The Reelfoot rift is a N45°E-trending Cambrian rift that has controlled late Pliocene and Quaternary deformation and current seismicity beneath the central Mississippi River Valley. Analysis of 557 well logs between 35°N and 37°N latitude reveals that the Pliocene Upland Complex of western Kentucky and Tennessee and the Upland Complex in Crowley's Ridge of eastern Arkansas have been displaced by east-striking normal faults that extend across the Eastern Lowlands of the Mississippi River Valley. The normal faults extend east and west of the margins of the Reelfoot rift, thus indicating that the region between the Commerce fault and the Big Creek–Ellendale fault has been subjected to right-lateral simple shear since the Pliocene.
An additional 3334 well logs in the lowlands of the Mississippi River Valley between 35°N and 37°N were analyzed for structural displacement of the unconformity at the base of the Quaternary-age Mississippi River alluvium. No Quaternary displacement is seen on the east-striking normal faults in the Eastern Lowlands identified in this study, suggesting their Quaternary displacement is below the resolution of the mapping. However, our base-of-Quaternary mapping has confirmed previously mapped structures and identified new structures that allow the estimation of uplift rates. In the Western Lowlands, the Quaternary unconformity (locally 24–27 ka) is displaced 30 m down-to-the-southeast across the northwestern Reelfoot rift margin, indicating a displacement rate of ∼1.2 mm/yr. In the Eastern Lowlands, within the last 12 k.y., the Charleston uplift has been lifted 36 m, resulting in an average uplift rate of 3 mm/yr; Lake County (Reelfoot North fault) uplift is 21 m (1.8 mm/yr), Blytheville arch uplift is 25 m (2.1 mm/yr), Joiner Ridge uplift is 20 m (1.7 mm/yr), and the Meeman-Shelby fault zone uplift is 28 m (2.3 mm/yr). Absence of a Quaternary fault scarp or surface monocline on the subsurface Reelfoot South fault suggests that this seismically active fault zone may have become reactivated during the earthquake sequence of 1811–1812.
The Reelfoot rift and its outboard faults have undergone right-lateral simple shear since the Pliocene, resulting in (1) right-lateral strike-slip occurring along the northeast- striking basement faults, (2) east-west compression across north-striking reverse faults (stepover zones), and (3) north-south extension across east-striking normal faults. The origin of these structures is illustrated within the strain field of a simple strain ellipse that should be considered in future field strain measurements of the New Madrid seismic zone region. Quaternary deformation in the Reelfoot rift region provides insights into intraplate strain and associated seismicity that may apply to intraplate rifts globally.
- Received 24 January 2013.
- Revision received 12 August 2013.
- Accepted 11 September 2013.
- © Geological Society of America