AbstractWe present a 3‐D model of upper mantle seismic discontinuity structure below Cascadia using a receiver function Kirchhoff migration method. A careful analysis of the primary and multiple reverberated phases allows imaging of the Juan de Fuca plate dipping below the North American continent. The subducting slab is observed as an eastward dipping signal at all latitudes. We associate this signal with a thermal gradient between the slab and surrounding mantle, rather than a sharp chemical discontinuity. Our model also shows along‐strike variations in the dipping angle and strength of this signal. To the southern and northern ends of the subduction system, the signal is clearly observed down to ~300 km. However, beneath central Oregon, this structure is missing below ~150 km depth. We propose that this gap is due to weakening of the slab beneath central Oregon possibly caused by deformation and hydration combined with plume‐slab interaction processes after subduction.