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

Abstract

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.

Seismic evidence for megathrust fault-valve behavior during episodic tremor and slip

“…At ETS depths (30 to 40 km), most fluids derive from prograde metamorphic dehydration reactions of subducting material (8). Studies of subduction zone forearcs suggest that fluids eventually migrate along the megathrust and into the overlying crust, near the mantle wedge corner (14)(15)(16). It has been suggested that these fluids may not only play a passive role in ETS by weakening faults via high P f but that ETS may also be coupled with cyclic fluid processes (14)(15)(16)(17)(18).…”

“…Studies of subduction zone forearcs suggest that fluids eventually migrate along the megathrust and into the overlying crust, near the mantle wedge corner (14)(15)(16). It has been suggested that these fluids may not only play a passive role in ETS by weakening faults via high P f but that ETS may also be coupled with cyclic fluid processes (14)(15)(16)(17)(18). However, observations of these cyclic processes are limited and indirect (17,18).…”

Earthquake swarms and slow slip on a sliver fault in the Mexican subduction zone

“…Similarly, a recent study in Kanto, Japan found that seis-micity rates and seismic attenuation above the plate interface changed following slow slip, suggesting slow slip releases trapped fluids that then migrate updip along the plate interface until they migrate into the overlying crust via an inherent permeable zone where they can trigger seismicity (79). Furthermore, in northern Cascadia, Savard et al (80) attribute vertically clustered, swarmlike seismicity that is concentrated in the forearc continental crust above the mantle wedge downdip of the tremor zone to trapped fluids released from the plate interface, while Wells et al (81) fluids released from the subducting crust. If the megathrust is well-drained, then the overpressurized fluids that flow along crustal faults would facilitate shallow seismicity due to the consequent increase in pore-fluid pressures and decrease in the shear strength of the crustal faults (49,83,84), which would explain the occurrence of our swarm seismicity and a-SSEs along the sliver fault.…”

Preferential Orthopyroxene Serpentinization and Implications for Seismic Velocity Interpretation in the Fore‐Arc Mantle

“…The porosity is shown near the cross symbols. (b) V p and V p / V s observed in the fore‐arc mantle wedge in the southern Western Alps (Solarino et al, ) and Cascadia subduction zones (Ramachandran & Hyndman, ; Savard et al, ) along with those of the serpentinized harzburgite.…”

“…In Cascadia, the temperature of the fore‐arc mantle wedge is estimated to be mostly higher than 450 °C due to the subduction of young and hot oceanic lithosphere (Wada & Wang, ) and thus preferential Opx reaction is possible as in the southern Western Alps subduction zone. However, seismic observations revealed the presence of low V p and high V p / V s anomalies in the Cascadia fore‐arc mantle ( V p = 7.2–7.6 and V p / V s = 1.76–1.81, Ramachandran & Hyndman, ; Savard et al, ), which cannot be explained by the presence of talc (Figure b). This anomaly is rather consistent with the stoichiometric serpentinization which produces only serpentine as a hydrous mineral.…”