Mantle helium along the Newport‐Inglewood fault zone, Los Angeles basin, California: A leaking paleo‐subduction zone

Abstract

Mantle helium is a significant component of the helium gas from deep oil wells along the Newport-Inglewood fault zone (NIFZ) in the Los Angeles (LA) basin. Helium isotope ratios are as high as 5.3 Ra (Ra 5 3 He/ 4 He ratio of air) indicating 66% mantle contribution (assuming R/Ra 5 8 for mantle), and most values are higher than 1.0 Ra. Other samples from basin margin faults and from within the basin have much lower values (R/Ra < 1.0). The 3 He enrichment inversely correlates with CO 2 , a potential magmatic carrier gas. The d 13 C of the CO 2 in the 3 He rich samples is between 0 and 210&, 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 extensional stress regime, lacks either recent magma emplacement or high geothermal gradients, and is modeled as truncated by a proposed major, potentially seismically active, d ecollement beneath the LA basin. Our results demonstrate that the NIFZ is a deep-seated fault directly or indirectly connected with the mantle. Based on a 1-D model, we calculate a maximum Darcy flow rate q 2.2 cm/yr and a fault permeability k 6 3 10 217 m 2 (60 microdarcys), but the flow rates are too low to create a geothermal anomaly. The mantle leakage may be a result of the NIFZ being a former Mesozoic subduction zone in spite of being located 70 km west of the current plate boundary at the San Andreas fault.

Tomography of Southern California Via Bayesian Joint Inversion of Rayleigh Wave Ellipticity and Phase Velocity From Ambient Noise Cross‐Correlations

“…There are clear velocity changes across the Newport‐Inglewood fault (B‐B′, 1‐3 km depth), which have previously been observed in more detailed studies (Lin et al, ). The basin deepens between the Newport‐Inglewood and Whitter faults, in line with previous geological (Shaw & Suppe, ) and geochemical (Boles et al, ) studies. Additionally, there is a strong contrast across the Whitter fault leading to a shallower basin NE of the fault as expected from the fault throw (Davis et al, ; Shaw et al, ).…”

Regional crustal-scale structures as conduits for deep geothermal upflow

“…Tapping of these fluids by shallow geothermal systems results in background, ∼0.2 Ra geothermal fluids. Relatively rapid (1-10 mm/year) upflow of fluids occurs in permeable (Ä = 10 −17 -10 −21 ) conduits controlled by crustal-scale structural zones allowing for less dilution by crustal 4 He (Boles et al, 2015;Kennedy et al, 1997). Tapping of these fluids by shallow geothermal systems results in anomalously high 3 He/ 4 He in these geothermal fluids.…”

“…6). Fluid upflow rates of ∼1-10 mm/year and a permeability of ∼10 −18 -10 −21 m 2 associated with similar anomalous 3 He/ 4 He and crustal-scale fluid upflow occur along the San Andreas fault system and in the Los Angeles Basin (Boles et al, 2015;Kennedy et al, Fig. 6.…”

“…1997). Boles et al (2015) calculate a permeability of ∼10 −17 m 2 associated with deep upflow of mantle helium along a paleosubduction zone in the Los Angeles Basin. Both of these are several orders of magnitude higher than bulk permeability of intact crystalline rock (∼10 −23 m 2 ; Ingebritsen et al, 2001).…”

Geostatistical analysis of tritium, groundwater age and other noble gas derived parameters in California

“…Besides addressing the research questions presented above, the purpose of this paper is to present the data set to the scientific community to solicit further research. Future research opportunities include calibration of numerical groundwater models (Castro et al, 1998;Ivey et al, 2008;Murphy et al, 2010;Sanford, 2010;Sheets et al, 1998), paleoclimate reconstruction (Stute et al, 1992;Aeschbach-Hertig et al, 2000b;Hall et al, 2005;Castro et al, 2007;Klump et al, 2008;Stute et al, 1995b;Corcho Alvarado et al, 2009), water quality vulnerability in relation to groundwater age (Wright et al, 2004;Johnston et al, 1998;Tesoriero et al, 2007;Visser et al, 2007), and identification of regions where mantle fluids are transported into shallow aquifers (Boles et al, 2015;Kulongoski et al, 2013;Castro, 2004;Kulongoski et al, 2003;Jenden et al, 1988;Poreda et al, 1986). The geostatistical analysis presented here is a first step in disseminating and visualizing the data set.…”