Abstract
Abstract The Yakima fold belt is a series of asymmetric, east-west anticlines separated by much broader synclines in the thick and otherwise horizontal sequence of Columbia River basalt flows. The folds have grown progressively over the last 17 Ma in response to regional north-south compression and east-west extension. This stress and strain pattern is reflected on a regional scale by the highly oriented north-northwest-south-southeast basalt feeder dykes, by the WNW- (right-lateral) and NNE- (left-lateral) trending conjugate fault system, and by the presence of a similar strain field measurable in the basalt flows today. Peripheral to the Yakima fold belt, pre-basalt structures of different orientations project under the basalt pile. Some of these structures have continued to develop during and after basalt extrusion and have influenced structures in the basalt flows which are, therefore, ‘forced’.
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