Timing of Iapetus Ocean rifting from Ar geochronology of pseudotachylytes in the St. Lawrence rift system of southern Quebec
Bibliographic record
Abstract
Laser ablation 40 Ar/ 39 Ar step-heating analyses for encapsulated and unencapsulated pseudotachylytes from a Neoproterozoic normal fault belonging to the St. Lawrence rift system (Canada) preserve the absolute timing of rifting and initial opening of the Iapetus Ocean. The total gas and retention ages for encapsulated pseudotachylytes from the Montmorency fault (Quebec City) are 610.3 ± 4.6 Ma and 619.0 ± 2.5 Ma. Ten unencapsulated analyses from two pseudotachylyte veins with varying matrix/clast ratios yield total gas ages of 634.7 ± 1.6–663.9 ± 1.8 Ma. These ages show an excellent linear relationship with the proportion of clast inclusions, resulting in lower intercept ages (i.e., no host rock) of 613.3 and 614.2 Ma. These statistically indistinguishable ages constrain major seismic faulting along the St. Lawrence rift system and significantly improve prior estimates for late Neoproterozoic rifting of Iapetus. The upper intercepts, reflecting host-rock ages, match cooling ages of Grenville basement in the area. We conclude that the time of major continental rifting along the northern Laurentian margin and initiation of the Iapetus Ocean occurred at 613–614 Ma, coeval with emplacement of the 615 Ma Long Range dikes of Labrador. This study also demonstrates that Ar geochronology of pseudotachylytes using varying clast/matrix ratios is a robust method to date ancient faulting.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| 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) | 0.003 | 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 teacher head, 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".