QUATERNARY FAULTING IN THE CAMBRIAN REELFOOT RIFT OF THE MISSISSIPPI RIVER VALLEY

New Madrid seismicity is occurring along reactivated Cambrian Reelfoot rift faults in the Mississippi River Valley. These faults include the New Madrid North, Reelfoot North, Reelfoot South, Axial, with minor seismicity along the southeastern Reelfoot rift margin faults. Some Reelfoot rift faults are seismically quiet yet show Quaternary movement such as the Commerce, Joiner Ridge, Meeman-Shelby, and Big Creek faults. Additionally, the Charleston uplift, Beedeville graben, and east-west striking normal faults that cross the Eastern Lowlands appear to have Quaternary displacement but have not been quantified by trenching or drilling. All these faults fit the contemporary plate tectonic stress field of approximately east-west horizontal maximum compression resulting in right-lateral shear across the Reelfoot rift. However, isostatic forces have also occurred and continue to occur in the southern Mississippi River Valley. Approximately 85 m of sediment in the southern Mississippi River Valley was eroded during Pleistocene glacial sea level low stands. This erosion caused a minimum isostatic uplift of 45 m at the latitude of Memphis, Tennessee. Isostatic uplift has centered on Crowley’s Ridge of eastern Arkansas with Quaternary erosion by the ancestral Mississippi and ancestral Ohio rivers migrating west across the Western Lowlands and east across the Eastern Lowlands, respectively. The eastern bluff margin of the Mississippi River Valley eroded eastward ~ 50 km during the late Pleistocene and Holocene. This erosion removed 70 m of sediment and reduced the vertical stress on underlying Reelfoot rift faults thereby promoting fault movement and New Madrid seismic zone earthquakes.

Many questions remain. Have the Reelfoot rift faults of Joiner Ridge shut down? What are the fault histories and seismic threats posed by the Charleston uplift near Cairo, Illinois, Beedeville graben near Little Rock, Arkansas, and the east-west striking normal faults that cross the Eastern Lowlands? Does the detailed Quaternary erosional history of the lower Mississippi River Valley mirror its Quaternary faulting history? Perhaps most interesting is how did, and how does, the plate tectonic stress field interact with Mississippi Valley isostatic uplift and valley erosion to cause Holocene faulting and earthquakes?