The Late Cretaceous Middle Fork caldera, its resurgent intrusion, and enduring landscape stability in east-central Alaska
Notice bibliographique
Résumé
Dissected caldera structures expose thick intracaldera tuff and, uncommonly, co genetic shallow plutons, while remnants of correlative outfl ow tuffs deposited on the preeruption ground surface record elements of ancient landscapes.The Middle Fork caldera encompasses a 10 km × 20 km area of rhyolite welded tuff and granite porphyry in eastcentral Alaska, ~100 km west of the Yukon border.Intracaldera tuff is at least 850 m thick.The K-feldspar megacrystic granite porphyry is exposed over much of a 7 km × 12 km area having 650 m of relief within the western part of the caldera fi ll.Sensitive high-resolution ion microprobe with reverse geometry (SHRIMP-RG) analyses of zircon from intracaldera tuff, granite porphyry, and outfl ow tuff yield U-Pb ages of 70.0 ± 1.2, 69.7 ± 1.2, and 71.1 ± 0.5 Ma (95% confidence), respectively.An aeromagnetic survey indicates that the tuff is reversely magnetized, and, therefore, that the caldera-forming eruption occurred in the C31r geomagnetic polarity chron.The tuff and porphyry have arc geochemical signatures and a limited range in SiO 2 of 69 to 72 wt%.Although their phenocrysts differ in size and abundance, similar quartz + K-feldspar + plagioclase + biotite mineralogy, whole-rock geochemistry, and analytically indistinguishable ages indicate that the tuff and porphyry were comagmatic.Resorption of phenocrysts in tuff and porphyry suggests that these magmas formed by thermal rejuvenation of near-solidus or solidifi ed crystal mush.A rare magmatic enclave (54% SiO 2 , arc geochemical signature) in the porphyry may be similar to parental magma and provides evidence of mafi c magma and thermal input.The Middle Fork is a relatively well preserved caldera within a broad region of Paleozoic metamorphic rocks and Mesozoic plutons bounded by northeast-trending faults.In the relatively downdropped and less deeply exhumed crustal blocks, Cretaceous-Early Tertiary silicic volcanic rocks attest to long-term stability of the landscape.Within the Middle Fork caldera, the granite porphyry is interpreted to have been exposed by erosion of thick intracaldera tuff from an asymmetric resurgent dome.The Middle Fork of the North Fork of the Fortymile River incised an arcuate valley into and around the caldera fi ll on the west and north and may have cut down from within an original caldera moat.The 70 Ma land surface is preserved beneath proximal outfl ow tuff at the west margin of the caldera structure and beneath welded outfl ow tuff 16-23 km east-southeast of the caldera in a paleovalley.Within ~50 km of the Middle Fork caldera are 14 examples of Late Cretaceous (?)-Tertiary felsic volcanic and hypabyssal intrusive rocks that range in area from <1 km 2 to ~100 km 2 .Rhyolite dome clusters north and northwest of the caldera occupy tectonic basins associated with northeast-trending faults and are relatively little eroded.Lava of a latite complex, 12-19 km northeast of the caldera, apparently fl owed into the paleovalley of the Middle Fork of the North Fork of the Fortymile River.To the northwest of the Middle Fork caldera, in the Mount Harper crustal block, mid-Cretaceous plutonic rocks are widely exposed, indicating greater total exhumation.To the southeast of the Middle Fork block, the Mount Veta block has been uplifted suffi ciently to expose a ca.68-66 Ma equigranular granitic pluton.Farther to the southeast, in the Kechumstuk block, the fl at-lying outfl ow tuff remnant in Gold Creek and a regionally extensive high terrace indicate that the landscape there has been little modifi ed since 70 Ma other than entrenchment of tributaries in response to post-2.7 Ma lowering of base level of the Yukon River associated with advance of the Cordilleran ice sheet.
Récupéré en direct depuis OpenAlex et désinversé. Les résumés ne sont pas conservés dans cette base de données : les index inversés représentent 8,6 Go des 9,3 Go de texte de la base, et le serveur dispose de 13 Go libres.
Comment cette classification a été obtenuedéplier
Prédiction machine sur la base complète
Imitation des enseignantsNi prévalence calibrée, ni vérité terrain. Validation humaine à venir. Le volet Gemma est une étiquette directe du modèle pour chaque travail de la base, lue sur la notice réduite au titre. Le volet Codex est un classifieur appris des 10 348 étiquettes directes de Codex et calibré sur les taux pondérés de l'échantillon; les champs sans appui suffisant ne portent aucun appel Codex. Le mode candidate est l'union des deux volets; le consensus est leur intersection. Ces sorties portent le statut machine_predicted_unvalidated et ne sont pas des étiquettes humaines.
Scores du classifieur distillé par catégorie (deux têtes)
| Catégorie | Codex | Gemma |
|---|---|---|
| Métarecherche | 0,000 | 0,000 |
| Méta-épidémiologie (sens strict) | 0,000 | 0,000 |
| Méta-épidémiologie (sens large) | 0,000 | 0,000 |
| Bibliométrie | 0,001 | 0,000 |
| Études des sciences et des technologies | 0,001 | 0,000 |
| Communication savante | 0,001 | 0,000 |
| Science ouverte | 0,000 | 0,000 |
| Intégrité de la recherche | 0,000 | 0,000 |
| Charge utile insuffisante (le modèle a refusé de juger) | 0,002 | 0,000 |
Scores machine (provisoires)
Les deux têtes enseignantes du modèle étudiant, lues sur ce travail. Un score ordonne la base pour la relecture; il n'affirme jamais une catégorie, et le statut de validation accompagne chaque rangée tel quel.
Scores de référence d'un modèle non mature (critères de maturité non atteints, 7 itérations). Un score ordonne; il n'affirme jamais une catégorie.
score_only:v0-immature-baseline · tel quel depuis la passe de notation : score_only signifie que le nombre peut ordonner les travaux, et qu'aucune étiquette de catégorie n'en découleClassification
machine, non validéePrédiction automatique; un appel candidat d’une seule source (Gemma direct ou Codex distillé), pas un consensus.
Le détail, modèle par modèle et score par score, se trouve en fin de page sous « Comment cette classification a été obtenue ».