Rapid synconvergent exhumation of Miocene-aged lower orogenic crust in the eastern Himalaya
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Bibliographic record
Abstract
Rare granulitized eclogites exposed in the eastern Himalaya provide insight into conditions and processes deep within the orogen. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb, Ti, and rare earth element (REE) data from zircons in mafic granulitized eclogites located in the upper structural levels of the Greater Himalayan Sequence in Bhutan show that zircon was crystallized under eclogite-facies metamorphic conditions between 15.3 ± 0.3 and 14.4 ± 0.3 Ma, within a couple million years of the later granulite-facies overprint. In conjunction with pressure estimates of the eclogite- and granulite-facies stages of metamorphism, the age data suggest that initial exhumation occurred at plate-tectonic rates (cm yr<sup>–1</sup>). These extremely rapid synconvergence exhumation rates during the later stages of the India-Asia collision require a revision of theories for the transportation and exhumation of crustal materials during continental collisions. In contrast to western Himalayan examples, the eastern Himalayan eclogites cannot be tectonically related to steep subduction of India beneath Asia. Instead, they more likely represent fragments from the base of the overthickened Tibetan crust. Based on the zircon age and trace-element data, we hypothesize that the protolith of the mafic granulites was middle Miocene mafic intrusions into the lower crust of southern Tibet, linked to Miocene volcanism in the Lhasa block. We suggest that a transient tectonic event—possibly the indenting of a strong Indian crustal ramp into crust under southern Tibet that had been weakened by partial melting—may have promoted exhumation of the eclogitized lower crust under Tibet. The mafic magmatism and volcanism themselves may have been related to the convective thinning of the lithospheric mantle triggered by a reduction in the India-Eurasia convergence rate during the middle Miocene, which in turn could have facilitated the rapid extrusion of the lower crust over the earlier-exhumed middle crust.
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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 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.087 | 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