Sierra Nevadas Formed in a “Geologic Instant” – Geologists Raise the Speed Limit for How Fast Continental Crust Can Form

The study suggests that parts of the Sierra Nevadas formed in a “geological instant”, more than twice as fast as previously thought.

Although we can’t see it in action, the earth is constantly producing new land. This happens in subduction zones, in which tectonic plates press against each other and plow up volcanic chains through which magma can rise. Some of this magma does not spit out, but mixes and transforms just below the surface. It then crystallizes out as a new continental crust in the form of a mountain range.

Scientists have thought that this process formed the Earth’s mountain ranges over many millions of years. But WITH Geologists have now found that the planet can create new land much faster than previously thought.

In an article published in the magazine geologyThe team shows that parts of California’s Sierra Nevada rose surprisingly quickly over a period of just 1.39 million years – more than twice as fast as expected for the region. The researchers attribute the rapid land formation to a massive flare-up of magma.

“The really exciting thing about our results is that with the new high-precision geochronology, we were able to determine how fast this crust formation process was, and we showed that this large volume of new crust was being stored extremely quickly.” says the study’s lead author, Benjamin Klein PhD ’19, who did the research as a PhD student at the MIT Institute for Earth, Atmospheric and Planetary Sciences (EAPS). “It was kind of a moment. It was a little over a million years ago, but in geological times it was super fast. ”

Klein’s co-authors are Associate Professor Oliver Jagoutz and Research Scientist Jahandar Ramezani, both at EAPS.

A complete cross-section

The Sierra Nevada is a product of the collision of two tectonic plates: the North American Plate, moving westward, and the then Farallon Plate, which slowly landed under the North American Plate and eventually slipped completely into the Earth’s mantle.

About 100 million years ago when the two plates collided, they created first a volcanic chain, then a towering mountain range that is now the Sierra Nevada.

“What is the west coast of the United States today probably looked like the Andes today, with high peaks and a chain of large volcanoes,” says Klein.

For their study, the researchers focused on a geological feature in the Sierra Nevada known as the intrusive suite – a large volume of rock that originally formed deep inside the earth. Once crystallized, the rocks form a new vertical column of continental crust.

In particular, they focused on the Bear Valley Intrusive Suite, a unique formation that represents the remnants of a new continental crust that is now exposed at the surface as 40 mile granite stretch. These rocks, which now lie horizontally along the mountain range, were originally designed as vertical columns. Over time, this new continental crust tower eroded, expanded, and toppled into its current horizontal configuration.

“The Bear Valley Intrusive Suite gives us a complete cross-section of what those magma plumbing systems looked like under large volcanoes that we would normally only have a limited snapshot of,” says Klein. “This allows us to think much more carefully about how quickly a new crust was built.”

A speed limit for new crust

The team collected rock samples from a region of the Sierra Nevada Batholith and brought them back to MIT to analyze their composition. They were able to determine the age of nine samples using uranium-lead geochronology, a high-precision dating technique developed by the late MIT professor Emeritus Sam Bowring. From each sample, the researchers isolated individual grains of zircon, a mineral found in rocks that contains uranium and some lead. Scientists can measure the ratio to get an estimate of the rock age.

In their analyzes, Klein and his colleagues found that the ages of all nine samples spanned a surprisingly short range of just 1.39 million years. The team calculated an estimate of the amount of magma that must be crystallized to form the new crust that the samples represent. They found that roughly 250 cubic kilometers of magma likely rose from the Earth’s interior and turned into a new crust in just 1.39 million years.

“That’s about two and a half times faster than previous estimates of crust formation in the sierras, which is quite a big difference,” says Klein. “It gives us a maximum speed limit for how fast these things can actually happen.”

Klein says that given the rate at which this new crust formation was taking place, the likely cause was a magma flare-up or a sudden burst of magmatic activity.

“The entire batholith was built in almost 200 million years, but we know there were periods during that time when it was very active and quieter times when less new material was added,” says Klein. “What we could show in this area was that, at least locally, the rate at which magma was introduced is much faster than the average rates documented in the sierras.”

Geologists have assumed that magma bursts occur as a result of unusual activity on Earth, such as when tectonic plates suddenly collide faster. However, from what researchers have documented about the Bear Valley Intrusive Suite, no such activity was occurring at the time the mountain range was formed.

“There’s no obvious trigger,” says Klein. “The system is going pretty well and then we see this big magma eruption. So this calls into question some basic notions in the field and should inform how people think about how quickly these things could happen today in places like the Andes or the volcanoes in Japan. ”

Reference: “Highly Accurate Geochronology Requires Ultra-Fast Magmatic Flux From The Mantle Build The Transcrustal Bear Valley Intrusive Suite In Sierra Nevada, California, USA” by Benjamin Klein, Oliver Jagoutz, and Jahandar Ramezani, September 18, 2020, geology.
DOI: 10.1130 / G47952.1

This research was supported in part by the National Science Foundation.