Geologists were surprised to find helium-3 leaking along a 30-mile stretch of a fault zone in the Los Angeles Basin.
The leak suggests the Newport-Inglewood fault is deeper than scientists previously thought. Considered primordial, helium-3 (3He) is a vestige of the Big Bang. Its only terrestrial source is the mantle.
“Time will tell what the true importance of all this is.”
“The results are unexpected for the area, because the LA Basin is different from where most mantle helium anomalies occur,” says Jim Boles, professor emeritus in the Earth science department at the University of California, Santa Barbara. “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.”
When Boles and his co-authors analyzed the 24 gas samples, they found that high levels of 3He inversely correlate with carbon dioxide (CO2), which Boles says acts as a carrier gas for 3He. An analysis showed the CO2 was also from the mantle, confirming leakage from deep inside the Earth.
Blueschist, a metamorphic rock, found at the bottom of nearby deep wells indicates that the Newport-Inglewood fault is an ancient subduction zone—where two tectonic plates collide—even though its location is more than 40 miles west of the current plate boundary of the San Andreas Fault System. Found 20 miles down, blueschist is only revealed when regurgitated to the surface via geologic upheaval.
“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 explains. “Then somehow that intersection jumped clear over to the present San Andreas Fault, although how this occurred is really not known.
What the discovery means
“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.”
The study’s findings contradict a scientific hypothesis that supports the existence of a major décollement—a low-angle thrust fault—below the surface of the LA Basin.
“We show that the Newport-Inglewood fault is not only deep-seated but also directly or indirectly connected with the mantle,” Boles says.
“If the décollement existed, it would have to cross the Newport-Inglewood fault zone, which isn’t likely,” he adds. “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.”
Study coauthors include Grant Garven of Tufts University; Hilario Camacho of Occidental Oil and Gas Corp.; and John Lupton of the National Oceanic and Atmospheric Administration’s Pacific Marine Environmental Laboratory.
The US Department of Energy’s Office of Science and Office of Basic Energy Sciences and the NOAA Pacific Marine Environmental Laboratory supported the work. Boles’s findings appear in the journal Geochemistry, Geophysics, Geosystems (G-Cubed).
Source: UC Santa Barbara