Seismicity, Metamorphism, and Fluid Evolution Across the Northern Cascadia Fore Arc

We invert traveltime data from regular seismicity and low‐frequency earthquakes (LFEs) on southern Vancouver Island to map fore‐arc structure and seismogenesis. Tomographic images reveal high Poisson's ratios associated with a previously mapped, dipping low‐velocity zone inferred to be overpressured, upper oceanic crust of the Juan de Fuca plate, where LFEs and other slow‐slip phenomena occur. Low Poisson's ratios (∼0.225) in the fore‐arc continental crust above the mantle wedge are accompanied by high Vp (∼6.65 km/s) and high levels of clustered microseismicity. This crustal anomaly is positioned above the slab, where focused expulsion of fluids is inferred based on independent evidence, suggesting an association between composition, fluids, and seismogenesis. We propose that the anomalous crust harbors a high percentage (≤15%) of quartz, characterized by extremely low Poisson's ratio of 0.1. Earlier studies have proposed that excess quartz in the crust is precipitated from fluids fluxed from the subducting slab. In contrast, we posit that quartz is produced and concentrated in situ by metasomatism in the fore‐arc crust catalyzed through focused ingress of slab‐derived fluids at high‐pore pressure. These fluids enable microseismic activation of high‐angle thrust faults through a fault‐valve mechanism that concentrates quartz via pore‐pressure cycling. Larger M ≥ 6 events like those recently proposed to occur along the Leech River Fault may nucleate deep in the brittle‐ductile transition as a result of these reactions. The fore‐arc crust in warm subduction settings may thus suffer extreme exposure to prolonged fluid flow, an inference with relevance to the genesis of orogenic gold deposits.