Plate tectonic models indicate subduction of the Kula-Farallon spreading ridge, and thus imply formation of the Kula-Farallon slab window, beneath western North America from Late Cretaceous to middle Eocene time. Seafloor magnetic anomalies, however, provide few constraints on the configuration of the subducted ridge and the location of the slab window. We use geochemical data from Eocene igneous rocks to provide an estimate of the slab window's position at 50 Ma. Contouring of the trace element ratios K2O/SiO2, Rb/Zr, and Ta/Ce for lavas of the Eocene Challis-Kamloops volcanic belt demonstrates a southward trend toward enriched geochemical character. This trend is consistent with a Pacific Northwest position for the slab window; this is supported by the presence of adakites (slab melts) near the Canada–United States border, and by ca. 51 Ma forearc intrusions and volcanic rocks on Vancouver Island that are regarded as products of ridge subduction. A new reconstruction of the slab window's geometry over time indicates that its chemically defined position is kinematically viable.

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