March 19, 2013
Researchers Piece Together Tectonic Puzzle Of California Plate Fragments
Brett Smith for redOrbit.com - Your Universe Online
A team of American scientists believe they have solved a geological mystery buried about 100 miles below California.
According to a report in the Proceedings of the National Academy of Sciences, geologists from Brown University, Columbia University, the University of Rhode Island and the University of Oregon identified the source of anomalous seismic readings as a fragment of the Farallon tectonic plate, which was pushed deep into the Earth´s mantle as the Pacific and North American plates slid together around 100 million years ago.
As the Farallon plate sunk into the Earth´s mantle, pieces of it broke off with some of them eventually becoming part of the Pacific plate. According to this latest report, some of the fragments are located deep under central California near the Sierra Nevada mountains.
“Many had assumed that these pieces would have broken off quite close to the surface,” said co-author Donald Forsyth of Brown University. “We´re suggesting that they actually broke off fairly deep, leaving these large slabs behind.”
Years ago, geologists identified a “high velocity anomaly” near the Sierra Nevada mountain range. Seismic tomography measures the speed of seismic waves underground, and the speed of these waves provides clues about the make-up and temperature of the material it travels through. Slower seismic waves tend to mean softer and hotter material, while faster waves mean harder and cooler material.
The unusually fast seismic wave readings in California, known as the Isabella anomaly, indicated to geologists a large mass of relatively cool and stiff material is present at a depth of around 100 miles below the surface. Experts were unsure of how this mass might have gotten there.
Scientists had originally theorized the mass could be a sunken part of the Earth´s crust that had broken off. That prevailing theory was recently called into question when scientists detected another anomalous reading just east of the known coastal remains of the Farallon plate under the Baja Peninsula. This led Forsyth to believe the anomaly represented another underground piece of the Farallon fragment.
After finding tell-tale signs of tectonic fragmentation in the form of high-magnesium andesite deposits near the eastern edge of the anomaly, Forsyth decided to take a closer look at the Isabella anomaly. He found the Baja and Isabella are similar to known Farallon pieces located underneath Washington and Oregon. Because all the anomalous readings occur at the same depth and line up nearly due east of known fragments from Farallon, the geologists concluded another fragment is responsible for the Isabella anomaly.
“The geometry was the kicker,” Forsyth said. “The way they line up just makes sense.”
“This work has radically changed our understanding of the makeup of the west coast of North America,” said co-author Brian Savage of the University of Rhode Island. “It will cause a thorough rethinking of the geological history of North America and undoubtedly many other continental margins.”
Forsyth added the study´s findings will cause geologists to re-evaluate the tectonic history of the West Coast, especially the area around the Sierra Nevada range.