Paleomagnetism of the Quottoon plutonic complex in the Coast Mountains of British Columbia and southeastern Alaska: evidence for tilting during uplift
Bibliographic record
Abstract
The Quottoon plutonic complex (part of the great tonalite sill) was emplaced into eastern parts of the Coast shear zone along the west flank of the Coast Mountains. UPb crystallization ages range from 72.3 to 55.5 Ma. A regional compilation of KAr hornblende dates from the tonalite sill indicates a west to east decrease from ~60 Ma to ~54 Ma across the intrusive suite. Paleomagnetic samples were collected at 160 sites ([Formula: see text] 8 samples per site) along six transects across the Quottoon complex between the Skeena River and Willard Inlet. Directions of characteristic remanent magnetization (ChRM) were successfully determined by principal component analysis of thermal demagnetization data for about half of the samples collected. Site-mean ChRM directions were determined for 56 sites from the Skeena River, Quottoon Inlet, Steamer Passage, and Wales Island transects, using criteria of [Formula: see text] 4 sample ChRM directions per site and 95% confidence limit (α95) [Formula: see text] 25°. For Filmore Island and Willard Inlet, few sites provided adequate determinations of site-mean ChRM directions, but many sample ChRM directions were well determined and consistent. The mean direction from the Skeena River Quottoon Inlet collection is nearly concordant with the expected Eocene direction. Mean directions from other transects have clockwise-rotated declinations and inclinations that are shallower than the expected direction by up to 23°. Consistent with expectations for east-side-up tilting of crustal panels, sites from the western part of each transect have mean directions farther from the Eocene expected direction than do sites from the eastern part of the transect. The magnitude of east-side-up tilt varies along strike: the Skeena River to Quottoon Inlet segment experienced little or no tilt; Steamer Passage south of Portland Inlet records 15° tilt; north of Portland Inlet, tilts are 40° for Wales Island and 35° for Filmore Island; farther north, tilt decreases to 12° for Willard Inlet. Tilting was driven by Eocene extension of the Coast Mountains, with tilted crustal panels bounded by northwest-striking, east-side-down normal faults and northeast-striking transfer faults or shear zones. The ACCRETE seismic transect imaged east-dipping reflectors down to ~15 km depth, which likely correspond to the east-side-down normal faults. The most significant of the northeast-striking transfer structures must underlie Portland Inlet. Based on a tilting domino model, about 30% extension is required to produce the 40° tilt of crustal blocks at Wales and Filmore islands.
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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 | 0.002 |
| Science and technology studies | 0.002 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 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".