GEOPHYSICALRESEARCH LETTERS, VOL. 17, NO.8, PAGES1101-1104, JULY 1990

THE DARWIN RISE: A SUPERSWELL?

M.K. McNutt, E.L. Winterer,W.W. Sager,J.H. Natland,and G. Ito

Abstract.The JapaneseGuyots, Wake ,and Mid- ?-•cificMountains are part of a broad area of Cretaceous volcanismin the westernPacific termedthe "Darwin Rise." Basedon Seabeam bathymetric data we classifythese drowned volcanicislands as: type "A," thosethat advancedto the ragebefore f'mal submergence; type "B," thosethat drowned atthe barrier reef stage;and type "V," thosewith little or no reefmaterial on theirvolcanic summits. Widespread evidence forkarst topography extending to depthsof 200 m on the summitsof A andB guyotssheds new light on eventsleading mthe synchronous extinction of reefson the Darwin Rise in t• mid-Cretaceous.We proposethat afterthe formation of the reefson the A andB guyots,the entireregion was elevatedat approximatelytheAptian- boundary (113 Ma) to forma I superswellsimilar to thatexisting now in FrenchPolynesia. 150øE 160øE 170øE 180øE •e type V guyots formed on this anomalouslyshallow I•thosphere.The demiseof thereefs was the direct result of the Fig. 1. Locationof guyotsof the northernDarwin Rise in the riseof this superswell,although climate factors may have W•sternPacific used in this study. tabventedreef recolonization following its later subsidence. The Data Inmoducfion Table 1 summarizesinformation on guyorsused in this The westernPacific contains numerousguyots whose study. The morphologicand sediment-thicknessdata for the originsand geologic histories are unclear. Most seemto have first 22 guyotswere collected during Roundabout Leg 10; the formedduring the Cretaceouson lithospherethat was too data for the otherfour guymsof the Mid-Pacific Mountains shallowatthe time of volcanismgiven its ageand have summit were taken from Kroenke et al. [1985] and Nemoto and depthsshallower than expected for theelapsed time since they Kroenke[1985]. For theJapanese and Wakes, present age of drowned[Menard, 1964]. It has beenproposed that these theplate was taken from Nakanishi et al. [ 1989]. For theMid- volcanoesformed during widespreadvolcanic events that Pacs, where Mesozoic series linearions have been obliterated waricedthe Apfian and later Cretaceous stages [Schlanger et aL, by volcanismon the MidPacificplateau, we assigngenerous 1981]associated with anomalouslyshallow seafloor (the boundson lithospheric age as bracketed by identifiedmagnetic "DarwinRise" [Menard, 1964]). A morerecent and uniformi- linearionson itsperimeter [Winterer and Metzler, 1984]. tarianview is thatthe Cretaceous guyots formed in a setting Lithosphericdepth is measuredat the base of eachguyot and s•nilarto present-dayFrench Polynesia [Menard, 1984; isostaticallycorrected for the thin pelagicsediments using McYut•and Fischer, 1987]. McNutt and Fischer.[ 1987] term watergun profiles. Correctionfor any flexural moats is thisregion of elevated seafloor in the South Pacific the unnecessary,because gravity and seismic data reveal that they "Superswell"and suggest a link to theDarwin Rise based on have been filled with acousticallyopaque material to the .thesimilar magnitude of theirdepth anomalies at thetime of regionaldepth of the surroundingseafloor. For the MidPats, volcanism[Menard, 1964]; their common high vulnerability to the depthis takenat theedge of theplateau upon which the volcanismalong short, frequently overprinting, radiogenically guyots are superimposed.The summitdepth is the last, enrichedhot-spot chains [Heezen et al., 1973; Duncan and highestsea-level stand recorded on each . Taikuyo-daini McDougalI,1976; Hart, 1984;Smith et al., 1989]; and andSeiko lie on theflexural arch of theJapan Trench. Their absoluteplate-motion models which tracethe sourceof the summitdepths and lithospheric depths have been increased by roMplateCretaceous volcanism to theFrench Polynesian hot 300 m to removethis effect as calculatedfrom unpublished spots[Gordon and Henderson, 1985]. multi-channelseismic profiles from Lamont-Doherty crossing In I988 we collectedmarine geophysical data on cruise thearch just north and just south of thesetwo guyms. RoundaboutLeg 10 of theR/V ThomasWashington in the Basedon summitmorphology, each guyot is classifiedas Japanese,Wake, and Mid-Pacific chains (Figure 1) type"A", for atoll(rim enclosinga shallowdepression); type whichcomprise the northern part of theDarwin Rise. Here we "B", for barrier reef (rim surroundingcentral volcanic addressthree issues with the data from this expedition: pinnacle);or type "V", for volcanic(gently convex-upward I) Doesthe paleodepth/age curve for the Darwin Rise support summit with no acousticpenetration). A velocityof 2600 m/s thehypothesis that it formedin a Cretaceoussetting very was assumedin converting airgun profiles to sediment s•milartothat of the present-day Superswe!l? If so, thickness.In no placedid we fail to observebasement beneath 2)Does the superswell model account for the sea-level history the reefs. AII of the B and severalof the A guyorsshow of the carbonatereefs of the northernDarwin Rise, evidence of limestone dissolution on their summits. On B particularlythe deepkarsting we observedon the reef guyms,the floor of the lagoonlies at least 180 m below the surfacesand their eventual drowning? reef rim, comparedto valuesof 20 to 40 m for modem-day ,suggesting that sediments have been dissolved or eroded 3)•Wh•e present-dayatis the long-term morphology fateofofthe the SuperswellDarwin Rise? asinferred from from thelagoon. The only sedimentdetected on B guyotsby the the profiling systemconsists of isolated pocketsin topographic1ows. Clear evidencefor sink holesand dolines Copyright1990 by the AmericanGeophysical Union, was observed on MIT (Figure 2), Vibelius, Huevo, and Allison (the lattereven has a low-standterrace 200 m below the Papernumber 90GL00746 reef rim), Given that no Seabeamdata exist for Sio South, 0094-8276/90/90GL-00746503.00 Harvey, Thomas,and Allen and thatsummit surveys for most 102 McNutt et al.: The Darwin Rise:A CretaceousSuperswell?

Table 1. Darwin Rise Guyots Lith Age Lith Summit Sed Smt Mag Paleomag Seamount Guyot (ma) Depm Depth Thick Depth Age Polarity Age Group N,'mue ½m) (m) T.• (m) (m) (Ma• * (Ma) Japanese Taikuyo-da/ni 142 5800 1750 B 50 180 N (7) Seiko 142 5800 1720 B 100 180 102+3 N 85-120 Winterer 146 5800 1450 V 0 N C Johnson 150 5900 1750 B 130 180 (9)M > 118 Stout 152 5800 1580 V 0 N 85-120 T Washington 152 5900 1500 V 0 N 85-120 Isakov 150 6000 1460 V 0 N 85-120 Makarov 157 6000 1390 V 0 94+1 N 85-120 MIT 160 6100 1390 A 650 200 R >118 Wakes Scripps 160-169 5800 1300 V 0 98+3 N 85-120 160-169 5600 1330 V 0 87+4 N Pot 160-169 5700 1390 V 0 N Vibelius 160-169 5800 2000 A 650 N Wilde 160-169 5600 1410 V 0 86+2 N 85-120 I WoodsLamont Hole 160-169 5600 1130 V 0 N 85-120 lVtid Pacs Darwin 155 5850 1350 A 500 none R > 118 Heezen 135-156 5850 1300 A 850 N (?) Huevo 135-156 5850 1320 A 1000 100 Unnamed 135-156 5850 1650 A 450 Caprina 135-156 5850 1650 A 780 Jacqueline 135-156 5850 1650 A 650 (?)M Allison 135-156 5850 1650 A 6OO 200 Sio South 1.56-169 5850 1270 A 750 no data (?)N >120 Harvey 156-169 5850 1050 A (?) no data N (?) Thomas 156-169 5850 1287 A 910 no data N >120 x / Allen 135-156 5850 1376 A 585 no data N >120 '*N=normal;'•-J--reversed; M=mix'ed of the other type A guyotsconcentrated on the more resistant Sager, 1989] were used as additionalage constraints.Based reef rim, it may be that karst surfacesare more widespread on palcopolepostions, the A guyotsare the oldest(>118 Ma), amongstthe drownedatolls than indicated by Table 1. whereasthe type V are the youngest(80-120 Ma). Similarly, Radiometricages in Table 1 arepresently available for only a their magneticanomalies fall into two categories.The V's have very few volcanoes [Ozima et al., 1970, 1977; R. Duncan, large amplitudeanomalies and normal polarities,implying pers.commun.]. Paleontologicalestimates of minimum ages formationduring the CretaceousLong Normal Period. The from rudists, molluscs, and planktonic assemblagesare A's have smaller, often complex anomaliessuggestive of reported in Matthews et al. [1974], who conclude that the formationduring the period of reversingpolarity that preceeded Japaneseand MidPacs are age or older and that the the Quiet Period. Of the threetype B guyots,Charlie Johnson Wakes are older than Eocene but younger than Aptian. has an anomaly typical of the A type, whereasSeiko and Magnetic character and palcopole positions relative to the Tailmyo-Dainihave anomalies similar to theV types. Pacific apparentpolar-wander path [Sager and PringIe, 1988; PalcodepthVersus Age

,t'•i,i•Course:230 ø, •j?eed:8.7 Knotsi[?;'i•[8 Nov. 88 Assumingthat these guyots were all formerlyislands eroded •.•-,••....,....••.;,•. :•,•,•_•....•,•':•<•g•;'•;•'i•;3,•'.•.•',•':•';E; at sealevel soonafter formation,the heightof the volcanic summit(beneath any sediment)above the surroundingseafloor is a measureof the depth of the lithosphereat the timeof volcanism[Menard, 1964]. Despitethe looseage constraiv•s as outlinedabove, the depth-agerelationship at the timeof volcanismfor the typeA andB guyotsusing informanon from Table 1 is consistentwith Parsonsand Sclater's [ 1977] depth- --2..I s age relationshipfor normal seafloorin the North Pacific (Figure 3). In contrast,the palcodepthsfor the typeV guyors --2.3s follow the present-daydepth versus age for the Superswell [McNutt and Fischer, 1987], well above the standardcurie. The foregoinganalysis assumes that the reefs on the guyots -2.5s are totallyuncompensated, i.e., the weightof a reef capdoes not cause a seamountto subside. This assumptiondoes not

2.7s affectthe palcodepth/age relationship for the type B andtype ¾ guyots,with little or no sedimentat present.Allowing for reboundof the summitsof the type A guyorsafter sediment removalincreases their palcodepths by severalhundred meters

...... (Figure3), andthus strengthens the conclusion that the type A I I I I I I guyotsdid not form on anomalouslyshallow lithosphere. Fig. 2, Processedseismic section (automatic gain control, bandpass40-110 Hz) across the summit of MIT guyot, History of the Darwin Rise showinglagoonal layers cut by an erosionalsurface with a total relief of 100 m. Maximum depthof sinkholesfrom Seabeam The evidencefrom palcodepths that a superswellmay ha•e recordselsewhere on this guyot is 180 m. We interpret the existedat thetime of formationof thetype V ,but strongreflector at 2.3 s to be the top of basalticbasement. notearlier, leads to thefollowing scenario for thevertical McNuttet al.:The Darwin Rise: A CretaceousSuperswell? 1• 03

2000 ß i ....- ß i - i ß ! ß i - i - period(110 to 120Ma), the type B guyotsformed. An earlier O Isostasyneglected datewould be inconsistent with their palcomagnetism andtheir 3000 A Correctedforisostasy . smallamount of subsidence.Reef caps formed on their sub- sidingsummits until time t2. Thereefs stopped building after timet2 and by t3, the reefs were being dissolved subaerially. A - B 4000 Webelieve that reef extinction was caused by a relative fall in sealevel that was tectonic, rather than eustatic inorgin, because duringthis time of long-term eustatic sea level rise [Haq et al., 5000 1987],the reefs of thecentral Pacific rose at least200 m above sealevel over an areawhich extends 40 ø across the Pacific. ß Thepresent-day Superswell inFrench Polynesia has similar 125-km Plate Model ' lateraldimensions [McNutt and Fischer, 1987], and the depth anomalyis composedof twopans [McNutt and Judge, 1989]. Thefirst component isdynamic uplift of 200-250m causedby 7000 ...... convectionin theasthenosphere, affecting the entire region of 0 20 40 60 80 100 120 140 160 180 theSuperswell. The uplift could occur in a timeperiod of less Age at Loading (Ma) than 10 m.y. (comparableto the rise time of the Hawaiian swell)and would lead to emergenceof existingshore lines. •• Palcodepthversus age of thelithosphere at thetime of The secondcomponent is lithosphericthinning caused by loadingfor the guyotsof the Darwin Rise. "A", "B", "V-Jap", enhancedheat flux from the mantle and reduced -Ascosity at the and"V-Wake" refer to the averagepalcodepth and agevalues baseof the lithospherethat leadsto convectiveinstabilities. forall guyotswith thatclassification. Error bars on palcodepth The lithospherethinned to approximately75 km subsidesat a reflectonly the scatterin the population. Error bars on age slowerrate and eventually leads to depthanomalies exceeding reflectscatter in the populationas well asuncertainty in the age 750 m with respectto the 125-kmplate model,but never ofthe lithosphere as shownin Table 1. actuallyelevates sea floor with respect to sealevel. We propose that a thermal disturbancesimilar to that of FrenchPolynesia occurred beneath the Darwin Rise at time t2, motionsin the CretaceousPacific (Figure4). Between135 and causing200-250 m of uplift with respectto sealevel for the 169Ma (time tO),the lithospherebeneath the Darwin Rise was preexistingatolls and barrier reefs. Duringthe period t3 to t4, createdand subsidedas a normal, 125-km-thickthermal plate. the Darwin Rise lithospheresubsided at the rate of a 75-km- Atsome later time tl (about120 to 140 Ma basedon their palc- thickplate, producing anomalously shallow palcodepths for the omagnetismand local compensation),the volcanicfoundations type V guyors,which erupted during this period on the thin, forthe type A guyotserupted. Volcanismwas predominantly vulnerablelithosphere. For theA andB guyors,this period is onthe MidPac plateau, probably in a settingmuch like that of marked by a 20- to 25-m.y. hiatusin sedimentationon their summits until the accumulated subsidence of the 75-km thick theTuamotu Islands in the early Tertiary. The Pacific plate wasmoving slowly southwest[Winterer and Metzler, 1984] plateexceeded the initial uplift. Becausethe DarwinRise does froma hot spotthat was located near Easter Island. Isolated not presently display geoid and surface wave anomalies volcanoes(including MIT and Vibelius) formed farther west. indicativeof a thinnedlithosphere and a hot, low-viscosity It tookbetween 10 to 20 my for their carbonatereefs to form asthenosphere[McNutt and Judge, 1989], at sometime r4 in (timeinterval t2-tj in Figure 4). Towards the end of this time the past the lithosphereof the Darwin Rise beganto thicken again and subsideat the rate of a 125-kin-thickplate. The changein thermalregime may have resulted from motion of the plate with respect to the upwelling asthenosphereor a t5 t4 13t2 tl 0 500 ..... - , - ', ß ',' - ...... reorganizationof the convectionpattern. - Sea•uel , ' ...... ' ....' 'l ' ,' ' ' The geologicaland geophysicaldata from the Darwin Rise allow us to constrainthe absoluteages of the eventst2, t3, and -500 ' t4, which must be synchronousfor all guyms. Lacking finn dateson the agesof the A andB guyms,the time t2 of reef -1500 extinctioncan be estimatedin two ways(Figure 4). For the A guyotson well-datedlithosphere, tO-tl emd tl-t2 are estimated • -2500 from the palcodepthand thicknessof reef cap, respectively. For A guyots on poorly dated lithosphere(most of the • -3500 MidPacs), the estimateof t2 is lesssensitive to the assumedtO if thepresent summit depth of theguyot is usedto estimatethe time intervalt3-tS. In backtrackingthe present summit depths -4500 D inRise .. • to time t2, we assumethat t2-t3 is small (only a few million years) and that t4 must be greaterthan 70 Ma basedon the present-daylithospheric depth and less than 80 Ma basedon ' theages and palcodepths for theyoungest type-V guyots. 0 20 40 60 80 100 120 140 The time of extinction of the reefs calculated in these two Time (Ma) ways is remarkablysynchronous at 113:t:8Ma (exceptfor Vibelius, which has an anomalouslydeep summitprobably • Schematicmodel for vertical motionson the Darwin causedby flexuralloading from theyounger nearby volcano, Risefor the specificcase of a 140-Mavolcano on 155-Ma Wilde). This resultis consistentwith the 118-Ma age for the lithosphere.Between times tO when the lithosphere forms and youngestguyot-derived, shallow-water sediments drilled at t2,the lithosphere subsides as a 125-1cm-thickthermal plate. At DSDP site 463 on a fan of Huevo [Sayre, 1981] and fits well timet2 the seafloor is upliftedto forma superswell.Between with the Cenomanianto Turonianage (97-87 Ma) for pelagic t:•and t4 the lithospheresubsides at the rate of a 75-km-thick chalks dredgedfrom JacquelineGuyot that could only be thermalplate with effectivethermal age at t3given by its depth depositedafter the 20- to 25-m.y. hiatus(Figure 4) following attime 0. Betweentimes t4 and t$ (the present),the plate uplift [Matthews et al., 1974]. Assumingt2=l 13 Ma and resumesthe subsidencerate of a 125-km-thick plate, with t4=70 Ma also providesa goodfit to the palcodepthsand effectiveage.at t4 given by its depth at t4. present-daysummit depths of thetype V guyotsof knownage. 110t• McNuttet al.:The Darwin Rise: A CretaceousSuperswell?

The existenceof an ancientsuperswell in the mid-Cretaceous Kroenke,L., J.N. Kellogg,and K. Nemoto,Mid-Pacific coincideswith the timeperiod when the DarwinRise lay over Mountainsrevisited, Geo-Marine Lens., 5, 77-81,1985. the sameregion of mantlepresently beneath Matthews,J.L., B.C. Heezen,R. Catalano,A. Coogan,M. [Gordon and Henderson,1985], and its initiation may be Tharp,J. Natland, and M. Rawson,Cretaceous drowning of linked to a shift in plate motion,perhaps even more dramatic reefson Mid-Pacific and Japanese guyors, Science, 184, than the onerecorded by the Hawaiian-Emperorbend. The 462-464, 1974. poleof rotationmigrated to a position90 ø awayfrom the South McNutt, M.K. and K.M. Fischer,The SouthPacific Pacific, the angularrate of rotationdoubled, and the plate superswell,in Seamounts,Islands, and Atolls,B.H. startedmoving rapidly to the northwest(~80 mm/yr) alongan Keating,P. Fryer,R. Batiza,and G.W. Boehlert, eds., azimuthdefined by thelapanese and Wake seamountlines. GeophysicalMonograph 43,American Geophysical Union, This modelstill doesnot explainwhy the A and B guyots Washington,D.C., 1987. did not later reestablishreef communities,why the Japanese McNutt,M. K., andA.V. Judge,The Superswell and mantle type-V reefs died almostimmediately, and why the Wakes dynamicsbeneath the South Pacific, Science, inpress, 1990. neverbore reefs. Subsidencerates due to coolingof the plate Menard,H.W., Marine Geology of thePacific, 271 pp., (0.01 mrn/yr), motion with respectto the dynamicmantle McGraw-Hill, New York, 1964. upwelling (0.1 mm/yr), and even collapseof the dynamic Menard,H.W., DarwinReprise, .L Geophys.Res., 89,9960- component(10 mm/yr)all occurat rates too slow to outstripthe 9968, 1984. present-dayestimate of growth underfavorable circum- Nakanishi,M., K. Tamaki, and K. Kobayashi,Mesozoic stances,although Cretaceous rudist reefs may have grown magneticanomaly lineations and seafloor spreading history moreslowly. Climaticfactors linked to sea-surfacetempera- of the NorthwesternPacific, J. Geophys.Res., 94, 15437- ture, species-dispersalpatterns, or true eustaticsea-level rise 15,462, 1989. may still be invokedto explainwhy, whensome 20 my earlier Nemoto,K. and L.W. Kroenke,Sio Guyot:a complex the type A reefsflourished, they were not ableto reestablish volcanicedifice in thewestern Mid-Pacific Mountains, Geo. communitieswhen they once again sank below the seasurface. Marine Letts., 5, 83-89, 1985. Ozima,M., M. Honda,and K. Saito,40Ar-39Ar ages of Conclusions guyots in the western Pacific and discussion of their evolution, Geophys.J.R. astr. Soc.,51,475-485, 1977. Ozima, M., I. Kanoeka,and S. Aramaki,K-Ar agesof The palcodepth/agerelationship for the typeV guyotsin the submarinebasalts dredged from seamountsin the western Japaneseand Wake seamountchains supports the hypothesis Pacific area and discussionof oceaniccrust, Earth Planet. that they formedon an ancientsuperswell. The uplift of the Sci. Letts., 8, 237-249, 1970. superswelloccurred at about113+8 Ma (Aptian-Albiantime) Parsons,B. and J. Sclater, An analysisof the variationof basedon thereef thicknesses and current summit depths of the oceanfloor bathymetry and heat flow with age,J. Geophys. guyots. The uplift of this Cretaceoussuperswell can explain Res., 82,803-827, 1977. the 200-m emergenceand karsting of the reefson the MidPacs Sager,W.W., Seamountages estimates from paleomagnetisrn andthe otherisolated older guyors. The present-daydepth of andtheir implications, in The Geologyand OffshoreMineral theDarwin Rise is shallowerthan normal lithosphere of similar Resourcesof the CentralPacific Basin, B. KeatingaM B• age, but the depthanomaly can be explainedas the residual Bolton, eds, submitted, 1989. effectsof mid-Cretaceousreheating. This result suggests that the depthanomaly in FrenchPolynesia is alsotransient. As Sager,W.W. and M.S. Pringle,Mid-Cretaceous to Early Tertiaryapparent polar wander path of the Pacificplate, J. soon as the plate drifts off the mantle upwelling (or the Geophys.Res., 93, 353-369, 1987. upwellingitself tums off), thelithosphere will beginto subside as a normal, 125-km-thick plate. The fact that the oldest Sayre, W.O., Preliminary report on the paleomagnefismof lithospherein the Pacific is also lithosphereaffected by AptJanand Albian limestonesand trachytesfrom theMid- superswell-typevolcanism may not be a coincidence. The PacificMountains and Hess Rise, DeepSea Drilling Project residualthermal effects of platereheating, as well asthe locally Leg 62, Init. Rep. DSDP, 62, J. Thiede and T. L. Va!Iier, thickenedcrust beneath the seamountsand plateaus,may eds., U.S. Gov. Printing Office, Washington,D.C., 983- counteractthe negativebuoyancy that facilitatessubduction of 994, 1981. extremelyold lithosphere. Schlanger, S.O., H.C. Jenkyns, and I. Premoli-Si!va, Volcanism and vertical tectonics in the Pacific basin related m Acknowledgements.We thankSy Schlangerfor comments. global Cretaceoustransgressions, Earth Planet. Sci.Letts., This work was supportedby NSF-ODP-8717826-OCE to 52,435-449, !981. MKM, NSF-ODP-8717079-OCE to ELW, and NSF-ODP- Smith, W.H.F., H. Staudigel,A.B. Watts, and M.S. Pringle, 8715733to WWS. GI was supportedby NSF's Reseach The Magellan Seamounts:early Cretaceousrecord of •e Experiencesfor Undergraduates Program through MIT. South Pacific isotopicand thermal anomaly,J. Geophys. Res., 94, 10501-10523, 1989. References Winterer,E.L. and C.V. Metzler, Origin and subsidenceof guyotsin Mid-Pacific Mountains,J. Geophys.Res., 89, 9969-9979, 1984. Duncan,R.A. and I. McDougall, Linear volcanismin French

Polynesia,J. Volcan. Geotherm.Res., 1,197-227, 1976. ,, Gordon, R.G. and L.J. Henderson,Pacific plate hot spot G. Ito, 6013 E. Mineral, Englewood,CO 80112 tracks,unpub. preprint, I985. M. McNutt, 54-826, MIT, Cambridge,MA 02139 Haq, B.U., J. Hardenbol, and P.R. Vail, Chronology of J. Natland and E.L. Winterer, GeoloicaIResearch Division, fluctuatingsea levels since the Triassic (250 million years ago ScrippsInstitution of Oceanography,La Jolla,CA 92093 to present),Science, 235, 1156-1167, 1987. W. Sager,Department of Oceanography,Texas A&M Hart, S.R., A large-scaleisotope anomaly in the Southern University,College Station, TX 77843 Hemispheremantle, Nature, 309, 753-757, 1984. Heezen, B.C., J.L. Matthews, R. Catalano, J. Natland, A. Coogan, M. Tharp, and M. Rawson, Western Pacific guyors, Init. Rep. DSDP, 20, B.C. Heezen and I.D. (Received November 30, 1989; MacGregor,eds., U.S. Gov. PrintingOffice, Washington, revised March ! 2, 1990; D.C., 653-723, 1973. acceptedMarch 15, 1990)