PROCESS AND TIMING OF UPLIFT IN THE OZARK PLATEAU, MISSOURI

The Ozark Plateau is a 150,000 sq km intracratonic uplift located in the Midcontinent region of the United States. Though the region has subdued topography, structural relief between the high point of the uplift and the low point of the adjacent Illinois basin, is over 7.5 km, comparable to the elevation of major mountain belts. Stratigraphic data suggests that the uplift has remained relatively high throughout the Phanerozoic, but it was probably submerged in a shallow marine environment as recently as the Mississippian. Thus, it has risen by at least a few hundred meters since the end of the Paleozoic. The origin of this uplift, and of comparable uplifts in other continental interiors, remains unknown. Unlike the Tibet Plateau or the Colorado Plateau, the Ozark Plateau does not occur at, or near, a tectonically active plate boundary. We suggest that formation and persistence of the uplift reflects multiple phenomena. First, the area of the uplift was a felsic intrusive/extrusive center during the Proterozoic, thickening the crust with low-density rock. Second, regional crustal shortening during the late Paleozoic shortened the region reactivating faults that outline the block, so it could tilt up like a foreland basement-cored uplift. Such uplift may have been enhanced by thrust-sheet loading in the Ouachitas to the south and stabilized by short-term flow of a mid-crustal weak zone. Explanations for possible post-Paleozoic uplift of the plateau are harder to formulate -- it is not clear whether the present-day elevations are a residual of earlier uplift, or are a consequence of renewed movement on border faults. Procedures that can be used to test uplift models of the Ozarks include reprocessing of seismic-reflection profiles to characterize fault displacement along the boundary of the Plateau and dating to create a thermal history of the area and constrain the age of the uplift. If the uplift is less than 250 m.y., then renewed displacement may be associated with Cenozoic mid-plate seismicity. By determining the structure of the block (by characterizing the architecture of faults) and its uplift history (by determining if the rocks exposed contain a record of post-Paleozoic uplift) we can help constrain the different origin theories.