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 inter iors(1-5) 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 dow Here we in vestigate thermal anomalies across the eastern margin of the North Ame rican continental keel by imaging the seismic discontinuities at depth s of 410 and 660 km with compressional-to-shear converted waves record ed by a 1,500-km-long seismometer deployment in the eastern United Sta tes. The thickness of the transition zone (the region nominally betwee n depths of 410 and 660 km) and the depth to the '410-km' discontinuit y indicate that cold keel material and sub-keel down-wellings 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 tha n similar to 150 K. A 20-km depression of the '660-km' discontinuity t o the south of the westernmost stations coincides with a region of fas t velocity in the deep transition zone(2) and may be associated with t he remnants of the subducted Farallon plate(1,2,4).