The sources of metamorphic heat during collisional orogeny: the Barrovian enigma
Bibliographic record
Abstract
The problem of the observed very rapid advection of heat into metamorphic thrust stacks is reviewed. Conductive models relying on the thermal relaxation of a thickened crust will not produce the observed Barrovian (medium temperature, medium pressure) assemblages within some short-lived orogens (e.g., western Ireland and Timor). Studies of the rate and timing of metamorphic mineral growth suggest that this is commonly faster than predicted by thermal relaxation. Barrovian assemblages are localised in some orogens (e.g., the Alps) but extensive in others (e.g., the Himalayas). Metamorphic mineral growth brackets deformation; consequently, slow growth is inconsistent with the rapid uplift of many orogens. Thus, no single mechanism can account for the development of Barrovian assemblages during collisional orogeny. The only mechanisms that can supply large amounts of heat for regional metamorphism quickly (<10 Myr) are: rapidly thinning the lithosphere without stretching it (e.g., by plume thermal erosion, slab drop-off, or delamination); by emplacing magma into the crust (modest deep mafic underplate and (or) very large amounts of mafic and silicic magma emplaced into the middle and upper crust); or obducting hot nappes of arc with a thin ophiolite forearc (“hot iron” mechanism). Frictional and viscous heating produces local rapid heating but not fast regional heating. Back-arc or any kind of lithospheric extension increases the geothermal gradient and heat flow but does not heat rocks up. We suggest that magmatic advection of heat-associated lithospheric thinning or “hot iron” overthrusting of an arc/ophiolite are the primary sources of heat in short-lived orogens.
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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 | 0.001 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.001 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.002 | 0.002 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.001 | 0.001 |
| 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".