Recent State of Stress Change in the Walker Lane Zone, Western Basin and Range Province, United States

Home , Basin and Range Province, Walker Lane

University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln

USGS Staff -- Published Research US Geological Survey

1995

Recent state of stress change in the Walker Lane zone, western Basin and Range province, United States

Oliver Bellier Centre National de la Recherche Scientifique

Mary Lou Zoback U.S. Geological Survey, [email protected]

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Bellier, Oliver and Zoback, Mary Lou, "Recent state of stress change in the Walker Lane zone, western Basin and Range province, United States" (1995). USGS Staff -- Published Research. 472. https://digitalcommons.unl.edu/usgsstaffpub/472

This Article is brought to you for free and open access by the US Geological Survey at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in USGS Staff -- Published Research by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. TECTONICS, VOL. 14, NO. 3, PAGES 564-593 , JUNE 1995

Recent state of stress change in the Walker Lane zone, western Basin and Range province, United States

Olivier Bellier CentreNational de la RechercheScientifique, URA D1369,Universit6 de Paris-Sud,Orsay Cedex

Mary Lou Zoback u.s. GeologicalSurvey, Menlo Park,California

Abstract. The NW to north-trending Walker Lane zone simple strain partitioning of deformation within a single (WLZ) is locatedalong the westernboundary of the northern regional stressfield suggestedfor the WLZ by Wesnouskyand Basinand Range province with the SierraNevada. This zoneis Jones [1994]. The location of the WLZ between the deep- distinguishedfrom the surroundingBasin and Range province seated regional extension of the Basin and Range and the on the basis of irregular topographyand evidencefor both right-lateral strike-slip regional tectonicsof the San Andreas normal and strike-slipHolocene faulting. Inversionof slip fault zone is probably responsiblefor the complex interaction vectors from active faults, historic fault offsets, and of tectonic regimes in this transition zone. In early to mid- earthquakefocal mechanismsindicate two distinctQuaternary Tertiarytime the WLZ appearsto havehad a similarlycomplex stressregimes within the WLZ, bothof whichare characterized deformational history, in this case as a back arc or intra-arc by a consistentWNW o3 axis; theseare a normalfaulting region, accommodating at least part of the right-lateral regimewith a meano3 axis of N85ø+ 9øW and a meanstress componentof oblique convergenceas well as a componentof ratio (R value)(R=(o2-ol)/(o3-ol)) of 0.63-0.74and a younger extension. strike-slipfaulting regime with a similarmean o3 axis (N65ø - 70øW) and R values ranging between - 0.1 and 0.2. This Introduction youngerregime is compatiblewith historicfault offsetsand earthquakefocal mechanisms.Both the extensionaland strike- The Walker Lane belt or zone (WLZ), as defined by Stewart slip stress regimes reactivated inherited Mesozoic and [1988], is a NW to north trending structuralzone, - 700 km Cenozoicstructures and alsoproduced new faults.The present- long and 200-300 km wide, locatedin the westernmostpart of day strike-slipstress regime has producedstrike-slip, normal the Northern Basin and Range province. This zone is oblique-slip,and normaldip-slip historicfaulting. Previous characterized by rather irregularly-shaped topography, in workers have explained the complex interaction of active contrastto the linear ranges and basins to the east (Figures 1 strike-slip, oblique, and normal faulting in the WLZ as a and 2), and has had a complex structuraland tectonicevolution simpleconsequence of a singlestress state with a consistent [Stewart, 1988, 1992; Bellier and Zoback, 1991]. WNW o3 axis and transitionalbetween strike-slip and normal Deformation within the zone was initiated during Late Triassic faulting (maximumhorizontal stress approximately equal to or Jurassictime, probably in responseto oblique subduction vertical stress,or R -- 0 in both regimes) with minor local which produced a broad right-lateral strike-slip fault zone fluctuations. The slip data reported here support previous along the magmatic arc and within the back arc region results from Owens Valley that suggestdeformation within [Stewart, 1988, 1992]. Cenozoic deformation styles within temporally distinct normal and strike-slip faulting stress the WLZ include both strike-slip fault zones and a range of regimes with a roughly constantWNW trending o3 axis extensional features including detachment faults and core (Zoback, 1989). A recent changefrom a normal faulting to a complexes (in highly extended areas) as well as normal to strike-slip faulting stress regime is indicated by the oblique normal faulting associatedwith both symmetric and crosscuttingstriae on faultsin basalts<300,000 yearsold and asymmetric basin-range blocks [e.g., Thompsonand Burke, is consistentwith the dominantly strike-slipearthquake focal 1973; Proffett, 1977; Stewart, 1978, 1979, 1983, 1988, mechanismsand the youngeststriae observedon faults in 1992; Zoback et al., 1981; Hardyman, 1984; Wallace, Plio-Quaternarydeposits. Geologic control on the timing of 1984a,b; Beanland and Clark, 1995]. the changeis poor; it is impossibleto determineif there has The study area for the current investigation is shown in been a singlerecent absolute change or if there is, rather,an Figure 1 and lies within the WLZ and adjacentregions of the alternatingor cyclicalvariation in stressmagnitudes. Our slip western Basin and Range and includes sites along the Sierra data, in particular,the cross-cuttingnormal and strike-slip Nevada frontal fault zone on the west and the north trending striaeon the samefault plane, are inconsistentwith postulated Nevada Seismic Belt on the east [e.g., Slemmonset al., 1979; Stewart, 1988, 1992] (note that this seismic belt is not limited to Nevada but extends southward into easternmost Copyright 1995 by the American GeophysicalUnion, California and includes Owens Valley). The results reported here are a subsetof a broaderstudy of stressstate in this region Paper number 94TC00596. reportedby Bellier and Zoback [ 1991] which demonstratedtwo 0278- 7407/95/94T C-00596510.00 phasesof late Tertiary normal faulting regime within the WLZ

564 BELLIER AND ZOBACK: STRESS CHANGE IN WALKER LANE ZONE 565

120'W 118'W 116'W

40'N illwater gap

Tahoe Lake

38øN Mono Lake

White Mountain

Owens2 Quaternarysedimentaryrocks L•,,?,,,•(;1 Quaternary !,'f, ,--1 volcanism

'] Pre-Quaternarybedrock Owens I. '•:'•?•1 Majorhistoric ••J seismiczone Southern S•erra gap: 36'N l96oJ-earthquakesMajor historic (table 4l SITE LOCATION

, .J_ 100Km 566 BELLIER AND ZOBACK: STRESS CHANGE IN WALKER LANE ZONE as follows: an older phasecharacterized by NE to ENE trending system in California. Extensive study of the state of stress extension directions (believed to correspondto a regionally adjacent to the San Andreas indicates that this strain defined "pre-basin-range"extensional phase [Zoback et al., partitioningis the consequenceof deformationwithin a single 1981]) and a younger deformationalphase corresponding with regional stress field, one in which the maximum horizontal WNW trending "modern"basin-range extension [Zoback and stressis oriented roughly 80-85ø clockwise to the strike of the Zoback, 1980, 1989; Stock et al., 1985; Zoback, 1989]. This San Andreas fault and in which the major strike-slipfault (the .-- 40 ø clockwise change in the o3 stress direction was San Andreas proper) has a shear strength substantiallylower previously recognized elsewhere in the Northern Basin and than the surroundingfaults and crust [Mount and Suppe,1987; Range province and is believed to have occurredbetween 10 Zoback et al., 1987; Oppenheimeret al., 1988]. and 7 m.y.B.P. in responseto superimposedshear related to The focus of the current study is fourfold; (1) to determine developmentof the San Andreastransform system [Zoback and the Quaternaryto present-daystate of stressacting in the WLZ Thompson,1978; Eaton et al., 1978; Eaton, 1979; Zoback et by inversion of both geologically and seismically determined al., 1981]. Thus the WLZ shows an extensionaldeformational slip vectors on minor and major faults within the zone, (2) to history similar to the rest of the Northern Basin and Range; test whether observed slip vectors can be explained by a however, within the WLZ the extensional deformation single, transitional stressstate, (3) to analyze implicationsof apparently preceded a complex early Tertiary deformational crosscuttingsets of late Cenozoic fault striae with rake angles episode characterized by a strike-slip to oblique-reverse which differ by more than 60 ø for possibletemporal variations tectonism [Bellier and Zoback, 1991]. in stressregime, and (4) to examine the hypothesisof strain Both geologic and earthquakefocal mechanismdata indicate partitioning in light of the temporal faulting relationships an abundance of strike-slip faulting in addition to normal and slip compatibility within a uniform regional stressfield. faulting within the WLZ. Many of these strike-slip faults have been active during late Tertiary and into recent time [e.g., dePolo et al., 1989, 1991; Wallace, 1979, 1987; Bell, 1981, Inversion of Fault Slip Data to Determine Stress State 1984a,b; Bell et al., 1984; Doser, 1988, Beanland and Clark, 1995]. A detailed field investigation of scarpsformed in the Kinematicsof a fault populationcan be definedusing the 1872 (M 7.6+4) Owens Valley earthquake established the striationsobserved on the fault planes.Often, however,more dominantlystrike-slip characterof that particularfault zone as than one set of striae are presenton a fault plane. Separation well as provided evidence for at least three large Holocene of distinct families of striations must be done on the basis of strike-slipevents along the zone [Beanland and Clark, 1995]. geological field data using relative chronology of the The Owens Valley fault zone is subparallel to the adjacent striations(crosscutting relationships) and their relationship dominantly normal Sierran frontal fault zone which, in this with regionaltectonic events. The initial analysisof the slip area, has demonstrated late Quaternary displacement but no data presentedhere was conductedusing graphicalmethods resolvable Holocene movement [Gillespie, 1982]. Previously, [e.g., Vergely et al., 1987] to distinguish,when necessary, Wright [1976] suggested that the complex interaction of distinct families of striations at individual sites. The active strike-slip, oblique, and normal faulting in the WLZ was methodologyof kinematicanalysis of fault slip data usedhere a simple consequenceof a single stressstate with a consistent to definestress state has been developed over the past20 years WNW 03 axis and transitionalbetween strike-slipand normal by Frenchstructural geologists [e.g., Carey, 1979; Carey and faulting (maximum horizontal stress{JHmax = vertical stress Brunier, 1974,Angelier, 1979, 1984]. This form of analysis Or) with minor local fluctuations.However, Zoback [1989] hasbeen applied to constraintemporal and spatialchanges in showed that the large difference in rake of slip vectors on the stress state in numerous regions such as in the Central these two subparallel faults can not be explained by a single Andes [e.g., S•brier et al., 1988; Bellier et al., 1991; Mercier stressstate with {JHmax= (IV, and suggesteda recent (post- et al., 1992]. 100,000 to pre-10,000 years) major temporal variation in If one assumesthat the slip vectors indicated by the stressregime. striations represent the direction of the maximum resolved Alternately, Wesnouskyand Jones [1994] have suggested shear stresson each fault plane [e.g., Bott, 1959], then the that the contrast in rake angles of slip on subparallel faults observationsof fault slip on multiple planescan be inverted both in the Owens Valley region and throughoutthe Walker to determine a mean best fitting stresstensor. To determine Lane is an example of "strain partitioning," a deformation the stressstate responsible for late Cenozoicfaulting in the style analogousto the contemporaneousstrike-slip and thrust WLZ, we performedsuch a quantitativeinversion of familiesof deformation on subparallel faults within the San Andreas fault slip data, determinedat individual sites, using a method

Figure 1. Simplifiedgeological map of the WalkerLane zone with locationsof fault slip measurementsites indicatedby solidcircles and identifiedby numbers(or lettersfor siteslocated along known historic active faultzones). Main SierraNevada eastern bounding fault zone is indicatedby a thick,solid line, hachured on the down-thrownside. Major basinsare indicatedby stipplingand the main range-boundingfaults are schematicallyindicated. The NevadaSeismic Belt (whichextends into easternmost California) is shownby the diagonallylined area, and the dateand approximate locations of the majorhistoric earthquakes within this belt are shownby the year in rectangles. BELLIER AND ZOBACK: STRESS CHANGE IN WALKER LANE ZONE 567

ß,,tli:•,•' ...... Pyramid Lake ' o•.•tl• -

,L'ab'•a•loe

...

i-'L ,l, '1

Figure 2. Digitally shadedrelief map of the Walker Lane zone and westernmostBasin and Range with several localities noted.

originally proposedby Carey [1979]. This fault slip inversion of the principalstress axes c•l > (72> (73of a meandeviatoric method computes a mean best fitting deviatoric stresstensor stresstensor as well as a "stressratio" R=(tj2-{51)/({53-{51), a from a set of striated faults by minimizing the angular linear quantitydescribing relative stressmagnitudes. It should deviation between a predicted slip vector 'c and the observed be notedthat R = 1-q•,q• being anothercommonly used stress striation s [Carey, 1979; Carey and Brunier, 1974]. The ratio [e.g., Angelier, 1984; Zoback, 1989]. Discussionof the inversion results include the orientation(azimuth and plunge) significance of stress ratio variations in interpreting 568 BELLIER AND ZOBACK: STRESS CHANGE IN WALKER LANE ZONE

inversionsresults is presentedin a recent paper by Ritz and Table 1. Specific Criteria for DistinguishingWell- Taboada [ 1993]. Constrained Inversion Solutions As defined, the stress ratio R varies between two end- Criteria Value memberuniaxial stress states (R=0 when(i2=(i1, and R=I when N, number of faults N>11 (i2=(i3). In a normal faulting stress regime (where vertical M.D., mean deviation angle M.D. < 13 ø stress(iv = (i1, and maximum horizontalstress (iHmax = (i2) an [Z(x,s)l/N R = 1 indicatesthat the two horizontalstresses are equal (radial where ('r,s) is angle between extension) and the predicted deformation is pure normal the predicted('r)and (downdip) slip on dipping faults of all orientations. In a observeds slip vectors M.D. < S.D. < 3/2 M.D. strike-slipfaulting stressregime ((iv = (i2 and (iHmax= (i1) the S.D., standard deviation R = 1 end-member represents a stress state transitional to [(Z('c,s)2)/NI1/2 thrustfaulting (in whichminimum horizontal stress, (ihmin = Plunge of subverticalstress 70-90 ø (iv). In contrast,R = 0 ((i2 = (i1) in either a normalfaulting or axes Plunge of minimum < 20 ø strike-slip faulting stressregime indicatesthat the maximum "horizontal" stress horizontal and vertical stressesare equal ((iHmax= (IV), Plunge of maximum < 20 ø implying a stressstate transitionalbetween strike-slipand "horizontal" stress normal faulting. Near-transitional strike-slip or normal faulting stressstates require only minor fluctuation of stress magnitudesto go from one stressregime to the other. The Results predicted deformation for the transitional or near-transitional stressstates (R<0.15 in either a normal or strike-slipfaulting Sites of fault slip measurementsin this study are shownin stressregime) can range between oblique-normal faulting to Figure 1, and the locations of each site and the age of the pure strike-slip faulting, dependingon the orientation of the faulted formationsin which striae are measuredare given in fault. Table 2. Detailedlocations for a numberof importantfault slip Results of stress inversions are generally considered localitiesare given in the appendix.As mentionedpreviously, the focusof this paperis the Quaternaryto present-daystate of reliable if 80% of the deviation angles (angle between the stressin the WLZ. Unfortunately, it is generally difficult to calculated slipvector 'r and the observed striation s) are less date the age of fault striationsany more preciselythan being than 20 ø and if the computed solution is stable (i.e., the younger than the age of the rocks cut by the faults. In this inversion tends toward the same solution regardless of the study we have included results of measurements made in initial given parametervalues). However, in our study we have Mesozoic to Plio-Quaternary age rocks. The inversion results established specific criteria for distinguishing well- from the youngest striae affecting the Mesozoic and early constrained inversion solutions (Table 1). The objective, Tertiary rocks generally agree with the results from the quantitative part of the quality assessmentis based on the relatively few striations on faults affecting the Plio- value of the mean deviation angle and its standarddeviation. Quaternary deposits. Equally important, but more subjective, parameters As shown in Figure 3, normal slip striationswere found to influencing the quality include the number of fault planes be crosscutby right-lateral shallow rake striationsalong the sampled and the distribution of their attitudes. All fault slip same major fault planes or adjacent faults at two localities inversion schemes are based on the assumption that the alongthe southernsegment of the NevadaSeismic Belt, along measuredslip direction on each plane representsthe direction the Owens Valley and Rainbow Mountain fault zones of the maximum resolved shear stresson that plane. In this (localitiesowens2 and rm in Figure 1). In OwensValley these case there are four unknowns (three defining the orientationof two contrasting sets of striae are observed on a fault zone the principal axes and one defining the stressratio R), and any inversion thus requires at least four independent fault sets. parallelto and directlywest (<1 km) of the main OwensValley fault zone (Figure 4). Both faults cut late Pleistocenebasalt Ideal data sets contain faults dipping in both directions with from Crater Mountain dated at 288 + 70 ka [Turrin and distinct strike directions, not just a continuum of strikes arounda single mean direction. Gillespie, 1986]. The younger, shallow rake deformationon both the Owens Valley and Rainbow Mountain fault zonesis We have adopted an alternate approachto deal with poorly in agreement with the dominantly strike-slip character of distributed fault sets, utilizing a "fixed" inversion, i.e., one in recent earthquakesalong this seismic belt [e.g., Doser, 1988; which the principal stressaxes are fixed to lie in horizontal Beanland and Clark, 1995; dePolo et al., 1987; 1991]. These and vertical planes (consideredgenerally to be the casein the data suggesta recentchange in stressregime (or equivalently, Earth's crust [see Zoback and Zoback, 1980] and consistent temporal fluctuations in relative stressmagnitudes); however, with results from most of the well-distributed data sets in this as discussed below, the orientation of the minimum horizontal paper.). In this fixed inversionthere are only two unknowns stress axes (the (I3 axis) has apparently remained (the orientation of one of the horizontal stressesand the stress approximately fixed. The youngest set of deviatoric stress ratio), thus requiring only two independentfault sets. As tensorsdetermined geologically from fault slip measurements discussed below, the inferred horizontal stress orientations are shown to be remarkably consistentwith the present-day from the fixed inversion are typically within 5-10ø of the strike-slip stressregime deduced from inversion of historic stress axes obtained from standard inversions of data sets earthquake slips. which lack the fault distributionsto define four independent Because of the evidence for temporal changes in stress fault sets. regime, we have divided the data into dominantlynormal and BELLIER AND ZOBACK: STRESS CHANGE IN WALKER LANE ZONE 569

Table 2. Location of Fault Striae Measurement Sites Table 2. (continued) Site Latitude,øN Longitude, øW A[[ea Reference Site Latitude,øN Longitude,øW Agea Reference 1 40.180 120.210 Q+Ne? 1 Rainbow Mountain Fault 1 c 40.156 120.283 Ne 1 rm 39.440 118.526 mM-1M 6 2 40.110 120.080 Mz 1 rm c 39.420 118.535 mM-1M 6 2 c 40.095 120.067 Mz 1 Owens Valley Fault 3 40.080 120.190 Mz-Ne 1 owensl 37.145 118.290 P1 18,20 3 c 40.098 120.219 Mz 1 owensl 37.108 118.280 P1 18,20 4 40.250 119.880 1M-P 2 OlinghouseFault 5 40.288 119.854 1M-P 2 oling 39.595 119.500 mP-1P 2 6 40.355 119.836 1M-P 2 olin•c 39.589 119.522 mP-1P 2 7 40.417 119.860 1M-P 2 8 40.406 119.850 Q 2 a Ages of faulted formations are as follows: Q, 9 40.035 119.485 mM-eP 2,3,4,5 undifferentiated Quaternary; P1, Pleistocene; Ne, 10 40.106 119.503 mM-eP 2,3,4,5 undifferentiated Neogene; eP, early Pliocene; mP, middle 10 c 40.170 119.510 mM-eP 2,3,4,5 Pliocene; 1P, lower Pliocene; P, undifferentiated Pliocene; eM, 11 40.027 119.301 Q 6 11 c 40.050 119.290 Q+IMz 6 early Miocene; mM, middle Miocene; 1M, lower Miocene; M, 11 c 40.065 119.285 1Mz 6 undifferentiated Miocene; 10, late Oligocene; O1, 12 39.680 119.380 M-eP 2 undifferentiated Oligocene; eMz, Triassic; mMz, Jurassic; 12 c 39.650 119.426 M-eP 2 eMz, Cretaceous; Mz, undifferentiated Mesozoic; and 1Pz, late 13 39.800 119.377 eM-mM 2,3 Paleozoic. 14 39.824 118.220 Ol-eM 2,7,8 b Referencesused to date faultedformations are as follows: 15 39.402 119.850 mM 9,10,11 1, Lydon et al. [1960]; 2, Bonham [1969]; 3, Everden and 16 39.240 119.850 Mz 6 James [1964]; 4, Axelrod [1966]; 5, Silberman and McKee 17 39.115 119.840 Mz 6,12 [1974]; 6, Stewart and Carlson [1976a, 1978]; 7, D. A. John 18 39.060 119.845 1Mz 6,12 19 38.997 119.844 1Mz 6,12,13 and E. H. McKee (written communications,1990); 8, Page 20 39.980 119.835 1Mz 6,12,13 [1965]; 9, Bonham and Rogers[1983]; 10, Vikre and McKee 21 38.555 119.504 1Mz 13 [1987]; 11, Whitebread [1976]; 12, Pease [1980]; 13, Stewart 22 38.680 119.550 e+mMz 13 et al. [1982]; 14, Stewart and Dohrenwend[1984]; 15, Noble 23 38.735 119.355 eM+mM 13,14 [1962]; 16, Proffett and Proffett [1976] and Proffett [1977]; 24 38.780 119.400 1Mz 14,15 17, Strand [1967]; 18, Matthews and Burnett [1965]; 19, 25 38.812 119.256 1P 13,14 Stewart and Carlson [1976b]; and 20, Beanland and Clark 26 38.812 119.244 1P 13,14 [ 1995] and Lubetkin and Clark [ 1988]. 27 38.814 119.225 mM-1M 13,16 c Multiple localitiesalong the samefault zone. Data from 27 c 38.814 119.216 mM-1M 13,16 all sites with same number or name are analyzed together. 27 c 38.840 119.194 mM-1M 13,16 28 38.845 119.185 M 13,16 29 38.875 119.170 1Mz 13 30 38.895 118.850 1Mz 13 31 38.810 118.765 1Mz 13 to38.830 118.770 1Mz+Q 13 32 38.787 118.756 1Mz 13 strike-slip faulting stress regime subsets. This distinction 33 38.710 118.768 1Mz 13 allows for determinationof stresstensors individually, to test 34 38.670 118.770 1Mz 13 whether the observed slip patterns are compatible with a 35 38.350 118.035 1Mz 13 single stress state (e.g. a stress state transitional between 36 38.372 118.047 eMz 13 normal and strike-slip faulting) or if they require distinct 37 37.900 118.340 Mz 6,17 38 37.800 118.382 Mz 17 stressstates. Results of the stressinversion on both geologic chalf 37.540 118.315 Pz-Mz 17 and earthquake slip data are tabulated in Tables 3 and 4, 39 36.590 118.190 Mz 18 summary stereoplotsof individual inversion axes are shown in 40 36.600 118.202 Mz 18 Figure 5, and stereoplotsof the actualfault slip data are given Pleasant Valley Fault in Figures6 and 11. The computed(53 orientationsare shown plvl 40.381 117.580 1Pz 6,16 in map view on Figures7 and 12 and are discussedbelow by ply2 40.360 117.601 1Pz 6,19 stressregime and locality. ply3 40.320 117.620 eMz 6,19 It is significantto note that we have strictly applied the ply4 40.300 117.630 eMz 6,19 quality criteriain Table 1 to our data. Only 10 of the 31 normal ply5 40.290 117.630 eMz 6,19 fault inversions given in Table 3 are considered well- Dixie Valley Fault dvl 39.555 118.212 10-eM 6,7,8 constrained,reliable results. Similarly, none of the results on dv2 39.620 118.175 10-eM 6,7,8 small individualstrike-slip data sets(Table 4) wasjudged to be dv3 39.790 118.105 M-PI? 6,7,8 well constrained;only the results on the combined data sets Fairview Peak Fault met the quality criteria in Table 1. We have chosento show fairy 39.260 118.135 mM-1M 6 inversionresults for all data setscollected and to highlightthe fairy 39.210 118.150 mM-1M 6 well-constrainedresults in Figures 5-6 and 11. 570 BELLIER AND ZOBACK: STRESS CHANGE IN WALKER LANE ZONE

Owens Valley Rainbow Mountain fresh-lookingscarps which mark the trace of the fault. A series Ng mg of large, fresh triangular facets in the footwall block provide slickensided bedrock fault exposuresalong portions of the fault zone. Examples of these striated bedrock fault exposures can be seen along the Genoa fault in Figure 8 (sites 17 to 20, Figures 1 and 7) and along the Independence fault (from northwestof site owensl to south of site 40, Figures 1 and 7) which forms the west side of Owens Valley. These border fault traces also cut late Quaternary fluvial and colluvial deposits, ' x • '• chronology producingsurface fault scarpswhich are sometimesarranged in an en echelon pattern such as along the Jack Valley zone Figure 3. Lower hemisphere stereographicprojection of (south of Carson, near and between sites 17 and 18 in Figures faults with crosscuttingfamilies of striae measuredalong the 1 and 7). The profoundrelative vertical relief acrossthe Sierra Owens Valley and Rainbow Mountain fault zones. Arrows on frontal fault zone and the large vertical offset of late fault planes point in the horizontal slip azimuth direction. The Pleistoceneglacial moraines with no detectablelateral offset older slip vector on each fault plane is indicated by .1 suffix; [Clark et al., 1984; M. M. Clark, oral communication, 1988] the younger is given by .2 suffix. The slip vectorsfor historic attest to the dominantly dip slip character of this fault zone earthquakesalong each fault zone (data given in Table 5) are and imply a Pleistocenenormal faulting stressregime. also plotted; 1872 is Owens Valley earthquake;June 6, 1954, Striations were measuredon the major fault planes of the and August 24, 1954, correspond to Rainbow Mountain Sierran frontal fault zone and on secondary, associated earthquakes. Also given on each stereonet are the results of fractures within the fault zone. The striations were typically stressinversions on young fault slip data measuredalong the located at the base of the major fault scarps,primarily within OwensValley and the RainbowMountain faults (Table 4); c•l is Late Cretaceous granitic bedrock. The slightly weathered given by a star; c•2, a diamond; and c•3 is shown by a circled slickensides showed well-preserved grooves and frictional triangle. striations. As indicated in Table 3, all inversions yielded a normal faulting stressregime with c•3 axes trending between WNW and about E-W. Only three of the datasets(sites 17, 21, Evidence for Basin-Range Normal Faulting Stress and 39) yielded well-constrained inversion results (indicated Regime by the crosses in Table 3); the stress axes for these were Evidence for a "basin-range"extensional stage is found similar to the overall trends, varying between WNW (N57øW, throughout the northern Basin and Range province. This site 17) and approximatelyE-W (N87øW to N98øW, sites 21 deformationalstage is characterizedby a generally WNW and 39 respectively). Stress ratio values R for the well- trending extension direction [Zoback et al., 1981; Zoback, constrained inversions range between 0.75 and 0.82, 1989].We foundthis deformationalstage recorded throughout indicating a stressstate in which the two horizontal stresses the WLZ by slip on both minor and major faults which affect are close in magnitude and much less than the vertical stress Plio-Quaternaryfluvial, lacustrine,and colluvialdeposits, and c•l. Thesehigh R valuesare a directresult of the fact that most Quaternaryvolcanic flows, as well as Cenozoicand Mesozoic deformation measured represents nearly pure dip-slip bedrock. The major faults borderingthe ranges,including the movementon fault planeswith a ratherwide rangein strike. normal fault zone forming the eastern boundary of the Sierra Only the fault kinematics of the northernmostSierran Nevada, also expose slickensides with striations, all frontal fault (Honey Lake region, inversionfrom the combined indicating a roughly WNW trending extension. sites 1, 2, and 3) are very discordantand indicatea NE trending Results of inversion of all fault slip data setsbelonging to c•3 axis. Note on Figure 1, however, that the major Sierran this extensional deformational stage are given in Table 3. boundingfault in this area strikesNW, thusthe NE trendingc• 3 Lower hemisphere stereoplotsof all slip data and inversion axis may be due to extension in a normal faulting stress results, including histograms of deviation angles (angle regime with R -- 1 (two horizontal stressesapproximately betweenthe observedslip directionand that predictedfrom the equal in magnitude),in which casethe c•3direction determined maximum shear direction), are plotted in Figure 6; the well- by the inversion would be strongly influenced by the overall constrained results are indicated by the solid c•3 arrows. orientationof the faults sampled.However, the NE trendingc• 3 Corresponding horizontal stress axes (with the well- axis indicatedby the inversionof the combineddata from sites constrained results also highlighted) are plotted on the 1, 2, and 3 (Figure 7 and site l&2&3 in Table 3) may also stereonet labeled NF/GEO in Figure 5, together with mean representa real rotation of c•3 orientationin the northernmost stress axes and their 95% confidence ellipses (determined Sierra Nevada/southern Cascade region. Volcanic vent independentlyusing Fisher statistics,as modified by Watson alignmentsfarther to the NW in the Lassen-Susanvilleregion [1960], on each subsetof axes). Individual site c•3 directions of northeastern California indicate (many <1.0 m.y. old) are shown on the geologic map in Figure 7. The data and indicatea NE to ENE c•3 directionin that region[Luedke and results of the stress inversion are described below by Smith, 1981]. geographic and tectonic setting, generally in the order they Range-bounding fault zones within the Basin appearin Tables 2 and 3. and Range province. Numerous Basin and Range-type Sierra Nevada frontal fault zone. The NNW trending mountain blocks within the WLZ have recent scarpsat their east facing Sierra Nevada frontal fault zone consistsof steep, base,for example,the Dry Valley borderfault shownin Figure BELLIERAND ZOBACK: STRESS CHANGE IN WALKERLANE ZONE 571

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.....•.;:•-;-:.:.• ...... •.:.;.½; ";}½:;• ...... :•:•,:.';:: ...•:• .... '*•..•...... ;:% ...... •...... '': '...... "'. • ...... •? -..:..:?'-i::.:-::'?: •, ;.•!..•;'::"::...:,...... ,...•..,;.::•: •..::;..?<•-•:-' •.;.•,..;•, ...... :•:•.. •'•?...... >.•;?:•;'-½ ½•:•}.'.,.....w•:.:...•..•.;•:;..

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.,•

'"½3

.;;

..::•

--::.:: 572 BELLIIERAND ZOBACK:STRESS CHANGE IN WALKERLANE ZONE

N NF/GEO N SS/GEO

N275ø-9•/0 ø

NSiic• i I I I NSilo • .•

2 4 6 8 10 R. 10 •

Figure 5. Late Neogeneto present-dayregional stressstates in the Walker Lane zone. NF/GEO and SS/GEO are lower hemisphere summary stereoplotsshowing local "horizontal" stressaxes determinedby inversions for each individual site. NF/GEO shows results for the normal faulting inversions given in Table 3, while SS/GEO gives the more limited resultsfor the strike-slipfaulting inversionsgiven in Table 4. Stars, squares, andtriangles refer to thec•l, c•2,and c• 3 axis,respectively. For NF/GEOthe resultsfrom well-constrained inversions(pluses in Table 3) are shown by solid symbols,while open symbolsrefer to resultsfrom poorly constrained inversions (minuses in Table 3). For SS/GEO the solid symbols refer to results of the fixed inversions (i.e., inversions in which the principal stressesare fixed to lie in horizontal and vertical planes; asterisksin Table 4), and open symbolsindicate standard,poorly constrainedinversions (minuses in Table 4). The encircledstar, square,and triangleindicate the meanhorizontal c•l, c•2,and c•3 axes, respectively, determinedby the Fisher statisticsmethod (modified by Watson [1960]), to each of the local horizontal stress axes. Dotted areas correspondto 95% confidencecones for the mean directions.Histograms at bottom show distribution of computed mean stress ratio R values for each individual inversion. Numbers inside the histogramsrefer to site numbers given in Table 2 and which correspondto labels outside of the individual stereoplotson Figures 6 and 11. The lettered sites are dv, Dixie Valley; pl, PleasantValley; f, Fairview Peak; r, Rainbow Mountain, f+r, combined Fairview Peak and Rainbow Mountain; ol, Olinghouse;ow, Owens 2; m, SS/g-minor;M, SS/g-major;fm, SS/fm. Dotted squaresin histogramsshow R valuesfor the well-constrained inversions(pluses in Tables3 and 4), while on the SS/GEOhistogram the minussuperscripts refer to R values from poorly constrainedstandard inversions and the asterisksuperscripts refer to R valuesobtained from the fixed inversions.

Figure 4. (a) Close-upphotograph of a fault plane within the OwensValley fault zone cuttinga late Pleistocenebasalt flow (age 288 + 70 ka, [Turrin and Gillespie1986]) and showingtwo crosscuttingstriae families(owens 2 site in Figures1 and4c). The olderstriae (2.1) indicatenormal slip andthe younger(2.2) showright-lateral slip, these data are plottedon Figure3. (b) A generalview of the fault planecontaining the crosscuttingstriae, large square shows area of the close-upphotograph in Figure4a. (c) Locationof the fault striaemeasurement localities in OwensValley on (left) a U.S. GeologicalSurvey aerial photograph and (right) on a geologicalmap after Beanland and Clark [1995].Rectangle on left insetindicates the owens2 faultzone whichcontains the fault planedepicted in Figures4a and4b. Legendfor geologicalmap on rightis: (1) pre- Cenozoic bedrock, (2) Pleistocenebasalt (<300,000 years in age), (3) Pleistocenelake sediments,(4) Pleistocenefanglomerates, (5) late Pleistoceneto earlyHolocene fan deposits,and (6) 1872fault rupturetrace. BELLIER AND ZOBACK: STRESS CHANGE IN WALKER LANE ZONE 573

9 (sites 4 and 5 in Figures 1 and 7). In most casesthis faulting values for four of the five well-constrained, high-quality occurs ,,•,,ly at or near the dl -" I .... U V :-'"--•'I ill/D•11 .... UUK '- range frolit inversions were very consistent, varying between 0.68 and boundary, and these faults bound asymmetric, graben-in- 0.77, similar to those determined from the well-constrained graben structural basins [e.g., Slemmons, 1957; Thompson inversions along the Sierran frontal fault zone. One well- and Burke, 1973; Anderson et al., 1983; Wallace, 1984a]. constrained inversion (site 24), however, yielded a much Fault slip data were collected along these major normal fault smaller R value, 0.24. planes and from secondaryfractures within the fault zone. The Minor faulting in Plio-Quaternary deposits. studied striations are mechanical striations in volcanic, Faults with minor displacements (1-2 m) cutting Plio- granitic, and sedimentaryrocks. Quaternaryfluvio-lacustrine and volcano-sedimentarydeposits Results of the inversions of all these range-boundingfault that infill the present-daybasin-range grabens were observed slip data are given in Table 3, and the actual fault data are at several sites within the WLZ (sites 1, 11, 25, 26, 28, and 31 shown in Figure 6. The inversion results indicate a normal in Figure 1). Unfortunately, striations are rarely preservedin faulting stressregime with •3 axestrending between WNW to such poorly consolidateddeposits. The best example of these about E-W (between N61øW and N95øW, with one site, 32, faults, exposed by a road cut in Smith Valley, is shown in somewhatdiscordant with a •3 axis trendingN109øW). The •3 Figure 10 (sites 25 and 26 in Figure 1). Here the faulting cuts axes for the well-constrained inversions covered the entire Pliocene sedimentary deposits of the Wilson Canyon range and includedthe somewhatdiscordant site 32. The R formation, a poorly consolidatedsilt, sand, and gravel unit

Table 3. Results of Stress Tensor Inversion for Slip Data RepresentingBasin-Range Normal Faulting Stress Regime Azimuth/Plungec, deg Sitea Nb (51 (52 (53 M.D.d S.D.e Rf Ageg Qualh Sierra Nevada Frontal Fault Zone 1&2&3 12 164/51 313/34 054/15 8.9 11.8 0.22 Q+Ne+M - 15 10 227/86 019/03 109/02 14.0 18.1 0.71 mM - 16 7 233/75 012/11 104/09 1.5 1.7 0.76 Mz - 17 14 335/83 213/03 123/06 11.1 13.3 0.82 Mz + 1 8 7 174/53 005/37 271/05 11.5 13.7 0.92 1Mz - 19&20 13 114/86 358/02 268/03 14.2 19.4 0.95 1Mz - 21 20 247/84 357/02 087/06 9.7 11.7 0.75 1Mz + 39 14 048/72 169/10 262/15 7.3 9.8 0.81 Mz + 40 8 324/63 188/20 092/18 12.1 14.1 0.40 Mz - Range-BoundingFault Zones Within the Basin and Range Province 4&5 24 031/85 207/05 297/0.3 12.4 16.3 0.75 1M-P + 6&7 13 302/81 200/02 110/08 7.3 9.8 0.75 1M-P + 10 15 042/80 209/10 299/02 8.3 9.9 0.68 mM-eP + 13 12 074/73 179/05 270/17 14.3 19.1 0.79 eM-mM - 14 9 318/76 180/10 089/09 18.2 33.4 0.75 Ol-eM - 24 18 204/73 356/15 088/08 5.6 7.3 0.24 1MZ + 29 19 235/74 017/13 110/10 14.7 19.0 0.26 1Mz - 32 23 139/71 343/18 251/07 12.3 16.7 0.77 1Mz + 33&34 19 165/59 359/30 265/06 11.6 14.1 0.75 1Mz - 35&36 21 115/68 005/08 272/20 10.5 14.3 0.75 1Mz - 37&38 6 085/72 194/06 286/17 3.2 3.9 0.34 Pz-Mz - Faulting in CenozoicBasin Fill Within the Basin and Range Province 1-Q 2 ENE-WSW P-Q 1 1 7 069/74 171/04 262/16 13.2 16.2 0.11 Q+IMz - 25 16 206/83 018/07 108/01 9.6 11.1 0.89 1P + 26 8 251/89 008/00 098/01 4.9 7.9 0.83 1P - • i'-•6 24 184/85 009/05 279/00 7 9 10 7 0 86 1P + 28 12 161/79 353/11 263/02 11.3 18.9 0.81 M - 3 1 14 296/84 181/03 091/06 13.5 15.9 0.34 1Mz+Q - Nevada Seismic Belt Faults plv 40 195/84 009/06 099/01 10.2 12.3 0.84 Pz-Mz + dv 31 219/64 026/25 119/05 16.0 18.4 0.41 10+M - owensl&2 8 200/85 358/05 088/02 9.7 13.0 0.99 P1 - owensl&2 8 131/90 356/00 266/00 9.6 13.8 0.82 P1 * 574 BELLIERAND ZOBACK:STRESS CHANGE IN WALKERLANE ZONE

Table 3. (continued) Azimuth/Plungec,deg Sitea Nb 15! 62 i53 M.D.d S.D.e 'i•f Ageg Qualh fairv 4 vertical ENE-WSW mM+IM rm 1 1 216/69 342/13 076/16 7.4 12.3 0.55 mM+iM - rm 1 1 153/90 355/00 265/00 10.2 14.6 0.66 mM+IM * fair&rm 15 154/90 344/00 254/00 15.0 17.2 0.31 mM+IM * All Data NF/GEOi 5+8/04 275+9/0.2 0.63 0.74Q + aAmpersands indicate an inversion solution computed from data from two or more different sites. For example, l&2&3 correspondsto an inversion computed from data set 1, 2, and3. Site1-Q is thePliocene-Qualernary age shriaeonly at site1. Sitesplv anddv representinversions of all normalstriae collected at all thesites along the PleasantValley andDixie Valleyfault zones (plvl to plv5 anddvl to dv3,respectively). Other named sites include:owens l&2; OwensValley Fault; fairv, Fairview Peak fault; rm, RainbowMountain fault. b N isnumber of striatedfault planes used to compute the solutions. c Deviatoricprincipal stress axes 15! > 152> 153,specified by azimuthsmeasured clockwise from north and plungesmeasured from horizontal. dM.D. is themean deviation angle (defined in Table1). e S.D. is the standarddeviation of deviationangle (defined in Table 1). f R equals(152-151)/(153-151), the"stress ratio" of thedeviatioric stress tensor. g Agesare sameas in Table 2. h Qualrefers to thequality of thestress inversion; crosses indicate a well-constrained inversion as defined in Table 1, minusesindicate inversion results which did not meetthe qualitycriteria in Table 1, andasterisks indicate a fixed inversion(stress axes constrained to lie in horizontaland vertical planes). i NF/GEOisthe mean an average regional deviatoric axes determined using Fisher statistics independently on the two subhorizontalstress axes from each of the individual inversions.The R value is the arithmetic mean of all sites,second R value(followed by Q) is themean for onlywell constrained sites (crosses in Qualcolumn).

[Stewartand Dohrenwend,1984]. The majorityof the fault Wallace,1984b; Bell andKatzer, 1990; dePolo et al., 1991]. planesstrike NNE to NNW and are imprintedby very thin Manyof theseNevada Seismic Belt fault zones were sampled normal (dip slip) frictional striations on silt-clayish in this study(see Figure 1 for localities):Pleasant Valley slickensideswhich agree roughly with a west to WNW (sitesplv l to plv5),Dixie Valley(sites dvl to dv3),Fairview trendingextension (see stereoplots in Figure10). As givenin Peak (site fairv), RainbowMountain (site rm), and Owens Table 3, inversionof thesestriations from two sitesyield Valley (site owensl&2). Thesefault zonestypically have consistentresults, indicating a normalfaulting stress regime recentscarps at their basewhich occurat, or near, the alluvial with153 axes which trend N82øW (site 26) andN72øW (site 25) andcolluvial/bedrock range front boundary. The Dixie Valley andR valueswhich vary between0.83 and 0.89 (although and the Rainbow Mountain fault zones also have associated only resultsfor site 26 with the smallmean deviation angle minorsurface ruptures which propagated outward into basins. and standarddeviation are consideredwell constrained). Onlythe Owens Valley fault zone is locatedcompletely within Results of inversion of the combined data from sites 25 and 26 a basin;this basin is boundedon the westby a partof the (25&26 in Figure 10 and Table 3) yield a well-constrained SierraNevada frontal fault zone, the Independence fault zone. solution which confirms the local individual site inversions Theslip data come from mechanical striations collected along and indicatesa normalfaulting stress regime with a N81øW themajor normal fault planes and from secondary fractures trending153 axis and an R value of 0.86. within the fault zone. The studiedstriae affect late Mioceneto Inversionsof fault slip data from the other sites in Plio- early Pleistocene volcanism and Paleozoic to Cenozoic Quaternarydeposits which showednormal faulting slip graniticand sedimentaryrocks. vectorsgenerally agree with an approximatelyE-W trending The actualfault data are shownin stereoplotsin Figure 6. 153axis (sites 11, 28, and 31 in Figure 6 and Table 3) and The inversionresults are given in Table 3 and indicatea yieldedR valuesranging between 0.11 and 0.81. However,all normalfaulting stress regime with an approximatelyWNW to these inversions are poorly constrainedbecause of the westtrending least principal stress. Only the Pleasant Valley restricteddistribution of faulttrends (all faultswere dipping in fault zonedata set (plv) yieldedwell-constrained results from approximatelythe samedirection). Only two striationswere thestandard inversion, a normal faulting stress regime with a measuredat site 1-Q so thesedata were not inverted. N81øW trending153 axes, and an R valueof 0.84. ThisR value Range-bounding fault zones within the Nevada is very consistentwith the otherR valuesdetermined from the Seismic Belt. Many of the range-boundingfault zones well-constrainedinversions within the WLZ, includingalong within the WLZ and adjacentparts of the westernBasin and boththe Sierranfrontal and range-boundingfault zones.Note Rangeprovince have been reactivated in historicearthquakes, that for several of the data sets (owens l&2, rm, and defining the Nevada Seismic Belt [see Slemmons, 1957; fairvw&rm), both fixed and standard inversions were BELLIER AND ZOBACK: STRESS CHANGE IN WALKER LANE ZONE 575

. 1Q ," 1&2&3 ,• 4 & 5 N297'-0'

N54ø-15 ø

5 5

fi=0.75 10 20 13', s} 10 20 30 40 IT, s)

"6 & 7 " 11

N299'-2

N262ø-1 Nl10O_8o 5

R=0.75 R=0.68 1020 ('T,s) 10213 ('T, s) 10 20 30 IT, s}

" 13 " 14 " 15

N270 o -17 89 ø 5 5 N109o_2o

10 20 31340 5OI'T, s) 10 21330 40 50 60 70 80 90('T, s) 10 20 30 I'T, s)

Figure 6. Lower hemispherestereoplots of normalfaulting slip data from the Walker Lane region,together with inversionresults presented in Table 3. Labelsoutside and to top right of stereoplotsrefer to site names and numbersshown in Figure 1 and describedin Table 2. Individualfault planesand measuredslip vectorsat eachsite are plotted,arrows on fault planespoint in the directionsof the horizontalazimuth of the slip vector. Solid lines on the individualfault planesgive the deviationangle between measured s and predicted• slip vectorson each fault plane. Stressaxes obtainedfrom the inversionsare given by diamonds((•]), triangles (•2), and squares(•3). Large arrowsoutside stereoplots give azimuthof least horizontalstress (•3). Results from well-constrainedinversions (pluses in Table 3) are shownby solid stressaxes symbolsand solid outer arrows. Resultsof eitherfixed or poorly constrainedstandard inversions are representedby opensymbols and arrows. Histograms below each stereplot show distribution of deviation angles. R values for the well- constrainedinversions are given outsideand to the bottomright of the stereoplots. performed(with the resultsof the fixed inversionindicated by zone (dv); however, the inversion results are rather the asterisk in Table 3); in all three cases the distribution of inhomogeneouswith a largemean deviation angle suggesting the sampledfaults did not satisfythe four independentfault set possiblesuperimposed slip eventsthat were, unfortunately, criteria. It is interesting to note that the fixed solutionsall not recognizedby field observations. have o3 azimuthswithin 10ø of the standard(but not well- Summary of geologic fault slip data on Basin constrained) inversion results. In contrast, a large and well- and Range extensional deformation stage. Striae distributeddata set was collected along the Dixie Valley fault corresponding to the "basin and range" extensional :576 BELLIERAND ZOBACK:STRESS CHANGE IN WALKERLANE ZONE

. 16 . 17 . 18

I]4 '-9'

N271'-5'

R=0.82N123'-6' 2• 10 20 30 I"r' ß s) 10 2013' ß s)

19 & 20 . 21 . 24

N268'-3 N88'-8' 5

R=0.75 R=0.24 10 20 30 40 I 7', sl lO 2o 10 20(?', s)

25 & 26 . 28 29

N279'-0

10- N263'-2'

5 5 5 Nl10'-10'

R=0.86 10 20 {'T, s) 10 20 30 4'050 I'T, s) 10 20 30 40 {'T, s}

Figure 6. (continued) deformationstage found in the four regionsdescribed above extension"stress state (characterized by R = 1; i.e., (52--(53). consistentlyindicate a normal faulting stressregime with an However, a few sites, including two with high quality (53direction between WNW and E-W, as shownin Figures5, 6, inversions,yielded very largeR values(R > 0.85), implyinga and 7. A Fisher statistics analysis done on the two sets of local stress state close to the uniform radial extension. Not individual site subhorizontal stress axes for the entire surprisingly,as shownin Figure5, the siteswith theselarge R geologicfault slip data set yields a mean(53 (or (shmin)axis values (notably from inversions 18, 19 & 20, 25, and 25 & trendingN95ø+9øW with a plungeof 0.2ø and a mean(52 (or 26) are characterizedby slip data with very steeprakes on (sHmax)axis trendingN5ø+8øE with a plungeof 4 ø (seesolution normalfaults with a wide rangein strike. NF/GEO in Figure 5 and Table 3). The 8-10ø uncertainties quotedin mean stressazimuths correspond to the radiusof the 95% cone of confidencein Fisher statistics.Stress ratios (R Evidence for Recent Strike-Slip Faulting Stress Regime values)obtained for this normalfaulting deformation stage are generally >0.50 (Table 3). The arithmetic mean value for all Both historic earthquakeslip and the youngestgeologic inversions listed in Table 3 is R = 0.63, which is similar to striae data measuredprovide evidencefor a contemporary, the mean R value, R = 0.74, determinedfor only the 11 well- dominantlystrike-slip stress regime in the WLZ. Along the constrainedinversions given in Table 3. Both R values are Nevada Seismic Belt in easternmost California and clearly distinctfrom a "transitionalto strike-slip"stress state westernmost Nevada, numerous historic and Holocene (characterized by R -- 0; i.e., (52--(51) and from a "radial earthquakescarps along the major faults are associatedwith en BELLIER AND ZOBACK: STRESS CHANGE IN WALKER LANE ZONE 577

31 . 32 N 33 & 34

N251'-7'

N265'-6'

N91o-6 ø _ 5

10 20 30 I'T, s) 10 20 30 40 50 I'T, sl 10 20 301'T, sl

35 & 36 . 37 & 38 . 39

N286 ø N272'-20 [• •' N262'-15 I•

= . 1 10 20 30 401'T, sl 101'T, sl 10 20 301'T, s)

. 40 PIv . dv (Pleasant V.) (Dixie Vail.)

10 Ngg'-I' Ng2'-18' -• 5 3 Nl19'-5'

R=0.84 R=0.41 10 20 301'r, sl 10 20 30 ('r', s) 10 20 30 ('r', s) Figure 6. (continued) echelon ruptures and discontinuous ruptures, strongly Owens Valley fault zone and located 600-700 m to the west suggestiveof a strike-slip componentin the last event [e.g., (Figure 4c). (See appendix for more a detailed descriptionof dePolo et al., 1989, 1991]. Major (at the fault scarp offset this site locality.) scale) and minor (at the slickensidescale) deformation along The young strike-slip striae along the Owens Valley fault the Nevada Seismic Belt shows evidence of a recent change in zone indicate a slip direction which is in excellent agreement stressstate. This changeis clearly demonstratedby familiesof with the historic (1872) seismic slip on the main fault zone, crosscuttingstriae which show differencesin rake angle of 65 as determinedby detailedsurface fault offset studies[Beanland + 15ø (Figure 3) measuredalong both the OwensValley and the and Clark, 1987, 1995] (Figure 3). Similarly, the youngest Rainbow Mountain fault zones. Along both fault zones the striae along the Rainbow Mountain fault zone are also in good older striae set (marked by a .1 at the end of the fault plane agreementwith slip determinedfrom focal mechanismstudies number in Figure 3) records a normal faulting slip episode of the 1954 earthquakes [Doser, 1986, 1988]. Thus slip overprinted by younger (marked by a .2 at the end of the fault vectors recorded both by the youngest near-surface striations plane numberin Figure 3), right-lateral strike-slipfaulting. As and historicearthquakes appeared to have occurredin response shown in Figure 4, this youngestslip is recordedgeologically to the same or very similar stress state. The clear in parallel, rough grooves and thin frictional strike-slip chronological relationship between an older normal faulting striations along small faults in basalts dated at 288 + 70 ka event and a younger strike-slip faulting event recorded by [Turrin and Gillespie, 1986] which are parallel to the main crosscutting striae in young basalts adjacent to the Owens 578 BELLIER AND ZOBACK: STRESS CHANGE IN WALKER LANE ZONE

. rm . Fairv Mount' w Pea

5 •/ N2•4

10 20 30 ('T, s) 10 20 30 ('r, s)

ns l& 2

N88ø-2 o

10 20 30('r, sl Figure 6, (continued)

Valley fault zone is consistent with the contrast between (plv, fairv, rm, oling and owens2 in Table 4) and one large-scale late Pleistocene pure dip-slip displacement and combined (fairv&rm in Table 4) data sets are shown on the dominantly strike-slip 1872 earthquake offset on subparallel SS/GEO lower hemisphere stereoplot in Figure 5. A mean major faults in the Owens Valley region time reported by regional stressstate was determinedfrom the horizontal stress Zoback and Beanland [ 1986] and Zoback [ 1989]. axes determined by both standardand fixed inversions using Geologic evidence for recent strike-slip the Fisher statistic method (modified by [Watson, 1960]). The faulting stress regime. Striae indicating very young Fisherian mean o3 axis for the results from these 12 strike-slip deformation were measuredat five sites, all along inversions(SS/GEO in Table 4 and Figure 5) trendsN69_+11 øW fault zones with historic offsets; the Olinghouse, Fairview (with a plunge of 1ø to the ESE), while the Fisherianmean ol Peak, Rainbow Mountain, PleasantValley, and Owens Valley axis trendsN21_+17øE (with a plungeof 8ø to the NNE). fault zones (Figure 1). The slip data were measuredboth on Although the inferred stress directions did not vary more major fault planes and within adjacent secondaryfaulting in than -10-15 ø between the fixed and the standard inversions, late Neogene and Quaternary volcanic rocks as well as in the R values were, in some cases,significantly different (Table Paleozoic and Cenozoic bedrock; the measured slip data are 4). Only the R values from the standard(not fixed) inversions shown on lower hemisphere stereoplots in Figure 11. The are considered reliable since the criterion of 4 independent results of the stress inversions are tabulated in Table 4. fault sets (the minimum to define the complete stresstensor Unfortunately, none of the inversionsfor the individual young and R value was not met by the smallerdata sets).The R values strike-slip sites can be consideredwell constrainedbecause all determined by the standard inversions are consistently of the data sets contained only a small number of striae on between 0.0 and 0.33, with a mean value of 0.25 (R(*) -- 0.25 fault planes with limited distributions, generally only in Table 4, also see histogram on Figure 5). Recall that an R defining three independent sets of fault planes. For this value close to 0.0 indicates a stress state transitional between reason, all of the data were also inverted using a fixed strike-slip and normal faulting. inversion in which the stress axes are restricted to lie in In an attempt to constrainthe parametersof this strike-slip horizontal and vertical planes; the results of the fixed stress state at a regional scale, we also did a combined inversions are indicated by asterisks in Table 4. Both the inversionincluding slip data from all five sites.The data were standard and the fixed inversions indicate a NW to west separated into two subsets, one containing striae trending Ohmindirection (03 axes varying betweenN41øW and measurements on the major fault planes and a second set N82øW). Interestingly, the orientation of the horizontal stress including striae on minor fault planes adjacent to the main axes were generally very similar betweenthe standardand the exposed fault plane. The inversion of the major fault fixed inversions (typically within 5-10ø). Individual site 03 population (SS/g-major in Figure 11 and Table 4) yields a and o• directionsare shownin Figure 12. well-constrained and stable result (mean deviation angle of The subhorizontalstress axes (03 and o•) determinedby 5.7ø + 8.3ø) with a horizontalN65øW trendingo3 axis and R = both the standard and fixed inversions of the five individual 0.11, indicating a regional strike-slip stressstate transitional BELLIER AND ZOBACK: STRESS CHANGE IN WALKER LANE ZONE 579

120øW 118øW !!6øW

40øN

ß o øFrei

CARS Tahoe Lak

Walke • 19' akex %

38'N ono Lake

Quaternary sedimentaryrocks owens

Quaternaryvolcanism

[ Pre-Quaternarybedrock Unconstrained(r 3 direction '•'U•()wensl.ake I (T 3 direction 36'N

Well-constrained (T 3 direction

PLIO (?)QUATERNARY EXTENSION m '

Figure 7. Azimuthsof (73 (least horizontalstress) axes for the normal faulting stressregime deducedfrom fault slip inversionsgiven in Table 3 and shownin Figure 6. Larger solid arrowsplot (73axes determinedfrom well-constrainedinversions (pluses in Table 3); smaller solid arrowsare deducedfrom the poorly constrained inversions(minuses in Table 3); and the dashedarrow is a graphicallydetermined (73 direction compatible with the few normalslip datameasured at Fairview Peak. 58O BELLIERAND ZOBACK: STRESS CHANGE IN WALKER LANE ZONE

NNW

Figure8. Viewlooking southwest ofthe Genoa fault scarp. Arrows point to the two fault slip measurement sitesalong this fault zone (sites 19 and 20; see Table 2 forprecise locations).

to normal faulting. Resultsof the combinedinversion using Present-day strike-slip stress regime inferred data from significantminor faults at all five sites(SS/g-minor from historic seismic offsets. A NE to NNE trending in Figure 11 and Table 4) yieldedvery similarresults with a zoneof activeseismicity crossing the WLZ region(the Nevada N70øW {53axis and with R = 0.19. seismicbelt in Figure1) hasbeen the locus of a seriesof large As can be discerned from Table 4, there is remarkable (M>6) earthquakesaccompanied by surface-faultingevents similaritybetween the three geologicallydetermined regional duringthe lastcentury [e.g., dePolo et al., 1989, 1991; strike-slipstress states, inversions SS/g-major and SS/gminor Wallace,1979, 1984b, 1987; Bell, 1981,1984a, 1984b; Bell as well as the Fisherian mean based on individual site et al., 1984]. Twelve NNW to NNE trendingfaults were inversions, SS/GEO. All three of these separately inferred reactivatedalong this zone duringearthquakes occurring stressstates are characterizedby {53 axes trendingN65-70øW between 1872 and 1986 (Table 5). These faults representa and R valuesranging between 0.11 and 0.25. varietyof structuralstyles from right-lateral strike-slip, and

Figure9. Viewlooking northeast of the eastern Dry Valley fault scarp. Arrows point to thefault slip measurementsites (site 5' seeTable 2 for location). BELLIER AND ZOBACK: STRESS CHANGE IN WALKER LANE ZONE 581 582 BELLIER AND ZOBACK: STRESS CHANGE IN WALKER LANE ZONE

Fairv rm •___•__.....(Fairv iew Peak) • ,•T•--•.(Rainbow Mountain) • Fairv& rm

102•0 30 (T, s) 1020 30(T, s) 1020 30(T, s) 0ling PIv Owens 2 (Pleasant V.) (Owens V.I N319ø-0 N306ø-0

10 20 30 401'T, s) 10 ('r, s) 10 20 30 40 50 60 SSlfm SSlg minor !gmajor •.(f•ocaI mechanisms)

1 N293"-0

Nl10ø-0 o

11 R=0.19 R=0.11 NILS"5ø•Li R=0.20 10 20 IT, s} 10 20('T, sl 10 2o 30 13' Figure 11. Lower hemispherestereoplots of strike-slip faulting slip data from the Walker Lane region, togetherwith inversionresults presented in Table 4. Labelsoutside and to top right of the stereoplotsrefer to site namesand numbersshown in Figure 1 and describedin Table 2. (See Figure 6 for a detailedexplanation of the stereoplotsand histograms.)Also given are stereoplotsshowing the data and inversion results for three combined"regional" data sets listed in Table 4. Abbreviationsinclude SS/g-major,combination of all strike- slip data measuredon major faults at all five sites;SS/g-minor, combination of all slip data measuredon more significantminor faults at eachof the five sites;and SS/fm, inversionof the best constrainedfault planesand slip vectorsfor historicearthquakes. The individualfault planesand slip data shownon SS/fm come from Table 5; the fault numberson this stereoplotrefer to the numbersin the secondcolumn of Table 5.

right-lateral oblique-slip to normal dip-slip faults. The Nevada SeismicBelt variesfrom predominantlynormal dip historicfaults are, from northto south(see Figure'l); the 1915 slip in the northernpart of the zone(the PleasantValley fault) Pleasant Valley, the 1954 Dixie Valley, the 1954 Rainbow to oblique right-lateral slip in the central part of the zone Mountain, the 1903 Wonder, the 1954 Fairview Peak, the (Dixie Valley/Fairview Peak/Rainbow Mountain faults) to 1932 Cedar Mountain, the 1986 Chalfant Valley, the 1980 nearly pure right-lateralstrike-slip events in the southernmost Mammoth Lakes, and the 1872 Owens Valley faults. part of the seismicbelt (ChalfantValley and OwensValley Geological field data and focal mechanisms [e.g., Doser, faults). 1986, 1988; Zoback, 1989; dePolo et al., 1989, 1991] In addition,several other historic earthquakes have occurred indicate that the style of slip in these earthquakesalong the in the WLZ with sparselydistributed surface faulting. These BELLIER AND ZOBACK: STRESS CHANGE IN WALKER LANE ZONE 583

Table4. Resultsof StressTensor Inversion for SlipData Representing Strike-Slip Faultin8 Stress Regime ...... •,,•,de• Site N • •2 •3 M.D. S.D. R Age Qual Pleasant Valley Fault plv 10 191/14 017/76 281/02 04.6 05.6 0.40 eMz - plv 10 188/00 042/90 278/00 06.7 07.2 0.33 eMz * Faimiew Peak and Rainbow Mountain Faults fairv 04 007/41 212/47 108/13 04.1 05.2 0.70 mM+IM - fairv 04 188/00 041/90 278/00 11.7 16.1 0.33 mM+IM * rm 06 033/52 199/37 294/07 09.3 10.3 0.23 mM-1M - rm 06 200/00 065/90 290/00 10.3 14.4 0.20 mM-1M * hirv & rm 10 016/21 185/68 285/04 11.5 14.4 0.31 10+M - hirv & rm 10 193/00 050/90 283/00 12.8 16.0 0.32 10+M * OlinghouseFault oling 12 222/11 352/74 129/12 09.6 12.9 0.50 mP-1P - oling 12 229/00 094/90 319/00 12.4 16.2 0.00 mP-1P * Owens Valley Fault owens2 37 016/20 212/70 108/05 12.1 16.2 0.96 P1 - owens2 37 216/00 106/90 306/00 18.9 24.9 0.32 P1 * Regional Results SS/GEOa 21!l 7/08 11!l 1/01 0.25 SS/g-minorb 12 017/18 229/70 110/10 08.4 10.1 0.19 Mz-Q + SS/g-majorc 19 025/02 272/85 115/05 05.7 08.3 0.11 Mz-Q + SS/fmd 13 199/38 034.51 295/07 10.8 13.7 0.28 Historic - SS/fmd 13 203/00 056/90 293/00 13.5 16.0 0.20 Historic * Descriptionof individual columns same as in Table3. Notethat plv representsinversion of strike-slipstiae measuredalong the Pleasant Valley hult zone(plv l to plv5). a SS/GEOis themean regional horizontal deviatoric stress axes determined using Fisher statistics independently on the two subhorizontalstress axes from eachof the individualinversions, arithmetic mean R valuecomputed onlyfrom fixed inversion results (* in Qualcolumn). bSS/g-minoris an inversion of all slip data measured onminor hults adjacent tothe main hult zone at each of the five sites. c SS/g-majoris an inversion of all slipdata measured onthe major faults zones at each of thesites. d SS/fmis aninversion of historichult slipalong the Nevada Seismic Belt and adjacent parts of theWalker Lane zonecomputed using parameters forhistoric hult slipreported in Table5 andshown in Figure12, both poorly- constrained(-) and fixed inversion(*) resultsgiven.

distributed surface ruptures include the ENE trending et al., 1985; Moos and Zoback, 1993]. For this reason, we Olinghouseand ExcelsiorMountain faults, the north trending have not included focal mechanismsof any of the Mammoth Fort Sage Mountain fault, and the NE trendingTruckee fault Lake earthquakeswithin this present-daystate of stressstudy. (Figure 1). The historic slip in these earthquakeswas a The availableslip vectorsfor the historicearthquakes along combinationof right-lateral and normal faulting on NNW to the Nevada Seismic Belt and within the WLZ deduced from NNE trending faults and predominantlyleft-lateral to normal publishedfocal mechanismsor from analysesof geological faultingon ENE to WNW trendingfaults. We havetabulated in surfacerupture are given in Table 5 and are plottedon the Table 5 all the available information on slip vectorsfor these stereonet SS/fm in Figure 11. The inferred, contemporary historic events. mean stress state deduced from a standard inversion of these Seismic activity has also been intense in the Mammoth data is a strike-slip stress regime characterized by Lake/Long Valley calderaregion just southof Mono Lake. In subhorizontalN65øW trending(53 (lJhmin) axis, with (51and (52 additionto a May 25, 1980, M=6.3 earthquakenear Mammoth lying in a well-constrainedplane perpendicularto this Lake, swarmlike sequencesof several (M>5) earthquakes direction,and with (52having the steepestplunge (51ø toward occurredin October 1978 and May 1980 in the MammothLake the NE) (Figure 12 and Table 4). An R valueof 0.28 for this region [Ryall and Ryall, 1981a,b;Hill et al., 1985]. Most SS/fm inversion indicates a stressstate where (51 is close in focal mechanismsobtained for these earthquakesshow a mean magnitudeto (52,explaining the clear definitionof the (5•-(52 N65øE trendingT (tension)axis, nearly40 ø to 50ø obliqueto T plane but not the unique definition of the individual axes. An axesof earthquakesin the surroundingregion [Lide and Ryall, inversion fixing one of the principal stressesto be vertical 1985; Vetter and Ryall, 1983; Vetter, 1990]. Deformationin yielded a (53 axis trendingonly 2ø differently and a similar R the Mammoth Lake and Long Valley calderaregion appearsto value (R=0.20) confirming that the present-day strike-slip be a local anomaly in the regional stress pattern and is stress state is very close to transitional between the strike- possiblyrelated to active magmaticresurgence superimposed slip and normal faulting stressregimes. on regionalextension of the Basin and Range province[Hill In conclusion, a recent strike-slip stressregime along the 584 BELLIERAND ZOBACK: STRESSCHANGE IN WALKERLANE ZONE

120'W 118'W 116'W

40øN

Tahoe Lake

38øN one Lake

Quaternary I:ø '- ;1

;•.• volcanism Quaternary

bedrock • Pro-Quaternaryo'1direction '•Oowens wens / a ke o'3direction i 36øN

RECENT STRIKE-SLIP REGIME

J 100Km

Figure12. Azimuthsof {71(maximum horizontal stress) and G 3 (leasthorizontal stress) axes for thestrike- slipstress regime deduced from the individual fault slip inversions given in Table4. Large,open arrows, representingthe{73 (least horizontal stress) axes determined from the normal fault slip inversions along the PleasantValley and Dixie Valley fault zones (from Table 3), aregiven for comparison. BELLIER AND ZOBACK: STRESS CHANGE IN WALKER LANE ZONE 585

Table 5. SeismicFault Slip From HistoricalEarthquakes Fault Fault Date Magnitude FaultPlane b Methodc Reference Numbera Strike Dip Rake de• de[[ deg Olinghouse 1 Dec. 27, 1869 M1 6.7 45 60 -30 G 1, 2 Owens Valley 2 March 26, 1872 Mw 7.25-8 160 85 - 170 G 3 Owens Valley March 26, 1972 Mw 7.25-8 195 89 - 160 G 3 Wonder 1903 ? M1 5.5-6.5 N-S G 4 Pleasant Valley 3 Oct. 2, 1915 Ms 7.6 25 59 -94 G 5, 6 Pleasant Valley Oct. 2, 1915 Ms 7.6 14 44 -119 FM 7 Cedar Mountain Dec. 21, 1932 M1 7.2 181 72 -176 FM 7, 8, 9 Cedar Mountain 4 Dec. 21, 1932 M1 7.2 167 81 -179 FM 7 Excelsior Mtn. 5 Jan. 30, 1934 M1 6.3 49 54 -53 FM 7, 10, 11 Excelsior Mtn. Jan. 30, 1934 M1 6.3 68 40 -90 FM 7 Fort Sage Dec. 14, 1950 M1 5.6 N-S G 5, 12, 13 Rainbow Mtn. 6 June 6, 1954 Mw 6.2 156 80 -140 FM 4, 5, 14, 15 Rainbow Mtn.-1 June 6, 1954 Mw 6.2 160 60 -115 FM 14 Rainbow Mtn.-2 7 June 6, 1954 Mw 5.9 165 60 -135 FM 14 Rainbow Mtn.-3 8 Aug. 24, 1954 Mw 6.5 175 50 -125 FM 4, 5, 14, 15 Fairview Peak 9 Dec. 16, 1954 Mw 6.9 170 60 -160 FM 4, 5, 14, 15 Dixie Valley Dec. 16, 1954 Mw 6.7 170 50 -90 FM 14-17 Dixie Valley 10 Dec. 16, 1954 Mw 6.7 169 62 -152 FM 18 Dixie Valley 1 1 March 23, 1959 Ms 6.3 ? 168 46 -168 FM 14 Truckee 12 Sept. 12, 1966 Ms 5.7 224 80 -1 FM 19, 20 Mammoth Lake May 25, 1980 M1 6.3 FM 21, 22, 23 Chalfant Valley July 21, 1986 Ms 6.2 335 60 -179 FM 24 ChalfantValley 13 July 21, 1986 Ms 6.2 335 59 -152 FM 25, 26 a Fault numbersrefer to the stereoplotin Figure 12. Where two fault slip parametersare given for the same earthquake,the numberedfault parametersare consideredto be the best determinedand are used in the inversion. The result of standardand fixed inversionsof this data set are given as solutionSS/fm in Table 4. b Faultplane slip vectors are specified by strike,dip, and rake using standard seismological convention. Strike azimuth measuredclockwise from north, 90 ø less than dip direction; dip measuredfrom horizontal; and rake gives the direction of motion of the hanging wall with respectto the footwall measuredcounterclockwise from the strike direction (positive rakes indicate thrust faulting, negative rakes indicate normal faulting, rakes with absolute values < 90 ø are left-lateral, rakes with absolutevalues > 90 ø are right-lateral). CMethodindicates how the mean slip vectorwas determined;G, determinedgeologically from surfacerupture and displacementsand FM, determinedfrom an earthquakefocal mechanismwhere the fault plane was selectedusing the surface fault trace. d Referencesare as follows: 1, Slemmons[1977]; 2, Sandersand Slemmons [1979]; 3, Beanlandand Clark [1995]; 4, Slemmons et al. [1959]; 5, Bonilla et al. [1984]; 6, Wallace [1984b]; 7, Doser [1988]; 8, dePolo et al. [1987, 1989, 1991]; 9, Molinari [1984]; 10, Ryall and Priestley [1975]; 11, Slemmonset al. [1965]; 12, Gianella and Callahan [1934]; 13, Gianella [1957]; 14, Doser [1986]; 15, Slemmons [1957]; 16, Bell and Katzer [1990]; 17, Zoback [1989]; 18, Romney [1957]; 19, Ryall et al. 1966]; 20, Tsai and Aki [1970]; 21, Ryall and Ryall [1981a]; 22, Hill et al. [1985]; 23, Clark et al. [1982]; 24, Cockerhamand Corbett [1987]; 25, Gross and Savage [1987]; 26, Lienkaemperet al. [1987] and dePolo and Ramelli [1987].

Nevada Seismic Belt and within the WLZ has been of a previousanalysis of well-constrainedmajor fault offsets demonstratedby fault slip inversions on both the youngest in Owens Valley by Zoback [1989]. Whether there is one, geologically measured striae and inferred earthquake slip single changein stressregime, or fluctuationsor temporal vectors. All inversionssuggest that the present-daystate of variations in stress regime, can not be resolved with the stressis a strike-slip stressregime close to transitional to a existing data. normal faulting stressregime (low R value), explaining the compatibility between the observed normal, oblique, and Explanations for Slip on Subparallel Strike-Slip strike-slip active faulting. Even the geologically determined, and Normal Faults dominantly normal slip (rake = 86 ø) in the 1915 Pleasant Valley earthquake is compatible with this strike-slip stress Wright [1976] explained the complex interaction of active tensor because the mean fault trend is approximately strike-slip, oblique, and normal dip-slip faulting within the perpendicularto the c53(lJhmin) direction. As discussedin the WLZ as a simple consequenceof a single stress state, one beginningof the Results section,slip-vector chronologies on transitionalbetween strike-slip and normal faulting OJHmax= fault planescutting recent depositssuggest a recent change c5v, or R ,• 0 in both regimes) with minor local fluctuations. from a normal to a strike-slipfaulting in stressregime with a However, as summarized by the data presentedabove, while consistentWNW trending c53OJhmin ) axis, supportingresults the WNW orientation of the least principal axis appears to 586 BELLIER AND ZOBACK: STRESS CHANGE IN WALKER LANE ZONE remain approximately constant, substantial changes in field, one in which IJHmaxis orientednearly perpendicular(80- relative stressmagnitudes are required to explain the observed 85ø) to the strike of the main strike-slipfault. In both areasa deformation in this region. The contemporarystress field in small component of right-lateral shear allows the "weak" the WLZ does appearto be characterizedby a strike-slipstress strike-slipfault to slip, while the normalcomponent of plate regime transitional to normal faulting (with R • 0.1-0.2); motion is accommodated on subparallel thrusts. In fact, however, this stress state is incompatible with the slip data Zoback et al. [1987] have argued that the regional from the best constrained normal fault inversions (which approximatelyfault-normal orientation of IJHmaxin an-100- requirea normalfaulting stressregime with R -- 0.7) as well as km-wide zone on both sides of the plate boundary in with the observations of crosscutting striae with highly California is a consequenceof the very low shearstrength of oblique rakes (65ø _+15 ø) observedon severalfault planes. the main San Andreasfault zone; the stressrotation occurring These data suggesttemporal changesin stressregime in the becausethe San Andreasfault zone is nearly a principalstress WLZ and, specifically,a recentchange (possibly post late plane, thus, the shear stressacross it must be reduced. Hence, Pleistoceneand pre-Holocenebased on the crosscuttingstriae while the strain field is partitionedto dip-slip movementon in Owens Valley) from a dominantlynormal faulting regime to dipping faults and strike-slip motion on subparallelvertical a strike-slip regime. These results are consistent with faults, the stressfield responsiblefor both setsof slip appears uniform. conclusionsbased on a previous analysis of large-scale late Pleistocene fault offsets in the Owens Valley region which In the contextof thesediscussions of the San Andreasfault, indicate a relatively recent (post-300,000 year, pre-10,000 it is clear that strain partitioning within a uniform stress year) changefrom a normal faulting stressregime with a high modelrequires marked variations in the relativeshear strength R value to a strike-slip faulting regime with a low R value of vertical strike-slip and subparallel dipping faults. [Zoback, 1989]. The change from a normal to a strike-slip Wesnousky and Jones' analysis suggests that the Independencenormal fault zone in OwensValley mustbe at faultingstress regime can be interpretedin termsof a temporal and/or lateral variation in the magnitudeof one or more of the least4 timesstronger than a verticalOwens Valley strike-slip stresses. fault (their figure 3). Unlike the manypapers written about the Recently, Wesnouskyand Jones[1994] suggestedthat the absoluteand relative weakness of majorplate-boundary faults combined normal and strike-slip deformation in the Owens like the San Andreasfault, Wesnouskyand Jonessimply Valley and surroundingWLZ can be explainedsimply as a assumethe weaknessof the strike-slipOwens Valley fault consequenceof strainpartitioning of dip-slip and strike-slip relative to the Independencefault without presentingany displacementon subparallel normal and strike-slip faults evidenceto supportthis assumption.This assumptionrequires that the discontinousstrike-slip faults within the WLZ are within a singleregional stress state (provided there are marked much weaker than the major, subparallel,continuous Sierra differencesin the frictional strengthof the dip-slip and strike- Nevada frontal fault system. Thus in rejecting possible slip faults). They argued that the shallow rakes (strike-slip temporalvariations in the stressfield to explaindeformation movement)inferred for the steeply-dippingOwens Valley fault in the WLZ, Wesnouskyand Jones [1994] offer a "simpler" zone could also be compatible with a number of stressstates hypothesis of strain partitioning that requires marked which would also permit the observeddip-slip movementon differencesof the strengthof faultswithout any independent the Independencefault (within the uncertainitiesof published substantiatingevidence. data) by correctlypoint out a problemwith the stressanalysis Consideringthe conceptof strainpartitioning in general,it based on slip on only these two faults: for horizontal and couldbe assumedthat the Sierranfrontal normal fault system vertical stress axes, a near-vertical fault (900+5ø) will always is a weak fault controlling the stress field. However, the exhibit nearly pure strike-slip motion, regardlessof the stress observednearly pure dip-slip movementon this fault would regime or R value, as long as there is shear stressin the requirelittle or no shearstress in the horizontalplane and/or horizontalplane (that is, IJHmax:/: IJhmin ). However, we think an IJHmaxoriented subparallel to the Sierran frontal fault that the crosscutting strike-slip and normal fault striae (-NNW). Neither case would be conduciveto slip on the reportedhere on the samefault planeswith moderatedips (dips subparallelOwens Valley fault zone,requiring that the strike- between40 ø and 58ø in Owens Valley and between56 ø and 59ø slipfault would also have to be weakin orderto slip! along the Rainbow Mountain fault zone) supportthe earlier Inversion of the slip data presentedhere as well as the conclusionof temporalvariations in stressregime, rather than alignmentof young volcanicvents suggesta NNE to north simple strain partitioning within a single regional stress trending IJHmaxorientation in this region (not NNW); the field. However, the conceptof strainpartitioning is currently nearlypure dip-slipmovement on the NNW trendingfrontal being widely discussedand its relationshipto regional stress fault requiresa normalfaulting stress regime in whichthe two state deserve more discussion. horizontalstresses are approximatelyequal (R .• 1.0). The The combined strike-slip and thrust deformation on northto NNE IJHmaxorientation is compatiblewith activation subparallelfaults within the San Andreasfault systemcould be of the NNW trendingOwens Valley strike-slipfault zone; viewed as an analog to deformation in the WLZ. Extensive however,a changein relativestress magnitudes is requiredin stressstudies [Mount and Suppe, 1987, 1992; Zoback et al., order to have enough horizontal shear stressto drive the 1987; Oppenheimer et al., 1988] indicate that the strain strike-slip faulting. That is, while the orientation of the partitioning along the San Andreas fault system and also principal stress axes remains constant,the apparently adjacent to the similar Great Sumatranright-lateral strike-slip youngerdominantly strike-slip deformation on subparallel fault (in an oblique subductionenvironment [see Mount and faultsoccurs in a stressregime transitional between strike-slip Suppe, 1992]) occur in responseto a uniform regional stress andnormal faulting (IJHmax • IJhmin or R -• 0.1-0.2). BELLIERAND ZOBACK: STRESSCHANGE IN WALKERLANE ZONE 587

Table6. An,eularDifference Between Measured and Predicted Slip Vectors on Owens Valley Fault FaultData SS/l•-major,del• SS/l•-minor,del• SS/GEO,de• SS/fm,de[• NF/GEO,deg 1872 slip 0 3 8 7 40 2.2 (younger) 15 14 17 18 7 2 2.1 (older) 61 62 58 58 4 Values are the deviationangles (the differencebetween predicted x and measureds slip vectors)for the various regionalstress states indicated, NF stressstate from Table 3, SS stressstates from Table 4. See text for discussion of significanceof resultspresented here.

Previouslymentioned crosscutting dip-slip and strike-slip stressmagnitudes. Because the two stressstates have the same striae are further evidence of variations in relative stress principal axis directions (coaxial), they can be easily magnitudeswithin this actively deformingregion. Striae sets represented on the same Mohr's circle. The stress values 2.1 and 2.2 (Figure 3) were measuredon a minor fault in a shown on the Mohr's circle in Figure 13 correspondto 7.5 km <300,000-year-oldbasalt flow adjacentto the Owens Valley depth, roughly half the thickness of the brittle layer in the fault zone and are an excellentexample of very youngchanges Basin and Range as defined by seismicity, and hence they in slip on the same fault plane. To quantitativelytest whether should represent approximate mean stress values within the theseslip data or the slip in the 1872 OwensValley earthquake brittle layer. As is clear on the Mohr's circle, the primary (from Table 5) could be explainedby strainpartioning in a difference between the two regimes is a roughly 80 MPa singleregional stress field, we computedpredicted slip vectors difference in the relative magnitude of the maximum on these fault planes using the regionally significant strike- horizontaleffective stress (ONNE-P, which is O1in the strike- slip stress states (SS/g-major, SS/g-minor, SS/GEO and slip regime, o2 in the normal faulting regime). SS/fm) as well as the "earlier"normal faulting (NF/GEO) stress The sourceof either a lateral or temporal variation in ONNE statedetermined in this paper.The deviationangles between (OHmax)magnitude is enigmatic.As Zoback [1989] pointed the predictedand observedslip are tabulatedin Table 6. Note out, tectonicprocesses related to the Sierra Nevada-Basinand that the deviation angles for the most recent deformation Range boundaryzone such as stresseffects due to the abrupt (Owens Valley earthquake and striae set 2.2) are very and high topographyof the Sierra or sheartractions due to the consistentwith the computedregional strike-slipstress states lateral contrast in crust/lithospherestructure and/or heat flow (deviationangles between 0 ø and 18ø) and are incompatible should primarily affect the magnitudeof the WNW horizontal with the "older" normal faulting stressregime (deviation stress (o3). The -80 MPa difference in ONNE magnitudes anglesbetween 40 ø and 72ø). In contrast,the slip represented between the two regimes is much greater than the stress by the older striae set, 2.1, is well predicatedby the normal change associatedwith an individual large earthquakein the fault stressstate (deviationangle = 4 ø) and is incompatible with the strike-slipstress states (deviation angles = 60ø___2ø).

These resultssuggest that the deformationalong the Owens 50- Valley fault zone can not be explainedby strainpartitioning within a single regional stressfield; the normal dip slip movementappears related to a normal faulting stressregime, while the strike-slip faulting is compatible with a recent • effective strike-slip faulting stressregime. tnorrn•alstress (MPa) The postulated change in stress regime from a normal (oWNW- P) 50 loo (Ov- P) (ONNE- P)SS faultingregime with R -- 0.75-1.0 to a strike-slipregime with ! R -- 0.1-0.2 could be accomplishedby a temporalvariation in the magnitudeof the maximumhorizontal stress (ONNE). To demonstratehow different these two stressstates are, we can examinechanges in the magnitudeof the maximumhorizontal normal faultingregime stress required by the "frictional strength of optimally- - ß - strike-slip faulting regime oriented faults" model to predict crustal stressesat depth Figure 13. Mohr's circle representationof effective stress [Jaegerand Cook, 1979, p. 80]): states(P-P) for the two coaxial stressregimes determined for the WLZ; the normal faulting stress regime (R=0.15) is (Oi_p)/(03 _ p) = [(g2+ 1)1/2+ g]2 (1) representedby the solidcircle, and the strike-slipstress regime (R=0.75) is given by the dashedcircle. In both regimesthe where P is pore pressure,and g is the sliding frictional minimumhorizontal stress is o3 and is givenby OWNW;the coefficient on the most well-oriented faults assumed to control maximumhorizontal stress is given by ONNE;and the vertical frictional strength.Using g=0.65 as a typical value of crustal stressis givenby Ov (equalso2 in the strike-slipcase, o l in friction (Byerlee's law [Byerlee, 1978]) and assuming thenormal faulting case). Stress magnitudes determined using hydrostaticpore pressure(P=(1/2.67)ov=O.3737ov), Ol/O 3 the frictional strengthof optimally orientedfaults model to valuescan be computed.Assuming the verticalprincipal stress predictcrustal stresses at depth(see text). Note that the primary is equalto the lithostat(pgz) and utilizingmean R valuesfor differencebetween the two stressstates can be explainedby a the two regimes (R=0.15 in strike-slip regime, R=0.75 for relative change in the maximum horizontal effective stress normal faulting regime), we calculatedthe predictedeffective (ONNE-P) of- 80 MPa. 588 BELLIER AND ZOBACK: STRESS CHANGE IN WALKER LANE ZONE

region(earthquake stress drops are typicallyof the orderof axis and with the maximum horizontal stressapproximately 0.1-10 MPa [Thatcher and Hanks, 1973]). The differencesin equal to vertical stress,or R -- 0 in both regimes). However, stressmagnitude between the two regimescould be much such a stress state is not consistent with observed post- smaller in the case of elevated pore pressurein the source Pleistocene, nearly pure dip-slip movement on the Sierran regionwhich would reducethe overallstress differences frontal fault zone. An alternative, strain-partitioning model (53)required to causefaulting [e.g., Zoback, 1992]. has been proposed to explain slip on the subparallelstrike- Unfortunately,the timing of the temporal variationsin slip and normal faults in the Walker Lane zone within a single stress state inferred from the geologic data is too poor to uniform regional stress tensor, this model requires that the determineif there has been a singlerecent absolute change in discontinuousstrike-slip faults be substantiallyweaker than stressmagnitude or if this variationis fluctuatingor cyclical. the main Sierran frontal fault system. Stewart [1992] noted that strike-slipdeformation has occurred Our slip data, including crosscuttingstriae on the same fault in the WLZ throughoutthe late Tertiary, as well during the planes, supportprevious results from Owens Valley [Zoback, Mesozic when the faultingmay havebegun as intra-arcor back 1989] that indicate deformation within temporally distinct arc strike-slipfaults. He also pointsout that the present-day extensional and strike-slip stress regimes with a roughly WLZ is largely coextensivewith the Miocene (20-24 Ma) constant WNW trending (53 axis. A recent change from a magmaticarc and attributesmuch of the pre-5.5m.y. old shear normal faulting to a strike-slip faulting stress regime is deformation in the WLZ to intra-arc strike-slip faulting indicated by the crosscutting striae on faults in rocks subparallel to the active offshore obliquely convergent <300,000 years old and is consistent with the dominantly subductionzone, a style of deformationobserved in a number strike-slip earthquake focal mechanismsand the youngest of present-day analogs [e.g., Fitch, 1972; Beck, 1983; striae observed on faults in Plio-Quaternary deposits. Jarrard, 1986; McCaffrey, 1991; Beck et al., 1993]. Thus Geologic control on the timing of the change is too poor to deformation in easternmost California and westernmost determineif there has been a single recentabsolute change or Nevadaappears to haveaccommodated a portion of the relative if there is an alternating or cyclic variation in stress plate motion along the westernplate boundarythroughout magnitudes. muchof the late Tertiary and possiblysince the Mesozoic. A change from a normal faulting stressregime with R -- The existenceof a present-dayNNW trendingzone of right- 0.75 to a strike-slip faulting stressregime with an R --0.1- lateralshear extending northward from the SanAndreas fault in 0.2, both with a WNW trending(53 axis, can be explainedby the south through the easternMojave Desert and up into an increase in IJNNE magnitude. Assuming that stress OwensValley carrying-8 mrn/yrof relativeplate motion has differencesin the crust are predictedby the "frictionalstrength been suggestedon the basis of geologicand geodeticdata of optimally-oriented faults" model [Jaeger and Cook, 1979, [Bird and Rosenstock,1984; Sauberet al., 1986; Dokka and p. 90] and using Byerlee'sLaw [Byerlee, 1978] and hydrostatic Travis, 1990a, 1990b; Savage et al., 1990] and recently by pore pressure, the primary difference between the inferred Ward [1990] using very long baselineinterferometry (VLBI) normal faulting regime and the strike-slip regime is an -80 measurements.Dokka and Travis [1990b] suggestthat this MPa difference in the magnitude of the effective maximum eastern California shear zone has carried between 18% and horizontal stress((5•4NE-P) for hydrostaticpore pressure.This 23% of Pacific-NorthAmerican plate motionover the last 5.5 value is far greater than typical earthquake stress drops, m.y. and they further suggestthat this intracontinentalshear suggestingthat the changes in stress regime are related to may continueto the north and includethe Walker Lane zone. something other than the simple stress changes related to These observationsall suggesta link between deformation individual earthquake cycles (or alternately, that stress alongthe southernmostpart of the NevadaSeismic Belt within differencesat focal depths may be smaller than predictedby the WLZ and broad-scale San Andreas plate motion frictional faulting theory, perhapsas a result of elevatedpore deformation. pressure).The location of the WLZ between the deep-seated regional extension of the Basin and Range and the right- Conclusions lateral strike-slip regional tectonics of the western plate boundary (presently the San Andreas fault) is no doubt Late Cenozoic tectonics in the WLZ are characterized by responsibleboth for the Plio-Quaternary tectonic regimes as normal,oblique, and strike-slipfaulting which reactivated well as much of the late Tertiary and Mesozoic deformational inherited Mesozoic structures and produced new faults. history of strike-slip faulting acting in this zone. The historic Inversionof fault slip data collectedalong both major and (possibly extending back through the Holocene) strike-slip minor fault zonesindicate two Plio-Quaternarystress states. A deformation in the WLZ may be in response to the normalfaulting stress regime with a mean(53 axis of N83øW developmentof a NNW trending zone of shearwhich may be anda meanR value(R=((52-(51)/((53-(51)) of 0.63 - 0.74 appears accommodatingroughly 20% of Pacific-North American plate to have been replacedin much of the region by a younger motion [Dokka and Travis, 1990b]. strike-slipfaulting stress regime with a (53axis of N65-70øW and a meanR value of 0.1 - 0.2. This youngerstrike-slip stress state is close to transitional to normal faulting and is Appendix: Detailed Description of Important consistent with focal mechanismsof the historic earthquakes Fault Slip Localities in the WLZ. Previous workers have explained all the complex patterns of strike-slip, oblique, and normal faulting in the Dry Valley (Sites 4 and 5) WLZ as a simple consequenceof minor fluctuationsabout a Sites 4 and 5 are located along the western and eastern such single transitionalstress state (with a WNW trending(53 borderfaults, respectively, of the Dry Valley graben.The data BELLIER AND ZOBACK: STRESS CHANGE IN WALKER LANE ZONE 589 were collected both on minor slickensides (centimeter to meter measured on inherited fractures which affect Cretaceaous •caic]•,• ...:,,•:w•tmn the c,au,t , zoncs and on the main t,•,,.,••,•,,•,e, ,. fault gr•n•tt•c and ctrnnr•rtlr•rit•c planes. All the sampled fault planescut late Miocene-Pliocene (17-6 Ma) basalt flows. The morphology of the western fault Smith Valley (Sites 25 and 26) scarp yields no indication of recent fault movement, whereas Sites 25 and 26 are located in Smith Valley along state the eastern fault scarp is exposed along a steep free face in highway 208, where the road crosseslow hills --4 to 6 km east basalts and colluvium which probably indicates recent of the Smith Valley airport. Minor normal fault zones (1 to 2 (Holocene) vertical fault displacement.Measurements for site m displacements) affecting Plio-Pleistocene fluvio-lacustrine 4 were made along a 3-km segmentof the east dipping range depositsare exposedin road cuts as illustratedin Figure 10. front scarplocated 5 to 8 km NNW of the Double Check Well near Flanigan. Site 5 representstwo localities along the west dipping range front fault zone adjacent to Dry Slick Hill and Wassuk Fault Zone (Sites 30 to 34) Mission Peak (which are < 1 km apart ); both localities are Sites 30 to 34 are located just east of U.S. highway 95, -10 km north of the Double Check Well. along the east dipping Wassuk range frontal fault zone. The striae measurementsrepresent deformation on small planes Winnemuca (Site 11) within the fault zone proper (sites 30 to 33) and on the actual Site 11 consists of three measurement localities on or close main bedrockfault plane (sites 33 and 34). Site 30 is -5 km to the west dipping Nightingalerange front fault escarpment north of Reese River Canyon; site 31 is between Penroc and which forms the easternedge of the Winnemucavalley. All DeadmanCanyons; site 32 is 3 km southof DeadmanCanyon; three localitiesare in canyonsalong a 5-km stretchof the fault site 33 is betweenCooper Canyon and Dry Creek; and site 34 (from -2 km north of the water tank of the "MGL mine" to 3 is along the main escarpmentjust north of CottonwoodCreek. km south of this water tank). The measured striae represent deformation on minor planes within the fault zone proper Pleasant Valley Fault Zone (Sites plvl-5) which affect Plio-Pleistocene fanglomerates and Late Sites plv l to plv5 are located along the central segmentof Mesozoic quartzites,rather than on the actual major fault the west dipping PleasantValley fault scarp formed in 1915, plane. specifically along the central part of the 30-km long Pearce scarp segment [see Wallace, 1984b]. Sites plv l to plv5 are Genoa Fault Zone (Sites 17 to 20) located from north to south where bedrock is exposed in the scarp face; plv l is at the latitude of the old Siard Ranch Sites 17 to 20 are located on or close to the east dipping damagedin 1915; plv2 is 4 km north of PearceRanch; plv3 is Genoa range front escarpmentof the Sierran frontal fault zone just south of Golconda canyon; and plv4 and plv5 are located which forms the western margin of Jack Valley. This Sierran -8 to 10 km southof PearceRanch. Data from plv l and 2 were frontal fault zone has a mean direction which trends N-S at collectedon minor (centimeterto meter scale) and major (more sites 18 to 20, while at site 17 the frontal fault trends NNE. Site 17 consists of several measurement localities of fault than 1 m scale) fault planes.Data from plv3 and plv5 represent deformation on small planes within the fault zone proper, planes affecting Cretaceousgranitic rocks along highway750, while plv4 data represent major slickensidesalong a major 2-3 km west of highway 395. One locality was in road cutsjust quartzite escarpment. north of the gauging station at Clear Creek, and the second was along natural exposuresalong the banks of Clear Creek. Dixie Valley Fault Zone (Sites dvl-3) Site 18 also consists of several measurement localities in Cretaceous granites in small canyons between Bennett and Sitesdvl to dv3 are locatedalong the centralpart of the east James Canyons, close to Jack Ranch. dipping Dixie Valley fault scarpformed in 1954 which forms Sites 19 and 20 record measurementsalong the main Genoa the easternborder of the Stillwaterrange, west of Dixie Valley range front fault zone and representtwo different kinds of fault road. Site localities from south to north are: in the vicinity of slip data. Site 19 is along the prominent main bedrock Sheep and Coyote Canyons (dvl), between Little Box and escarpmentexposed just west of state highway 206, -1 km Brush Canyons (dv2), and between Cottonwood and Hare south of the town of Genoa (this site is shown in Figure 8). In Canyons(dv3). The data were collectedon minor and major contrast, site 20 represents stiae measurementson minor slickensideswhich representdeformation just adjacentto the normal fault planes adjacent to the main range front fault fault front and along the major escarpment.The measuredfault which affects Cretaceaousgranitic rocks and are exposed in planes, at localities dvl and dv2, affect Oligocene to early road cuts of highway 206, 2 to 3 km southof Genoa. Miocene granites and granitic and metamorphicrocks with undifferentiated ages ranging between Jurassicand Miocene. Antelope Valley (Site 21) At site dv3, they affect Mio-Plioceneand early Pleistocene(?) volcanic rocks. Site 21 measurementswere madeon the eastdipping range front escarpmentforming the west margin of the Antelope Valley graben; this fault zone is part of the Sierran frontal Olinghouse Fault Zone (Site Oling) fault zone. Striae data were collected south of Coleville, This site is located along the range front fault, -2-3 km behind the Meadowcliff Motel, from both minor slickensides north of the "powerhouse"and gauging station near the along the range front fault scarpand on major fault planes Patrick interchange on Interstate 80, 26-28 km ENE of Reno. generally oblique to the frontal fault zone. The striae data were The slip measurements were made on major and minor 590 BELLIERAND ZOBACK:STRESS CHANGE IN WALKERLANE ZONE slickensidesalong a majorbasaltic escarpment within the relationshipsbetween an older normal striaeset and a younger southdipping segment of theOlinghouse fault scarp formed in strike-slip set. The majority of the data came from an en 1869. echelon, left-stepping east dipping scarp. However, the crosscuttingrelationships were observedalong a roughly1-m Rainbow Mountain Fault Zone (Site rm) high and 8-m wide west dipping fault scarp within this left- steppingen echelonscarp system (Figure 4). This site consists of several data localities along the RainbowMountain fault scarpformed in 1954,between 5 and Acknowledgments. This work was pan of the CooperativeResearch 10 km north of the U.S. highway 50 and just NNE of Salt Program between the U.S. Geological Survey (USGS) and the URA Wells. Measurements were made on minor fault planes CNRS G6ophysiqueet G6odynamiqueInterne. O.B.'s participationwas affectingboth rhyolitic and basaltic Miocene flows. made possiblein pan by a CNES (Centre National d'EtudesSpatiales) grant. Special thanks are due to J. H. Stewart, R. F. Hardyman,M. M. Owens Valley Fault Zone (Sites owens 1-2) Clark, and D. P. Schwartz,who providedmaps and expertiseon active faulting in the Walker Lane zone. We thank G. King for helpful The OwensValley sitesare all locatedalong the northern discussionsconcerning the stressstate changes in the Basin and Range segmentof the OwensValley fault zone(Figure 4c). At site province and Mark Zoback, Jack Stewart, and Michel S6brier for owensl (-2 km SSEof Big Pine)we observedonly normal slip helpful reviews which clarified our interpretations.We also thank E. on minor faults cutting the main east dipping major fault Carey-Gailhardis for modifying the orginal Carey stress inversion scarp.Site owens2, located about 6 km SSEof Big Pine,is the programto accommodatefixed inversionsand G. Roche for draftingof more importantlocality becauseit exposedcrosscutting someof the figures.

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of stressand style of tectonismof the Basin G6ophysiqueand G6odynamique Interne, Branchof Seismology,Mail (e-mail:zoback@ and Range provinceof tnc westernumtcu Universit6de Paris-Sud,Bat. 509, Orsay Cedex andrvas.wr.ust• States,Philos. Trans. R. Soc.London A, 300, 91405,France. (e-mail: [email protected] 407-434, 1981. psud.fr)Stop 977, 345 Middlefiedl Road, (ReceivedAugust 17, 1992; O. Bellier, URA D1369, Centre MenloPark, CA 94025. revisedFebruary 23, 1994; National de la Recherche Scientifique, M. L. Zoback,U.S. GeologicalSurvey, acceptedMarch 2, 1994.)