Refining the target for EarthScope in the central Midcontinent

Seismic reflection exploration of the lithosphere has focused preferentially on geologic settings dominated by large-scale boundaries, such as active or ancient plate margins, and has involved multi-institutional efforts of new data acquisition. Analogous exploration programs of continental interiors have lagged far behind due in part to the lack of a clearly defined geologic target. Researchers and funding agencies are understandably cautious about committing the necessarily large resources required for studying “intra-cratonic ” areas for which independent interpretational constraints for deeply buried Precambrian rocks are scarce. Results of reprocessing several hundred kilometers of industry and academic seismic reflection data [e.g., McBride and Kolata, 1999] from the Illinois basin region (data are from Illinois, Indiana, andKentucky; e.g., Fig. 1) can be employed to develop a more refined target in the crust and uppermost mantle for EarthScope in the central Midcontinent. One of the underlying aims of our research has been to “rescue ” geophysical data that might otherwise be permanently lost to the scientific community due to corporate mergers and petroleum companies leaving the Illinois basin [McBride, 1999]. Using extended vibroseis correlation and conventional techniques has provided new high-quality images of the crust and uppermost mantle on records that continue to as much as 20 s travel time—clear dipping reflections extend to near the bottom of records (e.g., Fig. 2). The quality of the resultant images is equivalent to or exceeds that of the COCORP transect [Pratt et al., 1989] that was surveyed over a decade ago through the Midwest (Fig. 1). The new images of deep structure, extending at least as deep as about 65 km, now represent the largest concentration of deep seismic reflection profiles between the Appalachians and the Rockies. One of the most remarkable results i