A composite oceanic and continental terrane, the Yakutat block, is currently colliding with and accreting to North America in the northern Gulf of Alaska. Marine geophysical data indicate that the block has moved with the Pacific plate during Pliocene and Quaternary time, but are indeterminant about its previous movement history. A model that explains some observed geologic and structural features of the block is that it originated as a result of subduction of the Kula-Farallon spreading center beneath North America. In this model, the spreading center that formed the basaltic basement of the Yakutat block was offset from a spreading center to the south by a left-lateral transform fault 400–500 km long. Subduction of the northern spreading center about 45 m.y. ago sliced off part of the North America continental margin, attached it to the Kula plate, and initiated northward movement of the composite terrane as the Yakutat block. During Eocene and Oligocene time, migration of the southern spreading center toward North America truncated the Yakutat block on the south and left a fossil fracture zone that is now seen as the Transition fault. The southern spreading center subducted about 25 m.y. ago, and the Yakutat block has since traveled with the Pacific plate to southern Alaska.

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