MessageToEagle.com - Southern California is seismically one of the most active areas in the world and
has been studied by many seismologists so far.
It's a very sensitive area because of the well-studied and carefully observed - Isabella Anomaly, which is the
best known example of a high seismic velocity anomaly beneath the southern Central Valley of California.
It's a piece of stalled oceanic lithosphere still attached to the unsubducted Monterey microplate.
The Pacific-North American plate boundary forms a broad zone of deformation extending across this area.
Isabella feature, which is in fact a surviving slab of the Farallon oceanic plate, extends down to about
300 km depth, and its main part extends to deeper than 200 km depth.
Most of the Farallon plate was driven deep into the Earth’s mantle as the Pacific and North American plates
began converging about 100 million years ago, eventually coming together to form the San Andreas fault.
Large chunks of an ancient tectonic plate that slid under North America millions of years ago are still
present under parts of central California and Mexico, according to new research led by Brown University geophysicists.
Around 100 million years ago, the Farallon oceanic plate lay between the converging Pacific and North American
plates, which eventually came together to form the San Andreas fault.
As those plates converged, much of the Farallon was subducted underneath North America and eventually sank deep into the mantle.
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.
But this new research suggests that large slabs from Farallon remain attached to these unsubducted fragments.
The researchers used seismic tomography and other data to show that part of the Baja region and part of central
California near the Sierra Nevada mountains sit atop “fossil” slabs of the Farallon plate.
Blue swath shows approximate position of Isabella Anomaly
“Many had assumed that these pieces would have broken off quite close to the surface,” said Brown geophysicist
Donald Forsyth, who led the research with Yun Wang, a former Brown graduate student now at the University of
“We’re suggesting that they actually broke off fairly deep, leaving these large slabs behind.”
Geologists had known for years about a “high velocity anomaly” in seismic tomography data near the Sierra
Nevada mountains in California. Seismic tomography measures the velocity of seismic waves deep underground. The
speed of the waves provides information about the composition and temperature of the subsurface. Generally, slower
waves mean softer and hotter material; faster waves mean stiffer and cooler material.
The Isabella anomaly (IA, above), is at the same depth (ca. 100 km) as other fragments of the Farallon plate under Oregon and Washington,
is on a line with fragments off the California coast, and has a similar seismic tomography signature.
Credit: Forsyth lab/Brown University
The anomaly in California, known as the Isabella anomaly, indicates that a large mass of relatively cool and
dehydrated material is present below the surface.
Just what that mass was wasn’t known, but there were a few theories. It was often explained by a process called delamination.
A few years ago, scientists detected a new anomaly under the Mexico’s Baja Peninsula, due east of one of the
known coastal remains of the Farallon plate. Because of its proximity to the Farallon fragment, Forsyth and Wang
thought it was very likely that the anomaly represented an underground extension of the fragment.
Carefull studies of the area suggest that the Isabella anomaly in California might also
represent a slab still connected to an unsubducted fragment of the Farallon plate, so scientists compared the
Baja and Isabella anomalies to anomalies associated with known Farallon slabs underneath Washington and Oregon.
The study found that all of the anomalies are strongest at the same depth — right around 100 kilometers. And
all of them line up nearly due east of known fragments from Farallon.
“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 delamination of the Sierra Nevada, which had been used to explain
the Isabella anomaly.
“However the Sierra Nevada was delaminated,” Forsyth said, “it’s probably not in the way that many people had
The findings are published in the Proceedings of the National Academy of Sciences.
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