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The Deep Structure of Continents VOL. 84, NO. B I3 JOURNAL OF GEOPHYSICAL RESEARCH DECEMBER 10, 1979 The Deep Structure of Continents DON L. ANDERSON SeismologicalLaboratory, California Institute of Technology,Pasadena, California 91125 The Lehmann discontinuityat 220-km depth is an important global feature which occursunder both oceansand continents.It is a barrier to penetrationby younglithosphere and marks the baseof seismic- ity in regionsof continent-continentcollision. The stronglateral variationin uppermantle velocitiesoc- cursmainly abovethis depth. Continentalroots extend no deeperthan about 150-200 km. The basalt- eclogitetransformation and eclogite-harzburgiteseparation may be responsiblefor the geometryof inter- mediatedepth earthquakes.Oceanic and continentalgeotherms converge above about 200 km and be- comeless steep than the meltinggradient at greaterdepth. This impliesa low viscositychannel near 250 km. This would give a decouplingzone of maximumshear beneath continental shields. The Lehmann discontinuitymay be the interfacebetween two distinctgeochemical reservoirs. The velocityjump, and the inferreddensity jump, at 220 km are consistentwith an increasein garnetcontent. The mantle may be garnetlherzolite above and eclogiteimmediately below the Lehmanndiscontinuity. The transitionre- gion may be mainly eclogiteand be the sourceregion for oceanictholeiites. INTRODUCTION nental values[Jordan and Anderson,1974; Jordan, 1975].Hart et al. [1977] determined an attenuation corrected free oscilla- Continental lithosphereis much thicker than oceanic litho- sphere,but the question of how thick a sectionof continent tion averageearth model, QM2, which is comparedwith the translatescoherently during continental drift has not yet been continentalmodel SHR14 of Helmbergerand Engen[1974] in adequatelyaddressed. The bottom of the low-velocity zone is Figure 2. A more direct comparisonis the Pacific-easternU.S. usuallyconsidered to be the bottom of the asthenosphereand curve which is from Cara's [1979] surfacewave study. In this casethe major differencesare above 250 km. it has been presumedthat coherent translation of both oceanic and continental plates takes place above some 200 km. This Okal and Anderson[1975] and Sipkin and Jordan [1976] basicassumption of plate tectonicsis contrary to the idea that studiedthe corereflected shear phase, ScS, with conflictingin- terpretations. Okal and Anderson concluded that all differ- ontinents have roots deeper than 400-500 km [MacDonald, 963; Jordan, 1975, 1979a, b]. ences could be explained in terms of known effects above Surfacewave studieshave shownthat there are large differ- --,180 km, while Sipkin and Jordan concludedthat differences races between oceans and shields above about 220 km [Dor- in velocity must persistto great depths,perhaps extending nan et al., 1960; Brune and Dorman, 1963; Anderson, 1967a; throughout the entire upper mantle. Okal [1977] concluded Kanamori, 1970; Dziewonski, 1971]. Recently, Cara [1979] has that surfacewave data, regionalizedto take accountof the age made a detailed study of regional differencesusing higher of the oceaniclithosphere, are incompatiblewith strong,deep mode surfacewaves. He found strong regional variations be- lateralinhomogeneity anddo notrequire any substantial structure variation below 240 km. tween the Pacific Ocean, western U.S., and eastern U.S. above 250 km and no resolvable difference below this depth. Eng- The ScS phase,of course,averages the velocity throughout the mantle and cannot resolve where the differences occur. land et al. [1978] made a direct comparisonof upper-mantle structure under the North Atlantic and Arctic oceans and the There are more direct ways to isolatethe effect.The velocity old shield of the Russianplatform. Even when maximum dif- structureof the upper mantle has been studiedin many re- ferencesbetween the regions were allowed the P wave data gionsby body wave and surface wave techniques. These stud- could be satisfiedby velocity modelswhich were substantially ies give remarkablyconsistent results when the averagetravel the samebelow 300 km. Cara's modelsare shownin Figure 1. times above 200 or 250 km are calculated.Table 1 presents these results. Shields are the fastest, about 3.5 s faster than Okal and Anderson [1975] used multiple ScS phases to samplethe earth under variousgeological provinces including youngocean. Oceanic models, on the average,are about 1.5-2 oceans and shields. They' concluded that the observations s slowerthan shields,but the differencedecreases with tl•e age of the ocean. were consistent with known differences above about 180 km. We have computed the differences between the $ wave There is therefore good agreement between the body wave and surfacewave data. Jordan [1975], however, proposedthat traveltimes above 200 km for the Rayleigh•ave models and ocean-continentdifferences extend deeper than 400 km and the Jeffreys-Bullenvalues. Figure 3 displaysthe computed ScS residuals for these oceanic and shield models as a func- that the region which translatescoherently in the course of plate tectonicsmay occupy the entire upper 700 km of the tion of crustal age. Measured residualsrelative to Jeffreys- Bullen, are alsoshown. It is clear that the ScStunes can be ex- mantle. This proposalhas reopenedthe questionof the deep structure of continents. plainedby known differences in the upper 200•250 km of the mantle. THE DEEP STRUCTURE OF CONTINENTS Theaverage one-way ScSresidual f•r 'ayerage age'ocean Prior to the recognitionthat attenuationwas important in (70-90 m.y.) is +0.8 s. This can be comparedwith the average interpreting free oscillationperiods, it was thought that aver- residualof +2.0 s for all oceanicdata combined,including the age earth shearvelocities were appreciablyslower than conti- very slow young oceans[Sipkin and Jordan, 1976].This latter value is a straightaverage of all data and ignoresthe variation Copyright¸ 1979 by the Americ.•nGeophysical Union. with age. Paper number 9B0991. 7555 0148-0227/79/009B-0991501.00 7 5 56 ANDERSON:DEEP STRUCTUREOF CONTINENTS Vs, km/sec TABLE 1. Upper Mantle Vertical Shear Wave Travel Times Above 4.0 4.5 5.0 200 and 250 km 0 Travel Time, s 200 km 250 km Reference ioo Shield 45.2 _+0.3 56.1 +_0.3 (1) • J/'%"EastU.S.- Continent 45.9 +_0.5 57.0 +_0.2 (2) 200 Ocean 46.7 _+0.4 57.7 +_0.3 (3) Minimum 49.2 60.6 (4) 15 m.y. 48.7 59.8 (5)* 70 m.y. 47.1 58.1 (5)* •00 100 m.y. 46.4 57.5 (5)* 150 m.y. 45.8 56.8 (5)* Maximum 46.0 57.2 (4) 400 • D• (1) •4nderson[1967a], •4ndersonand Harkrider [1968], Brune and Fig. 1. Shear velocity versusdepth for Pacific (average age, 90 Dorman [1963], }Vickens[1971], and Massb[1973]. m.y.) and easternU.S. from Cara [1979]. CANSD is Canadian shield (2) Helmbergerand Engen [1974],•4nderson and Julian [1969], Cara from Brune and Dorman [1963]. [19791. ' (3) •4nderson[1967a], Kanamori [19701,Schlue and Knopoff[1976], The residualsfor the oldestocean, > 120 m.y., are scattered, and N. R. Burkhard and D. D. Jackson (unpublished manuscript, 1979) (average ocean). but the mean is -0.7 s. This includesanomalously slow read- (4) Yoshida[1978]; minimum and maximum age groups. ings from the mid-Pacific mountains and the Bermuda Rise. (5) N. R. Burkhard and D. D. Jackson(unpublished manuscript, Therefore the one-way difference in travel times between 1979). shields and old oceans is about 1.3 s. The difference between *SV velocitiesfrom Rayleigh waves.Horizontally traveling SH averageocean and old ocean is about the sameas determined waves(Love waves) give travel times 1.6s (15 m.y.) to 0.5 s (150 m.y.) shorter.ScS waves should be comparedwith Rayleighwave, not Love by Duschenesand Solomon[1977] usingshear waves from Pa- wave,velocities. 'Average' ocean (--,70 m.y.) is about2 s slowerthan cific events. shield.If Love wavesare usedin the comparisonoceans would ap- From the raw ScS data the mean ocean-continent differen- pear to be about 3 s slow. tial time is + 1.5 + 2.8 s, about the same as the upper 250 km alone.Correcting for attenuationreduces the differentialtime sured shear wave velocities in the LVZ are 12-15% lower than slightly. The ScS shield data overlap the shield models,which the high-frequencyvelocities for mineralogiesin assemblages differ from the average earth only above 250 km, but average ranging from pyrolite to eclogite [Anderson,1977]. In addition about I s faster (Figure 3). to the temperatureeffect an additional severalpercent varia- From independent data (Table 1) the travel times above tion is allowed by variations in mineralogy. We take 12% as a 200-250 km in shieldsaverage 1.6 _+0.6 s faster than under plausible variation between shield and ocean mantles,take a average oceans and 0.9-2.0 s faster than 70-100 m.y. old travel time of 57 s above 250 km, and assume that velocities oceans.Therefore the ScS data are in good accord with the are the same at •250 km. This gives3.4 s as a conservativees- surfacewave studies,and it appearsthat all differencescan be timate of possible upper mantle shear wave vertical travel accommodated above 250 km. time variations. This would be the difference between a cold, The ScS data, when correctedfor crust and upper mantle garnet-rich upper mantle and a warm, relaxed, garnet-poor effects above 50 km, suggestthat the mantle under shields may be as much as about 3.4 s fasterthan under old oceans. +5 OC'E•NS At this point, it is instructive to estimate the maximum CONTINENTS plausiblevariations in the upper mantle.The shearvelocity in +z the low-velocity zone (LVZ) in oceansand tectonicregions is •,••yle•ghwaves about 10% lower than subcrustal velocities. This can be ac- c•+3 - \ - counted for by a difference in chemistryor by high-temper- --'+2 - ature stress relaxation mechanisms such as dislocation or grain boundary relaxation [Andersonand Minster, 1979}.Mea- a• -i- I - , \ - Sc \ +050 Average Earth OM2 I" -I •ELDS +025_[7 -2 - -3 I , \! I04 106 i08 109 -025 Age, yeors Fig. 3. ScS residuals (vemcal bars) as functio,• o! age of litho- sphere.Data are from Okal and Anderson[1975], Okal [1978a,b], and -O50 Sipkinand Jordan[1976].
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