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Recurrent movements on the northeast-trending Reelfoot rift and west-trending Rough Creek fault zone dominated southeastern Illinois tectonic history. Early Cambrian rifting along both zones created deep trenches that began to fill with sediments. Intermittent movements continued, but faults were quiescent by the Mississippian. Then renewed extension on the Reelfoot rift in the Early Permian produced high-angle normal faults in the Wabash Valley fault system and Fluorspar area fault complex, and the right-lateral Cottage Grove fault system. Igneous intrusions accompanied this action: upwelling magma formed Omaha dome; Hicks dome and associated concentric and radial faults appear to have been formed by explosive igneous activity.
After the Early Permian, recurrent up-and-down movements of several thousand feet reactivated the fluorspar area fault complex and created the present day Rough Creek and Shawneetown fault zones. Blocks bordering faults returned roughly to their original positions by the Late Cretaceous, leaving narrow slices of rock upthrown and downthrown along faults.
Faults in Illinois probably have been inactive since the Cretaceous Period, although the Reelfoot rift south of Cairo has been reactivated. Earthquakes in Illinois today apparently are caused by local east-west horizontal compressional stresses not related to known bedrock faults.
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