Abstract
Bathymetric and magnetic anomaly profiles show that the East Pacific Rise crest and both the Tamayo and Rivera Fracture Zones presently define the boundary of the Pacific and North American plates at the mouth of the Gulf of California. New oceanic crust is forming at the rise crest and transform fault slip is occurring along the fracture zones at the rate of 6.0 cm/yr. The Middle America Trench north of the Rivera Fracture Zone became nearly dormant as a plate boundary 8 to 10 m.y. ago when the rise crest changed from the Pacific-Farallon to the Pacific-Rivera plate boundary. Two m.y. ago, this section of the trench ceased crustal subduction as the Rivera plate became attached to the North American plate and the rise crest became the Pacific-North American plate boundary.
Baja California was dislocated from the North American plate 4 m.y. ago and began rafting away to the northwest at a velocity that has not been significantly altered since the initial breakaway. For the past 2 m.y., a coherent pattern of spreading has been recorded north of the Tamayo Frácture Zone.
