'Lost' Tectonic Plate Found Under California

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Ever have one of those days when you can’t find anything: your keys, your cell phone or that darned Farallon tectonic plate? I’m happy to report some progress on the plate. The Farallon plate, what’s left of it, is just fragments of oceanic crust off the West Coast. The lost part was pushed east and subducted under the North American plate, probably accompanied by a ultra-ultra-low frequency, multi-million-year-long munching sound that can’t even be heard by elephants.

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Scientists are now reporting they have found a big chunk of the Farallon plate still stuck in the subduction digestion system, so to speak, some 100 to 200 kilometers under California. The researchers explain it all in far more appropriate and scientific terms in a paper in this week’s issue of Proceedings of the National Academy of Sciences.

But if you’d rather get the short version, here it is: They found, specifically, evidence that a seismological blob called the Isabella anomaly (“IA” in the map above) is a slab of the Farallon plate that got stuck to the underside of the North American plate instead of dropping off into the Earth’s mantle.

The anomaly itself is a large slab of relatively cool and dried out rock. Geologists know this because of the way seismic waves pass through it: slower waves usually means softer, hotter material while faster waves mean stiffer and cooler rocks.

The Isabella Anomaly has been known for a while. But by comparing it with another seismological blob under Baja California, which, for a bunch of reasons, is even more certainly a piece of the Farallon plate, the researchers, led by Donald Forsyth of Brown University and Tun Wang of the University of Alaska, reexamined the seismic data of the West Coast and were able to make an argument that the Isabella anomaly is more of the same. It is the same depth, treats seismic waves the same and, very significantly, the blob is in the right place.

“The geometry was the kicker,” Forsyth said in a Brown press release. “The way they line up just makes sense.”

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Another kicker is that if Forsyth and his team are correct, it throws a wrench into another theory about how the Sierra Nevada and the Isabella anomaly are related. Oh well, that’s science for you: out with the old and in with the new.

IMAGE: The Isabella anomaly (IA) in California is in line with known remnants of the long-gone Farallon plate. (Forsyth lab/Brown University)

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