Chapter 22: Paleomagnetism of continental North America; Implications for displacement of crustal blocks within the Western Cordillera, Baja California to British Columbia

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Abstract
A compilation of 89 paleomagnetic poles for westernmost North America— southern British Columbia to northern Mexico—demonstrates that the Cordillera is composed to a very important extent of displaced crustal blocks. The distribution of paleopoles also shows (with decreasing degree of certainty) that (1) plate interactions along the leading edge of North America are responsible for terrane displacement; (2) since about 125 Ma, displacement relative to stable North America was northward, with clockwise rotation; (3) prior to 125 Ma (and beginning at perhaps 155 Ma), terranes moved generally southward relative to North America, and rotated counterclockwise; and (4) displaced terranes tend to fragment, the farther they travel. Complications in the interpretation of paleomagnetic data arise from several sources, principally uncertainty as to hemisphere of origin for displaced terranes older than middle Cretaceous, uncertainty as to the correct shape of the North American reference curve for rocks older than middle Cretaceous, and questions as to the reliability of the data themselves. No effective, immediate way to rid paleomagnetism of these questions and uncertainties exists. For that reason, interpretation of paleomagnetic data in terms of regional tectonics requires large data sets and tends to generate highly useful (but broad-brush) general models, not the detailed and specific reconstructions geologists might prefer.

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