ON THE BAJA BC HYPOTHESIS

The Baja BC hypothesis remains a major challenge to our understanding of the tectonic evolution of the western Cordillera of North America. This hypothesis states that the western parts of British Columbia and Washington State were transported northward 3000 km along the North America margin between 85 and 55 Ma. The idea gains much of its support from paleomagnetic data, but in turn, many have argued that it is in direct conflict with geology. This is not true given that there are no recognized geologic relationships that can prove that Baja BC was in place relative to North America at 85 Ma. Some have argued for a “conservative” view, that an in-situ interpretation should be preferred until evidence proves otherwise. However, this approach is weak given overwhelming evidence for fast motion between North America, Kula, and Farallon plates during the Late Cretaceous and early Tertiary. We contend that there is no preferred solution; the Baja BC debate will only be resolved by clear and consistent paleomagnetic and geologic data. Cowan et al. (AJS 1997) argue that the Mesozoic and Paleozoic geology of western North America permit only two restorations of Baja BC, either north of the Klamath Mountains (indicating a northward displacement <1000 km) or south of the Sierra Nevada (indicating a northward displacement >2500 km). The paleomagnetic data continue to provide the most consistent test for the Baja BC hypothesis, in that there is no definitive result indicating the northern option for Baja BC at 85 Ma. In fact, we will show that all high quality paleomagnetic data are consistent with transport of the Baja BC block from the southern option during the interval 85 to 55 Ma. There remains strong resistance to this interpretation, perhaps because it challenges many long held ideas. For instance, the Baja BC block contains a Late Cretaceous-early Tertiary magmatic arc, and thus might account for the cessation of magmatism in the Sierra Nevada at that time, which has traditionally been explained by shallow subduction of the Farallon plate.