Over 100 stable isotope and 45 tritium analyses from thermal and nonthermal waters of the Jemez Mountains region, New Mexico, have been used to define the hydrodynamics of the Valles caldera (Baca) geothermal system and related geothermal fluids of the region. Evaluation of 36 cold meteoric waters yields an equation for the Jemez Mountains meteoric water line of δD = 8δ18O + 12, while further evaluation of nine cold meteoric waters yields an equation relating recharge elevation to deuterium content of E(meters) = −44.9 (δD) ‐ 1154. Based on the deuterium content of five Baca well waters (223°–294°C), the average recharge elevation of the Valles geothermal system ranges from 2530 to 2890 m. This range of elevations falls between the elevations of the lowest point of the caldera floor (2400 m) and the summit of the resurgent dome inside the caldera (3430 m). Thus stable isotopes indicate that the caldera depression probably serves as a recharge basin for the deep geothermal system. Although cold spring waters of the Jemez Mountains region consist of meteoric water, tritium analyses show that most of them contain water between 20 and 75 years old. The two major streams draining the Valles moat zone, San Antonio Creek and East Fork Jemez River, may contain more than 50% of this relatively old groundwater depending on the season. In contrast, streams draining the central resurgent dome of the caldera contain present‐day meteoric water. Using piston flow and homogeneously mixed reservoir models as end‐member cases, the tritium contents of the Baca fluids (0.18–1.10 tritium units (TU)) indicate that the mean residence time of water in the reservoir is between 60 and 10,000 years old. Deep geothermal fluids display a positive oxygen 18 shift of not less than 2‰ because of rock‐water isotopic exchange at 220°–300°C. The Valles geothermal system is capped by a vapor zone that is roughly 600 m thick and best developed at Sulphur Springs. Fumarolic steam at Sulphur Springs is unusually depleted in oxygen 18, suggesting that it is probably derived by boiling of near‐surface groundwater at 200°C. Surface acid‐sulfate waters are mixtures of condensed steam and surficial groundwaters that display the isotopic processes of evaporation and exchange between H2O and CO2. A lateral outflow plume discharges from the Valles geothermal system down the Jemez fault zone and feeds two sets of thermal springs in San Diego Canyon. Isotopic evidence shows that these springs consist of three components: (1) deep geothermal fluid, (2) surficial and/or near‐surface groundwater, and (3) relatively old, but cold, mineralized water. This latter component presumably circulates in Paleozoic strata underlying the western caldera flank. Hot, Precambrian, pore fluid brine occurs beneath the main Valles caldera hydrothermal system and may be generated by metamorphic processes in the relatively impermeable conductive regime above the magmatic heat source of the caldera.

No abstract

Individual faults, faults linking at depth in flower structure zones and jogs bounded by faults are common structural elements in strike-slip fault systems and can play an important role in controlling thermal fluid flows. This paper explores the influence of these structures on the thermal circulations and fluid outflows of Terme di Valdieri, in the crystalline basement of the Argentera Massif (western Alps). In this site, thermal waters upwell at the tip of a NW-trending right-lateral fault, but exactly which structures control infiltration of meteoric waters and deep circulation is not clear from field surveys. Three-dimensional thermohydraulic numerical models calculated in steady-state and in transient regimes are presented for three alternative hypotheses. These account for circulations occurring: (i) within a single fault and adjoining host rocks; (ii) in faults intersecting at depth; and (iii) in faults interacting by means of a permeable step-over. The simulations show that advective flows can coexist with convective flows in models (i) and (iii), provided that the fault permeabilities are higher than 2 Â 10 213 m 2 , while advection prevails in model (ii) at all values of permeability. Model (iii) achieves the best fit to the data under the assumption of advective and convective flows. This finding provides a first quantitative estimate of the importance of jog structures bounded by strike-slip faults in favouring thermal outflows. Moreover, the numerical results suggest that thermal convection can coexist with advection also in mountainous settings.

Stratigraphic, temperature gradient, hydrogeochemical, and hydrologic data have been integrated with geologic data from previous studies to show the structural configuration of the Valles caldera hydrothermal ouflow plume. Hydrologic data suggest that 25–50% of the discharge of the Valles outflow is confined to the Jemez fault zone, which predates caldera formation. Thermal gradient data from bores penetrating the plume show that shallow gradients are highest in the vicinity of the Jemez fault zone (up to 190°C/km). Shallow heat flow above the hydrothermal plume is as high as 500 mW m−2 near core hole VC‐1 (Jemez fault zone) to 200 mW m−2 at Fenton Hill (Jemez Plateau). Chemical and isotopic data indicate that two source reservoirs within the caldera (Redondo Creek and Sulphur Springs reservoirs) are parents to mixed fluids flowing in the hydrothermal plume. However, isotopic data, borehole data, basic geology, and inverse relations between temperature and chloride content at major hot springs indicate that no single reservoir fluid and no single diluting fluid are involved in mixing. The Valles caldera hydrothermal plume is structurally dominated by lateral flow through a belt of vertical conduits (Jemez fault zone) that strike away from the source reservoir. Stratigraphically confined flow is present but dispersed over a wide area in relatively impermeable rocks. The Valles configuration is contrasted with the configuration of the hydrothermal plume at Roosevelt Hot Springs, which is dominated by lateral flow through a near‐surface, widespread, permeable aquifer. Data from 12 other representative geothermal systems show that outflow plumes occur in a variety of magmatic and tectonic settings, have varying reservoir compositions, and have different flow characteristics. Although temperature reversals are commonly observed in wells penetrating outflow plumes, reversals are not observed in all plumes. Less information is available on the absolute age of hydrothermal outflow plumes, although the data show that they can be as old as 106 years and display episodic behavior.

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