The Andes' Mountainous Paradox: so tall, so young

March 1st, 2009

By DeLene Beeland

Salla Formation

Sedimentary rocks of the Salla Formation in Eastern Cordiller, Bolivia, were deposited about 29 million to 25 million years ago. Paleoelevation constraints from these rocks suggest they were deposited at less than 500 meters. They are now at about 3,600 meters. © Science

 

When asked if mountains grow slowly and steadily versus in rapid spurts, most people intuitively gravitate to the “slow and steady” model. Mountains, we are taught, take an incomprehensibly long time to build up their scads of boulders, jagged peaks and high-altitude plateaus.

In fact, most known mountain building processes do require large amounts of time to complete their skyward climb. But for every rule there is an exception. Consider the Himalaya and Andes mountains — despite their relative geologic youth, these mountain belts rank among the world’s tallest peaks. And therein lies the mountainous paradox: How do geologically young mountains grow extremely tall in extremely short time periods?

diagram of Andes

This diagram shows the land formation processes of orogeny, subduction and accretion.

 

A recent study tracking the uplift of a central portion of the massive Andes Mountains in South America shows that mountain building — what geologists term “orogeny” — may actually occur in much faster fits and spurts than previously realized due to the rapid loss of large amounts of material from the mountain’s root.

Conventional geology tells us that as the earth’s tectonic plates collide and dive beneath one another, and these actions cause t he earth’s skin to crumple and fold. Below the surface, mountains have deep roots where dense material accumulates over time. It was previously thought that a gradual erosion of this root resulted in the gradual rise of the crust: As weight was worn off the bottom, the top rose.

This theory would predict that the Andes Mountains rose gradually and in sync with the scrunching of the Nazca plate beneath the South American plate, a process that caused dense material to accumulate up to 70 kilometers deep over thousands of years below South America’s western coast. But Florida Museum of Natural History paleontologist Bruce MacFadden, a co-author of last year’s June 6 study in Science, said that this did not happen.

students crossing river

Carmala Garzione, right, lead author of the paper, and one of her graduate students prepare to cross a river in the Andes. © Photo by Bruce MacFadden

“Instead of the Altiplano rising little by little each year, we found two phases of spasmodic or punctuated uplift interspersed by millions of years of stability,” MacFadden said.

The authors assert that as the crustal layer (which floats above the mantle and is known as the lithosphere) was squeezed under deforming pressures, large parts of the accreted material plummeted downwards into the more plastic upper mantle layer (known as the athenosphere). This loosening of the root load caused the surface crust layer to rise, buoyed upward like a released cork.

“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,” said lead author Carmala Garzione, a geologist at the University of Rochester.

Geochemical clues

Carmala Garzione

Carmala Garzione displays a fossil jaw found at the Andes site. © Photo by Bruce MacFadden

The researchers found that Andes uplift began between 30 million and 20 million years ago. It then leveled off into relative stability until “a pulse of rapid uplift” occurred between 10 million and 6 million years ago, when the landscape rose between 1.5 and 3.5 kilometers. To reconstruct the Altiplano’s sequential rise, the researchers examined several lines of proxy evidence including two different types of stable isotopes, fossil plants and ancient magnetic-bearing deposits.

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. 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.

“Carmie’s ability to put this study together shows her brilliance,” MacFadden said. “She’s synthesized research in theoretical geophysics, geochemistry, and paleontology and made a strong case for the timing and consequences of the Altiplano’s rise.”

A professor of earth and environmental sciences at Lehigh University who also researches ancient elevations 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,” Dork 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.”

Reconstructing ancient topographies, climates

fossil mammal teeth

These fossil mammal teeth were used for isotopic analysis associated with notungulates from Cerdas. © Photo by Bruce MacFadden

MacFadden, who has spent nearly three decades collecting and studying fossil mammals from Bolivia, contributed by leading the research team to several key fossil sites in the Altiplano where he had previously established geological age sequences. While Garzione’s interest was grounded in the geology, MacFadden’s interest in the project lay in understanding how the Andes’ birth affected South America’s ancient climate and animals.

“The big-picture question is: When did the Andes grow high enough to become drivers of the South American climatic regime?,” MacFadden said. “Because this event obviously had cascading effects upon plant and animal life across the continent.”

Based on their findings, MacFadden said this likely happened around 10 million years ago.

Today, the massive Andes 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, stretches between the Andes’ central western foothills and the Pacific Ocean. Six hundred miles to the east, across the Bolivian bulge at the Andes’ widest point, the world’s largest collection of wetlands form the Pantanal.

“If we could rewind a video of the Andes’ formation,” MacFadden said, “we’d see how they grew into an immense force, affecting the distribution and abundance of moisture across large portions of South America.”


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.


What’s in a name? Sometimes a genus

Old photo Bruce MacFadden

© Photo by Bruce MacFadden

The Journal of Vertebrate Paleontology published a description of the genus Brucemacfaddenia in May 2008. A longtime South American colleague of MacFadden’s, paleontologist Federico Anaya of the Universidad Autónoma Tomás Frías in Bolivia, also had a genus named for him in the journal article, Federicoanaya.

Since the early 1980’s, MacFadden and Anaya have collected and studied fossils across Bolivia and in the Salla Beds, which preserved many mammal species. These deposits were also sampled for MacFadden’s most recent study on mountain building (see main story).

An independent research team headed by Ralph Hitz of Tacoma Community College named the two new genera of notoungulates — an extinct biological order of small- to medium-hoofed herbivorous mammals endemic to South America — after the two paleontologists in honor of their extensive contributions to describing and studying the evolutionary history of South America’s ancient mammals.

Hitz, who first went on a field expedition with MacFadden as a graduate student in 1994, said that it was due to MacFadden’s efforts that the “full relevance of this important fauna” came to be recognized.

fossil

The fossil is an example of the super-family of extinct ungulates that includes Brucemacfaddenia. Note, this notungulate skull is upside down so that the fossil is seen right side up. © Photo by Bruce MacFadden

“I was also impressed that he was engaged in the place of research as much as the objects of research,” Hitz said, “by which I mean he fully involved local institutions and paleontologists in the program, and also spent a considerable amount of time in La Paz teaching at the university.”

Hitz said he chose to also honor Anaya because of his perseverance and dedication to studying paleontology in Bolivia.

While new species are often named after relevant scientists as an honorary gesture, it isn’t typically a common practice at the genus level. According to Florida Museum vertebrate paleontologist Richard Hulbert, an Argentinean paleontologist started the tradition in 1901 of naming new South American ungulate genera after famous paleontologists and mammalogists — stringing together both their first name and surname. This practice has since been used to describe more than 30 new South American genera, according to Hitz.

MacFadden said he was a little embarrassed when he learned of Brucemacfaddenia boliviensis, the genus and species described in the journal, but other colleagues in his field say the gesture is fitting due to the breadth of his contributions.

“Dr. MacFadden devoted a considerable portion of his career working in Bolivia, leading very successful fossil-collecting expeditions there, studying and dating the rock layers, producing the fossils, and assisting and training Bolivian scientists,” Hulbert said. “The most productive and significant region he worked in was the Salla Beds, which produced the fossils of Brucemacfaddenia.”


Published on Science Stories: March 2009.