Using paleomagnetically derived estimates of latitudinal displacement, within the northern Cordillera we identify two major tectonic domains that were active during Late Cretaceous through Eocene time (roughly 83 to 45 Ma). At 100 to 90 Ma, the Interior Domain (comprising much of interior British Columbia, central Yukon, and eastern and central Alaska) was situated in the latitudes of Oregon and northern California, and the Coast Domain (southeastern Alaska, much of the Coast Ranges and islands of British Columbia, and the Cascade Mountains of Washington) was in latitudes similar to those of northern Mexico. Subsequently, both domains moved northward, reaching their present positions before 45 Ma. Within each domain, displacement estimates from northern locations are greater than those from southern locations, as if the domains had been elongated in an orogen‐parallel sense. Evidently, during the latest Cretaceous through Eocene, the Cordillera was a huge dextral shear system, which (83 to 65 Ma) carried the Coast Domain about 2000 km northward until it combined with the Interior Domain and then (65 to 45 Ma) drove the combined domains an additional 1000 to 2000 km north northwestward, spreading them out along the continental margin. Following previous workers, we ascribe these motions to the rapid oblique subduction of the oceanic Kula Plate beneath the continental western margin of the North American Plate, and derive a model based, in part, on comparisons with the oblique subduction of the Nazca Plate beneath the central Andes. Between the Interior Domain and the craton, geologic evidence in the north has been interpreted as indicating displacements comparable in magnitude to paleomagnetic estimates; in the south, the record has been obscured by Eocene extension. Between the Interior and Coast Domains, geological relationships are complex, and most current geologically based interpretations propose that post‐mid‐Cretaceous relative motions between them did not exceed a few hundred kilometers, whereas paleomagnetic evidence indicates 2000 km. Our reading of the geologic evidence is that it neither requires nor precludes the very large displacements inferred from paleomagnetic observations. Hence we argue that although the paleomagnetically derived displacements far exceed those proposed by geologic interpretations, they are not inconsistent with the geologic evidence itself.

This study contributes the first palcomagnetic data from dated Cretaceous units of the Coast Belt (51.3øN, 123.8øW) in which palcohorizontal bedding surfaces can be identified with certainty. We sampled 38 sites through a 1.5-km stratigraphic interval of Cenomanian Silverquick sedimentary strata and Cenomanian to Campanian Powell Creek volcanics in the Tatlow syncline in the Tyaughton-Methow Basin. Magnetizations at 21 sites have high unblocking temperatures (TUB) and pass the tilt test (precision k = 5 before untilting, maximum k = 48 after 100% untilting). Magnetic minerals of volcanics show original textures, mean declinations in volcanic and sedimentary rocks are statisticall•lelltlCal, and---'•: ........ ' •-• : ^,: •,.• • •o •.4 o pmanty is consistent + , 1101111•1,1. 111• mean lil•11natlon ..• _ •, probability P -0.05) of this pretilting magnetization is 20.1 ø _+ 4.0 ø shallower than that expected if the rocks were deposited in their present position relative to cratonic North America. Field and laboratory observations rule out other causes for these anomalous shallow inclinations. These rocks therefore have been displaced northward by 3000 +_ 500 km relative to cratonic North America since mid-Cretaceous time. This displacement is almost 3 times greater than that estimated from mid-Cretaceous volcanics in the nowadjacent Intermontane Belt. Evidently the two belts were not amalgamated until latest Cretaceous or Early Tertiary time. These results are inconsistent with all previous geologically based tectonic reconstructions of the Cordillera. Magnetizations at eight sites in Silverquick and Powell Creek beds have low Tus and directional characteristics similar to those in three younger diorite intrusions. Reversed and normal polarities occur, the rocks are commonly highly altered, and maximum precision occurs after 40% untilting. The magnetizations therefore are interpreted as overprints of latest Cretaceous age acquired during folding. magnetization direction to be corrected to the palcohorizontal plane, and (2) by obtaining observations from Cretaceous stratified rocks in which accurate bedding attitudes can be determined. The first test has already been carried out for the 96-93 Ma Mount Stuart batholith (Figure 1); Ague and Brandon [1992], on the basis of hornblende geobarometry, have corrected the palcomagnetic observations [Beck et al., 1981] for tilt and obtained an estimate of motion from the south of 2900 + 700 km. We describe the first palcomagnetic results from Cretaceous bedded rocks of the Coast Belt, which allow us to carry out the second test. The sedimentary and volcanic sequence studied is particularly suitable for six reasons. They contain thinly bedded, planar-laminated siltstones and fine-grained sandstones of fluvial overbank origin from which paleohorizontal surfaces can be determined with precision. The nøAr/39Ar data provide the age of the Powell Creek volcanics. The strata occur in a syncline, the Tatlow syncline (Figure 2), with an interlimb 6073 6074 WYNNE ET AL.' MOUNT TATLOW PA...

The mid-Cretaceous Spuzzum and Porteau plutons of the Coast Plutonic Complex of British Columbia have two magnetizations, A and B. The A magnetization (eight sites, 83 specimens, D = 30.3", I = 56.7", ay, = 4.9", paleolatitude = 37 t SON, paleopole 65.0°N, 14.9OW, Ay, = 6.2") is considered to have been acquired in the age range 105-90 Ma. This result differs from the field established for cratonic North America in this time range. The difference could be caused either by previously undetected tilting about a horizontal axis of the plutons, or by their rotation about a vertical axis and lateral displacement relative to the craton. Previously observed mid-Cretaceous magnetizations from other rock units from the western Canadian Cordillera and the Cascades of Washington, United States, are similarly discordant with respect to the craton. This similarity over such a large area indicates that, although local undetected tilting could be partly responsible, it is unlikely to be the prime cause, and we argue therefore that lateral displacement and rotation have occurred. It would seem that much of the western part of the Canadian Cordillera has moved north by about 2400 km and rotated clockwise since the mid-Cretaceous. The paleolatitude of the southern Coast Plutonic Complex of British Columbia is statistically identical to that recently observed (39 * 3ON) for three plutons from the Central Sierra Nevada of California, which raises the possibility that the two complexes were much closer together at the time of their emplacement than at present. The second magnetization called B (four sites, 27 specimens, D = 5. lo, 1 = 67.6", a , , = 4.7", paleopole 86.S0N, 5 1.2"W) is parallel to the mid-Tertiary field, as previously determined from nearby intrusions, and is considered to be an overprint acquired during regional heating and low-grade metasomatism. Some earlier paleomagnetic studies of mid-Cretaceous rocks from the Coast Plutonic Complex indicated either an absence of displacement or uncertain evidence for it, and we attribute this to the nonrecognition, in this earlier work, of similar magnetically stable overprints of Tertiary age. Overprints in several Triassic rock units in the western Cordillera are parallel to the A magnetization, indicating that the mid-Cretaceous and the mid-Tertiary probably were periods of severe magnetic overprinting in British Columbia. Mid-Cretaceous and Late Triassic results from the western Cordillera of British Columbia are systematically different, indicating that movements relative to the craton occurred between these times.Les plutons de Spuzzum et de Porteau d'bge crktact moyen du Complexe plutonique cbtier de la Colombie-Britannique exhibent deux aimantations, A et B. L'aimantationA (huit sites, 83 spkcimens, D = 30,3", 1 = 56,7", ag5 = 4,9O, palkolatitude = 37 2 5"N, palkopole 65,0°N, 14,9"0, A,, = 6,2") semble avoir kt6 acquise entre 105 et 90 Ma. Ce rksultat differe du champ ktabli pour le craton de I'Amkrique du Nord relatif h cet Cpisode de temps. Cette diffkrence peut s'expli...

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Abstract:The 70 Ma Carmacks Group, a subaerial volcanic succession which once covered much of central southwest Yukon, has a paleomagnetic remanent direction which passes the fold test and the reversal test. A new collection of 13 sites, combined with 13 sites from a previous study, renders a pole (088.6°E, 78.4°N, A 95 = 7.8°) which is far-sided with respect to the pole for cratonic North America and implies a displacement from the south of 1900 ± 700 km. Late Triassic Mandanna Member red beds and Early Jurassic Nordenskiöld Formation tuffs, deformed in the Late Jurassic, fail the fold test and conglomerate test but pass a contact test with Eocene dykes. The postdeformational remanent direction is identical to that isolated from the Carmacks Group. The magnetic signature contained in these older formations is probably an overprint produced by an extensive hydrothermal system active during Carmacks extrusion. Geological work indicates that the Carmacks Group is plume related. Given its paleomagnetic latitude and geological nature, we hypothesize that the Carmacks Group is a displaced segment of the Yellowstone hot-spot track, and the hydrothermal system which remagnetized the older rocks was established by mantle upwelling below the region.Résumé : Le Groupe de Carmacks, âgé de 70 Ma, est une succession volcanique subaérienne qui couvrait jadis la majeure partie du centre sud-ouest du Yukon, il possède une direction paléomagnétique rémanente certifiée par le test du pli et le test des inversions. Une nouveau groupe de 13 sites, combiné à 13 sites d'une étude antérieure, fournit un pôle (088,6°E, 78,4°N, A 95 = 7,8°) qui est considérablement éloigné par rapport au pôle du craton de l'Amérique du Nord et qui implique un déplacement à partir du sud de 1900 ± 700 km. Les couches rouges du Membre de Mandanna, du Trias tardif, et les tufs du Formation de Nordenskiöld, du Jurassique précoce, qui subirent une déformation durant le Jurassique tardif ont échoué le test du pli et le test de conglomérat, mais ils ont passé avec succès le test de contact avec les dykes de l'Éocène. La direction rémanente post-tectonique est identique à celle isolée du Groupe de Carmacks. La signature magnétique qui caractérise ces formations plus anciennes est probablement une réaimantation produite par un système hydrothermal étendu qui fut actif durant l'extrusion de Carmacks. L'étude géologique révèle que le Groupe de Carmacks est apparenté à un panache. Étant donné sa latitude paléomagnétique et sa nature géologique, nous avançons l'hypothèse que le Groupe de Carmacks représente un segment déplacé du tracé du point chaud de Yellowstone, et que le système hydrothermal qui a réaimanté les roches les plus anciennes est dû à l'ascension du manteau sous la région étudiée.[Traduit par la Rédaction]