Continentalization of an intraoceanic arc as exemplified by the Jurassic Bonanza arc of Vancouver Island, Canada
Bibliographic record
Abstract
We present new U-Pb zircon ages, bulk rock chemistry, and field observations for volcanic and plutonic rocks of the Jurassic Bonanza arc of Vancouver Island, Canada, a proposed southern extension of the Talkeetna arc crustal section in Alaska. Rather than a synchronous and contiguous arc crustal section, the new field and geochronological data reveal three distinct periods in the arc development, wherein the oldest, “prima” (205−195 Ma), was built upon by the youngest, “dopo” (<180 Ma), with both preserving their volcanic and plutonic sections of the upper crust. In contrast, the middle “mezza” period of arc construction (195−180 Ma) has no volcanic expression and is represented almost exclusively as plutons in the middle crust emplaced below the brittle-ductile transition, and exhumed mainly along the western and southern coast of Vancouver Island. The new data and structural interpretation refute earlier models suggesting that the deeper, more mafic rocks of the middle crust (West Coast Complex) are either restite or the crystal cumulates that form the more silicic plutonic rocks (Island Plutonic Suite) in the upper crust. Both the middle and upper plutonic components are remarkably similar to average continental crust. The growth and development of the Bonanza arc in an oceanic realm—but on a thick, pre-existing marine plateau—promoted a “continentalization” of its bulk composition within ∼10 m.y., similar to what is proposed to have occurred in Costa Rica over the same timescale.
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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 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.002 | 0.003 |
| Science and technology studies | 0.003 | 0.001 |
| Scholarly communication | 0.002 | 0.000 |
| Open science | 0.000 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.003 | 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".