U-Pb zircon ages and Lu-Hf isotope systematics across northwestern Mexico: implications for Cretaceous to Paleocene tectonomagmatic evolution during Farallon subduction
Bibliographic record
Abstract
This study presents U-Pb zircon ages and Lu-Hf zircon isotope data for Cretaceous-Paleocene plutonic rocks along a W-E transect in northwestern Mexico. These data are combined with tectonic reconstruction that restores Late Cenozoic extensional deformation and shows the position of magmatism at 36 Ma. Zircon U-Pb ages results span from 142 to 58 Ma and demonstrate that the continental arc migrated northeastward at 1–2.5 km/Myr. These rates are slower than previously interpreted, but consistent with landward arc migration rates observed in the Andes. Weighted mean initial epsilon hafnium (εHf(t)) values of plutonic rocks along the transect range from + 8.8 to −9.1. The heterogeneity in the zircon εHf(t) is spatially related to the pre-Cretaceous basement provinces that the intrusive rocks were emplaced into. Zircon εHf(t) values of western Baja California display positive values ranging from + 8.8 to + 2.6 suggesting they were formed from a moderately depleted mantle and were emplaced into the Guerrero-Alisitos-Vizcaino terrane. Zircon εHf(t) values in the eastern part of Baja California and most of Sonora are heterogeneous ranging between −0.7 and −9.1 and may be formed from a relatively slightly more evolved mantle source and end up more evolved after crustal assimilation of metasediments. Zircon εHf(t) values ranging from + 8.7 to + 2.9 in Chihuahua are consistent with a depleted-mantle derived melt and assimilation of Grenville lithospheric province. Our results highlight how Hf isotopic signatures help to constrain the pre-Cretaceous basement configuration in northwestern Mexico despite the few exposed outcrops along the transect.
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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 | 0.001 | 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 | 0.000 |
| 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".