Stepwise breakup of supercontinent Nuna: A synthesis with new ca. 1236 Ma paleomagnetic data from the North China craton
Bibliographic record
Abstract
Abstract The timing and mechanisms by which the supercontinent Nuna broke up are still enigmatic, although the details have implications for global tectonic activity through Mesoproterozoic periods of alleged quiescence. The proposed breakup time of Nuna has ranged from ca. 1.6 to 1.2 Ga based on the episodic eruptions of large igneous provinces (LIPs) and associated large-scale continental rift systems. Paleomagnetic data from Proterozoic cratons can quantify their motions and directly test these competing inferences. In this study, new paleomagnetic results, which passed baked-contact tests and a reversal test, are reported from 32 dikes from the North China craton (NCC). An isotope dilution–thermal ionization mass spectrometry (ID-TIMS) baddeleyite date from one of these dikes constrains dike emplacement at 1235.6 ± 2.0 Ma, which provides a new, well-dated paleomagnetic pole (27.7°N, 168.5°E, A95 = 5.0°) for the NCC. Combined with the previously published 1.45–1.04 Ga paleomagnetic and geologic data from Laurentia, the NCC, Baltica, and Australia, the divergence of their apparent polar wander paths suggests that the core of Nuna (Laurentia, Baltica, and Siberia) broke up with East Nuna (Australia and the NCC) at ca. 1.38 Ga. Thereafter, the breakup of East Nuna, denoted by the separation of the NCC and Australia, occurred after ca. 1.32 Ga, and the breakup of the core of Nuna, denoted by the breakaway of Baltica from Laurentia and Siberia, occurred at ca. 1.26–1.22 Ga. The stepwise breakup process of Nuna was similar to that of Pangea, providing evidence for a robust tectonic regime in Earth’s middle age.
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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) | 0.001 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.004 | 0.003 |
| Science and technology studies | 0.001 | 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) | 0.002 | 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".