NEW GEOCHRONOLOGIC, STRATIGRAPHIC, AND STRUCTURAL CONSTRAINTS ON THE TIMING AND RATE OF DEFORMATION ALONG THE SADDLE MOUNTAINS ANTICLINE, CENTRAL WASHINGTON STATE

The U.S. Pacific Northwest is recognized as a region of seismic hazard, owing to subduction along the northwest margin of North America and deformation associated with clockwise rotation of upper crustal blocks. Geodetic data show that modern strain accumulation extends >500 km inboard of the Cascadia subduction zone, indicating that seismic hazard continues eastward into the backarc. The Yakima fold province (YFP), located in central Washington, is a region of active strain accumulation and deformation distributed across a series of east-west oriented fault-cored anticlinal ridges. Recent high-density geodetic data, paired with gravity anomaly data, suggest that the YFP is spatially segmented along strike into regions that accommodate varying magnitudes of strain. However geodetic data are unable to resolve shortening rates across individual structures in this low-strain-rate environment. Resolving fault geometries, slip rates, and timing of faulting in the YFP are of critical importance to seismic hazard assessment for nearby infrastructure, including the Hanford Nuclear Reservation and hydroelectric dams along the Columbia and Yakima Rivers.

Over 90 km in length, the Saddle Mountains anticline (SMA) is the one of the most prominent of the Yakima folds. It is unique within the YFP, in that Miocene–Pleistocene syntectonic strata are preserved in the hanging and footwalls of the Saddle Mountains fault and provide a record of local deformation and regional drainage reorganization. We investigated the timing and rate of deformation across the SMA by measuring 6 stratigraphic columns, dating offset tephra and deformed petrocalcic horizons with U-Pb zircon and U-series geochronology, respectively, and constructing two line-balanced cross sections. These new structural and stratigraphic constraints indicate that the SMA has accommodated 0.9–1.0 km of N-S shortening since ~10 Ma, that shortening increases westward along the SMA, and that the average slip rate has increased by 10% since ~7.5 Ma. New detrital U-Pb zircon provenance analysis suggests that the late Miocene fluvial deposits exposed on the Saddle Mountains were sourced from the Idaho Batholith and Challis Volcanic Group, located to the east, and that post-Miocene uplift of the Columbia plateau has caused regional drainage reorganization.