Insights into Midcontinent (USA) tectonics and seismicity from the OIINK EarthScope Flexarray Experiment

Newly acquired EarthScope seismic data refine our picture of lithospheric structure beneath North American's cratonic platform and determine its relationship to tectonic features and seismicity. These data include results from the OIINK (Ozarks, Illinois, INdiana, Kentucky) Flexible Array, a network of 140 seismometers at approximately 25 km spacing, which was deployed from 2011 to late 2015 to explore deep structure beneath a WNW-ESE trending swath of the Midcontinent. The study area extends from the Ozark Plateau in the west, across the southern Illinois Basin and Rough Creek Graben, to the Cincinnati Arch and Grenville Front in the east. Receiver function and surface-wave analyses, coupled with structural studies, emphasize the following: - Distinct tectonic domains can be identified on a DEM of the Great Unconformity; - Seismicity concentrates along the border of the Midcontinent domain (a region bounded by the Rocky Mountain front on the west, and by Iapetan rifting-related basins on the south and east). Other distinct seismic zones, following fault zones initiated during Proterozoic rifting, also exist; - The Ste. Genevieve seismic zone, which defines the boundary between the Ozark Plateau and the Illinois Basin, hosts more seismicity than previously recognized. Its activity is comparable to that of the Wabash Valley zone and exceeds that of the Rough Creek graben; - P- and S-wave receiver function analyses indicate that crustal thickness varies dramatically across the region. Anomalously thick crust (max: 62 km) underlies the central and southeastern Illinois basin, and thinner crust (min: 45 km) underlies the Ozark Plateau; - Analysis of fundamental mode Rayleigh waves and S-to-P receiver functions indicate that high crustal shear velocities characterize the Reelfoot Rift and the southern Illinois Basin, and that upper mantle low-velocity anomalies lie beneath intraplate seismic zones of the area. These data also indicate that distinct mid-lithospheric discontinuities underlie the midcontinent region. Overall, our observations emphasize the heterogeneity of the Midcontinent lithosphere and suggest that deeper structures influence upper-crustal tectonic features.