Pacific Section AAPG Convention

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Fault-Related Mantle Helium, Los Angeles Basin, California


Mantle helium with 3He/4He ratios as high as 5.3 x air ratios are found in gas from deep oil wells along the 50 km long Newport-Inglewood fault zone (NIFZ) in the Los Angeles (LA) basin. Up to 66% of the He gas is derived from the mantle. The 3He/4He ratios along the NIFZ are significantly higher than other faults and areas sampled within the basin. Surprisingly, the NIFZ is currently in a transpressional rather than extensional stress regime and lacks both high thermal gradients and recent magma emplacement, all of which typically characterize crustal areas with high 3He/4He values. Our results demonstrate that the NIFZ is a deep-seated fault in communication with the mantle, in spite of being modeled as truncated by a proposed major, potentially seismically active, decollement beneath the LA basin.