Seismicity, Metamorphism, and Fluid Evolution Across the Northern Cascadia Fore Arc
Bibliographic record
Abstract
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.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.002 | 0.001 |
| Science and technology studies | 0.001 | 0.000 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".