Detrital zircon record of the Mesoproterozoic Belt basin and implications for horizontal and vertical tectonic models
Bibliographic record
Abstract
ABSTRACT It is debated whether plate tectonics (horizontal tectonics) or single-lid tectonics (vertical tectonics) dominated the Mesoproterozoic Era. Either rifting of the Nuna/Columbia supercontinent or a localized vertical subsidence and tectonism mechanism within a single tectonic plate is likely recorded in Mesoproterozoic basins. This study summarizes detrital zircon samples from the Mesoproterozoic Belt and Purcell Supergroups and Lemhi subbasin of the western United States and Canada and tests competing rift and intracratonic basin models. Rift models take the observed detrital zircon trends to mean that a non-Laurentian (ca. 1.6–1.5 Ga) detrital zircon component becomes completely absent higher in the section, signifying rifting of the Nuna/Columbia supercontinent at ca. 1.4 Ga. Intracratonic models acknowledge this observed shift in provenance but interpret a long-lived intracratonic setting for the basin following an earlier failed rifting event. The fundamental question is whether the Belt basin represents a failed or successful rift. We used statistical comparison of 72 detrital zircon signatures, reported in the literature and presented in this study, to test the rift model. Samples are not evenly distributed across the basin or its stratigraphy. Non-Laurentian grains are spatially restricted to the northwest part of the basin but are present in all groups, suggesting that the apparent loss of the non-Laurentian population is an artifact of sampling bias. Like stratigraphic boundaries and facies changes, mixing trends are gradual, not sharp or sudden, signifying progressive reworking of Proterozoic zircons and transport from all sides. Archean zircons are localized near the edges of Archean blocks, signifying local down-dropping along cratonic margins. The rift model is therefore rejected in favor of the intracratonic model for the Belt basin on the basis of variable mixing between non-Laurentian and Laurentian sources in both pre–Missoula Group and Missoula Group strata. Far away from plate margins, sediment incrementally filled topographic depressions created by densified and thinned Proterozoic crustal blocks, resulting in vertical down-dropping along preexisting sutures with neighboring Archean blocks. More systematic detrital zircon studies are needed in order to accurately quantify provenance trends in space and time. Continued investigation of the Belt basin may reveal underappreciated or unrecognized vertical tectonic processes that may explain Mesoproterozoic rocks more accurately.
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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 | 0.001 | 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) | 0.001 | 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".