NEW GEOPHYSICAL OBSERVATIONS ALONG THE KLAMATH-BLUE MOUNTAINS LINEAMENT AND WALLULA FAULT ZONE, NE OREGON AND SE WASHINGTON

We focus on the tectonic significance and potential seismic hazard associated with the Olympic-Wallowa lineament (OWL) and Klamath-Blue Mountains lineament (KBL), two major physiographic and geophysical features that intersect near the border between northeast Oregon and southeast Washington. Geodetic and geologic observations show clockwise vertical axis rotation around a pole located near the juncture of these lineaments. West and northwest of this intersection, strain is accommodated as contraction along roughly E-W oriented fault-cored anticlines (Yakima fold province); to the south and southeast as extension along NE-SW oriented normal faults (Basin and Range province), and regionally as right-lateral displacement along NW-SE oriented faults that accommodate the northward translation of crustal blocks. This region is seismically active and capable of generating moderate to strong ground motions (e.g. 1936 M5.8 Milton-Freewater earthquake), but the local transfer of strain along the complex zone of faulting near the KBL-OWL intersection remains poorly resolved.

Potential-field geophysics is ideal for investigating tectonic features in northeast Oregon and southeast Washington, given the strong magnetic properties and density contrasts of the regional geology and extensive concealment by late Quaternary strata. We acquired 267 new gravity stations and 138 km of new ground-magnetic data along 12 transects that, combined with an extensive 2017 gravity survey (1384 stations) and high-resolution aeromagnetic data, better define geophysical anomalies. We use these data to construct three geologic cross sections across several notably steep gravity gradients of the KBL and two cross sections across the Wallula fault. Cross sections were developed using Geosoft GM-SYS software to forward model the potential-field data, and are constrained by published rock properties, existing geologic mapping, and field observations. The new data improve our knowledge of the location and magnitude of the steep, northeast-trending gravity gradient along the KBL, which we model as a NW-dipping normal fault, and the details of a moderate step in gravity anomalies and offset of 8.5 Ma Ice Harbor dikes across the Wallula fault, which we suggest show oblique right-lateral faulting.