Regional Tectonics and Seismicity of Southwestern Iowa: Final Report: 1978-1982 (NUREG/CR-3021)

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Publication Information

Manuscript Completed: October 1982
Date Published: November 1982

Prepared by:
O. J. Van Eck, R. R. Anderson, C. L. Cumerlato,
T. H. Faller, J. D. Logel

lowa Geological Survey
123 N. Capitol Street
lowa City, IA 52242

Prepared for:
Division of Health, Siting and Waste Management
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, D.C. 20555
NRC FIN B6312
Under Contract No. NRC-04-78-228

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Abstract

Utilizing gravity and aeromagnetic data, a series of geophysical profiles were constructed across the Midcontinent Geophysical Anomaly (MGA) which extends across the southwestern Iowa study area. By combining the information | provided by modeling techniques with limited deep well data, a map of the Precambrian basement was generated. The interpretation that emerged includes a central horst of igneous intrusives and extrusives, extensively faulted, and overlain in some areas by Keweenawan clastics. The horst is flanked by a series of high angle faults, with the majority of the faults displaying vertical displacement along two structural zones, the Thurman-Redfield Structural Zone, along the southeastern margin of the MGA, and the Northern Boundary Fault Zone which flanks the northwestern boundary. The total vertical displacement present along these fault zones is estimated to be a maximum of 9 Km. Two clastic-filled basins flank the horst, one of which has an interpreted depth of 10 Km. Seismic profiling indicates rather extensive folding and faulting of Paleozoic rock units, although the scale of deformation is minor when compared to Precambrian features in the same area. Although the Paleozoic Era was the most tectonically active of the post-Precambrian eras there are indications of post-Cretaceous movement. Recent tectonic activity is suggested by the numerous microearthquakes that were recorded by the monitoring array that was installed for this study.