Download Revised tectonic boundaries in the Cocos Plate off Costa Rica

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."?."• • •; .....
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:..
-. •:•:::'•
.:•...... :'---•.:-,-•::::.
• '--•,
-4:: .•zt..•.•.
•:;•' --t•..........
,'.:•.: •:•1::.•"•
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-t• -•-"
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'r•.':•-••• .,.•.
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:•'•'..::-•':'...'-•-"':
.........
•?•': --•
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•. --•.
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-'..•"-.
....•:•,E"
:--.;•:•..•-•
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......• : .' •"•Z•-..
• ':.•:'-:
•:• ",•,,-:•
.•-'--•
•.• •:'• t. :•....
.''•',.'.'-,.... ":.•-:•
'•;¾::-, t•::f•.:•:,•H•
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:•:.
:•"•T. ......
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:;•,.. .•
'
•:•.;:•;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. Rapalini,D. Wilson,and an anonymous
journal reviewer Protti, M., F. Gfiendel,and K. McNally, Correlationbetweenthe age
for helpfulcommentson thismanuscript.The DFG (GermanScience
of the subductingCocosplate and the geometryof the WadatiFoundation)gavesupportfor a visitingresearchfellowshipof U. B. at
Benioff zone under Nicaragua and Costa Rica, in Geologicand
the ScrippsInstitution of Oceanography.
TectonicDevelopmentof the CaribbeanPlate Boundaryin Southern
CentralAmerica,editedby P. Mann, Spec.Pap. Geol.Soc.Am., 295,
309-343, 1995a.
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[email protected])
S.C. Cande, ScrippsInstitution of Oceanography,Universityof
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(ReceivedJuly20, 2000;revisedFebruary16, 2001;
acceptedApril 21, 2001.)