alexander 1996 epsl

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EPSLELSEVIER Earth and Planetary Science Letters 139 (1996) 387-394

Small off-axis volcanoes on the East Pacific RiseRuss T. Alexander, Ken C. Macdonald *

Deparhnent of Geological Sciences and Marine Sciences Inst it ure, Univ ersity of California a t Santa Barbara, Sant a Barbara, CA 93106,USA

Received 24 July 1995; accepted 7 February 1996

A study of Sea Beam bathymetry and SeaMARC II side-scan sonar data allows a quantitative measure of the contributionof off-axis volcanism to the creation of abyssal hill topography on the East Pacific Rise (EPR) 915N-950N. In order toassess the role of off-axis volcanism, the distribution of volcanic edifices within 35 km of the ridge was determined. Wemeasure the size and location of 55 edifices defined as local highs greater than or equal to 40 m with aspect ratios less than2. The distribution of small volcanic edifices is notably different from that of larger volcanic constructions (seamounts) inthat the former do not appear to be organized into discrete chains. The volcanic edifices form 5-10 km off-axis and aretypically 40-70 m high. in contrast, the seamounts (2 200 m high) measured by Scheirer and Macdonald [ 11 fit formbetween 5 and 15 km off-axis and then continue to grow in volume and size over a distance 2-4 times larger. The smalleredifices make a minute contribution to crustal volume (0.02-0.03%) but can cover 7-11% of the mature seafloor.Seamounts contribute - 0.3- 1% to the volume of oceanic crust and cover only - 6% of the seafloor [1,2]. We propose thatwithin 10 km of the ridge, seamounts and smaller volcanic edifices compete for a limited magma supply. The mechanism ofmelt delivery to seamounts at these distances is not yet fully mature, and some magma may leak in a random fashion ontothe seafloor to form volcanic edifices typically less than 70 m high. Beyond 10 km, the mechanism of melt delivery toseamounts is mature and no further formation or growth of small volcanic edifices occurs.Keywords: East Pacific Rise; submarine volcanoes; abyssal hills; spreading centers

1. IntroductionWhile there has been considerable discussion ofthe abyssal hills of the Pacific [3,4] and linearseamount chains [1,5,6], little attention has beengiven to small off-axis volcanoes. Small volcanoes

lying on top of abyssal hills have been observed ondeep-tow profiles [7] and from submersibles [8].Off-axis volcanism has been inferred from geochem-

l Corresponding author. E-mail: [email protected]

ical [9,10], seismic [ll-131, magnetic 114,151 andside-scan sonar studies as well [16]. In this paper wemake quantitative estimates the off-axis volcaniccontribution to abyssal hill topography on the EastPacific Rise CEPR) 915N-950N (Fig. 1) usingSea Beam bathymetry and SeaMARC II side-scansonar records. The location and size of small, subcir-cuhu, closed-contour bathymetric highs are mea-sured. These features are presumed to be volcanicconstructions, and we determine the distance off-axisin which they are created. The volumetric contribu-tion of these volcanic edifices to the crust is calcu-

0012-821X/%/$12.00 0 1996 Elsevier Science B.V. All rights resexvedPIf SO01 2-82 1X(96)00028-3


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388 R.T. Alexander. KC h4acakaId/Earth and Planetary Science Letters 139 (1996) 387-394

lated, as well as the percentage of the seaflooroff-axis covered by them.Within the study area, the ridge has an averagestrike of 352 and spreading has been essentiallyconstant at - 110 km/my for the last 2 my [171.The 9N overlapping spreading center (OSC) haspropagated south at a rate of 42 km/my sinceanomaly 2 time, and a discordant zone of anomalousbathymetry, magnetization, and fault traces hasmarked its passage [171. The study encompasses anarea from 10436 to 10355W and 917-947N,and is bounded by the 9N OSC to the south and theLament seamounts to the north. Off-axis volcanicconstructions were measured out to the edge ofcomplete Sea Beam coverage at 35 km (0.65 my).Seabeam data collected on the RaitOl cruise [18],re-navigated by Wilcock et al. 1191, and gridded at77 m is available over virtually the entire study area.Sea Beam is a hull-mounted system that uses anarray of sixteen 2.67 by 2.67 beams to image theseafloor [20]. Kleinrock et al. [21] define the practi-cal resolution of Seabeam as a feature 25 m high and

-I-

200 m wide, and only slopes up to 30 can berepresented accurately. SeaMARC II side-scan sonarcoverage of most of the study area is provided by theRapaOl and MWave8706 cruises[22]. A swath mapmosaic was preprocessed and hand mosaicked at theHawaii Institute of Geophysics [22]. A map is gener-ated by towing the SeaMARC II vehicle (fish) be-hind a ship; the fish sends out acoustic energytransverse to the ship track and measures the energyreturned (back-scattered) from the seafloor. Inwardand outward scarps, facing toward and away fromthe ridge, respectively, can then be picked due totheir distinct signatures (Fig. 3).

2. Off-axis volcanismIn order to assess the role of off-axis volcanism inthe study area, we determined the distribution ofvolcanic edifices within 35 km of the ridge. Seafloor

topography is modified from relatively smooth, nearthe ridge, to rolling abyssal hill terrain off-axis by a

Overlapping Spreading Centers -

Propagating Ri -

CLIPPERTON

20N

10N

Fig. 1. ?he location of the study area on the East Pacific Rise.


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R.T. Alexmder, K.C. Mac&maM/ Earth and Planetary Science Letters 139 (1996) 387-394 389

combination of volcanic and tectonic processes. Thewhole axial ridge of the EPR between 23S- 18Nlacks any significant circular volcanoes [l], exceptfor a portion of the ridge near the Galapagos mi-croplate and the triple junction of the Pacific, Cocosand Nazca plates [23]. A 10 m Sea Beam contourmap shows gently sinuous contours within 2-3 kmof the axis becoming contorted and outlining localbasins and closed-contour highs at increasing dis-tances off-axis (20 m contours are shown in Fig. 2).We have assessed the role of faulting by correlatinglinear SeaMARC II reflectors to features in the SeaBeam bathymetry [22,24]. The topography createdby the faults is typically elongate, ridge parallel, andexhibits steep, relatively linear slopes. In contrast,local subcircular highs 2 40 m high (enclosed bythick, bold contours in Fig. 2) are first observed _ 5km off-axis. We interpret these highs as volcanicedifices based on the following: they lack crisplinear edges and typically have no clear expressionin the side-scan sonar; the highs are bounded by

sinuous contours in plan view; and the aspect ratio(length to width) is low (they are close to circular).These small volcanoes are similar in shape to thoseobserved by Smith and Cann [25,26] in the medianvalley of the Mid-Atlantic Ridge. No obvious rela-tion between the edifices and faults is evident, be-sides the fact that the volcanic constructions often lieon top of fault-bounded blocks.Scheirer and Macdonald [l] in a study between 8and 17N on the EPR, measured seamounts, definedas local highs greater than or equal to 200 m withaspect ratios less than 2. The zone of seamountcreation was found to be 5-15 km from ridge, ingood agreement with previous studies [27,28]. Wefound 55 edifices from the Sea Beam map using asimilar criteria to Scheirer and Macdonald [11,exceptthat the height cutoff was 40 m (Fig. 3). The distri-bution of small volcanic edifices is notably differentfrom that of larger seamounts in that the former donot appear to be organized into discrete chains.There are, however, two clusters of volcanic edifices

9 36

-104 16 -104 12 -104 08 -104 04 -104 ocFig. 2. Seabeam bathymetryof a near-axis regiou of the seafloor. ?he bathymetq is contouredevery 20 m and changes in shading are every100 m. (10 m charts were used in our analysis but the 10 m contours are too closely spaced to be shown here.) The axial summit caldera[42] is shown in a medium bold line. The thick bold lines eucompass subcircular ocal highs 2 40 m tall, which are interpretedas volcauicedifices. Note that volcanoes 2 40 m tall do not occur within 5 km of the axis.


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390 R.T. Alexander. K.C. Macdonald/Earth and Planetary Science Letters I39 (19%) 387-394

apparent in the northwest and southwest comers ofthe study area (shaded gray in Fig. 3). The construc-tions in the northwest are adjacent to and related tothe Lamont seamounts [29]. The edifices in thesouthwest lie just north of the disrupted crust of the9N OSC wake, and we interpret these features asgenetically related to the passage of that OSC. Previ-ous workers have noted that small volcanic cones areoften associated with the wakes of OSCs [30-321.The crust near the retreating ridge of an OSC may bea preferred site for constructional volcanism due toeruptions on a ridge that is no longer actively spread-ing [30,33,34].The histogram in Fig. 4A provides a measure ofthe relative abundance of volcanoes as a function ofdistance from the ridge. Those clusters of edificeswhich have been related to the Lamont seamounts or

Sea MARC I I Fault Picksand VolcanicEdifices>= 40m High

9 5ON

9 4ON

9 30N

9 2ON

9 1ON

10 ow 1043ow lO4.2oW 10410w 104bOWFig. 3. Map of scarps (interpreted as being caused by normalfaults) and volcanic edifices derived from interpretation of SeaMARC II and Seabeam bathymetry for the area of 100% SeaBeam coverage. lhm lines are interpreted fault locations and ticmarks show tbe downthrown side of the fault. Faults with ambigu-ous facing directions lack tics. Volcanic edifices are local highs2 40 m tall with aspect ratios 5 2. Thin Lines filled gray showlocations of volcanic edifices interpreted to be in association withthe Lamont Seamounts and the 9N overlapping spreading centerwake. Ihick bold lines delineate the remainder of the volcanicedifice population. The thick gray line shows the axis of the EastPacific Rise, and the axial summit caldera is the medium boldline.

Abundance of Volcanic Edttces, 5 Km owns

5 10 15 20 25 30distance from axis (km)Volume of Volcanic Edifces, 5 km bins

l-0 l-5 ;a 35distance from axis (km)Fig. 4. (AlAbundance of volcanic edifices versus distance fromthe ridge. The number of edifices per 5 km bin per 1000 km* isshown. Those volcanic edifices interpreted as being in associationwith the Lamont Seamounts or the 9W overlapping spreadingcenter wake are shown in gray. The remaining population ofedifices is shown in black. (B) As (A) except the volume of thevolcanic edifices versus distance from the ridge is shown.

the 9N OSC are shown in gray in order to highlightthe remaining population. Those edifices with noobvious source are shown in black and will be thefocus of the discussion below. The first edifice isobserved 5 km from the ridge, and the abundancedoes not appear to increase significantly beyond 10km. Assuming the formation of volcanic edifices is areasonably steady-state process, the zone of theirformation is limited to within 5-10 km of the ridgeon crust _ 0.1-0.2 my old, and the number ofedifices per 1000 km2 of seafloor reaches its finalvalue of N 11 over this range.The volumes of the volcanic edifices in this studyare estimated as a truncated right circular cone by


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measuring their average basal diameter, summitplateau diameter, and height. All of the edifices, withthree exceptions, are 40-70 m high and contain noobvious summit plateau. Three edifices range from110 to 140 m high, and two of them possess asummit plateau less than 0.5 km wide. Two of theselarger constructions are associated with the cluster ofedifices near the Lamont seamounts. The volume ofthe edifices per 1000 km of seafloor, excludingthose associated with the Lamont seamounts and the9N OSC, does not appear to increase significantlybeyond the 10 km in which they first form (Fig. 4B).We infer that these volcanic edifices are createdprimarily within 10 km of the ridge on crust < 0.2my old. In contrast, the seamounts measured byScheirer and Macdonald [l] first form between 5 and15 km off-axis and then continue to grow in volumeand size over a distance 2-4 times larger. The totalvolume of erupted material in the volcanic edifices is7.7 km3 or 4.5 km3, depending on whether theedifices near the Lamont seamounts and the 9NOSC are included. We estimate the volumetric con-tribution of this material to an average crustal sectionof 6 km [35] as 0.02-0.03% (Table 1). The contribu-tion to an extrusive layer (2A) 300 m thick [ 11-131is O&0.67%. These volumes calculated for off-axisvolcanism must be considered lower bounds, due tounmeasured sources such as smaller edifices, sheetflows and mass wasting deposits [29]. Volcanic con-structions which occur in narrow grabens [24] mayalso be systematically under-sampled by our measur-ing scheme, due to the finite beam width of SeaBeam.

Table 1W-axis volcanism

Other workers have estimated that seamounts con-tribute 0.3-l% to the volume of oceanic crust (Table1) [1,2]. The contribution of lava flows and volcanicdebris associated with seamounts at 16-18s on theEPR is estimated as 0.8 f 0.3% of the crustal vol-ume [2]. We use the data of Scheirer and Macdonald[l] to estimate that the volumetric contribution ofseamounts within 5-10 km of the axis is 0.13%.Small volcanic edifices, which contribute 0.02-0.03% to the crustal volume, thus account for u 13-19% of the off-axis volcanism within 10 km of theridge. Beyond 10 km, all detectable off-axis volcan-ism in this study area is confined to the largerseamount chains.

Although volcanic edifices contribute a minutevolume to the crust (0.02-0.03%) and extrusivelayer 2A (0.4-0.67%), they do cover at least 7-l 1%of the mature seafloor (Table 1). The coverage ofseafloor by volcanic edifices is calculated assuminga circular shape, with the radius equal to an averagemeasured value. Seamounts cover only - 6% of theseafloor (Table 1) [l], so small volcanic edifices areactually more extensive in this respect.

3. DiscussionBetween 16 and 18s on the EPR, seamountproduction in adjacent chains is noted to be nega-

tively correlated, suggesting a common plume-likemagmatic source for the chains [36]. Since seamountsand the small volcanic edifices measured in thisstudy form at about the same distance range from the

study AreaOntbeEFX9.3-P.80N9.3-P.80N

al7oNa17oN16-180s16-180s

Source Contribution CrustalVolume (%6)all volcanic edifices >= 40 .a3mhighselected volcanic editices .I?2>=4Omhighseamounts >= 200 m high .13within 10 km of axisseamounts >= 200 m high .3seamounklava flows proximal toseamounts

1.05f.05.&3

Area1 Coverage of ReferenceCNSt (%)

11 -his Study7

6

40-50

This StudyScheirer andMacdonald, 1995Scheirer andMacdonald, 1995Shen et al., 1993Shen et al., 1593


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392 R.T. Alexander, K.C. MacdonaId/ Earth and Planetary Science Letters 139 (19%) 387-394

ridge, they may tap the same magmatic source. Wepropose that, within 10 km of the ridge, seamountsand smaller volcanic edifices compete for a limitedmagma supply. The mechanism of melt delivery toseamounts at these distances is not yet fully mature,and some magma may leak in a random fashion ontothe seafloor to form volcanic edifices typically lessthan 70 m high. Beyond 10 km the mechanism ofmelt delivery to seamounts is mature and no furtherformation or growth of volcanic edifices occurs. It isalso possible that the lack of volcanic edifice forma-tion and growth beyond 10 km is due to thickeningof the lithosphere with age, which makes the litho-sphere impenetrable to small bodies of melt [11. Theproliferation of volcanic edifices within 5-10 km ofthe ridge to cover at least 7- 11% of the seafloor hasobvious implications for petrologic and geochemicalstudies of oceanic basalts. For example, approxi-mately 15% of the off-axis collection of dredgedbasalts in the area are enriched mid-ocean ridgebasalts (E-MORBS), which may have erupted off-axissince none are found along the present day axialsummit caldera [9].We embarked on two investigations which yieldednull results. We did not find any correlation betweenthe locations of small off-axis volcanoes and off-axismelt bodies detected seismically in the 9-10N area[37,38]. We tested the idea that the Smith-Jordanexponential distribution of seamount heights mightapply to these small volcanoes [39]. Unfortunately,our sample is far too small to successfully test thisidea. With 23 volcanoes at _ 40 m high, 20 at * 50m high, 7 at e 60 m high, 3 at N 70 m high, and 1each at 110 m, 120 m, and 140 m high, one couldvery loosely fit either an exponential or power lawcurve to the distribution.Abyssal hills in the 9N area of the EPR aremainly produced by faults bounding narrow, l-2 kmwide grabens [4,24]. The inward side of the hills,facing the ridge, consists of multiple fault scarps andtalus ramps [22]. The outward slopes are producedprimarily within N 2-7 km of the ridge by a combi-nation of fault growth and syntectonic volcanic drap-ing [4,40]. An alternation between fault slip andvolcanic burial produces a volcanic growth fault[4]. Volcanic additions to this topography coverscarps, fill grabens, and create numerous subcircularedifices. The source of volcanism is either the axial

summit caldera, neovolcanic zone vents or lava tubeswithin 5 km of .the ridge [g-15], or small volcanicedifices, typically 40-70 m high, formed between 5and 10 km of the ridge. Since outward-dipping faultslock within N 6 km of the ridge [4,24], the axialsummit caldera and neovolcanic zone eruptions areprimarily responsible for draping the outward-facingescarpments. Inward faults act to dam lava flows andsome accumulation of eruptive material in the grabensmay occur [16,41]. These eruptions do not produceedifices higher than N 30 m and, presumably, actmore to smooth topography than to create it. Incontrast, eruptions that build volcanic edifices 5-10km off-axis ornament abyssal hills with construc-tions generally less than 70 m high.

AcknowledgementsWe thank the captain, crew, and scientific partiesof the R/V Moana Wave 8706 cruise and the R/V77romus Washington RAPA 1 cruise for their assis-tance in collecting data. Discussions with SuzanneCarbotte, Doug Wilson and Dan Scheirer signifi-cantly improved this work. We thank F.N. Spiess for

a thoughtful and helpful review. Antoinette Padgettassisted with the drafting of figures. We thank theOffice of Naval Research for their support of thiswork through grants NO001490-J-1645, NO001493-l-0108, and NO001494-l-0678 and the NationalScience Foundation through grant OCE-88-17587.LMKI

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