UPPER MANTLE REFLECTIVITY BENEATH AN INTRACRATONIC BASIN: INSIGHTS INTO THE BEHAVIOR OF THE MANTLE LITHOSPHERE

Several hundred kilometers of seismic reflection profiles obtained for oil industry operations were reprocessed using extended vibroseis correlation from the initial 4-second correlated records, to the full 20 seconds (~70 km) listening travel time in order to image features throughout the entire crust and into the upper mantle beneath the Illinois basin. The initial 4-second records were acquired with a 16-second vibroseis sweep from 14 to 126 Hz. The frequency band width used included frequency components suitable for imaging structures from signals received from both sedimentary basin reflectors and those received from reflectors in the deep crust and upper mantle. Final results of the reprocessing show that boundaries with sufficient impedance contrast to produce prominent reflectivity exist within the upper mantle beneath the Illinois basin. The mantle reflectors are clearly imaged in records down to 18 seconds two- way travel time and are observed on intersecting profiles and generally dip to the southwest. The true dip of these reflectors is obtained from intersecting profiles (true dip to the southwest and a northwest-southeast strike). Occasional Moho reflections are also observed across the profiles (12 seconds or ~36-40 km) while reflectivity in the lower crust is generally marked by intermittent horizontal packages and short, gently dipping reflections and diffraction segments. The plausible origin of the mantle reflectors as suggested from available geological and geophysical data is either related to subduction along an ancient Proterozoic active margin or sub-crustal processes associated with lithosphere extension or compression such as delamination of the lower crustal lithosphere into the mantle. The presence of mantle reflectors beneath the Illinois Basin indicates either significant upper mantle heterogeneity or better signal penetration relative to other parts of the USA studied using seismic reflection methods. Our effort in using recorrelated industry seismic profiles to image mantle depths is one of the first of its kind and has provided valuable insights into the nature of the mantle lithosphere beneath the Mid-continent USA, as well as the probable geodynamic processes previously active in this region.