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et al., 1984). Nonetheless, the evidence sinistral-oblique south-directed thrusting The spatial configuration of these three
used to support west-dipping subduction is at 125–120 Ma (Labrado et al., 2015); tectonic elements requires north-dipping
that when North America is restored to its (4) a greywacke/conglomerate package (present coordinates) subduction beneath
mid-Cretaceous position, the Cordilleran from 100 to 90 Ma (Amato et al., 2013); the outboard margin of WCT (Fig. 2)
margin lay east of the deep mantle anoma- and (5) turbidites from 90 to 70 Ma throughout late Mesozoic time.
lies. Hence, a west-dipping subduction (Amato et al., 2013). Intermittent accre-
zone provides a simple explanation, albeit tion continues to the present day. Magmatic Arc Rocks (Talkeetna-
dependent on these assumptions. These data demonstrate a strong Chitina-Chisana-Kluane–Coast
temporal link between this accretionary Mountains Arcs)
GEOLOGIC OBSERVATIONS complex and the adjacent forearc basin In southern Alaska, the Jurassic arc
SUPPORTING EAST-DIPPING and arc. When younger strike-slip dis- system built on the WCT is the Peninsular
SUBDUCTION—THE TOP-DOWN placement is restored, this link has led terrane, or Talkeetna arc. This arc shows
APPROACH to the long-standing interpretation that a continuous magmatic record from ca.
Here we review the geologic evidence subduction polarity along what is now 200–150 Ma, but magmatism migrated
for subduction polarity in the northern the southern/western margin of Alaska northward in time with Early Jurassic
Cordillera using distributions of key to British Columbia and the Pacific rocks exposed in an upturned crustal-
tectonic elements. Northwest has been continuous from ca. mantle section to the south and an Early
210 Ma to present. The recent reference to Middle Jurassic granitic batholith on
Chugach Accretionary Complex to this interpretation as a “myth” (Sigloch the north (e.g., Clift et al., 2005; Hacker et
The Chugach accretionary complex and Mihalynuk, 2017) is perplexing, as al., 2011). Although early studies using
is exposed outboard of Early–Middle no other reasonable tectonic scenario has geochemical trends in the batholith
Jurassic plutonic rocks of the Jurassic been suggested to explain the presence allowed from south-dipping subduction
Talkeetna arc built on the northern WCT of blueschist-facies rocks located in the (Reed et al., 1983), those studies failed to
(Fig. 2). It records progressive outboard “backstop” of an accretionary complex recognize that the Early Jurassic rocks to
accretion of an ~60–100-km-wide pack- and coeval with an oceanic magmatic the south were part of the same arc sys-
age of sedimentary/volcanic rocks with arc in the adjacent terrane. tem. Thus, a broader view of geochemical
metamorphic or maximum depositional trends shows a pattern indicative of north-
ages that young consistently to the south, Forearc Basin Strata (Cook Inlet– ward subduction with mafic rocks to the
away from the arc (e.g., Plafker et al., Matanuska–Wrangell Mountains Basins) south and more silicic rocks to the north
1994; Amato et al., 2013). This age pro- Thick successions of Middle Jurassic and an age trend indicating northward
gression matches classic forearc accre- to Upper Cretaceous siliciclastic strata migration of the magmatic arc (Clift et al.,
tionary models with gaps in the record and minor volcanic rocks lie inboard 2005; Rioux et al., 2007). This pattern,
compatible with subduction erosion. (north) of the Chugach accretionary together with age-equivalent accretionary
The oldest rocks in the accretionary complex and outboard (south) of volcanic- complex rocks exposed to the south
complex from north (closest to the arc) plutonic belts attributed to arc magma- (Amato et al., 2013), leaves virtually no
to south (outboard) are blueschist-facies tism in south-central Alaska. These doubt that Jurassic subduction was north
fault-bounded slices of oceanic material, strata reflect deposition in intra-arc and dipping (Fig. 2A).
with 204–185 Ma crystallization ages forearc depocenters with respect to the Middle Jurassic to Late Cretaceous
(e.g., Sisson and Onstott, 1986; Roeske et Talkeetna-Chitina-Chisana arcs to the plutons and associated volcanic rocks
al., 1989). The accretion record is miss- north (Trop and Ridgway, 2007), and intrude and overlie much of the WCT in
ing between ca. 185–170 Ma, which cor- sediment was sourced chiefly from these south-central Alaska (Plafker and Berg,
responds to an inboard migration of the arcs (Stevens Goddard et al., 2018). 1994) and continue southward along the
arc, when subduction erosion destroyed Locally, sediment was eroded from coast to central British Columbia, where
part of the forearc (Clift et al., 2005) and sources within the Chugach accretionary they become the western Coast
the forearc basin became well estab- complex starting in early Late Cre- Mountains batholith (Gehrels et al.,
lished (Stevens Goddard et al., 2018). taceous time. U-Pb detrital zircon data 2009). A first-order observation con-
This lack of preservation is cited by show a shared source of magmatic-arc cerning the polarity of these arcs is that
Sigloch and Mihalynuk (2017) as evi- sediment for both the forearc basin and all segments record eastward migration
dence that the accretionary complex is accretionary complex during the Jurassic of magmatism at ~2 km/m.y. from ca.
not linked with the Jurassic arc system and Cretaceous, and this Mesozoic 120–80 Ma (Cecil et al., 2018). This rate,
despite the clear evidence globally that detrital link between the accretionary age, and direction of arc migration are
subduction erosion removes material complex, the forearc basin, and the mag- also shared by the Sierra Nevada and
from subduction complexes (e.g., von matic arc on the upper plate indicates a Peninsular Range batholiths, which are
Huene and Scholl, 1991). Continued kinship between these different elements interpreted to have faced to the west in
accretion and underplating produced (Stevens Goddard et al., 2018). Moreover, nearly all Cordilleran syntheses. These
(1) a mélange assemblage with maximum detrital zircon populations from Albian magmatic shifts are consistent with evi-
depositional ages (MDA) of 170–155 Ma; and younger strata reflect sedimentary dence in the accretionary complex for
(2) blueschists constrained by MDA to linkage with sources in the WCT and subduction erosion and ridge subduction
ca. 135–100 Ma (Day et al., 2016); (3) Intermontane terranes (Reid et al., 2018). (e.g., Amato et al., 2013).

6 GSA Today | November 2019