Abstract

The Baja British Columbia hypothesis holds that a large segment of the western edge of northern North America (Baja B.C.) was situated alongside California and northern Mexico in middle Cretaceous time, was displaced northward in the Late Cretaceous and Paleocene by north-oblique convergence of the Kula plate with North America, and arrived near its present location by the early Eocene. A consistent body of paleomagnetic data supports this hypothesis. However, doubt persists, and various crucial tests of a geologic nature have been proposed. One such test concerns the provenance of zircons in Cretaceous sedimentary basins of Baja B.C. In this paper we use both paleomagnetic data and zircon occurrences to reconfirm the Baja B.C. hypothesis. We first argue that the only truly crucial tests yet performed have been paleomagnetic, and that all such tests have been positive. Second, we show that detrital-zircon data from the Upper Cretaceous Nanaimo Group, although not a crucial test, provide valuable paleogeographic information. Available data demonstrate a change in detrital-zircon provenance in the Nanaimo Group that closely matches the position of these rocks predicted by the Baja B.C. hypothesis. Detrital zircons in the Nanaimo Group suggest a change from a southwestern North American source rich in Grenville and 1.4–1.5 Ga rocks to an increasing contribution from older parts of the craton such as the Wyoming province. Together, detrital zircons and paleomagnetic inclinations allow us to assemble a detailed schedule of northward tectonic transport of the Baja B.C. terranes.

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