Evidence for local variations in the depth to the 410 km discontinuity beneath Albuquerque, New Mexico

We stack data from the three-component broadband seismic stations at Pinon Flat, California (PFO); Ontario, Canada (RSON); and Albuquerque, New Mexico (ANMO) using the velocity spectrum stacking method. By doing so, we estima te the transition zone thickness (TZT) beneath southern California and Onta rio to be 236 and 263 km, respectively. Beneath ANMO we find a bimodal dist ribution in our estimates of the TZT. The transition zone west of ANMO appe ars to be similar in thickness to that beneath PFO; (232 km). The transitio n zone east of ANMO is 253 km, similar to that estimated for RSON. We concl ude that surface tectonics across the Rio Grande Rift are closely related t o the transition zone structure beneath it and that the rift, which separat es the region of active tectonism from the stable High Plains of eastern Ne w 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 discon tinuity reflects surface tectonics more closely than does the 660 km discon tinuity.