The western edge of North America has long puzzled geologists from around the world. Some of the most basic questions remain to be resolved: what caused the uplift of the Rocky Mountains? How did the Colorado plateau, a vast region of relatively undeformed rock, escape extensive folding and faulting? Further west, the Great Basin is a vast region where the land between the Sierra Nevada in California and the Colorado Plateau has been stretched to horizontally over twice its original extent. Why did this stretching occur?
Although it will take years of work to answer these questions, a recent study has cast some light on the relationship between plate tectonics, the Sierra Nevada, and the Basin and Range.
The geologic history as we understand it is complex, but the basic idea is simple, and revolves around a geologic plate that no longer exists on Earth’s surface – the Farallon Plate. Twenty million years ago, this oceanic plate was being subducted beneath the North American continent.
South America is currently experiencing this type of tectonic environment. There, the subduction of the Pacific Plate beneath the South American plate has resulted in the uplift of a vast plateau. The Altiplano, as the plateau is called, runs parallel to the coast and is, on average, 500 km wide and 3,800 meters high.
The American Altiplano
Now, scientists believe they have evidence that a similar plateau once existed in what is now Nevada. The University of Idaho team that announced the discovery used measurements of hydrogen isotopes to reconstruct the elevation of rocks now located in the Basin and Range region.
It may seem unusual that hydrogen can tell us anything about elevation. Here’s how it works:
20 million years ago, as moisture moved in from the west, it would have been forced upward over the high elevations of the Sierra Nevada and the highlands of Nevada. As the moisture rose, it cooled, resulting in rainfall in the mountains. The tiny difference in weight between water molecules containing heavy hydrogen isotopes and those containing light isotopes means that the heavy molecules come out of the clouds at a lower elevation.
The team was lucky to identify a large volcanic ash deposit that contained some of this ancient precipitation. By measuring the isotopes of hydrogen across the geographic range of the deposit, they were able to reconstruct the ancient topography (learn more about their methods here).
The results are startling: not only did Nevada contain a high plateau – in addition, with a maximum height of 3,500 meters, that plateau was much higher than any highland region in North America today.
How could such a large plateau have completely disappeared?
About 20 million years ago, the Farallon plate was completely subducted beneath North America. The resulting changes in the tectonic environment led to the formation of the San Andreas Fault and caused the collapse of the “Nevadaplano”.