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ABSTRACT We present the first detailed geodetic image of the entire western United States south of lat 42°N, merging both campaign and continuous Global Positioning System (GPS) and very long baseline interferometry (VLBI) data sets in a combined solution for station velocities having a single, uniform reference frame. The results are consistent with a number of features previously observed through local geodetic studies and very sparse space geodetic studies, including a dominant pattern of right-lateral shear associated with the San Andreas fault, rates of the westernmost sites (along the California coast) of 46–48 mm/yr relative to a North America reference frame, and some 11–13 mm/yr of deformation accommodated east of the Sierra Nevada in the Basin and Range province north of lat 36°N. South of 36°N, the solution also shows that the southernmost San Andreas fault system accommodates effectively all interplate motion and that the southern Basin and Range is not deforming significantly. At lat 37°N, the eastern California shear zone appears to exhibit simple shear oriented between ~N20°W and ~N40°W relative to North America, with a fairly well defined transition zone from localized shear to diffuse spreading in the Basin and Range. Enigmatically, this transition involves a significant component of contraction normal to the overall shear-zone trend; sites in the Great Basin move southwestward at up to ~5 mm/yr toward sites within the eastern California shear zone. To the north, in contrast, there appears to be a relatively smooth transition from east-west spreading within the eastern Great Basin to northwest-southeast shear across the westernmost Basin and Range.
INTRODUCTION Practical limitations on the scope of geodetic networks have resulted in either densely sampled local studies, with no means to tie the detailed observations to other densely sampled areas, or very sparse networks with just a few stations covering very large areas. Conventional (i.e., groundbased) networks are limited to apertures of 50–100 km, with no possibility to create network solutions in a common reference frame at larger scale. Even in the case of dense Global Positioning System (GPS) sampling, it has not been feasible until quite recently to attempt merging disparate data sets, collected at different times in different areas, into a single large-scale solution. This limitation has been a major obstacle in understanding active tectonic processes in diffusely deforming continental crust; such processes are of great scientific and societal interest. For example, in the western United States (Fig. 1), it has long been known that deformation of the northern Basin and Range province, as indicated by patterns of seismicity and Holocene faulting, amounts to ~20%–25% of the rate of relative motion between the Pacific and North America plates. Thus far, however, there have been only tantalizing glimpses of the large-scale pattern from very sparse, space-based networks (e.g., Dixon et al., 1995).
Geology; April 1999; v. 27; no. 4; p. 371–374; 3 figures; 1 table.
Figure 1. Pacific–North America plate boundary and major tectonic features of western United States. Magenta diamonds show seismicity from catalogues of California Institute of Technology, Centro de Investigación Cientifíca y de Educación Superior de Ensenada (Mexico), University of Utah, and University of Nevada. ISB—Intermountain seismic belt, CNSB—central Nevada seismic belt, ECSZ—eastern California shear zone, SBR—southern Basin and Range. Northern Basin and Range province stands ~1 km above sea level (dark brown) in contrast to low-lying southern Basin and Range province (green and yellow).
Figure 2. Estimates of horizontal velocities relative to stable North America for sites in western United States (arrows). Error ellipses represent 95% confidence level. Thin black lines represent mapped Quaternary faults. Pink lines show locations of profiles A and B in Figure 3 which are perpendicular to direction of NUVEL-1A Pacific–North America relative plate motion.Triangles show locations of points in profiles (zero) lying on common small circle to NUVEL-1A pole.
Figure 3. Velocities of sites within 200 km of profiles shown in Figure 2 projected onto directions parallel and perpendicular to direction of NUVEL-1A Pacific–North America relative plate motion. Blue lines show NUVEL-1A estimates of Pacific–North America rates. SA indicate locations of San Andreas fault zone within each profile.
the Canadian Geological Survey and Canadian Geodetic Survey, JPL, MIT, Caltech, the STRC consortium, the Yucca Mountain–Death Valley consortium, UNAVCO, Scripps Institution of Oceanography, the USGS, the Seismological Laboratory of UC Berkeley, and Trimble Navigation, Inc. We also made use of VLBI data products (from terrestrial reference frame solution number 1083c) provided by the NASA GSFC. Topography data are from NOAA. Figures were created with the GMT software. This research was funded by National Science Foundation grants EAR 94-18784 and EAR 95-12212, Nuclear Regulatory Commission grants NRC-04-92-071 and NRC-02-93-005, the California Institute of Technology, and the Smithsonian Institution. REFERENCES CITED Argus, D. F., and Gordon, R. G., 1991, Current Sierra Nevada–North America motion from very long baseline interferometry: Implications for the kinematics of the western United States: Geology, v. 19, p. 1085–1088. Argus, D. F., and Gordon, R. G., 1996, Tests of the rigid-plate hypothesis and bounds on intraplate deformation using geodetic data from very long baseline interferometry: Journal of Geophysical Research, v. 101, p. 13,555–13,572. Bennett, R. A., Rodi, W., and Reilinger, R. E., 1996, Global Positioning System constraints on fault slip rates in southern California and northern Baja, Mexico: Journal of Geophysical Research, v. 101, p. 21,943–21,960. Bennett, R. A., Davis, J. L., Elósegui, P., Wernicke, B. P., Snow, J. K., Abolins, M. J., House, M. A., Stirewalt, G. L., and Ferrill, D. A., 1997, Global Positioning System constraints on fault slip rates in the Death Valley region, California and Nevada: Geophysical Research Letters, v. 24, p. 3073–3076. Bennett, R. A., Davis, J. L., and Wernicke, B. P., 1998, Continuous GPS measurements of deformation across the northern Basin and Range province: Geophysical Research Letters, v. 25, p. 563–566. Beutler, G., Mueller, I., and Neilan, R., 1994, The International GPS Service for Geodynamics: Development and start of official service on January 1, 1994: Bulletin Géodesique, v. 68, p. 39–70. Blewitt, G., Bock,Y., and Gent, G., 1993, Regional clusters and distributed processing, in IGS position paper, IGS analysis center workshop: Ottawa, Canada, International GPS Service, October 12–14, p. 62–91. Bock, Y., Wdowinski, S., Fang, P., Zhang, J., Behr, J., Genrich, J., Agnew, D., Wyatt, F., Johnson, H., Marquez, S., Hudnut, K., King, R., Herring, T., Stark, K., Dinardo, S., Young, W., Jackson, D., and Gurtner, W., 1997, Southern California Permanent GPS Geodetic Array: Continuous measurements of regional crustal deformation between the Landers and Northridge earthquakes: Journal of Geophysical Research, v. 102, p. 18,103–18,033. DeMets, C., 1995, Reappraisal of seafloor spreading lineations in the Gulf of California: Implications for the transfer of Baja California to the Pacific plate and estimates of Pacific–North America motion: Geophysical Research Letters, v. 22, p. 3545–3548. Dixon, T. H., Stefano, R., Lee, J., and Reheis, M. C., 1995, Constraints on present-day Basin and Range deformation from space geodesy: Tectonics, v. 14, p. 755–772. Hearn, E., and Humphreys, E. D., 1998, Kinematics of the southern Walker Lane Belt and motion of the Sierra Nevada block, California: Journal of Geophysical Research, v. 103, p. 27,033–27,049.

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