Forearc Mantle Helium as a Constraint on Location of the Seismogenic Zone along the Cascadia Subduction Interface

Abstract:

The ratio between helium isotopes (³He/⁴He) provides an excellent geochemical tracer for investigating the sources of fluids sampled at the Earth’s surface. ³He/⁴He values observed in springs and wells along the Cascadia forearc document a significant component of mantle-derived helium above Juan de Fuca lithosphere, as well as variability in ³He enrichment across the forearc. The highest ratios in the Cascadia forearc coincide with slab depths (40–45 km) where metamorphic dehydration of young oceanic 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 46 mineral springs and water 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. 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. The tremor band is centered on the FMC, and we tentatively postulate that hydrated forearc mantle beneath Cascadia deflects a significant portion of slab-derived fluids updip along the subduction interface, to vent in the vicinity of its corner. Furthermore, high R_A values within the tremor band just arcward of the FMC, suggest that the innermost mantle wedge is relatively permeable. 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.