Teleseismic tomography images a large high-velocity “curtain” extending vertically beneath the ca. 50 Ma Challis magmatic trend to maximum depths of 230–600 km. We interpret this structure as subducted Farallon ocean lithosphere that stalled with the ca. 55 Ma accretion of the Siletzia microplate to North America within the Columbia Embayment, and we consider the regional tectonic implications. The abrupt switch ca. 53 Ma from Laramide thrusting and magmatic quiescence to extension and vigorous magmatism in the northwestern United States is evidence for foundering of the flat-subducting Farallon slab. To account for the imaged curtain, foundering apparently occurred by rollback after Siletzia accretion terminated subduction within the Columbia Embayment. The magnitude of the high seismic velocity curtain is approximately that expected for 20–50 m.y. old ocean lithosphere that stalled in the upper mantle ca. 50 Ma. We suggest that the stalled slab became nearly neutrally buoyant as a result of removal of its basaltic crust, the melting of which likely contributed to the short-lived vigor of Challis magmatism. After Siletzia accretion, normal dip Cascadia subduction initiated west of Siletzia, evidenced by arc volcanism in Oregon and Washington beginning ca. 45–40 Ma, while continued quiescence of the Sierra Nevada arc suggests persistence of flat subduction to the south. Kinematically, this requires a tear in the subducted Farallon slab near the latitude of southern Oregon. We propose that north to south propagation of this torn slab edge propagated the ignimbrite flare-up across what now is the northern Basin and Range and ended the Laramide orogeny.