Paper No. 8-6
Presentation Time: 9:00 AM-6:00 PM
KINEMATICS OF THE COLUMBIA HILLS ANTICLINE AND THE WARWICK STRIKE-SLIP FAULT, YAKIMA FOLD AND THRUST BELT, WASHINGTON, USA
Clockwise rotation produces deformation in the North American plate above the Cascadia Subduction Zone, with discrete fault systems accommodating both N-S contraction and dextral shear. In the backarc of Oregon and 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 and geochemical analyses of Columbia River Basalt Group (CRBG) flows combined with structural analyses to examine the kinematic relationship at the intersection of two of these structures: the Columbia Hills anticline, a south-vergent anticlinal fold stretching more than 150 km east from The Dalles, OR and the Warwick fault, a NW-trending dextral strike-slip fault zone. West of the Warwick, the core of the anticline exposes deformed Wanapum Basalt over upper Grande Ronde Basalt. East of the Warwick, the anticline deforms late Wanapum flows and the upper Grande Ronde Basalt. The youngest CRBG flow deformed by the anticline across the Warwick fault is the Priest Rapids Member 15.07–15.25 ± 0.11 Ma Rosalia flow. The topographic and strong aeromagnetic expression of the over 55 km-long Warwick fault suggest that it is a crustal scale fault. Deformation of loess deposits sampled for luminescence dating and right-lateral offset of an active stream across the Warwick suggest Pleistocene movement of the structure. A key dip-domain boundary on the backlimb of the anticline provides a marker for measuring the sense and magnitude of anticlinal separation across the Warwick. This boundary exhibits an apparent long-term right-lateral separation of approximately 1.2–1.4 km and is coupled with apparent clockwise rotation of the anticline, and a component of oblique separation of the anticline east of the Warwick fault. These new field data support the structural model that the Warwick fault and, possibly, other NW-trending strike-slip faults distributed in the YFTB are primarily manifestations of distributed regional shear and are not simply tear faults in individual YFTB structures.