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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