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)

Abstract

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.

Reply to “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)”

“…Yokelson et al () argue that this paleogeographic view of the western Gravina belt as a deep‐marine basin adjacent to the Alexander‐Wrangellia terrane poses a significant challenge for Sigloch and Mihalynuk's (, Figure 3b) proposal that an east facing subduction zone existed along the inboard margin of Alexander‐Wrangellia during Late Jurassic‐Early Cretaceous time. Our interpretation was (and remains) that the paleographic constraints developed above require Gravina strata to have accumulated on what is shown by Sigloch and Mihalynuk (, Figure 3b) and Lowey (, Figure 2b) as the Mezcalera ocean floor given that it is the only deep‐marine basin shown inboard of the Alexander‐Wrangellia terrane. If this interpretation is correct, the stratigraphic linkages described above preclude the existence of a subduction zone between the Mezcalera ocean floor and the Alexander‐Wrangellia terrane.…”

“…Lowey's () discussion of the eastern margin of the Gravina belt is puzzling, as it questions our interpretation that eastern Gravina strata accumulated on and were derived from rocks of the adjacent Taku and Yukon‐Tanana terranes, whereas Figure 2b of Lowey () shows eastern Gravina strata resting depositionally on rocks of the Yukon composite terrane (which includes the Taku and Yukon‐Tanana terranes). A comparison of Figure 2b of Lowey () and the 120 Ma reconstruction from Yokelson et al (, Figure 7) suggests that we are in perfect agreement on the primary relationship between the eastern Gravina belt and the inboard Yukon composite terrane. However Lowey () proposes that the eastern Gravina belt and correlative deposits of the Tyaughton‐Methow basin formed in a passive‐margin paleotectonic setting.…”

“…Although both sets of workers agree that western Gravina strata are stratigraphically linked to the Alexander‐Wrangellia terrane, there is disagreement on the paleogeographic position of the western Gravina belt relative to the Alexander‐Wrangellia terrane. The view presented by Lowey ( statement above), Yokelson et al (), and most previous workers is that western Gravina strata accumulated in a deep‐marine basin situated adjacent to the inboard margin of the Alexander‐Wrangellia terrane. For example, Berg et al (, p. D20) report that “The deep‐marine deposition of most of the Gravina‐Nutzotin sedimentary rocks and their great thickness imply rapid depression of a long, linear belt.” Lowey (, p. 275), in reference to strata of the Gravina‐Nutzotin belt, states that “The Dezadeash Formation was deposited during the Late Jurassic to mid‐Cretaceous in a collapsing back‐arc basin located between the Intermontane Superterrane, composed mainly of amalgamated Yukon–Tanana, Nisling and Stikine Terranes that represent the ancient margin of North America, and the docking Insular Superterrane, composed mainly of the amalgamated Alexander and Wrangellia Terranes….” Lowey (, Abstract), in a description of sediments of the Gravina‐Nutzotin basin, reports that “The Dezadeash Formation, a 3000m thick sequence of deepwater turbidites in southwestern Yukon, was deposited as a submarine fan in a backarc basin to the Chitina arc.” All of these descriptions are consistent with Berg et al's (, Figure 10) classic depiction of Gravina strata lapping onto the eastern margin of the Alexander terrane but accumulating mainly in a deep‐marine basin that extended for a considerable distance (e.g., width of the Alexander terrane) to the east.…”

“…The alternative model proposed by Lowey (, Figure 2b) argues instead that strata of the western Gravina belt accumulated as a thin veneer of sediment deposited in a fore‐arc basin that rests entirely on the Alexander‐Wrangellia terrane and does not extend into deep‐marine basinal settings to the east. Although this geometry supports the model of Sigloch and Mihalynuk (), we view it as unlikely because it violates the widely accepted view (e.g., Berg et al, ; Lowey, , ) of western Gravina strata accumulating in a deep marine basin east of the Alexander‐Wrangellia terrane.…”

“…We appreciate this opportunity to discuss the views expressed by Lowey () and Yokelson et al () regarding the origin of the Gravina belt.…”