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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

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