Revised Tectonic Boundaries in the Cocos Plate Off Costa Rica Implications for the Segmentation of the Convergent Margin And

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 convergent margin 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 oceanicCocos Plate subductingbeneath Costa Rica has a complexplate tectonichistory resulting in segmentation.New lines of magneticdata clearlydefine tectonicboundaries which separatelithosphere formed at the East PacificRise from lithosphereformed at the Cocos-Nazcaspreading center. They also define two early phase Cocos-Nazcaspreading regimes and a major propagator.In addition to these sharply definedtectonic boundaries are overprintedboundaries from volcanismduring passageof CocosPlate over the Galapagoshot spot.The subductedsegment boundaries correspond with distinctchanges in upper plate tectonicstructure and featuresof the subductedslab. Newly identifiedseafloor-spreading anomalies show oceanic lithosphere formed during initial breakupof the FarallonPlate at 22.7 Ma and openingof the Cocos-Nazcaspreading center. A revisedregional compilation of magneticanomalies allows refinement of plate tectonicmodels for the earlyhistory of the Cocos-Nazcaspreading center. At 19.5Ma a major ridgejump reshapedits geometry,and after -14.5 Ma multiplesouthward ridge jumps led to a highly asymmetricaccretion of lithosphere.A suspectedcause of ridgejumps is an interactionof the Cocos-Nazcaspreading center with the Galapagoshot spot.

1. Introduction continentalslope [von Huene et al., 1995;Hinz et al., 1996], and even forearc uplift nearshore[Fisher et al., 1998]. The down- The Central America convergentmargin off CostaRica and going slab along Central America changesdip significantly Nicaraguahas been an area of concentratedstudy during the [Prottiet al., 1995a],and the geochemistryof arcvolcanic rocks past decadebecause of its variable characterin a relatively and the alignmentof volcanoeschanges similarly [e.g., Carr small area and its well-imagedsubduction zone. Recent pub- and Stoiber,1990; Patino et al., 2000].A recentstudy shows the licationsreport a distinctivesegmentation of the upper plate degreeto which characterand relief of the subductinglower tectonicstructure and relate much of this to a corresponding plate relatesto upper plate tectonismand arc volcanism[von segmentationof the subductingCocos Plate. This segmenta- Huene et al., 2000]. However, precise age information and tion was recognizedin a progressionof studieseach contrib- identification of tectonic boundaries of the Cocos Plate are uting to an increasingunderstanding of the tectonicorigin of lacking.In this study,we focuson the integrationof new data each plate segment.The existenceof a rough and a smooth with previouslypublished compilations [Barckhausen et al., morphologicaldomain on the CocosPlate was noted in the 1998] that answersome of thesequestions. We presenta de- early 1960s [Fisher,1961] (Figure 1). In the early 1990s a tailed magneticanomaly map including-8000 km of new data comprehensivemultibeam bathymetric survey of ocean crust and analyzethe tectonicsetting of the studyarea in the frame- was made off Costa Rica [vonHuene et al., 1995]. This study work of the regionalmagnetic seafloor-spreading anomalies. showedsharp boundariesbetween three morphologicalseg- This constrainscrustal age and preciselocation of major tec- ments on the oceanicplate: (1) smoothseafloor facing the tonicboundaries. The crustalages permit us to revisethe plate tectonichistory of the Cocos-Nazcaspreading center (CNS) Nicoya Peninsula,(2) a segmentwith abundant(40%) sea- from the breakupof the Farallon Plate at 23 Ma to 10 Ma. mountsto the southeast,and (3) CocosRidge enteringthe subduction zone off Osa Peninsula on the southern Pacific coastof CostaRica. It becameclear that the roughnessof the 2. Previous Work seafloorsignificantly affects the shapeand the tectonicsof the The first consistent models of CNS evolution and the formation of the aseismicCocos and Carnegieridges were de- •Bundesanstaltfiir Geowissenschaftenund Rohstoffe,Hannover, rivedby Hey [1977]and Lonsdale and Klitgord [1978]. From the Germany. 2GeomarForschungszentrum fiir marine Geowissenschaften, Kiel, analysisof magneticand bathymetricdata they concludedthat Germany. the FarallonPlate broke into the Cocosand Nazca Platesalong 3Alsoat Departmentof Geology,University of California,Davis, a preexistingfracture zone in equatorial regionsat •27 Ma. California, USA. Accordingto this model the newlyformed CNS interactedwith 4ScrippsInstitution of Oceanography,University of California,San the Galapagoshot spot,which simultaneouslydeposited vol- Diego, La Jolla, California, USA. canic material on both sides of the CNS to feed the Cocos and Copyright2001 by the American GeophysicalUnion. Carnegie Ridges on the Cocos and Nazca plates. Magnetic Paper number 2001JB000238. seafloor-spreadinganomalies had been identifiedin the inner 0148-0227/01/2001JB000238509.00 regionof the CNS alongthe activespreading axis and southof

19,207 19,208 BARCKHAUSEN ET AL.: COCOS PLATE TECTONIC BOUNDARIES

105øW 100øW 95øW 90'W 85'W Ill IllIll Ill Ill Ill Ill I Ill I III 15'N =• • -.•-•.

ß .....ß"•' -.:'i: '":i:

.... •.... I .....:•L.;:ß ,.'.' .:'• •..,;.....:.:..•.-/."• ..:.•}:•...... •..... :..::..;.• •...• • " "

10'N Plate,

ß"' Nazca Plate

I / 105'W 100'W 95øW Figure 1. Bathymetricmap of the Cocos-Nazcaspreading region based on satellite altimetry of Smith and Sandwell[1997]; EPR, East PacificRise; CNS, Cocos-Nazcaspreading center; PFZ, PanamaFracture Zone; MAT, Middle America Trench. Arc volcanoesin Central America are shownas triangles.Arrows indicate absoluteplate motion vectors[DeMets et al., 1990].The rough-smoothboundary is expressedclearly only in the westernpart of the CNS region. The small rectangleoutlines the area in Plate 1 and Figure 4; the large rectangleindicates the area coveredby Plate 2. the CarnegieRidge. In the areasof the submarineridges that rough-smoothboundaries are geneticallydifferent and do not were overprintedby hot spot related volcanicactivity no lin- necessarilycoincide. Near the Middle America Trench (MAT) eated anomaliescould be identified.North of CocosRidge the the fine-scale topographyof the oceanic basement is fairly identificationof seafloor-spreadinganomalies was also impos- smooth, and the rough-smoothboundary was defined at the sible at that time becauseof the paucity of data and compli- limit between an oceanic domain with numeroushotspot re- cated anomaly pattern [Hey, 1977]. Later, Wilson and Hey lated volcanicedifices (ridges, conical volcanoes, and guyots) [1995] revisedthe magneticanomalies of the inner part of the [vonHuene et al., 1995]and a domainwith smoothtopography. CNS and carefully documentedanomalies younger than 10 Barckhausenet al. [1997] showedthat magneticanomalies of Ma, includinga pattern of propagatorsand small ridge jumps. the CNS continue north of the morphologicalrough-smooth Today,oceanic crust along the westand southboundaries of boundary, but the exact position of the boundary between the CocosPlate is generatedby activespreading along the East EPR- and CNS-generated crust was not clear from the mag- PacificRise (EPR) and the CNS. Oceaniccrust formed at the netic anomalydata. The magneticanomaly map compiledfrom EPR has the featurelessmorphology and low-amplitudemag- data acquired during cruise SO-76 [Barckhausenet al., 1998] netic anomaliescommon to fast-spreadingridges [Hey, 1977; showedtwo different patternsof seafloor-spreadinganomalies Wilson,1996]. Oceaniccrust currentlygenerated at the CNS off Costa Rica, both being attributed to the CNS. However, near the triple junctionwith the EPR is formed by slowspread- since the survey area was relatively small and the magnetic ing. It has a rough topographyand high-amplitudemagnetic signaturesof numerousseamounts are superimposedupon the anomalies[Wilson and Hey, 1995]. Hey [1977] mapped the linear anomalies,it was still impossibleto identify seafloor- resulting"rough-smooth boundary," separating two provinces spreadinganomalies and clearly define tectonicboundaries in formed at two different spreadingcenters within the Cocos the oceaniccrust. Wilson [1996] analyzedseafloor-spreading Plate (Figure 1). Hey [1977] projected the rough-smooth anomalieson the EPR-derived part of the CocosPlate along boundary from the seaward area where it is well expressed, the rough-smoothboundary between 94øW and 88øW but did landward to the southerntip of the Nicoya Peninsula.How- not extend the identificationof anomaliesand the triple junc- ever, sinceno seafloor-spreadinganomalies could be identified tion trace eastward to the MAT. in the area off Costa Rica, the location of the boundarywas The bathymetricrough-smooth boundary has been widely defined from bathymetricobservations [Hey, 1977]. In addi- usedby different authorsas the trace of the triple junction of tion, Hey [1977] pointed out that in the older part of the EPR and CNS off Costa Rica and inferred as a major lithos- CNS-derived CocosPlate crust the magneticand bathymetric phericfeature that explainspatterns in the configurationof the BARCKHAUSEN ET AL.: COCOS PLATE TECTONIC BOUNDARIES 19,209 subductingslab along the continentalmargin [e.g.,Pratti et al., following terms for the boundarybetween the EPR- and CNS 1995a;Marshall et al., 2000;Patina et al., 2000]. The boundary derived provinces:The younger part of the boundarywhich between the seamount domain and the smooth domain is formed after the breakup of the Farallon Plate at the Ridge- markedby a tectonicboundary that 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 boundaryalong the fracture zone ment and the baseof the crust [vanHuene et 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-smoothboundary associated with a the EPR- and the CNS-derived magnetic anomalies, and tectonic scarp and the trace of the triple junction between crustalages are 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 progressionalong the CNS side and increasingages along 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- [vanHuene et al., 1992], SO-107 [Mrazeket al., 1996], Revelle cordantcontact along a line that parallelsthe N70øEdirection. deliverycruise (R. Knox, unpublishedreport, 1996), SO-144/1 This is likely to be the result of a ridge jump breakingthrough [Bialaset al., 1999],SO-144/3 [Werner et al., 2000],and BGR-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 significantchange of the strike direc- cessedwell enoughto reducethe RMS crossovererror to <10 tion was discussedby Hey [1977] and is also proposedin a nT [Barckhausenet al., 1998]. The remainingdata were aver- model by Meschedeet al. [1998]. Justsouth of the discordance aged to produce along-trackvalues at 20 s intervals.The av- the N70øE striking anomaliesseem to be offset along a mor- eragingprocess included a procedure to eliminate spurious phologicaland tectonicfeature calledFisher Ridge [vanHuene values.Positions were correctedfor the distancefrom ship to et al., 2000] (Figure 2). The point where the two supposed sensor,and anomalieswere calculatedby subtractingthe geo- tectoniclines meet off the southerntip of the Nicoya Peninsula magneticreference field IGRF 95 [InternationalAssociation of coincideswith the prominent Fisher Seamount(Figure 2). Geamagnetismand Aeranomie(/AGA), 1996]. To correctfor The area of mapped magneticanomalies off Central Amer- magneticdaily variations arising from ionosphericcurrents, we ica has more than doubled since the SO-76 data were collected. digitizedanalogue magnetograms obtained from the geomag- Although the anomalypatterns have become much clearer, the netic observatoryTilaran, Costa Rica, which is located at a map is still not extensiveenough to clearly identify seafloor- distancebetween 50 and 350 km from the surveyedprofiles. spreadinganomalies. In order to decipher the complicated The scalarfield was calculated(AF = AZ sin I + AH cosI plate tectonicconfiguration it is instructiveto look at the mag- for smalldeclinations) and subtractedfrom the data. Sincethe netic anomaliesat a larger scale. sensorshad been towed at distancesexceeding three ship's lengthsfrom the researchvessels in all cases,corrections of headingeffects were not necessary.After processing,the new 4. Regional Framework data also have low crossover errors like the SO-76 data. The Seafloor-spreadinganomalies are reliably identified only in zero levelsof all data setswere adjustedto that of the SO-76 part of the CNS area. Between 96øW and 84øW, CNS-derived data at crossovers,and the data files were then merged. anomaliesfrom recent to 4A (0-10 Ma accordingto the mag- The resultingtotal field magneticanomalies (Plate 1) show netic polarity timescaleof Cande and Kent [1995]; this time- three zoneswith different magneticanomaly patterns: (1) In scaleis used throughoutthis paper) have been mapped thor- the northwestoffshore Nicaragua, 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 anoma- anomaliesgenerally parallel the MAT inasmuchas the pattern lies 5A through6A (12-21 Ma) have been identifiedbetween is observable.The high-amplitudeanomalies in 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, clearly defined than 4A which must be present in this gap have proven ex- linear anomalies trend N50øE and extend from the ocean basin tremelydifficult to correlate[Hey, 1977;Wilson and Hey, 1995]. landwardto the end of the profiles.(3) In the southeastern In order to fill as much as possibleof this area with reliable part, linear anomalieswith significantlystronger amplitudes magneticanomaly identifications we first extendedthe corre- strike N70øE. Superimposedare local anomaliescaused by lation of EPR-derived anomaliesalong a stripe north of the seamounts.In the area of Cocos Ridge 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, profilesincluding two profilesfrom our new data (Figure 3a). 1996] it seemsobvious that the oceaniccrust in the northwest- This more completeknowledge of crustalages along the triple ern part of the surveyarea wasformed at the EPR, whereasthe junction trace alsoprovides age controlfor CNS-derivedmag- two SW-NE trending anomalypatterns must 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 junc- separating 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 re- With the knowledgeof the position and age progressionof searchersin the area. Becausethe rough-smoothmorpholog- the triple junction trace on the northern side and the position ical proxy breaks down near the continent,we will use the of anomaly4 or 4A on the southernside it was possibleto 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 Sonnecruises 76, 107, and 144 and R/V Revelletransit. Tracks of multichannelseismic reflectiondata in Figures6a, 6b (thick segmentalong track line 7a), 6c, and 6d are shown.The scarpof the narrow ridge marks the boundarybetween the lithosphereformed at the East Pacific Rise and the Cocos- Nazca spreadingcenter. This boundaryof the oceanplate correspondsto a small landslidein the lower slope and is alsoimaged on trackline 7a as a stepin the plate boundarybeneath the slope(see Figure 6b). The ridge jump and the propagatorare explainedin the text. major parts of the gap betweenthe studiesof Wilsonand Hey 2). We found half-spreadingrates increasing from 20 mm/yr at [1995] and Wilson[1996]. We selectedseveral single magnetic 95øWto 30 mrn/yrat 90øW(Figure 3b). Thesespreading rates are profiles heading approximatelyparallel to the tectonic flow higher than those Wilsonand Hey [1995] found for the time lines from data availablefrom the National GeophysicalData period 5.23 Ma to 10 Ma (14 mm/yr at 95øWand 21 mrn•r at Center(NGDC) [1998] data compilationand from the Scripps 90øW),but the increasetoward the eastmatches the findingsin Institutionof Oceanography'sdatabase (2000). Researchves- the youngeranomalies very well. With the existingdata we have selspassing the surveyarea at different times have taken the not beenable to map propagatorsand transformfaults in detail. profiles,and the errors along them are not well documented. East of 89øW, reconstruction is difficult for two reasons. We examinedall data for spuriousvalues and subtractedthe First, the irregular pattern of magnetic anomalies associated mean value from each profile. Having two pointswith known with Cocos Ridge interrupts anomaliesin the south and the age on each of these profiles, it was possibleto identify the triple junctiontrace in the north, obstructingcorrelation of the seafloor-spreadinganomalies between 4A and the triple junc- seafloor-spreadinganomalies. Second, east of 89øW the dis- tion trace in the area between 96øW and --•89øW. The anomalies tances between the oldest identified anomalies south of the trend N70øE,like thosemapped off southernCosta Rica (Plate CocosRidge and the triple junction trace are muchlarger than BARCKHAUSEN ET AL.: COCOS PLATE TECTONIC BOUNDARIES 19,211

o o o I-. o o o o • o o

z z z z

rO ,0

""" 0 •N

z z Z Z Z 19,212 BARCKHAUSEN ET AL.: COCOS PLATE TECTONIC BOUNDARIES

5 5A 5B 5C 5D 5E 6 6A1 6A2 6AA1 6B 6C ,•1-• -•14 ß 96 mm/yr 75 mm/yr 65 mm/yr

(b) 5B 5AB 5A 5 4A (C) 6 5E 5D 5C 5B (d)

13 I 1 //+----Ridge Jump FZT20• 18....•

TJT •,A• gator

6B1 6AA 6AA1

Figure 3. (a) Correlationof magneticanomaly profiles taken north of the triple junctiontrace/fracture zone tracewith a syntheticprofile calculatedfrom a 500 m thick sourcelayer at 4000 m water depth.Spreading rates for ages<20 Ma are from Wilson[1996]. For location of the profiles,see Figure 3f; MAT, Middle America Trench.(b) Correlationof magneticanomaly profiles taken north of the CNS with syntheticprofiles calculated 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 profilessee Figure 3f. The spreadingrate is 35 mm/yr;RJ, ridge jump. (d) Correlationof magneticanomaly profiles taken off the southernhalf of the NicoyaPeninsula with a syntheticprofile calculatedfrom a 500 m thick sourcelayer at 3500 m water depth. For location of the profilessee Figure 3f; FZT, fracturezone trace.These magneticanomalies record the onsetof the opening of the CNS. On the CocosPlate they are only preservedin a small area off Costa Rica. (e) Correlationof magneticanomaly profiles taken west of the GalapagosIslands with syntheticprofiles calculatedfrom a 500-m-thicksource layer 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 bottom of the figure (cf. Plate 2). (f) Locationmap of the profilesmodeled in Figures3a-3d. All profilesare shownas wiggletraces in Plate 2. spreadingrates found farther west. Either the spreadingrate anomaly6B, and that instead,the boundarybetween EPR- and wasmuch higher (close to 60 mm/yrhalf-spreading rate) in this CNS-derived anomaliesparallels anomaly 6B1 off northern area or anothertectonic system prevailed. A likely candidateis Costa Rica. Therefore, for this part of the boundarythe term a significantridge jump at -14.5 Ma proposedin a model by "fracture zone trace" applies as it is the discontinuitythat Meschedeet al. [1998]. Despite these problems it was still representsthe initial Farallon Plate breakup. possibleto tie magneticanomalies at the triple junction trace In order to completethe picture the proposedidentifications with known ages.We were able to correlateanomalies 5B and of old CNS-derivedseafloor-spreading anomalies on the Cocos older in the area east of 89øW at spreadingrates of 35 mm/yr, Plate mustbe comparedto thoseon the Nazca Plate southof without an increaseof spreadingrates eastward(Figure 3c). the Carnegie Ridge. In this area, Hey [1977] identified sea- The oldestof the N70øE strikinganomalies is anomaly6. This floor-spreadinganomalies paralleling the northernflank of the anomalycan be traced into the mapped area off Costa Rica, Grijalva Scarpwhich separatesoceanic lithosphere formed at where it is in contactwith the N50øE strikinganomalies, thus the EPR and at the CNS on the Nazca Plate (Figure 1). He datingthe inferred ridgejump at -19.5 Ma. The N50øE strik- suggestedthose anomaliesmight be remnants of the initial ing anomaliescan easily be correlatedwith anomalies6AA openingof the CNS alonga preexistingfracture zone. As a test through6B1 at a half-spreadingrate of 50 mm/yr (Figure 3d). of this interpretation,Hey [1977] proposedthat a boundary We found no evidencefor anomaliesolder than 6B1 generated counterpartto the Grijalva scarpmust be found east of 88øW at the CNS. During cruiseBGR-99 one magneticprofile head- on the CocosPlate off Costa Rica. We reexaminedthe mag- ing parallel to the N50øE striking anomalieswas taken just netic anomaliesnorth of the Grijalva Scarp.Our interpretation north of the inferred position of the triple junction trace/ differs only slightlyfrom that given by Lonsdaleand Klitgord fracture zone trace. This profile clearly showsEPR-derived [1978],who correlatedthe oldest CNS anomalyjust north of magneticanomalies 6B and older (profile 7 in Figure3a). This the Grijalva Scarpwith anomaly6B. This matchesexactly the provesthat there was no triple junction configurationprior to oldest of the N50øE striking anomalies off Costa Rica. We BARCKHAUSEN ET AL.: COCOS PLATE TECTONIC BOUNDARIES 19,213

(e) 5A 5AA 5AC 5B

(t)

96'W 94"W 92'W 90'W 88'W 86'W 84'W 82 'W 12'N 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 However, the distance between anomalies 2A and 3 north of mirror image of those north of the Grijalva Scarp and thus the ridge and 5C south of it is clearly too small to allow a representthe remainingrecord of the initial openingof the continuousaccretion of crust between them. Hey [1977] had CNS on the Cocos Plate, located exactly where Hey [1977] alreadynoted that the highlyasymmetric accretion of oceanic suspectedthem to be. In agreementwith Lonsdaleand Klitgord lithospherealong the easternpart of the CNS cannotbe ex- [1978] we find that anomalies6B1 through 6A1 parallel the plainedby asymmetricspreading rates and discussedthe pos- Grijalva Scarp strikingN65øE at a half-spreadingrate of 45 sibilityof a seriesof southwardridge jumps. Wilsonand Hey mm/yr. Anomalies6 and youngertrend ---E-W and can be [1995]confirmed this explanationfor ages<10 Ma. It is likely correlatednorthward up to anomaly5C at a half-spreading that similarridge jumps occurred earlier. The questionis, was rate of ---40 mm/yr. This configurationleaves a blank wedge- there a seriesof small ridge jumps or one major ridge jump shapedpiece of crustbetween the N65øEstriking anomaly 6A1 betweenanomalies 5B and 5. The latter was proposedin the and the E-W strikinganomaly 6. Lonsdaleand Klitgord[1978] model by Meschedeet al. [1998]. Sincewe found nearly sym- called this a "region of rise jumps." Even though magnetic metricspreading rates in the time intervalbetween anomaly 6 anomaliesin this region are not clear without any new mag- and 5C on both sides of the CNS and no indications of small netic profilessince 1978, we suggestthat the wedge-shaped ridgejumps, we speculatethat a ridgejump similarto the one pieceof crustrepresents the missingpart of the N50øEstriking at 19.5 Ma occurred shortly after anomaly 5B at ---14.5 Ma. anomaliesoff CostaRica. The abandonedspreading axis that However, since these discordancesare buried under the Cocos has been transferredto the Nazca Plate by the ridge jump at 19.5 Ma discussed earlier would be included in that area. and CarnegieRidges, there is little evidenceto definitively West of the Galapagos Islands, CNS-derived magnetic answerthis question. anomalies5A through5B with a half-spreadingrate of ---35 Assuminga major ridgejump at 14.5 Ma in addition_tothe mm/yr (Figure 3e) were identifiedin somerecently acquired confirmedjump at 19.5 Ma splitsup the spreadinghistory of magneticprofiles. These anomaliesare the undisturbedcon- the CNS into three stageswhich have been termed CNS-1 tinuationof the youngeranomalies identified by Wilsonand (22.7-19.5 Ma), CNS-2 (19.5-14.5 Ma), and CNS-3 (14.5- Hey [1995], similar to those north of the CNS and west of recent)by Meschede et al. [1998].Since these terms have been 89øW.East of the GalapagosIslands the CarnegieRidge pro- usedalready by others[e.g., yon Huene et al., 2000], we con- hibits correlation of magnetic anomaliesyounger than 5C tinue usingthem here. Figure 4 is a schematicsketch that northwardinto the area mappedby Wilsonand Hey [1995]. summarizesthe three-stageevolution of the CNS. 19,214 BARCKHAUSEN ET AL.: COCOS PLATE TECTONIC BOUNDARIES

96'W 94øW 92'W 90'W 88'W 86'W 84'W 82'W 12øN !2'N 6B 500nT ßTdpteFracture JunctionTrace ZoneTrace RidgeJump Propagator 10'N 10'N i

8øN 8øN

6'N 6'N \\

4'N 4øN

2'N 2øN

ø I

o •

2'S 2'S

5D 5D 5E--

GalapagosIslands 4'S 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; ScrippsInstitution of Oceanographydatabase (2000); this study]in the areaof the CNS between96øW and 82øW shown as wiggles with positiveanomalies shaded above the trackline. Profileswith grayshaded anomalies are from the NGDC and Scrippssources; profiles with solid shadedanomalies are new data compiledhere. The interpretationof youngeranomalies at the centerof the map is from Wilsonand Hey [1995]; see legend in the lower left corner. Green dashedlines indicate seafloor-spreadinganomalies formed at the EPR. Red dashedlines mark magneticanomalies derived from CNS-1. Blue dashedlines indicatemagnetic anomalies formed at the CNS-2 after a ridge jump at 19.5 Ma. EPR-derivedmagnetic anomalies 6B through10 southof the Grijalva Scarpafter B. Eakins (unpublished data,2000). The interpretedmagnetic anomalies, paleo plate boundaries, ridge jumps, and propagators reveal a complicatedplate tectonichistory. BARCKHAUSEN ET AL.: COCOS PLATE TECTONIC BOUNDARIES 19,215

are also topographicfeatures observed in multibeam bathymetry and crustal structurein seismicsections: 1. The NW flank of the fracture zone where the CNS ini- tially openedcorresponds to a narrow ridge acrossthe oceanic plate (Figure 2). A seismicreflection image acrossthe fracture zone trace indicates lateral continuity even where sediment buriesthe ridge. The ridge is the culminationof a ---5 km wide tilted basementblock with a smoothupper surface (Figure 6a). 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 litho- sphericboundary shows only a minor changein crustal struc- 5. Discussion ture. Weak reflectionsfrom the crust-mantleboundary appear at about the same two-way time on either side of the seafloor 5.1. Crustal Ages and Tectonic Boundaries off Costa Rica scarp,and only a smallchange in lower crustreflections occurs Our revised magnetic anomaly map provides crustal ages acrossthe boundary(Figure 6a). The smoothupper surfaceof and definesthree tectonicboundaries off Costa Rica (Figure the oceanicigneous crust acrossthe boundaryindicates that 5). (1) The •80 km long fracturezone trace separatingEPR during opening, magmatismand deformation rapidly nucle- crustfrom CNS crustis orthogonalto the MAT off the central ated into the new spreadingcenter and that duringbreakup the NicoyaPeninsula (Plate 2 and Figure 5). (2) The ridge jump Farallon crust experiencedlittle vertical displacement.The from CNS-1 to CNS-2 resultsin a tectonicboundary parallel- ridge appearsto continue acrossthe trench beneath the con- ing anomaly 6. This boundarymarks an age jump of 1.9 m.y. tinental slopeand coincideswith seafloorinstability (Figure 2). (chron 6n to 6A2r) at the MAT. (3) A •35 km offset of A seismicreflection strike line acrossthe slopeand parallel to magnetic anomalies occurs along a structure that strikes the trenchshows an offsetat the plate boundary,indicating the obliqueto the ridgejump (Figure5). The obliquetrend of this subductedextension of the ridge (Figure 6b). The plate bound- structureand the lateral displacementof anomaliesindicate a ary reflections most likely come from subducted sediment 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 tectonicboundaries mapped with magneticdata to estimatethe dimensionsof the ridge. Although the expres-

88'W 87'W 86øW 85'W 84øW 83'W 12'N 12'N ..... TripleJunction Trace ...... Fracture Zone Trace ------,-,- Ridge Jump Arc volcanoes .... Propagator offset 11'N 11'N

DSDP 84 ODP 170

10'N 10'N

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 agesderived from identificationof seafloor- spreadinganomalies. Numbers indicate crustal ages in m.y. Tectonicboundaries and the locationsof the Deep Sea Drilling ProjectLeg 84 and Ocean Drilling ProgramLeg 170 drill holesare indicated.Triangles show the locationsof arc volcanoes;FS, Fisher Seamount;OSC, OuesadaSharp Contortion.The 100 km isobathof the Wadati-Benioffzone is added [after Protti et al., 1995a]. 19,216 BARCKHAUSEN ET AL.: COCOS PLATE TECTONIC BOUNDARIES

(a) NWkm 5 sill 10 top15igneous crust 20 Fracture25Zone Trace• 30 top igneous 35 crust 40 45 50 SW

......

(b) SE 2• km 20 lS •0 s NW 2' ' ' • .... t ...... • ..... t ...... I ...... • • , , ' 2 PoststackTime MigrationSonne 81 Line 7a 3 slopesed'• 3

•i.boundaryPlate

, ,

Figure 6. Poststackfinite differencestime migrationsof multichannelseismic reflection lines acrossseveral tectonicboundaries of the CocosPlate. For location,see Figure 2. (a) Line BGR99-45 acrossthe fracturezone where the initial opening of the Cocos-Nazcaspreading center took place. The crust-mantleboundary is definedby somefaint reflectionsat ---7 s two-waytime. Lower crustalreflectivity is also observed.(b) Line Sonne-81-7a.The profile runs acrossthe middle slopeoffshore Nicoya Peninsula.The seismicrecord shows the plate boundaryas a band of low-frequencyreflections occurring at 6 to 5.5 s two-waytime. The boundary betweenthe lithosphereformed at the GalapagosSpreading Center and the East PacificRise is observedas an offset in the plate boundarytopography. (c) Line Sonne-81-10.The seismicrecord imagesthe structure acrossthe Fisher Ridge and the trace of a ridge jump. Recent tectonicactivity has uplifted and folded the sedimentarycover and top basementnorthwest of Fisher Ridge. The fold can also be observedin the bathymetryof Figure2 and is coincidentwith the ridgejump mappedwith magneticdata (Plate 1), indicating a reactivationof this boundaryduring subduction processes. Note the onlap of sedimentstrata on the Fisher Ridge,indicating that the topographywas created before the depositof mostsediment. (d) Line Sonne-81-20. This line displaysthe lateral continuityof structuresdescribed in Figure 6c. It also displaysthe onlap of sediment strata on a seamount. sion of the boundarybetween EPR and CNS lithosphereob- 2. The ridgejump is coincidentwith a broadfold producing served at the Middle America Trench is a subduedfeature, the a gentle seafloorridge (Figure 2). Seismicreflection profiles subductedextension of the boundarymight have been more acrossthe ridge jump image folded sedimentand associated pronounced.Assuming spreading rates like thosemeasured for small-scalefaulting (Figures6c and 6d). The igneouscrust is the oldest EPR crust off northern Costa Rica and south of the slightlythicker to the south(e.g., line 10, Figure6c). No onlap Grijalva Scarpimplies that about 1500 km and 900 km of the of strata on the basementis observed,whereas other gently boundaryhave been subductedduring the last 22.7 m.y. be- dippingbasement features show strata onlap (e.g., km 10-15 in neath Central America and northern South America, respec- Figure 6d). The upper half of the sedimentsection is hemipe- tively.Thus therewas an agejump of at least20 m.y. acrossthe lagic [Kimuraet al., 1997],and therefore the distributionof the oldest portion of the fracture zone where the CNS formed. depositsis partially controlledby the topographyof the ocean Rejuvenationof a 20 m.y. old lithosphereduring the opening basement.For instance,the flat basementtop betweenFisher and formation of the CNS probably created a topographic Ridge and the seamountto the south(Figure 6d) has a thin feature more prominent than that currentlybeing subducted, sedimentcover, indicatingsome control from currentsduring and the effect of the collision of such a feature with the con- sedimentdeposit. The folding and the seafloorridge and the tinent might have involvedimportant tectonism. faulting and lack of onlap indicaterecent tectonism,perhaps BARCKHAUSEN ET AL.: COCOS PLATE TECTONIC BOUNDARIES 19,217

(c) NW 5 km 10 15 20 25 $0 35 40 45 50 SE ...... I ...... I . , , . t .... I , . . , ,. ? . .t ...... , I. . , ..... ! ...... I ..... ! . . ,:•: .: t . . . PoststackTime MigrationSonno 81 Line 10

.. Ridge Jump Fisher••Ridg•.•;;••.•.. e qnlap sed}menttop igneous ' crust fold sediment...... •.•.••.•.-- •!'i'.•...... -':•'•.•. .. . E•...••....-••:4• ..

• CMP •04• 10800 11200 11600 12000

(d) N 80km 7• 70 65 60 55 50 45 40 35 30 25 20 15 10 5 S PoststackTime MigrationSonne 81 Line 20 PropagatorTrace . - Ridge Jump fold sediment Fisher Ridge onlap E 4. topigneous crust • -.• \ onlap ..... ,/ ...... • •-..,•.. . :..-:-•.-..•-:-;::.-•..•r..--• -. . -- ... ., :3•-.• .... -....

*' ;•r.--- ' ' ' •" ':-•'-'- ': -"•"••••'••••••••••"• •..'.:..• ....:...... :. :....-..- -.•.-- ...:'-;.•' -••••••-•.•-- ....••••••'•••• '.... multiple ' '"""''•.'-•i'"':•';':'•' •'•...... '•'•"•"•- '• "'i ..... '" ' •' '•" 1" ''' ' ' .... ' ..... ! '"•...... '"' ' ' I "• • ...... '" ' 13200 128O0 •2• 120• 11600 1t2• t08• 1•0 CMP

Figure 6. (continued) created by reactivation along the structure during flexural probablyformed later than the propagatorduring Miocene bendingof the subductingcrust into the trench axis. volcanism.Seismic images show down-to-the-NW displace- 3. The propagatoris associatedwith the ridgeextending SW ment of all reflectivehorizons across the ridge (Figures6c and of FisherSeamount (Figure 7), calledFisher Ridge. This ridge 6d). Rock from FisherRidge is 19.2m.y. old, equivalentin age

86øW 85'W

I•:••, :%, •:•'"':•/•.• •>...... :.•.:•:•... ' .... .'"'":'...... •:'•. ! ;':"' /:•.•..•.;..-...'t..,•(•:/.'; ,.::," :".?...... :..... •--'.•: 10'N!=a: •',,• .. ',::•:-'•&."/...... •'•.>V•;....".,'"'•j• .'•':.:::;/'.,-* '-•...... :"•v...... : ::'- .... L'.-. •-.. '•...... ---:,:.:.: ...... :••"'-':. "!10'N ':'•::..':•-'• ...... :,--'•,5. 4•' ?'"•'"•'•,.•: :X:'•...... ':'...•/ •. -•:"ß •.?•.....,•:• ½•'"'•.;.•••:' ..... ,.;•' ".:• -<-½;•--•t:•-- :•'..F•.•):.•:-,..•.:.•:•.:•,.,,_•;N•• ;:.••::..:.,.':4::• •"D:.:"...... :....- '...... ½•:.•...... -- '-" ';':•'":..... ' ., ".•'7'.-'•.... . • ...... h."?."• • •; ..... ) :..

-. -t••:•:::'• -•-".:•-..:...... :'---• 'r •. ':•-•••.:-,-•::::. • '--•, .,.•. .•.• •.•-4:: .•zt..••.:.•:-..•. •...... •:;•':.•..... --t•...... •-,'.:•.:• •:•1::.•"•.....•:•.'• .....?...... •?•':•;-..-.•.:•, --•...... ":•-..'•.E.•'-.•. --•. • •:•'•'..::-•':- --'• -:.....?•-.'...'-•-"': -' .- -'•"-..... ,•::•'•.... •:•,E":--.;•:•..-.:•t...• •-•...... •.---...... '•'; .''•',.' .'-, .... ":.•-:•'•;¾::-, t •::f•.:•:,•H•"• •-•?" ...... • : .' •"•Z•-..• ':.•:'-: •:• ",•,,-:•.•-'--• •.• •:'• t. :•.... :•:.:• "•T...... :•:::: :;•,. . .• •:•.;:•;f."•"'-•.•""• •::•:g%;.:•-::• •'•:•.-..-.:.-:•':' .:..:'"'-'.•' '::•:.:: '--? 'E•':•=•'•-•:". '.• :::' -.. .•:•;-.:...•,"•$.• •:-•;•:•;.;-•:•::.....•:•.½,-,.•' ...... •"•:':.•...,.-, .....'•'•,, • .... .,- •,.•:.. -.•,..:•...... :•'•.:-., '•/• .•. ••'• ,-•::. ....• •;•.•:'•-•;•. • '>•:•':'* ;:;:• •'•:••-•2• ...... •:•.•:y:•, .•:•' ...•..?. -.•.

A•:A•'•-•.'-<'• •,.:• •;•.....';•'•'. •' ' -.•:'. • -'f:••.•-•.:•... •';•.:•.. . ':::t;•:• •- • .':. ----•-•.';• •. • ...... •-::•.-•:• •

...... - - :'fi

86'W 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 tectonicboundaries parallels the 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 BOUNDARIES to the adjacentmagnetic anomalies, and hasa mid-oceanridge short section(-80 km) of this isochronremains unsubducted basalt geochemistry[Werner et al., 1999]. Conversely,Fisher (Plate 2; anomaliesare markedwith red dashedlines), where Seamountis -14 m.y. old ocean islandbasalt. Thus the ridge it couldonly be detectedwith high-resolutionmagnetic map- involvesruptured oceancrust and extrusionof lava. Strata on ping (Plate 1). The oldest CNS magneticanomalies on the either side of Fisher Ridge and a seamountto the southhave Nazca Plate are poorly resolvedin the critical area between similar patterns of sedimentonlap, indicating a similar age 89øW and 86øW becausevery few magneticprofiles lacking (Figures 6c and 6d; km 10-20). The ridge forms the north- satellitenavigation were acquired.However, detailed mapping westernboundary of an area flanking CocosRidge containing of the mirror imageof thoseanomalies off CostaRica provides many seamountslike Fisher Seamount(Figure 2) with a Ga- the key to reconstructingthe early spreadinghistory of the lapagoshot spot geochemistry[Werner et al., 1999]. Fisher CNS. Spreadingalong CNS (CNS-1) began at 22.7 Ma, at a Ridge, Fisher Seamount,and the propagatorare alignedwith rate of -95 mm/yr and with an almostsymmetric accretion of subductedseamounts extending landward under the slopeand newlithosphere to both sidesof the rise(Figure 4). At 19.5Ma shelfto beneaththe coast(Figure 2) [yonHuene et al., 2000]. the ridgejumped south, changing its strike direction by -22 øto These findingsindicate that the fracture associatedwith the nearlyE-W (Plate 2; anomaliesare markedwith blue dashed propagator probably continuesacross the ridge jump from lines). On the CocosPlate, only the westernmostpart of the CNS-1 to CNS-2 and into already subductedCNS-1 litho- area where the CNS-2 broke throughthe old pattern remains sphere.The currenttopography of the Fisher Ridge probably unsubducted.The spreadingrate at the CNS-2 decreasedto formed as -5.5 m.y. old lithospherewas thermally thinned and -75 mm/yr and continuedto be almostsymmetric. The mag- domed during hot spot activity.It broke alongthe propagator, netic anomaliesyounger than C6 have higher amplitudesand whichprobably focused magma extrusion at FisherSeamount and are lessregular in shapethan the older anomalies,indicating related subductedseamounts in the chain.Vertical displacement an overall changein spreadingconditions. acrossFisher Ridge is the mostprominent of all three tectonic It is not clear exactly how long spreadingat the CNS-2 boundaries(Figures 6c and6d) and mayexplain the moreprom- continued.As explainedearlier, because of the overprintingof inent failureof the continentalmargin above its subductedexten- the originalpattern by the Cocosand CarnegieRidges, major sion.However, the propagatorwas inactive at the timethis part of gapsremain in the identificationof anomaliesbetween 4A and the CocosPlate moved over the Galapagoshot spot(14 Ma) due 5C on both sidesof the CNS. However, it is clear that during to an earlierridge jump to the south(CNS-2 to CNS-3).A plau- this period the accretionof crustwas very asymmetrical[Hey, siveexplanation is that the propagatorfracture was reactivated by 1977]. In a previousmodel [Meschedeet al., 1998] the aban- the hot spot event and extendedinto older lithosphere,where doned CNS-2 spreadingcenter is presumedto coincidewith a magmatismfocused and formed the seamountchain. The land- N70øEstriking bathymetric feature and gravitylow that can be ward projectionof FisherRidge developsinto the QuesadaSharp tracedfrom a positionat 6øN,88øW into the CocosRidge. The Contortion,a slabtear at a depthof 70 km beneathcentral Costa criticaltest of this model would be to find magneticanomalies Rica [Prottiet al., 1995a].Above it the volcanicarc is left-laterally mirrored along this line. Unfortunately, a reliable correlation offsetacross a gap (Figure5). of anomalieson the northern flank of CocosRidge is difficult The fracture zone trace and ridge jump are not alignedwith evenwith data from cruisesSO-144/1 and SO-144/3.We pre- suchprominent structuresin the continentalcrust as the prop- sumethat the originalpattern of seafloor-spreadinganomalies agator.This maybe due in part to smallercrustal displacement in this area was heavilyoverprinted during formation of Cocos and orientationwith respectto the plate convergencevector. Ridge.If the modelof Meschedeet al. [1998]were correct,then The collisionpoint of the ridgejump migratedalong the trench extrapolationfrom anomaly5B southwardprovides a crustal becauseits strike differs --•25ø from the convergencevector. It age of -14.5 m.y. alongthe abandonedspreading axis. Given migrated-90 km to the northwestalong the trenchduring the that anomaly5C is preservedon the Nazca Plate, the ridge past 2 m.y. (Figure 7). The propagatorparallels the conver- jump from CNS-2 to CNS-3 must have transferredonly -•60 gencevector and thushas subductedat the sameposition with km of crust (formed between16.0 Ma and 14.5 Ma at a half- respectto the continentfor sometime. The anglebetween the spreadingrate of 40 mm/yr to the southernside of the CNS-2) fracture zone trace and the plate convergencevector is -12 ø, from the Nazca to the CocosPlate. If the ridgejump occurred resultingin migrationto the northwestat -20 km/m.y. Migra- later than 14.5 Ma, the amount of crusttransferred may have tion can explain the lack of prominent structurein the conti- been larger. It seemsimpossible at the moment to resolvethe nental plate abovethe fracturezone trace and the ridgejump. questionsraised above. Therefore we suggestthat at -• 14.5 Ma However, the propagator subductedin the same area for at a seriesof intermediateand small ridge jumps began which least 1-2 m.y., therebyaffecting margin structure significantly. continuetoday [Wilsonand Hey, 1995] and that this produces A major offset and divisionof the volcanicarc betweenNica- the asymmetricalaccretion of lithosphereobserved along the raguaand CostaRica (Figure 5) [Can'and Stoiber,1990] have easternpart of the CNS. no counterpartin the CocosPlate and mustbe associatedwith On the basis of the identification of anomaly 6C in the an older continentalcrustal structure [von Huene et al., 2000]. northeasternPanama Basin,Lonsdale and Klitgord[1978] in- terpreted an older age for the initiation of Cocos-Nazca 5.2. Implications for CNS History spreadingthan the 22.7 Ma givenhere. We reinvestigatedthe The southern side of the CNS fracture zone trace coincides magnetic anomaliesin the area south of Panama with a few with the GrijalvaScarp (Plate 2). Hey [1977]stated that it is "an magneticprofiles from the 1980sand one profile from cruise isochron that marks the time of origin of the Cocos Nazca SO-144 (Figure 8). We find that the seafloor-spreadinganom- spreadingcenter and the rifting apart of the Farallon Plate to aliescan be reasonablycorrelated with anomalies6B1 through form the Cocos and Nazca plates." We confirmedits age at 6A1 (22.7-20.5Ma) at a half-spreadingrate of 50 mm/yr,which 22.7 Ma (chron 6B1) and found its mirror imageon the north- is in agreementwith the findingsoff Costa Rica. Hence we ern side of the CNS off Costa Rica. Off Costa Rica, only a 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 constantdirection over the last 23 m.y., there is evidencethat the situation,in general,has not changedduring this time period [Mammerickxand Klitgord,1982; Mayes et al., 1990].The identifiedridge jumps and 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 asymmetriccrustal accretion near hot spotsin the SouthAtlantic, Indian Ocean, and southeasternPacific. They concludedthat randomlyoccurring small-scale ridge jumps can be biased by a nearby hot spot into successiveridge jumps toward the plume. The Galapagoshot spot and the CNS ap- parently alsointeract in this way. The Galapagoshot spot has been activeduring the past 23 m.y. and probablymuch longer [Hauff et al., 1997]. It remains to be seen if the changefrom rare major ridgejumps in the early phaseof 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 boundaries in the Cocos Plate off Costa Rica. (b) 6B16AA 6A1 1. We revised the position of the triple junction trace/ fracture zone trace between EPR- and CNS-derived lithosphere.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 spreadingrates during the early phaseof CNS opening associatedwith the generation of smooth oceaniccrust. 6AA1 6A2 2. The revisedmagnetic anomalies revealed two spreading Figure 8. (a) Magneticanomalies [NGDC, 1998;this study] regimesin the early phaseof CNS openingthat are separated in the northeasternPanama Basin shownas wiggleswith pos- by a ridge jump at 19.5 Ma. The conjugateold CNS-derived itive anomaliesshaded above the track line. Profileswith gray magneticanomalies on the Nazca Plate confirm that a wedge- shadedanomalies are from the NGDC source;the profile with shaped piece of lithospherewas transferred from Cocos to solid shaded anomaliesis from the new data compiled here. Nazca Plate by the ridge jump. The magnetic anomalies are derived from crust that was 3. A prominent topographicfeature on the CocosPlate off formed in the first phaseof the CNS opening(CNS-1). (b) Costa Rica, Fisher Ridge, correspondswith a propagator.We Correlationof the magneticanomaly profiles shown in Figure find indicationsthat this propagatorwas reactivatedduring a 8a with a syntheticprofile calculatedfrom a 500 m thick source phaseof hot spot related volcanicactivity and is again reacti- layer at 3500 m water depth with a half-spreadingrate of 50 vated during the subductionof the CocosPlate. Overprinting mm/yr. of the CocosPlate by Galapagoshot spotvolcanism is a second processthat segmentsCocos Plate off Costa Rica along with the complex plate tectonic history. The tectonic boundaries, 5.3. Possible Influence of the Galapagos Hot Spot seamount chains, and thickened crust of the hot spot trace on Ridge Jumps define segmentsof the lower plate that appear to be related to Formation of Cocosand CarnegieRidges on the Cocosand a similar segmentationof upper plate tectonicsand arc volca- Nazca platesfrom the Galapagoshot spot is widely accepted. nism [yonHuene et al., 2000]. Thus the ridgesmark the azimuthsof plate motion relative to Detailed mapping of the oldest CNS seafloor-spreading the hot spot.In the simplestversion of this model the hot spot anomaliesoff CostaRica and additionalidentifications of mag- activityhas alwaysbeen at the head of both ridges.However, netic anomaliesolder than 10 Ma allow refinement of the Hey the magneticanomalies show that the spreadingcenter repeat- [1977] model for early evolution of the CNS. Breakup of the edlyjumped over considerabledistances. Other indicationsfor Farallon Plate occurredat 22.7 Ma alonga fracture zone strik- a more complicateddevelopment of the submarineridges are ing -65øE. The fracture zone trace separatingEPR and CNS the difference in age between oceanic basement and sea- lithospherescurrently intercepts the continentoff Costa Rica. mountson the flank of CocosRidge off Costa Rica, a depres- After a phaseof rapid and symmetricspreading the spreading sionin the CarnegieRidge between85øW and 87øW,and the centerjumped southat 19.5 Ma and changedits directionby presentdistance of- 170 km betweenthe activespreading axis 22ø . Spreadingremained symmetric at somewhatlower spread- and the center of hot spotrelated volcanicactivity. Today, the ing rates. At -14.5 Ma, another ridge jump to the south ini- Cocos Plate moves in a northeasterlydirection (N40øE), tiated a phaseof frequent smallersouthward ridge jumps that whereas the Nazca Plate moves nearly east [DeMets et al., still continues.Half-spreading rates are symmetricacross the 1990].This impliesthat as long as spreadingis symmetric,the CNS but increase eastward and let the Cocos Plate rotate 19,220 BARCKHAUSEN ET AL.: COCOS PLATE TECTONIC BOUNDARIES counterclockwise.The strongpreference for ridgejumps in the Meschede,M., U. Barckhausen,and H.-U. Worm, Extinct spreading southern direction implies that the Galapagoshot spot is a on the CocosRidge, TerraNova, 10, 211-216, 1998. Mrazek, J., T. Spangenberg,and R. von Huene, Geologischeund drivingforce for frequent ridgejumps and the resultingasym- geophysikalischeUntersuchungen vor CostaRica and Nicaragua-- metric crustal accretion. Beitrfigezum Verstfindnisdes aktiven ostpazifischenKontinental- randes,in FS SonneFahrtbericht S0-107, cruisereport, 172 pp., Ernst-Moritz-ArndtUniv. Greifswald,Germany, 1996. Acknowledgments. This study is based mainly on data obtained Mfiller, R. D., W. R. Roest, and J. Y. Royer, Asymmetricsea-floor duringR/V Sonnecruises SO-76, SO-81, SO-107,and SO-144,funded spreadingcaused by ridge-plumeinteractions, Nature, 396, 455-459, by the BMBF, and cruiseBGR-99. We thank the captains,the crews, 1998. and the scientificand technicalstaff of the participatinginstitutions for National GeophysicalData Center (NGDC), Marine Geophysical their efforts.The GeologicalData Center of the ScrippsInstitution of Data [CD-ROM], Nat. Oceanicand Atmos. Admin., Boulder,Colo., Oceanographyprovided unpublished bathymetric and magneticdata. 1998. Excellent recordsof the magneticvariations were provided by the Patino, L. C., M. J. Carr, and M.D. Feigenson,Local and regional geophysicalobservatory of Tilaran in CostaRica. Severalfigures were variationsin Central Americanlavas controlled by variationin sub- preparedwith GMT publicdomain software [Wessel and Smith,1995]. ductedsediment input, Contrib.Mineral. Petrol., 138, 265-283, 2000. We thank A. Rapalini, D. Wilson,and an anonymousjournal reviewer Protti, M., F. Gfiendel,and K. McNally, Correlationbetween the age for helpfulcomments on this manuscript.The DFG (German Science of the subductingCocos plate and the geometryof the Wadati- Foundation)gave support for a visitingresearch fellowship of U. B. at Benioff zone under Nicaragua and Costa Rica, in Geologicand the ScrippsInstitution of Oceanography. TectonicDevelopment of the CaribbeanPlate Boundaryin Southern CentralAmerica, edited by P. Mann, Spec.Pap. Geol. Soc.Am., 295, 309-343, 1995a. References Protti, M., et al., The March 25, 1990 (Mw = 7.0, Mz• = 6.8), earthquakeat the entranceof the NicoyaGulf, CostaRica: Its prior Barckhausen,U., H. A. Roeser, and R. von Huene, New insightinto activity,foreshocks, aftershocks, and triggered seismicity, J. Geophys. the structureof the Cocosplate offshoreCosta Rica from seamag- Res., 100, 20,345-20,358, 1995b. netic measurements, Zentralbl. Geol. Palaeontol. Teil 1, 385-391, Ranero, C. R., and R. von Huene, Subductionerosion along the 1997. Middle America convergentmargin, Nature, 404, 748-752, 2000. Barckhausen,U., H. A. Roeser,and R. von Huene, Magneticsignature Reichert, C., U. Barckhausen, S. Neben, and C. R. Ranero, Marine of upper plate structuresand subductingseamounts at the conver- geophysicalinvestigations along the active convergentmargin off gent margin off Costa Rica. J. Geophys.Res., 103, 7079-7093, 1998. CostaRica--BGR cruiseBGR-99, cruisereport, Bundesanstaltfiir Bialas, J., E. R. Flfih, G. Bohrmann, and G. Haass, FS Sonne cruise Geowissenschaftenund Rohstoffe,Hannover, Germany,2000. report SO144/1&2, Geomar Rep., 94, 437 pp., GEOMAR, Kiel, Smith,W. H. F., andD. T. Sandwell,Global seafloor topographyfrom Germany, 1999. satellite altimetry and ship depth soundings,Science, 277, 1956- Cande, S.C., and D. V. Kent, Revisedcalibration of the geomagnetic 1962, 1997. polaritytimescale for the Late Cretaceousand Cenozoic,J. Geophys. von Huene, R., E. Flfih, J. Bialas, E. Fabel, J. Hoffmann, and K. Res., 100, 6093-6095, 1995. Emeis, Pakomar 91/92--Fahrtbericht Sonne 76, cruise report, 59 Carr, M. J., and R. E. Stoiber, Volcanism, in The Geologyof North pp., GEOMAR, Kiel, Germany, 1992. America,vol. H, The CaribbeanRegion, edited by G. Dengo and J. E. von Huene, R., et al., Morphotectonicsof the Pacificconvergent mar- Case,pp. 375-391, Geol. Soc. of Am., Boulder, Colo., 1990. gin of Costa Rica, in Geologicand TectonicDevelopment of the DeMets, C., R. G. Gordon, D. F. Argus, and S. Stein, Current plate CaribbeanPlate Boundary in SouthernCentral America, edited by P. motions, Geophys.J. Int., 101,425-478, 1990. Mann, Spec.Pap. Geol. Soc.Am., 295, 291-307, 1995. Fisher, D. M., T. W. Gardner, J. S. Marshall, P. B. Sak, and M. Protti, von Huene, R., C. R. Ranero, W. Weinrebe, and K. Hinz, Quaternary Effect of subductingseafloor roughness on forearc kinematics,Pa- convergentmargin tectonicsof Costa Rica, segmentationof the cific coast,Costa Rica, Geology,26, 467-470, 1998. Cocos Plate, and Central American volcanism, Tectonics,19, 314- Fisher, R. L., Middle America Trench: Topographyand structure, 334, 2000. Geol. Soc. Am. Bull., 72, 703-720, 1961. Werner, R., K. Hoernle, P. van den Bogaard, C. R. Ranero, R. von Hauff, F., K. Hoernle, H.-U. Schmincke, and R. Werner, A mid- Huene, and D. Korich, A drowned14-m.y.-old Galapagos archipel- Cretaceousorigin for the Galapagoshotspot: Volcanological, pet- ago off the coastof Costa Rica: Implicationsfor tectonicand evo- rological, and geochemical evidence from Costa Rican oceanic lutionary models,Geology, 27, 499-502, 1999. crustalsegments, Geol. Rundsch.,86, 141-155, 1997. Werner, R., D. Ackermand,T. Worthington,and ShipboardScientific Hey, R., Tectonic evolution of the Cocos-Nazcaspreading center, Party, Cruise report SONNE 144-3, Ber. Rep. Inst. Geowiss.Univ. Geol. Soc. Am. Bull., 88, 1404-1420, 1977. Kiel 10, 177 pp., Univ. Kiel, Kiel, Germany,2000. Hinz, K., R. von Huene, C. R. Ranero, and PACOMAR Working Wessel,P., and W. H. F. Smith,New versionof the GenericMapping Group, Tectonicstructure of the convergentPacific margin offshore Tools released, Eos Trans.AGU, 76, 329, 1995. Costa Rica from multichannel seismic reflection data, Tectonics,15, Wilson,D. S., Fastestknown spreading on the Miocene Cocos-Pacific 54-66, 1996. plate boundary,Geophys. Res. Lett., 23, 3003-3006, 1996. InternationalAssociation of Geomagnetismand Aeronomy(IAGA), Wilson, D. S., and R. Hey, History of rift propagationand magneti- International geomagneticreference field, 1995 revision,Geophys. zation intensityfor the Cocos-Nazcaspreading center, J. Geophys. J. Int., 125, 318-321, 1996. Res., 100, 10,041-10,056, 1995. Kimura,G., E. Silver,P. Blum,and Shipboard Scientific Party, Pro- ceedingsof the OceanDrilling Program Initial Reports,vol. 170,Ocean U. Barckhausen and H. A. Roeser, Bundesanstalt ffir Geowissen- Drill. Program, College Station,Tex., 1997. schaften und Rohstoffe, P.O. Box 510153, D-30631 Hannover, Ger- Lonsdale,P., and K. D. Klitgord, Structureand tectonichistory of the many.([email protected]; [email protected]) eastern Panama Basin, Geol. $oc. Am. Bull., 89, 981-999, 1978. S.C. Cande, ScrippsInstitution of Oceanography,University of Mammerickx,J., and K. D. Klitgord, Northern East PacificRise: Evo- California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0220, lution from 25 m.y. to the present,J. Geophys.Res., 87, 6751-6759, USA. ([email protected]) 1982. C.R. Ranero and R. von Huene, Geomar Forschungszentrumfiir Marshall, J. S., D. M. Fisher, and T. W. Gardner, Central Costa Rica marine Geowissenschaften,Wischhofstrasse 1-3, D-24148 Kiel, Ger- deformed belt: Kinematicsof diffuse faulting acrossthe western many.([email protected]; [email protected]) Panama block, Tectonics,19, 468-492, 2000. Mayes, C. L., L. A. Lawver, and D. T. Sandwell,Tectonic history and isochronchart of the SouthPacific, J. Geophys.Res., 95, 8543-8567, (ReceivedJuly 20, 2000;revised February 16, 2001; 1990. acceptedApril 21, 2001.)