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and accumulated outboard of the eastern   and 1B). Thus, extrapolating this outward-   ophiolitic rocks associated with the sys-
          Coast Mountains batholith. Western facies   dipping subduction-collision system to all   tem, and their emplacement over rocks
          of the Gravina belt are interpreted to have   of the Cordilleran margin (e.g., Sigloch   that were clearly part of the North
          been juxtaposed against eastern facies of   and Mihalynuk, 2017) is not warranted.  American passive margin from southern
          the Gravina belt by Early Cretaceous sinis-                           British Columbia (e.g., Slide Mountain
          tral strike-slip followed by mid-Cretaceous   Summary of the Geologic Data  terrane; Roback et al., 1994) to Alaska
          structural imbrication (Monger et al.,   The most marked differences in the    (e.g., Seventy Mile terrane; Dusel-Bacon
          1994). Nowhere along British Columbia or   models (Fig. 1) are (1) the inferred polarity   et al., 2006), attest to a collision along a
          southeast Alaska have direct remnants of   of subduction zones during Late Jurassic–  west-dipping subduction zone. We follow
          subduction been observed within the   Early Cretaceous time; and (2) the nature    Monger (2014) in suggesting that this
          basinal strata or along these thrusts.  of the suture zone inboard of the WCT.    event provides a likely explanation of the
            Farther to the south, eastern facies   The upper-plate geology preserves abundant   geophysical observations of Sigloch and
          strata of the Gravina belt extend into    lines of evidence for an east-dipping sub-  Mihalynuk (2017). A scenario that incor-
          the Tyaughton-Methow basin (M on    duction zone beneath the outboard margin   porates these earlier events as an explana-
          Fig. 1), which also consists of Upper   of WCT during Jurassic–Late Cretaceous   tion of the tomographic anomalies is pro-
          Jurassic–Cretaceous marine strata and   time as well as south to north closure of a   vided in GSA Data Repository Fig. DR1 .
                                                                                                               3
          subordinate volcanic rocks. These basins   marine basin between the WCT and North
          are interpreted to record east-dipping   America along an east-dipping megathrust.   Reconciling the Top-Down Record with
          subduction during Late Jurassic–Early   There is virtually no evidence for west-   the Bottom-Up Record: Implications for
          Cretaceous time, followed by arrival of   dipping subduction anywhere along the   Whole-Earth Processes
          the Insular terrane along the Cordilleran   inboard margin of the WCT.  Reconciling these issues is an impor-
          margin during Albian time (e.g., Sur-                                 tant problem because it relates to whole-
          pless et al., 2014). These relations sug-  DISCUSSION AND CONCLUSIONS  earth processes of mantle convection and
          gest that most basin strata formed along                              past plate motion. We suggest that a chal-
          an east-dipping subduction zone con-  Alternative Explanation of Geophysical   lenge to the broader community is provid-
          structed on most of the western margin   Observations                 ing clear tests of the hypothesis that the
          of the North American continent. Deep   Although we have emphasized the geo-  deep anomalies are indeed subduction
          exhumation and strike-slip faulting   logic record relative to the WCT here, the   zone remnants, which will require clear
          obscure details (Figs. 1A and 1B), but   record of east-dipping subduction during   correlations to the geologic record.
          there is no evidence to support the inter-  Late Jurassic through Late Cretaceous   Conversely, assuming the interpretation
          pretation of Sigloch and Mihalynuk   time is even better established along the   of the deep anomalies as subducted litho-
          (2017) that these basins formed in a   continental margin of Oregon, California,   sphere is correct, the community must
          west-dipping subduction zone.      and northwestern Mexico by the Sierra   develop tectonic models that fit both the
                                             Nevada, Great Valley, and Franciscan   deep geophysical data and the geologic
          Evidence from Northern Alaska      assemblages. Any tectonic model calling   record. Resolving this conflict is a funda-
            The only part of the northern Cordillera   on west-dipping subduction during this   mental tectonic problem that requires
          that has a clear geologic signal of post-  time interval must address how these   integrated analysis between geologists
          mid Jurassic outboard (away from conti-  iconic tectonic relations have been misin-  and mantle observers/modelers.
          nental margin) subduction is in northern-  terpreted by generations of geologists   The diversity of tectonic models (Fig. 1)
          most Alaska (e.g., Moore et al., 1994).   (e.g., Dickinson, 1974). Ribbon continent   places the community in a quandary. New
          There, in the Brooks Range, structural   reconstructions of western North   models based on geophysics can stir debate,
          and metamorphic evidence shows sub-  America (e.g., Johnston, 2008) provide   but these insights must be consistent with
          duction of the continental margin beneath   alternate views, but represent even more   the geologic record and what the tomogra-
          a Late Jurassic–earliest Cretaceous island   glaring contradictions to generations of   phy data actually show (e.g., Liu, 2014).
          arc, the Koyukuk terrane (Box and   geologic studies (see discussion in Sigloch   Nonetheless, among a larger group of north-
          Patton, 1989). Fragments of an ocean   and Mihalynuk, 2017).          ern Cordilleran geologists, the evidence
          basin were emplaced on the continental   It is important to note that there is    from surface geology seems overwhelm-
          margin as the Angayucham complex dur-  evidence of west-dipping subduction in    ingly opposed to the tomography-based
          ing collision at ca. 145–135 Ma (Roeske et   the Cordillera, but it is clearly pre-Late   conclusions. How then can we proceed?
          al., 1989; Lemonnier et al., 2016). Possible   Jurassic and does not involve the WCT   One approach is to assemble working
          tectonic connections, if any, between the   (Monger, 2014). Instead, vast areas that   groups with broad knowledge that tackle a
          northern and southern Alaska Mesozoic   comprise the terranes inboard of the WCT   problem by integrating information from a
          arcs are highly uncertain, particularly   show evidence of Permo-Triassic ocean   wide range of approaches and attempt to
          given that this collision occurred prior to   basin closure along a west-dipping sub-  arrive at a solution that honors all observa-
          the opening of the Canada basin when the   duction interface that existed until Early–  tions. Alternatively, new insights might
          orogen faced north, not south (Figs. 1A   Middle Jurassic time. Widespread   arise from technological advances (e.g.,


          3 GSA data repository item 2019259, Figure DR1, showing the tectonic models discussed in the text in a global projection, is online at www.geosociety.org/
          datarepository/2019.

       8  GSA Today  |  November 2019
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