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

Abstract

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.