Structural and kinematic analysis of the Shagawa Lake shear zone, Superior Province, northern Minnesota: implications for the role of vertical versus horizontal tectonics in the Archean
Bibliographic record
Abstract
The steeply dipping Shagawa Lake shear zone, which strikes ENE through the Vermilion District of the Superior Province in northeastern Minnesota, is marked by zone-parallel foliation and a mineral elongation lineation (Le). Le includes (i) a broad population with pitch of 90° ± 60°, and (ii) a less abundant population with pitch of 15° ± 15°. Shallowly plunging Le is rare, and where it occurs, it overprints the pervasive steeply plunging Le. Shear sense indicators occur within the L–S tectonite motion plane, normal to foliation and parallel to Le. Microstructures define both south-side-up and north-side-up displacement domains, but no spatial patterns emerge across the shear zone. L–S tectonites with east-plunging Le indicate either south- or north-side-up shear parallel to Le, whereas L–S tectonites with west-plunging Le indicate predominantly north-side-up shear parallel to Le. Strike-slip L–S tectonites are rare, but consistently record sinistral shear. Overprinting relationships and structural-kinematic patterns can be attributed to sinking of the Vermilion District volcanic basin and relative rise of the southern region followed by rise of the northern region, as represented by the Giants Range Batholith and Vermilion Granitic Complex, respectively. Structural and kinematic evidence indicates that a process like sagduction–diapirism could explain the rising granitoids and sinking volcanic basin. The narrow width of the shear zone, the need for structural dates and radiometric dates of the surrounding plutons, and lack of documented strike-slip kinematic data make it difficult to evaluate when and how the shear zone transitioned to horizontal displacement.
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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 | 0.001 | 0.001 |
| Science and technology studies | 0.000 | 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".