There's a massive fault hidden under America's highest mountain — and we finally know how it formed

We finally know how a fault that gave rise to Denali, North America's highest mountain, first formed.

According to new research, the Denali Fault is actually an ancient suture mark where two land masses once joined together. Between 72 million and 56 million years ago, an oceanic plate called the Wrangellia Composite Terrane bumped into the western edge of North America and stuck there.

"Our understanding of lithospheric growth, or plate growth, along the western margin in North America is becoming clearer," Sean Regan, a geoscientist at the University of Alaska, Fairbanks and the lead author of a paper published in October in the journal Geology detailing the fault's history, said in a statement.

Massive fault

The Denali Fault is a strike-slip fault, a place where two chunks of continental crust slide past each other. On Nov. 3, 2002, the fault jolted, creating a magnitude 7.9 earthquake that knocked houseboats off their moorings more than 1,500 miles (2,414 kilometers) away in Seattle, according to an article in NASA's Earth Observatory blog.

Regan studied three sections of the fault: The Clearwater Mountains of southeastern Alaska, Kluane Lake in Canada's Yukon Territory, and the Coast Mountains near Juneau. These sites are hundreds of miles apart along the faultline. The sites are spread across about 620 miles (998 kilometers).

Research in the 1990s had suggested that despite this distance, these three fault sections were formed at the same time and place, only to be torn apart later as the two sides of the fault slid against one another. But no one had confirmed that finding.

Stitching of once-distant lands

In an attempt to do so, Regan studied a mineral called monazite at all three locations. This mineral, which is made of rare-Earth elements, changes as the rock hosting it is transformed or bent under pressure or high temperature, giving researchers a way to understand the rock's history.

“We showed that each of these three independent inverted metamorphic belts all formed at the same time under similar conditions," Regan said in the statement. "And all occupy a very similar structural setting. Not only are they the same age, they all behaved in a similar fashion. They decrease in age, structurally, downward."

This decrease in age is an effect of a phenomenon called inverted metamorphism, whereby rocks formed under high temperatures and pressures are found above rocks formed under lower temperatures and pressures — the opposite of the usual pattern, given that the deeper you go in the Earth's crust, the hotter and more pressurized it is. Inverted metamorphism is found in places where tectonic forces have warped the crust and pushed deeper rocks over shallower ones.

The findings reveal that these three regions formed at the same place and time. That place was the terminal suture zone between the North American plate and the Wrangell subplate, a mini tectonic plate that makes up part of the complex jigsaw of the northern Pacific coast.

"We’re starting to recognize those primary features involved in the stitching, or the suturing, of once-distant land masses to the North American plate," Regan said.