Rapid postorogenic cooling of the Paleoproterozoic Cape Smith foreland thrust belt and footwall Archean basement, Trans-Hudson orogen, Canada
Bibliographic record
Abstract
ABSTRACT Rapid midcrustal cooling (>10 °C/m.y.) is typical of Phanerozoic orogens, but it is less commonly reported from Precambrian orogenic belts. Abundant new 40Ar/39Ar (predominantly plateau) dates reveal a period of late, rapid cooling following slow postpeak metamorphic cooling during the evolution of the Paleoproterozoic Cape Smith belt, a greenschist- to amphibolite-facies foreland thrust belt in the ca. 1.83–1.76 Ga Trans-Hudson orogen. We conducted 40Ar/39Ar step-heating analyses on biotite, hornblende, and/or muscovite from 38 samples sourced from the thrust belt and its footwall basement, the Archean Superior craton. The 40Ar/39Ar dates from the Cape Smith belt and re-equilibrated Superior craton ranged ca. 1948–1708 Ma in biotite, ca. 1801–1697 Ma in muscovite, and ca. 1764–1694 Ma in hornblende. Of these, ~70% were ca. 1740–1700 Ma plateau dates, which we interpret as cooling ages following Cape Smith belt metamorphism; gas-release spectra of older outlying dates exhibit characteristics of excess Ar. Following the metamorphic thermal peak, the belt cooled at slow rates of up to ~1 °C/m.y. until ca. 1740 Ma. Concordant biotite, muscovite, and hornblende cooling dates of ca. 1740–1700 Ma require fast, late cooling of the belt (≥4 °C/m.y.) through upper midcrustal levels (~500–300 °C), and they allow for very rapid cooling rates (≤200 °C/m.y.). Accelerated cooling rates may have been triggered by uplift in response to detachment of lower crust or subcontinental lithosphere, facilitated by the postcollisional relaxation of isotherms and structural uplift in basement-involved folds. In Superior craton basement, ca. 2704–2667 Ma 40Ar/39Ar hornblende plateau dates reflect undisturbed cooling ages following Neoarchean metamorphism, whereas younger and wide-ranging 40Ar/39Ar biotite dates (ca. 2532–1743 Ma) with variable gas-release spectra suggest spatially heterogeneous degrees of Ar resetting in biotite during Cape Smith belt tectonism. Partially reset 40Ar/39Ar biotite dates in the Superior craton up to ~100 km south of the belt suggest that the pre-erosional thrust wedge extended at least that far south, and that it imposed a widespread low-temperature (<300 °C) and/or short-lived thermal overprint on the footwall basement. Integration of multimineral 40Ar/39Ar data with structural and metamorphic constraints for the Cape Smith belt indicates that modern-style postcollisional exhumation and rapid cooling were viable processes during the middle Paleoproterozoic.
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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 | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.001 | 0.001 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.001 |
| Insufficient payload (model declined to judge) | 0.016 | 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".