A new plate history for the northeast Pacific, formulated from compiled magnetic-anomaly data, indicates that the Kula-Pacific ridge ceased spreading in late Paleocene time (about 56 to 59 m.y. B.P.). Magnetic anomalies 32 to 25 (70 to 59 m.y. B.P.) show the configuration of the Kula-Pacific-Farallon triple junction. Between anomalies 24 and 22 (56 to 53 m.y. B.P.), the southeast- and northeast-trending limbs of this triple junction fragmented and formed one set of north-trending ridge segments separated by transform faults. This realignment of spreading centers requires the simultaneous deactivation of the west-trending Kula-Pacific ridge. The removal of the Kula-Pacific ridge as a topographic barrier allows derivation of Aleutian Abyssal Plain turbidites directly from southern Alaska during Eocene and Oligocene time. Cessation of spreading along the Kula-Pacific ridge may have been caused by partial subduction of the ridge beneath the Aleutian arc. This model of ridge demise limits the amount of subsequent underthrusting of the Pacific plate and apparently requires the independence of southern Alaska from North America during early Tertiary time.

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