Helium Leakage and Fault Zone: Los Angeles Basin Fault Runs Deep

The Newport-Inglewood Fault is one of several earthquake fault lines in Southern California, running from Culver City to Newport Beach, then south-southeast into the Pacific Ocean. Recently, a UC Santa Barbara geologist, Jim Boles, found evidence that this fault zone is very, very deep--deep enough to touch the Earth's mantle.

Boles discovered this connection with the planet's molten layer by finding evidence of helium leakage along a 30-mile stretch of the fault zone in the Los Angeles Basin, according to a release.  

In his research, Boles used samples of casing gas from two dozen oil wells ranging from LA's Westside to Newport Beach (Orange County), he found that more than one-third of the sites show evidence of high levels of helium-3 (3He), according to a release.  

This is pretty astounding, because 3He is considered primordial and a vestige of the Big Bang. Its only source on Earth is the mantle. The presence of 3He suggests that the fault is much deeper than scientists previously believed. Boles' report appears in the electronic journal Geochemistry, Geophysics, Geosystems (G-Cubed).

"The results are unexpected for the area, because the LA Basin is different from where most mantle helium anomalies occur," said Boles, professor emeritus in UCSB's Department of Earth Science, according to a release. "The Newport-Inglewood fault appears to sit on a 30-million-year-old subduction zone, so it is surprising that it maintains a significant pathway through the crust."

The study also found carbon dioxide in the gas analysis, and confirmed that it was also from the mantle. The scientists also located blueschist--a metamorphic rock revealed when brought to the surface during geologic upheaval. They found it at the bottom of nearby deep wells, 20 miles down, indicating that the Newport-Inglewood fault is an ancient subduction zone--where two tectonic plates collide.

"About 30 million years ago, the Pacific plate was colliding with the North American plate, which created a subduction zone at the Newport-Inglewood fault," Boles explained in a release. "Then somehow that intersection jumped clear over to the present San Andreas Fault, although how this occurred is really not known. This paper shows that the mantle is leaking more at the Newport-Inglewood fault zone than at the San Andreas Fault, which is a new discovery."

"Our findings indicate that the Newport-Inglewood fault is a lot more important than previously thought, but time will tell what the true importance of all this is," Boles said in a release. 

The research was supported by the U.S. Department of Energy's Office of Science and Office of Basic Energy Sciences and by the NOAA Pacific Marine Environmental Laboratory.

Follow Catherine at @TreesWhales