FOREARC MANTLE HELIUM AS A CONSTRAINT ON LOCATION OF THE SEISMOGENIC TRANSITION ZONE ALONG THE CASCADIA SUBDUCTION INTERFACE

The ratio between helium isotopes (³He/⁴He) provides an excellent geochemical tracer for investigating the sources of fluids sampled at the Earth’s surface. Primordial ³He has a mantle source, whereas ⁴He is mainly produced from radioactive decay of uranium- and thorium-series elements in the crust. Consequently fluids derived from a crustal source will exhibit low ³He/⁴He values of less than 0.02 R_A (R_A is the observed ³He/⁴He value divided by the atmospheric value). Conversely, ³He/⁴He values greater than 1.0 R_A may be interpreted as including a component of mantle-derived helium, once corrected for a potential ³He contribution from ³H decay_. In subduction systems where helium isotope data are available, two bands of enriched mantle-derived helium can typically be resolved—one along the volcanic arc and a second within the forearc. The highest ratios in the Cascadia forearc coincide with slab depths (40–45 km) where metamorphic dehydration of young Juan de Fuca lithosphere is expected to release significant deep fluids and where tectonic tremor occurs. Lower ratios coincide with slab depths (25–30 km) seaward of the forearc mantle corner (FMC) where little slab fluid is expected to be released. Specifically, ³He/⁴He values observed in 40 mineral springs and wells above the Cascadia forearc document ratios of 1.2–4.0 R_A within the tremor band arcward of the FMC as compared with ratios of 0.03–0.7 R_A seaward of the FMC and tremor. This spatial correlation between high R_A values and tectonic tremor (considered a marker for high fluid pressure) provides independent evidence that tremor is associated with deep fluids, and further suggests that high pore pressures associated with tremor may serve to keep fractures open for ³He migration through ductile upper mantle and lower crust. A similar pattern is observed along the Nankai subduction margin, where high ratios are documented along the tremor band—just arcward of the FMC. In both subduction systems, the spatial correlation of tremor, high R_A values, and the FMC coincide with the inferred down-dip edge of the seismogenic transition zone on the subduction interface, suggesting that they can serve as a priori constraints on its location.