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KUEHN, Stephen C., HOOPER, Peter R., THIESSEN, Richard L., WATKINSON, A.John, Department of Geology, Washington State Univ., Pullman, WA 99164-2812, firstname.lastname@example.org
The significance of strike-slip faulting on the Columbia Plateau, including displacement along the structural zone coincident with the Olympic-Wallowa Lineament (OWL-zone) and along the Hite fault System (HFS), has long been controversial, in part because of difficulty in determining strike-slip displacements in horizontal flows of the Columbia River Basalt Group (CRBG). To better understand OWL-zone and HFS structures, the flow-by-flow stratigraphy of the CRBG was determined for the area in the Blue Mountains of northeast Oregon where these structures intersect. A series of stratigraphic sections on opposite sides of major faults were sampled and mapped in detail. 347 samples of basalts were analyzed for 27 major and trace elements by XRF at Washington State University. Flows were correlated using detailed mapping, petrologic character, magnetic polarity, and chemical composition. Numerous faults, both those mapped by earlier workers and those observed for the first time in this study, were examined to determine striae, associated minor structures, and, with the aid of stratigraphy, displacements. The main conclusions are (1) that there is significant, albeit still somewhat circumstantial, evidence for at least 15 km of right-lateral offset on the OWL-zone and 80 to 100 km of left-lateral offset of the cratonic margin along the HFS (Sobczyk, 1994) prior to the eruption of the Columbia River basalts, (2) that there has been 300 meters of exposed syn- and post-CRBG vertical displacement of flows on the Hite fault, (3) that virtually all of the fault zones studied are dominated by horizontal striae in the basalts, and (4) that the OWL-zone and HFS probably represent conjugate fault systems with real, but very limited, post-CRBG strike-slip displacements. The eastern part of the OWL-zone is located near the northern margin of a broad zone of strike-slip and graben formation which extends south and eastward at least as far as the western Snake River Plain and Vale zone.