FAULTS AND FOLDS OF THE CRATONIC PLATFORM ALONG THE BOUNDARY BETWEEN THE OZARK DOME AND ILLINOIS BASIN: LINKING STRUCTURE TO TOPOGRAPHY AND TECTONIC HISTORY

A compilation of structures along the boundary between the Ozark Dome and the Illinois Basin on a digital elevation model (DEM) emphasizes that the mapped fault and fold traces in this portion of the cratonic platform display a rectilinear pattern. Two structural trends dominate (E-W to NW-SE and N-S- to NE-SW). This pattern suggests that the Ozark Plateau is a tilted crustal block, similar to the East Kaibab Plateau of Arizona. By making slight adjustments to the map location of some structures (assuming hand-drawn maps are not perfectly geo-referenced), they correlate with distinct topographic features. This association indicates that bedrock structure controls the details of topography. Patterns of the DEM also hint that the Eureka-House Springs structure (EHSS) links along strike, in the subsurface, with the Valmeyer monocline, and perhaps the Cottage Grove fault. In fact, taken together, this set of structures parallels the Ste. Genevieve Fault Zone (SGFZ) and control the trend of a reach of the Mississippi River. Cross-sections based on photomosaics of roadcuts crossing a monocline of the EHSS indicate that, at the surface, the fold faces SW (i.e., SW side down), and has a total stratigraphic throw at ground level of less than 90 m. Small thrust-sense faults with displacements of less than 20 cm cut through the hinge zone of the monocline. The EHSS lies a few km SW of the NW-trending Maxville fault, which is down to the NE. This relation suggests that the EHSS forms the SW edge of a box anticline. A regional-scale synoptic cross section model of the upper crust in the study area, taking into account the facing of monoclines, suggests that the E-W- to NW-trending features are flower structures, formed above synthetic and antithetic fault splays that merge with SW-dipping transpressional master fault of the SGFZ at depth. These structures are the surface manifestation of a crustal boundary that was active multiple times during the Paleozoic, locally accumulating up to 7.5 km of vertical displacement and contributing to the uplift of the Ozark block. The OIINK array indicates that the boundary remains seismically active today. (U-Th)/He thermochronology, currently in progress, may test whether uplift of the Ozarks, perhaps accommodated by movement on the boundary, has occurred since the Paleozoic.