous rocks of the fold-thrust belt are less resistant to erosion than the nearly flat-lying strata of the foreland, so exhumation rates of the fold-thrust belt are faster than that of the foreland basin and adjacent craton. Results demonstrate that flexural rebound in response to differential erosion yields the observed topographic inversion, as well as the escarpment that forms the hinterland edge of the plateau. Here, we extend this model to explain the regional-scale topography of the Catskill Plateau in New York and the Deep Valleys Plateau of Pennsylvania. Notably, the difference in widths of these two plateaus (measured perpendicular to the axis of the fold-thrust belt) reflects the width of the eroded fold-thrust belts. The widths are also sensitive to the elastic thickness of the lithosphere.