By Kimberly N.
In order to explore of the geology of our slice of the Rincon Mountains, we need to widen the lens a little and look at the regional geology of Tucson and the greater southwest. It can help with understanding the bigger picture of our Sonoran Desert, and its picturesque scenery.
It’s difficult for many of us to conceptualize geologic time. Human lives are generally limited to approximately a century, if we’re lucky. Thinking in terms of millions and billions of years can be mind boggling. Rocks are dated through radiometric dating methods that use the natural radioactive decay of certain elements such as potassium and carbon. The results have concluded that we are walking over rocks that are over a billion years old.
Our area has rocks as old as 1.7 billion years old and as young as a few thousand years. All phases of the rock cycle are represented, including igneous, sedimentary, and metamorphic rocks. Our rocks were originally derived from erosional processes like hot magmatic intrusions or deep and shallow marine environments. I will be referencing geologic time, so I encourage readers to take time to review the geologic time scale. This can be found online, under a basic keyword search.
Original Rock in Arizona
Arizona’s original crust is composed of the Pinal Schist. At the beginning of the Paleozoic Era, several millions of years of erosion occurred as this crust gently rose and fell. This led to the encroachment of seas into the area, which transgressed and regressed over time, resulting in layers of limestone we see in the Rincon Mountains today.
By the late Mesozoic Era, the Pacific Plate continued to descend, or subduct, under the North American Plate, creating extensive volcanic activity. This event, known as the Laramide Orogeny, resulted in mountain building processes in this area. These volcanic rocks form a majority of the Tucson Mountains to the west. During and following this volcanic period, masses of granite were emplaced 5 to 8 miles below the surface, where they slowly cooled.
In approximately 20-30 MYA, the Earth’s Crust in this region was stretched and sheared from the northeast towards the southwest. This period of stretching designates what geologists call the Basin and Range Geologic Province, causing the land, near and far, to stretch like geo-taffy. Over time, the stretching caused a series of other geologic events, including geologic arching of the crust.
Detachment and Block Faulting
As arching and stretching continued in the area, a huge slab of rock broke loose and slid to the northwest along a detachment fault. This movement took place over thousands of years, shattering the rocks along the fault zone. Above the fault, forming the upper plate, are Paleozoic limestones and the rocks forming the lower plate are highly altered and stretched granites that form a type of metamorphic rock called mylonite. These mylonites form the impressive Tanque Verde Ridge in the Rincon Mountains. Following the detachment of the upper plate rocks, their stresses finally relaxed. By about 8 million years ago, the pull-apart action stopped, the thinned crust cooled, and Basin and Range mountains and valleys stabilized.
Fun Fact! At one time, before detachment of the upper plate rocks, the Tucson Mountains were located further east of the present Rincon Mountains. The detachment is responsible for their current location.
Basins Are Created And Filled
The extension of this area created block faulting, where many blocks separated along steep, normal faults. This creating the basins that surround the Rincon Mountains and other similar ranges in the southwest today. At one time, valley floors may have been as much as eight to ten thousand feet below the mountain crests. Today relief is much reduced as alluvial (stream) deposits of gravel, sand, and mud have filled the basins to their present-day levels, providing the geologic containers for our desert aquifers.
Basin and Range country is unique. No other region of similar origin is identified on the planet thanks to the geologic events that shaped the unique Sonoran landscape. We will explore more about the geology and it’s effects in this space in the future! If you have any of your own questions, feel free to send a line.
Our friends and scientists at the Sonoran Desert Museum, U.S. Department of the Interior/Saguaro National Park, with a special mention of Dr. George H. Davis and Roadside Geology of Arizona by Halka Chronic, have provided helpful reconstructions of our local geology. References are provided below.
Arizona-Sonora Desert Museum, The Geologic Origin of the Sonoran Desert, 2020
Chronic, Halka. Roadside Geology of Arizona. Missoula, Montana: Mountain Press Publishing Company, 1983.
McPhee, John. Basin and Range. New York: Noonday Press, 1990.
U.S. Department of the Interior, National Park Service, Saguaro National Park, Dr. George H. Davis, Geology of the Rincon Mountain