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

Journal of Geophysical Research
By: , and 

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Abstract

The fault-related Yakima folds deform Miocene basalts and younger deposits of the Columbia Plateau in central Washington State. 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. 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. 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 26Al and 10Be at seven sites across the folds date episodes of strath terrace formation over the past ~2.9 Ma. 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). 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. 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.

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Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Differential uplift and incision of the Yakima River terraces, central Washington State
Series title Journal of Geophysical Research
DOI 10.1002/2015JB012303
Volume 121
Issue 1
Year Published 2016
Language English
Publisher American Geophysical Union
Publisher location Richmond, VA
Contributing office(s) Earthquake Science Center
Description 20 p.
Larger Work Type Article
Larger Work Subtype Journal Article
Larger Work Title Journal of Geophysical Research
First page 365
Last page 384
Country United States
State Washington
Online Only (Y/N) N
Additional Online Files (Y/N) N