Post‐12 million year clockwise rotation in the Central Columbia Plateau, Washington

Abstract

The Pomona flow (12 m.y. B.P,) of the Columbia River Basalt Group was sampled for paleomagnetic analysis at 32 sites in the Yakima Fold Belt of the central Columbia Plateau to assess the pattern of deformation. Field directions from anticlinal ridges are found to be rotated clockwise relative to sites from synclinal areas. Two geographic patterns of rotation are present. The primary pattern is one in which the greatest amount of rotation occurs in the crest‐hinge area of the anticlines and decreases into synclines. A secondary pattern, in which the amount of rotation is consistent within a geometric segment but varies among them, is superimposed on the primary pattern. The pattern of clockwise rotation is interpreted to be the result of local tectonic rotation occurring along a closely spaced north‐northwest trending primary shear system and northeast trending secondary shear system. The shear system and concurrent rotation are interpreted to have developed during the growth of the Yakima folds. The relationship between the paleomagnetic data and the geologic structures can be used to constrain tectonic models for the Columbia Plateau. The data reported in this study indicate that clockwise rotation can not be attributed to rigid block or microplate rotation, but support models that predict localized dextral shear. Furthermore, it is postulated that this mechanism may contribute to clockwise rotation observed in other parts of the western United States.