Fine-scale structure of the Precambrian beneath the Illinois Basin

Increasing our understanding of the heterogeneity of Precambrian crust continues to be a focus for deep seismic reflection studies. High-resolution two-dimensional (2D) seismic profiles and a high-resolution 3D seismic volume, all centered on Decatur (Illinois, USA), provide new insights on the structure and composition of Precambrian basement beneath the Illinois Basin of the central USA midcontinent. The new data reveal a pattern of strong and coherent reflections and associated diffractions deeply buried within the eastern Granite-Rhyolite Province. This pattern is dominated by a thick seismic stratigraphic sequence, which is wedge or bowl shaped in cross section and has an angular unconformity with the overlying Paleozoic section. Deeper intrabasement bowl-shaped sequences or series are also observed in the same area. We interpret these features to be a northward continuation of analogous basement sequences located 75 km to the south below the southern part of the Illinois Basin. This correlation indicates a vast Precambrian province with a north-south dimension of >200 km. Although multiple explanations are admissible for the Precambrian reflectivity, the most likely for our study area is igneous intrusion of broad mafic igneous (diabase?) sills possibly underlain by small plutons. The concentration of such mafic (or bimodal) igneous activity within or coeval with the eastern Granite-Rhyolite Province suggests an episode of Proterozoic crustal extension and rifting.