The location and timing of activity in marginal basins are considered in the context of relative plate motions. The motion of the Pacific and Farallon plates are reconstructed relative to the overriding Eurasian and South American plates for two models: Antarctica as one and as two plates (converging over the interval from 59 to 36 m.y. B.P.). The two-plate model (the more consistent) indicates that over the last 80 m.y. the motion of the Pacific plate relative to Eurasia was first westward (80--59 m.y. B.P.), then northward (59--36 m.y. B.P.), and then westward again (36 m.y. B.P. to present). For those Pacific marginal basins where magnetic anomalies have been identified, there is a pattern related to the direction of motion of the Pacific. Those basins active during the time period of northward motion of the Pacific opened in a north-south direction, and most of those which were active during the recent time period of westward motion of the Pacific opened a east-west direction. No basins were active during the time of change of direction. This strong correlation between the opening of marginal basins and the relative motions of the converging plates suggests that changes in these relative motions are a factor in the episodicity of marginal basins.