40Ar/39Ar geochronologic evidence of Eurekan deformation within the West Spitsbergen Fold and Thrust Belt
Bibliographic record
Abstract
ABSTRACT Eocene Eurekan deformation has proven to be an enigmatic sequence of tectonic episodes dominated by tectonic plate compression and translation in the circum-Arctic region. Prins Karls Forland on western Spitsbergen is composed of Neoproterozoic siliciclastic metasediments of Laurentian affinity regionally metamorphosed to greenschist facies conditions. A crustal-scale ductile to brittle deformation zone, here named the Bouréefjellet fault zone, contains the amphibolite facies Pinkiefjellet Unit exposed between the lower metamorphic grade, upper structural unit of the Grampianfjella Group and the Scotiafjellet Group in the footwall. A preliminary age for the amphibolite facies metamorphism (ca. 360–355 Ma) indicates Ellesmerian tectonism, unlike other higher-grade basement rocks on Svalbard. Ten metasedimentary rocks from within the fault zone were collected for multiple single-grain fusion 40Ar/39Ar geochronology, with up to ten muscovite crystals dated per sample. High strain in the rocks is evinced by mylonitic structure, mica fish, and C’ shear zones, and dynamically recrystallized quartz with significant grain bulging and subgrain rotation, indicative of >350 °C temperatures. There is notable dispersion in the 40Ar/39Ar ages between samples, with single muscovite dates ranging from ca. 300 Ma to as young as 42 Ma, reflecting recrystallization and resetting of the muscovite. Younger, reproducible ages were obtained from samples that possess chemically homogeneous muscovite, yielding dates of 55–44 Ma for the Eurekan deformation on Prins Karls Forland. We suggest that Ellesmerian structures on Prins Karls Forland were reactivated during the Eocene (commencing as early as 55 Ma) progressing under warm, yet brittle, conditions that continued to 44 Ma. These 40Ar/39Ar muscovite dates are the first documented Eurekan deformation ages from Svalbard and enable a better understanding of the stages of Eurekan deformation in the Eocene to improve correlations across the circum-Arctic region.
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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 | 0.001 | 0.000 |
| Open science | 0.000 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 0.001 |
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".