We propose to map in detail uppermost mantle Pn velocities beneath western North America and other elevated continental regions located behind convergent plate boundaries, such as the Tibetan, Turkish-Uranian, and the Altiplano-Puna plateaus on a scale comparable to the major tectonic provinces. This mapping will provide critical constraints on the structure and origin of continental plateaus. Such results can be used to answer an ongoing debate over the "double" Moho beneath the Great Basin. We propose to use the massive digital ISC data in a simple but effective way to obtain thousands of Pn velocity determinations, which will be based on the reports of Pn arrival times from about ten thousand events recorded at hundreds of seismic stations in western North America. We plan to use the two- station method in which the source is located at epicentral distances where Pn is the first arrival and the source is in, or very near, the azimuth of the two-station pair. The station pairs are selected to be within definable tectonic provinces. A pilot study to map Pn velocity beneath the Colorado plateau produced very encouraging results with about a thousand Pn velocity determinations. We plan to extend our efforts to the rest of the provinces in western North America, and to other elevated continental regions located behind convergent plate boundaries. We also propose to use all available Pn velocities and source and station delays, using the procedure of Hearn and Clayton. Because of the large variations in the spatial distribution of stations and sources, the resolution of this method may vary from one province to another. The combined results of the two-station and tomography methods will provide a detailed map of uppermost mantle velocities.
