DETECTION OF POST-PALEOZOIC DEFORMATION FROM SEISMIC REFLECTION PROFILES NEAR THE OLMSTED LOCK AND DAM, NORTHWESTERN “PRONG” OF NEW MADRID SEISMIC ZONE

High-resolution shallow seismic reflection profiles across the northwesternmost part of the New Madrid seismic zone (NMSZ) and northwestern margin of the Reelfoot rift, near the confluence of the Ohio and Mississippi Rivers in the northern Mississippi embayment, reveal intense structural deformation that apparently took place during the latest Paleozoic and/or Mesozoic up to near the end of the Cretaceous Period. We produced seismic profiles along the northeast-trending Olmsted fault, which is defined by displacement of the base of Cretaceous sediments on top of Mississippian bedrock. The trend of this fault is close to and parallel with an unusually straight segment of the Ohio River and is approximately on trend with the westernmost of two groups of northeast-aligned epicenters ("prongs") in the NMSZ. Initial data acquisition has revealed: (1) many high-angle normal and reverse faults expressed as narrow grabens and anticlines; (2) deformation due to thrusting; and (3) zones of steeply dipping faults that bound chaotic blocks similar to those observed previously from the nearby Commerce geophysical lineament. Our most recent (September 2002) compressional and shear wave data, although acquired under more challenging conditions, confirmed offset in shallow Neogene sediment and have imaged the location of the Olmsted fault. The results thus confirm the prevalence of complex deformation of Paleozoic bedrock (and reactivation of Paleozoic structures) continuing up into Tertiary strata near the northern terminus of the NMSZ. Plans for 2003 include drilling along seismic profiles and microgravity surveys over more intensely deformed zones.