The Influence of the Wallula Fault and Pasco Basin on the Tectonic Framework of South-Central Washington

The Yakima fold and thrust belt (YFTB) in eastern Washington manifests broad-scale regional deformation of the Cascadia backarc occurring prior, during, and after emplacement of Miocene flood basalts of the Columbia River Basalt Group (CRBG). CRBG basalts are strongly magnetic, and anticlines and faults of the YFTB are well displayed in high-resolution aeromagnetic surveys. Gravity anomalies, on the other hand, reflect in part the thickness of pre-Miocene sedimentary rocks beneath CRBG. The broad width of the YFTB in central Washington narrows eastward toward Idaho, with anticlines and faults merging into the narrow Wallula fault zone (WFZ). The WFZ is one element of the Olympic-Wallowa lineament (OWL), an alignment of topographic and structural elements extending from the Olympic Peninsula in Washington to the Wallowa Mountains in Oregon. The tectonic relevance of the OWL, particularly the degree to which dextral shear has contributed to its evolution, is still a matter of discussion. The Pasco basin, a late Cenozoic sedimentary basin atop CRBG, is associated with a broad gravity low. The thickness of post-CRBG sediments is insufficient to account for the entire gravity anomaly, suggesting the presence of a sediment-filled basin beneath CRBG. YFTB evolution and Quaternary deformation appear to have been influenced by the Pasco basin, as evidenced by potential-field anomalies. Northernmost faults of the YFTB (Frenchman Hills, Saddle Mountains, and Umtanum Ridge) abruptly terminate as they cross the western margin of the basin. Derivative maps (e.g., maximum horizontal gradient and tilt derivative) calculated from high-resolution magnetic anomalies show no evidence of these faults beyond their mapped extent in this area. Southern faults of the YFTB (Rattlesnake Mountain, Horse Heaven Hills, and Columbia Hills) in central Washington are on strike with the Pasco basin but veer abruptly southeastward at its southwestern margin to merge into the WFZ. Northwest-striking magnetic anomalies define a broad region of dike injection crossing the western Pasco basin north of the WFZ. The most prominent anomaly correlates with mapped dikes and vents of the Ice Harbor Member (8.5 Ma) of CRBG, and the entire anomaly pattern has been attributed to intrusive sources of Ice Harbor flood basalts. At their northern end, Ice Harbor magnetic anomalies connect with the Saddle Mountains thrust fault. At their southern end, the anomalies appear truncated by the WFZ, although a complex system of WNW-striking faults obscures magnetic evidence for dikes in this region. The episode of NE-SW extension that promoted dike injection in the Pasco basin 8.5 Ma apparently involved both the Saddle Mountains thrust fault and WFZ serving as restraining stepovers in a broad right-lateral fault system.