The Eocene accretion of the Siletzia oceanic plateau to the western margin of North America is the most recent accretion event for this area and coincides with the formation of the modern Cascadia subduction. The plateau formed offshore between 56 and 49 Ma, likely related to the Yellowstone hotspot, and it was added to the continental margin between southwest British Columbia and Oregon within 5 Myr during subduction of the young (~10 Ma) Farallon Plate. Using numerical models, we show that the age of the oceanic plate plays a dominant role in the interactions between an oceanic plateau and subduction zone. A plateau on an old plate is readily transported with the oceanic plate into the mantle, with little disruption to the subduction system. In contrast, plate convergence stalls for a young plate, as the buoyant plateau enters trench, causing the deeper part of the subducting plate to break off. The models show that the Siletzia plateau would have been readily accreted to western North America, but if the Cascadia subduction zone formed as a new plate boundary west of Siletzia, continued plate convergence may have been driven by the older subducting plate to the south of this area. The models also demonstrate that plateau accretion and slab-break-off may have caused changes in surface topography and stresses in the over-riding plate and induced mantle melting. These dynamics may explain the observed deformation and magmatism within the continent during the time of Siletzia accretion.