The continental margin off the Western United States changes abruptly from the subduction type north of lat 40°38’N to the translational type south of that latitude. Along the Cascadia subduction zone in the northern area a change in structural geometry at about the Columbia River separates the Oregon structural type of fore-arc basin-ridge-slope accretionary prism from the Washington type of low accretionary plateau. This change in gross morphology results from the dominantly landward-dipping thrust faults off Oregon to seaward-dipping thrusts off Washington. Gravity anomalies along the Oregon slope have steep gradients and relatively high amplitudes, whereas off Washington the gradients...