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Figure 4. Much simplified reconstruction of
GSA TODAY | NOVEMBER 2015 Cordilleran fold-thrust belt terminates just southwest of Las Vegas
and to the south resumes ~800 km farther east, as noted by King
(1969). South of the zone, terranes collided with North America
much farther east than to the north, but the overall geology is
similar in many respects simply because some of the same events
took place on both sectors of the continental margin. Others, such as Coast plutonic complex of British Columbia and the Yukon
deformational features related to the 125–110 Ma Sevier event, have
no recognized counterpart to the south because the impinging
block, which Stephen Johnston and I (Johnston, 2008; Hildebrand,
2013) argued to have been a ribbon continent, did not arrive there
until much later, as suggested by the coincidence of the distinctive
eastward-extending prong of the mantle fast-zone (Sigloch and
Mihalynuk, 2013), and the meridionally restored location of the
Great Basin region, at about 125 Ma (Hildebrand, 2014).
A major Late Cretaceous–Paleocene magmatic belt, interpreted
by Hildebrand (2013) to represent Laramide slab failure magma-
tism and metallogenesis, extends from Alaska to just south of the
Lewis & Clark transverse zone and from southern Mexico to the
Transverse Ranges (Figs. 1 and 2). It provides another robust
piercing point. The present-day magmatic gap in between the
two was perhaps the most important reason to ascribe Laramide
thick-skinned deformation to flat-slab subduction (Dickinson and
Snyder, 1978; Humphreys, 2009), but by reuniting the transverse
zones, the two belts of Laramide magmatism and their related
porphyry copper deposits are joined, obliterating the magmatic
gap and validating the overall reconstruction (Fig. 4).
It is worth noting that the Laramide magmatic belt has exhu-
mation ages of 70–50 Ma over its entire length (Miller and
Morton, 1980; Wells and Hoisch, 2008; Miller et al., 2009;
Armstrong, 1988) and that there are two bands of Laramide
deformation: the better known band, mostly without proximal
magmatism, located in the eastern Cordillera, and another with
associated high-grade metamorphism and generally rapid
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margin just prior to northward migration
illustrating continuity of Late Cretaceous–Early
Tertiary magmatic belt. The sinuous shape of
the magmatic belt reflects the shape of the
southwestern margin of North America
immediately after terminal collision of the
Cordilleran Ribbon Continent.
exhumation located farther west in or adjacent to the magmatic
belt. The deformation, magmatism, and exhumation are well
known throughout the Transverse Ranges and Mojave Desert
region of Southern California and Arizona (Haxel et al., 1984;
May, 1989; Needy et al., 2009), as well as in the Cascades and
(Miller et al., 2009; Rusmore and Woodsworth, 1991; Parrish,
1992; Evenchik et al., 2007; Johnston and Canil, 2007).
Overall, the proposed reconstruction resolves many long-
standing issues in Cordilleran geology and hints at solutions to
many more. Not only are the paleomagnetic data accounted for,
and the Baja-BC controversy resolved, but the currently dismem-
bered Laramide magmatic, deformational, and metamorphic
collisional belt is reunited and validates the reconstruction.
Hopefully, this initial first-order model will lead others to work
backward through time to better understand the development of
the Cordilleran orogen. Many more surprises are yet to come.
CONCLUSIONS
1. Similar relationships of well-dated and mapped units along
both sides of the Lewis & Clark zone and the Texas Lineament
suggest that the two features were formerly continuous.
2. By restoring 1300 km of dextral slip along the Cordilleran
fold-thrust belt—about the minimum indicated from paleo-
magnetic data—the Lewis & Clark transverse zone and the
Texas Lineament are aligned into a continuous structure. The
reconstruction is simple and clarifies many relationships that
were previously difficult to explain.
3. In the reconstruction (Fig. 4), the Laramide collision zone and
its exhumed upper-plate slab-failure rocks occur in a contin-
uous band from southern Mexico through the Transverse
Ranges into the Cascades and Coast plutonic complex. Thus,
there was no magmatic gap during the Laramide.