ACKNOWLEDGMENTS We thank the 100 volunteers who helped deploy seismographs, and personnel from the IRIS/PASSCAL Instrument Center and the Geological Survey of Canada (GSC) Instrumentation Group who achieved an almost perfect data collection record. Special thanks are due Ed Criley, Grey Jensen, and Tom Burdette of the US Geological Survey, Tim Cartwright and Peter Carroll of Lithoprobe for assistance with our shot points. Phelps-Dodge Tyrone, Inc., permitted us to use one of their copper mines as a shot point. We thank numerous private landowners, as well as state, provincial, and federal agencies in the US and Canada for access, in particular the US Forest Service, the New Mexico Department of the Environment, the US Bureau of Land Management, and Canadian Forces Bases Suffield and Cold Lake. Lastly we thank Michael Milstein of the Billings Gazette for accurate news coverage of the experiment. We thank reviewers Eric Erslev, Ron Bruhn, and Walter Mooney for many suggestions which improved the manuscript. This research was supported by the NSF Continental Dynamics Program, EAR- 94109021, EAR-9706841, AFOSR Grant F49620-J-0438, the Lithoprobe Project [funded by the GSC and the Natural Sciences and Engineering Research Council of Canada (NSERC)], and NSERC research grants. REFERENCES CITED Benz, H.M., and J. McCarthy, 1994, evidence for an upper mantle low velocity zone beneath the southern Basin and Range-Colorado Plateau transition zone, Geophysical Research Letters, v. 21, p.509-521. Burchfiel, B. C.., and Davis, G. A., 1975, Nature and controls of Cordilleran Orogenesis, western United States: Extensions of an earlier synthesis, American Journal of Science, v. 275, p. 363-396. Christensen, N. I. and W. D. Mooney, 1995, Seismic velocity structure and composition of the continental crust: A global view: Journal of Geophysical Research, v. 100, pp. 9761- 9788. Durrheim, R.J., and W.D. Mooney, 1994, Evolution of the Precambrian lithosphere: Seismological and geochemical constraints, Journal of Geophysical Research, v. 99, 15,359-15,374. Fuchs, K., and G. Muller, 1971, Computation of synthetic seismograms with the reflectivity method and comparison with observations, Geophysical Journal of the Royal Astronomical Society, v. 23, p. 417-433. Gorman, A.R., T.J. Henstock and 9 others, 1997, Southern Alberta Refraction Experiment (SAREX) and Deep Probe Refraction Experiment 1995: Field Acquisition and Preliminary Data Processing Report, LITHOPROBE Report No. 52, 26 pp, 11 apps. Grand, S.P., 1994, Mantle shear structure beneath the Americas and surrounding oceans, Journal of Geophysical Research, v. 99, p. 11591-11621. Grand, S.P., R.D. van der Hilst, and S. Widiyantoro, 1997, Global seismic tomography: A snapshot of convection in the Earth, GSA Today, v. 7, p. 1-7. Hoffman, P. F., 1988, United Plates of America, birth of a craton: Early Proterozoic assembly and growth of Laurentia: Annual Reviews of Earth and Planetary Sciences, v. 16, p. 543- 603. Hoffman, P.F., 1989, Precambrian geology and tectonic history of North America, in Bally, A.W. and A.R. Palmer, eds., The Geology of North America - An overview, Geological Society of America, The Geology of North America, v. A, p.447-512. Houston, R. S., ed., and 11 others, 1993, The Wyoming Province, in Reed, J. C., Jr., Bickford, M. E., Houston, R. S., Link, P. K., Rankin, D. W., Sims, P. K., and Van Schmus, W. R., eds., Precambrian: Conterminous U.S.: Boulder, Colorado, Geological Society of America, The Geology of North America, v. C-2, p. 121-170. Jordan, T.H., 1988, Structure and formation of the continental tectosphere, Journal of Petrology, Special Lithosphere Issue, 11-37. Karlstrom, K. E., and Bowring, S. A., 1988, Early Proterozoic assembly of tectonostratigraphic terranes in southwestern North America: Journal of Geology, v. 96, p. 561-576. Karlstrom, K. E.,and R. S. Houston, 1984, The Cheyenne belt: analysis of a Proterozoic suture in southern Wyoming: Precambrian Research, v. 25, p.415-446. Keller, G.R., P. Morgan, and W.R. Seager, Crustal structure, gravity anomalies and heat flow in the southern Rio Grande rift and their relationship to extensional tectonics, Tectonophysics, v. 174, p. 21-37. LeFevre, L. V., and D.V. Helmberger, 1989, Upper mantle P velocity structure of the Canadian shield, Journal of Geophysical Research, v. 94, p. 17749-17765. Luetgert, J. H., 1992, MacRay: Interactive two-dimensional seismic raytracing for the Macintosh: United States Geological Survey Open-File Report 92-356, p. 1-2. Pakiser, L. C., 1989, Geophysics of the Intermontane system, in Pakiser. L. C., and Mooney, W. D., Geophysical framework of the continental United States: Boulder, Colorado, Geological Society of America Memoir 172. Prodehl, C., and Lipman, P. W., 1989, Crustal structure of the Rocky Mountain region, in Pakiser. L. C., and Mooney, W. D., Geophysical framework of the continental United States: Boulder, Colorado, Geological Society of America Memoir 172. Ross, G.M., R.R. Parrish, M.E. Villeneuve, and S.A. Bowring, 1991, Geophysics and geochronology of the crystalline basement of the Alberta Basin, western Canada, Canadian Journal of Earth Sciences, v. 28, p. 512-522. Ross, G.M., D.W. Eaton, D.E. Boerner, R.M. Clowes, 1997, Geologists probe buried craton in western Canada, EOS Transactions of the American Geophysical Union, 78, 493-497. Rudnick, R. L., and D. M. Fountain, 1995, Nature and composition of the continental crust: a lower crustal perspective: Reviews of Geophysics, v. 33, pp. 267-309. Schneider, R.V., and G.R. Keller, 1994, Crustal structure of the western margin of the Rio Grande rift and Mogollon-Datil volcanic field southwestern, New Mexico and southeastern Arizona, in Keller, G.R., and Cather, S.M., eds., Basins of the Rio Grande Rift: Structure, Stratigraphy, and Tectonic Setting, Geological Society of America Special Paper 291, Boulder, Colorado, p. 207-226. Sheehan, A.F., G.A. Abers, C.H. Jones, and A. Lerner-Lam, 1995, Crustal thickness variations across the Colorado Rocky Mountains from teleseismic receiver functions, Journal of Geophysical Research, v. 100, p. 20319-20404. Sinno, Y.A., and G.R. Keller, 1986, A Rayleigh wave dispersion study between El Paso, Texas and Albuquerque, New Mexico, Journal of Geophysical Research, v. 91, no. B6, p. 6168-6174. Slack, P.D., P.M. Davis, W.S. Baldridge, K.H. Olsen, A. Glahn, U. Achauer, and W. Spence, 1996, The upper mantle structure of the central Rio Grande rift region from teleseismic P and S wave travel time delays and attenuation, Journal of Geophysical Research, v. 101, p. 16003-16023. Snelson, C.M., 1998, An Integrated Lithospheric Study of the Rocky Mountain Region Along the Deep Probe Seismic Profile, M.S. Thesis, University of Texas at El Paso, El Paso, Texas, 150 pages. Snelson, C.M., T.J. Henstock, G.R. Keller, K.C. Miller, and A. Levander, 1998, Crust and uppermost mantle structure along the Deep Probe seismic profile, accepted by Rocky Mountain Geology. Vand der Lee, S., and G. Nolet, 1997, The subducted Farallon plate, Nature, 386, 266-269. Van der Lee, S. and G. Nolet, 1997, Upper mantle S velocity structure of North America, Journal of Geophysical Research, v. 102, p. 22815-22838. Walck, M., 1983, The P-wave upper mantle structure beneath an active spreading center: The Gulf of California, Geophysical Journal of the Royal Astronomical Society, v.76, p. 697-723. Ye, H., L. Royden, B.C. Burchfiel, and M. Schuepbach, 1996, Late Paleozoic deformation of interior North America: The greater ancestral Rocky Mountains, American Association of Petroleum Geologists Bulletin, v. 80, p. 1397-1432. Zelt, C., and Smith, R. B., 1992, Seismic traveltime inversion for 2-D crustal velocity structure, Geophysical Journal International, v. 108, p. 16-34. Figures Figure 1 : Location map showing the Deep Probe corridor in western North America. Blue line shows seismograph stations occupied for the 1995 active source experiment. SP refer to Deep Probe shot points, S to SAREX shot points used in this study. Red lines denote borders of major geologic provinces labeled in green (Hoffman, 1989). VS - Vulcan structure between the Hearne and Wyoming Archean provinces; CB - Cheyenne Belt suture between Archean Wyoming province and the Proterozoic accreted terranes. Figure 2: Comparison of 1-D velocity profiles from reflectivity modeling with those determined previously from earthquake observations. The Proterozoic is compared with model GCA for the Gulf of California (Walck, 1983) and the Wyoming province with model S25 for shield North America (LeFevre and Helmberger, 1989). In each case the earthquake profiles are shifted vertically so that Moho depths match those in our model. Plate 1 : Top: two-dimensional P-velocity model along the Deep Probe corridor. Stars show shots used in this study, gray lines show locations of intracrustal and Moho reflection points. The one-dimensional velocity profiles derived from reflectivity modeling are superimposed. All velocities shown assume a flat earth: at 100km depth they are ~0.15km/s faster than in a spherical earth. Bottom: record sections from SP43, SP37, and SP49 are plotted beneath the shot location with calculated travel-time curves from the model superimposed in red. The blue shaded circles indicate the crossover where Pn becomes the first arrival. Timothy J. Henstock and Alan Levander, Department of Geology & Geophysics, Rice University, Houston, TX 77005 Catherine M. Snelson, G. Randy Keller, Kate C. Miller, and Steven H. Harder, Department of Geological Sciences, University of Texas at El Paso, El Paso, TX 79968 Andrew R. Gorman and Ron M. Clowes, Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada Michael J.A. Burianyk, Department of Physics, University of Alberta, Edmonton, Alberta T6G 2J1, Canada Eugene D. Humphreys, Department of Geological Sciences, University of Oregon, Eugene, OR 97403 17 % , "