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Present Day Deformation Across the Basin and Range Province,
Western United States
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.