REFLECTIVITY OF LOWER CRUST AND UPPERMOST MANTLE BENEATH THE ILLINOIS BASIN: A KEY TO UNDERSTANDING THE PROTEROZOIC LITHOSPHERE

Several hundred kilometers of seismic reflection profiles procured from the petroleum industry were reprocessed using extended vibroseis correlation from the initial 4-second correlated records to the full 20-second (~70 km) listening travel time in order to image features throughout the entire crust and into the uppermost mantle beneath the Illinois Basin. The frequency band width (14-126 Hz) included components suitable for imaging structures normally targeted by academic “deep seismic” profiles. Occasional Moho discontinuity reflections are observed across the profiles, while reflectivity in the lower crust is marked by intermittent horizontal packages (“layering”?) and short, gently dipping reflections and diffraction segments. Sub-Moho reflections, although rare in general on deep reflection profiles, occur frequently on the profiles and can clearly be correlated across two intersecting profiles making possible the measurement of a true dip and strike. Contoured values in two-way travel time for the mantle reflectors range from 12.5 s (just below the Moho) to approximately 18 s. The contour map indicates a NW strike and a SW dip to the most prominent mantle reflector. The total area of inferred mantle reflector coverage beneath the Illinois Basin exceeds the area of the state of Connecticut. The presence of newly observed mantle reflectivity beneath the Illinois Basin indicates significant upper mantle heterogeneity, relative to other parts of the USA studied using reflection methods. The isolated occurrence of individual reflections makes it difficult to uniquely infer their origin. However, available geologic and geophysical constraints, especially from geochemical and geochonological studies of drilled basement rocks, effectively limit the possibilities to: (1) remnants or “scars” of lithospheric delamination associated with the melting of the Proterozoic crust that led to the emplacement of the granite–rhyolite province that underlies much of USA Midcontinent; or (2) deformation caused by plate subduction associated with the hypothetical accretion of a juvenile arc to the pre-1.6 Ga southern margin of the Laurentian continent.