Using Oregon State University and Mexican Navy cruise data, along with maps previously published by other workers, we have compiled a set of regional maps of the marine magnetic anomalies and oceanic crustal isochrons off northwest Mexico and southern Alta California. We then examined magnetic anomaly profiles obtained in and near the mouth of the Gulf of California to determine spreading rates between the North American, Pacific, Rivera, and Cocos plates.

The post-4 Ma Pacific-North American spreading rate in the mouth of the Gulf of California, measured using magnetic anomaly profiles, is either 66 or 49 km/m.y., depending on which of two sets of subjective anomaly correlations is preferred. The post-4 Ma Pacific-Rivera spreading rate over the Rivera Rise increases along the ridge from 51 km/m.y. near the Tamayo Fracture Zone to 56 km/m.y. near the Rivera Fracture Zone. From this linear rate-of-change of spreading we determine that the Pacific-Rivera instantaneous pole of relative motion is about 21 arc-degrees northeast of the middle of the Rivera Rise. Early Gulf spreading on the northern Rivera plate at the Maria Magdalena Rise may have been diffuse, characterized by multiple axes of extension. The post-1.9 Ma Pacific-Cocos spreadi g rate south of the Rivera Fracture Zone is about 88 km/m.y. After a ridge-jump at about 5 Ma and prior to 1.9 Ma, the spreading rate here was only about 69 km/m.y. Prior to about 5 Ma, the spreading rate was about 76 km/m.y. Thus, although there may have been a change in the overall spreading rate coincident with the ridge-jump, it appears that about 3 m.y. were still required to completely transfer Pacific-Cocos spreading from the Mathematicians Rise to the East Pacific Rise.

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