Introduction
Arizona makes up a tiny fraction of the land surface on Earth (0.2%, or 295 thousand km2 compared to total land area of 149 million km2). However, it has had a comparatively larger impact on the evolution of geologic concepts, especially in three areas of geologic inquiry: (1) porphyry copper deposits, (2) metamorphic core complexes, and (3) evolution of large rivers. Below is a brief review of the significance of Arizona geology in development of concepts associated with these features, presented here in commemoration of the 125th anniversary of the Arizona Geological Survey and its predecessors.
Porphyry copper deposits
Porphyry copper deposits are the single largest source of copper, which is the metal of choice for most electrical applications because copper is both highly conductive to electricity and is malleable. Southeastern Arizona is the center of one of Earth’s three great clusters of porphyry copper deposits (the two others are in northern and central Chile). The geology department at the University of Arizona played an outsized role in understanding porphyry copper deposits, largely from studying Arizona deposits.
Porphyry copper deposits, in idealized form (Fig. 1), are each centered on an igneous intrusion composed of porphyry, which is a fine grained granitic rock with larger crystals of feldspar. The porphyry is surrounded by concentric shells containing different mineral assemblages that reflect the conditions of hydrothermal alteration and mineralization at the time that the magma was intruded and crystallized to form the porphyry. Copper is typically concentrated at a specific zone within these concentric alteration-mineral assemblages. This model has been used to help find porphyry copper deposits where only parts of the alteration zones have been located. The idealized porphyry copper model was identified by J. David Lowell, an exploration geologist working on the San Manuel porphyry copper deposit north of Tucson, and University of Arizona geology professor John Guilbert (Lowell and Guilbert, 1970).
Metamorphic core complexes
Geologic investigations in southwestern North America, beginning in the latter half of the 19th century, generally inferred that gneissic rocks (layered igneous and highly metamorphosed rocks) were very old, which was consistent with their location beneath thick sections of younger sedimentary rocks (for example, at the bottom of the Grand Canyon). The application of isotope geochronology techniques, which allow determination of the age and cooling history of rocks based on the slow decay of naturally occurring radioactive isotopes, revealed that some of these gneissic rocks were in fact young by geologic standards. In addition, some gneissic rocks were found to include a type of rock called “mylonite” in which the layered character of the rock was produced by shearing and smearing rather than by gradual recrystallization under conditions of great pressure and near-melting temperatures.
Field and geochronologic study of the South Mountains on the south flank of Phoenix determined that gently dipping mylonitic layering was superimposed on a granite that was only about 25 million years old (Reynolds and Rehrig, 1980; Reynolds, 1985), much younger than anticipated. Furthermore, geologists were able to determine that the mylonitic shearing occurred during horizontal extension of Earth’s crust, and during uplift of rocks from deep in the crust during this extension. The mylonite was produced by shearing as the granitic rocks of the eastern South Mountains were sheared below a gently inclined ductile shear zone and its up-dip continuation as a brittle fault (e.g., Davis et al., 1986).
This insight, that mylonites were produced during the uncovering and uplift of deep crustal rocks, was essential to the identification of metamorphic core complexes (Fig. 2). This term is now applied to mylonitic rocks uncovered during crustal extension in many parts of the world, including many undersea complexes.
River evolution
The Colorado River did not exist, at least not as we know it, before about 5 million years ago. The water that flowed off of the western flank of the southern Rocky Mountains and off of the Colorado Plateau flowed down a different, and still unknown, path to the ocean, or it was trapped in lakes. Geologists remain uncertain as to what caused drainages on the Colorado Plateau to change to form the modern Colorado River, which now flows through the Grand Canyon, across the eastern Mojave Desert, and into the Gulf of California (Fig. 3).
It had been thought, and many geologists still believe, that opening of the Gulf of California due to plate tectonic processes led to a lowering of base level for small streams entering the subsiding gulf, and that this base-level lowering caused headward erosion of these streams so that they expanded their reach, eventually extending headward to capture a pre-existing river on the Colorado Plateau and forming the modern Colorado River (Lucchitta, 1979). Headward erosion at this scale is problematic, however, partly because it is clearly so ineffective in many other cases over much smaller distances. For example, Red Lake Playa north of Kingman in northwestern Arizona has not been captured by the Colorado River for the past 5 million years, even though the Colorado River is only 40 km away (Spencer and Pearthree, 2001). Furthermore, top-down integration associated with lake spillover can be very effective at creating new drainage systems, and is likely a much more important factor in genesis of the modern Colorado River (Meek and Douglass, 2001). Lively and active debate over the origin of the Colorado River has brought a renewed focus on lake spillover and top-down integration in the origin of large river systems.
Conclusion
Arizona has been an instructive natural laboratory for understanding geologic processes, partly because of superb rock exposures in the arid Southwest, and also because geologically young tectonic and igneous processes have uplifted and exposed so much rock. New insights would not have been gained without active community of geologists, including those at the Arizona Geological Survey and its predecessors over the past 125 years. Arizona’s complex geologic history, diverse rocks types, and abundant mineral resources continue to be the target of ongoing studies.
References cited
Davis, G.A., Lister, G. S., and Reynolds, S. J., 1986, Structural evolution of the Whipple and South Mountains shear zones, southwestern United States: Geology, v. 14, p. 7-10.
House, P.K., Pearthree, P.A., and Perkins, M.E., 2008, Stratigraphic evidence for the role of lake spillover in the inception of the lower Colorado River in southern Nevada and western Arizona, in Reheis, M.C., Hershler, R., and Miller, D.M., eds., Late Cenozoic drainage history of the southwestern Great Basin and lower Colorado River region: geologic and biotic perspectives: Geological Society of America Special Paper 439, p. 335-353; doi: 10.1130/2008.2439(15).
Lowell, J.D., and Guilbert, J.M., 1970, Lateral and vertical alteration-mineralization zoning in porphyry ore deposits: Economic Geology, v. 65, no. 4, p. 373-408.
Lucchitta, I., 1979, Late Cenozoic uplift of the southwestern Colorado Plateau and adjacent lower Colorado River region: Tectonophysics, v. 61, p. 63-95.
Meek, N., and Douglass, J., 2001, Lake overflow: An alternative hypothesis for Grand Canyon incision and development of the Colorado River, in Young, R.A., and Spamer, E.E., eds., The Colorado River: Origin and evolution: Grand Canyon, Arizona, Grand Canyon Association Monograph 12, p. 199-204.
Reynolds, S.J., 1985, Geology of the South Mountains, central Arizona: Arizona Bureau of Geology and Mineral Technology Bulletin 195, 61 p., 1 sheet, scale 1:24,000.
Reynolds, S.J., and Rehrig, W. A., 1980, Mid-Tertiary plutonism and mylonitization, South Mountains, central Arizona, in Crittenden, M.D., Jr., Coney, P.J., and Davis, G.H., eds., Cordilleran metamorphic core complexes: Geological Society of America Memoir 153, p. 159-175.
Spencer, J.E., and Pearthree, P.A., 2001, Headward erosion versus closed-basin spillover as alternative causes of Neogene capture of the ancestral Colorado River by the Gulf of California, in Young, R.A., and Spamer, E.E., eds., The Colorado River: Origin and evolution: Grand Canyon, Arizona, Grand Canyon Association Monograph 12, p. 215-219.
Spencer, J.E., and Reynolds, S.J., 1989, Middle Tertiary tectonics of Arizona and the Southwest, in Jenney, J.P., and Reynolds, S.J., eds., Geologic evolution of Arizona: Arizona Geological Society Digest, v. 17, p. 539-574.
Spencer, J.E., Patchett, P.J., Pearthree, P.A., House, P.K., Sarna-Wojcicki, A.M., Wan, E., Roskowski, J.A., and Faulds, J.E., 2013, Review and analysis of the age and origin of the Pliocene Bouse Formation, lower Colorado River Valley, southwestern USA: Geosphere, v. 9, n. 3, doi:10.1130/GES00896.1
