Gravity anomaly map of the central and eastern Aleutian Island arc, from National Oceanographic and Atmospheric Administration (1990). Gravity anomalies are free-air at sea and Bouguer on land. Values greater than 150 mGal are indicated by a single color, as are values less than minus 150 mGal. The volcano Mt. Venidminof is indicated. Note the positive gravity anomaly begins to bear away from the arc near Unimak Island and branches sharply in the Shumagins. The portion of the positive anomaly that roughly parallels the ridge fades in the vicinity of Mt. Venidminof. It is postulated that these changes in the bearings and intensity of the anomaly relate to locking of the RTB-thrust in the Shumagin region (see text, Section 5).

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