GEOTHERMAL POTENTIAL of Anomalously Warm Ground WATER IN the AREA of the NEW Madrid Seismic Zone, Midcontinent U.S

The New Madrid Seismic Zone (NMSZ) is an area of intraplate tectonism that lies in the northern Mississippi embayment (NME). The NMSZ is seismically active, with an average of hundreds of recorded events per year, yet it lies in what is considered to be part of the stable interior of the North American continent; it has been the focus of tectonic research over much of the last third-century.

Of relevance to this theme session, a thermal anomaly in the McNairy-Nacotoch aquifer overlies the western margin of the rift bounding the NMSZ. Although the heat flux is much less intense and does not owe its origin to igneous sources such as most of the world’s significant geothermal areas, it has meaningful energy potential nonetheless at a local and regional scale, and represents a class of heat sources that may be overlooked.

Coupling hydrogeologic considerations with a hydrostratigraphic framework and tectonic boundaries, we have developed conceptual and numerical models that are consistent with available data, including water levels, thermal signatures, geochemical analyses, stable isotopes, and radionuclides. Heat in the system appears to be derived from advective ground-water flow down the geothermal gradient. Flow originates from meteoric recharge on outcrops in the Ozark uplands, and follows gently south- and west-dipping Paleozoic carbonate aquifers deep beneath the near-horizontal sediments of the NME. Rapid upward flow along fractures in the carbonate aquifers at the western margin of the NMSZ rift results in thermal disequilibrium, and mixing with distinctly different water types that have followed a slower, more geochemically evolved water flowing laterally through the McNairy-Nacatoch aquifer. Dimensions of the documented anomaly are ~35 kilometers wide by ~80 kilometers long, although the southern and eastern margins of anomaly are not well characterized by wells; maximum size of the anomaly could be much wider and longer. Maximum thermal anomalies in wells in the McNairy-Nacatoch aquifer in the NMSZ are in the range of 7-8^o C. We calculate that this hot water is suitable for low-level energy extraction. The main mechanisms of this model are similar to those of the flow system at Hot Springs National Park, although budget components of the two different systems are markedly different.