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Structure of the Lake County uplift: New Madrid seismic zone

Center for Earthquake Research and Information Department of Geological Sciences University of Memphis, Memphis, Tennessee 38152

Abstract

The central segment of the New Madrid seismic zone lies within a left step-over zone between two northeast-striking, right-lateral, strike-slip fault systems. Within this compressional step-over zone is the topographically and structurally high Lake County uplift, which includes the Tiptonville dome and Ridgely ridge. We believe these structures are a consequence of deformation in the hanging wall above the northwest-striking, southwest-dipping Reelfoot reverse fault. Reelfoot fault dips 73° from the surface to the top of the Precambrian at a depth of approximately 4 km. From 4 to 12 km depth, the fault dips 32° and is seismically active. Based on a fault-bend fold model, we believe that the Reelfoot fault becomes horizontal and aseismic at the top of the quartz brittle-ductile transition zone, at approximately 12 km depth. Our data indicate that the western margin of the Tiptonville dome-Ridgely ridge and the western margin of the Lake County uplift are bounded by east-dipping kink bands (backthrusts). Recent work suggests that the Reelfoot fault is responsible for the 7 February 1812, M 8 New Madrid earthquake. However, the Reelfoot fault has a surface area that is less than that necessary for an M 8 earthquake. A possible solution to this discrepancy between magnitude and fault plane area is that the associated backthrusts are seismogenic.

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