GEODYNAMIC ORIGIN OF THE SOUTHEASTERN US STRESS FIELD AND LARGE-SCALE APPALACHIAN GEOMORPHOLOGY

Although the intraplate stress field in the southeastern United States (SEUS) has generally been viewed as broadly uniform and controlled dominantly by far-field forces, stress indicators (data from the World Stress Map as well as additional earthquake focal mechanisms and Quaternary faults) show substantial variability throughout this region. For example, in the Central Virginia Seismic Zone, focal mechanisms and faults that show evidence for movement in the Pleistocene indicate a direction of maximum horizontal compression that is highly oblique to that inferred from regional World Stress Map data. We use CitcomS to model lithospheric stresses based on relatively simple input density, temperature, and viscosity fields. Our density field predominantly reflects crustal thickness variations, which are constrained by seismic data. We use a one-dimensional thermal field that is constrained by magnetotelluric (MT) observations. For viscosity, we use both a one-dimensional field and three-dimensional fields that are informed by MT imaging. With these simple models, we are able to reproduce many features of the heterogeneous SEUS stress field. Our results indicate that the modern observed stress field arises from the superposition of far-field forces and local body forces. Crustal compositional buoyancy, controlled predominantly by crustal thickness variations, appears to be most important in reproducing observations, although we slightly better match the observed stress field by including variable crustal viscosity. Additionally, in using our models to predict surface deflection, we find that variable crustal viscosity encourages a steepening of topography compared to the uniform viscosity (purely isostatic) case. Our models suggest that boundaries in crustal strength redistribute isostatic body forces so that relatively long wavelength crustal thickness variations can support comparatively short wavelength topography. Heterogeneous crustal viscosity may therefore be important in explaining apparently long-lived topographic features of the southern and central Appalachians, such as the Blue Ridge Escarpment.