Abstract

Two different sets of extinct spreading ridge segments (Pacific-Farallon and Pacific-Nazca) are identified from bathymetric and magnetic data in the southeast Pacific. The older set is composed of several topographically subdued segments of a fossil ridge (Pacific-Farallon) that trends northwest, parallel to and about 175 to 550 km distant from the younger side of anomaly 7 (26 m.y. B.P.) The younger set trends north-northeast subparallel to the present East Pacific Rise (EPR), at an angle of about 45° to the older fossil ridge. The younger ridge forms the Galapagos Rise and includes a set of west-northwest–trending fracture zones. This was the original site of the EPR before it jumped westward to its present position. Oligocene magnetic anomalies (7–12) were generated at the older northwest-trending ridge rather than from the younger northeast-trending Galapagos Rise. The change in ridge orientation from the older Pacific-Farallon to the younger Pacific-Nazca direction is likely to be associated with the birth of the Cocos-Nacza spreading ridge system, about 20 to 25 m.y. ago.

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