Microstructural investigations along a blueschist facies paleomegathrust: Implications for deformation mechanisms for deep slow slip behavior
Bibliographic record
Abstract
The subduction interface is characterized by downdip changes in seismic and aseismic behavior that is controlled by metamorphic reactions, deformation conditions, rheological properties, and fluid sources and pathways. Recent seismological evidence shows that a mixture of brittle-ductile deformation at the downdip limit of the seismogenic zone is responsible for producing slow slip and very low-frequency earthquakes. One way to address the possible mechanics and causes behind this dual deformation behavior is by observing exhumed rocks that provide us with the opportunity to make direct geological observations for microstructure and deformation mechanism analysis. Our project aims to understand the deformation mechanism hosted in the dominant mineral phases of metacherts and blueschist from an exhumed paleomegathrust in the Franciscan Subduction Complex. The Mesozoic Franciscan Subduction Complex consists mostly of underplated clastic sediment-rich terranes that are metamorphosed from prehnite-pumpellyite to blueschist facies. Angel Island, in the San Francisco Bay, is composed of blueschist facies metasedimentary and metabasic rocks containing potential paleo-megathrust faults that correlate to the source depths of slow slip earthquakes. Subduction-related faults and shear zones crop out in sea cliffs around the perimeter of the island and enable us to observe deformation features in characteristic subduction zone lithologies. We focus on a proposed subduction-related shear zone in blueschist-facies metachert juxtaposed against mafic/ultramafic rocks. Centimetric-scale field maps show coeval brittle-ductile features such as mutually cross-cutting extension fractures, kink folds, veins, and mylonitic foliations hosted in a wide variety of rheologically distinct lithologies. These lithologies contain synkinematic sodic amphibole and stilpnomelane demonstrating that this fault was active at blueschist facies. The main deforming mineral phases are sodic amphiboles, phyllosilicates, and quartz. The structural fabric comprises of shear fabrics, kink folding, dismembered veins, flattened grains, solution seams, and compositional layering. Amphiboles and phyllosilicates define the foliation of these rocks whereas quartz exhibits evidence of dislocation creep. Using EBSD-generated phase maps, we try to determine the deformation mechanisms hosted within the metacherts and blueschists along the paleomegathrust. Paleostress during megathrust creep will be constrained using EBSD-generated grain size statistics. Our study highlights the complexity of deformation within these lithologies while presenting evidence for possible deformation mechanisms witnessed at slow slip depths.
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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 | 0.002 | 0.001 |
| 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) | 0.001 | 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".