The Mesozoic convergence of the allochthonous Wrangellia composite terrane (WCT) with the western margin of Laurasia coincided with the construction of the Chitina magmatic arc on the WCT, and the dispersal of volcanic flows and sediment gravity flows into an adjacent flysch basin. The basin, preserved as the Gravina–Nutzotin belt, includes the Dezadeash Formation in southwest Yukon, the Nutzotin Mountains sequence in southern Alaska, and the Gravina belt in southeastern Alaska. The Dezadeash Formation is a submarine fan system comprising stacked channel-lobe transition and lobe deposits interposed with overbank deposits. Conglomerate pebble-counts, sandstone point-counts, detrital zircon ages, and major element, trace element, rare earth element, and Sm–Nd isotopic geochemistry of sandstone, mudstone, and hemipelagite beds suggests that the deposits consist mainly of first-cycle volcanogenic detritus shed from the undissected Chitina arc, in addition to material eroded from the WCT. The arc was constructed of undifferentiated magma sourced from the depleted mantel, as well as older crustal material attributed to the WCT proxying for continental crust. The compositional provenance results, together with published paleocurrent data for the Dezadeash Formation and compositional and directional provenance indicators from the Nutzotin Mountains sequence and Gravina belt, does not require a sediment source from Laurasia. The provenance record is compatible with deposition of the Gravina–Nutzotin belt in a convergent plate margin setting.

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Abstract.The Tatshenshini

Yokelson et al. (2015) present U‐Pb and Hf isotope analyses of detrital zircons from the Gravina belt showing that the western facies was derived from the west and the eastern facies was derived from the east and concluded that this is evidence that a west dipping subduction zone did not exist along the inboard margin of the Alexander‐Wrangellia terrane during Late Jurassic‐Early Cretaceous time as proposed by Sigloch and Mihalynuk (2013). However, the tectonic affinity of the eastern Gravina belt is open to debate, and Yokelson et al.'s (2015) claim that the Sigloch and Mihalynuk (2013) model of west dipping subduction was implausible is based on an incorrect assumption.

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