extension

Arizona Geological History Chapter 7: The Cenozoic Era

Cenozoic Era, Arizona was squeezed, then stretched; steamed and frozen.

The Cenozoic era represents the most recent 65 million years. (See the geologic time chart for the subdivisions.)

Paleomap 50

 

Construction of the Rocky Mountains, volcanism, and emplacement of our major copper deposits, all of which began in Cretaceous time,  continued in the Cenozoic Era 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.

The compression produced large thrust faults which led to mountain building. The Front Range of the Rocky Mountains in Colorado has a structure similar to the diagram below.

blindthrust

 

 

By about 20 million years ago, Arizona was covered with thousands of feet of volcanic rocks, locally punctured by calderas.

The photo below (from the Arizona Geological Survey) shows erosional remnants of a volcanic ash-flow in the Chiricahua Mountains. These rocks were expelled from the Turkey Creek caldera 27 million years ago. The spire forms, called “hoodoos,” result from mass wasting by ice and water.

Hoodoo

 

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.

Initially, crustal extension was characterized by widespread normal faulting and fault-block rotation. Movement occurred along high-angle normal faults some of which may flatten at depth into low-angle detachment faults. 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.

NormalFault

Detachment fault from Arizona Geology

All of this faulting sometimes makes the life of exploration geologists very interesting when hunting for porphyry copper deposits, because some of those deposits were cut and fanned out like a deck of cards. Finding all the pieces takes some geologic detective work.

Perhaps the most famous local case of geological detective work is that of John Guilbert and David Lowell who studied the San Manuel mine north of Tucson. They noticed that the arrangement of mineralization and alteration formed shells around the generating intrusive. But the model they constructed implied that the deposit was lying on its side, and half of it was missing. It was removed by faulting. By applying their model, Lowell and Guilbert found the other half.

South of Tucson, the Mission-Pima mine and the San Xavier mine seem to be slices removed from top of the Twin Buttes deposit by low-angle faulting.. The Sierrita mine, located on the opposite side of a major high-angle fault from Twin Buttes is still intact (we think).

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 (see map below). 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 most recent volcanism was at Sunset Crater near Flagstaff. It erupted about 1,000 years ago. The San Francisco field is considered active and the most likely place in Arizona to have another eruption. The map below, from the Arizona Geological Survey shows the extensive Cenozoic volcanism.

Volcanic-AZ-young

The Grand Canyon was formed during the late Cenozoic. The Colorado Plateau initially tilted to the northeast and rivers, including the ancestral Colorado River, flowed in that direction into Utah and Colorado. Beginning about 18 million years ago, crustal stretching formed the Basin and Range province west and south of the plateau. Also around this time, plate tectonic adjustment began to tilt the Plateau toward the southwest. Sometime around 10 million years ago, plate tectonic movement began to open the Gulf of California and a river at its north end began to cut northward. At about the same time, the northeastward flowing rivers of the Colorado Plateau reached the southern escarpment of the plateau and began to flow south forming lakes along what is now the course of the Colorado River. Actual cutting of the Grand Canyon probably began about 5.5 million years ago.

Climate in the early Cenozoic continued to be hot and steamy, about 18̊F 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 = 10,000 years.) 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 15̊F 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 400 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 hypotheses 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 64̊F 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 when global temperatures began to drop. Antarctica had separated itself from Africa, Australia, and South America which caused the southern circumpolar ocean current to be established which isolated Antarctica from warm tropical waters. Global 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.

References:

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.

Pearthree, P., House, K., (now with USGS), and Perkins, M., Stratigraphic evidence for the role of lake spillover in the inception of the lower Colorado River in southern Nevada and western Arizona, Geological Society of America Special Paper 439

Scarborough, R., 1989, Cenozoic erosion and sedimentation in Arizona.

 

Tucson Mountains Chaos

“Tucson Mountain Chaos is a formal geologic name, describing one of the more confusing, complex, and controversial areas in southern Arizona.” So says the newsletter of the Arizona Geological Society.

If you drive over Gates Pass and look closely at the road cuts, you will see a jumble of various-colored rocks. Within the beds of volcanic ash are big chunks of other volcanics, limestones, granites, and schists. The mountain range appears to be composed of a mega-breccia.

The origin of the Tucson Mountains is still subject to geologic debate. The following is what I think is the most probable chain of events. Like many stories in the very complex structural geology of the Western U.S., even the probable may seem fantastic.

[NOTE:  New evidence obtained since this article was written shows that the caldera did not form over the Catalina Mountains as postulated below. See:

https://wryheat.wordpress.com/2016/08/30/tucson-mountains-geology-an-update/

for updated information.]

 

Our story begins during the Laramide Orogeny, when the Rocky Mountains were being built about 70 million years ago. The North American continent was speeding westward at 2 inches a year and it was crashing into oceanic crust under the Pacific Ocean. The heavier oceanic crustal rocks dove under (were subducted beneath) the lighter continental crust. This caused compression, mountain building, and volcanism.

As subduction of the ocean crust continued, it reached a depth that was hot enough to melt it. Great blobs of magma rose like balloons through the continental crust. Some of these blobs became the copper deposits we have in Arizona, others reached the surface and became volcanoes.

One such volcano was formed where the Catalina Mountains are now, east of Tucson. It was a large volcano that erupted in violent explosions which eventually caused collapse of the volcanic edifice to form a caldera about 10 miles across.

Portions of the wall rocks fell into the caldera. This probably accounts for the chaotic mixture of rocks in the Tucson Mountains. But that’s just half of the story.

If the volcano was east of where Tucson is now, how did the rocks wind up to the west of the city?

The North American continent was still moving westward. Sometime between 40- and 20 million years ago it overrode a spreading center called the East Pacific Rise. This area was 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.

The compression that built the Rocky Mountains was turned into extension. The western part of the American continent began to be torn apart.

By about 30 million years ago, crustal stretching and heating from below, arched-up the area beneath the “Tucson Mountains” volcano. It may have looked something like Figure A.

tm-01

About 25 million years ago, stretching caused low-angle faulting to detach much of the volcanic edifice from underlying rocks. The volcano and its caldera began to slide to the west. (Figure B).

tm-021-300x110

By the way, detachment faults crop out along the western base of the Catalina Mountains, amid all that nice expensive foothills property.  Sometime between 12- and 6 million years ago, the on-going crustal stretching reached a limit and things started to break. High-angle faults formed. This produced the Basin and Range topography we have now. (Figure C.)

tm-03

As the valleys dropped, erosion filled them with debris from the mountains. The glacial epochs added water. (Figure D)

tm-04

 

The Tucson Mountains represent just one example of the consequences of crustal stretching. Picacho Peak has a similar history. Near Green Valley, the old Twin Buttes mine, the Mission mine, and the Eisenhower pit are a few pieces of what once was one deposit that got sliced up and fanned out like a deck of cards.

Many geologists disagree that the Tucson Mountain volcano formed over the Catalina Mountains, rather, they think the volcano is where the rocks now sit. The main evidence they cite is that the chemistry of the volcanics in the Tucson Mountains is incompatible with the proposed generating pluton in the Catalina Mountains. They also cite structural inconsistencies. Notice in the last cross-section the Tucson Mountain volcanics dip to the east (left) as would be consistent with the story above. However, other rocks in the Tucson Mountains dip to the west and the volcanics of Tumamoc Hill are horizontal. That may negate the detachment theory. If this latter hypothesis is correct, then the volcanics of the Tucson Mountains represent the west half of a caldera. The rest of it is buried in the Tucson Valley to the east.

In either case, it remains the Tucson Mountain Chaos.

References:

Lipman, Peter, 1993, Geologic map of the Tucson Mountains Caldera, southern Arizona, U.S.G.S. IMAP 2205.

Scarborough, Robert, The Geologic Origin of the Sonoran Desert, Arizona-Sonora Desert Museum, http://www.desertmuseum.org/books/nhsd_geologic_origin.php

The graphics used in this article came from this paper.