GEOLOGY OF THE NEW MADRID SEISMIC ZONE REGION OF THE CENTRAL UNITED STATES REVEALS SLEEPING GIANTS

New Madrid seismicity is occurring along reactivated basement faults within a small portion of the Cambrian Reelfoot rift. Peripheral to the seismic zone are major rift faults that have Quaternary displacement but are aseismic or have minor seismicity. These include the eastern and western margins of the Reelfoot rift, Charleston uplift, Joiner Ridge, Crowley’s ridge, Beedeville graben, and the Meeman-Shelby horst. All of these have less than 30 m of vertical Quaternary displacement, thus suggesting an uplift limit on individual faults. Contemporary right lateral simple shear across the N45E trending Reelfoot rift, due to plate tectonic forces, appears to be the principal driving force responsible for fault movement. Various triggering mechanisms have been proposed for Quaternary activation of the Reelfoot rift faults including Quaternary erosional entrenchment by the ancestral Mississippi River system and its alteration of the local stress regime. Current research indicates that entrenchment of the ancestral Mississippi River was more recent and more extreme than previously recognized. Perhaps as much as 120 m of sediment has been removed from above much of the Reelfoot rift within the last 3 Ma due primarily to sea level decline. Thirty meters of that denudation appears to have occurred within the last 20 ka from atop the most seismically active structures of the Reelfoot rift. The 30 m of sediment erosion occurred when the Mississippi River migrated eastward during the past 20 ka thereby erosionally shifting the Mississippi River bluffs ~50 km eastward to its current position. This rapid lateral migration and resultant erosion probably occurred during extreme floods as the Wisconsinan ice sheet melted. Our current structure and geomorphic evolution models may permit kinematic modeling of the Reelfoot rift faults.