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Forearc magmatic rocks were emplaced in a semicontinuous belt from Alaska to Oregon from 62 to 11 Ma. U-Pb and 40 Ar39 Ar dating indicates that the magmatism was concurrent in widely separated areas. Eight new conventional isotope dilution–thermal ionization mass spectrometry (ID-TIMS) U-Pb zircon ages from forearc intrusions on Vancouver Island (51.2 ± 0.4, 48.8 ± 0.5 Ma, 38.6 ± 0.1, 38.6 ± 0.2, 37.4 ± 0.2, 36.9 ± 0.2, 35.4 ± 0.2, and 35.3 ± 0.3 Ma), together with previous dates, indicate that southwestern British Columbia was a particularly active part of the forearc. The forearc magmatic belt has been largely attributed to ridge-trench intersection and slab window formation involving subduction of the Kula-Farallon ridge. Integration of the new and previous ages reveals shortcomings of the Kula-Farallon ridge explanation, and supports the hypothesis of two additional plates, the Resurrection plate (recently proposed) and the Eshamy plate (introduced herein) in the Pacifi c basin during Paleocene and Eocene time. We present a quantitative geometric plate-tectonic model that was constructed from 53 Ma to present to best account for the forearc magmatic record using ridge-trench intersection and slab window formation as the main causes of magmatism. The model is also in accord with Tertiary to present inboard magmatic and structural features.

Cenozoic to Recent plate configurations in the Pacific Basin: Ridge subduction and slab window magmatism in western North America