JOURNAL OF GEOPHYSICAL RESEARCH, VOL. tOO, NO. B5, PAGES 8097-8114, MAY t0, 1995 Crustal structure of the Tuamotu Plateau, 15øS, and implications for its origin Garrett Ito MIT/WHOI JointProgram in Oceanography,Woods Hole OceanographicInstitution, Woods Hole, Massachusetts Marcia McNutt Departmentof Earth,Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology,Cambridge Richard L. Gibson Earth ResourcesLaboratory, Department of Earth, Atmospheric,and Planetary Sciences MassachusettsInstitute of Technology,Cambridge Abstract. We investigatethe sedimentaryand volcanicstructure of the TuamotuPlateau with multichannelseismic, seismic refraction, and gravity data along a shiptrack crossingthe plateau near 15øS. The volcanicbasement of the centralportion of the plateauis cappedwith a 1 to 2- km-thicksediment layer composedof two compositionalsequences. The uppermostsequence, with semblance-derivedP wave velocities of 1.6-1.9 km/s and thicknessesof 0.2-0.9 km, is composedof pelagic sediments.The underlyingsequence, with velocities2.5-3.5 km/s and thicknessesof 0.5-1.5 km, is composedof limestoneand volcaniclasticsediments. Sonobuoy refractiondata showthe upper 1 km of the volcanicbasement to have velocities4.5-5.5 km/s. The gravity dataindicate that the platformis compensatedby an elasticlithosphere with effective thickness 5+5 km and that the volcanic thickness is 9-10 km thicker than normal oceaniccrust with a volumeof 2.0-2.6x106km 3. Theinferred eruption rates of 0.1-0.13km3/yr are comparableto thoseof the Hawaiian andMarquesas island chains but substantiallyless than thoseof many oceanicplateaus. Radiometric and paleontological ages for the plateauand geomagneticdates of the surroundingseafloor indicate that the northwesternportion of the plateauformed -600 km off the axis of the paleo-Pacific-Farallonspreading center, on lithosphereof age ~ 10-20 Ma. Linear volcanicridges and scarpsbounding deep sediment-filled basins,however, are similarto featuresof oceanicplateaus which formedat or near accretionary plateboundaries. We suggestthat these volcanic ridges and the grossplateau like morphology were formedby magmathat was channelledalong the lithosphericdiscontinuities left behindby a southwardpropagating rift segmentof the nearbyspreading center. We attributethe formation of the northwesternportion of the TuamotuPlateau to the passageof two hotspotsduring times 50-30 Ma as they migratedbeneath the Pacific platebut remainedwest of the Tuamotu propagator. Introduction areless than magnetic ages of theunderlying lithosphere [Jarrard and Clague, 1977; Henderson,1985], mostisland chains were Hotspotsin the mantle, are thoughtto be the sourcesof many formedmidplate, far from oceanicspreading centers. As a result, crustal anomaliesin the world's ocean basins. Oceanic hotspot most island chainsare compensatedby mechanicallycompetent features can be separatedinto two classes: island chains and lithospherewith effectiveelastic thicknesses (typically 10-25 km) oceanic plateaus. The morphological characteristics that thatthicken with age,consistent with a conductivecooling model distinguishthese two classesmay reflect differences in the for the lithosphere[Watts, 1978; Watts et al., 1980]. tectonic environments at which they formed and/or the mantle Oceanicplateaus, on theother hand, are defined by theirbroad sourceswhich producedthese melt anomalies. Ocean island elevated surfaces which, in many cases (e.g., Ontong Java, chains are composed of discrete volcanic edifices with ShatskyRise, and Manihiki), are nonlinear in planview andlack geographic age distributions reflecting the motion of the the geographicage distributions so typicalamong island chains. lithospherewith respectto the hotspotreference frame [Duncan Most oceanicplateaus are thoughtto have originatedfrom and McDougall, 1976; Clague and Jarrard, 1973; Morgan, hotspotssited near or at oceanicspreading centers or rifted 1972]. As demonstratedby isotopicand paleontologicalages that continentalmargins because (1) edificeages are close to thoseof the surroundingseafloor [Detrick et al., 1977]; (2) subsidence Copyright1995 by the AmericanGeophysical Union. histories of oceanic plateaus match closely those of the surroundingseafloor [Detrick et al., 1977]; and (3) oceanic Papernumber 95JB00071. plateausare compensated by Airy isostasyindicating they loaded 0148-0227/95/95JB-000710505.00 thin elastic lithospheresfound near ocean spreadingcenters 8O97 8098 ITO ET AL.' CRUSTAL STRUCTURE AND ORIGIN OF THE TUAMOTU PLATEAU [Detrick and Watts, 1979; Kogan, 1979; Sandwell and easternportion of TuamotuPlateau may have beenemplaced near MacKenzie, 1989]. In addition, plateaus such as Manihiki the paleo-Pacific-Farallonspreading center, as suggestedby the [Winterer et al., 1974] and Kerguelen [Munschy and Schlich, underlying weak lithospherewith effective elasticthickness of 2- 1987] show faulted structureswhich may have resulted from 6 km [Okal and Cazenave, 1985]. extensionalprocesses at an oceanicspreading center. The broad The nearly parallel alignment of the Line Islands-Tuamotu continuousextent of oceanicplateaus may result from efficient chain with the Hawaiian-Emperorchain led Morgan [1972] to penetrationof heat and magmathrough the weak lithosphereat hypothesize that the two were formed concurrentlyby two oceanicspreading centers [Vogt, 1974; Okal and Batiza, 1987]. stationary hotspots. He suggestedthat the northwesternmost Volcanic volumesof oceanicplateaus, representing 5-25% of the portionof Tuamotuplatform marked the changein plate motion world's total volcanicproduction for the past 150 m.y. [Larson, recordedby the bend in the Hawaiian-Emperorchain datedat 42- 1991], are typically much larger than the volumes of island 43 Ma [Clague and Jarrard, 1973]. Subsequentpaleontologic chainsand may be the productsof massiveflood basalteruptions and radiometric age constraints, however, indicate that the generatedat the onsetof mantleplume activity [Morgan, 1972, northernmost Tuamotu islands formed as late as 47-55 Ma 1981; Richards et al., 1989]. [Martini, 1976; Jacksonand Schlanger,1976; Schlangeret al., The Tuamotu Plateau, in the south central Pacific, is a volcanic 1984], thuspreceding the Hawaiian-Emperorbend by as muchas feature with characteristics of both island chains and oceanic 9 m.y. Several hotspotsare suggestedto have formed the Line plateaus(Figure 1). From a regionalperspective, the platform Islandschain, based on the geochronologicstudies of Schlanger appearsplateau like, with a broadelevated surface of width80- et al. [1984] and Winterer [1976], while two hotspots are 200 km. A closer look, however, reveals discrete volcanic hypothesizedto have formed the Tuamotu Islands from tectonic pinnacles,the largestof which are now reef-coveredatolls. The studiesof Okal and Cazenave[1985]. Thus, the origin of the platformis a linear featuretrending from northwestto southeast platform may be more complex than the single hotspot similar to Pacific island chains such as the Hawaiian chain, the mechanismoriginally suggestedby Morgan [ 1972]. Cook-Austral chain, and the Society Islands. However, the In this paper we investigate the sedimentary and volcanic a) b) -10ø / •.• '--;•--,. '- Marquesas.• ,, •- • • -• !"•'- •C•'•'•.•:......• Tnamotn '•, •'• • •- ! I• •s•:•:•:•.'•.-3 Islands • •,..•.'..,..... •" '.,'½•?, ;•.... • '•:.,> '• ½•' •.•'"-•.,-, • ', , x H •,•., • ....• •.i'• • x•' x_,•--•, '--,, ' x.... _20• • -,.%-.,. •h --W.-•.>.•,.-., .................. [•:' • Austral ".• • - •e/" •-'=•"..... • c• • ..... l I / 205 ø 210 ø 215 ø 220 ø 225 ø Longitude Figure 1. (a) Regionalmap surroundingthe TuamotuPlateau contouring the 3-km isobath. The rectangle outlinesthe bathymetrymap below. (b) Bathymetrymap of the Tuamotuplatform with 1-km contourinterval. The solidline marksthe surveyline whichcrosses the northernportion of the platformin the regionmarked by the rectangle. ITO ET AL.- CRUSTAL STRUCTURE AND ORIGIN OF THE TUAMOTU PLATEAU 8099 _14 ø _15 ø 211 ø 212 ø 213 ø 214 ø Longitude -5000 -4000 -3000 -2000 - 1000 -0 Depth(m) Figure2. Bathymetrymap of thesurvey region (rectangle in Figure lb) witha contourinterval of 500m. The locationsof thesonobuoy deployments are marked by X' s alongthe survey track shown as solid white. Sections A, B, andC of thesurvey line are sediment basins revealed in thereflection profile. Dashed lines mark lineations of volcanicridges and scarps inferred from the volcanic morphology observed in thereflection profile. structure of the northwestern end of the Tuamotu Plateau in order Gravity data were collectedwith a KSS-30 gravimeter,while to better understand the tectonic and magmatic processes bathymetrywas determined from the centerbeam readings of the controllingits formation. We imagethe shallowcrustal structure Hydrosweepmultibeam system. The bathymetrysurrounding our (upper-2 km) with multichannelseismic (MCS) reflectiondata shiptrack are digitized points from bathymetrymaps produced at obtainedalong a ship crossingof the northernportion of the the Institut Francois de Rechere pour l'Exploitation de la Mer platform. The velocitystructure used for stackingthe reflection (IFREMER) and are from the Digital BathymetricData Base 5 dataand for interpretationof sedimentarycomposition is derived (DBDB5) data set (Figure 2). Also shownin Figure 2 is the from semblanceanalyses of the MCS data and from sonobuoy locationof Deep Sea Drilling Project (DSDP) site 318, which refraction data. To constrainthe deep crustal structureand penetrated745