Nevada's Basin and Range: Down on its luck

The traditional story of the western United States reads like a geologic Horatio Alger tale. From a low birth, the landscape has pulled itself up by its bootstraps to reach its present high elevation. New studies, however, are deflating this story of the West's rising fortunes by showing that much of the landscape has actually lost elevation in the recent geologic past.

 The latest blow to the textbook account comes from an analysis of fossil leaves in Nevada's Basin and Range geologic province, which separates the Sierra Nevada from the Rocky Mountains and the Colorado Plateau. The Basin and Range region consists of high ridges and flat, sediment-filled valleys, the bottoms of which now lie at 1,000 to 1,500 meters above sea level.

 To probe the geologic history of this region, Jack A. Wolfe of the University of Arizona in Tucson and his colleagues studied fossil leaves at 12 sites in western Nevada. They analyzed each leaf for its shape, size, the number of notches around its margin, and other characteristics. Because moisture and temperature control these features, Wolfe and his colleagues could use the fossils to determine earlier climatic conditions of the Basin and Range. That climatic information was then fed into estimates of the landscape's previous altitude.

 According to the new study, reported in the June 13 Science, the Basin and Range reached some 3,000 meters above sea level 16 million years ago, then dropped to its present height by about 13 million years ago.

 Further back in time, the region apparently stood even taller, perhaps resembling the modern Andes, says Wolfe. Since then, the Nevada crust has stretched and thinned, causing the land to drop. "Presumably, in the long run, places like the Andes will show the same sort of pattern as the Basin and Range," he says.

 Brian Wernicke of the California Institute of Technology in Pasadena and his colleagues reached a similar conclusion last year in a study of the Sierra Nevada. Most mountain ranges have a thick foundation of buoyant crustal rock, which makes them float high above the denser, underlying mantle. The scientists' seismic analysis showed that the Sierra crust is only 30 to 40 kilometers deep, not nearly thick enough to explain the range's current elevation. The group inferred that the Sierra and the Basin and Range once stood taller and had a thicker crust, which has thinned in the last 20 million years.

"This is a hard-core about-face," says Wernicke. "The idea had been that this was a low-elevation area, and it had been rising until now."

 His team is seeking to test its ideas by studying the canyons in the Sierra Nevada. If the range reached much higher in the past, it would have contained major canyons cutting into the heart of the mountains and helping to cool their molten interiors. Such accelerated cooling should have left a fingerprint in the existing rocks of the mountain range.