BIRD: LITHOSPHERE DYNAMICS AND CONTINENTAL DEFORMATION Geochemical objections have been raised to this model Bird, P, and X. Kong, Computer leg, Livaccari and Perry, 1993: 1994, especially that nth of major faults. Geol. geochemical lithosphere(defined by certain element and sotope concentrations) is still present. But several Bird, P, Isotopic evidence for preservation of Cordilleran ithospheric mantle during the Sevier-Laramide orogeny recent studies have recently shown that the seismic lithosphere ( defined by high velocity and low 1994 United States: Comment, Geology, attenuation) has roughly the predicted structure Buck, W.R., Effect of lithospheric thickness on the formation of high- and low-angle normal faults, Geology, 21,933- Humphreys and Dueker[1994] performed a regional 936.1993 tomographic inversion which confirmed that upper Burgmann, R, Transpression along the southen San Andreas mantle seismic velocities are systematically slower in ult, Durmid Hill, California, Tectonics, 10. 1152-1163 the western U. S. than in the east, with the differences confined to the uppermost 300 km. In a profile from Byerlee, J, The change in orientation of subsidiary shears near Utah to Kansas. p-s conversions at the moho show that the crust thickens eastward from the colorado Byerlee, J, Model for episodic flow of high-pressure water in Plateau to the Great Plains, so that the high topography ult zones before earthquakes, Geology, 21, 303-306. of the former must be compensated in the mantle Chester. F.M. J. P. Evans. and R. L. Biegel Internal structure [Sheehan et al, 1992]. An inversion of teleseismic data and weakening mechanisms of the San Andreas fault, J. alderman and Davis, 1991] shows mantle lithosphere Geophys.Res,98,771-786,1993 is 80 km thicker on the east side of the rio grande rift Chiu, J-M, A. C. Johnston, and R. B. Herrmann, A than on the west. This compares well with analysis of collaborative research: Analysis of PANDA data and gravity data by Cordell et al. [1991] which shows continuation of PANDA experiment Madrid seismic mantle lithosphere thicknesses of 200 km to the East, but only 50-125 km to the west. Also, Beghoul et al. U.S. Geol. Sur. Open -File Rep, 93-195, 247-250, 1993 [1993]used teleseismic travel times to show that mantle Cordell, L, Y. A. Zorin, and G. R. Keller, The decompensative ithosphere is typically 20-50 km under the Basin/Range Grande rift, J Geophys. Res, 96. 6557-6568, 1991 and Colorado Plateau, but 150-190 km under the Great Craddock, J P, M. Jackson, B. A. van der Pluijm, and R. T. Plains. A tomographic image of uppermost-mantle(Pn) Versical, Regional shortening fabrics in eastern North velocity in the western U.S. [Hearn et al, 1991] shows he Appalachian- hat within the low-velocity region, local seismic Ouachita orogenic belt, Tectonics, 12 257-264, 1993 Dokka, R. K, Original dip and subsequent modification of a velocity is lowest in areas of Neogene extension, and along the Yellowstone plume track California, Geology, 21, 711-714, 1993 Teleseismic shear wave splitting and polarization Estabrook C.H. and K. h. jacob Stress ind provide an exciting new tool to determine the stretchi Neotectonics of North America direction of the upper mantle fabric. At 3 sites in the Slemmons. E. R. EngdahL. M. D. Zo Blackwell, pp. 387 west-central U.S.. these directions are east/northeast 1991 west/southwest [Silver and Chan, 1991]. If these Furlong, K. P, Thermal- rheologic evolution of the fabrics are in the lithosphere, they are inconsistent with mantle and the development of the San andreas Birds model; but if they are in the asthenosphere below, system, Tectonophysics, 223, 149-164 oldfinger, C, L D. Kulm, R S. Yeats, B. Applegate, M.E. they are entirely consistent ast shall MacKay, and G. F. Moore, Transverse structural trends subduction. Improved depth resolution should be a along the Oregon convergent margin: Implications for priority ntial and crustal rotations Halderman, T P, and P. M. Davis, Q beneath the Rio grande References nd B. L. Isacks, Lithosphe Harbert, W, Paleomagnetic database search possible, Eos structure of tibet and western North America: mechanisms Trans.AGU,74,100-101,1993. plift and a comparative study, J. Geophys. Res, 98 Hauksson, E, State of stress from focal mechanisms before Bergerat, F, C. Bouroz-Weil, and and after the 1992 Landers earthquake sequence, Bull er. Paleostress Seismol.Soc.Am,84,917-934,1994. Hearn, T, N. Beghoul, and M. Barazangi, Tomography of the U.S.A., Tectonophysics, 206, 219-2 Bird, P, Lateral extrusion of lower crust from under hgh hiceophys United States from re arrival times. Res,96,16,36%-16,381,1991 S.H., Stress in the lithosphere and 10,275-10,286,1991 Bird, P. Deformation and uplift of North America in the Union of Geodesy and Geophysics dited by M. A. Shea the IBM 1990 Contest Prize Papers, I, edited by K. R nd E. De ts, J F. Dolan, K.E. 105, Baldwin Press, Athens, Georgia, 1992a. ch, and G. J. Huftile, Wilshire fault: Earthquakes in Bird, P, Computer simulations of tectonics around Hollywood?, Geology, 22. 291-294, 1994 Alaskan syntaxis(abstract), Eos Trans. AGU, 73, Fall Humphreys, E. D, and K. G. Ducker, Western U.S.upper Meeting Suppl., 504, 1992b mantle structure, J. Geop/ys. Res, 99, 9615-9634, 1994BIRD: LITHOSPHERE DYNAMICS AND CONTINENTAL DEFORMATION 382 Geochemical objections have been raised to this model [e.g., Livaccari and Perry, 1993; 1994], especially that geochemical lithosphere (defined by certain element and isotope concentrations) is still present. But several recent studies have recently shown that the seismic lithosphere (defined by high velocity and low attenuation) has roughly the predicted structure. Humphreys and Dueker [1994] performed a regional tomographic inversion which confirmed that upper￾mantle seismic velocities are systematically slower in the western U.S. than in the east, with the differences confined to the uppermost 300 km. In a profile from Utah to Kansas, P→S conversions at the Moho show that the crust thickens eastward from the Colorado Plateau to the Great Plains, so that the high topography of the former must be compensated in the mantle [Sheehan et al., 1992]. An inversion of teleseismic data [Halderman and Davis, 1991] shows mantle lithosphere is 80 km thicker on the east side of the Rio Grande rift than on the west. This compares well with analysis of gravity data by Cordell et al. [1991] which shows mantle lithosphere thicknesses of 200 km to the East, but only 50-125 km to the west. Also, Beghoul et al. [1993] used teleseismic travel times to show that mantle lithosphere is typically 20-50 km under the Basin/Range and Colorado Plateau, but 150-190 km under the Great Plains. A tomographic image of uppermost-mantle (Pn) velocity in the western U.S. [Hearn et al., 1991] shows that within the low-velocity region, local seismic velocity is lowest in areas of Neogene extension, and along the Yellowstone plume track. Teleseismic shear wave splitting and polarization provide an exciting new tool to determine the stretching direction of the upper mantle fabric. At 3 sites in the west-central U.S., these directions are east/northeast￾west/southwest [Silver and Chan, 1991]. If these fabrics are in the lithosphere, they are inconsistent with Bird's model; but if they are in the asthenosphere below, they are entirely consistent with past shallow-angle subduction. Improved depth resolution should be a priority. References Beghoul, N., M. Barazangi, and B. L. Isacks, Lithospheric structure of Tibet and western North America: Mechanisms of uplift and a comparative study, J. Geophys. Res., 98, 1997-2016, 1993. Bergerat, F., C. Bouroz-Weil, and J. Angelier, Paleostresses inferred from macrofractures, Colorado Plateau, western U.S.A., Tectonophysics, 206, 219-243, 1992. Bird, P., Lateral extrusion of lower crust from under high topography, in the isostatic limit, J. Geophys. Res., 96, 10,275-10,286, 1991. Bird, P., Deformation and uplift of North America in the Cenozoic era, in Scientific Excellence in Supercomputing: the IBM 1990 Contest Prize Papers, 1, edited by K. R. Billingsley, H. U. Brown, III, and E. Derohanes, pp. 67- 105, Baldwin Press, Athens, Georgia, 1992a. Bird, P., Computer simulations of tectonics around the Alaskan syntaxis (abstract), Eos Trans. AGU, 73, Fall Meeting Suppl., 504, 1992b. Bird, P., and X. Kong, Computer simulations of California tectonics confirm very low strength of major faults, Geol. Soc. Am. Bull., 106, 159-174, 1994. Bird, P., Isotopic evidence for preservation of Cordilleran lithospheric mantle during the Sevier-Laramide orogeny, western United States: Comment, Geology, 22, 670-671, 1994. Buck, W. R., Effect of lithospheric thickness on the formation of high- and low-angle normal faults, Geology, 21, 933- 936, 1993. Burgmann, R., Transpression along the southern San Andreas fault, Durmid Hill, California, Tectonics, 10, 1152-1163, 1991. Byerlee, J., The change in orientation of subsidiary shears near faults containing pore fluid under high pressure, Tectonophysics, 211, 295-303, 1992. Byerlee, J., Model for episodic flow of high-pressure water in fault zones before earthquakes, Geology, 21, 303-306, 1993. Chester, F. M., J. P. Evans, and R. L. Biegel, Internal structure and weakening mechanisms of the San Andreas fault, J. Geophys. Res., 98, 771-786, 1993. Chiu, J.-M., A. C. Johnston, and R. B. Herrmann, A collaborative research: Analysis of PANDA data and continuation of PANDA experiment in the central New Madrid seismic zone, National Earthquake Hazards Reduction Program, Summaries of Technical Reports, 34, U.S. Geol. Surv. Open-File Rep., 93-195, 247-250, 1993. Cordell, L., Y. A. Zorin, and G. R. Keller, The decompensative gravity anomaly and deep structure of the region of the Rio Grande rift, J. Geophys. Res., 96, 6557-6568, 1991. Craddock, J. P., M. Jackson, B. A. van der Pluijm, and R. T. Versical, Regional shortening fabrics in eastern North America: Stress transmission from the Appalachian￾Ouachita orogenic belt, Tectonics, 12, 257-264, 1993. Dokka, R. K., Original dip and subsequent modification of a Cordilleran detachment fault, Mojave extensional belt, California, Geology, 21, 711-714, 1993. Estabrook, C. H., and K. H. Jacob, Stress indicators in Alaska, in Neotectonics of North America, edited by D. B. Slemmons, E. R. Engdahl, M. D. Zoback, and D. D. Blackwell, pp. 387-399, Geol. Soc. Am., Boulder, Col., 1991. Furlong, K. P., Thermal-rheologic evolution of the upper mantle and the development of the San Andreas fault system, Tectonophysics, 223, 149-164, 1993. Goldfinger, C., L. D. Kulm, R. S. Yeats, B. Applegate, M. E. MacKay, and G. F. Moore, Transverse structural trends along the Oregon convergent margin: Implications for Cascadia earthquake potential and crustal rotations, Geology, 20, 141-144, 1992. Halderman, T. P., and P. M. Davis, Qp beneath the Rio Grande and East African Rift zones, J. Geophys. Res., 96, 10,113- 10,128, 1991. Harbert, W., Paleomagnetic database search possible, Eos Trans. AGU, 74, 100-101, 1993. Hauksson, E., State of stress from focal mechanisms before and after the 1992 Landers earthquake sequence, Bull. Seismol. Soc. Am., 84, 917-934, 1994. Hearn, T., N. Beghoul, and M. Barazangi, Tomography of the western United States from regional arrival times, J. Geophys. Res., 96, 16,369-16,381, 1991. Hickman, S. H., Stress in the lithosphere and the strength of active faults, in U.S. National Report to International Union of Geodesy and Geophysics, 1987-1990: Contributions in Tectonophysics, edited by M. A. Shea, pp. 759-775, Am. Geophys. U., Washington, 1991. Hummon, C., C. L. Schneider, R. S. Yeats, J. F. Dolan, K. E. Sieh, and G. J. Huftile, Wilshire fault: Earthquakes in Hollywood?, Geology, 22, 291-294, 1994. Humphreys, E. D., and K. G. Dueker, Western U.S. upper mantle structure, J. Geophys. Res., 99, 9615-9634, 1994