How the West was Done

Tectonic history of North America’s Pacific Rim gets even more jumbled

By Erin Wayman

Web edition: April 3, 2013
Print edition: May 4, 2013; Vol.183
#9 (p. 19)

Scientists used seismic waves to create this 3-D image of the Farallon plate extending 1,800 kilometers beneath North America (each color represents a 200-kilometer change in depth). Credit: K. Sigloch

The building of western North America wasn’t a simple construction job. Multiple sections of seafloor slid beneath the continent and each other like conveyor belts, researchers suggest, bringing islands in from different directions and pasting them to the western edge of North America in a jumble of rugged terrain.

“It’s a major change in how we view the tectonic history of North America,” says geophysicist Don Forsyth of Brown University in Providence, R.I., who wasn’t involved in the work. “It’s a mini-revolution.”

North America west of the Rocky Mountains is a patchwork of different islands and other fragments of crust that the continent accumulated over the last 200 million years. The accumulation began when North America broke free from Pangaea and started drifting west.

During this time, geologists had thought, a section of seafloor crust known as the Farallon plate dove beneath North America’s western edge and into the mantle. Like a bulldozer, the continent scooped up islands sitting on top of the subducting Farallon plate, which were too buoyant to sink into the mantle.

Three-dimensional pictures of Earth’s interior expose a more complicated geological history, say Karin Sigloch of Ludwig Maximilian University in Munich and Mitchell Mihalynuk of the British Columbia Geological Survey in Canada. The pair describes the new scenario in the April 4 Nature.

Because an earthquake’s seismic waves travel faster through remnant slabs of subducted crust than the surrounding mantle, Sigloch could use the vibrations to create images of slabs extending about 2,000 kilometers beneath North America. The pictures reveal that two cycles of subduction helped stitch together the western part of the continent.

Sigloch and Mihalynuk noticed that the shape of one slab, previously interpreted from fuzzier images as the eastern section of the Farallon plate, doesn’t match what you’d expect to find if it had simply Scientists used seismic waves to create this 3-D image of the Farallon plate extending 1,800 kilometers beneath North America (each color represents a 200-kilometer change in depth). Credit: K. Sigloch subducted beneath North America. Instead, the researchers say, this slab is the remains of two newly discovered sections of seafloor crust that once extended off the west coast of North America.

As North America drifted westward, these seafloor sections plunged beneath the Farallon plate and perhaps other plates to the west, the researchers propose. Islands began accumulating around that subduction zone. Once these seafloor slabs completely disappeared into the mantle, the Farallon plate began sliding east beneath North America. First the islands from the initial period of subduction were pasted to the continent, then islands on the Farallon plate began piling up along its edge.

Sigloch and Mihalynuk tested the scenario by reconstructing North America’s trajectory over the last 200 million years. They say the timing of geological events on the surface, such as the rise of the Canadian Rockies, matches their reconstructed timeline of when North America should have collided into islands, based on the rate at which seafloor slabs subducted.

The new research does reveal that the subduction of the Farallon plate was not as simple as geologists had previously thought, says Lijun Liu, a geophysicist at the University of Illinois in Urbana-Champaign. But he’s not ready to accept the idea that there are two previously unrecognized sections of ocean crust that were attached to North America. The problem, he says, is that the researchers’ finding is based on some overly simplified assumptions about how the seafloor subducted, which may not hold up to further scrutiny.

Only further testing, Forsyth says, will determine whether the new hypothesis better explains all of western North America’s complex geologic features than the traditional view.

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Reprinted with permission from Society for Science & the Public