The east-central Pacific between 20° N. and 45° S. records several patterns of sea-floor spreading. Three distinct spreading centers are now active in the area, and total spreading rates across one of them, the East Pacific Ridge, are the fastest observed anywhere.

The present East Pacific Ridge between 20° N. and 45° S. has maintained its present configuration only during the last 9 m.y. Prior to 10 m.y. ago, a north-northwest-trending ridge was active throughout the east-central Pacific, and the fossil crest of this ridge can be identified in the basin east of the present ridge axis between 10° S. and 30° S. and west of the ridge axis between 20° N. and the equator. Between the equator and 20° N., the north-northwest-trending ridge system has been continuously active since at least the Late Cretaceous, but 5 to 10 m.y. ago the position of the axis jumped to the east. South of the Chile Fracture Zone at 36° S., the north-northwest-trending Chile Ridge appears to be a still active segment of this old ridge system.

Spreading on the north-northeast-trending East Pacific Ridge began more than 50 m.y. ago at 55° S. and grew northward so that opening at 35° S. began about 20 m.y. b.p. Between 10° N. and 10° S., spreading about the present axis was initiated only during the last 10 m.y.

Analysis of magnetic anomalies associated with north-trending ridges near the equator is of limited value because of the extremely low amplitudes of the anomalies generated by the ridge. However, although identification of individual anomalies is tenuous at best, comparison of relative amplitudes and shapes with computed models offers limited support of the hypothesis of Francheteau and others (1970) that the Pacific Plate has migrated northward since the Late Cretaceous.

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