Mantle Fluids in the San Andreas Fault System, California

• Published in: Science (American Association for the Advancement of Science) Vol. 278; no. 5341; pp. 1278 - 1281
• Main Authors: Kennedy, B. M., Kharaka, Y. K., Evans, W. C., Ellwood, A., DePaolo, D. J., Thordsen, J., Ambats, G., Mariner, R. H.
• Format: Journal Article
• Language: English
• Published: Washington, DC American Society for the Advancement of Science 14.11.1997; American Association for the Advancement of Science; The American Association for the Advancement of Science
• Subjects: Chemical composition; Chemistry; Crystalline rocks; Earth; Earth sciences; Earth, ocean, space; Earthquakes; Exact sciences and technology; Fault zones; Fluid dynamics; Fluid pressure; Fluids; Geology; Helium; Hot springs; Igneous and metamorphic rocks petrology, volcanic processes, magmas; Mantle; Mantle (Geology); Nuclear chemistry; Organic Chemistry; Rocks; Tectonics. Structural geology. Plate tectonics; Water; San Andreas Fault; Southern California; Central California; California; United States; United States > US; fluid pressure; isotopes; mantle origin; fluid phase; noble gases; helium; He-4/He-3; North America
• ISSN: 0036-8075; 1095-9203
• DOI: 10.1126/science.278.5341.1278

Fluids associated with the San Andreas and companion faults in central and south-central California have high $^3$He/$^4$He ratios. The lack of correlation between helium isotopes and fluid chemistry or local geology requires that fluids enter the fault system from the mantle. Mantle fluids passing through the ductile lower crust must enter the brittle fault zone at or near lithostatic pressures; estimates of fluid flux based on helium isotopes suggest that they may thus contribute directly to fault-weakening high-fluid pressures at seismogenic depths.