PROTEROZOIC ASSEMBLY OF THE CENTRAL GREAT PLAINS REGION, USA – CURRENT KNOWLEDGE AND OPPORTUNITIES FOR EARTHSCOPE SCIENCE

Tectonically, the Central Plains region of Kansas, Missouri, Nebraska, and Iowa has been relatively stable since continental assembly in the Proterozoic. However, direct evidence of the assembly process is obscured by Phanerozoic cover. The entire region is covered by Paleozoic sedimentary strata and some areas are further obscured by younger deposits. Knowledge of the Precambrian terranes must be derived from sparse drill data and extrapolated using potential field geophysical models controlled by outcrop in surrounding states. Drill data, including rare cores, are from shallow basement penetrations by petroleum exploration wells, or from mineral exploration cores, which may provide deeper penetrations, but are typically drilled into unusual lithologies marked by bulls-eye gravity and/or magnetic anomalies. These do not tend to reflect the typical ages or lithologies of the basement terranes. Active source seismic data sets available for the area are optimized to image the Paleozoic section. Lines that do image tectonic targets, such as the Kansas Cocorp experiment, tend to focus on the Middle Proterozoic Midcontinent Rift System, and thus do not address the north to south accretionary trends. North-south oriented experiments crossing the enigmatic terrane boundaries are necessary to solve the geologic questions stated as goals of the EarthScope Project. The recent CD-ROM experiment in the Rocky Mountains showed that all the major lithospheric boundaries in that area were complicated three-dimensional features, even the ‘sharp' boundary of the Cheyenne Belt. These boundaries are thought to correlate with the Archean/Proterozoic accretion belts in the northern Central Plains. The Yavapai/Penokean/Archean boundary appears to have undergone a similar reversal of subduction direction in the two areas. The Mazatzal/Yavapai CD-ROM boundary was shown to be a complex, three-dimensional, east-west trending, 150-km wide zone. This zone probably extends across all of the southern Central Plains in the subsurface. Additional EarthScope science is needed to clarify Central Plains crustal accretion history and improve understanding of the deep structure of the Midcontinent Rift System.