Western U.S. Created From Geologic 'Pileup' Says New Study

Scientists have long thought the land features of the Western North American continent were caused when two large tectonic plates shifted together, with geologic material pushed together in a relatively smooth manner --- as if on a moving conveyor belt --- to create the mountain ranges that stretch from Alaska to Mexico.

New research, however, suggests the landforms resulted when several plates smashed into one-another, similar to a multicar pileup.

That activity created the Cascade Mountains, the Sierra Nevada, Sierra Madre and Rocky Mountains, said a report published in the journal Nature.

The new observations suggest the continental collisions happened between 200 million years ago, during the so-called Jurassic period, and 50 million years ago, the Eocene era.

And, even though such a rethinking of geologic history has been proposed before, the theory has taken on new life, thanks to the work of Karin Sigloch, a seismologist at Munich University in Germany, whose study team used a scanning program that produces 3-D images from the energy waves produced by earthquakes.

"Now it fits together," said Sigloch. "We've come up with a pretty different solution that I think will hold up."

Sigloch looked deep into the mantle, at the remnants of oceanic crust beneath North America.

In a subduction zone, two of the planet's tectonic plates collide and one slides into the mantle. The plates are colder and denser than the surrounding mantle rock. Seismic waves change their speed as they pass through the plates, revealing their location.

But Sigloch discovered a vertical pile of crust segments, stacked on top of one another. The massive accumulation runs from northwest Canada to Central America, measures 250 to 375 miles wide and extends from about 500 to 1,200 miles beneath the surface.

Just as important, the new research provides much more accurate detail about what happened when the involved plate segments met up with the mass that eventually became the North American continent

"What this does is provide us with a time machine. The slabs are telling us the sites of past ocean trenches and the locality of island arcs, which are the building blocks of continents," said Mitch Mihalynuk, a geologist with the British Columbia Geological Survey in Canada.

Western North America "consists of scores of continental jigsaw pieces of different origins, sizes and ages," said Saskia Goes, an earth scientist at the Imperial College London, who added the region was birthed through "a slow dance of tectonic plates over the past 350 million years."