STRUCTURE OF THE LAKE COUNTY UPLIFT - NEW MADRID SEISMIC ZONE

The central segment of the New Madrid seismic zone lies within a left step-over zone between two northeast-striking, right-lateral, strike-s lip fault systems. Within this compressional step-over zone is the top ographically and structurally high Lake County uplift, which includes the Tiptonville dome and Ridgely ridge. We believe these structures ar e a consequence of deformation in the hanging wall above the northwest -striking, southwest-dipping Reelfoot reverse fault. Reelfoot fault di ps 73 degrees from the surface to the top of the Precambrian at a dept h of approximately 4 km. From 4 to 12 km depth, the fault dips 32 degr ees and is seismically active. Based on a fault-bend fold model, we be lieve that the Reelfoot fault becomes horizontal and aseismic at the t op 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 a re bounded by east-dipping kink bands (backthrusts). Recent work sugge sts that the Reelfoot fault is responsible for the 7 February 1812, M 8 New Madrid earthquake. However, the Reelfoot fault has a surface are a that is less than that necessary for an M 8 earthquake. A possible s olution to this discrepancy between magnitude and fault plane area is that the associated backthrusts are seismogenic.