Geologic setting and geochemistry of thermal water and geothermal assessment, trans-Pecos Texas

Hot springs and wells in West Texas and adjacent Mexico are manifestations of active convective geothermal systems concentrated in a zone along the Rio Grande between the Quitman Mountains and Big Bend National Park. Maximum temperatures are 47C and 72C for hot springs and wells in Texas and 90C for hot springs in Mexico within 5 km of the border. The area lies along the eastern margin of the Basin and Range province in what may be an extension of the Rio Grande Rift. The heat source for the thermal waters is deep circulation of ground water in an area of relatively high thermal gradient. Recent volcanism is not a source of heat because, based on both paleontologic and isotopic evidence, the youngest observed igneous activity in West Texas is Miocene. Most hot springs lie on or immediately basinward of normal faults, at the edges of late Tertiary basins formed by east-west extension. This setting implies that faults are permeable channelways which allow thermal water to rise from below. Recharge for the thermal systems probably occurs in adjacent highlands. Hot springs are not restricted to faults with large displacement or to particular rock types. However, many faults do show evidence of recent movement, which may be important in keeping fracture systems permeable. The setting of hot springs in basins composed of permeable sediments implies that, although thermal circulation could occur along other faults, the water does not discharge to the surface. For example, several wells tap hot water at depths of about 20 to 1,000 m.