Jun 10, 1995 - The marbles are primarily .... Marble. 5.0 + 3.0. 4.6 + 3.0. 4.6 + 2.9. 4.6 + 2.9. Felsic granulite .... provinces originally defined by Roy et al. [1968] ...
JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 100, NO. B7, PAGES 9761-9788, JUNE 10, 1995
Seismicvelocitystructure and compositionof the continental crust: A global view Nikolas I. Christensen Departmentof EarthandAtmosphericSciences,PurdueUniversity,West Lafayette,Indiana
Walter D. Mooney U.S. Geological Survey, Menlo Park, California
Abstract. Seismictechniquesprovidethe highest-resolution measurements of the structureof the crust and have been conducted on a worldwide basis. We summarize the structure of the
continentalcrustbasedon the resultsof seismicrefractionprofilesandinfer crustalcomposition as a functionof depthby comparingtheseresultswith high-pressure laboratorymeasurementsof seismicvelocityfor a widerangeof rocksthat are commonlyfoundin the crust. The thicknessandvelocity structureof the crustare well correlatedwith tectonicprovince,with extendedcrustshowingan averagethicknessof 30.5 km andorogensan averageof 46.3 km. Shieldsandplatformshavean averagecrustalthicknessnearlyequalto the globalaverage. We havecorrectedfor the nonuniformgeographical distributionof seismicrefractionprofiles by estimatingthe globalarea of eachmajor crustaltype. The weightedaveragecrustalthicknessbasedon thesevaluesis 41.1 km. This value is 10% to 20% greaterthanpreviousestimateswhichunderrepresented shields,platforms,andorogens.The averagecompressional wave velocity of the crustis 6.45 km/s, andthe averagevelocityof the uppermostmantle(Pn velocity)is 8.09 km/s. We summarizethe velocity structureof the crustat 5-km depthintervals, bothin the form of histogramsandas an averagevelocity-depthcurve,and comparethese determinationswith new measurements of compressional wave velocitiesand densitiesof over 3000 igneousand metamorphicrock coresmadeto confiningpressures of 1 GPa. On the basis of petrographicstudiesand chemicalanalyses,the rockshavebeenclassifiedinto 29 groups. Averagevelocities,densities,and standarddeviationsare presentedfor eachgroupat 5-km depthintervalsto crustaldepthsof 50 km alongthree differentgeotherms.This allowsus to developa modelfor the compositionof the continentalcrust. Velocitiesin the uppercontinental crustare matchedby velocitiesof a largenumberof lithologies,includingmany low-grade metamorphicrocksandrelatively silicic gneissesof amphibolitefaciesgrade. In midcrustal regions,velocity gradientsappearto originatefrom an increasein metamorphicgrade,aswell as a decreasein silica content. Tonalitic gneiss,graniticgneiss,andamphiboliteare abundant midcrustallithologies. Anisotropydueto preferredmineral orientationis likely to be significantin upperand midcrustalregions. The bulk of the lower continentalcrustis chemically equivalentto gabbro,with velocitiesin agreementwith laboratorymeasurements of marie granulite. Garnetbecomesincreasinglyabundantwith depth,andmarie garnetgranuliteis the dominantrock type immediatelyabovethe Mohorovicicdiscontinuity.Averagecompressionalwavevelocitiesof commoncrustalrock typesshowexcellentcorrelationswith density.
Themeancrustal density calculated fromourmodel is2830kg/m 3,andtheaverage SiO2content is 61.8%.
Introduction
gradientmodels,oftenwith lateralvariabilities, whichemphasize local structuralcomplexities.Seismicreflectivityalso
Knowledgeof the structureandpetrologyof the continental varies widelyin different geologic provinces, providing addicrustis of fundamental importancein understanding crustal tional supportfor the existenceof crustalheterogeneity.These
generationand evolution. The last two decadeshave witnessed
new seismicimages of the continentalcrustare consistentwith a complex origin involving multiple episodesof accretion,deformation,metamorphism,plutonism,andvolcanism. Even though a variety of geologicand geophysicalstudies haveprovidedan importantframeworkfor our understanding of
a remarkable increase in seismic measurements of crustal
structure. In the past, seismic refraction models of crustal
structure wereusuallypresented in termsof rathersimplelayer models. Recently,however,layeredcrustalmodelshavebeen generallydiscarded in favor of more geologically reasonable the continental crust, serious deficiencies remain in our knowl-
Copyright 1995bytheAmerican Geophysical Union. Papernumber95JB00259. 0148-0227/95/95JB-00259505.00
edge of the geologicalprocessesthat create and rework continental crust. Only when sufficiently detailed information on the distributionof rock typeswith depth,aswell as their lateral variability, is obtained,will an understandingof the geologic processesthat form continentalcrustbe possible. 9761
9762
CHRISTENSEN
AND
MOONEY:
CONTINENTAL
CRUST
Thispapersummarizes the seismicstructure of the continen- Seismic Refraction
Observations
tal cruston the basisof a newworldwidedatacompilation.The term continentalcrust, as used here, includesall land masses A total of 560 determinations of the velocity-depthstructure above sea level, with the exceptionof oceanicvolcanicpla- of the crusthavebeencompiledin this study(Figure1). Every teaus,suchas IcelandandHawaii. Continentalmarginsare not effort has been made to include all global seismicrefraction included,sincetheseareasusuallyhavea seismicstructurethat measurements of the continental crust which we have deteris intermediate between oceanic crust and continental crust. In mined to be reliable. Continentalmargins have not been orderto interpretseismicobservations in termsof crustalpe- included,but measurementsin inland seasand lakes, such as trologyandchemistry,we presentvelocitiesof compressionalHudson'sBay, the Great Lakes of North America, and Lake waves as functionsof depthand temperaturefor major rock Baikal,havebeenincluded.Thesedatawere compiledfromthe typesbelievedto be significantconstituents of the continental publishedliteratureandfrom a limited numberof unpublished crustanduppermantle. Thiscompilation of laboratory velocity reports. The publishedliteraturecoversthe years 1950-1993 datafor a wide varietyof continentaligneousandmetamorphic andincludesjournal articles,monographs, and specialpublicarocktypesprovidesa basisfor thenewinterpretation of crustal tions such as meeting proceedings,governmentalopen file composition.In thispaper,we donot attemptto presenta sur- reports,and annualreportsof researchinstitutions. Unpubvey of all previouswork relatingto the composition andstruc- lished literature consistsprimarily of high-quality technical ture of the continentalcrustbut insteadfocuson the integration reportsof recentfield measurements thathavebeenissuedprior of the compilations.For comprehensive discussions of previ- to journalpublication. Table 1 liststhe previouscompilations ousideason crustalcomposition, the readeris referredto re- of seismicrefractiondatathat havebeenincludedin this compidistriviewarticlesby Kay andKay [1981],FountainandChristensen lation. It is evidentfrom Figure 1 that the geographical [1989], Percivalet al. [1992],Holbrooket al. [1992],Rudnick butionof seismicrefractionprofilesis uneven,with manyprofiles in central North America, westernEurope, Eurasia, and [1992], andDowns[1993].
80 ø
0ø
170 ø
0ø
Figure 1. Locations (solidtriangles) of 560individualvelocity-depth functions compiled for thisstudy.Only theresultsof seismicrefraction/wide-angle reflection profileshavebeenused.Individualtrianglesarelocated at the midpointof individualcrustal-scale profiles. A singledetermination in Antarcticais not shown.Lines of trianglesat uniformspacingareindicativeof long-range seismicrefractionprofileswith detailedcrustalinformation.No efforthasbeenmadeto averageindependent neighboring determinations into a singlemeasurement;ratherthis averaginghas been accomplished throughthe use of histogramsof largenumbersof measurements. Principaldatasources arelistedin Table1. Dataselection andinterpretation uncertainties are discussed in the text.
60 o 180 o
CHRISTENSEN
AND
MOONEY:
Table 1. SeismicRefractionData Compilations Reference
Numberof Profiles
Region Covered
Tuveet al. [1954] Press [ 1966] JamesandSteinhart[ 1966] McConnellet al. [ 1966] WarrenandHealy [ 1974] Gieseet al. [ 1976] Christensen [1982] Allenbyand$chnetzler[ 1983] Sollet et al. [ 1982] Prodehl [1984] Meissner [ 1986] Meissneret al. [ 1987] Braile et al. [1989] MooneyandBraile [1989] Collins[1988] Mechie and Prodehl [ 1988] Beloussovet al. [ 1991] Holbrooket al. [ 1992] Kaila andKrishna [ 1992] GEON Center[ 1994] Li andMooney [ 1995]
15 30 30 100 40 80 278 200 297 200 150 100 200 220 50 20 120 90 25 200 25
North America global North America global North America Europe global North America global global global Europe North America North America Australia Afr-Arabia formerUSSR global India formerUSSR China
Total numberof profiles compiledhere
560
global
Australia. There is an underrepresentation of datafrom Africa, South America, northern Canada, Antarctica, and Greenland.
CONTINENTAL
CRUST
9763
Muller [1971], which can provide the completeseismicresponseof an elasticmedia,includingseismicvelocitygradients and low velocity zones. Inversemethods,which provide seismic velocity modelsbasedon the direct inversionof the data, have been developedin the last 10 yearsbut have been applied in lessthan 5% of the resultscompiledhere. Data quality control is an importantconsiderationin any compilationof global geophysicaldata. In the presentcase, theseseismicprofileshave been collectedover a period of 40 yearsby more than 100 separateinvestigators.Thereforecareful guidelineshave been adopted. First, where coincidentdata sets are available, a selectionhas been made of the higherqualitydatawith a moremodeminterpretation.Second,where only onedataset is available,an assessment hasbeenmadeof the qualityof the publishedinterpretation prior to inclusionin this compilation. Major considerations included(1) signal-tonoiselevelsand spatialdensityof the data;(2) clarityof secondmy phasesusedto interpretcrustallayeringand Moho depth; and (3) use of appropriatemethodsto comparetheoreticaland observedtravel times and, in many cases,seismicamplitudes. However,for someolderpublicationstheseguidelinesare difficult to apply as thesepapersdo not includemany examplesof the recordeddata and the methodof data interpretationis not fully described. In thesecases,which constituteonly 10% of the data compiledhere, a critical assessment of the data reliability has been made basedon the written descriptionof the
dataacquisition andinterpretation methods. In the seismicrefractionmethod,apparentseismicvelocities are directly measured,while the depthsof refractinghorizons are successivelycalculatedfrom the uppermostlayer to the deepestlayer measured. Thus seismicvelocity determinations generallyhave lower percent errorsthan depthdeterminations. For the seismicprofile data compiledhere, seismicvelocities
Despitethesegapsin geographicalcoverage,the availabledata includemorethan adequatecoverageto allow us to accurately are accurateto 3%, or about + 0.2 km/s. Velocities are 2 or 3 times lessaccuratefor the few unreversed profilesthatwe have characterize continental crest. includedfor geographicalcompleteness.The 3% velocity uncertainty is conservativeand is discussedby Mooney [1989] Reliability and Precision andHolbrook et al. [1992]. It is basedon suchfactorsas the The reliability of publishedcrustalvelocitymodelsis critical accuracyof seismicarrival-time determinations,chronometer to the usefulnessof the compilationusedhere, especiallyas no corrections,field surveyinguncertainties,typical seismograph new crustal models have been recalculatedfor this study. andshotpoint spacings,andeffectsof lateralvariationsin nearRather,publishedinterpretations have been evaluatedfor relisurfacelow-velocity anomalies,especiallysedimentarybasins. ability. All boundarydepths(includingthe Moho) are accurateto about A variety of methodshavebeenusedto interpretthe seismic 10% of the depth. Thus a reportedcrustalthicknessof 40 km refractiondatausedin this compilation[cf. Mooney,1989]. By typicallyhasan uncertaintyof + 4 km. far the most commonmethod is the interpretationof seismic traveltimes (but not amplitudes)using either one- or twoCrustal Thickness,Average Crustal Velocity, dimensionalmodelingmethods. Sinceabout1980, most interand Pn Velocity pretationshave involvedthe applicationof raytracingto comPerhapsthe mostbasicparameterregardingthe continental pare the traveltimesthrougha two-dimensional (2-D) velocity model with observedtraveltimes. Prior to 1980, most crustal crustis total thickness(surfaceto Mohorovicicdiscontinuity). modelswere developedin termsof eitherplane-dippinglayers Becauseall previousestimatesof averagecrustalthickness or by interpolatinga set of 1-D modelsinto a 2-D model. In have been basedon the simple averagevalue from reported the former Soviet Union and India, very densedata setswere measurements,we have also calculatedthis unweightedaverrecorded,and complex2-D modelscouldbe constructed from age. In addition,we have calculateda more accurateaverage of five well-recordedwide-anglereflectionsthat imagedmajor crustal thatis weightedby thepercentareaof crustthatconsists basic crustaltypes describedbelow. This weightedaverage velocitydiscontinuities. The modelingof seismicamplitudes,in additionto travel corrects for biases that result from the numerous measurements times,becameroutinelypossiblewith the developmentof effi- that are available for the thin extendedcrustof westernEurope cient computercodesfor the calculationof amplitudesin one- and the westernNorth America, in contrastto the sparsemeasand two-dimensional media. For 1-D media, the most com- urementsavailable for the thicker crust of Africa, SouthAmermonly usedmethodis the reflectivitymethodof Fuchs and ica, Antarctica, and Greenland.
9764
CHRISTENSEN
AND MOONEY:
CONTINENTAL
CRUST
100 90
Worldwide (n o
80
Compilation
7O
Average 39.17 krn
;:' 60 O
5o
o
4o
•
30
Std. Der.
8.52 krn
E
z
2o lO o
16
20
24
28
32
36
40
44
48
52
56
60
64
68
72
Crustal Thickness (km) Figure 2. Histogramof crustalthickness.Locationsof dataare shownin Figure1. The thicknessshownin theboxis the averageof 560 measurements. Theweightedaveragecrustalthickness, basedon estimated proportionsof tectonicprovincesby area,is 41.0 km + 6.2 km.
Our worldwide compilationshowsthat the mean crustal tire rift zones,suchas the Kenya rift, wherePn velocitiesof
thicknessexpressedas the unweightedaverageof reliable 7.6-7.7 km/s havebeenreliablymeasuredalongthe axis of the observationsis 39.2 km, with a standarddeviationof 8.5 km
rift [Mechieet al., 1994]. The existenceof seismicanisotropy in theuppermantlehasbeenwell determined from oceanicand that have consistently shownan azi(Afar Triangle,Ethiopia,recentlytransitional from oceanic) continentalinvestigations of the Pn velocity[Raitt et al., 1969;Barnandthethickest(asdetermined fromseismic refraction data)is muthaldependence 72 km (TibetanPlateau,China). Recallingthe estimated10% ford, 1977]. error in depth determinations,the continentalcrustcan be said to vary in thicknessbetween14 and 80 km. However,more that 95% of all measurements fall within two standard devia- SeismicVelocitiesas a Functionof Depth
(Figure2). The thinnestreportedcontinentalcrustis 16 km
tions, between 22 km and 57 km.
In order to comparefield measurements of crustalseismic
Our estimateof meancrustalthickness is higherthanmany velocitieswith laboratorymeasurements, we havemadehistopreviousestimates(Table 2). This can be attributedto the gramsof crustalvelocitiesat 5-km depthintervalsto a maxiinclusionof a substantial amountof newlyavailabledatafrom mum depthof 50 km (Figure5). The velocitiestakenat each the formerSovietUnionwhichincludes 40-50 km thickplat- depthwere point values,not averagesover a depthrange,and form and shield crust. exact values were determinedin the case of velocity-depth Average crustalvelocity (Figure 3) is a parameterthat is gradientzones. Thereareinsufficientdatafor the histograms to well determined from seismicrefractiondata. Our datapro- be meaningfulat greaterdepths(Figure2). The histograms at videsa value of averagecrustalvelocityof 6.45 km/s, with a 5-km and 10-km depthsare sharplypeakedat 6.0-6.2 km/s, standarddeviationof 0.23 km/s. This unweightedvalue is corresponding to seismicvelocitiesthat are typicallyreported higherthansomeearlierestimates [e.g.,Smithson et al., 1981], for the crystallineuppercrust.At 15-kmand20-km depths,the but is the same as a previousestimatefor the conterminous velocitydistributionbroadensconsiderably and seismicvelociUnitedStates[Braile et al., 1989]. tiesgreaterthan6.3 km/sbecomemorecommon.At a depthof We haveadoptedthe definitionof the top of the upperman- 25 km, the velocitydistributionis peakedonceagain,at a value tle as that depthwherethe seismicvelocityexceeds7.6 km/s. of 6.6 km/s, indicating that typical middle crustal seismic This uppermantle velocity is frequentlyreferredto as the Pn velocities have been reached. In terms of common crustal velocity,for "normalP (compressional)wave". Our compila- nomenclature,by 25 km depth,we have passedthroughthe tion of the Pn velocitiesshowsa rangeof valuesfrom 7.6 to 8.8 "Conraddiscontinuity,"a gradationalboundarythat separates km/s (Figure4). The unweightedaveragePn velocityis 8.07 the upper crust(6.0-6.3 km/s) from the intermediate-velocity kin/s, and the standarddeviation is 0.21 km/s. This estimate (6.6-6.8 km/s) middle crust(or in somecaseslower crust)in compares well with previouscompilations of Pn velocity(Table many crustalsections. 2). There are two factorsthat appearto play a major role in The histogramsbetween30 km and 50 km showa continued determiningupper mantle velocity: temperatureand anisot- trend to higher crustalvelocitiesand at 40 km showa distinct ropy. The effect of temperaturecanbe clearlymeasuredin ac- bimodalpattern,with peaksat 6.8-6.9 km/s and 7.2 km/s. At
CHRISTENSEN
AND
MOONEY:
Table 2. ContinentalSeismicProperties Reference
Region
Value
Estimatesof ContinentalCrustal Thicknesses
Woollard [ 1959] Woollard [ 1959] Press[ 1966] Pavlenkova[1979] Garland [1979] Braile et al. [1989] Fowler [1990] TwissandMoores [1992]
westernEurope 35 km easternCanada 40 km globalrange 28-65 km Eurasia 42 km global average 40 km North America(average) 36 km globalaverage 35 km globalaverage 35 km
This paper
unweightedglobal average weightedglobal average
This paper
39 km 41 km
Estimatesof AverageCrustal Veloci.tv*
Mohorovicic[ 1909] Press[1966] Smithson et al. [ 1981] Braile eta/. [ 1989] Beloussov et al. [ 1991]
Yugoslavia global global NorthAmerica Eurasia
This paper
unweightedglobal average weightedglobal average
This paper
5.7 km/s 6.4 km/s 6.3 km/s 6.44 km/s 6.55 km/s
6.45 km/s 6.45 km/s
Estimatesof Uppermost Mantle VelocityPn
Mohorovicic[ 1909] Press[1966] Garland[1979] Braile et al. [1989] Beloussov et al. [1991]
Yugoslavia 7.75 km/s global 7.9-8.2 km/s global 8.0 km/s North America(average)8.0 km/s Eurasia 7.7-8.6 km/s
CONTINENTAL
CRUST
9765
genssuchasthe Urals,Appalachians, andthe Tien Shan,China. Continentalarcsincludethe trans-Mexicanvolcanicbelt, Cascades of North America, and active volcanic terrane of the westernPacific. Extendedcrustincludessuchregionsas the Basin and Range of the western United Statesand much of westernEurope. Rifts includeEastAfrica, Lake Baikal, andthe Rio GrandeRift. Extendedcrustand rifts are distinguished
from eachotherin this studybecausethey are traditionally consideredseparatelyin the geologicalliteratureand because thesetwo typesof crusthavebeenthe subjectof intensivegeophysicalinvestigationwhich have distinguished differencesin their seismic structure.
Shieldsandplatformsshowan averagecrustalthicknessthat is closeto the worldwideaverage,41.5 km. Extendedcrust,as the name implies, has been thinnedand showsan average thickness of 30.5km with a standard deviationof 5.3 km (Table 3). Orogensshowa wide rangeof crustalthicknesses, ranging from about 30 km to 72 km. Rifts, both active and inactive, also showa broadrange, from 18 km to 46 km. It shouldbe emphasizedthat some of these rangesare obtainedwithin a singletectonicprovince. For example,the crustalthickness within the Alps variesfrom about35 km to as muchas 60 km, and the Kenya rift showscrustalvariationsalongstrikeof the rift that amountto 20-36 km. Thus significantvariationsoccur alongthe strikeof major geologicfeatures. As was mentionedabove, the available global seismicrefractiondatahavea nonuniformgeographic distribution, which resultsin a stronggeographical bias in suchquantitiesas average crustal thickness. In order to correctthe nonuniform data
distribution,we have calculatedcrustalpropertiesusing a weightedaverage. Our approachis similar to that usedby Holbrook et al. [1992] to calculateaveragecrustalvelocity. We estimatethe followingproportionsof continentalcrustby area: 69% shieldand platform; 15% old and youngorogens; 9% extendedcrust;6% magmaticarc; 1% rift. Mean values of crustalthickness,averagecrustalvelocity, and Pn (upper mantle) velocity are listed in Table 3 for eachof thesecrustal types, as are the weighted values determinedfrom the five
crustaltypes. This procedurecorrectsfor the overrepresentation of crustalmeasurements in regionsof extendedcrust,such 8.07 km/s as western Europe and the Basin and Range of the western This paper United States,andthe lack of measurements from Africa, South 8.09 km/s America, andAntarctica. The weightedmeancrustalthickness of 41 km is 2 km thickerthanthe simplearithmeticmeanof 39 *No unconsolidated sediments. km. We believethat the weightedvaluesmoreaccuratelycharacterizeaveragecontinentalcrust. Vast portions of the continental crust are Precambrian 45 km and 50 km depths,the modalvelocityis 7.3 km/s, but a shieldsandplatforms(Figure 8), whereasmuch seismicexplobroad distributionof velocitiespersists(from 6.1-7.5 km/s), rationhas concentrated in the youngercrustof westernEurope andthe bimodalpatterncanbe weakly discerned.The increase andthe coastalregionsof North America. Thus someprevious in crustalvelocitywith depthin the crustcan be seenin a perstudieshave overemphasizedthe significanceof the crustal spectiveplot of these10 histograms(Figure6). structure of Phanerozoic crust. Precambrian crust is more This paper
unweightedglobal average weightedglobal average
"typical"of the continentalcrustthan is Phanerozoiccrust. We notethat the crustalstructureof Precambriancrust,which spans the period0.6 Ga to 3.8 Ga, appearsto showvariationswith age So far our discussionhas been limited to a compilationof the entireworldwide data set. However,it has long beenrec- of the crust. Drummond [1988] and Durrheim and Mooney ognizedthat there are importantcorrelationsbetweencrustal [1991, 1992]presentevidencethat mostProterozoiccrusthasa high-velocity(7 km/s) structureandtectonicprovince. We havethereforedividedour thicknessof 40-55 km and a substantial datainto five tectonicprovinces(Figure7) and have examined layer at its base,while Archeartcrustis only 27-40 km thick (exceptat collisionalboundaries)andgenerallylacksthe basal the propertiesof the crust in these provincesto searchfor trends.Shieldsandplatformsoccupyby far the largestarea of high-velocitylayer. Durrheirnand Mooney [1994] arguethat continentalcrust(Figure8). Orogensincludethe young,active there are also differencesin major and minor trace elements mountainbelts of the Alps, Andes,and Tibet and ancientoro- betweenArcheanand Proterozoiccrust. The secularchangein Tectonic
Provinces
9766
CHRISTENSEN
AND MOONEY:
CONTINENTAL
CRUST
lOO 90
Worldwide
Compilation
(t) 80 o
Average 6.45 km/s
•
70
(t)
60
0
5o
Std. Dev.
0.23
km/s
o
4O
IE
30
z
2O 10 o
5.5
5.7
5.9
6.1
6.3
6.5
6.7
6.9
7.1
7.3
7.5
Average Crustal Velocity (km/s) Figure3. Histogram ofaverage crustal velocity.Locations ofdataareshown inFigure1. Theweighted averagecrustalvelocityis 6.45 km/s+ 0.21 km/s.
crustalpropertiesis attributedby them to a declinein mantle temperature,which plays a major role in the magmaticand rheologicprocesses of crustalevolution. The seismicstructureof the crustdescribedaboveprovides importantconstraintson the compositionand evolutionof the crust. We rely on laboratorymeasurements of rock types commonlyfoundin the continentalcrustto guideour interpretation of crustalcomposition. In orderto comparevelocitypressuredata with velocity-depthdata, a weighted reference velocitydepthfunctionhasbeenmadefrom the averageof the
five tectonicprovinces(Table 3 and Figure 9). Note that the velocity-depth functionsof Figure 9 do not indicatevelocity discontinuities in the crust, because discontinuities are
smoothedby the averagingprocess. It is perhapssurprising thatthe calculatedstandarddeviationsare consistent with depth in view of the expecteddecreasing resolutionof seismicvelocity in the lower crust. This consistency may be attributedto greaterlithologicdiversityin the upperandmiddlecrust(where velocityresolutionis higher)as comparedto the morehomogeneous lower crust.
140 130
.............• :•• ..........•...: Worldwide
120 110
•
100
Compilation
iiiiiiiiiiiiiiiiiiiiiiiil AveraDge;.8'0.0;1k•l/js
9O 8O 7O 6O 5O
4O 3O 2O 10 0
7.6
7.7
7.8
7.9
8.0
8.1
8.2
8.3
8.4
8.5
8.6
8.7
8.8
8.9
Upper Mantle Velocity (krn/s) Figure 4. Histogramof uppermostmantle velocity (Pn). Locationsof data are shownin Figure 1. The weightedaveragePn velocityis 8.09 km/s + 0.20 km/s.
CHRISTENSEN
AND MOONEY:
CONTINENTAL
lOO
CRUST
9767
80
c 80 o
õ 60
o 40
o
40
d 20 5.0 I
I
I I I5.4I I I I5.8I I I I6.2I I I I 6.6I I I I 7.0I I I 171.41 I
I
I
I
5.0
I
I
I
I
5.4
CompressionalWave Velocity (km/s)
I
I
I
5.8
I
I
I
6.2
I
I
I
I
6.6
I
I
I
I
I
7.0
I
I
I
I
7.4
Compressional Wave Velocity (km/s) 60
lOO
-
k
= 80
o
o
so o 40
20
5:0 5.4 '5:"'•::•2' ' '6'6 70 ' '7'.4''
0 510'' '5'4'' '5'.8'' '6'2'' '6'.6'' '7'0'' '7'4'' ....
Compressional Wave Velocity (km/s)
Compressional Wave Velocity(km/s) 40
IO0 = 80 .m
o
- I 15krn I
c 3o o
_
so _
_
o 40
20
o
ß
20
(51o
_
z
-
_1 _ i
i
i _1 . i
i
i _1 _ i
....
i
i _1 _1
i
•
. i
.i
i
_1 _ i
i
i
i
i _1 _ i
i
i _1 .......
i
7.4
20
lOO
õ16
c 80
t•
i
Compressional Wave Velocity(km/s)
Compressional WaveVelocity(km/s)
._.
i
5.4
_
60-
•
o 4o
8
2o _
o
i
5.0
5.4
5.8
i
i
6.2
i
i
i
6.6
i
i
i
i
i
7.0
i
i
i
i
i
i
i
i
i
5.0
7.4
i
i
i
i
i
....
5.8
6.2
6.6
7.0
i
7.4
Compressional Wave Velocity(km/s)
Compressional Wave Velocity(km/s) 10
lOO = 80 o
i
5.4
• [2 km
• 8
_
60 0
o 40
20 5,0 ,
,5141 . I5•8 - -6:2
6.(• ' '7'0' . ' '7'.4' '
Compressional Wave Velocity(km/s)
Figure5.
4
_ I
5.0
I
I
I
I
I
5.4
I
I
I
5.8
I
I
_1 .....
6.2
6.6
I
7.0
compressional Wave Velocity(km/s)
Histograms of crustal velocityat 5-kmdepthintervals.Locations of dataareshown in
Figure1. Data havenot beensortedby geologicor tectonicsettingor crustalage. Shallowcrustalvelocities lessthan 5.0 km/s, corresponding to sedimentary rocks,and sub-Mohovelocitiesgreaterthan 7.5 km/s have been excluded.
7.4
9768
CttRISTENSEN
AND
MOONEY:
CONTINENTAL
CRUST
I 5km •::-:•10km • 15km • 20km kXr•t25km lOO
:• •:•. 30km •
35krn
"'""".'.'.':• ß 40km [•1 45km • 50km
8o
6o
4o lO 2o
--20 3o 5O
Depth (km)
CompressionalWave Velocity(km/s) Figure 6. Perspectiveplot of the samedata as shownin Figure 5, with the deepcrust(50 km) in the foreground. A bimodal distributionin seismicvelocity is visible at depthsof 40-50 km, as are the increasing numberof observations andthe decrease in averagecrustalvelocityat shallowdepths.
Orogens Shields& Continental Platforms
Riffs
Arcs
Extended
10
2O
40
50
60
Vp (km/s)
,e-
0
O•
e,O
0'>
xl'
•0
0
0
•
0
•0
I'-.
x$'
0 •--
• 6
•0 t-.
'•d
'•. •
(',,I c5
•0 0 ,no
I•. • co d
• eq co o
0 •--
0 o
e.Z> 0 e.. o
'•. •0
'•. o
•
xl'
o ,-:. e.. o
0• ,•-- c5
eq. • • c5
0• q ,no
• ,co d
o> e,! co o
•. t-.
o o
e... o e.. o
• ,-: ca o
e.. o
•
c5
,n
u5 c5
c6 o
co o
•
o
e.. o
•0
c5
O.
0•
,-:.
o
•
•
o
•0
q
•
,-
I•
0
xl'
0
(e>
0')
•
,-. •0
,.-:. o
0
c,i ,-
t-.
o •
d
•0
q
,-
•0
•0
I'-.
,.-:. o
• e..
d
,-; o
•
o c•
•
d
,e-
0
• •
eq. o
•
o
o
e,O I'--
o o
i,,. ,c6 c5
•0
eq
I---
e,,I
(',,I
e,O •0
0
'•.
(e> I'--
•0
0
eq
•
o
,n
c5
u5 o
I'-.
eq (e>
o
• •
e.. o
0') '•.
•
0') 0
•
•
o
•0 • co o
•
o •.
o
q o
•
o
•o o r-.. o
•
0
•
0') e,l
0
0
0
0')
0')
•
0
co o
•
o
•..
o
•0
c5
C,,I
,e-
e,,I
co d
0
r,O 0
e,,I ',-
O•
•0
I•
,e-
0
Xl'
I'--
Xl'
0
,e-
O•
,""1 0
i'-.
o
i',.. d
•
o
u•
o
r,o c:5
r,o o
I,,.. o
I,,.
o
•0
o
•
O0
,e-
I'-.
0')
0
0
I'-..
•0
e,O 0
O0
•0
0')
0
0
I-,.
o
•0
o
•
o
e.z> o
I-,. o
•0
o
,-:. ,--:
o.
,-:
cq. •
o
o
(.o o
•
eq
,.-
e,.l:>d
0
'•.
cn •
0
,'z. o
e-
o
i'-.. o
cn ,-:
9776
CHRISTENSEN
AND
MOONEY:
CONTINENTAL
CRUST
E
o
E
CHRISTENSEN
AND MOONEY:
CONTINENTAL
CRUST
9777
40
Mafic Granulite
36
20 km 32
28 • 24 o 20
-• 16
• 12 8
4 0
6.2
__ ,
!
6.'4
6.6
6.8
7.0
7.2
7.4
7.6
CompressionalWave Velocity (km/s)
Figure10. Maficgranulite velocities atpressures corresponding to 20 km depth. sionbecomesimportantin laboratoryvelocity measurements. ally do not take this shorteninginto account,resultingin veVelocities are commonlycalculatedfrom transit times meas- locitieswhich are high, by as much as 0.05 km/s [Birch, 1960]. ured at elevatedpressuresand corelengthsmeasuredat atmos- Thesecorrections,which have beenappliedto the dataof Table calculatedfrom vephericpressure.At high pressures, corelengthschangedueto 4, were determinedfrom compressibilities samplecompression.Velocitiesreportedin the literatureusu- locity measurementsand an iterative techniquefirst described
Table5. AverageCompressional WaveAnisotropies andStandard Deviations RockCatagory
5 km
15 km
25 km
35 km
Granite-granodiorite
1.5 + 1.0
1.3 + 1.0
1.3 + 0.9
1.3 + 1.0
Diorite Gabbro-norite-troctolite Andesite Basalt Diabase Zeolite facies basalt
1.3 2.0 1.1 1.7 0.8 0.9
1.2 1.9 1.0 1.4 0.7 0.7
1.1 1.9 0.9 1.3 0.7 0.7
1.1 1.9 0.9 1.3 0.7 0.8
Prehnite-pumpellyite faciesbasalt
+ + + + + +
0.8 1.6 0.7 1.3 0.8 0.8
+ + + + + +
0.8 1.5 0.6 1.1 0.7 0.6
+ + + + + +
0.6 1.4 0.5 1.0 0.6 0.6
+ + + + + +
0.6 1.4 0.5 1.0 0.6 0.6
3.2 + 2.9
3.0 + 2.9
2.9 + 0.8
2.9 + 2.7
Greenschist facies basalt Slate
5.2 + 4.2 21.2 + 6.4
4.5 + 3.9 19.3 + 6.2
4.2 + 3.7 18.1 + 6.1
4.1 + 3.5 17.2 + 6.0
Metagraywacke Phyllite
4.8 + 3.5 12.4 + 7.7
4.1 + 3.1 10.6+ 6.2
3.6 + 2.5 9.9 + 5.6
3.3 + 2.1 9.5 + 5.2
Marble
5.0 + 3.0
4.6 + 3.0
4.6 + 2.9
4.6 + 2.9
Felsicgranulite Mafic granulite Mafic garnetgranulite Paragranulite Anorthositic granulite Amphibolite Mica quartzschist Granitegneiss Tonaliticgneiss
2.5 + 1.5 3.7 _+2.8 5.7 + 4.7 4.9 + 4.8 2.9 + 1.8 10.2+ 4.4 16.0+ 8.7 5.2 + 4.0 9.7 + 6.1
2.4 + 1.5 3.3 + 2.7 5.2 + 4.3 4.8 + 4.3 2.8 + 1.7 9.7 + 4.2 14.0+ 8.1 4.2 + 3.6 8.9 + 5.7
2.5 + 1.6 3.3 + 2.6 4.9 + 4.2 4.7 + 4.1 2.7 + 1.8 9.4 + 4.1 13.4+ 8.0 3.8 + 3.5 8.5 + 5.5
2.5 + 1.6 3.3 + 2.6 4.7 + 4.2 4.7 + 3.9 2.8 + 1.9 9.3 + 4.1 13.0+ 7.8 3.9 + 3.5 8.3 + 5.3
Anorthosite
3.7 + 2.1
3.4 + 2.1
3.4 + 2.1
3.4 + 2.2
Eclogite
2.2 + 1.1
1.9+ 0.9
1.8+ 0.9
1.9 + 0.8
Dunite
8.1 + 3.9
8.1 + 3.9
8.0 + 3.9
8.0 + 3.9
Serpentinite Pyroxenite
4.5 + 1.6 3.6 _+2.1
4.4 _+1.6 3.4 + 1.8
4.3 + 1.6 3.4 + 1.7
4.2 +1.6 3.3 + 1.6
Hornblendite
4.3 + 1.3
3.7 + 0.8
3.6 + 0.9
3.7 + 0.9
Quartzite
2.4 + 0.8
2.1 + 0.8
1.9+ 0.9
1.9+ 0.9
In percent.
9778
CHRISTENSEN
AND
MOONEY:
CONTINENTAL
With the exceptionof a few granitesand basalts,velocities
DiMbase
"'
for each rock were measured in three directions from cores
Andesite
o
n,'
CRUST
Diorite
takenin mutuallyperpendicular directions.To a first approximation, simple averagesof velocitiesin three directionsgive c Granite-Granodiorite reasonablevelocitiesfor isotropicmineral aggregates[Birch, G a b b ro-N o rite -Tro cto lite 1961]. Averageanisotropies and standarddeviationsare given Zeolite Facies Basalt at four depthintervalsin Table 5. The changein anisotropy • PrehnitePumpellyite Facies Basalt with depthis minimal for most rock types. The origin of the • Metagraywack. e anisotropies is preferredmineralorientation. • Greenschist FaciesBasalt Compressional wave anisotropies at 1 GPa, corresponding to ..3 .......:........ ...'!:. Phyllite Slate approximately35 km depth,are shownin Figure 11 for several of the rocks of Table 4. The igneousrocks are to a first approximationisotropic,whereasmany of the metamorphic rocksshowsignificantanisotropy.Anisotropyis a particularly •o :•E= !:: :::.:':::::::::::::::i ::!::i iii::;:i ?:i!iiiii::!::i?::f:i::!?iiif:!if:i!f:iiiiii!ii::!ii::iiiiiif:f:ii!i!::i?:f:ii! ITonalitic Gneiss importantparameterin the low-gradepelitic rocks, reaching •:•2./•;::::Z.•:.:.:.:•:=========================:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::•::•:•i:::::i::::::::::::::::::::::::::::::::::::::::::::::::::::: averagevaluesof 9.5% in phyllite and 17.2% in slate. For the medium-grademetamorphicrocks,tonaliticgneiss,amphibolite and mica quartz schist have average anisotropiesof 8.3%, -o !!i::!ii i!ill i!!iii iiii:•i] Anorthositic Granulite 9.3%, and 13.0%, respectively.Thesevaluesare all higherthan the 8.0% averageof mantle dunite. Low velocitiescorrespond to propagation normal to cleavage, foliation or banding -r Paragranulite [Christensen,1965]. The high grade granuliteand eclogite ::::::::::::::::::::::::::::::::::::::::::::::::::: Eclogite facies rock are relatively low in anisotropy. Thus seismic • cr i::i::i?:iiii:::::: :::::::::j:::::::::: ::i::11 i::::i!i ::?:i:: !::iiii::!::i! i :i:.i!::i::i?.!i!!i I D•unite• • • , anisotropyis likely to be a more significantpropertyof upper 0 3 6 9 12 15 18 andmidcrustalregionsthanthe lower crust. This is illustrated Average Vp Anisotropy(%) at 1 GPa for mafic rocksin Figure 11 by followingthe changein anisotropy accompanyingprogressivemetamorphismof basalt. Figure 11. Averageanisotropies, 100 (Vma x-Vmin)/Vave, at presAnisotropy increasesfrom the zeolite facies through the surescorresponding to 35 km depth. prehnitepumpellyitefacies and greenschistfaciesto a maximum value at amphibolitefaciesconditionsandthen decreases in mafic granulitesand eclogites. Crustaltemperatureis an importantparameterbearingon the by Christensen andShaw[ 1970]. The densitiesin Table4 have models alsobeen correctedfor compression.For convenience in com- interpretationof seismicvelocities. Temperature-depth paring the laboratoryvelocitieswith field velocities,pressures have been calculatedby severalauthorsfor three heat flow havebeenconvertedto crustaldepthsat 5-km intervals,usinga provincesoriginally definedby Roy et al. [1968], the Sierra meancrustal density of2830kg/m 3. Nevada, eastern United States, and Basin and Range, corre•
Basalt
o
ø I =i
i:, ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: MicaQua=Schist
Ii::111::i!!::!iiiii::i::::i[[::i::i::::[::::i!iiii!;i[::!::::i::] Felsic Granulite
o Granulite .-•I IMafic iii iiIU•fi• Gamut Gr•nulite
Temperature (øC) 0
0]•
5
200
i
i
400
i
i
600
i
800
I
i
i
1000
L,• • i
Water Saturated ,,'
2o
• %X '•'•:...••
•
•
i
1400
i
•1
[ •_,•:i••Granite Solidus •, 15 k
1200
ii
,, Dq Basalt
Solid us•,
'
•
',
25
35
40
---
-
45 50
Sierra Nevada Eastern U.S. (Low Heat Flow) (Avg. Heat Flow)
Basin & Range (High Heat Flow)
Figure 12. Temperature-depth curvesfor threeheatflow provincesafterBlackwell[1971] (triangles,circles, andsquares)andLachenbruch andSass[1977] (curvesA, B, C, andD). Averagecrustalthicknessis shown as the dashed line "M".
CHRISTENSEN
AND
MOONEY:
Table 6. TemperatureCoefficients Rock
OVp/OT kms"øC"
Granite- granodiorite,graniticgneiss, tonaliticgneiss
-0.39 x 10'3
Gabbro-
-0.57 x -0.39 x -0.40 x -0.52 x -0.49 x -0.55 x -0.41 x -0.56 x -0.53 x -0.68 x -0.54 x -0.41 x
norite-
troctolite
Basalt, metabasalt
Slate,phyllite, quartzmicaschist Marie granulite,marie garnetgranulite Felsicgranulite,paragranulite Amphibolite Anorthosite
Dunite,pyroxenite,hornblendite Eclogite Serpentinite Quartzite Marble
CONTINENTAL
CRUST
9779
graywacke,andesite,andbasalt. A largevarietyof rockshave average velocitiesfalling between6.0 km/s and6.5 km/s, ineludinggranite,diorite, slate, phyllite, zeolite facies basalt, prehnite-pumpellyite faciesbasalt,amphibolitefaciesgranitic gneiss,tonaliticgneissand schist,and felsic granulitefacies rocks. With the exceptionof marble and anorthosite,rocks with velocities between 6.5 km/s and 7.0 km/s are mafic
10'3 10'3 10'3 10-3 10'3 10'3 10'3 10'3 10'3 10-3 10'3 10'3
FromChristensen [1979, alsounpublished data,1980].
sponding to low, average,andhighheatflow, respectively.To illustratethe range of possiblecrustaltemperatures, the preferredmodelsof Blackwell[1971] andLachenbruchand Sass [1977] are comparedin Figure 12. Thesestudiesand more recent continentalheat flow summaries[e.g., Morgan and Gosnold,1989] concludethat temperature differences between thethreeprovincesaremuchlargerthantemperature uncertainties introducedby differentmodelingassumptions.The temperatures in Table4 at 5-kmdepthintervalsarefromBlackwell [1971].
In comparison with measurements at elevated pressures, the influenceof temperature on seismicvelocitieshasreceivedonly limited attention. To avoid formation of cracks during the
heatingof rocks,whichleadsto irreversible decreases in velocity, it is necessary to raisethepressure a few hundred megapascals beforeheating[Birch, 1943]. Temperaturecoefficients obtainedat elevatedpressures showa fairly narrowrangeof
in
composition.Theseincludediabase,greenschist faciesbasalt, amphibolite andmariegranulite.Rockswith averagevelocities between7.0 km/s and 7.5 km/s includegabbro,homblendite, and marie garnetgranulite. In the classification adoptedfor Table 4, the distinctionbetween marie garnet granulite and mafic granuliteis basedsolely on the presenceor absenceof garnet. Velocitiesabove7.5 km/s are limited to pryoxenite, eclogite,anddunite. Lithologieswith relativelylargestandard deviations(e.g., metagraywacke, quartzmica schist,and the
low-grade metabasalts) haveextremely variablemineralogies andare sometimessignificantlyanisotropic.Thewide rangein velocitiesin basalt is also a consequence of glass content, porosity,often in the form of small vesicleswhich do not close at elevatedpressures,and secondaryalteration. As earlier observedby Birch [1961], diabaseis significantlylower in velocity than gabbro. This is likely related to mineralogical differences,sincemany of the gabbroscontainabundanthighvelocity olivine, whereasthe diabasesin this compilationare usuallyolivine-free. The order of increasingvelocity illustrated in Figure 14 showsthat velocity is not simply a functionof metamorphic grade. This is demonstrated in Figure 15 where histogramsof rock velocitiesfor the major metamorphicfaciesare compared with velocitiesat 20 km depthfor plutonicigneousrocksranging in compositionfrom graniteto gabbroandultramarierocks. Figure 15 illustratesthat crustalvelocitiesin the rangeof 6.0 to 7.5 km/s can be matchedwith low-, medium-, and high-grade metamorphicrocks, as well as commonigneousrocks. The narrowrangeof high velocitiesobservedin the eclogitefacies rocksis dueto the limited compositionalrangegenerallyfound o
Mafic Garnet Granuli e
valuesfor mostcommonrocks[e.g.,Fielitz, 1971;Kern, 1978;
Christensen, 1979].Valuesof (OVp/OT)p usedto obtainvelocitiesfor thethreeheatflowprovinces of Table4 aresummarized in Table 6. Thesevalueswere appliedto the velocity-pressure dataobtainedat room temperature.
lO
Theimportance of applyingtemperature corrections to laboratoryvelocitydatapriorto makingcomparisons with continen- '2o
High
-D-Avg' HF I
½&
-O-Low HF•
tal crustalseismicvelocitiesis illustratedin Figure 13 for aver-
agegranite/granodiorite andaverage mariegranulite.The correctedcurvesfor the threeheat flow provincesincreasein ve-
locityuntil depthsof 10 to 15 km are reached,wheregrain r• 30 boundary cracksareclosed,anddecrease at greaterdepths.For geothermal gradients differentfromthoseusedin Table4, it is possible to arriveat reasonable velocityvaluesby simpleex4O trapolation.For velocitiesalongthehighheatflow geotherm, thereadershouldbe cautioned thatpartialmelting,especially in the more silicic water saturatedrocks,may causesignificant
departures from the velocitiesin Table 4. Also, aboveap50 ' proximately 500øC,velocities in quartz-rich rocksarelikelyto 5.4 5.6 5.8 6.0 6.2 6.4 6.6 6,8 7.0 7.2 7.4 be affectedby thequartzc•-15 transition [e.g.,Fielitz, 1971]. Compressional Velocity(km/s) Averagevelocitiesand standarddeviationsfor the major rocktypes,arranged in orderof increasing velocityat pressures Figure 13. Velocityversusdepthandheatflow provincefor andmarie garnetgranulite. For corresponding to 20 km depth,are illustratedin Figure14. averagegranite/granodiorite Velocities under 6.0 km/s are limited to serpentinite,meta-
temperatures, seeTable 4.
9780
CI-[RIS•NSEN AND MOONEY: CON2TNENTALCRUST '
I
'
I
'
I
I
'
I
'
I
'
in eclogitefaciesmineral assemblages.The wide rangeof ve-
I
-•-Dunite
20 km Depth; Average Heat Flow
=
locities observed for the ultramafic
Eclogite
-e- Pyroxenite =
Mafic Garnet Granulite
free.
ß Hornblendite --
rocks is due to variable ser-
pentinization. Serpentinitehas the lowest velocities,whereas duniteswith velocitiesabove8.3 km/s are virtually serpentine-
Gabbro
Velocity-Density Relationships = --•-•
t
Amphibolite
--
Mafic Granulite
= =
--e--
Facies Basalt
Diorite
Prehnite-Pump Facies Basalt
-e--
Felsic Granulite
---
Zeolite Facies Basalt
--
-•--
Greenschist Diabase
•
--•-
Correlations between compressionalwave velocity and densityare importantbecausethey allow estimatesof crustal densityto be madefor gravitymodelingfrom seismicrefraction velocitiesand, conversely,becauserock densitiescan be used to predict seismicvelocities. Examplesof velocity-density relations in wide use include the Nafe-Drake curve [Nafe and Drake, 1957], Birch's[ 1961] law relatingvelocity,density,and mean atomicweight, and regressionline solutionsfor oceanic rocks[Christensenand Salisbury,1975]. In Figure 16, average velocitiesfor our 29 rock categoriesat a depthof 20 km and a temperatureof 309øC, corresponding to an averagegeotherrn, areplottedagainstdensities. In Table 7, the parametersof least squaressolutionsof the forms p = a + bVp and Vp = a + bp are given at severaldepths. Velocitieshave been correctedfor temperaturescorresponding to the averageheat flow temperaturevaluesof Table 4. We have chosentwo sets of solutions,one set for all rock types includedin Table 4 and a subsetshownas solid circlesin Figure 16 with a least squaresstraight-linefit, which doesnot in-
Anorthositic Granulite Marble
Mica Quartz Schist
Paragranulite Granite-Granodiorite
_-
Tonalitic Gneiss
+
Phyllite
-•-
Slate
-e-
Granitic Gneiss Basalt
-•-
_-
Quartzite
Andesite
=
Metagraywacke Serpentinite
I
5.0
,
I
5.5
6.0
,
I
6.5
I
,
I
7.0
I
•
7.5
I
8.0
I
I
8.5
•.0 clude monomineralic rocks andbasalt andandesite. Weprefer
the latter set of solutions,sinceit is unlikely that the unmetamorphosedvolcanic rocks or the monomineralicrocks are Figure14. Averagecompressional wavevelocities andstan- abundantcrustallithologiesat depth. Coefficientsof determi-
CompressionalWave Velocity(km/s)
darddeviations at 20 km depthand309øC(averageheatflow) nationsare high (> 90%) for this subset. The monomineralic rocks quartzite,serpentinite,and hornblenditehave relatively
for major rock types.
• Plutonic (Granite - Gabbro) •.......Zeolite-Greenschist Facies "'•'"""•'=• Amphibolite Facies • Granulite Facies
8O
c 60 o
•
EclogiteFacies
•
Ultramafic
o 40 o
E = 20
z
0
4.7
5.1
5.5
5.9
6.3
6.7
7.1
7.5
7.9
8.3
8.7
compressionalwave Velocity (km/s) at 20 km Figure15. Velocitiesat 20 km depthfor plutonicigneous rocksrangingin composition fromgraniteto gabbro,metamorphic rocksof variousfacies,andserpentinized to unaltered ultramafic rocks.Notethatrocksof all themajormetamorphic facies,plutonicigneous rocks,andpartiallyserpentinized peridotites havevelocitieswithin the commonlyobserved crustalvelocitiesof 6.0 km/sto 7.5 km/s.
CHRISTENSEN
AND
MOONEY:
CONTINENTAL
CRUST
9781
8.5
8.0
7.5
6.0
5.5[ o SER
5.0 2500
I
I
I
2700
I
I
2900
I
I
3100
I
I
3300
I
3500
Density(kg/m 3) Figure16. Averagevelocityversusaveragedensityfor a varietyof rocktypesat a pressure equivalent to 20 km depthand309øC.Rockabbreviations aregivenin Table4. Regression lineparameters at variousdepths aregivenin Tables7 and8. Thenonlinear solutions arerecommended for crust-mantle calculations (seetext). low velocities for their densities, whereas the anorthosites, is predominantlyperidotite,andthuswe have removedeclogite marble,pyroxenite, anddunitchavehighvelocities.Thepo- data. The best correlationsbetweendensityand velocity for lymineralic lithelogics functionally average thesehighandlow relativelysimpleequationsare of the form p = a + b/Vp for calvelocitiesand densitiesof the mineral constituents,so that bulk culating density fromvelocity andVp'1= a + bp3forobtaining
velocityfrom density. The parametersof thesecurvefatsare velocitiesanddensities producea betterlinearfat. Note that the linear solutionin Figure 16 for the crustal givenin Table 8 at variousdepths. The 20-km solutionfor the rocksdepartssharplyfor the mantlerockspyroxenite (PYX) latter equationis shownas the dashedcurvein Figure 16. The anddunitc(DUN). Thusthelinearsolutions of Table7 arenot solutionsin Table 8 are recommendedfor usein gravity calcurecommended for comparing crustandperidotitcmantle. For lationsof crust-mantledensitycontrasts.
velocity-density solutions applicable to crust-mantle sections, the data have been fat to severalhundrednonlinearequations. SeismicConstraints on Crustal Composition The data setwhich we have selectedfor theseregressionline The compilations presentedin the previoussections provide fatsincludesthe mantlerocks dunitcand pyroxeniteand the a new basis for generalizations on continental petrology and polymineralic crustal rocks.We assume thattheuppermantle
Table 7. LinearVelocity-DensityRegression Line Parameters p=a+ bVp
Depth, a, km kgm-3
b, kgm'a/km s'1
Vœ=a+ bp
S(O,Vp), kgm'3
rg, %
a, kms'1
b, kms4/kgm'3
S(Vp,p), kms-1
rg, %
0.348 0.333 0.326 0.326 0.339
75 76 76 75 71
0.183 0.159 0.143 0.130 0.110
88 91 92 93 94
All Rocks
10 20 30 40 50
989.3 947.3 946.6 964.5 1078.3
289.1 296.6 299.7 300.5 299.0
116.3 113.3 112.5 113.3 120.3
75 76 76 75 71
-0.924 -0.836 -0.802 -0.764 -0.775
0.00259 0.00256 0.00252 0.00249 0.00238
AHRocksExceptVo•anieRocksandMonominera•cRocks 10 20 30 40 50
540.6 444.1 381.2 333.4 257.1
360.1 375.4 388.0 398.8 431.4
70.2 62.8 57.8 53.8 49.1
88 91 92 93 94
-0.566 -0.454 -0.377 -0.318 -0.192
0.00245 0.00241 0.00237 0.00232 0.00218
Vpis compressional wavevelocity;p, density;$(p, Vp),standard errorof estimate of p on Vp;S(Vp,p), standard errorof es-
timateof Vponp;r2,coefficient ofdetermination.
9782
CHRISTENSEN AND MOONEY:
CONTINENTAL CRUST
Table 8. NonlinearVelocity-DensityRegression Line Parameters
p =a+ b/Vp
Depth,
Vp4 =a+ bp3
a,
b,
3'(0,Vp),
r2,
a,
b
$(Vp,p),
km
kgm'3
kgm'•/kms4
kgm'•
%
km/s 4
-
kms4
10 20 30 40 50
4929 5055 5141 5212 5281
-13294 -14094 -14539 -14863 -15174
69.30 62.20 57.36 53.63 50.51
87 90 91 92 93
0.2124 0.2110 0.2115 0.2123 0.2130
-2.4315x1042 -2.3691x1042 -2.3387x1042 -2.3155x1042 -2.2884x1042
0.19 0.17 0.15 0.14 0.13
r2, 91 92 93 94 95
Vpis compressional wavevelocity;p, density; S(p,Vp),standard errorof estimate of p on Vp;
S(Vp,p),standard errorof estimate of Vponp;r2,coefficient of determination.
chemistry. In Figure 17, averagevelocity-depthcurvesfor a varietyof majorigneousandmetamorphic rocktypesare superimposed on our average crustal velocity model. We have dividedthe rocktypesinto five major groups(monomineralic, igneous, and low-, medium-, and high-grademetamorphic rocks). All velocity-depthcurvesare at temperaturescorrespondingto the averagegeothermof Table 4. It shouldbe notedthatin all five diagrams,velocitiesfor the upper5 km are not shown,sinceat theseshallowdepths,velocitiesare often loweredsignificantlyby cracks. Monomineralic
Rocks
The monomineralicrock averagesareperhapsmostusefulin understanding the contributionsof selectedmineralsto rock velocities.Velocitiesof serpentinite andquartzitefall significantly below middle and lower crustal velocities, whereas velocitiesof pyroxeniteand duniteare muchhigherthan even the fastest lower crustal velocities.
Various combinations of
anorthosite, hornblendite, andpyroxenitecanreadilybe equated to observedlower crustalvelocities,suggesting that plagioclase,amphibole,andpyroxeneare abundantlower crustalminerals. Note alsothe relativelyhigh velocitiesof marblewhich match our weighted averagecrustalvelocitiesat 25 to 30 km depths. On the basisof surfaceexposures of medium-to highgrademetamorphicterranes,marble doesnot appearto be a particularly abundantmidcrustallithology. Calc-silicatesare, however,commonsuggesting a breakdownof carbonates with lossof CO2andthe productionof silicateminerals. Igneous Rocks
Early seismicstudiesofteninterpretedcrustalseismicdatain termsof igneouslithologies. Figure 17 showshow an igneous crustalmodelis in agreementwith our averagecrustalvelocitydepthcurve. Uppermostcrustalvelocitiesof this modelmatch either an assemblageof volcanicrocks or fracturedplutonic rocks. At greaterdepthsthe velocitiescanbe simplyexplained by a gradualchangefrom granite/granodiorite throughdioriteat 20 km to gabbro at 40 km. Diabase dikes have velocities equivalentto average crustal velocities at 25 km, and anorthosite velocities are similar to crustal velocities at 35 km.
Metamorphic Rocks
The conceptof a crustalsectionconsisting of simpleigneous lithologieshas been under attackfor many yearsas Earth scientistsgraduallyturned to models consistingof metamorphic
assemblages[e.g., Christensen,1965; Ringwood and Green, 1966]. In particular,therehasbeenalmostuniversalagreement that the lower crustin stableplatformsmustcontainhigh-grade metamorphicrocks in the granulite and/or eclogite facies. However,the specificchemistryof theserockshasbeenwidely debated,with several studiesconcludingthat the lower crust consistson the averageof rocks of intermediaterather than mafic silica composition[e.g., Ringwoodand Green, 1966; Smithsonet al., 1981]. Average upper crustalvelocitiesare in the range of many metamorphicrock types. Observedvelocitieswithin the top 15 km of the crustmatchlaboratoryvelocitiesof slatesandphyllites andinterlayeredmetagraywacke andlow-grademetabasalt. Surfaceexposures of similarassemblages are commonin many forearc regions, such as the Franciscanof California [e.g., Hamilton and Meyers, 1966] or the Chugachof Alaska [e.g., PlaJkeret al., 1989],whereprocesses involvingaccretionhave been active along continentalmargins. In older shieldsand many orogenicregions,where erosionhas exposedrocksconsistingof graniticgneissor an assemblage of quartzite,quartz mica schist, tonalitic gneiss, and minor amphibolite,upper crustal velocities match velocities of these medium-grade metamorphicrocks. Note that the high-grademetamorphic rockshave velocitieshigherthan averageuppercrustalvelocities, which is consistentwith the rare occurrencesof granulite andeclogitefaciesrocksin surfaceexposures. At depthsbetween15 km and 30 km, averagecrustalvelocities increasefrom approximately6.3 km/s to 6.7 km/s. Within many crustalsections,this is undoubtedlya depthrangewhere metamorphic grade increases from amphibolite facies to granulitefacies. Progressive metamorphism is likely responsible for somevelocityincreaseoverthis depthrange. However, the velocitycomparisons in Figure 17 also showthat this middle crustalregion must also changecompositiongraduallyto higherpercentagesof mafic mineral assemblages.At greater depths(from 30 km to 50 km), velocitiescan be simplyexplainedas originatingfrom a predominately mafic lower crust, with a lowermostsectionof mafic garnet granulite. Alternatively, the lower crustcouldcontainfelsicgranulitefaciesrocks if mafic eclogitesarepresentin similarproportions.However, we wishto emphasize that on the basisof our field andlaboratory comparisons, the bulk of the lower continentalcrust is likely of mafic composition. Estimatesof crustalcompositionhave been derivedfrom a wide variety of observations, includingaveragechemistries of crystallinerocks from surfaceexposures[e.g., Clarke and
CHRISTENSEN
AND MOONEY:
CONTINE••
CRUST
9783
o
Igneous
Low Grade
lO
'•i:•:,',':• •
15
10
[ '•ii ,•BZE i ::",ii: mBPP ....... : rnBGR
2o
•.
oSLTI
15 20 •.
30
:
35
BAS ß GRA
ß DIA E]ANO
oGAB
30 35 40
4o
:
45
45 0 5o
Medium
Grade
5 55
I
I
I
I
I
I
I
I
I
I
I
I
5.0 5.5 6.0 6.5 7.0 7.5 10 Vp (km/s) 15 2O .• E
25
•
30
a)
35
.QTZI
oGGNI ßBGNI IQSC I [,ML I
I
I
I
5.5 6.0
I
I
6.5
I
I
7.0
I
I
7.5
I
I
8.0
I
8.5
Vp (km/s)
4O 45 50
0
0
nomineralic 515 6.06.57.0 5 5-
15
Jio iiii!i i .......... High 15
20T
..... :;:'
35 o •
•i? i'i!•.
20 c) 35
30 I[(•ETz R '•{i';i!i• •iii.•30 ,,
40 ,•ANO : ::• :•:: •
40
0[•••• ••••
50
55
I I I I I .....
I
5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5
Vp (km/s)
55
5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5
Vp (km/s)
Figure17. Average continental velocity structure compared to average laboratory measured velocities in major¾ock types.Rockabbreviations aregiven inTable4.
9784
CHRISTENSEN
5.6
A 100
6.0
6.4
AND MOONEY:
6.8
7.2
2.7
CONTINENTAL
2.9
3.1
40
CRUST
60
I
o/o• o I •.I i I i I i I
80
0
I
I
'
100
I
I
•
80
0
I
ranitic gneiss
•, 75% nitic gneiss 0 25% tonalitic gneiss 10
[] 5 % phibolite •, 45% graniticgneiss ¸ 50% tonalitic gneiss 15
•- 20 Q•)
25
[]35% a•mphibolite
•, 15% granitic gneiss 0 50% tonalitic gneiss
0 20% •nalitic gneiss [] 40% amphibolite 0 40% mafi½granulite
30
35
O 50% !afic granulite X 50% mafic garnet granulite
40
Base of Crust
I 5.6
• I 6.0
•
I 6.4
•
I 6.8
Vp(km/s)
•
I
I
7.2
2.7
•
I 2.9
•
I
I
3.1
40
Density(kg/m3x 10-3)
•
I
•
60
% SiO2
I
I
100
Reflection Density
Figure 18. A model for averagecrustalpetro!ogyversusdepthconsistentwith our averagevelocity depth profile (solidcircles)andvelocitydepthcurvesfor commonrocktypes(opensymbols).Variationsof density and SiO2 contentwith depthare from rock percentages shownon the left. Histogramsof reflectiondensities are normalizedwith reflectionsoriginatingfrom metamorphiclayering.
Washington,1924], comparisonsof crustalseismicvelocity profiles with laboratory measurements[e.g., Birch, 1958; Fountain and Christensen,1989], estimatesof mean crustal velocity [Smithsonet al., 1981], Poissoffsratios of the crust [e.g., Christensen and Fountain,1975; Holbrooket al., 1992], studiesof xenolithsfrom volcanic and kimberlitic eruptions [e.g., McGetchin and Silver, 1972; Kay and Kay, 1981], and models of crustal structurebasedon the geologyof exposed crustalsections[e.g., Schmidtand Wood, 1976; Fountain and Salisbury,1981; ]½fi'llerand Christensen,1994]. Each of these methodshas provided important constraintson the chemical andmineralogicalcompositions of the crust;however,eachapproachhas seriouslimitationsdueto a wide varietyof assumptions. In the following discussion,we presenta model for crustalpetrologyand chemistrybasedon comparisons of our crustalvelocity-depthprofile with velocitiesof major crustal lithologies. This approachperhapsmost closelyfollows an early geochemical estimateof bulk crustal compositionby Pakiserand Robinson[1966], in that we are dealingwith observedaveragevelocity structureand laboratorymeasuredvelocities. Pakiserand Robinsondividedthe crustunderlyingthe United Statesinto an uppergraniticlayer and a lowerbasaltic
assemblages possessing velocitiessimilarto our averagecrustal velocity-depth curve. Ourestimateof crustalpetrologyis based on a selectionof rock assemblages which are commonly observedin exposuresof crustalsections. For example,the association of graniticgneiss,tonaliticgneiss,andamphibolite is commonin many metamorphicterranesand their combined seismicpropertiesare similar to those of many midcrustal regions[Christensen, 1989]. Also, crustalsectionsoriginating from greater depthsoften contain abundantmafic granulite [e.g., Percival et al., 1992]. Our weightedaveragecrustalvelocity profile is shownin Figure 18 as solid circleswith horizontal lines representing + one standarddeviation. Velocitydepthcurvesfor graniticgneiss,tonaliticgneiss,amphibolite, maficgranulite,andmaficgarnetgranulitearesuperimposed on the crustalprofile. Percentages of theserock typesconsistent with averagecrustalvelocitiesover selecteddepthintervalsare shownon the left of Figure 18. This model assumesthat the upper crustalsectioncontains graniticgneisswhich becomesincreasinglytonaliticat depth. In addition, amphibolite becomes abundant at midcrustal
depths. The region between25 km and 30 km marks a transition from amphibolitefacies rocks to granulitefacies assemlayer. Volumes of thesetwo layers were estimatedfrom seis- blages. At greaterdepths,garnetbecomesincreasinglyabunmic data, and crustal chemistrywas calculatedfrom average dant and mafic garnetgranuliteis the dominantrock type imchemicalanalysesof graniticandbasalticrocks. mediatelyabovethe Mohorovicicdiscontinuity. The crustal ofthismodelincreases from2660kg/m 3atthesurface In Figure 18, we have modeledthe petrologyof the average density 3atthebase ofthecrust.Alsoshown inFigure 18 continentalcrustbasedon relatively simplemetamorphicrock to3100kg/m
CHRISTENSEN
AND MOONEY:
CONllNENTAL
CRUST
9785
Table 9. Bulk CrustalChemicalComposition
Oxide, wt %
SiO2 TiO2 A1203 Fe203
Clarke
Pakiser and Robinson
Taylor and Mclennan
Weaverand Tarney
[1924]
[1966]
[1981]
[1984]
59.0 1.0 15.2 3.1
57.9 1.2 15.2 2.3
58.0 0.8 18.0 ......
63.2 0.6 16.1
61.7 0.9 14.7 1.9
This Work*
FeO
3.7
5.5
7.5
4.9
5.1
MgO
3.5
5.3
3.5
2.8
3.1
CaO
5.1
7.1
7.5
4.7
5.7
MnO
0.1
0.2
0.1
0.1
0.1
Na20 K20 P205 H20
3.7 3.1 0.3 1.7
3.0 2.1 0.3 .........
3.5 1.5 ...
4.2 2.1 0.2
3.6 2.1 0.2 0.8
*Bulk continentalcrustfrom averagecrustalvelocity-depthcurve(Figure 18).
is the variationin weightpercentof SiO2 with depth,calculated nozoic in age (either highly extendedcrust or thickenedorofrom averagechemicalanalysesof the rock suitesat 5-km in- genic crust). This observationimplies that very thick crust, tervals.
such as in Tibet and the Andes, will not remain thick but will
A histogramof reflectiondensity[iVteissner,1986] for our crustalmodel, normalized accordingto the maximum number of reflectionsper kilometerdepth,is shownin Figure 18. The reflectionhistogramis basedon syntheticmodelingwith horizontal layering. Reflectioncoefficientswere determinedfrom the rock velocitiesand densitiespresentedin Table 4. The ini-
evolvetoward crustwith a more typical thicknessof about40 km. The primaryprocesses involvedareprobablyisostaticuplift and erosion and the lateral flow of warm, rheologically weak middle and lower crustalrocks [Englandand Houseman, 1989]. An additionalcrustalprocessis the delaminationof the lower crustbeneaththick orogens[Kay and Kay, 1991], as is hypothesized to be occurringbeneaththe Alps basedon seismic
tiationof majorreflectionsat midcrustal depthsoriginates from the large contrastsin acousticimpedanceof amphibolitewith
images. Continental crust that is thinner than 24 km, whether it has granitic gneissand tonalitic gneiss. Seismicmodeling of rebeen highly extendedor formed at an active margin, will be flectivity from a similar sequenceof rocks, obtainedfrom a continuousdrill core in the southernAppalachianPiedmont, underlainby a weak, thin lithosphereand is thereforelikely to substantiatesthe significant reflectivity of this assemblage be subjectto compressionalcollapseand thickening. Long[Christensen, 1989]. At deeperlevels,garnet-richlayers,with lived, stablecrust,the Precambrianplatformsand shieldshave high densitiesand velocities,are likely to be significantreflec- cold, 40-km thick crust, with a mafic lower crust that is
rheologicallystrongdueto its lowersilicacontent(Figure18). It is generally agreed that the rocks that constitute the We have calculatedan averagechemistryof the continental with crustfrom the volumepercentages of the variousrock typesin Earth's crustoriginatedfrom igneousprocessesassociated extraction of material from the mantle. The processes of crusourmodel(Figure18) andtheiraveragechemicalcompositions. Becausemost models have assumedthat the upper crust is tal growth involve not only magmatic contributionsfrom the
tors.
"granitic" and overlies a "mafic" lower crust, our average mantle but also accretion of tectonic terranes, as well as crustal (Table 9) is quitesimilarto manypreviousestimatesof crustal recycling,the latter resultingin negativegrowth. The ultimate chemistry.Our basicapproachin obtainingaveragechemistry fate of most newly formed crust is to becomeattachedto old is unique, however, in thatwehaveidentified likelymetamor- nuclei,which we have includedin our compilationas shields phic rock assemblages at variouscrustallevelsbasedon com- andplatforms. The averagetectoniccolumnsof Figures7 and 9 provideinparisonsof our averagecrustalvelocitycoluntowith the new databaseon rock velocities. The bulk continentalchemistry sightinto the natureof the tectonicand magmaticprocesses presented in Table 9 is probablya reasonable approximation for required to evolve continentalarcs and orogensinto stable of the velocmanycrustalsections.Regionalvariationsare almostcertainly shieldorogensandplatforms.First, a comparison ity structuresof continentalarcs with shieldsand platforms importantbutpoorlyunderstood at thistime. showsthat they have similar lower crustalvelocity profiles. Continentalarcs,however,appearto have significantlyhigher
Implications for Crustal Evolution
The resultspresentedin this paperhave broadimplications for modelsof crustalevolution. Our histogramof crustalthickness(Figure2) showsthatvery little continental crustis either thickerthan 58 km or thinnerthan 24 km. Nearly all continental crustwith a thicknessoutsidethe 24-56 km rangeis late Ce-
velocitiesin their upper20 km, suggesting that the uppersectionsof continental arcsmustbe modifiedby the intrusionof silicic magmas, or by the incorporationof metasedimentary rocks through a complex tectonic history, before being convertedto shieldsand platforms. Second,the low velocitiesof orogenscan be convertedto shieldand platform velocitiesby
9786
CHRISTENSEN AND MOONEY: CONTINENTAL CRUST
the intrusionof a significantvolumeof maficmagmaor simple andmeta-basalt.In midcrustal regions,at depthsof 10 km to isostaticuplift of approximately5 km associated with surficial 25 km, amphibolite faciesrocksarelikelyto comprise thebulk erosion. Suchuplift is alsorequiredto satisfycrustalthickness of the crust. Withinthisregion,thereis a gradualchange in observations,as mentionedabove. composition from graniticgneissandtonaliticgneissto mafic We have presentedevidencethat the lower crustof the continents is typically composedof mafic granuliteswith about 47% SiO2 and that the silica contentincreasesto greaterthan 70% in the uppercrust(Figure18). This observation is consistent with at least two hypothesesregardingthe evolutionof stablecrustthat the crustgrowsprimarily eitherby the accretion of continentalmagmaticarcsor by the accretionof diverse geologicalterrainsthat undergosecondarydifferentiationsoon after accretion[3deissnerand Mooney, 1991]. The first model
phylliteanddeepercrustalsections of amphibolite andquartz
is consistentwith our data. In the latter model, the lower crust
micaschist.Presently, it is notpossible to takeintoaccount the
is meltedbyheatprovided by'intruding maficmagmas, anddif-
influence of anisotropy on ourcrustalaverages sincethepres-
ferentiationproducesgraniticand dioriticplutonsthat moveup into the upperandmiddle crust. This processof differentiation will alsoproducea mafic/ultramaficresiduumat the baseof the crust,andthe ultramaficmaterialwill form a new, youngMoho at a depthof about40 km. We note that either of theseprocesses,accretionof intactmagmaticarcsor differentiationof diverseaccretedterrains,predominantlyinvolvesmagmaticprocessesto producestablecontinentalcrust.
enceof crustalanisotropy has only recentlybeendocumented [e.g.,Brocherand Christensen, 1990] We look forwardto the
Summary and Conclusions Any model of the continentalcrustmust be consistentwith
threefundamentalobservations: (1) the seismicstructureof the continentalcrust, (2) the petrology of surfaceexposuresof crustalsectionswhich have previouslybeen buried to various depths,and (3) laboratorymeasurements of seismicvelocity throughthe commonlyobservedlithologiesof surfaceexposures. In this paperwe havepresentedmodelsfor the seismic structureand compositionof the continentalcrustbasedon a worldwide data set of seismicrefractionprofiles, as well as a comprehensive summaryof compressional wave velocitiesas a function depth for a wide variety of continentaligneousand metamorphicrocks. The thicknessand velocitystructureof the crustarewell correlated with tectonic province and reflect the processesthat have formed and modified the crust. Shields and platforms have an averagecrustalthicknessof 41.5 km, nearly equal to the globalaverage. Extendedcrusthas an averagethicknessof 30.5 km and a low averagecrustalvelocityof 6.2 km/s. Rifts and orogensshow a wide range of crustalthickness,18-46 km and 30-72 km, respectively. The availableglobal seismicrefractiondatahave a nonuniform geographicdistribution,which resultsin a stronggeographicalbias in suchquantitiesas averagecrustalthickness andaveragecrustalvelocity. In orderto correctthe nonuniform data distribution,we have calculatedaveragecrustalproperties using a weighted averageof five tectonicprovinces: shields and platforms, orogens,extended crust, magmatic arcs, and rifts. Mean weightedvaluesof crustalthickness,averagecrustal velocity, andPn velocity are 41.0 km, 6.45 km/s, and 8.09 km/s, respectively. Average velocities, densities,and standarddeviationspresentedfor 29 igneousand metamorphicrock typesbelievedto be abundantin the continentalcrust showa wide range of velocitiessimilar to the rangereportedin crustalrefractionstudies. Averageupper crustalvelocitiesmatchlaboratorymeasured velocitiesof a variety of rocks,includinggraniticgneiss andinterlayeredassemblages of metagraywacke, slate,phyllite,
mineralassemblages richin amphibole.At depths greater than 30 km, the continental crustcontainsabundant granulitefacies rocksof maficcomposition. Observed velocitygradients in this regionindicatethatgarnetcontent increases with depth. Seismicanisotropy originating frompreferredmineralorientationis likely to be an important propertyof upperandmidcrustalregions. Maximumanisotropy is expectedin upper crustalmetamorphicterranescontainingabundantslate and
findingsof futureseismic fieldprograms designed specifically to investigate the magnitude andsymmetry of anisotropy at variouscrustallevels.Thesestudies will notonlyrevealimportant information on crustal structure and deformation but will
alsoaddmuchto ourunderstanding of crustalcomposition.
Crustal density forourmodel increases from2660kg/m 3 at
thesurface to3100kg/m 3at40klI1.Average crustal density is 2835kg/m 3. SiO2decreases withdepth fromover70%in the upper 5 km of the crustto 47% at the Mohorovicic discontinu-
ity. The averageweight percent SiO2 calculatedfrom our crustalmodelis 61.8%,in goodagreement with manyprevious estimates.
In the modelpresented in thispaper,we haveattempted to explainseismicvelocityobservations of the continentalcrustin
termsof a petrologicmodel consistent with new laboratory velocity measurementson common continentalrocks. The
crustalmodeladoptedwasfoundedon simplepetrologicobservationsbasedprimarilyon studiesof deepexposures of continental crust, but to obtain information on the relative abun-
dancesof rocksat crustaldepthsit was necessary to compare the velocities with seismic structure. We stress that our model
is only an averagerepresentation of crustalpetrologyand chemistryand variationsfrom this averageare certain. These variationsin crustalcompositionare poorlyunderstood at this time, but of prime importancein understanding continental crustalgenesisand evolution. Aclmowledgments.The manuscriptwas greatlyimprovedby the perceptivecommentsof GeorgeThompson,Roy Johnson,Randy Keller, Bill Hinze, Tom Brocher,and Jill McCarthy. K. Wilcox, D. Kingma, W. Wepfer, and JianpingXu providedvaluabletechnical supportfor the experimentalphaseof this study. Financialsupport for the laboratory studieswas provided by the Office of Naval Researchandthe NationalScienceFoundationContinentalDynamics Program.The field compilationhasbeensupported by the USGS Deep ContinentalStudiesProgram.
References Allenby, R. J., and C. Schnetzler,United Statescrustalthickness, Tectonophysics, 93, 13-31, 1983. Bamford,D., Pn-velocityanisotropyin a continentaluppermantle, Geophys.J. R.. Astron.$oc., 49, 29-48, 1977. Beloussov,V. V., N. I. Pavlenkova,andA. V. Egorkin,Deep Structure of the Territory of the U$$R, 224 pp., Nauka,Moscow,1991. Birch,F., Elasticityof igneousrocksat hightemperatures andpressures,Geol. $oc. Am. Bull., 54, 263-286, 1943.
Birch, F., Interpretationof the seismicstructureof the crustin light
CHRISTENSEN
AND MOONEY:
of experimentalstudiesof wave velocitiesin rocks, in Contributions in Geophysicsin Honor of Beno Gutenberg,edited by H. Benloft, pp. 158-170, Pergamon,New York, 1958. Birch, F., The velocity of compressional wavesin rocksto l0 ldlobars, l, J. Geophys.Res., 65, 1083-1102, 1960. Birch, F., The velocity of compressional wavesin rocksto l0 ldlobars,2, J. Geophys.Res.,66, 2199-2224, 1961.
CONTINENTAL
CRUST
9787
England,P. and G. Houseman,Extensionduringcontinentalconvergencewith applicationto the TibetanPlateau,J. Geophys.Res., 94, 15,561-15,569, 1989.
Blackwell, D. D., The thermal structure of the continental crust, in
Fielitz,K., Elasticwavevelocitiesin differentrocksat highpressure andtemperatures up to 750øC,Z. Geophys.,20, 943-956, 1971. Fountain,D. M., andN. I. Christensen, Compositionof the continental crustand uppermantle;A review, in GeophysicalFramework of the ContinentalUnited States,editedby L. C. Pakiserand W.
The Structureand PhysicalPropertiesof the Earth's Crust, Geophys.Monogr. Ser., vol. 14, editedby J. G. Heacock,pp. 169184, AGU, Washington,D.C., 1971.
Fountain,D. M., and M. H. Salisbury,Exposed cross sections throughthe continentalcrust;Implicationsfor crustalstructure,
D. Mooney, Mere. Geol. Soc.Am., 172, 711-742, 1989.
Braile, L. W., W. J. Hinze, R. R. B. yon Frese, and G. R. Keller,
petrology, and evolution, Earth Planet. Sci. Lett., 56, 263-277, 1981. Seismicpropertiesof the crustand uppermostmantle of the conterminousUnited States and adjacent Canada, in Geophysical• Fowler, C. M. R., The Solid Earth: An Introductionto Global GeoFramework of the ContinentalUnited States, edited by L. C. physics,472 pp., CambridgeUniversityPress,New York, 1990. Palriserand W. D. Mooney,Mere. Geol. Soc.Am., 172, 655-680, Fuchs,K., and G. Muller, Computationof syntheticseismograms 1989. with the reflectivity methodand comparisonwith observations, Brocher,T. M., andN. I. Christensen,Seismicanisotropydueto preGeophys.J.R. Astron.Soc., 23, 417-433, 1971. ferred mineral orientation observed in shallow crustal rocks in Garland,G. D., Introductionto Geophysics: Mantle, Core and Crust, southernAlaska,Geology,18, 737-740, 1990. 384 pp., W. B. Sanders,Philadelphia,Pa., 1979. Center of Regional Geophysical and Geoecological Research Giese,P., C. Prodehl,and A. Stein,ExplosionSeisinologyin Central (GEON), RussianMinistry of Geology,Atlas of seismicrefracEurope, 429 pp., Springer-Verlag,New York, 1976. tion profiles, Moscow, 1994. Goodwin,A.M., PrecambrianGeology:The DynamicEvolutionof Christensen,N. I., Compressionalwave velocities in metamorphic the Continental Crust, 666 pp., Academic, San Diego, Calif., 1991. rocksat pressuresto 10 kbar, J. Geophys.Res., 70, 6147-6164, 1965. Hamilton,W. B., and W. B. Meyers, Cenozoictectonicsof the westChristensen, N. I., Compressional wave velocitiesin possiblemantle ernUnited States,Rev. Geophys.,4, 509-549, 1966. rocks to pressuresof 30 kbar, J. Geophys.Res., 79, 407-412, Holbrook,W. S., W. D. Mooney, and N. I. Christensen,The seismic 1974. velocity structureof the deep continentalcrust, in Lower ContiChristensen,N. I., Compressional wave velocitiesin rocks at high nental Crust, edited by D. M. Fountain,R. Arculus and R. Kay, temperaturesandpressures,criticalthermalgradients,and crustal pp. 1-43, Elsevier,New York, 1992. low velocity zones,J. Geophys.Res.,84, 6849-6857, 1979. Hughes,D. S., and C. Maurette, Variation of elasticwave velocities Christensen,N. I., Seismic velocities, in Handbook of Physical in granites with pressure and temperature,Geophysics,21, Properties of Rocks,vol. 2, edited by R. S. Carmichael,pp. 1277,284, 1956. 228, CRC Press,Boca Raton, Fla., 1982.
Christensen,N. I., Measurementsof dynamicpropertiesof rock at elevatedpressuresand temperatures,in Measurementof Rock Propertiesat ElevatedPressuresand Temperatures, editedby H. J. Pincusand E. R. Hoskins,pp. 93-107, American Societyfor TestingandMaterials,Philadelphia,Pa., 1985. Christensen,N. I., Reflectivity and seismicpropertiesof the deep continentalcrust,J. Geophys.Res.,94, 17,793-17,804, 1989. Christensen,N. I., and D. M. Fountain, Constitution of the lower continentalcrust basedon experimentalstudiesof seismicvelocitiesin granulite,Geol.Soc.Am.Bull., 86, 227-236, 1975. Christensen,N. I., and M. H. Salisbury,Structureand constitutionof the lower oceaniccrust,Rev. Geophys.,13, 57-86, 1975. Christensen,N. I., and G. H. Shaw, Elasticity of mafic rocks from the Mid-Atlantic Ridge,Geophys.J. R. Astron.Soc.,20, 271-284, 1970.
James,D. E., and J. S. Steinhart, Structure beneath the continents:A
critical review of explosion studies 1960-1965, in The Earth Beneath the Continents,Geophys.Monogr. Set., vol. 10, edited by J.S. Steinhartand T.J. Smith,pp. 293-333, AGU, Washington, D.C., 1966.
Kaila, K. L., and V. G. Krishna, Deep seismicsoundingstudiesin India and major discoveries,in Seismologyin India, An Overview, editedby H.K. Gupta,CurrentSci., 62, 1992. Kay, R. W., and S. M. Kay, The nature of the lower continental crust: Inferencesfrom geophysics,surface geology and crustal xenoliths,Rev. Geophys.,19, 271-297, 1981. Kay, R. W., and S. M. Kay, Creationand destructionof lower continental crust, Geol. Rundsch., 80, 1-20, 1991.
Kern, H., The effect of high temperatureandhigh confiningpressure on compressional wave velocitiesin quartz-bearingand quartzfree igneousand metamorphicrocks, Tectonophysics, 44, 185-
203, 1978. Clarke,F. W., andH. S. Washington,The composition of the Earth's Lachenbruch,A. H., and J. H. Sass,Heat flow and the thermal recrust,U.S. Geol. Surv.Prof. Pap., 127, 117 pp., 1924. gime of the crust,in The Earth's Crust:Its Nature and Physical Collins,C. D. N., Seismicvelocitiesin the crustand uppermantle of Properties,Geophys.Monogr. Ser., vol. 20, editedby J. G. HeaAustralia,Rep. 277, Bur. Miner Resour.,Geol. and Geophys., cock,pp. 626-675, AGU, Washington,D.C., 1977. Canberra,159 pp., 1988. Downs,H., The natureof the lower continentalcrustof Europe:Pet- Li, S. L., and W. D. Mooney, Review of the crustal structureof Chinafrom seismicrefractionprofiling, Tectonophysics, in press, rolotic and geochemicalevidencefrom xenoliths,Phys. Earth
Planet. Inter., 79, 195-218, 1993.
1995.
Drummond,B. J., A review of crust/uppermantle structurein the McConnell,R. K., Jr., R. N. Gupta,and J. T. Wilson, Compilationof deepcrustalseismicrefractionprofiles,Rev.Geophys.,4, 41-100, Precambianareasof Australiaand implicationsfor Precambrian 1966. crustalevolution, Precambrian Res., 40/41, 101-116, 1988. mantlemodelfor Durrheim,R. J., and W. D. Mooney, Archcanand Proterozoiccrus- McGetchin,T. R., andL. T. Silver,A crustal-upper the Colorado Plateau based on observationof crystalline rock tal evolution: evidencefrom crustal seismology,Geology, 19, fragementsin the MosesRock Dike, J. Geophys.Res., 77, 7022606-609, 1991. 7057, 1972. Durrheim,R. J., andW. D. Mooney, Archcanand Proterozoiccrustal evolution:Evidencefrom crustalseismology--Reply, Geology, Mechie, J., and C. Prodehl, Crustal and uppermostmantle structure beneaththe Afro-Arabianrift system,Tectonophysics, 153, 10320, 665-666, 1992. 121, 1988. Durrheim,R. J., and W. D. Mooney,Evolutionof the Precambrian lithosphere: Seismological and geochemical constraints, J. Geo- Mechie, J., G.R. Keller, C. Prodehl, S. Gacii, L.W. Braile, W.D. Mooney,D. Gajewski,andK.J. Sandmeier,Crustalstructurebephys.Res.,99, 15,359-15,374, 1994.
9788
CHRISTENSEN
AND
MOONEY:
neaththeKenyaRift fromaxialprofiledata,in CrustalandUpper MantleStructureof theKenyaRift, editedby G.R.Kellerand
CONTINENTAL
CRUST
edited by S. P. Clark Jr., Mer• Geol. Soc. Am., 97, 195-222, 1966.
M.A. Khan, Tectonophysics, 236, 179-200, 1994.
Prodehl, C., Structureof the Earth'scrust and upper mantle,LanMeissner, R., TheContinental Crust;A Geophysical Approach, 426 doldt Bornstein, 11A, 97-206, 1984. pp., Academic,SanDiego, Calif., 1986. Raitt, R. W., G. G. Shot Jr., T. J. G. Francis, and G. B. Morris, AniMeissner, R., T. Wever,andE. R. Flueh,The Mohoin Europe-sotropyof the Pacific uppermantle,J. Geophys.Res., 74, 3095-
Implications for crustaldevelopment, Ann. Geophys., BS, 357364, 1987.
Meissner,R. O., and W. D. Mooney,Speculations on continental crustalevolution,Eos Trans.AGU., 72, 585, 590, 1991.
Miller,D. J., andN. I. Christensen, Seismic signature andgeochemistryof an islandarc: A multidisciplinary studyof the Kohistan accretedterrane,northernPakistan,J. Geophys. Res.,99, 11,62311,642, 1994.
Mohorovicic, A., Dasbebenvom 8, X, Jahrb.Meterol.Ohs.Zagreb, 9, 1-63, 1909.
Mooney, W. D., Seismic methodsfor the determinationof earth-
quakesource parameters andlithospheric structure, in GeophysicalFramework of the Continental UnitedStates,editedby L. C. Pakiserand W. D. Mooney,Mere. Geol. $oc. Am, 172, 11-34, 1989.
Mooney, W. D., and L. W. Braile, The seismicstructureof the con-
tinentalcrustanduppermantleof NorthAmerica, in TheGeology ofNorthAmerica- An Overview, editedby A. W. BallyandA. R. Palmer, pp. 39-52, GeologicalSociety of America, Boulder, Colo., 1989.
Morgan,P., andW. D. Gosnold,Heat flow andthermalregimesin the continental UnitedStates,in Geophysical Frameworkof the ContinentalUnited States,edited by L. C. Pakiserand W. D. Mooney,Mere. Geol.$oc.Am, 172, pp. 493-522, 1989. Nafe, J. E., and C. L. Drake, Variationwith depthin shallowand deepwatermarinesediments of porosity,densityandthe velocitiesof compressional andshearwaves,Geophysics, 22, 523-552, 1957.
Nelson,K. D., A unifiedview of cratonicevolutionmotivated by recentdeepseismicreflectionandrefractionresults,Geophys. J. Int., 105, 25-35, 1991.
Pakiser,L. C., and R. Robinson,Composition and evolutionof the continentalcrustas suggested by seismicobservations, Tectonophysics,3, 547-557, 1966.
3109, 1969.
Ringwood,A. E., and D. H. Green,An experimentalinvestigationof the gabbro-eclogite transformation and somegeophysicalimplications,Tectonophysics, 3, 383-427, 1966. Roy, R. F., E. R. Decker,D. D. Blackwell,andF. Brich, Heat flow in the United States,J. Geophys.Res., 73, 5207-5222, 1968. Rudnick,R.L., Xenoliths--Samples of the lower continentalcrust,in Lower ContinentalCrust, editedby D.M. Fountain,R. Arculus andR. Kay, pp. 269-308, Elsevier,New York, 1992. Schmidt, R., and B. J. Wood, Phase relationshipsin granulitic metapelitesfrom the Ivrea-VerbanoZone (northernItaly), Contrib. Mineral. Petrol., 54, 225-279, 1976. Smithson,S. B., R. A. Johnson,and Y. K Wont, Mean crustalvelocity: a critical parameterfor interpretingcrustalstructureand crustalgrowth,Earth Planet. Sci. Lett., 53, 323-332, 1981. Sollet, D. R., R. D. Ray, and R. D. Brown, A new global crustal thicknessmap, Tectonics,1, 125-150, 1982. Taylor, S. R., and S. M. McLennan,The compositionand evolution of the continental crust; rare earth element evidence from sedimentaryrocks,Philos. Trans. R. Soc. London,A, 301, 381-399, 1981.
Tuve, M. A., H. E. Tatel, and P. J. Hart, Crustal structure from
seismicexploration,J. Geophys.Res.,59, 415-422, 1954. Twiss, R. J., and E. M. Moores,Structural Geology,532 pp., W. H. Freeman, New York, 1992.
Warren,D. H., and J. H. Healy, Structureof the crustin the conterminousUnited States,in The Structureof the Earth's Crust,Der. Geotecton.,vol. 8, editedby St. Mueller, pp. 203-213, Elsevier, New York, 1974.
Weaver,B. L., andJ. Tarney,Major andtraceelementcomposition of the continental lithosphere, in StructureandEvolutionof the ContinentalLithosphere,edited by H. N. Pollack and V. R. Murthy, pp. 39-68, Pergamon,New York, 1984.
Windley,B. F., TheEvolvingContinents, 399 pp.,JohnWiley,New
York, 1984. Pavlenkova,N. I., Generalizedgeophysical model and dynamic propertiesof the continentalcrust,Tectonophysics, 59, 381-390, Woollard,G. P., Crustalstructurefrom gravity and seismicmeas1979.
urements,J. Geophys.Res., 64, 1521-1544, 1959.
Percival,J. A., D. M. Fountain,andM. H. Salisbury, Exposedcrustal cross sections as windows on the lower crust, in Continental
N. I. Christensen,Departmentof Earth and AtmosphericSciences,PurdueUniversity,West Lafayette,IN 47907-1397. (e-mail: pp. 317-362, Elsevier,New York, 1992. chris•geo.purdue.edu) Plafker,G., W. J. Nokleberg,and J. S. Lull, Bedrockgeologyand W. D. Mooney, U.S. GeologicalSurvey,345 Middlefield Road, tectonicevolutionof the Wrangellia,Penninsular, and Chugach MS 977, Menlo Park, CA 94025. (e-mail: mooney• anterranesalongthe Trans-AlaskaCrustalTransectin the Chugach dreas.wr.usgs.gov)
LowerCrust,editedby D.M. Fountain,R. ArculusandR.W. Kay,
Mountainsandsouthern CopperRiverBasin,Alaska,J. Geophys. Res., 94, 4255-4295, 1989.
(ReceivedAugust30, 1994;revisedJanuary19, 1995;
Press,F., Seismicvelocities,in Handbookof PhysicalConstants, accepted January23, 1995.)
