Differential uplift and incision of the Yakima River terraces, central Washington State

The fault-related Yakima folds deform Miocene basalts and younger deposits of the Columbia Plateau in central Washington State. Geodesy implies similar to 2mm/yr of NNE directed shortening across the folds, but until now the distribution and rates of Quaternary deformation among individual structures has been unclear. South of Ellensburg, Washington, the Yakima River cuts a similar to 600m deep canyon across several Yakima folds, preserving gravel-mantled strath terraces that record progressive bedrock incision and related rock uplift. Here we integrate cosmogenic isochron burial dating of the strath terrace gravels with lidar analysis and field mapping to quantify rates of Quaternary differential incision and rock uplift across two folds transected by the Yakima River: Manastash and Umtanum Ridge. Isochron burial ages from in situ produced Al-26 and Be-10 at seven sites across the folds date episodes of strath terrace formation over the past similar to 2.9Ma. Average bedrock incision rates across the Manastash (similar to 88m/Myr) and Umtanum Ridge (similar to 46m/Myr) anticlines are roughly 4 to 8 times higher than rates in the intervening syncline (similar to 14 m/Myr) and outside the canyon (similar to 10m/Myr). These contrasting rates demonstrate differential bedrock incision driven by ongoing Quaternary rock uplift across the folds at rates corresponding to similar to 0.13 and similar to 0.06mm/yr shortening across postulated master faults dipping 30 +/- 10 degrees S beneath the Manastash and Umtanum Ridge anticlines, respectively. The reported Quaternary shortening across the anticlines accounts for similar to 10% of the similar to 2mm/yr geodetic budget, suggesting that other Yakima structures actively accommodate the remaining contemporary deformation.