Geologists discover missing tectonic plate under Canada

What did the northwestern North American continent look like 40 to 60 million years ago? 

A team of geologists used advanced tomography and computer computation to reconstruct a map of tectonic plates from that early period, and to identify a missing plate that fits neatly between two plates known today. 

The missing plate is named Resurrection for the mountainous Resurrection Peninsula in southern Alaska, a few miles southeast of Seward. It fits between the two known ancient plates in the Pacific, Kula and Farallon, in the reconstructed map.

Geologists Spencer Fuston and Jonny Wu, from the University of Houston Department of Earth and Atmospheric Sciences, published their research in the Geological Society of America Bulletin on Oct. 19.

The area has puzzled geologists as today's geological surface features in the Northwest could have arisen in different ways. The ongoing debate is important because knowing how these plates actually shifted can help us understand ancient climate and predict future climate changes and volcanic movement.

“Volcanoes form at plate boundaries, and the more plates you have, the more volcanoes you have,” said Wu in a University of Houston news release regarding their discovery. “Volcanoes also affect climate change. So, when you are trying to model the earth and understand how climate has changed since time, you really want to know how many volcanoes there have been on earth.”

One scenario for how geology in the region evolved is that the chain of volcanic mountains on the coast of Alaska, located approximately 1,500 miles from the volcanic mountains on the coast of Washington and Oregon, were once connected, and the two chains moved apart over time.

An alternative explanation posed by the new research is that another tectonic plate once connected them, but over years moved under an adjoining plate (a process called subduction) in the Pacific Ocean.

“We believe we have direct evidence that the Resurrection plate existed," co-author Fuston said. "We are also trying to solve a debate and advocate for which side our data supports.” 

Fuston and Wu discovered the existence of the Resurrection plate by taking existing evidence from seismic waves caused by earthquakes, or explosions, in the region and creating a 3D map of the waves moving through the Earth's crust and lithosphere. 

Their technique of seismic tomography is analogous to a CT scan in medicine. Here, the different travel times and pathways of seismic waves indicate the structures in the Earth's subsurface.

The researchers selected particular slabs from the Pacific Ocean crust and upper mantle (lithosphere) to model. Next, they worked backwards from the data and "retro-deformed these slabs to their pre-subduction position at Earth's surface," the paper explains. This process is known as slab unfolding.

“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,” Wu explained.

The research paper also discusses correlations between their reconstructed Resurrection plate and observed geological data today, which they say "is unlikely to be coincidental."

Their research analyzed 10 cross-sections from southern California to the Aleutian Islands. An animation of how the Resurrection plate was subducted over time appears on this YouTube video.