STRATIGRAPHY AND STRUCTURE OF GRANDE RONDE BASALT AT THE WESTERN MARGIN OF THE COLUMBIA PLATEAU: INSIGHT INTO ORIGIN OF THE YAKIMA FOLDS

Tracing and correlating lava flows by extensive sampling and chemical analyses shows that seven flows in the upper section of Grande Ronde Basalt (GRB) lap out on the eastern flank of the Cascade Range, in the upper Naches River basin, Washington. Working out the stratigraphy flow by flow has been the key to determining the structures in the area. Flows are folded in two directions, axes striking ENE and NNW, and broken and displaced by N- to NW-striking steeply dipping normal and reverse faults and ENE-striking, shallowly S-dipping thrust faults. The structure in the Naches River basin defines three domains: to the west the Cascade Range of NW-striking folds and faults; to the east the OWL of NNW-striking folds and faults, mainly reverse and thrust; and a middle, northward-narrowing domain of ENE-striking open, upright Yakima folds and thrust faults. Analysis of these structures indicates that the major compressional stress is horizontal approximately N-S, the intermediate stress horizontal E-W, and the minimal stress vertical. In this structural regime, the steeply dipping N- to NW-striking faults cutting GRB are possibly bounding deep-seated, wedge-shaped blocks of subcrustal rock uplifted differentially (pop-up structures) by a northward, horizontally directed compressive stress. The Yakima folds, whose axes strike easterly, could represent a chain of blocks that have been squeezed up. The thrust faults in the flanks of the folds are breaks in the brittle basalt, where they are unable to conform to the compressional squeezing of the underlying ductile crustal rocks. The folds of the Cascade Range and OWL are transpressional shear folds, subparalleled by faults developed 15-30° to the northerly directed compressive stress. The orientation of these structures, together with the regional extent of similarly oriented structures, suggests that the deformation is caused by a northward underflow of the mantle toward a resisting buttress of deeply seated crustal rocks in northern Washington. Thickness of this deformation would be commensurate with the depth of the northern-lying crustal rocks. Because of the lack of Yakima folds in the eastern part of the plateau, the western part has likely been transposed northward relative to its eastern part by the mantle-controlled regional structure regime.