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The southern flank of the Black Hills uplift is characterized by high geothermal gradients, which exceed 40°C/km at many places and attain a maximum estimated value of 84°C/km. Aeromagnetic, gravity, geologic, geothermal, and Na-K-Ca-Mg geother-mometer data are correlated to attempt to understand better the geothermal system of the upper crust of this region. Analyses of the gravity and magnetic fields include compilation of derivative maps to enhance the expressions of lithologic and structural boundaries.

In our model of the geothermal system of the southern Black Hills, ground waters in the topographically high areas descend to a depth of roughly 1.6 km along southward- and southeastward-trending fault zones, migrate upward primarily along faults after becoming heated, and spread outward through permeable sedimentary units. Several interpreted gravity and magnetic features appear to be related to the structural control of the thermal convection. For example, fault-formed lithologic boundaries expressed in the gravity and magnetic data may act as conduits for the southeastward-trending ground waters. These boundaries are abruptly truncated at their southeastern edges by northeast-trending features that presumably represent fault zones. Thermal gradients are significantly higher southeast of the northeast-trending fault zones. Of particular interest are two broad structures, an antiform and a basin, that appear to influence the channeling of warm water.

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