Pre-Earthscope Workshop Proposal: Tectonic Targets for Earthscope in the Midcontinent

University Of Illinois At Urbana-Champaign, Urbana IL

Investigators

Abstract

This grant funds a workshop to develop EarthScope-related research projects concerning the geologic character and tectonic evolution of the crust and mantle beneath the central Midcontinent of the United States. The specific region of focus includes two major continental blocks, the Ozark Plateau and the Illinois Basin. Interactions at the workshop can foster partnerships among researchers familiar with the geologic context of the target region (which encompasses Illinois, Indiana, Missouri, and Kentucky), those familiar with existing geophysical data sets covering the region, and those with experience in design and implementation of geophysical experiments that can take advantage of the instrumentation associated with EarthScope. Participants come from research universities, state geological surveys, four-year colleges, and the U.S. Geological Survey. The region of interest is part of North America's cratonic platform. A cratonic platform is long-lived continental province in which a veneer of Paleozoic sedimentary rocks overlies metamorphic and igneous rocks. (The Paleozoic rocks in the region are less than half a billion years old, whereas the Precambrian rocks are 1 to 3 billion years old.) The U.S. Midcontinent provides one of the most fascinating examples of cratonic-platform lithosphere anywhere in the world. Specifically, this block includes a major uplift (the Ozark Plateau) and a major basin (the Illinois Basin). The contact between Paleozoic and Precambrian rocks on the crest of the Ozark Plateau is 700 m above sea level, whereas the same contact at the base of the Illinois Basin is about 7 km below sea level. Put another way, hidden beneath the Midwestern cornfields of today, geologic movements have produced relief comparable to that of mountain belts. The region of interest also includes the northern end of the Mississippi Embayment (a region that remained anomalously low during the past 100 million years), three major Precambrian sutures along which new crust attached to North America, one of the world's largest granite provinces, and the intersection of several significant fault-and-fold zones. These zones possibly originated as rifts, due to stretching of the crust in the Precambrian, and have remained weak and susceptible to displacements ever since. As a result, the target region is one of the most seismically active regions of cratonic platform lithosphere anywhere?numerous earthquakes have happened in the central Midcontinent during historic time. Study of the target region can address issues of craton formation and evolution, intraplate seismicity, and the formation of regional basins and domes. In cratonic platforms, coherent and coupled motions have continued from the Precambrian to the present. Surprisingly, decades after the advent of plate tectonics, there is no universally accepted theory to explain controls on tectonism in cratonic platforms, even though these regions account for over 25% of continental lithosphere. The limitation on understanding comes largely from lack of data on the deeper character of the crust and mantle beneath these regions. Such data will become available through EarthScope-related measurements, so the unique opportunity to organize a coordinated scientific agenda for studying a particularly illustrative example of cratonic platform is not to be missed. Improved understanding of this region of the United States is important for understanding seismic hazard in populated areas away from plate boundaries, for the evolution of reservoirs for energy resources and for carbon sequestration, and for the development of a general understanding of the geological evolution of continents. Since the midcontinent does not have abundant surface exposures of geologic features, EarthScope related work in the midcontinent also provides the opportunity to attract public interest in geologic processes and phenomena.