Miocene–Pleistocene deformation of the Saddle Mountains and ongoing work in the Cascadian backarc with implications for seismic hazard in Washington and Oregon

Lydia Staisch

USGS, GMEG, Menlo Park

Date & Time
Building 3, Rambo Auditorium
Tom Brocher

The Yakima fold province, located in the backarc of the Cascadia Subduction Zone, is a region of active strain accumulation and deformation distributed across a series of fault-cored folds. The geodetic network in central Washington has been used to interpret large-scale N-S shortening and westward-increasing 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 Yakima fold province are critically important to seismic hazard assessment for nearby infrastructure and population centers.

The Saddle Mountains anticline (SMA) is one of the most prominent Yakima folds. It is unique within the Yakima fold province in that the syntectonic strata of the Ringold Formation are preserved and provide a record of deformation. Here we present new stratigraphic columns, U-Pb zircon tephra ages, U-series caliche ages, and geophysical modeling that constrain two line-balanced and retrodeformed cross sections. These new constraints indicate that the SMA has accommodated 1.0–1.3 km of N-S shortening since 10 Ma, that shortening increases westward along the anticline, and that the average slip rate has increased 6-fold since 6.8 Ma. Using new slip rates and structural constraints, we calculate the strain accumulation time, interpretable as a recurrence interval, for earthquakes on the Saddle Mountains fault (SMF) and find that large magnitude earthquakes could rupture along the SMF every 2–11 k.y.

While new data provide improved constraints on earthquake hazards along the Saddle Mountains, many other poorly understood structures exist in Washington and Oregon. Ongoing work in the region includes addressing slip rates along other Yakima fold structures, characterizing fault interactions and the paleoseismic history of the Wallula and Hite faults, and understanding on-fault and off-fault seismicity in the region.

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