MIOCENE TO QUATERNARY DEFORMATION HISTORY OF THE YAKIMA FOLD PROVINCE AS DETERMINED FROM GEOMORPHIC AND GEOLOGIC RECORDS: IMPLICATIONS FOR SEISMIC HAZARD IN CENTRAL WASHINGTON

The Yakima fold province (YFP), located in the back-arc of the Cascadia subduction zone, is a region of distributed deformation. Here, east-west trending fault-cored folds actively accommodate north-south oriented contraction. Seismicity in the YFP is elevated relative to surrounding regions east of the Cascadian arc. However, because geodetically measured strain rates are relatively low, seismic and GPS data are not of sufficient resolution to resolve deformation on individual structures. The geologic and geomorphologic records of the Yakima folds, on the other hand, span sufficient time to investigate the longer deformation history of each structure. In this work, I exploit longitudinal stream profiles from tributaries to the Yakima River to extract a continuous, long-term history of deformation on individual structures. Stream profiles are inverted to analytically solve for a linear solution to relative uplift rate for the Manastash, Umtanum, Selah Butte, and Yakima anticlines. Results indicate that the uplift rates of the Manastash and Umtanum anticlines increased around ~2 Ma and have since remained elevated, that the Selah Butte anticline has gradually increased in uplift rate since ~3.5 Ma, and that uplift rate of the Yakima anticline has decreased gradually since middle Miocene time. Stream profile inversion results are comparable to the magnitude of uplift experienced by Pliocene fluvial gravels in the Yakima River canyon, suggesting that the relative uplift rates derived from this analysis are robust. I construct a structural cross section based on new and published geologic mapping, field measurements, borehole observations, and seismic reflection surveys to compare with stream profile inversion results. I find that deformation of the Yakima folds likely initiated prior to middle Miocene time and that the ~2 Ma common onset of relatively rapid uplift along the Manastash and Umtanum anticlines may be due to tectonic shortening along a common décollement at depth. Overall, this work documents a previously unidentified increase in the rate of deformation along individual Yakima folds in Quaternary time and highlights the potential seismic hazards in central Washington.