Heat flow in VC-2A and VC-2B, and constraints on the thermal regime of the Valles Caldera, New Mexico
Paul Morgan, John H. Sass, and Ronald D. Jacobson
Shallow thermal gradient data from the Quaternary Valles caldera in the Jemez Mountains of New Mexico indicate that heat loss from the caldera is concentrated in its western half. The Sulphur Springs area, near the western margin of the resurgent dome, was the site of the earliest intensive exploration for geothermal resources in the caldera. In an earlier study we demonstrated that the conductive heat flow near the western margin of the Valles caldera averages between 300 and 400 mW m -2. A combined total of 150 determinations of thermal conductivity on core segments from research wells VC-1. VC-2A, and VC-2B are sufficient to characterize the thermal conductivity of the major rock types in the area. A high degree of alteration of tuffs from VC-2A and VC- 2B relative to VC-1 is reflected by significantly higher thermal conductivities, generally greater than 2 W m -1 K-1, for the tuffs in the VC-2 wells relative to an average tuff conductivity of 1 W m -1 K-1 for VC-1. Temperature data from VC-2A and VC-2B confirm surface indications that the Sulphur Springs area is the locus of vigorous hydrothermal activity. The hydrothermal systems in both wells are capped by relatively thin (12 and 200 m, respectively) near-surface conductive thermal regimes. Heat flows from these near-surface caps are 7200 ± 370 mW m -2 (±standard error) for VC-2A and 1340 ± 60 mW m-2 for VC-2b, several times the very high background heat flux in the western portion of the caldera of 250 to 350 mW m -2. The probable source of these high heat-flow values is the intrusion of one or more shallow magma bodies during the post-collapse history of the caldera.
- Morgan, Paul; Sass, John H.; Jacobson, Ronald D., 1996, Heat flow in VC-2A and VC-2B, and constraints on the thermal regime of the Valles Caldera, New Mexico, in: The Jemez Mountains Region, Goff, Fraser; Kues, Barry S.; Rogers, Margaret Ann; McFadden, Les D.; Gardner, Jamie N., New Mexico Geological Society, Guidebook, 47th Field Conference, pp. 231-236.