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The Balcones and Luling-Mexia-Talco fault systems in central Texas delineate the deep reaches of several aquifers that yield low-temperature (up to 65°C) geothermal water. This geothermal region also coincides with a tectonic province that bisects Texas. Besides the normal faults that mark the boundary between the Edwards plateau uplands and Gulf coastal plain, there is at depth the foundered Ouachita structural belt, a hinge zone that repeatedly affected Mesozoic and Cenozoic sedimentation and structures. For example, crustal adjustments across the buried Ouachita trend apparently controlled the location of the strand throughout much of the Cretaceous time. The change from fluvial and deltaic terrigenous systems to open marine and lagoonal systems has an important e fect on aquifer properties; dip-oriented sand trends are preferred pathways for groundwater flow. Other geologic features that occur along this tectonic and geothermal trend are igneous plugs, loci of hydrothermal mineralization, and ongoing (aseismic or microseismic) adjustments across faults. In short, just as active tectonic areas are the present geothermal "hot spots" of the world, "relict" tectonic areas are distinguished by a coincidence of geologic features that suggest the local occurrence of low-temperature geothermal resources.
Along other parts of the Gulf Coast basin and within the Mississippi embayment, tectonic, igneous, and hydrothermal features converge in a similar manner. Hot Springs, Arkansas, is a notable example. Other areas also show a similar coincidence, but they have not been recognized as having geothermal waters. Areas such as the headward part or the axis of the Mississippi embayment, and the zone of intersection of the deep Appalachian and Ouachita structural trends are potential targets for production of low-temperature geothermal waters. Tectonic trends commonly coincide with population trends, such as along the inner Gulf coastal plain of Texas and along the fall line of the eastern United States. Hence, there may be an established market for direct use of the low-grade geothermal water from aquifers superjacent to the inner margin of the Gulf Coast basin.
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