EFFECT OF A TRANS-LITHOSPHERIC WEAK ZONE ON THE MAGNITUDE OF PALEOZOIC FLEXURAL UPLIFT: A CASE STUDY OF THE OZARK DOME

The mechanism of uplift of the Ozark Dome in the United States Midcontinent, part of North America's cratonic platform, remains poorly understood. The existence of an unconformity between Mississippian and Pennsylvanian strata implies that the region underwent a phase of uplift roughly at the time of the Ouachita Orogeny (an event that produced a north-verging fold-thrust belt to the south of the Ozark Dome). Previous authors have suggested that flexural loading by the Ouachita fold-thrust belt not only contributed to the subsidence of the Arkoma Basin, but also promoted uplift of the Ozark Dome region as an outer swell. Subsequently, the dome has remained abnormally high and, at times, has undergone renewed uplift despite the lack of significant tectonic stress in the region; consequently, the region is an intracratonic plateau today. The Midcontinent region has been cut by numerous fault zones, some of which may behave as lithospheric weak zones. Could the presence of such a weak zone motivate additional uplift of the Ozark Dome? A recent magnetotelluric investigation by DeLucia et al. (2019) imaged the Missouri High Conductivity Belt, a lithospheric transecting fault zone, which flanks the Ozark Dome and appears to be a potential weak zone. We present a suite of mechanical finite element models using COMSOL Multiphysics to test the effect of a trans-lithospheric weak zone on the magnitude of foreland uplift due to flexural loading during the Ouachita Orogeny. The goal of this modeling is to determine whether a weak zone will amplify or dampen flexural uplift when other variables are held constant. Numerical models indicate that flexural loading alone results in a ~ 370 m flexural bulge. However, when a trans-lithospheric weak zone is included in the model, an additional 500 m of uplift is predicted, comparable to the height of the present-day Ozark Plateau. The magnitude of dome uplift is highly sensitive to parameters such as weak zone width and rock stiffness indicating that pre-existing heterogeneities in the midcontent may play a critical role in shaping present day topography.