Location of the Border Ranges fault southwest of Kodiak Island, Alaska

A positive magnetic anomaly extends discontinuously from the south side of the Copper River basin, along the southeast side of Cook Inlet and Shelikof Strait, and southwestward from Kodiak Island to near Sutwik Island. Regionally, this anomaly parallels the Border Ranges fault and is within 10 km of it along the Kodiak Islands. The most probable source for the anomaly near Kodiak Island is mainly Early Jurassic plutonic rocks and locally ultramafic rocks that are associated with a melange. The Border Ranges fault lies between these two rock types, and given the regional parallelism of the fault and the magnetic anomaly, the fault can be traced offshore southwest of Kodiak Island to near Sutwik Island using marine magnetic data. Lithologic evidence suggests that the fault continues southwest of Sutwik Island toward the southwest end of the Alaska Peninsula.