Page 7 - i1052-5173-29-11
P. 7

Backarc/Retroarc Basin Strata
          (Kahiltna-Nutzotin-Dezadeash-
          Gravina–Tyaughton/Methow Basins)
            From Alaska to Washington, a belt of    NE                       NE
          Jura-Cretaceous marine assemblages
          separates the WCT from terranes that
          had previously been attached to the conti-
          nental margin (Fig. 1). This basin consists   Former continental margin  Former continental margin
          of an outboard belt that was deposited
          on and derived from the WCT and an
          inboard belt that was deposited on and
          derived from the Intermontane terranes.
          These stratigraphic ties are accepted by      Nutzotin and Kahiltna basins, melange
          Sigloch and Mihalynuk (2017), but in                   LK - Lower Cretaceous plutons
          their interpretation the outboard belt                  UJ - Upper Jurassic plutons MJ - Middle Jurassic plutons LJ - Lower Jurassic plutons
          formed in the forearc of a west-dipping                     CHA - Chisana arc
          subduction zone located along the inboard
          margin of the WCT (Figs. 1C and 2B).
            In south-central Alaska, the Upper
          Jurassic to Upper Cretaceous marine           CHA  LK
          clastic strata are referred to as the Kahiltna
          assemblage (K on Fig. 1) (Hults et al.,    Wrangellia                    Trench
          2013). Prior to final closure, Kahiltna                                         LK - Lower Cretaceous plutons UJ - Upper Jurassic plutons MJ - Middle Jurassic plutons LJ - Lower Jurassic plutons
          assemblage strata along the southern                                                  CHA - Chisana arc
          margin of the basin were sourced from        uplift, unconformity development  UJ
          WCT rocks in a backarc position. Northern         MJ
          Kahiltna assemblage strata were sourced
          from the Intermontane terrane in a forearc   Peninsular terrane        Nutzotin and Kahiltna basins, melange
          basin position related to north-dipping sub-
          duction beneath inboard terranes. Meta-         LJ                                       Figure 2. Schematic cross sections showing inferred tectonic framework of the south-central Alaska segment of the Insular terrane during Late Jurassic–Early Cretaceous time.  (A) East-dipping model. Note inboard-dipping subduction beneath the Wrangellia-Peninsular terrane (i.e., Insular terrane) based on accretionary complex, forearc basin, magmatic arc  elements, narrowing/clos
          morphic rocks, mélange, and submarine
          fan strata are all part of the Kahiltna                             Wrangellia
          assemblage and represent a zone of crustal
          thickening with south-vergent structures            spreading ridge
          (e.g., Brennan et al., 2011). Results from     Chugach  acc.   prism   CHA  LK
          these studies indicate an inboard- (north-)     Trench
          dipping subduction zone along the north-
          ern margin of the Kahiltna basin that
          closed during Late Cretaceous time         oceanic crust
          (Hampton et al., 2010). In eastern Alaska
          and the Yukon Territory, age-equivalent                                uplift, unconformity development  UJ
          basinal strata belong to the Nutzotin                                Peninsular terrane  MJ
          Mountains sequence and the Dezadeash
          Formation (N on Fig. 1). Sedimentological
          and detrital data reflect a provenance                                   LJ                    the marginal basins such as the Kahiltna. Modified from Trop and Ridgway (2007).
          chiefly from Mesozoic arc sources within
          the WCT (e.g., Lowey, 2018).
            In southeastern Alaska, age-equivalent
          basinal strata, referred to as the Gravina
          belt, accumulated along the inboard mar-                              Chugach  acc.   prism
          gin of the Insular terrane and the outboard
          margin of the Intermontane terrane (G on
          Fig. 1) (McClelland et al., 1992). Western            Early Cretaceous   (Berriasian-Aptian)  oceanic crust  Early Cretaceous   (Berriasian-Aptian)
          Gravina strata depositionally overlie and           144-112 Ma
          were derived chiefly from the WCT to the                                      144-112 Ma
          west (Yokelson et al., 2015). In contrast,   A  SW               B  SW
          Jurassic–Cretaceous strata of the eastern
          Gravina belt depositionally overlie Middle
          Jurassic or older rocks of inboard terranes

                                                                                       www.geosociety.org/gsatoday  7
   2   3   4   5   6   7   8   9   10   11   12