POTENTIAL-FIELD ANOMALIES ILLUMINATE CASCADIA TECTONIC HISTORY, FROM REGIONAL TO OUTCROP SCALE: WORMS, WIGGLES, AND A STRING OF PEARLS

A two-decade program of airborne data acquisition has yielded 150,300 km² of high-resolution magnetic data over the Cascadia forearc and backarc of Oregon and Washington. Mostly flown at 400-m line spacing and as close to the ground as safely possible, these data provide a rich tapestry of information about subsurface geology and tectonic history when interpreted in combination with modern geologic mapping, LiDAR topography, and other geophysical methods. Gravity anomalies, for example, illuminate seven sedimentary basins aligned along the forearc trough, consistent with a subduction origin. Magnetic anomalies and other observations over the intervening uplifts, however, indicate very different modes of basin evolution. The 9-km-deep, asymmetric, Seattle basin forms the footwall of the Seattle fault, a N-vergent thrust fault responsible for a M7 earthquake circa 900 AD. In contrast, the 5-km-deep Tualatin basin formed in the transtensional zone between the active, NW-striking Gales Creek and Oatfield-Sylvan faults. Magnetic anomalies and geologic mapping suggest that some forearc faults are kinematically linked to structures in the backarc. The Southern Whidbey Island fault (SWIF) in the forearc, for example, aligns with faults of the Yakima fold belt (YFB) in the backarc. Forearc-backarc linkage would imply that dextral shear on the SWIF continues into the backarc, as supported by observations of late Quaternary right-lateral slip from recent paleoseismic studies in the YFB, including on a strand of the Wallula fault zone (WFZ). We previously conducted a detailed study of magnetic anomalies over the mostly concealed 8.5 Ma Ice Harbor dikes north and south of the WFZ and argued on the basis of anomaly patterns that four dikes (or swarms) exhibit right-lateral offsets along one strand of the WFZ ranging from 1300 to 2200 m. Here we use spectral analysis to design filters matched to specific source depths and flight-line data normal to the dikes to identify correlative characteristics of individual dike or dike-swarm anomalies. A single Ice Harbor dike (or swarm) is confidently identified on opposite sides of the fault strand with 1980 m of right-lateral offset, indicating an average slip rate of 0.23 mm/y over the last 8.5 Ma. This rate should be considered minimum, given that the WFZ has multiple strands.