Active displacement transfer and differential block motion within the central Walker Lane, western Great Basin. J.S. Oldow Department of Geological Sciences, ...
Active displacement transfer and differential block motion within the central Walker Lane, western Great Basin J.S. Oldow Department of Geological Sciences, University of Idaho, Moscow, Idaho 83844-3022, USA C.L.V. Aiken |− Department of Geosciences, University of Texas at Dallas, Richardson, Texas 75080, USA J.L. Hare | J.F. Ferguson R.F. Hardyman 195 Moonshine, Reno, Nevada 89523, USA ABSTRACT Velocities determined for 50 global positioning system sites within the central Walker Lane indicate differential motion among tectonic blocks forming a boundary zone between the Great Basin extensional province and the Sierra Nevada. The velocity field is related to displacement transfer from the Owens Valley and Furnace Creek fault systems of eastern California to transtensional structures of the Walker Lane and extensional faults of the central Nevada seismic belt. Block boundaries are sharp and appear to be inherited from pre-Tertiary crustal structure. The block geometries exert strong influence on differential displacements concentrated along boundaries as belts of divergent, transcurrent, and convergent motion. The aggregate velocity accounts for about 25% of the relative motion between the Pacific and North American plates. About 5 mm/yr of the motion is localized along the eastern margin of the Sierra Nevada, whereas about 10 mm/yr is stepped 100 km east along a belt of east-northeast–trending transtensional faults that merge with northwest-trending transcurrent structures of the Walker Lane. About 6 mm/ yr of the velocity field is transferred to north-northeast–trending extensional faults of the central Nevada seismic belt. The heterogeneous distribution of motions is consistent with partitioning of a regional velocity field formed by westward extension and N408W-directed shear. Keywords: global positioning system geodesy, active tectonics, Walker Lane. INTRODUCTION Late Cenozoic deformation is broadly distributed across the North American plate margin of the western United States (Fig. 1) from the San Andreas fault system eastward into the Basin and Range (e.g., Hamilton and Meyers, 1966). Comparison of plate motion and geodetic measurements indicates that the San Andreas system accommodates about 75% of the velocity between the Pacific and North America plates (e.g., Argus and Gordon, 1991). The residual, 10–14 mm/yr (Savage et al., 1990; Miller et al., 1993), passes northeast from the Gulf of California and crosses the Mojave Desert via the eastern California shear zone (Dokka and Travis, 1990). The displacement is carried north, east of the southern Sierra Nevada, by the southern Walker Lane in a narrow zone of deformation bound on the west and east by the Owens Valley and Furnace Creek fault systems (Fig. 1), respectively (Dixon et al., 1995). North from the latitude of the central Sierra Nevada, the zone of deformation broadens to include the central and northern Walker Lane and the central Nevada seismic belt (Wallace, 1984) in the northwestern Great Basin. The Sierra Nevada behaves as a coherent tectonic block with a northwest-directed motion of 10–14 mm/yr and forms the western boundary of a zone of distributed deformation in the Great Basin (Dixon et al., 2000).
The distribution of seismicity (Rogers et al., 1991) and spatial pattern in estimated strain rates (Eddington et al., 1987) suggesting that active deformation is concentrated at the margins of the Great Basin is supported by wideaperture global positioning system (GPS) geodetic surveys (Bennett et al., 1999; Thatcher et al., 1999). Motion along the eastern margin of the province accommodates a 2–3 mm/yr westward velocity of the central Great Basin with respect to stable North America (Bennett et al., 1999; Thatcher et al., 1999). Along the western margin, the first significant change in velocity field occurs at the north-northeast– trending central Nevada seismic belt, where the velocity increases to about 6 mm/yr. Farther west, as the northern Sierra Nevada is approached, the velocity progressively increases to ;9 mm/yr and becomes more northwesterly (Thatcher et al., 1999). CENTRAL WALKER LANE Earthquakes in northwestern Nevada are concentrated in the north-northeast–trending central Nevada seismic belt and the northwesttrending Walker Lane (Wallace, 1984). The seismic belts merge in the central Walker Lane, where seismicity is deflected west toward the Sierra Nevada (Ryall and Priestly, 1975) and thence south parallel to the Sierran front. Earthquakes with magnitudes M $ 6
have a restricted distribution and together with late Quaternary fault scarps define the locus of active deformation in the central Walker Lane (Fig. 2). Earthquakes with magnitude M $ 3 (Rogers et al., 1991) have focal mechanisms consistent with geologically determined offsets and record left-lateral, right-lateral, and extensional first motions. Late Cenozoic faults (Fig. 2) of the central Walker Lane form a complex array of variably oriented structures characterized by coeval strike-slip and dip-slip motions (Oldow, 1992). The eastern margin of the Walker Lane is a pronounced physiographic boundary (Locke et al., 1940) across which north-northeast–trending extensional faults of the central Great Basin and northwest-trending right-lateral faults of the Walker Lane are juxtaposed (Stewart, 1988; Oldow, 1992). In the northern part of the central Walker Lane, northwesttrending dextral transcurrent faults form a belt ;50 km wide that is bound on the west by a region of north-northwest–striking faults extending to the Sierran front (Fig. 2). Farther south, the northwest-trending transcurrent structures swing westward and form an eastnortheast–trending belt of left-lateral and extensional faults (Fig. 2). Near the California border, the east-northeast–trending transtensional structures swing south and merge with the White Mountains and Fish Lake Valley faults, which constitute the northern extension of the Owens Valley and Furnace Creek fault systems, respectively (Fig. 2). GPS RESULTS The Central Walker Lane Network consists of 50 GPS sites spanning the boundary zone between the Sierra Nevada and the central Great Basin from Tonopah to Carson City, Nevada (Fig. 3). The GPS network was occupied in 1994, 1996, and 1999. In 1994, sites were occupied for at least two sessions of 10–12 h each, but in the 1996 campaign, each site received a minimum of 24 h of continuous measurement. In 1999, one site was continuously monitored and all other sites were occupied for 24–96 h. Data from all campaigns were transformed to the ITRF96 coordinate realization and processed with the software BERNESE. Network data were processed together with data from International GPS Service sites
q 2001 Geological Society of America. For permission to copy, contact Copyright Clearance Center at www.copyright.com or (978) 750-8400. Geology; January 2001; v. 29; no. 1; p. 19–22; 4 figures.
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Figure 1. Tectonic map of southwestern conterminous United States showing belts of active deformation. CNSB—central Nevada seismic belt; NWL—northern Walker Lane; CWL—central Walker Lane; SWL—southern Walker Lane; ECSZ—eastern California shear zone; sSAF—southern San Andreas fault; nSAF—northern San Andreas fault; OVF—Owens Valley fault; FCF—Furnace Creek fault.
in Quincy and Goldstone, California, which served as reference locations. Resultant velocities (Fig. 3) are residuals; the North American plate motion determined from the model NUVEL-1A (DeMets et al., 1994) is subtracted. Uncertainties shown in Figure 3 are 95% confidence ellipses estimated by scaling formal errors by RMS coordinate uncertainties. The velocity field is complex but shows systematic variations in orientation and magnitude that define regions of essentially constant motion. Velocity domain boundaries (Fig. 4) are located by the spatial correspondence between zones of greatest differential motion and throughgoing fault systems. In many cases the velocity domains coincide with physiographic boundaries, but in others no surface manifestation of the differential velocity exits. In general, the velocity domains are spatially coincident with tectonic domains characterized by different orientations and/or senses of slip on late Cenozoic faults. All velocities are presented in a reference frame fixed on a site located in the southeast part of the Central Walker Lane Network. The reference site was selected because it is outside the seismically active Walker Lane and near the western margin of the central Great Basin extensional domain (Fig. 2). In this reference frame, three additional sites along the eastern margin of the network show no statistically significant motion (Fig. 3). By comparison with velocities recorded in continuous and campaign GPS networks farther north (Bennett et al., 1999; Thatcher et al., 1999),
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Figure 2. Digital shaded relief map (Sterner, 1999) of central Walker Lane (Fig. 1), extending from Tonopah to Carson City and bound on west by Sierra Nevada and on east by central Great Basin. Physiographic boundaries are marked with yellow dashed lines. Traces of late Cenozoic transcurrent and extensional faults are shown in white, and epicenters of 1860 to 1999 earthquakes M $ 6 are shown by red dots. Northern extension of Owens Valley and Furnace Creek fault zones are White Mountains fault zone (WMFZ) and Fish Lake Valley fault (FLVF), respectively. Central Nevada seismic belt (not labeled) is north-northeast–trending belt of earthquake epicenters located east of Carson Sink.
the four sites probably have a small (2–3 mm/ yr) westward motion relative to stable North America. The Sierra Nevada block moves northwest at ;15 mm/yr relative to a fixed central Great Basin (Figs. 3 and 4). In the western part of the central Walker Lane, site velocities are directed north-northwest at 8–10 mm/yr (Wassuk domain), yielding a differential motion with the Sierra Nevada of more than 5 mm/ yr. In the region east of Lake Tahoe, differential motion is not concentrated along the Sierran front but is more distributed and partially accommodated by an intervening region (Topaz domain) with a northwest-directed motion of about 10 mm/yr. The Gillis domain, east of the Wassuk domain, is underlain by northwest-trending right-lateral and east-northeast–trending leftlateral faults. In this part of the central Walker Lane, site velocities are 6–9 mm/yr to the northwest. The velocities are consistently smaller than those of the Wassuk domain and are oriented more to the west, indicative of relative convergence. A northern margin of the Gillis domain (Fig. 4) is not recognizable on the basis of velocities. A tentative boundary is drawn to separate the Gillis domain from the northern Walker Lane, where a complex history of differential block rotation is interpreted from paleomagnetic data acquired
from late Cenozoic volcanic rocks (Cashman and Fontaine, 2000). To the east and south of the Gillis domain, northwest-directed velocities are substantially reduced, to 3–4 mm/yr (Monte Cristo domain). The boundary between the Monte Cristo and Gillis domains is well determined by the distribution of GPS sites and forms a rectilinear pattern of relatively high and low northwestward displacements (Fig. 4). The steps correspond in a general way with curved fault systems that transform motion from the east-northeast– trending belt of transtensional structures to northwest-trending transcurrent and extensional faults. The curved faults have kinematically coordinated slip and behave as relays between the two fault systems (Oldow, 1992). The most complex pattern of displacement is found in the boundary zone between the curvilinear transcurrent faults of the Gillis and Monte Cristo domains and the central Great Basin. In this region, displacements are small but vary substantially in azimuth (Fig. 4). In the southern part of the Monte Cristo domain, differential motion is directed to the north at 1–3 mm/yr but changes to northwest-directed motion as the boundary with the Gillis domain is approached. To the northeast along the boundary with the central Great Basin, displacements are to the west but progressively shift to northwest (Toiyabe domain) near the
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northeastern side of the central Walker Lane. The patterns of displacement are consistent with anticlockwise and clockwise rotation of the Monte Cristo and Toiyabe domains, respectively. Intervening between the convergent velocities of the Monte Cristo and Toiyabe domains is the Royston domain, which is characterized by east-northeast– and northdirected velocities of 4–8 mm/yr. The velocities of the Royston domain, which are determined by four GPS sites, are internally divergent and are discordant to those of adjacent domains. Motion of the Royston domain indicates shortening, particularly with respect to the Toiyabe domain. The region of discordant displacement is bound on the northeast by the central Nevada seismic belt, which has a velocity of about 6 mm/yr to the northwest. Northwest from the intersection of the central Nevada seismic belt and the central Walker Lane, little differential motion is recorded, which may explain the reduced seismicity observed in this area (Oldow, 1992). DISCUSSION AND CONCLUSIONS The velocity field in the central Walker Lane is complex but well organized (Fig. 4). The velocities show a stepwise increase to the northwest across the region from the central Great Basin to the Sierra Nevada and accommodate about 25% of the relative motion between the Pacific and North American plates. About one-third of the motion is concentrated along the northwest-trending Sierran front. Surprisingly, this area is not characterized by large (M $ 6) earthquakes. The locus of large earthquakes is more closely associated with the curvilinear array of faults that steps motion east to the northwest-trending transcurrent structures of the central Walker Lane via eastnortheast–trending sinistral-transcurrent and extensional faults (Figs. 2 and 4). Much of this displacement is passed to the central Nevada seismic belt, the residual continuing northwest to the northern Walker Lane. Motion on the central Nevada seismic belt accounts for nearly half of the displacement observed between the Sierra Nevada and central Great Basin in regions farther south. The GPS velocity field documents that the central Walker Lane acts as a distributed zone of displacement transfer linking the eastern California shear zone with the northern Walker Lane and central Nevada seismic belt (Oldow, 1992) and has displacements consistent with westward extension and superposed N408W-directed shear. Displacement transfer is not accommodated by a smooth, spatial transition in velocity, however. Rather, the velocities exhibit a pattern consistent with differential motion of tectonic blocks forming a boundary zone between the central Great Basin and the Sierra Nevada. The block bound-
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Figure 3. Global positioning system sites of Central Walker Lane Network. Velocities are in reference to a site fixed in ITRF96 at southeast margin of Central Walker Lane Network (triangle) and are residuals with North American velocity from NUVEL1A NNR subtracted. Ellipses represent 95% confidence.
aries are characterized by significant differences in rates and/or by direction of the velocity field. The shape and distribution of the tectonic blocks are not consistent with a simple velocity gradient and reflect the influence of pre-
existing crustal anisotropy on the displacement field. In the eastern part of the central Walker Lane, the curvilinear pattern of Cenozoic faults and seismicity together with the velocity domains mimics the regional distribution of Paleozoic and Mesozoic sedimen-
Figure 4. Velocity domains of central Walker Lane in fixed central Great Basin reference frame. Global positioning system sites are shown as yellow dots. CNSB—central Nevada seismic belt.
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tary facies, isotopic boundaries, and middle Paleozoic to late Mesozoic contractional structures (e.g., Oldow et al., 1989; Burchfiel et al., 1992). The preponderance of geologic evidence supports the conclusion that the northnortheast and east-northeast trends of the eastern part of the central Walker Lane reflect the original morphology of the early Paleozoic miogeosynclinal margin, which controlled subsequent Paleozoic and Mesozoic depositional and structural patterns (Oldow et al., 1989). Superposed on these old preferred orientations is a northwest-trending structural grain. Northwest-trending velocity-domain boundaries in the western part of the central Walker Lane coincide with middle to late Mesozoic transcurrent structures formed during intraplate deformation associated with Mesozoic transpression along the North American plate boundary (Oldow et al., 1989). The architecture of the late Cenozoic displacement transfer system is strongly influenced by structures formed during a protracted history of pre-Tertiary deformation. Differential motion between coherent blocks with irregular boundaries may explain some of the discordant displacements observed in the central Walker Lane. Zones of relative extension and shortening are well organized and follow domain boundaries. The greatest extension is localized along the Sierran front and between the Topaz and Wassuk domains, and farther east, extension is concentrated along east-trending segments of the dogleg boundary between the Gillis and Monte Cristo domains (Fig. 4). Divergent displacement between blocks is not unexpected. As others have noted in regions north and south (Hearn and Humphries, 1998; Bennett et al., 1999; Thatcher et al., 1999), this area marks the transition from westward extension of the Great Basin to shear associated with the northwest-directed passage of the Sierra Nevada block. Zones of velocity convergence are more difficult to explain, however. Convergence is observed in belts traced for tens of kilometers along the eastern flank of the Wassuk domain and along the eastern boundary of the Royston domain (Fig. 4). At this time it is unclear to what degree the belts of velocity convergence are produced by local displacement incompatibility stemming from the irregular block boundaries or produced by incompatibility of the extensional and shear components of the regional velocity field. It is possible that shortening within the central Walker Lane is an expression of regional northeast-southwest contraction suggested for the Sierra Nevada–
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Great Basin boundary zone on the basis of fault kinematic models (Hearn and Humphries, 1998) and wide-aperture GPS studies (Bennett et al., 1999). The northwest-trending Pacific–North American transform boundary may inhibit southwestward motion of the Sierra Nevada block (Hearn and Humphries, 1998) which, together with westward motion of the central Great Basin (2–3 mm/yr), could produce a component of shortening across the boundary zone. ACKNOWLEDGMENTS This research was funded by National Science Foundation grants EAR-9405486 and EAR95962257 (to Oldow) and EAR-9405583 (to Aiken). We thank the undergraduate and graduate students of Rice University, the University of Texas, Dallas, and the University of Idaho who worked on various GPS surveys. Without their diligent efforts, this research would not have been possible. Early versions of this manuscript were improved by reviews and discussions with L. Ferranti, W. McClelland, and P. Cashman. The manuscript was greatly improved by the critical review of T.W. Gardner. 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. Bennett, R.A., Davis, J.L., and Wernicke, B.P., 1999, Present-day pattern of Cordillera deformation in the western United States: Geology, v. 27, p. 371–374. Burchfiel, B.C., Cowan, D.S., and Davis, G.A., 1992, Tectonic overview of the Cordilleran orogen in the western United States, in Burchfiel, B.C., et al., eds., The Cordilleran orogen: Conterminous U.S.: Boulder, Colorado, Geological Society of America, Geology of North America, v. G-3, p. 407–479. Cashman, P.H., and Fontaine, S.A., 2001, Strain partitioning in the northern Walker Lane, western Nevada and northeastern California: Tectonophysics, v. 20 (in press). DeMets, C., Gordon, R.G., Argus, D.F., and Stein, S., 1994, Effect of recent revisions to the geomagnetic reversal time scale on estimates of current plate motions: Geophysical Research Letters, v. 21, p. 2191–2194. Dixon, T.H., Robaudo, S., Lee, J., and Reheis, M.C., 1995, Constraints on present-day Basin and Range deformation from space geodesy: Tectonics, v. 14, p. 755–772. Dixon, T.H., Miller, M., Farina, F., Wang, H., and Johnson, D., 2000, Present-day motion of the Sierra Nevada block and some tectonic implications for the Basin and Range province, North American Cordillera: Tectonics, v. 19, p. 1–24. Dokka, R.K., and Travis, C.J., 1990, Late Cenozoic strike-slip faulting in the Mojave Desert, California: Tectonics, v. 9, p. 311–340. Eddington, P.K., Smith, R.B., and Renggli, C., 1987, Kinematics of Basin and Range intraplate extension, in Coward, M.P., et al., eds., Conti-
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