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Abstract

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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