Known as the Isabella anomaly, the large mass of cool, dehydrated material is a "fossil" slab of the Farallon oceanic plate, linked to known Farallon slabs at similar depths under Washington, Oregon and Baja California in Mexico, Brown University geophysicists reported Monday.
It is also due east of a known fragment still off the California coast, they said.
Most of the Farallon plate has been driven deep into the Earth's mantle since the Pacific and North American plates began converging about 100 million years ago, forming the San Andreas Fault at their boundary.
Off the west coast of North America, the Farallon plate fragmented, leaving a few small remnants at the surface that stopped subducting and became part of the Pacific plate, researchers said.
"Many had assumed that these pieces would have broken off quite close to the surface," Brown researcher Donald Forsyth said.
"We're suggesting that they actually broke off fairly deep, leaving these large slabs behind."
The findings could force scientists to re-examine the tectonic history of western North America, Forsyth said.