Geologic interpretation of an aeromagnetic map of the west-central Columbia Plateau, Washington and Oregon

A low altitude, total intensity aeromagnetic map of the ?west-central Columbia Plateau, underlain principally by the Yakima Basalt,. shows Positive and negative anomalies that stand out from a moderate intensity .background reflecting .interbedded flows of normal and reversed magnetic polarity. One set of anomalies is related to anticlinal ridges, nother follows the traces of known or inferred faults, and a third set coincides with a swarm of feeder dikes for the Ice Harbor flows of Swanson and others (1975b), the youngest unit of basalt in the area. A fourth set of narrow, sinuous anomalies is related to flows that filled ancient valleys during the late stage of Yakima volcanism. The magnetic map suggests that the Ice Harbor dike swarm is offset by left-lateral strike-slip displacement along the Rattlesnake-Wallula fault, a segment of the Olympic-Wallowa lineament. The aeromagnetic map is of great help in delineating the extent of the dike swarm, mapping the ancient drainage system, and understanding the structural history of this part of the Columbia Plateau.