CRATONIC PLATFORM ASSEMBLY AND REACTIVATION IN THE OZARK PLATEAU / ILLINOIS BASIN REGION, MIDCONTINENT UNITED STATES: UNRESOLVED ISSUES

In the continental-interior platform of North America, a region which accounts for over a quarter of the land area in the conterminous United States, a veneer of Phanerozoic strata covers a variety of Precambrian basement terranes and the extensive ? 1.4 Ga granite-rhyolite province. The region also hosts several regional-scale structures (intracratonic basins, domes, and arches), as well as "Laramide-style" basement-cored fold-and-fault zones, some of which are seismically active. Landscape features (e.g., bedrock-incising streams) imply that epeirogenic movements have continued to affect the region during the Cenozoic. A crustal section incorporating portions of the Ozark Plateau, the Illinois Basin, and the Mississippi Embayment provides examples of key tectonic features of the cratonic platform. Specifically, it includes the Midcontinent's highest uplift (the Ozark Plateau), one its deepest basins (the Illinois basin), the Mississippi Embayment, several fault-and-fold zones including the seismically hazardous New Madrid zone, possible major Precambrian sutures, a major Proterozoic igneous province, and a variety of small Phanerozoic intrusions. Though the region's landscape is subdued, the vertical elevation difference between the highest exposure of the Cambrian/Precambrian unconformity in the Ozarks, and the same contact at the floor of the Illinois basin reaches 7.5 km. Numerous tectonic questions pertaining to the area remain incompletely answered: How did the uplifts and basins originate and why do they persist? What initiated fault-and-fold zones and how were these zones reactivated? What controls pulses of epeirogenic subsidence and uplift? What is the history of lithospheric assembly? How was this region of continental crust cratonized? Does the locus of seismic activity in continental interiors migrate over time? What is the relation between seismic activity and lithospheric structure? Progress in answering these questions will depend on acquisition of new data that characterizes the underlying mantle and deep-crustal structure, relates this structure to shallow and surficial crustal features, defines regional patterns of microseismicity in 3-D, and constrains current crustal movements.