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1995 Constraints on Present-Day Basin and Range Deformation from Space Geodesy Timothy H. Dixon University of Miami, [email protected]

Stefano Robaudo University of Miami

Jeffrey Lee California Institute of Technology

Marith C. Reheis U.S. Geological Survey

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Scholar Commons Citation Dixon, Timothy H.; Robaudo, Stefano; Lee, Jeffrey; and Reheis, Marith C., "Constraints on Present-Day Basin and Range Deformation from Space Geodesy" (1995). School of Geosciences Faculty and Staff Publications. 495. https://scholarcommons.usf.edu/geo_facpub/495

This Article is brought to you for free and open access by the School of Geosciences at Scholar Commons. It has been accepted for inclusion in School of Geosciences Faculty and Staff ubP lications by an authorized administrator of Scholar Commons. For more information, please contact [email protected]. TECTONICS, VOL. 14, NO. 4, ]'AGES 755-772, AUGUST 1995

Constraints on present-day Basin and Range deformation from space geodesy

TimothyH. Dixonand Stefano Robaudo • RosenstielSchool of Marine and AtmosphericSciences, University of Miami Miami, Florida

JeffreyLee 2 Division of Geologicaland Planetary Sciences, California Institute of Technology,Pasadena

Marith C. Reheis U.S. GeologicalSurvey, Lakewood, Colorado

Abstract. We use new spacegeodetic data from very long extension.A slip rate budgetfor major strike-slipfaults in baselineinterferometry and satellite laser ranging combined our studyarea based on a combinationof local geodeticor with other geodetic and geologic data to study late Quaternary geologic data and the regional space contemporarydeformation in the Basinand Range province geodeticdata suggeststhe following rates of right-lateral of the western United States. Northwest motion of the slip: Owens Valley fault zone, 3.9+1.1 mm/yr; Death central Sierra Nevada block relative to stable North Valley-FurnaceCreek fault zone, 3.3+2.2 mm/yr; White America, a measure of integrated Basin and Range Mountains fault zone in northernOwens Valley, 3.4+1.2 deformation,is 12.1+1.2 mm/yr orientedN38øW+5 ø (one mm/yr; Fish Lake Valley fault zone, 6.2+2.3 mm/yr. In the standarderror), in agreementwith previous geological last few million yearsthe locusof right-lateralshear in the estimateswithin uncertainties.This velocity reflectsboth regionhas shifted west andbecome more north trendingas east-west extension concentrated in the eastern Basin and slip on the northweststriking Death Valley-Furnace Creek Range and north-northwestdirected right lateral shear faultzone has decreased and is increasinglyaccommodated concentratedin the westernBasin and Range. Ely, Nevada onthe north-northwest striking Owens Valley faultzone. is moving west at 4.9+1.3 mm/yr relativeto stableNorth America, consistentwith dip-slip motion on the north Introduction striking Wasatch fault and other north striking normal faults. Comparison with ground-basedgeodetic data It haslong been recognized that Basin and Range extension suggeststhat most of this motion is accommodatedwithin is an important componentof deformationwithin the N50 km of the Wasatch fault zone. Paleoseismic data for Pacific-NorthAmerica plate boundaryzone [Atwater, the Wasatchfault zone and slip rates based on seismic 1970]. In the last decadespace geodetic techniques have energyrelease in the regionboth suggestmuch lower slip helpedto clarifythe kinematicsof Basin and Range rates. The discrepancymay be explained by some deformationand its role in Pacific-NorthAmerica plate combinationof additional deformationaway from the interaction[Minster and •1ordan, 1984, 1987; Ward, 1990; Wasatchfault itself, aseismicslip, or a seismicrate that is Argus and Gordon, 1991]. There are neverthelesssome anomalouslylow with respectto longer time averages. remainingpuzzles. For example,why do recentspace Deformationin the westernBasin and Range provinceis geodeticestimates of the rotationvector describing Sierra also largelyconfined to a relativelynarrow boundary zone Nevadablock-stable North America relative motion [Argus and in our study area is partitioned into the eastern and Gordon,1991], a measureof integrateddeformation Californiashear zone, accommodating10.7+1.6 mm/yr of acrossthe Basin and Rangeprovince, differ in direction north-northwestdirected right-lateral shear, and a small fromgeological estimates of Basinand Range deformation component(-1 mm/yr) of west-southwest- east-northeast averagingover longertimes [Minsterand Jordan,1987; Wernicke,1988]? Does this imply rapid evolutionin deformationgeometry? Which faultsaccommodate surface •Nowat Ecology and Environment, Ft.Lauderdale, Florida. deformationassociated with the recentlyrecognized eastern 2Nowat Departmentof Geology,Central Washington California shear zone, and what is the total slip rate University, Ellensburg. [Sauberet al., 1986;Dokka and Travis,1990; Savage et al., 1990; Sauber et al., 1994]? What is the relation between the eastern California shear zone and overall Copyright1995 by the AmericanGeophysical Union. deformationof the Basin and Range? Is the Basin and Range province deforming as a continuum, or is Paper number 95TC00931. deformationlargely restricted to the boundaryzones? In 0278-7407/95/95TC-00931 $10.00 this paperwe addressthese questions with new space

755 756 DIXON ET AL.' BASIN AND RANGE DEFORMATION

Excelsior Quaternary Faulting ' In"'•)'• Network ...... Western Boundary Zone ,•CratYers•. Central Basin & Range 0 20 40 60km I I I I \\ '<'\"*' •..ong Valley. • \" • •Caldera•] • Normalfault, ball on downthrown side • Strikeslip fault with sense ofshear '• Fault,slip oblique or unknown • M >6.0 Earthquakesince 1978 ii'•'}•: ...... Surface rupture 1872 earthquake .':iiii•:Sprin Significant present day deformation, Eastern -.• ..... :... '-:!::•: California shear zone

Sierra .... E! Space geodetic site Nevada Block

Owens Valley Trilateration Network

Cottonwood Mountains

119 ø

Towne [] ass

...... ======•..... BASIN Platteville AND Fault

STABLE

ave 36

o • [] Yuma PACIFIC • • • -- --NDavisFort PLATE •xico []

Figure 1. Major Quaternaryfaults and selectedearthquakes for the boundaryzone betweenthe Basin and Range province and the Sierra Nevada block near Owens Valley Radio Observatory(OVRO). Surface rapture of 1872 earthquakeand outline of Owens Valley and southernExcelsior trilateration networks [Savageand Lisowski, 1980, 1995] are also shown. Fault abbreviationsare DVFCFZ, Death Valley- FurnaceCreek fault zone; FLVF Fish Lake Valley fault; HSF, Hartly Springs fault; HCF, Hilton Cr•k fault, MLF, Mono Lake fault, RVF, Round Valley fault; SLF Silver Lake fault. SAF, San AndreasFault. Focal mechanismis shown for the May 17, 1993, Eureka Valley earthquakefrom Harvard Centroid Moment Tensor (CMT) solution [Dziewonski et al., 1994]. Light stipple is postulatedsurface trace of major faults presentlyaccommodating the easternCalifornia shear zone. Inset shows location of most spacegeodetic sites used in thisstudy. Figure 1 is modifiedfrom Hill et al. [ 1985]and California Division of Mines and Geology[ 1992]. DIXON ET AL.: BASIN AND RANGE DEFORMATION 757 geodeticdata combined with other geodetic and geological presented by Dixon et al. [1993]. This paperexpands and data. updatesthe earlier interpretationand correctsan error in the uncertaintyassigned to the stationvelocities. The space geodetic data have two important Space Geodetic Data characteristics.First, they describedeformation relative to Previousspace geodetic studies of Basin and Range an external reference frame. This will allow us to link local deformation[ Ward, 1990; Argus and Gordon,1991 ] were deformationestimates based on geological and ground basedon very long baselineinterferometry (VLBI) data geodeticobservations to more regionaldata and processes. collectedby NASA's CrustalDynamics Project (CDP) up Second, they define integrateddeformation over a broad to the endof 1989. Our studyimproves on earlierstudies region. For example OVRO's velocity relative to stable byhaving a longertime span of dataand by incorporatingNorth America approximatesintegrated Basin and Range additionaldata types. We useVLBI datacollected by the deformation,absent only a small componentof extension CDP from 1979 to the end of 1991 (GLB 868) [Ryan et acrossthe easternSierra Nevada range front fault to the west al., 1993]and incorporate satellite laser ranging (SLR) data (Figure 1). The velocity of Ely defines integrated to the Lageossatellite from 1976 to the end of 1990 deformationacross the easternBasin and Range and by (Universityof Texas long arc solutionLLA 9101) vector differencewith OVRO provides informationon the [Watkins,1990]. Whereappropriate we alsoincorporate magnitude and style of present day deformationin the geologicalobservations and local ground geodetic data to westernBasin and Rangebetween OVRO and Ely. Thus helpinterpret the velocitiesof the spacegeodetic sites. we can describeBasin and Range deformationin a transect Thisallows us, for example,to betterrelate the velocityof roughly orthogonalto a small circle describingPacific- OwensValley Radio Observatory(OVRO), locatedin North Americamotion, connecting the Wasatchfront in the OwensValley east of theeastern Sierra Nevada range front easternBasin and Rangeto the San Andreasfault in central fault and also east of the sur-facetrace of the Owens Valley California (Figure 1, inset). faultzone (Figure 1), to the motionof the stableSierra In a study like this it is important to determine the Nevada block to the west. influenceof the referenceframe on the site velocity For analysisof the spacegeodetic data, we combine estimates. While our choice of referenceframe ("stable baselinelength and (for VLBI) transverse rates of changefor North America") is logical, the ensembleof stations used a globalnetwork of stations,similar to the technique to defineit is arbitraryand necessarilyintroduces random describedby Ward[ 1990]and Argus and Gordon[ 1991] and possibly systematicerror. For example, our six- for VLBI dataonly. Detailsof the analysisare givenby station referenceframe encompassesregions undergoing Robaudoand Harrison [1993]. Velocities of sites of differentialpostglacial rebound and includes three sites interest,Ely (Nevada),Hat Creek, Quincy,OVRO, and (Fairbanks, Platteville, and Yuma) arguably close to Mojave(California) (Figure 1, Table 1), are determineddeforming zones. We opted to use this larger group of relativeto stableNorth Americaby minimizing in a least referenceframe-defining stations in order to minimize the squaressense the velocitiesof six stations:Fairbanks influenceof errorsat any one station,but it is usefulto ask (Alaska),Platteville (Colorado), Fort Davis (Texas), Yuma how results might vary with different combinations of (Arizona), Westford(Massachusetts), and Richmond referenceframe stations.In an otherwisesimilar analysisof (Florida). Platteville,Fort Davis,and OVRO haveboth VLBI data from 1979 to 1991, Gordon et al. [ 1993] used a SLR and VLBI data; the remainingstations have VLBI differentdef'mition of stableNorth America,eliminating all data only. A preliminaryversion of theseresults was stationswest of the MississippiRiver and fixing only two

Table1. Velocityof SelectedSites in WesternUnited States Relative to StableNorth America

Northand West Velocities* Rate,Azimuth and Error Ellipse* '• North, West, Rate, Azimuth, c•1, 01, c•2, mm/yr mm/yr mm/yr deg mm/yr deg mm/yr Ely -0.7+1.1 4.9_+0.7 4.9_+1.3 262 1.2 12 0ø9 Hatcreek 4.5_+0.4 7.7_+0.3 8.9_+0.5 300 0.7 343 0.6 Mojave 7.5_+0.3 4.2_+0.2 8.6_+0.4 331 0.7 342 0.6 OVRO 8.0_+0.4 6.2_+0.3 10.1_+0.5 322 0ø7 347 0.6 Quincy 3.6_+0.4 8.1_+0.3 8.9_+0.5 294 0.8 338 0.6

*Uncertainties are one standard error. ?C•land 01 arethe length and orientation (degrees clockwise from north) of thesemimajor axis of the velocityerror ellipse at one standarderror; c•2 is the lengthof the semiminoraxis. For 95% confidenceellipse, multiply g l andg2 by 2.45. 7:58 DIXON ET AL.: BASIN AND RANGE DEFORMATION

easternseaboard stations, Westford and Richmond. Their Basin and Range deformation. First, the kinematic resultsare generallysimilar to ours within uncertainties. boundarycondition for Basin and Range deformation For exampletheir velocityfor Ely (5.6 mm/yr-k-_l.0mm/yr (Pacific-NorthAmerica relative plate motion) has been at an azimuth of 274ø or N86øW) overlapsour result essentiallyconstant for at leastthe last 3.4 million years (4.9+1.3 mm/yr at an azimuth of 262ø or W8øS; Table 1; [Harbertand Cox, 1989], conœmnedby goodagreement all errorsquoted at onestandard error) within uncertainties. betweengeologic and geodeticestimates of the plate This suggeststhat the referencetime definitionwe have motionvector within theirjoint uncertainties[e.g., Ward, chosenhas not unduly influencedthe site velocities of 1990;Argus and Gordon,1990; Dixon et at, 1991;Feigt interest. et at., 1993]. Second,the present-daystate of stressas Our results for most stations are also similar to those of indicatedby earthquakesindicates least principal stress Ward [1990] and Argus and Gordon [1991] within directions between east-west and east-southeast - west- uncertainties.The azimuthof OVRO's velocity obtained northwestfor most of the Basin and Rangeprovince in our study (322ø or N38øW; Table 1)is 10ø more [Zoback,1989] in agreementwith longer-termgeological westerlycompared to earlierresults [Argus and Gordon, indicatorsof extensiondirection [Zoback and Zoback, 1991] (332ø or N28øW) though not as westerly as that 1980;Minster and Jordan,1987; Wernickeet at., 1988]. obtainedby Gordon et at. [1993] (314ø or N46øW). Third,there are several problems with a right-lateralsimple Results for Ely have not been discussedin any detail shearmodel for Basin and Rangeextension, noted in the previouslybecause of few observationsand correspondingprevious paragraph. Motivated by theseobservations, we large error. However, there are now sufficientdata to reexaminethe spacegeodetic data and relevantterrestrial warrantanalysis. Our velocityestimate for Ely is basedon data and considerimplications for deformationon the 12 observationstaken betweenApril 1984 and October easternand western boundaries of theBasin and Range and 1990. the location, kinematics, and evolution of the eastern Californiashear zone. We thendefine a vector(valid in the Discussion generalvicinity of OVRO) describingmotion of the Sierra Nevadablock with respectto stableNorth America,a Argus and Gordon [1991] used data then available to measureof integratedBasin and Range deformation,•r predictnorth-northwest motion of the SierraNevada block comparisonto geologicalestimates and considersome relativeto stableNorth America,approximately parallel to tectonicimplications. the block's easternboundary with the Basin and Range, and suggestedthat Basin and Range deformationreflected Deformation in the Eastern Boundary this motionvia a right-lateralsimple shear model. In other Zone words, west-northwest- east-southeastextension in the Basinand Range,taken as a continuum,was a directresult Ely's westwardmotion is consistentwith previous of right lateralshear on the westernboundary. There are suggestionsthat the north strikingWasatch range front threeimportant implications of this model. First, while fault(Figure 1, inset)and relatedeastern boundary faults motionof the SierraNevada block predictedby the Argus primarilyexperience dip slip motion [Bestand Hamblin, and Gordon[ 1991] model(N28øW at OVRO) would yield 1978; Minster and Jordan, 1984; Zoback, 1989]. a maximum stretching direction oriented-N70øW, in However,the rate estimatefor Ely (4.9+1.3 mm/yr) agreementwith geologicalindicators of Basin and Range suggestsfaster extension across the eastern Basin and extension,it also predictsorthogonal shortening for which Rangethan is indicatedby mostother data (see below). If there is no geologicalevidence. Second,a right-lateral correct,the new spacegeodetic data have important simple shear model for Basin and Range deformation implicationsfor seismic hazard and earthquake process in predictsno crustalthinning. However,elevated heat flow, the region. gravityand seismic reflection and refraction data all suggest Beforecomparing our resultto otherdata, we first assess that relatively thin crust is characteristicof much of the the possibleimpact of our choiceof referenceframe on the Basin and Range province [e.g., Smith, 1978; Eaton, extensionrate estimate. Gordon et al. [1993] discussan 1982; Knuepfer et at., 1987]. Third, the amount of apparenteast-west lengthening of- 2 mm/yr acrosseastern extensionpossible in right-lateralsimple shearis fairly North America based on VLBI data. Whether this limited, difficultto reconcilewith the large magnitude representsa physicalprocess or a systematicerror in the extensioncharacteristic of the province on geological VLBI datais beyondthe scope of this paper,but it clearly timescales[Wernicke et at., 1988]. Either the rate or impactsour interpretationof Ely's velocity. The same geometryof Basin and Range deformationhas recently physicalprocess or dataartifact manifested in the Gordonet changedsuch that geodeticobservations do not agreewith at. [1993] result will affectour results as well since we longer-termgeological indicators of deformation,or there is usedessentially the samedata. Gordonet at. 's [1993] a problemin one or both data sets or their respective velocity for Ely (5.6 mm/yr) is fasterthan ours because interpretations. they def'medstable North America by fixingtwo siteson Several observationssuggest that it is worthwhile the easternseaboard. If lengtheningacross the stable reexaminingthe spacegeodetic data and some assumptions interiorof the UnitedStates is occurring,it wouldadd to concerninglocal site geology and strain distribution within thewestward velocity of Ely relativeto the referenceframe the Basinand Rangeprovince in order to betterunderstand by an amount unrelated to extension acrossthe eastern comparisonsbetween geologic and geodeticmeasures of boundaryzone of the Basinand Range province. Thus our DIXON ET AL.: BASIN AND RANGE DEFORMATION 7259 choice of referenceframe may be more appropriate•)r zone and give consistentresults, this seems unlikely. investigatingBasin and Range deformation, as the effectof Another possibility is that significant strain is thislengthening is reducedwith a referenceframe of widely accommodatedaseismically and thusis not recordedin the separatedsites includingsites in both eastemand central paleoseismicsites. A third possibilityis that otheractive North America. But we are still left with the questionof but less well studied faults accommodate significant referenceframe dependence; that is, how much is our rate extensionin the eastemBasin and Range. Numerousfaults for Ely affected? with late Pleistoceneoffset have been mapped both east and One way of minimizingthis frame dependence is to look west of the Wasatchfault [e.g.,Nakata et al., 1982], most at the velocity of one station with respectto another. lying within about +100 km of the Wasatchfault itself. While this is generally avoided becausethe station Ely lies about 250 km west of the Wasatchfront, thus its velocitiesof interestare unduly sensitiveto errorsat the velocity relative to stableNorth A•hericaintegrates across single referencestation, it does get aroundthe reference roughlythe easternthird of the provinceand would include frameproblem noted above. The velocity of Ely with slip on theseadditional faults if they are presentlyactive. respectto Platteville (Figure 1) in our solution (see also Other geodetic data discussed below suggest strain Gordon et al. [1993])is 3+1 mm/yr to the west, still accumulationin the generalvicinity of the Wasatch fault significantly higher than most previous estimates •)r zone (distances<50 km) at rates sufficientto explain our deformationin the region (next paragraph). We conclude dataassuming simple elastic strain models. Assumingno that our choice of reference frame does not affect the basic systematicerror in eitherthe spaceor groundgeodetic data, result, and becauseof the beneficialeffect of using six this agreementwould seem to limit the location of the broadlydistributed stations to definea referenceframe, we most active faults to no more than about 50 km from the take our velocityfor Ely with respectto this referenceframe Wasatch fault. as the most appropriateestimate available at this time •)r Ground-basedtrilateration data indicate about 2 mm/yr of investigatingeastern Basin and Range deformation. extensionacross a- 70-km-wide networkroughly centered Rates of seismic energy release based on summing on the central Wasatch fault [Savage et al., 1992]. seismic moment tensorsin a given region over a given However,fitting thesedata to an elasticstrain accumulation periodcan be used to infer recentdeformation rates. This model and solvingfor fault slip and horizontalextension at approachestimates brittle strain release,i.e., strain release depth(the latter for comparisonto the "far-field"horizontal associatedwith earthquakes,but underestimatestotal strain rate measuredat Ely) implies significantlyfaster rates of if aseismicdeformation is occurring. Studiesin the general horizontal extension. Savage et al. [1992] fit the vicinity of the Wasatchfault zone on the easternboundary trilaterationdata to two models (planar and listric normal of the Basin and Range province(Figure 1) wheremodem faults) and obtained east-westextension rates of 5.3_+2.0 seismicityis concentratedindicate very low ratesof seismic and 7.6_+1.6mm/yr respectively. Both resultsagree with energyrelease, equivalent to deformationrates <0.5 mm/yr, our spacegeodetic data at Ely within one standarderror. If except for one area (Hansel Valley) west of the northern correct,this implies that all surfacedeformation manifested Wasatchfault zonewhere the rate is 1.5 mm/yr [Eddington by Ely's velocity with respectto stableNorth Americacan et al., 1987]. These rates are significantlyless than the be accommodatedwithin a relatively narrow region total extensionrate observedat Ely by spacegeodesy. The centered on the Wasatch fault zone. Additional active discrepancybetween the higher geodetic rate and lower faultsnear the Wasatchfault are allowed by thesedata, but seismicrates could be explainedif aseismicstrain release they would have to be locatedwithin the-70-km aperture accountsfor a significantfraction of overall deformation. of the trilateration network, or less than-50 km from the Alternately,or in addition, theremay be an anomalously Wasatch fault. low rate of seismicityduring the short period coveredby PreliminaryGlobal Positioning System data spanninga historicaldata comparedwith longertime averages.In fact, broad aperturethat includesthe Wasatch fault zone also modem earthquakespredict anomalously low rates of slip suf•portthe conceptof rapid extension(4+1 mm/yr) across along most of the Wasatch front even comparedto the this region [Martinez et al., 1994]. In summary,all recent paleoseismicrecord (itself low comparedto the space geodeticdata are in rough agreementand are consistent geodetic result; see below), which has important with relatively rapid (3.0-7.6 mm/yr) east-westextension implicationsfor futureseismic activity and hazard [Smith, acrossthe Wasachfault zone and eastemBasin and Range. 1978; Eddingtonet al., 1987]. The discrepancybetween high geodeticrates and low rates Paleoseismicstudies on the Wasatchfault zone suggest inferredfrom both paleoseismicityon the Wasatch fault Holocene rates of horizontal extension less than about 1.0 zone and seismicmoment tensorsummation in the region mm/yr [Schwartzand Coppersmith,1984; Eddington et may be explainedby activefaulting away from the Wasatch al., 1987; Machetteet al., 1992; McCalpin et al. 1994], fault zone itself, aseismicdeformation, and/or anomalously againsignificantly less than the spacegeodetic result which low ratesof currentseismic activity comparedto long-term integratesacross a broaderregion. One explanationis that rates. the paleoseismicstudies underestimateslip rate because some fraction of slip is accommodatedby nonbrittle coseismicmechanisms such as warping and rotationwhich Deformation in the Western Boundary Zone do not lead to discrete,easily measuredoffset [Salyards et al., 1992]. Given that paleoseismicstudies have been Data. The followingfour setsof geologicaland geodetic conductedat numerouslocations along the Wasatch fault observations are relevant to our discussion of the 760 DIXON ET AL.: BASIN AND RANGE DEFORMATION

VLBI/SLR datafor the westernboundary zone of the Basin Clark, 1995]) or White Mountains (0.5-1.2 mm/yr and Rangeprovince: [dePolo, 1989]) fault zones. As with the geological 1. Geological evidence suggests that the eastern estimates,the ground-basedgeodetic data describeonly Californiashear zone has accommodated northwest trending localrelative motion. The spacegeodetic data will allow right lateralshear since late Miocenetime (-10-6 Ma) with us to link these local data sets to an external reference frame a significant fraction accommodatedalong the Death such as stable North America. Valley-FurnaceCreek fault zone (6-12 mm/yr [Dokka and 3.Deformation along the westernboundary of the Basin Travis, 1990]). Geomorphicevidence indicates this fault andRange province in thevicinity of OVRO at the present zone is still active, but present day rates are poorly time is apparentlypartitioned into nearly orthogonal strike- constrained. Other active fault zones in the region slip and extensionalcomponents on subparallelfaults, as accommodatingright-lateral shear include the Hunter suggestedby the simultaneousoccurrence of dominantly Mountain-Panamint Valley fault zone [Smith, 1979; right-lateraldisplacement on the OwensValley fault and Burchfielet al., 1987], the Owens Valley [Lubetkinand nearbyactive normal faulting along the easternSierra range Clark, 1988] and White Mountains fault zones, and the front [e.g, Stewart, 1988; Zoback, 1989; Jones and Fish Lake Valley fault zone [Reheis, 1994a] (Figure 1). Wesnousky,1993]. Howeverthe partitioning is not perfect. Zhang et al. [1990] reporta Holoceneslip rate of 2.4_+0.8 For example, vertical as well as horizontal offsetoccurred mm/yr for the southernPanamint Valley fault. Beanland alongthe surface break of the 1872 earthquakewith a ratio and Clark [1995] report a late Quaternaryslip rate of of about 1:6 [Beanlandand Clark, 1993]. Differencesin 2.0_+0.5mm/yr for the Owens Valley fault zone. North of the thicknessof valley fill on either side of the Owens OVRO, geologic mapping indicatesthat both the Fish Valley fault zone [Hollett et al., 1991] indicatethat this Lake Valley fault zone [Reheiset al., 1995] and White faulthas accumulated significant vertical offset in the past. Mountains fault zone [dePolo, 1989] are active. dePolo Note that OVRO lies eastof both the OwensValley fault [1989] reportsa Holoceneslip rate of 0.5-1.2 mm/yr for the zoneand the eastern Sierra Nevada range front faults (Figure White Mountains fault zone (Figure 1). Reheis [1994b] 1). The White Mountainsfault zone, immediately north of suggestsa minimum Holoceneslip rate of 4 mm/yr for the OVRO, has a right stepping,en echelonrelation with the Fish Lake Valley fault zone. OwensValley fault zone(Figure 1) but differssomewhat in Note that the rate and direction of individual faults or character.While both are obliqueslip faults,the White even the shear zone as a whole do not define motion of the Mountainsfault zone has a higher normal component Sierra Nevada block relative to stable North America; the comparedto strike-slipcomponent [dePolo, 1989] in geological indicatorsonly describerelative motion in a contrastto the OwensValley fault zone wherethe strike- local reference frame. slip componentdominates. 2. Ground geodeticdata led Savage e! al. [1990] to 4. A seriesof northto north-eaststriking normal faults suggestthat the southernpart of the shear zone in the cuts across the White and Inyo Mountains, the Mojave Desertpresently accommodates -8 mm/yr of north- CottonwoodMountains and the SalineRange (Figure 1). northwest directedright-lateral shear and extends north Thesefaults may haveacted in the past as transferfaults from the Mojave Desert into Owens Valley, west of both linkinga seriesof en echelon,right stepping,northwest to the Death Valley-FurnaceCreek and Hunter Mountain fault north-northweststriking right-lateralstrike-slip faults. zones. A recentanalysis of theseand other geodeticdata Geomorphicand seismic evidence suggest that the from the Mojave Desertsuggests that the rate may be as northernmostof the normalfaults are active. Fresh,steep high as 12 mm/yr [Sauberet al., 1994]. A recentanalysis faultscarps, vertically offset drainages, and offsetHolocene of the Owens Valley data suggeststhat -7 mm/yr is alluvialfans indicate recent activity on the DeepSprings accommodatedhere [Savageand Lisowski, 1995]. New fault [Bryant, 1989] (Figure 1). The May 17, 1993, data from the Excelsior trilateration network (the M=6.1 EurekaValley earthquakesimilarly indicates active southernmostpart of which is shownin Figure 1) and other normalfaulting immediately south of DeepSprings Valley networksnorth of OwensValley along the centralNevada (Figure 1). We suggestthat the normal faultsin Eureka seismic zone show strain rates about 50-75% of those Valley provide a kinematic link between the Hunter observedin Owens Valley, with orientationsconsistent Mountain-PanamintValley fault systemand the Fish Lake with north-northwesttrending right-lateral shear [Savage et Valleyfault zone (we areaware of no otherfaults that might al., 1995]. Thus right-lateralshear continuesnorth of transferslip northward from the Hunter Mountain fault; see Owens Valley, perhaps at reduced rate, or perhaps is Figure1). If correct,this linkageimplies a minimum rate partitioneddifferently compared to regionsto the south, for the Fish Lake Valley fault zone of 2.4_+0.8mm/yr. with some shear occurring in regions not covered by Thisrate is a minimumbecause it doesnot accountfor any trilateration. These data also indicate that unlike the slip transferredfrom the Death Valley-FurnaceCreek fault Owens Valley area where most right-lateral shear is zone southeastof Fish Lake Valley nor for any slip accommodatedwithin -100 km of the easternSierra range transferredfrom the OwensValley faultzone via the Deep front, significantright-lateral shear is accommodatedin Springsfault (Figure 1). centralNevada 150 km or moreeast of the rangefront. Note that slip ratesestimated from groundgeodetic data Strain concentrationin the western boundary zone: in OwensValley (-7 mm/yr;Savage and Lisowski [1995]) Evidence and implications. The kinematic model are significantlyfaster than geologicalestimates for slip presentedin this sectionattempts to reconcilethe different rateson the Owens Valley (2.0_+0.5mm/yr Beanlandand observationslisted above,using the new spacegeodetic DIXON ET AL.: BASIN AND RANGE DEFORMATION 761 data as an additional constraint. OVRO's northwest velocityrelative to stableNorth America(10.1+0.5 mm/yr) is roughly comparableto the expectedrate of shearfor the eastemCalifomia shearzone [Savageet at., 1990; Sauber 1o et at., 1994; Savage and Lisowski, 1995], but to make a c rigorouscomparison we must first accountfor the fact that OVRO's velocity is deftned relative to stable North America and thus includes deformation in the interior of the Basin and Rangeplus the easternboundary zone, whereas 6 the terrestrialsurvey data describeonly local deformation within part of the westernboundary zone in a reference ECSZ VNORTH (mm/yr) frame deftnedby the localnetwork. Also, we must correct 8.8mm/yr for elastic strain accumulation since OVRO is located near NNW 4 seismically active faults comprising the shear zone (Figure 1). If A, B, and C representpoints in stableNorth America, the centralBasin and Range (e.g., Ely) and the westem Basin and Range(e.g., OVRO), respectively,then vector AC (motion of OVRO relative to stableNorth America) equalsthe sum of vectorAB (motion of Ely relativeto stableNorth America)plus vectorBC (motion of OVRO relativeto Ely)(Figure 2). VectorBC (8.8+1.3 mm/yr at 8 6 4 A N9øW+5ø, i.e., north-northwestdirected right-lateral motion) reflects motion associated with the eastern VWEST B ¾ California shear zone. Because of elastic strain (mm/yr) accumulationit doesnot exactlyrepresent the far-fieldrate, i.e., the rate that shouldbe comparedto geologicrates which averageover many earthquakecycles, although the uncorrectedand correctedvalues are actuallyvery similar (seenext section).The generalsimilarity betweenvector Figure 2. Velocity of Ely, Nevada (B), and OVRO, BC (or its value correctedfor elasticstrain accumulation) California (C), relative to stable North America (A). and the rate and directionof shear(8-12 mm/yr north- Ellipsesare at 95% confidence.BC (8.8 mm/yr at N9øW) northwest)for the eastemCalifomia shear zone inferredby representsintegrated deformation between Ely and OVRO Savageet al. [ 1990] and Sauberet al. [ 1994] from ground (absent only a small amount due to elastic strain geodeticdata is importantand suggeststwo key points. accumulation) and is similar to right-lateral shear First,the relativelynarrow aperture of the groundgeodetic deformation across the eastern California shear zone networks, including the one outlined in Figure 1, measuredby ground-basedgeodetic data [Savageet at., apparentlycaptures a significantfraction of the deformation associated with the shear zone. This is consistent with the 1990; Sauber et at., 1994]. observationby Savageet at. [1994] of negligible strain accumulation in the Yucca Mountain trilateration network in westemNevada, just east of the shearzone. Second, the Wasatchfault zone on the easternboundary of the most of the deformationbetween Ely and OVRO must in province. This generalizationdoes not apply to "pull fact be restrictedto the shearzone. This suggeststhat apart"basins such as Death Valley or PanamintValley deformationacross the westem half of the Basin and Range associatedwith rightsteps in northweststriking strike-slip provinceis not uniformly distributedbut ratheris restricted faults[Burchfiet and Stewart,1966]. Finally, the factthat to a relatively narrow western boundary zone. The theshear zone at the latitudeof OVRO maintainsroughly deformationrepresented by BC (motion of OVRO relative the same rate recordedby geodetic data to the south to Ely) must includevirtually all deformationassociated suggeststhat the rate is not decreasingsignificantly with the eastemCalifomia shear zone and little else (we northward.Pezzopane and Weldon[ 1993] traceshear zone- arguebelow that some extensionlikely occurswest •' related deformation into Oregon and Washington, OVRO, but this is a relativelysmall effect). SinceOVRO consistentwith this observation,and discussthe regional apparentlyrecords all or mostof the motioncharacterizing tectonics. the shearzone, we also surmisethat the shearzone must lie The concentration of surface strain in the western entirelyto the east (perhapsjust east) of OVRO. The boundaryzone of the Basin and Rangenear OVRO is an OVRO-Ely velocity also indicatesnegligible east-west important observation. It is consistentwith the similar extensionbetween Ely and OVRO. Other data described concentrationof seismicity[Eddington et at., 1987], but belowlimit extensionon the easternSierra Nevada range sincethe seismicrecord is short and perhapsanomalous front(the westernboundary of the Basinand Range)to with respectto longertime averages,geodetic evidence [or about1 mm/yr. Apparently,significant Basin and Range strain concentrationis important. Present-daystrain extensionat the presenttime is limitedto a regionclose to concentrationalong this part of the westernboundary zone 762 DIXON ET AL.: BASIN AND RANGE DEFORMATION

is also consistentwith concentrationof late Quaternary the lower (8 mm/yr) rate suggestedby Savageet al. [1990] faultingin the region[Wallace, 1984]. Perhapsthis strain andthe higher(12 mm/yr) rate suggestedby $auber et al. concentrationis relatedto the anomalouslylow-velocity [1994]. As an aside, the rate need not be constantwith (hot and weak?) upper mantle observedhere by Biasi and latitude,though data to be presentedsuggest no significant Humphreys [1992] using P wave travel times; that is, rate variationfrom the southernMojave desertto northern strain has concentrated in a weak zone. OwensValley; belowwe makethe assumptionof constant The spacegeodetic data suggestthat simple models of rate over our limited studyarea. Basin and Range deformation such as unidirectional If most slip is conf'medto the Owens Valley-White extensionor right-lateralsimple sheartell only part of the Mountains fault zone (Figure 1), then the OVRO-Ely story; at least both thesemodes of deformationapparently velocity can be fit to an elasticstrain model with a far-field occur simultaneously. Northwest motion of the Sierra rateof about12 mm/yr. If, however,slip also occurseast Nevadablock with respectto stableNorth Americaconsists of Owens Valley (Figure 1), the situation is more of two main components,with two correspondingstyles of complicated,and the lower (8 mm/yr) rate is allowed by Basin and Rangedeformation: east-west extension on north the data. Evidencediscussed below suggestssignificant striking normal faultsin the easternpart of the province, right-lateralmotion on the Fish Lake Valley fault zone. probablyconcentrated near the easternboundary, and right- We can fit the OVRO data to a model where slip is lateral shear on northwest to north-northweststriking partitionedbetween the Owens Valley-White Mountains strike-slipfaults concentrated in the westernboundary zone. fault zoneto the west and the Fish Lake Valley fault zone Assuming these two modes of deformation have to the east, using additional informationto help define continuedfor severalmillion years,we can seewhy it has some of the model parameters. For two locked, parallel been difficultto reconcilegeologic indicators of extension strike-slipfaults in an elastichalf-space, the velocity field directionwith spacegeodetic measurements made on or in a referenceframe defined by the perpendiculardistance near the SierraNevada block. Only the first (extensional) from the first fault is givenby: mode of deformation leads to creation of significant amountsof new crust, leaving a large arealfraction of the 12-- -- 12atan-' + v•ø tan-' - (]) Basin and Range province with north-south striking •r D• normal fault scarpsand north trendingbasins and ranges indicative only of the east-westextensional component. These data by themselveswould lead to a biasedview of wherev is the velocityat perpendiculardistance x fromthe Sierra Nevada-stableNorth America motion. Similarly, firstfault (a), vao and vt,o arethe far field velocitiesof the the integratedmotion measured by spacegeodesy at OVRO first and secondfaults with lockingdepths D a and D/•, by itself is not very descriptiveof critical details of respectively,and the faultsare separatedby distance$. deformationin the interiorof the province,especially the Using the bestestimate for slip partitioningon the faults exensionalcomponent. (derivedbelow) and assuminglocking depths of 8 and 12 km for OwensValley andFish LakeValley, respectively, A Kinematic Model for the Eastern California Shear we obtainan estimate of 10.7+1.6mm/yr for thevelocity of Zone. The OVRO-Ely velocity (8.8+1.3 mm/yr) is a easternCalifornia shear zone (Figure 3), 1.9 mm/yrhigher minimum estimate for the total rate of shear across the thanthe measured OVRO-Ely velocity. The uncertaintyis easternCalifornia shear zone because OVRO lies very close the root sum square(rss) of the spacegeodetic error (1.3 to one of the faultscomprising the shearzone (the Owens mm/yr)and additional uncertainty introduced by the model Valley-WhiteMountains fault zone) and thus is affectedby (estimatedat 1.0 mm/yr). Our rateestimate is intermediate elasticstrain accumulation. OVRO wouldalso "miss" any betweenother publishedgeodetic estimates (-8 mm/yr; right-lateralshear accommodated on strike-slipfaults that [Savageet al. 1990];-12 mm/yr,[Sauber et al. 1994])and lie to the west between OVRO and the Sierra Nevada is equivalentto them within 95% confidencelimits. In the block, but these are probably insignificant(Figure 1). kinematic models derived below for individual faults Simpleelastic half-space models for a lockedvertical strike- comprisingthe shear zone we arbitrarily assumethat slip fault [Savageand Burford, 1973] allow us to estimate 10%+5%(1.1+0.5 mm/yr) of thetotal 10.7mm/yr slip rate the range of far-fieldrates for the easternCalifornia shear for the shear zone is accommodated east of Fish Lake zoneconsistent with the spacegeodetic data for comparison Valley, sincethere is geologicalevidence for minor right- to geologicdata averagedover many earthquakecycles. lateralfaulting immediately east of Fish Lake Valley. Sauberet al. [1994] modeleddeformation in the Mojave Remaining slip (9.6 mm/yr) is assumed to be Desert as a broad zone of shear. However, north of the accommodated"locally," i.e., in the regionbetween Fish Garlock fault, geologicevidence suggests that slip is LakeValley andnorthern Owens Valley, or betweenDeath concentratedon a few major faults. We model the Valley andsouthern Owens Valley (Figure 1). deformationaccordingly with discretefaults, albeit ones The discrepancybetween ground-based geodetic and that are sufficientlyclose that their elastic strain fields geologicslip rate data in OwensValley bearson the issue overlap. of where most of the deformation associatedwith the shear Because of ambiguities associatedwith the exact zoneis presentlyconcentrated. On onehand, geologic data partitioningof slip amongthe activefaults comprising the indicatesignificant motion across the DeathValley [Dokka shearzone and uncertainties in the faultlocking depths, the and Travis, 1990] and HunterMountain [Zhang et al OVRO-Ely velocitydata alone cannot distinguish between 1990] fault zones(Figure 1) and lower ratesof motion on DIXON ET AL.: BASIN AND RANGE DEFORMATION 763

g - 8 • 6 •- • FishLake _-• • • FaultParameters •,.. ValleyFault: ..• • 4

0 -60 -40 -20 0 20 40 60 80 Distance from White Mountains Fault (km) Figure 3. The measuredvelocity of OVRO relativeto Ely (8.8 mrn/yrNNW) is a functionnot only of the total slip rate acrossthe easternCalifornia shear zone but also, becauseof elasticstrain accumulation, the distributionof slip on variousfaults comprisingthe shearzone and their respectivelocking depths. Theseeffects can be approximatedwith elastichalf-space models (1), seetext. The modelsare non unique (1 datapoint, 6 adjustableparameters !), andvarious velocity distributions, with far-field ratestotaling about 9 to 13 mm/yr, fit the spacegeodetic data equally well. The model shownhere, with a total far-fieldrate of 10.7 mm/yr, is consistentwith both the spacegeodetic data and other data, as discussedin text. Slip is partitionedbetween the White Mountainfault (3.4 mm/yr), the Fish Lake Valley fault (6.2 mm/yr), and a hypotheticalfault (1.1 mm/yr) 80 km eastof the White Mountain fault. the OwensValley fault zone [Beanlandand Clark, 1995]. southernhalves of the Owens Valley network, limiting the On the otherhand, ground-basedtrilateration data suggest amountof such sheartransfer to less than about 1 mm/yr. that the locus of present day right lateral shear is Thus we need another explanation for the discrepancy concentratedin OwensValley [Savageet al., 1990; Savage between ground- and space-basedgeodetic results, and Lisowski,1995]. The fact that our spacegeodetic data preferablyone that alsoaddresses the differencebetween the requirethat OVRO lie west of the easternCalifornia shear ground-basedgeodetic rates and geologicrates in Owens zone appearsat first glance to agree with geological Valley. evidencefor significantshear acrossthe Furnace Creek- The trilateration data record 2.9+0.4 mm/yr of right- DeathValley fault zoneand Hunter Mountain fault zones. lateral shearacross Owens Valley and strongly imply (via One complicationin comparingthe spaceand terrestrial elasticmodels) a higherfar-field rate [Savageand Lisowski, geodeticdata is that OVRO lies nearthe northernlimit of 1995], but it is usefulto ask whether these data actually the OwensValley trilaterationnetwork. OVRO alsolies at requirethat the major locus of shearbe centeredin Owens the northernlimit of the surfacerupture associated with the Valley. If right-lateralshear is distributedamong the Death 1872 earthquakeon the Owens Valley fault (Figure 1), Valley-FurnaceCreek, Hunter Mountain-PanamintValley consistentwith a stepor changein trendof the surfacetrace andOwens Valley fault zones,we can investigatehow slip of the shearzone at thispoint. If strainwas concentrated in might be partitionedamong these systems using available the southern and central part of the Owens Valley geologicand geodeticdata combinedwith elastic strain trilaterationnetwork and the shearzone steppedeast of models (1). Considera model whereright-lateral motion OVRO, then at leastthe availableground- and space-based occurson threeparallel, locked faults, all with slip ratesof geodeticdata could be reconciled(we would still have a 3 mm/yr, separatedby 25 and 50 km, respectively,roughly discrepancybetween ground-basedgeodetic rates and similarto the situationfor the Death Valley-FurnaceCreek, geologicrates in Owens Valley). Dixon et al. [1993] Hunter Mountain-PanamintValley, and OwensValley fault suggestedthat at the presenttime right-lateralshear was zones (Figure 1). Clearly the situation is more largelyrestricted to south-centralOwens Valley fault zone complicatedin our studyarea, since the three faultsare not andthe Death Valley-FurnaceCreek fault zone,and most of exactly parallel and extension as well as right-lateral the OwensValley componentstepped east of OVRO to the motion is accommodatedin the region. However, if FishLake Valley fault via the activeDeep Springsnormal obliqueextension is largelypartitioned into strike-slipand fault (Figure 1). However,recent analyses of additional normal components,treating the strike-slip component trilaterationdata [Savageand Lisowski,1995] suggestno independentlyis probablyan adequateapproximation. The significantstrain rate differencesbetween the northernand key point is that a velocitygradient will be observedacross 764 DIXON ET AL.: BASIN AND RANGE DEFORMATION the westernmost (Owens Valley) fault even though zone (3.9+1.1 mm/yr)differs from the geologic estimate significantshear occurs to the east becauseof elasticstrain (2.0+0.5 mm/yr) at one standarderror, but the values are effects. The magnitudeof the observedvelocity gradient equivalentat two standarderrors (95% confidence).For the dependson the actualslip distributionand the fault locking Death Valley-Furnace Creek fault zone, the total error depth assumedin the models. For a locking depth of 15 dependson the errorfor the OwensValley fault zone (1.1 km on all faults,the differentialvelocity across a distanceof mm/yr), the error for the Hunter Mountain fault (0.8 25 km (the dimensionof the trilaterationnetwork) centered mm/yr), and the error in our estimate of total slip rate on the westernmostfault is about 1.9 mm/yr, less than the accommodatedin the region (rss of 1.6 mm/yr and 0.5 2.9+0.4 mm/yr observedby Savageand Lisowski [1995] mm/yr), for a total error of 2.2 mm/yr. acrossOwens Valley. However, Owens Valley has been The slip rate model can be extendednorth if we assume the locusof abundantlate Quaternaryvolcanism, and there that slip on the Hunter Mountain system is transferred is seismic evidence far anomalous crust consistent with completely to the Fish Lake Valley fault. This is a magmaticactivity at depthat somelocations [e.g., Sanders reasonable assumption since there are no other et al., 1988]. Thus it is reasonableto assume that the throughgoing faults in this region capable of crest here is weaker,hence the fault locking depth should accommodatingthe additionalslip. Thus the slip rate on be shallowerthan regions to the east. Assigninga locking the Death Valley-Furnace Creek system must increase depthof 8 km to the OwensValley fault and 12 and 16 km northward,to 5.7+2.3 mm/yr north of its intersectionwith to the middle and easternfaults, respectively, again all with thenorth striking system of normalfaults in EurekaValley slip ratesof 3 mm/yr, resultsin a differentialvelocity of 2.4 (Figure 1) at the south end of Fish Lake Valley (here the mm/yr acrossOwens Valley, still less than the observed Death Valley-FurnaceCreek fault zone becomesthe Fish value but equivalentwithin uncertainties. Lake Valley fault zone;Figure 1). We are now in a positionto derive a slip rate budgetfar Activity on the Deep Springs fault implies that the major elementscomprising the easternCalifornia shear additionalslip may be transferredto the Fish LakeValley zone that better matches available data, with the space fault zonenorth of its intersectionwith the Deep Springs geodeticdata constrainingintegrated slip rate and local fault,perhaps from the OwensValley fault zone(Figure 1). geodeticor late Quaternaryor Holocene geologic data However,precise slip rate estimatesfor the Deep Springs constrainingslip ratesfor individualfaults. In constructing fault are not available. Bryant [1989] suggestedthat the this model we ignorethe small obliquitiesbetween major minimum vertical rate throughlate Quaternarytime was strike-slip faults, treating their slip rates as additive far 0.24 mm/yr, basedon offsetstream channels containing comparison to the overall slip rate. Given other depositscorrelated with the Bishop Tuff. Wilson[1975] uncertainties,this approximationis reasonable.The model measureda 40ø fault dip in the subsurfacedeposits of Deep we deriveis non unique, and otherslip rate distributions Springs Lake, giving a minimum rate of horizontal could satisfy the data nearly as well. However, our extensionfor the Deep Springs fault of 0.29 mm/yr. example illustrates some key points and is a useful Trilaterationdata limit the amountof slip rate reductionin working hypothesisto be tested and revised with future northernOwens Valley to lessthan 0.8 mm/yrcompared to data. the sliprate in centraland southernOwens Valley [Savage Assuming locking depthsof 8 km, 12 km, and 16 km and Lisowski, 1995; J.C. Savage, personal (west to east) and settingthe rate for the middle (Hunter communication,1995]. This suggestsan upperlimit to Mountain),fault equalto the Holoceneestimate, then slip thehorizontal component of extensionon theDeep Springs rates of 3.9 mm/yr (Owens Valley), 2.4 mm/yr (Hunter fault, presuminga kinematicconnection between this fault Mountain)and 3.3 mm/yr (Death Valley-FurnaceCreek) fit andthe OwensValley fault (Figures1 and4). We therefore the available data very well. Specifically, this slip assumethat 0.5+0.3 mm/yr slip transfersfrom the Owens distributiongives a velocitygradient across Owens Valley Valley fault zoneto the FishLake Valley fault zonevia the that exactly matchesthe trilaterationdata (2.9 mm/yr), Deep Springs fault, implying that the slip rate on the minimizesthe discrepancybetween geologic and geodetic White Mountains fault zone, north of the intersectionof the data in OwensValley, and meetsthe geologic(Holocene) OwensValley fault zone and the southwestextension of the constraintfor the Hunter Mountain fault. The slip rate far Deep Springfault, decreasesto 3.4+1.2 mm/yr, while the the DeathValley-Furnace Creek fault is constrainedby the sliprate on the Fish Lake Valley fault zonenorthwest of its fact that the total slip rate acrossthe three fault systems intersectionwith the Deep Springs fault increasesto must sum to 9.6 mm/yr, basedon the spacegeodetic data 6.2+2.3mm/yr. Thusnorth of OVRO, our modelrequires and assumptionsoutlined earlier. If our model is that more than 50% of the right-lateralshear associated approximatelycorrect, more than 50% of right-lateralslip with the eastern California shear zone is accommodated east associated with the eastern California shear zone is of Owens Valley, mainly in Fish Lake Valley. Reheis accommodatedeast of OwensValley, but the OwensValley [ 1994a,b] and Reheiset al. [ 1995] foundthat the likely fault zone is still the singlemost active fault in this part of rangeof late Pleistoceneand Holoceneslip ratesfor the the system. Fish Lake Valley fault zoneis 4-7 mm/yr with an upper Uncertainties are estimated as follows. For the Owens limit of 12 mm/yr, consistentwith this suggestion. Valley fault zone, the uncertainty is the rss of the Our modelsatisfies the spacegeodetic data at OVRO (it trilateration error (+0.4 mm/yr) and the model error, placesmost right-lateralshear to the east), satisfiesthe estimatedat 1.0 mm/yr, for a total of 1.1 mm/yr. Thus combinedspace geodetic data at OVRO and Ely (the total our estimatefor present-dayslip on the OwensValley fault "far-field"rate of right-lateralshear accommodated between DIXON ET AL.: BASIN AND RANGE DEFORMATION 765

thesestations is 10.7mm/yr; we haveaccommodated 90%, OwensValley, can be reconciledwith the observationof or 9.6 mm/yr, within a relativelynarrow zone), and Dokka and Travis [1990] that the southernDeath Valley- eliminatesthe discrepancy between ground-and space-based Furnace Creek fault zonewas the principal locus of shear geodeticdata in Owens Valley. The model does not averagedover the last 5-10 million years,if the systemis satisfygeological evidence for low (0.5-1.2 mm/yr) slip evolvingrapidly and the shearzone is in the processof rateson the White Mountainsfault zone [dePolo, 1989]. migratingwest. This evolutionarymodel predicts that: Coseismicdisturbances associated with Long Valley 1. While activity still persistson the Death Valley- activity beginningin 1979 limit the ability of ground FurnaceCreek fault zone, it has slowed in the last few geodeticdata here to characterizethe secularslip rate million years as slip is increasinglytaken up to the west, [Savageand Lisowski,1995]. Perhapsseveral millimeters particularlyon the OwensValley fault zone. per year of slip are accommodatedinstead to the west, on 2. North to northeast striking normal faults, faultscloser to the easternSierra rangefront ratherthan the accommodatingright stepsin the shearzone, should young White Mountains fault zone. to the northwest. Our model for slip distribution on major faults North to northeaststriking normal faultsbetween Owens comprisingthe easternCalifornia shearzone north of the Valley and the Furnace Creek-DeathValley fault zone Garlockfault includesan importantkinematic role for north include, from south to north, the Towne Pass and to northeaststriking, west to northwest dipping normal Emigrantfaults, faults on the west side of the Cottonwood faults such as the Deep Springsfault and faults in Eureka Mountains [Reheis, 1990], a group of faults in the Saline Valley, which accommodateright stepsin the shearzone Rangethat connectSaline Valley with EurekaValley, and and transferslip eastwardfrom Owens Valley and Saline the Deep Springsfault (Figure 1). We speculatethat all of Valley to Fish Lake Valley. Right steps in the Death thesefaults have, at varioustimes, transferredright-lateral Valley and PanamintValley fault zonesare associatedwith slip from the Panamint Valley-HunterMountain fault or the Death Valley and Panamint Valley pull apart basins, OwensValley fault zoneto the DeathValley-Furnace Creek respectively[Burchfiel and Stewart, 1966;Burchfiel et al., or Fish Lake Valley fault zones,in a mannersimilar to the 1987]. The origin and kinematic developmentof Deep "displacementtransfer" mechanism of Oldowet al. [1994]. SpringsValley, northof and adjacentto Deep Springsfault Seismicity, geomorphicevidence for young slip, and (Figure 1), and EurekaValley may be analogousto these better known basins. preservationof surfacefault traces increase northward fi'om the Towne Pass area to Deep Springs Valley, consistent Savageet al. [1990] note that the southernand central with our suggestionthat as right-lateral shear migrated partsof the shearzone are right steppingwith respectto a small circle about the Pacific-North America pole of' progressivelywest, right stepsin the shearzone migrated progressivelynorth. The DeepSprings fault would be very rotation. Our proposedconfiguration suggests that the young in this model, consistentwith available age patternof right stepscontinues northward. A right step constraints.Reheis [1993] pointsout that streamchannels betweenthe Fish Lake Valley fault zone and the Walker Lane fault system to the north [Stewart, 1988], also on present-daydivides between Deep Springsand Eureka accommodatedby northeaststriking normal faults, was Valley (Figure 1), abandoneddue to activity on the Deep noted by Reheis and Noller [1989] and Kohler et al. Springs fault and consequentchanges in topography, [1993]. We suggestthat the northwesttrending Walker containstream gravels overlain by reworkedBishop ash. Lanebelt accommodatessignificant right-lateral shear based This suggeststhat activity on the Deep Springs fault on its connectionto the Fish Lake Valley fault zone and initiatedor increasedsharply sometime after 760,000 years thusis the northerncontinuation of the most activepart of' ago,the age of the ashunit [Izett and Obradovich,1994]. the eastern California shear zone. Offset Holocene features Dokka and Travis [1990] suggestthat the shearzone in [Hardyman,1978] supportour contentionthat this long- the south-centralMojave jumped west between1.5 and 0.7 lived belt ofdeformationis still active. Earthquakedata are Ma, basedon the initiation age of northweststriking faults also consistentwith this interpretation. The 1932 Cedar in the San BernardinoMountains [Meisling and Weldon, Mountain earthquake(M=7.2) occurredwithin the Walker 1989]. Hodgeset al. [ 1989] describeprogressive westward Lane belt and had a strongright-lateral component on a migrationof faultingand sedimentarydeposition within the north-northweststriking fault plane [Doser, 1988]. In Emigrant, Towne Pass, and Panamint Valley fault contrast,the 1915 Pleasant Valley earthquake(M=7.6) systems,with movementof the earliest (Emigrant) fault occurred 175 km northeast of the Walker Lane and was a bracketed between 6.1 and 3.6 Ma and the Towne Pass and normalfault earthquake on a north-northeaststriking fault PanamintValley faultsbecoming active sometimeafter 3.6 plane [Doser, 1988], similar to the 1993 EurekaValley Ma (Figure 4). event(Figure 1). If our interpretationis correct,it suggests Finally, our model for the evolution of the shearzone is that some right-lateral shear also occurs east of Walker consistentin a qualitative way with large (40-100 km) Lane, with normalright obliquefaults of the centralNevada right-lateraloffset for the Death Valley-FurnaceCreek fault seismiczone accommodatingthe implied right step (see zone [Stewart, 1967; McKee, 1968; Saleebyet al., 1986; alsoPezzopane and Weldon[1993]). Reheis, 1993] comparedwith smaller (2-20 km) right- lateral offsetsuggested for the Owens Valley fault zone Evolution of the Eastern California Shear Zone over [Stewart, 1988; Beanland and Clark, 1995]. While the Last Few Million Years. The present-day OwensValley may be the principal locus of right-lateral configurationof the shearzone, with significantshear in shearat present,it hasnot beenactive for nearlyas long as 766 DIXON ET AL.: BASIN AND RANGE DEFORMATION

4 Ma 2 Ma ...... Present Day

/ Young Volcanics (<0.5 Ma)

GARLOCKFAULT GARLOCKFAULT iii-. '...... GARLOCK FAULT

Figure 4. Cartoonsummarizing possible evolution of the easternCalifornia shear zone over the last few million yearsconsistent with availabledata. Shadingindicates most active faults at indicatedtime. Subsidiaryslip undoubtedlyoccurs on numerousminor faults not shown. DSF is Deep Springs fault, DVFCFZ is Death Valley-FurnaceCreek fault zone, EF is Emigrant fault, EVFZ is Eureka Valley fault zone, FLVF is Fish Lake Valley fault, HMF is Hunter Mountain fault, PVFZ is Panamint Valley fault zone, OVFZ is OwensValley fault zone, TPF is Towne Pass fault, and WMFZ is White Mountain fault zone. On diagramfor present-dayconfiguration, we also indicateregions of extensiveyoung volcanism (age lessthan 500,000 years)from Jenningsand Saucedo[ 1994].

the DeathValley-Furnace Creek system and thus has not that mainly lie west of the strike-slip faults, such as the accumulated as much offset. easternSierra Nevada range front (Figure 1). Due to its Why the shearzone shouldmigrate rapidly west is location at the step over betweenthe Owens Valley fault unclear,but young igneousactivity in Owens Valley zone to the southwest and the White Mountains fault zone (Figure4) may be relevant. Perhapsupper crustal faults to the northeast,east of any rangefront faulting but west of migratesuch that slip is accommodatedin the hottest, most strike-slipfaulting, OVRO's velocity with respectto weakestcrust. Anotherpossibility discussed in the next stable North America mainly reflects the strike-slip sectionis thatthe migrationis relatedto changesin overall component of western boundary zone deformation but plate motion. misses the extensional component (Figure 1). Any extension east of OVRO is already reflectedin OVRO's Significanceof Normal Faulting on the Sierra Nevada velocity due to the referenceframe we have chosen,except RangeFront. Doesthe observationthat OVRO lies west perhaps for a minor componentassociated with elastic of the easternCalifornia shear zone imply that OVRO lies strain accumulationon the White-lnyo Mountains frontal on the stable Sierra Nevada block7 We think not, fault (Figure 1). Savage and Lisowski [1995] measured primarily becauseactive normal faultsalong the Sierra 1.0+0.3 mm/yr extensionnormal to the axis of Owens Nevadarange front lie west of OVRO (Figure 1). OVRO Valley acrossthe -25 km width of the Owens Valley alsolies eastof the OwensValley fault, which experiences trilateration network (Figure 1) and suggestedthat this a minordip-slip component. But what arethe magnitudes mainly reflectedextension on the Owens Valley fault and of these effects? Can we correctthe velocity of OVRO to the easternSierra Nevada range front fault, both west of reflectthis dip-slip deformation? And what is the relation OVRO. This is consistentwith the OVRO-Ely space of thisnormal faulting to the easternCalifornia shear zone? geodeticdata which precludesignificant east-west extension A useful model for oblique extensionin the western acrossthis baseline(Figure 2). boundaryzone is one in which deformationis largely Becauseof elastic strain accumulationand the relatively partitionedinto a strike-slipcomponent, accommodated on narrowaperture of the OwensValley trilaterationnetwork, faultssuch as the Owens Valley or Death Valley-Furnace it is difficult to determine the far-field extension rate fi'om Creek fault zones, and an extensional component, trilaterationdata alone. A modeljointly fitting trilateration accommodatedon north-northweststriking normal faults and levelingdata and assumingdip-slip motion is divided DIXON ET AL.: BASIN AND RANGE DEFORMATION 767 betweenthe OwensValley fault andthe easternSierra range fault segments(Figure 1) and active normal faultsjust east front fault suggestsa total of 3.6 mm/yr of normalslip west of the Hilton Creek fault in the volcanic tableland [Pinter, of OVRO, equivalentto about 1.2 mm/yr of far-field 1995]. We have not includedthese in our estimates. horizontalextension [Savage and Lisowski,1995]. Since OVRO lies near the transition in range front Geologicalestimates for late Quaternaryextension am behavior, we take 1.0+0.7 mm/yr in the direction also available for the range front faults and the Owens W20øS+10 ø as our estimate of extension west of OVRO, Valley fault(Figure 1). Gillespie[ 1991] estimatesthat in intermediate between the geologic value northwest of the pastas muchas half of the total subsidenceof Owens OVRO and the mean of geodetic and geologic values Valley may have occurredon the Owens Valley fault. southwest of OVRO. The sum of this vector and OVRO's Since OVRO is locatednear a transition in range front velocity correctedfor elastic strain accumulationon the behavior,with the rangefront steppingwest at the latitude easternCalifornia shearzone (obtainedby adding a vector of OVRO, and is also within the step over betweenthe 1.9 mm/yr oriented N9øW, the differencebetween the Owens Valley and White Mountains fault zones which observedand calculatedOVRO-Ely velocity; Figure 3) partition normal and right-lateralslip quite differently,we provides a good estimate of present-daymotion of the separateour discussionaccordingly. South of OVRO, SierraNevada block with respectto stableNorth America Martel [1989] investigatedone strand of the OwensValley in the vicinity of OVRO. This vector is 12.1+1.2 mm/yr fault zonethat had experiencedmainly verticalmotion and orientedN38øW+5 ø (Table 2). In computingthis value we foundthe verticalrate to be 0.24+.02 mm/yr averagedover haveignored the effectof strain accumulationon the range late Quaternarytime. Assuminga 60ø dipping fault, this front dip-slip faults, since the fault geometry is poorly is equivalentto 0.14+0.02 mm/yr of horizontalextension. knownand the effect is very small. The Independencefault southeastof OVRO is the range Our rateestimate is equivalentwithin one standarderror front fault. Gillespie [1982] suggested that it to the geological(primarily Holocene) estimate of Minster accommodatesno more than 0.1 mm/yr extension at and Jordan [1987] for Basin and Range deformation present. Thus south of OVRO, geological evidence (Model "A," Table 2). The correspondingazimuth suggestsno more than 0.24 mm/yr extension summed estimates differ at the level of one standard error but are acrossthe Owens Valley and Independencefaults; we take equivalentat two standarderrors (Table 2). This is not to the value to be 0.2+0.1 mm/yr. The averageof the saythat Basin and Range deformation is not evolvingwith geologicaland geodeticrate estimatesfor extensionacross time. For example, Wernickeet al. [1988] discussa the OwensValley andeastern Sierra range front faults south reductionin extensionrate and a changein orientationover of OVRO is 0.7 mm/yr. the last 10-15 millionyears. Rather,we cannotresolve the Northwest of OVRO, the range front consists of en possiblysubtle changes that may haveoccurred over the echelon,left steppingnormal faults, includingthe Round last 1-2 million years or less becauseof limitations in Valley, Hilton Creek, and Mono Lake faults(Figure 1). availabledata. For example,our discussionof the eastern Clark and Gillespie[1993] measured2.3+1.3 mm/yr slip Californiashear zone suggests that certainparts of the plate at McGee Creekon the Hilton Creekfault averaged over the boundaryzone actually evolve quite rapidly, with the main last -100,000 years. Assuming a 60ø dip and that locus of right-lateral shear accommodatedon more extensionoccurs perpendicular to the local strike of the northerlyfaults compared to the meantrend severalmillion normal faults (N20øW+10ø) gives 1.2+0.7 mm/yr of yearsago. Unfortunately,it is difficultsay whetherthis horizontal extension oriented W20øS+10 ø. Extension evolvingfault mosaictakes place in the contextof more or acrossa singlefault segmentunderestimates total extension lessuniform far-field motionor may in fact be a responseto acrossthis part of the rangefront becauseof overlapping changingfar-field conditions. Below, we discussone

Table 2. Estimatesof IntegratedBasin and Range Deformation Near OVRO

Source Rate,* Azimuth,* mm/yr degrees

Minsterand dordan [1987] ? 9.2+2.6 N64W+9 (Model A, GeologicalData) Argus and Gordon [ 1991] 11+1 N28W+3 (VLBI to 1989) ThisStudy õ 12.1+1.2 N38W+5 (VLBI/SLR to 1991)

* Uncertainties are one standard error ? Recalculatedto a pointon the eastern Sierra Nevada range front west of OVRO õ OVROdata corrected for elastic strain accumulation and Sierra Nevada range front faulting. 768 DIXON ET AL.: BASIN AND RANGE DEFORMATION possiblescenario relating the evolvinggeometry of the years,from exclusive exploitation of the northweststriking easternCalifornia shear zone to changingfar-field plate Death Valley-FurnaceCreek fault zone to the present motion. configurationthat includesa major componenton the Our revised estimate for Sierra Nevada block motion has north-northweststriking OwensValley fault zone(Figures implicationsfor convergencenormal to the San Andreas 1, 2, and 4), could make averageSierra Nevadablock fault in California. The vector describingPacific-North motion more northerly and reduce overall fault-normal Americamotion can be comparedto the vector sum o[ convergenceacross the plate boundary.The beginningof Basin and Range deformationand motion on the San the westwardmigration of the southernpart of the shear Andreasfault. The resulting"discrepancy vector" [Minster zone(and thus the changeto a more northerlytrend) dates and Jordan, 1984, 1987] can be resolvedinto components from the first movementon the Emigrantfault (between6.1 paralleland perpendicular respectively to the San Andreas and 3.6 Ma) and Towne Pass and PanamintValley fault fault. Usingdata then available and assuming that OVRO systems(both after 3.6 Ma) [Hodges et al., 1989], lies on the stableSierra Nevada block, Argus and Gordon consistentwith the timing of Pacificplate motion change [ 1991] usedtheir predictedvalue for motion of the Sierra (3.4-3.9 Ma; [Harbert and Cox 1989]). Nevada block and the NUVEL-1 plate motion model [DeMetset at., 1990]to predictconvergence normal to the Conclusions San Andreas fault in central California at a rate of 2+_2 mm/yr. Our predictionof morewesterly motion for the 1. Combined SLR and VLBI data indicatethat Ely, central Sierra Nevada block implies a higher rate of Nevada, is moving west with respect to stable North convergencein centralCalifornia, but without an accurate America at 4.9+1.3 mm/yr, while Owens Valley Radio SierraNevada-Pacific or SierraNevada-North America pole Observatory(OVRO) in easternCalifornia just east of the we canonly makea crudeestimate. Recent geodetic and Sierra Nevada block is moving northwest at 10.1+0.5 seismicdata seemto requirefault-normal convergence in mm/yr. OVRO's motion with respectto Ely is north- centralCalifornia at ratesof-3-6 mm/yr [e.g., Sauber et northwestat 8.8+1.3 mm/yr. Northwest motion of the at., 1989; Feigt et at., 1993, Wakabayashiand Smith, Sierra Nevada block relative to stable North America 1994] consistentwith this suggestion.Lisowski et al. consistsof two main components,east-west extension in [ 1991] foundno convergencewithin errors(<2mm/yr), but the easternboundary zone of the Basinand Rangeprovince most of their trilateration networks are within about 30 km andright-lateral shear on north-northweststriking faults in of the San Andreasfault, and geologicand seismicdata the westernboundary zone. suggestthat convergenceis oftenaccommodated at larger 2. The spacegeodetic data for Ely suggestthat east-west distances. G PS data from a network northeast of San extensionin the easternBasin and Range is significantly FranciscoBay also show no fault-normalconvergence fasterthan previousestimates based or• paleoseismicdata within errors(<2 mm/yr) [Williamset at., 1994],but most for the Wasatchfault zone or ratesbased on summingthe of the stationsare locatedwest of the Coast Range-Central seismicmoments of earthquakesacross the region. This Valley Thrust, which Wakabayashiand Smith [1994] suggeststhat there are other active faults accommodating suggestis the locusof convergentdeformation in the significantextension, that currentseismic slip rateson the region. Wasatch fault are anomalouslylow comparedto longer- Theconvergence rate normal to the SanAndreas fault is term averages, or that some deformation occurs sensitive to the mean azimuth of Basin and Range aseismically.There is approximateagreement between our deformation. Over geologic time, Basin and Range spacegeodetic data and ground geodeticdata combined deformationmay evolve to minimize convergencein with an elasticstrain model, consistentwith locking and California,i.e., minimizingthe fault-normalcomponent of elastic strain accumulation on the Wasatch fault zone and the SanAndreas discrepancy and perhapsminimizing the nearby faults and strain concentrationin the eastern work done in plate boundaryzone deformation[e.g., boundaryzone. Lachenbruchand Thompson,1972], sincerocks are weaker 3. Most Basin and Range deformationwest of Ely is in shearthan in compression. The changein absolute accommodatedwithin a relatively narrow (N50-150 km motion of the Pacific plate between 3.4 and 3.9 Ma wide) westernboundary zone. Most deformationassociated [Harbert and Cox, 1989], which increasedconvergence in with the easternCalifornia shear zone lies east of OVRO, California,may be significanthere. Relative plate motion whose rate of motion with respectto Ely corrected has been essentiallyconstant since then [e.g., Argus and elastic strain accumulation is 10.7+1.6 mm/yr at Gordon 1990, DeMets et at., 1990, Dixon et at. 1991; N9øW+5 ø. Slow (N1 mm/yr) west-southwestextension Feigt et at., 1993] as has the orientation of the San acrosseastern Sierra Nevada range front faults and the Andreasfault in California [e.g.,Powell, 1993]. Thus any Owens Valley fault zone occurswest of OVRO. We subsequentkinematic changes in the plateboundary system proposea kinematicmodel for the presentconfiguration in responseto the changein Pacific plate motion likely the shearzone includinga slip rate budgetfor the Owens involve the Basin and Range component. The western Valley, White Mountains, Hunter Mountain-Panamint boundaryof the Basin and Range provincemay be best Valley, DeathValley-Furnace Creek, and Fish Lake Valley ableto accommodatechanges since it is hotter and weaker fault zones. One predictionof our model is relativelyfast comparedto adjacentregions [Biasi and Humphreys, (6.2+2.3mm/yr) slip on the Fish Lake Valley fault zone. 1992]; (seeFigure 4). The rapidly changinggeometry of North to northeaststriking normal faultssuch as the Deep the eastern California shear zone over the last few million Springs fault and faults in Eureka Valley accommodate DIXON ET AL.: BASIN AND RANGE DEFORMATION 769

rightsteps in the north-northweststriking strike-slip faults to make SierraNevada block motion more northerly, andlead to developmentof pull-apartbasins. minimizing the rate of convergencenormal to the San 4. Motion of the SierraNevada block with respectto Andreasfattit, possibly in responseto a changein overall stableNorth America (a measureof integratedBasin and plate motion between3.4 and 3.9 Ma. Range deformation)inthe vicinity of OVRO occursat a rateof 12.1+1.2 mm/yr orientedN38øW+5 ø, in agreement with previousgeological estimates at the level of two Acknowledgments.We thankAllan Gillespieand Gene standarderrors. Humphreysfor discussionand sharingunpublished data 5. Over the last few million years,the easternCalifornia and Humphreysand Warren Hamilton for comments. Jim shearzone has evolved, with significantright-lateral shear Savageand Wayne Thatcher provided thoughtful reviews migratingwest from the southempart of the northwest whichgreatly improved the manuscript.Bernard Minster strikingDeath Valley-Furnace Creek fault zone to the generouslyprovided computer programs and adviceto north-northweststriking Owens Valley fault zone. Activity recalculate"Model A" to a newfiducial point. Part of this on north to northeaststriking normal faults has workwas done while JL wassupported by NSF grant correspondinglymigrated northwest to maintainconnection EAR-92-96102 awardedto J. Stock. This reseachwas with the northernDeath Valley-Furnace Creek and Fish supportedby grantsfrom NASA's Solid Earth Science Lake Valley fault zones. These changesin geometrytend Program.

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