THE INFLUENCE OF FLEXURAL UNLOADING AND ROCK FRACTURES ON LANDSCAPE EVOLUTION AT THE BOUNDARY BETWEEN A CRATONIC PLATFORM AND AN OROGEN: A CASE STUDY OF UPLIFT IN THE SOUTHERN OZARK PLATEAU

The mechanism that maintains the present elevation of the Ozark Plateau in Missouri and Arkansas (midcontinent United States) remains poorly understood, for the plateau lies within the cratonic platform of North America and is not currently within or adjacent to a tectonically active province. The Boston Mountains, which define the southern margin of the Ozark Plateau and now reach an elevation of 780 m, consist of flat-lying late Paleozoic strata. They lie directly north of the Ouachita Mountains, an eroded remnant of a late Paleozoic fold-thrust belt. Elevations in the Ouachitas are generally lower than 500 m but locally rise to 840 m. The east-flowing Arkansas River defines the boundary between the two provinces. We propose that the flexural unloading by preferential erosion of the Ouachita fold-thrust belt contributed to the uplift of the Boston Mountains. Specifically, the contrast in resistance to erosion between relatively strong undeformed beds of sandstone and limestone in the Boston Mountains and highly fractured, relatively soft flysch of the Ouachita fold-thrust belt caused the Ouichitas to erode much more deeply than the Ozarks. Overall flexural rebound due to the erosion of the fold-thrust belt, has driven Cenozoic uplift of the entire region. The relatively strong strata of the Boston Mountains have remained high, relative to the Ouachitas. To test this hypothesis, we developed a group of numerical landscape evolution models using the Landlab modeling platform to test the effect of flexural unloading and erodability contrasts on the landscape evolution of an inactive orogen and its adjacent foreland basin. The fluvial erosion process is modeled using the stream power law. The relative weakness of rocks in the fold-thrust belt, due to their composition and fracturing, is simulated with an erodability coefficient in the stream power law. When the erodability coefficient of the orogen is five times as large as the foreland basin, the model predicts a high foreland basin and a low orogen, and the height contrast is comparable to the current topography of the Boston Mountains and the Ouachita Mountains. A factor of five difference in the erodability coefficient is reasonable given the difference in both lithology and structure that distinguishes the Boston Mountains from the Ouachitas.