Aulacogens of the Neoproterozoic to Ordovician Laurentian Iapetan Margin
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Bibliographic record
Abstract
The Ottawa and Saguenay grabens are aulacogens (failed rifts) formed on the Ediacaran Laurentian Iapetan continental margin. They contain a record of syn-rift evolution and passive margin reactivation through an entire Wilson Cycle, yet their origins, the role they played in continental margin evolution, and insights they can provide on breakup processes and passive margin structural inheritance are not known. This review contextualizes the magmatic, stratal, and structural histories of the Ottawa and Saguenay grabens by integrating details of their evolution with a review of the coinciding stratal and plate tectonic evolution of the Laurentian margin from New York to Newfoundland. The results demonstrate that these aulacogens provide a sensitive record of the coeval plate tectonic evolution of the Laurentian continental margin, from rifting processes to peripheral subduction zone evolution. The Ottawa and Saguenay grabens formed as rift branches of the larger St. Lawrence rift system, analogous the East African Rift System, during 620–570 Ma rifting and breakup between Laurentia and Baltica. The much greater volume of magmatism in the Ottawa Graben suggests that it was a more axial part of the rift system, albeit distal to the margin. During subsequent hyperextension leading to the opening of the Taconic Seaway (ca. 570–515 Ma), the Ottawa and Saguenay grabens developed into margin-normal rifts along the final manifestation of the Laurentian passive margin. Alkaline flood basalts erupted at the mouth of the Ottawa Graben and felsic stocks intruded inboard parts graben, likely as the result of asthenosphere upwelling due to edge-driven mantle convection below the thinned crust of the Ottawa Graben. Corresponding deltaic progradation at the mouth of the Ottawa Graben records renewed rifting that facilitated focused sediment transport through to the margin. Following Middle Cambrian margin breakup, reactivation of the Ottawa and Saguenay grabens illustrate a connection to inherited margin-transverse faults and fracture zones and a significant influence of inherited rifted margin architecture on strain partitioning and subduction propagation. During the ca. 510–505 Ma post-rift “Hawke Bay event” while rifting and breakup intensified in the southeastern Laurentian margin, the Missisquoi transverse fault zone and collinear Ottawa Graben and Franklin Basin were reactivated, taking up regional margin-normal extension via transtension. Subsequent Furongian (ca. 500–495 Ma) reactivation of the Ottawa and Saguenay grabens remains enigmatic, perhaps responding to distant fore-arc spreading in the Iapetus Ocean or to broad cratonic uplift. Tremadocian (ca. 485–482 Ma) aulacogen reactivation is linked to the coeval onset of segmented fore-arc spreading in the Taconic Seaway, which made use of fracture zones to accommodate localized extension via strike-slip deformation, which, in part, was terminated in the Ottawa and Saguenay grabens. Finally, Late Tremadocian to Floian reactivation of the Ottawa Graben (ca. 480–475 Ma) is linked to flexural strain from the onset of tectonic loading on the most distal parts of the Laurentian margin.
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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 | 0.001 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.003 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.013 | 0.002 |
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 it