LITHOSPHERIC STRUCTURES IDENTIFIED BY THE OIINK ARRAY REVEAL INSIGHTS INTO THE EVOLUTION OF THE MIDCONTINENT

Epiorogenic structures within midcontinent of the United States indicate that the cratonic interior of North America has not behaved as a single strong rigid block. The existence of the Illinois Basin, the Ozark Plateau, the Rough Creek Graben, and the Reelfoot Rift emphasize that the region has undergone episodes of uplift, subsidence, and extension. Has this deformation modified the crust and lithosphere? The combined sampling of the EarthScope Transportable Array and OIINK FlexArray, which involved seismic stations at over 150 locations spaced ~25 km apart from eastern Missouri to central Kentucky, allowed us to construct a detailed seismic model of the crust and upper mantle to explore relationships between lithospheric structures and styles of deformation. Our model reveals areas of elevated shear-wave velocities in the crust of the southern Illinois Basin, however crustal velocities of the central and northern portions of the basin appear lower. Differences in seismic velocities from the southern to northern extent of the Illinois Basin follow a similar pattern to changes in crustal thickness. Specifically, the southern portion of the basin possesses thinner crust (~40 km) while the central portion is underlain by thicker crust (>50 km). The transition between thin crust and high velocities to the south and thicker crust and lower velocities to the north aligns with the trace of the Cottage Grove fault system, suggesting a link between surface deformation and structures at depth. Differences in the north-to-south structure of the Illinois Basin highlight the possibility that, while extension in the Reelfoot Rift and Rough Creek Graben may have led to the formation of the southern Illinois Basin, another factor likely contributed to subsidence in the northern Illinois Basin. Areas of thinner crust also continue to the west and east of the southern Illinois Basin into the Ozark Plateau and Rough Creek Graben. At greater depths within the upper mantle, reduced velocities underlie a southwest-to-northeast oriented swath at depths from ~75-150 km beneath the Reelfoot Rift, the Rough Creek Graben and the Wabash Valley Seismic Zone. The locations of the New Madrid and Wabash Valley Seismic Zones above these low upper-mantle velocities may indicate that intraplate seismicity has roots in a weakened upper mantle.