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Stagnant Slabs in the Upper and Lower Mantle Transition Region

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Stagnant Slabs in the Upper and Lower Mantle Transition Region STAGNANT SLABS IN THE UPPER AND LOWER MANTLE TRANSITION REGION Yoshio Fukao Sri Widiyantoro MasayukiObayashi • EarthquakeResearch Institute Departmentof Geophysics Seismologyand Volcanology Universityof Tokyo and Meteorology ResearchDepartment Tokyo,Japan BandungInstitute of Technology MeteorologicalResearch Bandung,Indonesia Institute Tsukuba,Japan Abstract. We made a region-by-regionexamination of with present slab subduction appears to be blocked subductedslab images along the circum-Pacificfor some stronglyto turn into predominantlyhorizontal flow in of the recentglobal mantle tomographicmodels, specif- the transitionregion. Recent globaltomographic models ically for two high-resolutionP velocitymodels and two show also a group of lithosphericslabs deeply sinking long-wavelengthS velocity models. We extractedthe throughthe lower mantle, typicallythe presumedFaral- slab imagesthat are most consistentamong different lon slab beneath North and Central America and the models. We found that subducted slabs tend to be sub- presumedIndian (Tethys) slab beneathHimalaya and horizontally deflected or flattened in the upper and the Bay of Bengal. These remnant slabs are not con- lower mantle transitionregion, the depth range of which nectedto the surfaceplates or to the presentlysubduct- correspondsroughly to the Bullen transition region ing slabs and appear to sink independentlyfrom the (400-1000 km). The deflectedor flattenedslabs reside latter. The presenceof thesedeeply sinking slabs implies at different depths,either above or acrossthe 660-km that the pre-Eocene subductionoccurred in much the discontinuityas in Chile Andes, Aleutian, Southern sameway asin the presentday to accumulateslab bodies Kurile, Japan,and Izu-Bonin; slightlybelow the discon- in the transitionregion and that the consequentunstable tinuity as in Northern Kurile, Mariana, and Philippine; downflow occurred extensivelythrough the transition or well below it as in Peru Andes, Java, and Tonga- region in the Eocene epoch to detach many of the Kermadec. There is little indication for most of these surfaceplates from the subductedslabs at depths and slabsto continue "directly" to greater depthswell be- hence to cause the reorganizationof global plate mo- yond the transitionregion. Mantle downflowassociated tion. 1. INTRODUCTION tensen, 1989; Ringwood, 1994; Carlson, 1994; Tackley, 1995a;Ogawa, 1997]. Since the discoveryof the so-called Bullen [1963] called the outermost1000 km of the 660-km discontinuity[Niazi and Anderson,1965; John- Earth's mantle the upper mantle,which was dividedinto son, 1967], however, the boundary between the upper the B and C layers at a depth of about 400 km. The C and lower mantle has been placed at this discontinuity layeris the transitionregion between the B layer (above and the transition zone has been defined as the depth 400 km) and D layer (below1000 km). Figure la shows range between the 410- and 660-km discontinuities.In the depth variations of P and S wave velocities in Figure la we add a typical model having the 410- and Bullen's model. While the gradientsin seismicvelocity 660-km discontinuities,the preliminaryreference Earth and in densityin the B and D layers are steady and model (PREM), at a referencefrequency of 1 s which positivewith respectto increasingdepth, those in the C has the first-order discontinuities at 400- and 670-km layer are anomalouslylarge. These large gradients imply depths[Dziewonski and Anderson, 1981]. The lower half gradualor multiplechanges of composition,or of phase, of the mantle transitionregion in its original definition or of both [Birch, 1952]. Such compositionaland/or structuralchanges in mineral assemblageshould have has sincethen been regarded as the uppermostpart of played a significantrole in convectionprocesses and the lower mantle, where the velocity and densitygradi- evolutionhistories of the mantle [Anderson,1979; Chris- ents remain steadyand normal. This consensushas recently been challenged by Kawakatsuand Niu [1994],who confirmedthe existence Now at Japan Scienceand TechnologyCorporation, of a seismicdiscontinuity beneath Tonga at a depth of Kawaguchi-shi,Japan. 920-950 km to suggestthat the bottom of the transition Copyright2001 by the AmericanGeophysical Union. Reviewsof Geophysics,39, 3 / August2001 pages291-323 8755-1209/01/1999RG000068515.00 Papernumber 1999RG000068 ß 291 ß 292 ß Fukao et al.' SLABS IN THE MANTLE TRANSITION REGION 39, 3 / REVIEWS OF GEOPHYSICS (a) Velocity(km/s) (b) RMS Amplitude (%) 4 6 8 10 12 14 0 1 .o 2.0 S ;•-•-...P 500 - 1000 - 500 - 2000 - Bullen 2500 - 3000 0.3 0.6 0.9 Correlation Figure 1. (a) P and S wavevelocities of the sphericalmodels of Bullen and the preliminaryreference Earth model(PREM) asa functionof depthin the mantle.The depthrange of the Bullentransition region is shaded. (b) A comparisonof the three-dimensional(3-D) seismicmodels constrained by seismicdata alone (SH8M/ WM13) and by seismicand gravitydata (modelA) after Forteand Woodward[1997]. The root-mean-square (RMS) amplitudesof seismicshear velocity heterogeneity described by SH8M/WM13 andmodel A are shown by the dotted and solidcurves, respectively. Units are percentperturbation relative to the sphericalreference velocityat the givendepth. The dashedcurve indicates the globalcross correlation between SH8M/WM13 and model A. region may be defined by this discontinuityso that it mappingof seismicvelocities). Tanirnoto [1990b] inves- deepensback to its originalplacement by Bullen [1963] tigated correlation of lateral velocity variation among and Gutenberg[1959]. Niu and Kawakatsu[1997] further different layers in the three-dimensional(3-D) model extended their work to show a large depth variation MDLSH of Tanirnoto[1990a] and a combinedmodel of (900-1100 km) of this discontinuity.Its presencehas M84A [Woodhouseand Dziewonski,1984] and L02.56 now been reported in the areas of New Britain [Reve- [Dziewonski,1984]. He found that the correlationsbe- naughand Jordan, 1991], Japan-Kurile-Kamchatka[Pe- tween layersin the upper 1000 km and betweenlayers in tersenet al., 1993], Tonga [Kawakatsuand Niu, 1994], the rest of the mantle are high but that the correlation and Japan-Izu-Boninand Java-Banda-Flores[Niu and betweenthe upper 1000 km and the rest of the mantle is Kawakatsu,1997]. More recently, Vinnik et al. [1998] either low or even negative,as later confirmedby Mon- observedmultiple discontinuities (at 860, 1070,and 1170 tagner [1994]. On the basis of this finding, Tanirnoto km) and the persistentappearance of the 1070-kmdis- [1990b] suggestedplacing the boundary between the continuitybeneath the Sundaarc. These reportsindicate upper and lower mantle at a depth of 1000 km. Wenand that a thickness of several hundred kilometers below the Anderson [1995] found that the correlationsbetween 660-km discontinuityis far more complicated,at least in seismicheterogeneity and graveyardsof subductedslabs the subductionzones, than has ever been thought. [Engebretsonet al., 1992]peak near 1000-kmdepth. They The significanceof separatingthe upper 1000 km of interpreted this result as indicatinga significantbound- the mantle from the rest of the mantle is also indicated ary between800 and 1100 km depth acrosswhich vertical by analysesof previous seismic tomographic models flow of the mantle is stronglyinhibited. (Earth-mantle models obtained by three-dimensional In the present paper we maintain the view that the 39, 3 / REVIEWSOF GEOPHYSICS Fukaoet al.: SLABSIN THE MANTLETRANSITION REGION ß 293 nominalboundary between the upper and lower mantle convectionmodels constrainedby seismictomography lies at 660-km depth.We then refer to the "transition [see alsoButler and Peltier,2000]. These resultsimply region"as the transitionallayer between the upperand that the transitionregion acrossthe 660-km discontinu- lower mantle across the 660-km discontinuityrather ity maybe understoodas a relativelyimpermeable layer than the lower half of the upper mantle.The transition of mantle circulation.Li and Romanowicz[1996] noted regionin thisview maybe understoodas a depthrange in their 3-D model SAW12D that the midmantle is geodynamicallyaffected directly by processesacross the characterizedby the "white" characterof the heteroge- 660-kmdiscontinuity. Assuming that the mantleconvec- neity spectrumand that this "white" heterogeneityis tive circulationis driven by densityheterogeneity in- sandwichedby the degree-2dominant heterogeneity at ferred from global seismictomography models, Forte depthsof 500-800 km and the even strongerdegree-2 and Woodward[1997] carriedout a joint inversionof dominantheterogeneity in the D" layer. On the basisof seismicand gravitydata while simultaneouslyminimiz- thisobservation they suggesteda thermalboundary layer ing verticalflow acrossthe 660-kmdiscontinuity. This nature not only in the D" layer but alsoin the upper and inversionrevealed a family of mantle modelsthat pro- lower mantle transitionzone (500- to 800-km depth). vide as goodfit to the data as the whole mantleflow Downflow of mantle convection occurs mainly by modelsbut generatea stronglyor partiallylayered man- platesubduction. If thisdownflow is stronglyor partially tle flow.Figure lb [Forteand Woodward,1997] compares inhibited at 660-km depth and at other possibledepths a modelof this family (modelA) to the modelinverted in the transitionregion, vertical fluxes of subductedslabs from the seismicdata alone (SH8M/WM13 [Forteet al., mayconvert largely into horizontalfluxes in this region. 1994]).The solidand dottedcurves indicate
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