Hot springs can occur in amagmatic settings, but the mechanisms of heating are often obscure. We have investigated the origin of the Truth or Consequences, New Mexico low-temperature (approximately 41°C) hot springs in the southern Rio Grande Rift. We tested two hypotheses that could account for this amagmatic geothermal anomaly: lateral forced convection in a gently dipping carbonate aquifer and circulation through high-permeability crystalline basement rocks to depths of 8 km that is then focused through an overlying faulted hydrologic window. These hypotheses were tested using a regional two-dimensional hydrothermal model. Model parameters were constrained by calibrating to measured temperatures, specific discharge rates, and groundwater residence times. We collected 16 temperature profiles, 11 geochemistry samples, and 6 carbon-14 samples within the study area. The geothermal waters are Na + /Cl À -dominated and have apparent groundwater ages ranging from 5500 to 11 500 years. Hot-spring geochemistry is consistent with water/rock interaction in a silicate geothermal reservoir, rather than a carbonate system. Peclet number analysis of temperature profiles suggests that specific discharge rates beneath Truth or Consequences range from 2 to 4 m year À1 . Geothermometry indicates maximum reservoir temperatures are around 170°C. Observed measurements were reasonably reproduced using the deep circulation permeable-basement modeling scenario (10 À12 m 2 ) but not the lateral forced-convection carbonate-aquifer scenario. Focused geothermal discharge is the result of localized faulting, which has created a hydrologic window through a regional confining unit. In tectonically active areas, such as the Rio Grande Rift, deep groundwater circulation within fractured crystalline basement may play a more prominent role in the formation of geothermal systems than has generally been acknowledged. . (2015) 15, 139-160 Garven 1995), contact metamorphism (Gerdes et al. 1995), and metasomatism (Fritz et al. 2006). Crustal permeability is widely believed to decay with depth due to mechanical loading and fluid-rock geochemical reactions. used geothermal and metamorphic data to suggest that crustal permeability decreases from 10 À12 to 10 À16 m 2 at 1 km depth to 10 À15 Geofluids

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Water samples were collected from 31 springs and analyzed for general chemistry, trace metals, stable isotopes, and for several relative age-dating analyses, including tritium, chlorofluorocarbons (CFCs), and carbon-14. The report contains maps of the locations of inventoried springs, data tables that characterize the springs, the results of geochemical sampling and laboratory analysis of spring water chemistry and a variety of tracers, and a brief summary of the location and field parameters of each major cluster of springs.