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

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Relationship of the Central Indian Ridge Segmentation with the Evolution of the Rodrigues Triple Junction for the Past 8 Myr 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 To cite this version: 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 and gentleslopes 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
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