Magnetization age from paleomagnetism of the Copper Harbor red beds, Northern Michigan, USA, and its Keweenawan geologic consequences
Bibliographic record
Abstract
The Copper Harbor Formation on Lake Superior’s Keweenaw Peninsula records the transition from volcanic to sedimentary infilling of North America’s 1.1 Ga Keweenawan rift. Radiometric dating shows that the formation’s primary mafic sediments and interbedded “Lake Shore” flows were deposited between ∼1092 and ∼1082 Ma. Our regional paleomagnetic results for the Copper Harbor’s red beds yield a dominantly prefolding normal-polarity secondary chemical characteristic remanent magnetization in hematite at 18 of 21 sites with a mean direction of declination = 274.9°, inclination = +10.9° (k = 69.5, α95 = 4.2°), and a paleopole at 7.4°N, 181.7°E (A95 = 3.3°). Using paleopoles from Keweenawan volcanic rocks with U–Pb zircon age dates, an apparent polar wander path is constructed from 1106 ± 2 to 1087 ± 2 Ma. Extrapolation of this path dates oxidation of the Copper Harbor’s primary gray beds to red beds at 1060 ± 5 Ma. The path implies an apparent polar wander rate of ∼18 cm per year from ∼1108 to 1096 Ma and of 6.8 cm per year from 1096 to 1087 Ma, along with a consistent clockwise rotation of 0.30 ± 0.05°per million years for the Laurentian Shield from ∼1108 to ∼1160 Ma. Further, most Keweenawan volcanic rocks around the Lake Superior region carry an endemic ∼1060 Ma normal-polarity hematite remanence overprint, acquired during the initial stages of Grenvillian tectonic uplift, that has caused asymmetry in a unit’s normal and reverse paleopoles. Also, the Copper Harbor paleopole dates emplacement of the White Pine stratiform sedimentary copper mineralization more precisely at 1060 ± 5 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".