Rayleigh–Wave Tomography of the Ontong–Java Plateau

Physics of the Earth and Planetary Interiors 118Ž. 2000 29±51 www.elsevier.comrlocaterpepi

Rayleigh±wave tomography of the Ontong±Java Plateau

W. Philip Richardson a,b, Emile A. Okal a,), Suzan Van der Lee c,d a Department of Geological Sciences, Northwestern UniÕersity, 1847 Sheridan Road, EÕanston, IL 60201, USA b CheÕron Petroleum Technology, 935 GraÕier Street, New Orleans, LA 70112, USA c Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Brach Road, N.W., Washington, DC 20015, USA d Institut furÈÈ Geophysik, Eidgenossische Technische Hochschule, Honggerberg, È CH-8093 Zurich, È Switzerland Received 6 April 1999; accepted 6 July 1999

Abstract

The deep structure of the Ontong±Java PlateauŽ. OJP in the westcentral Pacific is investigated through a 2-year deployment of four PASSCAL seismic stations used in a passive tomographic experiment. Single-path inversions of 230 Rayleigh waveforms from 140 earthquakes mainly located in the Solomon Trench confirm the presence of an extremely thick crust, with an average depth to the MohorovicicÏÂ discontinuity of 33 km. The thickest crustsŽ. 38 km are found in the southcentral part of the plateau, around 28S, 1578E. Lesser values remaining much thicker than average oceanic crustŽ 15±26 km. are found on either side of the main structure, suggesting that the OJP spills over into the Lyra Basin to the west. Such thick crustal structures are consistent with formation of the plateau at the Pacific±Phoenix ridge at 121 Ma, while its easternmost part may have formed laterŽ. 90 Ma on more mature lithosphere. Single-path inversions also reveal a strongly developed low-velocity zone at asthenospheric depths in the mantle. A three-dimensional tomographic inversion resolves a low-velocity root of the OJP extending as deep as 300 km, with shear velocity deficiencies of ;5%, suggesting the presence of a keel, dragged along with the plateau as the latter moves as part of the drift of the Pacific plate over the mantle. q 2000 Elsevier Science B.V. All rights reserved.

Keywords: Ontong±Java Plateau; Rayleigh waveform; Solomon Trench

1. Introduction and background Basin. It rises an average of 2000 m above the basin floor, reaching soundings of less than 4000 m over We present the results of a 2-year passive tomo- an area of 1.6=106 km2, and culminating at 2000 graphic experiment over the Ontong±Java Plateau m below sea level in its southcentral section, around Ž.OJP , a massive feature of the western Pacific Basin, 38S, 1578E. This makes the OJP the largest ``Large located between the Caroline Islands to the north and Igneous Province''Ž. LIP on Earth. These provinces the Solomon Islands to the southŽ. Fig. 1 . The have generally been described as resulting from rapid plateau is generally L-shaped, with the horizontal episodes of massive basaltic floodingŽ Coffin and arm of the ``L'' extending into the south Nauru Eldholm, 1994. , and as such may be conceptually similar to planetary structures such as the lunar ) Corresponding author. Tel.: q1-847-491-3194; fax: q1-847- maria, the Tharsis region of Mars, the Beta±Atla± 491-8060; e-mail: [email protected] Themis Province of Venus, and possibly the smooth

Fig. 1. Location map for the OJP seismic experiment. The plateau is defined by the 4000-m isobathŽ. labeled ; the 2000-m one is also shown. The four island stations deployed are shown as solid circles, with three-letter codes: TKKŽ.Ž Chuuk , PNI Pohnpei . , KOS Ž Kosrae . and NAU Ž.Nauru . The triangles show the DSDPrODP sites where drilling returned basaltic samples. The following islands mentioned in the text are identified: MalaitaŽ. M , Santa Isabel Ž. SI , San Cristobal Ž SC . , Guadalcanal Ž. G , New Georgia Ž NG . , Bougainville Ž. B and New Britain Ž.NB . Ramos is a small islet between Malaita and Santa Isabel which cannot appear on the scale of this figure. The inset replaces the main mapŽ. box in the local geographical context. plains of MercuryŽ. Head and Coffin, 1997 , although ceous extinctions, remain highly speculativeŽ e.g., significant differences exist among the topographic Sleep, 1990; Arthur et al., 1991; Larson, 1991; Lar- expressions and the rates of extrusion on each planet. son and Olson, 1991; Head and Coffin, 1997. . In this Such massive eruptions must involve relatively general framework, the investigation of LIPs is of violent episodes of accelerated mantle dynamics, prime importance to our improved understanding of themselves representative of major changes in the the internal dynamics of the Earth and its sister pace of the Earth's internal engine, and in turn planets. potentially impacting its surficial environment. How- In very general terms, two classes of models have ever, the exact association, and causal relationship been proposed to explain the origin of LIPs, follow- between LIP eruptions and such disruptions as the ing ideas originally put forth by MorganŽ. 1981 . In onset of the magnetic quiet interval, or the Creta- the passiÕe model, LIPs are generated during conti- W.P. Richardson et al.rPhysics of the Earth and Planetary Interiors 118() 2000 29±51 31 nental rifting over a zone of permanent upwelling Nova±Canton Trough through a lateral sublitho- Ž.White and McKenzie, 1989, 1995 . In the actiÕe spheric ponding mechanismŽ. Sleep, 1996 . model, massive igneous outpouring occurs upon ini- Major element petrological signatures of the tiation of a hotspot trackŽ. Richards et al., 1989 . available samples are typical of oceanic tholeiites Laboratory experimentsŽ Whitehead and Luther, Ž.Stoeser, 1975; Mahoney et al., 1993 , and their 1975; Olson et al., 1988. have suggested that a large geochemistry is that of a MORB contaminated by a head can develop on an ascending diapir, giving rise hotspot sourceŽ Mahoney, 1987; Mahoney and to a flooding event when it punctures the lithosphere; Spencer, 1991.Ž , confirming probable on-ridge or present-day hotspots are then interpreted as the more near-ridge.Ž emplacement of the OJP e.g., Nakanishi sedate and steady-state expression of the tail of the and Winterer, 1996. . mantle plume. In general, the passive model has been applied to continental flood basalts, such as the 1.2. Crustal structure Ethiopian TrapsŽ. Courtillot et al., 1999 , and the active one to oceanic plateaux. As supported by the The OJP was targeted for crustal studies using early observations of MorganŽ. 1972 and Winterer seismic refraction in the early 1970s. Failing to

Ž.1976 , the active model has been proposed as the detect Pn as a first arrival over profiles as long as mechanism of choice for the formation of oceanic 150 km, Furumoto et al.Ž. 1976 and Hussong et al. plateaux emplaced on or near a ridgeŽ Richards et Ž.1979 argued for a crustal thickness of at least 39, al., 1991; Winterer and Nakanishi, 1995; Gladczenko and possibly 45, km. These datasets were recently et al., 1997. , and in the case of the OJP, as express- reinterpreted by Gladczenko et al.Ž. 1997 and com- ing the original phase of activity of the present-day plemented by a new refraction experiment involving Louisville hotspot, despite differences in geochemi- the deployment of ocean-bottom seismometers cal signaturesŽ. Neal et al., 1997 . Ž.Miura et al., 1997 . In the next few paragraphs, we review briefly a Similarly, anomalous values of crustal thickness number of geological and geophysical properties of have been reported along other oceanic structures, the OJP, as they were known mostly from investiga- such as the Tuamotu PlateauŽ 25±32 km, Talandier tions predating our experiment. and Okal, 1987.Ž , the Iceland±Fñrú Ridge 25±30 km, Bott and Gunnarsson, 1980. , or the Nazca Ridge 1.1. Age and nature of Õolcanism Ž.18 km, Woods and Okal, 1994 , all features generally interpreted as having been generated on or near The age of formation of the OJP is known from a mid-oceanic ridge. 39Arr40Ar dating of basement rocks recovered at On the other hand, and since such crustal thick- DSDPrODP Sites 289Ž. on the center of the plateau ness is more typical of continental structures, Nur and 803 and 807Ž.Ž at its northern edge Mahoney et and Ben-AvrahamŽ. 1982 have speculated that al., 1993. , as well as of basalt formations on Malaita, oceanic plateaux, and in particular the OJP, could be Ramos and Santa Isabel in the Solomon Islands, wandering pieces of a lost continent. This, however, which are interpreted as obducted fragments of the is contradicted by the basaltic nature of the drilled bottom layers of the OJPŽ e.g., Hughes and Turner, and obducted samples, and by the crustal stratigra- 1977; Tejada et al., 1996. . These studies suggest two phy revealed by the refraction campaigns, which is episodes of emplacement: a major one at 121"1 found to be typically oceanic, except for a general Ma, followed by a less intense one at 92"2 Ma. thickening by a factor of ;5Ž Hussong et al., 1979; The younger samples, found at Site 803 and on Santa Gladczenko et al., 1997. , and, hence, the continental Isabel, suggest that the eastern section of the OJP model is now abandoned. was formed during a later stage of volcanismŽ Maho- ney et al., 1993; Yan and Kroenke, 1993; Parkinson 1.3. Geoid et al., 1996; Kroenke and Wessel, 1997. . Elaborating on this dual pulse scenario, LarsonŽ. 1997 also pro- The OJP has only a minor signature in the geoid, posed linking the Manihiki and OJ plateaux with the indicating a large degree of isostatic compensation. 32 W.P. Richardson et al.rPhysics of the Earth and Planetary Interiors 118() 2000 29±51

The geoid-to-topography ratio of 1.65=10y3 re- tage of modern communications, this approach pro- ported by Sandwell and McKenzieŽ. 1989 could be vides a dense coverage of seismic rays across the accommodated by a 25-km thick crust, this ratio structure, allowing the inversion of Rayleigh wave being intermediate between those of other oceanic dispersion for crust and mantle structure, initially plateauxŽ Manihiki: 0.52=10y3 ; Tuamotu: 0.69= under individual seismic paths, and in a second step 10y3 .Ž and mid-plate hotspot swells Hawaii: 3.76= through a full three-dimensional tomographic experi- 10y3 ; Tahiti: 2.47=10y3. . Recently, and posterior ment. We chose not to use Love waves, because of to our deployment, the joint inversion by Gladczenko the generally stronger level of noise on horizontal et al.Ž. 1997 of the reexamined 1970 refraction data components in the relevant frequency band at island and of new gravity data yielded a crustal thickness of sites. 36 km along the deepest section of the OJP transects. 2.1. Deployment 1.4. Heat flow Portable PASSCAL stations using three-compo- nent STS-2 seismometers were deployed in January Twenty-five values of heat flow measurements 1994 on the islands of Chuuk, Kosrae and Nauru. It are available on the OJP, principally in its westcen- was initially hoped to avail ourselves of the perma- tral sectionŽ C.A. Stein, personal communication, nent POSEIDON station being installed contempora- 1999.Ž . They range from 0.4 to 1.9 HFU 17±80 neously at Pohnpei. Difficulties in this respect man- mWrm2 .Ž , with an average of 1.1 HFU 47 mWrm.2 . dated the deployment of a fourth station at Pohnpei These values do not reveal any systematic heat flow in July 1995. The four stations were shut down anomaly over the plateau, and are if anything, lower during the first quarter of 1996. than in the nearby Nauru Basin to the northeastŽ on the average of 1.25 HFU or 52 mWrm.2 . 2.2. Siting All these previous investigations then suggest the broadly consensual model of the generation of the ChuukŽ. 12±15 Ma is the oldest member of the OJP at or near a Mid-Oceanic RidgeŽ the Pacific± Caroline chainŽ. Keating et al., 1984 ; its erosion has Phoenix one according to the general reconstruction reached the stage of a 70-km wide lagoon in which of LarsonŽ.. 1976 , during an outburst Ž probably a only a few basaltic islets remain above sea level. The dual one. of plume head activity, resulting in a StationŽ. TKK; 7.4478N, 151.8878E was installed on crustal thickness of ;35 km. However, most of the the island of Moen, in an abandoned tunnel entrance evidence for this figure is largely circumstantial, located under Xavier High School, a fortress-like providing little if any insight into any lateral hetero- building originally built as a communication head- geneity in structure, and no resolution into the under- quarters for the Japanese Navy during World War II. lying mantle. This clearly warrants a more detailed PohnpeiŽ. 5±8 Ma is a large circular island reach- seismological investigation, which is the subject of ing 772 m above sea level. Most of the strongly the present study. eroded interior consists of inaccessible jungle. The stationŽ. PNI; 6.9698N, 158.2108E was deployed in an outbuilding of the Micronesian Seminar in Kolo- 2. The experiment: layout and deployment nia. KosraeŽ. 1.4±2.6 Ma , the youngest member of the Because of the scarcity and inaccessibility of Caroline chain, is already strongly eroded and im- islands on the OJP itself, its crustal structure could penetrable in most of its interior. The stationŽ KOS; not be resolved through the now classical approach 5.3248N, 163.0098E. was installed in an abandoned of receiver functionsŽ. e.g., Owens and Zandt, 1985 . government warehouse outside Topol. In October Rather, we targeted four islands around the OJP for a 1995, the building was reclaimed by the State of passive experiment using sources in the Solomon± Kosrae, and the station moved 200 m to the old radio Santa Cruz seismic belt. Besides the logistical advan- station office. W.P. Richardson et al.rPhysics of the Earth and Planetary Interiors 118() 2000 29±51 33

Fig. 2. Path coverage of the OJP experiment. One-dimensional inversions were carried out along each of the great-circle segments plotted, and the results subsequently inverted for the three-dimensional model. In this respect, the density of paths in any given region gives a qualitative estimate of the local resolution of the three-dimensional inversion. The plateau structure is outlined in white along the 4000-m isobath. The four small maps show the location of the four stations on their respective islands; note that the scales are different for each map. 34 W.P. Richardson et al.rPhysics of the Earth and Planetary Interiors 118() 2000 29±51

Nauru is an uplifted atoll of unclear geological 2.3. Dataset historyŽ. Hill and Jacobson, 1989 , now covered with phosphate deposits, which have been mined to near We targeted for study all earthquakes occurring in exhaustion. The stationŽ. NAU; 0.5098S, 166.9328E the Solomon and Vanuatu subduction systems at was deployed in a World War II Anti-Aircraft gun depths less than 210 km, and with at least one bunker dug into the limestone cliffs at an altitude of reported magnitude Ž.mbs; M above 5, resulting in 60 m above sea level. the initial processing of 256 seismograms. A number Despite several problemsŽ flooding at Chuuk, of complications in the waveforms resulting from vandalism at Nauru. , the stations achieved a 77% probable source complexity, as well as uncertainties up-time record, which must be considered good given in hypocentral depthsŽ. as discussed below , resulted the difficulties inherent in their operation from 11,000 in a final dataset of 230 seismograms inverted from km away. 140 events, of which a full list is given by Richard-

Fig. 3.Ž. a One-dimensional inversion for the event of 2 February 1996 in East Bougainville recorded at Pohnpei. The box at upper left shows the poor match between the observed seismogramŽ. solid trace and a synthetic trace computed using Model JV30 Ž dashed line . . The bottom box shows the fit for the final model resulting from the inversion, itself described in the vertical box at right. The latter shows the shear velocity profile of the starting modelŽ. JV30; solid line and of the final one-dimensional model for the path Ž. dashes . Note the thicker crustŽ. 34 km and the development of a strong low-velocity zone between 55 and 150 km. The inset summarizes the geometry of the path and of the event. W.P. Richardson et al.rPhysics of the Earth and Planetary Interiors 118() 2000 29±51 35

Fig. 3.Ž. b Same as Fig. 4a for the event of 23 September 1995 at San Cristobal, recorded at Chuuk. This model has the thickest crust among one-dimensional inversionsŽ. 37 km , and a strongly developed mantle low-velocity zone.

sonŽ. 1998 . The spatial coverage of the OJP by the here. In this method, the local deviation from a various paths is shown in Fig. 2. The time series reference Earth model E of the shear velocity along were windowed to include arrivals between group the j-th path, db Ž j.Ž.r , is sought as an expansion: velocities of 4.8 and 3.5 kmrs, which include all body phases extending from SS to the fundamental Rayleigh wave, which can also be regarded as a M db Ž j.Žr s g j.hr 1 complete set of Rayleigh overtones. Ž.Ý ii Ž. Ž. is1

on M basis functions hi sampling the crust and 3. One-dimensional inversions upper mantle in a resolvable domain of depthsŽ which in the present study we take as independent of the We first carried out one-dimensional inversions of path j.. For each path j, the vector g Ž j. is found by individual records, using the first step of the parti- an inversion procedure minimizing the misfit be- tioned waveform inversionŽ. PWI technique intro- tween the observed seismogram, dtŽ j.Ž.and its syn- duced by NoletŽ. 1990 , which we briefly summarize thetic counterpart, stŽ j. Ž ;g Ž j.., the latter expressed 36 W.P. Richardson et al.rPhysics of the Earth and Planetary Interiors 118() 2000 29±51

Fig. 3.Ž. c Same as Fig. 4a for the Guadalcanal event of 5 December 1994 recorded at Nauru. Note the much thinner crustŽ. 23 km for this path sampling the eastern portion of the OJP. in the frequency domain as a superposition of N where Fourier transformation is expressed by capital- surface wave modes: ization. In practice, the time series stŽ.and dt Ž.are Ny1 subjected to filtering, and the inversion is stabilized Ž j.Žv g j. s 0 v v S Ž.; Ý AnjŽ.exp ikŽ n Ž. by the addition of a Bayesian covariance term; fur- ns0 ther details can be found in the work of Nolet qd j vD Ž.1990 . The procedure is then iterated until a satis- knjŽ.. Ž.2 factory level of convergence is reached. where Dj is the epicentral distance along path j, In the present study, we used Ns30 branches knŽ.v is the wavenumber of the n-th mode for the and Ms11 basis functions extending the perturba- j Earth model E, and dknŽ.v is the perturbation to tion to a depth of 300 km, and inverted the seismic knŽ.v resulting from the change in shear velocity signal in the frequency range 0.008Ffsvr2pF Ž.1 . In its simplest form, the misfit function F Ž j. 0.125 Hz. would take the form:

2 3.1. Reference models F Ž j.ŽŽ.g sH stj.Ž Ž;g .ydtj. Ž.dt We used a variety of starting Earth models E. UU s2p

Fig. 3.Ž. d Same as Fig. 4a for the New Britain event of 19 October 1994, recorded at Chuuk. Note the 26-km thick crust obtained for this path crossing the Lyra Basin to the west of the OJP. to a depth of 210 km, and the PEM-O profile called ``JVUU '' models. was developed, featuring a Ž.Dziewonski et al., 1975 at greater depths. These low-velocity zone expressed as a triangular b profile mantle models were combined with various crustal with a minimum velocity of 4.2 kmrs at 120 km, structures, with MohorovicicÏÂwx Moho depths ranging together with a two-layer parameterization of the from 15 to 45 km. For example, Model OJ30 has a crust. The latter was introduced in the hope of 30-km-thick crust. Such variability in crustal thick- recovering information on a proposed high-density ness allowed a lesser degree of perturbation, and cumulate in layer 3Ž. Neal et al., 1997 . Full details of generally resulted in an improved convergence of the the parameterization of the various OJUU and JVUU iterations and in reduction of residual variance. We models are given in the paper of RichardsonŽ. 1998 . verified, however, that the final inverted crustal Because the spectral phase of the seismogram is thicknesses were robust with respect to a change in controlled primarily by the epicentral distance Dj in the starting model. The near-surface structure of the Eq.Ž. 2 , source mislocation could be a serious issue crustal models included a 3-km water columnŽ the in this method. Three sources of source locations observed average over the OJP. , a 1-km sedimentary were generally available: the Preliminary Determina- layer, and a basaltic crust. The velocity, density and tions of EpicentersŽ. PDE catalogue of the National attenuation in the sedimentary structure were taken Earthquake Information CenterŽ. NEIC , the recent from Mosher et al.Ž. 1993 . Based on early inversion catalogue by Engdahl et al.Ž. 1998 , and the Centroid models, a second class of refined modelsŽ the so- Moment TensorŽ. CMT locations of Dziewonski et 38 W.P. Richardson et al.rPhysics of the Earth and Planetary Interiors 118() 2000 29±51

Fig. 3.Ž. e Same as Fig. 4a for the New Georgia event of 19 April 1995, recorded at Chuuk. The resulting model has a particularly strong low-velocity anomaly in the mantle, with b reaching 4.0 kmrs at a depth of 110 km.

al.Ž.Ž 1994 and subsequent quarterly updates . . A excitation of the various overtones making up the number of tests showed that, for the combination of seismogram; in particularŽ and depending on the path lengths and frequencies involved, the epicentral initial model E chosen. , a change in depth may variations resulting from the use of the various cata- move the source across the Moho discontinuity, thus logues were negligible factors in waveform inver- strongly affecting the excitation. Our procedure was sions, a conclusion supported by concurrent work by to use the PDE depthŽ. if not constrained to 33 km , Lebedev et al.Ž. 1997 and Xu and Wiens Ž. 1997 . and in doubtful cases, to perform inversions for all As detailed by NoletŽ. 1996 and Van der Lee three reported hypocenters, keeping whichever Ž.1998 , uncertainties in depth could be significantly achieved the best variance reduction. more challenging: first, it is well-known that the resolution of depth is poorer than that of the horizon- 3.2. Results tal epicentral coordinates, with many published depth estimates remaining constrained at either 10 km These are presented in Fig. 3a±e for a variety of Ž.Ž.PDE, oceanic basins ; 33 km PDE, other regions representative paths. In each of these figures, the or 15 kmŽ. CMT . Second, depending on focal geom- boxes at left compare the observed seismograms etry, depth will control both the amplitude and phase Ž.deconvolved ground motion; solid trace with the W.P. Richardson et al.rPhysics of the Earth and Planetary Interiors 118() 2000 29±51 39 relevant synthetic for the starting modelŽ dotted line; km to the west of 1508E. The latter figure is in top.Ž. and the final inverted model bottom . The right agreement with the results of Gladczenko et al. box compares the initial shear-velocity profileŽ solid Ž.1997 for the northeastern part of the basin in the line.Ž. with the final model dashes . The inset at right vicinity of ChuukŽ. 11 km , while the average figure sketches the geometry of the particular path in- of 23 km under the Lyra Basin essentially supports volved. the suggestion by KroenkeŽ. 1972 , Erlandson et al. The most important result, as shown in the five Ž.1976 and Gladczenko et al. Ž. 1997 that the OJP figures presented and confirmed on all other paths, is structure spills over into the Lyra Basin. the extreme crustal thickness of the OJP. The aver- To the east, along the east±west branch of the age depth to the Moho discontinuity for all our paths OJP ``L'', crustal thicknesses of 25"2 kmŽ. Fig. 3c over the OJP is 33 km, with individual values rang- suggest a general thinning of the plateau structure, ing from 23 to 38 km. Such very high values for the consistent with the suggestion of a laterŽ. 90 Ma crustal thickness are however comparable to those emplacement of this portion of the structure, as also found in such oceanic provinces as the Tuamotu supported by the hint of a different crustal structure PlateauŽ. 25±32 km, Talandier and Okal, 1987 , the featuring an enhanced high-velocity layer 3Ž Gladc- Iceland±Fñrú RidgeŽ 30±35 km, Bott and Gunnars- zenko et al., 1997; Neal et al., 1997. . This province son, 1980.Ž , and Nazca Ridge 18 km, Woods and could have been built on somewhat mature litho- Okal, 1994. . By analogy with such regions, this sphere, capable of offering some amount of elastic means that the OJP was formed in the immediate support, as part of the 90-Ma reactivation of the vicinity of a Mid-Oceanic Ridge, when the plate was magma source, possibly in relation to an earlier split still too young and hot to provide any elastic support, of the plume into several discrete blobs upon ascent and the province was emplaced in a fully compen- through the transition zone discontinuitiesŽ Bercovici sated geometry, as discussed, for example, by and Mahoney, 1994. . Cazenave and OkalŽ. 1986 . The range of values of crustal thickness found in 3.4. Mantle structure the present study are generally less than the 35±42 km advocated by Furumoto et al.Ž. 1976 , and more A common feature to all profiles inverted in this in line with the 32-km average of Gladczenko et al. study is the development of a strong low-velocity Ž.1997 resulting from a reinterpretation of the early zone between depths of 50 and at least 150 km, with seismic profiles. It should be borne in mind that our shear wave speeds b dropping to as low as 3.95 method yields an average thickness, integrated over kmrsŽ. Fig. 3e . This is in contrast to typical values the whole path from epicenter to stationŽ typically of bs4.2±4.3 kmrs for unperturbed oceanic re- 1600 km. , while seismic refraction experiments were gions of Late Jurassic ageŽ Mitchell and Yu, 1980; intrinsically limited to a smaller regionŽ typically Nishimura and Forsyth, 1989. , indicating that the less than 200 km. . perturbation in structure associated with the presence of the plateau reaches significantly deeper than the expected thickness of the lithospheric plate Ž;100 3.3. Lateral Õariation km. . The general level of heterogeneity revealed by the Fig. 3a±e show that the crustal structure of the family of one-dimensional inversions warrants a full OJP exhibits lateral heterogeneity. The thickest crusts three-dimensional inversion before discussing further Ž.38 km are obtained for paths linking the southeast- the nature of deeper structures. ern Solomon IslandsŽ. e.g., San Cristobal to Station TKK, and thus sampling the westcentral part of the plateau around 28S, 1578E. To the west, the crust is 4. Three-dimensional inversion found to thin to 29 km as the path samples the western border of the OJP, 26 km under the Lyra We apply here the second step of the PWI method BasinŽ. New Britain to TKK; Fig. 3d , and finally 15 of NoletŽ. 1990 summarized below, using a slightly 40 W.P. Richardson et al.rPhysics of the Earth and Planetary Interiors 118() 2000 29±51 W.P. Richardson et al.rPhysics of the Earth and Planetary Interiors 118() 2000 29±51 41

Fig. 4. Checkerboard resolution tests for the three-dimensional inversion.Ž. a Original checkerboard model used as input. The checkers are vertical dykes extending from the surface to 200 km.Ž. b Result of inversion at 30 km; note smearing along the NE±SW directions, and loss of resolution south of Nauru and in the Lyra Basin.Ž. c Same as Ž. b at 60 km depth. Ž. d Same as Ž. b at 100 km depth. Ž. e West±east cross-section of the checkerboard model along latitude 38SŽ. vertical exaggeration factor: 7 ; the horizontal line represents the unperturbed Moho discontinuity; a cross-section of the bathymetry is shown above the Earth structureŽ.Ž. vertical exaggeration factor: 35 . f Cross-section of the inverted model along the same parallel; the perturbed Moho discontinuity is also shown. different notation, which reflects both the simplifica- Ž.1990 , and within the domain of perturbation the- tion of using a common unperturbed modelŽ. JV30 ory, the deviation db Ž j.Ž.r found in the first step of for all paths j, but, on the other hand, the additional the inversion along the j-th path, Pj, is expected to complexity of allowing for a variable depth HŽ.u,f be simply the path average of the three-dimensional to the Moho discontinuity. perturbation: We now envision a three-dimensional deviation, 1 dbŽ.r sdb Žr,u,f .of the shear-wave velocity from db Ž j. Ž.r s db Žr,u ,f .d D Ž.4 D H the reference model JV30. As explained by Nolet j Pj 42 W.P. Richardson et al.rPhysics of the Earth and Planetary Interiors 118() 2000 29±51

For each of the J paths Pj, and for each of the M The final equations used in the present study, components of the expansion of db Ž j.Ž.r onto its 11a db Ž.r,u ,f basis functions, Eq.Ž. 4 is equivalent to: d D grŽ.dr D HHs Äi j Pj a 0 b 1 a drgÄ Ž. r d D db Žr,u ,f .sh ,5 Ž. aD HHiidHŽ.u ,f j 0 Pj q g H sh "Dh s Äiii a series of M=J linear equations for the unknown H model dbŽ.r,u,f . In Eq. Ž. 5 , a is the Earth's radius, reflect the introduction of the discrete parameteriza- and the vector h is derived from g in Eq.Ž. 1 by tion of the Moho discontinuity through its depth rotation into the principal directions of the Hessian HŽ.u,f , and of weighting factors and error estimates E2 r E E Ž F.Žgabg .of the misfit function Ž. 3 at its ex- in order to stabilize the inversion; details are found gsg Ž j. s Ž. tremum . Similarly, grkŽ., Ä4k 1, M is in the papers of Nolet 1990 and Van der Lee and s Ž. rotated from hriŽ., Ä4i 1, M , and the functions Nolet 1997a . grÄiŽ.are a dual basis such that: In practical terms, the three-dimensional model dbŽ.r,u,f is discretized along a Cartesian grid, for 1 a sd which we selected a 70-km spacing in the horizontal H grgrÄkiŽ. Ž.dr ik Ž.6 a 0 directions, and a 40-km one vertically for b.A

Fig. 5. Test of the resolution of a synthetic ``plume'' conceptually similar to the structure detected under the OJP.Ž. a Map view of the input modelŽ.Ž. at 60 km depth . b, c and d Same as Fig. 4c±e for the inversion of the plume model. Ž.Ž.Ž. e and f Same as c and d for a plume composed of depth layers alternatively fast and slow. Note the practical absence of vertical smearing. W.P. Richardson et al.rPhysics of the Earth and Planetary Interiors 118() 2000 29±51 43

Fig. 5Ž. continued . 44 W.P. Richardson et al.rPhysics of the Earth and Planetary Interiors 118() 2000 29±51

Fig. 6. Results of the inversion of the dataset shown in Fig. 2. FramesŽ. a±d show horizontal sections of the seismic structure of the OJP at depths of 20, 60, 100 and 150 km, respectively. FramesŽ. e and Ž. f present west±east and south±north vertical cross-sections Ž in the latter, the vertical exaggeration factors are 4.6 for the structure, and 23 for the bathymetric profile. . W.P. Richardson et al.rPhysics of the Earth and Planetary Interiors 118() 2000 29±51 45

Fig. 6Ž. continued .

25-km vertical spacing was also tested as a verifica- the northern side; the geometry of the other studies tion of the stability of the inversion. The crustal allowed a more homogeneous distribution of sources thickness HŽ.u,f is similarly discretized along a and receivers, resulting in particular in better az- triangular tesselation on the spherical Earth. imuthal coverage. Two series of resolution tests were performed. In 4.1. Resolution tests the first oneŽ. Fig. 4 , we perturbed the JV30 model It is especially important to test the resolution of with a harmonicŽ. checkerboard pattern centered at our inversion since other applications of the PWI 60 km depth and extending from the surface to a methodŽ e.g., Zielhuis and Nolet, 1994; Lebedev et depth of 200 km, with alternately fast and slow, al., 1997; Van der Lee and Nolet, 1997a,b. utilized square, ``dykes'' of 58 Ž.Ž.556 km width Fig. 4a . The larger datasets, especially regarding the number of amplitude of the heterogeneity in the checkers reaches receivers. We use here an asymmetric distribution "0.6 kmrs. The inverted structure shown here at a where all sources are on the southern side of the series of depths ranging from 30 to 100 kmŽ Fig. structure, with a small number of stationsŽ. 4 , all on 4b±d. suffers some level of smearing along the 46 W.P. Richardson et al.rPhysics of the Earth and Planetary Interiors 118() 2000 29±51

Fig. 7. Map of crustal thicknesses obtained in the three-dimensional inversion. W.P. Richardson et al.rPhysics of the Earth and Planetary Interiors 118() 2000 29±51 47

NE±SW direction, which is the predominant path redŽ. ``slow'' cells simply confirm a crust thicker azimuth in our dataset. In addition, and as character- than 30 km, while the outer, green or blueŽ. ``fast'' istically observed in similar tests, the maximum am- cells indicate a crust thinner than 30 km, in agree- plitude of the model anomalies is not fully restored. ment with the results of the one-dimensional inver- Finally, and as expected, the inversion cannot re- sions. The deeper cells centered at or below 60 km solve the frontiers of the study area, such as the Ž.Fig. 6b±d indicate a uniformly slow velocity easternmost tip of the OJP ``L'', where path cover- anomaly, covering the center of the plateau and age is low. However, and within these limitations, reaching both into the Lyra Basin and south of the polarity of the anomaly is recovered in a robust Nauru at depths greater than 100 km. The cross-sec- fashion through the most of the OJP structure, espe- tions presented in Fig. 6e±f also show the resulting cially at the shallower depths, as illustrated in the perturbations in Moho depth, from 21 to 34 km cross-sections shown in Fig. 4e and f. We note in along the W±E section, and from 28 to 36 km along Fig. 4f that there is some trade-off between perturba- the S±N one. These fluctuations are much larger tions in shear velocity and in Moho depth: columns than those observed in the synthetic testsŽ. Fig. 4f , uniformly slow in mantle and crust result in a artifi- while the perturbations in velocity are generally cial sinking of the Moho, while fast ones yield a smaller. Also, and as shown in Fig. 7, the map of raised discontinuity. The amplitude of this effect is inverted crustal thickness is in general agreement "2 km in the example selected. with the results of the individual inversions pre- In the second series of resolution testsŽ. Fig. 5 , sented in Section 3: 28±36 km along the north±south we seek to reproduce a more realistic Earth structure, arm of the ``L'', but only 22±28 km along the namely, a plug or root of the nature suggested by the east±west one. one-dimensional inversions. We define the plug as a Fig. 6e±f also confirm that the anomalous struc- vertical dyke, centered at the bathymetric high of the ture has the shape of a grossly cylindrical root plateauŽ. 48S, 1588E , with a square cross-section of centered around 38S, 1578E, and approximately 600 800 km side, but limited to the southwest at the km in radius, although this number may be enhanced Solomon Trench. It is modeled as having a fast by smearing artifacts. In its upper section, the struc- Ž.q400 mrs crust down to 30 km, and a slow ture expands sideways under the Lyra and south Ž.y400 mrs mantle from 40 to 300 km, with a Nauru BasinsŽ where the ``fast'' anomalies mapped neutral layer between 30 and 40 kmŽ. Fig. 5c . Fig. at shallow depths simply express the thinning of the 5b and d show that all three elements of the structure crust with respect to the reference model. , and under are well restored by the inversion, despite some the Caroline Islands near Pohnpei to the north. The lateral smearing which increases the width of the root is present and resolved down to at least 300 km. plume to 1000 km, and lessens the velocity anomaly. Inverted velocity anomalies in the root are consis- Finally, a vertical anomaly alternating with depth tently y250 to y300 mrs in its upper partŽ above was testedŽ. Fig. 5e±f , with the result that no signifi- 150 km depth. and y200 mrs deeper. Since our cant vertical smearing could be detected for depth resolution tests have generally recovered anomalies bands of 100 km. Fig. 5d and f suggest a depth of smaller in absolute value than were injected into the 300 km as a realistic bound of the resolution of the model, these numbers could represent a lower bound dataset. on <Cretaceous, and which must have Trampert and WoodhouseŽ. 1995 : only at their low- accompanied the plateau as it drifted passively over est Rayleigh frequencies, will the OJP root con- the mantle as part of the Pacific Basin. While such tribute a coherent, detectable anomaly in phase ve- deep-rooted structures have indeed been proposed locity. for continental regionsŽ. Jordan, 1975 , they had not Among global tomographic studies, we note that been previously described under oceanic formations. the model S12_WM13 of Su et al.Ž. 1994 does Other low-velocity bodies underlying LIPs were exhibit a 1.5%±2% low-velocity signal at 200 km identified under the ParanaÂ flood basalts by VanDe- depth in the general area of the OJP. Other models car et al.Ž. 1995 and under northwestern India by based on spherical harmonics expansionsŽ e.g., Li Kennett and WidiyantoroŽ. 1999 ; in both instances, and Romanowicz, 1996. do not reveal a root to the these structures were associated with fossil plumes. OJP, which can be explained by their lack of resolu- There are however significant differences with the tion for a structure of such relatively small horizontal present study. First, the root under the OJP is much dimensions: the degree presently resolvable is lf12, larger, extending 1200 km across as opposed to no corresponding to a wavelength of 3200 km. As for more than 300 km; next, the perturbation in shear body-wave investigations involving a finer, but irreg- velocity reaches 5%, as opposed to 1.5% under India ular, parameterizationŽ e.g., Grand et al., 1997; Bi- and 2.4% under ParanaÂ Ž although Kennett and jwaard et al., 1998; Vasco and Johnson, 1998. , they Widiyantoro caution that the anomaly may be obviously suffer from a lack of sampling over a stronger than reported according to their checker- poorly instrumented region. board resolution tests; different amounts of smearing The existence of a low-velocity root under the in the two studies may not result in directly compa- OJP is supported independently by an analysis of rable anomalies. . Finally, the OJP is attached to an shear-wave splitting at our four portable stations oceanic plate, whereas the other sites are continental; W.P. Richardson et al.rPhysics of the Earth and Planetary Interiors 118() 2000 29±51 49 in the oceanic environment, we also note that a Micronesia was supported by the National Science low-velocity lid, suggestive of a chemical anomaly, Foundation, under grant EAR-93-16396. was found to underlie the crust of the Walvis Ridge, although no anomalously low wavespeeds were found deeper in the low-velocity zoneŽ. Chave, 1979 . We regard it as extremely unlikely that this struc- References ture could be thermally maintained. The observed 5% deficiency in b would require a temperature Arthur, M.A., Kump, L.R., Dean, W.E., Larson, R.L., 1991. anomaly of 350±700 KŽ. Duffy et al., 1995 , which Superplume, supergreenhouse?. EOS Trans. Am. Geophys. would easily melt the mantle and should result in Union 72Ž. 17 , 301, abstract. Bercovici, D., Mahoney, J.J., 1994. Double flood basalts and massive present day volcanism, while not even a plume head separations at the 660-km discontinuity. 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