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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. B9, PAGES 19,207-19,220, SEPTEMBER 10, 2001 Revised tectonic boundaries in the Cocos Plate off Costa Rica: Implications for the segmentation of the convergentmargin and for plate tectonic models Udo Barckhausen,• Cesar R. Ranero, 2 R. von Huene, 2,• Steven C. Cande,4 and Hans A. Roeser t Abstract. The oceanicCocosPlate subductingbeneathCostaRica has a complexplate tectonichistoryresultingin segmentation.New lines of magneticdata clearlydefine tectonicboundarieswhich separatelithosphereformed at the East PacificRise from lithosphereformed at the Cocos-Nazcaspreadingcenter.They alsodefine two early phase Cocos-Nazcaspreadingregimesand a major propagator.In additionto thesesharply definedtectonicboundariesare overprintedboundariesfrom volcanismduringpassageof CocosPlate over the Galapagoshot spot.The subductedsegmentboundariescorrespond with distinctchangesin upper plate tectonicstructureand featuresof the subductedslab. Newlyidentifiedseafloor-spreading anomaliesshowoceaniclithosphereformedduringinitial breakupof the FarallonPlateat 22.7 Ma and openingof the Cocos-Nazca spreadingcenter.A revisedregionalcompilationof magneticanomaliesallowsrefinementof platetectonicmodels for the earlyhistoryof the Cocos-Nazca spreading center.At 19.5Ma a majorridgejump reshapedits geometry,and after -14.5 Ma multiplesouthwardridgejumpsled to a highly asymmetric accretionof lithosphere. A suspected causeof ridgejumpsis an interactionof the Cocos-Nazca spreadingcenterwith the Galapagoshot spot. continentalslope[vonHueneet al., 1995;Hinz et al., 1996],and even forearcuplift nearshore[Fisheret al., 1998]. The downThe Central America convergentmargin off CostaRica and going slab along Central America changesdip significantly Nicaraguahasbeen an area of concentratedstudyduringthe [Prottiet al., 1995a],andthe geochemistry of arcvolcanicrocks past decadebecauseof its variable characterin a relatively and the alignmentof volcanoeschangessimilarly[e.g., Carr smallarea and its well-imagedsubductionzone. Recent pub- and Stoiber,1990;Patinoet al., 2000].A recentstudyshowsthe licationsreport a distinctivesegmentationof the upper plate degreeto which characterand relief of the subductinglower tectonicstructureand relate much of this to a corresponding plate relatesto upper plate tectonismand arc volcanism[von segmentationof the subductingCocosPlate. This segmenta- Huene et al., 2000]. However, preciseage information and tion was recognizedin a progressionof studieseach contrib- identification of tectonic boundaries of the Cocos Plate are uting to an increasingunderstandingof the tectonicorigin of lacking.In this study,we focuson the integrationof new data et al., each plate segment.The existenceof a rough and a smooth with previouslypublishedcompilations[Barckhausen 1998] that answer some of these questions. We present a demorphologicaldomain on the CocosPlate was noted in the tailed magnetic anomaly map including -8000 km of new data early 1960s [Fisher,1961] (Figure 1). In the early 1990s a comprehensive multibeambathymetricsurveyof ocean crust and analyzethe tectonicsettingof the studyarea in the frameanomalies. was made off CostaRica [vonHuene et al., 1995]. This study work of the regionalmagneticseafloor-spreading This constrains crustal age and precise location of major tecshowedsharpboundariesbetweenthree morphologicalsegtonic boundaries. The crustal ages permit us to revise the plate ments on the oceanicplate: (1) smoothseafloorfacing the tectonichistoryof the Cocos-Nazcaspreadingcenter (CNS) Nicoya Peninsula,(2) a segmentwith abundant(40%) seafrom the breakupof the Farallon Plate at 23 Ma to 10 Ma. mountsto the southeast,and (3) CocosRidge enteringthe 1. Introduction subduction zone off Osa Peninsula on the southern Pacific coastof CostaRica. It becameclear that the roughnessof the seafloorsignificantlyaffectsthe shapeand the tectonicsof the 2. Previous Work The first consistent models of CNS evolution and the for- mation of the aseismicCocosand Carnegieridgeswere de•Bundesanstalt fiir Geowissenschaften und Rohstoffe,Hannover, rivedbyHey [1977]andLonsdaleandKlitgord[1978].From the Germany. 2GeomarForschungszentrum fiir marineGeowissenschaften, Kiel, analysisof magneticand bathymetricdata they concludedthat the FarallonPlatebrokeinto the CocosandNazcaPlatesalong Germany. 3Alsoat Departmentof Geology,Universityof California,Davis, a preexistingfracture zone in equatorialregionsat •27 Ma. California, USA. Accordingto thismodelthe newlyformed CNS interactedwith 4Scripps Institution of Oceanography, University of California, San the Galapagoshot spot,which simultaneously depositedvolDiego, La Jolla, California, USA. canic material Copyright2001 by the American GeophysicalUnion. Paper number 2001JB000238. 0148-0227/01/2001JB000238509.00 on both sides of the CNS to feed the Cocos and CarnegieRidges on the Cocosand Nazca plates. Magnetic seafloor-spreading anomalieshad been identifiedin the inner regionof the CNS alongthe activespreadingaxisand southof 19,207 19,208 BARCKHAUSEN 105øW 15'N =• • -.•-•. ET AL.: COCOS PLATE TECTONIC 95øW 100øW Ill IllIll Ill Ill Ill BOUNDARIES 90'W Ill I 85'W Ill I III ..... ß"•'-.:'i: '":i: ß I .... ..... :•L.;: ß,.'.' .:'• •.... •..,;.....:.:.. •.-/."• •..... •...• • ..:.•}:•... ..... :..::..;.• " " 10'N Plate, ß"' Nazca Plate I 105'W 100'W / 95øW Figure 1. Bathymetricmap of the Cocos-Nazcaspreadingregion basedon satellite altimetry of Smith and Sandwell[1997];EPR, East PacificRise; CNS, Cocos-Nazcaspreadingcenter;PFZ, PanamaFracture Zone; MAT, Middle America Trench. Arc volcanoesin Central America are shownas triangles.Arrows indicate absoluteplate motionvectors[DeMetset al., 1990].The rough-smoothboundaryis expressedclearlyonly in the westernpart of the CNS region.The small rectangleoutlinesthe area in Plate 1 and Figure 4; the large rectangleindicatesthe area coveredby Plate 2. the CarnegieRidge. In the areasof the submarineridgesthat were overprintedby hot spot related volcanicactivityno lineated anomaliescouldbe identified.North of CocosRidge the identificationof seafloor-spreading anomalieswas alsoimpossible at that time becauseof the paucity of data and complicated anomaly pattern [Hey, 1977]. Later, Wilson and Hey [1995] revisedthe magneticanomaliesof the inner part of the CNS and carefully documentedanomaliesyounger than 10 Ma, includinga pattern of propagatorsand smallridgejumps. Today,oceaniccrustalongthe westand southboundariesof the CocosPlate is generatedby activespreadingalongthe East PacificRise (EPR) and the CNS. Oceaniccrustformed at the EPR has the featurelessmorphologyand low-amplitudemagnetic anomaliescommonto fast-spreadingridges [Hey, 1977; Wilson,1996]. Oceaniccrustcurrentlygeneratedat the CNS near the triple junctionwith the EPR is formedby slowspreading. It has a rough topographyand high-amplitudemagnetic anomalies[Wilsonand Hey, 1995]. Hey [1977] mapped the resulting"rough-smoothboundary,"separatingtwo provinces formed at two different spreadingcenterswithin the Cocos Plate (Figure 1). Hey [1977] projected the rough-smooth boundary from the seaward area where it is well expressed, landwardto the southerntip of the Nicoya Peninsula.However,sinceno seafloor-spreading anomaliescouldbe identified in the area off Costa Rica, the location of the boundarywas defined from bathymetricobservations[Hey, 1977]. In addition, Hey [1977] pointed out that in the older part of the CNS-derivedCocosPlate crust the magneticand bathymetric rough-smoothboundariesare geneticallydifferent and do not necessarilycoincide.Near the Middle America Trench (MAT) the fine-scaletopographyof the oceanic basement is fairly smooth, and the rough-smoothboundarywas defined at the limit between an oceanicdomain with numeroushotspot related volcanicedifices(ridges,conicalvolcanoes,and guyots) [vonHueneet al., 1995]and a domainwith smoothtopography. Barckhausenet al. [1997] showedthat magneticanomaliesof the CNS continue north of the morphologicalrough-smooth boundary, but the exact position of the boundary between EPR- and CNS-generatedcrust was not clear from the magnetic anomalydata. The magneticanomalymap compiledfrom data acquiredduring cruise SO-76 [Barckhausenet al., 1998] showedtwo different patternsof seafloor-spreading anomalies off Costa Rica, both being attributed to the CNS. However, since the survey area was relatively small and the magnetic signaturesof numerousseamountsare superimposedupon the linear anomalies,it was still impossibleto identify seafloorspreadinganomaliesand clearly define tectonicboundariesin the oceaniccrust. Wilson [1996] analyzedseafloor-spreading anomalieson the EPR-derived part of the CocosPlate along the rough-smoothboundarybetween94øW and 88øWbut did not extendthe identificationof anomaliesand the triple junction trace eastward to the MAT. The bathymetricrough-smoothboundary has been widely usedby different authorsas the trace of the triple junction of EPR and CNS off Costa Rica and inferred as a major lithosphericfeaturethat explainspatternsin the configurationof the BARCKHAUSEN ET AL.: COCOS PLATE TECTONIC BOUNDARIES 19,209 subductingslabalongthe continentalmargin [e.g.,Pratti et al., following terms for the boundarybetweenthe EPR- and CNS 1995a;Marshallet al., 2000;Patina et al., 2000]. The boundary derived provinces:The youngerpart of the boundarywhich between the seamount domain and the smooth domain is formed after the breakup of the Farallon Plate at the Ridgemarkedby a tectonicboundarythat is shownin seismicdata as Ridge-Ridge triple junction will be called "triple junction an abrupt but smalljump in the depth of the top of the base- trace." The older part of the boundaryalongthe fracture zone ment and the baseof the crust[vanHueneet al., 2000]. There- where the Farallon Plate broke up is called "fracture zone fore there are at least two tectonic boundariesin our study trace." The triple junction trace has an oblique angle to both area: the traditional rough-smoothboundaryassociatedwith a the EPR- and the CNS-derived magnetic anomalies, and tectonic scarp and the trace of the triple junction between crustalagesare equal to both sidesof the trace. The fracture Pacific,Cocos,and Nazca plates. zone trace parallels the CNS magnetic anomalies and is a discontinuitywith no age progressionalongthe CNS side and increasingagesalong the EPR side of the trace. 3. New Data Looking at the map (Plate 1) in more detail, it is apparent Our studyis basedon magneticdata from the cruisesSO-76 that the N50øE and the N70øE striking anomaliesare in dis[vanHueneet al., 1992],SO-107 [Mrazeket al., 1996], Revelle cordantcontactalonga line that parallelsthe N70øEdirection. deliverycruise(R. Knox, unpublishedreport, 1996), SO-144/1 This is likely to be the result of a ridgejump breakingthrough [Bialaset al., 1999],SO-144/3[Werneret al., 2000],andBGR-99 the old pattern during the early history of the CNS. Such an [Reicherret al., 2000]. The SO-76 data were previouslypro- early ridge jump with a significantchangeof the strike direccessedwell enoughto reducethe RMS crossovererror to <10 tion was discussedby Hey [1977] and is also proposedin a nT [Barckhausen et al., 1998]. The remainingdata were aver- model by Meschedeet al. [1998].Justsouthof the discordance aged to produce along-trackvalues at 20 s intervals.The av- the N70øE striking anomaliesseem to be offset along a moreragingprocessincluded a procedureto eliminate spurious phologicaland tectonicfeature calledFisherRidge [vanHuene values.Positionswere correctedfor the distancefrom ship to et al., 2000] (Figure 2). The point where the two supposed sensor,and anomalieswere calculatedby subtractingthe geo- tectoniclinesmeet off the southerntip of the NicoyaPeninsula magneticreferencefield IGRF 95 [InternationalAssociationof coincideswith the prominentFisher Seamount(Figure 2). Geamagnetism and Aeranomie(/AGA), 1996]. To correctfor The area of mappedmagneticanomaliesoff Central Amermagneticdailyvariationsarisingfrom ionosphericcurrents,we ica has more than doubled since the SO-76 data were collected. digitizedanaloguemagnetogramsobtainedfrom the geomag- Althoughthe anomalypatternshavebecomemuchclearer,the netic observatoryTilaran, Costa Rica, which is located at a map is still not extensiveenoughto clearly identify seafloordistancebetween50 and 350 km from the surveyedprofiles. spreadinganomalies.In order to decipher the complicated The scalarfield was calculated(AF = AZ sinI + AH cosI plate tectonicconfigurationit is instructiveto look at the magfor smalldeclinations)and subtractedfrom the data. Sincethe netic anomaliesat a larger scale. sensorshad been towed at distancesexceedingthree ship's lengthsfrom the researchvesselsin all cases,correctionsof headingeffectswere not necessary. After processing,the new 4. Regional Framework data also have low crossover errors like the SO-76 data. The Seafloor-spreading anomaliesare reliably identified only in zero levelsof all data setswere adjustedto that of the SO-76 part of the CNS area. Between 96øWand 84øW,CNS-derived data at crossovers, and the data files were then merged. anomaliesfrom recentto 4A (0-10 Ma accordingto the magThe resultingtotal field magneticanomalies(Plate 1) show netic polarity timescaleof Cande and Kent [1995]; this timethree zoneswith different magneticanomalypatterns:(1) In scaleis usedthroughoutthis paper) have been mappedthorthe northwestoffshoreNicaragua,northern Costa Rica, and oughlyon both sidesof the activeCNS [Wilsonand Hey, 1995]. the northern half of the Nicoya Peninsula,relatively weak Along the northern triple junction trace EPR-derived anomaanomaliesgenerallyparallelthe MAT inasmuchasthe pattern lies 5A through6A (12-21 Ma) have been identifiedbetween is observable.The high-amplitudeanomaliesin a small zone 94øW and 88øW [Wilson,1996]. The gap between the areas northwest of Nicoya Peninsula are derived from a shallow coveredby these two studiesis <50 km wide at 95øW and upper plate basement.(2) In a strip only -90 km wide that broadens to -550 km at 88øW. CNS-derived anomalies older facesthe southernhalf of the NicoyaPeninsula,clearlydefined than 4A which must be present in this gap have proven exlinear anomalies trend N50øE and extend from the ocean basin tremelydifficultto correlate[Hey,1977;WilsonandHey, 1995]. landwardto the end of the profiles.(3) In the southeastern In order to fill as much aspossibleof this area with reliable part, linear anomalieswith significantlystronger amplitudes magneticanomalyidentificationswe first extendedthe correstrike N70øE. Superimposedare local anomaliescausedby lation of EPR-derived anomaliesalong a stripe north of the seamounts.In the area of CocosRidge in the southeastern- triple junction trace and eastwardto the MAT. We found a most corner of the surveyarea the pattern becomesirregular. reasonable correlation for anomalies 6A to 6C on a number of On the basis of previous investigations[Hey, 1977; Wilson, profilesincludingtwo profilesfrom our new data (Figure 3a). 1996] it seemsobviousthat the oceaniccrustin the northwest- This more completeknowledgeof crustalagesalongthe triple ern part of the surveyareawasformed at the EPR, whereasthe junctiontrace alsoprovidesagecontrolfor CNS-derivedmagtwo SW-NE trendinganomalypatternsmust be attributed to netic anomaliesjust south of the triple junction trace. It is a oceaniccrust formed at the CNS. The EPR-CNS boundary basic requirement for a Ridge-Ridge-Ridge-typetriple juncseparating the lithospheric provinces formed at different tion configurationthat crustal ages along the triple junction spreading ridges deviates from the morphological rough- trace are equal on both sidesof the boundary. smoothboundary.This fact has led to confusionamong reWith the knowledgeof the positionand age progressionof searchersin the area. Becausethe rough-smoothmorpholog- the triple junctiontrace on the northern sideand the position ical proxy breaks down near the continent,we will use the of anomaly4 or 4A on the southernsideit waspossibleto fill 19,210 BARCKHAUSEN ET AL.: COCOS PLATE TECTONIC BOUNDARIES 86 øW 10ON= ". 9øN 9øN 86øW Figure 2. High-resolutionmultibeam bathymetry map of the study area (100 m grid). The data were collectedduring R/V Sonnecruises76, 107, and 144 and R/V Revelletransit.Tracks of multichannelseismic reflectiondata in Figures6a, 6b (thick segmentalongtrack line 7a), 6c, and 6d are shown.The scarpof the narrow ridge marks the boundarybetween the lithosphereformed at the East Pacific Rise and the CocosNazca spreadingcenter.This boundaryof the oceanplate correspondsto a smalllandslidein the lower slope andis alsoimagedon trackline 7a asa stepin the plate boundarybeneaththe slope(seeFigure6b). The ridge jump and the propagatorare explainedin the text. 2). We foundhalf-spreading ratesincreasing from 20 mm/yr at 95øWto 30 mrn/yrat 90øW(Figure3b). Thesespreading ratesare higher than those Wilsonand Hey [1995] found for the time period 5.23 Ma to 10 Ma (14 mm/yr at 95øWand 21 mrn•r at 90øW),but the increasetowardthe eastmatchesthe findingsin the youngeranomaliesverywell. With the existingdatawe have not beenableto map propagators and transformfaultsin detail. major parts of the gap betweenthe studiesof Wilsonand Hey [1995]and Wilson[1996].We selectedseveralsinglemagnetic profiles heading approximatelyparallel to the tectonic flow lines from data availablefrom the National GeophysicalData Center(NGDC) [1998] data compilationand from the Scripps Institutionof Oceanography's database(2000). Researchvesselspassingthe surveyarea at different times have taken the profiles,and the errors along them are not well documented. We examinedall data for spuriousvaluesand subtractedthe mean value from each profile. Having two pointswith known age on each of these profiles,it was possibleto identify the seafloor-spreading anomaliesbetween4A and the triple junc- First, the irregular pattern of magnetic anomaliesassociated with Cocos Ridge interrupts anomaliesin the south and the triple junctiontracein the north, obstructingcorrelationof the seafloor-spreading anomalies.Second,east of 89øW the dis- tion trace in the area between 96øW and --•89øW. The anomalies tances between trendN70øE,like thosemappedoff southernCostaRica (Plate CocosRidge and the triple junction traceare muchlarger than East of 89øW, reconstruction is difficult for two reasons. the oldest identified anomalies south of the BARCKHAUSEN o o z ET AL.: COCOS PLATE TECTONIC o o o I-. • o o BOUNDARIES o o z z 19,211 z f, .J rO ,0 """ 0 •N z z Z Z Z 19,212 BARCKHAUSEN 5 5A ET AL.: COCOS PLATE TECTONIC 5B 5C 5D BOUNDARIES 5E 6 6A1 ,•1-• 96 mm/yr (b) 5B 5AB 5A 5 4A (C) 6A2 6AA1 6B -•14 ß 75 mm/yr 6 5E 5D 5C 6C 65 mm/yr (d) 5B 13 I //+---Ridge Jump 1 FZT 20• 18....• TJT •,A• gator 6B1 6AA 6AA1 Figure 3. (a) Correlationof magneticanomalyprofilestakennorth of the triple junctiontrace/fracturezone tracewith a syntheticprofilecalculatedfrom a 500 m thicksourcelayerat 4000m water depth.Spreadingrates for ages<20 Ma are from Wilson[1996]. For locationof the profiles,see Figure 3f; MAT, Middle America Trench.(b) Correlationof magneticanomalyprofilestakennorthof the CNS with syntheticprofilescalculated from a 500 m thick sourcelayer at 3000 m water depth. Spreadingrates are 20 mm/yr at the bottom and 30 mm/yr at the top of the figure. For location of the profiles,see Figure 3f; TJT, triple junction trace. (c) Continuationof Figure 3b. For locationof the profilesseeFigure3f. The spreadingrate is 35 mm/yr;RJ, ridge jump. (d) Correlationof magneticanomalyprofilestaken off the southernhalf of the NicoyaPeninsulawith a syntheticprofile calculatedfrom a 500 m thick sourcelayer at 3500 m water depth. For locationof the profilesseeFigure 3f; FZT, fracturezone trace.Thesemagneticanomaliesrecordthe onsetof the opening of the CNS. On the CocosPlate they are only preservedin a small area off Costa Rica. (e) Correlationof magneticanomalyprofiles taken west of the GalapagosIslandswith syntheticprofiles calculatedfrom a 500-m-thicksourcelayer at 3000 m water depth. Spreadingrates are 30 mm/yr at the bottom and 42 mm/yr at the top of the figure.The westernmostprofile is at the bottomof the figure(cf. Plate 2). (f) Locationmap of the profilesmodeledin Figures3a-3d. All profilesare shownaswiggletracesin Plate 2. spreadingrates found farther west. Either the spreadingrate wasmuchhigher(closeto 60 mm/yrhalf-spreading rate) in this area or anothertectonicsystemprevailed.A likely candidateis a significantridge jump at -14.5 Ma proposedin a model by Meschedeet al. [1998]. Despite these problems it was still possibleto tie magneticanomaliesat the triple junctiontrace with knownages.We were able to correlateanomalies5B and older in the area east of 89øWat spreadingratesof 35 mm/yr, without an increaseof spreadingrates eastward(Figure 3c). The oldestof the N70øE strikinganomaliesis anomaly6. This anomalycan be traced into the mapped area off Costa Rica, where it is in contactwith the N50øE strikinganomalies,thus datingthe inferred ridgejump at -19.5 Ma. The N50øEstriking anomaliescan easilybe correlatedwith anomalies6AA through6B1 at a half-spreadingrate of 50 mm/yr (Figure 3d). We found no evidencefor anomaliesolder than 6B1 generated at the CNS. During cruiseBGR-99 one magneticprofile heading parallel to the N50øE striking anomalieswas taken just north of the inferred position of the triple junction trace/ fracture zone trace. This profile clearly showsEPR-derived magneticanomalies6B and older (profile7 in Figure3a). This provesthat there was no triple junctionconfigurationprior to anomaly6B, and that instead,the boundarybetweenEPR- and CNS-derived anomaliesparallels anomaly 6B1 off northern Costa Rica. Therefore, for this part of the boundarythe term "fracture zone trace" applies as it is the discontinuitythat representsthe initial Farallon Plate breakup. In order to completethe picturethe proposedidentifications of old CNS-derivedseafloor-spreading anomalieson the Cocos Plate mustbe comparedto thoseon the Nazca Plate southof the Carnegie Ridge. In this area, Hey [1977] identified seafloor-spreading anomaliesparallelingthe northernflank of the Grijalva Scarpwhich separatesoceaniclithosphereformed at the EPR and at the CNS on the Nazca Plate (Figure 1). He suggestedthose anomaliesmight be remnantsof the initial openingof the CNS alonga preexistingfracturezone.As a test of this interpretation,Hey [1977] proposedthat a boundary counterpartto the Grijalva scarpmustbe found eastof 88øW on the CocosPlate off Costa Rica. We reexaminedthe magneticanomaliesnorth of the Grijalva Scarp.Our interpretation differs only slightlyfrom that givenby Lonsdaleand Klitgord [1978],who correlatedthe oldestCNS anomalyjust north of the Grijalva Scarpwith anomaly6B. This matchesexactlythe oldest of the N50øE striking anomalies off Costa Rica. We BARCKHAUSEN ET AL.: COCOS PLATE TECTONIC (e) 5A 5AA 5AC BOUNDARIES 19,213 5B (t) 96'W 12'N 94"W 92'W 90'W 88'W 86'W 84'W 82 'W 12'N 10'N 10"N 8øN 8øN 6'N 6"N 4'N 4'N 96'W 94'W 92'W 90'W 88'W 86øW 84'W 82'W Fig. 3. (continued) conclude that the N50øE anomalies off Costa Rica are the mirror image of those north of the Grijalva Scarp and thus representthe remainingrecordof the initial openingof the CNS on the Cocos Plate, located exactlywhere Hey [1977] suspected themto be. In agreementwithLonsdaleandKlitgord [1978] we find that anomalies6B1 through6A1 parallel the Grijalva ScarpstrikingN65øE at a half-spreadingrate of 45 mm/yr. Anomalies6 and youngertrend ---E-W and can be correlatednorthwardup to anomaly5C at a half-spreading rate of ---40 mm/yr. This configurationleavesa blank wedgeshapedpieceof crustbetweenthe N65øEstrikinganomaly6A1 and the E-W strikinganomaly6. Lonsdaleand Klitgord[1978] called this a "region of rise jumps." Even though magnetic anomaliesin this region are not clear without any new magnetic profilessince 1978, we suggestthat the wedge-shaped pieceof crustrepresents the missingpart of the N50øEstriking anomaliesoff CostaRica. The abandonedspreadingaxisthat has been transferredto the Nazca Plate by the ridge jump at 19.5 Ma discussed earlier would be included in that area. West of the Galapagos Islands, CNS-derived magnetic anomalies5A through5B with a half-spreadingrate of ---35 mm/yr (Figure 3e) were identifiedin somerecentlyacquired magneticprofiles.These anomaliesare the undisturbedcontinuationof the youngeranomaliesidentifiedby Wilsonand Hey [1995], similar to those north of the CNS and west of 89øW.East of the GalapagosIslandsthe CarnegieRidge prohibits correlation of magnetic anomaliesyounger than 5C northwardinto the area mappedby Wilsonand Hey [1995]. However, the distancebetween anomalies 2A and 3 north of the ridge and 5C south of it is clearly too small to allow a continuousaccretionof crust betweenthem. Hey [1977] had alreadynotedthat the highlyasymmetricaccretionof oceanic lithospherealongthe easternpart of the CNS cannotbe explainedby asymmetricspreadingrates and discussed the possibilityof a seriesof southwardridgejumps. Wilsonand Hey [1995]confirmedthisexplanationfor ages<10 Ma. It is likely that similarridgejumpsoccurredearlier.The questionis, was there a seriesof small ridge jumps or one major ridge jump betweenanomalies5B and 5. The latter wasproposedin the model by Meschedeet al. [1998]. Sincewe found nearly symmetricspreadingratesin the time intervalbetweenanomaly6 and 5C on both sides of the CNS and no indications of small ridgejumps,we speculatethat a ridgejump similarto the one at 19.5 Ma occurredshortly after anomaly 5B at ---14.5 Ma. However, sincethese discordancesare buried under the Cocos and CarnegieRidges,there is little evidenceto definitively answerthis question. Assuminga major ridgejump at 14.5 Ma in addition_tothe confirmedjump at 19.5 Ma splitsup the spreadinghistoryof the CNS into three stageswhich have been termed CNS-1 (22.7-19.5 Ma), CNS-2 (19.5-14.5 Ma), and CNS-3 (14.5recent)byMeschede et al. [1998].Sincethesetermshavebeen usedalreadyby others[e.g.,yon Hueneet al., 2000], we continue usingthem here. Figure 4 is a schematicsketchthat summarizesthe three-stageevolutionof the CNS. 19,214 BARCKHAUSEN ET AL.: COCOS PLATE TECTONIC BOUNDARIES 94øW 96'W 12øN 92'W 90'W 88'W 86'W 82'W !2'N 84'W 500 nT 6B ßTdpteJunctionTrace Fracture Zone Trace Ridge Jump Propagator 10'N 10'N i 8øN 8øN 5A 6'N 6'N \\ 4'N 4øN 2'N 2øN ø I • o c 2'S 2'S 5D 5D 5E-- 4'S GalapagosIslands 4'S \\ 6'S 6'S %,% 96'W 94'W 92'W 90'W 88'W 86'W 84øW 82øW Plate 2. Magnetic anomalies[NGDC, 1998; ScrippsInstitutionof Oceanography database(2000); this study]in the areaof the CNS between96øWand82øWshownaswiggleswith positiveanomaliesshadedabove the trackline. Profileswith grayshadedanomaliesare from the NGDC andScrippssources; profileswith solid shadedanomaliesare new data compiledhere. The interpretationof youngeranomaliesat the centerof the map is from Wilsonand Hey [1995]; see legend in the lower left corner. Green dashedlines indicate seafloor-spreading anomaliesformedat the EPR. Red dashedlinesmark magneticanomaliesderivedfrom CNS-1. Blue dashedlines indicatemagneticanomaliesformed at the CNS-2 after a ridgejump at 19.5 Ma. EPR-derivedmagneticanomalies6B through10 southof the GrijalvaScarpafter B. Eakins(unpublished data,2000).The interpretedmagneticanomalies, paleoplateboundaries, ridgejumps,andpropagators reveal a complicatedplate tectonichistory. BARCKHAUSEN ET AL.: COCOS PLATE TECTONIC BOUNDARIES 19,215 are also topographicfeaturesobservedin multibeam bathymetry and crustal structurein seismicsections: 1. The NW flank of the fracture zone where the CNS ini- tially openedcorrespondsto a narrow ridge acrossthe oceanic plate (Figure 2). A seismicreflectionimageacrossthe fracture zone trace indicates lateral continuity even where sediment buriesthe ridge. The ridge is the culminationof a ---5 km wide tilted basementblockwith a smoothuppersurface(Figure6a). Figure 4. Schematicsketch of the evolution of the CNS in Overlying strata onlap the tilted block, indicating basement three stages. tectonismprior to sedimentdeposition.Surprisingly,this lithosphericboundaryshowsonly a minor changein crustalstructure. Weak reflectionsfrom the crust-mantleboundaryappear 5. Discussion at about the same two-waytime on either side of the seafloor 5.1. Crustal Ages and Tectonic Boundaries off Costa Rica scarp,and only a smallchangein lower crustreflectionsoccurs Our revised magnetic anomaly map provides crustal ages acrossthe boundary(Figure 6a). The smoothupper surfaceof and definesthree tectonicboundariesoff Costa Rica (Figure the oceanicigneouscrust acrossthe boundaryindicatesthat 5). (1) The •80 km long fracturezone trace separatingEPR during opening, magmatismand deformation rapidly nuclecrustfrom CNS crustis orthogonalto the MAT off the central atedinto the new spreadingcenterand that duringbreakupthe NicoyaPeninsula(Plate 2 and Figure 5). (2) The ridgejump Farallon crust experiencedlittle vertical displacement.The from CNS-1 to CNS-2 resultsin a tectonicboundaryparallel- ridge appearsto continueacrossthe trench beneath the coning anomaly6. This boundarymarks an age jump of 1.9 m.y. tinentalslopeand coincideswith seafloorinstability(Figure 2). (chron 6n to 6A2r) at the MAT. (3) A •35 km offset of A seismicreflectionstrikeline acrossthe slopeand parallel to magnetic anomalies occurs along a structure that strikes the trenchshowsan offsetat the plate boundary,indicatingthe obliqueto the ridgejump (Figure5). The obliquetrend of this subductedextensionof the ridge (Figure 6b). The plate boundstructureand the lateral displacementof anomaliesindicate a ary reflections most likely come from subductedsediment propagatorsimilar to those in young crust on either flank of coverof the oceanplate and perhapsmaterial eroded from the the CNS [Wilsonand Hey, 1995] (Plate 2). upper plate [Raneroand yon Huene, 2000], making it difficult The three tectonicboundariesmappedwith magneticdata to estimatethe dimensionsof the ridge. Although the expres- 88'W 87'W 86øW 85'W 84øW 83'W 12'N 12'N Arc volcanoes offset ..... TripleJunctionTrace ......... Fracture Zone Trace --- ---,-,.... Ridge Jump Propagator 11'N 11'N DSDP 84 ODP 170 10'N 10'N 23 g'N g'N 6 8'N 8øN 88'W 87'W 86'W 85'W 84'W 83øW Figure 5. Isochronmap of the studyarea off Costa Rica with agesderivedfrom identificationof seafloorspreadinganomalies.Numbersindicatecrustalagesin m.y.Tectonicboundariesand the locationsof the Deep Sea Drilling ProjectLeg 84 and OceanDrilling ProgramLeg 170 drill holesare indicated.Trianglesshowthe locationsof arcvolcanoes;FS, Fisher Seamount;OSC, OuesadaSharpContortion.The 100 km isobathof the Wadati-Benioffzone is added [after Protti et al., 1995a]. 19,216 BARCKHAUSEN (a) NW km 5 sill 10 ET AL.: COCOS PLATE TECTONIC BOUNDARIES topigneous crust 20 Fracture 25ZoneTrace • 30topigneous 35crust40 15 45 50 SW ......... (b) SE 2' ' 2• ' km • .... 20 lS t ...... •0 • ..... t .......... s I....... • • NW , , '2 PoststackTime MigrationSonne81 Line 7a 3 3 slope sed'• •i. Plate boundary 7 , , Figure 6. Poststackfinite differencestime migrationsof multichannelseismicreflectionlines acrossseveral tectonicboundariesof the CocosPlate.For location,seeFigure2. (a) Line BGR99-45acrossthe fracturezone where the initial openingof the Cocos-Nazcaspreadingcenter took place. The crust-mantleboundaryis definedby somefaint reflectionsat ---7 s two-waytime. Lower crustalreflectivityis alsoobserved.(b) Line Sonne-81-7a.The profile runs acrossthe middle slopeoffshoreNicoyaPeninsula.The seismicrecordshows the plate boundaryas a band of low-frequencyreflectionsoccurringat 6 to 5.5 s two-waytime. The boundary betweenthe lithosphereformed at the GalapagosSpreadingCenter and the East PacificRise is observedas an offset in the plate boundarytopography.(c) Line Sonne-81-10.The seismicrecordimagesthe structure acrossthe Fisher Ridge and the trace of a ridgejump. Recent tectonicactivityhas uplifted and folded the sedimentarycover and top basementnorthwestof Fisher Ridge. The fold can also be observedin the bathymetryof Figure2 and is coincidentwith the ridgejump mappedwith magneticdata (Plate 1), indicating a reactivationof this boundaryduringsubductionprocesses. Note the onlap of sedimentstrataon the Fisher Ridge,indicatingthat the topographywascreatedbeforethe depositof mostsediment.(d) Line Sonne-81-20. This line displaysthe lateral continuityof structuresdescribedin Figure 6c. It also displaysthe onlap of sediment strata on a seamount. sion of the boundarybetweenEPR and CNS lithosphereobserved at the Middle America Trench is a subduedfeature, the subductedextensionof the boundarymight have been more pronounced.Assumingspreadingrateslike thosemeasuredfor the oldest EPR crust off northern Costa Rica and south of the Grijalva Scarpimpliesthat about 1500 km and 900 km of the boundaryhave been subductedduring the last 22.7 m.y. beneath Central America and northern South America, respectively.Thustherewasan agejump of at least20 m.y.acrossthe oldest portion of the fracture zone where the CNS formed. Rejuvenationof a 20 m.y. old lithosphereduringthe opening and formation of the CNS probably created a topographic feature more prominentthan that currentlybeing subducted, and the effect of the collision of such a feature with the con- tinent might have involvedimportanttectonism. 2. The ridgejump is coincidentwith a broadfold producing a gentleseafloorridge (Figure 2). Seismicreflectionprofiles acrossthe ridge jump image folded sedimentand associated small-scalefaulting (Figures6c and 6d). The igneouscrustis slightlythickerto the south(e.g.,line 10, Figure6c). No onlap of strata on the basementis observed,whereasother gently dippingbasementfeaturesshowstrataonlap(e.g.,km 10-15 in Figure6d). The upperhalf of the sedimentsectionis hemipelagic [Kimuraet al., 1997],and thereforethe distributionof the depositsis partiallycontrolledby the topographyof the ocean basement.For instance,the flat basementtop betweenFisher Ridge and the seamountto the south(Figure 6d) has a thin sedimentcover,indicatingsomecontrol from currentsduring sedimentdeposit.The foldingand the seafloorridge and the faulting and lack of onlap indicaterecent tectonism,perhaps BARCKHAUSEN (c) NW 5 ........ I km 10 ...... I ET AL.: COCOS PLATE TECTONIC 15 . , , . t 20 .... I 25 , . . , ,. ? . .t PoststackTime MigrationSonno81 Line 10 $0 ....... , I. BOUNDARIES 35 . , ..... 19,217 40 ! ............... 45 I ..... Fisher••Ridg e qnlap •.•;;••.•.. ! 50 . . ,:•: .: t fold . . ' .. Ridge Jump SE . sed}ment topigneous crust sediment ...... .....•.•.•• .•.- •!'i'.• .................. -':•' •.•. • CMP (d) N 80km7• •04• 70 10800 65 60 55 .. . E•...••....-••:4• 11200 50 45 40 .. 11600 35 30 25 12000 20 15 10 5 S PoststackTime MigrationSonne81 Line20 PropagatorTrace Ridge Jump sediment fold E 4. -.• top igneous crust • .. \ ...... ........ • •-..,•.. ' ' ' •" onlap ..... ,/ . -- -••••••-•.•-- ....•'•••• ••••• '.... ., multiple '"""' '•.'-•i'"':•';':'•' •'•......... '•'•"•"•- '• "'i ..... '" ' •' '•" 128O0 ... :3•-. • .... -.... ':-•'-'- ': -"•"••••'••••••••••"• •..'. :..• .... :....... :. :....-..-.•.-- ...:'-;.•' 13200 - Fisher Ridge onlap . :..-:-•.-..•-:-;::.-•..•r. .--• -. *' ;•r.--- . •2• 120• 1" ''' ' ' .... ' ..... ! 11600 '"•.......'"' 1t2• ' ' I ' t08• "• • ....... '" 1•0 ' CMP Figure 6. (continued) created by reactivation along the structure during flexural bendingof the subductingcrustinto the trench axis. 3. The propagatoris associated with the ridgeextendingSW of FisherSeamount(Figure7), calledFisherRidge.This ridge probablyformed later than the propagatorduring Miocene volcanism.Seismicimages show down-to-the-NW displacement of all reflectivehorizonsacrossthe ridge (Figures6c and 6d). Rock from FisherRidgeis 19.2m.y. old, equivalentin age 86øW 85'W 10'N =a: ,,• ',:: ....... •'•.>V•;....".,'"'•j• '-•. ...... :"•v.. •-......... ---:,:.: .......... :•• "! 10'N I•:••, :%, •:•'"': •/•.• •>...... .:.•.:•:•... ! ;':"' /:•.•..•.;..-...'t..,•(•:/.'; ,.::," :".? ...................... ' .....'"'":' ............ ..•:'•. :..... ! •' .. •:-'•&."/.'•':.:::;/'.,-* .......:::'-.... L'.-. •--'. •: '• .: "'-':. ':'•::.. ':•-'•....... ...... :,--'•,5.4•' ?'"•'"•'•,.•: :X:'• ....... ':'...•/ •. -•:" •.?•.. ½•'"'•.;.....,.;•' -<-½;•--•t:•--• ;:.• •::..:.,.':4::• •"D:.:".. '...... ½•:.• ...... ß ...,•:• .•••:' ".:• :•'..F•.•):.•:-,..•.:.•:•.:•,.,,_•;N• ........ :....................... -- '-" ';':•'":.....' ., ".•'7'.-'• ..... • ........ h."?."• • •; ..... ) :.. -. •:•:::'• .:•...... :'---•.:-,-•::::. • '--•, -4:: .•zt..•.•. •:;•' --t•.......... ,'.:•.: •:•1::.•"• .... -t• -•-" -..: ..":•-..'•.E.•'-. 'r•.':•-••• .,.•. .•.•..... •.• •.:.•:-. •-' ......,•::•'•.... :.• ..... .:•t...• •-• ..... •:•.'• .?'•'; ............. •;-..-.• .:•, .... :•'•'..::-•':'...'-•-"': ......... •?•': --• ....... •. --•. • • --'• -:?•-. -'..•"-. ....•:•,E" :--.;•:•..•-• •.---. ..... ......• : .' •"•Z•-.. • ':.•:'-: •:• ",•,,-:• .•-'--• •.• •:'• t. :•.... .''•',.'.'-,.... ":.•-:• '•;¾::-, t•::f•.:•:,•H• "• •-•?" :•:. :•"•T. ...... :•:::: :;•,.. .• ' •:•.;:•;f."•"'-•.•" "• •::•:g%;.:• -::• •'•:•.-..-.:.-:•':' .:..:'"'-'.•' '::•:.:: '--?'E•':•=•'•-•:". '.• "• •' •"•: '•'•....:•:•'• '•/••'• •'>•:•':'* •'2......... ' :: ':.•...,.•,, .,-•,.•:.. .:-.,.•.• ,-•:: •-• ...•..?. : -...•:•;-.:.. .•,$.• •:-•;•:•;.;-•:•:: ..... •:•.½,-,. .......... , ..... -.•,.. ................... . ....•:'•-•;•. •;•.•;:;:• •:• • •:•.•:y:•, .•:• -.•. A•:A•'•-•.'-<'• •,.:••;•.....';•'•'. •' ' -.•:'.• ...... 86'W -'f:• •.•-•.:•... •';•.:•.. . ':::t;• :• •- ......... • .':. ----•-•.'; • •. • ....... .- - •-::•.-•:• • :'fi 85'W FiSure ?. Mo•cmc•t of tectonicboundaries• the CocosPlate alongthe Middle •cdca T•c•ch o•c• the last 2 m.y. Th• a•ows •d•catc the mod•g d•cct•o• of the boundaries;the thick a•ow showsthe plate co•c•gc•cc dkcct•omO•c of the th•cc tectonicboundariesparallelsthe plate co•c•gc•cc •cctor a•d has •cmai•cd stablew•th •cspcctto the uppc• plate. 19,218 BARCKHAUSEN ET AL.: COCOS PLATE TECTONIC to the adjacentmagneticanomalies,and hasa mid-oceanridge basalt geochemistry[Werneret al., 1999]. Conversely,Fisher Seamountis -14 m.y. old oceanislandbasalt.Thus the ridge involvesruptured oceancrustand extrusionof lava. Strata on either side of Fisher Ridge and a seamountto the southhave similar patterns of sedimentonlap, indicatinga similar age (Figures6c and 6d; km 10-20). The ridge forms the northwesternboundaryof an area flanking CocosRidge containing many seamountslike Fisher Seamount(Figure 2) with a Galapagoshot spot geochemistry[Werneret al., 1999]. Fisher Ridge, Fisher Seamount,and the propagatorare alignedwith subductedseamountsextendinglandwardunder the slopeand shelfto beneaththe coast(Figure 2) [yonHueneet al., 2000]. These findingsindicate that the fracture associatedwith the propagator probably continuesacrossthe ridge jump from CNS-1 to CNS-2 and into already subductedCNS-1 lithosphere.The currenttopographyof the FisherRidge probably formed as -5.5 m.y. old lithospherewasthermallythinnedand domed duringhot spot activity.It broke alongthe propagator, whichprobablyfocusedmagmaextrusionat FisherSeamountand relatedsubductedseamountsin the chain.Vertical displacement acrossFisherRidge is the mostprominentof all three tectonic boundaries (Figures6c and6d) andmayexplainthe moreprominentfailureof the continentalmarginaboveits subducted extension.However,thepropagator wasinactiveat thetimethispartof the CocosPlatemovedoverthe Galapagos hot spot(14 Ma) due to an earlierridgejump to the south(CNS-2 to CNS-3).A plausiveexplanation is that thepropagatorfracturewasreactivated by the hot spot event and extendedinto older lithosphere,where magmatismfocusedand formedthe seamountchain.The landward projectionof FisherRidgedevelopsinto the QuesadaSharp Contortion,a slabtear at a depthof 70 km beneathcentralCosta Rica [Prottiet al., 1995a].Aboveit the volcanicarcis left-laterally offsetacrossa gap (Figure5). The fracture zone trace and ridgejump are not alignedwith suchprominentstructuresin the continentalcrustasthe propagator.This maybe due in part to smallercrustaldisplacement and orientationwith respectto the plate convergencevector. The collisionpoint of the ridgejump migratedalongthe trench becauseits strike differs --•25ø from the convergencevector. It migrated-90 km to the northwestalongthe trenchduringthe past 2 m.y. (Figure 7). The propagatorparallelsthe convergencevector and thushassubductedat the samepositionwith respectto the continentfor sometime. The anglebetweenthe fracturezone trace and the plate convergence vectoris -12 ø, resultingin migrationto the northwestat -20 km/m.y.Migration can explainthe lack of prominent structurein the continentalplate abovethe fracturezone trace and the ridgejump. However, the propagatorsubductedin the same area for at least1-2 m.y., therebyaffectingmarginstructuresignificantly. A major offset and divisionof the volcanicarc betweenNicaraguaand CostaRica (Figure 5) [Can'and Stoiber,1990]have no counterpartin the CocosPlate and mustbe associated with an older continentalcrustalstructure[vonHuene et al., 2000]. 5.2. Implications for CNS History The southern side of the CNS fracture zone trace coincides with the GrijalvaScarp(Plate2). Hey [1977]statedthat it is "an isochron that marks the time of origin of the Cocos Nazca spreadingcenter and the rifting apart of the Farallon Plate to form the Cocosand Nazca plates." We confirmedits age at 22.7 Ma (chron6B1) and found its mirror imageon the northern side of the CNS off Costa Rica. Off Costa Rica, only a BOUNDARIES shortsection(-80 km) of this isochronremainsunsubducted (Plate 2; anomaliesare markedwith red dashedlines),where it couldonly be detectedwith high-resolution magneticmapping (Plate 1). The oldest CNS magneticanomalieson the Nazca Plate are poorly resolvedin the critical area between 89øW and 86øW becausevery few magneticprofiles lacking satellitenavigationwere acquired.However,detailedmapping of the mirror imageof thoseanomaliesoff CostaRicaprovides the key to reconstructingthe early spreadinghistory of the CNS. Spreadingalong CNS (CNS-1) beganat 22.7 Ma, at a rate of -95 mm/yr and with an almostsymmetricaccretionof newlithosphereto both sidesof the rise(Figure4). At 19.5Ma theridgejumpedsouth,changing itsstrikedirectionby -22 øto nearlyE-W (Plate 2; anomaliesare markedwith blue dashed lines). On the CocosPlate, only the westernmostpart of the area where the CNS-2 broke throughthe old pattern remains unsubducted.The spreadingrate at the CNS-2 decreasedto -75 mm/yr and continuedto be almostsymmetric.The magnetic anomaliesyoungerthan C6 have higher amplitudesand are lessregularin shapethan the older anomalies,indicating an overall changein spreadingconditions. It is not clear exactlyhow long spreadingat the CNS-2 continued.As explainedearlier,becauseof the overprintingof the originalpatternby the Cocosand CarnegieRidges,major gapsremain in the identificationof anomaliesbetween4A and 5C on both sidesof the CNS. However, it is clear that during this period the accretionof crustwasvery asymmetrical[Hey, 1977]. In a previousmodel [Meschedeet al., 1998] the abandoned CNS-2 spreadingcenter is presumedto coincidewith a N70øEstrikingbathymetricfeatureand gravitylow that canbe tracedfrom a positionat 6øN,88øWinto the CocosRidge.The criticaltest of this modelwould be to find magneticanomalies mirrored along this line. Unfortunately,a reliable correlation of anomalieson the northern flank of CocosRidge is difficult evenwith data from cruisesSO-144/1and SO-144/3.We presumethat the originalpattern of seafloor-spreading anomalies in this area washeavilyoverprintedduringformationof Cocos Ridge.If the modelof Meschede et al. [1998]were correct,then extrapolationfrom anomaly5B southwardprovidesa crustal age of -14.5 m.y. alongthe abandonedspreadingaxis.Given that anomaly5C is preservedon the Nazca Plate, the ridge jump from CNS-2 to CNS-3 must have transferredonly -•60 km of crust(formed between16.0 Ma and 14.5 Ma at a halfspreadingrate of 40 mm/yr to the southernsideof the CNS-2) from the Nazcato the CocosPlate. If the ridgejump occurred later than 14.5 Ma, the amount of crusttransferredmay have been larger. It seemsimpossibleat the moment to resolvethe questionsraisedabove.Thereforewe suggestthat at -• 14.5 Ma a seriesof intermediateand small ridge jumps beganwhich continuetoday [Wilsonand Hey, 1995]and that thisproduces the asymmetricalaccretionof lithosphereobservedalongthe easternpart of the CNS. On the basis of the identification of anomaly 6C in the northeasternPanamaBasin,Lonsdaleand Klitgord[1978] interpreted an older age for the initiation of Cocos-Nazca spreadingthan the 22.7 Ma givenhere. We reinvestigatedthe magnetic anomaliesin the area south of Panama with a few magneticprofilesfrom the 1980sand one profile from cruise SO-144(Figure 8). We find that the seafloor-spreading anomaliescanbe reasonablycorrelatedwith anomalies6B1 through 6A1 (22.7-20.5Ma) at a half-spreading rate of 50 mm/yr,which is in agreementwith the findingsoff Costa Rica. Hence we suggestthat the entire CNS opened at 22.7 Ma. BARCKHAUSEN ET AL.: COCOS PLATE TECTONIC BOUNDARIES 19,219 CNS axis migratesnorthward. Even though both plates have not movedin a constantdirectionover the last 23 m.y., there is evidencethat the situation,in general,hasnot changedduring this time period [Mammerickxand Klitgord,1982;Mayeset al., 1990].The identifiedridgejumpsand the asymmetryof crustal accretionat the CNS showa tendencyfor the CNS axisto jump southwardtoward the Galapagoshot spot,thus compensating for the northwardmigration.Mtiller et al. [1998] found many examplesof asymmetriccrustalaccretionnear hot spotsin the SouthAtlantic, Indian Ocean, and southeasternPacific.They concludedthat randomlyoccurringsmall-scaleridgejumpscan be biased by a nearby hot spot into successive ridge jumps toward the plume. The Galapagoshot spot and the CNS apparentlyalsointeractin this way. The Galapagoshot spothas been activeduringthe past 23 m.y. and probablymuch longer [Hauff et al., 1997]. It remainsto be seenif the changefrom rare major ridgejumpsin the earlyphaseof the CNS historyto more frequent small-scaleridge jumps after -14.5 Ma is related to a changein the activityof the Galapagoshot spot. 6. Conclusions With the data compiled here we located three tectonic (b) 6B16AA boundaries 6A1 fracture 6AA1 6A2 Figure 8. (a) Magneticanomalies[NGDC, 1998;this study] in the northeasternPanamaBasinshownas wiggleswith positive anomaliesshadedabovethe track line. Profileswith gray shadedanomaliesare from the NGDC source;the profile with solid shadedanomaliesis from the new data compiled here. The magnetic anomalies are derived from crust that was formed in the first phaseof the CNS opening(CNS-1). (b) Correlationof the magneticanomalyprofilesshownin Figure 8a with a syntheticprofile calculatedfrom a 500 m thick source layer at 3500 m water depth with a half-spreadingrate of 50 mm/yr. 5.3. Possible Influence of the Galapagos Hot Spot on Ridge Jumps Formation of Cocosand CarnegieRidgeson the Cocosand Nazcaplatesfrom the Galapagoshot spotis widely accepted. Thus the ridgesmark the azimuthsof plate motion relative to the hot spot.In the simplestversionof this model the hot spot activityhas alwaysbeen at the head of both ridges.However, the magneticanomaliesshowthat the spreadingcenterrepeatedlyjumpedoverconsiderabledistances.Other indicationsfor a more complicateddevelopmentof the submarineridgesare the difference in age between oceanic basement and seamountson the flank of CocosRidge off CostaRica, a depressionin the CarnegieRidge between85øWand 87øW,and the presentdistanceof- 170 km betweenthe activespreadingaxis and the centerof hot spotrelatedvolcanicactivity.Today, the Cocos Plate moves in a northeasterlydirection (N40øE), whereasthe Nazca Plate movesnearly east [DeMets et al., 1990].This impliesthat as long as spreadingis symmetric,the in the Cocos Plate off Costa Rica. 1. We revised the position of the triple junction trace/ zone trace between EPR- and CNS-derived litho- sphere.The positionof this boundaryis well definedwith the new data, and it departsfrom the morphologicalrough-smooth boundaryin the older part of the CocosPlate. This is explained with high spreadingratesduringthe earlyphaseof CNS opening associatedwith the generationof smoothoceaniccrust. 2. The revisedmagneticanomaliesrevealedtwo spreading regimesin the early phaseof CNS openingthat are separated by a ridge jump at 19.5 Ma. The conjugateold CNS-derived magneticanomalieson the Nazca Plate confirmthat a wedgeshapedpiece of lithospherewas transferred from Cocos to Nazca Plate by the ridge jump. 3. A prominent topographicfeature on the CocosPlate off Costa Rica, Fisher Ridge, correspondswith a propagator.We find indicationsthat this propagatorwas reactivatedduring a phaseof hot spot related volcanicactivityand is again reactivated during the subductionof the CocosPlate. Overprinting of the CocosPlate by Galapagoshot spotvolcanismis a second processthat segmentsCocosPlate off Costa Rica along with the complex plate tectonic history. The tectonic boundaries, seamount chains, and thickened crust of the hot spot trace definesegmentsof the lower plate that appearto be related to a similar segmentationof upper plate tectonicsand arc volcanism [yonHuene et al., 2000]. Detailed mapping of the oldest CNS seafloor-spreading anomaliesoff CostaRica and additionalidentificationsof magnetic anomaliesolder than 10 Ma allow refinement of the Hey [1977] model for early evolutionof the CNS. Breakup of the Farallon Plate occurredat 22.7 Ma alonga fracturezone striking -65øE. The fracturezone trace separatingEPR and CNS lithospherescurrentlyinterceptsthe continentoff Costa Rica. After a phaseof rapid and symmetricspreadingthe spreading centerjumped southat 19.5 Ma and changedits directionby 22ø. Spreadingremainedsymmetricat somewhatlower spreading rates.At -14.5 Ma, another ridge jump to the southinitiated a phaseof frequentsmallersouthwardridgejumps that still continues.Half-spreadingrates are symmetricacrossthe CNS but increase eastward and let the Cocos Plate rotate 19,220 BARCKHAUSEN ET AL.: COCOS PLATE TECTONIC BOUNDARIES counterclockwise. The strongpreferencefor ridgejumpsin the Meschede,M., U. Barckhausen,and H.-U. Worm, Extinct spreading on the CocosRidge, TerraNova, 10, 211-216, 1998. southern direction implies that the Galapagoshot spot is a Mrazek, J., T. Spangenberg,and R. von Huene, Geologischeund drivingforce for frequentridgejumps and the resultingasymgeophysikalische Untersuchungen vor CostaRica and Nicaragua-metric crustal accretion. Beitrfigezum Verstfindnisdes aktiven ostpazifischen Kontinentalrandes,in FS SonneFahrtberichtS0-107, cruisereport, 172 pp., Ernst-Moritz-ArndtUniv. Greifswald,Germany,1996. Mfiller, R. D., W. R. Roest, and J. Y. Royer, Asymmetricsea-floor spreadingcausedby ridge-plumeinteractions,Nature,396, 455-459, Acknowledgments. This study is based mainly on data obtained duringR/V SonnecruisesSO-76, SO-81, SO-107,and SO-144,funded 1998. by the BMBF, and cruiseBGR-99. We thank the captains,the crews, and the scientificand technicalstaffof the participatinginstitutionsfor National GeophysicalData Center (NGDC), Marine Geophysical their efforts.The GeologicalData Center of the ScrippsInstitutionof Data [CD-ROM], Nat. OceanicandAtmos.Admin.,Boulder,Colo., 1998. Oceanographyprovidedunpublishedbathymetricand magneticdata. Excellent recordsof the magneticvariationswere providedby the Patino, L. C., M. J. Carr, and M.D. Feigenson,Local and regional geophysical observatoryof Tilaran in CostaRica. Severalfigureswere variationsin Central Americanlavascontrolledby variationin subpreparedwith GMT publicdomainsoftware[Wessel and Smith,1995]. ductedsedimentinput, Contrib.Mineral.Petrol.,138, 265-283, 2000. We thankA. 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