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Threedimensional Seismic Model of the Sierra Nevada Arc, California

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Threedimensional Seismic Model of the Sierra Nevada Arc, California 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.
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