NEW AEROMAGNETIC SURVEYS REVEAL THE CRUSTAL STRUCTURE OF THE CASCADE RANGE IN OREGON AND WASHINGTON

New high-resolution aeromagnetic surveys over the Cascade Range fill a critical data gap and complete seamless coverage of a ~500 km-wide swath across the Cascadia convergent margin, from the continental shelf off Oregon and Washington to near the Laurentian craton. Clockwise rotation and northward motion of Cascadia is accommodated by Yakima folds, northwest oriented right-lateral faulting, and a northward propagating rift, all of which overlap in the volcanic arc. Previously, we linked WNW oriented Yakima folds in the Washington backarc across the Cascade Range to active faults in the Puget Lowland. The new surveys show that other Yakima fold structures continue SW through the Cascade Range at the latitude of the Columbia River into the greater Portland area. These Yakima structures appear crosscut by active oblique and normal faults within and subparallel to the arc, such as the Mount Hood fault zone. Active NW-striking dextral faults, notably the Gales Creek, Mount Angel, and Oatfield-Sylvan faults, kinematically link dextral transtension in the arc and dextral transpression in the forearc to the westward-increasing clockwise rotation and northward convergence across Cascadia. To the north, match-filtered magnetic anomalies reveal a sigmoidal magnetization low that crosses the Washington forearc, following the SE-trending synclinal Chehalis basin and a restraining bend along the ~E-W Doty reverse fault, then SE toward Mt. St. Helens along a prominent right-lateral offset in the Coast Range gravity high. The magnetic low may reflect a structural trough disrupting the forearc at a prominent north-south change in megathrust tremor density, suggesting that, like the Doty fault, it has roots penetrating deep into, and perhaps through, the upper plate.