Capitalizing on the success of a pilot study conducted in 2008, and exploiting the advantages of marine acquisition (time effective, low cost), we will acquire ~600 km of high-resolution marine seismic reflection data along the Mississippi River to identify the location and timing of deformation. This study will image with unprecedented resolution the system of active faults of the NMSZ that cross the Mississippi River. The data will characterize the style of deformation and the long-term history of the presently seismogenic faults. Outside the NMSZ we will identify locations of other recently active faults, especially those that are responsible for liquefaction features distinct from the documented NMSZ ruptures. In those areas we will use fault/stratigraphic relations to begin deciphering their rupture histories. As we document where and when recent faulting has occurred, this study will contribute to the understanding of how the long-term deformation is partitioned among the structures buried under the Mississippi embayment, and will characterize the style (thrust, normal, strike-slip) and the timing of deformation of the imaged faults.

Phase II

Thebes, MO to Caruthersville, MO profile (~310 km)

The northern profile crosses the system of active faults that are responsible for the historic and prehistoric NMSZ seismicity and overlaps with part of the USGS 1981 seismic acquisition (Fig 4a). The deformation in this area is dominated by the Reelfoot thrust (RFT), which represents the compressional step-over zone that connects the two southwest trending dextral strike-slip segments of the Cottonwood Grove fault and the New Madrid North fault. The RFT is the only seismogenic fault in the NMSZ with clear surface expression, represented by the Reelfoot scarp and by the Lake County uplift, a composite topographic and structural high located on the hanging wall of the RFT. Historic accounts confirm that the Reelfoot scarp ruptured the Mississippi riverbed in at least two locations during the last of the 1811-1812 events, and the RFT has been mapped intermittently at the surface for ~32 km. The subsurface geometry of the RFT is less clear. At depths of 5-25 km, the tabular trend of hypocenters defines a ~31°- 45° southwest dipping fault, which projects to the surface east of the Reelfoot scarp. Some authors suggest that the Reelfoot scarp is a vertical splay off of a blind thrust, while others propose a sharp change in the dip of the fault near the surface (~4.5 km depth), which would result in reverse kink band (backthrust) faults in the hanging wall controlling the rise and the extent of the Lake County uplift. Because the Mississippi crosses the Lake County uplift and the Reelfoot scarp, we expect to identify and characterize the structures responsible for the present deformation, and to test the two alternative interpretations. In addition, we expect to image and date erosional and uplift structures associated with the Lake County uplift, and to obtain age, amount of motion and fault slip rates for the Reelfoot thrust. The age of deformation of the NMSZ is crucial information for the understanding of the long-term evolution of this fault system and to discriminate between proposed hypotheses.