Ancient Tectonic Plate Under California Prompts Re-Thinking Of American Tectonic History

An ancient tectonic plate along the American west coast long thought to have been subducted beneath North America is causing scientists to re-examine the entire tectonic history of western North America.

Researchers from Brown University have determined that large chunks of the ancient Farallon oceanic plate, which is believed to have sunk deep into the Earth's mantle around 100 million years ago, are still present under parts of California and Mexico.

The old thinking goes that the Farallon plate, which laid between the converging Pacific and North American tectonic plates, was forced beneath the two converging plates as they came together to form the San Andreas fault, leaving only a few small remnants at the surface that stopped subducting and became part of the Pacific plate.

But new research suggests that large slabs of the Farallon plate remain attached to those unsubducted fragments. The geophysical research shows that part of the Baja region of Mexico and part of Central California near the Sierra Nevada mountains sit atop "fossil" slab of the Farallon plate.

"Many had assumed that these pieces would have broken off quite close to the surface," said Brown geophysicist and research leader Donald Forsyth, in a statement. "We're suggesting that they actually broke off fairly deep, leaving these large slabs behind."

The research is published in the Proceedings of the National Academy of Sciences.

The findings could force scientists to re-examine the tectonic history of western North America, Forsyth said.

Study co-author Brian Savage at the University of Rhode Island agrees.

"This work has radically changed our understanding of the makeup of the west coast of North America," Savage said. "It will cause a thorough rethinking of the geological history of North America and undoubtedly many other continental margins."