Timing and anatomy of granitic strain gradients in the Grenville Front tectonic zone, Ontario, Canada
Bibliographic record
Abstract
Observations of physical and chemical changes across strain gradients can provide information about the processes that lead to localization and therefore provide better tools for prediction of spatial and temporal strain patterns. In contrast to the many chemical and microstructural studies of natural shear zones in metapelites and mafic lithologies, few have described kilometer-scale granitic strain gradients despite the fact that granitoid bodies make up much of the orogenic crust. This study reports microstructural and compositional data across two amphibolite-facies strain gradients from middle crust of the Grenville orogen. The kilometer-scale gradients, defined by stretched enclaves and foliation intensity, form parts of the Boundary and Bad River shear zones, located along the east and west borders of the Bad River granite in the southwestern Grenville Front tectonic zone in Ontario, Canada. The granite is bounded on both sides by older orthogneiss (ca. 1720 Ma igneous crystallization age). Zircon U-Pb ages indicate that the granite crystallized from a magma at ca. 1465 Ma, with metamorphic growth at ca. 1020 Ma. Zircons in the orthogneiss record metamorphic growth during these two younger episodes. These age determinations are consistent with other regional dates and indicate that the strain gradients in the Bad River granite formed during the Grenville orogeny. Whole-rock analyses reveal minor heterogeneity in major-element distribution in the granite that can be attributed to igneous processes, and homogeneity in the trace elements, indicating that strain did not affect the bulk-rock composition. In contrast, microstructural and chemical analyses across the two strain gradients indicate some correlation with strain: slight changes in mineral compositions, development of crystallographic preferred orientations in quartz, an increase in recrystallized fraction, a reduction in recrystallized grain size, and the development of a mixed-phase matrix. However, most of these variations are subtle and occur at different positions across the gradients. The spatial distribution of the microscale changes suggests a change in deformation mechanisms toward the margin, accompanying increased localization. The rheological weakening at the Bad River granite margins was the product of microstructural, rather than mineralogical, change, in contrast to nearby shear zones in more mafic units.
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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) | 0.007 | 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".