Giant slabs of tectonic plate suspended far beneath North America reveal that the continent took a hand in feeding its own growth over the past couple of hundred million years.
With the advent of plate tectonics in the 1970s, geologists realized that slices of odd-looking rock from southern Alaska to western Mexico had been stuck on to the western edge of North America, enlarging it and pushing up mountains. Many geoscientists had assumed that great sheets of ocean tectonic plate like the ones underlying the Pacific Ocean today had carried scraps of crust—like the volcanic islands of today's Southwest Pacific or even Japan—to North America and stuck them to the continent as the plates dove into the interior.
Those tectonic plates are still down there, suspended in the 2900 kilometers of rocky mantle. They can provide a record of 100 million years or more showing when and where those plates had "subducted" into the mantle.
So seismologist Karin Sigloch of Ludwig Maximilians University in Munich, Germany, and geologist Mitchell Mihalynuk of the British Columbia Geological Survey in Victoria, Canada, created the sharpest picture yet of subducted plates near North America, called slabs, hanging in the mantle. They drew on recordings of seismic waves that had passed through the deep mantle on their way from earthquakes to seismometers, including instruments in the dense network recently deployed across the continental United States. They then formed a 3D image of the slabs using the latest techniques, the way doctors create a CT x-ray image of the human body.
What the pair found was surprising. By filling in more of the deep mantle picture, they could see that a great "wall" of slab is dangling just off the coast of California. That doesn't jibe with the conventional picture of how plate tectonics worked back then, says seismologist Eugene Humphreys of the University of Oregon in Eugene. Because this slab is vertical, the deep-sea trench it subducted into and thus the spot where subduction-induced eruptions created new crust must have been stationary while subduction occurred. But that spot is now just off the coast of North America, which has been moving westward for more than 150 million years. So, much subduction and volcanic creation of crust must have happened far out in midocean, not near the coast, the team reports online today in Nature.
Instead of the so-called Farallon Plate—a bit of which is still subducting under the Pacific Northwest—carrying all those bits and pieces to North America, Sigloch and Mihalynuk suggest, the continent overran them and scarfed them up where they formed. "It's really quite neat," Humphreys says. And the new perspective might also explain the dearth of volcanic activity along much of western North America the past 80 million years or so, he says. There just wasn't much eruption-inducing subduction going on at the coast.