______O_ C_E_A_N_O_L_O_G_'C_A__ A_ C_T_A_, _'9_8_' _, _N_' _S__ P ~ ______~
East j'itcifk Risc T eclO n k ~ Fine scale morphological Accrelinll pl ate boundary EX l e n ~ ion VOk:ulis rn
I>ors,alc Est·Pacifiquc and structural Tectonique Zone d 'accrétion EXIé:n,io n analysis of the East Pacific Rise, Volcanisme 21 °N (Rita project)
C . Ka ngin ", J . Franchcteau b • LaboratoÎre: de Géoteçtoni que. Uni versité I~erre et Marie Curie . 4. place Jussieu. 75230 Paris Cedex 05. b Centre Océanologique de Bret ABSTRAC T M:mned submersible observatio ns o n ri transcct (rom the axis o f Th e East Pacifie Rise (21"N) 10 the af CU of the Brunhes-Matu ya ma magnet Îc inve rsion werc made during the Cyamex and Rist diving phases of the Rita project. In the nOrlhern pari of the study lu ca. axial. lincar . 400 10 600 m-widc IOpogmphic highs are formed by relatively unfaulted fresh pillow-Iava·flows (zone 1). Flanking the axis of reccnl volc;tnism is a 1.200 10 1.500 m-wide zone of hi ghl y fractured volcanic hill y terrain Ihllt represents the main region of extension as expressed on the surface (zone 2). In the soulhe rn part of Ihe siud y area, the volcani c hi ghs of zones 1 a nd 2 are rlooded by fluid lava flows formi ng a gentle topogntphy. In both areas, this central pan of Ihe Ri sc is bordered by o Ul ward· lilted flau ish blocks that are mai nl y formed by fluid lavas. This zone, zone 3, could represenl a former, large floodi ng episode of the sea floor tII the axis of the Ri se. TeclonÎC acti vil y is particularly evident in zone 2, decreases rapidl y across slrike, a nd vf! ni shes 12 km from the a ", is o f the East Pacifi c Rise . The morphology of the East Pacifk Ri se is controll cd by normal faulting. :Ind by the relative 'Ibunda nce of flu id lavas and pill ows - I , l v ,, ~ which are emplaced al the axis of the Ri se. The differing types of volcanism coul d reflect important variations in magma chamber size or depth or variations in the inSla n!rlneOus accreti on rates al ong the a", is of the East Pacifi e Rise. Ocellrwl. A c/(! . 1981 . Proceedings 26 '~ Int ernational Geologiçal Congress. Geology of ocean!> ~ympo~ IU!ll. l'am. 7- 17 Jul y. 1 ~ {). 15-.24. RE SUM E A na lyse mo rpho logiq ue e t structura le dé taillée de la do rsa le ESI-Pac ifiq ue. 21 °N (projet Rita). Des observati ons par submersible habité ont été réalisées dura nt l e ~ ph,l ses de plongée Cyamex CI Rise du projet Rita. au long d' un tf C"nlr,hUli"n n" 716 ,Ju Cenne Océll ilui oil ique de Il relll" ne. 15 C. RANGIN , J. FRANCHETEAU Cene zone (zone 3) pourrait représenter un ancien épisode d'ennoyage de la dorsale. L'activité tectonique est particuJiërement marquée en zone 2, et décroît rapidement dans [es zones plus externes pour venir mourir à 12 km de ['axe de la dorsale Est-Pacifique. La morphologie de la dorsale est contrôlée par des failles normales et par l'importance relative des laves flu ides et des laves en coussins qui se sont mises en place à J'axe de la dorsale. Les différents types de volcanisme pourraienl refléter d 'importanles variations de la Taille ou de la profondeur de la chambre magmlitique. ou encore des variations du taux d'accrétion instantané au long de l'axe de Ifi dorsale. Oceanol. Aera. 1981. AC le s 26" Congrès International de Géologie. Colloque GéOlogie des océans. Paris. 7- 17 juil. 1980. 15-24. Angus). Information from these tools must be complemen INTRODUCTION ted by direct observ o Z O ~[ 1 - ' : ~~'- J M ~~h cYra'6 " 8 ,. Figure 1 LOCUlÎun of Cytlmex d;"n lU EPR 21 ~N. Locali~ation des plongées Cy:.unex-EPR 2, oN. Fig 1 • , . 16 TECTONICS OF THE EAST PACIFie AI SE AT 21 (Rise) using the submersible Alvin. focused one year later intensive ly fraclUred zone (zone 2) marked by ils horst and o n Ihe southern extension of the region explored by Cyana graben morphoteetonic style (Fig. 2 h). (Rise projett Group. 1980), The extrusion zone (zone 1) is limited to young voJcanic The purpose of this paper is to presc nI the fine seale highs. up to 1,300 10 long. and 400 10 800 m wiùe. morphology and teetonics of crust accreted al a medium The axial volcanic highs (li. h, c on Fig. 2). are not strictly rate spreading center using field observati ons made frorn aligned along a theorelical ri dge axis. Volcanic high (Il) wa!; Cyana on Ihe sea floor, T he study leads to a model for the identified during dives CY 78-03. 04 and 05 (Fig. 3). Stan evolution of a moderately fast sprcading center. II can be ding big her than adjacenl topograpbie higbs it ro rms a compared 10 similar studics eonducled on cither slow simple dome. with a eonSI.lnt widlh o n dives C V 78·04 and (A rcyan'I. 1975 ; Ballard. V'1n Andel, 1977): or muderately 05, but much narrower toward the south on di ve CY 78-03. fast spreading ridges (Van Andel. Ballard. 1979). This volcani c high disappears along strike around 20o_5 4'N. The observations of the shallow teetonies lInd styles of on the southern branch of d ive CY 78·03. On the southern volcanis m provide necessary conlrainsts fo r any model of part o r Ihis dive (Fig. 2 B). the more recent volcanic high is Ihe slructure .. -- ri~ ur e 2 li: Gellerafited Cross Sel"fio,r of ErR lION fm'" Deep ToI!' profUn Ulld selecud Cyamex di\'l'S. b : Cyum ex tl i~es lopogruphk profiles - EPR axis lI·N. "E,r eelre/ol!" axial ~o/c (mic hiRIt.f urt lube/ed Il. b. c. li : Coupe Ilo!nérale de E PI{ 21 N réil l i~é.:: i. p 17 C, AANGIN, J. FAANCHETEAU » ' " - " -' ~'''' CY 78 05 , ~ ~...... " ...... "- ...... ~ ... = .... -:; 1 ,,, 1 ,/-h ' , ,".\rJ in1 r~yI v 1 ~' N"N '" , ,. i, 1 " .'- ~." . w ...... ESE "'w -, cr 78 04 ~ .- ,, ~ t , •• ' ' ! " ~~~.: : " " • D " rc=:::::: CY 78 · 02 ...... '. \'.:,:'":,'" ,., ~ ':" NW cy 18 , 09 . FigJ Figure 3 Le~ hat:hure, ~ou, ch~que profil représentent le, lave, f!uide~. '1 Gt'ological Helio,ls (II Il,1' (ais af EPf/ 11 ",v (""nhe"l di";"g u,eu). indique les talus et P les lacs de lave eHondrés soulignés par ue~ Ver/leu! txugger(llioli .. 4. piliers. Cmsl"t's l/lldtr l'udl pm/il, repfl')'elll f!uid III l'as. T illdica/es wltl! Au·ue~su~ de chilql.le profil. et ue haut en has. sont figl.lré s: 1) ulld P ('o/lupsed lara laku ",arked fi)' pillars. l'échelle de temps de lil plongée ; 2) la prhen~e ue colorillion, AbOt" f'ach profile. fmm /Op III hUllom: 1) dil'i"g rime; hyurolhermale~ (hachures verticales) ; 3) la rraîcheur du ,erre 1) pre, 18 TECTONICS OF THE EAST PACIFIC AISE AT 21 "N The volcanie highs are not faulled. fmcllIres are Iimiled 10 C V 7K-05, Ihis zone cxhibils a sirong asymmelry in morpho· fis ~ ures {dive C V 78-03 : Fig.4}. Faults transverse to the logy. Wcst of Ihe axial high. the pillow·lava rrows are CUI by main direction (N.30) of the ridge are ubiquitous in the a series of faults, 30 to 100 m apltrl resulting in a horst·Jike region of offset of the axial volcanic highs (C V 78-02 : morphology. The top of the faulted blods are cO mprised of Fig. 3), or al Ihe fool of recenl volcanic hi ghs (C V 78·05 : well-preserved trap door and pillow.type basalls. In Fig. 2 band 3). Il is not demonsnated whcther the trans· Co ntnlSt, cast of the axial high. fr acturing is denser. withoui verse faulls represenl the same teclonie regime as the faults lmy apparent vertic.. 1 offset, and none of the primitive pllrHlleJ to the ridge. volcanic hi ghs arc wdl preservcd. SimiJ .. r observll iions The fnlctured extension zone (zone 2). were made on dives CV 78-03, 04. 06. 07 and 10 (Fig. 3 and 4). - Morphology and tettonies. Numerous lalus piles. eilher oorc or slighlly sedimenled are The axial volcanic highs are fJ anked by an intensively present al the foot of Ihe walls. Near Ihe ridge axis, fresh u:: clonized zone with open fi ssures and near vertical faults. I;IJUS wilhout sediment is çommon at Ihe fool of both inward resulting in a horst and gntben terrain , and cXlending 1.2 10 and outw;lrd flu; ing sçarps. Further ,tW,tY from the axis of 1.5 km on either side of the axial VOlcllOi c highs. On dive extrusion. talus piles presenl at the b..1se of outwanl.f;lçÎng , ,, . ... _. : 1 : J ; , ',,, , '''.' ., "" , ., ...... ,,, , , " " .,... .; " " "! " -- ~~"~"-- --- e. " · 06 ...... , ... ,~ ...... ~ .. -~ ..~' ,,·1 - l'fil''''-r,n- ,,"'f -""" 'l-='"-r----r- .\'>yry 'r , , '" ------ ,u »:.= , ", , .. , . ; ."," .". " ... ,'." '", , ,. .•. ,-" ;.;"" ,'. • """, "J' " !", '. , ...... J ...... "' .;. :. f """""" '"'' 1\~~,1'-·" J " " , ,... ' ":;~' .; 'r " . -,~.. "" . ~ 1;1' 18 10 ...... ,, = ...... , .!. , .. . , • •.. , •. , .... SS E ~ , '. < -,' '...... , >" "".,-'- ( .'" '- - , , , , , .. , • f q • ·• i --, - 1 •1 ...... ~ '!,". & .. --, © " " Cl' 7 8 ·03 Fig4 Figure 4 COl1l/llele /;'f!Qlogiclllsuflm,s IlrmuMh lire C)"lIIue.f Mf" , I rom Z ()"~ 1 Coupes géologiques ~omp l èlC S :. Ir;o vers le s e~ leur Cymllex ue 1;0 IQ zo ue J. Sallie caplim, lU" Figure J . 7.tlne l :. la wne J. Même I ~t:ellde que 1:. Figure J. 19 C. RANGIN, J. FRANCHETEAU ~carp s are more and more sedimented, indicati ng a rapitl ln the Famous area. McI)onald and Luyendyk (1 977) decay in the lecloni c activit y o f thcse {aults. In contrast, propo!;ed , on Ihe bllSis of the Deep-Tow datll. a value of 4 tul us assodated with inwrlrd-facing scarps. often shows and 6 percent for the ex t en~ i o n on eilher side of the axial superposition of fre sh talus on top of slightl y sedimcnted zone. The extension reaches l K perccnt o n the in ner walls of talus. a clear indication for re newal of tecloni c .. ' - e-)'.78.06. - " ...... e)'.78.9 • :!::::.: .: u~_ i_.• i .~...... " •• , ••• : •..•..•., ...;;-.; . r-.7-~.. ~. _·~ .~lo=--~======~ .. ' ey.7El..03 - - '-:'- ' • ~_ • • , •• , • , , t:Lt:o:n:o::.-..--.-.-.-.-,-...-.l :" •• ,., .. .. " di· _ -liuilM", " " ". • • • cy•.18_• 10 . C\l-18.o7 ! ,--.~=--- -, ~ ... ~ ..... , ...... ,n.' , ••'DU" ,.T<' TJTr:O:U •• •• •• e)'.18.04 , , ' .- -ln .,•' :,.~.-;<.~. , . ~-- . e)'.78.05 •• •• y.'/~ . ·' ... ",' . . • UUH_U ._O:.:acii:i:uo-' _ .-.-...... _ ·' •• L~ - ,...... Figure 5 tJ "fIlt diagram shows tht ,",uill/i",. of fault thro ...s in ZOlle} ; /lm Le diagramme montre la va riation de s rejets des failles en .tOne 2. domllins can ln distillguishtd ; - II purtl)' txttnsia,.al doma;n. [)eux domaines peuvent être distÎniUé) : - un domainc à extension "'htrt Ihro ..'s 0 11 out ...ard a/td in",ard-Iodng loulls llrfl Cflllrpar/lblt pure où les rejets des {ailles 11 regard interne ou e:>.terne ont des ;n ,'a/ut. This sub:ont (figurtd b)' d()u) is .. ·tll dtfilled tilS/ of the villelln com parables. Cette 5OU)·:.o;one (représenté e par un pointillé) axial "alranic highs : _ IUfllttr U"'U)' from tht axis. :ont 1 is est bien dérinie 11 l'e~t des reliefs volcaniques ax iaux; - cn d,ura r ltri~td b)' a doma;n "'I,ert Iht' /hro .. 's suddtn/y inutaJt 011 ~ 'éloignanT de l'axe. la tone 2 c~t caraclêrisée par un domainc ou le ;II,,·ard·facing lUI/IlS (c) and /,. 0 Insu IWlOllm ail aU /"' llrd-fo rin~ rejet des billes 11 regard interne croît rapidement. alors que cdui laul/.f (d). de s failles à reprd e~tcrnc croit de maniè re plus modeste. a: fissures; b: jll ...ard-facil1g lili/ill ; C : /Jull.'oro-/ocing loults : a ; fissures; b : faille~ 11 regard interne : c : failles à regard externe ; d : IrllllSi"trSt loulls. d : failles transverses. l'lit preStn'O/iQ" QI l'()/cullic g/aH wrd tlt~ reloti,·t impur/an(', 20 TECTONICS OF THE EAST PACIFIC AISE AT 21 eN fig5 ~ Fiw:ure 5 b Diagram s showing /Ill l'oriation al txltnsion in W llt}. Tilt txlension auodaud wilh n/JrmullaullS is compllltJ ussu· min,; an 8(J" dip. Diagramme montrant la variation dc l'extcnsion cn zonc 2. L'extcnsion associée aUIl faille s normales est calcul ée pour un-e inclinaison ue 80" du miroir dc ccs railles. FiGure 5 c l..UWtr righ/ Inu/ : schema/ic rtprtSfn/utlun 01 /tcl/mÎt" proctsse$ /n axial w ne. PU rt u /ens/on is replaced i" Ollftrmos/ url'as hy /ill;n1(. Enca rt inUrieur droit: représen13 tÎon sc hémntiquc de s processus tcctoniqucs dans la zone ax iale . L'cxtension pure CSI prise en relais uans Ics scctcurs plus ellternes par un ba~u lcmcnt. extrusion were in a steady state (permanent axis of extru' intact la va pools (12 h 13 and 11 h 20) which can be detected sio n) : but good preservation of old volcanie highs west of by their roofs of lobate pillows. Along dive profile the elltrusion zone. demonstrate the non·steady state nature C V 711-18 , the smooth nearly horizontal surface of the axial of the extrusion proçess. This fine seale zonation i5 also high (between 13 h 00 and 13 h 35) compriscd of lob."lte locally blurred in zone 2, by sneet flows and old la va pools pillows. coul d represent a large intact lava pool. In the emplaced in zone 2. An expia nati on for the origin of the lava northern area studied by Cyamex. lava pools were ont y pools has been proposed by Ballard et u/. (1979) and rarely present in zone 2, close to the alliai volcnnic high s Franchcteau e/ ,,/. (1979). (C V 78-06, 08 and 12). I n contras\, in tne soUlhern area. lava pools and sheeted lavas are widespread. and the fJows have invaded both zones 1 and 2. The dives conducted during the Rise l'rogram, south of the Cyame ll area (Rise Project The axial zone in the sou/hem s/udy areu Group, 1980), have confirmed the lar8e southward develop ment of fluid lavas in the shallower southern axial zone . We Alo ng dive traverses CY 78-18 and 19.60 % o f the fresh lava believe that the increasing fluid-Iava to pillow flow rati o as flows :He round in lava pits (Fig. 6). Along dive traverse one aces south along the axis re veals 11 fundamental change C V 711-19, the 1,200 m wide extrusion zone is marked by 1I10ng strike in the magmatic rej!ime. f ~ r ;~,t: ::~I =~ :=~ ; . ... ;. : ••• ~2,. , -- , , ...1 1 Il -- " o a" '.' ! I ! 1 1 - _ • "dI« cv 78 le 4&4# •• "_;' -",,,"'~" "'"' " "' "" "'-O"'O'"'"' " O' "" O' " ""~-----~~".. o··· 7;;ë:O:,;:·:~:.ë· :ë;j· <• • , . ... ••••• -l' ,'" ,... ••••••• u ~ , ...... , '- - r-~.,..-·'-. • ". Figurc 6 Gtulo/{icul CTUSS sec/ions in the suu/hem dil,jllg (/Tf/I. Same cup/iUII Ils ;/1 ~---- ~---'----_ " .. Fr Rurl J. _. Coupes ,,'!'alogiqucs dans Ic seC leur m,!,ridional dc pl ongée. Même légendc - - que ce lle de la Figure 3. c:y (G 19 .... ml'---",,' .~....., "," ...... <...... '. ">0 _____"0. ____ ...... ___.... ·"·'-_-""00... ..'0>- __l'" ...." ! " . 21 C. RANGIN . J . FRANCHETEAU The e)l: lermtl zone (zo ne J ) Matuyama reversai area (Fig . 2 a) reveals IWO large o ut er lopographic hil!hs. bounded by inward-facing fault )c arp ~. The a)l: ial zone if flanked on bolh sides by OUI ward lihed block:.. wilh wide flanish surfaces. ThÎ:. area of monolo Diw CV 78· 17 was made in the area betwcen these IWO nous lopography was observed over a wid th of 2 km on topographie highs (Fig. 2 a and 7 {l). Moderately ~edimented either side o f Ihe ridge . tal us was observed at the fool o f the main (aults searps and Dives CY 78·07 and la (Fig . 4) focused on the transi,ion in the depressio n belween the two h i gh~ (1 2 h 47 : CY 7H·17, between zones 2 and 3. Di ve~ C Y 78-03 and 06. reached the Fig. 7 a). The presence of e xposed talus in this area wh e ~e inward-facing fault :.carps at the boundary bctween Ihe IWO sedimen[ cover is important (a few melers). demon ~ trate s zone!>. whcre the offset is amons the largest ob~erv c:d in the very recenl teeto nie :lctivity along Ihese faul ts at a distrt nce Cyame)l: area. The inward-facins fault scarps are arranged of 12 km from the ridge axis. The large offset 1Iiong the symetrically on either side of Ihe axial zone. and show faults (up [050 ml. indiealeli a prolonged f:wlling hislory for pillow basalts alternating with sheeted I,jvas ,jlong the scarp. this fragment of oceanic cru~t. T hc tOp surface of the fiat and smooth lil tcd blocks i~ ln Ihe Brun hes-;\Iatuyama re"t'rsal aN'a (dive traverses Intl Înly formed by slightl y sedimented lobate pillows. Along C V 78- 15 and 16). the predomi nfl nt inward-fadng scarp ~ M C di ve tr;wcrse CY 78·07. 200 m wc SI of the M undary free of any talus (Fig. 7 b). ln this area every fnult is bctween zones 2 :lOd 3. sediments li re 40cIn thick. ,md inac tive. We concJ ude that [eclOnic aClivit y extcnds al lens! lobale pillows d i~a ppea r progressively beneath Ihe sed iment 12 km from Ihe ridge axis. in the regio n of dive CY 78-17. eover. but is clearly absen[ 20 km from [he axis in Ihe reversai Topographie highs comprised o f pillow nows similar to area. those present in zone 2. we re nOI seen in zone 3. The thi n sediment eover in this area C;j nnol account for the f1 attish morpholoSY present in zone 3. The steady increase in sediment cover lowards the external !.One of the ridge. DISCUSSION allows us 10 rejecl lhe hypothesis of off-axis volcanism. The tabular morphology and Ihe large amount of lobate p ill ow~. suggesl a magma ti c regime domina[ed by fl uid lavas. Se veral observations made d urinS the Cyana dives are The numerous fresh talus piles present at Ihe base of the worth poi nting out : faul t scarps. suggest a continuous teetoni c activity in Ih is a) the East Pacifie Ri st: at 21°N ca n be subdivided inlo an J:one. In eon[rast, outward-facing faults a re scarce. and ac[i ve teetonic zone from the ridgc ax is \() li distance of open fissures :Ire frequentl y pre ~ e n t bclween inward-fa cing 12 km awtl y from the aX Îs. and ;10 inacli ve tectoni c zone faults scarps a nd talus piles. Nenr the boundary of zo n e~ 2 beyond : ;md 3. the fnults are sp:jced 200 m apart in a direction no ~m ,,1 to the uxis. Fnrther out. the frcqucncy of fiOul ti ng h b) thc acti ve [ectonic zone is characterizcd by IWO principal deerensing progressively. The ex t en~ i on;11 process. seems 10 styles of morphology, modi fied lalcrall y by tecto nic proees bc concen[rated along a few faults in the outward most area ses: rough terrain formed by volcanie highs wit h Sleep of zone J. pillow basa l! fl ow-fronts. domi naling in the norl hern Cya mex area and smooth tGrrain formed by fluid lavas a nd partiall y coll apsed 13v1l pools, as in the southern :uialzone GEOLOGY O F T HE OUTER I NACT I VE ZONE and in I.o ne 3 : A Ocep Tow profile ( K . MeDonald . pers. comm.) a c ro~ ~ Ihe d the extension due to faul!i ng is ,mail (less than la '/f) in Ea ~t Pacifie Ri se at 11 N. fmm the axis ln the Orunhcs- zone 2. bu[ ~ hows im portant vitria tions from place 10 place r-- Figure 7 Geulugieul crQ$$ stC/iolls ilr /Ire ou/t r :ont. Il : Ste/ion IUI'u/t d l:!km [rom /ht (..... u tI[ EPR. Sli~lrlly stdiml'flItd falus suggt~1 fl.'twnic ac/i";I)' jus/ ('tastd. b; Sec/rem s in I l,,~ Brullhes A!MII)'ama ma~ lI tlie rel'usal urea. Coupes géologiques dan~ l~ 1.onc clI:terne. Il : Coupe ~ ituée • 12 km de l'axe de EPR. Le ~ mlus légèrement ~é dimen t és § um.~rent que L,nét ut! 1' .. ~tr\lIC le(tonlque ",,, re,;crU . 1> c,)upcs d:m) Je ""'<:\"'Ut d'm\crsion !Iu!;ne",!u,", Ur ul\hc~- M atu).un ... '.' 22 TECTONICS OF THE EAST PACIFIC AISE AT 2t in this Zone (3 to 6 % ). The incre,lse in the extension EVOLUTION MO DEL. AND CONCL USIONS outward in zone 2. could be related to outward lilting. Two main cycles of vo!canÎsm a! ternate along the axis of the Continuous and discontinuous regimes arc ruling the geody EPR (Fig. K h), We arbitrarily consider on Figure 9, tha! the n ; ,.. - Mter flooding (Fig, 9 8). a new pillow basait cycle is ~- ini tiilted, TopOgraphie highs with steep flow fronts and made up of piliOW lavas, are built upon the subhorizontal sheet flows : - during a third stage (Fig, 9 q, 23 C. AANOIN , J . FAANCHETEAU Figurc 9 , , , E"olulioll of Ih/;' ElUI P Acknowledgemcnts colleagues m,Ide observatio ns crÎtÎ cal to thÎs paper. A nthony Laughton a nd Bob Ballard read the manuscript and made val uable commenls_ We Ihank Alain GrOlle for Wc tha nk the Centre Natio nal pour rExploitatio n des Océans (CNEXO) and the Comité Scientifique des Submer d ra fting. s ibles for e ncouraging French participation in project Rita The expeditÎon was funded by CNEXQ with contributio ns and for sponsoring the Cyamex expedition. Wc thank from NSF and usas. Woods Hole Oceanographie Institu o fficers, c rew, pilots, engineers and technicians who contri tion. N;J !lon;J1 Gcogmphic Socie t y and the Mexic;J n govcr buted to the success o f the Cyana dives. Our sdentific nmenl. REFERENCES Allmendlnger R. w. , Riis t·., 1979. The Ga!apa8o~ Rift al 86"W. 1. Klil gord K. D .• Mudle J. O., 1974. The Galapagos spreading cen· Re gional morphoJogical and structure analysis. J. Geopltys. 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