Andes Mountains grew in rapid spurts, not slowly, UF researcher says

June 5, 2008

GAINESVILLE, Fla. — Mountain building may occur in faster fits and spurts than previously realized, according to a new study tracking the uplift of a central portion of the massive Andes Mountains in South America.

Using multiple techniques of geochemical analysis, scientists reconstructed 28 million years of the Altiplano portion of the central Andes’ ancient upward march due to movements in the earth’s tectonic plates miles below. But the peaks didn’t grow slowly and steadily, according to University of Florida researcher Bruce MacFadden, a co-author of the study published June 6 in the journal Science.

“Instead of the Altiplano rising little by little each year, like conventional theory might indicate, we found two cycles of spasmodic or punctuated uplift interspersed by millions of years of stability,” said MacFadden, a vertebrate paleontology curator at the Florida Museum of Natural History.

Geologists have long known mountains are born when continental crust shortens and thickens as one tectonic plate slides beneath another, so conventional theory held that the Altiplano rose gradually in sync with the moving of the earth’s plates. But the study reports an unexpected process. Lead author Carmala Garzione, a geologist at the University of Rochester, said the Nazca tectonic plate sliding beneath the South American continental plate caused the dense lower crust to accumulate material at deep depths.

“But when the dense material is removed rapidly — by downward dripping which is a convective process, or by another process called delamination — it caused rapid surface uplift,” Garzione said. “Our findings will force geologists to acknowledge that removal of lower lithosphere material could be an important process that causes rapid surface uplift in different mountain belts worldwide and over geologic time.”

To reconstruct the Altiplano’s sequential rise, the researchers coaxed geochemical clues in the form of oxygen isotopes from ancient soil nodules made of calcium carbonate. The nodules were sampled from layered soil deposits between 5 million and 28 million years old. Oxygen isotopes serve as reliable proxy indicators for the actual temperatures in which they formed — so the researchers used them to reconstruct ancient temperature records, and then linked these records to known temperature clines associated with vertical elevation gain. They also analyzed magma and sediment as additional proxies.

Dork Sahagian, a professor of earth and environmental sciences at Lehigh University who did not participate in this study, said that while weaknesses were inherent when single proxy methods were used, the multiple methods used in this study made the results robust.

“Remarkably, the rapid recent uplift scenario presented here is similar to what I found for the Colorado Plateau,” Sahagian said. “The greatest novelty in their study is the number of proxies they brought to bear on the problem. This is the right way to go about it.”

In 2005, Sahagian organized a national workshop to refine and strengthen paleoelevation techniques. Garzione presented the beginnings of her Altiplano work there and later, she contacted MacFadden to tap his three decades of research on stratigraphy and fossils from the Bolivian Andes. Over the past few years, MacFadden led Garzione and her team to several key fossil sites in the Altiplano where he had established geological age sequences decades ago. While Garzione’s interest was grounded in geology, MacFadden was interested in understanding how the birth of the Andes may have affected South America’s ancient animals and climate.

“The big-picture question is: When did the Andes grow high enough to become drivers of the South American climatic regime? Because this event obviously had cascading effects upon plant and animal life across the continent,” MacFadden said. “If we could rewind a video of their formation, we’d see how they grew into an immense force, affecting the distribution and abundance of moisture across large portions of South America.”

Today, the Andes’ massive mountain belt snakes 4,400 miles along the continent’s western edge and is the longest unbroken terrestrial chain on the planet, with peaks soaring to 22,841 feet. The world’s driest desert, the Atacama, lies to its west and the world’s largest collection of wetlands form the Pantanal to its east.

Additional study co-authors include: Gregory Hoke, University of Rochester; Julie Libarkin and Saunia Withers, Michigan State University; John Eiler, California Institute of Technology; Prosenjit Ghosh, Center for Atmospheric and Oceanic Science; and Andreas Mulch, Universität Hanover in Germany.