JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 105, NO. B5, PAGES 10,899-10,921, MAY 10, 2000
Three-dimensional seismic model of the Sierra Nevada arc California and its implications for crustal and upper mantle composition Moritz M. Fliednet• and SimonL. Klemperer Department of Geophysics, Stanford University, Stanford, California
Nikolas I. Christensen Department of Geology and Geophysics,University of Wisconsin, Madison
Abstract. A three-dimensionalP wave velocity model of south-central California from the Coast Rangesto the Sierra Nevada showsthat the crust under most of the southernSierra Nevadabatholith has seismicvelocities (5.9-6.3 kin/s) below the continentalaverage. The crustis not muchthicker (on averageabout 35 kin) than in the adjacent Great Valley and Basin and Range province apart from a small, northwa,rd thickening, crustal root under the western Sierra Nevada that reachesa depth of 42 kin. Crustal velocities above the continental average are observedbeneath much of the Great Valley due to a high-velocity body underlying the sedimentarybasin and the Foothillsmetamorphic belt (6.4-7.0 kin/s). Upper mantlevelocities are generallylow (7.8 kin/s) but spana widerange (7.4-8.2 kin/s). We display the velocity model in several crosssections a, nd maps of Moho depth and averagecrustal velocity. The measuredvelocities in the upper and mid crust of the Sierra Nevada batholith are in good agreementwith laboratory measurements on Sierra Nevada tonalites after correctionsfor density a.ndtemperature. Peridotite xenoliths from the eastern Sierra Nevada suggeststrong upper mantle anisotropy, which could explain someof the velocity heterogeneityin the Sierra Nevada mantle. By the time Creta,ceous subduction-related magmatism ceased,the Sierra Nevada arc must have had a thick marie lower crust; yet a principal result of our work is that today the batholith has a crust of mainly felsic compositionthroughout. A subcrustallayer with velocitiesbelow normal P• velocities(<7.6 kin/s) may indicate the presenceof lower crustal material in eclogitefacies.
1. Introduction Nevada, and to the south bounded by the Garlock Fault The modern tectonic framework of central California in southern California, lies the extensional Basin and is dominated by the San Andreas strike-slip fault sys- Rangeprovince (Figures 1 and 2). Our seismicinvesti- tem, but its basic geologicframework is related to its gation of the crust from the Pacific coastto the western earlier subductionregime, which continuestoday north Basin and Range province was designed to show the structural consequencesof this tectonic development. of the Mendocino Triple Junction in northern Califor- In summer 1993 the Southern Sierra Nevada Conti- nia.. The three major geologicprovinces of central Cal- nental Dynamics (SSCD) project shot two seismicre- ifornia, the Coast Ranges,the Great (Central) Valley, and the Sierra.Nevada, representthe accretionary-prism fraction lines in central California [Malin el al., 1995; complex, the forearc sedimentary basin, and the ex- Wcrnick½el al., 1996]. The first ran from the Coast Rangesacross the Sierra.Nevada and westernBasin and humed batholith of a magmatic arc system generated during Mesozoic subduction, respectively. To the east Rangeprovince (west-east line, length: 400 km), and and south, bounded by the eastern scarp of the Sierra the second in Owens Valley east of the Sierra. Nevada (north-southline, length: 325 km). In addition,a modi- •Now at Bullard La.boratories, University of Cambridge, fied west-eastline recordedthe NPE (Non-Proliferation Cambridge, England. Experiment) shot on the NevadaTest Site over a length of 480 km into the Coast Ranges(Figure 2). With a Copyright 2000 by the A•nerican GeophysicalUnion. shot spacing of 50 km and a receiver spacingof 500 m Paper number 2000JB900029. this survey is the most detailed active seismicstudy of 0148-0227/00/2000JB9000 29509.00 the Sierra. Nevada batholith to date.
10,899 10,900 FLIEDNER ET AL.' CRUSTAL STRUCq'URE OF SOUTHERN SIERRA NEVADA
124ø 122ø 120ø 118ø 116ø 114øW 42 ø N '-• Mesozoicbatholiths
Klamath ••...... FranciscanAssemblage Mountains • GreatValley Sequence major faults
40 ø Basin and
Lake Range 'Tahoe
Study Area (Figure 2) 38 ø Vlono Lake ß ,• San • Francisco ! !
o Fresno
Visal• 36 ø Death Valley
! ! ', Mojave PACIFICOCEAN ',,, Desert 34ø :. '••=:• ß
,%
I I I 0 200km ',• •:• San
Figure 1. Overviewmap of Californiaand its tectonicprovinces (modified fi'om Fuis and Moony [1990];and Saltusand Lachenbruch[1991]). WWF, White Wolf Fault' KCF, Kern CanyonFault. The outlined study area is shown in detail in Figure 2.
Analysesof the first arrivalsand wide-angleMoho re- mic refraction surveys in the Coast Ranges and Great flections(P,•P) fi'om this data set have beenreported Valley and (2) a set of earthquake recordingswithin by Fliednetet al. [1996]and Ruppertel al. [1998].This the Sierra Nevada, batholith published by Savage el studyexpands on the earlierresults in severalways. (1) al. [1994] and here referred to as "the earthquake We have modeled a strong secondarycrustal refracted data." We have not included the pioneering data of arrival in order to resolvethe velocity gradient in the Eaton[1966] and Carder[1973],which s[arted the Sierra lower crust. (2) We remodel some of the older seis- Nevada controversy,because [hey are too sparseto be mic data collectedin the regionin order t.oimprove the inverted together with the much denser modern data three-dimensional picture of the crust in central Califor- and also because they fall partially outside our study nia, updatingthe reviewby Mooneyand Weaver[1989]. area (their competingmodels are discussedwith regard (3) We present laboratory data on velocitiesin rocks to the SSCD data of Fliednet et al. [1•6]). Most of from the regionto better understandcrustal and upper the refraction data have been analyzed and published mantle composition. We note that the major contro- before [Colburn and Walter, 1984; Murphy and l/l/al- versywe resolve,namely, the seismicvelocities (and by ter, 1984; Uolburn and Mooney, 1986; Holbrook and implicationthe composition)of the Sierranlower crust, Mooney,1987; Howie et al., 1993]. is complicatedfor the simple reason that is is notori- We compare the seismic observations with labora- ously di•cult to derive accurate lower crustal velocities tory measurementson crustal and mantle rock samples from surface seismic data. from the same area. Our seismic results imply a sili- The additionaldata setsfall into two groups:(1) pre- cic composition of the crust, in the Sierra Nevada. This vious, densely sampled two- and three-dimensionalseis- supports the conclusion from observationsof tonalitic FLIEDNER ET AL.' CRUSTAL STRUCTURE OF SOUTHERN SIERRA NEVADA 10,901
X (km) gorithm of Hole [1992], which computestravel times -50 0 50 100 150 200 250 300 350 400 using a finite differencecode based on the method of Vidale[1990]. The codeis able to handleboth first and 350 ß % + -• 38ø later arrivalsincluding reflections [Hole and Zelt, 1995]. '• '-, .Sh-lO•::•'•.•."• •42 Basinand 300 •'• '•...... •:s%2-•fiX• -. • ". RangeProvince Refractedarrivals (first arrivalsof Pg and P•, and sec- "'•: , •' -'* '' Nevada ondaryPa) wereinverted for crustaland uppermantle 250 ,••' , •.... *'•P4'•: [ '. site• -{--37 ø velocities, and the Moho reflection P,•P was inverted + -• •:-•-'::..•,.,+ [ '•'---.•+ -,,+ • for the locationof the reflector[Hole et al., 1992]and 200 lower crustal velocities[Zel! et al., 1996]. The reflec- •5o tion Moho was used to enforce a velocity discontinuity -[- 36ø between crust and mantle in order to trace P• more re- alistically: a pure refraction inversionwould smooth out lOOi.....•.•,.• .... - N '• '• '••• ...... :•;•i'"..LOa -- X':•.:..•.•••.•• •o Mojave the crust-mantle transition over that part of the lower crust which is unconstrainedby turning rays. -•- 35øN + , •; ...... '+ + In Figure 3, we display six shot gathers from the 121ø 120ø 119ø 118ø 117ø 116øW SSCDdata. set. The NPE recording(Figure 3a; reduced Figure 2. Area coveredby 3-D seismicvelocity model. at 8 km/s) showsP• as a clearfirst arrivalfrom 170km Black triangles are shot point locations;numbered shot offsetto the end of the line at over 450 km offset (note locations are SSCD: 1-11 west-east line, 12-19 north- the delay due to Great Valley sediments in the offset south line, and 40-43 fan shots. Stars are earthquake rangeof 300 to 380 km). Pa is a strongsecondary ar- epicenters. Black dots are receiver locations. Gray rival out to offsetsof about 400 km. The following two lines, labeled P1 to P7 are profiles in Plate 1. White records(Figures 3b and 3c) arefrom the sameshot point circles are locations of tonalire samplesSN-1 and SN-2 location in the western foothills of the Sierra Nevada, of Figure 9 and Table 1. Arrow points to Oak Creek, recorded as in-line shot 4 in the SSCD west-east receiver collection site of mantle xenoliths. SLO is San Luis Obispo. The Sierra Nevadabatholith is light gray. line and as fan shot. 41 in the north-south receiver line. Note the absence of short offsets due to the distance of shot 41 from the receiverline. The in-line record(Fig- lower crustal exposuresat the southernend of the Sierra. ure 3b), shot 4, showsthe great differencein quality Nevada[Pickett and Sakeby,1993] that the batholith of later arrivalsin the east (Basin and Range)where doesnot becomemore mafic with depth. The geologi- P,•P is visible from precritical offsetsof about 5 km to cal evidencefor a siliciccrust and the consequencesfor post-critical offsets at the end of the line, and in the the development of the Sierra. Nevada arc have most west (Great Valley) where no deep reflectionscan be recentlybeen discussed by Duceaand Saleeby [1998a]. picked.The fan record(Figure 3c) showsarrivals from The upper crustalstructure of the Basinand Range the Sierra Nevada batholith approximately beneath the to the east of the Sierra Nevada is discussed in more highest part of the mountain range. P,•P arrives about detail by Duran [1997].This work is alsobased on the 1 s later in the north than in the south at equal offsets SSCD seismicdata. A magnetotelluricsurvey in the due to northward deepeningof the Moho. The other fan SSCD program[Park et al., 1996]found low electric shot (Figure 3e; the in-line recordingof the sameshot resistivitiesin the lower crust and upper mantle of the is Figure 3d) was recordedin the west-eastline from a Sierra Nevada. This is attributed to partial melt at shot to the north in Owens Valley. It showsthe deepest least in the eastern Sierra Nevada. Moho arrival in the study area (seebelow). Shotsfrom the Great Valley (e.g., shot 2; Figure 3f) sufferfrom 2. Seismic Data Analysis reverberationswithin the sedimentarybasin (reflected refraction following the first arrival with 3.5 s delay), We have used the publishedfirst-arrival travel times but show still a recognizableP• signal and a strong from the data setsof Colburnand Walter[1984] and long-offsetPa' Additional data sectionsare presented Murphyand Walter[1984]; we usedour ownpicks from by Fliednet [1997]. the originalseismic records from the threeGreat Valley Figure 4 showsfive different views of the three-dimen- strikelines, of whichonly two have been published [Col- sional ray coverageof our velocity model. The first four burn and Mooney,1986; Holbrook and Mooney,1987] are views from the side through the transparent model and the San Luis Obispo2-D and 3-D data set [see, cube: Figures 4a and 4c are views in the Y direction, e.g., Howie et al., 1993]. From the earthquakedata set that is, rays projected along the Y axis into the X-Z we used those events and stations that fall within our plane; Figures 4b anc 4d are views in the X direction, studyarea as pickedby Savageet al. [1994]. that is, rays projected along the X axis into the Y-Z Although most of the seismicdata we discussare in- plane; Figure 4e is an aerial (map) view of the first- line recordings,the use of broadsidefan shotsfrom the arrival ray coverage. High areal coverageis achieved SSCD data set and the integrationof earthquakedata wherefan recordingsare available(Figure 4e), both in and crossingprofiles make 3-D analysispossible. For the center of the SSCD data set (X - 100-250 km, our travel time analysiswe use a 3-D tomographyal- Y - 150-250 km) and in the San Luis Obispo area 10,902 FLIEDNER ET AL' CRUSTAL STRUCTURE OF SOUTHERN SIERRA NEVADA Southwest NPE(Nevada Test ..I ,t 'i......
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'•:'•-..... •::.:..:..:....'::..:.•:•..•:..•/'":::'":':-:':'.: .:"•'•':'•'•'-'.:.:.•...... :... 4• 4• •0 •0 320 2• 2• 230 1• 1• 120 90 61 offset(km) •i•ure 3a. 88CD •t, her from •P• shot,displayed with tr•vel time reduced•t 8 km/s. •hot point loc•tio• is i•dic•ted by i•verted tria,n•le. shot point 4 W9st • East
160 130 100 70 40 20 0 20 5.0 80 110 140 170 200 230 offset(km) Figure 3b. SSCDgather from shot point 4 displayedwith travel time reduced at, 6 km/s. Shot point location is indicated by invert,ed triangle. Shot point 4 is coincidentwith shot point 41. FLIEDNER ET AL.- CRUSTAL STRUCTURE OF SOUTHERN SIERRA NEVADA 10,903
shot point 41 North South
160 140 120 100 80 72 80 100 120 140 16.0 offset(kin)
Figure 3c. Sameas Figure 3b, exceptfor shot point 41. The invertedtriangle marks the projection of the shot point location into the displayedreceiver line. Gather is displayedwith constanttrace spacing(non-linear offset scale).
(around X - 0 km, Y - 50 km). The other data rive measure of the details that can be resolved with sets are essentiallytwo dimensionalor, in the case of our data set.. Figure ,5 shows t,he inversion results of the earthquakedata, have only sparsecoverage. The two checkerboardson a sectionthrough the SSCD west- upper 15 km of the crustare well coveredby P• first east.line correspondingwith the section of Ruppcrt arrivals(Figuress 4a and 4b), as is the regiondirectly al. [1998]. The amplitudeof the velocityperturbation beneath the Moho at about 35 km depth where most is -t-0.2 km/s and the anomalysize (half wavelength) P., rays turn. No first arrivals turn in the lower 15 km is 12.,5km vertically/125km horizontallyin Figure of the crust. This regionis coveredby later arrivals, and 7.,5km vertically/62.5 km horizontallyin Figure 5b. secondaryP.q and P,•P (Figures4c and 4d). Prop rays The long-wavelengthanomalies (Figure 5a) are well re- are only plotted between the Moho reflector and a mid- solvedthroughout the crust both in location (except crustaltransition at 10-1.5km depth,the depthrange in the deepest.part of the crustal root) and amplitude. over which P,•P was used for velocit,yinversion. This Smaller amplitudes can only be resolvedin the upper transitioncan be traced in someareas, particularly the crust..The streakingin Figure 5b indicates[hat [he lim- Basin and Range, as a reflector[Ruppcrt el al., 1998; its in resolvingpower of the data set have been reached. Duran, 1997]but is not usedotherwise in this study. An important questionis how well the velocity in the The spatial resolutionof the velocity model can be lower crust,can be constrained. If the velocity gradient testedby invertingtravel times calculatedfrom a syn- in the lower crust.is low, there is a wide gap between thetic modelof sinusoidalvelocity anomalies (checker- [he turningdepths of the first arrivalsof Pg and P,.. board test) superimposedon a backgroundmodel t,hat is not very sensitive to the velocities in the lower crust. reflectsthe general (long wavelength)features of the and for Prop there is a trade-off between lower crustal real model (we use a smoothedversion of our veloc- velocity and reflector(Moho) depth. The most direct ity model). The degreeto whichanomalies of different measure,though, is the secondarycrustal P• arrival, sizes(wavelength of the sinusoidvelocity pert,urbation) which convergesat long offsets with the P,,P reflec- can be reconstructedby the inversiongives a qualit.a- tion. The SSCD data show a strong secondarycrustal 10,904 FLIEDNER ET AL.: CRUSTAL STRUCTURE OF SOUTHERN SIERRA NEVADA
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60 30 0 30 60 90 120 150 180 210 240 offset(km) Figure 3d. SSCDgather fi'om shot point 13 displayed with travel time reduced at 6 km/s.Shot pointlocation is indicatedby invertedtriangle. Shot point 13 is coincidentwith shotpoint 43. phasewith a phase-velocityof about 6 km/s evenat the is well constrained, overlaid with the location of the largestoffsets (Figures 3a, 3d, and 3f) wherepostcritical Moho and for comparisonthe Moho from Mooneyand P,•P asymptoticallyconverges with P•. This suggests Weaver[1989], which is a contourmap basedon older, that velocities stay more or less constant throughout separatedeterminations of crustalthickness by multi- the crust or may even decreaseover some interval. The ple authors.The first two profilesfollow the two SSCD sensitivity of the different arrivals to a mid or lower receiverlines (Plates la and lb). The west-eastprofile crustal low-velocity zone is shown in Figure 6 by com- shows that the crust thickens from the Coast Ranges paring the travel time fit between our velocity model (25 km) to the easternedge of the GreatValley (40 km), (Figure 6a) and one that enforcesa. nonnegative veloc- which marks the center of the western Sierra crustal ity gradientthroughout the crust (Figure 6b). Figure 6 root [Fliednetet al., 1996],and then thins againto- displaysthe shotsrecorded in the SSCD west-eastline, ward the Basin and Range to just under 30 km (see which traverses the Sierra Nevada and shows therefore also contourmap of P,•.P reflectorin Plate 2). Under the strongesteffect. The misfits in precritical P,•P and the westwarddeepening sediments of the San Joaquin P,• (not shown)can be accommodatedin a positivegra- (southernGreat) Valley, a high-velocitybody stands dient model due to the additional free paraIneters of out at 12-15 km depth. FollowingGodfrey et al. [1997] Moho depth and mantle velocity as given by Ruppert and Godfreyand Kleinpeter[1998], we interpret this et al. [1998]. In contrast,the misfitsat far offsetsin body as Great Valley Ophiolite. It is probablycon- P• can only be reducedin a near-zerogradient model; tinuous with the Coast Range Ophiolite, which crops these misfits are most clearly seenon the NPE shot and out intermittently along the Coast Range thrust sys- the eastern branches of shots 2 and 3. tem but doesnot have a clear velocity signaturedue to mixingwith low-velocityFranciscan material [Godfrey 3. Profiles Through the 3-D Velocity et al., 1997; Godfreyand Kleinpeter,1998]. Ahnostthe entire rest of the crust under the Great Valley Ophio- Model lite and in the Sierra Nevada has unusually low seismic Plate I showsseven crustal crosssections through the velocities(about 6 kin/s). It is unclearwhether this 3-D velocitymodel (seeFigure 2 for locations)where it near-homogeneityin velocityreflects a homogeneityin FLIEDNER ET AL.- CRUSTAL STRUCTURE OF SOUTHERN SIERRA NEVADA 10,905
shot point 43 West East
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Figure 3e. Same as Figure 3d, except for shot point 43. The inverted triangle marks the projection of the shot point location into the displayed receiver line. Gather is displa.yedwith constant trace spacing (non-linear offset,scale). coinposition. A tongue of slightly higher velocitiesnear lock Fault; Plate lb). In general,the Basin and Range the eastern edge of the San Joaquin Valley (at 125 km crust can be divided into an upper crust with a velocity on P1, Plate la) reaching down to a depth of 25 km of about.6.0 km/s and a lower crust with a velocityof could indicate a buried tectonic boundary between the about.6.4 km/s, separatedby a discontinuityat about Franciscanassemblage and the Sierra Nevada arc (W. 17 km depth. Ray tracingof intra.crustal phases [Duran, Hamilton, personal communication, 1996). It seemsto 1997]puts the boundarybetween low upper crustalve- us, however, more plausible to regard it, as a feature locities and higher lower crustal velocitiesas shallowas within material of Sierran affinity, perhaps a transition 11 km beneath Death Valley and 13 km at the eastern from the main Cretaceous batholith to Jurassic, pre- end of the profile. Nevadan arc crust. In any case, the strong contrast Velocitiesof lessthan 7.6 km/s just belowthe Basin betweenhigh and low velocitiesis smearedout in the to- and Range Moho probably indicate magmatic under- mographicinversion (Figure 5). A more detailed inter- plating of the crust. There is no consensuswhether to pretation of domainswithin the low-velocityarea must treat this thin layer (up to 5 km) as lowermostcrust thereforedepend on additionalgeological or geophysical or uppermostmantle. Catchingsand Mooney [1991] evidence. It. is clear, however,that. the high velocities argue for a 7.4 km/s lower crustal layer and against under the Great Valley sediments do not continue all older studies that identified this layer with the upper- the way to the Moho (Figure 6). most mantle [see Catchingsand Mooney, 1991, and OwensValley (at 230 km on P1, Plate la) separates referencestherein] on the groundsthat (1) there is a the low-velocitySierra Nevada from the on-averagehigh- deeper8.0 km/s layer and (2) the top of the 7.4 km/s er velocity Basin and Range crust near the intersection layer does not correspondto the Moho as identified by with the north-south profile (P2, Plate lb). Except.for near-verticalreflections [Klcrn, perer et al., 1986]. To be a 40-70 km wide blockunder the Panamint Rangewith consistent within our study, but, in contrast to Catch- velocitiesunder 6 km/s downto a depthof about 18 km, i•gs and Mooney[1991], we placethis layer belowthe crustal velocities below 5 km depth increasedownward Moho as defined by the onset of the wide-angle P,•P from 6.0 to 6.8 km/s (up to 7.0 km/s near the Gar- reflection. This is not the only place where we find 10,906 FLIEDNER ET AL.: CRUSTAL STRUCTURE OF SOUTHERN SIERRA NEVADA
shot point 2 West • Pn East
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'::"z:::::.. •" ':'.:•-.'.-:?..?•:"":,:'•:•::::'.•:•:•:'•e•?•::•c:;•:...•."•:•:::: ..'.:•;':.-:..::?'•:.-:-:....:/:'•.'•.•"•.:::.'.":.....•'..:::.".•:-• .... :.../•:.•::.:'.-e•:::..-.:.:,.:•: ' .'.•.•.:.:•:•:,"::':' -'::"• .... •"•'ff'•.?t•:'--•.•:..?-..•-,.:.•,-,.•.•,'"•:':.••::.•: •...:.'*•:•"•:'•••-,'::. ",".•-•:•.'::4.•: .-,. ...:...... -...... ::...... :..... -..•:.•'• ...... -,• ..... •."'•2:b':•:?• ...... '• ß .• .....-...... -..:•..-c.•:::•:::...-:.-.•-.-• •., • ...... -.. 60 20 7 20 60 100 150 180 210 240 270 3.00 33.0 offset(km) Figure 3f. SSCD gather from shot point 2 displayedwith travel time reducedat 6 km/s. The inverted triangle marks the projection of the slightlyoff-line shot point location into the displayed receiver line. (in commonwith previousauthors [Savage et al., 1994; The central and southernSierra. Nevada (Plate ld, Jonesel, al., 1994]) velocitieswell below the definition bounded by the Garlock Fault in the south) in contrast of P•. (>7.6 kin/s), but locatedbeneath the P,,P re- reachesvelocities of at most6.4 km/s in the lowercrust. flector. VVe use P,,.P as primary criterion for locating On this profile, we also see the most dramatic disagree- the Moho because we have a better distribution of ment of our results with some of the older data: Moone• reflection points than of P,,, measurementsand it marks a•d Weaver[1989] overestimatedthe thicknessof the almost everywhere the •najor velocity discontinuity be- Sierran crust by roughly 10 km with respect to our tween typical crustal velocitiesand (usually low) upper model. This difference is partially a matter of seman- mantle velocities. This deftnition has problems of its tics as explained above. The differencedecreases signifi- own (see below), but is in our judgment the simplest cantly when we base the comparisonon velocitiesalone. and most consistent.A differentquestion is the geo- The conventionaldefinition of the refraction Moho (in logicalmeaning of a 7.x km/s layer (terminologyfrom absence of a clear discontinuity between crustal and Savageet al. [1994],and Joneset al. [1994]).We return mantle velocities)is the 7.6 km/s velocitycontour. On to this problem in the final section. the central Sierra profile 4, velocities between the Prop The two profiles in Plates l c and l d are joined at reflectorand the Moho of Mooneyand Weaver[1989] SSCD shot point 19 at the southern end of our model. are about 7.6 km/s. Material with such seismicve- Lack of short-offsetrays from the NPE recordingat locities cannot easily be attributed to either crust or NevadaTest Site (NTS) preventsus from resolvingde- mantle but should be considered a mixture of both. If tails of the upper crust in the NE half of the Basin and one takes into account about 10% error in the estimate Rangeprofile (P3, Plate lc), but it seemsto havegener- of absolute Moho depth by either method, we are then ally lowervelocities than the SW half (6.0 km/s versus left with only one unequivocal area of disagreement: 6.2-6.4 kin/s). Velocitiesin the lower crust increase where profiles I and 4 intersect. We find there a man- from 6.0 km/s to a maximumof 7.0 km/s (the veloci- tle high with normal mantle velocities,whereas Mooney ties above6.6 km/s near the Moho cannotbe considered and Weaver [1989] place there the deepestpart of a well resolved). Sierran crustal root. The again quite different interpre- FLIEDNER ET AL' CRUSTAL STRUCTURE OF SOUTHERN SIERRA NEVADA 10,907 numberof rays in x axis gridcolumn numberof raysin x axisgrid column 8 , , ?•....'•x•';!?...... • E • ?:-.••' :•':':':•?•:);•-'7•;;•:.-: . '%...... • ...... • • ;•'"• :•?•:'::;•?•'•:'::•;•?:•:•:'.... •':"•••C 'w'•'"•'-•*:••••:• •':•'•m..:•:::: • -- '.'...... •.•;•':'::':•::: • • • • • • .• '"• ..• :?"• ..• -•?•...... •:•-4• ..•;':•:•?*•• ..-:-•:;:•::;;•:•*:•½•::•x•j•.... •-...... •,•5•" .....:-.::'" ...... :.:;:;'.-,,:-. ....-gx½ •.,-;::.:::-: ..... ...... ß-:::.:•:. (w•t) qldop I I I o umnlooppõ s• /• u! s/•m1o •eqtunu uLunlooppõ S• u! $/[l•JJO JequJnu 10,908 FLIEDNER ET AL.' CRUSTAL STRUCTURE OF SOUTHERN SIERRA NEVADA and north-south profilesare forcibly reconciledat their intersectionand agreewithin errorsboth with Mooney and Weaver[1989] and with this study. 10 The last threesections in Plate1 tracesome of •he older seismicprofiles to the west of the Sierra Nevada. The Great Valley profile (Plate le) followsthe axis of the San Joaquin Valley. The crust generally thickens 3o from north to south from 20 to 30 kin. The high veloc- ities of the Great Valley Ophiolite [Godfreyand Klein- 4o peter, 1998]can be tracedalmost continuously along the entire section southward up to the White Wolf Fault, 0 50 100 150 200 250 300 350 which separates the Great Valley from the southern bend of the Sierra Nevada batholith. Although poorly I constrained,our results suggesta velocity decreasebe- low the Great Valley Ophiolite correspondingto older continentalcrust below the obductedophiolite [God- frey and Klemperer,1998]. The Great Valley Ophiolite also shows up as a 10 km thick west,dipping body in 50 100 150 200 250 3• 350 0istance (kin) the profile from the Pacific coast.to the eastern edge of the Sierra Nevada(Plate lf). This is the only area the synthetic model where our seismic data cross the San Andreas Fault,. •.;]•• detailfrom At shallow depth the San Andreas Fault appears as a Figure 5. Checkerboard test of model resolution steep boundary betweenthe low-velocityFranciscan as- for a cross section along profile Pl (see Figure 2 semblageon the eastern side (Temblor Range of the and Plate l a). Sinusoidalvelocity perturbations of CoastRanges) and highervelocity Sierran granitoids in 4-0.2 km/s (detail shownbelow the crosssections) the Salinian block. This velocity contrast vanisheswith and two different wavelengthsare superimposedon a depth(see also the discussionof the Franciscan/Sierran smoothed version of the final P wave velocity model. boundaryon profile 1 above). Upper crustalvelocities The reconstruction started with the unperturbed fi- decreaseagain to the west of the Salinian block in the nal velocity model and inverted the travel times cal- culated from the synthetic model as observations. Dis- Sur-Obispo terrane of the western Coast Ranges. A played are the differencebetween the starting model •nd the synthetic(i.e., correct),nodel(contour lines) a in comparisonwith the differencebetween the starting 14 model and the inversionresult (i.e., our best estimateof '--•--•.•. Fitwith midcrustal low-velocity zone the synthetic model given the distribution of available 12 data; grayscale). "Success"would be alternatingdark •1o and light bull's-eyesas in the detail from the synthetic model, below left. Contour lines are 0.0 (rectangular grid of nodal planes),0.05, 0.1, and 0.15 km/s (positive ..• ! solid, negativedashed) deviation of the syntheticmodel 4 from the starting model. 2 0 i ß' 0 50 100 150 200 250 300 350 400 b tation of Ruppertet al. [1998]does not resolvethat dis- •1..,. , , i , , , ß i .... i .... [ .... i .... i .... i .... 14 •""--• .. Fitwithout midcrustal low-velocity zone crepancy, but offers one possiblesolution: a laminated Moho, that is, a mixture of crustal and mantle material 12"'"-"•' "%.,%. '";,,..yPE over a depth range of severalkilometers. Another pos- sibility, explored further below, is mantle anisotropy: seismicenergy that travels along the spineof the Sierra Nevada encountersmarkedly loweredP,• velocitiesthan 4'-% that travelingacross it. If so, longitudinalprofiles would giverise to a crustalroot interpretation[Eaton, 1966], 0 , whereastransverse profiles would yield a rootlessinter- 0 50 100 150 200 250 300 350 400 pretation [Carder, 1973]. Our isotropicanalysis that model x coordinate(km) incorporatessparse arrivals from many azimuthsthere- fore results in a heterogeneousupper mantle with the Figure 6. Comparisontravel time fit,of Ps phasewith (a) and without (b) midcrustallow-velocity zone for best trade-off between the two extremes for the data. shots recorded in the SSCD west-east line. Black crosses Rupper!e! al. [1998]escapes the Eaton-Carderdiscrep- are travel time picks, gray solid lines the modeled travel ancy becausetheir 2-D interpretations of the west-cast times. Traveltimesare reducedat 8 km/s. FLIEDNER ET AL.- CRUSTAL STRUCTURE OF SOUTHERN SIERRA NEVADA 10,909 West-East Profile (P1) Basin and Range Province Diablo San Joaquin Valley Sierra Nevada Range ._ , -5 "•-1 I 2 • '• 4 8 n 10 11 '- 0 5.0 5 5.0 . •- 6.2 _ 6.2 10 ::------_ -- 6.4 15 • -I-•,o---•-•!--:ss7.0 6.2160 6.2 20 60 I - 6-6 - .6.,, - ' .- 62 6;6 25 I --.:-•-' _._6.8-••- -•-- • 6.2 6.0 30 I - .••__•""•-7• • --F_ 7,8 • I - -.• 17.8 7.6"== 35 80 40 45 5O •{ •1 vertical.•0'. 55 • •1 exaggeration2.5:1 i,,,,i,•,1,,,,i,,,,i,,,,1',,,,'i,,,,i,, ''1'''' i''''i'''•1'''' ' ' I''''1''''1 0 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375 w distance(km) E i""•"'i,,,,•....i.... I',,,,r,•'q ....•.... •.... •.... •.... •.... 2.02.53.03.54.04.55.05.56.06.57.07.58.0k•s ChalfantValley OwensValley -5 -=iF12 13 14 15.b 16•b. ' 17J., 18.E' o 4.0•... • -"'"--•-5:0 -•- -.4:0 5 • -• .• • -- 6.0 lO • -> 6.2 • •.. 64 15 2o 25 . ,• _ 6•4 . North-South . 6.26.0 .• 6.4 r • • 3o Profile (P2) '. 4•Oho• ' 6.6•, •/ 6 8 .• -• 8.0 7.0...... 35 ß.6 8 0 7,8, '•••7,6 4o •., 7.8 • • 7,8 • 45 • 80 • 5o o 55 ..... - i'l''''l' ' b 0 25 50 75 100 125 150 175 200 225 250 275 300 N distance(km) S Plate 1. Cross-sectionsthrough the 3-D velocitymodel along profiles (a) P1, (b) P2, (c) P3, (d) P4, (e) P5, (f) P6, and (g) P7 in Figure2 (seealso inset maps). The shadedand contouredareas are constrainedin the inversionwith nearby rays. Locationsof intersectingprofiles are marked by vertical lines, inverted triangles mark the locationsof shot points (SSCD labeled), and stars mark earthquake hypocenters. Dashed squares in Plate l d mask unreliable velocities that are solely controlled by direct rays from the earthquake hypocentersbelow and therefore sensitiveto errors in hypocentral depth determination. The thick black line marks the Moho as determined by the P,•P reflector inversion. The white dashedline marks the Moho of Mooney aa.d Weaver [1989]. 10,910 FLIEDNER ET AL.- CRUSTAL STRUCTURE OF SOUTHERN SIERRA NEVADA Basin and Range Profile (P3) " Pahute Mesa <{: •3 n =. • m Indian Wells Valley -5• 'N•S 17 . • 6:4 '. E 15 ' •• • ]ß•-•• -':-6:•6.0 162 • - ' •. • .-•, • 8•0 •.• ',• .... :.0 7.8 ' 40 8.0s,• 50 ve•ical - - •aggeration 2 5:1 55 ,,,,i,,,,1•,,,i,,, ''''1''''1''''1''''1'' 'I''''1''''1' 0 25 50 75 100 125 150 175 200 225 250 275 300 NE distance(km) sw """'!" "rl/'" [I """l '[" "l]""l "'""l'"" "l'"'[I ' "' '1' "" I""'1' 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 km/s c: 0 P4 c: _1 c: '*'" o o 0 6.2 %- 6.O r --- 6.0 10 • 6.0 • 6.0 *' 25-.: 64 '-6.0 /j 7.6 8.0 7'8 55- 75 100 125 150 1 Z5 200 225 250 275 300 325 350 N distance(kin) S Central Sierra Nevada Profile (P4) Plate 1. (continued) FLIEDNER ET AL.- CRUSTALSTRUCTURE OF SOUTHERNSIERRA NEVADA 10,911 e GreatValley Profile (P5) WhiteWolfFault -5•. ,i,2 J• 10- •.• • .... ' •;•-6 2 ,• :•• ••• i •6 8 • • ' • 6,8 :• •••'• •e,s -•'• ..• '•N½-• I .'• ,.,•e,s .•.7.o - "' , E 15 7•0 .• '" % • • •• • •.o 8•8 ••-• I • •.• ' •-• • • 6,4 -• 20 6•"u . 66 •/ / '" ' ' I 0 '• • *• 25- q •'-' - ' • 7.6 • , • • / • I • •t • • '•_ 13 30- 35- - ..... 0' [ .• ,•• 40 - 0' - 50 verticalexaggeration 2.5:1 • 55 iii,i ,i,i, lll,, •,,i,,i ,•,l,ri,l,i' •'" •ll,,i,i,, ,,,t,, ,l[,i,,[i,, i,,,,,, I .... •,•,1i,i,,,, ll,,,,,•,,,t•,!,, i ,,,,,,,,,•,,,,•• •l•l, ½,,,,, 0 25 50 75 100 125 150 175 2• 225 250 275 300 325 350 375 NW distance(km) SE =-- m "• • • • •1' "i • •1" I""I • •'1""1 '• 1TM _ L L m I •• • = z.uz.o •.u 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 • o-, m I =•XX PSi, P km/s •m.- • m o I I • I > •oZE .-- • • I i • I o= • m - •.•• • I I •. •Iß •E "I SanValleyJoaquin I Sierra--•I Nevadal I16 • - -5 • ,Coa•nga g 154 •••a',e%•a•ø2'• I :• .•.7•o•:[I 'e;-2'-,• I • 15 i d.a 'e.2 25• • ••,• - • • '•- .• i• I• 1• 25 • -;...... '• 35- • 7:s-•• 35 o o 78' "•. 50-.5: •a •a •'.... ',• • { N distance(kin) ss. ", ,"•",',,, ,, •, ,,, •,',• ,','•',,,, •,',m ',""i",,"',",'"t',", ,',,'", ,',, •,,,, ,, ', •"'m• 0Coast 25 50Ranges 75 1 f W0 25 50 75 • distance•25aso (kin) •75 200 22s 250 275 E Profile(P7) Coast to Sierra Profile (P6) Plate 1. (continued) 10,912 FLIEDNER ET AL.: CRUSTAL STRUCTURE OF SOUTHERN SIERRA NEVADA X (kin) -50 0 50 100 150 200 250 300 350 400 350t ]I ii • [ I • • l••• ]• I i • • i I i , , , I .... 1.... I.... 'Jr-38 ø 300 \, 250- ß 25 • A • ' ' JI- 370 200 40 '•, ; • 35 , 150 , 100 ,'• [-[-36ø 5O ,,.. • -iF'35øN 121ø 120ø 119ø 118ø 117ø 116øW 20 25 30 35 40 45 km below sea level Plate 2. Map of depth to the P,•P reflector. Areas not constrained with nearby reflection points are masked. The Sierra Nevada batholith is outlined in white. Locations of points with P,•P picks are marked with triangles, and P,•P reflection points are marked with gray dots. Thin black lines are Moho depth contours; contour interval is 5 km. small body at 12 km depth with a velocityof 6.4 km/s On the Coast-to-Sierraprofile (Plate lf) under the could be another fragment of ophiolitic material, which western Sierra Nevada (between 160 and 225 kin), we alsocrops out at the surfacein this area (for a more de- lack P,,.P data to determine the depth of the Moho, but tailed study of this profile,see Howie et al. [1993]). A the refraction data of the earthquake data set require a slight velocity decreasein the lower Salinian crust could layerof 7.0-7.2 km/s (probablythe sameas the crustal indicate the presenceof Franciscanvelocity material un- 7.x layer of Savageet al. [1994])above normal mantle der the Salinian block, a possibility that was discussed velocitiesof 7.8-8.0 km/s. We havetherefore forced the by Howie el al. [1993]. Lowercrustal velocities, as far interface inversion for Moho depth to place the Moho as they are resolved,reach 6.7 km/s, indicating more between these two layersinstead of extrapolating freely marie material either of ophiolitic (subcretedoceanic from the nearestreflection points (Plate 2). crust) or lower arc crust origin. The continuityof the ophiolitic materiM under the Franciscan of the eastern 4. Crustal Thickness and Average Coast Ranges cannot be determined with the available Velocities data (Plate lg): this short Coast Rangesprofile, which links the two west-eastrunning profiles1 (Plate l a) The map of Moho depth in Plate 2 is an updated and and 6 (Plate lf) throughthe area of the 1983Coalinga extendedversion of the map of Fliednet e! al. [1996]. earthquake[Wentworth and Zoback,1990], shows a ve- It incorporates picks from the Great Valley and San locity increaseto only 6.4 km/s except at its northern Luis Obispo data sets and the inversion was allowed end whereit clearly cuts the Great Valley (or Coast to produce a somewhat rougher Moho topography to Range) Ophioliteat 15 km depth. LackingP,•P re- allow a better fit with the travel time data without flection data, the lower crustal structure between the abandoning the underlying concept of the Moho as a San Joaquin Valley and the Salinian block remainsun- simple reflector. In the final iteration of our inver- resolved. sion we applied horizontal smoothing acrossa length FLIEDNER ET AL- CRUSTAL STRUCTURE OF SOUTHERN SIERRA NEVADA 10,913 scale of 25 kin. The inclusion of the earthquake data. in 5. Velocity-Depth Functions of the the mantle velocity inversion (and to a lesserdegree Sierra Nevada the lowered estima, te of lower crustal velocities from The velocity structure of t,he batholith seemsto be secondaryPa) requiresa significantreduction (up to ,5km) in Moho depth in the area,of shot points 4 and 5 very uniformthroughout/.he crust with a velocitythat (Plate la) comparedwith t,he interpretationsof Flied- deviatesnot more than 0.2 km/s from 6.0 km/s (we her et al. [1996] and Ruppertet al. [1998],creating a ignorefor t,he sakeof simplicitythe upper 5 km of local mantle bulge with velocit.ies of 7.8-8.0 km/s (pro- the crust). As discussedabove, this interpretationde- file 1, 170 to 220 kin). The SSCD data,alone did not. pendson the appearanceof the long-offsetPa arrival, require this surprising feature, but the reverberatory which is not,compatible even with the low positiveve- nature of the P,•P arrival led Ruppert½t al. [1998]to locity gradientin the Sierranlower crust that is al- infer a laminated Moho (interlayeringof mat,erial with lowedby the first-arrivaltravel times [Ruppcrtctal., mantle and crustal velocities) on the basisof synthet,ic 1998]. We investigatethe implicationsof this unusu- seismogrammodeling. Although our interpretation sat- ally low crustalvelocity gradient, for the composit,ion of isfies the travel time constraints of all the data better the Sierra Nevadaby comparingaverage velocity-depth than Fliednet ½! al. [19961and Ruppcrt½! al. [1998], functions of the Sierra, Nevada from our model and that the introduction of a laminated layer that separates the of Ruppcrtet al. [1998](Figure 8) with velocitiesmea- base of the crust from the top of the mantle allows an suredin the laboratoryon two samplesof tonalite from overall simpler shape of the Moho and may therefore be the centralSierra, Nevada (Table 1 and Figure9). the preferable interpretation. Our samples(see Table 2 for chemistryand miner- Crustal thickness increases from less than 25 km at alogy)were collected from fresh road cuts a.t locations the coast to 35-42 km in the western Sierra, Nevada and shownin Figure2. Acousticvelocities of the samples decreasesto 30-35 km in the Basin and Range. There were measured at room temperature and hydrostatic seems to be a systematic thinning of the crust from confiningpressures to 1000MPa (equivalentto about north to south in the Basin and Range reflecting both 35 km depth) usingthe pulsetransmission t,echnique the decreasein elevation[Saltus and Thompson,1995] describedby Christensen[1985]. Bulk densitieswere and the increasein Bouguergravity betweenthe north- obtainedfrom the weightsand dimensionsof the cores ern Basin and Range (Great Basin) and the southern used for the velocity measurements. Basin and Range (Mojave-SonoranDesert). The acoustic velocities were corrected for temper- The major tectonic provincesare also reflectedin the ature assumingan averagecrustal geothermal gradi- average crustal velocities, calculated as a vertical av- eragebetween the 5 km/s contour(in order to exclude X (kin) sedimentarybasins) and the Moho (Figure 7). Both the -50 0 50 100 150 200 250 300 350 400 Sierra,Nevada, batholith and the Basin and Range show •tl,,,,l,•,,I .... I .... I .... I .... I .... I .... I .... I crustalaverages (6.0 to 6.2 km/s) well belowthe world- wide continentalaverage of 6.45 km/s [Christensenand Mooney, 1995]. These low averagecrustal velocities have important implications for crustal composition, 250 which will be discussedlater in this paper. Similar val- ues are found for the FranciscanCoast Rangesand the 200 westernside of the Great Valley (at X = 50 km) where 150 low-velocityFranciscan assemblage overlies (and under- -•- 36ø lies?) a very thin Great Valley Ophiolite (Plate la, 1 oo westernmost50 km of the profile). The rest of the 5o Great Valley is closerto the continentalaverage (6.3- 6.6 km/s). The northward increasein velocity is prob- -Jr-35øN ably due to an increasein thicknessof the Great Valley 121o 120ø 119ø 118ø 117ø 116øW Ophiolite. contours:average crustal velocity between 5.0 and 7.6 km/s The highest averagevalues are observedat the north- 5.9 6.0 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 km/s western edge of the Sierra Nevada batholith, at the averagecrustal velocity above Pm P reflector southern end of the Foothills metamorphic belt and closeto a large outcrop of ultramafic and metavolcanic Figure 7. Map of averagecrustal velocities. Velocities rocksnear Fresno(X = 80 km, Y = 220 km). Con- are smoothedaverages calculated from the 3-D veloc- ity modelin the verticalcolumns between the 5 km/s sidering that velocities here are mainly determined by contourand the P,•P reflector(grayscale map) or the the sparse earthquake data, this correlation could be 7.6 km/s contour(contour map; contourinterval 0.2 fortuitous. Miller and Mooney[1994] find similar val- km/s). The maskoutlines roughly the area covered uesfor averagevelocity (6.6 kin/s) and crustalthickness by first arrivals (see Figure 4e). The Sierra Nevada, (32 kin). batholith is outlined in white. 10,914 FLIEDNER ET AL.' CRUSTAL STRUCTURE OF SOUTHERN SIERRA NEVADA Vp (km/s) The midcrustal region extending from 12 to 22 km has 0 0 0 0 0 0 0 0.07 km/s lowervelocities than the laboratorymeasure- ments. Lower crustal velocities match the laboratory _ values. However, if this region is granitoid in compo- sition, the batholith rocks have likely recrystallized to granulite facies assemblages,the equilibrium faciesun- 5: ß--- this study der lower crustal conditions. A felsic granulite under --- Ruppert et al. theseconditions (25 km depth, averageheat flow) is ex- 10- pected to have a higher than observedP wave velocity (6.327-t-0.131km/s; Christensenand Mooney[1995]). The low-velocity zone in the middle crust could be 15 i explained by lowering velocities in granitoid rocks by i i (1) a loweraverage density of 2660kg/m a. Thiswould i i i produce a mechanically unstable crustal column and re- ....i ...... i i quires more radiogenic granite in the batholith than is ! i compatiblewith the low surfaceheat flow [Lachenbruch, i I'3 25 i i 1968]. The low-velocityzone could also be explainedby i (2) increasedporosity. In igneousrocks where poros- i . • i 30÷ ity consistsprimarily of microcracks with high aspect ratios, 1% porositylowers •0 by about 0.2 km/s at, X(kin) ' 100 MPa confiningpressure [Christensen, 1989]. A -50 0 50 IO0 150 2OO 25O 3OO 35O 4OO differential pore pressure of 20 MPa would produce a '•.•:,: 0.05-0.1 km/s decreasein 1/•0at midcrustalconfining 250 ;•::'•"•*•'•;•:':;;•- • pressures(Stephenson County Granite given by Chris- tense•[1989]). The velocity decreaseis largestwhen the pore pressure approaches the confining pressure. . i Vp(km/s) 50 I , Figure 8. Averagedvelocity-depth functions of the 0 .... I .... I .... i,,,,1,],, Sierra Nevada batholith, extracted from the 3-D ve- •• labmeasurements '•, ' ---' withrange ingray locity model(square outline on insetmap) and the 2- hi • modelaverage D model of Ruppertet al. [1998](rectangular outline / • ' • withbounds of around P1 on inset map). Inset map showslocation upper cru variabili• and averagingareas of the velocity functionsand loca- 10- tions of tonalitic samples(Figure 9); the Sierra Nevada. batholith is shadedin dark gray, profilesof Plate 1 are marked in black. middlecrustal • • ent [Blackwell,1971] and a temperaturederivative of low-velocityzone• • -0.39 x 10-a km/s/øC.The densitiesof the tonalitesin Table1 (2740kg/m a) areslightly higher than the aver- lower crust agedensity of 2690kg/m a for SierraNevada granodior- ites measuredby Oliver et al. [1993] for about 6000 30 -• ...... :.:.:.;;•}:..:.:..::::.:. ::...... •. :...... samples collected from surface exposuresthroughout Moho...... •...... 5 the batholith (although our densitiesagree with Oliver 35 et al. [1993] measurementsin the westernportion of the batholith). For the comparisonof averagefield and Figure 9. Comparison of velocity-depth function laboratory velocity-depth functions, we have lowered from the 3-D velocitymodel (Figure 8) and corrected our velocitiesby 0.12 km/s using the linear velocity- (seemain text for details) laboratorymeasurements on densityrelationship of Christensenand Mooney[1995] Sierra Nevada tonalite samples(Table 1). The shaded (Figure 9). This adjustsour tonalitevelocities to those area around the average tonalire values (thin dashed of granodiorite with the average granodiorite density line) showsthe range of values measuredon the two determinedby Oliver et al. [1993]. samples. The thin solid outline around the average velocity-depth function from the 3-D velocity model In the upper 12 kin the agreement between field shows the variation of velocities within the area over and laboratory measurementsis very good, indicat- which the average has been calculated. The depth to ing that the granitoids extend to at least this depth. the Moho varies over the averagingarea. FLIEDNER ET AL.' CRUSTALSTRUCTURE OF SOUTHERNSIERRA NEVADA 10,915 10,916 FLIEDNER ET AL.: CRUSTAL STRUCTURE OF SOUTHERN SIERRA NEVADA An increasein pore pressurealso producesan increase crustalbeit with a biggervelocity contrast with the rest in Poisson'sratio, but this is precluded by results of of the crust than with the rest of the upper mantle. A Jo•cs and Phin•,½!l[1998] which showno increasein velocityof about 7.8 km/s seemsto be regionalaver- Poisson's ratio for the 12 to 22 km depth range, but age. We refrainfrom actuallycalculating an average only below 24 km depth. The low-velocityzone could becauseof the nonuniform ray coverage,both areally be explainedby (3) increasedtemperature. An in- and in depth,and the biastoward low, possiblycrustal creasein temperature of 180ø(7over the assumedaver- velocities,that our definitionof the Moho as the P,•P agegeotherm would bring the laboratorymeasurements reflectorentails. In the followingparagraphs we referto into agreement,with the velocitymodel and wouldmake "P• velocity" rather than upper mantle velocitywhen felsicgranulite (0.09 km/s reductionin P wavevelocity) we mean to excludethe velocitiesbelow 7.6 km/s. rather than unmet.amorphosed granitoids compatible The pattern of the velocitydistribution, however, al- with the observed seismic velocities in the lower crust lows the possibilitythat at least someof the velocity (felsic granulitesas defined and measuredby C,ht'is - extremesindicate strong seismic anisotropy rather than tensen and Mooney [1995]). Poisson'sratio is not af- a heterogeneous(but isotropic)mantle as we have mod- fected by an increase in temperature. A heat. pulse eled;almost all the low velocitiesderive from ray paths from the mantle that has reached the low-velocity zone alongthe strikeof the SierraNevada batholith (espe- as givenby Croughand Thompson[1977] could be the cially the earthquakedata set), whereasthe high ve- source for this temperature increase. It would only locitiesderive from ray paths acrossthe Sierra Nevada take 2 to 4 million years for a thermal disturbanceto (whichalso sample the Basinand Range province). The travel from the base of the crust to the top of the low- sparsenessof the P•, ray coveragepermits a separation velocityzone (using the estimateof Croughan Thomp- of the high and low velocitiesboth horizontally(low son [1977]),considerably less than the 10 millionyears under the batholith, high under the Basin and Range) assumedin their uplift, model for the Sierra Nevada. and vertically(high and low directlybeneath the Moho, The low-velocityzone could be explainedby (4) crustal averageat greaterdepths) in our isotropicmodel. anisotropy.Assuming/,hat the granitoidsbecome gneis- a•,d Jackson[1992] report low mantlevelocities (7.4- sic with depth, the low-velocity zone could be the re- 7.7 kin/s) underthe southernmostSierra Nevada near sult of waves traveling perpendicular to the prevailing White Wolf Fault (seePlate le); they alsofound signifi- foliation of midcrustal rocks. Since the paths of the cant P, anisotropyof about3% (0.224-0.08km/s) with rays that determine the low-velocity zone are mainly a fast direction N75ø4-4øW. Anisotropy in the mantle east-westand mainly horizontal, the foliation would be can be producedby aligningthe fast axesof olivine vertical and parallel to the batholith. Tonalitic gneiss grains(aaaxis). Measurementson uppermantle xeno- at 15 km depth typically exhibits a P wave aniso/ropy liths from the central Sierra Nevada produce a P wave of 9% in laboratory measure•nents[C, hrisl½•,scn and anisotropyof 6% [b'cottand Christet,set,, 1996], which Mooney, 1995], more than enoughto accountfor t,he can easily explain the entire range of observedPn ve- 1-2% needed to explain the low-velocity zone. Ducca locities. and b'aleeb!l[1996] observedgneissic texttire in xeno- Studies from two xenolith localities suggestthat the liths of granodioriticcompositions in the easternSierra uppermantle is anisotropicbeneath the SierraNevada Nevada. batholith. Pesel,ck el al. [1977]report anisotropiesof 0.5-0.6 km/s for two peridotitexenoliths collected from a basalt pipe near Big Creek, FresnoCounty, on the 6. Upper Mantle Velocities western flank of the central Sierra Nevada. In addition, Measurements of P,• velocities are sparseand far froin we have measured olivine petrofabrics for three peri- uniformly distributed in our data set,. They colne from dotire xenoliths collected froin the Oak Creek region the shots at the ends of the two SSCD profiles, the (Figure 2) located on the easternflank of the Sierra NPE shot at Nevada Test Site and the earthquake data Nevada [Ducea and ,S'aleeb!l,1996]. The peridotites set. The inversion of these data points for an isotropic show weak foliation and average90% olivine. [Ising velocity model results in a highly heterogeneousupper a 5-axis universal stage, 100 grains were selectedfor mantle. The very generalized pattern is a southward orientation from each specimen. Contoured axis orien- velocity increasein the Sierra Nevada (Plates lb and tations with foliations oriented parallel to the page are ld) and (arguably)in the Great Valley (Plate le) and shown in Figure 10. a westward increase across the Sierra Nevada into the The fabric diagramsshow that olivineorientation is westernmostBasin and Rangeprovince (Plat, e la). The relatively uniform for the three peridotites. Olivine highestvelocities (8.0-8.2 kin/s) are observeddirectly crystallographica axesshow strong concentrations and beneath the Moho of the southwestern Sierra Nevada lie in the foliation planes. The maxima of the olivine and westernmostBasin and Range(Plates la-lc). The crystallographicb axestend to fall approximatelynor- lowermostvelocities of 7.4-7.6 km/s beneaththe central mal to the foliations. A strongb axesmaximum is con- Sierra Nevada (Plate ld) and someparts of the Basin sistentwith an origin by high-temperatureplastic flow and Range(Plate la,) couldequally well be calledlower with slipon olivine (010) (planeperpendicular to b axis) FLIEDNER ET AL.: CRUSTALSTRUCTURE OF SOUTHERN SIERRA NEVADA 10,917 a b a b c Vp /•Vs [100] [010] [001] OC- Max. = 8.8 km/s Max. = 0.31 km/s OC- Max. = 8.7 km/s Max. = 0.27 km/s 0C-04 Max. = 8.7 km/s Max. = 0.22 km/s Figure10. Anisotropymeasurements onOak Creek xenolith samples OC-02, OC-03, and OC- 04(eastern Sierra Nevada). Host rock is a 150,000year old volcanic pipe [Ducea and Saleeby, 1996].Xenoliths are spinel peridotires; depth estimates based on clinopyroxene barometry are 29 to 34kin. (a) Equal-area.lower hemisphere projections ofa, b, andc axesof olivine. Contour intervalis 2% per 1% area. Mininmm contours (2%) are shown as dashed lines. (b) Compressional velocityanisotropy and shear wave splitting anisotropy. Contour intervals are 0.2 km/s for Vp and 0.1km/s for AI/•. Minimumcontours areshown asdashed lines and maximum values correspond to solid squares. in the [100](a axis) direction[Carter and Ave'Lalle- peratureand presence of up to 20%pyroxene (olivine mant, 1970; Nicolaset al., 1973]. The olivinecrystal- 8.45km/s andorthopyroxene 7.85 kin/s). lographicc axesshow slightly weaker orientations than Our study cannotconstrain to what degreethe P• the a- and b axes and tend to subparallel foliations. velocityvariations in thisregion are a resultof compo- We have calculated velocity anisotropiesof the xeno- sitional and thermal variations or of seismicanisotropy liths at a confiningpressure of 1000 MPa from the fab- in peridotite(eclogite in contrastis largelyisotropic ric diagramsusing the computerprogram of Crosson [Christensenand Mooney, 1995]), but wehave shown and Lin [1971]. From the single-crystalelastic con- that the measuredanisotropy in the Oak Creek xeno- stants of olivine, their pressure derivatives, and the liths,if representativeof a regionallydeveloped mantle universal-stageorientation data of eacholivine crystal, fabric,is on its ownsufficient to explainour seismic the program calculatesthe contributionof each min- observations. eral to the rock velocities in specified directions. One compressionalwave velocity and two shear wave ve- 7. Tectonic Implications and locities,with perpendicularpolarization directions, are Conclusions givenas output for eachspecified propagation direction. These velocities were then contoured to show the total A schematiccross section through California is shown anisotropypatterns in three dimensions.In Figure 10, in Figure11. It combinesdetails from the two west- contoureddiagrams are given for compressionalwave eastprofiles i and6 (Platesla andle). To the westof velocity(Vp) and shearwave splitting (AV•). Actual the San Andreas Fault are terranes moving northward velocitiesand anisotropieswill be lowerbecause of tern- with the Pacificplate. To the eastof the SanAndreas 10,918 FLIEDNER ET AL.- CRUSTAL STRUCTURE OF SOUTHERN SIERRA NEVADA i- Basin and Range Province o ._• o z E Sierra Nevada o San Joaquin Valley -5 0 'GmatValley --:'• -"' '-/-". .... 5 .. ',:',, ';". ( ,,Mesozoicarcplutons '• .:.. . 10 o . .. :-,:.-i--:':':•'•:.•!!;•!•,:•,•;•:•i;:!•.:::•:"-':-"--';, ,.... :,, _',-,,.... :, _',-,,-' o ., :,_,.[,,., ,._,_,.-. .... 15 :'.'.' .'..?'• ' .•:.':.::•:•..':5'!::::..:.::)i'i•i... ':'•:::"q•' midcrust (Ouran, h(•9'•)' Jurassicarc? cratonal ..• 20 North ß,.., 25 material? America granulitic lower •X,•"• uncertainboundary arc crust 35 oceanic crustal magmatic underplating • (ophiolitic)material 40 •_• maficlower crust 45 peridotite mantle ....•"'•"'•""•'"•-'.•.'..'{'•---ß Franciscan assemblage 10øø ø , ?:•::;:.::::'!:;i-i:'i::•sedimentarybasin 20 ø 55 Sri = 0.706line vertical exaggeration 2.5:1 1.... I .... I .... I .... I .... I .... I .... I .... I .... I .... I .... li'''l''•'l .... I .... I .... I'''•J .... I 0 25 50 75 1O0 125 150 175 200 225 250 275 300 325 350 375 400 425 450 distance (km) Figure11. Schematiccross section through south-central California (at roughly360 latitude) combiningthe velocityprofiles P1 (Platela) andP6 (Platelf). Thickblack line marks seismic Moho.Location of the lineof initial87Sr/8eSr - 0.706,thought to be roughlycoincident with edgeof NorthAmerican craton, from Kistler and Peterman [1978]. CRO+GVS is Coast Range ophioliteand Great Valley sequence [Godfrey and Kleinpeter, 1998]. The extentof faultsin the lower crust is uncertain. Faultare the formersubduction zone and arc complex, lowercrust on the westside of the Great Valley is not representedby the Franciscanassen•blage and Great visible in our data. ValleySequence/Great Valley Ophiolite in the forearc, Figure ll showsthe SierraNevada arc continuingbe- the Sierra.Nevada as the formervolcanic arc, and the neaththe Great ValleyOphiolite. Since there is no sig- Basin and Range provincein the backarc. nificant differencein velocity betweenthe deep crust There is a relatively clear divisionbetween the ter- of the Franciscanassemblage and the Sierra Nevada, runeswest of the Great Valley. Crustalvelocities gen- it is not possibleto draw the boundary between the erallydecrease from west to east. In the Sur-Obispotwo on the basis of the present seismicdata. Neither terranethe high and low velocitiesare probablydue is there a clear seismicexpression of the boundarybe- to a mixtureof ophioliticor intrudedmaterial (in the tween the Precambrian North American craton and the middlecrust) and remnant oceanic crust (in the lower younger accreted terranes to the west. This bound- crust)with low-velocity Franciscan assemblage [Howi½ ary is usually thought to coincidewith the Sri = 0.706 ½t al., 1993]. The Salinian block is believedto be a line [Kistler and Peterman,1973, 1978]in the Sierra pieceof Sierra.Nevada. arc transportednorthward by Nevada batholith, which coincides with an isostatic the SanAndreas Fault [Whiddenet al., 1998,and ref- residualgravity gradient [Jachcns and Griscom,1985]. erencestherein; James, 1992]. On the eastside of the This means that, in order to satisfy the seismicve- SanAndreas Fault, the Franciscan assemblage has lower locities, at least in the easternSierra. Nevada parts of velocitiesthan the Salinianin the upper15 km but is the lowercrust consist of continentalfelsic granulites or indistinguishablefrom the Salinianbelow that depth. metapeliteswith protolithsthat predatethe formation There is no seismicvelocity contrast between the Fran- of the arc. The only surface-seismicindication of struc- ciscanand the GreatValley Sequence or CoastRange ture withinthe Sierra,Nevada, crust is the slightvelocity Ophioliteabove the high-velocity body of theGreat Val- decreasein the midcrust.Although seismic velocities re- leyOphiolite (Godfrey and Klempercr [1998] found the quirea felsiccomposition throughout the crust,the low samein thenorthern Coast Ranges/Great Valley). The surfaceheat flow in the Sierra,Nevada, the linear heat FLIEDNER ET AL.: CRUSTAL STRUCTURE OF SOUTHERN SIERRA NEVADA 10,919 flow-heat production correlation observed by œachen- suring the regionaldistribution of sub-Mohoanisotropy bruch[1968] and surficialdensity variations [Saltus and would make an important contribution to resolvingthis Lachenbruch,1991; Oliver e! al., 1993]suggest a baseof problem. granitoids(with heat-generatingradiogenic U, Th, and In conclusion, we can make a rough estimate how K) at a depthof about 10 km [Lachenbruch,1968]. The thick the completeSierra Nevada arc was originally. We midcrustal low-velocity zone is therefore probably the assumethe sameproportions of upper/middle and lower baseof the plutons that characterizesthe Sierra Nevada crust as in the averagearc of Holbrooket al. [1992] at the surface. In the middle and lower crust, gneisses applied to the Mesozoic Sierra. Nevada although that or felsicgranulites (whether older continentallnaterial, "average arc" may be biased toward island arcs. The or radiogenicallydepleted arc plutons)with a low heat depth of crystallization exposed at the surface at the production must prevail. Increased lower crustal tem- latitude of our crosssection is 8 to 15 km [Ague and peratures that have not yet affected the low surfaceheat Brimhall, 1988]. So if we add 10 km of eroded up- flow in most parts of the Sierra Nevada would indeed per crust to the existing 35 km of low-velocity crust we make felsic granulite the compositionthat matches the form an original arc upper crust and midcrust of 45 km. observedvelocities (see section 5). This includes, at least in the eastern Sierra Nevada, 'an In the Mesozoicthe arc must have lookedvery differ- unknown amount of preexistingNorth American conti- ent from the present-day Sierra Nevada crust if it was nental crust and can therefore be consideredan upper then a typical continentalarc. Holbrooket al. [1992] bound. Forty five kilometersof original felsic crust im- showthe crustal columnof an averagearc to consistof ply an original mafic lower crust of about 30 km and a 22 km of upper and middle crust with a bimodal veloc- Mesozoic crustal column of 75 km thickness, compara- ity distribution(6.1 and 6.6 km/s) and 16 km of lower ble to the modern Andes [Zand! et al., 1994]. Only crust with a velocityof 6.8-7.0 km/s. The exposedKo- about a third of this conjectural lower crust can be histan arc sectionin Pakistan[Miller and Christensen, accounted for by the up to 10 km thick layer of ex- 1994]consists of a 15 km thickgranodioritic upper crust tremely low sub-Moho velocities found beneath much followed by 3 km of diorite with velocities of 6.0 to of the central and western Sierra Nevada that is most 6.9 km/s and a 25 km thick metagabbroicand amphi- likely to represent an eclogite facies lower crust. This bolitic lower crust with velocitiesof 7.0 to 7.5 km/s. layer balanceshardly more than the now eroded top of The modern Sierra Nevada seemsto have retained only the arc. Furthermore, the higher upper mantle veloci- the upper crustal and midcrustal part of that column ties and the presenceof peridotite xenolithssuggest that (at the low end of the velocitydistribution). A high- such a subcrustal layer is absent in the eastern Sierra velocity lower crust is completely absent. This miss- Nevada. The remainder of the original lower arc crust ing lower crust would have had a mafic composition if still present at all is now indistinguishablefrom the (metagabbro,mafic granulite, or amphibolite)and must surrounding mantle. have been present during arc formation accordingto current models of the generation of continental crust Acknowledgments. This study was supported by Na- [e.g., Taylor and Mcœennan,1995]. At least for the tional ScienceFoundation (NSF) ContinentalDynamics Pro- gram grant EAR-93-16008. The laboratory studies were deepestparts of the arc crust, the problem of the "miss- funded by NSF Continental Dynamics Program grant.EAR- ing" mafic residuemay be partially semantic:Rapp and 93-17522. The petrofabrics were measured by Kim Schram Watson[1995], among others, point out that the residue at Purdue University, West Lafayette, Indiana. We thank would be a garnet pyroxenite, an eclogitefacies rock. the reviewers, especially R. Saltus, C. Jones, G. R. Keller, Eclogiteshave seismic velocities above 7.6 km/s [Chris- and M. Ducea, and the Associate Editor W. Mooney for their detailed and insightful (even if sometimestough) com- tensen and Mooney, 1995], placing them in the seis- ments. mic mantle. Xenoliths of the appropriate composition havebeen found in the centralSierra Nevada [Ducea and References $aleeby,1996, 1998b](see below). The up to 10 km of sub-7.6 km "mantle" found in the central and western Ague, J. J., and G. H. Brimhall, Maõm•tic arc •symme- try and distribution of anomalous plutonic belts in the Sierra Nevada must be composedof garnet granulite, batholiths of California: Effects of assimilation, crustal pyroxenite,eclogite, or a mixture of suchlithologies as thickness, and depth of crystallization, Geol. Soco Am. the velocitiesare too low for peridotiteunless the upper Bull., 100, 912-927, 1988. mantle is partially molten. Partial melt as an explana- Blackwell, D. D., The thermal structure of the continen- tal crust, in The Structure and Physical Properties o.f the tion for low upper mantle and lower crustal velocities Earth's Crust, Geophys. Monoõr. Ser., vol. 14, edited by is supportedby magnetotelluricevidence for enhanced J. G. Heacock, pp. 169-184, AGU, Washington, D.C., conductivity in the lower crust and upper mantle of the 1971. Great Valley,western Sierra Nevada., and OwensValley Carder, D. S., Trans-California seismicprofile, Death Valley [Park et al., 1996].Since the thermalconditions in the to Monterey Bay, Bull. S½ismol. Soc. Am., 63, 571-586, 1973. upper mantle are so uncertain, seismicvelocities alone Carter, N. L., and H. G. Av• Lallemant, High temperature are insufficientto infer its composition.As peridotires flow of dunite and peridotite, Geol. Soc. Am. Bull., 81, in contrastto eclogitescan be stronglyanisotropic, mea- 2181-2202, 1970. 10,920 FLIEDNER ET AL.: CRUSTAL STRUCTURE OF SOUTHERN SIERRA NEVADA Catchings, R. 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