Fragments of 'Lost' Land Mass Identified Under California, Northern Mexico

Remains of an ancient tectonic plate that scientists say was pushed under the North American continent millions of years ago have been identified under central California and Northern Mexico.

New research led by Brown University geophysicists suggests the so-called Isabella anomaly - evidence of a large mass of cool, dehydrated material lying approximately 100 kilometers, or, 62 miles, beneath central California - is a surviving chunk of the Farallon oceanic plate, which researchers believe was driven deep into the Earth's mantle when the Pacific and North American plates started converging about 100 million years ago, eventually meeting to form the San Andreas fault.

Off the west coast of North America, the Farallon plate fragmented, leaving a few small chunks that later became part of the Pacific plate instead be being thrust below.

The new findings suggest other Farallon slabs remained attached to the fragments that remained at crust level. Researchers discovered areas in Mexico's Baja region and central California sit on top of older "fossil" pieces --- which they now say were also part of the Farallon plate.

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

The study found that all of the abnormal slabs are most evident at around the same depth -100 kilometers (62 miles). And all of them line up almost directly east of other known Farallon fragments.

"The geometry was the kicker," Forsyth said. "The way they line up just makes sense."

The findings could force scientists to re-examine the tectonic history of western North America, Forsyth said. In particular, it forces a rethinking of the physics behind the formation of the Sierra Nevada mountains, which had been used to explain the Isabella anomaly.

Said study co-author Brian Savage of the University of Rhode Island: "This work has radically changed our understanding of the makeup of the west coast of North America...It will cause a thorough rethinking of the geological history of North America and undoubtedly many other continental margins."