Nature and Composition of the Continental Crust: A
NATUR COMPOSITIOD EAN CONTINENTAE TH F NO L CRUST LOWEA : R CRUSTAL PERSPECTIVE
Robert . RudnickaL 1 David M. Fountain Research School Earthof Sciences Department of Geology and Geophysics The Australian National University, Canberra University Wyoming,of Laramie
Abstract. Geophysical, petrological geochemid an , - surement a wid r efo s variet f deeo y p crustal rocks cal data provide important clues abou composie th t - provide a link between crustal velocity and lithology. tion of the deep continental crust. On the basis of Meta-igneous felsic, intermediat mafid ean c granulite, seismic refraction data divide w , cruse eth t into type and amphibolite facies rock distinguishable sar e th n eo sections associated with different tectonic provinces wav5 .d ean basivelocitiesP f so metamorphoset bu , d Each shows a three-layer crust consisting of upper, shales (metapelites) have velocities that overlap the middle lowed an , r crust whicn i , hwavP e velocities complete velocity range displaye meta-igneoue th y db s increase progressively with depth. There is large vari- lithologies. The high heat production of metapelites, ation in average P wave velocity of the lower crust coupled with their generally limited volumetric extent between different type sections, but in^general, lower in granulite terrains and xenoliths, suggests they con- crustal velocities are high (>6.9 km s"1) and average stitute onl ya smal l proportio e loweth f ro n crust. middle crustal velocities range between 6.3 and 6.7 km Using average P wave velocities derived from the s"1. Heat-producing elements decrease with deptn hi crustal type sections e estimateth , d areal extenf o t cruse th t owin theigo t r depletio felsin i c rocks caused each type of crust, and the average compositions of by granulite facies metamorphis increasn a d man n ei different type granulitesf so estimate w , average eth e the proportion of mafic rocks with depth. Studies of lowe middld an r e crust composition lowee Th . r crust crustal cross sections show that in Archean regions, is composed of rocks in the granulite facies and is 50-85 e heath t%f o flowin g fro e surface mth th f eo lithologically heterogeneous. Its average composition Earth is generated within the crust. Granulite terrains is mafic, approaching that of a primitive mantle- that experienced isobaric coolin representative gar f eo derived basalt, but it may range to intermediate bulk middl r loweeo r crus havd an te higher proportionf o s composition n somi s e regions e middlTh . e cruss i t mafic rocks than do granulite terrains that experienced composed of rocks in the amphibolite facies and is isothermal decompression lattee probable Th . rar t yno intermediate in bulk composition, containing signifi- representative of the deep crust but are merely contentsuppeU d ran , .can Th Averag , tK e continental crust crustal rocks that have been through an orogenic cy- is intermediate in composition and contains a signifi- cle. Granulite xenoliths provide deepese somth f eo t cant proportion of the bulk silicate Earth's incompat- samples of the continental crust and are composed ible trace element budget (35-55% of Rb, Ba, K, Pb, largely of mafic rock types. Ultrasonic velocity mea- Th, and U).
1. INTRODUCTION wate Eartn ro h [Campbell Taylor,d an 1985]. Whereas the upper crust is accessible to geological sampling Continents cover 41% of the Earth's surface [Cog- and measurements deepee th , r portion cruse th f to s ley, 1984] and sit at high elevations compared to the relativele ar y inaccessible dateo deepese T . th , t drill ocean basins owinpresence th o gt lower-densityf eo , hol penetrates eha d onl crusf o y 1m t2 k [Kremenetsky evolved rock types. ("Evolved defineds "i , along with and Ovchinnikov, 1986]. Nevertheless, these deep por- other specialized terminology, in the glossary follow- tions of the crust contain important information re- ing this introduction. e evolveTh ) d rocks that domi- lated to the bulk composition of the continental crust nate the upper portions of the Earth's continental crust as well as how it forms. unique r solaar ou rn e i system [Taylor,e 1989ar d ]an The lower crust (below -20-25 km depth) is be- probably ultimately linked to the presence of liquid lieve consiso dt metamorphif to c granulitrocke th n si e facies (referre simplo dt granulites ya s throughout this paper), which are accessible either as large tracts of !Now at Department of Earth and Planetary Sciences, surface outcrop (terrains) or as tiny fragments carried Harvard University, Cambridge, Massachusetts. from great depths in volcanic conduits (xenoliths). The
Copyright 1995 by the American Geophysical Union. Reviews of Geophysics, 33, 3 / August 1995 pages 267-309 8755-1209/95/95 RG-01302$75.00 Paper number 95RG01302 267 26 8• Rudnic Fountaind kan : LOWER CONTINENTAL CRUST 33, 3 / REVIEWS OF GEOPHYSICS
middle crust (i.e., between 10-15 and 20-25 km depth) Acoustic impedance produce th : f velocito t d yan may contain rockamphibolite th n si e facies, whice har density. also found in surface outcrop or as xenoliths. Amphib- Amphibolite: a mafic rock consisting dominantly olite facies rocks may also be important in the lower- of amphibole. most crust in areas of high water flux (such as in island Amphibolit f metamorphico t e se facies e th : min- settingc ar s where hydrous oceanic lithospher subs ei - eral assemblage whicn si h mafic rock composee sar d ducte dewatered dan d [e.g., Kushiro, 1990]. of amphibole and plagioclase. The facies is typical of rol e lowee eth Th r crust play continentan si l tecton- regional metamorphism at moderate to high pressures poorls i s ic y understood r example rheoe Fo . th e - ar , and temperatures (i.e., >300 MPa, 450°-700°C). logical and compositional differences between upper Anisotropy: see seismic anisotropy. and lower crust sufficient to promote delamination of Anorthosite: a plutonic igneous rock composed lowee th r crus continent-continent a t t collision zones? almost entirely of plagioclase feldspar (see Ashwal How much lower crust migh recyclee tb d back inte oth [1993] for an excellent review). mantl t convergenea t margin mucsettingsw ho h d an , Constructive interference: see interference. remains withi e crusth n t under condition f higho s - Continent-continent collision zone: a special type grade metamorphism? of convergent margin where a continent on the sub- Our understanding of the deep continental crust has ducting plate collides with another on the overriding improved dramatically ove lase rth t decad resula s ea t plate. of detailed seismological studies and numerous studies Convergent margin zone th e: wher tectonio etw c of lower crustal rocks. However, the composition of plates converge and one is subducted beneath the the deep crust remains the largest uncertainty in de- other. termining the crust's overall composition. This is due Craton aren a f crusa:o t tharemaines ha t d stable e largth ) e (1 compositiona o t l differences between for very long periods of time. granulites that occur in surface tracts (granulite ter- Critical angle: the angle of incidence of a seismic rains, in which felsic rocks dominate) and those that wave at which a head wave (or refracted wave) is are carried as small fragments to the Earth's surface in generated. rapidly ascending magmas (xenoliths, whic dome har - Critical distance: offset at which reflection time inate y mafidb c e rocks)verth y) (2 heterogeneou, s e naturloweth f ro e crus s observea t granulitn i d e equals refraction time. terrains, and (3) the difficulty in determining rock Cumulate: an igneous rock formed by accumula- type(s) from average seismic velocities derived from tio f crystallizinno g phases. refraction studies. Delamination: a process by which dense seg- thin I s contributio reviee nw knowledgr wou e th f eo ments of the lower crust (and lithospheric mantle) sink deep continental crust from both geophysical-based into the convecting asthenosphere as a result of their and sample-based studies. Of the various geophysical negative buoyancy. methods (seismic, thermal, electrical, potential field), Ductile: generally regarded as the capacity of a seismological data and heat flow studies reveal most material to sustain substantial change in shape without about the composition of the crust. We will focus on gross faulting [see Paterson, 1978], though there ear these two methods here. (The interested reader is numerous d sometimean , s conflictinge , th use f o s referre Joneso t d [1992 Shived an ]t al.e [1992r fo ] word. review f electricao s magnetid an l c propertiee th f o s Ductile shear zones: a fault zone in which the lower crust, respectively.) We then integrate both data deformatio ductilenis . set orden i s o derivt r a bule k compositioe th r nfo Eclogite high-pressura : e mafic rock composef do lower, middle, and bulk continental crust. Our subdi- garne Na-ricd an t h clinopyroxene (omphacite); alsoa vision of the crust into upper, middle, and lower is metamorphic facies defined by the appearance of these base observationn do s from seismic studie summas sa - phases in mafic rocks. rize Holbrooky db . [1992]al t e . Eu anomaly: Eu/Eu* = 2Eu /(Sm Gd )° , where n n n 5 subscriptee th indicatedn s tha valuee tth normale sar - ize chondritio dt onle c th meteorites ye raron es i u E . . 2 GLOSSARY earth element (REE) that can occur in the 2+ valance state under oxygen fugacity conditions found in the Sources of definitions are Bates and Jackson [1980], Earth. Eu2+ is larger than its REE3+ neighbors and has Fowler [1990] Sheriffd an , [1991]. a charge and radius similar to that of Sr. It therefore Accessory phase: mineral present in low abun- substitutes for Sr in feldspars, and fractionation of dances in rocks but which may contain a significant feldspar wil anomalyu E l leaa o dt . proportion of the incompatible trace element inven- Evolved: said of an intermediate or felsic igneous tory of the rock. Examples include monazite and al- metamorphoses rocit r k(o d equivalent) tha derives ti d lanite. from a rock of a more mafic composition through the 33, 3 / REVIEWS OF GEOPHYSICS Rudnick and Fountain: LOWER CONTINENTAL CRUST • 269
processe f magmatiso c differentiatio r partiao n l melt- Peridotite a roc: k containing >40% olivin- ac e ing. companie y Cr-diopsidedb , enstatite n alumia d -an , Felsic: roca sai f do k composed mainl f lightyo - nous phase (either spine garnetr lo , dependin presn go - colored minerals. sure) uppee Th . r mantl believes ei composee b o dt d Granulite: a rock that exhibits granulite facies mainly of peridotite. mineral assemblages. Poisson's ratio: the ratio of elastic contraction to Granulite facies: the set of metamorphic mineral elastic expansio materiaa f no uniaxian i l l compres- assemblage whicn si h mafic rock representee sar y db relatee b n sion elastio dca t t I . c wave velocitie 0.y s5b 2 diopsid hypersthen+ e plagioclase+ e faciee Th .s i s {1 - l/[(Vp/Vs) - 1]}, where Vp is P (primary, or typica f deep-seateo l d regional dynamothermal meta- compressional) wave velocity and Vs is S (secondary, morphism at temperatures above 650°C. Although or shear) wave velocity. granulite fory t relativelmma a s pressuresw ylo e w , Prograde: mineral reactions occurring under in- tere th m e herus implo et y pressures greater tha0 n60 creasing temperature and pressure conditions. MPa. Reduced heat intercepflowe th lineae : th f rto sur- Granulite terrain: a large tract of land composed face heat production-heat flow relationship. of rocks at the granulite facies. Reflection coefficient: measure of relative amount Granulite xenolith a foreig: n rock fragmenn i t of reflected energy from an interface, determined as granulite facies that is carried to the Earth's surface by RC = [Z2 - ZJ/[Z2 + ZJ, where Zi is acoustic rapidly ascending, mantle-derived magmas. impedance of layer i (Zt = p/V/, where p is density and Head wave: wave characterize enteriny db d gan velocity)s i V . leaving high-velocity mediu mt criticaa l angle heaA . d Refraction surveys: experiments that utiliz- re e wave corresponds to a refracted wave that travels fracted waves and/or wide-angle reflected waves from alon interface gth velocita f eo y discontinuity. different layercruse th r mantl o n ti s deduco et e eth variation in seismic velocity with depth. Heat-producing elements (HPE): those elements Restite: material remaining behind after extrac- that generate heat as a result of their rapid radioactive tion of a partial melt. . U) decad an , y (i.e.Th , K , Retrograde: mineral reactions occurring unde- rde Interference: superposition of two or more wave- creasing temperature and pressure conditions. forms. Constructive interference occurs when wave- Seismic anisotropy: variation of velocity as a phasformn i destructivd e esan ar e interference occurs function of direction, usually reported as a percent. when waveforms are 180° out of phase. Either A = 100(V - V )/V or A = 100(V Island arc: an arcuate string of volcanic islands max min max max ~~ ^ min/' * mean• formed above zones of descending oceanic crust (sub- Shot-to-receiver offset (offset) e distancth : - be e duction zones). tween the energy source (explosion, earthquake, etc.) Isobaric cooling: pressure-temperature path fol- anseismometere dth . lowed by metamorphic rocks in which temperature Stacking: addin seismif go c trace increaso st e eth decreases while pressure remains high. signal-to-noise ratio. Isothermal decompression: pressure-temperature Subcritical energy: energy receive t distanceda s path followe metamorphiy db c rock whicn si h pressure less than the critical distance. decreases while temperature remains high. Supercritical energy: energy received at distances Lithospheric mantle: that portioe Earth'th f no s greater tha criticae nth l distance. mantle that immediately underlies convectivels i d an , y Supracrustal: a rocsai f o kd that e formth t a s coupled to, the crust. Earth's surface. Mafic: adjective describing a rock composed Trondhjemite plutonia : c rock compose sodif do c mainly of ferromagnesian, dark colored minerals. plagioclase, quartz, biotite, and little or no potassium Metapelites: metamorphosed shales (fine-grained feldspar. detrital sedimentary rocks composed largely of con- Underplating: intrusio f magmano s nea base rth e solidated clay, siltmud)d an , . of the crust. M : g# 100Mg/(Me 2Fe)ar g+ e F ,d wheran g eM Xenolith: a rock that occurs as a fragment in an- expresse moless da . other, unrelated igneous rock (literally, foreign rock Modal mineralogy e volumetrith : c proportiof no fragment). minerals in a rock. Mu (ji): the ratio of 238U to 204Pb. Orogenic belt: a linear or arcuate belt that has 3. SEISMIC PROPERTIES OF LOWER been subjecte foldino t d deformatiod an g n duringa CONTINENTAL CRUST mountain-building event. Pelitic: sedimentara sai f do y rock composef do In this section we review the major findings related clay. to the structure and composition of the lower conti- 27 0• Rudnic Fountain:d kan LOWER CONTINENTAL CRUST / REVIEW 3 , 33 GEOPHYSICF SO S
nental crust derived from seismic surveys. We com- dimensional (2-D) forward modeling methods that at- pare these with ultrasonic velocities measure difdin - tempt to match both observed travel times and the ferent deep crustal rock types. In a later section we amplitude wide-angle th f so e reflectio refractiod nan n use these comparisons to infer the bulk composition of arrivals with theoretically determined travel times and the deep crust. amplitudes. Such approaches have been improved by travel time inversion methods coupled with amplitude 3.1. Methods modeling [e.g., ZeltSmith,d an 1992; Hole al.,t e 1993; Much of our knowledge about the physical proper- Holbrook et al., 1994b]. e continentath tie f o s l crus s derivei t d from various Near-vertical reflection surveys record subcritical seismic refractio d reflectioan n n methods (see Hoi- energy at very small offsets (2-10 km), small receiver brook et al. [1992] and Mooney and Meissner [1991] spacing (25-10closd an e, sho0m) t spacing (50-50) 0m for recent reviews). In these studies, seismic energy relative to refraction surveys. Although subcritical re- generated by natural (e.g., earthquakes) or artificial flections have lower amplitudes than critical to super- (e.g., explosions, vibrator trucks, air guns) sources is critical reflections acquisitioe th , n geometry produces recorde seismometery db s place t variouda s spacings data a redundancy that wida permit f o e e arrasus f yo (receiver spacing t lona ) g distances fro energe mth y signal processing methods (e.g., stacking r amplifo ) - source (shot-to-receiver offset) refraction I . n surveys, tude enhancement and correlation of reflected phases the offset is generally long (200-300 km) in order to over large distances. Additionally e higher-freth , - record arrivals fro uppee mth r mantl Mohod ean , shot quency content of energy sources in near-vertical sur- spacin generallgs i y receive20-10d an , 0km r spacings i veys (e.g., air guns and vibrators) provides higher highly variable. Receiver spacin n oldei g r surveys resolutio cruse th f tn o tha n available with lower fre- commonl morn i y t exceedee bu recen, km 0 td 1 studie s quency sources used in refraction surveys. Vertical generalls . iReceivei t km 5 y1- r spacings less than1 resolution of reflection data is generally regarded as km have been achieved in surveys that use marine one-quarte signae th r l wavelength crustaa r - Fo .ve l profiling techniques where airguns towed by ship frequenc, verticaa s"sd Hz m 1k an 5 2 locit0 lf 6. yo f yo shoot frequently into hydrophone streamers, fixed resolution is 60 m [Mooney and Meissner, 1991]. De- ocean bottom seismometers, or land-based stations spite this potentia resolvinr lfo g structure deee th pn si [e.g., BABEL Working Group, 1993; Holbrook t al.,e crust with near-vertical reflection methods, ther- ere 1994b]. Small receiver spacing, although expensive in main numerous problems in the interpretation of re- land-based surveys, is highly desirable because of the flectors in the deep crust (for instance, complications improved correlatio phasef no recorn so d sections. arise from the effects of geometrical spreading, scat- The variety of waves recorded in typical refraction tering, attenuation, interference effects d inadean , - surveys provide the basis for seismic velocity models quate knowledg f deeeo p crustal velocities user dfo cruste ofth addition I . direce th o nt wave that travels migration and stacking). In addition, near-vertical in- through the uppermost crust (Pg), various waves are cidence methods provid direco en t measur seismif eo c reflected and refracted (head wave) from what are velocities of reflectors, and the reflection coefficients traditionally interpreted as first-order velocity discon- of reflecting horizons remain unconstrained. tinuities (see texts of Bott [1982 Fowlerd ]an [1990r ]fo review). For a simple, layered crust, only reflections 3.2. Lower Crustal Structure are recorded from an interface at small offsets .e mos Ath sf t o remarkabl e On e feature f reflectioo s n offset increases criticaa , l angl incidencf eo attaines ei d survey e presencth s i s f numerouo e s subhorizontal where energ partitiones yi d int owide-angla e reflected reflectors in the lower crust of many regions, which in wave and a head wave (refracted wave) that travels most cases disappea Mohe th t ora (see Barnes [1994] along the interface. For the Moho these waves are for discussio f sub-Mohno o reflectors) mann I . y cases referred to as PmP and P'„, respectively. In principle, the onset of reflectivity within the crust appears at the measurement of travel time of refracted arrival lowese ath sf ro a crusp continued to tan s throughoutn i d an , functio f offseno t allows determinatio apparenn a f no t other cases it is confined to the upper and lower velocite heath dr yfo wave . Because amplitudef o s boundarie lowee th f rso crust (see Mooney Bro-d an these arrival generalle sar y smaller than amplitudef so cher [1987] for a review). A detailed discussion of the wide-angle reflection t offseta s s greater tha critie nth - voluminous literatur e causeth n f deeo eo s p crustal l distanceca , the difficule yar identifo t t correlatd yan e reflectivity is provided by Mooney and Meissner in record sections. Wide-angle reflections from other [1992], who summarize the large number of models for interfaces can also mask the refracted arrival of inter- e origith f reflectiono n s froe deemth p continental est. Because of this complication, most current inter- crust. These include (1) acoustic impedance contrasts pretations rely heavil n criticao y d supercriticaan l l cause solidifiey db d igneous intrusions within crustal wide-angle reflection o constrait s e positioth n d an n rocks of differing physical properties; (2) fine-scale velocity of deep crustal layers. In the last decade, lithologic layerin metamorphif go c rocks where reflec- most analyses of refraction data employ various two- tions can be caused by lithologic variations, seismic 33/ REVIEW 3 , GEOPHYSICF SO S Rudniek and Fountain: LOWER CONTINENTAL CRUST • 271
anisotropy (variatio velocitf no functioa s ya direcf no - strongly influence both the amplitude and travel time tion) constructive interference combinatioa r o , f no wide-angle oth f e reflections that figur heavilo s e n yi these; (3) faults that juxtapose different rock types; (4) interpretatioe th refractiof no n data [see Levanderd an localized ductile shear zones where reflections origi- Holliger, 1992; Long et al., 1994; and references there- nate because of seismic anisotropy withi e sheanth r potentiae in]Th . l severit thif yo s proble millustrates i d zone, metamorphic recrystallization (prograd r reteo - by Levander and Holliger [1992], who show that ran- rograde) withi sheae nth r zone tha absens i t t outside dom velocity variations superimpose first-ordea n do r zonee th , constructive interference from enhancement discontinuity produc same eth e seismic responst e(a of lithologic layering related to high strain, or the intermediat largd ean e offsets velocita s a ) y gradient complex interaction of all these effects; (5) local zones with the same magnitude of superimposed fine-scale containing fluids under high pore fluid pressure (for a velocity variations. Levander Holligerd an [1992] con- critiqu f thieo s hypothesi Mooneye sse Meissnerd an clude that fine-scale velocity variation lowee th n i rs [1992] and Frost and Bucher [1994]); (6) pervasive crust may make it difficult to detect large-scale veloc- ductile flow in the deep crust that enhances layering, ity changes in the absence of clear refraction arrivals. anisotropy constructivd ,an e interference mol) (7 -d an ; However, we note that although Levander and Hol- ten or partially molten bodies in the lower crust [Suet- liger [1992] produce similar theoretical seismic profiles nova et al., 1993]. Support for the second hypothesis is for very different models, the average velocity of the provided by seismic modeling studies where geologic lower crus botn ti h sanlemodele th s . sAccordinglywa , maps of exposed deep crustal terrains (see below) and we assume that the average velocity of a lower crustal laboratory measurement rocf so k velocit usee yar o dt layer is indicative of the average properties of the calculate theoretical reflection profiles [Burked an constituent rock types while recognizing that sucn ha Fountain, 1990; Fountain, 1986; Fountain Salis-d an averagcausee b heterogeneoua y y deb ma s packagf eo bury, 1986; Hale and Thompson, 1982; Holliger and lithoiogies. Levander, 1992; Hurich Smithson,d an 1987; Reston, 1990] generaln I . modele th , s simulat reflectivite eth y 3.3. Seismic Velocitie Cruse th n sti patterns observe reflection do n profiles. Opinion no mose Somth f te o usefu l parameters determinen di fourte th h hypothesi mixeds si . Some studies indicate seismic refraction surveys are average P (compres- that strain gradient diminisy sma h ductile shear zone sionalS (shear d an )) wave velocities ,d VVsan ,p reflectivity [Rey et al., 1994] and that compositional respectively for different crustal layers. Velocities are changes associated with retrograde or prograde meta- relate physicae th o dt l environmen cruse th f t o t(tem - morphism (synkinematic w mineragrowtne f o hl perature, pressure, porosity, fluid content, etc.) and phases) may be required for ductile shear zone reflec- e intrinsith c propertie e rockth f so s (mineralogical tivity [Fountain t al.,e 1994a; Kern Wenk,d an 1990]. composition, chemical composition, metamorphic Other investigations [Barruol et al., 1992; Christensen grade, crystallographic preferred orientation of con- and Szymanski, 1988; Jones and Nur, 1984; McDon- stituent minerals, etc.) through which the seismic ough and Fountain, 1988] show examples where duc- waves pass. Reviews of the relative importance of tile shear zones may be reflective. these parameter crustan si l rock givee s ar Chris-y nb Consideratio near-verticae th f no l reflection datr afo tensen and Wepfer [1989], Fountain and Christensen deee th p continental crust raise importano stw t points [1989], and Jackson [1991]. An excellent review of concerning interpretation of seismic velocity models seismic constraint lowen so r crustal composition using derived from refraction data. First prominene th , - re t seismic refraction data and laboratory measurements flections observe mann di y lower crustal section- sim of rock properties is given by Holbrook et al. [1992]; ply thae loweth t r crus s compositionalli t y and/or r analysiou s follows from theirs. structurally heterogeneous at a variety of scales. For In order to summarize the general characteristics of example, modelin Holligery gb Levanderd an [1992] the lower continental crust of differing tectonic affini- illustrates that acoustic impedance contrasts from ties usee highest-qualite w , dth y seismic studies com- lithological variations with fractal dimensions of 2.7 piled by Holbrook et al. [1992, Table 1-1] and supple- can cause deep crustal reflectors of the type observed mented these with more recent studies. Because these on deep seismic profiles. Thus average velocities de- studies are confined mainly to North America and termined from refraction surveys represent onln a y Europe includee w , d dat lessef ao r quality (e.g., wide average velocity of the rock units in the layer. More- receiver spacing, one-dimensional interpretationso s ) over, the absence of reflectors does not imply th eachievo t s a moreea thorough samplin contie th f g-o absenc compositionaf eo structuralr (o l ) variabilit- ybe nents. We divide the crust into the following catego- cause many rock types have similar acoustic imped- ries Precambria) (1 : n platform shieldd an s s (compris- ances (see below) and geometric effects (destructive ing areas of exposed Precambrian crust plus interference, scattering, etc.) may diminish reflectivity sedimentary platforms covering Precambrian crust), inheterogeneoua s medium. Second presence th , f eo (2) Paleozoic orogenic belts (e.g., Varisca westerf no n small-scale heterogeneities within the lower crust may Europe, Appalachians), (3) Mesozoic-Cenozoic exten- 272 • Rudnick and Fountain: LOWER CONTINENTAL CRUST / REVIEW 3 , 33 GEOPHYSICF SO S
WAVP E VELOCITY (km/s) with depth r hotteFo . r geotherms, Vp decreases with 6.0 6.5 7.0 7.5 depth. So that the various refraction models in our 0 compilation coul e effectiveldb y compared, e corw - rected all values to a standard pressure of 600 MPa and 10 room temperature. To do so, we estimated the tem- perature fro famile mth conductivf yo e geotherms pre- 20 sented by Chapman and Furlong [1992], determined the pressure of each layer assuming an average crustal 3 g 30 density of 2800 kg m~ , and recalculated the velocities for each layer usin derivativee gth s given abovey B . w 40 means of these corrections, the average velocities can D also be compared with laboratory measurements of 50 ultrasonic velocities, which are generally performed at room temperature (see below) assume W . e thae th t 60 crus isotropis i t thad can t other factors that influence Vp, suc higs ha h pore fluid pressures t regionno e -ar , 70 ally important. Tabl list1 ethicknessee th s - re d an s ported velocitie profilee th r sfo s compila e useth n di - Figure 1. Variation of compressional wave velocity for tion as well as the velocities corrected to room typical lower crustal felsi mafid can c rock functioa s sa f no temperature and 600 MPa confining pressure. All spe- depth in the crust for geothermal gradients corresponding to 2 cific refraction velocity values mentioned henceforth surface hea tm0 9 floW d wm~ an value 0 4 [Chapmanf o s wil correctee b l d velocities. and Furlong, 1992]. Mineralogies are constant throughout illustratee th d pressure interval. Although there is considerable and important vari- ation within each crustal type t appeari , s that some type f cruso s t share comparable velocity structure from place to place. For example, Precambrian shields sional regions (e.g., U.S. Basin and Range), (4) Meso- and platform e generallar s y characterize y thicb d k zoic-Cenozoic contractional orogenic belts (e.g., Alps, crust (43 km) having a high-velocity layer in the lower Himalayas, Canadian Cordillera), (5) continental arcs one third of the crust. (We find no significant differ- (e.g., Cascades) ) activ(6 , e rifts (e.g., Rheingraben), ences between Archea d Proterozoian n c shieldn i s (7) rifted margins (e.g., eastern continental shelf of North America), and (8) forearc regions (e.g., coast of either crustal thickness (43 km for both) or velocity British Columbia) obvioun A . s proble sucmn i hcoma - structure [cf. Durheim Mooney,d an 1994]. conn I ) - pilatio thas ni t many continental regions remain only trast, other types of crust are more highly variable. poorly studied from a seismological viewpoint (i.e., Paleozoic fold belts in Europe appear to have thinner mos Africaf to , Asia, South America Antarctica)d an , . crust with lower velocity than do non-European Pa- thus i t I s picturuncleae th w crustaf eo r ho l structurn ei leozoic fold belts (this may be due to postorogenic different tectonic settings derived from this database extension [Menard and Molnar, 1988]). As might be might change when more data become available. expected, Cenozoic-Mesozoic contractional orogenic Because pressur temperaturd ean strongln eca - yin belts have the greatest variability, with crustal thick- fluenc e seismicth e velocitie f crustao s d mantlan l e ness varyin nearly g b lowed factoyan a 2 r f crustao r l 1 necessars rockwa t si correco yt crustae tth l refraction velocity varying by nearly 1 km s" . data to a common pressure and temperature for pur- These velocity models were average orden di o t r poses of comparison. The potential magnitude of these derive crustal type sections (Figure 2, Table 2). All effects is illustrated in Figure 1 for two different con- show a three-layer crust with increasing P wave ve- ductive geotherms (correspondin heao gt t0 4 flo f wo locity with depth. With the exception of Precambrian and 90 mW m~2) and two different lithologies, a deep shields, the upper crust is characterized by a layer crustal mafic gneiss and a deep crustal felsic rock (see having low velocities (<6.2 km s" ); shield upper crust 1 1 below) usee W .dtypica a l pressure derivativ r Vpefo ha a highers averag e s"em k Th velocit. 3 6. f o y determined at confining pressures above 400 MPa average velocity for the middle crust is variable from (~ 2x 10~ s" m 1k 4 MPa" typicad 1an ) l temperature sectio sectiono nt . Paleozoic fold belts, Cenozoic-Me- derivative (~4 x 10~4 km s"1 °C~1) for crustal rocks sozoic areas, and rifted margins all exhibit middle (see references in Table 3 and summaries by Kern crusts having velocities between 6.2 and 6.5 km s"1, [1982], Christensen and Wepfer [1989], Fountain and whereas shield platformsd an s , continental arcd an s Christensen [1989], Jackson [1991], and Christensen rifts hav higheea r velocity middle crust, betwee5 n6. and Mooney [1995]). For a crust of constant composi- and 6.9 km s"1. The lower crust in all profiles exhibits m~s i tioW s whicn 2m na i , 0 heae (4 hth tw flolo s wi the highest average velocity, between 6.9 and 7.2 km 1 observe n cratonii d c regions) Vp increases slightly s" generaln I . e typth , e section Figurn i s bea2 e r TABL . E1 Summar Refractiof yo n Studies Use Compilation di f Crustano l Type Sections
Upper Middle Lower Lowest Moho Heat Flow 73 Thick- Thick- 1 Thick- 1 Thick- km ?-* Flow, m Vp, km s~* s-Vp>km Vp, km s' vvp , Km s Fluff, ness, ness, ness, ness, ————— — Depth, mW Refer- Place Reference km km Obs. Cor. km Obs. Cor. km Obs. Cor. km Obs. Cor. km m~2 ence* (S) 0 Cenozoic-Mesozoic Extensional Terranes -n Basi Rangd nan e Stauber [1983] 3.00 16.00 6.00 6.22 11.00 6.64 6.89 30.00 90 1 o Basi Rangd nan e Valasek et al. [1987] 3.00 9.50 6.10 6.30 7.00 6.35 6.57 8.50 6.60 6.86 3.00 7.35 7.60 31.00 90 1 om Basi Rangd nan e Goodwin McCarthyand 10.00 5.50 5.50 10.00 6.00 6.21 12.00 6.60 6.86 32.00 90 1 transition., Arizona [1991] Quesnellia terrane, B.C. Zelt et al. [1993] 2.00 22.00 6.20 6.35 7.00 6.65 6.84 3.00 7.70 7.90 34.00 73 2 Basin and Range ZeltSmithd an [1992] 9.00 5.75 5.90 12.00 6.25 6.46 8.00 6.85 7.11 29.00 90 1 Q Basin and Range Holbrook [1990] 8.00 6.00 6.00 10.00 6.10 6.30 8.00 6.60 6.85 2.00 6.80 7.07 28.00 90 1 Omineca belt Kanasewich et al. 15.00 5.90 6.06 10.00 6.15 6.36 10.00 6.75 6.98 35.00 83 2 [1994] Colorado River corridor McCarthy . [1991al t e ] 5.00 5.80 5.90 15.00 6.13 6.30 10.00 6.55 6.77 30.00 80 1 Mean 1.00 11.81 5.91 6.03 8.00 6.16 6.37 9.31 6.66 6.90 1.00 7.28 7.52 31.13 Standard deviation 1.41 5.46 0.22 0.27 5.42 0.12 0.13 1.71 0.10 0.10 1.41 0.45 0.42 2.42 N 3 8 8 8 6 6 6 8 8 8 3338 Arcs Cascades, Oregon Leavert . [1984al t e ] 7.60 6.00 6.00 19.40 6.45 6.65 15.00 6.90 7.20 4.00 7.40 7.70 46.00 90 3 Coast plutonic belt, B.C. Zelt et al. [1993] 1.00 12.00 6.30 6.50 8.00 6.80 7.00 15.00 7.03 7.25 36.00 80 2 Trans-Mexico belt Valdes . [1986al t e ] 4.00 6.00 6.00 12.00 5.90 6.10 30.00 6.90 7.16 46.00 90 1 Honshu, Japan Asano et al. [1985] 19.00 6.00 6.00 20.00 6.80 7.06 39.00 90 4 73 Northern Honshu, Japan Iwasaki . [1994bal t e ] 12.00 6.20 6.46 13.00 6.45 6.74 8.00 6.95 7.22 33.00 90 4 Q. Mean 10.92 6.40 6.62 17.60 6.92 7.18 n* 6.10 6.19 13.10 40.00 7T Standard deviation 5.62 0.14 0.26 4.72 0.37 0.38 8.14 0.08 0.07 5.87 O> N 5 5 5 4 4 4 5 5 5 5 Q. •n Forearcs O Coast plutonic complex Yuan et al. [1992] 4.00 4.00 6.35 6.48 7.00 6.50 6.63 12.00 6.85 6.97 0.50 7.45 7.57 27.50 50 2 Queen Charlotte I Coast plutonic complex Hole et al. [1993] 7.00 5.95 6.07 11.00 6.60 6.75 11.00 6.85 6.97 29.00 50 2 13 Queen Charlotte Coast plutonic complex Mackie et al. [1989] 10.00 5.70 5.82 13.00 6.48 6.60 6.00 6.84 6.95 29.00 50 2 Hecate Strait 73 Coast plutonic complex Spence and Asudeh 7.00 5.60 5.73 8.00 6.40 6.50 14.00 7.05 7.15 29.00 50 2 n Hecate Strait [1993] o Coast plutonic complex Spence Asudehand 4.00 4.00 6.10 6.22 12.00 6.48 6.60 6.00 6.90 7.02 26.00 50 2 Hecate Strait [1993] Vancouver Island, B.C. . [1993Zeltal t e ] 1.00 12.00 6.30 6.43 9.00 6.65 6.75 8.00 7.00 7.09 30.00 40 2 S. California margin, Howie et al. [1993] 3.00 5.20 5.33 12.00 6.10 6.25 12.00 6.70 6.88 27.00 70 1 Salinia S. California margin Howie et al. [1993] 4.00 10.00 5.63 5.78 6.00 6.90 7.07 20.00 70 1 n S. California margin, SLO Howie . [1993al t e ] 4.00 4.90 5.02 11.00 5.80 5.95 8.00 6.90 7.07 23.00 70 1 73 TABL . E1 continue d Rudnic k an d F
Upper Middle Lower Lowest Moho Heat Flow
1 1 Thick- Vpf km s~* Thick- y^^ £m s-i Thick- s'Vkm , Thick- s'Vkm , Flow, Fluff, ness, ness, p —p ————— Depth, mW Refer- Place Reference km km Obs. km Cor.Obs. Cor. km Obs. Cor. km Obs. Cor. km m~2 ence* o Forearcs (continued) D Great Valley, Calif., west Colburn and Mooney 5.00 8.00 5.75 6.00 7.00 6.70 6.87 7.00 7.20 7.38 27.00 70 1 sr [1986] 5' Great Valley, Calif., Holbrook and Mooney 4.00 4.00 6.00 6.12 11.00 6.66 6.77 8.00 7.00 7.10 27.00 40 1 central [1987] \ Diablo Range, Calif. MacGregor- Scott and 4.00 3.10 3.22 10.00 5.75 5.86 13.00 6.87 6.97 27.00 40 1 m Walter [1988] 73 N. California Beaudoin . [1994al t e ] 1.00 5.00 4.00 4.13 8.00 5.50 5.63 7.00 6.85 6.98 21.00 50 3 8 NE Japan Suyehiro and Nishizawa 1.50 2.50 3.80 3.93 13.50 5.99 6.11 4.00 6.90 7.02 21.50 50 17 Z [1994] H Z Mean 1.32 8.54 5.74 5.79 8.25 6.42 6.54 7.79 6.95 7.09 25.89 m Standard deviation 2.30 4.25 0.78 0.76 4.26 0.35 0.35 2.75 0.13 0.13 3.25 Z AT 8 14 14 14 12 12 12 14 14 14 14 > Active Rifts n 73 Black Fores/ t Gajewski Prodehland 13.00 6.00 6.00 12.00 6.65 6.80 25.00 70 5,6 C Rheingraben [1987] C/5 Rheingraben Zucca [1984] 8.00 6.00 6.00 17.00 6.25 6.47 25.00 90 6 Kenya rift south Mechie et al. [1994] 2.00 4.00 5.80 5.95 21.00 6.43 6.64 9.00 6.83 7.07 36.00 90 7 Kenya rift north Mechie . [1994al t e ] 1.00 3.00 5.80 5.95 6.00 6.30 6.48 10.00 6.61 6.83 20.00 90 7 Salton Sea Fuisetal. [1984] 13.00 6.00 6.00 1.00 6.90 7.11 6.00 7.35 7.58 20.00 90 3 Rio Grande rift Sinno et al. [1986] 12.00 6.00 6.10 8.00 6.10 6.34 10.00 6.67 6.93 30.00 90 3 Dead Sea Ginzburg et al. [1981] 3.00 15.00 6.20 6.40 10.00 6.60 6.84 5.00 7.20 7.43 33.00 80 7 Dead Sea, Jordan El-Isa. [1987aal t e ] b , 19.00 6.15 6.31 8.00 6.60 6.81 5.00 7.30 7.52 32.00 80 7 Mean 0.75 10.88 5.99 6.09 5.75 6.47 6.68 9.63 6.77 6.99 27.63 Standard deviation 1.16 5.49 0.14 0.17 7.34 0.30 0.30 3.74 0.35 0.36 5.66 N 3 8 8 855 5 8 8 8 8 Rifted Margins . NorwaW y9 Mutter and Zehnder 8.00 6.00 6.00 9.00 6.00 6.13 4.00 7.00 7.15 21.00 60 8 ^OJ [1988] uo W. Norway 10 Mutter Zehnderand 13.00 6.00 6.00 4.00 6.50 6.65 3.00 7.50 7.60 20.00 60 8 73 [1988] m Lofoten, Rost high Mjelde [1992] 6.00 6.00 6.00 12.00 6.40 6.54 9.00 6.80 6.95 27.00 60 8 m Lofoten, Voring plateau Mjelde [1992] 10.00 6.00 6.00 2.50 6.40 6.56 2.50 6.80 6.97 15.00 60 8 ^ Goban spur, E. Atlantic Horsefield et al. [1993] 2.00 2.00 5.05 5.18 10.00 6.29 6.45 8.00 6.80 7.00 22.00 70 9 0 Biscaf o y yBa Ginzburg et al. [1985] 9.00 6.00 6.00 6.00 6.00 6.15 6.00 6.50 6.66 21.00 60 8 -n Hatton Bank Morgan et al. [1989] 6.00 6.00 6.00 10.00 6.50 6.65 9.00 7.00 7.23 25.00 70 9 n Hatton Bank Morgan . [1989al t e ] 7.00 6.00 6.00 3.00 6.85 7.00 11.00 7.26 7.43 21.00 70 9 0 Hatton Bank A Fowler et al. [1989] 7.00 6.00 6.00 3.00 6.35 6.51 17.00 7.00 7.29 27.00 70 9 -0 Hatton Bank C Fowler et al. [1989] 2.50 2.00 6.00 6.00 3.00 6.75 6.92 3.00 7.25 7.41 14.00 7.50 7.67 24.50 70 9 Hatton BankE Fowler . [1989al t e ] 4.50 6.00 6.00 3.00 6.20 6.31 14.50 7.40 7.51 22.00 70 9 n1 C/5 Carolina trough Holbrook et al. [1994b] 8.00 6.00 6.00 2.00 6.30 6.45 24.00 6.65 6.83 34.00 70 9 ^ Carolina trough Holbrook et al. [1994b] 11.00 6.00 6.00 6.00 6.30 6.47 15.00 7.40 7.58 32.00 70 9 w Carolina trough Trehu et al. [1989] 13.00 6.00 6.00 9.00 6.60 6.77 13.00 7.35 7.54 35.00 70 9 TO Virginia coast Holbrook et al. [1994a] 8.00 10.00 6.50 6.66 4.00 6.70 6.88 11.00 7.50 7.69 33.00 70 9 m Offshore Georgia Lizarralde et al. [1994] 2.00 12.00 6.09 6.24 23.00 6.70 6.89 4.00 7.20 7.40 41.00 70 9 m Granf o E dS Banks, Reid [1994] 2.00 1.00 5.05 5.18 21.00 6.13 6.29 4.00 6.74 6.93 28.00 70 9 C/} Newfoundland 0 Grand Banks Reid [1993] 4.00 6.00 6.00 20.00 6.10 6.30 6.00 6.73 6.92 30.00 70 9 -n O Jeanne d'Arc basin Reid and Keen [1990] 28.00 6.00 6.00 7.00 6.73 6.91 35.00 70 9 m Gulf coastal plain Lutter and Nowack 11.00 6.00 6.00 5.00 6.20 6.31 12.00 6.85 6.94 28.00 40 3 0 [1990] 5 Baltimore Canyon LASE [1986] 8.00 6.00 6.00 11.00 6.05 6.22 9.00 7.25 7.43 28.00 70 9 35 . GreenlanE 3 d2 Mutter and Zehnder 8.00 6.00 6.00 7.00 7.00 7.15 7.00 7.50 7.68 22.00 70 9 n [1988] C/5 E. Greenland 16 Mutter and Zehnder 13.00 6.00 6.00 3.00 6.50 6.67 4.00 7.00 7.17 20.00 70 9 [1988] SW Greenland Gohl and Smithson 1.00 15.00 6.00 6.007.00 6.70 6.87 5.00 7.00 7.19 28.00 70 9 [1993] SW Greenland Chian and Louden 5.00 6.00 6.00 10.00 6.33 6.46 14.00 6.75 6.88 29.00 10 [1992] Lincoln Sea, N. Forsyth . [1994al t e ] 4.00 8.50 5.88 6.14 5.00 6.50 6.67 5.00 7.50 7.68 22.50 70 9 Greenland Red Sea Mechie et al. [1986] 4.00 6.00 6.00 8.00 6.20 6.38 6.00 7.20 7.36 18.00 80 11 Red Sea, Egypt Gaulier et al. [1988] 7.60 6.00 6.00 4.40 6.33 6.52 4.70 7.20 7.44 16.70 80 11 Red Sea, Sudan Egloffet al. [1991] 6.00 6.00 6.00 3.00 5.95 6.11 5.00 6.60 6.78 14.00 80 11 Red Sea, Yemen Egloffetal. [1991] 2.00 6.00 6.00 12.00 6.10 6.26 15.00 6.90 7.11 29.00 80 11 70 Mean 0.72 8.32 5.95 5.98 7.53 6.38 6.54 8.59 7.05 7.22 25.62 Q. Standard deviation 1.70 5.21 0.26 0.25 5.69 0.27 0.28 5.12 0.30 0.30 6.46 n' N 7 30 30 30 29 29 29 30 30 30 30 cu 3 Paleozoic Orogens Q. •n Swabian Jura Gajewski et al. [1987] 5.00 6.00 6.00 14.00 6.05 6.15 8.00 6.35 6.47 27.00 70 5,6 O . GermanS y (WF-Z1) Zeis et al. [1990] 3.00 6.00 6.00 15.00 6.05 6.20 12.00 6.65 6.89 30.00 70 5,6 C 3 Rhenohercynian Aichroth et al. [1992] 13.00 6.10 6.20 19.00 6.65 6.83 32.00 70 5,6 Saxothuringian Aichroth et al. [1992] 21.00 6.10 6.20 8.00 6.80 6.98 29.00 70 5,6 3* Moldanubian Aichroth et al. [1992] 23.00 6.00 6.15 10.00 6.70 6.89 33.00 70 5,6 . GermaN n basin Aichroth . [1992etal ] 11.00 6.00 6.00 10.00 6.20 6.37 6.00 6.90 7.09 27.00 70 5,6 1 Ireland Jacob et al. [1985] 10.00 6.20 6.30 13.00 6.50 6.67 8.00 6.90 7.09 31.00 70 6,8,12 m Scotland Bamford et al. [1978] 10.00 6.10 6.20 10.00 6.43 6.59 8.00 7.00 7.18 28.00 70 6,8,12 70 . EnglanN d Bott et al. [1985] 4.00 6.00 6.00 24.00 6.48 6.63 28.00 70 6,8,12 Islandsf o y Ba , Nfld. Marillier et al. [1991] 4.00 20.00 6.40 6.51 15.00 7.20 7.35 39.00 60 9 8 Caledonians, Norway Iwasaki . [1994aal t e ] 1.00 5.00 6.30 6.42 13.00 6.43 6.59 15.00 6.80 6.98 34.00 70 6,8,12 zH Appalachians-91-1 A Marillier et al. [1994] 4.00 15.00 6.33 6.45 16.00 6.68 6.82 35.00 60 9 m Appalachians-91-lB Marillier et al. [1994] 4.00 17.00 6.39 6.53 12.00 6.86 7.01 33.00 60 9 Z Appalachians-91-2 Marillier . [1994al t e ] 9.00 7.00 6.34 6.40 7.00 6.54 6.65 17.00 6.89 7.03 40.00 60 9 Maine 1 Luetgert et al. [1987] 1.00 5.00 6.00 6.10 20.00 6.30 6.42 10.00 6.70 6.81 36.00 53 3 nr- Maine 2—SE Luetgert et al. [1987] 1.00 5.00 6.00 6.12 15.00 6.30 6.42 17.00 7.00 7.11 38.00 53 3 Gander 1 Hennet . [1991al t e ] 1.00 11.00 6.05 6.18 13.00 6.40 6.52 15.00 6.80 6.91 40.00 53 3 c/> 276 • Rudnick and Fountain: LOWER CONTINENTAL CRUST 33/ REVIEW 3 , GEOPHYSICF SO S
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Cor. km m~2 ence* Shields Platformsd an (continued) Mean 0.90 12.50 6.17 6.27 13.43 6.50 6.63 14.77 6.98 7.09 1.80 7.36 7.48 43.37 Standard deviation 2.21 5.86 0.15 0.19 8.97 0.14 0.148.23 0.32 0.23 5.74 0.12 0.18 5.19 AT 6 30 30 30 24 24 24 29 29 29 5 5 5 30 Archean Shields Mean 0.09 10.00 6.14 6.22 16.73 6.45 6.57 13.09 6.90 6.99 3.09 7.34 7.36 43.00 Standard deviation 0.30 4.00 0.15 0.20 9.38 0.16 0.16 7.84 0.19 0.19 9.01 0.23 0.27 6.34 N 1 11 0 11 0 11 0 1 11 10 10 11 2 2 11 Proterozoic Shields Mean 1.39 13.95 6.19 6.30 11.53 6.54 6.67 15.74 7.03 7.15 1.05 7.37 7.56 43.58 Standard deviation 2.70 6.35 0.16 0.17 8.38 0.12 0.11 8.50 0.24 0.23 2.55 0.03 0.04 4.57 N 5 19 19 19 14 14 14 19 19 19 3 3 3 19 aAbbreviations are Obs., observed; Cor., corrected to room temperature and 600 MPa; N, number of profiles; A, Archean; and P, Proterozoic. Heat flow references are 1, Blackwell and Steele [1991]; 2, Lewis [1991]; 3, Morgan and Gosnold [1989]; 4, Nagao and Uyeda [1989]; 5, Chapman et al. [1979]; 6, Cermdk [1993]; 7, Nyblade etal. [1990]; 8, Cermdk [1979]; 9,Jessop [1991]; 10, Chian and Louden [1992]; 11, Gettings et al. [1986]; 12, Brock [1989]; 13, Cull [1991]; 14, Shen [1991]; 15, Huang and Wang [1991]; 16, Gupta . [1991]al t , Okuboe 17 ;Matsunaga d an [1994]. C/l O -n O om Q / REVIEW 3 , 33 GEOPHYSICF SO S Rudnic Fountaind kan : LOWER CONTINENTAL CRUS9 27 T• pr • Paleozoic Paleozoic ._ A .. Platforms Orogens: Orogens: Mes./Cen. Arcs Forearcs ^d Mes./Cen. Active ° European others extensional Margins Contractional Rifts P atf rmS — 10 n=10 Ke velocitieso yt : <6.2 km/sec 6.2-6.5 km/sec 6.5-6.9 km/sec > 6.9 km/sec Figur . 2 eTyp e section continentaf so l crust roovelocitiel d an Al .m a temperature reportee MP sar 0 numbe60 e t dth a s i ;n r of profiles use construco dt t each type section. similarit thoso yt e presente Mooneyy db Meissnerd an However, various complex heat production models [1991 Holbrookd ]an t ale [1992]. can satisfy the surface heat flow and heat production observations [Fountain et al., 1987; Jaupart, 1983]. Whatever the exact distribution of heat-producing 4. HEAT FLOW STUDIES elements, the abundances observed at the Earth's sur- face canno maintainee b t d throughou crustae th t l col- Heat flow (thermal conductivity times thermal gra- umn. If they were, all of the heat flow measured at the dient) throug continentae hth l crus determines i t y db surfac continente th f eo s woul producee db d withie nth measurement of the thermal gradient and conductivity crust, leaving no heat input from the Earth's mantle, in shallow boreholes. Values typically range froo t 0 m3 or possibly requiring a negative input from the mantle 100 mW m~2, and mean values vary systematically (i.e. e crustath , l abundances accoun r morfo t e heat depending upon tectonic province [see Morgan, 1984, flow tha observed)s ni abundanceU d an , . ThuTh , s sK and references therein]. Recognitio lineaa f no r rela- must decrease with increasing depth in the crust, but tionship between surface heat flow and surface radio- the nature and cause of this decrease are debated. genic heat productio heat-producino t e ndu g elements Both changing composition and increase in meta- al.,t e e [e.g.) th y U 1968o t d ,Ro d an ]le , (HPE Th , ;K morphic grade could accoun decrease E th r HP tfo f eo view that heat production decreases with depth in the with deptcruste th n hi . Mafic granulites generally have exponentialn crusa n i t , linear r steo , p wise manner. lowecontentE HP r s than metasedimentar r moryo e TABLE 2a. Average Properties of Middle Crust in Different Type Sections 1 Thickness, Vp , km s' Tectonic Province N km Observed Corrected* Cenozoic-Mesozoic extensional regions 8 8.0 ± 5.4 6.2 ± 0.1 6.4 ± 0.1 Continental arcs 5 13.1 ± 4.7 4 0. 6. 4± 4 0. ± 6 6. Forearcs 14 3 4. 8. 2± 6.4 ± 0.3 6.5 ± 0.4 Active rifts 8 3 7. 5. 8± 3 0. 6. 5± 6.7 ± 0.3 Rifted margins 30 6 5. 7. 5± 3 0. 6. 4± 3 0. 6. 5± Paleozoic orogens 23 4 12.16. 1± 2 0. 6. 4± 2 0. 6. 5± Europe only 7 8 7. 9. 9± 2 0. 6. 3± 2 0. 6. 5± Non-European 16 2 13.5. 4± 1 0. 6. 4± 6.5 ± 0.1 Cenozoic-Mesozoic contractional orogens 8 1 11.7. 4± 1 0. 6. 3± 6.5 ± 0.1 Shields and platforms 24 0 13.9. 4± 6.5 ± 0.1 1 0. 6. 6± Archean shields 10 16.7 ± 9.4 6.5 ± 0.2 2 0. 6. 6± Proterozoic shields 14 4 11.8. 5± 6.5 ± 0.1 1 0. 6. 7± Values are given as averages ± 1 standard deviation. N is number of profiles. aAll velocitie correctee sar rooo dt m temperatur MPa0 60 .d ean 280 • Rudnick and Fountain: LOWER CONTINENTAL CRUST 33, 3 / REVIEWS OF GEOPHYSICS TABL . EAverag2b e Propertie f Loweso r Crus Differenn i t t Type Sections Thickness, Vp, km s Moho Tectonic Province N km Observed Corrected* Depth,km Cenozoic-Mesozoic extensional regions 8 7 1. 9. 3± 6.7 ± 0.1 1 0. 6. 9± 31.1 ± 2.4 Continental arcs 5 17.6 ± 8.1 1 0. 6. 9± 7.2 ± 0.1 40.0 ± 5.9 Forearcs 14 7.8 ± 2.7 7.0 ± 0.1 1 0. 7. 1± 25.9 ± 3.2 Active rifts 8 9.6 ± 3.7 6.8 ± 0.4 7.0 ± 0.4 27.6 ± 5.7 Rifted margins 30 8.6 ± 5.1 7.0 ± 0.3 3 0. 7. 2± 5 25.6. 6± Paleozoic orogens 29 13.4 ± 5.0 6.8 ± 0.2 2 0. 7. 0± 34.8 ± 5.0 Europe only 10 2 5. 9. 4± 6.8 ± 0.2 6.9 ± 0.2 29.9 ± 2.5 Non-European 19 5 15.3. 5± 6.8 ± 0.2 7.0 ± 0.2 9 37.3. 4± Cenozoic-Mesozoic contractional orogens 10 21.0 ± 12 3 0. 6. 7± 3 0. 6. 9± 52.4 ± 13 Shields and platforms 29 14.8 ± 8.2 7.0 ± 0.3 7.1 ± 0.2 43.4 ± 5.2 Archean 10 13.1 ± 7.8 6.9 ± 0.2 2 0. 7. 0± 43.0 ± 6.3 Proterozoic 19 15.7 ± 8.5 7.0 ± 0.2 2 0. 7. 2± 43.6 ± 4.6 Values are given as averages ±1 standard deviation. N is number of profiles. bAll velocities are corrected to room temperature and 600 MPa. evolved meta-igneous granulites. In particular, mafic k lithospherif mo c mantl requires ei d beneat crae hth - cumulates, which for bule m th mostf ko lower crustal ton accouno st difference th r fo t hean i e t flow. Lesser xenoliths, hav(mediaE eHP verw n ylo hea t produc- thicknesses would require a compositional contrast tio mafif no c granulite xenolith 0.0s si 6 jxW m~ ). Thus between Archean and post-Archean crust. Thus al- lowee ith f r crus predominantls i t y mafic (cumulates3 , thoug hcompositionaa l contrast between Archead nan restites r simplo , y metabasalt metagabbros)d an s t i , post-Archean crust is permitted by the heat flow data, would have intrinsically low heat production. In addi- it is not required to explain the differences in heat flow. tion, granulite facies metamorphism causes pervasiv d unifore an w mBecauslo heae th - tf eo Ar flo n wi depletion of U ± Th due to loss of grain boundary chea m~nW ture 2provincem )w thes,o 2 nt ± e0 (4 s fluid breakdowd an s f accessorno y phase t highea s r regions in order to evaluate the abundances and depth pressure P and temperature T [Rudnick et al., 1985]; distribution of HPE in the continental crust. Heat whether or not K is depleted by metamorphism is still productio f Archeano n crus s beeha t n estimater dfo debated {Rudnick and Presper, 1990]. So it is possible five regions where deeper crustal level exposede sar . for HPE depletion due to metamorphism to occur These findings are summarized in Table 3. without partial melt removal, hence withou signifia t - e VredeforTh 1. t dome, an upended section cant change to the bulk composition of the crust. As an f Archeao throug m k 0 nh2 crystalline rocks s comi , - exampl f thiso e , felsic granulites fro e Scouriamth n posed of granitic gneisses, with mafic rocks becoming complex in Scotland record some of the lowest HPE more prominen lowermose th n i t sectioe t th par f o tn concentration whan i sstile ar tl evolved compositions [Hartetal., 1981; Nicolayson etal., 1981]. The crustal [Rudnick et al., 1985; Sheraton et al., 1973]. contribution to heat flow, estimated from the radio- A second observation from heat flow studie thas si t genic heat production acros e overlyinth s g Witwa- Archean cratons, and adjacent Proterozoic belts (with- tersrand basi Vredeford nan t dome m~-2s i ,W 82m . in 400 km of Archean cratons), have significantly average Th e observed 6 hea ± basite 1 flo5 th s wn i i lower heat flow than post- Archean regions [Morgan, mW m~2 [Jones, 1988] implying a mantle heat flux of 1984 d referencean , s therein; Nyblade Pollack,d an 12-22 mW m~2. 1993]. This may be due to (1) a compositional differ- 2. In the Lewisian complex of NW Scotland, am- ence between Archea postd nan - Archean crust, with phibolite facies and granulite facies gneisses consist of the former having markedly lower K, Th, and U con- felsic meta-igneous rocks with subordinate mafid an c centrations [Morgan, 1984; Taylor and McLennan, ultramafic compositions and minor metasediments 1985] and (2) the presence of a thick lithospheric man- [Weaver Tarney,d an 1980] Archean A . n crustal sec- tle beneath Archean cratons that effectively insulates tion base averagen do thesf so e rock typeheaa s tsha the crust from asthenospheric mantle heat flow in production of 0.85 jjiW m~3 [Weaver and Tarney, these regions [Jones, 1988; Nicolayson t al.,e 1981; 1984], correspondin crustaa o gt l contributio heao nt t Nyblade Pollack,d an 1993]. m~W 2m , 4 assuminflo 3 thic f m wo k k0 4 crus ga d an t The latter hypothesis is supported by the observa- proportions of upper, middle and lower crust defined tion that cratonic mantle xenolith coolen o e sli r geo- by Weaver Tarneyd an [1984]averagn a r Fo . e Archean therms (Siberia [Boyd, 1984]; Kaapvaal [Finnertyd an hea m~ W mantle t m )floth 0 w(4 e contributio heao nt t 2 Boyd, 1987]) than mantle xenoliths from post- Archean flow woul m~ W e onlt m db shoulI . y6 e noteddb , regions. Nyblade and Pollack [1993] calculate that 400 however, 2 thahige th t h crustal heat productioe th n ni 33, 3 / REVIEWS OF GEOPHYSICS Rudnic Fountaind kan : LOWER CONTINENTAL CRUS1 28 T• TABL . E3 Summar f Heayo t Productio Crosn i s Section f Archeaso n Crust Heat Flow 2 Surface Heat Contribution, m mW Crustal Exposure Flow, mW Exposed Mantle Heat Flow Reference* Thickness, km Depth,km m'2 Crust (Estimated) From Crust, % Vredefort 1 36 20 51 ±6 28 12-17b 67-76 Lewisian 2 40C 20C 40C 34 6 85 Kapuskasing 3 43 25 41 22 12-17b 58-70 Pikwitonei 4 40C 30 41 19 18-21b 50-57 S. Norway 5 28 7 21 11 10 52 References are 1, Nicolayson et al. [1981]; 2, Weaver and Tarney [1984]; 3, Ashwal et al. [1987]; 4, Fountain et al. [1987]; 5, Pinet and Jaupart [1987]. bRange represents two assumptions about the heat production of the unexposed lowermost crust. The upper estimate assumes that unexposed crust has heat production equal to that of mafic granulites; the lower estimate assumes a mixture of mafic and HPE-depleted felsic granulites. Estimated. Weaver and Tarney model results not from the felsic regional heat flow, with a mantle heat flow of only 10 granulites (which hav verea hea w ylo t productiof no mW m~2. 0.12 juuW m~ , due to their strongly depleted charac- n summaryI , these observations from Archean teristics), but3 from the relatively thick upper crustal crustal sections show that although HPE decrease layer (fully one third of the crust), which has a high with depth in the crust, the middle crust may contain heat production (3.25 jjiW m~3). significant abundance HPEf so . Moreover, radiogenic e Kapuskasinth n I 3. g structural zone (KSZ)a heat production withi cruse nth t account 50-85r sfo % 25-km-thick crustal cross section is composed of of the surface heat flow in Archean regions. greenstone metasedimentary and volcanic rocks in the Recent studies in Archean regions have shown that upper crust (Michipocoten greenstone belt), tonalitic reduced heat flow (the intercep lineae th f ro t relation- amphibolite facies gneisses in the middle crust, and ship between surface heat production and heat flow) is tonalitic, mafic anorthositid an , c granulite facies rocks higher than the estimated mantle heat flux in shield in the lower crust. Heat flow from the exposed crustal regions [Furlong Chapman,d an 1987; Pinet Jau-d an m"W sectio[Ashwalm 2 2 s n i t al., e 1987], yieldinga part, 1987]. Thi bees sha n attribute effecte th f o dt o s mantl2 e heat flu f 12-1m~xo W 8m (depending upoe nth lateral heterogeneities withi crusnthe t lea thadto tmay compositio remainine th f no g unexpose2 d crust usind )an g underestimatio e deeth pn i e E amounth HP f no f o t averagn a e m~ heaW 2m t[Pinet 1 flo4 f t al.,wo e 1991]. crust. The studies of crustal profiles discussed above 4. The Pikwitonei subprovince, northern Superior l estimatal e mantle heat fluW m x 0 betwee2 d an n6 province, Manitoba, expose crustasa l section —3m 0k m~2 (or 15-50% of total heat flux), which is in contrast thick. The upper to middle crust (God's Lake subprov- to earlier models that suggested a uniform mantle heat ince) is composed of greenstone belts surrounded m~ bW y m 2 [Vitorello 8 flo2 f w o Pollack,d an 1980] irre- felsic gneisse d granitian s c batholiths, which grade spectiv f crustaeo l age. Mos f theso t e profiles show into deeper crust that is dominated by granulite facies increasing proportion mafif so c rocklowermose th n si t tonalitic gneisse d lessean s r amount f o mafis c crust, suggesting that lithologic changes s wela ,s a l gneisses, quartzites, trondhjemites anorthositesd an , . metamorphic depletions, are responsible for the de- Heat productio f cruso uppehighle s m i t k th 0 n ni 1 ry crease of HPE with depth. It is also apparent that variabl t averageebu s aroun jx9 Wd0. m~3 [Fountaint e many granulite facies terrains contain large amountf so al., 1987]. The middle crust is characterized by lower felsic granulites and have very low heat production and less variable heat production, averaging 0.6 (xW (0.4 mW m~2 is typical). Thus the low heat flow of heathe m~ t productio3and , lowethe nrof cruslow is t Archean cratons canno usee b t infeo dt r large propor- uniford |x4 an W0. t mm"a 3. Heat flo- w ex fro e mth tions (up to 2/3) of mafic lithologies in the crust [cf. posed crusta m~lW sectiom , 9 wit 1 mantle s nhi th e Taylor and McLennan, 1985]. The heat flow data are heat flow a m~ lyinmW for 2 g221 betweeand 18 n consistent with the presence of significant volumes of 40-km-thick crust (again, depending upo compoe nth - intermediate and felsic granulites in the middle and unexposee sitioth f no d lowermost crust). lower crust. 5. A section through the deep crust in southern Norwa a hea t has yproductio jxW1.6 m~of n for 3 amphibolite facies assemblage jji4 W0. d m"r an s fo 3 . 5 GRANULITES granulites lattee th , r containing significant proportions of mafic rocks [Pinet and Jaupart, 1987]. The crustal Granulites form under high temperatur presd ean - component of heat flow was estimated to be 75% of the sure conditions withi Earth'e nth s continental crust. 282 • Rudnick and Fountain: LOWER CONTINENTAL CRUST / REVIEW 3 , 33 GEOPHYSICF SO S r thiFo s reason, the e widelyar ye believeth e b o dt counted for (they presumably were tectonically em- dominant rock type in the lower continental crust, and placed int deee oth p crust durin earlien ga r event, such studies of granulites allow inferences to be made re- s tectonia c buria t convergena l t plate margins [von garding lower crustal composition and physical prop- Huene and Scholl, 1991]). In contrast, isothermal de- erties. compression is generally considered to result from Granulites that equilibrate t lowea d r crustaT P- l double thickenin f continentago l crust [Harley, 1989; condition e availablar s e fromaio mtw n sources) (1 : Newton andPerkins, 1982], wit granulitee hth s begin- surface outcrops (terrains), covering area hundredf so s ning and ending their travels at the Earth's surface. to thousands of square kilometers, and (2) xenoliths, This model accounts for both the presence of su- or small rock fragments carried rapidly to the Earth's pracrustal assemblage occurrence th d gransan e th f e-o surface in magmas. The large compositional contrast ulites presently at the Earth's surface. between thes o possibltw e e lower crustal sample Most of the P-T paths for granulite terrains re- suitefundamentae th s si l problem encountere- de n di viewed by Harley [1989] record isothermal decom- termining a robust estimate of the composition of the pression thereford an , e thestrule b t ye no rock y ma s bulk lower crust. Rudnick [1992 Rudnickd an ] d an representativ e loweth f ro e a significancrust t Ye . t Presper [1990] reviewed many of the chemical and proportion, about 35%, record isobaric cooling and physical propertie thesf o s einterestee rocksth d an , d therefor havy ema e reside lonr dfo g time periode th n si reader is referred to these papers. Below we summa- lower crust. Geochronologic constraints, where avail- rize the main features of each type of granulite, incor- able, support long-term residence of these terrains in porating recent data from the literature, and attempt to the deep crust (e.g., Enderby Land, Pikwitonei, Ad- put these rocks intcontexe oloweth e th f rto crust. irondacks [see Mezger, 1992], and references therein). These different terrains show an apparent difference 5.1. Terrains in their overall compositions (Figur . Those3) e expe- Major question relatinn si g granulite facies terrains riencing isothermal decompression are dominated by present-dae th o t thee yar o yn lowey wh ) r crus(1 e ar t felsic compositions, whereas those that have cooled longer in the lower crust, and (2) by what means were isobarically hav esignificantla y larger componenf o t they uplifted? Both these questions relate to the larger mafic lithologies, resulting in a bimodal distribution of question of whether the rocks found in granulite ter- rock types. The higher proportion of mafic lithologies rains represent actual sections of lower crust or were in granulite terrains that represent deep crustal sec- merely lower crustal transients, passing through high tion s consisteni s t wit n increasingla h y more mafic pressure-temperature condition thein so r return trio pt crust with depth. the surface, such as may occur during a continent- continent collision. Additional questions pertaining to 5.2. Xenoliths both terrains and xenoliths are what pressures (hence It is well established that mafic lithologies predom- depths thed )di y equilibrat thers i d esystema an , eat - inate in granulite xenolith populations (Figure 3). Sev- atic depth difference between the two [e.g., Bohlen eral questions arise from this contrast with granulite and Merger, 1989]? These latter questions wile b l terrains granulite Ar ) (1 . e facies xenoliths representa- addressed in section 5.2. tive of the lower crust? They may not be representa- In order to answer the first set of questions posed tiv f the ei e simplyar y deep-seated manifestationf o s above, investigators attemp defino t t e pressureth e - the volcanism that transports them to the surface. temperature-time (P-T-t) path followe granulitey db s Relate thiso dt felsie ar , c xenoliths underrepresented during their evolution. To date, the temporal con- in xenolith populations because of their dissolution in straintpathT P- sn s o hav e been determine onlr dfo ya the host basalt? (2) How do xenoliths relate to isobar- few terrains [see Mezger, 1992]; thus we will confine ically cooled granulite terrains? Do they represent our discussio pathT theid P- an so nt r possible inter- different crustal levels? pretations. Different tectonic setting givy esma riso et Mafic granulite xenoliths coul e relatee db th o dt different P-T paths, and although a particular P-T path host volcanism in two ways: (1) they could be cognate t necessarilino s y uniqu givea o et n tectonic setting, inclusions (i.e., cumulates of the magmas that carry certain generalization e madb n e ca s[Bohlen, 1991; them), or (2) they could represent earlier pulses of the Harley, 1989]. Granulites that cooled at lower crustal basaltic magmatism that intruded the lower crust and depths (isobaric cooling regardee ar ) products da f so equilibrated there. The metamorphic textures and min- magmatic underplating [Bohlen, 1991; Bohlen and erals found in the majority of mafic granulite xenoliths Mezger, 1989; Mezger, 1992; Wells, 1980], although preclude the cognate inclusion hypothesis. The second more complicated scenarios have been suggested alternative is less easily evaluated: basaltic provinces [Harley, 1989, Figure 14] thesn I . e models, upliff o t typically have lifetimes spanning several millionf o s granulite Earth'e th o st s surface results from tectonic years [e.g., Duncan McDougall,d an e 1989]th o s , processes unrelated to their formation, and the pres- earliest intrusions in the deep crust have sufficient time ence of supracrustal rocks in the terrains is not ac- to reequilibrate under the generally hot conditions / REVIEW 3 , 33 GEOPHYSICF SO S Rudnic Fountaind kan : LOWER CONTINENTAL CRUS3 28 T• prevalent there. Determining xenolith age therefors si e Xenolith ages crucia evaluatinn i l secone gth d possibility (see Rud- nick [1992 d Hancharan ] Rudnickd an [1995a r fo ] OOO discussion of dating techniques and problems encoun- tered in dating lower crustal xenoliths). Comparison between age of earliest volcanic activ- 100 ity versu sxenolith agethe sof s (where they have been established) shows that most mafic xenolitht no e sar 10 related to their host basalts (Figure 4). Because mafic xenoliths are generally interpreted to have formed by basaltic underplating of the crust [Rudnick, 1992], 0.1 1.0 10 100 1000 these results demonstrate that basaltic underplating is Initiation of volcanism (Ma) a recurrent process and thus may be an important means by which the crust grows [Bohlen and Merger, Figure 4. Timing of earliest volcanism associated with host pipe compared to ages of granulite facies xenoliths from 1989]. Moreover, geochronological studies have estab- these pipessummarizes a dat e e aar Ag . Rudnicky db [1992], lished that episodes of basaltic underplating correlate with newer additions from Wendlandt et al. [1993] and Chen with major regional geologic events [e.g., Rudnickd an etal. [1994]. Williams, 1987; Chen t al.,e considee 1994]w o s , t i r unlikely that significant underplating occurs without manifestation at the Earth's surface (e.g., volcanism r tectonism)o . 50 Another concern is whether felsic rocks are under- Isothermally uplifted 40 represente xenolitn di h suite resula s sa f thei o t r dis- 1 =n 25 solution in the host basalts. Thermal diffusion is very 30 rapid, so even though transport times are short (esti- ~c d o mated to be a few hours based on entrainment of dense u 20 peridotites [Spera, 1980] and fluid dynamical calcula- tions of dike propagation speeds [Lister and Kerr, 10 1991]), most xenoliths will be heated above their so- 50 somn i lidid e an ,case s liquidi, during transport from Isobarically cooled lowee th r crust [Tsuchiyama, 1986]. Recent calcula- 40 n = 330 tions suggest, however, that the degree of dissolution of xenolith s e limitediffusioi s th y b d e n th rat f o e +- 30 chemical components, such that minerals >100 jjimn i diameter within xenoliths will survive transport in ba- u 20 saltic magmas over the short period between entrain- 10 ment and eruption [Kerr, 1995]. We conclude that felsic xenoliths sampled from the lower crust will sur- vive transpor rapidly b t y moving basaltic melte th f si lower crus belos i t solidus w timit e th xenolitf eo t sa h entrainment. If the lower crust is instead partially molten, transport of these materials could lead to their disaggregation. Given that most mafic xenolith oldee sar r than their host volcanics, how do they relate to the isobarically cooled granulite terrains, which contain significantly greater amount f felsio s c components? Bohlend an 50 60 70 Mezger [1989] suggested basie th equilibratiof sn o o , n SiO% . 2 wt pressures, that thes typeo etw granulitef so same sar - ple differenf so t levelcruste th f so : isobarically cooled Figur . 3 eSiO histogra r isothermallmfo y uplifted granu- 2 granulite terrains equilibrate t middlda lower-mido et - lite terrains (Limpopo, southern Africa; southern India; dle crustal level sGPa) 8 (0.0. o 6t , whereas xenoliths Prydz Bay, Antarctica; Rayner complex, Antarctica; Mus- grave Ranges, Australia), isobarically cooled granulite ter- represen lowermose th t t crus GPa)t5 (1.1. o 0t . More rains (Pikwitonei, Canada; Scourian, Scotland; Napier com- recent review granulitf so e terrains suggest that many plex, Antarctica; Furura complex, Tanzania; Ivrea zone, have equilibrated at high pressures, similar to those Italy; Adirondack Mountains, U.S.A.) granulitd an , e xeno- derived from granulite xenoliths [Harley, 1989, 1992]. liths. Geochemistry compiled by Rudnick and Presper Moreover, pressures may be calculated only for xeno- [1990]; P-T paths compiled by Harley [1989]. liths that contain garnet, so it is likely that garnet-free 284 • Rudnlck and Fountain: LOWER CONTINENTAL CRUST / REVIEW 3 , 33 GEOPHYSICF SO S mafic xenoliths equilibrate t pressureda s below 1.0- summarize their average propertie Tabln si fula ( le4 a (e.g.1.GP 5 , garnet-free mafic granulite xenoliths compilation is available from the authors). Metamor- from northern Mexico are estimated to have equili- phic rock f apparenso t igneous origi thin ni s compila- brated at <0.72 GPa [Cameron et al., 1992]). Thus it is tion are grouped by rock composition following the not possible, on the basis of geobarometry, to distin- International Union of Geological Sciences (IUGS) guish difference derivation si n depths between terrains scheme for igneous rocks [Le Bas and Streckeisen, and xenoliths. 1991], where felsic rocks have >63 wt. % SiO2, inter- The geophysical constraints discussed above, how- mediate rocks have 52-6SiO % mafi d . 23an , wt c rocks ever, lend suppor a chemicall o t t y stratified crust. have 45-52 wt. % SiO2. Pelitic rocks are metamor- Seismic velocities increase with depth and typically phosed shales, having high A12O3 contents and SiO2 reach 6.9 km s" or greater, typical of mafic granulites generall peliti e . %Th .wt y6 c betwee 6 rock d an s5 n5 (see below), at 20-31 0 km depth in the crust. Likewise, have been further subdivided into two groups: those heat production must decrease with deptcruste th n hi , for which muscovit biotitd ean absen e ear r nearlo t y although it is not clear whether the requisite decrease absent ("granulite facies" thosd an )whic n ei h biotite can be accomplished strictly by depletion of HPE due and muscovite are abundant phases ("amphibolite fa- high-grado t e metamorphism r whetheo , r mafic lithol- cies"). The former are interpreted as residues (res- ogies become more abundant with depth. Thus the tites) f partiao l meltin f pelitigo c e lowerockth n i sr model of a stratified crust put forward by Bohlen and crust. Merger [1989] is an attractive one, if not entirely The summary in Table 4 includes only laboratory supported by their original arguments. measurements made in at least three orthogonal direc- tions for dry, unaltered samples where confining pres- r greateo a rGP wer6 sure0. ef o sattained , assuring . 6 TOWAR DBULA K DEEP CRUST COMPOSITION complete or near-complete crack closure. In some case e reclassifiew s d sample a manne n i s r different Give diversite nth rocf yo k type granulitn si e facies from that of the original investigators based on the terrains, the large compositional contrast between ter- reported mineralogy and/or chemical composition ni rains and xenoliths, and the nonunique interpretation e consistenordeb o t r t wite categorizatioth h - de n of seismic dat termn ai rocf so t surprisk no type s i t i -, scribed above. Becaus limitee th f eo d numbe meaf ro - ing that the composition of the lower crust, hence bulk surements made on intermediate composition granu- crust, is difficult to quantify. In this section we utilize lite facies rocks, we also include calculated time existine th g seismic, heat flow geochemicad an , l data averag waveP e velocities determine e Voigtth y db - in orde estimatto r e lower, middle totaand , l crust Reuss-Hill (VRH) method using modal mineralogies compositions. Usin type gth e sections derived above, reported by Lambert [1967] and Coolen [1980]. Veloc- we assign rock types to match the observed velocities, ities calculate methoH d VR wit de hgenerallth y agree based on ultrasonic measurements in lower crustal with measured values [Dhaliwal and Graham, 1991], rock types. Utilizing the granulite geochemical data- provided tharocke th tunalteree sar contaid dan o nn bas f Rudnicko e d Presperan [1990] e obtaiw , n a n glas r decompressioo s n features s soma , e xenolith average chemical compositio eacr nfo h laye wels ra s la sample [Parsonso sd t al.,e 1995; Rudnick Jack-d an whole th e crus eacr fo t h type section thee W .n assign son, 1995]. We believe the latter features are not rep- each type sectio arean na l extent base previoun do s resentative of in situ lower crust, so these xenolith geochronologica tectonid an l c thise studieus n i , d san data are not included in our compilation. All mean conjunction with the average crustal thickness of each rood valuean m a Tabln e reportei sGP ar 6 e4 0. t da section, to calculate volumes. From this we calculate temperature, the same common reference used for the e averagth e compositio e lowerth f o n, middled an , seismic refraction tabulation. total continental crust. resultine Th g mean value thin si s compilation differ in several ways from result previoun si s compilations 6.1. Linking Velocity to Lithology: The Laboratory [e.g., Holbrook et al., 1992], perhaps because of the Datt aSe restrictive criteri e use r samplw a dfo e selectionn I . In orde interpreo rt seismie tth c section termn si f so general, the range of velocities for each category is rock type, we need to know the seismic velocities of narrower somfor e categorieand , averagsthe e veloc- the different rock types that may exist in the deep ities are slightly different. For example, Holbrook et 1 wavS d crustean velocitieP . r appropriatsfo e rock . [1992al ] repor averagn a t e 7.0Vf o p 0.2 s"± 8m 7k types have been determined by either laboratory ul- for mafic granulite facies rocks (this value is derived trasonic measurements [e.g., Christensen Foun-d an by subtracting out the temperature correction Hol- tain, 1975] or calculations from modal analyses and brook et al. applied to their average velocities in their single-crystal elastic properties [e.g., Jackson et al, Table 1-3), wherea obtaie w s n 7.23 0.1± s"m 81k 1990]. We compiled the available Vp and Vs data for (both given at 600 MPa and room temperature). Our possible deep crustal and upper mantle rock types and values compare favorably with those determiner dfo / REVIEW 3 , 33 GEOPHYSICF SO S Rudnick and Fountain: LOWER CONTINENTAL CRUST • 285 TABLE 4. Average Properties of Lower and Middle Crustal Rocks at 600 MPa and Room Temperature All Samples Samples Whichfor VAvailableIs s Density, g Vp, SiO2, Density, Vp, Vskm Poisson's, SiO2, s~km cm~3 wt. % g cm~3 km s~ % References*wt. Ratio Amphibolite Fades Felsic gneisses Mean 2.690 6.355 70.34 2.733 6.329 3.633 0.254 69.88 2,3,4,6,7,9, Standard deviation 0.065 0.145 3.22 0.063 0.146 0.104 0.016 2.81 10,11,12,13,> Minimum 2.570 6.060 63.23 2.654 6.060 3.470 0.215 64.10 16,18,27,35,> Maximum 2.857 6.630 77.84 2.857 6.554 3.837 0.281 74.40 36 N 59 59 49 17 17 17 17 13 Pelitic gneisses Mean 2.801 6.477 64.78 2.799 6.506 3.638 0.271 63.77 1,2,7,9,12, Standard deviation 0.090 0.195 5.38 0.075 0.132 0.136 0.023 4.65 13,14,15,27 Minimum 2.645 6.050 55.40 2.645 6.295 3.363 0.233 55.40 Maximum 3.058 6.868 76.74 3.002 6.782 3.893 0.312 71.50 TV 35 36 30 15 15 15 15 13 Mafic gneisses Mean 3.028 7.018 48.61 3.043 7.039 3.964 0.267 48.55 1,2,3,4,7,10, Standard deviation 0.050 0.169 1.75 0.053 0.170 0.154 0.016 1.69 12,13,15,16, Minimum 2.939 6.720 45.91 2.968 6.738 3.717 0.241 46.23 17,18,23,27 Maximum 3.120 7.310 52.39 3.120 7.310 4.270 0.304 51.87 31,35 N 24 24 17 12 12 12 12 12 Granulite Fades Felsic Mean 2.706 6.529 68.46 2.718 6.526 3.714 0.260 67.52 3,4,5,6,7,16, Standard deviation 0.057 0.108 2.93 0.065 0.125 0.088 0.015 2.98 18,19,20,34, Minimum 2.619 6.289 63.01 2.619 6.289 3.563 0.238 63.01 35 Maximum 2.816 6.870 74.20 2.816 6.870 3.860 0.284 74.20 N 26 26 20 15 15 15 15 12 Intermediate: measured Mean 2.859 6.727 57.40 2.836 6.661 3.626 0.288 57.56 3,5,6,18,19,> Standard deviation 0.095 0.185 3.73 0.086 0.122 0.151 0.026 3.85 20 Minimum 2.738 6.536 35.32 2.728 6.536 3.434 0.242 53.32 Maximum 3.020 7.140 62.19 2.992 6.901 3.840 0.322 62.19 N 9 9 6 7 7 7 7 7 Intermediate: calculated, VRH time-average Mean 2.909 6.657 57.25 2.909 6.657 3.767 0.264 57.25 Standard deviation 0.090 0.203 3.62 0.090 0.203 0.124 0.014 3.62 Minimum 2.706 6.035 50.27 2.706 6.035 3.477 0.237 50.27 Maximum 3.139 7.002 63.57 3.139 7.002 3.998 0.283 63.57 N 26 26 22 26 26 26 26 22 Pelitic gneisses Mean 3.007 7.091 55.06 3.038 7.069 3.980 0.264 54.37 1,2,5,6,14, Standard deviation 0.108 0.345 6.29 0.108 0.337 0.125 0.022 6.34 16,21,35 Minimum 2.781 6.417 45.38 2.781 6.417 3.766 0.226 45.38 Maximum 3.240 7.742 66.56 3.240 7.742 4.297 0.302 66.56 N 19 19 20 13 13 13 13 13 Maficl al : Mean 3.038 7.226 47.85 3.031 7.209 3.970 0.282 47.74 1,2,3,4,17, Standard deviation 0.091 0.181 1.97 0.096 0.189 0.127 0.014 1.91 18,19,20, Minimum 2.899 6.787 43.88 2.899 6.787 3.721 0.243 43.88 26,34 Maximum 3.279 7.500 52.70 3.279 7.500 4.221 0.302 51.99 TV 33 33 27 27 27 27 27 21 Mafic: garnet-bearing Mean 3.099 7.326 48.09 3.113 7.328 4.049 0.280 47.80 1,2,3,4,17, Standard deviation 0.085 0.136 2.09 0.097 0.152 0.108 0.013 1.95 20,26 Minimum 3.001 7.030 43.88 3.001 7.030 3.838 0.263 43.88 Maximum 3.279 7.500 52.70 3.279 7.500 4.221 0.302 50.05 TV 12 12 12 7 7 7 7 7 Mafic: garnet-free Mean 3.003 7.169 47.67 3.002 7.167 3.943 0.283 47.71 1,2,4,17,19, Standard deviation 0.075 0.179 1.84 0.077 0.183 0.121 0.014 1.89 20,34 Minimum 2.899 6.787 45.20 2.899 6.787 3.721 0.243 45.20 Maximum 3.120 7.450 51.99 3.120 7.450 4.154 0.301 51.99 N 21 21 15 20 20 20 20 20 286 • Rudnick and Fountain: LOWER CONTINENTAL CRUST / REVIEW 3 , 33 GEOPHYSICF SO S TABL . E4 Continued All Samples Samples Whichfor VAvailableIs s V V V , km Pois son's Density, g P> Si02 , Density, P> s Si02 , cm~3 s * km wt. % g cm~3 km s ] s~J Ratio wt. % References^ Granulite fades (continued) Anortho sitesl al : Mean 2.798 7.108 52.28 2.752 6.949 3.728 0.298 53.69 3,7,8,18,20, Standard deviation 0.098 0.241 1.72 0.036 0.047 0.047 0.009 0.84 22,34 Minimum 2.707 6.810 48.32 2.707 6.888 3.666 0.286 52.52 Maximum 3.058 7.548 54.42 2.785 7.033 3.801 0.313 54.45 N 14 14 10 5 5 5 5 3 Anorthosites: garnet- bearing Mean 2.893 7.405 51.94 22 Standard deviation 0.096 0.107 0.81 Minimum 2.789 7.257 50.86 Maximum 3.058 7.548 53.22 N 5 5 5 Anorthosites: garnet-free Mean 2.743 6.944 52.63 2.752 6.949 3.728 0.298 53.69 3,7,8,18,20, Standard deviation 0.033 0.088 2.24 0.036 0.047 0.047 0.009 0.84 34 Minimum 2.707 6.810 48.33 2.707 6.888 3.666 0.286 52.52 Maximum 2.785 7.120 54.45 2.785 7.033 3.801 0.313 54.45 N 9 9 5 5 5 5 5 4 Upper Mantle Eclogites Mean 3.432 8.095 47.35 3.445 8.127 4.583 0.266 47.26 7,16,20,23, Standard deviation 0.108 0.226 2.09 0.096 0.205 0.151 0.018 2.00 25 Minimum 3.235 7.714 44.52 3.270 7.836 4.277 0.236 44.63 Maximum 3.585 8.582 51.68 3.585 8.582 4.817 0.288 50.24 N 24 24 18 12 12 12 12 12 Ultramafic rocks Mean 3.290 8.199 42.60 3.297 8.185 4.681 0.256 42.75 1,2,7,17,24, Standard deviation 0.028 0.198 2.81 0.026 0.196 0.127 0.021 2.77 27,28,29,30, Minimum 3.239 7.802 38.40 3.246 7.889 4.430 0.205 39.5 32,33,37 Maximum 3.369 8.590 47.95 3.335 8.590 4.890 0.288 47.95 N 41 41 12 14 14 14 14 7 Abbreviations are N, number of samples; VRH, Voight-Reuss-Hill. References are 1, Fountain [1976]; 2, Burke and Fountain [1990]; 3, Fountain et al. [1990]; 4, D. M. Fountain and M. H. Salisbury (manuscript in preparation, 1995); 5, Kern and Schenk [1985]; 6, Kern and Schenk [1988]; 7, Birch [I960]; 8, Birch [1961]; 9, Hughes et al. [1993]; 10, McDonough and Fountain [1988]; 11, McDonough and Fountain [1993]; 12, Christensen [1965]; 13, Christensen [1966b]; 14, Burlini and Fountain [1993]; 15, Padovani et al. [1982]; 16, Kern andRichter [1981]; 17, Miller and Christensen [1994]; 18, Salisbury and Fountain [1994]; 19, Christensen and Fountain [1975]; 20, Manghnani et al. [1974]; 21, Reid et al. [1989]; 22, Fountain et al. 1994a; 23, Simmons [1964] , Christensen;24 [1974] Kumazawa, ;25 . [1971]al t Fountain, e ;26 . [1994b]al t e Kern, Tubiad ;27 an [1993] , Christensen;28 and Ramananantoandro [1971]; 29, Christensen [1971]; 30, Christensen [1966a]; 31, Siegesmund et al. [1989]; 32, Peselnick and Nicolas [1978] , Babuska33 ; [1972] , Fountain34 ; [1974] . , [1993]Kernal 35 ; t , Mooneye 36 ; Christensend an [1994] , O'Reilly . 37 ;[1990] al t e . felsic granulite and amphibolite facies rocks by Chris- We observe the widely reported trend that seismic tensen Mooneyd an [1995]; however r mafiou , c gran- velocity increases with rock densit r crustayfo d an l ulite velocitie slightle sar y higher than theirs, although upper mantle rocks (Figure 5) but the distribution of still within the 1 a limits (e.g., 7.17 ± 0.18 versus data n detaili , , doe t follono s w well-defined linear 6.9 ± 0.14 s"r garnet-frem 8k fo 1 7.3d an e0.1± 3 4 trends reported in earlier studies [e.g., Birch, 1961]. versus 7.2 ± 0.13 s"r garnet-bearinm 8k fo 1 g mafic The data cluster in regions around a mean velocity and granulites). The discrepancy for garnet-bearing sam- density for each lithology and are not dispersed along differenca o t averag e e th du n i ee eb modapley sma l linear array samrockr e sth fo f eso mean atomic weight garnet content of the small population in Table 4 (N = anticipates a Birchy db [1961]. Ther soms ei e overlap 12) compared with Christensen and Mooney's (N = between the groupings, but the concentrations at par- 90). The reason for the discrepancy in garnet-free ticular mean values are strong. The calculated values samples is more difficult to determine. However, be- intermediate foth r et show rockno e thin ni ar s s dia- cause the main phases of mafic granulites have high P gra t woul mp bu betwee ga d e e filfelsith d nth l an c wave velocities (e.g., 6.6-7. s~plagioclasem n i 0k , mafic rocks. The various fields for metapelitic rocks 7.7-8.0 km s"1 in pyroxenes)l , we believe the higher overla felsice pth , intermediate mafid an , c rocks. averag waveP e value correcte sar . Our analysis (Figure 5) lends support to the idea 33/ REVIEW 3 , GEOPHYSICF SO S Rudnick and Fountain: LOWER CONTINENTAL CRUST • 287 8.5 \ Ultramafic rocks 8.0 t Figure 5. Compressional wave velocities (de- termined at 600 MPa and room temperature) 6.0 versus density for (a) felsic and mafic amphibo- lit granulito et e facies rocks, ultramafic rocks, 6 3. 4 3. 2 3. 0 3. 8 2. 2.6 and eclogites and (b) metapelitic rocks. Fields Density (g/cm3) are for each category in Table 4 and are shown contourea s a d distributio s 2.5na % (lightest), 5%, 10% (darkest% totae 15 , th f l o )populatio n of the lithological category for Figure 5a and 5%, 8.5 totaf o 10%l% populatio15 , Figurr nfo . Alse5b o shown are curves of constant acoustic imped- ance (x 106 kg m~2 s"1) and lines of constant 8.0 mean atomic weight (at 1.0 GPa) from Birch [1961]. 7.0 6.5 6.0 2.6 2.8 3.0 3.2 3.4 3.6 Density (g/cm3) that there is a bimodal acoustic impedance distribution Compressional wave velocity generally decreases (the product of velocity and density) in the deep crust with increasing SiO r high-gradfo 2 e metamorphic [Goff t al.,e 1994; Levander t al.,e 1994]. Acoustic rock e relationshisth (Figurt t bu simpl no ) 6 es i pe impedance typicar sfo l felsi mafid can c rock aboue sar t [Fountain et al., 1990]. For laboratory data (Figure 6 2 1 6 2 1 1 x 810m~g k d 22.s"an 5 x 10m~g k s" , 6a), Vp is nearly constant for granulite facies rocks respectively differenca , e that correspond refleca o st - SiO% 75 2 . betweed Mafian c% rockn65 s sho wwida e tion coefficient greater tha nfirst-ordea 0.r 1fo r discon- range of velocities, a range that would be larger yet if tinuity s illustrate. Thuwa s sa Goffy t al.db e [1994], mafic eclogite facies rocks were included in this dia- strong reflection e expecteb y ma sd from deep crust gram [see Fountain et al., 1994a, Figure 4]. Calculated characterize layerey db d sequence mafif so felsi d can c velocitie r granulitfo s e facies rocks show the same rocks. The situation is not much different if we postu- general tren t exhibidbu t lower valuee felsith cn i s late that pelitic rock abundane sar lowee th n ri t crust range (Figure 6b). The linear trend shown for the becaus strone th f eo g overla thesf po e fields with those calculated velocities is not evident for the measured of felsic, intermediate mafid an , c rocks. Modele th f so values generaln I . , ther onle ear y small differencen i s reflectivit e Ivreth f ao y zone [Burke Fountain,d an Vp between amphibolit d granulitan e e facies rocks 1990; Holliger et al., 1993] show that lower crust (Figure 7). reflectivit expectes yi d where mafic rock interlaye sar - ered with upper amphibolite-lower granulite facies 6.2. Comparison With Refraction Data metapelites (metamorphosed shales) but diminishes The velocities given in Table 4 are compared with when mafic rocks are interlayered with higher-grade the mea d rang an nf pressure o e d temperaturean - - metapelites. corrected lower crustal velocities observe eacr dfo f ho 28 8• Rudnic Fountaind kan : LOWER CONTINENTAL CRUST 33, 3 / REVIEWS OF GEOPHYSICS 8.0 cies rocks overlap with amphibolite facies mafic a MP Measure0 60 t a p dV gneisse anorthositesd san lattee th d r overlaan , p mafic 7.5 granulite facies rocks. Perhap e mosth s t important example of this is the overlap between mafic rocks and line from calculated data granulite facies pelites. These metapelites contain rel- 7.0 atively low mica and quartz contents, large amounts of high-density phases such as garnet, and Al2SiO5 poly- 6.5 morph havd an s e probably los mela t t fraction [e.g., Mehnert, 1975] (see below). Inclusio lowef no r crusta wavlS e data does littlo et 8.0 help resolv e aforementioneeth d overlaps (Figur, 8 e B a MP Calculate 0 60 t a p dV Tabl . Ther e5) cleaa s ei r distinction between mafic • Tanzanian granulites felsid an c rocks; intermediate rocks generally fill this 7.5 Australiao n granulites gap (see Figur . However6) e , metapelites exhibia t rang velocitief eo s that overlap felsic granulitee th t sa I 7'° lower end and mafic granulites at the higher end. There itendenca s e high-Vth r yfo ^ metapelite exhibio t s t 6.5 slightly higher Vs than mafic granulite facies rocks, but the difference is subtle, and a number are indistin- S wav d guishablean velocitiee basiP th f o sn o e s 6.0 alone. Metapelites with hig wavhP e velocities com- 40 50 60 70 80 SiO (wt. monly exhibit strong preferred orientation of anisotro- 2 pic phases resulting in high seismic anisotropy [e.g., Figure 6. Compressional wave velocities at 600 MPa ver- Burlini and Fountain, 1993]. This may ultimately pro- sus silica for (a) granulite facies rocks measured in the vid meanea f distinguishinso g them from mafic gran- laborator ) granulit(b d an ye facies rocks calculated from ulite facies rocks, which generally have lower anisot- modal analyses given by Lambert [1967] and Coolen [1980]. ropy. At present, however, there are no measurements lease Th t square sslopa lin s f —0.03eo eha s"m 81k (wt. of lower crustal anisotropy. 1 %)~ 9 ^-intercept of 8.91 km s, and correlation coefficient of The limited number of S wave data for the lower 0.89. crust are plotted in Figure 8. Average 5 wave veloci- tie lowef so r crus shieldn i t s generall yfiele ploth dn i t of mafic rocks, whereas rift intermediate li s e between the type sections in Figure 7. The diversity of the mafic and felsic rocks. All other crustal types show a lower crus s immediatelyi t evident from these dia- largwavS d ee an rang valuesP n ei , highlightine gth grams meae Th . n valuel typal er ssectionfo s teno dt heterogeneity of the lower crust from place to place. cluster between 6.9 and 7.2 km s"1, but the ranges can summaryn I , distinguishing between meta-igneous be large, overlapping many rock types. In general, the rocks of variable compositions is possible on the basis lower crust of most type sections overlaps mafic S wav od f ean thei velocitiesP r . Distinguishing rocks, anorthosites high-gradd an , e metapelitese W . metapelitic rocks from meta-igneous t rockposno -s si reemphasize, however, that near-vertical reflection sible on the basis of P and S wave velocities alone. data and geological observations indicate that the Such a distinction is crucial, however, for determining lower crust mus compositionalle b t y heterogeneous. bulk lower crust composition because of the large Moreover, mean lower crustal refraction values are differences in major and trace element compositions of usually slightly lower than average velocities of mafic these rock types (see below). Evaluation of the volume rocks (Table 4), suggesting the presence of more of high-grade metapelites and mafic granulites in the evolved granulite facies rocks smalA . l percentagf eo deep crust must therefore rely upon integration of these rocks in the deep crust can explain the reflectiv- additional data such as seismic anisotropy and heat ity observe near-vertican di l incidence profiles. Mean flow. value r severafo s l type sections (e.g., contractional orogenic belts, extended terrains) correspone th o dt 6.3. Assigning Lithologies to Type Sections lower range of mafic rocks and the upper range of The first step in our calculation is to assign rock intermediate rocks. The bulk composition of these types to the different layers in our type sections. This lower crustal sectionmore b y e intermediatsma e than is perhaps the most difficult aspect of the exercise. those with higher mean velocities. 6.3.1. High V layers (>6. e typsm Th 9k )e . p -1 One problem in interpreting P wave velocities in sections show Figurn ni contaie2 nlowermosa t layer 1 terms of lithology and chemical composition, apparent characterized by high Vp (>6.9 km s" ). Only four of from these histograms, is that different rock types do the lower crustal rock types shown in Figure 7 exhibit t havno e unique velocities. Intermediate granulit- efa P wave velocities greater than 6.9 km s" ; these are 1 33/ REVIEW 3 , GEOPHYSICF SO S Rudnic Fountaind kan : LOWER CONTINENTAL CRUS9 28 T• 25 Felsic amphibolite fades Felsic granulite facies 20 — z crogeCz/M n shield • Cz/Mzext IS -rifted margins 1- i • • • • i 0 8. 5 7. 0 7. 6.0 5 6. 6.5 7.0 7.5 8.0 P wave velocity (km/s) P wave velocity (km/ s) 25 25 Intermediate granulite facies Anorthositic granulite facies Pz — Pz — 20 .Cz/Mz orogen -Cz/Mz orogen 20 shield . shield . Cz/Mzext _ Cz/Mz ext- -rifts .rifts 15 ——rifted margins . rifted margins 15 cZ CQ garnet-free w 10 calculated 10 garnet-bearing measured 6.0 6.5 7.0 7.5 0 8.8. 0 5 7. 0 7. 5 6. 8.5 P wave velocity (km/s) P wave velocity (km/s) 25 25 Pelitic amphibolite facies Pelitic granulite facies Pz—- 20 —— Cz/Mz orogen _ Cz/Mz orogen 20 shield • shield • ^ Cz/Mz ext. - Cz/Mz ext. rifts rifts ——o——— rifted margins .rifted margins z 15 15 c CO w 1. 10 0 8. 5 7. 0 7. 5 6. 5 8. 0 8. 8.5 5 7. 6.0 0 7. 6.5 P wave velocity (km/s) P wave velocity (km/s) Figure 7. V histograms for lower crustal rock types at room temperature and 600 MPa for each lithologic category (Table 4) compared witp meae hth n velocity (circlestandars it d )an d deviation eacr (barfo h) crusta rood an m a l typMP e0 als60 t oa temperature (Table. 2) sd 1an mafic granulites, mafic amphibolite facies rocks- an , felsic granulite xenolitn si h suites argue against signif- orthosites high-gradd an , e metapelites. Another possi- icant amount f felsio s c granulite e lowermosth n i s t bility is that these layers are composed of a mixture of crust, (2) where inferred crust-mantle boundaries are peridotite (or eclogite) and felsic granulite. However, exposed (e.g., Ivrea zone), the lowermost crust is e considew r this unlikelo reasonse tw th r ) fo y(1 : composed of mafic granulite and high-grade predominanc f mafieo c granulite near-absencd san f eo metapelite ,interlayeree whicb y hma d with peridotite. 29 0• Rudnlc Fountaind kan : LOWER CONTINENTAL CRUST 33/ REVIEW 3 , GEOPHYSICF SO S 25 —i 25 Mafic amphibolite facies Mafic granulite facies 20 —Cz/Mzorogen -Cz/Mzorogen 20 shield shield . . Cz/Mz ext. -Cz/Mz ext. rifts . rifts w 15 -rifted margins - rifted margins - 15 Z to I garnet-free D | garnet-bearing 10 1iu0 6.5 5 8. 7.0 0 8. 7.5 5 7. 8.0 0 7. 5 6. 6.0 P wave velocity (km/s) P wave velocity (km/s) 25 25 Mafic eclogite facies Ultramafic rocks Pz _ Pz 20 . Cz/Mzorogen _ Cz/Mzorogen 20 shield . shield .arc Cz/Mz ext. . Cz/Mz ext. —rifts rifts 15 -rifted margin- s 15 Z w -rifted margins — c CO S 10 10 6.0 6.5 7.0 P wave velocity (km/s) P wave velocity (km/s) Figure 7. (continued) We therefore r turattentioou n o discriminatint n g (and high Vp) phases suc garnes ha Ald 2an tSiO 5 poly- which of the rock types listed above are most likely to morphs (kyanite or sillimanite) [Vielzeuf and Hollo- presene b thesn i t e layers. way, 1988]. However, removal of partial melt in a Although anorthosites hav requisite eth e velocities metapelite does not cause uniform depletion of heat- for high-Vp crustal layers, we take the rarity of an- producing elements, as might be expected. This is due orthosites in the geologic record, coupled with their to the retention of Th and U in accessory phases, shallow emplacement depths [Ashwal, 1993], to mean which may remain stable in the residue during partial that they form only minor constituent e loweth f ro s melting [e.g., Sawka Chappell,d an explain1986d an ] s crust. We therefore did not incorporate anorthosites the lack of correlation between Al2O3/SiO2 and heat intcrustar oou l estimate uniqua s sa e entity. However, production in Figure 9. numbea lowef ro r crustal xenolith geochemicae th n si l Heat productio n metapelitei n s aboui s time9 t s compilation approach anorthosite compositions (e.g., higher than heat production in mafic granulites (Table Chudleigh province, Queensland, Australia [Rudnick 6; Figur . Thise9) conjunction i , n wit relativele hth y et al., 1986]) anorthositio s , c rocks have been aver- low heat flow in Archean shields, allows us to place aged with mafic granulite database th n si e (see below). limits on the amount of metapelite in the high-velocity hige Th h Vsomf o p e metapelites suggests that they lower crust of these regions. The Kapuskasing struc- may be important constituents of the lower crust. The tural zone is the best place to perform this calculation wide range of P wave velocities exhibited by granulite because regional heat flow, heat production to 25 km facies metapelites generally correlates with Al2O3/SiO2 depth seismid an , c structur cruse knownth e f tar e o A . (Figure 9), whic functioa amouns e hi th f no f mel o t t limiting case is made if we assume that the 23 km of depletion they have experienced. Partial meltin- gre unexposed, high-V^ (7.0-7. s"m 61k ) crust beneate hth moves micas, some quartz, and lessea o ,t r extent, KSZ is composed entirely of high-grade metapelite (an feldspar and leaves the rocks enriched in high-density assumption that coul consistene db t wit measuree hth d 33, 3 / REVIEWS OF GEOPHYSICS Rudnic Fountaind kan : LOWER CONTINENTAL CRUS1 29 T• Ultramafi Eclogitc& e 45 Mafic rocks A Rifted Margins • Extended Regions 35 e Rift • Shield ° Paleozoic Orogens • Arc I I 65 7 75 85 wavP e velocity (km/s) B Ultramafi Eclogitc& e •3 4 01 £ Rifted Margins £ s Extended Regions cn 35 e Rift • Shield ° Paleozoic Orogens MetapeHtes A ^ Arc I I I I 65 7 75 8 85 P wave velocity (km/s) Figure 8. Compressional wave velocity versus shear wave velocity (at 600 MPa and room temperature) for (a) felsic, mafic, ultramafic, and mafic eclogite facies rocks and (b) metapelitic rocks compared with data from refraction experiments (also rood an m a temperaturcorrecteMP 0 60 o dt e (Table 5)). Fields e (seth f eo Figur% explanationr 15 fo e5 d , 10%an 5% , e ar ) 2 total population. Lines of constant Poisson's ratio (-0.5(1 - l/[(Vp/Vs) - 1]}) are superimposed. Also shown are results for individual refraction profiles listed in Table 5. Vp and Vs of these layers [Boland and Ellis, 1989]). Because average seismic velocities of the lower Using the median heat production of metapelites (0.57 crus Archeaf o t n shield lowee sar r than both average jjiW m~3), we calculate a heat flow of 35 mW m~2 for mafic granulite and granulitic metapelite (7.0 km s"1 presene th t crustal thicknes surface km)8 Th s(4 . e heat versus 7.2 or 7.1 km s"1, respectively), it is likely that W Abitibe flom th 8 n wi 3 is i bel Z t adjacenKS e th o t t felsi d intermediatan c e granulites alsoe th exis n i t m~2 [Pinet t al,e 1991]deee th pf .o crus Thul al f tsi lower crust of Archean cratons (—30% based on aver- were metapelite, heat productio e crusth n tni would velocities)e ag therefore W . e conclude thalowee th t r accoun r nearlfo t e entiryth e surface heat flow. This crustal layer n Archeai s n shield s predominantli s y obviously cannocasee th e . b tPinet t al.e [1991] esti- compose f mafido c granulite r amphibolites(o s ) with mate e mantldth e contributio heao 2 nt 1 te b flo o wt only 5% metapelite and 30% intermediate or felsic mW m~ i2 n the Superior province. Using this value, a granulites. Post-Archean lower crus contain ca to t p nu maximu metapelit% e presen20 b e f mth y o n i ma et 10% metapelit high-velocite th n ei y layers. This pro- lower crust underlyin presentle gth y exposed KSZ. portion is consistent with the generally small, but 292 • Rudnick and Fountain: LOWER CONTINENTAL CRUST 33, 3 / REVIEWS OF GEOPHYSICS TABLE 5. Compressional and Shear Wave Velocities for Middle and Lower Continental Crust Middle Crust Lower Crust Reference Location Paleozoic Orogens Banda et al. [1981] Iberian peninsula 6.56 3.57 7.04 4.01 Holbrook . [1988al t e ] SW Germany 6.50 3.79 Luetgert et al. [1987] N. Appalachians 6.41 3.70 6.79 3.93 Luetgert et al. [1987] . AppalachianN s 6.41 3.70 7.09 4.10 Shalev . [1991al t e ] Appalachians, N. H. 6.60 3.86 Shield!Platforms Boland Ellisd an [1990] Superior province 6.80 3.90 7.24 4.13 Grad and Luosto [1987] Baltic shield 7.07 4.02 7.32 4.15 Grad and Luosto [1987] Baltic shield 6.79 4.03 7.15 4.15 Grad Luostod an [1987] Baltic shield 7.07 4.02 7.32 4.15 Luosto et al. [1990] Baltic shield 6.72 3.78 7.14 4.07 Epili and Mereu [1991] Grenville front 6.82 4.02 Rifts El-Isa et al. [1987a, b] Jordan 6.89 3.71 El-Isa et al. [1987a, b] Jordan 6.78 3.51 Holbrook et al. [1988] Rheingraben 6.80 3.79 Rifted Margins Chian and Louden [1992] SW Greenland 6.86 3.86 Extended Regions Goodwin and McCarthy [1991] W.Arizona 6.18 3.62 6.82 3.83 Keller . [1975al t e ] Great basin 6.71 3.62 Braile . [1982al t e ] Snake River plain 6.60 3.61 7.10 3.93 All velocities are adjusted to room temperature and 600 MPa. significant, numbe f metapeliteo r s observe postn i d - heat flow data described t supporabovno o d ea t Archean lower crustal xenolith suites [Rudnick, 1992]. metapelite-dominated layer in the deep crust (e.g., The lower crust of each type section was modeled KSZ, Ivrea). Alternatively, these layers coul come db - accordin s averagit o gt waveP e velocity. Although pose f mixedo d mafi felsid an c c amphibrocke th n si - garnet-bearing mafic granulites have higher average olite or granulite facies. In terms of chemical compo- velocities than garnet-free mafic granulites, it is not sition t makei , s little difference whether these layers possible to determine independently their relative pro- modelee ar intermediats da equar eo l mixture mafif so c portion e loweth n ri s crust therefore W . e usee dth and felsic rocks (Figure 10). We therefore modeled average mafic granulite velocity (7.2 s"m 3rep1k o )t - these layers as 45% intermediate amphibolite facies resent mafic lower crustal rocks thee W .n assumea gneisses mixe% 45 , d amphibolite felsid an c amphibo- constant proportion of metapelite (as described in the lite facies metapelitegneisses% 10 d an , . 1 preceding paragraph calculatd an ) amoune - eth in f o t V6.3.3w p layerLo . s (6.2-6. s~m 5k ) . Low-ve- termediate and felsic granulites present in the lower locity layers typically occur at mid-crustal levels in crust of each type section (assuming equal proportions Paleozoi mord can e recent fold belt rifted an s d mar- of both). Thus a lower crust having an average velocity gins. They also constitut e uppermosth e t crusn i t of 6.9 km s"1 is composed of 40% mafic granulite, 10% shields and platforms (Fig. 2). Such layers are likely to metapelite, and 25% each of intermediate and felsic be composed of felsic rocks in the granulite and am- granulite. At the higher end of the velocity scale, a phibolite facies (Fig. 7), and we have modeled them as lower crust wit wavP ha es" m velocit1k woul2 7. f ydo such. Several of these low velocity layers have aver- be composed of 90% mafic granulite and 10km/sec5 %6. Vf e o p ag , nea boundare rth y between felsic metapelite wit intermediato hn felsir eo c granulites. and intermediate granulites. These layers are inter- 1 6.3.2. Intermediate Vp layers (6.5-6. s"km 9) . prete s equaa d l mixture f felsi o sd intermediat an c e Layer intermediatf so e velocitie importane sar sevn ti - granulites. eral of the crustal sections shown in Figure 2. These lie directly abov lowermose eth t crus shieldn i t platd san - 6.4. Chemical Composition f Granuliteso s forms, arcs riftsd an , . Rock types wit wavhP e veloc- nexe Th calculatior t steou n pi assigo t s ni nchema - itie n thii s s rang e intermediatar e e granulited an s ical composition to the mafic, intermediate, felsic, and metapelite s. Crusta (Figure7) d an l cros6 s s sections pelitic granulites and amphibolite facies rocks. To do [e.g., Fountain and Salisbury, 1981], as well as the this, we used the data base of Rudnick and Presper 33, 3 / REVIEWS OF GEOPHYSICS Rudnic Fountaind kan : LOWER CONTINENTAL CRUS3 29 T• portion of the middle crust [e.g., Stosch et al., 1992]. 8.0 _ Metapelites f granuliteo e us e modeo t s Th e middlth l e crust will have little effect on the major element composition • Granulite Facies because amphibolit granulitd ean e facies rock simf so - 7.5 D Amphibolite Facies ilar bulk compositions generally have similar P wave velocities (Figure 7). It will have an effect, however, on elements that are depleted by granulite facies meta- n o morphism. Most specifically, we expect U, Cs, and He a Granulite fades metapelites with a time-integrated JJL ( U/ Pb) of ~8, which correspond U/Pa o st b rati f 0.13oo . Using this ratio 4 0. 3 0. 2 0. 1 0. 0.5 abundanceb P e 0.differenth e 6 d th r an sfo t type felsif so c and intermediate granulites, we calculated the U con- Al2O3/SiO2 ten f theio t r amphibolite facies counterparts. OnceU ) FigurAl(a O/SiO . 9 e versu P wavs e velocitr fo y 3 2 2 abundance is established, the Th abundance can be metapelites Tabl e datr Se .fo ae4 sources Hea) (b , t produc- derived from Th/ Uabundanc= 3.8K e ,th e from tion versus Al2O3/SiO r metapeliti2fo c xenoliths (data from K/U = 10,000, the Rb abundance from K/Rb = 260 Rudnick Presperd an [1990]). Solid symbols represent aver- age post-Archean shale compositions [Krauskopf, 1967; Tay- and the Cs abundance from Rb/Cs = 25 [McDonough lor and McLennan, 1985]. Arrow marks the average heat et al., 1992; Taylor McLennan,d an 1985] r felsiFo . c productio r mafinfo c granulite xenoliths. granulites (both Archean and post-Archean), the cal- culations given above result in values for some mobile elements being higher in the granulites than in the [1990], supplementing the granulite xenolith data with amphibolite facies gneisses. This suggests that Pb may more recent data from the literature [Halliday et al., have been deplete granulity db e facies metamorphism 1993; Hanchar et al., 1994; McGuire and Stern, 1993; in these rocks thesn I . e case increasee sw contenb dP t Mengel, 1990; Wendlandt t al.,e 1993] subdividee W . d amphibolite inth e facies rock mobile th s f untio e l al l the database into averages of mafic granulites, inter- element abundance e amphiboliteth n i s e consisar s - mediate granulites, felsic granulites, and metapelites, tently higher than granulitesthose th f eo resultine .Th g eac f whic ho turn i ns hi subdivided into those occur- compositions of amphibolite facies gneisses (Table 6) rin xenolithss ga , post-Archean terrains Archead an , n were used to model midcrustal layers. terrains (Tabl. e6) generaln I averagee th , major sfo r elemente th d san 6.5. Areal Exten f Crustao t l Type Sections trace elements thacompatible ar t r moderateleo - yin There are two schools of thought regarding the areal compatible agree well with the medians, indicating extent of different crustal provinces (i.e., Precambrian thapopulatione th t s follo wnormaa l distributionr Fo . shields, Paleozoic orogenic belts Cenozoic-Med an , - highle th y incompatible trace elements, howevere th , sozoic regions (Table 7)). One school [Condie, 1989; medians can be significantly lower than the average Hurley and Rand, 1969; Sclater et al, 1980] argues e hav(TablW e. chose6) e median r us ou o nt n i s that 60-70% of the crust is Precambrian in age. This calculations rather than averages in order to minimize estimat areae bases ei th ln do exten f inferreo t d Pre- e effectth f outliero s r smalfo s l sample populations cambrian crust (i.e., all crust of inferred Precambrian [e.g., Rock, 1988]. thae exposes ag ti coverer do shalloy db w sedimentary Althoug e middlth h e composeeb crusy ma tf o d deposit continents)e th n so secone Th . d school [Good- rockgranulite th n si e facie somn si e regions likels i t i , y win, 1991; Sprague and Pollack, 1980] contends that that amphibolite facies rocks maka significan p u e t e continentath f o onl% l50 y crus s Precambriani t . 294 • Rudnick and Fountain: LOWER CONTINENTAL CRUST 33, 3 / REVIEWS OF GEOPHYSICS TABLE 6. Average and Median Values for Different Types of Granulite (and Amphibolite) Facies Rocks Mafic Archean Terrains Post-Archean Terrains Xenoliths* Archean Terrains Ave Med N Ave Med N Ave Med N Ave Med N SiO2, wt. % 49.7 49.8 101 49.2 49.3 96 49.1 49.5 270 58.3 58.4 106 TiO2, wt. % 0.95 0.96 101 1.32 1.25 95 1.00 0.84 266 0.83 0.78 106 A12O3, wt. % 14.5 14.8 100 15.8 15.9 95 16.5 16.7 269 16.3 16.3 106 FeOT, wt.% 11.3 11.1 101 9.2 9.4 65 9.02 8.91 269 7.3 7.2 106 MnO, wt.% 0.19 0.20 96 0.19 0.18 95 0.15 0.15 255 0.11 0.10 102 MgO, wt. % 8.53 7.25 100 8.14 7.3 95 8.56 8.24 269 3.6 3.3 106 CaO, wt. % 9.72 9.99 101 9.31 9.7 95 10.89 10.9 269 6.2 6.1 106 Na2O, wt. % 2.56 2.59 100 2.39 2.51 94 2.45 2.51 269 4 4.1 106 K2O, wt.% 0.70 0.58 100 0.85 0.62 94 0.38 0.28 236 1.55 1.1 (1.3) 106 P2O5, wt. % 0.19 0.12 98 0.3 0.22 87 0.20 0.12 258 0.27 0.23 105 Mg #, mol 57.4 53.8 56.2 53.6 62.8 62.2 46.8 45 Li, ppm 15 13 7 6 6 14 11 9 9 Sc, ppm 39 41 23 45 43 33 35 34 92 17 17 15 m pp , V 216 195 67 261 257 55 216 219 173 128 121 81 Cr, ppm 464 229 92 345 153 74 353 252 192 155 103 90 Com ,pp 48 47 34 52 50 39 53 42 105 26 22 45 Nim ,pp 227 112 84 126 80 61 150 105 175 71 46 88 Cu, ppm 44 38 49 56 48 21 49 28 152 27 22 43 Zn, ppm 99 93 64 152 110 42 90 78 132 73 76 49 Gam ,pp 18 17 53 21 19 23 15 14 21 19 18 38 Rb, ppm 12 5 84 22 14 68 7 3 197 30 19 (41) 96 Sr, ppm 227 156 89 378 232 80 421 365 207 519 454 100 m pp , Y 25 22 73 39 31 47 16 13 151 22 20 85 Zr, ppm 94 64 86 196 106 78 76 40 181 163 130 96 Nb, ppm 6 5 71 9 6 33 9 4 102 7.5 7 76 Csm ,pp 1 0.11 32 0.9 0.2 (0.9) 5 Ba, ppm 294 137 75 332 184 66 275 175 179 691 512 97 Lam ,pp 31 8.5 70 25 14 43 8 5 121 38 30 68 Cem ,pp 37 20 72 48 24 41 19 12 122 74 58 67 Pr, ppm 5.5b 2.9b 2 6.7b 3.2b 2.5b 1.6b 37 8.7b 6.4b 4 Nd, ppm 27 14 25 31 14 26 11 8 15 31 20 32 Sm, ppm 5.4 3.1 37 9.0 5.8 28 2.81 2.3 135 5.7 4.1 36 Eum ,pp 1.0 0.9 37 2.3 1.5 35 1.07 0.96 123 1.5 1.2 37 Gd, ppm 5.5 3.9 16 6.9 4.7 21 3.06 2.71 63 4.4 3.5 24 Tb, ppm 1.0 0.6 27 1 0.8 27 0.47 0.41 87 0.66 0.5 19 m pp , Dy 7.8 4 13 4.2 3.6 7 2.99 2.67 40 4.3 2.9b 23 m pp , Ho 1.6b 0.8b 4 0.9b 0.8b 0.63 0.59b 54 0.81 0.60b 6 Erm ,pp 4.3 2.2 13 2.5 2.5 6 1.84 1.68 41 2.1 1.6b 24 Ybm ,pp 3.1 2.2 37 3.4 2.7 34 1.53 1.17 120 1.7 1.4 36 Lu, ppm 0.5 0.2 12 0.6 0.5 26 0.24 0.19 71 0.29 0.25 23 Hf, ppm 1.8 2 21 5.2 2.5 26 1.87 1.01 75 3.8 3.4 10 Ta, ppm 0.3 0.2 8 0.6 0.2 12 0.55 0.5 18 0.62 0.63 5 Pb, ppm 6 5 59 8.5 6 27 2.58 1.8 47 14 10 51 Th, ppm 1.8 1 39 6.4 1 23 0.6 0.3 55 2.3 1.0 (4.0) 37 U, ppm 0.7 0.5 24 2 0.55 13 0.21 0.08 36 0.54 0.4(1.6) 24 #,d jjiW rrT3 0.37 0.25 1.06 0.27 0.13 0.06 0.43 0.27 (0.80) Numbers in parentheses represent estimated values for amphibolite facies rocks (see text for method of calculation). aK, Sr and Ba values for kimberlite-hosted xenoliths were omitted owing to alteration effects. These estimates assume that the 30% of submerged The areal extent of shields and platforms is taken as continental crust [Cogley, 1984 youngers ]i theree W . - the amount of Precambrian crust. In both models this fore constructe modelo dtw s using different crustae lag constitutes the largest single portion of crust. The distributions (Table 7). It is important to stress that the proportions of Archean to Proterozoic crust (14:86) ages we are discussing are the observed crystallization are taken from Goodwin. The Phanerozoic crust in cruste age th t growt f sno o , h ages (argument conf so - both model divides si followss da Paleozoi% 46 : c crust tinental freeboard require tha larga t e proportion(at and 54% Cenozoic-Mesozoic crust (following subdivi- least -70%) of the crust was in place by the end of the sions of Condie and Sprague and Pollack). In model 1, Archean [Armstrong, 1991; Taylor and McLennan, cruse 30th assigne%s f i to rifteo dt d margins, whereas amounte 1995]th t extanf ;ye o t Archean crus mucs i t h in model 2, rifted margins constitute only 10% of the lower than this). crust. In both models rifted margins are assumed to be / REVIEW 3 , 33 GEOPHYSICF SO S Rudnic Fountaind kan : LOWER CONTINENTAL CRUS5 29 T• Intermediate Felsic Metapelite Post-Archean Terrains Xenoliths Archean Terrains Post-Archean Terrains Xenoliths Ave Med N Ave Med N Ave Med N Ave Med N Ave Med N 58.3 58.3 138 54.1 53.2 48 70.5 70.7 379 70.2 70.2 246 60.5 60.3 78 0.97 0.95 132 0.81 0.79 48 0.40 0.36 379 0.48 0.43 222 1.00 8 1.07 2 16.8 16.6 132 17.5 17.8 48 14.4 14.4 379 14.0 14.2 224 17.6 17.6 78 8.1 8 132 7.2 7.1 48 3.1 2.9 379 4 3.9 224 7.84 8 7.57 1 0.14 0.13 132 0.13 0.13 47 0.05 0.04 348 0.07 0.05 220 0.13 0.11 77 3.8 3.6 132 5.5 8 5.4 3 1.1 0.88 378 1.3 1.2 223 3.12 3.03 78 5.7 5.9 132 9.2 8.8 48 2.8 2.7 379 2.8 2.5 224 2.97 2.4 78 3.4 3.4 132 3.6 3.8 48 3.8 3.8 379 3.4 3.5 224 2.4 2.48 7 9 1.55 0.96(1.7) 138 0.78 0.65 48 2.8 2.6 (3.2) 379 2.52 1.9(2.5) 246 1.99 1.90 78 0.26 0.24 120 0.19 6 0.14 6 0.11 0.09 358 0.11 0.09 186 0.12 0.08 7 9 45.4 44.5 57.7 57.1 38.7 35.1 36.7 35.4 41.5 41.8 9 10 5 10 10 2 15 15 10 7.4 7 21 27 27 41 22 19 19 6 5 91 13 13 59 23 19 32 157 160 77 155 154 29 38 29 244 50 40 138 129 129 50 80 56 79 122 95 30 47 16 249 41 22 134 135 6 5 117 35 30 65 33 29 17 12 10 76 22 15 52 26 21 43 35 24 69 76 52 30 20 15 263 16 8 128 52 33 52 50 25 43 42 22 24 15 8 196 22 13 116 26 16 48 110 90 64 72 72 18 38 34 243 54 48 130 96 90 47 22 22 44 17 17 2 17 16 183 19 18 133 42 22 (54) 111 12 5 33 71 57 (103) 366 78 58 (80) 204 51 42 72 408 346 126 549 435 41 285 257 353 222 185 215 331 284 69 30 24 86 23 17 25 24 11 286 35 22 151 44 33 35 188 120 112 127 54 38 229 184 332 254 156 163 248 210 48 11 8 37 10 7 15 8 5 232 13 8 78 21 14 29 3.2 2(1.1) 5 0.07 0.04 3 0.62 0.5(5.1) 42 1.7 1 (4.0) 18 0.73 0.9 23 470 400 105 498 380 33 823 693 361 568 453 170 730 700 53 23 18 71 14 10 29 44 34 276 31 19 127 36 24 43 56 50 65 32 22 29 71 54 274 82 56 143 71 50 44 7.4b 6.4b 4 3.7b 2.9b 8 9.7b 5.1b 15 10b 7.4b 8.4b 6.1b 3 31 26 31 13 12 23 43 18 74 39 32 45 32 23 47 6.4 5.5 35 3.7 2.8 30 5.7 2.9 110 4.2 4.6 13 5.9 5 50 2.2 1.6 33 1.4 1.2 29 1.4 1.3 110 2.3 1.9 13 1.48 1.4 44 4.9 4.1 20 3.1 2.8 18 4.0b 2.7b 32 7.7 4.4b 7 6.85 81 2 7. 0.8 0.60b 22 0.58 0.48 17 0.56 0.32 91 0.84 0.6 11 1.05 0.90 40 4.5b 3.8b 13 2.8 2.8 8 3.5b 1.84b 52 5.1b 4.19b 7.75 6.8 22 0.82 0.75b 7 0.64 0.54 13 0.75b 0.36b 32 1.0b 0.93b 4 1.60b 1.08 11 2.2 1.95b 10 1.8 1.5 10 2.0b 0.84b 55 2.9 2.8b 6 4.52 1 b 9 2. 2.3 2.2 32 1.5 1.5 28 1.6 0.6 111 3.1 2.9 12 4.43 7 3.3 6 0.53 0.5 19 0.22 1 0.12 9 0.26 0.13 69 0.47 0.44 12 0.67 0.63 3 0 4.8 2.9 22 3.8 1.6 16 4.5 3.9 61 8.3 6.2 10 6.88 6 42 1.4 1.4 4 0.6C 1 0.58 0.35 34 0.8C 0.5C 1 1.3 1 15 15 13 55 2.9 2.9 11 17 14 (25) 226 22.5 16 (20) 129 12.3 12 28 9.6 2.2 (5.3) 27 1.4 0.25 14 12 5.5 (10) 217 12 7.7 (8.0) 53 7.53 4 41 0.95 0.8(1.4) 21 0.15 0.08 10 1.44 1 (2.7) 121 0.81 0.7(2.1) 48 0.58 0.5 35 1.06 0.45 (0.89) 0.20 0.09 1.50 0.87(1.7) 1.27 0.89(1.33) 0.85 0.57 Interpolated fro rare mth e earth element (REE) pattern wher numbee eth observationf o r relativw lo adjacenn s si thaa o e t f o t t REE. cCalculated assuming Nb/Ta =16. dHeat production. e dividear Phanerozoi d dan equalle ag n ci y between into large errors when considering the whole crust. Paleozoi Cenozoic-Mesozoid can c crust remaine Th . - ing Paleozoic crust is assigned to Paleozoic orogeni Estimatw c Ne 6.6 A f Cruseo . t Composition belts, and the remaining Cenozoic-Mesozoic crust is Using the type sections, areas, lithological assign- further subdivided between extensional areas (3-3.5% ments to P wave velocities and chemical compositions of the total crust), contractional areas (2-2.5%), active given above e estimatew , e compositioth d e th f o n rifts (1%) arcd ,an s (6-7%) lattee .Th r assignmente sar lower, middle, and bulk continental crust. Table 8 somewhat arbitrary but based mainly on Condie's sub- give e lowe th stotad an r l crust compositioe th r nfo divisions. The small areal extent of these regions make various type sections, whereas Tabl givee9 bule sth k them of less significance when the entire crust is con- lower, middle, and total crust compositions. Table 10 sidered erroro s , thein i s r area shoul t propagatdno e presents some relevant trace element ratios. 29 6• Rudnic Fountaind kan : LOWER CONTINENTAL CRUST 33, 3 / REVIEWS OF GEOPHYSICS 1000 performed relatively recently, using analytical tech- niques that have better detection limit d highean s r precision than older techniques, especialle th t a y lower concentrations thatypicae ar t f theso l e rocks. For the upper crust we used Taylor and McLennan's [1985] value for post-Archean regions and their aver- age Archean upper crust for the Archean regions. Both models of areal distribution gave similar results for the lower and whole crusts, so only one value is presented in Table 8. In our model the bulk lower crust is mafic and simila previouo t r s estimate Poldervaarty b s [1955], 1000 Pakiser and Robinson [1966] and Rudnick and Taylor Figure 10. Comparison of major and trace element compo- [1987 t significantl]bu y less evolved tha estimatee nth s sitions between average intermediate (;c axisd mixean ) d of others (see revie Fountainy wb d Christensenan felsic-mafic (y axis) granulites. Each data point represents an individual element; points that deviate significantly froe mth [1989]). (The lower crustal estimate of Taylor and 1:1 linlabelede ear . McLennan includes that portio f crusno t beloe wth upper crust (—upper third of the crust in their model) and abov Mohoe eth . Thus their "lower crust" corre- We combined lower crustal rock types in the pro- sponds to our combined middle and lower crusts, so portions dictated by the crustal type sections in Figure apparentle th y large difference composition i s t no e nar 2 and as discussed above. We used the average com- as great as they seem.) The lower crust approaches the positio f mafino c granulite xenolith o modet s l mafic compositio primitiva f no e basalt, havin # higga g hM granulite reasonso believe tw w r ) sfo (1 :e that xeno- (100Mg/(Mg + SFe)), high Ni and Cr contents, and liths best represent lowermos compare) t (2 crus d an t d low abundance f heat-producino s g elements (HPE) with terrains, the majority of xenolith analyses were lowee (TablTh . r e9) crus lighs i t t rare earth element TABL . E7 Areal Percen f Differento Continentae Ag t l Crust Hurley and Condie Goodwin Sclateret Sprague and Rand [1969] [1989] [1991] al [1980] Pollack [1980]* Model lb Model 2 Archean 5.6 7.0 20.0 5.2 7.0 9.0 Proterozoic 65.0 43.0 48.0 43.7 43.0 56.0 Paleozoic 19.0 8.0 24.0 23.0 16.0 Cenozoic-Mesozoic 23.0 24.0 27.1 27.0 19.0 Total 70.6 42.0 50.0 100.0 100.0 100.0 100.0 Total Precambrian 70.6 57.0 50.0 68.0 48.9 50.0 65.0 Total Phanerozoic 30.0 42.0 32.0 51.1 50.0 35.0 Breakdown of Cenozoic-Mesozoic Crust (27% and 19% of Total Crust) Model lc Model 2 Extensional areas 3.0 3.5 Contractional orogens 2.0 2.5 Active rifts 1.0 1.0 Rifted margins 15.0 5.0 Arcs 6.0 7.0 Total 27.0 19.0 Breakdown of Paleozoic Crust (23% and 16% of Total Crust) Model lc Model 2 Paleozoic orogens 8.0 11.0 Rifted margins 15.0 5.0 Total 23.0 16.0 a Continental shelve includee sar Paleozois da c crust. b Precambrian crustal estimates from Goodwin; proportions of Paleozoic and Cenozoic-Mesozoic crusts from Sprague and Pollack. c Rifted margins assumed to represent 30% of crust by area [Cogley, 1984] and are shared between Paleozoic and Cenozoic-Mesozoic ages. 33, 3 / REVIEWS OF GEOPHYSICS Rudnic Fountaind kan : LOWER CONTINENTAL CRUS7 29 T• TABLE 8. Major and Trace Element Concentrations of Lower and Total Crust for Dififerent Type Sections Mesozoic- Mesozoic- Platform/ Paleozoic Cenozoic Continental Rifted Active Cenozoic (Ar- Extensions Shield chean) Orogen Contractions Arcs Margins Rifts Lower Total Total Lower Total Lower Total Lower Total Lower Total Lower Total Lower Total Major Elements SiO% . 2wt , 52.4 57.8 55.2 55.0 61.0 58.0 62.5 50.57.63 50.6 60.2 55.0 60.5 58.0 64.8 TiO2, wt. % 0.8 0.7 0.7 0.8 0.7 0.8 0.7 0.9 0.8 0.9 0.7 0.8 0.7 0.8 0.6 A12O3, wt. 16.5 15.8 15.4 16.4 15.7 16.1 15.6 16.816.2 16.8 15.8 16.4 15.9 16.1 15.2 FeOT, wt. % 8.2 6.9 7.3 7.9 6.3 7.3 6.0 8.8 7.2 8.8 6.3 7.9 6.3 7.3 5.2 MnO, wt% 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 MgO, wt. % 7.1 4.7 4.9 7 7. 6.0 3 3. 9 8 4. 5.1 7.7 4.3 6.0 4.0 4.8 2.6 % . CaOwt , 9.5 6.7 6.9 8.0 5.8 6.7 5.1 10.1 7.2 10.1 6.3 8.0 6.0 6.7 4.4 Na% . 2Owt , 2.7 3.1 3.1 5 2. 8 4 3. 2 0 3. 3.1 2.5 3.2 2.8 3.3 3.0 3.5 K2O, wt. % 0.6 1.7 1.2 0.8 1.9 1.0 2.3 0.4 1.6 0.4 2.1 0.8 2.1 1.0 2.7 P% 2 . O5wt , 0.1 0.2 0.2 0.1 0.2 0.1 0.2 0.1 0.2 0.1 0.2 0.1 0.3 0.1 0.2 Mg #, mol 61 55 54 57 53 54 49 61 56 61 55 57 53 54 47 Trace Elements Li, ppm 6 11 11 5 6 11 01 7 10 5 11 6 12 7 12 Sc, ppm 30 22 20 3 3 28 9 1 21 6 2 24 33 21 28 20 26 16 V, ppm 194 141 162 174 118 151 105 210 150 210 122 174 120 151 82 m pp , Cr 213 128 162 175 9 23 101 6 7 132 141 239 114 175 100 132 57 m pp , Co 37 26 27 34 0 4 24 1 2 30 28 40 24 34 23 30 18 Ni, ppm 88 55 77 71 42 53 32 98 59 98 49 71 43 53 25 m pp , Cu 26 24 22 24 7 2 22 2 2 22 25 27 23 24 24 22 21 Zn, ppm 77 75 69 77 9 7 70 1 7 75 77 79 71 77 75 75 66 m pp , Ga 13 15 15 14 16 16 17 13 15 13 16 14 16 16 17 Rbm pp , 12 52 29 18 59 26 72 7 49 7 65 18 65 26 85 m pp , Sr 349 331 317 327 304 307 307 357 340 357 320 327 332 307 291 m Ypp , 16 19 16 18 20 20 22 15 19 15 20 18 21 20 22 m pp , Zr 69 119 105 7 5 86 4 14 129 6 10 111 57 127 86 136 106 156 Nb, ppm 5 11 7 6 11 7 13 5 11 5 13 6 14 7 15 Cs, ppm 0.3 2.2 0.7 0.6 3.5 0.9 3.2 0.2 2.1 0.2 3.0 0.6 2.9 0.9 4.2 Ba, ppm 263 379 339 8 22 303 3 4044 3 3 35 364 228 397 303 425 353 467 La, ppm 9 17 18 11 18 14 20 7 16 7 18 11 20 14 22 m pp , Ce 21 40 36 28 6 1 44 0 5 36 37 16 43 28 46 36 54 m pp , Pr 2.6 4.8 4.1 3.6 1 2. 5.4 1 6. 4.7 4.5 2.1 5.1 3.6 5.5 4.7 6.5 Nd, ppm 11.4 19.0 15.4 15.6 22.2 19.9 24.3 9.2 17.9 9.2 20.5 15.6 21.5 19.9 26.2 Sm, ppm 2.8 3.8 3.3 6 2. 3.4 4 4. 1 9 3. 3.7 2.6 3.8 3.4 4.1 3.9 4.4 m pp , Eu 1.1 1.1 1.1 1.2 0 1. 1.3 3 1. 1.4 1.1 1.0 1.2 1.2 1.1 1.4 1.3 Gd, ppm 3.1 3.5 3.1 3.4 0 3. 3.8 9 3. 3.7 3.5 3.0 3.6 3.4 3.7 3.7 4.0 m pp , Tb 0.5 0.6 0.5 0.5 5 0. 0.6 6 0. 0.6 0.5 0.5 0.6 0.5 0.6 0.6 0.6 Dy, ppm 3.1 3.4 2.9 0 3. 4 7 3. 6 6 3. 3.4 3.0 3.5 3.4 3.5 3.6 3.8 m pp , Ho 0.7 0.7 0.6 7 0. 0.7 8 0. 8 8 0. 0.7 0.7 0.8 0.7 0.8 0.8 0.8 Er, ppm 1.9 2.1 1.8 2.1 2.3 2.2 2.3 1.9 2.1 1.9 2.2 2.1 2.2 2.2 2.4 Yb, ppm 1.5 1.9 1.5 1.8 2.2 2.1 2.3 1.4 1.9 1.4 2.0 1.8 2.1 2.1 2.4 m pp , Lu 0.2 0.3 0.2 2 0. 3 4 0. 4 4 0. 0.3 0.2 0.3 0.3 0.3 0.4 0.4 m Hfpp , 1.9 3.5 2.6 2.6 5 1. 4.1 5 4. 3.3 3.3 1.5 4.0 2.6 4.1 3.3 5.1 Ta, ppm 0.6 1.1 0.6 6 0. 0.7 2 1. 0.9 8 0. 1.1 0.6 1.1 0.7 1.3 0.8 1.2 Pb, ppm 4.3 11.4 8.5 6.6 14.2 2 9. 15.8 92. 10.6 2.8 13.7 6.6 13.6 9.2 18.5 Th, ppm 1.2 5.1 3.0 2.1 7 0. 5.8 1 7. 3.0 4.7 0.7 6.3 2.1 6.3 3.0 8.3 U, ppm 0.2 1.3 0.7 0.3 1.4 0.5 1.8 0.1 1.2 0.1 1.6 0.3 1.6 0.5 2.1 Heat Production, 0.18 0.84 0.51 0.30 0.96 0.42 1.17 0.120.79 0.12 1.05 0.30 1.04 0.42 1.38 Flow,a mW 36 22 36 61 32 27 29 43 m-2 Crustaa l contributio heao nt t flow calculated with average crustal thicknesses give Tabln ni . e2 298 • Rudnlc Fountaind kan : LOWER CONTINENTAL CRUST 33, 3 / REVIEWS OF GEOPHYSICS TABLE 9. Major and Trace Element Composition o fconten E th middle e HP th f e o te Th crus higs i t lowe hbu r Continental Crust tha uppene thath f ro t crust. All existing models indicate that average continen- Lower Middle Upper* Total tal crust is intermediate in composition, and our result Major Elements is no exception (Table 11). This result is due more to SiO% . 2wt , 52.3 60.6 66.0 59.1 mixin f mafigo c (lower crust felsid an ) c (upper crust) TiO% . 2wt , 0.8 0.7 0.5 0.7 lithologies rather than a predominance of intermediate A12O3, wt. % 16.6 15.5 15.2 15.8 crustrocke th detailn n sI i . r crustaou , l mode less i l s FeOt, wt. % 8.4 6.4 4.5 6.6 evolved than most other estimate mors i t esbu evolved MnO% . wt , 1 0. 0.10 0.08 0.11 MgO, wt. % 7.1 3.4 2.2 4.4 than Taylor McLennarfsd an [1985] average crustn I . CaO, wt. % 9.4 5.1 4.2 6.4 addition, our estimate is considerably lower in FeO Na% . 2Owt , 2.6 3.2 3.9 3.2 than that of Taylor and McLennan. It is significant that K2O, wt. % 0.6 2.01 3.40 moder highese estimatey ou th f an #s 1.8 g o i lf o 8tM e , th P2O5, wt. % 0.1 0.1 0.4 0.2 reflectin e generallth g y f higmafi#g o M hc lower Mg #, mol 60 48 47 54 crustal xenoliths (Tabl. e6) Trace Elements Trace element compositions of the different crustal Li, ppm 6 7 20 11 models are plotted in Figure 12. In contrast to the m pp , Sc 31 22 11 22 m pp , V 6 19 118 60 131 major elements, our average crustal model bears clos- m pp , Cr 215 83 35 119 t similarites y estimatee toth s of Weaver Tarneyd an Co, ppm 38 25 10 25 [1984] and Wedepohl [1994] and has markedly higher Ni, ppm 88 33 20 51 incompatible trace element contents than the average m pp , Cu 26 20 25 24 crust deduced by Taylor and McLennan. Using our Zn, ppm 78 70 71 73 Ga, ppm 13 17 17 16 continental crust composition and the composition of Rb, ppm 11 62 112 58 the primitive mantle from McDonough and Sun [1995], m pp , Sr 348 281 350 325 we calculat proportioe eth incompatiblf no e trace ele- Y, ppm 16 22 22 20 ments that are contained within the continental crust m pp , Zr 68 125 190 123 c o (Figure 13). It is apparent from this figure that the Nb, ppm J O 25 12 Cs, ppm 0.3 2.4 5.6 2.6 largest differences between alkal e estimateth r i fo e sar Ba, ppm 259 402 550 390 elements, Ba, Pb, Th, and U. Our estimate places m pp , La 8 17 30 18 35-55% of the silicate Earth's budget for these ele- Ce, ppm 20 45 64 42 ments withi continentae nth l crust. Pr, ppm 2.6 5.8 7.1 5.0 returquestioWe w th e no o nt heaf no t production i Nd, ppm 11 24 26 20 Sm, ppm 2.8 4.4 4.5 3.9 m pp , Eu 5 1. 1 1. 0.9 1.2 m pp , Gd 3.1 4.0 3.8 3.6 m pp , Tb 0.48 0.58 0.64 0.56 TABL . ESelecte10 d Trace Element Ratio Head san t Production in Continental Crust m pp , Dy 3.71 3.1 oQ 3.5 3.5 Ho, ppm O .Df-Qo U.o(\ O-"Z) 0.80 0.76 m pp , Er 1.9 2.3 2.3 2.2 Lower Middle Upper* Total Yb, ppm 1.5 2.3 2.2 2.0 m pp , Lu 0.25 0.41 0.32 0.33 Zr/Hf 36 31 33 33 Hf, ppm 9 3. 1.9 8 3. 4.0 9 3. 5.9 85. 3.7 Th/U 3 Ta, ppm 0.6 0.6 2.2 1.1 K/U(10 ) 23.8 10.7 10.0 10.1 Pb, ppm 2 4. 15.3 20 12.6 K/Rb 413 270 250 252 Th, ppm 1.2 6.1 10.7 5.6 Rb/Cs 35 25 20 22 m pp , U 0.2 1.6 2.8 1.42 Rb/Sr 0.033 0.22 0.32 0.18 • Sr/Nd 30 12 13 16 aFrom Taylor McLennanand [1985], phosphorus data estimated, La/Nb 1.6 2.2 1.2 1.5 cesium from McDonough et al. [1992]. Sm/Nd 0.25 0.18 0.17 0.19 Eu/Eu* 1.14 1.09 0.66 0.96 U/Pb 0.047 0.103 0.140 0.113 1 7. 9 4 6. 9 2. ,jib (LREE) enriched wit a hsmal l (~ 14%) positive Eu Heat production, 0.18 1.0 8 1. 2 0.93 5 anomaly (Eu/Eu* = 2EuA1/(SmwGdJ°- ), where the c 2 subscripted n indicates the values are normalized to Heat flow, mW rrT 37 chondritic meteorites containd an ) s higher concentra- aFrom Taylor and McLennan [1985], except for Cs value from McDonough . [1992]al t e . tions of the compatible trace elements than the middle "Calculated assuming bulk crust 207Pb/204Pb - 15.5, 206Pb/ 208 204 upped an r crus ; Figurt11 (Table d e an 11)0 1 s. [Rudnick8 pb/P 3 b18.P= 204 bd = Goldstein,d 2an an 1990] 204 208 204 206 The middle crust is intermediate in composition. It upped an r crusPbP b=t/ 207 P b 19.315./ P. bP= d b7/ an , P b39.= 1 [Zartman Doe,d an 1981]. 204 contains high concentration smala d lan pos-E RE f so Crustac l contributio heao nt t flow, assuming 40-km-thick crust itiv anomalu eE s LREE-enriche i d yan d (Figure 11). with average cm~g densit 8 2. 3 .f yo 33, 3 / REVIEWS OF GEOPHYSICS Rudnic Fountaind kan : LOWER CONTINENTAL CRUS9 29 T• the continental crust. Whereas our estimate of shield- platform crustal composition has a rather high heat productio f 0.9no 2 jxW m~3 (which corresponda o st heat flow of 40 mW m~2 for a 43-km-thick crust (Table 8))Archeae th , n componen f thio t s heaa crus s tha t productio onlf no y 0.66 jjiW m~3. This correspondo st TABLE 11. Compositional Estimates of the Continental Crust Shaw Weaver Taylorand et al. Wedepohlu L Tarneyb Y and m T r E o H y McLennanD b T d G u E Thism S m P d N r P e C U [1986] [1994] [1984] [1985] Study 10 Intracrustal differentiation Sc Q V o Cr SiO% . 2wt , 63.2 61.5 63.2 57.3 59.1 Euji^ 1• TiO% . 2wt , 0.7 0.68 0.6 0.9 0.7 A1% 2. O3wt , 14.8 15.1 16.1 15.9 15.8 FeO% . Twt , 5.60 5.67 4.90 9.10 6.6 MnO% . wt , 0.09 0.10 0.08 0.18 0.11 MgO, wt. % 3.15 3.7 2.8 5.3 4.4 RbTh< o Lower/Upper 0.1 ° Uo CaO, wt. % 4.66 5.5 4.7 7.4 6.4 Cso • Middle/Upper Na2O, wt. % 3.29 3.2 4.2 3.1 3.2 K% . Owt , 2.34 2.4 2.1 1.1 1.9 2 values normalized upperto crust P205, wt. % 0.14 0.18 0.19 0.2 Mg #, mol 50.1 53.4 50.5 50.9 54.4 Li, ppm 17 13 11 Figure Rar) 11(a e. earth element pattern uppef so r [Taylor m pp , Sc 13 16 30 22 and McLennan, 1985], middle lowed an , r continental crust, V, ppm 96 101 230 131 (b) Relative enrichment/depletio middlf no lowed ean r crusts m pp , Cr 90 132 56 185 119 compared with upper crust. m pp , Co 26 26 29 25 m pp , Ni 54 66 35 105 51 Cum pp , 26 26 75 24 Zn, ppm 71 66 80 73 2 Ga, ppm 15 18 16 a crustal heat flow component of 28 mW m~ , a value Rbm pp , 76 76 61 32 58 consistent wit heae hth t flow studie f Archeao s - nre Sr, ppm 317 334 503 260 325 gions reviewed above. This suggestd an w s lo tha e th t Y, ppm 26 24 14 20 20 uniform heat flow observed in Archean shields is due m pp , Zr 203 201 210 100 123 Nb, ppm 20 18 13 11 12 to the combined effects of low mantle heat flow (due to Cs, ppm 1 2.6 the insulating effects of a thick lithospheric mantle?) m pp , Ba 764 576 707 250 390 and low crustal heat production. La, ppm 25 28 16 18 Figur present4 e1 hypotheticasa l continental cross Ce, ppm 60 57 33 42 m pp , Pr 3.9 5.0 section derived from the seismic data reviewed here. Nd, ppm 27 23 16 20 The crust is divided vertically into three layers on the Smm pp , 5.3 4.1 3.5 3.9 basi averagf so e seismic velocities determine eacr dfo h Eu, ppm 1.3 1.09 1.1 1.2 typsmoota f eo sectione hus pattere Th . illustrato nt e m pp , Gd 4.1 3.3 3.6 crustae th l layert meanno s indicato si t t e lithological m pp , Tb 0.65 0.53 0.6 0.56 m pp , Dy 3.7 3.5 homogeneit cleas i t y(i r fro mnumbea studief ro s that m pp , Ho 0.78 0.78 0.76 e deeth p crus s lithologicalli t y diverse) e shadinth ; g Er, ppm 2.2 2.2 only indicates the average velocity of each layer (see Yb, ppm 2.0 1.5 2.2 2.0 Figure 2). Lu, ppm 0.36 0.23 0.3 0.33 diversite Th averagn yi e velocity, hence lithology, Hf, ppm 5 4.9 4.7 3 3.7 Ta, ppm 4 1.1 1 1.1 lowee foth r r crus reflectes i t change th y averdb n ei - m pp , Pb 20 14.8 15 8 12.6 age SiOd K2an O2 contents. These elements were Th, ppm 9 8.5 5.7 3.5 5.6 chosen to illustrate chemical diversity because they U, ppm 1.8 1.7 1.3 0.91 1.42 concentratee ar igneouy db s differentiatio thered nan - Heat pro- 1.31 1.25 0.92 0.58 0.93 ductiona, fore are sensitive indicators of the proportions of mafic IxWnr3 to evolved rock types in the lower crust. Potassium Heat flow,b 52 50 37 23 37 contents of the lower crust vary by a factor of 3, with mWm-2 highese th t values presen lowee th n i rt - crustex f so aAssumes an average density of 2.8 g cm~3. tensiona contractionad an l l tectonic e settingth d an s bAssumes a 40-km-thick crust. lowest values present in arcs and rifted margins, where 300 • Rudnick and Fountain: LOWER CONTINENTAL CRUST 33, 3 / REVIEWS OF GEOPHYSICS . 7 CONCLUSIONS 100 The seismic, petrologic, and geochemical data reviewed here were use develoo dt picturpa e th f eo deep continental crust. Increasing average seismic I velocities with depth indicate increasing proportions of mafic lithologies and increasing metamorphic U 10 O) grade. The lower crust consists of rocks in the granu- • This work lite facies and has an average composition that varies I + Taylor & McLennan c/OSi between different tectonic provinces bule Th k. lower • Others crust has a mafic composition, approaching that of a primitive basalt. Felsic and intermediate litholo- j gies are locally important in the lower crust and may La Ce Pr NdPmSm Eu Gd Tb Dy Ho Er Tm Yb Lu give rise to seismic reflections that are observed in e loweth r crus f somo t e regions. High-grade meta- morphosed shales are also present in the lower crust, but their high heat production, coupled with their generally limited occurrence in lower crustal xenolith suites, suggest f o smino thee rar y volumetric significanc n i mane y arease highest-gradTh . e 3*V4- metapelites, which have los granitia t c melt fraction, 10 ^^*=\ have high seismic velocities and are seismically indistinguishable from mafic granulites. Such rocke sar \*» unlikely to be the cause of seismic reflections in the lower crust. We have modeled the middle crust as consisting of I I I 1 I I e amphibolitrockth n i s e facies, although granulites CsRb BaTh U K NbLaCePb Pr SrNdZrHf SmEuTi Y HoYb may also be present. Average middle crust P wave Figure 12. Comparisons of trace element content o f explainee contib o t - w dominantly velocitiedlo b o to e sar y nental crust determined from our model (solid circles) versus mafic lithologies; thu middle sth e crus modeles i t a s da tha f Tayloro t McLennand an [1985] (pluses d otheran ) s mixture of mafic, intermediate, and felsic amphibolite (Weaver and Tarney [1984], Wedepohl [1994], and Shaw et facies gneisses. This crusta l a significanlaye s ha r t al. [1986]; shaded). amount of incompatible trace elements, including the heat-producing elements. bule Th k continental crust, modeled fro date mth a presented here and using the upper crustal estimates of the lower crust appears to be dominated by mafic rock Taylor d McLennanan n intermediat[1985]a s ha , e types. Shields and platforms, which constitute the compositio containd nan significana s t proportiof no largest volumetric proportio entire th f neo continental the bulk Earth's highly incompatible trace element lower crust, also have relativel contentsK w ylo . Nev- ertheless, in comparison with oceanic crust (as sam- ple mid-oceay db n ridge basalts) mafie th , c lower con- Proportion of Earth's Budget in Continental Crust tinental crus time 0 —1s 3 tha o 0st mor . SiOeK 2 varies 80 sympathetically with K, but the overall variation is 70 les factoa s( f 1.1)o r . 60 Delamination of the lower continental crust has 50 been suggeste importann a e b o dt t proces whicy sb h continental materia s recyclei l d inte convectinoth g % 40 mantle [Arndt and Goldstein, 1989; Kay and Kay, 30 1991]truee b r thi o ,Fo st . however lowee th , r crust 20 must hav overaln ea l mafic composition thao ca ns t i t 10 transfor eclogitmo t e wit hdensita y exceeding thaf o t 0 the underlying mantle f thiI . s proces shows si e b o nt Rb Pb Th U K Ba La Ce Mb Sr Sm 'logically important, then our data suggest that sig- Figure 13. Proportion of elemental budget of bulk silicate nificant quantities of incompatible trace elements may Earth that is contained within the continental crust. The lose b t fro continente mth thin si s manner owine th o gt shaded regio rangs ni modef eo l compositions liste Tabln di e presenc f interlayereeo d evolved rock types withina 11, circle valuee sar s from this study crossed an , valuee sar s predominantly mafic lower crust. from Taylor McLennand an [1985]. / REVIEW 3 , 33 GEOPHYSICF SO S Rudnlck and Fountain: LOWER CONTINENTAL CRUST • 301 65 % SiO2 Total Crust 60 Andesite 55 Lower Crust MORB K2O Total Crust Andesite - Lower Crust MORB Rift Paleozoic A te Margin Contractional shield & Platform Orogen Extensional Forearc 20 Depth 40 60 L km Vp (km/s) 6.2 6.4 6.6 6.8 7.0 7.2 Figur . Hypothetica14 e l cross sectio continentaf no l crust depicting averag waveP e velocities with dept differenr hfo t type sections (scale given at bottom). Vertical exaggeration is approximately 17 times. An attempt was made to show the relative areal proportions of the different type sections, but to do this in absolute terms was not feasible owing to the very small areal exten f somo t e type sections (e.g., active rifts constitute only \%f totao l crustal area Table se ;. Diversite7) n yi lithological assemblage lowef so r crus reflectes i t average th n di e K SiOd 2Oan 2 contents, shown above. Average normal mid-ocean ridge basalt (MORB) composition taken from Hofmann [1988]; average andesite composition taken from Taylor and McLennan [1985]. budget (35-55%). The basaltic lower continental crust Thomas Torgersen was the editor responsible for this s significantlha y greater incompatible trace element paper. He thanks Steve Bohlen and an anonymous referee content compared with oceanic crust lowef I . r crustal for their technical reviews. 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