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A  Large ocean basin:  Two  B  Back arc basin +/- oblique  C  West-dipping subduction
                       parallel subduction zones   opening and closure      zone collisions



                                       Brooks Range
                                                               K  Brooks Range

                                    K                           N
                            K                                                 “Alaskan Arcs”
                              Distance
                              Uncertain                        G
                              N     N
                                     G                           M
                                 G
                                       M






                                     ?                         ?                          ? ?
                                      ?                         ?
                                       ?                         ?                         ?
                                          ?                         ?                         ?
                             Wrangellia Composite/Insular  western Kahiltna/Gravina  older accreted terranes
                             Chugach accre onary complex  eastern Kahiltna/Gravina  Cordilleran Miogeoclinal rocks
                                  Ac ve subduc on zone      Backarc closure megathrust
                      Figure 1. End-member models for Early Cretaceous paleogeography. (A) and (B) are both east-dipping subduction
                      models distinguished by the magnitude of the east-dipping megathrust between the Insular (Wrangellia composite
                      terrane [WCT]) and Intermontane terranes (purple); either as a normal subduction zone (A) or a megathrust closing a
                      backarc basin (B) that opened in the Jurassic after an earlier collision of the Insular terrane. Marine basins:
                      K—Kahiltna; N—Nutzotin; G—Gravina; M—Methow. (C) West-dipping subduction zone between the Insular (WCT)
                      and Intermontane terranes (purple) after Sigloch and Mihalynuk (2017). In this model, the Insular terrane migrates
                      from an offshore position during Late Jurassic time and collides far to the south with a north-to-south closure during
                      mid-Cretaceous time. Note also the inferred polarity of north Pacific subduction zones in this model (labeled Alaskan
                      arcs) and distinctions with subduction polarities in models A and B. Figure modified from Kapp and Gehrels (1998).



          and that a second east-dipping subduction   1973; Dickinson, 1974). These interpreta-  much of the suture zone , exhumation
                                                                                                  2
          zone existed along the outboard margin of   tions are complicated by the potential for   along this contact reaches lower-crustal
          the WCT (Figs. 1A and 1B [see footnote   large-scale displacement along strike-slip   depths in the hanging-wall (e.g., Hollister,
          1]). A second group of models emphasizes   faults within and between the various    1982), confounding any attempts to recon-
          collision along a west-dipping subduction   convergent margin assemblages, and by   struct the eroded material.
          zone on the inboard margin of the WCT   removal of elements by subduction erosion   The bottom-up interpretation of polarity
          (Sigloch and Mihalynuk, 2017; Fig. 1C)    or exhumation during collision. These   is based on tomographic images of large,
          or between the entire terrane collage and   complications are the reasons for discrep-  near-vertical features in the mantle inter-
          North America (Johnston, 2008; Hilde-  ancies among existing models based on   preted as subducted slabs (Sigloch and
          brand, 2009).                      geology (Fig. 1). For example, a minimum   Mihalynuk, 2017). These slabs are now in
            The top-down interpretation of sub-   of 700–1500 km of post-latest Cretaceous   the mantle more than 3000 km from their
          duction polarity is based on (1) structural    dextral strike-slip is known from geologic   presumed paleotrench. To restore the path-
          vergence in accretionary prisms; (2) the   relationships alone in the northern   way over this distance requires multiple
          presence and position of high-P/T mineral   Cordillera (Stamatakos et al., 2001), and   assumptions, including the nature of the
          assemblages; (3) the location of forearc   the total dextral slip could be far larger   mantle anomaly, uncertainties in slab
          versus backarc strata; and (4) age and geo-  (e.g., Garver and Davidson, 2015).   sinking rates, and models of absolute plate
          chemical patterns within the magmatic arc.   Similarly, the boundary between the WCT   motion. Problems with absolute plate
          These features have been used to infer sub-  and the continent records closure of an   motion models based on hot spots have
          duction polarity since the advent of plate   ocean basin, a relationship first established   been known since the first plate recon-
          tectonics (e.g., Miyashiro, 1972; Ernst,   by Richter and Jones (1973), but along   structions that used them (Engebretson


          2 We use the terms suture or suture zone as nongenetic terms for areas showing demonstrable evidence of the closure of a deep ocean basin, regardless of basin size;
          i.e., open ocean versus marginal basin.

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