Twin Active Spreading Ridges in the North Fiji Basin (Southwest Pacific)

Twin active spreading ridges in the North Fiji Basin (sou t hwest Pacific)

Jean-Marie Auzende* IFREMER (Instliut Franqais pour la Recherche et I Exploitation de la Mer)/CB, BP 70, 29280 Plouzane cedex, France Bernar Pelletier ORSTOM (Instltut FranGais pour la Recherche en Cx” operation), BP A5, Nournea cedex, Nouvelle-Caledonie Yves Lafoy Service des Mines et de I Energie, BP 465, Nournea, Nouvelle-Calbdonle

AB srux Recent work on the North Fiji Basin itxial spreading ridge and on the active propagating ridge located west of the Fiji Xslands has established the existence of two active spreading ridges that have been functioning synchronously in the basin since at least 1 or 1.5 &fa.‘lhis Iiypothc~sis is tested and tlie kineinafics of tlie t.i+in actiw ridges are refined further on the basis of analysis of hathymetric and magnetic data.

Figure 1. Simplified bathymetric map of North Fiji Basin. Boxes indicate multibeam full-coverage areas. CSR = central spreading ridge; WFR = west Fiji ridge. Bathymetric contour interval = 1 km. Inset shows geody- namic setting of North Fiji Basin. Box represents surface covered by bathymetric map. SI = Solomon Islands; 0 = Vityaz Trench; PP = Pacific plate; KT = Kermadec-Tonga Trench; NZ = New Zealand; AiP = Australia-India plate; NCR = New Caledonia; NHT =

New Hebrides Trench; DEZ 2 D’En- treCaSteaUX zone; A = Australia.

0. R. s. 7.0. M. onds Documeataire, N” i 43463 17"IO' I I I I I, I S

I I at 176"OO' E 176'40'

250 nt 6 __1

Figure 3. a: Simplified bathymetric map of west Fiji propagating ridge. Contour interval = 250 m. Depth values every 0.5 km. PR = propagating rift axis; SR = southern rift; FR = failed rift; PT = propagating tip; TZ = transform zone; TL = transferred lithosphere; IPF = inner pseudofault; OFF = outer pseudofault. AB and CD = magnetic profiles shown in b. Stipple indicates anomaly 1; J = Jararnillo. Random stipples represent transform zone. b: Magnetic profiles across west Fiji ridge.

Figure 2. Simplified bathyrnetry (contour interval = I km) and magnetic lineations along central spreading ridge (from Auzende et al., 1990). In this p;ipcr we propose the existence of segment is similar to that of slow spreading Shading represents anomaly 1. J-Jararnillo ridges; there is u central graben III anomaly (1 Ma); 2-anomaly 2 (1.98 Ma); 2A- twin ;icti\-e ridges in the North Fiji Basin on >3000 anomaly 2A (3 Ma). Ages of anomalies are after the basis of inorphological a~idm;igiietic ev- dccp bounded by 25fhi-high ridges or-i- Cande and Kent (1992). idcnce ;ind discuss the implicatiolls of such ented 005". a dual system. 2. Between lat 21"OO'S and 18"1O'S, the north-south segment is offset -80 kin to the CENTRAL SPREADING RLIX I: \vest from the southemmost segment, and The central spreading ridge is mapped by its rnorphology is typic:iI of fast-sprcading multibeam over an area of inore than 1" of ridges (Macdonaid, 1982), with 311 8-km- longitudc and 8" of latitude (Fig. I), and c;ln wide, 200-300-m-high axial dome. This be divided into four niajor segments (Fig. 2). donie is cut locally in its middle by ;+X- 1. 13ctween lilt 22"s and 20"10'S, the ac- 500-m-w,ide and 20-50-in-dccp gnbcn. The tive spreading axis is dclinccl only by thc ;utial dome is bounded on both sides by iclciitilicstion of the axial iii;igiictic :uiomaly noltli-lrcncling abyssiil hills representative (M;iillet et al., 1986). 'I'hc moi-pliologyofthis of an c:ist-\vcst sprcxling direction. 'Tliis been reinterpreted as a propagating rift (Auzcndect al., 1993).

MorphostnIctural Units ,.1he propagating ridge (Fig. 321) is located in the \vcsterri part and consists of a central 6" ridge bounded by two y:ihcns. The eastern S gralicii is \wll dcvclopccl, and has U constant 10 kni width :ind 4000 in depth. The western gr;iben is ii;ii-rowcr (2 to 3 kin) and s1i;ilIowcr (3000 in). 'The ccntrnl ridge is 7-8 km widc and 2750 in tfccp. Its north-south to 00.5" 8" trend chaiipx to 155" at its southern tip. The \vidth of crust iiisicic the pscudof:ir~ltsvaries froin 40 km in thc north to X km in the south. South of 17"44'S, the ridge clisappcxs rind is replaced by ;I 3000-1-n-dce11pi-cqxigating tip (1,Iey et ;il., 19Sb) within converging pscudofaults. 2 0" 'rhc o~ter1)s2tlCi0C;lllit ~O~-CS~IO~KISto ;I 15i"-trcniling kiiilt 1li:rt 1~c)undsthe CI-LIS~ foi-rued by the propigating ridge. '1-his fault sep:iratcs the pi-Lipag:itiiig ridge from the occ;inic hottom grain, which trciicls 015"- 020". Norih of 17'.?5'S, thc inner pscudof~iult 2 2" trciitls 015" ;ind converges with the outer l~~~L~(i~)f;~~l~t:It :I 40" :II~~IC;south of I 7"15'~, the 005" inner pscudofiiult coiivcrgcs \\.it11 the Olltcr pScc"c1oi:lult at ;I 20" angle. x less ~cllm;irkcd scarp cast of the main inner pscuLlof~lultcoultl be ;I sccondxy ]7"CUdO- fault that convcrgcs ;it a 55" nnglc \vith the aure 4. Kinematic sketch of twin ridges. NFFZ = north Fiji fracture zone; CR= central Fiji frac- outcr pscll'lof~iult. re zone; SFFZ = south Fiji fracture zone; CSR = central spreading ridge; WFR = west Fiji ridge; The southern ridge (,Fig. 3) is charactcr- 6 = spreading rate (in cmiyr) calculated from magnetic data; 0-2? = inferred spreading rate izcd by a 3000-n-clccp, 7-kn-\vidc axial rift 1 cmiyr). Arrows at tips of ridge segments indicate direction of propagation. Contour interval = v:illcy ti-enciing 010". In the eastern part of tin. A = western North Fiji Basin plate; B = intermediate microplate; C = north Fiji (Pacific?) plate; the doimin the failed rift is coinposctl of ;I = southeast Fiji (Australian?) plate. ion of OlO"-Ol5" ridges abutting ;I 151)" Pault. ~I'licscridges are 2-3 kin widc and .gmcnt is propagating noi-thward, its dcrn- iictic aiiomaly is present aiid well dcfiiicd 2750 in deep, :id arc probably the 01~1 istratcd by Alteris et al. (1993). along the entire axis; the J (Jaraniillo) :iiioin- spi-eading:ixis, shut down wlieii the westcrn 3. 13ctwccnlat Is"10'S and the 16"50'S tri- aly definitely exists on the two soutlici-11scg- active rift propagated sciurli. c juiiction (Lafoy et al., 1990), the sprc:iri- ments and probably on the 015" scgmcnt and The junction hetwcen the prop~ig:itingrift g riclgc trends 015". In tlie northcrri part, it 011 the southern part of the 100" segment. ;ind the southern I-idgcis coniposcd oP a widc charactcrizcd by a high double ridge cul- The 2 ;ind 2R anoni;ilics are clearly dcliiied zoiic of xcuatc siriail i~itfgesinterpreted :IS ;I inating at <2500 in depth, bounding a 1-2- all along the centr:1l north-south scgmcnt. transform zone :is tldined by I-Icy et nl. n-wide, 100-111-deepgrabcii. The wis is The cwlculateti average spreading rates, c31- (19%). Transferred lithosphere is locatcd :gruented about every 30 km by transverse culated with 3 070" spreading trend, vary north of the ti-msforni zone in the central ructures. from 50 km/m.y. on tlie 160" and the 015" area, and comprises fan-shaped structures 4. North of the 1V50'S triplejunction, the segments to 80 km/m.y. on the central and that arc :is deep ;IS 3000 m in the northern ircading ridge consists of 3 succession of southcrnmost segments (Auzcnde et al., part. irec 3500-4000-m-decp grabens aligned on 1990). The spreading r:ite of the west Fiji ridge 160" direction. This 1110rph010~9yis typical was estimated to be 30 kni/m.y. (Pellcticr f slow sprc;iding ridges (M:icdonald, 19Y2), WEST FIJI PROPAGATING RIDGE and Louat, 1989), taking into account the kin- 'though the spreading rate calculated froin The west Fiji propagating ridge was ematics of the Junction bctwceii the North iagnctic lincatio~ianalysis is 50 to 60 kmi mapped during the Seapso I11 cruise of the Fiji and Lau Ixisiiis. The magnetic anomaly 1.y. The northcrii tip of tlie 160" axis con- R~/V.TemCharcot (December 1985) (Figs. 1 lineations and profiles in Figure 3 (a ant1 17) sts of a triple junction connccted to the and 321). It is associated with significai~tshal- also support thc existencc of a11 active orthcrii a1111 northwestern sprcding cell- low seismicity (Hamburger and Tsacks, spreading axis. In Ihc no~-lhwcstci-iipart of 'rs of the North Fiji I3;isin. 1988; I'cllctier and IBuat, 19s')). The focal the suivcy rliei-c is :i well-ticfined anomaly The m~rgneticlincatioiis (Fig. 2) have bccn nice h ;in i sms are noi-iii~il or s tri ke-sl i p f;iu I t- col-rcsp~~ndingto the pi.op;ig:iting rift sys- ;:ippcd :ill along the ccntr;il I-idgc of the ba- ing solutions. I'rcviously intcrprctcd ;IS :I tem; the ;inoiii;~lyis iiltcrpl-etcd ;IS ;inom- ll (/\rlzcI1lic et cli.> iwo). n~cclltl-;li lll:lg- strike-slip dcformation mnc, thc ridge h;is :rly I, irnd its width dccrcascs from north to south. On both sides, especially in the west, lapping spreading centers (hlacdonald, 1982) Hamburger, M.W., anti Isucks, I3.L., 19S8, Dif- and propagating rifts (Hey, 1977) or at the fuse back-arc deformition in the south\vest- a liiagnctic lineation could be identified ;is ern Pacific: Nature, v. 332, p. 599--603. anomaly J (Jaramillo, 1 h4a) (Fig. 3, a and b). same scale on the East Pacific Kise in the Hey, K.N.,1977, A new CIXS of psc~do-C:l~~lts rile magnetic pattern coincides well with tlie ai-ca of the Easter (Searle et al., 1989; Naar and their bearing on plate tectonics: A prop- structural interpretation (Fig. 3a). Anom- ct al., 2991) and Juan Fernandez (Franchc- agating rift Inodel: Earth and PlanetaIy Sci- aly 1, and possibly anomaly J, exists in tlie (eau et al., 1987; L.;irsoii et al., 1902) mi- ence Letters, v. 37, p. 321-325. croplates. This implies that the central Hey, R.N.,Kleinrock, M.C., Miller, S.P., Atxva- south-central part of the survey, coinciding tcr,'T.M., and Searle, R.C., 1086, Sea Ikinli with the southern rift. Hetwccn hoth axes ;i spreading ridgc and west Fiji ridge are prop- deep-tow investigation of an active oceanic large transverse c;ist-west negative magnetic agatiiig in oppositc sense. The result is the propagttins rift system, Gxlapagos 95.5"W: lineation can be intcrpretcd as the traiisfcmn individualizaticiIi of a microplate bctwccn Journal of Geophysic;!l Research, v. 91, 1'. 3309-3393. zone between thc propagating rift iiiid the both ridgcs since the generation of tllc west

Lafov.>, Y..I Auzende, J.M., Ruell;in, E., lIuchor1, southern rift. Tlie spreading ratc of the wcst Fiji ridge, 1 or 1.5 hla. I)., a11d I-IcwL~,' E., 1390, The 16'1CIS ti-iplr Fiji ridge, givcn by the magnetic data iritcr- Other csainplcs of twin ridge systems, ai- junction in the North Fiji Basin (SW 1'3- pretation, is .?5 kn1;rn.y. on both the prop+ though not yct very well clocunientctl, can cific): Marine Geophysical Research, V. 12, gating and soiitliern rifts. Considering tlic be foiind in the south\vest Pacific marginal 1'. 285-296. Larson, R.L., and eight others, 1992, Roller-hear- basins, e.g.. in the northern Liiu Basin, 41)" a~id20" aiiglcs ixtwccn the outcr and in- ing tectonic evolution of the Ju;ln Fernandez iicr pscudofinilts, the calculated propiityting Lv1ici-c thc Pezgy ridge and tlic 175"W ridge n3icroplate: Nature., v. 356, p. 571-276. velocity ob1aincd using the forniul:~of I Icy :ire acti\,e [Pellcticr and Louiit, IWI), mid in Macdonald, K.C., lOXZ, Mid-oce:tir ridges: Fine et al. (1986) is 42 kiii/iii.y., and incrcascs up the Maniis 13asin [?'a).lur et al., 1992),where sc;iIi: tectonic, volcanic :incl hydrothermal to 96 km/m.y. for thc southern tip. the accretion cciuld also be locatcd on at proc~~cswithin the plate boundaty zone: Exth and Planetaiy Science Inttel-s, V. 167, Icast ILVO ;id\ e spi-cading ccntcrs. p. 155-19(1. DISCUSSION AND CONCIAusIONS hlaillet, P.,Eisscn, J.P., Lapouille, A., hloiizier, The con$ la tion of the s t TUCt U rill a11d ACKNO~~I.Ei)G~lES'lS M.,I~;ileiv:iriualal:i, V., 13iitscher, J., Giliois, inagiletic data allows us to coniiriii the c~x- We thank aiioiiyiious rr\+n.ers for (lieii- corn I:., and I~rtly;M., 1986, 1L;i dorsule active du iiii'n iid i.5 Inssin No Fidj ien e n t I-e20'00' S et 2(1"23 ' S ; istencc in the North Fiji Basin of two p:iral- slriictive ci~iii 1s. a he scien t t s iiivolverl in id- tlic Se:apso :iiiil Stiir~nercruises. sigmtui-e magriCtique et iiioi-pholrigie: Paris, Icl, synchronously ;ictive sprcading ridges. Acad2niie des Sciences Coniptes I?eridus, The itiagnctic lineation analysis dcmon- KEFEHESCES crm sCr. 2, v. 302, p. 1.15-130. stratcs thc gcneratio~iof the ccntr;il sprcacl- Alteris. G.di.. mil eight others, lc193>1)ropag;iling hlalaliolf, A., Ftden, 1i.I J., snd Fleming, I-I.I.'., ing ridge to its prescnt-day position froin 3 rifts in tilt SLirtli Fiji I3asin (south\\wt Pa- 1982, Magnetic anomalies and tectonic fiibric cific): (itology. V. 21, p. of inrirginal basins north of New Zedand: Ma, at an average spreading ratc of kin/ SH3--586. 00 hrielle, Y., Schmf, A., Journal of Geophysical Research, v. 57, n1.y. The identification of thc J (Jarmillo) en, J.P., l9S6,Tectonicjue p. 4109-4125. anomaly on both sides of the west Fiji ridge intra-ocianiqiie dr'.crochsiite I l'oiicst des iles Naar, D.P., h?artiiiez, F., Hey, R.N..Reed, T.B,, implies that propagation bcgaii at Icast 1 hla, Fidji (I3assin Nord Fidjien). Cmpagne SEA- and Stein, S., 1991, I'ito rift: Ffow a large- and was related to the left-lateral strike-slip PSO III du N.O. JLWIICharzuf: Pxis, Xcad- offset rift propagates: Marine Ceupliysicsl Cmie drs Sciences Colilptes Rendus, V. 303, Research, v. 13, p. 287 -309. motion of the north Fiji fracture zonc (Lafoy p. 241--216. Pelletier, R., and Louat, R., 19S9, Mouvenient et al., 1990). Auzende, J.II.. Lafoy,Y., and hlarssct, 13., 1YS8, relatif des plaques dans le Sd-Ouest Paci- The southern termination of the west Fiji Recent scodynaniiic evolution of the North fique: Paris, AcadCmie drs Scienccs ridgc (Fig. 4) is formed by a11 oblique feature Fiji 13asin (south~vest Pacific): Gec~logy, Comptes Rendus, sbr. 2, v. 308, p. 123-130. trending 060", the ccntral Fiji fracture zone v. 16. p. 95-929. Searle, K.C., and eight others, 1989, Comprehen- Auzende, J.Il.. and 29 others, 1990, Active sive sonar imaging of the Easter microplate: (Pelletier and 1-ouat, 1989), which could bc spreading and hydrothermnlism in North Fiji Nature, V. 331, p. 701-705. interpreted as a transform zone between Basin (southxmt Pacific). Results of Japa- Taylol.,n.,Crook,K.A.W., Sinton, J.M.,andPe- both the central spreading ridge and west nese-French cruise Kaiyo 57: Marine Geo- tersen, L., 1992, Manus Basin, Papua New Fiji ridge axes. If we assume that tlie sprcxi- physical Research, v. 12, p. 269-283. Guinea, it1 Pacific seafloor atlas, SOPAC rc- Auzende, J.hl.. Hey, R.N., Pelletier, and gion: IUG Sheets 1-7. ing rate in the North Fiji Basin is constant I3., Lafoy, Y.,1993, Propagation d'une zone south of the north Fiji fracture zone, the sum d'accrCtion 6 I'est de In dorsale ciu bassin Manuscript received July 6, 1993 of the rates of the central spreading ridge and Nord Fidjien: Paris, AcadCniie des Sciences Revised manuscript received October 6, 1993 the west Fiji ridge north of the central Fiji Coniptes Rendus, sCr. 2, p. 1536-1552. hlanuscript accepted October 12, 1993 fracture zone inus be cquivalent all along Cande, S.C., and Kent, D.V., 1992, A new geo- magnetic polarity time scale for the Late Cre- their length. Taking into account the spread- taceous and Cenozoic: Journal of Geophys- ing rates deduced from m:ignetic anomaly ical Research, v. 97, p. 13,917-13,951. identifications, a rate of 0 to 20 kdlll.y. is Chase, C.G., 1971, Tectonic history of the Fiji inferred for the southcrnniost part of the Plateau: Geological Society of Arnerica Bul- west Fiji ridge. This low rate is col11p;itjble letin, v. 82, p. 3087-3110. Franchrteau, J., Yelles-Chaouche, A., and Craig, with the near absence of scisrnicity along I]., 1987, 'The Jukin Fernandez microplate this southern part. Following this sc-ficnlc, iiort h of the N3zcn-Pacific-,l\ntarctic plate spreading rates increase to the south aio11g junction at 35's: Earth and Planetary Science the central spreading ridge segmcllts 2111ti to Letters, v. 86, 1). 253-286. the north along the west Fiji ridgc. These characteristics can be coinpared directly to those observed :it a ~i~iallerscale on over-