LITHOLOGY OF THE CRUST AND UPPER MANTLE ACROSS EARTHQUAKE SOURCE ZONES, MEMPHIS, TENNESSEE, TO ST.LOUIS, MISSOURI AS INFERRED FROM SEISMIC AND LABORATORY MEASUREMENTS

Crustal seismic velocity models (Vp, Vs, Vp/Vs ratio, and Poisson’s ratio) and a density model of the crust and upper mantle are developed along a 400-km-long seismic refraction profile extending from Memphis, Tennessee, to St. Louis, Missouri (MSL profile, Catchings, 1999). The seismic profile crossed the Reelfoot rift and the western Illinois basin. Clear differences in crustal properties are determined across these structures. Laboratory measurements show strong, but non-unique correlations between specific rock types and their seismic properties (Vp, Vs, Poisson’s ratio, and density; Christensen, 1996); we estimate crustal and upper-mantle rock types by using the measured and modeled values along the MSL profile. Six layers are recognized. The surfical layer along the MSL profile is known to be sediments. Beneath the western Illinois basin and the Reelfoot rift, our correlations suggest that basement rocks are most consistent with the properties of a biotite gneiss or a phyllite, but paragranulite is also a possibility for the basement rocks beneath the Illinois basin. These layers are near the surface under Illinois basin, and extend to a depth of at least 10 km under the Reelfoot rift. The third layer, underlying the basement rocks, probably consists of a 35-km-thick felsic granulite beneath the Illinois basin and a 24-km-thick mica-quartz schist beneath the Reelfoot rift. The fourth layer, a "rift pillow layer", is consistent with a hornblendite or a mafic garnet granulite that is about 19 km thick beneath the Reelfoot rift but only about 6 km thick beneath the western Illinois basin. The fifth layer, the upper-most mantle, extends in depth from about 43 km to about 60 km and is most consistent with a mafic eclogite, The sixth layer, a layer within the upper mantle, is most consistent with dunite. We do not observe significant lateral variations in velocity and depth of the upper mantle layers. Variations in rock types probably arise from a combination of the differing rock types that make up the continent, continental rifting within the Reelfoot rift, and faulting within the New Madrid seismic zone.