Abstract

Recently acquired magnetic data have allowed better definition of the linear magnetic-anomaly pattern in the Gulf of Alaska. Based on anomaly offsets, the Aja fracture zone has been located precisely and three additional fracture zones have been identified. The Aja fracture zone undergoes a change in trend that reflects a major change in spreading direction that occurred about 30 m.y. ago. Along the northwestern margin of the Gulf of Alaska, magnetic anomalies can be traced across the Aleutian trench and up to 50 km into the continental margin; whereas to the northeast, the anomalies lose their identity several tens of kilometers before encountering the continental margin. This latter zone, paralleling the continental margin between 135° W. and 143° W., probably was caused by compressive stresses within the continental margin and oceanic crust related to recent plate convergence along a coupled margin. Also, it may reflect the future location of a transform fault that will short-circuit the present Juan de Fuca Ridge–Aleutian trench transform fault and greatly simplify the plate boundary bordering the Gulf of Alaska.

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