Study by Yale Researchers Addresses Longstanding Debate on Evolution of Mountains

Steady tectonic motions are causing the European Alps to grow at the same rate that they are eroded, a study by Yale researchers concludes.

The result is that this classic mountain belt has maintained its present size for more than 15 million years.

“In geology it has been an issue for at least 100 years whether mountains are formed during short-lived events or persist over long periods of time,” said Mark Brandon, associate professor of geology and geophysics and co-author of a study recently published in the journal Geology. “Our results indicate prolonged steady growth.”

The researchers arrived at their finding by determining the cooling ages of zircon, one of the minerals shed from the Alps in the last 15 million years. Zircon retains a record of the last time it cooled to temperatures below about 240 C and therefore tells of how mountains evolve with time.

“We can judge the rate of erosion by looking at the amount of time since cooling from 240 C until the rock reached the surface,” Brandon said. “In each sample, we date many zircon grains with each zircon providing an estimate of the longterm erosion rate in the mountain range.”

The samples came from sandstone that was shed off the Alps and deposited in nearby basins in Northern Italy. The researchers also analyzed sediment from modern rivers draining the Alps.

“What we find is that the lag time (the cooling age minus the age of erosion of the zircon grains) distribution is constant over the last 15 million years,” Brandon said. “This means that the average erosion rate in the Alps has been constant over that period of time. A steady erosion rate is considered strong evidence of a ‘steady state’ balance between uplift and erosion.”

The term “steady state” means that the average volume of the mountain range has remained the same, despite the fact that huge volumes of sediment shed were being shed from the Alps into rivers draining north and south. Thus, the balance between uplift and decay of the mountain range was maintained.

Previous studies of exhumation in the Alps have focused on reconstructing time-temperature-depth histories for currently exposed bedrock. Brandon and his colleague, John Garver of the Geology Department at Union College, Schenectady, N.Y., have spent about 10 years exploring this novel approach, which relies on using the sedimentary record to reconstruct the evolution of nearby mountains. Their approach has given new insight into how erosion and faulting work to shape and sculpt mountains.

“Exhumational steady state has been observed in some modern mountain belts, such as Taiwan, the Southern Alps of New Zealand, and northern Cascadia in the northwestern United States,” Brandon said. “Erosion dominates exhumation in those cases. The Alps appear to be an example where erosion and tectonic exhumation are operating together in an approximately steady state fashion.”

Other researchers on the project in addition to Brandon and Garver were Matthias Bernet of the Department of Geology and Geophysics; Massimiliano Zattin, of the University of Bologna, Bologna, Italy, and Joseph Vance of the Department of Geological Sciences, University of Washington, Seattle, Wash.

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