NEW CONSTRAINTS ON INTERPRETING THE ORIGIN OF THE EASTERN GRANITE-RHYOLITE PROVINCE BENEATH THE ILLINOIS BASIN

New reflectivity images of the crust beneath the Paleozoic Illinois Basin have been derived from reprocessing several hundred kilometers of industry seismic reflection data using extended vibroseis correlation. The reprocessing results are highly successful--prominent dipping reflections continue to near the bottom of the 20 s (~70 km) records. The quality of the resultant images in the crust and upper mantle exceeds that of previous profiles acquired in this area using a dedicated deep reflection acquisition approach. Three highly coherent "stratigraphic" sequences are detected beneath the Paleozoic Illinois Basin (i.e., sub-Mt. Simon Sandstone (Late Cambrian)) that extend down to mid-crustal depths. These sequences, some of which suggest unconformity-bounded depositional sequences, lie within the area of the “eastern granite-rhyolite province”, which is a major Proterozoic (1.48-1.45 Ga) igneous terrane composed of A-type felsic rocks with subordinate mafic components. The sequences are bounded by strong, laterally continuous reflectors mappable over distances of at least 225 km and are expressed as broad "basinal" packages in plan view that become areally more restricted with depth. Normal-fault reflector offsets that progressively disrupt the sequences with depth are possibly related to volcanic caldera development and/or possible Proterozoic rifting connected to the original thermal event that produced the granite-rhyolite province. Filtered gravity and magnetic data for the study area indicate that the deeper parts of the seismic sequences are generally composed of low-density and low-magnetization rocks, relative to the surrounding region. Because the potential-field analyses preclude a large concentrated mafic igneous component to the crust here, any Proterozoic volcanic episode must not have tapped deeply into the lower crust or upper mantle during the heating event responsible for the granite-rhyolite. However, the outer margins of the sequences, especially to the west and south, are marked by prominent magnetic and gravity highs, indicative of mafic igneous sources. These sources are expressed in a general way as highly diffractive zones on the deep reflection profiles, which further suggests that they represent major intrusive boundaries in the crust.