Thermochronology of the Needle Falls Shear Zone: a post-collisional high-strain zone of the Trans-Hudson Orogen
Bibliographic record
Abstract
The Needle Falls Shear Zone is a major Paleoproterozoic discontinuity separating an Archean craton from accreted Paleoproterozoic terranes in the western Trans-Hudson Orogen. It is a 1 km-wide subvertical mylonite zone forming the southwestern sector of a northeast-trending shear system extending for more than 400 km. Kinematic data indicate oblique, east-side-up dextral strike-slip displacement throughout its evolution from ductile through late brittle deformation. For most of its exposed length, it marks the northwestern margin of the Wathaman Batholith. The oldest Pb-evaporation ages of zircons from mylonite average 1857 Ma, consistent with published zircon ages of the Wathaman Batholith, confirming the batholith is protolith for much of the shear zone. The synkinematic metamorphic assemblage of amphibolebiotitequartzfeldspar signifies deformation under amphibolite-facies conditions (500630°C). Oxygen isotope fractionations between quartzhornblende and quartzbiotite pairs in textural equilibrium give temperatures from 430°C to 610°C, with a mode at 550°C. For hornblendebiotite pairs, 40 Ar/ 39 Ar plateau ages of hornblende cluster at 1780 Ma and of biotite at 1757 Ma. Given textural and oxygen isotope equilibrium between hornblende, biotite, and quartz, these ages are interpreted as cooling ages; consequently, this sector of the Trans-Hudson Orogen cooled from 500°C to 300350°C over about 23 million years following ductile deformation. The timing of ductile deformation cannot be well constrained, but occurred after emplacement of the Wathaman Batholith at 1857 Ma and prior to cooling of amphibole below its Ar blocking temperature at 1780 Ma.
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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 | 0.001 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| 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".