Paleomagnetism of the Columbia plateaus

The natural remanent magnetism (NRM) in Miocene lavas of the Columbia plateaus was surveyed in two stages corresponding to a natural subdivision of the plateaus into the Oregon plateaus and Owyhee upland in the south, and the Columbia River area in the north. In general, the lavas of the southern area are magnetically stable, and large sections often feature only a single magnetic polarity. In the northern area, sections of similar thickness are divided into several successive polarity zones and magnetically unstable lavas are common. By comparison with the inferred lava extrusion rate and the grouping of NRM directions in a well-defined transition from normal to reversed magnetic polarity across 19 lavas, stratigraphically separated groups of NRM directions in two other single sections in the southern area are interpreted to represent pauses during otherwise relatively frequent extrusions of lava. Alternatively, these observations may be explained by sudden movements of the geomagnetic field superimposed on a regular rate of lava extrusion. The NRM for the survey as a whole shows a striking relationship to a gigantic structural feature. The NRM of the Miocene lavas of the Columbia plateaus is thus consistent with previous suggestions of a tectonic origin for the broad structural pattern of the Pacific Northwest, and further implies that much of the associated rotation has taken place since the Miocene period.