Mantle discontinuities and temperature under the North American continental keel

A ubiquitous feature of upper-mantle seismic velocity models has been the presence of high-velocity ‘keels’ beneath stable continental interiors1,2,3,4,5. Uncertainty remains, however, regarding the maximum depth to which continental keels extend, the degree to which they have cooled the mantle that surrounds them and their role in mantle flow. Here we investigate thermal anomalies across the eastern margin of the North American continental keel by imaging the seismic discontinuities at depths of 410 and 660 km with compressional-to-shear converted waves recorded by a 1,500-km-long seismometer deployment in the eastern United States. The thickness of the transition zone (the region nominally between depths of 410 and 660 km) and the depth to the ‘410-km’ discontinuity indicate that cold keel material and sub-keel downwellings must be largely confined to the upper mantle and may impinge on the transition zone only in localized regions and with thermal anomalies of less than ∼150 K. A 20-km depression of the ‘660-km’ discontinuity to the south of the westernmost stations coincides with a region of fast velocity in the deep transition zone2 and may be associated with the remnants of the subducted Farallon plate1,2,4.