GEOMORPHOLOGY, GEOPHYSICS, AND GEOLOGY ALONG THE WALLULA FAULT ZONE, WASHINGTON AND OREGON

The Wallula fault zone is an active transpressional structure located in southeast Washington and northeast Oregon in the backarc of the Cascadia subduction zone. The fault zone is ~120 km long and a part of the Olympic-Wallowa lineament (OWL), a collection of NW-trending seismically active structural and physiographic features that obliquely crosses the Cascade Range and is considered an important regional tectonic feature. At the southeastern end of the OWL, the NW-trending Wallula fault zone occurs in a region of disparate fault orientations that record spatial variability in strain accommodation in the Cascadia backarc. West and northwest of the Wallula fault zone, strain is accommodated along the roughly E-W-oriented Yakima folds, whereas west and southwest, this strain is accommodated along NW-oriented right-lateral strike-slip faults and N-S-oriented normal faults. Although the geology, geophysics, and geomorphology along the Wallula fault zone preserves evidence of right-lateral strike-slip and contractional deformation, the evolution of its dominant deformation mode and slip rate remains poorly resolved.

In this study, we use geomorphic mapping, basalt geochemistry, potential-field geophysics, and seismic reflection to investigate the long-term and recent history of deformation along this enigmatic structure and characterize the relative components of strike-slip and contractional deformation. We target the 8.5-Ma Ice Harbor dikes to quantify long-term strike-slip offset using ground and airborne magnetic anomaly data and compare these results with geomorphic evidence of strike-slip faulting at several locales along the fault zone. We integrate magnetic and gravity anomaly measurements, seismic reflection profiles, and geologic mapping to construct 2 cross sections that provide insights into the long-term shortening and vertical offset across the fault. Together, these multidisciplinary datasets reveal low angle structures along the Wallula fault zone that accommodated north-directed thrust faulting as well as steep faults that accommodated dominantly right-lateral deformation. Preliminary cross sections suggest that steep faults may offset low angle structures.