A 3-D structural model of the Saddle Mountains, Yakima Fold Province, Washington, USA: Implications for Late Tertiary tectonic evolution of the Columbia River Flood Basalt Province

The Yakima Fold Province (YFP) is a series of asymmetric, north-verging anticlines in the Columbia River Flood Basalt Province of south-central Washington. Ridges represent anticlinal folds produced by tectonically active thrust faults, and their proximity to the Hanford nuclear facility has motivated many studies on the timing and extent of Quaternary deformation within the folds; however, multiple generations of faults and folds in the region have recorded a history of deformation spanning similar to 50 Ma. In this work, we utilize similar to 44 km of seismic profiles, structural orientation measurements collected in the field, USGS geologic maps, and stratigraphic information collected from 13 well logs to investigate the subsurface structure of the Saddle Mountains in the northern section of the YFP. We developed a three-dimensional structural model of the mountain range and estimated displacement and strain accommodated by faulting and folding. Our resulting model requires a multistage deformational sequence, and indicates that similar to 1800 m of uplift is generated by two parallel listric faults, soling into decollements at similar to 4 and similar to 8 km depth. Patterns of displacement and orientations of strain suggest that the deeper fault is older and that it may reflect a history of clockwise rotation, making the YFP the shortening counterpart to the extensional northern Basin and Range. The model presented has implications for understanding deformation in flood basalt provinces more broadly, and the methodology can be applied to interpret multi-stage deformation and reactivation histories of long-lived fault systems.