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Wayne Thatcher
(U. S. Geological Survey, MS/977, Menlo Park, CA 94025; thatcher@usgs.gov)

The distribution of deformation within the Basin and Range has been determined from 1992, 1996 and 1998 surveys of a dense 800-km aperture Global Positioning System network. Internal deformation generally follows the pattern of Holocene fault distribution and is concentrated near the western extremity of the province, with lesser amounts focussed near the eastern boundary. Little net deformation occurs across the central ~500 km of the network in western Utah and eastern Nevada. Concentration of deformation adjacent to the rigid Sierra Nevada block indicates external plate driving forces play an important role in driving deformation, modulating the extensional stress field generated by internal buoyancy forces due to lateral density gradients and topography near the province boundaries.

The northern Basin and Range is an actively deforming intracontinental plateau lying between the stable blocks of the Sierra Nevada and Colorado Plateau ( Fig. 1). The province has extended (increased in area) about a factor of two in the last ~20 Ma (1, 2) and extension continues, with ongoing seismic activity and slip along numerous faults distributed across a zone ~800 km wide (3-5). Constraints on the internal deformation of the province are limited. Geologic studies delineate regions of Holocene and late Quaternary fault slip (3, 4). Space geodetic measurements broadly define movements across the province (6-8) and local surveys map concentrated deformation in several seismically active zones (9-11). The detailed pattern is important because it defines the current seismic hazard, with regions of high velocity gradient having more frequent damaging earthquakes than regions of low gradient. In addition, the spatial pattern constrains the fundamental processes driving active continental deformation, here suggesting that external plate motions are more important than internal buoyancy forces in deforming the province.

Here we show the detailed velocity field mapped from a dense Global Positioning System (GPS) network that spans the Basin and Range. The GPS network consists of 63 stations, most of which were occupied on two or more days during surveys in 1992, 1996 and 1998 (12). The velocity of each station relative to stable North America was determined (Fig. 1) and velocity magnitude and vector orientations were calculated (Fig. 2).

Several first-order features are immediately apparent from Figs. 1 and 2. First, deformation is strongly concentrated in two regions, the westernmost ~200 km and easternmost ~100 km of the network, with little internal deformation of the intervening ~500 km of the central Basin and Range. Locally high velocity gradients (see Fig. 2a) are associated with fault zones near 111.8° (Wasatch fault), 113° (Drum Mountain fault), 117.9° (Central Nevada seismic zone, CNSZ), and across a more diffuse zone of conjugate strike-slip and normal faults between 119.1° and 120.2° (Sierra Nevada transition zone, SNTZ). This pattern is broadly consistent with existing geologic, seismic and space geodetic data. Reconaissance geologic mapping (3, 4) and seismicity compilations (5) show evidence for Holocene fault slip and historical seismic activity in central Utah and western Nevada but pre-Holocene slip and low seismicity levels in the central Basin and Range. Widely spaced VLBI (Very Long Baseline Interferometry) and continuous GPS station data are consistent with our results (14).

The distribution of deformation across western Nevada suggests that the 8-12 mm/yr of ~310°-oriented relative motion across the eastern California shear zone (7, 15-17), which lies south of our network near longitude 118° W, is partitioned between two fault zones. Average velocities of 2.8 ± 0.5 mm/yr between 114.9° and 117.7° W increase to 6.5 ± 0.7 mm/yr between 118° and 119.2° W and to 12.5 ± 1.5 mm/yr between 119.9° and 120.2° W. Thus 2.8 ± 0.5 mm/yr of relative motion occurs across the Wasatch and related faults in central Utah; 3.7 ± 0.8 mm/yr of relative northwestward motion occurs across the CNSZ; and an additional 6.0 ± 1.6 mm/yr is accomodated within the SNTZ. The latter value is within the range of the ~3-6 mm/yr of 300°-oriented motion inferred across faults in northwestern California and central Oregon (18), suggesting that much of this deformation may be accomodated through western Nevada.

Velocity vectors within the Basin and Range show the superposed effects of extensional stresses due to lateral density gradients in the lithosphere and tractions exerted by the relative motions of the bounding stable blocks. The average trend in velocity vector orientations across the province (Fig. 2b) is close to 310°, the direction of relative motion of the Sierra Nevada microplate with respect to stable North America (6, 7, 14), immediately suggesting the influence of this motion on internal deformation of the province. However, local variations in vector orientations provide clues that internal driving forces also affect the deformation.

The ~295° orientation of velocities in central Utah would seem to suggest deformation due largely to the motion of the Colorado Plateau (essentially stable North America) relative to the eastern Great Basin. However, the ~1 km increase in elevation and 15 km increase in crustal thickness across the Basin-Range/Colorado Plateau transition zone is expected to produce extensional stresses perpendicular to the Wasatch fault zone in central Utah (19). Geodetic measurements across the Wasatch zone are consistent with this stress field orientation. Velocity vectors and extensional strains are nearly normal to the local N20°E trends of the faults across our network (Fig. 1) and to the north-south striking Wasatch zone near Ogden, 200 km farther north (9, 10). These orientations are also consistent with least principal stress orientations inferred from various stress indicators near the Wasatch front (20).

Between 118° and 120° W, the orientation of velocities is within ±15° of the vector defining the relative motion of the Sierra Nevada block with respect to stable North America. This orientation, along with the high velocity gradients across the region, suggest that Pacific-plate-coupled motion of the Sierra Nevada microplate is responsible for much of the deformation of western Nevada. However, the large component of normal faulting present in this region suggests the perturbing influence of extensional stresses caused by buoyant, low density upper mantle beneath the Great Basin (21, 22). The local 295° orientation of velocity vectors across the Sierra Nevada-bounding Genoa fault, a pure dip-slip north-south-striking normal fault near 120° W, may be due to the perturbing effects of stresses generated by topographic gradients across this transition zone (see 19). These stresses would tend to rotate velocity vectors towards the normal to the Genoa fault in the elevated Sierra Nevada and away from this direction in the lower-lying Basin and Range, as observed.

The velocity field measured across active faults provides estimates of fault slip rate and constraints on the mechanism of elastic strain buildup in the adjacent crustal blocks (23). An elastic half-space dislocation model with a normal fault dipping 60° that does not slip between the surface and some fixed depth (H) but slides freely at a constant slip velocity below that depth yields the horizontal velocity expected due to elastic strain accumulation across the fault (24) (Fig. 3). The locked zone depth is taken to be the depth to which seismic fault slip or small earthquake hypocenters extend, 10-20 km in the Basin and Range. The high velocity gradient in the model is ~3H wide, or 30-60 km for locking depths appropriate here. For the area covered by our network the expected pattern of horizontal velocity across an individual fault thus is represented well by a smoothed step and local peak or trough, with the net offset equal to the horizontal component of the fault slip rate. Strain accumulation across a series of widely spaced faults should resemble an irregular staircase, with steps being the zones of elastic strain accumulation and flats representing the intervening undeformed blocks.

The observed pattern of deformation across most of Nevada is similar to these expectations, with high velocity gradients near 118° and 119.6° W and nearly constant velocities elsewhere. Fig. 4 shows that buried faults beneath the CNSZ and SNTZ can explain the main features of the data. Both normal and right-lateral strike-slip faulting occur across each zone, so the modeled fault slip rate is the resultant of these two components. Since a number of active faults are exposed at the surface in each zone the models are undoubtedly oversimplified. For example, the SNTZ contains both strike- and dip-slip faults, and the width of the deforming zone suggests several sub-parallel faults locked to ~10-15 km would match the data at least as well as a single fault locked to 30 km.

Movements in central Utah and eastern Nevada are more complex than those shown by the simple buried fault slip model. Horizontal velocities (Fig. 2a) increase near the Wasatch fault zone as expected, but then abruptly decrease west of it. Velocity subsequently increases near the Drum Mountain faults, decreases to the west, and finally reaches a stable value of ~3 mm/yr west of 113.3°W. There is suggestive evidence that a local velocity decrease similar to those shown in central Utah occurs near the Schell Creek Range (at 114.6°, see Fig. 2a). The changes there are small, about 2 mm/yr, and are supported by very few observations. However, it may be noteworthy that this is the only mapped Holocene fault that crosses our network between the Utah-Nevada border and the central Nevada seismic zone (4). Models like those in Fig. 3 match general features of the Utah and eastern Nevada data (25) but cannot reproduce the near-fault variations, which may be due to changes in slip along fault strike (26).

Our results suggest that forces exerted on the Basin and Range by motions of the bounding plates are more important than internal buoyancy forces in driving large scale deformation. Mappings of gravitational potential energy available to cause deformation (22) show high gradients in eastern Nevada that do not correlate with velocity gradients obtained by us. It may be that the stresses implied by these mappings are supported by the local strength of the lithosphere, with little resulting deformation. In contrast, the concentration of deformation near the western edge of the province indicates an important role for boundary forces due to motion of the Sierra Nevada microplate. Crustal thickness variations near the province boundaries may be important in causing deformation near the Wasatch fault and in complementing plate drag forces near the Sierra Nevada. However, the effects of all driving forces may be significantly modulated by lateral variations in lithospheric rheology, the relation between applied forces and resulting displacements. Rheology is very sensitive to temperature and rock type (27), so we might expect that the varied structural and thermal history of the western U. S. would generate strong lateral variations in strength, channeling deformation into zones that are intrinsically weaker than their surroundings.

REFERENCES AND NOTES
1. W. Hamilton, in Continental Extensional Tectonics M. P. Coward, J. F. Dewey, P. L. Hancock, Eds. (Geological Society, London, 1987), vol. 28, pp. 155-176.
2. B. Wernicke, in The Cordilleran Orogen: conterminous U. S. The geology of North America Volume G-3, P. W. Lipman, B. C. Burchfiel, and M. L. Zoback (Editors), Ed. (Geol. Soc. Am., Boulder, Co, 1992) pp. 553-581.
3. S. Hecker, "Quaternary tectonics of Utah with emphasis on earthquake-hazard characterization" Bulletin 127 (Utah Geological survey, 1993).
4. J. C. Dohrenwend, B. A. Schell, C. M. Menges, B. C. Moring, M. A. McKittrick, "Reconaissance photogeologic map of young (Quaternary and late Teritary) faults in Nevada" Open-File Report 96-2 (Nevada Bureau of Mines and Geology, 1996).
5. D. M. dePolo, C. M. dePolo, "Earthquakes in Nevada 1852-1996" Map 111 (Nevada Bureau of Mines and Geology, 1998).
6. D. F. Argus, R. G. Gordon, Geology 19, 1085-1088 (1991).
7. T. H. Dixon, S. Robaudo, J. Lee, M. C. Reheis, Tectonics 14, 755-772 (1995).
8. R. A. Bennett, B. P. Wernicke, J. L. Davis, Geophys. Res. Lett. 25, 563-566 (1998).
9. L. J. Martinez, C. M. Meertens, R. B. Smith, Geophys. Res. Lett. 25, 567-570 (1998).
10. J. C. Savage, M. Lisowski, W. H. Prescott, J. Geophys. Res. 97, 2071-2083 (1992).
11. J. C. Savage, M. Lisowski, J. L. Svarc, W. K. Gross, J. Geophys. Res. 100, 20,257-20,269 (1995).
12. The October 1992 survey was carried out with Turborogue GPS receivers with choke-ring antennas, September 1996 and September 1998 measurements with Ashtech Z12 receivers and Ashtech choke-ring antennas. The network consists of 47 stations strung about 15-30 km apart along US Highway 50 and 16 stations spaced between 60 and 120 km apart to the north and south. The 16 bounding network stations and 8 additional Highway 50 stations were generally occupied on 4 consecutive days in each survey. Of the other Highway 50 sites, 23 were occupied on 2 consecutive days and the remaining 16 were observed on a single day. Most Highway 50 sites used National Geodetic Survey leveling benchmarks installed as long ago as 1932. Other sites employed stainless steel plugs we cemented into bedrock outcrops. Data were reduced with Gipsy software release 4. Station positions for each epoch were derived in the ITRF96 reference frame and velocity vectors determined relative to stable North America. All station coordinates and site velocities may be accessed via the U. S. Geological Survey website http://quake.wr.usgs.gov/QUAKES/geodetic/gps/.
13. Errors were assigned assuming station-day position uncertainties of 3 mm (north), 5 mm (east) and a random walk component of benchmark motion of 1mm per ÷year. See J. Langbein, H. Johnson J. Geophys. Res. 102, 591-603 (1997).
14. The VLBI-derived estimate of Sierra Nevada-stable North America relative motion near 37° N, 118° W is 12.1 ± 1.2 mm/yr at a azimuth of 322° ± 5° (7). The average of GPS determinations from 4 stations within the Sierra Nevada block (open circles in Fig. 2) is 11.8 ± 1.6 mm/yr at 308° ± 5 °. Values between 119.8° and 120.3° W average 12.5 ± 1.5 mm/yr at 299° ± 5°. The velocity of a VLBI station near Ely, Nevada is 4.9 ± 1.3 mm/yr at an azimuth of 262° ± 13° (7). The VLBI site lies roughly equidistant and ~12 km from two of our stations (near 115° W, Fig. 2) whose average velocity is 3.2 ± 1.6 mm/yr at 303° ± 19 °. Although the two azimuth estimates are consistent at the two standard deviation level, our results from other GPS stations in the region suggest velocities are generally directed more northwest and are smaller in magnitude than the nearly west-oriented value determined for the Ely VLBI site. Initial results from a 13 station continuous GPS array (8) located north of our network and roughly centered on 40° N show an increase in velocity west of the central Nevada seismic zone, consistent with our data. An inferred linear east to west increase in the east component of velocity (8) is not supported by our data. Instead, our data show an abrupt increase of east and north velocity components near 118° W.
15. R. K. Dokka, C. J. Travis, Tectonics 9, 311-340 (1990).
16. J. Sauber, W. Thatcher, S. Solomon, J. Geophys. Res. 91, 12,683-12,693 (1986).
17. J. C. Savage, M. Lisowski, W. H. Prescott, Gephys. Res. Lett. 17, 2113-2116 (1990).
18. S. K. Pezzopane, R. J. Weldon, Tectonics 12, 1140-1169 (1993).
19. Isostatically balanced lateral differences in topography result from lateral density gradients that generate horizontal forces capable of causing deformation. For a two-dimensional structure across which the difference in topographic elevation is h and isostatic compensation occurs entirely within the crust, the force per unit length, F, normal to the structure is given by

where rc is crustal density, rm mantle density, g is the gravitational acceleration and ycco crustal thickness of low lying region. If ycco=35 km, rm=3300 kg m-3, rc=2750 kg m-3 and h=1 km, then F=2x1012 N m-1, comparable to many plate-driving and resisting forces. This force produces extensional stress in the elevated region and compressional stress in the adjacent lower lying crust. See D. L. Turcotte, in Mountain Building Processes K. J. Hsu, Ed. (Academic Press, London, 1982) pp. 141-146. The magnitude of these forces will differ if lateral density contrasts extend into the mantle. For example, the driving force will be smaller than computed above if mantle lithosphere is colder and denser beneath the Sierra Nevada and Colorado Plateau than it is beneath the Basin and Range.
20. We assume that principal stress orientations estimated from earthquake fault plane solutions, borehole elongations and slip directions on faults are coincident with incremental principal strains determined by GPS. M. L. Zoback, J. Geophys. Res. 94, 7105-7128 (1989) has estimated least principal stress orientations that are nearly east-west across the Wasatch fault near Ogden and N90°E to N120°E across our GPS network.
21. A. H. Lachenbruch, and P. Morgan, Tectonophys. 174, 39-62 (1990).
22. C. H. Jones, J. R. Unruh, and L. J. Sonder, Nature 381, 37-41 (1996) calculate that the driving force contribution of buoyant upper mantle in the northern Basin and Range (termed gravitational potential energy by them) averages 1.5x1012 N m-1, comparable to the effects of province margin topography given in (19). However, these authors also point out significant uncertainties in the computed force gradients due to poor constraints on the exact upper mantle density structure and its lateral variations.
23. L. B. Freund, D. M. Barnett, Bull. Seismol. Soc. Am. 66, 667-675 (1976). The horizontal displacement pattern in Fig. 3 is for a west-dipping fault. If the fault has an eastward dip the pattern is reversed, with all local peaks replaced by troughs and vice versa (turn Fig. 3 upside down to visualize). For dips shallower than 60° the local peaks and troughs become progressively less prominent for the same slip distribution as that of Fig. 3.
24. J. C. Savage, R. O. Burford, J. Geophys. Res. 78, 832-845 (1973); J. C. Savage, J. Geophys. Res. 88, 4984-4996 (1983).
25. The central Utah models use 2 60°-dipping faults locked from the surface to 15 km depth. Slip of 4 mm/yr is required across the Wasatch fault and 2 mm/yr across the Drum Mountain fault. These slip rates are surprisingly high, and may be related to a discrepancy noted from the Wasatch fault near Ogden, where geologically-estimated late Holocene slip rates are 1-2 mm/yr (D. P. Schwartz and K. J. Coppersmith, J. Geophys. Res. 89, 5681-98 (1984)) and geodetic estimates are ~5 mm/yr (9, 10).
26. Some of the local velocity decreases seen at fault crossings of our network may be due to a sampling bias. Both coseismic fault slip and topographic height are expected to be a maximum near the center of each range and decrease towards its ends. Interseismic strain accumulation rate (velocity gradient normal to the fault) should generally mimic this pattern. Because highways engineers site roadways through the lowest available topographic gradient, most of our range crossing stations are located either near the ends of active ranges or between adjacent ones. This bias will produce local along strike velocity gradients and velocity minima near the ends of active ranges, qualitatively consistent with patterns seen in our data across central Utah and easternmost Nevada.
27. Brace, W. F., and D. l. Kohlstedt, J. Geophys. Res. 85, 6248-6252 (1980); Sonder, L. J., and P. C. England, Earth Planet. Sci. Lett., 77, 81-90 (1986).
28. This work was supported by NASA's Dynamics of the Solid Earth Program. Help with GPS fieldwork was provided by G. Hamilton, J. Sutton, C. Stiffler, G. Marshall, R. Stein, K. Hodgkinson, M. Hofton, N. King, T. Sagiya, and B. Kilgore. Discussion with W. Hamilton, A. H. Lachenbruch, T. Parsons, W. H. Prescott, J. C. Savage, R. Simpson, G. Thompson, R. Wells, and M. L. Zoback is gratefully acknowledged. Careful reviews of the manuscript were provided by Drs. James C. Savage, Mary Lou Zoback, Robert B. Smith, and an anonymous reviewer.