Large chunks of a tectonic plate, thought to have sunk into the Earth's mantle about 100 million years ago, have been found to exist under parts of California and Mexico.
It was previously believed that the tectonic plate, called the Farallon plate, was forced under the converging Pacific and North American plates. The convergence resulted in the formation of the San Andreas fault, which left just a few remnants that did not subduct and became part of the Pacific plate.
Researchers from Brown University, however, have determined that large slabs of the Farallon plate are still connected to the unsubducted fragments. According to new research, the Farallon plate slab is located beneath the Baja region of Mexico and part of Central California near the Sierra Nevada mountains.
"Many had assumed that these pieces would have broken off quite close to the surface," Brown geophysicist and research leader Donald Forsyth said in a statement. "We're suggesting that they actually broke off fairly deep, leaving these large slabs behind."
The researchers discovered the remnants of the plate by mapping out the area underneath Earth's surface using seismic waves recorded from earthquakes or created by dynamic charges and other methods. They found a large amount of cool, dry material between 62 and 124 miles below the surface which they termed the "Isabella anomaly."
Later, they discovered a similar mass beneath the Baja Peninsula east of portions of the Farallon plate. This led them to believe that the two anomalies might be part of the same plate. Volcanic rock deposits were also found toward the eastern side of the mass. Such deposits are often concurrent with the melting of oceanic crust. This lends credence to the theory that the Farallon plate broke off at that location.
The discovery could hold implications for the understanding of Earth's formation, especially North America's west coast.
"This work has radically changed our understanding of the make-up of the west coast of North America," says co-author of the study Brian Savage of the University of Rhode Island. "It will cause a thorough rethinking of the geological history of North America and undoubtedly many other continental margins."
The study was published on Monday March 18 in the journal Proceedings of the National Academy of Sciences.
