TECTONIC ACTIVITY AT THE BOUNDARY BETWEEN BORDERING BASINS AND THE MIDCONTINENT BLOCK IN THE USA CRATONIC PLATFORM: HINTS FROM ANALYSIS OF THE OZARK DOME - ILLINOIS BASIN BOUNDARY

A basement-topography map (displaying elevation of the Great Unconformity, relative to sea level) across the cratonic platform of the USA, completed as part of the OIINK EarthScope project, emphasizes the subsurface structural relief of the basement-cover contact. The Midcontinent region—between the Appalachian and Rocky Mountain fronts—consists mostly of a coherent block that displays only broad epeirogenic warps. Its east and south edge is delineated by a series of bordering Proterozoic-early Paleozoic rift basins, some of which, amplified by flexural loading, evolved into foreland basins. Intracontinental seismicity of the USA is spatially associated with these bordering basins. Orogenic-scale structural relief accumulated during the Phanerozoic at the boundaries between the bordering basins and intracratonic domes and arches. Specifically, between the Ozark Dome and the Illinois Basin, the elevation of the Great Unconformity rises from a depth of 7 km below sea level (beneath the southern Illinois Basin) to an elevation of 0.5 km above sea level (on the Ozark Plateau). This relation hints that the Ozark Dome is a tilted crustal block whose uplift was accommodated, at least in part, by displacements along fault zones that form its NE and SE boundaries. The SE boundary correlates with the well known New Madrid seismic zone. OIINK results indicate that the NE boundary also correlates with a belt of seismicity which, at the ground surface, coincides with the Ste. Genevieve fault zone. Field studies of this fault zone indicates that it encompasses several discrete faults, some of which die out up-dip in monoclines. Previously published mapping of micro-stratigraphic features along the zone suggests that much of its movement occurred during the Paleozoic, associated with pulses of subsidence in the Illinois Basin. LIDAR maps of landscape features (e.g., incision of meandering trunk streams bordered by non-meandering tributaries) hint that landscape does not date entirely back to the Paleozoic. Seismicity suggests that subtle movements are continuing within the zone. This realization hints that some of the elevation of the Ozark Plateau, relative to sea level, may reflect tectonic uplift relative to the Illinois Basin, during post-Paleozoic time. This concept is being tested by (U-Th)/He thermochronology.