On the origin of Umtanum Ridge: kinematics of Neogene slip

Abstract

Constraining the geometry of active faults is an important step in determining accurate estimates of regional seismic hazard. Where subsurface data are lacking, the shape of hanging wall folds can be used to interpret fault geometry at depth. In order to better understand the seismic hazard posed by the Yakima folds in central Washington, I produced a new geologic map and employ kinematic modeling techniques to probe the geometry of a reverse/thrust fault controlling folding of the Columbia River Basalt flows at Umtanum Ridge near Ellensburg, WA. Depending on the technique used, Umtanum Ridge may have formed from either thin or thick-skinned deformation. My preferred model, using a combination of fault-bend and trishear fault-propagation folding, is consistent will measured flow top attitudes along a line of section through the ridge. This model implies that up to 520 m of slip has occurred since 15.6 Ma on a fault soling in a detachment ~ 4 km deep. The rate of deformation derived from the model underestimates (by a factor of 2) previously published rates derived from both geodetic and geomorphic data, implying that current rates of deformation may be higher than the long term average.