Colorado Plateau uplift related to warming of the North American plate since 40 million years ago
It’s been the subject of a long-standing geological debate. Some of the biggest names in American geology, Clarence Dutton, Charles Hunt, and John Wesley Powell, all wondered what forces were/are responsible for the uplift of the Colorado Plateau, a broad region of average elevation of 2 km centered in the Four Corners area of Arizona, Colorado, New Mexico and Utah.
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Diagrams illustrate Cenozoic magmatic patterns in the western United States, showing magmatic encroachment onto the Colorado Plateau. |
Shallow-marine and coastal rocks of that are 65 million years old drape the Colorado Plateau (for example, the Mancos Shale in New Mexico) so we know that the region was near sea level at that time. Today, these same rocks are uplifted by an average of 2.2 km above sea level.
Researchers at the University of New Mexico, University of Southern California and Utah State University, feel they have deciphered the mystery. Their work appears in a paper titled, “Colorado Plateau magmatism and uplift by warming of heterogeneous lithosphere,” published in the June 18 2009 issue of Nature magazine.
Researchers Mousumi Roy, UNM Earth and Planetary Sciences Department, Thomas Jordan, USC Department of Earth Sciences and Joel Pederson, USU Department of Geology, say the primary mechanism driving rock uplift of the plateau is the warming of the thicker, more iron-depleted Colorado Plateau lithosphere over a 35-40 million year period following the mid-Cenozoic removal of the Farallon plate from beneath North America.
“Basically, there are three main processes that could have uplifted the rocks of the Colorado Plateau,” said Roy. “Sixty-five million years ago during the Laramide event the plate was being shortened or squished together by compression, which can lead to uplift of rocks. But there’s not a lot of evidence that the Colorado Plateau underwent a great deal of shortening in this time. It could be that denser parts of the North American plate were removed just as dense blobs sink in a lava lamp. However, xenoliths, which are fragments of mantle rock found in volcanoes, show evidence the plate in the Colorado Plateau was intact at least three million years ago.
“Our mechanism for driving the uplift is the heating of rock, which causes expansion and a decrease in density. Because the North American plate floats on the flowing mantle below it, when the plate expands it floats higher and rises relative to its initial position.”
Roy and her colleagues’ propose that a thermal perturbation or disturbance associated with the removal of a subducting plate, the Farallon plate, from beneath North America about 40 million years ago caused warming and a subsequent re-equilibration which drove rock uplift in within the western U.S. The uplift of the Colorado Plateau region is enhanced by the fact that the plate is thicker in this region and so it protrudes into the flowing mantle below. As a result, the protruding region is heated laterally from the sides and from below.
“There’s pretty good evidence for this lateral warming of the Colorado Plateau region because Cenozoic magmatism (volcanic activity) encroaches onto the plateau, moving inward at rates of 4-6 km per million years, consistent with a process of slow warming” Roy explained.
“This suggests that the removal of the Farallon plate from beneath North America around 40 million years ago, is responsible for the uplift of the rocks of the Colorado Plateau. The removal of the plate is also accompanied by a huge pulse of volcanism, the “ignimbrite flare-up” that includes the Mogollon-Datil and the San Juan volcanic fields,” said Roy.
The research, funded by the National Science Foundation, is of particularly high interest. It helps to explain why rocks of the Colorado Plateau are above sea-level, which has been a longstanding problem among geologists.
“This conundrum has stirred the imagination of geologists for a long time,” said Roy. “Whatever raised the rock uplift didn’t cause the crust to get severely disrupted in the process. The dramatic landscape of the Colorado Plateau is a combination of this regional thermally-driven uplift that we propose began about 40 million years ago and continued to the present, and erosion of the mostly flat-lying strata.”
To view the paper visit: Nature. For more information on Roy’s research and her lab visit: Mousumi Roy.
Media Contact: Steve Carr, (505) 277-1821; e-mail: scarr@unm.edu
Posted by scarr at June 17, 2009 03:57 PM