Cooling and exhumation of the Shuswap Metamorphic Core Complex constrained by 40 Ar/ 39 Ar thermochronology
Bibliographic record
Abstract
The exhumation history of the Shuswap Metamorphic Core Complex (Shuswap MCC) is deciphered from structural analysis and 40Ar/39Ar thermochronology. With the exception of a few samples contaminated by excess argon, the analytical results indicate a consistent range of early Tertiary 40Ar/39Ar ages throughout the area. Hornblende yields ages ranging from ∼59-54 Ma; muscovite and biotite ages are clustered between 49.5 and 48 Ma, and are consistent with the timing of closure of K-feldspars, typically ranging from 50 to 43 Ma, except for samples in the immediate footwall of the Columbia River fault that yield ages as young as 26 Ma. These 40Ar/39Ar ages, combined with existing U/Pb ages on zircons and monazites, constrain the cooling history at several localities in the Shuswap MCC between Trans Canada Highway 1 and the Thor-Odin dome. The migmatitic core of the complex is affected by a rapid cooling event (∼700°C to ∼300°C), from crystallization of zircons through closure of argon diffusion in hornblende and micas, between ∼56 and ∼48 Ma. Based on our previous structural analysis, we propose that this first period of rapid cooling follows an exhumation event associated with the formation of the Shuswap MCC by activation of a low-angle detachment and ductile thinning of a previously thickened and partially molten crust. Multidiffusion-domain thermal modeling of K-feldspar constrains the lower temperature history from ∼350°C to ∼150°C and indicates a period of thermal stability from ∼48 to ∼45 Ma, and a cooling event at ∼45 Ma, except for the K-feldspars sampled in the immediate footwall of the Columbia River fault, which indicate rapid cooling at ∼33 Ma. These cooling events are attributed to exhumation accommodated by local activation of high-angle normal faults. Therefore, we distinguish two periods of exhumation in the Canadian Cordillera: (1) Exhumation in Paleocene time related to crustal thinning and tectonic denudation by activation of detachment zones at ∼60-50 Ma, which corresponds to gravitational collapse of the thickened and partially molten Cordilleran crust; and (2) Exhumation related to Eocene regional extension, Basin- and-Range style, at ∼45 Ma, with reactivation of the eastern boundary fault of the core complex, the Columbia River fault, in Oligocene time at ∼33 Ma.
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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 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.001 |
| 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".