We seek to determine the mantle lid thickness of the lithosphere and to constrain the mechanisms that are responsible for the Cenozoic uplift of the Tibetan plateau located behind the Himalaya collision zone and the western United States located behind the San Andreas transform/convergent plate boundary. Selected first P arrivals of International Seismological Centre (ISC) data at distances less than 22¿ are analyzed by the two-station method to constrain the upper mantle structure of the Basin and Range (BR), Colorado Plateau (CP), Great Plains (GP) and Tibetan Plateau (TP). The data are presented as plots of apparent velocity versus distance to the second station. We show that these observations of apparent velocities versus distance as well as those that concern the attenuation of Sn provide estimates of the mantle lid thicknesses. We obtain the following mantle lid thicknesses: BR 20--40 km; CP 35--50 km; GP 150--195 km; and TP 135--180 km. Sn is not observed for paths beneath the BR and CP, while Sn efficiently propagates beneath the GP; Sn is also observed beneath most of Tibet except in the north central part of the TP. A possible interpretation of our observations is that Indian continental lithosphere underthrusts only about the southern 2/3 of Tibet. ¿ American Geophysical Union 1993