T67 Crustal Deformation: How and When? Linking Long-Term Landscape Signatures with Earthquakes and Seismic Hazard (Posters)

Sunday, 9 October 2022
2:00 PM - 6:00 PM
Exhibit Hall F

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

Paper number: T67-2
Program: Technical Sessions
Time: 2:00 PM – 6:00 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 54, No. 6

doi: 10.1130/abs/2022AM-381799

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