The face of the Earth has changed drastically over its life, with plates shifting and sinking. Now, geologists at the University of Houston claim to have found the remains of an ancient tectonic plate beneath Canada that was pushed under the surface tens of millions of years ago.
It’s long been known that in the early Cenozoic Era – around 60 million years ago – there were two major tectonic plates, called Kula and Farallon, in the Pacific Ocean off the western coast of North America. But debate has raged about whether they were joined by a third, oddly named Resurrection, which would have since sunk beneath the surface. And now, the geologists on the new study say they’ve found this missing plate.
When plates push against each other, one of them will often end up sliding underneath the other, sinking down into the mantle – a process called subduction. Over time, the intense heat and pressure there will melt it and change its shape. To try to find the remains of the Resurrection plate, the team started by studying existing tomography images of the mantle below North America.
Next, they applied a technique called slab unfolding, which can identify subducted plates, then unfold and stretch them to roughly figure out their original shapes. Using this method, the team mapped out several large chunks of rock, and worked backwards to identify the original plates they came from.
Two of the objects they studied are already known – the Alaska and Cascadia slabs, which are still attached to each other. The Alaska slab sits beneath the Aleutian Islands and is thought to be the remains of the Kula plate, while the Cascadia slab lies under southern California and represents the remains of Farallon.
But intriguingly, the team identified a third slab detached from the others, about 400 to 600 km (250 to 370 miles) below the surface of northern Canada. They call it the Yukon Slab, and when the clock is wound back it appears to fit the calculated shape of the old Resurrection plate.
“When ‘raised’ back to the earth’s surface and reconstructed, the boundaries of this ancient Resurrection tectonic plate match well with the ancient volcanic belts in Washington State and Alaska, providing a much sought after link between the ancient Pacific Ocean and the North American geologic record,” says Jonny Wu, co-author of the study.
The team’s model suggests that the Yukon slab gradually made its way northeast after the Resurrection plate was first subducted some 40 million years ago.
“We believe we have direct evidence that the Resurrection plate existed,” says Spencer Fuston, co-author of the study. “We are also trying to solve a debate and advocate for which side our data supports.”
The research was published in the Geological Society of America Bulletin. The movements of the ancient plates can be seen in the animation below.
Source: University of Houston