Crustal geology of Arizona as interpreted from magnetic, gravity, and geologic data
Published:January 01, 1985
John S. Sumner, 1985. "Crustal geology of Arizona as interpreted from magnetic, gravity, and geologic data", The Utility of Regional Gravity and Magnetic Anomaly Maps, William J. Hinze
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Regional aeromagnetic and gravity maps of Arizona show a variable pattern of basement-fabric trend directions, commonly correlating with physiographic and structural geologic features in the state. The most prominent correlation is seen in the two major physiographic provinces, the Basin and Range and the Colorado Plateau, where the tectonism appears to be controlled by the underlying trend directions of the basement fabric.
At the southern edge of the Colorado Plateau province an ancient buried intrusive paleorift is identified from its distinctive magnetic signature. Also in that province and to the east near the state border there is a major 30-mGal low. This gravity low is on the Datil volcanic pile and measures about 100 km in diameter. The Mesa Butte magnetic anomaly and gravity gradient is a major linear feature in the state, which extends from southeastern Utah to west-central Arizona.
In the Basin and Range province, geophysical patterns are obvious for a northwesterly basement-fabric trend, whereas the more recent normal faulting typical of this Cenozoic tectonic province is generally north-northwesterly in direction.
Several of the gneiss-dome “metamorphic core complexes” exist in the Basin and Range portion of the state. Evidence is offered that these bodies are structural remnants of northeast-trending Precambrian basement blocks now being assimilated into the Basin and Range structural regime.
Some of the larger porphyry copper districts are related to arcuate magnetic lows, probably caused by the destruction of magnetite in the deep intersecting crustal fractures that controlled the circulating hydro-thermal fluids.
The Phoenix arc, identified from magnetic data, is at least 400 km long and concave to the south. It is interpreted as a late Precambrian transform fault at the south end of the White Mountains intrusive body, the 50- x 100-km paleorift feature mentioned above. Most of Arizona's porphyry copper deposits are found in the lithospheric dilatational stress regime along and south of the Phoenix arc.Gravity modeling of all alluvial basins within the state has been carried out in a program that integrated all available subsurface data. The resulting depth-to-bedrock map infers the Basin and Range fault strike and throw, and also quantitatively assesses the ground-water resources in this region.
Other regional geophysical techniques, including deep reflection seismic, electrical and electromagnetic, and heat-flow studies, have assisted in understanding the crustal geology of the state.
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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.