Developing an inverted Barrovian sequence; insights from monazite petrochronology
Why this work is in the frame
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Bibliographic record
Abstract
In the Himalayan region of Sikkim, the well-developed inverted metamorphic sequence of the Main Central Thrust (MCT) zone is folded, thus exposing several transects through the structure that reached similar metamorphic grades at different times. In-situ LA-ICP-MS U–Th–Pb monazite ages, linked to pressure–temperature conditions via trace-element reaction fingerprints, allow key aspects of the evolution of the thrust zone to be understood for the first time. The ages show that peak metamorphic conditions were reached earliest in the structurally highest part of the inverted metamorphic sequence, in the Greater Himalayan Sequence (GHS) in the hanging wall of the MCT. Monazite in this unit grew over a prolonged period between ∼37 and 16 Ma in the southerly leading-edge of the thrust zone and between ∼37 and 14.5 Ma in the northern rear-edge of the thrust zone, at peak metamorphic conditions of ∼790 °C and 10 kbar. Monazite ages in Lesser Himalayan Sequence (LHS) footwall rocks show that identical metamorphic conditions were reached ∼4–6 Ma apart along the ∼60 km separating samples along the MCT transport direction. Upper LHS footwall rocks reached peak metamorphic conditions of ∼655 °C and 9 kbar between ∼21 and 16 Ma in the more southerly-exposed transect and ∼14.5–12 Ma in the northern transect. Similarly, lower LHS footwall rocks reached peak metamorphic conditions of ∼580 °C and 8.5 kbar at ∼16 Ma in the south, and 9–10 Ma in the north. In the southern transect, the timing of partial melting in the GHS hanging wall (∼23–19.5 Ma) overlaps with the timing of prograde metamorphism (∼21 Ma) in the LHS footwall, confirming that the hanging wall may have provided the heat necessary for the metamorphism of the footwall. Overall, the data provide robust evidence for progressively downwards-penetrating deformation and accretion of original LHS footwall material to the GHS hanging wall over a period of ∼5 Ma. These processes appear to have occurred several times during the prolonged ductile evolution of the thrust. The preserved inverted metamorphic sequence therefore documents the formation of sequential ‘paleo-thrusts’ through time, cutting down from the original locus of MCT movement at the LHS–GHS protolith boundary and forming at successively lower pressure and temperature conditions. The petrochronologic methods applied here constrain a complex temporal and thermal deformation history, and demonstrate that inverted metamorphic sequences can preserve a rich record of the duration of progressive ductile thrusting.
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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 | 0.000 | 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 | 0.001 |
| Scholarly communication | 0.000 | 0.001 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 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 it