The Cenozoic era represents the most recent 65 million years. (See the geologic time chart for the subdivisions.) Arizona was squeezed, then stretched; steamed and frozen.
Construction of the Rocky Mountains, volcanism, and emplacement of our major copper deposits, all of which began in Cretaceous time, continued in the Cenozoic until about 40 million years ago. During this time, the oceanic crust of the Pacific Ocean was being subducted beneath the westward-moving North American continental plate. The resulting compression caused southern and western Arizona to be topographically higher than the Colorado Plateau, the opposite of current topography.
By about 20 million years ago (mya) Arizona was covered with thousands of feet of volcanic rocks, locally punctured by calderas. Sometime between 30- and 20 million years ago the north American tectonic plate overrode a spreading center called the East Pacific Rise. This area is similar to the spreading center of the Mid-Atlantic ridge that gradually separated Africa from South American, and Europe from North America. Today, this western spreading center runs up the Gulf of California and separates Baja from mainland Mexico. It is also the driver of the San Andreas fault in California. By over-riding the spreading center, the tectonic regime changed from compression to extension. Arizona began to be pulled apart to form the Basin and Range physiography of today.
Arizona is divided into three physiographic provinces: the Colorado Plateau in the northeast, a transition zone extending from northwest to southeast Arizona (the Mogollon Rim), and the Basin and Range province in the south and west. Currently the Basin and Range extends from the Snake River plain in Idaho, through Nevada and Arizona, into Mexico.
Initially, crustal extension along a northeast-southwest axis was characterized by widespread normal faulting and fault-block rotation. Movement occurred along high-angle normal faults which at depth flattened into low-angle detachment faults (see figure below from Spencer and Reynolds). Displacement along these faults is several tens of kilometers. The present location of the Tucson Mountains is a direct result of this extension. (See Tucson Mountain Chaos) Later extension resulted in high-angle faults which bound our valleys and make some of the valleys as much as 15,000 feet deep to bedrock.
This extension sometimes made the life of geologists very interesting when exploring for porphyry copper deposits, because some of those deposits were cut and fanned out like a deck of cards. Finding all the pieces took some geologic detective work. For instance, the Twin Buttes mine, the Mission-Pima mine, and the San Xavier mine south of Tucson, together with buried mineralization between them, represent slices of a once-intact mineral deposit. The Sierrita mine, located on the opposite side of a major fault from the others is still intact.
Middle Cenozoic veins host gold, silver, and base-metal deposits. Copper-gold mineralization is associated with the detachment faults. Manganese and uranium deposits occur in the basins resulting from the extension.
Volcanic activity resumed 2- to 3 million years ago with eruption of basalt which produced flows and cinder cones. The rocks of the San Francisco volcanic field near Flagstaff, the Springerville-Show Low field, the San Bernardino field east of Douglas, and the Pinacate field in Mexico are examples of this episode.
The Grand Canyon was formed during the late Cenozoic. The Colorado River existed as long ago as 20 million years, but it flowed to the northeast across the Colorado Plateau. Crustal extension disrupted this flow pattern and caused the formation of several lakes similar to the Great Salt Lake (i.e., the lakes filled by interior flow from rivers that did not flow to the sea). As the Gulf of California opened, drainage into the Gulf gradually worked its way north and eventually “captured” the interior drainage of the Colorado River system. The lower river began its development about 5.5 mya; by 1.2 mya it was at its present grade in the western Grand Canyon.
Climate in the early Cenozoic continued to be hot and steamy, about 18 F warmer than today, even though atmospheric carbon dioxide had been decreasing for 80 million years due to coal formation in the Cretaceous. Around 55 mya, there was a sudden temperature spike that lasted for about 100,000 years. (That’s geologically sudden, i.e., 10,000 years to form.) The spike is known as the Paleocene-Eocene Thermal Maximum (PETM). Data, derived from drill cores brought up from the deep seabed in the Atlantic and Pacific Oceans, show that the surface temperature of the planet rose by as much as 15 F over the already warm temperatures. The cause is controversial.
Carbon dioxide levels rose from 1000 ppm to 1700 ppm–more than four times higher than today’s level of 385 ppm, but that rise began after the start of the temperature spike.
Isotopic analysis of carbon suggests that the culprit was methane, which is 65 times more powerful as a greenhouse gas than carbon dioxide. There are two hypothesis as to the source of methane: microbially generated methane buried in sediments along the slopes of the continental shelves; and methane clathrates. Methane clathrates are crystalline structures of methane bound to water. They form at near freezing temperatures under high pressure. They are stable up to 64 F under high enough pressure. This form of methane exists along our coasts today, frozen in the sediment at low temperatures and high pressures. They are being investigated as a source of energy.
It is speculated that volcanism and tectonic disturbance released pressure that was holding the methane in clathrates or in sediments themselves. This “sudden” release of methane caused the temperature spike. (There is nothing to prevent this from happening again.)
After that temperature spike subsided, temperatures remained warm until about 34 mya. At that point the Antarctic ice sheet began to form. Temperatures continued to drop. About 2.6 mya, continental ice formed at lower latitudes and initiated the glacial epochs and interglacial periods of our current ice age. (I will write in detail about our ice age and its cosmic connection in a future blog.)
See other chapters in this series:
Throughout this series I have been using paleomap reconstructions of where continents have been. The continents are still moving. Here’s where the continents might be 50 million years from now.
Shellito, Cindy, 2006, Catastrophe and Opportunity in an Ancient Hot-House Climate, Geotimes, October 2006.
In Arizona Geological Society Digest 17:
Lucchitta, Ivo, 1989 History of the Grand Canyon and of the Colorado River in Arizona.
Lynch, D.J., 1989, Neogene volcanism in Arizona.
Menges, C. M., 1989, Late Cenozoic Tectonism in Arizona and its impact on regional landscape evolution.
Scarborough, R., 1989, Cenozoic erosion and sedimentation in Arizona.
Spencer, J.E. and Reynolds, S.J., 1989, Middle Tertiary tectonics of Arizona and adjacent areas.
Spencer, J.E., and Welty, J.W., 1989, Mid-Tertiary ore deposits in Arizona.