Model of the geothermal system in southwestern South Dakota from gravity and aeromagnetic studies
Published:January 01, 1985
T. G. Hildenbrand, R. P. Kucks, 1985. "Model of the geothermal system in southwestern South Dakota from gravity and aeromagnetic studies", The Utility of Regional Gravity and Magnetic Anomaly Maps, William J. Hinze
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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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The Utility of Regional Gravity and Magnetic Anomaly Maps
The first composite magnetic-anomaly map of the conterminous United States and adjacent offshore areas has been published at a color-contour interval of 200 gammas and at the scale and projection of other national geologic and geophysical maps for easy comparison. This map, despite the inconsistent characteristics of the surveys from which it was compiled, is useful in providing a regional framework for the interpretation of magnetic studies of limited areas, in selecting areas for more detailed magnetic investigations, and in studying the distribution and character of regional geologic features.
The map has a wide variation of magnetic-anomaly patterns, trends, and types, thus reflecting the diversity of the geologic terranes of the United States. In general, the anomaly pattern east of the Cordillera in the craton and in the Appalachian Mountains consists of more and greater intensity anomalies. The muted nature of the anomalies of much of the Cordillera is a result of several factors but appears to be primarily related to a decreased crustal magnetization caused by an abnormally shallow Curie isotherm. The anomalies of the Appalachian Mountains and the Cordilleran system primarily reflect the major structural patterns of the orogens, but important exceptions occur, such as those associated with rocks underlying thrust sheets in the Appalachian Mountains and westerly-striking anomaly trends in the Cordillera, which are correlated with igneous intrusives, faults, and mineral deposits.
The buried southern and eastern edges of the Pre-cambrian craton are indicated by changes in the magnetic anomalies and their dominant trends. Within the central United States, numerous regional magnetic-anomaly provinces are observed that reflect the long, complex history of the Precambrian basement rocks of the craton. These provinces are transected by conspicuous, intense, long, generally linear anomalies that originate from mafic extrusive or shallow intrusive igneous bodies within failed rifts, such as the Midcontinent rift system, the Southern Oklahoma aulacogen, and the Reelfoot rift buried beneath the Mississippi embayment. These are only a few of the many interesting regional geologic features that are observed on the composite magnetic-anomaly map of the United States.