Scientists have discovered that an ancient tectonic plate is still lodged under central California. The plate appears to be 100 km long, and is supposed to be a mass of dehydrated material that is a surviving part of the ancient Farallon plate, which was driven under the Earth's mantle when the Pacific and North American plates collided about 100 million years ago. That convergence of the two plates constitute the boundaries that make up the St. Andreas fault.
Much of the Farallon plate was subducted during the collision, and sunk deep into the Earth's mantle. But off the West Coast, the Farallon plate broke into smaller plates at the surface that acted as a break to the subduction process and become eventually imbedded into the Pacific plate.
The primordial Farallon plate however, is attached to the un-subducted 'pieces' that linger above. In fact, entire portions of central California and the Sierra Nevada could be sitting atop these 'hinged' fragments, like fossils of the ancient plate.
What scientists are now contending is that the fragments left behind did not separate from the ancient plate at the surface, but at a considerable depth, leaving these slabs behind that sit below the modern areas of central CAlifornia.
What prompted this discovery was the observation of a phenomenon called 'high velocity anomaly', in the seismic tomography data of the Sierra Nevada mountains. The tomography measures the velocity of seismic waves at great depths. The speed of a seismic wave provides information on the temperature of the subsurface. The slower the wave, the softer and hotter the material below, while faster waves indicate a more rigid subsurface and cooler composition.
The anomaly of the seismic speed observed is known to seismologists as the Isabella anomaly, and it told scientists that a large mass of the substrata was cool and dehydrated at about 100 to 200 km depth. But they did not know what that mass was. At first it was thought that the phenomenon was caused by a partial separation, or de-lamination, of the mountain range that sits atop the thin and hot mantle below it.
But then, scientists observed the same phenomenon in the Baja Peninsula area, east of what are the coastal remains of the Farallon plate. That gave clues to the observers that this part was an extension of the fragment observed under the Sierra Nevada mountains.
Geological analysis of the area showed unusual amounts of magnesium andesite, at the surface near the eastern edge of the area that was showing the anomaly. But these deposits are volcanic in origin, and are produced when the oceanic crust melts. These gave them clues that the eastern edge of the anomaly might be where the Farallon plate broke off, pushing andesite to the surface as the subducted plate melted.
This also led the researchers to believe that the area of the Isabella anomaly might be somehow still connected with an unsubducted fragment of the Farallon plate. An analysis of the tomography showed that the anomalies in both areas are strongest at the same depth, i.e. 100 km,and that they both line up due east of the known separation fragment from Farallon.
This finding could lead scientists to change the tectonic evolution history of the American West.
Source: Science daily 3.20.13
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