Refinements of the “Baja British Columbia” Plate-Tectonic Model for Northward Translation Along the Margin of Western North America

Paleomagnetic data suggest that a large part of northern Washington, British Columbia, the Yukon, and southern Alaska (as a whole block named Baja British Columbia or Baja BC) was at the paleolatitude of Mexico at about 90 Ma. Additional paleomagnetic data show that Baja BC was in place relative to cratonic North America by about 55 Ma. We assume that Baja BC was adjacent to North America by middle Cretaceous time, and was driven northward by Kula (and/or Farallon)-North America plate interactions from 90 to 56 Ma. Terrane-travel analysis based on the fixed hot-spot hypothesis shows that the Kula plate could have driven Baja BC the distance inferred from the paleomagnetic data from the Spuzzum and Mount Stuart plutons in the interval from 90 to 68 Ma. A model with a north-south boundary between the Kula and North America plates produces sufficient latitudinal displacement even when the component of movement parallel to the coast is as low as 50%. A north-south plate boundary between the Farallon and North America plates also produces sufficient latitudinal displacement to move Baja BC the inferred distance. The Farallon-North America model, however, only is viable in the extreme case when all of the plate motion was coast-parallel and the entire 90 to 56 Ma time interval is used. In our preferred scenario for translation of Baja BC, the southern end of the block was at the paleolatitude of west central Mexico at 85 Ma, after a few hundred kilometers of translation along the Farallon-North America plate margin. Baja BC then moved along a north-south transform fault(s) between the Kula and North America plates from 85 to 74 Ma, and along an oblique-convergent margin from 74 to 66 Ma. By about 70 Ma the southern part of Baja BC was at the paleolatitude of Oregon, just north of the Klamath Mountains. A triple-junction analysis shows that the Kula-Farallon-North America triple junction would also move rapidly northward in the wake of Baja BC. Most of the possible variations in the type of triple junction place it along northern California by 70 Ma, suggesting that the Kula plate would be offshore of the region from California to southwestern Mexico for part of the 85- to 70-Ma interval, and that the Farallon plate would be adjacent to southwestern North America both preceding and following the interaction of the Kula plate.

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