Lithospheric structure and composition of the Archean western Superior Province from seismic refraction/wide‐angle reflection and gravity modeling
Bibliographic record
Abstract
In 1996, LITHOPROBE acquired two ∼600 km long seismic refraction/wide‐angle reflection profiles oriented parallel and orthogonal to the regional geologic strike of the Archean western Superior Province of the Canadian Shield. We present results from ray‐based travel time inversion of these data, combined with modeling of the long‐wavelength gravity field and independent estimates of Vp/Vs (γ), all of which constrain the structure and composition of the lithosphere to depths of 120 km. Large variations in lower crustal Vp (6.7–7.5 km s−1), lower crustal γ (1.72–1.86), upper mantle Vp (8.0–8.8 km s−1), and Moho depth (32–45 km) are determined. Velocities along orthogonal profiles indicate 8% azimuthal Vp anisotropy (fast propagation normal to strike) in part of the lower crust and >6% azimuthal Vp anisotropy (fast propagation parallel to strike) in a 15–20 km thick layer (layer H) in the upper mantle which dips northward at ∼10° from a minimum depth of 48–50 km. Wide‐angle reflections identify a deep boundary at ∼110–120 km depth along both lines. Lower crustal chemical compositions estimated from Vp, γ, and density are generally intermediate (57–66% SiO2) requiring the presence of 25–60% granitic rocks, which is significantly higher than previous estimates (30% intermediate or felsic granulite) for average Archean crust. The anisotropic lower crustal layer characterized by anomalously high Vp, high γlc, and low (−7%) relative density values is interpreted as an amphibolite (basic‐intermediate composition) having a subhorizontal lineation to account for 8% azimuthal Vp anisotropy. The high Vp and intermediate anisotropy of upper mantle layer H requires a harzburgite peridotitic composition with the a axis of olivine aligned E‐W. We suggest that both of these layers are relic oceanic lithosphere accreted at the base of the crust during the final stages of lithospheric assembly in the western Superior Province at ∼2.7 Ga.
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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 | 0.001 | 0.000 |
| Scholarly communication | 0.001 | 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".