STRUCTURAL SETTING AND LONG-TERM DEFORMATION OF THE NEW MADRID SEISMIC ZONE, CENTRAL U.S

The New Madrid Seismic Zone (NMSZ) area of the Central U.S. has seen Cambrian to Ordovician rifting and subsequent compression, subsidence and shearing. The southern arm of the NMSZ is coincident with the anticlinal Blytheville Arch (BA), which seismic reflection data presented here indicate is a pop-up structure within a strike-slip fault system along the Cambrian graben’s axial fault. The BA has perhaps the densest concentrations of faults in the region, but numerous faults that cut shallow (Eocene?) strata outside of the BA indicate that earthquakes have not been restricted to the NMSZ. Modern seismicity outlines the Reelfoot fault stepover in the strike-slip New Madrid fault system (NMFS), and comparison with the fault pattern in analog models of stepovers and restraining bends suggests that deformation caused by this stepover controls the extent of NMSZ seismicity. About 4.3 to 5 km of post-Eocene(?) slip on the southern arm of the NMSZ can be estimated from apparent shearing of the southern portion of Crowley’s Ridge, a north-trending, fault-bounded topographic ridge. This apparent displacement of Crowley’s Ridge suggests that slip extends south of the NMSZ. Only a small fraction of this long-term slip is expressed as uplift above the Reelfoot reverse fault, suggesting that strike-slip motion on the southern arm of the NMSZ continues north of the Reelfoot fault. The NMFS thus appears to be part of a broad set of strike-slip faults that spans the Reelfoot rift, with long-term slip continuing north and south of the modern seismic zone.