TERTIARY-HOLOCENE TECTONIC ACTIVITY IN SOUTHERNMOST ILLINOIS

Much of the New Madrid seismic zone (NMSZ) has been intensively studied using a variety of geophysical and geological techniques. In contrast, the area just to the north-northeast in southern Illinois and adjacent western Kentucky, over the western “prong” of the northern NMSZ, has received relatively little attention. The study area overlies the northwest margin of the Reelfoot rift, a northeast-trending failed rift of latest Precambrian or early Cambrian age. Previous drilling and new seismic reflection data suggest that this area has undergone recurrent Cambrian to Holocene faulting. Recently we acquired 12.7 km of high-resolution seismic reflection profiles on both sides of the Ohio River near Olmsted, Illinois. Preliminary interpretation of one of the profiles shows several high-angle normal and reverse faults that produce large displacements of the McNairy Formation (Maastrichtian; Late Cretaceous) and underlying Mississippian bedrock, but apparently only small displacements of overlying Paleocene strata. Similarly, nearby boreholes indicate a fault having 120 m of displacement on the McNairy and none on the Paleocene. Other workers document Holocene faulting in the Benton Hills, Missouri about 30 km west and Pleistocene faulting in Massac County, Illinois 30 km east of Olmsted. More to the point, Olmsted is directly in line with the northern prong of active seismicity in the NMSZ as projected from the better defined epicentral zone in southeastern Missouri. In February 1984, a swarm of more than 150 earthquakes having body-wave magnitudes up to 3.6 occurred near Olmsted. Plotting the epicenters yields a northeast-oriented ellipse centered at Ohio River Lock and Dam 53, five km northeast of Olmsted. The linear reach of the river here also trends northeast and is parallel with the major faults. Our tentative explanation for a latest Cretaceous tectonic event is that it represents a far-field effect of the Cretaceous-Eocene Laramide orogeny, which was centered in the Rocky Mountain region several hundred kilometers to the west. Further work in progress is designed to pinpoint the timing of deformation in the northernmost NMSZ and to test the hypothesis that the 1984 earthquake swarm is related to pre-existing faults.