This study presents new seismic imaging results of the mantle lithosphere beneath the Cordillera-Craton transition region using S-to-P converted waves. The S-receiver functions from 107 stations in southern Alberta/British Columbia and northern Montana suggest a regional average lithosphere thickness of 207 km beneath the Precambrian crust and approximately 75 km under the Cordillera. The 100+ km lithosphere step, located within 50-100 km of the Rocky Mountain trench, is considerably greater than recent estimates from tomographic inversions. The Cordilleran Lithosphere-Asthenosphere Boundary (CLAB) at depths above 75 km tapers off near the Rocky Mountain Trench, forming a west-dipping geometry with consistent dips as constrained by recent surface wave analyses. The most surprising and important observation is a robust (and flat) Lower Lithosphere-Asthenosphere Boundary (LLAB) at approximately 180 km depth beneath the Cordillera, extending westwards far beyond the Rocky Mountain Trench. This interface correlates well, in both location and strength, with the horizontal shear-velocity gradient zones within the cratonic lithosphere. The presence of both CLAB and LLAB, which follows, respectively, the top and bottom of the high-velocity cratonic lithosphere, collectively form a dual-lithosphere scenario beneath much of the southeastern Cordillera. We conjecture that the Laurentian lithosphere subducted during the Cordillera-Craton convergence underplates the southeastern Cordilleran lithosphere. Based on observations of ~30 km uplift of the LLAB, vigorous mantle convection associated with the Farallon subduction may have caused partial erosion and uplift of the intruded cratonic lithosphere in the past 60 myrs.