Lithospheric Discontinuities in Illinois Basin and Their Tectonic Implications: Results from the EarthScope OIINK Experiment

Abstract:

We present new results from a regional study of lithospheric structure along a swath from Missouri to central Kentucky crossing the Illinois Basin, a major intracratonic basin in the Central U.S. We analyzed 27501 receiver functions for a dense, 140-station Flexible Array of the EarthScope OIINK (Ozark, Illinois, INdiana and Kentucky) experiment, combined with 65 USArray Transportable Array stations, 8 Cooperative New Madrid Seismic Network stations, and station CCM of the Global Seismographic Network. We used Common Conversion Point (CCP) stacking and 3-D plane wave migration techniques to image lithospheric discontinuities across the southern Illinois basin. The results reveal features in crustal and uppermost mantle structures beneath the study area that have not been described previously. The Ste. Genevieve fault zone in southeastern Missouri and southwestern Illinois has been mapped from northern flank of the Ozark Plateau in southeastern Missouri to southwestern edge of the Illinois basin. Our results indicate that this fault zone might be the surface expression of a profound crustal velocity boundary. Crustal thickness changes across this boundary. The Moho lies at depths of about 50-55 km beneath the transition from the Ozark Plateau to the Illinois basin, and shallows eastward and westward to about 45 km. The anomalously deep Moho beneath the central and southwestern basin might be explained by a process of crustal thickening (e.g., magmatic addition or crustal flow), at some point during subsidence of the basin and uplift of the Ozark Plateau. A distinct mid-crustal layer is observed beneath the Ozark Plateau and the southwestern Illinois Basin, at depths from 10 km to 25 km, and the thickest part (~ 15 km thick) is located beneath the Ozark Plateau. This layer is absent beneath the central Illinois Basin. The spatial expansion of this middle-crustal layer may explain the contrasting tectonic histories between the plateau and the basin, which were uplifted and subsided, respectively. In addition, previous studies have shown that earthquakes in the Ste. Genevieve seismic zone around the Ste. Genevieve fault zone are constrained within the top 25 km within the crust. This is comparable with the inferred depth of base of the middle-crustal layer in this region, indicating that this layer is seismogenic.