THE STE. GENEVIEVE FAULT ZONE OF THE CRATONIC PLATFORM, USA: A SURFACE MANIFESTATION OF THE LONG-LIVED AND STILL-ACTIVE CRUSTAL BOUNDARY BETWEEN THE OZARK PLATEAU AND ILLINOIS BASIN

The NW-SE-trending Ste. Genevieve fault zone (SGFZ) can be traced from southern Illinois, along a 100 km-long reach of the Mississippi River, into east-central Missouri. It is one of several NW-SE trending fault zones in the region. Recent field study of the fault, coupled with new results from the OIINK EarthScope seismic-array project, demonstrate that the structure is a surface manifestation of a profound crustal boundary separating the northeastern edge of the Ozark Plateau ("Ozark dome") from the Illinois basin. A new subsurface basement-topography map (i.e., a structure-contour map of the Great Unconformity, the surface between basement Precambrian rock and overlying Phanerozoic strata) emphasizes that about 7 km of vertical displacement accumulated across the boundary between the Ozark Plateau and the Illinois Basin during the Phanerozoic. (Only about 0.9 km of this displacement has been recognized across the SGFZ itself.) Field studies of mesoscopic deformation bands and fault-slip lineations in exposures of the fault in the Ozark Plateau show that dip-slip deformation has been dominant. But mapping of the southeastern end of the structure suggests that during its later phases of growth, the SGFZ propagated to the southeast as a strike-slip fault that dies out in a small pull-apart basin. The OIINK array of seismometers has recorded numerous small earthquakes that define a seismic belt encompassing the SGFZ. This seismicity suggests that the activity of the SGFZ, or other blind faults related to it, is comparable to that of the better-known Wabash Valley seismic zone, and is about one-third the activity of the prominent New Madrid seismic zone. OIINK receiver functions suggest a ~ 6-km step in the Moho may occur beneath the Ozark Plateau/Illinois Basin boundary, and teleseismic P-wave travel-time residuals suggest a change in lithospheric velocity structure across the boundary. Taken together, field structural analysis and seismological analyses imply that the SGFZ is the surface manifestation of a significant crustal boundary that has been active through the Phanerozoic and may continue to pose seismic risk today. In that the SGFZ delineates a transpressional boundary between a plateau province and a basin, the structure may be a buried, and more subtle, analog of the Altyn Tagh fault of central Asia.