Interpretation of a new compilation of magnetic anomaly data for north of the Mendocino fracture zone and younger than chron 15 (39 Ma) indicates a transition from a pattern of stable transform faults to a pattern of persistent ridge propagation. Reorganizations of the propagation pattern occurred at 30 Ma in response to a counterclockwise change in direction of relative motion, and at 19 Ma in response to a clock-wise change. In addition, there are significant changes in the gradient of the spreading rate, apparently without major changes in motion direction, at 27 and 24 Ma. The change at 30 Ma is apparently the result of the separation of the Juan de Fuca plate from the rest of the Farallon plate along the Pioneer fracture cone. The pattern of absolute motion after 30 Ma does not generally support Menard's pivoting subduction model and suggests that Juan de Fuca plate motion is being significantly affected by forces transmitted across the Medocina transform fault. ¿ Americal Geophysical Union 1988