THE OZARK-ILLINOIS-INDIANA-KENTUCKY (OIINK) EarthScope PROJECT: CHARACTERIZING CRATONIC-PLATFORM LITHOSPHERE IN NORTH AMERICA'S MIDCONTINENT

The central portion of the Midcontinent of North America (between latitude 36°N and 38°N) provides a type example of cratonic-platform lithosphere, a relatively stable portion of the continent where a veneer of Paleozoic sedimentary strata overlies Precambrian crystalline basement. We have initiated a multi-year project incorporating instruments of the Flexible Array (FA) to densify instrumentation in a region that encompasses the Ozark Plateau, the southern Illinois Basin (including the Wabash Valley seismic zone and the Rough Creek graben), and western Kentucky. The study area encompasses dramatic subsurface structural relief—its western portion includes the largest intracratonic uplift in North America, whereas its central portion includes a major intracratonic basin. The difference in elevation between the Cambrian-Precambrian unconformity at the crest of the Ozarks and the same horizon at the base of the Illinois Basin (< 100 km to the east) is over 7.5 km, comparable to relief between foreland basins and orogenic peaks. Our study area lies just north of the Mississippi embayment (which hosts the New Madrid seismic zone), crosses three major Proterozoic lithosphere accretionary boundaries (borders of the Yavapai, Mazatzal, and Grenville belts), one of the world's largest anorogenic igneous provinces (the 1.47 Ga Eastern Granite-Rhyolite Province), and several significant fault-and-fold zones (possibly formed by inversion of Proterozoic rifts) some of which remain seismically active. We have installed an initial cluster of broadband and short-period seismographs across the Ozark Plateau / Illinois Basin boundary to determine if micro-seismic activity is localized along this first-order crustal feature. Data from the FA, once installed, will provide further constraints on structural control of microseismicity, and will provide sufficient tomographic resolution to determine if deep-crustal or mantle features underlying the Ozark Plateau differ from those beneath the Illinois Basin. Understanding such differences will help to constrain models of epeirogeny in continental interiors. FA data will also be used to determine if the formation of Proterozoic sutures and of the granite-rhyolite province left a signature at depth in the lithosphere.