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The provenance of Jurassic to Cretaceous fl ysch along the northern boundary of the allochthonous Wrangellia composite terrane, exposed from the Lake Clark region of southwest Alaska to the Nutzotin Mountains in eastern Alaska, suggests that the fl ysch can be divided into two belts having different sources. On the north, the Kahiltna fl ysch and Kuskokwim Group overlie and were derived from the Farwell and Yukon-Tanana terranes, as well as smaller related terranes that were part of the paleo-Alaskan margin. Paleocurrent indicators for these two units suggest that they derived sediment from the north and west. Sandstones are predominantly lithic wacke that contain abundant quartz grains, lithic rock fragments, and detrital mica, which suggest that these rocks were derived from recycled orogen and arc sources. Conglomerates contain limestone clasts that have fossils matching terranes that made up the paleo-Alaskan margin. In contrast, fl ysch units on the south overlie and were derived from the Wrangellia composite terrane. Paleocurrent indicators for these units suggest that they derived sediment from the south. Sandstones are predominantly feldspathic wackes that contain abundant plagioclase grains and volcanic rock fragments, which suggest these rocks were derived from an arc. Clast compositions in conglomerate south of the boundary match rock types of the Wrangellia composite terrane.The distributions of detrital zircon ages also differentiate the fl ysch units. Flysch units on the north average 54% Mesozoic, 14% Paleozoic, and 32% Precambrian detrital zircons, refl ecting derivation from the older Yukon-Tanana, Farewell, and other terranes that made up the paleo-Alaskan margin. In comparison, fl ysch units on the south average 94% Mesozoic, 1% Paleozoic, and 5% Precambrian zircons, which are consistent with derivation from the Mesozoic oceanic magmatic arc rocks in the Wrangellia composite terrane. In particular, the fl ysch units on the south contain a large proportion of zircons ranging from 135 to 175 Ma, corresponding to the age of the Chitina magmatic arc in the Wrangellia terrane and the plutons of the Peninsular terrane, which are part of the Wrangellia composite terrane. Flysch units on the north do not contain signifi cant numbers of zircons in this age range. The fl ysch overlying the Wrangellia composite terrane apparently does not contain detritus derived from rocks of the paleo-Alaska margin, and the fl ysch overlying the paleo-Alaskan margin apparently does not contain detritus derived from the Wrangellia composite terrane.The provenance difference between the two belts helps to constrain the location of the northern boundary of the Wrangellia composite terrane. Geophysical models place a deep, through-going, crustal-scale suture zone in the area between the two fl ysch belts. The difference in the provenance of the two belts supports this interpretation. The youngest fl ysch is Late Cretaceous in age, and structural disruption of the fl ysch units is constrained to the Late Cretaceous, so it ap...

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The results of more than 20 years of geochronological studies in the Yukon Crystalline Terrane in east-central Alaska and the western Yukon Territory suggest at least six igneous and thermal (metamorphic?) events. Plutonism during Mississippian, Early Jurassic, mid-Cretaceous, Late Cretaceous, and early Tertiary times is indicated. Evidence also indicates that Mississippian, Early Jurassic, late Early Cretaceous, and late Cretaceous thermal (metamorphic?) events have affected parts of the terrane. The western part of the terrane was affected by a significant regional metamorphic event in late Early Cretaceous time, followed by a terrane-wide mid-Cretaceous plutonic event. The pattern of K-Ar ages allows division of the terrane into domains, bounded by northeast-trending lineaments.L'examen des rksultats de plus de 20 annkes d'ktudes gkochronologiques de la plate-forme cristalline du Yukon rkvkle au moins la prCsence de six kvknements ignks et thermiques (mktamorphiques?). Le plutonisme fut actif durant le Mississippien, le Jurassique infkrieur, le Crktack moyen, le Crktack supkrieur et le Tertiaire infkrieur. On observe que des manifestations thermiques (mktamorphiques?) ont affectk des parties de la plate-forme cristalline du Yukon durant le Mississippien, le Jurassique infkrieur, le CrktacC tardi-infkrieur et le Crktace supkrieur. Durant le Crktack tardi-infkrieur est survenu un important kvknement de mktamorphisme regional qui a affectk la partie occidentale de la plate-forme cristalline, lequel fut suivi au CrktacC moyen par un kvknement plutonique englobant toute la largeur de la plate-forme cristalline. Le motif d'ktalement des Lges K-Ar permet de diviser la plate-forme cristalline en domaines, lesquels sont limitks par des lineaments de direction nord-est.[Traduit par le journal]Can.

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