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Dismemberment and northward migration of the Cordilleran
orogen: Baja-BC resolved
GSA TODAY | NOVEMBER 2015 Robert S. Hildebrand, Dept. of Earth & Planetary Sciences,
University of California, Davis, California 95616-8605, USA;
rshildebrand@ucdavis.edu
ABSTRACT
Paleomagnetic results indicate that much of the North
American Cordillera migrated more than 1000 km northward
during the 80–58 Ma Laramide event, yet geologists cannot find
either the faults along which such movement might have taken
place or readily identifiable piercing points to document offset.
Here, I suggest that the sinistral Texas Lineament, which extends
west-northwest from the Gulf of Mexico to the Cordilleran fold-
thrust belt southwest of Las Vegas, and the sinistral Lewis & Clark
transverse zone, located about 1300 kilometers to the north, and
extending from southern Vancouver Island east-southeast to the
thrust belt in the Helena salient, can be restored to one through-
going zone to provide a piercing point that constrains meridional
migration. I interpret the zone as the result of plate interactions on data to resolve the longstanding Baja-BC controversy.
a left-stepping transform margin formed along the southern
margin of North America during Jurassic opening of the Atlantic
Ocean. The structure was dismembered and partly transported
northward along faults in and/or adjacent to the Cordilleran fold-
thrust belt. The proposed restoration also reunites two conspic-
uous bands of Late Cretaceous–Paleocene slab-failure plutons and
porphyry copper deposits into a single zone extending continu-
ously along western North America. This reconstruction obviates
the need for Laramide flat slab subduction.
INTRODUCTION
One of the more contentious aspects of North American
Cordilleran tectonics is the possible meridional migration, based
mostly on paleomagnetic evidence, of large sections of crust (Kerr,
1995). This is the so-called Baja-BC controversy, which was born
when paleomagnetists discovered anomalously shallow paleomag-
netic inclinations in Cretaceous rocks of the Canadian Cordillera
relative to those obtained from rocks of cratonic North America
(Beck and Noson, 1972; Irving, 1979, 1985). The data imply that a
major portion of the coastal Cordillera of British Columbia
migrated northward >1000 km between about 90 and 60 Ma
(Irving, 1985; Irving et al., 1996; Enkin, 2006).
Geologists soon developed models that incorporated the
paleomagnetic data (Umhoefer, 1987; Johnston, 2001, 2008;
Butler et al., 2001; Umhoefer and Blakey, 2006; Hildebrand,
2013) but failed to present obvious matches between rocks of
British Columbia and those much farther south. So, even
GSA Today, v. 25, no. 11, doi: 10.1130/GSATG255A.1.
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though (1) paleomagnetic data were compelling (Beck 1991);
(2) the method worked well elsewhere in the world (Mac Niocaill
et al., 2003); and (3) the long-standing northerly orientation of the
Cordilleran margin would seem to be ideal for paleomagnetic
studies, the geological community hasn’t accepted that thousands
of kilometers of translation had occurred because piercing points
weren’t readily located and because geologists couldn’t identify
the faults along which such large displacements took place (Kerr,
1995; Mahoney et al., 1999; Nelson et al., 2013).
In this contribution, I show that meridional migration within
the Cordillera was not confined to narrow slivers along the coast,
but instead involved the entire width of the Cordillera, from the
Laramide fold-thrust belt westward, as hypothesized by Enkin et
al. (2006a), Johnston (2008), and Hildebrand (2009, 2013). By
utilizing simple cross-cutting relationships and two piercing
points to constrain and support large-scale meridional migration,
I bring the paleomagnetic data into consilience with the geological
GEOLOGY
Decades ago Phil King (King, 1969) divided the Cordillera into
three along-strike sectors—northern, central, and southern
—based on geological differences across two transverse bound-
aries: the Lewis & Clark transverse zone of Montana and Idaho
and the Texas Lineament, which was considered to extend from
the Transverse Ranges of California to the Gulf of Mexico.
Regarding the southern boundary, he wrote (p. 72):
The zone is a strip of country as much as 160 km (100
miles) wide that separates two parts of the Cordillera
with different topographies, geologic histories, and styles
of deformation. South of the zone the Cordilleran fold-
belt extends 800 km (500 miles) farther east than on the
north side, and for long distances its deformed rocks
closely adjoin little deformed rocks in the Colorado
Plateau and the block mountains of New Mexico, which
are reactivated or disrupted parts of the former craton.
These contrasts have not been produced by transverse
faulting, and the Texas Lineament is not a through-
going fault zone, as has sometimes been assumed.
In my earliest paper on the Cordillera (Hildebrand, 2009), I
noted the many changes along the southern margin of the
Colorado Plateau and hypothesized that there must be a fault,
which I called the Phoenix fault, separating the non-extended
Colorado Plateau from the extended zone to the south. At the
time, I was unaware of King’s boundaries and was flummoxed
because I could not decide whether the fault was transform or