Magnetic anomalies and their relation to the Aleutian Island Arc

Magnetic lineations trending approximately east-west are found south of the Aleutian trench and are mapped as far west as 173°E. The lineations can be identified and correlated with a sequence of anomalies that have been mapped on a worldwide scale and have been interpreted as resulting from sea-floor spreading. Near 165°W the lineations are found to be continuous with those of the Great Magnetic Bight of the northeast Pacific. The lineation pattern is offset in a north-south direction in one or more places. The most prominent offset is along a line at about 173°W, where the offset of the lineation pattern is 250 km in a left-lateral sense. Magnetic lineations are also found northeast of the Japan trench but apparently are not continuous with those south of the Aleutian trench. The lineations northeast of the Japan trench were probably formed before Late Cretaceous, prior to the formation of the lineations adjacent to and south of the Aleutian trench. Magnetic anomaly profiles across the Aleutian trench are characterized by a `quiet zone' of small-amplitude anomalies about 100 km wide and centered near the trench axis. This quiet zone apparently does not result solely from the geometry, of the trench itself. A two-dimensional magnetic model is presented that could very nearly account for the observed anomaly pattern. With qualifications, the model can be interpreted as being consistent with the hypothesis of sea-floor spreading.