MESOZOIC-CENOZOIC DEFORMATION NEAR THE NORTHERN TIP OF THE NEW MADRID SEISMIC ZONE

Although the New Madrid seismic zone (NMSZ) has been intensely studied for many years, it remains an enigmatic zone of seismicity in the midcontinent USA. This is especially true for the northernmost part of the zone, near the confluence of the Ohio and Mississippi rivers within the northern Mississippi embayment in southernmost Illinois and adjacent western Kentucky, where epicentral patterns appear to split into multiple strands and then disperse into a diffuse zone of seismicity in the southern Illinois basin. Until recently, this area has been neglected even though it may hold the key for unraveling the neotectonic history of the NMSZ. As part of an ongoing program of research sponsored by the National Earthquake Hazards Reduction Program of the USGS and by the National Science Foundation-funded Mid-America Earthquake Center, we have acquired high-resolution shallow seismic reflection profiles over several sites at the northern tip of the NMSZ that show intense structural damage beginning in the Paleozoic and continuing through the end of the Cretaceous and into overlying sediments. The seismic profiles reveal steeply dipping faults associated with reactivated grabens and anticlines displaying varying structural styles along with offset of overlying sediment as recent as Holocene. Our interpretation of the seismic profiles is constrained by two boreholes drilled over the seismic lines that were used to determine P-wave and SH-wave velocities, as well as by pre-existing borehole data. The coverage of geophysical data is now sufficient to furnish a 3-D picture of deformation. Future plans for summer 2003 include micro-gravity profiles over selected seismic lines that will be used to confirm our structural interpretation; and further seismic acquisition in the area that will include a marine-style seismic survey in the Ohio River. The detection and mapping of Paleozoic faults that have been subsequently reactivated will provide valuable clues in constraining the study of earthquake recurrence near the northernmost extent of the NMSZ.