In Nevada and western Utah, Cenozoic igneous rocks within several age increments crop out in arcuate, generally east-trending belts, each successively younger to the south. Broad aeromagnetic highs with superimposed short-wavelength anomalies are associated with some of these outcrop belts. Mineral deposits are aligned along the belts in easternmost Nevada and western Utah. The east-west patterns are the result of a southward-migrating front of igneous activity that, in Nevada and Utah, started about 43 to 34 m.y. ago near lat 40°N and ended about 17 to 6 m.y. ago near lat 37°N. During any one time interval, igneous activity was concentrated near the leading edge of the east-trending front. The volcanic front may be related to igneous activity localized along a southward-propagating transverse break or structural warp in a subducting plate.

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