Holocene to Neogene tectonics in central Washington: assessing seismic hazards from the geomorphic, geologic, and paleoseismic record of the Yakima Folds

Abstract:

In the Pacific Northwest, clockwise rotation of upper crustal blocks and far-field subduction-related strain accumulation results in active north-south shortening in the back-arc of the Cascadian subduction zone. Recent paleoseismologic and geomorphic studies in the Yakima Folds (YF) of central Washington document Quaternary fault activity and differential river incision, suggesting that strain is accommodated on multiple structures and that seismic hazard may be underestimated. To investigate the tectonic evolution and potential seismic hazard of central Washington, we focus on constraining Holocene to Neogene tectonic activity across the YF using geomorphic, geologic, and paleoseismic records of deformation. We extract stream profiles from the Manastash and Umtanum anticlines from airborne lidar data and invert them for the post-middle Miocene uplift rate. Preliminary inversion results are suggestive of an increase in uplift rate along the Umtanum anticline after the late Neogene and are consistent with the Quaternary uplift rates determined from incised strath terraces. We present new age data for offset Miocene–Pliocene sedimentary units and construct cross-sections to compare to inversion results and to published incision rates. The modeled differential uplift between the Kittitas Valley and the Umtanum anticline closely matches the vertical displacement measured from Pliocene river gravels deposited in both localities. To constrain Holocene (?) deformation rates in the YF, we trenched a ~4–5 m-high scarp along the Burbank fault, a potential back-thrust related to the formation of Umtanum anticline. Together, these data provide a quantitative estimate for kyr- to Myr-scale uplift rates and horizontal shortening accommodated in the YF. We compare our Holocene to Neogene rates with the regional geodetic strain rate to assess the relative seismic hazards of individual tectonic structures in central Washington.