Differential Incision and Uplift of the Yakima River Terraces

"The Yakima fold belt comprises fault-related folds deforming Miocene basalts and younger deposits of the Columbia Plateau in central Washington State. Geodesy implies ~2 mm/yr of modern, NNE-directed regional shortening; however the distribution of Quaternary deformation among individual structures remains unclear. South of Ellensburg, Washington, the Yakima River cuts a ~600-m deep canyon across several of the folds, preserving flights of strath terraces that record the progressive incision. Graded alluvial basins at the head and mouth of the canyon imply that terrace incision also records differential rock uplift. We integrate lidar analysis, field observations, and cosmogenic burial dating of eight strath terraces in the canyon to quantify Quaternary incision across two folds, Manastash Ridge and Umtanum Ridge. Isochron burial ages from in-situ 26Al and 10Be characterize four terrace-forming intervals at ≤0.5 Ma, 0.7 -1.3 Ma, 1.5-1.7 Ma, and 2.8-3.0 Ma. Along with the burial ages, we use lidar-derived strath heights to calculate time-averaged bedrock incision rates of ~10-3 mm/yr through synclinal lows, and ~10-2 within the Manastash and Umtanum Ridge anticlines (~0.07 mm/yr from 0.2-0.4 Ma and ~0.04 mm/yr from 1.5-1.7 Ma, respectively). Collectively, the results demonstrate Quaternary differential bedrock incision and uplift of the Manastash and Umtanum Ridge anticlines. Incision rates permit horizontal shortening at ~0.08-0.12 mm/yr across master faults (dip 30±10° S) beneath the folds, indicating that other compressional structures in the region likely take up the remaining ~1-2 mm/yr of modern regional geodetic shortening."