U-Pb and Hf isotope analysis of detrital zircons from Mesozoic strata of the Gravina belt, southeast Alaska

Abstract

The Gravina belt consists of Upper Jurassic through Lower Cretaceous marine clastic strata and mafic-intermediate volcanic rocks that occur along the western flank of the Coast Mountains in southeast Alaska and coastal British Columbia. This report presents U-Pb ages and Hf isotope determinations of detrital zircons that have been recovered from samples collected from various stratigraphic levels and from along the length of the belt. The results support previous interpretations that strata in the western portion of the Gravina belt accumulated along the inboard margin of the Alexander-Wrangellia terrane and in a back-arc position with respect to the western Coast Mountains batholith. Our results are also consistent with previous suggestions that eastern strata accumulated along the western margin of the inboard Stikine, Yukon-Tanana, and Taku terranes and in a fore-arc position with respect to the eastern Coast Mountains batholith. The history of juxtaposition of western and eastern assemblages is obscured by subsequent plutonism, deformation, and metamorphism within the Coast Mountains orogen, but may have occurred along an Early Cretaceous sinistral transform system. Our results are inconsistent with models in which an east-facing subduction zone existed along the inboard margin of the Alexander-Wrangellia terrane during Late Jurassic-Early Cretaceous time.

Comment on “U‐Pb and Hf Isotope Analysis of Detrital Zircons From Mesozoic Strata of the Gravina Belt, Southeast Alaska” by Yokelson Et Al. (2015)

“…Generally, metamorphism and deformation increase toward the east where the terranes are engulfed by the Coast Mountains batholith. The phyllite, schist, and metaconglomerate assemblage was originally interpreted as part of the Taku terrane (Berg et al, ) but reinterpreted by Rubin and Saleeby () as part of the Gravina belt and named the Gravina sequence (the eastern Gravina belt of Yokelson et al, ).…”

“…Yokelson et al () update the narrow marine basin, sinistral juxtaposition model of Gehrels () by moving the eastern Gravina belt adjacent to the Tyaughton and Methow strata so that all three units occupy a fore‐arc position along the western margin of the Yukon composite terrane, and shifting the hypothetical 1000 km long sinistral fault zone from within the Yukon composite terrane westward to the contact between the Yukon composite terrane and the back‐arc basin assemblages (presumably corresponding to the inboard margin of the Wrangellia composite terrane).…”

“…The hypothetical fault that is invoked to duplicate the arc and back‐arc basin is the essence of the sinistral juxtaposition model of Yokelson et al (). The absence of evidence for this fault is not a problem per se, but Yokelson et al () employ a double standard for evaluating competing hypothesis.…”

“…Yokelson et al () present U‐Pb and Hf isotope analyses of detrital zircons from the Gravina belt to infer that the western facies (western Gravina belt, Dezadeash Formation, and Nutzotin Mountains sequence) was derived from the west and the eastern facies (eastern Gravina belt) was derived from the east, refine the narrow marine basin, sinistral juxtaposition model of Gehrels () for the tectonic setting of the western and eastern facies, and promote their paper as an important test of the Sigloch and Mihalynuk () hypothesis of west dipping subduction beneath the Alexander and Wrangellia terranes. Yokelson et al () conclude that the U‐Pb and Hf isotopic results are 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 ().…”

“…Yokelson et al () conclude that the U‐Pb and Hf isotopic results are 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 (). However, the tectonic affinity of the eastern Gravina belt is open to debate because Yokelson et al () overstate the geologic certainty of this unit, the sinistral juxtaposition model is based on a hypothetical fault that Yokelson et al () use as a double standard to dismiss competing hypotheses for the tectonic setting of the western and eastern facies, and Yokelson et al's () conclusion that the Sigloch and Mihalynuk () model of west dipping subduction was implausible is based on the incorrect assumption that the western Gravina belt was deposited in the Mezcalara basin.…”