1. Fluvial analysis of active deformation in the Yakima Fold and Thrust Belt, WA
Skyler J. Sorsby, Colin B. Amos, Harvey Kelsey, Paul Bierman, Kathryn Hanson, and Dylan Rood
Late Quaternary structural activity in the Yakima Fold-and-Thrust Belt (YFTB) of Central
Washington presents a potentially significant yet poorly characterized seismic hazard
for nearby critical facilities. We use robust methodology and high resolution fluvial
geomorphic data to provide new constraints on the timing and relative activity of Yakima
structures. As sensitive recorders of deformation, stream channels gain the necessary
erosional power to defeat uplifting topography by adjusting channel width and slope.
Diagnostic channel narrowing is present on the Yakima River as it transects the
structures of Manastash Ridge, Umtanum Ridge, Selah Butte, Ahtanum Ridge, and
Rattlesnake Hills. Similar but smaller-scale adjustments are present on the Columbia
across Gable Mountain structures, perhaps indicative of the greater erosional ability of a
larger river to mute tectonic signals. Channel steepening appears to correlate poorly
with structural relief of Yakima folds, likely illustrating instead the propagation of blind
faults beyond fold axes. Additionally, we mapped a series of Yakima fluvial terraces
between Kittitas Valley and Roza Dam from GeoEarthScope lidar at a nominal scale of
1:5000. We assigned a preliminary sequence of six age groups to these surfaces,
based on average elevation above the modern river channel. Cosmogenic dating of
quartz cobbles from several representative terraces across the Yakima Canyon place
formation and uplift of these surfaces in the mid Quaternary, with dates ranging from 1.1
(+0.75/-0.45) Ma to 1.6 (+0.9/-0.45) Ma . Umtanum Ridge represents a likely focus of
this uplift, as older terraces display progressive tilting closer to the anticlinal axis. When
taken together, analysis of channel geometry and fluvial terraces suggest modest late-
Quaternary deformation of the YFTB, concentrated along structures controlling
Manastash Ridge, Umtanum Ridge, Selah Butte, Ahtanum Ridge, Rattlesnake Hills, and
Gable Mountain.