Page 5 - i1052-5173-29-11

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.

www.geosociety.org/gsatoday 5