Detailed geologic map and cross sections of the Big Horn and Belmont Mountains, West-central Arizona

The principal geologic feature of the Big Hom and Belmont Mountains is a complexly faulted and tilted series of mostly Miocene volcanic rock that record a period of Middle Tertiary magmatism and extension. These volcanic rocks vary widely in composition, but basaltic and rhyolitic rocks are most abundant (Figure 1). Intrusive equivalents of these volcanics exposed in the Belmont Mountains are dominantly granitic. Despite the large volume of rhyolite erupted, small, coalescing flow and dome complexes were formed in preference to large-volume ash-flow tuffs, and no collapse calderas were formed. These rocks lie in the upper plate ofthe regional Whipple-Buckskin-Bullard detachment fault [Rehrig and Reynolds, 1980], at its southeastern tip [Richard et aI, 1990a]. A regional boundary between major tilts domains in Tertiary strata follows an irregular course from northwest to southeast through the range [Rehrig et aI., 1980]. Northeast of this boundary, strata dip to the northeast, and southwest of the boundary, Tertiary strata dip to the southwest. Pre-Tertiary rocks are exposed in several parts of the range. These rocks consist of a Laramide-age (71-63 Ma)hornblende-biotite granodiorite, informally referred to as the Big Hom granodiorite, which intrudes a complex of Proterozoic igneous and metamorphic rocks. Metamorphic grade in the Proterozoic rocks increases from southeast to northwest, and associated igneous rocks become more abundant to the northwest. In the southeast the Proterozoic rocks consist of highly deformed, greenschist facies, mafic and pelitic schist. These grade to heterogeneous mafic-tointermediate- composition gneiss to the northwest. Report with geologic map and cross sections of the Big Horn and Belmont Mountains, west-central Arizona. Scale 1:50,000.Documents in the AZGS Document Repository collection are made available by the Arizona Geological Survey (AZGS) and the University Libraries at the University of Arizona. For more information about items in this collection, please contact azgs-info@email.arizona.edu