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Helium and Carbon Isotope Systematics in Groundwaters along the Southern San Andreas Fault System

  • Author(s): Evans, Tyler John
  • et al.
Abstract

In this study, 38 groundwater wells and hotsprings were sampled in three different areas (Salton Sea, Coachella Valley, and San Bernardino) along the southernmost portion of the San Andreas Fault System (SAFS) and analyzed for helium and carbon abundances and isotopic signatures. Concentrations of ⁴He in the groundwaters, corrected for air saturated water contributions, vary from 0.32 to 55.0 (x 10⁻⁷ cm³ STP/g H₂O). Concentrations of CO₂ in the groundwaters vary from 2.62 to 38.4 (x 10⁻² cm³ STP/g H₂O). ³He/⁴He ratios vary from 0.18 to 2.23 RA (where RA = air ³He/⁴He), with Salton Sea, Coachella Valley and San Bernardino containing median ³He/⁴He ratios of 1.17 RA, 0.64 RA, and 1.37 RA, respectively. CO₂/³He ratios vary from 3.06 to 200 (x 1010) and [delta]¹³C (CO₂) values range from -16.8 ⁰/₀₀ to -4.6 (vs PDB). Using flux calculations developed by Kennedy et al. (1997), the Salton Sea, Coachella Valley, and San Bernardino segments of the fault have calculated fluid flux rates of 1190 mm/ year, 395 mm/year, and 622 mm/year, respectively. Total 3He fluxes in Salton Sea (segment length 80 km), Coachella Valley (100 km), and San Bernardino (50 km) are 0.013 mol ³He/year, 0.0058 mol ³He/year, and 0.0050 mol ³He/year, respectively. Using these calculated flux estimates and CO₂/³He ratios, this report makes a comparison between Mid Ocean Ridge and SAFS carbon fluxes. The Salton Trough represents a transition zone between the East Pacific Rise -related CO₂ degassing and SAFS fault-related degassing. This report shows a positive correlation between mantle involvement and earthquake activity

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