The paleomagnetic record for the western edge of North America shows consistent, systematic discordance. Virtually none of roughly three dozen high-quality paleomagnetic studies of pre-Pliocene rock units located within several hundred kilometers of the continental margin are lying anywhere near their appropriate reference poles (constructed from high-quality paleomagnetic data from the craton). Nor do these discordant paleomagnetic studies show simple random scatter, as might be expected for rock bodies whose postmagnetization histories include reheating, complex tectonic deformation, or chemical change. Instead, their paleomagnetic poles are clearly displaced systematically away from the reference curve into the general area of the Atlantic Ocean. To accomplish this tectonically requires that the magnetized rock body move northward, rotate clockwise, or both, in relation to North America. The consistency of paleomagnetic record argues that most of the western edge of North America has undergone such block movement and therefore is allochthonous, at a scale at least as large as the area of a typical paleomagnetic study. It further argues that northward transport and clockwise rotation have been prime elements in shaping the Cordillera. Differences between paleomagnetic records for separate parts of the Cordillera probably reflect differences in specific platetectonic histories. For instance, although northward transport of allochthonous blocks is important south of Cape Mendocino and north of southern Vancouver Island, in the area between (the Pacific Northwest), only clockwise rotations are found. This is consistent with steady under-thrusting of the Pacific northwest coastline by the Farallon plate for most of the Tertiary, which is in contrast to the steady or intermittent transform activity to the north and south. Batholith belts, including the Peninsular Range batholith of Southern California and the Coast Plutonic Complex of British Columbia, also seem to have been involved in the general northward transport and clockwise rotation, although the possibility of undetected tectonic tilts clouds the record. However, consideration of the thermal behavior of large, tilted plutonic blocks suggests that post-magnetization tilts probably are small. If so, then most western Cordilleran Cretaceous batholiths (not including the Sierra Nevada) also are allochthonous. Northward transport and clockwise rotation could be accomplished by a variety of mechanisms, involving recognized plate tectonic processes. Some of these mechanisms are discussed. However, dextral shear between North America and plates to the west is central to each mechanism. It seems likely that shear also has disrupted and internally modified older accreted terranes, producing internal block rotations. Such older, disrupted allochthonous terranes appear to be drawn out into highly attenuated tectonic laminae plastered on the western edge of the continent.

Paleomagnetic record of plate‐margin tectonic processes along the western edge of North America

Directions of magnetization observed from rock-units from a given continental block are conveniently summarized by a path of apparent polar wander (apw) relative to that block. This article documents apw paths for the major continental blocks spanning the interval from Late Carboniferous (approximately −300 Ma) to the present. It is shown that by passing moving windows of 30 or 40 Ma duration over the data an apw path can be obtained with little or no loss of features through smoothing. It also is shown that mean pole positions obtained using Fisher's statistics or by calculating modes do not differ significantly. In the analysis ‘poor’ results are screened out by applying certain minimum reliability criteria. Unit weight is then given to each of the remaining results. No attempt has been made to weight the results differentially because of the risk of added subjectivity.

Apparent polar wander paths carboniferous through cenozoic and the assembly of Gondwana

The Great Alaskan Terrane Wreck: reconciliation of paleomagnetic and geological data in the northern Cordillera

[1] Because of paleomagnetic inclination error (I error) in sedimentary rocks, we argue that previous estimates of Triassic and Jurassic paleolatitudes of the North American craton have generally been too low, the record being derived mostly from sedimentary rocks. Using results from all major cratons, we construct a new composite apparent pole wander (APW) path for Triassic through Paleogene based on 69 paleopoles ranging in age from 243 to 43 Ma. The poles are from igneous rocks and certain sedimentary formations corrected for I error brought into North American coordinates using plate tectonic reconstructions. Key features of the new APW path are a 25° northward progression from 230 to 190 Ma to high latitudes (off northernmost Siberia) where the pole lingers until 160 Ma, a jump to the Aleutians followed by a hook in western Alaska by ∼145 Ma that leads to the 130–60 Ma stillstand, after which the pole moves to its present position. As an example of the application of this new path we use paleomagnetic results to determine that southern Wrangellia and Stikinia (W/S), the two most westerly terranes in the Canadian Cordillera, lay 630 to 1650 km farther south than at present relative to the craton during the Late Triassic and Early Jurassic. This is consistent with an exotic Tethyan origin as paleontological and mantle geochemical evidences imply. During the Late Triassic through Early Cretaceous, W/S moved northward more slowly than the craton, implying oblique sinistral net convergence over this 130 Myr interval. This was followed by dextral shear in latest Cretaceous through Eocene.

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Influence of inclination error in sedimentary rocks on the Triassic and Jurassic apparent pole wander path for North America and implications for Cordilleran tectonics

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.

Large (1000 to 4000 km) northward movements of tectonic domains in the northern Cordillera, 83 to 45 Ma

IN a recent study of the distribution of fusulinaceans in western Canada, Monger and Ross1 interpreted their observations in terms of three plate-tectonic models, two of which invoke or might involve large northward translations of portions of the western Cordillera relative to cratonic North America. In this communication we describe anomalously low palaeomagnetic inclinations in radiometrically dated batholithic rocks exposed in the Cascade Range of central Washington. If interpreted in accordance with the usual axial dipole hypothesis of palaeomagnetism2, our results suggest that these rocks originated roughly 25° further from the Cretaceous pole than their present location relative to the North American craton suggests. A simple application of plate tectonics based on existing models1,3,4 can tentatively account for our results. Alternative explanations seem unlikely.

Anomalous Palaeolatitudes in Cretaceous Granitic Rocks

Paleomagnetic data have been obtained from granitic rocks of the Miocene Grotto and Snoqualmie batholiths in the Central Cascades, Washington. Alternating field magnetic cleaning demonstrated that a wide range of magnetic stability was present and that many specimens were unsuitable for paleomagnetic analysis. A simple statistic (Qd′), defined as the ratio of remanent magnetic intensity of the entire sample after af cleaning to weak-field magnetic susceptibility, proved effective in discriminating between specimens of marginal stability. Experiments with saturation isothermal remanent magnetism indicated that stable NRM is dominated by magnetic grains with single-domain characteristics, whereas grains with multidomain characteristics dominate the NRM of the unstable samples. After cleaning, a Miocene pole was obtained for the Central Cascades at 208.5°E, 85.5°N (δp = 4.5, δm = 5.4, and k = 360.4). This pole is statistically indistinguishable from other Miocene poles of North America, which suggests that any major tectonic movement postulated for the Central Cascades relative to North America must have occurred before the Miocene.

Linda Noson

Papers

Anomalous Palaeolatitudes in Cretaceous Granitic Rocks

Paleomagnetism of the Miocene Grotto and Snoqualmie Batholiths, Central Cascades, Washington