The final step in forming the dramatic Santa Catalina and Rincon mountain ranges occurred as recently as six million years ago.
The finding puts a new spin on scientists' understanding of similar mountain ranges in the western United States.
A team of University of Arizona geoscientists report that movement along faults bordering what are now mountains let huge blocks of rock spring up about 9,000 feet into the air, creating Tucson's impressive skyline. Geologists had previously thought the major activity that formed the Santa Catalina and Rincon mountains had occurred 20 million years ago or earlier.
"Now other geoscientists in the western United States may look even harder at the relationships between the most recent faulting and what has existed before," said George H. Davis, Regents' Professor of Geosciences and Provost of the University of Arizona in Tucson. "The magnitude and the influence of the basin-and-range faulting will be a surprise to some people."
Davis is the lead author of the research study appearing in the current issue of the Geological Society of America Bulletin. The other authors are Kurt Constenius, a former UA graduate student who is now a geological consultant in Tucson, William R. Dickinson, UA emeritus professor of geosciences, and former UA graduate students Edna P. Rodriquez and Leslie J. Cox.
Southern Arizona is part of what geologists call the "Basin-and-Range province," an area extending from southern Oregon to western Texas. The region was created when, about 15 million years ago, the Earth's crust was stretched, breaking into hundreds of blocks that form mountain ranges that run north-south.
In the 1970s, the Santa Catalinas and Rincons, like 40 or 50 other mountains in the basin-and-range region, were considered "strange," because they didn't fit into the categories of mountains that geologists already knew about, Davis said. He and other UA structural geologists began figuring out how the Catalinas, and ultimately other such mountain ranges, came to be.
By 1993, Davis and his colleagues knew that 20 to 25 million years ago, movement along faults running under the Tucson valley floor had shifted rocks from about 7 miles under the earth to the present site of the Rincon and Santa Catalina mountains. But those ancient movements would not have lifted the rocks high enough to create the tall mountain ranges that bracket Tucson today.
So the researchers focused on more recent faults, including the Pirate Fault along the western side of the Catalinas and the Martinez Ranch Fault on the eastern side of the Rincons.
Much of the work used time-honored geological techniques: the scientists and their students went into the field and looked at the rocks. Dickinson said, "I walked along the Pirate Fault from the northern end to the southern end -- crashing through the bushes and the cactus -- to map the fault." Dickinson did the same with the Martinez Ranch Fault.
Once the scientists had gathered and examined all the data, they realized that rocks at those faults had moved much more than previously thought.
Between 12 and 6 million years ago, the western portion of the Catalinas popped up "almost like opening a trap door" at the Pirate Fault, said Davis. The raised edge of the trap door is now known as Mount Lemmon, at 9,157 feet the highest point in the Catalina range. The same thing happened at the Martinez Ranch Fault, with the leading edge of another trap door becoming the highest point in the Rincons, the 8,664-foot Mica Mountain. The rocks in Reddington Pass, the low spot between the two ranges, stayed in place, like the hinged side of a door.
"It closes a big chapter in trying to describe the step-by-step-by-step movements that created the fundamental geometry and landscape we see in the Catalinas and the Rincons," Davis said.
The finding also will help explain the evolution of other U.S. core-complex mountains, including the Bitterroot mountains in Montana and the South Snake Range in Great Basin National Park in Nevada.
To further his understanding of such mountain-building, Davis will go to Greece this summer to study similar mountains at a much earlier stage of development.
"A modern example of all this is the Aegean region in Greece," he said. "The faulting in the Aegean a million years ago is a dead ringer for what happened in the Catalinas 20 million years ago."
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