HR: 08:15h
AN: T21D-02    [Abstracts]
TI: Slow Mid-Lithosphere Beneath the Mid-continent, USA
AU: * Bedle, H
EM: heather@earth.northwestern.edu
AF: Northwestern University, 1850 Campus Dr, Evanston, IL 60208 United States
AU: van der Lee, S
EM: suzan@earth.northwestern.edu
AF: Northwestern University, 1850 Campus Dr, Evanston, IL 60208 United States
AB: The Illinois basin is one of several basins within the North American craton. Despite years of study, the formational mechanisms of the basin remain largely unknown. While the crustal structure of the basin is well-defined from seismic studies, it remains unclear whether the basin is expressed in the sub-crustal structure; and, if the mantle structure provides clues towards an understanding the basin. We study the S-velocity structure of the upper-mantle using seismic tomography beneath the Illinois basin and its surrounding area. Since this area of the North American craton has been relatively stable since 1.4 Gy, significant deviations in S-velocities within the uppermost mantle are not expected. Nevertheless, a slower S-velocity in the upper-mantle of the Illinois basin was previously modeled in the 3D model NA04 (van der Lee and Frederiksen, 2005). To improve resolution and verify the existence of this seismically slow region, we fit seismic waveforms from 11 mid-continent events, using the methodology of Partitioned Waveform Inversion (Nolet, 1990). In doing so, we also incorporate constraints that are representative of the Illinois basin's crustal and sedimentary layering. The resultant 3D model, IL05, confirms the existence of a slow S-velocity structure in the uppermost mantle beneath this region. This anomalously slow region exists from the base of the crust to depths of ~100 km, and is slower than a cratonic average by about 200 m/s. The slow uppermost-mantle beneath the Illinois basin is then underlain by a faster lithosphere typical of the North American craton to depths of ~200 km. The coherency of this deeper cratonic lithosphere rules out a lithospheric delamination origin of the anomaly. The model IL05 points to a heterogeneous mantle beneath the mid-continent. Our imaging agrees with evidence of a heterogeneous mantle recently found in the form of sub-Moho reflectors from reprocessed seismic reflection surveys (McBride and Okure, 2004). The observed S-velocities can possibly be explained by a relatively enriched uppermost mantle, which could also be responsible for the high residual gravity anomaly in the mid-continent (Kaban and Mooney, 2005). The cause of this enrichment could also have initiated melting of the Proterozoic crust, and the emplacement of the granite-rhyolite province, which underlies much of the mid-continent.
DE: 7218 Lithosphere (1236)
DE: 7255 Surface waves and free oscillations
DE: 7270 Tomography (6982, 8180)
DE: 9350 North America
SC: Tectonophysics [T]
MN: Fall Meeting 2005