New detrital zircon U-Pb and Hf isotope constraints for the Kluane Schist, southwestern Yukon, Canada, and their implications for Cretaceous Insular terrane accretion: COMMENT

McKenzie et al. (2025) report the results of U-Pb age and Hf isotope analyses of detrital zircons from the Late Cretaceous Kluane Schist, a metasedimentary assemblage in southwestern Yukon, Canada, that formed during accretion of the Insular terrane with the Mesozoic margin of western North America. McKenzie et al. (2025) combine their detrital zircon data with similar datasets from other Jurassic and Cretaceous strata in the northern Cordillera of Canada and Alaska and present a model for the tectonic evolution of the Jurassic−Cretaceous basins in which accretion of the Insular terrane was facilitated by east-dipping subduction beneath the North American continent. McKenzie et al. (2025) conclude that their results are not consistent with the model of Sigloch and Mihalynuk (2013, 2017), in which accretion of the Insular terrane allegedly involved the closure of a 1000-km-wide Jurassic−Cretaceous ocean basin bounded by a west-dipping subduction zone. McKenzie et al. (2025) misrepresent the “western” subduction model of Sigloch and Mihalynuk (2013, 2017). In particular, McKenzie et al. (2025, p. 963) state that, according to the intraoceanic archipelago, westward subduction model proposed by Sigloch and Mihalynuk (2013, 2017), “the progressive accretion of this archipelago to the western North American paleomargin occurred diachronously between ca. 155 Ma and ca. 50 Ma. The closure of the intervening Mezcalera-Angayucham Ocean basin and the consequent accretion of the archipelago is thought to have been largely driven by slab pull of a west-dipping subduction system beneath the inboard margin of the archipelago.” In actuality, Sigloch and Mihalynuk (2013, 2017) proposed contemporaneous westward and eastward subduction, as well as westward-then-eastward subduction, as clearly indicated in figures 3b and 3d in Sigloch and Mihalynuk (2013), figure 2C1 in Sigloch and Mihalynuk (2017), and table 2 in Sigloch and Mihalynuk (2017), which states that collision of North America with intraoceanic arcs resulted in a flip in the polarity of subduction from westward to eastward at ca. 125 Ma. Furthermore, Sigloch and Mihalynuk (2020) stated that their model contains as much eastward subduction as the (more popular) long-term eastward subduction Andean-style model, and they provided a new figure correcting any misinterpretation of their model (Sigloch and Mihalynuk, 2020, fig. 1A). Although the controversy regarding the subduction polarity along the Mesozoic margin of North America is apparently well known, the misinterpretation of the model proposed by Sigloch and Mihalynuk (2013, 2017) as a westward subduction model also appears in other studies (e.g., Pavlis et al., 2019, 2020). McKenzie et al. (2025) also ignore the caveat expressed by Sigloch and Mihalynuk (2017, fig. 1B) that the size of the ocean separating the intraoceanic archipelago from the North American margin is a “negotiable region’” due to the width of the area being unconstrained by marine surface observations. Lastly, McKenzie et al. (2025) is based on the dissertation by McKenzie (2023), and McKenzie (2023) acknowledges that east-dipping subduction occurred on the outboard margin of the Insular terranes in the late Cretaceous according to the model of Sigloch and Mialynuk (2013, 2017). In conclusion, Sigloch and Mihalynuk’s (2013, 2017) model is not restricted to the closure of a “1000-km-wide Jura-Cretaceous, ocean basin bounded by a west-dipping subduction zone” (McKenzie et al., 2025, p. 984). Rather, the model proposed by Sigloch and Mihalynuk (2017) includes a flip in the polarity of subduction from westward to eastward at ca. 125 Ma, thus constraining the evolution of Cretaceous basins—such as the Kluane Schist basin—to an eastward subduction setting. Therefore, the results presented by McKenzie et al. (2025) are compatible with the westward-then-eastward subduction model proposed by Sigloch and Mihalynuk (2013, 2017). To sufficiently advance our understanding of the Cretaceous Insular terrane accretion, the westward and eastward subduction components of Sigloch and Mihalynuk’s (2013, 2017) model must be acknowledged.