LEAKAGE OF MANTLE HELIUM ALONG THE NEWPORT-INGLEWOOD FAULT, LOS ANGELES BASIN

Faults are known to strongly control fluid flow in the deep crust, the migration and accumulation of petroleum in young sedimentary basins, the reactions of diagenetic and hydrothermal fluids, and the natural seepage of hydrocarbons from offshore reservoirs. In southern California, migration of petroleum and noble gases can be used to constrain fault permeability. For example, in the Santa Barbara basin, observed petroleum flow rates from a submarine seep area constrains off-shore fault permeability k~30 millidarcys for the km-scale vertical migration of fluids within the South Ellwood Fault. In the Los Angeles Basin (LAB), mantle helium is a significant component of the helium gas from deep oil wells along the Newport-Inglewood Fault zone (NIFZ). Helium isotope ratios are as high as 5.3 Ra indicating 66% mantle contribution, and most values are well above background crustal numbers (Boles et al., in press). Other samples from basin margin faults and from within the basin have much lower values (R/Ra < 1.0). The ³He inversely correlates with CO₂, a potential magmatic carrier gas. The δ¹³C of the CO₂ in the ³He rich samples is between 0 and -10 per mil, suggesting a mantle influence. The strong mantle helium signal along the NIFZ is surprising considering that the fault is currently in a transpressional rather than an extensional stress regime, lacks either recent magma emplacement or high geothermal gradients, and is modeled as truncated by a proposed major, potentially seismically active, décollement beneath the LAB. Our results demonstrate that the NIFZ is a deep-seated fault connected with the mantle. We calculate a maximum Darcy flow of 2.2 cm yr^-1 and permeability of 160 microdarcys (1.6 x 10 ^-16 m²), vertically averaged through the 30-km thick crust. This flow rate is far too slow to perturb the regional heat flow around the fault. The mantle leakage may be a result of the NIFZ being a former paleo-Mesozoic subduction zone in spite of being located 70 km west of the current plate boundary at the San Andreas Fault.