Inherited Cross‐Strike Faults and Oligocene‐Early Miocene Segmentation of the Main Himalayan Thrust, West Nepal
Bibliographic record
Abstract
Abstract The geometry of orogenic basal detachments controls the location and partitioning of seismicity, yet how it influences past episodes of orogenesis is often overlooked. We integrate recently published microstructural, phase equilibria modeling, monazite U‐Th/Pb petrochronology, and white mica 40 Ar/ 39 Ar geochronology data from the Greater Himalayan sequence to characterize the geometry of the Himalayan basal detachment, the Main Himalayan thrust (MHT), during the Oligocene and early Miocene. The Karnali and Jajarkot klippen, located at equivalent along‐strike positions in the direction of tectonic transport in the foreland of west Nepal, record contrasting pressure‐temperature‐time‐deformation paths. Units in both klippen reached similar peak pressures with a 5‐ to 10‐Myr time difference: Units in the Karnali klippe were exhumed while units in the Jajarkot klippe were buried. We interpret the contrasting metamorphic pressures between 35 and 25 Ma to reflect the influence of a vertical or steeply east dipping lateral ramp (or tear fault) in the MHT between the two klippen. The location of this putative Oligocene MHT ramp coincides with other evidence for a MHT lateral ramp since the Pleistocene to the present. Segmentation of the MHT in west Nepal is therefore an active and protracted phenomenon that started in the Oligocene or earlier. We interpret this offsetting of the MHT to be linked to the Lucknow basement fault, a spatially coinciding inherited cross‐strike structure in the subducted Indian lithosphere. Our study highlights the important applications of metamorphic petrology and petrochronology to seismotectonics by illustrating the timescales of lateral segmentation of an orogenic basal detachment.
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How this classification was reachedexpand
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 | 0.001 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 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 | 0.001 |
| 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 itClassification
machine, unvalidatedMachine predicted; a candidate call from one teacher head, 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".