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The Mimbres basin of south-central New Mexico is the westernmost of the deep sedimentary basins which comprise the Rio Grande rift, a tectonic province of high heat flow trending northward from northern Mexico to central Colorado. The Mimbres basin is bounded on both sides by normal faults that have been active during the Quaternary. Quaternary basalt flows are also present in the area. Heat flow values in the basin are typically 60 to 110 mWm-2 and shallow geothermal gradients are typically 30 to 50°C/km.
Detailed temperature logs from 17 shallow boreholes less than 120 ft (40 m) deep have been obtained from wells located within and adjacent to the Mimbres basin near the international border with Mexico. The geothermal gradients from these wells have been continued downward through a typical "basin and range" conductivity model in order to estimate the deep, steady-state thermal structure within and beneath the basin. The predicted temperatures have been checked against temperatures measured in two separate oil tests at depths of 6,624 ft (2,019 m) and 9,435 ft (2,876 m) and found to agree to better than 5°C (9°F). Temperatures within the upthrown blocks adjacent to the basin are higher than those within the basin, a phenomenon that results from thermal refraction. Within the basin, the 120°C and 150°C isotherms are encountered at depths of 13,000 ft (4,000 m) and 18,000 ft (5,500 m). In the adjacent ranges, these isotherms are encountered at depths of 11,000 ft (3,400 m) and 16,500 ft (5,000 m), respectively. These depths are substantially deeper than might be inferred on the basis of the basin's tectonic setting or on the basis of the high gradients measured in the shallow boreholes. The temperatures in the deeper parts of the basin as well as part of the underlying basement are well within the liquid window of hydrocarbon stability. The downward continuation of shallow temperature gradient data appears to be an adequate tool for resolving the deep thermal structure of the Mimbres basin.
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