Driving mechanisms for >40 km of exhumation during contraction and extension in a continental arc, Cascades core, Washington
Bibliographic record
Abstract
In the NW North American Cordillera, the Cascades core region of the Coast Plutonic Complex underwent Late Cretaceous (>96 Ma to locally 73 Ma) SW‐NE contraction and crustal thickening followed by dextral transpression (∼73 to 55 Ma), then transtension (<55 Ma). Exhumation occurred during all three phases. During contraction, slow exhumation (∼0.6 mm/yr) occurred along the margins of the core, driven by isostatic rebound and erosion, and faster exhumation (>3 mm/yr) by local thrusting in regions undergoing crustal thickening. In the central part of the core (Chelan block), >40 km of exhumation occurred between 91 and 45 Ma, about half of which occurred during early contraction (driven by thrusting) and half during top‐to‐north, arc‐oblique shear during reactivation of a midcrustal Cretaceous thrust, the Dinkelman decollement. The footwall of this thrust consists of the Swakane Biotite Gneiss, a Cretaceous, metaclastic assemblage with recorded pressures of 10–12 kbar, no arc‐related magmatism, and structures dominated by pervasive top‐to‐north shearing. The hanging wall consists of the Napeequa Complex, an oceanic assemblage with recorded pressures of 6–12 kbar, voluminous arc‐related magmatism, and complex structures indicating early top‐to‐WSW shearing, younger top‐to‐north shearing, and widespread folding. In the Napeequa, top‐to‐north shearing started by 73 Ma during melt‐present conditions at pressures ≥6 kbar. Top‐to‐north shearing in both hanging wall and footwall continued during exhumation (∼1.6 mm/yr) and cooling to greenschist facies conditions during which slip became increasingly localized, eventually resulting in formation of pseudotachylite on discrete slip surfaces. We suggest that arc‐oblique extension was driven by along‐arc heterogeneity in displacements/erosion, initially during transpression and underplating of continental sediments, and later during transtension.
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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 | 0.001 | 0.000 |
| Science and technology studies | 0.001 | 0.000 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 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".