Graduate Thesis Or Dissertation


Kinematics of the Columbia Hills Anticline and the Warwick Strike-Slip Fault, Yakima Fold and Thrust Belt, Washington, USA Public Deposited

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  • Discrete fault systems accommodate both N-S contraction and dextral shear in concert with clockwise rotational deformation of the North American plate above the Cascadia Subduction Zone. In Washington, the Yakima Fold and Thrust Belt (YFTB) accommodates N-S contraction as a series of ENE-WSW trending thrust faults and folds. NW-striking, predominantly dextral strike-slip faults intersect YFTB structures at acute angles. We present new geological mapping, geochemical data, and structural analyses reveal the kinematic relationship between the Columbia Hills anticline (CHA) and thrust fault, a south-vergent anticlinal fold stretching more than 150 km east from The Dalles, OR, and the Warwick fault zone, a 54 km long, NW-trending dextral strike-slip fault zone. West of the Warwick fault, the Columbia Hills thrust fault juxtaposes upper Grande Ronde Formation basalt in the hanging wall over flat-lying upper Wanapum Formation basalt in the footwall. East of the Warwick fault, the Columbia Hills thrust fault thrusts mid-to-upper Grande Ronde Formation basalt in the hangingwall directly over flat-lying upper Wanapum Formation basalt in the footwall. The youngest CRBG flow deformed by the Columbia Hills structure across the Warwick fault is the Priest Rapids Member 15.1-15.3 ± 0.11 Ma Basalt of Rosalia. East of the Warwick fault, the Columbia Hills thrust fault cuts a ~1 Ma lava flow known as the Haystack Butte basalt. Line-balanced cross-sections across the CHA to the west and east of the WFZ, provide comparison of the geometry of the CHA across the Warwick fault. Alternative cross-sectional models based on ~30° and 45°N thrust fault dip angles indicate that approximately ~1 km of shortening accrued on the Columbia Hills thrust fault since emplacement of the youngest CRBG flow (~15.3Ma). The maximum post-15.3 Ma shortening rates on the Columbia Hills thrust fault are 0.06 ± 0.01 mm/yr and 0.07 ± 0.01 mm/yr to the west and east of the Warwick fault, respectively. Offset of the Haystack butte basalt yields a post-1 Ma slip rate of 0.008 ± 0.001 mm/yr. Deformed loess deposits sampled for luminescence dating and right-lateral offset of active and abandoned stream channels suggest Pleistocene movement of the Warwick fault in the past ~110,000 years. These new field data support the structural model that the NW-trending Warwick strike-slip fault is not kinematically or geometrically related to the Columbia Hills thrust fault. Given that the shortening and slip rates related to the thrust fault are about half of those predicted by regional crustal deformation models, the strike-slip faults likely account for the missing deformation. Combining the fault structural geometries and historical catalog seismic data suggests that the Warwick and the Columbia Hills thrust faults are capable of producing Mw 6.9 to 7.3 earthquakes. Both the Columbia Hills thrust fault and the Warwick fault display evidence of Quaternary age movement and must be considered in future seismic hazard assessments.
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