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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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