LARGE (1000 TO 4000 KM) NORTHWARD MOVEMENTS OF TECTONIC DOMAINS IN THE NORTHERN CORDILLERA, 83 TO 45 MA

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 Ala ska) was situated in the latitudes of Oregon and northern California, and the Coast Domain (southeastern Alaska, much of the Coast Ranges an d 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 locati ons 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 sh ear 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. Follow ing previous workers, we ascribe these motions to the rapid oblique su bduction of the oceanic Kula Plate beneath the continental western mar gin of the North American Plate, and derive a model based, in part, on comparisons with the oblique subduction of the Nazca Plate beneath th e 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 an d Coast Domains, geological relationships are complex, and most curren t 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 l arge displacements inferred from paleomagnetic observations. Hence we argue that although the paleomagnetically derived displacements far ex ceed those proposed by geologic interpretations, they are not inconsis tent with the geologic evidence itself.