Erosion during repeated late Pleistocene Lake Missoula flooding limited the preservation of landforms and deposits that recorded earlier Quaternary deformation in low-lying areas. Paleoseismic investigations of tectonic scarps, offset landforms, and faulted stratigraphy reveal evidence for late Pleistocene to Holocene activity on several Yakima folds, but rates remain poorly constrained. Geologic evidence for recent tectonic deformation among the Yakima folds is limited. Inset also depicts contemporary NNE shortening across the Cascade fore arc and back arc regions implied by geodesy-for GPS station locations and geodetic analysis see Figures 9–10 and related text in McCaffrey et al. Inset shows the relationship between the Pacific plate (PAC), Juan de Fuca plate (JDF), Cascadia subduction zone (CSZ), Yakima folds (YF) and faults, and the Olympic-Wallowa Lineament (OWL). Thorp Gravel (TG) fission track age and type location from Figure 2 in Waitt. Thin dark grey lines represent anticline axes, white lines represent known or suspected Quaternary active faults with reverse motion from Washington Department of Natural Resources ( ). Map depicts study area in the Yakima River Canyon across Manastash (MR) and Umtanum Ridge (UR) structures south of Kittitas Valley (KV) and west of the Hanford Nuclear Reservation. Structural and topographic map of the Yakima folds in central Washington. The rates of shortening distributed among specific Yakima fold structures remain unknown. Geodesy indicates that the rate of contemporary N-S shortening diminishes to ~2 mm/yr east of the Cascade arc, where fault-related folds developed in the Miocene Columbia River Basalts-the Yakima folds (Figure 1)-appear to accommodate the deformation. East-west and northwest striking fore arc faults between northwestern Oregon and Washington record Quaternary to Holocene reverse faulting cumulatively accounting for most of the ~5 mm/yr of geodetically observed regional shortening. Upper plate deformation along the Cascadia margin incorporates oblique Pacific-North American plate motion, along with basin and range extension, resulting in clockwise rotation of the western Oregon fore arc and consequent shortening on both sides of the Cascade volcanic arc in Washington (Figure 1). Pacific Northwest represents a locale where geodesy quantifies crustal strain far inboard of the subduction zone over several decades (Figure 1 inset), but concomitant rates of geologic strain at 10 2–10 6 year intervals lack equivalent spatial coverage. The Cascadia convergent margin in the U.S. Relatively few areas have complete spatial and temporal records of deformation, however, resulting in knowledge gaps that pose critical issues for regional seismic hazard analyses. Key PointsĪ comprehensive understanding of upper plate deformation along active convergent margins requires measurements of the deformation spanning 10 0 to 10 6 year intervals and over spatial scales sufficient to capture the influence of key tectonic structures.
The reported Quaternary shortening across the anticlines accounts for ~10% of the ~2 mm/yr geodetic budget, suggesting that other Yakima structures actively accommodate the remaining contemporary deformation. These contrasting rates demonstrate differential bedrock incision driven by ongoing Quaternary rock uplift across the folds at rates corresponding to ~0.13 and ~0.06 mm/yr shortening across postulated master faults dipping 30 ± 10°S beneath the Manastash and Umtanum Ridge anticlines, respectively. Average bedrock incision rates across the Manastash (~88 m/Myr) and Umtanum Ridge (~46 m/Myr) anticlines are roughly 4 to 8 times higher than rates in the intervening syncline (~14 m/Myr) and outside the canyon (~10 m/Myr). Isochron burial ages from in situ produced 26Al and 10Be at seven sites across the folds date episodes of strath terrace formation over the past ~2.9 Ma. 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. South of Ellensburg, Washington, the Yakima River cuts a ~600 m deep canyon across several Yakima folds, preserving gravel-mantled strath terraces that record progressive bedrock incision and related rock uplift. Geodesy implies ~2 mm/yr of NNE directed shortening across the folds, but until now the distribution and rates of Quaternary deformation among individual structures has been unclear. The fault-related Yakima folds deform Miocene basalts and younger deposits of the Columbia Plateau in central Washington State.
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