The Tucson Mountains form the rampart on the west side of the city. If you drive over Gates Pass, take a look at the road cuts, especially near the top, you will see a chaotic jumble of different rocks, mainly volcanics. According to the Arizona Geological Survey: “Tucson Mountain Chaos is a formal geologic name, describing one of the more confusing, complex, and controversial areas in southern Arizona.”
Like many mountain ranges in Southern Arizona, the Tucson Mountains have experienced several episodes of volcanic eruption. Major eruptions occurred during Triassic-Jurassic time (~190-200 Ma), early Laramide (74 Ma), later Laramide (62 Ma), and one late Tertiary (~20 Ma). (Ma means million years ago.) There were also several interspersed minor eruptions.
The early Laramide (74 Ma) eruption was very explosive and produced great volumes of rhyolite tuff (Cat Mountain tuff). The rapid eruption caused the volcano to collapse in on itself to form a caldera. That collapse produced megabreccia called the Tucson Mountain Chaos. (Breccia is simply a bunch of angular fragments cemented together.) Within that breccia are small to very large fragments of other rocks including house-sized blocks of limestone. The breccia could have formed in three ways (and there are proponents of each way): moat in-filling of the caldera, landslides, or fluidized material brought up from below.
The Tucson Mountain caldera is not a typical caldera with equal subsidence all around. Rather, it is a “trap door” caldera with the “hinge” area on the southeast and major subsidence on the west. The western ring fault (called the Museum Fault) parallels Kinney Road from about Old Tucson to just past the Arizona-Sonora Desert Museum, then swings east around a granite pluton. This structure was first proposed by Peter Lipman of the United States Geological Survey in 1994 and later supported by geophysical investigations.
Beginning about 25 to 30 million years ago, Arizona and the West experienced crustal stretching which began to tear things apart. It was proposed, about 10 years ago, that the Tucson Mountain volcano and caldera formed over where the Santa Catalina Mountains now stand on the east side of Tucson. It was posited that crustal stretching slid the caldera to its present location. (You can see an explanation and cross-sections of that story in a 2009 article from my Wryheat blog.) That was such a neat story that the Arizona-Sonora Desert Museum constructed a mechanical model which Docents (including me) used to interpret the story. That particular “kit” has been retired because subsequent evidence shows that the story is probably in error. The main evidence against the sliding story is that the chemistry of the volcanics in the Tucson Mountains is incompatible with the proposed generating pluton in the Santa Catalina Mountains. There are also some structural inconsistencies.
One other thing: there was a Tucson Mountain dinosaur. Dinosaur bones were found within one of the blocks of megabreccia about 1800 feet NNW of Gates Pass. This dinosaur is classified as a large Hadrosaur (duck-billed dinosaur). This dinosaur lived in Tucson some time between 72 and 83 million years ago.
Kring, D.A., 2002, Desert Heat – Volcanic Fire, The Geologic History of the Tucson Mountains and Southern Arizona, Arizona Geological Society Digest 21
Lipman, Peter, 1993, Geologic map of the Tucson Mountains Caldera, southern Arizona, U.S.G.S. IMAP 2205. (link)
Lipman Peter W., 1994: Tucson Mountains caldera; a Cretaceous ash-flow caldera in southern Arizona. U S (link)
Marshall, L. and Stokes, P., 2012, the Tucson Mountains Caldera: Using Gravity and Magnetic Anomalies to Test Trapdoor Subsidence and Locate Subsurface Plutonic Bodies. (link)
Spencer, G.L. et al., 2005, The late Cretaceous Tucson Mountains dinosaur, New Mexico Museum of Natural History and Science Bulletin, 29 (link)
Arizona Geologic History: Chapter 1, Precambrian Time When Arizona was at the South Pole
Arizona Geological History: Chapter 2, Cambrian and Ordovician Time
Arizona Geological History: Chapter 3: Devonian to Permian Time
Arizona Geological History Chapter 4: Triassic Period
Arizona Geological History Chapter 5: Jurassic Time
Arizona Geological History Chapter 6, The Cretaceous Period
Arizona Geological History Chapter 7: The Cenozoic Era