We stack data from the three-component broadband seismic stations at Pi¿on Flat, California (PFO); Ontario, Canada (RSON); and Albuquerque, New Mexico (ANMO) using the velocity spectrum stacking method. By doing so, we estimate the transition zone thickness (TZT) beneath southern California and Ontario to be 236 and 263 km, respectively. Beneath ANMO we find a bimodal distribution in our estimates of the TZT. The transition zone west of ANMO appears to be similar in thickness to that beneath PFO (232 km). The transition zone east of ANMO is 253 km, similar to that estimated for RSON. We conclude that surface tectonics across the Rio Grande Rift are closely related to the transition zone structure beneath it and that the rift, which separates the region of active tectonism from the stable High Plains of eastern New Mexico, is also associated with a boundary in transition zone structure. Most of this 20 km change in TZT appears to be due to topography on the 410 km discontinuity. This implies that beneath New Mexico, lateral variations in temperature do not cross the transition zone and that the 410 km discontinuity reflects surface tectonics more closely than does the 660 km discontinuity. ¿ 2000 American Geophysical Union