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