Magnetic anomaly patterns in the northeast Pacific Ocean combined with plate theory indicate that a trench existed offshore from western North America during mid-Tertiary time and that the present episode of strike-slip motion in the San Andreas fault system originated after the cessation of subduction, not earlier than 30 m.y. ago. At present, the American and Pacific plates appear to be moving past one another parallel to the San Andreas fault at a rate of 6 cm/yr. Data concerning the late Cenozoic history of motions between these plates is inconclusive, and so 2 probable models are examined. One assumes a constant motion of 6 cm/yr throughout the late Cenozoic, whereas the other assumes that the 2 plates were fixed with respect to one another until 5 m.y. ago, at which time they broke along the San Andreas fault system and began moving at 6 cm/yr. The second model implies that the San Andreas fault took up all the motion at the boundary between the North American and Pacific plates, while the first model suggests the broader view that much of the late Cenozoic tectonic activity of western North America is related to this boundary deformation. The models make testable predictions for the distribution of igneous rocks and for the total amount and timing of deformation expected. Extrapolation of the model of constant motions to the early Cenozoic suggests an era of slightly compressional strike-slip at the edge of North America. A major change in plate motions in late Mesozoic time is suggested.