GEOPHYSICALRESEARCH LETTERS, VOL. 24, NO. 15,PAGES 1955-1958, AUGUST 1, 1997

Seamountabundances and abyssalhill morphology on the eastern flank of the East Pacific Rise at 14øS IngoGrevemeyer, 12Vincent Renard, 3Claudia Jennrich, • and Wilfried Weigel I

Abstract. Bathymetricdata from a Hydrosweepmultibeam hills. Abyssalhills in the PacificOcean form. primarily sonarsurvey of a 720 km longtectonic corridor on theeast througha complexcombination of volcanicconstructional flank of the southernEPR at 14ø14'S coveredabout 25,000 processesand faulting that occur at or nearthe ridgeaxis km2 of zero-ageto 8.5 m.y. old crust(magnetic anomaly [e.g., Golf, 1991; MacdonaMet al., 1996]. Stochastic 4A). In this corridorwe documenta strongcorrelation of analysisof abyssalhills have shownthat ridge flank robustalong flowline changes in abyssalhill morphologyroughness increases with decreasing spreading rate [Menard, and seamountsize distributionwith spreadingrate changes 1967;Malinverno, 199l; Goff, 199l]. Nevertheless,seafloor deducedfrom our magneticdata. Indeed, we find thatboth roughnessvalues show a largevariation along a single rmsheight of abyssalhills andabundance and height of spreadingsegment [Golf, 1991; Goffet al., 1993],suggesting seamountsincrease significantly as spreadingrate changes that spreadingrate cannotbe the solefactor governing from ~ 75 mm/yrto over 85 mm/yr(half rate). Moreover, variationsin abyssalhill morphology. we identified 46 seamountstaller than !.00 m. Previous From November 8 to December 30, 1995 the R/V Sonne studieson the southernEPR reporteda larger densityof carriedout the EXCO-cruise,a geophysicalsurvey on zero- seamounts,organized primarily in chains.Our investigation, age to about8.5 m.y. old seafloor created at the"superfast" however, revealed seamountsnot associatedwith major spreading(full rate >140 mm/yr) East Pacific Rise south of chains,leading us to theconclusion that different forms of the Garrettfracture zone [Weigelet al., 1996]. The cruise off-axisvolcanism occur along the spreadingcenter. exploreda 720 km long and 20-45 km wide tectonic corridor on theeast flank whichintersects the ridge axis 60 km south of the Garrett transformbetween 14øS and a minor ridge Introduction axisdiscontinuity at 14027 ' S. Hydrosweepmultibeam bathy- Seamountsare common featuresin the marine environment metry,coveting approximately 25,000 km 2 along a flowline (Figure2), wereused to delinatevariations in seamount andrepresent a significantpercentage of the seafloor[e.g., Smith and Jordan, 1987; Scheirerand MacrohaM, 1995; abundancesand seafloor roughness. Magnetic reversals were identifiedto determineseafloor spreading velocities. Scheirer et al., 1996a]. In the Pacific Ocean, two main classesof seamountshave been found: large intraplate volcanoesforming seamount and islandchains which are Magnetic Data and Interpretation associatedwith hotspots [Wilson, 1963; Morgan, 1971], and small volcanoes which are often found near mid-ocean Magneticanomalies were obtainedafter removalof the IGRF from total field data, recorded with a proton spreadingridges (Figure 1). Bathymetricdata along the East Pacific Rise (EPR) reveal that a large numberof these precessionmagnetometer towed along the profilesof the seamountsareorganized in chains[Scheirer and MacdonaM, 1995' Scheirer et al., 1996a]. Statistical studies on the -15.2 ø distributionof near-axisseamounts indicate that the bulk of the volcanicactivity occurs between the ridgeaxis and ~ 1

Ma [Scheirerand MacdonaM, 1995; Scheirer et al., 1996b], _ suggestingthat near-axis seamounts are related to large-scale mantleupwelling and the developmentof lithospherenear seafloorspreading centers [Wilson, 1992; Scheirerand -15.25 ø MacdonaM,1995]. However,some seamount chains on the SouthernEast Pacific Rise (SEPR) exhibit fresh lava flows on 5.0-6.5m.y. old crust[Scheirer et al., 1996b]. Despitethe large abundanceof seamounts,the most prevalentgeomorphic structures on theseafloor are abyssal -15.3 ø

•Zentrum ffir Meeres- und Klimaforschungder Universitfit Hamburg,Institut ffir Geophysik,Hamburg, Germany. 2now at Universitfit Bremen, FB Geowissenschaften,Bremen, Germany. -15.35 o 3IFREMER, Centre de Brest,Plouzan6, France. .108 ø -107.95 ø -107.9 ø -107.85 ø Copyright1997 by the American Geophysical Union. Figure 1. Exampleof a near-axisseamount superposed on Papernumber 97GL01820. an abyssalhill. We identifiedas seamountsall local highs 0094-8534/97/97GL-01820505.00 havingplan aspectratios < 2 and relief > 100 m.

1955 1956 GREVEMEYER ET AL.: SEAMOUNT ABUNDANCES AND ABYSSAL HILL MORPHOLOGY

112øW 111 øW 110øW 109øW 108øW 107øW 106øW

20os 17øS 120øW 100øW 80øW Figure2. Thestudy area of theEPR south of theGarrett fracture zone. Complete bathymetric coverage is indicatedby grayshading, and seamounts _>100 m highare shownas filled circles.

survey. Four extendedprofiles normal to the ridge axis tallerthan 200 m [Scheirerand.Macdonald, 1995; $cheirer (Figure 3) were used to match the anomalies with the et al., 1996b].We tabulated34 seamounts> 200 m highin geomagneticpolarity reversalsscale obtained from Cande our 25,000km 2 area.These seamounts have an average and Kent [1992]. These authors showed that south of the abundance(number of seamountsper areaof bathymetric Garrett fracture zone previously published higher rates coverage)of 1..4stats/1000 km 2. towardsthe east (100 mm/yr betweenanamaly 3 and 4A) Seafloorroughness can be characterized in many ways; we could be eliminated by matchingconjugated sets of data usedhere the roughnessdefined as the square-rootof the with best fitting poles of rotation. Becauseof asymmetric averagesquared deviation about a linear trend [Malinverno, spreading,however, these higher rates could be real. Indeed, 1991],i.e., the seafloor roughness is given by rmsheight of using magnetic anom0,1iesfrom Cormier et al. [1996], it abyssalhills. Two linearprofiles were analysed (Figure 3). appearsthat up to anomaly3 seafloorspreading towards the Each of these profiles, derivedfrom swath-mapping eastis around86 mm/yr, while it is only 68 mm/yr towards bathymetryinterpolated on a 100 m gridusing continuous the west, giving an averagehalf rate of 77 mrn/yr. Our data curvaturesplines in tension[Smith and Wessel,1990], is were modeledwith correctionfor the topographyand paleo composedof 16 40-50 km long profile segments.These coordinatesidentical to present. Our best fitting model profile segmentsdo not crossany of the 46 seamounts confirms this high eastwardrate. Furthermore,the good identified above. quality of the data allows an unambiguousidentification of the sequenceof reversalsand points to a still higher rate Results and Discussion duringanomaly 3 (up to 116 mm/yr) followedby an abrupt drop (down to 70 mm/yr) occuringbefore anomaly 3A and The seafloor within 200 km of the ridge crest is lastingup to anomaly4A where our surveyends. Deduced characterizedby arms heightof abyssalhills of 40-50 m. spreadingrate versus distanceis shown in Figure 4. To Typical seamountssuperposed on the crustare lessthan 300 verify whether this pattern on the easternflank (i.e. the m high. At larger distancesfrom the axis, estimatedrms Nazca plate) is due to asymmetricspreading, a conjugate heightincreases, with valuesup to 110 m. In theseregions, datasetfrom the westernflank (i.e. the Pacificplate) would several seamounts rise more than 600 m above the be neededbeyond anomaly 3 and up to 4A. suroundingseabed. Only near the end of the tectonic corridorbeyond 650 km doesthe rms heightdecrease to Bathymetric Data valuesof 40 m with the completeabsence of seamounts higherthan 100 m (Figure4). Wc used 1:250,000Mercator projection maps to identify Of particularinterest is the positivecorrelation between 46 seamountstaller than 100 m. Thesemaps arc basedon increasingrelief of abyssalhills and increasingabundance bathymctrygriddcd at a 100 m spacingand contouredat a and height of seamounts(Figure 4). Other field studies, depth interval of 20 m. Wc identifiedas seamountsall local however,do not supportthis trend: maximum abundance and highshaving map-view aspect ratios <2 and relief >100 m size of seamountsare generallyassociated with shallower (Figure 1). For each seamountwc tabulatedthe positionof and broaderridge crest [Scheirerand Macdonald, 1995], its center, the scamount's distant from the EPR, its summit while abyssalhill rms heightis negativelycorrelated with depth,and its height.Figure 4 displaysthe distributionand the width of the ridge crest[Goff et al., 1993]. In addition height of seamountsversus distant from the ridge axis. In rms heightincreases from the middleof a segmenttoward general, recent studieson the EPR used only seamounts the ends[Goff et al., 1993],though Scheirer and Macdonald GREVEMEYER ET AL.' SEAMOUNT ABUNDANCES AND ABYSSAL HILL MORPHOLOGY 1957

surveydoes not provide completecoverage of the ridge flank, it is reasonableto hypothesizethat the along flowline variationsin seafloormorphology are almost dictated by J 2 2a 3 3a 3b 4 4a variationsin spreadingrate. 200 Althoughthe qualityof bathymetricmaps is very good,the 100 quantity of coverageis rather poor. In total, our survey o provided 4200 line kilometers of Hydrosweepmultibeam -100 bathymetry(-- 25,000km 2) coveringzero-age to 8.5 m.y. -200 '' I'' I'' I'' I'' I'' I'' old crust.Using 1 Myr bins,only about3000 km2 seafloor of the same age have been explored. Obviously, our lOOO statistical estimation with 1.4 smts/1000 km 2 taller than 200 900 m is only an initial assessmenton the size and the 800 distribution of seamounts south of the Garrett fracture zone. 700 Southwards between 15-19øS Scheirer et al. [1996b] 600 estimateda densityof 2.9 smts/1000km 2 whichare higher 500 than 200 m. Most of the observed seamounts are in the Rano 400 Rahi seamountfield on the Pacific plate. But the seamounts 300 on the Pacific flank are not distributeduniformly - thereare 200 twice as many edifices >_200 m tall south of 17øS (4.4 lOO smts/1000km 2) as there are to the north (2.2 smts/1000 o kin2). Size and distributionof volcanicedifices in the Rano -lOO Rahi field on the westernflank are quite different from the eastern flank. Most of the volcanic edifices on the Pacific 2750 flank are arrangedin major chains, while data from the 3000 Nazca plate provided only evidencefor a short seamount 3250 chain near 109ø30'W (Figure 2) and three short chains 3500 between 17ø-18øS [Scheirer et al., 1996a]. But the bulk of edifices eastwards from the EPR is not associated with 3750 4000 chains.It is possiblethat both populationsare causedby different mechanisms.Several mechanismmay accountfor 4250 the emplacementof near-axisseamounts [see $cheirer et al., 45oo 1996b]. However, we argue for a model where short 75 150 225 300 375 450 525 600 675 750 seamount chains are generally caused by independent distancefrom ridge axis [km] buoyant upwelling mini plumes [Barone and Ryan, 1990; Figure 3. (top) Geomagneticreversals scale [Cande and Kent, 1992] and computed magnetic anomalies; (middle) magneticanomalies recorded along the flowline. (bottom) topographicprofiles used to derive seafloor roughnessas 120 spreadingrate describedby Malinverno[1991 ]. Also shownis a subsidence lOO ß lll trendof 360 m (m.y.)-1/2 t1/2. 60 401,,i,,i,, i,,i,, i,,i,, i,,i,, i,,

[1995] found a relativelyuniform distribution of seamounts 1000L, i I,, Iii I,,I, along axis, showing no strong preference for more 800 seamounts seamountsnear discontinuitiesor near midsegments. 600 Furthermore,seamount abundance and seafloorroughness 400 show no common correlationwith spreadingrate. The rms 200 ' '- ' heightof abyssalhills decreasesas spreadingrate increases [Malinverno, 1991; Goff, 1991], while the abundanceof seamountsincreases as the spreadingrate increases[Scheirer 160 L, , I , , I , i I , • I , , I , , I , , I , , I , , I , , I and MacdonaM, 1995]. But seafloorroughness also increases 120 • abyssalhills • going from fast spreading (112 mm/yr) to "ultrafast" spreadingrate (171 mm/yr) [Goff, 1991]. If we considerdata from Goff et al. [1993], the thresholdbetween increasing spreadingrates and decreasingabyssal hill relief and again increasingrelief is near 150 mm/yr (full rate). In Figure 4 0 75 150 225 300 375 450 525 600 675 750 we have plotted rms height and distributionof seamounts alongwith picksof magneticanomalies used to calculatethe distancefrom ridge axis [km] spreadingrate. Indeed, there is a strongcorrelation between Figure4. Seafloorstatistics versus distance from ridge axis' increasingspreading rate, increasingseafloor roughness, the (top) spreadinghalf rate, (middle) height of > 100 m tall emplacementrate and height of seamounts.Although our edifices,and (bottom)rms heightof abyssalhills. 1958 GREVEMEYER ET AL.: SEAMOUNT ABUNDANCES AND ABYSSAL HILL MORPHOLOGY

$hen et al., 1993], while seamountsnot associated with Goff, J. A., A global and regionalstochastic analysis of near- chains may imply melting of mantle heterogeneities ridgeabyssal hill morphology,J. Geophys.Res., 96, 21,713- embedded in the upwelling system beneath the ridge 21,737, 1991. [Wilson,1992]. Moreover,the major seamountchains on the Goff, J. A., A. Malinvemo, D. J. Fomari, and J. R. Cochran, Pacificplate are probablyrelated to someother cause due to Absssalhill segmentation:quantitative analysis of the East unusuallithospheric and/or asthenospheric properties beneath Pacific Rise flanks 7øS-9øS,J. Geophys.Res., 98, 13,85!- 13,862, !993. the Pacific seafloor[$cheirer et al., 1996b].Recently, some Grevemeyer,I., Hotspot-ridgeinteraction in the Indian Ocean: authors argued that patterns of off-axis volcanism and constraintsfrom Geosat/ERMaltimetry, Geophys. J. lnt., 126, asymmetric subsidencemay be related to ridgeward 796-804, 1996. asthenosphericflow from off-axis plume source[Phipps Macdonald,K. C., P. J. Fox, R. T. Alexander,R. Pockalny,and Morgan et al., 1995; Greverneyer,1996]. Asymmetric P. Gente, Volcanic growthfaults and the origin of Pacific subsidenceis evidentalong the SEPR [Cochran,1986], and abyssalhills, Nature,380, 125-129,1996. PhippsMorgan et al. [1995] providedgeophys!cal evidence Malinvemo, A., Inverse square-rootdependence of mid-ocean for a flow connectionbetween the Society hotspotplume ridgeflank roughnesson spreadingrate, Nature, 352, 58-60, and the SEPR. Clearly, much more data from the ridge 1991. flanks will be needed to addressthese questionsin more Menard,H. W., Seafloor spreading,topography and the second detail. layer,Science, 257, 923-924, 1967. Morgan,W. J., Convectionplumes in the lower mantle,Nature, 230, 42-43, 1971. Conclusions PhippsMorgan, J., W. J. Morgan, Y.-S. Zhang,and W. H. F. Smith, Observationalhints, for a plume-fed, suboceanic With swath-bathymetrycoverage of a 720 km longtectonic asthenosphereand its role in mantleconvection, J. Geophys. corridor on the east flank of the SEPR, we characterized Res., 100, 12753-12767, 1995. abyssalhill morphologyand the populationof seamounts Scheirer, D. S., and K. C. Macdonald, Near-axis seamounts on alonga flowline.Modeling of magneticdata suggest that the the flanks of the East Pacific Rise 8øN to 17øN,J. Geophys. Nazcaspreading rate hasbeen between 70-116 mm/yr (half Res., 100, 2239-2259, 1995. Scheirer,D. S., K. C. Macdonald,D. W. Forsyth,S. P. Miller, rate) since magnetic anomaly 4A (about 8.5 Ma). The D. J. Wright, M.-H. Cormier, and C. M. Weiland, A map averagespreading rate has been 85 mm/yr. We foundthat series of the southern East Pacific Rise and its flanks, Mar. rms height of abyssal hills, abundanceand height of Geophys.Res., 18, 1-12, 1996a. seamountsincrease by goingfrom "superfast"spreading rates Scheirer,D. S., K. C. Macdonald,D. W. Forsyth,and Y. Shen, (~ 75 mm/yr) to '•ultrafast"spreading rates (> 85 mm/yr). Abundant seamounts of the Rano Rahi seamount field near There are 46 seamounts in our area taller than 100 m. the southernEast Pacific Rise, 15øSto 19øS,Mar. Geophys., Previousstudies on the west flank revealeda larger density Res., 18, 13-52, 1996b. of seamounts,arranged primarily in major chains[Scheirer Shen,Y., D. W. Forsyth,D. S. Scheirer,and K. C. Macdonald, et al., 1996b]. Of Interest is that we found no evidencefor Two forms of volcanism:implications for mantle flow and largeseamount chains, suggesting that different forms of off- off-axiscrustal production on the westflank of the southem EastPacific Rise, J. Geophys.Res., 98, 17,875-17,889,1993. axis volcanismoccur along the SEPR. Smith, D. K., and T. H. Jordan, The size distribution of Pacific seamounts,Geophys. Res. Lett., 14, 1119-1122,1987. Acknowledgements.We thank the captain,officers, crew and Smith,W. H. F., and P. Wessel,Gridding with continoussplines scientificstaff of the R/V Sonne(leg 105) for their supportat in tension,Geophysics, 55, 293-305, 1990. sea. KayhanAric, ChristlaneHaft, BernhardHustedt, Matthias Weigel,W., I. Grevemeyer,N. Kaul, H. Viilinger,T. Lfidmann, Lurati, Friedrich Roth and Andreas Stracke stood watch and andH. K. Wong,Aging of oceaniccrust at the southernEast greatly assistedwith data collectionat sea. We thank Dan Pacific Rise, EOS, 77 (50), p504, 1996. Scheirerand two anonymousreferees for very carefulreviews Wilson, J. T., ContinentalDrift, ScientificAmerican, 208, 86- which muchimproved the manuscript.This work wasfunded by 100, 1963. the GermanFederal Ministery of Education,Science, Research Wilson, D. S., Focusedmantle upwelling beneathmid-ocean and Technologythrough grant 03G0105. ridges: evidence from seamountformation and isostatic compensationof topography,Earth Planet. Sci. Lett., 113, References 41-55, 1992.

Cande,S.C., andD. V. Kent,A new geomagneticpolarity time I. Grevemeyer,FB Geowissenschaften,Universitfit Bremen, scalefor theLate Cretaceousand Cenozoic, J. Geophys.Res., Postfach 33 04 40, 28334 Bremen, Germany. 97, 13917-13951, 1992. ( greve meyer@ geo ph y s 2. uni-bre men .de ) Cochran,J. R., Variationsin subsidencerate along intermediate V. Renard, IFREMER, Centre de Brest, B.P. 70, 29280 and fast spreadingmid-ocean ridges, Geophys. J. Roy. astr. Plouzan6, France. (Vincent.Renard@ ifremer. fr) Soc., 87, 421-454, 1986. C. Jennrich,W. Weigel, Institut fur Geophysik,Universit/it Cormier, M.-H., D. S. Scheirer,and K. C. Macdonald,Evolution Hamburg,BundesstraBe 55, 20146 Hamburg,Germany. of the East Pacific Rise at ! 6ø-19 ø S since 5 Ma: bisection of overlappingspreading centers by new, rapidly propagating (ReceivedOctober 25, 1996; revisedApril 7, 1997; accepted segments,Mar. Geophys.Res., 18, 53-84, 1996. May 22, 1997.)