CASCADIA BACKARC-FOREARC TECTONIC LINKS INTERPRETED FROM POTENTIAL-FIELD ANOMALIES, GEOLOGIC MAPPING, AND PALEOSEISMOLOGY

Analysis of potential-field anomalies allows us to link faults and folds of the Yakima fold belt (YFB) in the Cascadia backarc through the Cascade Range to similar structures in the Cascadia forearc, as proposed by Beeson and Tolan (1990). Our analysis is based on >150,000 km² of high-resolution aeromagnetic data from Oregon and Washington acquired by the USGS over the past two decades. Previous work in central Washington used these data to map structural connections between the YFB and active faults and folds to the NW, in the Puget Lowland. Here we focus farther south on relations between the YFB and active NW-striking dextral faults and seismic zones near the Oregon-Washington border. YFB structures that deform Miocene basalts of the Columbia River Basalt Group (CRBG) produce pronounced WSW-striking magnetic anomalies along the fault-bounded Horse Heaven Hills and Columbia Hills anticlines. Boundary analysis and other interpretive techniques applied to the aeromagnetic data allow us to extend these structures to the WSW through the Cascade Range and into the Portland and Tualatin forearc basins, where they abut active, NW-striking dextral faults, including the Gales Creek and Portland Hills fault zones. These NW-striking faults and seismic zones offset YFB anticlines and thrust faults in the forearc into shorter, E-W oriented, YFB-style structures, a pattern that continues N into Washington (e.g., Grays River, Doty, Rochester, and Tacoma fault zones). Dextral slip on NW faults may be pervasive in the forearc and arc, given ~9 km of right-lateral offset of Miocene CRBG recently documented on the Gales Creek fault. The distribution of Quaternary volcanism narrows sharply S of our interpreted WSW continuation of the Columbia Hills anticline, suggesting that this structure is an important tectono-magmatic boundary. Gravity anomalies show that the Washington forearc is more fragmented N of this boundary than in the Oregon forearc to the S. This fragmentation may continue eastward into the arc where NW-striking transtensional structures appear to facilitate volcanism. In late 2021, the USGS will conduct a large-scale, high-resolution aeromagnetic survey of the Columbia River Gorge and adjacent parts of the Cascade Range to further support ongoing geologic mapping and constrain backarc-forearc linkages.