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ABSTRACT

Upper Jurassic to Upper Cretaceous forearc basin strata exposed in south-central Alaska record variations in sedimentary basin development associated with late Mesozoic terrane accretion and translation of the Insular terranes (Alexander-Wrangellia-Peninsular) along the western North American margin. Upper Jurassic strata contain a dominant population of Late Jurassic (ca. 150 Ma) detrital zircons with juvenile to intermediate isotopic compositions (εHf[t] >5) that reflect sediment input from the adjacent oceanic Talkeetna arc, located to the north in a marine forearc basin. By Early Cretaceous time, Early–Late Jurassic (ca. 160 and 198 Ma) and minor Late Devonian–Permian detrital zircon grains that have juvenile to intermediate hafnium compositions (εHf[t] >5) indicate continued exhumation of the Talkeetna arc and a new sediment contribution from erosion of Insular terrane basement. Upper Cretaceous strata record final suturing of the terrane, resulting in an influx of sediment from the inboard Intermontane terrane, represented by Ordovician–Permian detrital zircon grains with a wide range of hafnium compositions (εHf[t] >–25). Late Cretaceous detrital zircon ages (ca. 90 and 70 Ma) in these strata reflect establishment of a coeval volcanic arc. Trends toward more-evolved hafnium isotopic data within our data record ~40 m.y. orogenic cycles of crustal thickening during the Late Triassic–Early Jurassic and Late Jurassic that can be linked to Cordilleran-scale deformational events. Overall, we prefer a model where the forearc basin in south-central Alaska, the part of the Intermontane terranes that is today located in northern British Columbia, Yukon, and eastern Alaska, and the part of the Chugach–Prince William terrane found on Kodiak Island were all juxtaposed by Late Cretaceous (Turonian) time.

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