“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.
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.
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).
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.)
As the valleys dropped, erosion filled them with debris from the mountains. The glacial epochs added water. (Figure D)
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 east half of a caldera. The rest of it is buried in the Altar Valley to the west.
In either case, it remains the Tucson Mountain Chaos.
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.