Relationship of the Central Indian Ridge Segmentation with the Evolution of the Rodrigues Triple Junction for the Past 8 Myr

Home , Central Indian Ridge, Southeast Indian Ridge, Southwest Indian Ridge

Relationship of the Central Indian Ridge segmentation with the evolution of the Rodrigues triple junction for the past 8 Myr. Véronique Mendel, Daniel Sauter, Philippe Patriat, Marc Munschy

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Véronique Mendel, Daniel Sauter, Philippe Patriat, Marc Munschy. Relationship of the Central Indian Ridge segmentation with the evolution of the Rodrigues triple junction for the past 8 Myr.. Journal of Geophysical Research : Solid Earth, American Geophysical Union, 2000, B7 (B7), pp.16563-16575. 10.1029/2000JB900098. hal-00104237

HAL Id: hal-00104237 https://hal.archives-ouvertes.fr/hal-00104237 Submitted on 26 May 2020

HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 105, NO. B7, PAGES 16,563-16,575,JULY 10, 2000

Relationship of the Central Indian Ridge segmentation with the evolution of the Rodrigues Triple Junction for the past 8 Myr

VtroniqueMendel • andDaniel Sauter Ecoleet Observatoiredes Sciences de la Terre, Strasbourg,France

PhilippePatriat Institutde Physiquedu Globe,Paris

Marc Munschy Ecoleet Observatoiredes Sciences de la Terre, Strasbourg,France

Abstract. Locatednear 25ø33'S, 70ø00'E, the RodriguesTriple Junctionis thejoining point of the intermediate-spreadingSoutheast Indian and Central Indian Ridges with the ultraslowspreading SouthwestIndian Ridge.Bathymetric data and magneticanomalies are usedto analyzethe relationshipbetween the evolutionof the CentralIndian Ridge segmentationand the evolutionof the RodriguesTriple Junctionfor the past8 Myr. The CentralIndian Ridge domainexhibits a complexmorphotectonic pattern dominated by ridge-normaland obliquebathymetric lows interpretedas the off-axis tracesof axial discontinuities.The short-livednontransform discontinuitiesas well asthe segmentsthat lengthenor shortenalong the ridge axis revealthat the CentralIndian Ridge segmentationis unstablenear the RodriguesTriple Junction.The combined studyof the CentralIndian Ridge and SoutheastIndian Ridge domainsshows that the triple junctionevolves between two modes:a continuousmode where the CentralIndian Ridge and SoutheastIndian Ridge axes are joined and a discontinuousmode where the two ridge axesare offset.Owing to spreadingasymmetry, and differences in axis directionor in lengtheningrates of the CentralIndian and SoutheastIndian ridges,the continuousmode is unstableand evolves rapidly (<2 Myr) into a discontinuousmode. This last one is more stableand can evolve into a continuousmode only throughthe formationof a new CentralIndian Ridge segment,which takes placefacing the northernSoutheast Indian Ridge segment.The evolutionof the RodriguesTriple Junctionconfiguration and the evolutionof the CentralIndian Ridge segmentationare thus closelyrelated.

1. Introduction bathymetricsurvey of a 100km x 100 km areaaround the triple junction. From GLORIA side-scansonar images,Mitchell The RodriguesTriple Junction(RTJ), alsocalled Indian Ocean [1991a] suggestedthat the hypothesis of rigidplate tectonics may Triple Junction (25ø33'S, 70ø00'E) joins the intermediate notapply in a fine scaleat theRTJ because extension of the crust spreadingCentral Indian Ridge (CIR; 50 krn/Myr) and Southeast across the SWIR is distributed over a 10 km or more wide zone. Indian Ridge (SEIR; 60 km/Myr) and the ultraslow spreading Sea Beam and GLORIA data of the trace of the RTJ on the SouthwestIndian Ridge (SWIR; 16 km/Myr) (Figure 1). The RTJ Antarcticplate (0-20 Ma) alsoshowed sections with seriesof en was .first recognizedby McKenzie and Sclater [ 1971]. It is a 6chelon escarpmentsthat contrast with long linear scarps, stableridge-ridge-ridge (RRR) type triple junction at a regional suggestingtwo modes,discontinuous and continuous, of thetriple scaleand has existedsince at least65 Ma [Patriat and S•goufin, junctionevolution [Patriat and Parson, 1989;Mitchell and 1988]. For the last 10 Myr, the triple junction evolutionis well Parson, 1993]. Recently,the analysisof Simradmultibeam data describedby a RRR-type velocity diagram with the CIR and over the SWIR domainfor the last 8 Myr revealedtwo ways of SWIR lengtheningobliquely and the SEIR keeping a constant lengtheningof the SWIR: (1) a continuousand progressive length[Tapscott et al., 1980]. propagationof the SWIR with distributeddeformation in pre- The topographiccharacteristics of the three median valleys existingcrust of the CIR and(2) a discontinuouspropagation of weredescribed by Munschyand Schlich[ 1989]using a SeaBeam the SWIR with focusingof the deformationin a rift zonewhen the triplejunction migrates rapidly to the north[Sauter et al., 1Nowat Southampton Oceanography Centre, Challenger Division for 1997]. SeafloorProcesses, Southampton, England, United Kingdom. Mitchelland Parson[1993] andHonsho et al. [1996] related changesin the spreadingprocesses of the CIR to theevolution of Copyright2000 by theAmerican Geophysical Union. the RTJ. Periods of asymmetric spreadingon the CIR may Papernumber 2000JB900098. possiblyindicate that the CIR and SEIR havebeen offset by a 0148-0227/00/2000JB900098509.00 fracturezone at the triplejunction during one of the two modes

16,563 16,564 MENDEL ET AL.: EVOLUTION OF THE RODRIGUES TRIPLE JUNCTION

Table 1. Compilationof All BathymetTicData Usedfor This Study

Cruise Research Vessel Year Location Echo Sounder References

RodriguezI andII JeanCharcot 1984 RTJ SeaBeam Schlichet al. [ 1987] GeminoIII Sonne 1987-1988 CIR SeaBeam Pliigeret al. [ 1990] MD61 MarionDufresne 1989 CIR Monobeam Schlichet al. [ 1989] Hydrotmnc Sonne 1993 CIR Hydrosweep Halbachet al. [1995] Capsing L 'Atalante 1993 SWIR Simrad Patriatet al. [ 1997] KH93-3 HakuhoMaru 1993 SEIR SeaBeam Honshoet al. [ 1996] Hydrock Meteor 1995 CIR Hydrosweep Lochteet al. [1995]

The SeaBeam bathymetric survey system uses a multinarrowecho sounder (12 kHz) whichallows a maximumswath widthcorresponding to three fourths of thewater depth [Renard and Allenou, 1979]. Expanding on the Sea Beam design, the Hydrosweepsystem (15.5 kHz) providesa widerswath width (twicethe waterdepth) [De Moustier,1988]. The SimradEM12D formspart of thenew generation of deepsea sonar surveying tools [Bourillet et al., 1996].It is a 13 kHz multi-narrow-beamecho sounding system which can cover a sectorof 150ø, which represents a swath width of up to 7 timesthe water depth.

[Mitchelland Parson,1993]. However,the lack of dataover the asymmetricbathymetric highs with steepscarps facing toward the triplejunction trace on the Capricornplate could not confirmthe SWIR andgentle slopes facing away. Thesebathymetric features ridge-ridge-transformfault (RRF) evolutionof the RTJ. Honsho correspond,at a regionalscale, to the RTJ traceswhich separate et al. [ 1996]presented a new conceptualevolution model for the the seafloorgenerated at the SWlR from the seafloorgenerated at discontinuousmode that includesa short-living(<0.5 Myr) the SEIR and the CIR on the Antarctic and African plates northwardpropagation of theSEIR (of-25-30 km) followedby respectively.These RTJ tracesdraw a large V pointingtoward lengtheningof theSWlR, failing of theSEIR propagation rift and thenortheast and showingthe lengtheningof the SWlR [Mitchell, restorationof the CIR in less than -0.1 Myr. However, the 1991a]. The African RTJ trace is relatively continuousfrom pseudofaults associated with thosepropagating riffs werenot 68ø00'E to 69ø50'E except a small dextral offset at 68ø27'E. The observedon the bathymetricchart of the triple junction area strike of the African RTJ trace varies between N65øE and N95øE; [Honshoet al., 1996].How the segmentation of the CIR changes it is different from the mean spreadingdirection of the CIR whilethis ridge is lengtheningsouthward and whereas the triple (N60øE), indicatingthat the CIR lengthenedtoward the southeast junction evolves with two modes was thus still unclear because at different rates. By contrast,the Antarctic RTJ trace is only bathymetTicand magnetic data were lacking on theflanks of the continuous and linear from 69ø29'E to 69ø49'E near its eastern CIR. In thisstudy we describethe evolutionof theCIR domain,and to a lesserextent the SEIR domain, since anomaly 4 (8 Ma) using a compilationof bathymetricand magnetic data collected during severalcruises in theRTJ area. We firstanalyze the variations of the spreadingrates on the flanks of the CIR and SEIR. We then proposea location for the southeasternlimit of the CIR domain (the Capricorntrace of the triplejunction) and confirmthat an •i,,••;•."..' o"MD61 offsethas existed between the CIR andSEIR duringone of the modesof evolutionof the triplejunction. We thendetermine the - evolutionin timeand space of thesegments and discontinuities of the CIR domain.Finally, we show that the evolutionof the HYdrockcruies•• cruise segmentationalong the CIR and the evolutionof the triple junctionare stronglyrelated. Rod•'tgtte:•rztt,•e v 2. Main TopographicCharacteristics 26øS _-- of the Indian Ridgesat the RTJ We only give here a summaryof the characteristicsof the threeaxial valleys in the RTJ area,as theywere extensively describedby Munschyand Schlich [ 1989],Mitchell [ 1991a, b], Briais[1995], Honsho et al. [1996],and Sauter et al. [1997].We 27øS ; , , , thenfocus on the segmentationevolution of the CIR and SEIR 68øE 69øE ?0øE ?IøE 72øE from magneticand bathymetricdata. We usedmultibeam data collectedduring eight cruises (1984-1995; Table 1 andFigure 1) Figure 1. Trackchart of the studyarea. Ship tracks from Simrad, Hydrosweep,and Sea Beam multibeam bathymetrycruises are with various bathymetricsystems (monobeam, Sea Beam, indicated by solid bold, dashed bold, and solid thin lines, Hydrosweep,and Simrad) to producethe most complete respectively(see Table 1). Ship track from the MD61 cruise is bathymetTicgrid of theRTJ area (Figures 2a and2b). indicatedby a thin dottedline. CIR, CentralIndian Ridge;RTJ, The topographyof the RTJ area (Figures2a and 2b) is Rodrigues Triple Junction; SWIR, SouthwestIndian Ridge; dominatedby the deep SWlR domain boundedby two SEIR, SoutheastIndian Ridge. MENDEL ET AL.' EVOLUTION OF THE RODRIGUES TRIPLE JUNCTION 16,565 -

25a' ø 00'S I

25 ø 30'S

26 ø Offs

26 ø 30'S

[ 50km

27 ø 00'S

68 ø 00'E 68 ø 30'E 69 ø 00'E 69 ø 30'E 70 ø 00'E 70 ø 30'E 71 ø 00'E

Figure 2. Bathymetryof the RodriguesTriple Junctionarea. Contour interval is 200 m. Grids were computedwith a spatial resolutionof 120 m using the multibeam bathymetricdata and merged with an other grid that was computedwith a pixelspacing of only1 km usingthe monobeam bathymetric data of theMD61 cruise.

end. Elsewhere, it correspondsto a series of en 6chelon fault northwesternsegment of the SEIR, whichjoins the RTJ, is 85 km scarpswith 3-5 km offsetsto the northevery 10-28 km [Sauteret long and trendsN140øE [Honshoet al., 1996]. To the south,at al., 1997]. The strike of the Antarctic RTJ trace varies between about 26ø05'S, it is boundedby a left-lateral transform fault N4 IøE andN59øE with a meanvalue, N48øE, parallel to the mean which offsetsthe SEIR axis by -26 km [Honshoet al., 1996]. spreadingdirection of the SEIR. These small variationsindicate that periods of slight lengtheningand shorteningof the SEIR 3. Variation of the Spreading Rates have alternatedand that this ridge kept an almostconstant length on the Flanks of the CIR and SEIR since 8 Ma. The SWIR domainbounded by thesetwo RTJ tracesis deeper Magnetic anomalies have been identified by comparison (mean depth -4200 m) and more rugged than the seafloor betweenall available magneticprofiles in the area, collected generated at the CIR or the SEIR (mean depth -3100 m). during18 cruises(1960-1995), and synthetic magnetic anomaly Between 68ø10'E and 69ø25'E, the southern flank of the SWIR profileswhich were computedfor the CIR andthe SEIR (Figure domainis wider than the northernflank and is characterizedby a 3). We pickedeach magnetized block boundaryat anomalies1 seriesof en 6chelonbathymetric highs similar in shape,size, and (0-0.78 Ma), Jaramillo (0.99-1.07 Ma), 2 (1.77-1.95 Ma), 2a orientationto the high centeredat 69ø30'Enear the present-day (2.581-3.58 Ma), 3 (4.18-5.23 Ma), 3a (5.894-6.567 Ma), and 4 triplejunction [Sauteret al., 1997]. The easternmostsegment of (7.432-8.257 Ma) (geomagneticreversal timescale of Cande and the SWIR in the studyarea is centeredat 68ø22'E,about 200 km Kent [1995]; Figure 4). These crossings are shown on the from the RTJ, over a bathymetrichigh toppedby a volcanicridge structuralmap of the RTJ area (Figure 5). Owing to their very [Rommevaux-Jestinet al., 1997; Mendel et al., 1997]. low amplitudes,magnetic anomalies over the SWIR east of Within the survey area the CIR axial domain consistsof two 68ø30'E are difficult to interpret: the Brunhes anomaly is not segments(segments 1 and 2 in the nomenclatureof Briais [ 1995] recognizableuntil fartherwest at 68ø15'E[Tapscot.: ct al., 1980]. startingfrom the RTJ). They are trendingN 150øEand are 40 and There are two explanationsfor this [Patriat and Parson, 1989]: 50 km long, respectively [Briais, 1995]. A nontransform first, the SWIR spreadingrate is very slow (<8 km/Myr)and the discontinuity(NTD) offsets them, at 25ø10'S,by -20 km. The magneticreversal is more difficult to interpret;second, the SWIR 16,566 MENDEL ET AL.' EVOLUTIONOF THE RODRIGUESTRIPLE JUNCTION

24 ø 00'S Depth - - 1507

- -2400

- -2600

-2800

24 ø 30'S L -3000

-3200

- -3400

- -3600

- -3800 25 ø 00'S

-4000

' -4200

-5607m

25 ø 30'S

69ø 30'E 70ø 00'E 70ø 30'E 71ø 00'E 71ø 30'E 72ø 00'E

Figure 2. (continued)

SoutheastIndian Ridge CentralIndian Ridge Westflank Eastflank Westflank Eastflank vyi•,i•••N/ :•;V"•:•:•'•- Model•y •4 •a 3 [2a• J• I •J2 2a• 3• 3a 4 [4 •ai3 •2a• $•l•J • 2a•3 3a 4• MarionD,[besne<•• •h• • h •fi •i:•;•:.•:•:?• i t ••, /x...... 2• , • •.'":•'•"'•'•'•'•'•"• [] n• . Atlantis H

RodriguezJeanCharcot1(1984) __ .•-'•7•:•I•••:•5i5•½72dfi•:t [ ' ' • -4• m.•.;• / • •'•:•'Vi '• 2 •• MarionDufresne ::.•,•5..d•?p•t: , • w• v • "•'"'"-"•'•'•••i•:U• V • 61(19R9)

•Jeanleg 06Charcot (1976)' 'l,c':•'••%F'"•':•'•$ • F: t .... • , '-• , ' ' '*'"•'•:••:•:'-'••••• • • RobertCon,'ad or,gue9.Otl•n•isH, , , 94) •...... '•:iI h• • •Aff•, ,½'•[••• :'•:,•:::::::::•?•L:•(••:. •••'•''•[•'.... :,, 93 Atlantisllleg .06(1976) .. Rodriguez•ean Charcot1(1984) ' •i:•.•:•4{•I•• '"• • A.:n ;n ['•'":-'••__fi ' :,..;• ,. A .....Otarles Darwin 931eg06(1976)Atlantis II [1, •[• 'I, • '""':"••{?•.'.../•g ] • •'[ lndomedMelvilleleg07 (1978) - -k• -'• ...... :'"':'"•':'•'•...... i• • .... . [• k •[•"•'•'•1•,•il::.•::::<•:•:•:,.:;•• /•J[./( A,. " --93 Atlantisleg 06 (1976) II ist.ncrom {km - ' ' "" t • •:':::'"•'•:• [gz; (1984) -2• - 1• 0 1• 2• Distancefrom axis (km) Figure3. (boSom)Inte•retation ofsome ma•etic profiles across the Southeast Indian Ridge and the Cen•al IndianRidge. (top) The s•thetic profiles derive from the geoma•etic reversal timescale of Candeand Kent [1995]and a •o-dimensionalmodel consisting ofa flat layer of 300 m thickness ata depth of 3000 m with a spre•ingrate of 47 •• forthe Cenml Indian •dge and of 52 •M• forthe Southeast hdian Ridge. Each profileisidentified bythe name of the research vessel, thename of the cruise, and the ye• whenthe profile was acquired. MENDEL ET AL.: EVOLUTION OF THE RODRIGUES TRIPLE JUNCTION 16,567

J.y (>10 km). Whereverpossible, we measureddistances between iJ.o 2:0 isochronsin several locations.The half spreadingrates given in 1 i:: 2'Yi 2a.y 2a.o3.y 3.0 3a.y3a.o 4.y 4.0

.. Table 2 are averages of these different measurements. .. I • "2 2a 3 3a 3b 4 4a Uncertaintieson meanhalf spreadingrates average 2.0 km/Myr for the SEIR and 2.3 km/Myr for the CIR. The variationof full spreadingrate and asymmetryare presentedin Figures6a, 6b, and 6c. Concerningthe SEIR, the mean full spreadingrate is 52 km/Myr for the last8 Myr. Someperiods of slowerspreading, like the A2a.o-A3.yand A3a.o-A4.y periods,and periodsof fasterspreading, like theA3.y-A3.o and A3a.y-A3a.o periods, are Figure 4. Magnetic anomaliesand selectedisochrons (1-4.o) observed(Figure 6a). Spreadingof the SEIR is symmetricsince after the geomagneticreversal timescale of Cande and Kent A3.y and asymmetricbetween A3.y and A4.o, with a half [1995]. spreadingrate 13% greater for theeast flank than for thewest one (Figure6c). Note that this asymmetry was determined with few anomalies,especially from A3a.o to A4.o for the east flank. axis has a generaltrend which is not orthogonalto the spreading However,these results are in agreementwith thoseof Royerand direction and may be cut by closely spaced short offsets or Schlich[1988], who observean asymmetryof spreadingbefore characterizedby oblique spreadingsections. Hereafter we will A3.y on the SEIR between25ø30'S and 27øS. Concerning the refer to the differentanomalies by precedingtheir namesby a A CIR, the mean full spreadingrate is 47 km/Myr for the last for Anomaly. 8 Myr. Like the SEIR,periods of slowerspreading (A2a.y-A2a.o The half spreadingrates of the SEIR and the CIR have been andA3a.o-A4.y) alternate with periodsof fasterspreading (A2.y- determined between successiveisochrons along great circles A2a.y and A4.y-A4.o) (Figure 6a). The CIR was spreading oriented N48øE and N60øE, respectively. In order to smooth asymmetricallysince A1, generating,on average,33% morecrust errors due to uncertainties in both the location of magnetic on the west flank than on the east one, and between A2a.o and boundariesand the magneticreversal scale, we only considered A4.y, generating21% morecrust on the eastflank thanon the distancesbetween isochrons separated by at least 0.6 Myr westone (Figure 6b).

68ø00'E 69ø00'E 70ø00'E 71 ø00'E

25ø00'S

Figure 5. Structuralmap of the RodriguesTriple Junctionarea. The darkshaded areas correspond to bathymetric lows, while the light shadedareas correspond to bathymetrichighs and limit the bathymetricsurvey area. The mediumshaded areas correspond to alfimetriclows. Nontransformdiscontinuities associated to theselows are shownas bold dashedlines. Lettersidentify the CentralIndian Ridge spreadingsegments discussed in text. The axesof Indianridges are shown as bold lines. The anomaliesidentified are shownby opencircles and solid circles whichcorrespond to youngestand oldest boundaries of magnetizedblocks, respectively. Thin solidlines represent the isochrons.Solid lines correspond to the African andAntarctic triple junction traces and to the locationof the Capricorntriple junction trace deduced from the rotationof the Antarctictriple junction trace onto the Capricorn plate.The stippledarea reflects the uncertaintiesof the locationof the Capricorntriple junction trace. The large troughsobserved on theCentral Indian Ridge domain and interpreted-as traces of CentralIndian Ridge axis jumps are shownby thin dottedlines. Abyssal hills disruptingnontransform discontinuities (cross-hatched) are observed on either sideof the CentralIndian Ridge axis. 16,568 MENDEL ET AL.: EVOLUTION OF THE RODRIGUES TRIPLE JUNCTION

Table 2. AnomalyInterval Half SpreadingRates on the SEIR andthe CIR

Anomalies At, Myr SEIR CIR Half SpreadingRate, km/Myr Half S13readingRate, km/Myr

West Flank East Flank West Flank East Flank

Axis - 1 0.780 28.3 25.5 Seg.A: 30.3 19.5 Seg. B: 35.9 13.9 1 - 2.y 0.990 27.7 25.7 Seg.C: 33.2 13.1 Seg. D: 24.2 22.1 2.y - 2a.y 0.811 29.7 26.5 Seg.C: 29.4 25.5 Seg. D: 29.4 25.5 2a.y- 2a.o 0.999 27.1 26.2 Seg.C: 13.4 27.1 Seg. D: 20.9 19.6 2a.o - 3.y 0.600 20.9 23.0 18.6 24.1 3.y- 3.0 1.050 28.1 32.4 25.4 24.2 3.0- 3a.y 0.664 24.8 27.0 13.8 33.0 3a.y- 3a.o 0.673 23.0 36.8 20.7 29.8 3a.o-4.y 0.865 13.2 21.8 8.9 26.5 4.y- 4.o 0.825 22.6 28.4 26.5 25.0

Half spreadingrates of the SoutheastIndian and Central Indian Ridges have been determinedbetween successiveisochrons in a N48øEand N60øE direction respectively, where the time spanbetween these isochrons was >0.6 Myr. For the CentralIndian Ridge, wherever possible, half spreadingrates were determinedover each segment(A, B, C, etc.) shownin Figure5. Beforeanomaly 2a old, half spreadingrates on the CentralIndian Ridgewere measured over segmentsE andF (Figure5). The meanerror of half spreadingrates is -2.0 km/Myr for the SoutheastIndian Ridge and 2.3 krn/Myr for the CentralIndian Ridge.

4. Southeastern Limit of the CIR Domain: Between the isochrons, the age of the trace is linearly Capricorn Trace of the Triple Junction interpolated.We thenused rotations given by Royerand Schlich [1988], which allow to obtain a satisfyingfit betweenthe Owing to their close spreadingrates, the CIR and the SEIR crossingsof the SEIR on the Capricornplate and the rotated ones have generatedseafloor with similar characteristicsof depthand of the Antarcticplate. We have not usedthe projectionof the roughness.For the last 1 Myr, small changesof SEIR abyssal African RTJ traceto confirmthe positionof the RTJ traceon the hills direction toward the CIR domain can be used to locate the Capricornplate becausethe lack of magneticprofiles near the boundary area between the CIR and SEIR domains on the African RTJ trace does not allow a well-constrained age Capricornplate (Figures 2a and 2b). These small strike changes determinationfor this trace. Moreover, it has been previously are associatedwith the deepeningof the SEIR and CIR abyssal suggestedby Mitchelland Parson [1993] and Sauter et al. [1997] hills toward the CIR and SEIR domains(Figures 2a and 2b), that northward relocalizations of the SWlR rifting may have respectively.Unfortunately, the strike of the abyssalhills older enclosedpart of the seriesof normalfaults, which characterized than 1 Myr cannotbe preciselydetermined because multibeam the African RTJ trace closeto the RTJ [seeSauter et al., 1997, bathymetricdata are lackingfurther to the northeast.The position Figure11], withinthe Antarctic plate, leaving little evidencefor a of the Capricorn RTJ trace must therefore be determined preciselocation of thetriple junction trace on the Africanplate. arbitrarilyin order to indicatethe probablelocations where the BetweenA3.o and the presentday, GLORIA imagesshow that SEIR and CIR should meet between 8 Ma and 1 Ma. For this the predictedCapricorn RTJ tracefalls within a wide transition purpose, we have rotated the Antarctic RTJ trace onto the areawith, to the south,bathymetric features parallel to the SEIR Capricornplate. Platereconstruction vectors, age, and locationof abyssalhills, and to the north,lineations striking parallel to the the Antarctic RTJ trace are needed to perform this rotation. CIR abyssalhills [Mitchelland Parson,1993]. Before A2a.o the Becausethe SWlR domain near the RTJ results mostly from predictedCapricorn RTJ trace falls at thesouthern edge of a wide tectonicextension of SEIR and CIR preexistingcrust [Sauter et andcontinuous bathymetric low that we have interpretedas the al., 1997], it is difficultto locateaccurately the boundarybetween off-axis traceof an axial discontinuity(Figure 5). Sincewe used the SWIR domain and the CIR and SEIR domains.Moreover, the the top of the escarpmentsof the AntarcticRTJ trace,the triple foot of the seriesof the largenormal fault scarpscorresponding to junctiontrace on the Capricornplate shouldbe located-2-3 km the RTJ traces at a regional scale is buried by numeroustalus more to the northwestalong the deepestpart of the bathymetric producedby masswasting processes. We thereforechose to pick low. The bathymetrictrace of thisNTD to the westof theRTJ is the positionof the triple junction traceson the summitof these presentin theCIR domainfor the sameages but partly buried by large escarpmentsthat can be followed with ease on the theuplift of the structuresnear the AfricanRTJ trace(Figure 2a) bathymetricmap. The resultingerror of locationis -2-3 km (the dueto isostaticcompensations [Mitchell and Parson,1993]. The mean half width of the deformation zone in the SWlR domain sectionof the predictedCapricorn RTJ trace before A2a.o is near the triplejunction [Sauteret al., 1997]). A similar offsetwill associatedwith 14-23 km long offsets,measured along the trace, thus exist between the predictedtriple junction trace on the betweenthe CIR and SEIR isochrons(Figure 5). These offsets Capricornplate andthe positionof the traceitself. The ageof the are difficult to identify on the western CIR and SEIR flanks by Antarctic RTJ trace has been determined by lengtheningthe rotatingthe AntarcticRTJ traceonto the African plate usingthe isochrons of the SEIR until the trace in a N138øE direction. rotationsof Chu and Gordon [1999] for the SWIR. We suggest MENDEL ET AL.: EVOLUTION OF THE RODRIGUES TRIPLE JUNCTION 16,569 that since the distributed deformation prevents an accurate •4.7 to •-3 Ma (betweenA2a.o andA2a.y), segmentE, whichwas locationof the triple junction traces,especially for the African initially 50 km long, haslengthened to the southby -15 km. At RTJ trace,it alsoprecludes accurate reconstructions of the offsets •-3 Ma a new NTD separatedthis segment E in two new segments on the Africanand Antarctic plates. (D and C). From •-3 to •-1.3 Ma (between A2.y and AJ.o), We thusconclude that as in the present-dayconfiguration of segmentD haskept a constantlength, 45 km, while the lengthof the RTJ where there is a small •-5 km offset between the CIR and SEIR axes [Munschyand Schlich, 1989], an axial discontinuity has existed at the triple junction before A2a.o. By contrast, between A1 and A J, the isochrons are continuous across the 2.y 2a.o 3.0 3a.o 4.0 predictedCapricorn RTJ trace showingonly a slight changeof 1 2a.y 3.y 3a.y 4.y strike (Figure 5). Between AJ and A2a.y, the lack of CIR

_ well-identified magnetic anomalies does not allow us to be certainthat there is no offset.However, the predictedRTJ trace crossesa bathymetricdomain where the abyssalhills show a slightchange of strikebut neitherinterruption nor offset(Figures : 2a and 7).

5. Evolution of the CIR Segmentation The CIR domain is boundedto the north by the off-axis traces of the NTD (24ø30'S)between segments 2 and 3 that existssince 30 •-10 Ma (bold dotted line in Figure 5). This domain exhibits a complexmorphotectonic pattern dominated by ridge-normaland oblique bathymetric lows that disrupt isochrons and are interpreted consequently as the off-axis traces of axial b. CIR discontinuities(bold dotted lines in Figure 5). These troughs partitionthe ridge flanks into rhomb-shapedareas of relatively 601;. seg...... :...... B seg..:...... C i West.flan.k higherelevation which can be associatedto the off-axis tracesof segmentcenters. These areas are formed by a seriesof abyssal hills, lineatedmore or lessparallel to the presentCIR axis (N 150- 20':';':""'• seg.D1...... }seg. D • : 160øE),and spaced by •-2-8 km. Near the off-axisdiscontinuities, someabyssal hills curvetoward the troughsshowing the senseof the offsets. •-•:•i::•:a•:•:a•s•...... ß J. ' We have identifiedsix major rhomb-shapedareas on the west flank with their conjugateson the east flank (A-F in Figure 5). The oldestone, i.e., areaF, is limited to the northby the 24ø30'S -60 ...... •"••'•'•••••We•tfiank NTD off-axistraces and to the southby the RTJ tracesfrom 9 to 0 2 4 6 8 •-7 Ma (betweenA4.y and A3a.o) and by a N60øE NTD from -7 to •4.7 Ma (between A3.o and A3.y). SegmentF was •-25 km c. SEIR long at---9Ma (A4a.o). It has lengthenedto the southby---25 km 60 during •-2 Myr and has kept a constant length afterward :: Westflank : (-50 km). It is separated from area E by a north-south 40 bathymetrictrough (Figure 5) that we interpretas beingproduced i Eastflank : 20 : duringa major tectonicextensional period at about4.7 Ma. From : 0 ...... !...... ::•.--'.::•:•::4":::.::i:"'...'-'"2.;...•:,: _.

Figure 6. (a) Evolution of the full spreadingrate along the : : :: ."•...... :"• Eastflank Central Indian and SoutheastIndian Ridges for the last 8 Myr. -40 i Dashed lines are mean spreading rates. Shaded stripes are i Westflank -60 : : i E : : : associatedto the uncertainties. (b) and (c) Evolution of the spreadingasymmetry along the Central Indian and Southeast 0! ' 2I ' 4I * 6I ' Indian Ridges, for the last 8 Myr. This percentagehas been calculated using mean half spreadingrates (Table 2) and is d., positivewhen there is more crustcreated on the west flank than on the east flank and negativein oppositecase. Shadedstripes correspondto the error of percentagerelative to errorsof distance measurementsbetween isochrons. (d) Evolution of the apparent offset between the SoutheastIndian Ridge and Central Indian • 20 Ridge isochrons.These offsets are measuredalong the predicted • 10 Capricorntriple junction trace. Betweenanomaly 2a young and anomaly 1, the curvature of the Central Indian Ridge and 0 SoutheastIndian Ridge isochronsclose to the Capricorntriple I junctiontrace compensates for the real offsetsleaving apparent 0 2 •, 6 8 zero offsets. Age (Ma) 16,570 MENDEL ET AL.: EVOLUTION OF THE RODRIGUES TRIPLE JUNCTION

25 ø 20'S - •}:•::- . .[,,......

25 ø 30'S .. ß..

25 ø 40'S

25 ø 50'S

26 ø 00'S "•?•:'X...... • .....•N'" •'•::::•::"• -" '%'"•'"" •¾•:'•:•:•::":'"" "': "'

•.•' • ....-..'V¾ •...... •. ' :'::•5-5•.ß '- •:• ...... •. q"•e--:5• ...:.....SEIR'•:• "..: - ,. -..... '•.. - .. .,..>:• ......

I10km I ...... •X••••ß •' ...... '•'" •'...... • • "• '•.•'••••• • '%7 26 ø 1O'S 69 ø 50'E 70 ø 00'E 70 ø 10'E 70 ø 20'E 70 ø 30'E 70 ø 40'E

Figure7. Theabyssal hill system of theSoutheast Indian Ridge enhanced by calculatingthe principal curvatures at eachpixel of thebathymetric grid. Shaded areas correspond toridges. The shading scale is related to theamplitude of theridge: solid areas correspond toridges that are well marked in thetopography. Successive ridges and troughs are foundat aboutthe sameage on eachflank.

segmentC hasvaried between 16 and27 km.Discontinuity D-C A2a.y, to the south of discontinuityD-C (thin dotted line in wasa smallright-stepping offset (about 8 km). It disappearedat Figure 5), which has no conjugateon the oppositeflank. This -1.3 Ma, whensegments C andD joinedtogether to createa new bathymetrictrough can be interpretedas a relicof an axialvalley 60 km longsegment, segment B, whichcorresponds to the off- andso suggeststhat the offsetD-C resultsfrom a CIR axisjump axistrace of the present-dayCIR segment2. Duringthe last to thesouthwest of-8 km ratherthan a continuousasymmetry. In 1 Myr, thissegment has shortened by-10 km. On the westflank, the same way, betweenA2.y and AJ.o, an asymmetryof segmentB is characterizedby thepresence of a topographichigh spreadingis observedto the southof discontinuityD-C, while the (centeredat 69ø46'E-25ø17'S),which was interpreted by Mitchell spreadingis symmetricto the north (Figure 6b). A large et al. [ 1998]as a corrugatedsurface. At about2.6 Ma (A2a.y)a bathymetrictrough is also observedon the west flank, between new segment(A) tookplace to the southof segmentC, nearthe A2.y andAJ.o andto the southof discontinuityD-C (thin dotted RTJ.Segment A correspondsto the off-axis trace of thepresent- line in Figure5), withouta conjugatefeature on the eastflank. As dayCIR segment1. Since-2.6 Ma, it haslengthened to thenorth previously,we can interpretthis troughto be a part of a former andthe southreaching 40 km longat the presenttime. Before axialvalley. So, we suggestthat theoffset D-C wassuppressed -1.3 Ma (betweenA2.y andAJ.o), segmentA wasbounded to by a CIR axisjump, of-8 km, to the northeastrather than by a the northby discontinuityC-A, which is probablya small continuousasymmetry. Such CIR jumps have been already right-steppingoffset. However, for thisperiod the lack of well- proposedto explainthe asymmetryobserved between anomaly 1 constrainedmagnetic anomaly on segmentA doesnot allow an andthe CIR axis[Honsho et al., 1996].On segmentA thejump accurateestimation of this offset. At-1.3 Ma,discontinuity C-A' hasleft, on the west flank, a largetopographic trough interpreted becamediscontinuity B-A, whichoffsets segments A andB in a as a fossilaxial valley andcharacterized by highmagnetization right-steppingmanner by-16 km fromAJ.o to A1 andby 20 km [Honshoet al., 1996]. The asymmetry,which is greaterfor at the presenttime. segmentB (i.e., segment2) thanfor the segmentA (i.e., segment Theanalysis of bathymetricand magnetic data has shown that 1) (Figure6b), hasincreased the offsetA-B. thediscontinuity D-C is a veryshort-lived feature. It appearedat -3 Ma (betweenA2a.o and A2a.y) anddisappeared at-1.3 Ma (betweenA2.y andAJ.o). Its "birth"and "death" are associatedto 6. SEIR AbyssalHill System spreadingrate contrasts between segments C andD. The distance Althoughthe bathymetricand magnetic coverage of the SEIR betweenisochrons A2a.o and A2a.y showsan asymmetryof domainnear RTJ is more restrictedthan along the CIR, the spreadingto thesouth of discontinuityD-C, whilethe spreading structure of the SEIR domainlooks less complex than along the to thenorth is symmetric(Figure 6b). Furthermore, bathymetric CIR. The linear isochronsof the SEIR, especiallyon the west data show a large troughon the east flank, betweenA2a.o and flank, showthat the first segmentof the SEIR southof the RTJ is MENDEL ET AL.: EVOLUTION OF THE RODRIGUES TRIPLE JUNCTION 16,571

a long-livedfeature, which was not disruptedby NTDs since "normal"seafloor spreading on the SWIR nearthe RTJ, the first 9 Ma at least. Owing to the limited bathymetriccoverage, we spreadingsegment closest the RTJ is at only200 km from it, and haveanalyzed the SEIR abyssalhill systemonly for thissegment tectonicprocesses are predominant in this domain [Sauter et at., and for the last 1 Myr. 1997].Sauter et at. [1997]proposed that the evolutionof the The patternof abyssalhills (Figure7) overthe SEIR hasbeen SWIR domainis probablyrelated to differentlocalizations and enhancedby calculating the principal curvaturesaround each distributionsof strainwhich are in mm controlledby changesof pixel of thebathymetric grid followingShaw and Lin [ 1993]. The thetriple junction configurations. We thussuggest that both the abyssalhills have a mean strike of N138øE and are regularly evolutionof the segmentationof the CIR andthe SEIR andthe occurringevery 4-5 km. Like the axial volcanic ridge on the modesof propagationof theSWlR domain are strongly related to SEIR inner valley floor, they are crowned by numerousconic- changesof thetriple junction configuration. shapedpeaks, indicating that they result to some extent from volcanic processes.Close to the axis, the abyssal hills are 7.1. Capricorn RTJ Trace: A Record asymmetricin shapewith steepinward facing faults and more of the Past RTJ Configuration gentle slope facing toward the flanks. Outward facing faults The velocitytriangles [Patfiat and Courtillot,1984] of the becomeas numerousas inward facing faults at 18 km (-0.7 Ma) RTJ describe the offset evolution between the CIR and the SEIR. from the axis, producinga typical horst-and graben-typerelief. To built thesetriangles, we usedthe CIR and SEIR spreading The most striking features in the organizationof the off-axis rates(Table 2) andthe directions of theisochrons and of theRTJ terrainis the almostperfect symmetry between the ridgeson each traces(Figure 5). The CIR and SEIR spreadingdirections are side of the SEIR axis. Figure 7 showsthat the successiveridges givenby the rotationsof Royeret at. [1997] for the CIR and are found at almost the same age on each flank. Although the Royerand Schtich [1988] for the SEIR. Owingto the lackof southwesternflank hasbeen uplifted near the AntarcticRTJ trace, recognizablemagnetic anomalies, the SWIR spreadingrate, as the similaritybetween the amplitudeof the abyssalhills on each well asthe SWIR spreadingdirection, is givenby therotation of flank is also noticeable (dotted lines in Figure 7). This spatial Chu and Gordon [ 1999]. relationshipis in agreementwith the work of Kappetand Ryan BetweenA2a. o andA4.o, the predictedCapricorn RTJ traceis [1986] andPezard et al. [1992], who explainedthe generationof associatedwith 14-23 km long offsets(Figure 6d). The velocity the rift flankingtopography of the intermediate-spreadingJuan de trianglebuilt for theperiod between A3.o andA2a. o (Figure8a) Fuca Ridge and GalapagosRidge, respectively. They proposeda showsa 1.8 km/Myroffset lengthening rate between the CIR and cyclic processin which the volcanicconstruction of a crestal SEIR axes. This rate is small but remains significant. When ridge within the rift valley, accompanying active crustal integratedbetween A3a.o and A2a. o (-3 Ma), it is equivalentto a accretion,is followed by a collapseand splittingof the summit finite -5.4 km longoffset between the two ridges.Adding this region of this ridge when there is a reducedmelt supplyto the offset to the initial 14.2 km long offset at A3a.o (Figure 6d) axial magmachamber. Each part of the axial ridgeis thenshifted resultsin a 19.6km longoffset at A2a.o,which is consistentwith on the flank, producingthe observedsymmetric organization of the 19.5km longoffset measured at thistime (Figure 6d). abyssalhills. The topographyof the SEIR flanksprobably results For the periodbetween A2a.y and A1, thereis no offset from suchcyclic processes.The formationof a new SEIR crestal betweenthe SEIR and CIR isochrons(Figure 6d), whereasthe ridge, due to an increaseof volcanic constructions,is often velocitytriangle (Figure 8b) showsan offset lengthening rate of associatedwith a slight northward lengthening of the ridge, 6 km/Myrbetween the CIR andSEIR axes, which means that the creating the en 6chelonpattern of the Antarctic RTJ trace [see offset increasedby 10.8 km between A2a.y and A1. One Sauter et al., 1997, Figure 10). This small lengtheningof the possibilityis thatthe progressive curvature of theSEIR and CIR SEIR is coincident with the redistribution of the deformation at isochronsand abyssal hills nearthe CapricornRTJ trace(Figure the RTJ when it is relocatedto the north [Sauteret al., 1997]. We 5) compensatesfor smalloffsets (<6-7 km), leavingapparent zero thus suggestthat both the increaseof SEIR volcanic processes offsets.From -2.6 Ma (A2a.y) to -0.5 Ma, theabyssal hills of the and the redistribution of deformation at the RTJ favor small SEIR andthe CIR curvegently near the predictedCapricorn RTJ lengtheningof the SEIR. traceto connecteach other (Figure 7). During this period the predictedCapricorn RTJ trace corresponds to a relativelow along 7. Results and Discussion thesecontinuous abyssal hills. The abyssalhills beginto curveat -5 km from thepredicted Capricorn RTJ traceat AJ.o,whereas The analysisof the bathymetricand magnetic data of the CIR theycurve farther away from this trace (-10 km) at 0.5 Ma asthe domain near RTJ has shown that since at least 8 Ma, the CIR was offset at the RTJ is increasing.Removing this curvatureby lengtheningcontinuously, more or lessrapidly, to the southeast straighteningthe CIR andSEIR isochronsresults in a 6 km long and that periodsof asymmetricand symmetricspreading have offsetalong the Capricorn RTJ trace at A1. In thesame way, the alternated. Furthermore, the short-lived NTDs as well as the offsetsat AJ.y and AJ.o shouldbe -6 and-5.5 km long, segmentsthat lengthen or shortenalong the ridge axis have respectively.The lack of well-constrainedmagnetic anomalies on revealedthat the CIR segmentationis unstable. CIR segmentA, fromA2a.y to A2.y,does not allow estimation of The triple junction configurationalso changesthrough time. the offsetfor thisperiod. However, taking into accounta 6 km We have shown that an axial discontinuityhas existedbetween longoffset at A1 andthe offset evolution given by the velocity the CIR and SEIR axis from 9 to 3 Ma and disappeared trianglefor theA2a.y-A1 period, the CIR axisshould have been afterward. Moreover, the SEIR is characterized by frequent offsetby 4.8 km to the southof the SEIR axisat A2a.y. The occurrenceof periods of discontinuouslengthening, which velocitytriangle built for the periodbetween A2a.y andA2.y generatedthe en 6chelonpattern of the AntarcticRTJ trace. (Figure 8b(left)) showsthat this small left-steppingoffset The third branch of the triple junction, i.e., the SWIR, remainedroughly the sameduring this period.This unusual propagatesin a preexistingseafloor created at the CIR and SEIR configurationof the CIR and SEIR axes couldexplain the axes. However, contraryto the SEIR and the CIR, there is no unusualdirection of the isochronsand abyssalhills of the SEIR, 16,572 MENDEL ET AL.: EVOLUTION OF THE RODRIGUES TRIPLE JUNCTION

Capricorn which curveto the westtoward the predictedCapricom RTJ trace at A2a.y-A2.y (Figures 5 and 7). The transition between this a. A3.o- A2a.o configurationand the presentone, wherethe CIR axis is offsetto the north of the SEIR axis, occurredbetween A2.y and AJ.o, when the CIR spreading was highly asymmetric (Figure 8b(right)).This asymmetryis probablyrelated to a CIR axisjump to the northeastat the sametime than anotherridge jump on CIR segmentC. Finally, when the offset is too large (>6-7 km) to be

Afric compensatedby a long-wavelengthcurvature of the abyssalhills of the SEIR and the CIR, the deformation is focused in a NTD that takes place at the RTJ, and thus the axes are no longer connected.However, a curvatureof the abyssalhills of the CIR and the SEIR toward the NTD can still be observed in some places.At the presenttime, for example, the CIR axis curvesto the west towardthe RTJ, and a 5.5 km long apparentoffset can Antarctica be measured(Figures 5 and 6d). If we straightenthe CIR axis,the i i offsetbetween the CIR and SEIR axesis -9 km long. 10km/Myr Capricorn At aboutA3.o, for a shortperiod of time, offsetsof isochrons are no longerassociated with a bathymetriclow correspondingto a NTD but with a bathymetric high characterized by CIR b. A2a.y - A1 axis-parallel structures that cross the discontinuity. Similar featureshave been previously described by Grindlayet al. [ 1991] for the NTDs of the southMid-Atlantic Ridge. We suggestthat the CIR featureswere producedduring a period of high melt delivery and lengtheningtrough the NTD toward the SEIR flank. 6.okn,y This lengtheningoccurred when the offset of the RTJ between Africai.• ...•....'""w&/X A2a.o and A4.o was the smallest. 7.2. Two Configurations for the RTJ The morphologyof the Antarctic RTJ trace on the GLORIA images was one of the major reasonfor Mitchell and Parson [1993] to consider two modes of evolution for the RTJ: a Antarctica discontinuous mode when the Antarctic RTJ trace is associated with a series of en 6chelon faults and a continuous mode when the AntarcticRTJ tracecorresponds to a long linearscarp. The (left) A2a.y - bathymetricdata over the triple junctiontraces have shownthat the most recent period of continuousmode is shorter than (right),A2.y-A••,• previously defined by Mitchell and Parson [1993]: it extends q"•%0.9kngMyr • /•.'.,/ 11.7kin/My, from A2a.y to A1. Duringthis period, the CIR and SEIR axesare •//\ \ OLR connected,the SEIR is spreadingsymmetrically, and the CIR spreadingis mostlysymmetric but is asymmetricnear the triple li)km/M•/r junction trace, producingmore crust to the westernflank. As previouslymentioned, owing to spreadingasymmetry and to differencesin axis directionor in lengtheningrates of the CIR Figure 8. Velocity trianglesbuilt for (a) the periodbetween andthe SEIR, this RTJ configurationevolves rapidly (<2 Myr) anomaly3a old andanomaly 2a old and(b) the periodbetween into anotherconfiguration where there is a small offset between anomaly2a youngand anomaly 1. The CentralIndian Ridge and the CIR and SEIR axes (Figure 9b). The "appearance"of this SoutheastIndian Ridge spreadingdirections are given by the offset,just afterA l, coincidesto a slightlengthening of the SEIR rotationsof Royeret al. [1997] and Royerand Schlich[1988], respectively.The SouthwestIndian Ridge spreading direction and to the northwestas shownby the en 6chelonpattern of the rate are givenby the rotationof Chu and Gordon[1999] (long- Antarctic RTJ trace. Thus this suggeststhat a new period of dashedline). The short-dashedlines are the 0.8-2.6Ma (Figure discontinuousmode began just after A1. This period also 8a) and3.6-6.6 Ma (Figure8b) meanazimuths of the African and correspondsto an asymmetricspreading period of the CIR (since Capricorntriple junction traces taken from Figure5. The offset A1) that increasedthe offset between the SEIR and CIR axes betweenthe CentralIndian Ridge and SoutheastIndian Ridge (Figure9b). In contrastwith the findingsof Honshoet al. [1996], axes increasesat different rates, varying between 0.9 and we considerthat this asymmetry of the CIR spreadingis the result 11.7km/Myr (OLR, offsetlengthening rate). The shadedarrows of a CIR axis eastwardjump and not of a very short-lived indicatethe general displacement of theCentral Indian Ridge axis (<0.5 Ma) northwardpropagation of the SEIR (of >25 km into with respectto the SoutheastIndian Ridge axis. The Rodrigues Triple Jucntiontrace on the Africanplate is not shownbecause CIR segment 1). We argue that no pseudo-fault of this northwardrelocalizations of the SouthwestIndian Ridge rifting propagatingrift is observedon the bathymetricmap (Figures2a mayhave enclosed part of CentralIndian Ridge preexisting crust and2b) andthat not only CIR segment1 but alsoCIR segment2 withinthe Antarcticplate, leavinglittle evidencefor a precise showsthe sameasymmetric pattern (Figure 6b). locationof the traceon the Africanplate [Mitchelland Parson, The main characteristic of the discontinuous mode is the offset 1993;Sauter et al., 1997]. betweenthe CIR and SEIR axes.In the surveyedarea this offset MENDEL ET AL.: EVOLUTION OF THE RODRIGUESTRIPLE JUNCTION 16,573

(a) Continuous mode

120km• NTD creation

CIR segmentcreation

I 20 km I I 20krn I (b) Discontinuous mode

• 20 km i Figure9. Schematicpatterns of thetwo modes of evolutionof theRodrigues Triple Junction: (a) thecontinuous modeand (b) thediscontinuous mode. During the continuous mode period, the Central Indian Ridge and Southeast IndianRidge axes are connected. Owing to the spreadingasymmetry and differences in axisdirection or in lengtheningrates of theCentral Indian and Southeast Indian ridges, this triple junction configuration evolves rapidly(<2 Myr)into another mode where there is anoffset between the Central Indian Ridge and Southeast Indian Ridgeaxes, which is thebeginning of a discontinuousmode period. During this discontinuous mode period, the SoutheastIndian Ridge propagates slightly (<5 km) towardthe northwest every 0.5-1.1 Myr accordingto theen 6chelonpattern of theAntarctic triple junction trace. The transition from a discontinuousmode to a continuous modecorresponds to the formation of a newCentral Indian Ridge segment. See text for further details. never exceeds23 km (Figure 6d). During this mode, which extends from A4.o to A2a.y, periods of symmetric and asymmetric spreading of the CIR and the SEIR alternated (Figures 6b and 6c). When these ridges are spreading asymmetrically,both are systematicallyproducing more crustto the easternflank by contrastto the continuousmode. Between A4.y and A2a.o, the asymmetryof CIR spreadingallows to reduce the offset between the SEIR and CIR axes. At aboutA2a.y the transitionbetween the discontinuousmode and the continuousmode is associatedwith the appearanceof a new CIR segmentnear the RTJ. The reasonwhy this segment took place, at that time, is still unclear. However, we have observed that the "birth" of this segment is preceded by a southwardaxis jump in CIR segmentE which allowsreduction, but not removal, of the offset at the RTJ. Moreover, at about the sametime (A2a.y), segmentC lengthensthrough the NTD at the triplejunction (crossed area in Figure5). Boththe southwardaxis jump andthe lengtheningin the southernmostsegment of the CIR favor the connection between the CIR and the SEIR. This connectionmay resultfrom a relocationof the southerntip of the CIR at the northerntip of the SEIR by anothersingle small axis jump or by successiverelocations of the southerntip of the CIR. In the latter case, the relocalizations are associatedwith a series of alternatingsmall jumps and propagationsof the axial volcanic zonein the NTD at the triplejunction. The lack of well-identified magneticanomalies and fine-scale bathymetricdata does not allow us to better constrain the birth of segment A and its Figure 10. Schematicdrawing of thetriple junction traces, the evolutionuntil A2. Once the new segmentA has been created, threesegments of the CIR andthe segment of the SEIR,partly basedon theGLORIA mapover the SWIR valley(modified from the discontinuityA-C movesaway from the RTJ andbecomes the Mitchell and Parson[ 1993]) andthe SeaBeam maps over the discontinuitybetween today's CIR segments1 and2. At a longer CentralIndian Ridge [Briais, 1995] and the SoutheastIndian timescalethe morphologyof the AntarcticRTJ trace(Figure 10), Ridge[Honsho et al., 1996].Anomalies along the Antarctic triple mapped with GLORIA side-scan sonar data [Mitchell and junctiontrace are shownby solidtriangles: anomalies 2 to 4 Parson, 1993] and our bathymetricdata, showsanother example correspondto theyounger edge of theanomalies shown in Figure of transitionbetween periods of continuousand discontinuous 5; anomalies4a to 6 are from Mitchell and Parson[1993] and mode.Figure 10 showsthat as for the discontinuitybetween CIR correspondto thecenters of theanomaly maxima. 16,574 MENDEL ET AL.: EVOLUTION OF THE RODRIGUES TRIPLE JUNCTION segments 1 and 2, the discontinuity between today's CIR propagationand asymmetry, which result in thecreation of a new segments2 and 3 hasmoved away from the RTJ at the beginning segment,are short (<1.5 Myr) suggestingthat they result more of a period of continuousmode, when a new CIR segmentwas from variationsof shallowmagmatic processes than from deep- takingplace near the RTJ. seatedones. As themantle upwelling beneath the CIR andSEIR To summarize, the two modes of RTJ evolution are associated axesis continuousbut changesstrike at the triplejunction [West with the opening and the closure of offsets between the two et al., 1995], the creation of a new CIR segmentmay be adjacent spreading segments of the CIR and the SEIR, as associatedwith the creationof a new magmareservoir which is previously suggestedby Mitchell and Parson [1993]. The not in a vertical line with the mantle upwelling on the CIR but continuous mode where the CIR and SEIR axes are joined rather is aligned with the SEIR axis. Then the observed (Figure 9a) is unstableand evolves rapidly (<2 Myr) into the asymmetrycould correspondto displacementsof this magma discontinuousmode (Figure 9b) becausethe offset increases sourcewith respectto the CIR mantleupwelling [Palmer et al., rapidly(N6 km/Myr; Figure8b). This transitionis associatedwith 1993],and/or by relocationsof theneovolcanic zone [Grindlay et the formationof a NTD at the RTJ. The presenceof a suchNTD al., 1991]. In the first case,the migrationof the magmasource probablyfavors small discontinuouslengthening of the SEIR, would have the effect of heatingthe overlyinglithosphere and which createsthe en 6chelonpattern of the Antarctic RTJ trace creatingnew zonesof weaknesswhere spreading and crustal (Figure 9b). This NTD stays at the RTJ, following the accretionrelocate [Palmer et al., 1993]. In the secondcase, the lengtheningof the CIR, until a new CIR segmenttakes place and melt freezingin shallowcrustal level magma chambers, between the offset between the SEIR and CIR closes. This transition to the phasesof magmasupply, creates a barrier for the ensuing continuousmode is related to the magmaticbudget beneath the magmaticpulse which may be deflectedto the olderand faulted adjacent segments that controls the lengthening of these crust,resulting in a relocationof the neovolcaniczone [Grindlay segments,their propagationthrough the small nontransform et al., 1991]. Whateverthe origin of the asymmetry,shallow discontinuity, and their connection at the triple junction. magmaticprocesses probably play a dominantrole in controlling Althoughthe continuousmode is not stableand evolvesquickly theinitiation of the CIR segmentationat thetriple junction. to the discontinuousmode, the nontransform offset between the CIR and SEIR axes never exceeds 23 km for the last 8 Myr 8. Conclusions because small CIR axis jumps and asymmetric spreading counterbalancethe spreadingrate differencebetween the SEIR The analysisof multibeambathymetric data and magnetic data and the CIR. Reconstructionsof the junction of the two ridges of the three Indian ridge domainsaround the RTJ revealsthe show that no large offset has occurredat the RTJ for the last evolutionof the CIR segmentation,in its southernpart, and its 35 Myr [Patriat, 1987]. We suggestthat the formation of new relationsto the evolutionof the RTJ sinceanomaly 4 (8 Ma). It CIR segmentsfacing the SEIR segmenttakes place to fit on the suggeststhe followingconclusions. relative continuityof the mantleupwelling beneath the CIR and 1. The CIR was lengtheningcontinuously, more or less SEIR axes. Therefore the succession of continuous and rapidly,to the southeastwith alternatingperiods of asymmetric discontinuousmodes allows to keep a regionalRRR (ridge-ridge- and symmetricspreading. The short-livedNTDs as well as the ridge) configuration for the triple junction which is an segmentsthat lengthenor shortenalong the ridge axis have energetically more favorable configuration [Patriat and revealedthat the CIR segmentationis unstable. 2. The RTJ evolves between two modes: a continuous mode Courtillot, 1984]. where CIR and SEIR axes are connected and a discontinuous 7.3. Further Implications on Mid-Oceanic Ridge mode where the two ridge axesare offset. Thereforethese two Segmentation modesare controlledby the openingand the closingof offsets betweentwo adjacentspreading segments of the CIR and the Two main hypotheseshave been proposedto explain the SEIR. evolution of ridge segmentation: a tectonic control and a 3. Owing to spreadingasymmetry and differencesin axis magmaticcontrol [e.g., Tucholkeet al., 1997]. Accordingto the directionor in lengtheningrates of the CIR and the SEIR, the magmatic hypothesis,segments are positionedabove mantle continuousmode is unstableand evolvesrapidly (<2 Myr) into a upwelling centersor focusedzone of rising melt [Whiteheadet discontinuous mode. This transition is associated with the al., 1984; Lin et al., 1990]. The temporaland spatialvariability of formationof a NTD at the RTJ. The presenceof sucha NTD along-axismagmatic input may thus control the evolution of probablyfavors small discontinuouslengthening of the SEIR spreadingsegments and in the sameway the evolutionof NTDs. which createthe en 6chelonpattern of the Antarctic RTJ traceø By contrast,the tectonichypothesis suggests that ridge segments This discontinuous mode is more stable and can evolve to a fragment,forming new NTDs, and adjust in orientationmainly continuousmode only through the formation of a new CIR following changesin the pole of relative motion [Mdnardand segment,near the RTJ, whichtakes place facing the northern Atwater, 1968;Lonsdale, 1985]. SEIR segment.The evolutionof the RTJ configurationand the Althoughthe triple junctioncontext is particular,the studyof evolutionof the CIR segmentationare thus closely related. the origin and evolutionof the segmentationof the southernCIR 4. The succession of continuous and discontinuous modes gives some clues to testing these competing hypotheses. allowsto keep a regionalRRR (ridge-ridge-ridge)configuration Accordingto our model a new CIR segmenttook placenear the for the triplejunction. RTJ following a southwardpropagation of a CIR segmentand an asymmetryof spreading.The restrictionof this asymmetryto the Acknowledgments.We wouldlike to thankKensaku Tamaki and southerntip of the propagatingsegment probably excludesa HiromiFujimoto, who allowedus to usethe SeaBeam data collected changein the pole of relative motion, which shouldaffect the duringthe KH93-3 cruiseby the R/V HakuroMaru in 1993.We want to thankPeter Herzig, who kindly gaveus a copyof digital SeaBeam data entireplate boundaryrather than only a smallpart. We therefore collectedduring the R/V Sonneduring the Geminocruises. 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(mmtmschy•eost.u-strasbg.fr; dsauter•eost.u-strasbg.fr) Palmer, J., J.-C. Semp6r6,D. M. Christie, and J. Phipps-Morgan, P. Patriat, Institut de Physiquedu Globe de Parri, 4 Place Jussieu, Morphology and tectonicsof the Australian-AntarcticDiscordance F-75252Parri Cedex05, France.(ppa•ccr.jussieu. fr) between123øE and 128øE,Mar. Geophys.Res., 15, 121-152,1993. Patriat,P., Reconstitutionde 1' 6volutiondu syst•mede dorsalesde (ReceivedJune 14, 1999;revised February 21, 2000; l'oc6anIndien par la m6thodede la cin6matiquedes plaques, m6moire acceptedMarch 23, 2000)